Experiment assessment of the ballistic response of ... · Experiment assessment of the ballistic response of composite pyramidal lattice truss structures Christian J. Yungwirth a,*,

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Composites: Part B 39 (2008) 556–569

Experiment assessment of the ballistic response of compositepyramidal lattice truss structures

Christian J. Yungwirth a,*, Darren D. Radford a,b, Mark Aronson a, Haydn N.G. Wadley a

a Department of Material Science and Engineering, School of Engineering and Applied Science, University of Virginia, Charlottesville, VA 22903, USAb Department of Engineering, Cambridge University, Trumpington Street, Cambridge CBZ 1PZ, United Kingdom

Received 22 July 2006; accepted 22 February 2007Available online 23 March 2007

Abstract

Sandwich panels with lattice cores have attracted significant interest as multifunctional structures. The lattices consist of 3D repeatingunit cells constructed from plates or trusses oriented to efficiently support applied stresses. These systems show promise for supportingstructural loads and mitigating the blast effects of explosions. Here, a preliminary study has been conducted to investigate the ballisticbehavior of a model lattice and to explore the effect of filling the lattices void spaces with polymers and ceramics. A sheet folding andbrazing method has been used to fabricate pyramidal lattice truss structures from 304 stainless steel. The impact response of the variouspanels was assessed after impact by spherical, 12 mm diameter, 6.9 g projectiles with an incident, zero obliquity velocity of �600 m/s.Empty lattice sandwich panels with an areal density of 27.7 kg m�2 do not prevent the perforation of the sandwich panel. The impactwith proximal face sheet reduced the projectile velocity to �450 m/s (by about 25%). Interactions with the lattice trusses and the distalface sheet further slowed the projectile resulting in an exit velocity at the distal face sheet of �360 m/s. The projectiles energy was dis-sipated by face sheet plastic dishing and fracture by petaling, and by truss plastic deformation. Infiltration of the lattice with polyure-thane elastomers further reduced the projectile exit velocity. The strength of the effect depended upon the modulus of the polymer (andtherefore its glass transition temperature, Tg). Only high modulus (high Tg) elastomers fully arrested the projectile. The energy of theprojectile in this case was dissipated by a combination of face sheet stretching and polymer deformation and fracture. Low moduluselastomers reduced the projectile exit velocity by about 45% (to �250 m/s) and also resulted in resealing of the projectile path withinthe sandwich panel core. The incorporation of ballistic fabric within the polymer infiltrated systems had little effect on the ballistic resis-tance. A hybrid sample containing metal encased Al2O3 prism inserts provided the greatest resistance to penetration. In this case theprojectiles were arrested within a sphere diameter of the sample front surface. Several of these hybrid systems offer promise as multifunc-tional, ballistic resistant, load-bearing structures.� 2007 Published by Elsevier Ltd.

Keywords: A. Honeycomb; A. Hybrid; B. Fragmentation; B. Impact behaviour

1. Introduction

Cellular metal structures, with either stochastic (metalfoam) [1–3] or periodic topologies [4–6], are beginning tobe utilized for a variety of structural [6–12], thermal [13–16], and acoustic damping [1] applications. They show sig-nificant promise as multifunctional structures where func-tions such as structural load support and thermal

1359-8368/$ - see front matter � 2007 Published by Elsevier Ltd.

doi:10.1016/j.compositesb.2007.02.029

* Corresponding author.E-mail address: [email protected] (C.J. Yungwirth).

management are simultaneously exploited [13,15,17]. Cel-lular metals have also been shown to possess excellent,impact energy absorption performance [17–22]. Recentwork suggests some periodic topologies are useful for mit-igating high intensity dynamic loading such as that associ-ated with the impingement of blast created shock waves[19–23]. However, little is known about their ability toimpede the penetration of projectiles that often accompanyexplosive events in air.

One study has explored the ballistic characteristics oflow strength metal foams made from an aluminum alloy

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C.J. Yungwirth et al. / Composites: Part B 39 (2008) 556–569 557

for integral armor concepts and found a modest enhance-ment in protection [24]. The compressive strength andenergy dissipating properties of cellular structures dependsupon the topology and relative density which control themode of failure during loading, and the strength of thematerial used to fabricate the structure. Periodic latticestructures consisting of 3D space filling unit cells witheither honeycomb [6,25,26] or lattice truss topologies[6,7,25,27] are significantly more structurally efficient thantheir equivalent relative density metal foams [1,2]. The fab-rication routes developed for these periodic cellular systemsalso enable much higher strength alloys to be used. As aresult, lattice structures are more than an order of magni-tude stronger than metal foams of the same relative density[25,28].

Fig. 1 shows three examples of open cell lattice trusstopologies that can be used as the cores of an efficient load-ing, supporting sandwich panel structure. Recent studiesindicate the possibility of significantly reduced impulsetransfer and greatly decreased peak pressure transmissionduring the high intensity loading of these structures[29,30]. However, it is unclear if lattice truss structures orsandwich panels constructed from them offer significantresistance to the propagation of projectiles.

The two solid faces of the sandwich panels must eachindividually provide some projectile propagation resis-tance. The penetration of a metal sheet (i.e. rolled homog-enous armor) by a normal incidence projectile has beenwidely studied and the effects of plate thickness andmechanical properties fully assessed [31,32]. However, theexperimental studies of Almohandes et al. [33] indicatedthat redistributing the mass of a plate amongst a pair ofplates of equivalent areal density resulted in a slight lower-ing of the ballistic resistance. Theoretical studies byBen-Dor et al. [34] and experimental studies by Radinand Goldsmith [35] indicated that the distance betweenthe pair of laminae had little or no effect upon the ballisticresistance of such systems.

However, the lattice truss structure core of a sandwichpanel is likely to have some effect upon the propagationof a projectile and might increase the ballistic performanceby deflecting (tipping) the projectile or causing some of itsenergy to be dissipated by plastic deformation/fracture ofthe trusses. Projectile kinetic energy losses during penetra-tion of the face sheets and the truss structures are likely tobe increased by utilizing metals with high strength, ductil-

Fig. 1. Isometric view of (a) pyramidal lattice truss (b) tetrahedral lattice truspyramidal lattice truss possesses triangular, prismatic voids running orthogonalsimilar voids (0�/60�/120�). The 3D Kagome lattice truss possesses two sets o

ity, fracture toughness and strain/strain rate hardeningcoefficients. Many austenitic and super austenitic stainlesssteels have medium strength levels but high toughnessand strain and strain rate hardening coefficients [36].Recent developments in the fabrication of lattice structuresnow enable lattice fabrication from such alloys using perfo-rated metal folding and brazing techniques [4,6].

More significant ballistic resistance might be achievedby exploiting the interior void spaces in lattice truss struc-tures. Lattice truss structures provide easy access to theinterior of the sandwich panel and enable other materialsto be added that might improve ballistic resistance. Forexample, the voids could be infiltrated with polymers todissipate a projectiles kinetic energy [37], or with ballisticfabrics to arrest fragments [38,39] or with hard ceramicsthat fragment and erode projectiles and otherwise impedetheir penetration [40,41]. The merits of such approachesare presently unclear and no experimental assessment ofthese ‘‘hybrid’’ lattice truss structures has been reported.

Here we describe a preliminary experimental study thatdevelops simple methods for the fabrication of ‘‘hybrid’’lattice truss systems and initiates an exploration of theirballistic response to moderate velocity impact by a spheri-cal projectile. It is not the intent of the study reported hereto conduct a full ballistic evaluation of each concept [42];instead the relative performance against a spherical projec-tile of constant diameter and velocity (�600 m/s) is deter-mined. We show that the air filled pyramidal latticestructure can be relatively easily penetrated by such projec-tiles when the face sheets are thin and the core relative den-sity is low (<3%). We find that some polymer infiltrated‘‘hybrid’’ lattices have greatly improved ballistic resistancecompared to their unfilled counterparts. A system consist-ing of metal encapsulated ceramics in a polymer infiltratedlattice is found to have the highest ballistic resistance. Sub-sequent papers will delve more thoroughly into the ballisticperformance of each concept.

2. Fabrication

Seven system concepts were constructed and evaluated.Six utilized a single layer system and one a double latticelayer concept whose cell size was a half that of the others.Various materials were then introduced into the structuresvoid space. Fig. 2 and Table 1 summarize the conceptsevaluated.

s and (c) 3D Kagome lattice truss structure between solid face sheets. Thely (0�/90�) through the lattice. The tetrahedral lattice truss has three sets off void orientations (0�/60�/120� and 30�/90�/150�).

Fig. 2. Schematic illustrations of pyramidal lattice truss concepts evaluated in the study: (a) empty pyramidal truss lattice; (b) polymer filled in trusslattice; (c) ballistic fabric interwoven between trusses with polymer filling remaining air space; (d) 304 SS encased alumina inserted in triangular prismaticvoids and remaining air space filled with polymer.

Table 1Physical descriptions of composite lattice truss systems fabricated

System Layer Polymer Fabric Encasedceramic

Areal density(kg/m2)

1 Single None None None 27.72 Single PU1 None None 54.83 Single PU1 None None 54.64 Single PU2 None None 55.95 Single PU1 Yes None 53.76 Single PU2 None Yes 105.17 Double PU1 None None 53.1

558 C.J. Yungwirth et al. / Composites: Part B 39 (2008) 556–569

2.1. Lattice truss structure construction

A perforated sheet folding process [4] was used to create abaseline pyramidal truss sandwich panel structure with acore relative density ð�qÞ of 2–3%. A diamond perforationpattern was die stamped into a 1.9 mm thick (14 G) 304stainless steel sheet, Fig. 3. The individual perforations were5.46 cm in length and 3.15 cm wide. The patterned sheet wasthen bent as shown in Fig. 3 to create a single layer pyrami-dal truss lattice with a cell size of �25 mm. A double layerpyramidal truss lattice was created by stacking two layersof the lattice truss structure made from a 1.5 mm thick(16 G) 304 stainless steel using an intermediate solid sheetof 0.5 mm thick (20 G) 304 stainless steel. The diamond per-foration punch for the double layer lattice was 2.65 cm inlength and 1.53 cm wide and resulted in a pyramidal latticecell size of �12 mm. The bent perforated sheets were thenassembled to make nodal line contacts as shown in Fig. 3b.

The single layer samples were cut to a 3 · 3 pyramidalcell array and placed between 16 G 304 stainless steel face

sheets 12.07 cm · 12.70 cm. The double layer samples werecut to a 5 · 7 pyramidal cell array and also placed betweenidentical (16 G) pairs of 304 stainless steel face sheets. Thelattice truss layer(s) were bonded to these face sheets usinga transient liquid phase bonding process [43,44]. Thisinvolved spray coating the face and the intermediate solidsheets with a NICROBRAZ� alloy 51 (Wall Colmonoy,Dayton OH). This alloy is composed of 25 Cr, 10 P, 0.03C (wt%) with the balance consisting of Ni. The powderwas contained in a polymer binder. These brazing alloyscontain melting point depressants (e.g. boron, phospho-rous, silicon) to achieve desirable liquid flow and adequatewetting behavior. The sandwich structure was placed in ahigh-temperature vacuum furnace for brazing. The sampleswere heated at 10 �C/min to 550 �C, held for 20 min toremove any residual polymer binder, then heated to1050 �C, for 60 min at a pressure of less than 1.3 ·10�2 Pa before furnace cooling to ambient temperature at25 �C/min.

2.2. Lattice relative density

The relative density of the pyramidal lattice truss corescan be simply calculated from a unit cell analysis. Fig. 4shows the single and double layer pyramidal unit cells.The relative density, �q, is a non-dimensional ratio of thevolume occupied by metal to that of the unit cell. For thesingle layer pyramidal topology:

�q ¼ 2wt

l2 cos2 x sin xð1Þ

Fig. 3. Manufacturing process for making pyramidal lattice truss cored sandwich panels: (a) process sequence; (b) single layer system; (c) double layersystem.

Fig. 4. Unit cell geometries used to derive the relative density for (a) thesingle layer pyramidal topology (b) double layer pyramidal topology.


where x = 54.7� is the included angle (the angle betweenthe truss members and the base of the pyramid), w is thetruss width, t is the truss thickness, and l is the truss length.Similarly, the relative density of the double layer pyramidaltopology is:

�q ¼ 8lwt þ liwiti

4l3 cos2 x sin xþ liwiti

ð2Þ

where li is the length of the intermediate sheet, wi is thewidth of the intermediate sheet and ti is the thickness ofthe intermediate sheet. The 14 G 304 stainless steel, witht = 1.9 mm and l = 31.75 mm, results in a relative density,

�q ¼ 2:6%. The 16 G 304 stainless steel, with t = 1.5 mmand l = 14.73 mm, resulted in a relative density, �q ¼10:2% (including the 20 G intermediate layer).

The uniaxial tensile response of 304 stainless steel sub-jected to the same thermal history as the lattice truss struc-tures has been previously measured and reported [7]. Theelastic modulus and 0.2% yield strength were 203 GPaand 176 MPa, respectively. Significant work hardeningoccurred in the plastic region.

2.3. Polymer infiltrated concepts

Several of the lattice structures were infiltrated withpolyurethane (PU) elastomers. The effect of polymermechanical properties was explored using two differentpolyurethanes with widely differing elastic moduli andcompressive strengths. One of the polyurethanes (PU1)was PMC-780 Dry formulated by Smooth-On (Easton,Pa) [45]. PU1 was a two component, pliable, castable elas-tomer with an approximate 24 h cure time at room temper-ature. Part A was composed of polyurethane pre-polymerand a trace amount of toluene diisocyanate. Part B con-sisted of polyol, a proprietary chemical (NJ Trade Secret#221290880-5020 P), di(methylthio)toluene diamine andphenylmercuric neodecanoate. PU1 had a low elastic mod-ulus and tensile strength but a very high elongation to fail-ure [46].

The second polymer system (PU2), was CLC-1D078supplied by Crosslink Tech, Inc. (Mississauga, Ont., Can-ada) [47]. It had a high elastic modulus and high tensilestrength but low elongation to failure. PU2 is a two com-

Table 2Manufacturer reported properties for the two polyurethane systems

Property PU1 PU2

Manufacturer Smooth-on(Easton, PA)

Crosslink Technology Inc.(Mississauga, Ont., Canada)

Product name PMC-780 Dry CLC-1D078Tensile

modulus(MPa)

2.76 1120

Tensile strength(MPa)

6.21 68.9

Elongation tobreak (%)

700 16

Shore hardness 80 A 78 D

Fig. 5. Predicted values for (a) the storage (E 0) and (b) the loss (E00)modulus for PU1 and PU2 at a reference temperature of 25 �C as afunction of frequency.


ponent, rigid, rapid prototyping polyurethane system witha cure time of approximately 5 min at room temperature.Part A was composed of polyether polyol. Part B was com-posed of diphenylmethane-4,4 0-diisocyanate. Table 2 liststhe manufacturer’s specifications for each of the two poly-urethane systems.

The glass transition temperature (Tg) of each polyure-thane (PU1 and PU2) was determined with modulated dif-ferential scanning calorimetry (MDSC�) using a Q1000Modulated DSC (TA Instruments-Waters, LLC). Eachsample was heated over a temperature range of �80 to240 �C with a heating rate of 3 �C/min and a modulationof ±0.5 �C/60 s period. The Tg of each sample was takento be the inflection point of the step-change in the mea-sured heat capacity. Based on this definition, the Tg ofPU1 was �56 �C, while the Tg of PU2 was +49 �C.

The elastic and rheological properties of the two poly-urethanes were characterized by dynamic mechanicalanalysis (DMA) using a Q800 DMA system (TA Instru-ments-Waters, LLC). Measurements were made on eachsample at three different frequencies (1, 10, and 100 Hz)with a strain amplitude of 0.1% over a temperature rangeof �100 to 40 �C in 5 �C increments. The data obtainedover the entire temperature range were transformed bytime–temperature superposition using a reference tempera-ture of 25 �C. The result of this data manipulation is a mas-ter curve of predicted storage (E 0) and loss (E00) modulusvalues over a frequency range of 10�1 to 1010 Hz for eachsample at the reference temperature. Fig. 5 is a plot ofthe predicted storage (E 0) and loss (E00) modulus valuesfor PU1 and PU2 over a frequency range of 1–106 Hz ata reference temperature of 25 �C. As can be seen in this fig-ure, the predicted storage and loss modulus values of PU2are more than 100· greater than the storage and loss mod-ulus values of PU1 over the entire frequency range.

2.4. Ballistic fabric inserts

A ballistic fabric was integrated into the one of cellularstructure systems to investigate if it provided additionalresistance to projectile penetration. The ballistic fabric cho-sen was a high strength 2.54 cm wide ribbon composed of

woven Spectra� fiber. It was interwoven between everyother cell to create a 0–90� lay-up orientation. The ribbonwas a 1200 denier with a 21 · 21 plain weave impregnatedwith 20 ± 2% resin. A single layer of interwoven ribbonwas located inside the sandwich structure proximal to theback face sheet.

2.5. Metal encased ceramic prisms

Metal encased alumina prisms were inserted in one ofthe single layer samples. The AD-94 alumina rods weremanufactured by CoorsTek (Golden, CO). They were equi-lateral triangular prisms with a base length of 2.54 cm,12.07 cm long. The apexes of the prisms were ground toremove 0.32 cm of material so they could be slipped into0.05 cm thick (20 G) 304 stainless steel triangular tubes.The 304 stainless steel equilateral triangular tubes had aninterior base length of 2.54 cm. The AD-94 grade of alu-mina consists of 93.3 Al2O3, 4.1 SiO2, 0.8 BaO, 0.7 MgO,0.7 ZrO2, 0.3 CaO, 0.2 Fe2O3 and 0.1 Na2O (wt%). Themechanical properties for AD-94 alumina reported byCoorsTek are summarized in Table 3. The 304 stainlesssteel tubes were then capped with 304 stainless steel plugsand sealed by brazing utilizing the NICROBRAZ� alloy

Table 3Physical properties of AD-94 Al2O3 triangular prisms

Material Density(g cc�1)

Elastic modulus(GPa)

Flexural strength(MPa)

Tensile strength(MPa)

Compressive strength(MPa)

Fracture toughness(MPa m1/2)

Hardness(GPa)

AD-94 3.97 303 358 221 2068 4–5 11.5


51 and an identical process to that used to fabricate thetruss structures.

3. Ballistic testing

A series of ballistics experiments with spherical projec-tiles were conducted using the University of Cambridgegas gun facility [19]. Compressed nitrogen was used toaccelerate the projectile which then impacted the samplesnormal to their surface, Fig. 6. The gas gun fired 12 mmdiameter, 6.9 g, 420 stainless steel ball bearings at thespecimens at impact velocities m0 � 600 m s�1. For theseexperiments, the gas gun was fitted with a 12 mm bore,4.5 m long barrel designed for ballistic testing. The load-ing configuration, Fig. 6, shows that the projectileimpacted the center of the panels with zero obliquity.The panels were simply supported over an approximate

Fig. 6. Ballistic testing configuration. Ball bearing projectiles with a radius of600 m s�1 was used for all the tests.

Fig. 7. Projectile impact location for (a) the single and (b)

60 mm diameter hole located in a rigid, 25 mm thick back-ing plate. The specimens were adhesively attached to abacking plate with double-sided tape as shown in Fig. 6.Specimens used in evaluations of systems 1 through 6 wereorientated such that impact occurred at the center of thefront faceplate, which was supported centrally by thepyramidal core, as sketched in Fig. 7a. The specimen usedin evaluation of system 7, which consisted of the double-layered pyramidal core, was loaded in the configurationillustrated in Fig. 7b.

A Hadland Imacon-790 image-converter high speedcamera was used to monitor the responses of the (empty)system 1. It enabled observation of the projectile impactwith the sandwich panel and the sequence of subsequentdeformation. An inter-frame time of 10 ls and an exposuretime of 2 ls were used. Additionally, X-ray tomographywas performed on system concept 4.

6 mm and weight of 6.9 g were use. An impact velocity of approximately

double layer pyramidal lattice truss sandwich panels.


4. Experimental

4.1. Single layer empty system

Fig. 8 shows cross sectional views of a truss sandwichpanel (system 1) before and after impact by a spherical pro-jectile with an incident velocity of 598 m s�1. Examinationof the sample after impact indicated significant bucklingand plastic deformation of the trusses, Fig. 7b. This waslimited to one cell in the lattice and appears to be a conse-quence of proximal face sheet compression of the trussesbeneath the impact site. Node fracture was observed ateach of the four truss-face sheet contact points of theimpacted cell; two of these detached trusses can be seenshown in Fig. 7b. The fracture of the nodes at the bottomface sheet was consistent with large tensile deformationsapplied to the lattice as the projectile exited the distal facesheet. The entry hole on the front face sheet was 15 mm indiameter and suffered a plastic dishing deflection of 5 mm.The exit hole on the back face sheet was approximately18 mm in diameter and deflected 11 mm. Both face sheetsfailed by petalling, a phenomenon common observed whenthin, ductile metal plates suffer low velocity impact.

A time sequence of high-speed photographs was takenduring the test and these are shown in Fig. 9a–h. The timeafter impact is indicated for each frame, and it is seen thatimpact occurred between Fig. 9b and c. Subsequent frames,

Fig. 8. Test 1: (a) cross section of the single layer empty pyramidal truss lattictruss lattice after a projectile impact of 598 m s�1.

Fig. 9d–h, show the projectile penetrating the front faceand propagating towards the back face. Fig. 10 showsthe position of the projectile as a function of position fromthe front face sheet as a function of time. The distance trav-eled between frames, Fig. 9c and h, was approximately22.5 mm. Since the elapsed time between these frameswas 50 ls, the average velocity of the projectile as it prop-agated between the front and back faces was approxi-mately 450 m s�1. Since the measured impact velocity was598 m s�1, the reduction due to penetration of the frontface was �148 m s�1. The projectile then interacted withthe lattice trusses and deformed and perforated the proxi-mal face sheet. Additional experiments have indicated theexit velocity at the distal face sheet is �360 m/s.

As indicated in Fig. 9, the projectile impacted at a node-face sheet contact. There was no observable lateral deflec-tion of the projectile and no residual core compressioncould be detected. The lack of core compression in the bal-listic experiments is considerably different to that observedfor beams [19,20] and plates [21–23] with pyramidal latticecores impacted with large diameter metal foam projectiles.

4.2. Soft polymer filled system

Fig. 11 shows the cross section of a single layer pyrami-dal truss lattice filled with polymer PU1 before and afterimpact with a spherical projectile whose contact velocity

e along a nodal line; (b) Cross section of the single layer empty pyramidal

Fig. 9. High-speed photography of a projectile impact with the empty single layer pyramidal lattice (system 1). Each figure (a)–(h) depicts a frame of thehigh-speed photography. The time in microseconds (ls) is labeled from the initial impact of the projectile with the front face sheet.

-1

-0.5

0

0.5

1

1.5

2

2.5

-20 -10 0 10 20 30 40 50 60

Posi

tion

from

impa

ct (

cm)

Time (μs)

Time of Impact

Inside of Sample

Initial Velocity

Fig. 10. Position of a spherical projectile from the front face sheet of theempty single layer pyramidal lattice truss as a function of time. To the leftof the time of impact is before the impact of the projectile and the right ofthe time of impact is after impact of the projectile.


was 616 m s�1. Other experiments have indicated the exitvelocity for this configuration was �250 m/s. Examinationof the sample after impact showed an entry hole on thefront face sheet that was 12 mm in diameter and a localdishing deflection of 2 mm. This dishing was significantlyless than that observed with the empty lattice. Someupward deformation of the face sheet indicative of a dilata-tional deformation was also evident. The exit hole on the

back face sheet was 15 mm in diameter and it suffered adeflection of 12 mm which was comparable to that of theempty lattice. The reduced front face deflection is consis-tent with polymer constrained deformation of the proximalface sheet. The similar distal face sheet deflections and exithole diameters of systems 1 and 2 indicates that the poly-mer provides little constraint to distal face sheet bendingand perforation.

It is interesting to note that the PU1 infiltrated systemresealed the void space created by the projectiles penetra-tion. A remnant of the (brass) breech rupture disk usedby the gas gun can be seen in the upper part of this space.

4.3. Double layer filled with PU1

Fig. 12 shows cross sections the double layer pyramidaltruss lattice filled with PU1 before (a), and after impact (b)by a projectile with an impact velocity of 613 m s�1. Exam-ination of the sample after impact indicates an entry hole of12 mm diameter and a local dishing deflection of 2 mm.Evidence of outward deformation of the face sheet can alsobe seen. The exit hole was 16 mm in diameter and the distalface sheet suffered a deflection of 14 mm. Back face sheetpenetration occurred by petalling. The intermediate metalsheet and attached polymer suffered a displacement ofabout 5 mm. Nodal fracture of the impacted and adjacentcells occurred at the front face sheet. On the back facesheet, nodal failure extended to a radius of two to threecells in all directions. Only partial resealing of the PU1elastomer occurred in this system. This is consistent with

Fig. 11. Test 2: (a) cross section of the single layer pyramidal truss lattice filled with PU1; (b) cross section of the single layer pyramidal truss lattice filledwith PU1 after a projectile impact of 616 m s�1. Notice the brass breech rupture disk (b) remaining in the polymer while the projectiles path resealed.


restraint of the polymers reverse displacement by the smal-ler cell size lattice.

4.4. Hard polymer filled system

The PU2 infiltrated sample was impacted by a projectilewith an incident velocity of 632 m s�1. Fig. 13 shows thesample cross section before (a), and after impact (b). Thesystem was not penetrated by the projectile. Examinationafter impact shows a proximal face sheet entry hole thatwas 12 mm in diameter. Almost no detectable dishing couldbe observed. The rear face sheet suffered extensive bulgingdeformation with a maximum deflected of 8.5 mm. It wasnot penetrated and arrested the projectile. The projectileitself remained intact and can be seen in Fig. 13b. All ofthe distal and some proximal face sheet node failures nearthe impact site occurred in this system. Significant crackingand some pulverization of the polymer also occurred withina conical region originating from the impact site.

It was difficult to determine the deformation of thetrusses in this sample and so X-ray tomography was per-

formed on the test specimen after testing. Fig. 14 showstwo of the X-ray projections. These reveal that one of thepyramidal trusses had been severely plastically deformedand then severed.

4.5. Single layer filled with PU1 plus fabric

Fig. 15 shows cross sections of the single layer pyrami-dal truss lattice filled with fabric and PU1 before (a), andafter impact (b) by a projectile with an incident velocityof 613 m/s. Examination of the front face after impactshowed an entry hole 12 mm in diameter, slight outwarddilation of the front face and minor dishing with a maxi-mum deflection of 2 mm. The exit hole in the distal facesheet was also 12 mm in diameter and suffered a maximumdeflection of �8 mm. Significant resealed of the void spacecreated by the projectile again occurred in this system.Complete nodal failure occurred at the front face sheetpresumably because of the significant dilatational strainscreated upon entry of the projectile into this incompress-ible system. Only the impact cell at the back suffered node

Fig. 12. Test 7: (a) cross section of the double layer pyramidal truss lattice filled with PU1; (b) cross section of the double layer pyramidal truss lattice filledwith PU1 after an impact of 613 m s�1.


debonding. By comparing Figs. 11 and 15, it is apparentthat the addition of the fabric to the soft filled polymersystem appears not to have significantly altered itsperformance.

4.6. Ceramic plus PU2 filled system

Fig. 16 shows cross sections of the ceramic insert system6 before and after impact by a projectile with an impactvelocity of 613 m s�1. The system was highly effective atresisting penetration by the projectile. Examination of thesample cross section after impact shows the projectile wasarrested and fractured by the center metal encased aluminaprism, Fig. 16b. The metal casing was breeched and thealumina was fragmented. However, damage to the ceramicwas confined to just the single center prism. Minimal frac-turing of the surrounding hard PU2 polymer was evident.There was some nodal debonding near the entry locationof the projectile. The proximal face sheet had an entry holethat was 12 mm in diameter and the front face sheet wasdeflected outwards with a maximum deflection of 3 mm.The rear face sheet also bulged causing an outward deflec-tion of �3.5 mm.

5. Discussion

Sandwich panels with cellular metal cores have beenrecently shown capable of significantly reducing the trans-mitted impulse from a blast created shock wave [29]. Thepreliminary study conducted above has investigated theresponse of a model stainless steel sandwich panel with alow relative density pyramidal lattice core to impact by astandard, 12 mm diameter steel projectile with an impactvelocity of �600 m/s at zero obliquity. The baseline emptylattice system had a cell size of approximately 25 mm whichwas twice the projectile diameter. The empty pyramidal lat-tice truss sandwich panel had an aerial density of �54 kg/m2 and did not prevent the penetration of the projectile,Fig. 8. High speed photography indicated that the velocityof the projectile was reduced by �25%. The kinetic energywas absorbed in the plastic deformation (stretching) of thetrusses and by plastic bulging and fracture (by petalling) ofthe two face sheets.

Three samples were infiltrated with a polyurethane elas-tomer (PU1) with a low glass transition temperature belowthe test temperature. This polymer had both low storage(1–10 MPa range) and loss (�2 MPa) moduli at the test

Fig. 13. Test 4: (a) cross section of the single layer pyramidal truss lattice filled with PU2; (b) cross section of the single layer pyramidal truss lattice filledwith PU2 after a projectile impact of 632 m s�1 showing �8.5 mm deflection of the rear face panel. Note that the projectile is visibly arrested in (b).

Fig. 14. X-ray tomography of specimen 4. Side profile (left). Elevated view above front face sheet (right).


temperature. Infiltration with PU1 increased the aerial den-sity to �55 kg/m2 but failed to prevent penetration by theprojectile, Fig. 11b. The polymer impeded plastic bulgingof the front face sheet but was effective at coupling dilata-tional deformations to a larger volume of the sandwichpanel. A PU1 infiltrated, double layer pyramidal truss lat-tice that had a cell size approximately equal to the projec-tile also appeared to be easily penetrated by the referenceprojectile, Fig. 12b. Significant truss-face sheet disbondinglimited plastic stretching of the trusses. Adding ballistic

fabrics had little effect upon the penetration resistance ofthe PU1 infiltrated system. The PU1 elastomer was ableto elastically dilate sufficiently to allow passage of the pro-jectile through the structure. It sprang back behind the pro-jectile providing an effective sealing of the projectiles paththrough the sample.

Infiltration of the baseline lattice structure with a poly-urethane elastomer (PU2) with a glass transition tempera-ture above the test temperature had a very large effect uponthe propagation of the projectile within the sandwich panel

Fig. 15. Test 5: (a) cross section of the single layer pyramidal truss lattice filled with the interwoven fabric and the PU1; (b) cross section of the single layerpyramidal truss lattice filled with interwoven fabric and the PU1 after a projectile impact of 613 m s�1.


and prevented full penetration, Fig. 13b. This polymer hadboth high storage (1–3 GPa range) and loss (�100 MPa)moduli at the test temperature. The PU2 elastomer infil-trated system had almost the same aerial density(�56 kg/m2) as the PU1 system. Penetration of this systemrequired significant plastic dissipation in the metal part ofthe system and appears to have caused large amounts ofineleastic deformation and fracture of the polymer. Failureof all the truss-face sheet nodes appears to have prema-turely terminated energy dissipation by truss plasticstretching.

The insertion of metal encased Al2O3 prisms into the tri-angular cross section prismatic voids of the pyramidal lat-tice was a highly effective method of increasing thepenetration resistance of the lattice but was accompaniedby a significant increase (to 105 kg/m2) in aerial density,Fig. 16. The projectile was arrested by impact with the apexof one of the prisms. The impact caused extensive fractureof the impacted ceramic prism but had no effect on the sur-rounding inserts. It resulted in significant deformation andfracture and redirection of the projectile along the inter-

prism channeling direction. Confinement of the fragmentedceramic pieces appears to have improved the penetrationresistance of the structure. Accommodation of the volumeof the projectile within the structure and dissipation of theprojectiles kinetic energy was achieved in part by plasticbulging of the front and rear face sheets.

6. Conclusions

A series of experiments were conducted to assess thepotential use of various polymeric and ceramic inserts forincreasing the projectile penetration resistance of stainlesssteel sandwich panels with pyramidal lattice cores. Thispreliminary study utilized a standard spherical 12 mmdiameter spherical steel projectile with an initial velocityof 600 m/s to compare changes in performance as variousmaterials were added to the lattice core. We have foundthat:

1. An empty 304 stainless steel pyramidal truss sandwichpanel with an aerial density of 27.7 kg/m2 was able to

Fig. 16. Test 6: (a) cross section of the single layer pyramidal truss lattice filled with 304 stainless steel prisms and PU2; (b) cross section of the single layerpyramidal truss lattice filled with 304 stainless steel prisms and PU2 after an impact of 613 m s�1.


reduce the projectiles velocity by approximately 25% forimpact velocities in the 600 m/s range. Energy dissipa-tion occurred by front and back face sheet dishing andtruss stretching.

2. Filling the lattice with a low Tg elastomer and ballisticfabric increased the aerial density to �54 kg/m2 andbut failed to prevent projectile penetration. It did resultin projectile path resealing and an additional reductionof the exit velocity.

3. The use of a polyurethane elastomer with a glass transi-tion well above the test temperature successfully arrestedthe projectile. Truss face sheet nodal disbondingappeared to have impeded energy dissipation by latticeplastic stretching.

4. The insertion of the metal contained alumina prisms intothe lattice increased to aerial density to 105 kg/m2 andarrested the projectile just below the proximal face sheetwith minimal deflection of the back face sheet.

5. The study indicates that hybrid lattice concepts offer sig-nificant potential for manipulating the ballistic proper-ties of cellular core sandwich panel systems.

Acknowledgements

This research has been supported by the Office of NavalResearch under Grant N00014-01-1-1051. The Programmanager was Dr. Stephen Fishman.

References

[1] Ashby MF, Evans AG, Fleck NF, Gibson LJ, Hutchinson JW,Wadley HNG. Metal foams: a design guide. Butterworth–Heineman;2000.

[2] Queheillalt DT, Katsumura Y, Wadley HNG. Synthesis of stochasticopen cell Ni-based foams. Scr Mater 2004;50:313–7.

[3] Brothers AH, Scheunemann R, DeFouw JD, Dunand DC. Process-ing and structure of open-celled amorphous metal foams. Scr Mater2005;52:335–9.

[4] Wadley HNG. Cellular metals manufacturing. Adv Eng Mat Int JSolids Struct 2002;4(10):726–33 [special issue].

[5] Evans AG, Hutchinson JW, Fleck NA, Ashby MF, Wadley HNG.The topological design of multifunctional cellular metals. ProgrMater Sci 2001;46:309–27.

[6] Wadley HNG, Fleck NA, Evans AG. Fabrication and structuralperformance of periodic cellular metal sandwich structures. ComposSci Technol 2003;63:2331–43 [special issue].

[7] Queheillalt Douglas T, Wadley HNG. Cellular metal lattices withhollow trusses. Acta Mater 2005;53:303–13.

[8] Rathbun HJ, Wei Z, He MY, Zok FW, Evans AG, Sypeck DJ, et al.Measurement and simulation of the performance of a lightweightmetallic sandwich structure with a tetrahedral truss core. J ApplMech 2004;71:368–74.

[9] Kooistra GW, Deshpande VS, Wadley HNG. Compressive behaviorof age hardenable tetrahedral lattice truss structures made fromaluminum. Acta Mater 2004;52:4237–99.

[10] Wang J, Evans AG, Dharmasena K, Wadley HNG. On theperformance of truss panels with Kagome cores. Int J Solids Struct2003;40:6981–8.

[11] Donev A, Torquato S. Energy-efficient actuation in infinite latticestructures. J Mech Phys Solids 2003;51:1459–75.


[12] Hutchinson RG, Wicks N, Evans AG, Fleck NA, Hutchinson JW.Kagome plate structures for actuation. Int J Solids Struct2003;40:6969–80.

[13] Tian J, Kim T, Lu TJ, Hodson HP, Queheillalt DT, Wadley HNG.The effects of topology upon fluid-flow and heat-transfer withincellular cooper structures. Int J Heat Mass Transf 2004;47:317–3186.

[14] Evans AG, Hutchinson JW, Ashby MF. Multifunctionality ofcellular metal systems. Program Mater Sci 1999;43:171–221.

[15] Gu S, Lu TJ, Evans AG. On the design of 2D cellular metals forcombined heat dissipation and structural load capacity. Int J HeatMass Transf 2001;44:2163–75.

[16] Kim T, Hodson HP, Lu TJ. Fluid-flow and endwall heat-transfercharacteristics of an ultra light lattice-frame material. Int J Heat MassTransf 2004;47:1129–40.

[17] Tian J, Lui TJ, Hodson HP, Queheillalt D, Wadley HNG. Thermal-hydraulic performance of sandwich structures with crossed tube trusscore and embedded heat pipes. In: 13th International heat pipeconference, Shanghai, China, September 21–25; 2004.

[18] Hanssen AG, Enstock L, Langseth M. Close-range blast loading ofaluminum foam panels. Int J Impact Eng 2002;27:593–618.

[19] Radford DD. An experimental technique to study the response ofstructures to shock loading. Exp Mech [submitted].

[20] Radford DD, Deshpande VS, Fleck NA. The response of clampedsandwich beams subjected to shock loading. Int J Impact Eng2006;32:968–87.

[21] Rathburn HJ, Radford DD, Xue Z, Yang J, Deshpande VS, FleckNA, et al. A performance of metallic honeycomb-core sandwichbeams under shock loading. Int J Solids Struct 2006;43:1746–63.

[22] Radford DD, McShane GJ, Deshpande VS, Fleck NA. The responseof clamped sandwich plates with metallic foam cores to simulatedblast loading. Int J Solids Struct 2006;43:2243–59.

[23] McShane GJ, Radford DD, Deshpande VS, Fleck NA. The responseof clamped sandwich plates with lattice cores subjected to shockloading. Eur J Mech A–Solid 2006;25(2):215–29.

[24] Gama BA, Bogetti TA, Fink BK, Yu C, Claar TD, Eifert HH, et al.Aluminum foam integral armor: a new dimension in armor design.Compos Struct 2001;52:381–95.

[25] Wadley HNG. Multifunctional periodic cellular materials. PhilosTrans Royal Soc A 2006;364:31–68.

[26] Goldsmith W, Louie DL. Axial perforation of aluminum honey-combs by projectiles. Int J Solids Struct 1995;32:1017–46.

[27] Wicks N, Hutchinson JW. Optimal truss plates. Int J Solids Struct2001;38:5165–83.

[28] Evans AG. Lightweight materials and structures. MRS Bull2001;26(10).

[29] Wadley HNG, Dharmasena KP, Chen Y, Dudt P, Knight D,Charette R et al. Compressive response of multilayered pyra-

midal lattices during underwater shock loading. Int J Impact Eng,in press.

[30] Wei Z, Evans AG, Dharmasena KP, Wadley HNG. Analysis andinterpretation of a test for characterizing the response of sandwichpanels to water blast. Int J Impact Eng, in press.

[31] US Army Research Laboratory Technical Report ARBRL-MR-03097, Armor-Penetrator Performance Measures, Konrad Frank;1981.

[32] Backman ME, Goldsmith W. The mechanics of penetration ofprojectiles into targets. Int J Eng Sci 1978;16:1–99.

[33] Almohandes AA, Abdel-Kader MS, M Eleiche A. Experimentalinvestigation of the ballistic resistance of steel-fiberglass reinforcedpolyester laminar plates. Composites B 1996;27B:447–58.

[34] Ben-Dor G, Dubinsky A, Elperin T. On the ballistic resistance ofmulti-layered targets with air gaps. Int J Solids Struct 1998;35:3097–103.

[35] Radin J, Goldsmith W. Normal projectile penetration and perfora-tion of layered targets. Int J Impact Eng 1988;7:229–59.

[36] ASM Metals Handbook, vol. 1., 10th ed., ASM International; 1990.[37] Yadav S, Ravichandran G. Penetration resistance of laminated

ceramic/polymer structures. Int J Impact Eng 2003;5:557–74.[38] Naik NK, Shrirao P. Composite structures under ballistic impact.

Compos Struct 2004;66:579–90.[39] Zhu G, Goldsmith W, Dharan CK. Penetration of laminated Kevlar

by projectiles – I Experimental investigation. Int J Solid Struct1992;29:399–419.

[40] Subramanian R, Bless SJ. Penetration of semi-infinite AD995alumina targets by tungsten long rod penetrators from 1.5 to3.5 km/s. Int J Impact Eng 1995;17:807–16.

[41] Woodward RL, Gooch WA, O’Donnell RG, Perciballi WJ, BaxterBJ, et al. A study of fragmentation in the ballistic impact of ceramics.Int J Impact Eng 1994;15:605–18.

[42] Military Standard V50 Ballistic Test for Armor, MIL-STD-662E,January 22; 1987.

[43] Source book on brazing and brazing technology. American Societyfor Metals; 1980.

[44] Brazing handbook. Miami: American Welding Society; 1991.[45] PMC 780 Dry and PMC 780 Wet Industrial Liquid Rubber

Compounds (January 2002). Retrieved on April 4, 2005, fromhttp://www.smooth-on.com/PDF/780.pdf.

[46] American Gas Association Engineering Glossary (February 2005).Retrieved on May 10, 2005, from http://www.aga.org/Content/NavigationMenu/About_Natural_Gas/Natural_Gas_Glossary/Natural_Gas_Glossary_(E).htm#E.

[47] Crosslink Technology Inc. Formulated Epoxies, Urethanes – CustomCast Parts (September 9, 2002). Retrieved on April 4, 2005, fromhttp://66.146.131.130/data/PDFFiles/clc1d-078a_clc1d-078b.pdf.

http://www.smooth-on.com/PDF/780.pdf

http://www.aga.org/Content/NavigationMenu/About_Natural_Gas/Natural_Gas_Glossary/Natural_Gas_Glossary_(E).htm#E



http://66.146.131.130/data/PDFFiles/clc1d-078a_clc1d-078b.pdf

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