Energy Absorbing Foam
Foam is an object formed by trapping pockets
of gas in
a liquid or solid.[1][2][3] A bath sponge and
the head on a glass of beer are examples of foams. In most foams,
the volume of gas is large, with thin films of liquid or
solid separating the regions of gas. Soap foams are also known
as suds.
Solid foams can be closed-cell or open-cell. In closed-cell
foam, the gas forms discrete pockets, each completely surrounded by
the solid material. In open-cell foam, gas pockets connect to each
other. A bath sponge is an example of an open-cell foam: water
easily flows through the entire structure, displacing the air.
A camping mat is an example of a closed-cell foam: gas
pockets are sealed from each other so the mat cannot soak up
water.
Foams are examples of dispersed media. In general, gas is
present, so it divides into gas bubbles of different sizes (i.e.,
the material is polydisperse)—separated by liquid regions that
may form films, thinner and thinner when the liquid phase drains
out of the system films.[4] When the principal scale is
small, i.e., for a very fine foam, this dispersed medium can be
considered a type of colloid.
Foam can also refer to something that is analogous to foam,
such as quantum foam, polyurethane foam (foam
rubber), XPS foam, polystyrene, phenolic, or many
other manufactured foams.
Polyurethane Energy Absorbing Foams for Automotive
Applications
The use of rigid polyurethane (PU) foams in automobiles for
energy management applications is not a new concept. It is well
documented that its low density, moldability, and energy absorbing
characteristics have had advantages for use in knee bolsters, door
panel bolsters, steering wheel columns, head-rests, instrument
panels, and other interior as well as exterior automotive
applications.
Developing an “ideal” energy absorber, one that produces a
square wave when dynamically impacted and statically crushed, thus
maximizing the energy absorption is the goal of energy management.
This ideal energy absorber would minimize the amount of vehicle
occupant compartment space loss by reducing the energy absorbing
bolster size required to decrease the impact loads imparted on the
occupants during a collision.
When considering polyurethane rigid foams for energy absorbing
materials, elimination of CFCs or HCFCs (hydro chlorofluorocarbons)
has become an environmental and economic necessity. BASF
Corporation's Urethane Applications Development Group in Wyandotte,
Michigan has developed new CFC-free, water blown, low density,
rigid, Energy Absorbing (EA) polyurethane foams to accomplish these
goals. They are unique in their ability to efficiently absorb
energy during an impact, rather than storing it as potential energy
and then transferring this potential energy to the occupant which
could increase the probability of injury.
The high efficiency of these energy absorbing foams minimizes
interior space loss compared to other EA materials by reducing part
size. These low density foams can be molded efficiently into a
variety of shapes using existing PU processing equipment.
What is Energy-Absorbing Foam?
The foam capable of absorbing the impact energy due to crash by
its nature of elasticity is called as Energy-Absorbing Foam
Two different types of energy-absorbing foam:
1) EPS or Expanded Polystyrene foam, or
2) EPP or Expanded Polypropylene foam.
Both EPS and EPP are closed cell, lightweight and resilient
foamed plastics designed to cushion you from impact forces.
The Energy-Absorbing Foam located along the inside of the shells
and head restraints on the car seat Energy-absorbing
foam is used to absorb some of those crash forces, keeping
passengers from absorbing them and therefore minimizing injury.
The energy-absorbing foam deforms during a crash when your body
makes contact with it. At the point of contact, the foam crushes;
the energy required to crush the foam is absorbed by the foam
instead of your body. The foam also acts as a barrier between you
and the rigid seat shell and any other intruding objects such as
the vehicle door or window.
In the life span of a car seat, it is possible that components
may be affected by wear and tear or breaking. The seat is still
structurally safe to use as long as the foam is positioned in the
same place in which it was intended. Sometimes taping the foam in
place is suggested in order to prevent it from wiggling. Taping the
foam does not compromise the safety of the car seat.
Head impact protection (FMVSS 201)
Thin energy absorbing inserts protect the head against impact
forces. Parts made from PUR, fulfill the US standard for head
impact protection even at limited wall thickness.Fabrics and vinyl
skins can be insert-molded as an A-, B-, or C- pillar, providing a
"one-shot-part" with the interior surface, energy absorbing
function and insert-molded clips, eliminating the outer shell
(injection molded) part entirely.
Knee impact protection
In the IP or steering column an EA insert is used to protect the
knee against impact forces. An insert molded sheet metal further
distributes impact energy.
Side impact protection
Energy Absorbing PUR Crashpads substantially reduce side impact
forces. They protect occupants against hip- and thorax injuries and
can function in combination with side airbags. Hardness for hip- or
thorax impact protection requirements can be individually
adjusted.
Bumper inserts
Various EA parts made of a recoverable PUR foam are integrated
into the bumper. The foam compresses upon impact at 30 - 40% and
recovers back to 97% of its original thickness.
Pedestrian protection
New European regulations for pedestrian protection are currently
being established. THIEME offers a solution for "soft" bumpers by
insert molding "Class A" surfaces with recoverable EA foam that
drastically improve leg injury protection.
Internal Plant Logistics Containers
For the internal logistics at the automotive manufacturers`and
their suppliers`assembly lines, such as
· transport and storage containers
· workpiece holders
PUR material systems from the leading raw material
manufacturers Bayer and BASF . The materials are purchased as
preformulated systems and processed into high-quality shaped
part.
PUR integral hard foamWhy this foam?
+ Sandwich-style design with high inherent rigidity
+ Extraordinary design flexibility
+ Good thermal and acoustic insulation properties
+ Variable wall sections in a part without sink or
distortion
Fields of application
· Medical technology
· Handling units
· Access systems
· Bank and payment stations
· Consumer electronics
· Beverage industry
· Workpiece carriers
Technical data
Properties
Unit
Standard
IHS
Density
kg/m³
ISO/R 1183 / DIN 53479
600
Pulling strength
MPa
ISO/R 527 / DIN 53455
19
Ultimate elongation
%
ISO/R 527 / DIN 53455
6
Bending strength (max.)
MPa
ISO 178 / DIN 53452
38
Bending E-module
MPa
DIN 53457
900
Impact strength at 23 °C
kJ/m²
ISO/R 179 / DIN 53453
14
Surface hardness
Shore D
67
Dimensional stability (HDT)
°C
ISO 75-2
97
Values not absolute some may be altered.
Foam is an object formed by trapping pockets
of gas in
a liquid or solid.[1][2][3] A bath sponge and
the head on a glass of beer are examples of foams. In most foams,
the volume of gas is large, with thin films of liquid or
solid separating the regions of gas. Soap foams are also known
as suds.
Solid foams can be closed-cell or open-cell. In closed-cell
foam, the gas forms discrete pockets, each completely surrounded by
the solid material. In open-cell foam, gas pockets connect to each
other. A bath sponge is an example of an open-cell foam: water
easily flows through the entire structure, displacing the air.
A camping mat is an example of a closed-cell foam: gas
pockets are sealed from each other so the mat cannot soak up
water.
Foams are examples of dispersed media. In general, gas is
present, so it divides into gas bubbles of different sizes (i.e.,
the material is polydisperse)—separated by liquid regions that
may form films, thinner and thinner when the liquid phase drains
out of the system films.[4] When the principal scale is
small, i.e., for a very fine foam, this dispersed medium can be
considered a type of colloid.
Foam can also refer to something that is analogous to foam,
such as quantum foam, polyurethane foam (foam
rubber), XPS foam, polystyrene, phenolic, or many
other manufactured foams.
Structure
A foam is, in many cases, a multi-scale system.
Order and disorder of bubbles in a surface foam
One scale is the bubble: material foams are
typically disordered and have a variety of bubble sizes.
At larger sizes, the study of idealized foams is closely linked to
the mathematical problems of minimal surfaces and
three-dimensional tessellations, also called honeycombs.
The Weaire–Phelan structure is considered the best
possible (optimal) unit cell of a perfectly ordered
foam,[5] while Plateau's laws describe how
soap-films form structures in foams.
At lower scale than the bubble is the thickness of the film
for metastable foams, which can be considered a network
of interconnected films called lamellae. Ideally, the lamellae
connect in triads and radiate 120° outward from the connection
points, known as Plateau borders.
An even lower scale is the liquid–air interface at the surface
of the film. Most of the time this interface is stabilized by a
layer of amphiphilic structure, often made
of surfactants, particles (Pickering emulsion), or more
complex associations.
Applications : Liquid foams
Liquid foams can be used in fire retardant foam, such as
those that are used in extinguishing fires, especially oil
fires.
In some ways, leavened bread is a foam, as
the yeast causes the bread to rise by producing tiny
bubbles of gas in the dough. The dough has traditionally been
understood as a closed-cell foam, in which the pores do
not connect with each other. Cutting the dough releases the gas in
the bubbles that are cut, but the gas in the rest of the dough
cannot escape. When dough is allowed to rise too far, it becomes an
open-cell foam, in which the gas pockets are connected. Now, if the
dough is cut or the surface otherwise broken, a large volume of gas
can escape, and the dough collapses. The open structure of an
over-risen dough is easy to observe: instead of consisting of
discrete gas bubbles, the dough consists of a gas space filled with
threads of the flour-water paste. Recent research has indicated
that the pore structure in bread is 99% interconnected into one
large vacuole, thus the closed-cell foam of the moist dough is
transformed into an open cell solid foam in the bread.[13]
The unique property of gas-liquid foams having very high
specific surface area is exploited in the chemical processes
of froth flotation and foam fractionation.
Solid foams
Solid foams are a class of lightweight cellular engineering
materials. These foams are typically classified into two types
based on their pore structure: open-cell-structured foams (also
known as reticulated foams) and closed-cell foams. At high
enough cell resolutions, any type can be treated as continuous or
"continuum" materials and are referred to as cellular
solids,[14] with predictable mechanical properties.
Open-cell-structured foams contain pores that are connected to
each other and form an interconnected network that is relatively
soft. Open-cell foams fill with whatever gas surrounds them. If
filled with air, a relatively good insulator results, but, if the
open cells fill with water, insulation properties would be reduced.
Recent studies have put the focus on studying the properties of
open-cell foams as an insulator material. Wheat gluten/TEOS
bio-foams have been produced, showing similar insulator properties
as for those foams obtained from oil-based
resources.[15][16] Foam rubber is a type of open-cell
foam.
Closed-cell foams do not have interconnected pores. The
closed-cell foams normally have higher compressive strength due to
their structures. However, closed-cell foams are also, in general
more dense, require more material, and as a consequence are more
expensive to produce. The closed cells can be filled with a
specialized gas to provide improved insulation. The closed-cell
structure foams have higher dimensional stability, low moisture
absorption coefficients, and higher strength compared to
open-cell-structured foams. All types of foam are widely used as
core material in sandwich-structured compositematerials.
The earliest known engineering use of cellular solids is with
wood, which in its dry form is a closed-cell foam composed of
lignin, cellulose, and air. From the early 20th century, various
types of specially manufactured solid foams came into use. The
low density of these foams makes them excellent as
thermal insulators and flotation devices and their
lightness and compressibility make them ideal as packing materials
and stuffings.
An example of the use of azodicarbonamide[17] as a blowing
agent is found in the manufacture of vinyl
(PVC) and EVA-PE foams, where it plays a role in the
formation of air bubbles by breaking down into gas at high
temperature.[18][19][20].
The random or "stochastic" geometry of these foams makes them
good for energy absorption, as well. In the late 20th century to
early 21st century, new manufacturing techniques have allowed for
geometry that results in excellent strength and stiffness per
weight. These new materials are typically referred to as engineered
cellular solids.[14]
Syntactic foam
A special class of closed-cell foams, known as syntactic foam,
contains hollow particles embedded in a matrix material. The
spheres can be made from several materials, including glass,
ceramic, and polymers. The advantage of syntactic
foams is that they have a very high strength-to-weight ratio,
making them ideal materials for many applications,
including deep-sea and space applications. One particular
syntactic foam employs shape memory polymer as its
matrix, enabling the foam to take on the characteristics of shape
memory resins and composite materials; i.e., it has the
ability to be reshaped repeatedly when heated above a certain
temperature and cooled. Shape memory foams have many possible
applications, such as dynamic structural support, flexible foam
core, and expandable foam fill.
Integral skin foam
Integral skin foam, also known as self-skin foam, is a type
of foam with a high-density skin and a low-density core. It can be
formed in an open-mold process or a closed-mold
process. In the open-mold process, two reactive components are
mixed and poured into an open mold. The mold is then closed and the
mixture is allowed to expand and cure. Examples of items produced
using this process include arm rests, baby
seats, shoe soles, and mattresses. The closed-mold
process, more commonly known as reaction injection
molding (RIM), injects the mixed components into a closed mold
under high pressures.[21]
Foam, in this case meaning "bubbly liquid", is also produced as
an often-unwanted by-product in the manufacture of
various substances. For example, foam is a serious problem in
the chemical industry, especially
for biochemical processes. Many biological substances,
for example proteins, easily create foam
on agitation or aeration. Foam is a problem because
it alters the liquid flow and blocks oxygen transfer from air
(thereby preventing microbial respiration
in aerobic fermentationprocesses). For this
reason, anti-foaming agents, like silicone oils, are
added to prevent these problems. Chemical methods of foam control
are not always desired with respect to the problems
(i.e., contamination, reduction of mass transfer) they
may cause especially in food and pharmaceutical industries, where
the product quality is of great importance. Mechanical methods to
prevent foam formation are more common than chemical ones.
Gallery[edit]
Close-up of sea foam(decomposing plankton) on
a tide pool
Foamed aluminium
Micrograph of temper(memory) foam
Silicone foampenetration seal
Diet Coke and Mentosfoam "geyser"
Industrial CT scanning of a foam ball
Polystyrene foam cushioning
Aluminium foam sandwich
Aluminium foam sandwich
Aluminium foam sandwich (AFS) is a sandwich panel product
which is made of two metallic dense face sheets and a metal
foam core made of an aluminium alloy. AFS is an engineering
structural material owing to its stiffness-to-mass ratio and energy
absorption capacity ideal for application such as the shell of
a high-speed train.[1]
Metal foam
A metal foam is a cellular structure consisting of a
solid metal (frequently aluminium) with
gas-filled pores comprising a large portion of the
volume. The pores can be sealed (closed-cell foam) or
interconnected (open-cell foam). The defining characteristic of
metal foams is a high porosity: typically only 5–25% of the
volume is the base metal, making these ultralight materials.
The strength of the material is due to the square-cube
law.
Metallic foams typically retain some physical
properties of their base material. Foam made from
non-flammable metal remains non-flammable and can generally be
recycled as the base material. Its coefficient of thermal
expansion is similar while thermal conductivity is
likely reduced.[1]
Foamed aluminium
Regular foamed aluminium
Open-cell metal foam
CFD (numerical simulation) of fluid flow and heat transfer on an
open cell metal foam
Open-cell foam
Open-cell metal foam:
Open celled metal foam, also called metal sponge,[2] can be
used in heat exchangers(compact electronics
cooling, cryogen tanks, PCM heat exchangers), energy
absorption, flow diffusion, and lightweight optics. The high
cost of the material generally limits its use to advanced
technology, aerospace, and manufacturing.
Fine-scale open-cell foams, with cells smaller than can be seen
unaided, are used as high-temperature filters in the
chemical industry.
Metallic foams are used in compact heat exchangers to increase
heat transfer at the cost of reduced
pressure.[3][4][5][clarification needed] However, their use
permits substantial reduction in physical size and fabrication
costs. Most models of these materials use idealized and periodic
structures or averaged macroscopic properties.
Metal sponge has very large surface area per unit weight
and catalysts are often formed into metal sponge, such
as palladium black, platinum sponge, and spongy
nickel. Metals such as osmium and palladium
hydride are metaphorically called "metal sponges", but this
term is in reference to their property of binding to hydrogen,
rather than the physical structure.[6]
Manufacturing
Open cell foams are manufactured by foundry or powder
metallurgy. In the powder method, "space holders" are used; as
their name suggests, they occupy the pore spaces and channels. In
casting processes, foam is cast with an
open-celled polyurethane foam skeleton.
Closed-cell
Closed-cell metal foam was first reported in 1926 by Meller in a
French patent where foaming of light metals, either by inert gas
injection or by blowing agent, was suggested.[7] Two
patents on sponge-like metal were issued to Benjamin Sosnik in 1948
and 1951 who applied mercury vapor to blow liquid
aluminium.[8][9]
Closed-cell metal foams were developed in 1956 by John C.
Elliott at Bjorksten Research Laboratories. Although the first
prototypes were available in the 1950s, commercial production began
in the 1990s by Shinko Wire company in Japan. Closed-cell metal
foams are primarily used as an impact-absorbing material, similarly
to the polymer foams in a bicycle helmetbut for
higher impact loads. Unlike many polymer foams, metal foams remain
deformed after impact and can therefore only be deformed once. They
are light (typically 10–25% of the density of an identical
non-porous alloy; commonly those of aluminium)
and stiff and are frequently proposed as a lightweight
structural material. However, they have not been widely used for
this purpose.
Closed-cell foams retain the fire resistance and recycling
potential of other metallic foams, but add the property of
flotation in water.
Manufacturing
Foams are commonly made by injecting a gas or mixing
a foaming
agent into molten metal.[10] Melts can be
foamed by creating gas bubbles in the material. Normally, bubbles
in molten metal are highly buoyant in the high-density liquid and
rise quickly to the surface. This rise can be slowed by increasing
the viscosity of the molten metal by adding ceramic powders or
alloying elements to form stabilizing particles in the melt, or by
other means. Metallic melts can be foamed in one of three ways:
· by injecting gas into the liquid metal from an external
source;
· by causing gas formation in the liquid by admixing
gas-releasing blowing agents with the molten metal;
· by causing the precipitation of gas that was previously
dissolved in the molten metal.
To stabilize the molten metal bubbles, high temperature foaming
agents (nano- or micrometer- sized solid particles) are required.
The size of the pores, or cells, is usually 1 to 8 mm.
When foaming or blowing agents are used, they are mixed with the
powdered metal before it is melted. This is the so-called "powder
route" of foaming, and it is probably the most established (from an
industrial standpoint). After metal (e.g. aluminium) powders
and foaming agent (e.g.TiH2) have been mixed, they are compressed
into a compact, solid precursor, which can be available in the form
of a billet, a sheet, or a wire. Production of precursors can be
done by a combination of materials forming processes, such as
powder pressing,[11] extrusion (direct[12] or
conform[13]) and flat rolling.[14]
Composite
Composite metal foam (CMF) is formed from hollow beads of one
metal within a solid matrix of another, such as steel within
aluminium, show 5 to 6 times greater strength to density ratio and
more than 7 times greater energy absorption than previous metal
foams.[15]
A less than one inch thick plate has enough resistance to turn a
7.62 x 63 mm standard-issue M2 armor piercing
bullet to dust. The test plate outperformed a solid metal
plate of similar thickness, while weighing far less. Other
potential applications include nuclear
waste (shielding X-rays, gamma
rays and neutron radiation) transfer and thermal
insulation for space vehicle atmospheric re-entry, with twice the
resistance to fire and heat as the plain metals.[16]
CMF can replace rolled steel armor with the same protection for
one-third the weight. It can block fragments and the shock waves
that are responsible for brain injuries. Stainless
steel CMF can block blast pressure and fragmentation at 5,000
feet per second from high explosive incendiary (HEI)
rounds that detonate 18 inches from the shield. Steel CMF plates
(9.5 mm or 16.75 mm thick) were placed 18 inches from the
strikeplate held up against the wave of blast pressure and against
the copper and steel fragments created by a 23×152 mm HEI
round (as in anti-aircraft weapons) as well as a 2.3mm
aluminum strikeplate.[17]
Stochastic and regular foams[edit]
Stochastic
A foam is said to be stochastic when the porosity distribution
is random. Most foams are stochastic because of the method of
manufacture:
· Foaming of liquid or solid (powder) metal .
· Vapor deposition (CVD on a random matrix )
· Direct or indirect random casting of a mold containing beads
or matrix.
Regular
Manufacturing process of a regular metal foam by direct molding,
CTIF process
Foam is said to be regular when the structure is ordered. Direct
molding is one technology that produces regular foams
[18][19] with open pores. In the alternate, regular metal
foams can be produced by additive processes such as selective
laser melting (SLM).
Plates can be used as casting cores. The shape is customized for
each application. This manufacturing method allows for "perfect"
foam, so-called because it satisfies Plateau's laws and
has conducting pores of the shape of a truncated octahedron Kelvin
cell (body-centered cubic structure).
Kelvin cell (Similar to the Weaire–Phelan structure)
Regular foams gallery
Heat sink with copper foam
Crash box including Aluminium foam
Aluminium foam with big porosity
Aluminium foam with aluminium sheet
Header - steel metal foam
machined metal foam
Design heatsink with regular foam[21]
coffee table with large pored aluminium
Kelvin cell (Similar to the Weaire–Phelan structure)
Applications
Design
Metal foam can be used in product or architectural
composition.
Mechanical
Orthopedics
Foam metal has been used in experimental
animal prosthetics. In this application, a hole is drilled
into the bone and the metal foam inserted, letting the
bone grow into the metal for a permanent junction. For orthopedic
applications, tantalum or titaniumfoams are common
for their tensile strength, corrosion resistance
and biocompatibility.
The back legs of Siberian Husky named Triumph received
foam metal prostheses. Mammalian studies showed that porous metals,
such as titanium foam, may allow vascularization
within the porous area.[22]
Orthopedic device manufacturers use foam construction or metal
foam coatings[23] to achieve desired levels
of osseointegration.[24][25][26]
Automotive
The primary functions of metallic foams in vehicles are to
increase sound damping, reduce weight, increase energy
absorption in case of crashes, and (in military applications) to
combat the concussive force of IEDs. As an example, foam
filled tubes could be used as anti-intrusion
bars.[27] Because of their low density (0.4–0.9 g/cm3),
aluminium and aluminium alloy foams are under particular
consideration. These foams are stiff, fire resistant, nontoxic,
recyclable, energy absorbent, less thermally conductive, less
magnetically permeable, and more efficiently sound dampening,
especially when compared to hollow parts. Metallic foams in hollow
car parts decrease weakness points usually associated with car
crashes and vibration. These foams are inexpensive to cast with
powder metallurgy, compared to casting other hollow parts.
Compared to polymer foams in vehicles, metallic foams are
stiffer, stronger, more energy absorbent, and resistant to fire and
the weather adversities of UV light, humidity, and
temperature variation. However, they are heavier, more expensive,
and non-insulating.[28]
Metal foam technology has been applied to
automotive exhaust gas.[29] Compared to
traditional catalytic converters that
use cordierite ceramic as substrate, metal foam substrate
offers better heat transfer and exhibits excellent mass-transport
properties (high turbulence) and may reduce the quantity
of platinum catalyst required.[30]
Energy absorption
Aluminium crash graph
Metal foams are used for stiffening a structure without
increasing its mass.[31] For this application, metal foams are
generally closed pore and made of aluminium. Foam panels are glued
to the aluminium plate to obtain a resistant composite sandwich
locally (in the sheet thickness) and rigid along the length
depending on the foam's thickness.
The advantage of metal foams is that the reaction is constant,
regardless of the direction of the force. Foams have a plateau of
stress after deformation that is constant for as much as 80% of the
crushing.[32]
Thermal
Heat conduction in regular metal foam structure
Heat transfer in regular metal foam structure
Tian et al.[33] listed several criteria to assess a foam in
a heat exchanger. The comparison of thermal-performance metal foams
with materials conventionally used in the intensification of
exchange (fins, coupled surfaces, bead bed) first shows that the
pressure losses caused by foams are much more important than with
conventional fins, yet are significantly lower than those of beads.
The exchange coefficients are close to beds and ball and well above
the blades.[34][35]
Foams offer other thermo physical and mechanical features:
· Very low mass (density 5–25% of the bulk solid depending the
manufacturing method)
· Large exchange surface (250–10000 m2/m3)
· Relatively high permeability
· Relatively high effective thermal conductivities (5–30
W/(mK))
· Good resistance to thermal shocks, high pressures, high
temperatures, moisture, wear and thermal cycling
· Good absorption of mechanical shock and sound
· Pore size and porosity can be controlled by the
manufacturer
Commercialization of foam-based compact heat exchangers, heat
sinks and shock absorbers is limited due to the high cost of foam
replications. Their long-term resistance to fouling, corrosion and
erosion are insufficiently characterized. From a manufacturing
standpoint, the transition to foam technology requires new
production and assembly techniques and heat exchanger design.
Titanium foam
Titanium foams exhibit high specific strength, high
energy absorption, excellent corrosion resistance
and biocompatibility. These materials are ideally suited for
applications within
the aerospace industry.[1][2][3] An inherent
resistance to corrosion allows the foam to be a desirable candidate
for various filtering applications.[4][5] Further, titanium's
physiological inertness makes its porous form a promising candidate
for biomedical implantation
devices.[6][7][8][9][10][11] The largest advantage in
fabricating titanium foams is that the mechanical and functional
properties can be adjusted through manufacturing manipulations that
vary porosity and cell morphology. The high appeal of
titanium foams is directly correlated to a multi-industry demand
for advancement in this technology.
Microstructure
Titanium foams are characterized structurally by their pore
topology (relative percentage of open vs. closed
pores), porosity (the multiplicative inverse of relative
density), pore size and shape, and
anisotropy.[13] Microstructures are most often examined
by optical microscopy,[14]scanning electron
microscopy [15] and X-ray tomography.[16]
Categorizing titanium foams in terms of pore structure (as
either open- or close-celled) is the most basic form of
differentiation. In close-celled foams, pores are composed of
bubbles entrapped in the metallic solid. These foams consist of a
continuous network of sealed pores wherein interconnections between
the pores are virtually non-existent. Alternatively, in open-celled
foams, the pores are interconnected and solid struts allow fluid to
pass through.[17]
Most manufactured foams contain both types of pores, although in
many cases the subtype is minimal.[18] According to
the IUPAC, pore sizes are classified into three categories:
micro (less than 2 nm), meso (between 2 and 50 nm) and
macro (larger than 50 nm) pores.[18]
Mechanical properties
As with other metal foams, the properties of titanium foams
depend mostly on the properties of the starting material and the
relative density of the resultant foam. Thermal properties in foams
– such as melting point, specific heat and expansion coefficient –
remain constant for both the foams and the metals from which they
are composed. However, the mechanical properties of foams are
greatly influenced by microstructure, which include the
aforementioned properties as well as anisotropy and defects within
the foam's structure.[19]
Sensitivity to impurities
The mechanical properties of titanium foams are
sensitive to the presence of interstitial solutes, which present
limitations to processing routes and utilization. Titanium has a
high affinity for atmospheric gases. In foams, this is
evidenced by the metal's tendency to trap oxides within cell
edges.[20][21][22] Micro-hardness of cell walls, elastic
modulus, and yield strength increase as a result of
interstitial solutes; ductility, which is a function of the
quantity of interstitial impurities, is consequently
reduced.[23] Of the atmospheric gases, nitrogen has the most
significant impact, followed by oxygen and carbon.[24] These
impurities are often present in the precursor mixture and also
introduced during processing
Applications
Potential structural applications for titanium foams include
their general incorporation into light-weight structures and as
components for mechanical energy absorption. The most important
considerations for the use of titanium foams in structural
applications includes their porosity, specific strength, ductility
in compression and cost. Because of low manufacturing costs, most
metal foams marketed for structural applications are of a
close-celled aluminum variety.[66] In comparison, titanium
foam manufacturing incurs a higher cost, but this cost is
defensible in space applications where the material offers an
otherwise incomparable reduction in overall weight. The lower
thermal conductivity of titanium may also be appreciated in rocket
construction.[1] The specific strength, overall energy
absorbing capability and high melting point all reinforce
titanium's superiority to aluminum in aerospace and military
applications.[3] When used for aerospace applications, levels
of porosity close to 90% are desired.[53]Titanium foams are capable
of retaining their high tensile strength at temperatures up to
400 °C; a limit imposed by the metal's low resistance to
oxidation.[36]
Aerospace applications
The driving force for titanium foam's replacement of existing
materials in the aerospace sector results from the following five
factors:[36]
· Weight reduction: as a substitute for steels and nickel-based
superalloys;
· Application temperature: as a substitute for aluminum and
nickel-based alloys and steels
· Corrosion resistance: as a substitute for aluminum alloys and
low-alloyed steels
· Galvanic compatibility: with polymer matrix composites as
substitutes for aluminum alloys
· Space constraints: as substitutes for aluminum alloys and
steels
The most urgent problem of engineering and its advanced branch
of aerospace engineering is the efficient use of materials as well
as increased service life.[1]
Sandwich panel cores[edit]
Model of a sandwich panel assembly
Sandwich panel cores are used throughout the aerospace industry;
they are integrated within aircraft bodies, floors and internal
panels. Sandwich constructions consist of two faces separated by a
thick, light-weight core and are most commonly composed of
balsa-wood, foamed polymers, glue-bonded aluminum or Nomex (paper)
honeycombs. Typically, the cores are combined with reinforcing
fibers to increase their shear modulus.[67] Indeed, carbon
fiber-reinforced polymers exhibit the highest specific stiffness
and strength of these materials.[68][69] However, polymers
decompose at low temperatures; thus employment of the
aforementioned materials pose inherent challenges due to the
limited range of temperature they may be utilized within as well as
their moisture-dependent properties.[13] The largest and most
inadequately predicted failure within the core results from strain
localization. Strain localization refers to the development of
bands exhibiting intensive straining as a result of the
localization of deformations in the solid.[70] [71] For
the best performance, the structure should exhibit low peak
response force and high total energy absorption.[18] Titanium
foams are lightweight, stiff, and possess the capability to resist
blast. Furthermore, the use of titanium-based foams exhibiting
homogenous porosity distribution would significantly decrease the
risks associated with strain localization. The high
strength-to-weight ratio of titanium foams offers an opportunity to
provide increased bending and shearing stiffness as well as energy
absorption capabilities during periods of
bending.[67][71][72] Titanium foams may be utilized in
environments with elevated temperatures (up to 400 °C).
Composite structures may also be produced; the incorporation of
silicon carbide monofilaments into Ti-6-Al-4V foams was shown to
exhibit an elastic modulus of 195 GPa and tensile strength of 800
MPa.[73]
Titanium foamed with argon
Titanium powder
Titanium sponge cylinder produced via powder metallurgy, 120
grams, 3×4 cm
