Page 1

Abstract

Due to manufacturers desire to make lighter and more comfortable personal armour

systems, combined with the availability of higher energy ballistic impacts, non-

penetrating injuries such as BABT and BFS are likely to rise. This project, titled ‘The

Development of a Ballistic Trauma Pack’, investigates the concept whether foam can

be used in trauma attenuation protection equipment that can be either: 1) flexible

without a great deterioration in certain mechanical properties, and 2) improve the

energy absorption characteristics of the foam by preventing fracture without

increasing the plateau stress of the foam.

Results confirmed that the method of modelling the investigation was wrong. Too

many variables entered experimentation, altering the results desired. Flexible trauma

packs were successful in allowing bending to occur within one direction and not the

other, although there were slight deteriorations with the energy absorption properties

with the trauma packs. These were caused by the damaged cell faces, when the

foam was organized into a layer format. These damaged cell faces can also explain

the significant deterioration in strength and energy absorption qualities of the trauma

packs designed to improve them, although a reduction in fracture was also evident

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List of Objectives

• Investigate on the advantages of expanded polystyrene foam and how foam

would work within a trauma pack

• Analysis on expanded polystyrene foam. This would include standard

mechanical testing and includes analysis on a cellular level.

• Quasi-static testing with the EPS foamed material.

• Exploration on how to change mechanical properties of a material without

altering other specific characteristics. Specifically, the improving the fracture

properties of the expanded polystyrene foam, without making a dramatic

fluctuation in the samples density.

• To create a flexible trauma pack without drastic deterioration in strength and

energy absorption properties compared to a monolith sample.

• To create a sample which can absorb as much energy, if not more, than a

monolith sample, and also demonstrate an improvement in shear

characteristics.

• To understand the concept of deformation that occurs within an indentation

test and a ballistic impact test

Acknowledgements

I would like to acknowledge several people for their help, support, resources and

patience, throughout the first semester and helping me complete this report.

My project advisor: Dr Phil Harrison

Technical supervisors: Mr John Davidson, Mr Kaz Piechowiak, Mr Brian Robb and Mr

Alan Campbell

Phd supervisor: Mr Qusai Jebur

Page 3

Table of Contents

Introduction 6

Foams 9

Classification 9

Expanded Polystyrene Foam 10

Manufacture of EPS Foam 11

Characteristics of EPS Foam 12

Further Investigation into Energy Absorption 14

Microstructure of Cellular Materials under Compression 16

Concepts of the Project 17

Objective 1 17

Objective 2 18

Execution 20

Fabric Backing Material 21

Adhesive 21

Foam Characterisation 22

Cell Size 22

Density Variation 25

Inhomogeneous Structure 28

Standard Compression Test 29

Monolith vs. Layered 32

Fast Rate vs. Slow Rate 34

Young’s Modulus 38

Three-point Bend Test 41

Plane-Strain Deformation Impact Tests 44

Test Results & Analysis 53

Fracture 66

Adhesion Failure 68

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Improved Samples 70

Limitations on Experiments 74

Further Analysis 76

Conclusion 77

References 79

List of Figures

Figure 1 Differences between Closed and Opened Cell Structures 9

Figure 2 EPS Foams Beaded Nature 10

Figure 3 Other Applications of EPS Foam 13

Figure 4 Common Response of Cellular Plastics in Uniaxial Deformation 14

Figure 5 Side View of Tiled Trauma Pack Specimen 17

Figure 6 Example on Sample Sides Unable to Deform Compressively 19

Figure 7 Individual Cells that make up the EPS Foam 22

Figure 8 Image Manipulation with image analysis equipment 23

Figure 9 Bead Boundaries of the EPS Foam 24

Figure 10 Preparation for Density Variation Experiment No 1 25

Figure 11 Test Results of Density Variation Experiment No 1 26

Figure 12 Preparation for Density Variation Experiment No 2 26

Figure 13 Test Results of Density Variation Experiment No 2 27

Figure 14 Differences Between Initial Starting Positions 28

Figure 15 Test Results of Inhomogeneous Structure Experiment 28

Figure 16 Force-Displacement Response Curves of EPS Foam 29

Figure 17 Stress-Strain Response Curves of EPS Foam 31

Figure 18 Stress-Strain Response Curves of EPS Foam 32

Figure 19 Typical Behaviour of Foams with Increased Strain Rates 34

Figure 20 Stress-Strain Response Curves of EPS Foam 35

Figure 21 Average % Difference of Slow & Fast Rates of Loading 35

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Figure 22 Stress-Strain Response Curves in the Linear Elastic Regime 38

Figure 23 Young’s Modulus Automated Calculations 39

Figure 24 Results of the 3-Point Bending Tests with Tiled Specimens 42

Figure 25 Illustrations of Different Specimen Types 50

Figure 26 Results of Compressive Plane Strain Deformation Tests 53

Figure 27 Results of Small Indenter Plane Strain Deformation Tests 53

Figure 28 Results of Small Indenter Plane Strain Deformation Tests continued 54

Figure 29 Results of Large Indenter Plane Strain Deformation Tests 54

Figure 30 Foam only Deformation vs. Specimen Deformation 58

Figure 31 Monolith vs. Layered Compressive Plane Strain Deformation Tests 59

Figure 32 Monolith vs. Layered Uniaxial Deformation Tests 59

Figure 33 Results of Small Indenter Plane Strain Deformation Tests 61

Figure 34 Adhesion Failure 62

Figure 35 Monolith under Small Indenter Plane Strain Deformation Tests 66

Figure 36 Fractures Experienced by Double Bonded Sample 67

Figure 37 Results of Peel Testing with Wood Resin Adhesive 68

Figure 38 Results of Peel Testing with other Adhesives 69

Figure 39 Small Indenter Plane Strain Deformation Results 71

Figure 40 Small Indenter Plane Strain Deformation Results 72

Figure 41 Failures of the Double Bonded by Tape Specimen 73

Figure 42 Highlights of Tensile and Shear Deformation 76

List of Tables

Table 1 List of Cell Dimensions 23

Table 2 Standard Compression Test Specifications 29

Table 3 Accuracy between Methods for Calculating Young’s Modulus 40

Table 4 Variations of Loading in Plane Strain Deformation Experiments 44

Table 5 Guidelines for Specimen Parameters 45

Table 6 Sample Dimensions used in Uniaxial Compression Tests 46

Table 7 Sample Dimensions used in Small Indenter Tests 47

Table 8 Sample Dimensions used in Large Indenter Tests 48

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Introduction

Although ballistic armour is able to stop projectiles from penetrating through the

armour, injuries from the process of capturing the projectile can still occur. The rapid

jolting force of an impacting bullet is contrasted with the usually encountered

mechanisms producing blunt trauma injury. Vital organs and areas such as the

heart, lungs, spleen and spinal chord, are all vulnerable to injury, despite the

projectile not perforating the armour, and in extreme circumstances, these injuries

have recorded cases that have resulted in death [1].

These non-penetrating injuries can be caused by two distinct mechanisms; the

deformation of the surface of armour in contact with the body wall and the energy

transfer from the projectile. The depth of depression in the backing material that

results from a non-penetrating projectile is known as the ‘Backface Signature’ or

BFS. The deformation is part of the retardation and energy absorbing process that

captures the projectile. It is also referred to as ‘Backface Deformation’ or ‘Trauma

Signature’. The backface signature is measured from the plane defined by the front

edge of the backing material fixture and can be formally classed as “the greatest

extent of indentation in the backing material caused by a non perforating impact on

the armour” [2].

The energy deposited in the armour by the retarded projectile may be transferred

through the armour backing and body wall. The protective vest may impede the

projectile, but some of the kinetic energy is transferred to the body. It may produce

serious injury to the thoracic and abdominal contents behind the plate. Pressure

waves propagate through the body which can also affect the brain [3], even though

the head is not hit. With very high energy bullet impacts, the internal thoracic injuries

may result in death.

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Currently, there are two types of non penetrating injuries categorized: Behind armour

blunt Trauma and Backface signature. Behind Armour Blunt Trauma (BABT) is

defined as the spectrum of non-penetrating injuries to the torso resulting from the

rapid deformation of projectiles on personal armours, covering the body. The classic

behind armour blunt trauma injuries, historically have consisted of moderate to

severe bruising and rib fractures. These blunt trauma injuries occur when the vest

distributes the impact energy over a large area, causing a global deformation of the

chest without localized deformation. Usually, such injuries are not life-threatening to

the wearers, although lung contusions can also occur.

The backface signature injuries can be described as a more localized injury, when it

is in comparison to behind armour blunt trauma injuries. Although the vest is

successful in containing the round, it is not effectively dissipating the energy enough

to prevent large amounts of vest deformation at the area of impact. Therefore, the

increased deformation of the vest is causing a penetrating injury, as well as a blunt

trauma injury due to the localized nature of the impact. As a practical, real-life

interpretation, the backface signature can be defined as open, penetrating wounds

that occur even though the projectile did not penetrate the vest. The deformation of

the tissue exceeds the threshold of skin and penetrating wound results.

Cellular materials such as polymeric foams are often employed in shock mitigating

applications. Polymeric foam materials are widely used for impact protection and

energy absorption applications, such as the automotive crash safety systems. In the

automotive field compressive strain rates will reach up to 500-800/s [4]. Cellular

plastics may also be considered for higher rate applications. However, due to the

insufficient knowledge of compressive response of polymeric foams at such high

rates of strain (where ballistic impacts and blast waves can achieve rates of

deformation that exceed 1000/s [4]), such materials are not used yet in the

development of improved protection equipment.

Page 8

A trauma pack can be easily confused with a ballistic vest. A ballistic vest is where

the bullet is captured and where the majority of the energy absorption processes

takes place. There are various ballistic resistant materials ranging from the more

familiar Kevlar® (DuPont), Spectra®fiber (Honeywell), Goldflex® (Honeywell), Twaron®

(Teijin Twaron), Dyneema® (DSM) and Zylon® (Toyobo) [5]. The Nation Institute of

Justice certify that in order for ballistic resistant vests to be circulated for practical

use, then a tolerance of no more than 44 mm of backface indentation depth, must

take place when shot with a certain calibre of gun [6].

Although there are solutions to reduce the backface signature, which includes

inserting ceramics, metals or reinforced fabric behind the armour, even though the

indentation depth may be reduced, the energy transferral from the projectile to the

armour then to the body is unaffected. These solutions describe the aim of a passive

trauma pack where the aim is to keep the backface signature away from the wear.

However, with an escalation of the available energy of bullets and the desire of

armour designers to minimise the weight and bulk of the personal armour systems,

this will in all likelihood, increase the number of BABT and BFS or force the body

armour to become heavier and less flexible. This is where an active trauma pack can

play a pivotal role, where not only these trauma packs keep the backface signature

away from the wear, but also provide some reduction in depth of the backface

signature and to provide some energy absorption.

Page 9

Foams

Techniques which cause tiny bubbles to form within a plastic material such that when

the plastic solidifies, the bubbles, or at least the holes formed by the bubbles remain

within the material, are called foaming. This unique internal structure, with the

solidified bubble-containing material, are generally though of as a cellular structure.

Cellular solids are made up of interconnected networks of solid struts or plates that

form the edges and faces of cells. Products made by these processes are referred to

as cellular plastics or more commonly, foams. Advancements in technology also

mean that this foaming process can also occur in metals and natural materials. This

project will only refer to foams that are created with a plastic material base.

Classification

In the field of cellular plastics, there are two possible ways to categorize the types of

plastic foams: cell structure and wall rigidity [7]. Cellular structure is the most

common method to classify plastic plastics and there are two types: closed and open

cell structure. In a closed cell structure, each individual cell is impermeable, meaning

no fluid can pass between each cell. Each cell is a separate and discrete entity,

which means each cell can hold an individual gas. In an open cell structure, cells are

linked to each other due to the holes found in the cell walls. As a result, fluids easily

move within and throughout the entire plastic foam and the open cell foam is filled

with whatever fluid it is surrounded with (figure 1).

(a) (b)

Figure 1 (a) Microscopic close up of open celled Polyurethane foam. Figure 1(b) is a microscopic close

up of a closed cell Low Density Polyethylene foam (LDPE) [1].

Page 10

Wall rigidity describes the reaction of the cell walls when they are under

compression. In rigid foams, cell walls remain stiff, whereas in flexible foams, cell

walls collapse when they pressed. Both open and closed cell foams can have either

flexible or rigid walls.

Even though this project is interested in investigating using foam as an active trauma

pack, a certain type of foam must be chosen in order to represent the material.

Although there may be certain types of cellular plastics that are more suited to

become a part of an armoured system, expanded polystyrene foam has been

favoured for this project. Being cheap and commercially available, the main reason

why expanded polystyrene foam has been chosen is because of the cell size the

EPS foam has (see chapter ‘Cell Size’).

Expanded Polystyrene Foam

Pure solid polystyrene is a colourless, hard plastic with limited flexibility. It can then

be cast into moulds with fine detail, which the final product can be transparent or be

made to take on various colours. It is economical and can be found in uses such as

plastic modelling assembly kits, plastic cutlery or CD ‘jewel’ cases. However,

polystyrene’s most common use is as expanded polystyrene foam.

Expanded Polystyrene foam (EPS) are made up of expanded polystyrene beads and

usually white. Polystyrene is a polymer made from the monomer styrene, a liquid

hydrocarbon which is commercially made from petroleum by the chemical industry.

Close examination of the EPS foam will permit identification of the individual beads

that have been fused to form a continuous part, as in figure 2.

Figure 2 is taken from cushioning equipment from Wikipedia page

http://en.wikipedia.org/wiki/File:Polistirolo.JPG

Page 11

Polystyrene foams are closed cell foams, which mean that they are generally denser

than their open cell counterparts but more expensive to produce, since they require

more material. Their advantage over open cell foams are that in general, closed cell

foams acquire more compressive strength due to their wall structure.

Manufacture of EPS Foam

The most common method for making EPS foam involves the use of pre-foamed

polystyrene beads. The resin manufacturer makes these beads by adding an inert

gas, such as pentane or carbon dioxide, during polymerization. Polymerization is a

process of reacting monomer molecules together in a chemical reaction to form a

polymer chain or three-dimensional networks. Under proper conditions, which

usually involve a water suspension environment, polymerization occurs with the

formation of small, internally foamed polystyrene beads with the inert gas trapped

inside. These beads are then shipped to the part moulders. The part moulders

convey the beads from the shipping container to the mould by air pressure or

vacuum. During this conveying step, the beads are often heated and will expand up

to 20 times their original volume, however this is only a partial expansion. This

heating causes the beads to further expand, often doubling their size over the

partially expanded size and to fuse together. The moulds are then cooled and the

parts are removed. It is because of this manufacturing process, that variations in

density can be seen throughout the foam, which will be further discussed in the

‘Foam Characteristics’ chapter.

The EPS foam that would be used throughout this project would be the ‘Jablite

Flooring Insulation Polyboard White’. The material was bought at a local home store,

which was originally intended for home insulation purposes. The EPS foam came in

packs of 4 and in dimensions of (L) 1200mm x (W) 50mm x (T) 2400mm.

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Characteristics of EPS foam

Plastic foams have some physical characteristics that are valuable for several

important applications. The advantages of EPS foam over competitive materials

include the following:

· Low heat flow, making good insulation.

· Good energy absorption for packaging delicate instruments and other impact

applications.

· High buoyancy

· High stiffness to weight so that parts can be self-supporting and lightweight

· Low cost per volume

An advantage that is relevant to this project, of plastic foams, is the low weight it has

which can be traced back to its cellular structure. Due to the open structure of the

plastic foam, this means that the material is inherently lightweight. Weight can have

a very significant factor with ballistic protection wearers. Obviously, the lighter the

trauma pack, the ease of mobility for the wearer and therefore foam would be an

excellent choice. Other applications that take advantage of this light weight would

include flotation devices used in boats and planes such as life jackets, buoys and

pontoons.

Not only are plastic foams lightweight, they can also have excellent specific

properties. This high stiffness/load-bearing strength to weight ratio are not found in

all plastic foams and are somewhat a surprising attribute. The reason for this is

because of the cell walls within the foam, acting like many tiny columns, which

support the heavy loads. As a result, rigid foams can be found as structural parts or

cores for structural parts in areas such as aerospace, automobile and retail. In other

words, foams can be found in industries were weight is especially important and

applications can capitalize on the load-bearing capability of rigid foams. Examples

include airplane wings, space structures and furniture frames. By having this high

load bearing strength to weight ratio will also allow trauma packs to be not only

lightweight, but convenient, thus allowing sufficient protection without having a bulk of

clothing restricting the movements of the wearer. Many other applications require

that the foam material occupy space to give a desired shape and resist moderate

impacts, such as would occur in automobile dashboards.

Page 13

However, the characteristic of EPS foam that this project is most interested in, would

be the foams ability to absorb the high energy associated with impacts on the foam.

This energy absorption can be accounted by due to the internal structure of the foam

being able to collapse, when the material is crushed. The micro-mechanics of foam

energy absorption and their crushability properties will be further discussed in

another chapter. EPS foam can be found in many applications where they are used

to provide protection against high energy impacts such as furniture cushioning,

delicate instrument packaging and even in the transport industry with examples such

as seating, carpet padding, shock absorbers, crash pads and helmets. Combined

with their light weight and cheap production costs, these reasons further enhance

and reinforce the use of EPS foam within the advanced trauma attenuation protective

equipment area.

(a) (b)

Figure3 (a) bicycle helmet with EPS foam liner (b) EPS foam furniture cushioning

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Further Investigation into Foam Energy Absorption

As discussed in the previous chapter, due to their internal cellular structure, this

endows foams with several favourable properties such as low density, relatively high

strength-to-weight ratio, low heat flow and a significant degree of crushability.

Crushability is defined as compressing a material with a force, until it is deformed or

even destroyed. The Polymer handbook [9] states that ‘the term crushable implies

permanent plastic deformation or fracture of a compressed foam’. With the cellular

structure being collapsible, this high crushability is due to the presence of the large

void ratios within the foam. When the foam is impacted, the cell walls are able to

collapse, which enables the cell walls to flex, via buckling and bending, and therefore

absorb some energy of the impact.

With the foam under uni-axial deformation through compression, a common stress-

strain response would include three phases: (1) linear elastic region, (2) plateau

regime and (3) the final densification phase, as shown in figure 4.

Figure 4: Stress-strain response curve of an open-celled polyurethane foam under uniaxial compression

[static impact crushing layered], with the three common response regimes highlighted.

Linear elastic - occurs at low strain (a few percent, usually <5%) due to the uniform

cellular wall elastic bending and stretching throughout the whole foam structure. The

stress increases linearly with deformation and the phase deformation is recoverable.

This region defines the foam’s elastic modulus of the material.

Page 15

Plateau - corresponds to plastic yielding for rigid PS foams. By continuing the

permanent deformation at a relatively constant stress, this stage provides the

majority of the energy absorption capabilities of the material. This regime is the

dominant characteristic response of cellular materials when they are crushed. The

cellular buckling under compression commences a long stress plateau and happens

via the plastic buckling, yielding and rupture of cell walls and edges. With open

celled foams, the plateau region is generally responds as a level, constant force

(figure 4). The closed cell EPS foam, however, will exhibit a rising stress plateau

because of the compression of air within the closed cells (figure 6). The enclosed

gas pressure and membrane stretching will increase the level and slope of the

plateau.

Densification - where the cellular structure within the material has completely

collapsed and further deformation requires compressing the solid foam material. In

other words, the foam will have to behave as a compacted, homogeneous solid,

since the voids within the structure have been completely eliminated. The cell walls

are crushed together and there is a tight compaction of cell material. As a result, a

steeply rising hardening regime (a sharp increase in force) due to the consolidation of

the foam will be experienced. Energy dissipation via further deformation is

accompanied by a steep rise in force.

The ideal energy absorber is quoted as ‘one that minimizes force while dissipating a

given energy within a given energy within a specified stroke length’ [10]. This implies

that during the compressive deformation of the foam, the ideal energy absorber will

have a stress-strain response of a constant force. In practice, this is only achieved

in the middle stage, the plateau phase, when crushing the cellular material, although

this is also the predominant phase when a cellular plastic is being compressed.

It is only this plateau region and linear elastic area that is of interest to this project. If

any further deformation is experienced by the foam and the densification stage is

entered, then the steep rise in force that accompanies the deformation will also be

transmitted to the wearer. That is why through this project, emphasis in the analysis

will be on the linear elastic and plateau regimes.

Page 16

Microstructure of Cellular Materials under Compression

Crushing of cellular materials is characterized by the occurrence of localized

deformation, as described before. When a cellular plastic is being crushed, yielding

and collapse of the cell walls, originate where cells are the weakest. Weak cells are

determined from the structural imperfections, examples being randomly occurring

larger voids generated by coalesced gas bubbles and broken cells at surfaces

created by incisions. In turn, the collapse of these initial sites of failure reduces the

structural integrity of neighbouring cells, thus encouraging localisation of deformation

within the region. The collapsing nature from these locations are then transmitted

throughout the rest of the structure. This progression of deformation results in a

force response which is relatively constant and can account for the long post-yield

plateau in the stress-strain curve.

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Concepts of the Project

There are two main objectives for this project in order to create a successful active

trauma pack with foam:

1. to create a flexible trauma pack that can bend in one direction and not the

other (asymmetric flexing properties) without a great deterioration in strength

and energy absorbing characteristics.

2. to create a trauma pack that shows an improvement in fracture properties and

maintain, if not improve upon, the strength and energy absorbing

characteristics

Objective 1

In terms of comfort, flexible trauma packs are required in order to provide adequate

protection without restricting or hindering the movements of the user. To make an

active trauma pack more flexible, blocks of foam will have to be sliced into thin layers

of foam. How thin these layers can be, depend on how small the cell sizes the foam

possesses and are further investigated in the chapter, ‘Cell Size’. However,

enabling the trauma pack to flex in one direction and not the other will require further

manipulation of the EPS foam. Further incisions to enable these layers to turn into

tiles will allow these asymmetric bending qualities, and by making further incisions

will require a fabric backing layer, in order to maintain these tiles not becoming

individual small monolithic blocks and fall apart (figure 5).

Figure 5: Side view of the one sided bond with polypropylene, tiled structure.

By trying to create the sample shown in figure 5 not only includes the problem of

deciding which fabric backing layer to use, but also what bond will maintain a firm

hold between the EPS foam and fabric backing layer.

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Objective 2

To show an improvement in shear properties means that there has to be a reduction

in crack depth. By bonding these layers of EPS foam to a denser textile material on

one side, this will allow the specimen’s deformation in the tensile direction to

improve, and hence an improvement in shear characteristics of the overall specimen.

Rather than deforming further by fracturing, the deformation will be distributed in the

other directions, more specifically in the tensile direction. Hence, if a material which

is denser than the EPS foam is chosen to become the fabric backing layer, then both

flexible trauma packs and fracture reduction trauma packs can both be made with the

same material.

Once again, the bonding situation is brought up between the two materials. A firm

bond must be established between the two materials in order the sample to achieve

an improvement on energy absorption results compared a monolith sample. By

having a firm bond between the foam and fabric backing material will allow more

backing material to be drawn into the compact zone, thus when the foam is being

impacted, more foam can be involved in the deformation and allow more energy to

be absorbed. This will be further discussed in the ‘Adhesives’ chapter.

In order to take a step closer in modelling a ballistic environment, then a point load

must be applied to the samples. By introducing this point load, samples will

encounter three types of deformation: compression, tensile and shear. Mills &

Gilchrist modelled the behaviour of impact response of foam under various forms of

indentations (figure 6) and discovered that it was difficult to model certain

deformation fields, especially if the foam unloaded, or stressed in shear, or tension

[11]. From their report, it can be concluded that certain areas of the foam are being

left relatively unstrained, due to the presence of fractures that occur.

The compressive behaviour will be thoroughly investigated through uniaxial

compression testing; where only compressive deformation can happen. Out of the

two remaining deformations, it is failure in tensile direction that interest this project,

since it is this behaviour that influence the fracture behaviour within the specimens.

Page 19

Figure 6: A plane-strain deformation impact testing showing an extruded polystyrene foam block under

loading by a falling, large cylindrical indenter striker mass [11]. Figure shows pieces of the foam being

ejected laterally after 7ms under impact. This type of deformation will reduce energy absorption

capabilities of the foam (with less material to compress with) and one of the objectives is to reduce this

fracture behaviour

Considering the deformation processes that the foam will undergo when the material

encounters a ballistic environment. For illustration purposes, image a 22’’ calibre

solid rifle round impacting the foam. With speeds of at least 330s encountered, the

response of the foam is limited to a localised area within the region of impact. The

foam simply does not have enough time to transmit the responding information to the

rest of the material in order for it to react, as shear deformation would. As a

compromise, further tensile stretching is more likely to occur, since there are a limited

number of ways the foam can deform. Hence, it is most likely that a majority of

deformation through tensile stretching occurs, and not shear, along with compression

during a ballistic impact.

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Execution

First and foremost, to discover how thinly EPS foam slices can be made, cell size

investigation must first be completed. Due to the EPS foams beaded nature, an

investigation into density variation and inhomogeneous structure will also have to be

completed to confirm that all foam sections cut from the initial slab of EPS foam will

have consistent properties and that direction of loading does not influence response

results, respectively.

To be enable comparison between the different types of specimens, a standard

uniaxial compression test with a cube of monolith block of foam, will then be tested.

This will enable a set of results which can be referred upon as a reference, and

possibly obtain values the EPS foam three phases, if the EPS foam follows the

typical response of cellular plastics.

Exploration will then be focused in finding if by cutting the cube of monolith block of

foam into equal layers, will change the results. The reference monolith block of foam

can also be used to compare results tested with changes in strain rate. Young’s

Modulus calculations will also be attempted to find the elastic modulus of the EPS

foam between the two different loading rates.

To confirm the trauma pack can achieve asymmetric flexure qualities, three-point

bending tests will have to be employed, along with the quasi-static plane strain

deformation experiments.

The quasi-static plane-strain impact experiments are employed to view the

specimens in deformation through compression, tensile and shear. These

experiments will model closely to the experiments Mills & Gilchrist done to explore

foam impact responses [11]. These experiments will differ from uniaxial compression

tests, since uniaxial tests will only deform in a compressive manner, but the

deformation field experience on the samples surface is the same as that as the

interior. By testing with an indenter loading head, this will allow quasi-static testing

to take a step closer towards a point load produced by a bullet during ballistic impact.

Quasi-static testing is also easier to control and analyze than ballistic testing.

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Fabric Backing Material

The material chosen to be the fabric backing material of the specimens was

polypropylene. Polypropylene is a stronger, stiffer and denser material than the EPS

foam, thus by using the polyefin as a fabric backing material, this should reduce the

fracture behaviour of the specimen reducing crack length. Due to the woven nature

of the polyefin, polypropylene is also a difficult material to handle as well as a difficult

material to bond with, as discussed in the next chapter ‘Adhesives’.

Adhesives

Adhesion between the polypropylene and EPS foam layers provide a pivotal role

within this project. However, selecting the correct adhesion for this project may

prove to be difficult, which will be explained below.

Polypropylene is a very difficult surface to bond with and is best bonded using an

acrylic adhesive rather than an epoxy adhesive. As with all polyolefins,

polypropylene is very difficult to bond on account of their non-polar, non-porous and

chemically inert surfaces. The low surface energy polypropylene may need a coating

of primer, one such example being RS 108-722.

Expanded polystyrene foam is another material which is not straight forward to bond.

Any adhesives containing solvents will tend to melt polystyrene. Expanded

polystyrene also has low cohesive strength; therefore it is most likely that if a

successful bond with the expanded polystyrene is achieved, the adhesive will only

hold the polystyrene surface to the polypropylene.

One successful approach to adhesive bonding of these materials involve proper

surface pre-treatment prior to bonding, along with the proper adhesive used. The

options of different adhesives include:

• Araldite 2011- which will bond polystyrene

• Araldite 2018 - a highly flexible Polyurethane adhesive

• Wood Resin Adhesive – this will enable to seal the EPS to give a smooth flat

finish that can be bonded

• Double-sided adhesive tape – examples being 555-033 or 512-884

Page 22

Foam Characterisation

Cell Size

It is important that to have a rough estimate of the average of the sizes of the

individual cells, that make up the EPS foam. For polymer foams, cell sizes dictate

how small, or in this case how thin, the EPS foam can be made without size effects

occurring. Size effect can be defined as the material‘s dimensions having an

influence on its mechanical properties. In the case of polymer foams, size effect will

only be factor, if they do not go beyond the cell size by 20 times [9].

The cell shapes inside a moulded bead can vary. Cells which are close to the

surface of the bead skin tend to have brick-like shapes, with two of their faces

parallel to the bead boundary. Cells which are found closer to the core of the bead

have equiaxed polygonal shapes [9]. A skin-core morphology variation can influence

the mechanical properties of the moulded foam, as the denser skins are of higher

modulus (this is discussed further in the ‘Density Variation’ chapter).

To examine how small the specimen sizes could be without encountering the

phenomenon of size effect, microscopic testing was performed on the EPS foam. An

Optical Microscope Bressier Biolux AL 20X-1280X was used to provide figure 7 with

a magnification setting of x10.

Figure 7: Expanded Polystyrenes foams cells with the Optical Microscope Bressier Biolux AL 20X-

1280X at a setting x10 magnification

With an image analysis software Image-J, a more accurate approach in determining

the boundaries of the individual cells could be accomplished, as shown in the figures

below. Measurements from a rule could then be achieved, which a list of the cell

sizes could then be complied and are presented in figure 8 and table 1, shown below.

Page 23

Figure 8: Image manipulation with image analysis software Image-J

Cell No x/y (x+y)/2 x true (mm) y true (mm) 1 0.80597 60.5 0.17614 0.2273 2 0.929577 68.5 0.2216 0.2386 3 1.454545 67.5 0.2614 0.1818 4 0.586207 46 0.108 0.19318 5 1.414634 49.5 0.19318 0.13068 6 0.903226 59 0.1818 0.2045 7 0.783333 53.5 0.1591 0.19886 8 1.102941 71.5 0.25 0.227 9 1.196078 56 0.19886 0.17045 10 1.24 56 0.2045 0.16477 11 1.107692 68.5 0.2386 0.2159 12 0.983871 61.5 0.2045 0.2045 13 1.068182 45.5 0.1534 0.1477 14 0.734375 55.5 0.1591 0.2045 15 1.098361 64 0.22159 0.2045 16 0.756757 32.5 0.0909 0.125 17 1.090909 57.5 0.19886 0.1818 18 0.939394 32 0.10227 0.1136 19 0.766667 53 0.1534 0.2045 20 1.130435 49 0.17045 0.1534

Average 1.004658 55.35 0.1823825 0.184627 Table 1: Cell Sizes. The cell number refers to figure 14

From table 1, it is clear that the average cell size is 0.182mm in the horizontal (x)

direction, and 0.185mm in the vertical (y) direction, with a standard deviation of 0.047

and 0.036 respectively. In relation to calculating the smallest possible EPS foam

specimen without encountering size effect, the largest cell size dimension

encountered was 0.26mm. With this guideline, this meant that the EPS foam was

limited to being cut at minimum of 5mm, in order to dispose of size dependence.

x

y

1 mm 1 mm

Page 24

The sizes of the individual beads could also play an important part in the EPS foams

response. Again with the Optical Microscope Bressier Biolux AL 20X-1280X and a

magnification setting of x4, evident beads can be observed in figure 9. Nevertheless,

with the lack of information with bead sizes, bead size influence will not known or

taken into account during these experiments.

Figure 9: Microscopic image with the same microscope taken at a x4 magnification. Bead boundaries

can be viewed as darkened regions of the picture

Rao and Hofer stated that the yield stress of a cellular material can be enhanced by a

decrease in the cell size [12]. Be that as it may, the importance of the investigating

the EPS foams cell size, is to find out how small specimens of the EPS foam can be

made, without the phenomenon size effect occurring.

2.5m

x

y

Page 25

Density Variation

With bead foam mouldings, the Young‘s modulus and strength of a specimen can

vary with the position cut from the moulding. This is a result of the EPS foams

inhomogeneous microstructure. Bead foam mouldings usually vary in density, shape

and size from the skin to the core. Most beads are distorted spheres, with flat

patches in contact with neighbouring beads. The near spherical shapes are due to

the bead having a solid skin. Beads near the surface of the moulded product have

not deformed sufficiently to eliminate the inter-bead channels. If a polystyrene bead

is at a higher pressure than its neighbour, the pressure differential causes the inter-

bead boundary to be curved. Hence curved boundaries indicate either that the

beads contain differing amounts of blowing agent or that they have expanded by

different amounts. There is likely to be a density variation from moulded bead to

moulded bead, since the space for expansion is variable and the beads cannot move

relative to their neighbours once they begin to fuse at the boundaries.

A simple analysis in exploring the structural variation within the foam was completed

by measuring the weight throughout the foam, in different sections. In theory, by

increasing the material density, the effect of an increase crushing stress plateau and

a decreasing strain to densification, with an increasing strain rate, should be seen

[13].

Two different variations of these tests were done: one where the density variation

was tested throughout the whole slab of the EPS foam and the other where the

density variation was tested throughout an individual layer.

By taking a whole slab of Jablite EPS foam material, 20 identical pieces (dimensions

of (l) 0.45 x (w) 0.06 x (t) 0.05 m) were made as shown in figure 10. The individual

segments were then weighed and the density could be calculated from these values

(figure 11). The density variation in this test was measured along the thickness of

the foam.

Figure 10: EPS foam preparation for density variation analysis within the whole structure (slab) of

material.

= Length Reference

Surface Width

Thickness

Page 26

Density Variation (Slab)

0

2

4

6

8

10

12

14

16

18

20

0 0.2 0.4 0.6 0.8 1 1.2

Distance away from Reference Surface (m)

Den

sity

(K

g/m

³)

Figure 11: Test Results of density variation experiment no 1. The reference surface is highlighted in

previous figure 10

The other density variation test would be measured along the length of the foam.

Using an individual layer cut from the EPS foam, this segment of foam would also be

made into several identical pieces (dimensions (l) 20 x (w) 50 x (T) 6 mm, shown in

figure12. Again, variations in density were measured by weighing the individual

segments (figure 13).

Figure 12 EPS foam preparation for density variation analysis within a single layer of material

Reference Surface Length

Width

Thickness

Page 27

Density Variation (Single Layer)

0

2

4

6

8

10

12

14

16

18

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Distance away from Reference Surface (m)

Den

sity

(K

g/m

³)

Figure 13: Test Results of density variation experiment no 1. The reference surface is highlighted in

previous figure 12

Results confirmed that there was indeed a density variation throughout the EPS

foam. However, due to the magnitudes of the variation, around 2 Kg/m3, the

properties of the EPS foam is treated as a constant with calculations of the individual

specimens not requiring to specifically refer back to which area they were cut from,

calculating the exact mechanical properties they should hav

Page 28

Inhomogeneous Structure

Further testing was done to explore EPS foams inhomogeneous structure. Even

though the beaded nature of the EPS foam was found not to have a uniform structure

throughout the material, confirmation was required that test data was not influenced

by the direction of compression of the foam.

Further slow rate compression testing was continued, with differences on how the

cube samples were positioned, were attempted. Specimens 1 and 2 were placed on

the machine as in figure 20(a), while specimens 3 and 4 were mounted on the

machine as in figure 20(b)

(a) (b)

Figure 14: Figures depicting the method of testing a sample through different directions of loading.

Even though the machine was constricted to vertical testing, specimen samples could be altered in their

boundary conditions to achieve this.

Results showed that the direction of loading did not matter, with all four samples

achieving identical results (figure 15).

-100000

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Strain

Str

ess

(MP

a) 1

2

3

4

Figure 15: Stress-Strain response curve of foams tested in a uniaxial compressive manner. Variations

between results were in the initial starting position of the samples as shown in figure 20.

Length

Thickness Thickness Width

Width

Length

Page 29

Standard Compression Tests

A standard cyclic compression test was applied to cubed (50mm x 50mm x 50mm)

EPS foam specimen, in order to investigate the EPS foams properties going through

compressive deformation only. Specimens are cubed, to avoid buckling and to

ensure a uniform stress distribution. Testing was achieved by a vertical testing

machine, the Zwick Roell Z2.0 Uni-axial Tension Compression Machine. The

following parameters used for the analysis can be given in the table 2 and the results

are shown in figure 16.

Strain Rate Number of Cycles Displacement

5mm/min 1 cycle 45mm deformation (90% compression)

Table 2: Standard compression testing specifications

Figure 16: Force-displacement response curve of the EPS foam tested within a standard compression

test of strain rate 8.33 x 10-5 m/s and 90% deformation.

The response of the EPS foam matches the common description of polymeric foams,

which include three phases. The first and second phase, which is the linear elastic

response and stress/collapse plateau respectively, are almost identical within all

specimens. Even though the three regimes can be observed, there is difficulty in

determining at what exact values these regimes start and end. By eye, all 3

specimens enter the stress/collapse plateau under 200N force and remain almost

identical, including a rising plateau due to the trapped air associated with closed cell

Page 30

foams, until they enter the final phase, which is around 400N. It is this final phase

(‘densification’ or ‘hardening’) of the compression response where, apart from first

specimen, all other specimens retrieved almost identical results with 0.1mm

difference between them. Although the first specimen was able to compress and

unload under a force of under 2000N (1780N), the other three required a

compression force of over 2kN to reach the 45mm deformation target. Hence a

machine which could exert and measure a force of over 2kN was required.

Subsequently of the incomplete analysis obtained with the Zwick Roell Z2.0, the

Zwick Roell Z250 was employed. With an upper force limit of 250kN, in comparison

to 2kN the Z2 could achieve, complete full cycle data could be retrieved. Using the

same criteria as Table 2, the results of the compression tests with the Zwick Roell

Z250 are given in figure 17.

Again, a significant difference is seen between the fifth specimen tested and the

others at the final stages of compression response. Mirroring the first machine, the

fifth specimen was only 20N under the 2kN force to load and unload, whereas the

other two specimens passed the 2000N mark by a sizeable amount. Again, all 3

specimens went through the first two stages just like the previous four specimens

(i.e. a linear elastic response until reaching a mark under 70kPa and a

stress/collapse plateau of around 200kPa). Similarities are also spotted between

specimens 2 and 3 within the final stage, just like the above; however, they were not

nearly as identical. Unlike the above, comparisons on the unloading phase could

now be made, with all three following a similar trend. All three test specimens

recorded no reaction force after a strain of 30mm, which incidentally, is also true for

the very first specimen tested in the first machine. This signifies that the EPS foam

had permanently deformed under a height of 30mm.

Results that were directly recorded from the testing were the data of ‘Standard Force’

in Newtons and ‘Deformation’ in mm. To analyze data that is independent from the

size and shape of the specimen measurements of stress and strain are preferable.

Equations on how the two were obtained are given below. The work energy can also

be calculated with the recorded data by determining the area under the graph. Units

of work energy would therefore be in N/mm.

Page 31

Engineering Stress = Area

Force

Engineering Strain= lengthOriginal

lengthOriginallengthinChange −

Where Area = length x width = 0.05 x 0.05 = 0.0025m2 & Original length = 0.05m

0

200000

400000

600000

800000

1000000

1200000

0 0.2 0.4 0.6 0.8 1

Strain

Stre

ss (P

a)

Figure 17: Standard compression tests results, in terms of stress and strain, completed on the Zwick

Roell Z250 machine. Pink lines indicate a cyclic compression testing was done, whereas green lines

represent the data obtained in the compression only testing.

Compression only evaluations were then completed with the Zwick Roell Z250, to

ensure data repeatability (figure 17). Another aim of these tests was also to provide

information to allow Poisson’s Ratio calculations, although this will be discussed in

another chapter. Maintaining the same strain rate of 5mm per minute and displacing

the EPS foam 45mm (90% deformation),

Once again, repeatable results were produced within the first two stages of

compression response with results show identical similarities with the cyclic test

results. However, this time the EPS foam for both specimens peaked at a value of

below 2kN (1920N and 1960N respectively).

Page 32

Monolith vs. Layered

As mentioned within the ‘Concepts of the Project’ section, compression results

between a monolith block of foam compared to a block of foam that contains layers

of the same material, should obtain the same strength and energy absorption results.

To confirm this, more static crushing tests would be employed on both kinds of

samples. Monolith samples would be represented by a 50mm x 50mm x 50mm block

of foam whereas layered samples would correspond to five 50mm x 50mm x 10mm

pieces of foam stacked on top of one another. All cutting was performed by using a

band saw. Together with the Zwick Roell Z250 Uni-axial Tension Compression

Machine and with the same criteria in Table 2, the results produced can be viewed in

figure 18.

0

500000

1000000

1500000

2000000

2500000

3000000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Strain

Stre

ss (

Pa)

Figure 18: Stress-Strain compressive response of monolith (black) vs. layered samples (red)

The uniform layered width system has a similar force-deformation response to a

monolithic block. The three characteristic phases, initial linear elasticity, a post-yield

plateau and densification, are evident. However, there were obvious differences

between the monolith and layered samples. These differences were evident in the

first and third stage of results i.e. linear elastic response and densification regime of

response.

Page 33

During the first characteristic phase, linear elastic, the layered foam showed less

resistance, although similarities can be seen between the layered and monolithic

foam when the response begins to enter the plateau phase around the 50 kPa mark.

For the monolith, the strain recorded when the linear elastic region ends is roughly

0.04, whereas for the 1cm layered foam, the strain was measured around the 0.09

mark.

During the densification stage, it was clear that the layered specimens needed over

twice the force to compress the specimen 45mm, compared to the monolith

specimens.

However, despite these differences, this project is only interested in the first and

second stage of the characteristic phases, especially the plateau stage, since this is

where most of the energy absorption occurs. Taken these factors into consideration,

then it can be concluded that the results are similar to a monolith block of EPS foam.

Unloading also shows signs of slight discrepancies, although once again, this is an

area of little interest to this project.

Differences are thought to be due to be to the difference of boundary conditions of

the layers and monolith. Due to the specimen now made up of layers, there is a

slight increase in shear stress when the specimen is compressed. A phenomenon

called barrelling is also more likely to occur, due to the lack of lubrication between the

plate and specimen. To achieve more similar results, concluding that there is no

difference in compressive properties between a monolith and a layered block of

foam, the test can be repeated with lubrication, such as Teflon powder or oil with low

viscosity (WD-40 or hydraulic oil being such examples of low viscosity oil).

Page 34

Fast Rate vs. Slow Rate

Polymer foams are renowned for their strain rate dependence. By increasing the

strain rate, many polymeric foams have exhibited the same behaviour through an

increased elastic modulus, increased plateau stress level and a decreased

densification strain (figure 19). Yield stress increases with strain rate. The general

trend of cellular plastic seems to be the faster the strain rate, the stiffer the material

responds with a higher likelihood of fracturing.

Figure 19: A stress-strain response curve of a polymer foam under uniaxial compressive loading with

the variation in results expected by increasing the strain rate [4]

The examination of the effects of different strain rates would require the Zwick Roell

Z2.0 Uniaxial Tension Compression Machine, due to its faster loading rate compare

to that of the Z250 machine. With previous knowledge and experience gained by

using the machine in previous testing, it was clear that certain alterations, and not

only the strain rate, would have to be made. By operating the machine at the fastest

strain rate, 15000mm/min, or 0.25m/s, a displacement of 36.5mm was decided, in

order to keep within safe operating limits of the machine. The 73% deformation

‘calibration’ was agreed upon, through trial and error runs (see Limitations chapter).

Monolithic specimens were employed during this procedure and compression only

testing, not cyclic, was completed.

Page 35

0

200000

400000

600000

800000

1000000

1200000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Strain

Stre

ss (P

a)

Figure 20: Results of loading a monolith EPS foam with a slow rate (8x10-5m/s) which is shown in light

blue and a fast loading rate (0.25m/s), shown in pink

The results, given in figure 20, confirm that EPS foam, like all other polymers, is

strain rate dependent. Even through the initial stages of the foam response, there

are discrepancies between the fast and slower loading rates. Not only does the

faster loading rate require a great deal more strength in deforming the foam at the

same displacement (signifying that the faster the strain rate, the higher the stiffness

response of the EPS foam will become), the displacement where the response

regime changes as well. For the 5mm/min strain rates, the linear elastic region ends

at around a strain of 0.025. Contrasting this with the 15000mm/min strain rates, the

plateau regime seems to start at a strain value around 0.002, taking a tenth of the

distance of the slower strain rate. To summarise, the EPS material exhibited

increasing crush stress plateaus, and decreasing strain to densification, with

increasing strain rate.

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Strain

Ave

rage

Per

cent

age

Diff

eren

ce

Figure 21: Average percentage difference between the slow and fast loading rates within the plateau

region

Page 36

From figure 21, it can be distinguished that there is a maximum of 23% difference of

results, between the faster and slower rates of loading within the plateau region of

foam response. By loading the EPS foam at a higher rate, it can be seen that more

energy can be absorbed, and more force is required to reach the densification stage.

The material is overall, in other words ‘stiffer’ and stronger, as is true with all other

polymer foams. The stiffness is quite an important parameter since this will affect

how the shock waves propagate through the material. Although unrelated to the area

of interest, by being restricted to use the quicker, smaller machine, the peak rates of

fast loading of the specimen were immeasurable.

For many materials, a significant increase in the loading rate often results to higher

rates of deformation. Foam mechanical properties are extremely rate sensitive.

Generally the higher the loading rate, the stiffer the cellular material reacts, with the

consequence that it is more likely to fracture. The cause of this is fundamentally due

to the base polymer material from which the foam is made from, which is also strain

rate sensitive. This is also true in regards to cellular materials temperature

dependence properties, polymers are very temperature-sensitive materials.

Nevertheless, fluid within the voids of the cellular structure can also play a role in the

foams strain rate sensitivity.

In cellular materials, the fluid or gas contained within the voids is compressed and

expelled by flowing through the cells, as the material is compressed or crushed. As a

result, viscous forces generated when the fluid is pushed out of the foam cells during

the deformation increases when the deformation rate increases, leading to an

increased rate sensitivity. If the cells are relatively large and the deformation rate is

reasonable small, then the contained fluid usually has no effect on the material and is

generally ignored, especially in the analysis of quasi-static tests to determine the

mechanical properties of foams at low compression rates. This is especially true in

open-celled structures, which allows fluid to flow to one cell to the next with little

internal resistance.

Since the entrapped fluid is compressed locally within the cell for closed cell foams,

the effect of rate sensitivity is even more evident within these types of cellular

plastics. Combining closed-cell materials together with high loading rates, resistance

against the dynamic compression of fluid within cells becomes significant and will

affect characteristics such as the yield strength of the material to be strain rate

Page 37

sensitive.

Not much knowledge or research has been completed with polymeric foams

responding under high rates and significant deformation, although there are a few to

note. Altering the strain rate will also lower the densification strain [14]. This can be

attributed to the reduced ability of the foam cells and minimize the volume of the

compressed material. Stress wave effects, stress which now include inertia forces

and accelerations, will also become more evident within cellular materials when the

loading speed is increased [15]. Explosive responses have been reported by Green

et al [16], when the foam is impacted at sufficiently high velocity.

Page 38

Young’s Modulus

The Young’s modulus, or also known as the modulus of elasticity, is the mechanical

description of a subject’s tendency to deform along an axis when opposing forces are

applied along that axis, or ‘stiffness’. The elastic modulus of a material is defined as

the uniaxial stress over the uniaxial strain of a material, from the numerical evaluation

of Hooke’s Law.

To find the Young’s modulus of the EPS foam, data from the Fast vs. Slow Rate

chapter is adopted. The Young’s modulus can be found by the gradient of the slope

within the initial linear elastic regime. There are two approaches that can be

employed to find the gradient of this area. The first is by drawing a gradient by eye.

However, this technique is slightly subjective being based upon an individual’s

judgement (figure 22). Young’s Modulus values are given as, for the slow strain rate,

2.01 – 2.532MPa and for the faster strain rate, values range massively around 22.31

– 89.43MPa, although data repeatability suggests the value to be nearer the 27MPa

mark.

0

10000

20000

30000

40000

50000

60000

70000

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

Strain

Str

ess

(Pa)

Figure 22: Stress-Strain response curve of EPS foam within the linear elastic region. Strain rate of

loading is 8.33x10-5s-1. Blue line represents test data and the Pink line the arbitrary Young’s modulus

gradient by eye

Page 39

To avoid this subjectivity a second method is used. The second method is an

automated process and is not influenced by an individual’s preferences. By plotting a

graph of the foams gradient vs. the strain, as shown in figure 23, the variations in

gradients, along with the maximum gradient, can be observed. By isolating certain

amounts (an example of 30%) of data neighbouring and including the maximum

value, by averaging these figures, a straight line equation can be formulated. This

straight line is the elastic modulus of the foam, which passes through the point of

maximum gradient of the measured results.

0

0.5

1

1.5

2

2.5

3

0 0.01 0.02 0.03 0.04 0.05

Strain

You

ngs

Mod

ulus

(MP

a)

Figure 23: The elastic modulus within the linear elastic region with a step increment of three individual

samples tested with a strain rate of 0.25s-1. By averaging a certain region within the maximum value, an

accurate automated value of the Young’s Modulus of the foam can be obtained

Although the second approach can be considered a more reliable method, it is a

method which can only be applied to specimens tested in the slow, 5mm/min strain

rate. This is due to the number of data points that is obtained from the slow strain

rates (thousands), compared to the 100 or so data points gathered from testing with

the quicker strain rates. Attempts to gather more data points with the faster rate of

loading experiments, however none were successful. Table 3 shows the accuracy of

the automated system compared to the first approach by eye, with specimens loaded

at the slower, 8.33 x 10-5s-1 rate of strain.

Page 40

Method Cut-off Young’s Modulus (MPa) % Difference of Eye

Eye Na 2.01 - 2.532

Automated 30% 1.9 - 2.378 1.933 - 6.07

Automated 20% 2.00 – 2.49 0.148 – 1.6

Table 3 Accuracy between Naked Eye Observations and Automated Calculations

As stated in the previous chapter, the EPS foam is confirmed to be strain rate

dependent. Results clearly indicate that the EPS foam is rate dependent as the

plateau stress level is increased and the densification decreasing with an increasing

rate. Comparing the foam response of a monolithic block of EPS foam from a slow

loading rate of 8.33 x10-5s-1, to faster loading rate of 0.25s-1, will see a massive

difference from 2MPa and 27MPa in elastic modulus respectively. However, the

number of data points collected by the faster loading rate causes concern on the

reliability of the data, as well as determining the elastic modulus by readers

perception. This behaviour is confirmed through Young’s Modulus calculations.

Page 41

Three-Point Bend Tests

Three-point bend tests can only be performed on rigid foams. Given the low Young’s

modulus of polymer foams compression tests can easily generate stress-strain data.

With the foams brittle behaviour in the tensile direction, three point tests are usually

used to determine the tensile strength of EPS bead foams, since bead foam products

often fail in bending. Failure in a three-point bend test initiates a small, high stress

region, so the results are usually less affected by the random location of large flaws.

The three-point bending tests are employed to confirm that the tiled specimen

samples are flexible in one direction and not the other. To recap, tiled specimens

were constructed in order to investigate whether comfortable trauma packs could be

created with the EPS foam. To achieve this comfort, the rigid EPS foam would have

to be flexible and bend easily in one direction and not the other. It was assumed that

by tiling the one sided bond samples with polypropylene with the minimum adhesive

technique, that the tiled samples would have a slight reduction in performance than

the un-tiled samples. This assumption will be confirmed or rejected through the main

quasi-static strain deformation experiments.

If these tiled samples are used within the trauma pack, then the tiles must be placed

in manner so that they can flex in the direction away from the body armour easily and

do not flex towards the body armour. To test these tiled samples, three-point

bending tests were carried out as close to the British Standard Methods for Test for

Rigid Cellular Materials - Determination of flexural properties [17].

There are several notable differences from the method 14 of BS 4370-4:1991

compared to the three-point testing that was executed for this project. Firstly, BS

4370 recommends using a beam of span 300mm, depth 25mm and a loading point of

radii of 15mm. Hence, layers were made 300mm x 50mm x 5mm, and samples were

organized with 4 layers stacking on top of one another, giving a totally depth of

25mm in both samples. A total of 10 Incisions made were, 30mm apart. Skin and

moulded surfaces were removed from the samples. Cylinder supporting edges also

differed from BS4370, measuring a diameter of 19.6mm

The loading point also had to be altered. An initial attempt was given with the Zwick

Roell Z2.0 to complete the three-point bending tests. However, with the 20mm

loading head, it was found that the forces measured were miniscule and heavily

Page 42

influenced by background noise, in order to give clear results to formulate from. A

more successful approach was with the Zwick Roell Z250 machine, equipped with a

more sensitive 5N load cell. With the 5N load cell, the loading head became only a

single point. To distribute the load more evenly, a rectangular metal plate with a

length of 13mm and width of 5mm was placed between the specimen and 5N load

cell. The metal plate measured a weight of 3.4989g, which would have to be

acknowledged when calculations on elasticity of flexure and flexural strength are

applied.

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-50 0 50 100 150

Displacement (mm)

For

ce (

N) Polypropylene Top

(Flexible)Polypropylene Bottom(Stiff)

Figure 24: Results from three-point bending of tiled samples.

Even without calculations, examination with figure 24 can clearly see there is a

significant difference in rigidness between the two directions. The phenomenon

sagging was encountered when the flexible direction of the tiled samples were being

tests, which is why there is a delay in recording displacement. No fracture was found

with any of the samples tested, however this was expected due to no end

constrictions were employed on the samples to could contribute to the fracture. No

crushing was found throughout the foam, enabling flexural calculations to be

attempted.

Page 43

Equations for elasticity in flexure (E) and flexural strength (R) can be found from

BS4370 and is given below:

T

T

xbd

FLE

3

63

4

10×=

2

6105.1

bd

LFR R ×

=

where TF is the force Tx is the corresponding

deformation

RF is the maximum force applied L is the span

b is specimen width d is specimen thickness

With values given by drawing a tangent against the steepest gradient of the curve,

values of force = 1.00084N, displacement = 30.3913mm and maximum force = 1.35N

are found for the tiled specimen in the stiff direction. Considering the force generated

by improvised loading head = 3.4989 x 10-3N, this will enable values of elasticity in

flexure and flexural strength to be calculated.

( )3913.3025504

100000035.000100084.03503

63

××××−×=E = 450.24kPa

( )2

6

2550

103500000035.000135.05.1

×××−×=R = 22.6212kPa

The identical procedure was done for the flexible direction of the tiled samples.

Using values of force = 0.0308021N, displacement = 26.5346mm and a maximum

force of 0.0321675, values of elasticity in flexure and flexural strength are given

below.

5346.2625504

10)0000035.0000030802.0(3503

63

××××−×=E = 14.117kPa

( )2

6

2550

103500000035.0000321675.05.1

×××−×=R = 5.345kPa

Values confirm the conclusions that were made in the observations from figure 37, in

that flexibility has been achieved in one direction, while remaining stiff in the other.

Combining this data with the quasi-static plane strain deformation tests, where tiled

samples showed no dramatic deterioration from monolith samples, it can be

confirmed that indeed flexible active trauma packs can be made and are successfully

through testing. Future investigations on in testing even higher strain rates, and

maybe even ballistic experiments, can be continued with tiled samples.

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Plane-Strain Deformation Impact Tests

The main focus of this project is exploring the concept of foam being used in a

trauma pack with specific properties and the analysis of the behaviour of the trauma

pack when it is under impact. Although ballistic testing would seem the most

appropriate for this project, there are reasons why quasi-static tests was chosen

ahead of ballistic testing to investigate the material. One key reason was that quasi-

static testing would be easier to set up, control and analyze than the ballistic

experiments. Quasi-static simple geometry experiments with plane strain deformation

in the foam would allow the deformation field on the foam surface to be the same as

that in the interior.

A venture to recreate the Mills & Gilchrist’s plane-strain deformation impact

experiments [11] was attempted. In their experiments, simple geometry experiments

were tested on extruded polystyrene foam. The source of impact was created by

dropping a striker mass between 1.0 and 2.0 m to achieve their desired impact force

on the foam and their analysis heavily emphasised on the fracture behaviour of the

foam.

In this experiment, variations of the foam block would be subjected to three different

types of compression tests with the machine Zwick Roell Z2.0. All experiment strain

rates will be travelling at 15000mm/min (0.25m/s) and with simple geometry

variations of the loading head, given in table 4.

1 Uni-Axial Compression Several Final Compression Strains

2 Small Cylindrical Indenter (18mm diameter) 70% compression

3 Large Cylindrical Indenter (100 mm diameter) 70% compression

Table 4: Variations of loading heads along with final compression strain

By using cylindrical indenters, instead of having only compression deformation, as

the uniaxial experiments, indenter testing will allow other forms of deformation to be

introduced. The indenter testing will allow quasi-static testing to take a step closer

towards a point load produced by a bullet during ballistic impact. The logic behind

the two different types of indenters is due to the fact that impacts with indenters can

be divided into two cases. Assuming that a flat topped block of foam is being tested,

indenter geometries are classified according to strain field symmetry, where:

Page 45

1. The cylinder diameter is smaller than the original foam block thickness, so the

principle compressive stress field tends to radiate from the cylinder axis.

2. The cylinder diameter is larger than the original foam block thickness, and the

stress field in the foam tends to be vertical.

Despite recognizing the limitations on the strain rate and allowable force on the

machine, as previously with the foam characterisation tests, it needs to be noted that

there is another key difference between the these experiments and the Mills &

Gilchrist’s plane-strain deformation impact experiments. With Mills & Gilchrist’s

experiments, a variation of impact velocities were recorded due to the method they

deployed to achieve their impact force; by using a striker to fall between 1.0 and 2.0

meters with twin wire guidance.

However, due to the method the strain rates are applied in these experiments, the

same problems do not arise, since there is no variation in the striker mass.

Experiments are simply compression experiments with a different loading shaped

head.

A high speed camera - Optimas UK Kodak MotionCorder Analyzer model 1000 - was

also employed during the testing. Settings on the high speed camera included a

frame rate of 240 frames per second and a shutter speed of 1/500 sec. This allowed

analysis of the failure as specimens were tested.

The purpose of having variations in foam specimens is to change some the foam

properties without deteriorating others. The cross-sectional areas of the foam layers,

polypropylene and adhesion inserts in all foam plate systems are kept constant.

Guidelines of specimen sizes and dimensions are given in tables 5, 6 , 7 & 8. At

least two of each specimen was constructed so that experiments could be tested for

data repeatability.

Experiment Specimen Length Specimen Width Specimen Height

Small Indenter 100mm 50mm 50mm

Large Indenter 200mm 50mm 50mm

Table 5: Guidelines for specimen parameters

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Specimens for Plane-Strain Impact Experiments

The range of the specimens that were made and tested is given below, along with a

description on how they were made. The purpose and ideas of every manipulation

with the specimens can be seen in the ‘Concepts of the Project’ chapter.

Monolith

The monolith samples should obtain if not the best, then one of the best results in

terms of strength and energy absorption under deformation through compression

only. External interactions with these samples will only be on the top and bottom

surfaces of the monolith samples (figure 25(a)). With no other influences, the

monolith samples should be able to put up more resistance against the compression.

The data from the monolith will be used as a reference in comparison to other

specimen types.

In terms of fracture, on the other hand, the monolith samples should obtain the most

evident cracks. Foams break due to their brittle behaviour in tension and it is this

weakness in the tensile direction that must be altered, in order for foams to absorb

more energy. However restrictions into changing the foam property includes that

altering foam density is to be kept as minimum as possible.

Layered Specimens

There are three types of specimens that use this layered structure:

• Layered – where the samples only contain EPS foam.

• Single bonded – where samples are bonded with polypropylene on one side

of the EPS foam only. Further distinctions can be made through the amount

of adhesive used to bond the two materials together: minimum and generous.

Tiled specimens are samples which are made from single bonded with

minimum adhesive samples with a further step induced (figure 6).

• Double bonded – where samples are bonded with polypropylene on both

sides of EPS foam. Again, further distinctions can be made through the

amount of adhesive used to bond the two materials together: minimum and

generous.

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(a) (b) (c) (d)

Figure 25: Illustrations of the different specimen types. Figure 25(a) represents a monolithic foam block,

where Figure 25(b) shows how 3 foam layers where organized in the layered specimens. Figure 25(c)

illustrates the singled sided bonded with polypropylene (represented by the grey layers) with the yellow

surfaces indicating where adhesion was positioned in the samples. Graphical representation of the tiled

samples can be seen in Figure 25(d)

Layering Foam Only

By layering the foam, this will allow a flexibility of the trauma pack to increase, yet

see the compressive properties of the foam to maintain. By having the foam in this

layered structure will also require constraints to be put on the ends of the specimens.

This is to enable no bending to occur with any of the foam layers during testing. With

the string attachment, this will enable the whole specimen system to respond to the

impact loading. However by having this string attachment, a small flexion during the

test will also be recorded. This small flexion is considered insignificant and is

ignored.

Organizing the foam into layers meant that the foam slices had to be cut from the

original EPS foam slab. This was done through a ‘band saw’ cutting tool, which as a

consequence, exact layers of 5mm thickness were not able to be obtained. On

average, the thickness of foam was 5.38325mm, with a standard deviation of

0.00361.

One Sided Bonded with Minimum Wood Resin Adhesive between Polypropylene and

EPS Foam

Although layering the EPS foam will allow some flexibility, by adding polypropylene to

the foam, as shown in figure 25 (c), this should further reduce fracture depth yet

allow the compressive components of the layered foam to remain still remain the

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same. By introducing the polypropylene, this denser material will alter the whole

specimens shear properties, making the material stiffer and show less signs of

fracture, compared to the monolith samples.

Through the advice from adhesive specialists and cases of trial and error, Evo-stik

Wood Resin adhesive was judged to provide a decent bond between the two

materials with no pre-treatment necessary. Specimens of this type were also made

using the adhesion through double sided adhesive tape.

One of the objectives of this project was ideally, to change the fracture properties of

the EPS foam, without changing the yield plateau of the cellular plastic. With the

polypropylene being predetermined, the only option left to keep the density from

differing away from its original value as possible, was to use the minimal amount of

adhesive possible. This was achieved by applying a small and controllable amount

of adhesive to the EPS foam layer, spreading it across the foams cross sectional

area and scrapping the excess adhesive away before the polypropylene was applied.

No weights were used to ensure the adhesion of both materials, since it would be

likely that the weights would alter the performance of the foam by damaging the

unique cellular structure of the foam and hence achieve poorer results when tested.

One Sided Bonded with Generous Wood Resin Adhesive between Polypropylene

and EPS Foam

With the minimum adhesion technique, although this ensures that the specimen

density is kept as low as possible, the technique did not ensure that the firmest bond

possible with the two materials was applied. To establish the polypropylene and EPS

foams firm bond with one another, samples were made where a generous amount of

adhesive was applied. To enable measurability, the same amount of volume of

adhesive was applied through a syringe. For the specimens that were to be tested

with the small indenter, 2.5ml of wood resin adhesive was applied between the foam

and the polypropylene. For the specimens to be tested for the large indenter, the

amount was altered to 4ml.

One Sided Bond with Minimum Wood Resin Adhesive between Polypropylene and

EPS foam, Tiled

Tiling the trauma pack will enable flexibility to be achieved in one direction, and

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inflexibility in the other. In terms of this project, comfort is defined as the foams

flexibility. To judge, whether these tiled samples are more flexible or not, and

therefore more comfortable, tiled samples will have to be made to be subjected to

three-point bend testing.

The tiled samples were made in the identical manner as the one sided bond with

polypropylene with the minimum adhesion technique, only a further step was

introduced once the adhesive had fully dried. Once given enough time to dry within

the manufacturers’ specifications, incisions were made into the EPS foam, cutting it

into effectively ten sections of EPS foam bonded to the polypropylene (figure 25 (d)).

Incisions were made by a pen-knife, 10 and 20 mm apart for the small and large

indenter tests respectively.

Both Sided Bond with Minimum Wood Resin Adhesive between Polypropylene and

EPS foam

With uni-axial compression, there is no shear behaviour occurring within the foam

and the whole foam is reacting to the compressive force, indiscriminately. However,

with the indenter tests, only a certain region of foam will be compressed. By bonding

the foam on both sides, although there may be a loss in flexibility, the energy

absorption results should improve considerably with the indenter experiments. This

is a result of enabling to introduce more foam material into the indenter compression

region of the foam.

Both Sided Bonded with Generous Wood Resin Adhesive between Polypropylene

and EPS Foam

These samples were built in order to achieve the same purpose as the single sided

bond with generous amount of adhesion. The exact same measuring method of

using a syringe was adopted, with samples created for the small indenter having

2.5ml of wood resin adhesive in between layers of polypropylene and EPS foam, and

4ml for specimens to be tested with the large indenter.

However, the purpose of these specimens, were not only to see the effects of a firm

bond, but to also try and introduce more foam into the indenter compression region.

In turn, by allowing more material to be deformed by the indenter, these samples

should record more energy absorbing properties and higher strength.

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Test Results and Analysis

0

500

1000

1500

2000

2500

0 10 20 30 40

Displacement (mm)

For

ce (N)

MonolithLayeredOne Sided Bond MinTwo Sided Bond Min

Figure 26: Stress-strain response curves of different specimens under uni-axial compression.

Specimens all had approximately uniform dimensions of 100(l) x 50 (w) x 50 (t).

-100

0

100

200

300

400

500

600

700

-10 0 10 20 30 40

Displacement (mm)

For

ce (

N)

One Sided Bond MinLayeredTiled

Figure 27: Force-displacement compressive response curves under the small indenter testing

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0

100

200

300

400

500

600

700

800

900

0 10 20 30 40

Displacement (mm)

For

ce (N

)

Monolith

Double BondedMin

Double Bonded2.5ml

Figure 28: Force-displacement compressive response curves under the small indenter testing

(continued)

0

200

400

600

800

1000

1200

1400

1600

0 10 20 30 40

Displacement (mm)

For

ce (N

)

MonolithLayered1 Sided Bond Min1 Sided Bond TiledDouble Sided Bond MinDouble Sided Bond 4ml

Figure 29: Force-displacement compressive response curves under the large indenter testing

Page 55

With mechanical and quasi-static tests, it is preferred that the results are expressed

in terms of stress and strain, since the results will, and should be, independent of the

size and shape of the test specimen. However, with the indenter tests, it is unclear

whether the whole test specimen is involved with the experiment and results. Due to

the shape of the indenter, it is obvious that the loading head does not interact directly

with the whole foam block, unlike the uniaxial compression tests. As a result, when

referring to the data for the indenter tests, force and displacement units are given.

Another notable difference between the compression and indenter tests is that unlike

the compression tests, all tests with the indenters were able to record results within

the 70% compression without recording an error within the Zwick testing machine i.e.

none of the samples recorded a force over the 2000N limit. Again, this can be

explained by uni-axial compression, implying that the whole foam is involved in the

crushing deformation and the results. Contrasting this with the indenter tests, regions

on both sides of the samples can be considered uninvolved with the test, at least not

in terms with the strength and energy absorption. This may also explain the difficulty

in determining the different stages of the foam response and why a typical polymer

compressive response curve is hard to determine, like the ones discussed in the

‘Energy Absorption’ and ‘Standard Compression Test’ chapters (figures 4 & 26).

As mentioned previously, not only are there difficulties in determining at what exact

values the samples enter or leave a certain regime, there are difficulties in

distinguishing the three phases of deformation within the indenter tests. Analysis will

emphasize only on the interested regions, which are the first two regimes of foam

response, linear elastic and plateau stage, and results will not refer to densification or

the maximum force obtained by the samples.

Mechanical strain is a measurement in deformation. If a material is considered to be

incompressible, then when it is under a uniaxial compressible test, where the only

deformation is compression, then it cannot suffer deformation. Hence no strain

differences will be measured. With an assumption that the fibre backing material

(polypropylene) and adhesive (wood resin) are incompressible, then it is possible that

the strain measured during the plane-strain impact tests, is not the actual value of

foam deformation but the foam deformation plus incompressible layers. With a

second assumption that the adhesive thickness is half of the polypropylene

thickness, values of only foam deformation can be seen in figure 30.

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Figure 30 showing overlapping, identical results of strain, despite the pink line only showing foam

deformation and the blue line showing deformation of the whole sample. This figure represents the

thickness of incompressible textile and adhesive layer makes little difference to strain calculations

As seen in figure 30, the difference between both values is miniscule and it can be

stated that the inclusion of polypropylene and adhesive substance layers are

insignificant, in terms of the strain measurements.

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Monolith

Uniaxial tests

With uniform uniaxial loading up to 70% compression, even though the specimens

have doubled in their length, the monolith samples show the same type of

compressive stress-strain response as typical polymeric foam and with the high

strain rate compressive tests done previously with the cubed EPS foam (figures 18 &

26 respectively). This is an expected result, since no other variables have changed

from the high strain rate experiments, other than specimen dimensions, specifically

length.

The three phases of deformation can be clearly observed and the results of all three

monolith specimens are almost identical to one another, showing data repeatability,

at least within the areas of interest (linear elastic and plateau regime). The plateau

regime starts and ends at strains 0.03 and 0.6, respectively or in terms of

displacement, 2mm and 27mm respectively. In all three tests, results showed that

within the linear elastic and plateau regime, the monolith samples required the most

strength, in order to deform, although there was an exception.

Indenter tests

Unlike the uniaxial tests, however, with both of the indenter experiments, the typical

polymer foam response under compression could not easily, if at all, be recognized

(figures 27 & 29).

The energy absorption properties of the foam were calculated by using Microsoft

Excel and fitting a polynomial trend line against the interested curve. By gaining the

equation of the trend line, integration could then be performed and a value of the

work energy was calculated. Since it is unclear where the plateau region ends,

where most of the energy absorbing properties is concerned with, estimation on the

strain was done in order to calculate the work energy.

The most noticeable fractures that came from the indentation tests were from the

monolith specimens. Fractures were more evident with the small indentation tests,

than the large indentation tests. Fracture analysis will be discussed in the ‘Fractures’

chapter.

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Layered Foam Only

Uniaxial test

Samples that showed the least strength and least energy absorption in all three

different experiments were the layered EPS foam structures (figures 26, 27 & 29).

Uniaxial compression tests were performed up to 70% compression and even though

both samples showed signs of data repeatability, both did not show typical foam

behaviour under uniform loading. Results showed signs that there was no linear

elastic regime within layered samples and the densification stage began at a strain

value of 0.6 or displacement value of 30mm.

These results highly differed from the results obtained in the ‘Monolith vs Layered’

chapter. In that chapter, the linear elastic and plateau regime of both samples

(monolith and 10mm layered samples) showed nearly identical results and no

significant differences. However, with the clear figures below, it can be clearly shown

that there are differences between the 5mm layered samples with the monolith

and/or 10mm layered samples, even though both the rate of loading and deformation

is kept the same, 0.25m/s and 90% compression respectively.

0

500

1000

1500

2000

2500

0 10 20 30 40

Displacement (mm)

For

ce (

N)

MonolithLayered

Figure 31:Force-displacement response curves with samples under uniaxial compression testing. The

Page 59

layered samples were 5mm thick. Specimen dimensions were 100 (l) x 50 (w) x 50(t) mm and strain

rate was at 0.25s-1

0

500

1000

1500

2000

2500

0 10 20 30 40 50

Displacement (mm)

Sta

ndar

d F

orce

(N)

1cm Layered

Monolith

Figure 32: Force-displacement response curves with samples under uniaxial compression testing

completed in the initial stages of the project. The layered samples are 10mm with sample dimensions

50 x 50 x50 mm, straining at a rate of 8.33x10-5s-1

The significant deterioration between the layered samples and the rest of the

samples can be explained by the introduction of more damaged cell faces within the

layered specimens. In the monolith samples, damaged cell faces caused by the

knife incision would only appear on the top and bottoms surfaces specimens.

Theoretical analysis states that “an exterior of weak, cut surface cells on the sample,

of thickness approximately half the mean cell diameter, combined with the

neighbouring cells affected, and the contribution of weaker cells on the total stress

should be less than 5% of the total” [9].

Layered samples, on the other hand, contained 7 times more damaged cell faces

than the monolithic samples. With damaged cell faces located roughly 5mm apart,

failure is more easily initiated within the sample and hence the weaker response.

The overall density of the specimen must also be considered, with the layered

samples weighing slightly less than the monolith counterparts. Again surface

roughness plays an important part, as discussed before in the Specimens chapter,

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where even though both types of samples measuring roughly the same height, the

layered samples do not contain the same amount of material as the monolith, due to

debris contamination.

Indenter tests

The pattern with the specimens showing least strength and least energy absorption is

continued with both the indenter experiments. Instead of having no apparent linear

elastic region, both indenter experiments seem to demonstrate an initial reaction of

low resisting force for a displacement of 3-5mm. This could possibly be explained by

the surface roughness that is present between the layers of the specimen. The

presence of the debris can create space between the layers of foam and when a

compressive force interacts with the whole specimen, the primary reaction of the

specimen is to eliminate these gaps of space.

Due to an indistinguishable typical polymer response shape, that is usually observed

when the polymeric foam is under compression, the plateau region is hard to identify.

It could be possible that the plateau stage is not encountered with both indentation

tests with the layered specimens, due to the amount of force the machine is limited to

operate at. With both indentation responses being similar to the monolith response

data, this would suggest that the previous statement was true.

Compared to the monolith, the layered samples tested with the big indenter

experiments, almost halved in value of the work energy, the specimen could absorb

(figure 29).

One Sided Bond (minimum adhesive technique)

Uniaxial Tests

These samples showed slightly better results by having slightly better in the strength

and energy absorption than the layered un-bonded specimens (figures 26). Uniform

loading was both performed at 70% and 60% compression due to force limitations

within the machine. The response curve of the uniaxial compressive showed a curve

which the three regimes could barely be recognized.

Indenter Tests

Although no similar trend was followed in the uniaxial compression tests, this

remained untrue for both indenter tests. Showing similar performances in strength

Page 61

and energy absorption for identical samples, the one sided bond with polypropylene

with minimum adhesion samples also exhibited a similar pattern of results within the

layered specimens in the indenter tests. Once again the three typical responses

were difficult to identify or signify if they were even present during testing.

0

100

200

300

400

500

600

700

0 10 20 30 40

Displacement (mm)

Forc

e (N

)

Figure 33: One sided bonded with minimum wood resin adhesive between polypropylene and EPS foam

in small indentation tests. Red shows the sample without constraint on the foam, whereas blue

indicates string was used to constrain the sides of the sample deflecting upwards.

A total of three of these samples were tested with the smaller indentation tests, with

two showing very similar results and the other fairing slightly worse (figure 33). The

latter result was due to the fact that this specimen was not constrained vertically,

allowing some of the specimen layers to deflect upwards. By not constraining the

ends of the foam and isolate the effect of bending, the ends of the top layers of foam

will deform by bending and the ends of the bottom layers of foam will not be involved

with the foam response. By bounding the layers of foam together, the string will

allow samples to act like a whole structure and involve more foam to respond, when

the sample is deforming.

As with the small indenter, the samples tested with the large indenter faired only

slightly better than the layered specimens. Energy absorption results indicated that

these samples were 22% better than the layered specimens but 34% worse off than

the monolith samples.

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Due to the discovery of the 5mm layers not responding in the identical manner as the

10mm layers, it is unfair to compare any samples that use 5mm thick EPS foam with

the monolith samples. However, a comparison with these samples can be made with

the layered samples. Figures 26, 27 and 29 showed that these samples had better

strength and energy absorption properties than the layered samples, and this can be

explained by the wood resin adhesive. When the adhesive is applied onto the EPS

foam layers, due to its fluid form, the wood resin adhesive is able to create infiltrate

the layer of damage cells and make them stronger. The adhesive essentially

reinforces the damaged cells, allowing them to perform better during testing.

Two Sided Bond (minimum adhesive technique)

Uniaxial Compression

In the field of uniaxial compression testing, the two sided bond with polypropylene

with minimum wood resin samples obtained the best results, in terms of strength.

Once again, application of the wood resin adhesive can explain this by reinforcing

virtually all the damaged cells caused by cutting the foam into layers. Almost twice

the amounts of damaged cells were altered compared to the single sided bond

samples. However, this increase in strength and energy absorption with double

bonded samples did not repeat the same kind of results in the indenter tests.

Uniform compression testing was applied up to 50% compression. Once again,

although the sample did not express the results in the typical fashion of polymeric

foam compression response results, three distinguishable regimes are thought to be

identified (figure 26). By eye, the linear elastic region ends at roughly 2mm of

deformation, along with the monolith samples, although slightly larger forces are

required to achieve the same displacement. The biggest difference between

monolith samples and two sided bond samples were the displacement when

densification occurred. Monolithic samples recorded densification occurred at

roughly a displacement of 30mm compression, whereas the two sided bond samples

are thought to record densification around an earlier displacement of 18mm.

Indenter Tests

With the small indenter tests, the same pattern that the double sided bond samples

with minimum adhesion was not followed, and instead faired worse in both strength

and energy absorption parameters within the interested region of results, than the

monolith samples. The shape of the curve made it hard to determine the three

Page 63

regimes of a typical response polymeric foam. Experiments with the large indenter

showed similar results. Evident fracture marks could be seen in both indentation

tests, although once again this will be discussed in a future chapter.

Expectations of the double sided bonded with polypropylene with minimum adhesive

specimens were that this sample would match, if not improve upon, the energy

absorption properties of the monolith samples, in the indenter tests. The explanation

on how why they did not achieve the same work energy consumption could be

justified due to the way the double sided bonded specimens fail. Although the high

speed camera was intended to analyse the fracture behaviour of the bond, the

equipment managed to unintentionally capture another kind of failure that occurred

during the indenter testing: failure to bond. By failing to manage a firm bond between

the two materials (figure 34), this revealed that the wood resin adhesive was not

doing its job by trying to introduce more foam material into the indentation region and

thus absorbing more work energy. As suggested by the low forces recorded by the

doubly-bonded samples in figures 27 and 29, failures of this nature happened within

both indentation samples. The chapter titled ‘Adhesion Failure’ further investigates

the bonding failure.

Figure 34: Two sided bonded with polypropylene specimen with minimum adhesives under large

indentation. Adhesion failure highlighted, thus not allowing as much strength and energy absorption

characteristics

One Sided Bonded with Polypropylene, Tiled

Small Indentation Tests

Tiled samples did not undergo uniaxial testing and only were experimented on using

the indentation tests. Results showed that these tiled samples performed better than

the un-tiled one sided bonded with polypropylene with minimum adhesive specimens.

Even though these samples performed the best within the one sided bond range,

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they absorbed (18% less work energy than the monolith samples and more work

energy than the one sided bond with minimum adhesive with polypropylene and

foam). Figures 27 and 29 show that the shape of the compressive response of the

tiled samples follows the same trend as the layered and one sided bonded un-tiled

samples.

The improvement of results can be traced back to the application of wood resin

adhesive onto the samples. There is an uncontrollable element on the amount of

wood resin adhesive that is applied. Even though, initially a syringe is used, the

scrapping process takes off an unknown amount of debris and wood resin adhesive.

As a result, it is possible there was more adhesion in tiled samples than the others. It

is also impossible to determine which damaged cells can become strengthened by

the adhesive.

Large Indentation Tests

The tiled samples performed the best out of the one sided bonded with polypropylene

specimens, although they still did not compare to the monolith samples within the

linear elastic and plateau region, in the large indenter experiments. The flexible

samples absorbed around 17.5% more work energy than the one sided bonded with

polypropylene with minimum adhesive (significantly more than the small indenter

tests) and 21% less work energy than the monolith samples. Again, the better

results are explained previously, by the affect of the wood resin adhesive.

Tiled samples have shown that with added flexibility they perform just as well, if not

better, over their un-tiled counterparts in terms of strength and energy absorption

properties. The difference between these tiled samples and un-tiled samples would

be explained by the wood resin spreading.

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Both Sides Bonded with Polypropylene (Generous Amount of Adhesive)

Indenter Tests

These samples showed the greatest strength and energy absorption within the small

indenter experiments (figure 27). Both samples showed similar trends, suggesting

the plateau regime begins and ends at displacements 3-4mm and 20-23mm

respectively.

Unlike the small indenter tests, the samples bonded on both sides with more

adhesive, performed worse than both the samples bonded on both sides with

minimum adhesives and monolith samples (figure 29). This is only true during the

linear elastic and plateau region. The energy absorption however, remains better

that the both sided bonded with polypropylene with minimum adhesive, absorbing a

value around 10% more work energy.

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Fracture

For the small diameter cylinder indenter, cracking can be observed in three types of

specimens: monolith and both double sided bond with polypropylene with minimum

and generous adhesion. Other samples

For the Monolith samples, cracking could be clearly noticed from the foam surface of

the monolith specimen when observed through the high speed camera or by eye, as

shown in figure 35. With the high speed camera, it confirmed that the crack initiates

outside the contact area. The fracture is caused by the local bending of the foam

surface, which in turn provides a maximum tensile stress. Foams, as with the EPS

foam, react like a brittle material, when they are in tension. Tensile results are

affected by the random location of large flaws. Fracture initiates on the tensile

surface where the stresses are highest. As the bending moment increases, the

compressive stress may exceed the foam yield stress at locations on the

compressive surface, often near the central loading point. The crack stops with

lengths 15-18 mm with an angle of 22-30° to the ver tical.

Figure 35: High speed camera image of fracture with the monolith samples in the small indentation

tests.

The only other specimens that showed signs of fracture would be with the 2.5ml

adhesive. Although not that evident in the high speed camera, close inspection of

the sample after the specimen was tested confirmed that fracturing did occur within

the top layer of the EPS foam. Due to the size of the fractures and the viewing

position, the cracking is difficult to be seen with the high speed camera and is difficult

to describe with dimensions (figure 36)

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Figure 36: Fractures with double sided specimens with generous adhesion, tested with the small

indenter

Among the large diameter cylinder indenter, the only samples to encounter fracturing

failure were the monolith samples. The crack can be best described as a surface or

an edge crack, within the contact region, 1-2.5mm in length with a 52° to the vertical.

No other fractures were determined within other samples, tested with the large

indenter. In comparison to the Mills and Gilchrist experiments which this project was

aiming to build upon, the larger diameter cylinder indenter caused the largest

fracture, by able to create cracks that were long enough to separate the foam into

three isolated segments. The strain rate and different type of low-density polystyrene

foam was judged to be the cause of the difference in results, where with Mills and

Gilchrist experiments were able to achieve velocity impact rates of 4.4 to 6.3m/s,

compared to strain rates of 0.25m/s acquired in the above experiments.

Nevertheless, fracture processes tend to be less strain rate dependent than yielding

processes, therefore high-speed impact causes a more brittle-like response.

Whether or not cracks form and propagate depends solely on the strain energy

release rate associated with the crack propagation. Predictions and further analysis

on fracturing with EPS foam can be intestate further through the method of the J

integral [9]. Analysis cannot be done by some other method, due to the

inhomogeneous structure of the beads of EPS foam.

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Adhesion Failure

After the analyzing the results, it was found that samples were not responding in the

behaviour that was expected. Through the high speed camera, it was indicated that

this was due to the bonding method used. However, with the alternative bonding

method (the double sided tape method), this suggested that due to the similarity in

results, the reason for failure was not the adhesive, but the region of foam beside the

adhesive.

Further of investigation was done to find out which of the above statement was true.

By taking a used sample, a layer of polypropylene was peeled off a layer of EPS in

order to be inspected by the naked eye. Figure 38 shows the result of the peel

testing. With the lack of foam being attached to the polypropylene combined with the

ease the polypropylene was peeled off, it was determined that indeed that the failure

was due to the adhesive and not the region of foam neighbouring the adhesive.

Figure 37: Results of the wood resin adhesive bonding strength between EPS foam and polypropylene

after a peel test.

With the failure being adhesive related, an investigation began on how to achieve a

sturdier bond between the polystyrene and EPS foam. A surface pre-treatment guide

was provided by the company R.D Taylor, which provided useful surface pre-

treatments for Epoxy Araldite 2015 and polymer foams [18].

Pre-treatment on the polypropylene was attempted by sand-blasting the material and

by deforming it using emery paper. Both results found that this only frayed the

Page 69

polypropylene into an unusable condition. Different adhesives were also attempted,

and were also subjected to a peel test: epoxy araldite 2015 and two different types of

double sided tape.

(a) (b)

(c)

Figure 38: Results of peel testing. Bonds between the two materials were made by (a) Epoxy Araldite

2015, (b) Ultratape double sided tape and (c) Sellotape double sided tape

Analysis showed that the epoxy araldite 2015 maintained the same results of the

wood resin adhesive (figure 39 (a)). Although figure 39 (b) shows that there was

debris recorded with the clear double sided tape (Ultratape), which proved a slightly

better adhesion than the two previous adhesive substances, the clear double sided

tape did not perform as well as the white double sided tape. The white double sided

tape, mentioned previously as the Sellotape product) showed that it was possible for

the neighbouring foam region and not the adhesive, to fail first (figure 39 (c)).

Evident of foam failure can be observed with several beads attached to the tape,

along with fracturing of the foam from the resulting peel test.

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Improved Samples

Due to the knowledge gained by the initial plane-strain deformation experiments,

especially the way certain specimens achieved failure; newer ‘improved’ samples

were created in order to achieve the second objective. Samples which contained the

double sided tape were initially created to observe the effect of different adhesion.

Double sided tape was another adhesion method that was recommended by [22].

Two different brands of double sided tape were bought in order to be experimented

on. The first was Sellotape double sided tape, which can be described as white and

had a width of 15mm, which therefore allowed three slices of tape of length 100mm

to cover the cross-sectional area of the EPS foam. The second brand was Ultratape

double sided tape. This tape was clear and had a width of 12mm, which would allow

four slices of tape of length 100mm for the same purpose as the previous tape.

By applying adhesion with the double sided tape would provide an ease of

manufacturing with the specimens i.e. these samples would be simpler to make and

control than the wood resin adhesive.

One Sided Bond with Both Types of Double Sided Tape between Polypropylene and

EPS Foam

Samples bonded by the white double sided tape achieved slightly worse results than

the one sided bonded with polypropylene with minimum adhesive (figure 26). The

clear tape samples faired the worse out of the one sided bonded with polypropylene

samples, but again the difference between these samples were not that significantly

great with similar trends in all three samples. The similarity in results suggested that

no matter what the type of bonding was, the area that would fail first under enough

compressive strength with the small indenter, would be the weakened foam area

neighbouring the adhesive. However, investigation into the adhesion effects found in

the ‘Adhesive Failure’ chapter confirmed that failure was not with the weakened foam

area neighbouring the adhesive but the adhesive itself.

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0

100

200

300

400

500

600

700

0 10 20 30 40

Displacment (mm)

For

ce (N

)

One Sided Bond Tape(White)

One Sided Bond Tape(Clear)

One Sided Bond Min 2

Figure 39: Force Displacement response curves of the one sided bonds with double sided tape between

polypropylene and EPS foam, tested with the small indenter. Similarity in curves suggested that

neighbour foam area caused failure, but was disproved in the ‘Adhesive Failure’ chapter by peel testing.

When creating these samples, it could be seen that by even adding a minimum

amount of wood resin adhesive, with weight would almost double. Thus another

advantage of testing specimens with double sided tape would be the possible

reductions in weight. Although reductions on weight are hardly significant when

compared to the wood resin adhesive, further reductions can still be made. Currently

these tape samples have tape covering the whole cross-sectional area of the foam to

provide the adhesion. This can be considered overkill, with firm bonds between the

EPS foam and polypropylene being achieved by taping only the outer surfaces of the

foam, as shown in the figures below. By using less double sided tape, the density of

the whole specimen will lower.

Double Sided Bond with Sellotape Double Sided Tape between Polypropylene and

EPS foam

Through investigation of firm bonding between EPS foams and polypropylene, the

second objective of this project was then able to continue. By bonding EPS foam

and polypropylene on both sides, even though there was a loss in flexibility, it was

hoped that these specimens were able to make an active trauma pack absorb as

Page 72

much, or even more energy, than the monolithic samples, with a reduction in fracture

length, during the indenture tests. These samples were essentially a combination of

the double bonded with polypropylene samples but bonded with the white, Sellotape

double sided tape. Once again, the whole cross-sectional area of the EPS foam was

covered in tape, to ensure a maximum, firm bond.

0

100

200

300

400

500

600

700

800

900

0 5 10 15 20 25 30 35 40

Displacement (mm)

For

ce (N

)

Figure 40: Force-displacement response curves between monolith and the double side bonded

specimens in the small indentation tests. Key: Monolith, Double bonded with minimum adhesive, Double

bonded with 2.5ml adhesive and Double bonded with tape.

As figure 41 shows, in expectations of these samples performing the best, in truth

they performed the worse. No clear typical polymeric foam compression response

shape can be seen with the double bond tape specimens. Regards to creating an

EPS specimen that has improved shear properties than a monolith sample, yet able

to absorb as much, if not more, work energy has failed.

Figure 41 shows the fracture behaviour the foam responded to. Out of all of the

samples which had signs of fracture, the double sided bond with polypropylene with

Sellotape double sided tape had the smallest cracks which could be viewed by the

naked eye.

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(a) (b)

Figure 41: Failure analysis of the double side bonded with Sellotape double sided tape between

polypropylene and EPS foam. Figure 45 (a) shows the smallest fractures with the double sided bond

samples. Figure 45 (b) confirms that unlike specimens bonded with the wood resin adhesive,

specimens bonded with the double sided tape did not fail due to unable to maintain a firm bond between

polypropylene and EPS foam.

The energy absorption results, or lack of, are believed to be accounted for in tensile

direction compromised by damaged cell faces. Although successful in improving the

shear properties of the specimen, by reorganizing the EPS foam in this layered

manner affects too much foam cells by damaging them and not able to contribute

effective to the overall performance of the foam.

The different types of bonding also have affect on the plane-strain deformation tests.

Taped samples are unable to perform better than samples bonded with the wood

resin due to the reinforced damaged cells the wood resin samples received when the

adhesive is applied to the foam. Because the tape is not in a fluid substance, this will

not allow the adhesive to seep into the foam and ‘mend’ the damaged cells caused

by cutting the foam into layers.

A closer look at the incision method could be employed. Cutting the method using

the band saw clearly creates too many damaged cell faces and debris, affecting the

results. Future investigations into achieving a smoother surface finish and maybe

less damaged cell faces, may be found through the incision method with the lathe,

along with the blade and attachment.

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Limitations on Experiment

As mentioned before in this report, there are several limitations and restrictions with

the quasi-static plane-strain deformation impact experiments, which do not truly

reflect the foams performance within a ballistic environment. Therefore, the results

and conclusions that are derived from this report cannot be considered as great

models for dynamic experimentations. Further investigation and experiments will

have to be done in order for foam to advance, if foam is considered to be a worthy

material for protection. Once again, it is stressed that the project of ‘developing a

ballistic trauma pack’ is to investigate the basic idea whether or not foam can be

used as an active trauma pack.

There are three main limitations on the quasi-static plane-strain deformation impact

experiments, all of which are related to the machine that is used to obtain these

results, the Zwick Roell Z2.0. These restrictions include: speed, sampling rates and

force limitations.

Speed

All quasi-static plane strain deformation experiments are operated at the Zwick

Roells fastest strain rate, which is 15000mm/min. Conversion puts this strain rate at

0.25m/s. Comparing this to the speed of a .22’’ solid long rifle round which travels at

roughly 330m/s, the quasi-static experiments done in this project are estimated to

travel 1320% slower than the slowest .22’’ long rifle ammunition range. Recalling

back on the introduction, strain rates of up to m/s may even be encountered due to

blast and shock waves. As a result, only an observation can be made on how higher

strain rate affects the strain dependent foam and only assumptions can be made

from the trends or patterns about how the foam will respond to ballistic conditions.

Further analysis and investigation will have to involve ballistic testing.

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Sampling

A specimen which is going through a standard compression test with the Zwick Roell

Z2.0, at a strain rate of 5mm/min can obtain several thousand points of data. This

allows a very accurate stress-strain response curve to be drawn and precise

calculations, such as the automated method for calculate the foams Young’s

Modulus. Comparing this to the strain rate of 15000mm/min, only a limited number of

around a hundred data points were retrieved. The outcome of this lack of data points

were that even though a stress-strain response could be acquired from the test data,

the results were not as detailed as the results done with the slower strain rate. It also

signified that due to the lack of amount of sampling data, the automated Young’s

Modulus calculations could not be determined. Attempts to solve this restriction have

all failed, and are an impediment that has to be abided by.

Force

With the Zwick Roell Z2.0, an upper force limit of 2000N or 2kN cannot be exceeded

without causing an error with the machine. Already mentioned and encountered

before during the standard compression tests, the upper force limit restriction

hindered analysis of the EPS foam response and sometimes even impeded on

retrieving the data obtained during testing. Initial attempts to solve the problem of

data retrieval was done by asking the software recognize breach of lower force limits

proved to be unsuccessful when strain rates of 1000mm/min and above where

applied. The cause of this unsuccessful attempt is believed to be related to switching

relay delays within the machine. The feedback from the apparatus to the software of

the Zwick Roell Z2.0 is not quick enough when travelling at such high strain rates and

encountering such erratic signals from the force. The restriction however, was

eventually resolved manually, through trial and error methods. By using dummy

specimens, the specimen was deformed at the desired strain rate by a certain

displacement and a force reading was given. Judging from the force readout, it was

decided whether the deformation should increase, decrease or remain.

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Further Analysis

There is evidence of all three deformations that happen with the specimens when

they are being tested in the plane strain deformation experiments (figure 46). The

compressive response is evident with the data and graphs that were able to be

collected in the experiments. Tensile deformation is apparent due to the presence of

fractures that lie within the samples, especially the monolith and double bonded

samples. Not only the tensile deformation is evident through observations with the

high speed camera, but shear deformation is also noticeable. A possible method of

obtaining the data of both tensile and shear deformation would be manipulation with

image analysis software, of the images taken from the high speed camera.

(a) (b)

Figure 46: Deformation analysis with a high speed camera on a monolith EPS foam under small

indenter testing at a strain rate of 0.25m/s. Figure 46(a) has imprecise green line, showing elongation

and extension within the top surface of specimen foam cells, whereas Figure (b) gives a clear

It can be concluded that in terms of achieving the two objectives set out has neither

been achieved nor failed. Due to unexpected interferences, such as the layer of

damaged cells and wood resin reinforcement effects, these undesired variables alter

the results of the foam response and a conclusion is unable to be made in terms of

the objectives. Further investigation in order to achieve both objectives will have to

either done by ‘Analytical Analysis’ methods or testing with an ‘Aero-gel’ foam, where

cell sizes of this material are in nanometres.

Page 77

Conclusion

� The Jablite EPS foam was able to demonstrate the common response of

polymeric foam under uniaxial compression with the three regimes: linear plastic,

plateau and densification stages.

� By reorganizing the foam in a layered manner, compared to a monolithic block,

initially it was confirmed that these were able to achieve almost identical strength

and energy absorption results with a variation in elastic modulus only. However,

but cutting the foam layers even thinner, but not surpassing limits which dictate

when ‘size effect’ occurs, changed the identical response, to a deteriorating one.

This is due to the layer of damaged cell faces introduced when cutting the foam

into thin layers. It is this layer of damaged cells.

� Polymeric foams are indeed strain sensitive materials. By altering the uniform

loading during compression testing from 8.33e-5m/s to 0.25m/s changes in the

mechanical properties of the EPS foam can be witnessed. Dramatic differences

in the foams Young’s modulus can be observed, when changes of values from

2.01-2.532MPa to 22.31-89.43MPa for respective strain rates prove the strain

rate dependence. Response of when the foam enters the plateau stage can also

be observed by changing the strain rate.

� Flexible specimens were made by tiling the samples (cutting them into segments)

after they were fixed onto a fabric backing layer. The compressive data showed

that the results did not suffer any deterioration in strength or energy absorption

properties when compared to identical un-tiled samples, although there was still a

discrepancy between monolith samples due to the layer of damaged cells

explained above. Three-point bend tests confirmed that these samples were

indeed flexible in one direction and not the other, validating the creation of a more

comfortable trauma pack.

� Fracture propagation was indeed restrained by putting a denser material behind

the foam. However, samples were not able to obtain the same amount of energy

absorption properties, as shown by monolith samples, due to the interference of

damaged cell layers and adhesive reinforcement of these damaged cells.

� With the unforeseen impedance of unexpected variables, such as the layer of

damaged cells and adhesive reinforcement affects, it can be concluded that in

order to investigate the two objectives properly, ‘Analytical Analysis’ will have to

be employed. It may also be possible to investigate these objectives with the an

‘aero-gel’ foam, where the cell sizes of this material are in nanometres.

Page 78

References

[1] Major Gryth D. SF Swedish Armed Forces, Severe Lung Contusion and Death

after High-Velocity: Behind-Armour Blunt Trauma: Relation to Protection Level,

MILITARY MEDICINE, 172, 10; 1110. 2 f) 07

[2] Ballistic Resistance of Body Armour National Institute of Justice (NIJ) Standard-

0101.06, U.S. Department of Justice, Office of Justice ProgramsNational

[3] Drobin D, Gryth D, Persson J K E, Rockse´ n D, Arborelius U P, Olsson L,

Bursell J, and Kjellstro¨ m B. Electroencephalogram, Circulation, and Lung Function

After High-Velocity Behind Armor Blunt Trauma. The Journal of TRAUMA Injury,

Infection, and Critical Care. Volume 63 number 2, pp 405-413.

[4] Ouelleta S, Croninb D, Worswickb M, Compressive response of polymeric foams

under quasi-static, medium and high strain rate conditions, Polymer Testing 25

(2006) 731–743

[5] Wilhelm M, Bir C, Injuries to Law Enforcement Officers: The Backface Signature

Injury, Forensic Science International 174 (2008) 6–11

[6] National Institue of Justice. Ballistic resistance of personal body armour, NIJ

Standard 0101.06.

[7] A.Brent Strong, Plastics: Materials and Processing, 2nd Edition, Foams

[9] Mills, N J. (2007), Polymer Foams Handbook: Engineering and Biomechanics

Applications and Design Guide, Butterworth-Heinemann

[10] N. C. Hilyard, Shock mitigation material behaviour. Mechanics q[Cellular

Plastics, p. 180. Applied Science Publishers, Barking, U.K. (1982).

[11] Gilchrist A, Mills, N J. (2001), Impact deformation of rigid polymeric foams:

experiments and FEA modeling. International Journal of Impact Engineering 25

(2001) 767–786.

Page 79

[12] Rao P N, Hofer K E, The mechanical behaviour of flexible polyurethane foams under high rate loading. J. Mater.6 (3), 704~717 (1971)

[13] Ma G W, Ye Z Q, Shao ZS, Modelling Loading Rate Effect on Crushing Stress of

Metallic Cellular Materials, International Journal of Impact Engineering 36 (2009)

775–782

[14] Magkiriadis Q M Li I, Harrigan J J, Compressive Strain at the Onset of

Densification of Cellular Solids, Journal of Cellular Plastics 2006

[15] Shim V EW, Yap K Y, Modelling Impact Deformation of Foam-Plate Sandwich

Systems, Int. J. Impact Engng, Vol. 19, No. 7, pp. 615-636, 1997

[16] S. J. Green, F. L. Schierloh. R. D. Perkins and S. G. Babcock, High-velocity

deformation properties of polyurethane foams. Exp. Mech. el, 103 109 (1969}.

[17] Methods for Test for Rigid Cellular Materials: Part 4: Method 14. Determination

of Flexural Properties, British Standard, BS-4370-4

[18] Araldite® Bonding: Surface Preparation and Pre-treatment, Huntsman

[19] Sriram R, Vaidya U K, Blast Impact Response of Aluminimum Foam Sandwich

Composites, J MATER SCI 41 (2006) 4023–4039

[20] SHIM V. P. W, YAP K. Y., Static and Impact Crushing of Layered Foam-Plate

Systems, Int. I. Mech S¢i VoL 39, No. 1, pp. 69-86. 1997