3

Mechanisms of Penetration in Protective Materials

In designing armor, materials high in hardness, strength, and toughness have traditionally been sought, since common sense would dictate that such materials should be most resistant to attack by a projectile. However, according to Shockey et al., ballistic tests often show that the best-performing armor material is not necessarily the strongest, the toughest, or the hardest. Are there other properties that reliably offer guidance in choosing and developing armor materials, if such conventional bulk properties do not? Does ballistic behavior depend on some vague or unknown property or combination of properties, and, if so, how can they be identified, measured, and even enhanced? Can the chemistry and processing of materials be manipulated to achieve microstructures that exhibit nonconventional mechanical properties once they have been identified? Can such manipulation improve penetration resistance?

To answer these questions, armor development should be looked at not from the perspective of conventional bulk material properties but from that of micromechanical mechanisms.1An understanding of the mechanisms operating in a target during a penetration event can suggest microstructures—including those that characterize the chemical and phase composition of the building blocks—that are more resistant to penetration and that will lead to protective materials with better performance (see Box 3-1). Moreover, by identifying penetration-induced failure mechanisms and quantifying their activity, mathematical damage models can be developed that may allow what is termed computational armor design.

Penetration mechanisms are perhaps best revealed by post-test examination of penetrated targets. Ejected or otherwise separated target material contains telltale signs of the failure modes that operated during penetration, as does the material in the vicinity of the penetration cavity. The collection of loose material and the sectioning of penetrated material, followed by unaided visual inspection and inspection under a microscope, show the damage features, helping to uncover the mechanisms of material failure. In situ,

BOX 3-1
Microstructural Options for Influencing Failure Mechanisms in Metals, Ceramics, and Polymers

The nucleation, growth, and coalescence of cracks and shear instabilities in metals and ceramics could be suppressed by manipulating the grain structure or by adding second phase particles. The size, shape, and orientation of the grains could be configured to disrupt failure mechanisms. The mechanical properties of the grain boundaries can, moreover, dictate a transgranular or intergranular failure mode. And the chemical and phase composition of the grains themselves and their crystalline structure can be specified to affect deformability, mode of deformation (dislocation activity, twinning, phase changes), and propensity to rupture.

Likewise, the size, shape, orientation, crystal structure, spatial distribution, and mechanical properties of second-phase particles as well as the strength of particle and matrix interfaces can be manipulated to deter failure mechanisms. Second-phase particles such as coherent nanocrystallites have been shown to improve the ballistic performance of glasses, although there is not yet a detailed understanding of their effect on failure mechanisms. Pores can also inhibit cracks, and judicious open-architecture geometries may provide a lightweight solution to a penetration or blast problem.

Microstructural variables in polymers include chemical makeup, length and degree of branching of molecular chains, degree of alignment and entanglement, and extent of cross-linking. The types and strengths of bonds in the chains and between chains affect polymer strength and deformability (for instance, in thermosets versus thermoplastics) and can be expected to affect failure mechanisms.

______________

1Shockey, D.A., J.W. Simons, and D.R. Curran. 2010. The damage mechanism route to better armor materials. International Journal of Applied Ceramic Technology 7(5):566-573.



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3 Mechanisms of Penetration in Protective Materials In designing armor, materials high in hardness, strength, collection of loose material and the sectioning of penetrated and toughness have traditionally been sought, since common material, followed by unaided visual inspection and inspec- sense would dictate that such materials should be most resis- tion under a microscope, show the damage features, help- tant to attack by a projectile. However, according to Shockey ing to uncover the mechanisms of material failure. In situ, et al., ballistic tests often show that the best-performing armor material is not necessarily the strongest, the toughest, or the hardest. Are there other properties that reliably offer BOX 3-1 guidance in choosing and developing armor materials, if such Microstructural Options for Influencing Failure conventional bulk properties do not? Does ballistic behavior Mechanisms in Metals, Ceramics, and depend on some vague or unknown property or combination Polymers of properties, and, if so, how can they be identified, mea- sured, and even enhanced? Can the chemistry and process- The nucleation, growth, and coalescence of cracks and shear ing of materials be manipulated to achieve microstructures instabilities in metals and ceramics could be suppressed by ma- that exhibit nonconventional mechanical properties once nipulating the grain structure or by adding second phase particles. they have been identified? Can such manipulation improve The size, shape, and orientation of the grains could be configured to penetration resistance? disrupt failure mechanisms. The mechanical properties of the grain To answer these questions, armor development should boundaries can, moreover, dictate a transgranular or intergranular be looked at not from the perspective of conventional bulk failure mode. And the chemical and phase composition of the grains material properties but from that of micromechanical mecha- themselves and their crystalline structure can be specified to affect nisms.1 An understanding of the mechanisms operating in deformability, mode of deformation (dislocation activity, twinning, a target during a penetration event can suggest microstruc- phase changes), and propensity to rupture. tures—including those that characterize the chemical and Likewise, the size, shape, orientation, crystal structure, spatial phase composition of the building blocks—that are more distribution, and mechanical properties of second-phase particles as resistant to penetration and that will lead to protective ma- well as the strength of particle and matrix interfaces can be manipu- terials with better performance (see Box 3-1). Moreover, lated to deter failure mechanisms. Second-phase particles such as by identifying penetration-induced failure mechanisms and coherent nanocrystallites have been shown to improve the ballistic quantifying their activity, mathematical damage models can performance of glasses, although there is not yet a detailed under- be developed that may allow what is termed computational standing of their effect on failure mechanisms. Pores can also inhibit armor design. cracks, and judicious open-architecture geometries may provide a Penetration mechanisms are perhaps best revealed lightweight solution to a penetration or blast problem. by post-test examination of penetrated targets. Ejected or Microstructural variables in polymers include chemical makeup, otherwise separated target material contains telltale signs of length and degree of branching of molecular chains, degree of align- the failure modes that operated during penetration, as does ment and entanglement, and extent of cross-linking. The types and the material in the vicinity of the penetration cavity. The strengths of bonds in the chains and between chains affect polymer strength and deformability (for instance, in thermosets versus ther- 1Shockey, D.A., J.W. Simons, and D.R. Curran. 2010. The damage moplastics) and can be expected to affect failure mechanisms. mechanism route to better armor materials. International Journal of Applied Ceramic Technology 7(5): 566-573. 24

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25 MECHANISMS OF PENETRATION IN PROTECTIVE MATERIALS real-time, high-speed dynamic observations can in principle both plate and rod determine which failure processes operate. provide even better indications of failure modes. However, Typically, however, it is a combination of simultaneously ac- it is difficult to simultaneously achieve both high spatial and tive failure modes that governs the outcome of the encounter. high temporal resolution. Future advances in instrumentation Other failure modes may be invoked at higher velocities. will bring new insights to the complex interplay of deforma- Figure 3-2 shows a polished and etched cross section through tion and failure mechanisms during penetration. the crater in a 1-in.-thick steel plate that has been impacted at 6 km/s by a 12.7-mm-diameter polycarbonate sphere.2,3 Partially penetrated targets are particularly useful for Adiabatic4 shear bands can be seen as white-etching bands determining failure mechanisms. A close examination of areas where the damaged material remains in place and of of hard, untempered martensite extending into the plate (1), polished cross sections taken on a plane containing the shot surfaces of strain localization that look like bands when line demonstrates how damage varies with distance from the seen edge-on. The path of the bands is followed by brittle side and distance ahead of the penetrating object. Such obser- cracks (2), which intersect with other cracks and liberate vations also suggest how damage evolves, thereby providing fragments. Just below the crater are spherical voids (3), a notions for equations describing damage development. The manifestation of ductile tensile failure; these are linked by next section illustrates the failure mechanisms invoked by a shear bands. Homogeneous plastic flow (4) is made clear by penetrator by presenting damage observations in penetrated the deviation of the process rolling lines from the horizontal. and partially penetrated targets of metals and alloys, ceram- Ultimately, the hemispherical volume of dark-etching mate- rial just below the point of experienced α↔ε polymorphic ics and glasses, and polymeric materials. This is followed by a short discussion on the damage mechanisms in cellular phase change brought about by pressure (5). The grain size materials invoked by blast loads. is refined and the transformed material is significantly hard- ened. The boundary of the dark-etching material is a 130 kbar isobar. Thus, five failure modes operated at once, with the PENETRATION MECHANISMS IN METALS AND stress relaxation effect of each mode affecting the behavior ALLOYS of the others. Consider the case of a rod impacting a steel plate (Fig- The damage beneath the crater in Figure 3-2 is com- ure 3-1). If the plate is relatively soft compared to the rod, plex and seems at first nearly impossible to interpret, yet perforation may occur by homogeneous plastic flow of the it reveals how the material is failing. Such damage “hiero- plate, with little or no damage to the rod. A hardened plate, glyphics” must nevertheless be read and understood in order on the other hand, may fail by shear banding and consequent to predict penetration behavior and design microstructures liberation of a plug of material pushed out by the projectile with enhanced protective capabilities. Key to developing a (Figure 3-1). Reflected stress waves from the rear surface deeper understanding are laboratory experiments that isolate of the plate may produce tensions large enough to nucleate, each specific damage mechanism. This would allow each grow, and coalesce voids or microcracks, causing spallation. failure mode to operate under a range of well-controlled Thus, the result of an encounter between a rod and plate is de- rate, temperature, and stress state conditions, providing the termined by microscopic failure processes such as homoge- opportunity to study and quantitatively describe its evolution neous plastic flow, shear banding, and tensile fracture in the by means of real-time observation or post-test analysis of plate and in the rod. The impact conditions and properties of tests interrupted at various stages of damage development. Finding 3-1. Ballistic penetration of metals can occur by five failure modes—adiabatic shear bands, cracks, voids, plastic deformation, and phase changes—more than one or all of which can occur simultaneously. 2Shockey, D.A., D.R. Curran, and P.S. DeCarli. 1975. Damage in steel FIGURE 3-1 Impact on steel plate. Rod impacting a plate at 90 plates from hypervelocity impact, I: Physical changes and effects of projec - degrees (left), and cross section showing simple plugging of rolled tile material. Journal of Applied Physics 46(9): 3766-3775. homogeneous armor by shear instabilities and back surface spalling 3Bertholf, L.D., L.D. Buxton, B.J. Thorne, R.K. Byers, A.L. Stevens, and by nucleation, growth, and coalescence of voids and cracks (right). S.L. Thompson. 1975. Damage in steel plates from hypervelocity impact SOURCE: Erlich, D.C., L. Seaman, D.A. Shockey, and D.R. Cur- II: Numerical results and spall measurement. Journal of Applied Physics ran. 1980. Development and Application of a Computational Shear 46(9): 3776-3783. Band Mode. Menlo Park, Calif.: SRI International. 4“Adiabatic” refers to any process which occurs without heat transfer.

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26 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE 3-2 Polished and etched cross section through the crater in a steel plate that was impacted at 6 km/s by a 12.7-mm-diameter poly - carbonate sphere. Five damage modes operated. SOURCE: Reprinted with permission from Shockey, D.A., D.R. Curran, and P.S. De Carli, Journal of Applied Physics, 46, 3766, (1975). Copyright 1975, American Institute of Physics. PENETRATION MECHANISMS IN CERAMICS AND GLASSES Penetration of thick sections of ceramics and glasses oc- curs by damaging the target material at the leading surface of the projectile and then pushing the damaged material out of the projectile path.5 Here, too, an understanding of the dam- age mechanisms is key to developing ceramics and glasses with improved ballistic performance. The damage mechanisms are readily revealed in experi- ments in which the projectile does not penetrate—that is, at velocities and test conditions sufficient to initiate the damage process but insufficient for ingress. Such tests produce ring cracks and radial cracks on the impacted surfaces, as well as the well-known Hertzian cone cracks, which extend into the target at divergent angles from the shot line. More important when considering penetration mechanisms, however, is the microdamage produced in the target directly ahead of the projectile, since it is the material in this location that must FIGURE 3-3 Polished cross sections through the shot line of a SiC be extruded from the projectile path to permit penetration. (A) and a TiB2 (B) target, showing typical microdamage immedi- The polished cross sections taken through the shot lines ately below the impact site after a no-penetration experiment with after tests on SiC and TiB2 (Figure 3-3) show the Hertzian a long rod tungsten projectile. SOURCE: LaSalvia, J.C., and J.W. cone cracks and, often, an obvious zone of damaged material McCauley. 2010. Inelastic deformation mechanisms and damage immediately beneath the impacting projectile.6 in structural ceramics subjected to high-velocity impact. Inter- national Journal of Applied Ceramic Technology 7(5): 595-605. 5Shockey, D.A., A.H. Marchand, S.R. Skaggs, G.E. Cort, M.W. Burkett, See also LaSalvia, J.C., R.B. Leavy, J.R. Houskamp, H.T. Miller, and R. Parker. 1990. Failure phenomenology of confined ceramic targets D.E. MacKenzie, and J. Campbell. 2010. Ballistic impact damage and impacting rods. International Journal of Impact Engineering 9(3): 263- observations in a hot-pressed boron carbide. Pp. 45-55 in Advances 275. Also in Shockey, D.A., A.H. Marchand, S.R. Skaggs, G.E. Cort, M.W. in Ceramic Armor V. J.J. Swab, D. Singh, and J. Salem, eds. New Burkett, and R. Parker. 2002. Failure phenomenology of confined ceramic York, N.Y.: John Wiley & Sons. targets and impacting rods. Pp. 385-402 in Ceramic Armor Materials by Design, Ceramics Transactions Column 134. J.W. McCauley, A. Rajendran, W. Gooch, S. Bless, S. Wax, and A. Crowson, eds. Westerville, Ohio: The American Ceramic Society. 6LaSalvia, J.C., and J.W. McCauley. 2010. Inelastic deformation mecha - nisms and damage in structural ceramics subjected to high-velocity impact. International Journal of Applied Ceramic Technology 7(5): 595-605. See also LaSalvia, J.C., R.B. Leavy, J.R. Houskamp, H.T. Miller, D.E. MacK- enzie, and J. Campbell. 2010. Ballistic impact damage observations in a

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27 MECHANISMS OF PENETRATION IN PROTECTIVE MATERIALS FIGURE 3-4 Damage mechanisms (see arrows) observed in several ceramics: Nanoscale amorphization bands in ballistic-generated B 4C fragments (a); shear “band” in a sphere-impacted B4C (b); shear-induced amorphization in a sphere-impacted b-SiC (c); multiscale frag - mentation and shear localization in the same material (d); twins in sintered Al 2O3 shocked above its Hugoniot elastic limit (e); aluminum oxynitride (AlON) fracture surface showing multiscale cleavage (f); AlON fragment showing microcleavage (g); and stacking faults in a SiC (h). SOURCE: LaSalvia, J.C., and J.W. McCauley. 2010. Inelastic deformation mechanisms and damage in structural ceramics subjected to high-velocity impact. International Journal of Applied Ceramic Technology 7(5): 595-605. See also LaSalvia, J.C., R.B. Leavy, J.R. Hous - kamp, H.T. Miller, D.E. MacKenzie, and J. Campbell. 2010. Ballistic impact damage observations in a hot-pressed boron carbide. Pp. 45-55 in Advances in Ceramic Armor V. J.J. Swab, D. Singh, and J. Salem, eds. New York, N.Y.: John Wiley & Sons. Closer examination of the damage zone, known as the 1-3 μs, and can be observed with high-speed photography. Mescall zone (MZ),7 provides valuable details of the mate- Penetration can proceed only after material in the MZ has rial failure process. Figure 3-4 shows the variety of damage failed and has been pushed from the projectile path. Thus the mechanisms observed beneath the projectile impact sites in projectile dwells on the target surface before beginning to several ceramics. These mechanisms include intergranular penetrate. If a projectile did not reach a certain velocity—the and transgranular macro- and microcracking; shear localiza- transition velocity—and if the ceramic held together suf- tion; solid-state amorphization; dislocation activity; twin- ficiently—that is, if it exhibited “target confinement”—the ning; stacking faults; and phase transformations.8 projectile would not penetrate and “interface defeat” would The damage process requires some time, typically be said to have occurred. Projectiles with sufficient kinetic energy to fully de- hot-pressed boron carbide. Pp. 45-55 in Advances in Ceramic Armor V. J.J. velop the MZ penetrate by continuously damaging the Swab, D. Singh, and J. Salem, eds. New York, N.Y.: John Wiley & Sons. target material at the projectile tip and extruding the frag- 7The Mescall zone—first defined in Shockey, D.A., A.H. Marchand, S.R. ments to the side of the shot line. Insight into the extrusion Skaggs, G.E. Cort, M.W. Burkett, and R. Parker. 1990. Failure phenomenol- process is obtained by examining cross sections of partially ogy of confined ceramic targets and impacting rods. International Journal of penetrated target blocks. Monolithic targets of a soda lime Impact Engineering 9(3): 263-275—is named after John Mescall, a scientist at the U.S. Army Materials and Mechanics Research Center, who deduced glass impacted by a hemi-nosed steel rod at velocities suf- the existence of the finely comminuted volume of target directly beneath ficient to penetrate partway through the target9 retain the the nose of an advancing projectile from his computational simulations cracking pattern and fragments produced during penetration (Mescall, J., and C. Tracy. 1986. Improved modeling of fracture in ceramic (Figure 3-5a). The cracks and fragments are revealed by armors. Pp. 41-54 in Army Science Conference Proceedings, 17-19 June infiltrating a damaged target with a low-viscosity epoxy, 1986, Volume III. Washington, D.C.: Department of the Army, Deputy Chief of Staff for Research, Development & Acquisition; Mescall, J., and V. Weiss. then sectioning the target with a diamond saw, usually on a 1983. Materials Behavior under High Stress and Ultrahigh Loading Rates. plane through the shot line. Next, the surfaces of section are New York, N.Y.: Plenum Press). polished and examined by optical and scanning electron mi- 8LaSalvia, J.C., and J.W. McCauley. 2010. Inelastic deformation mecha - nisms and damage in structural ceramics subjected to high-velocity impact. International Journal of Applied Ceramic Technology 7(5): 595-605. See 9Shockey, D.A., D. Bergmannshoff, D.R. Curran, and J.W. Simons. 2008. also LaSalvia, J.C., R.B. Leavy, J.R. Houskamp, H.T. Miller, D.E. MacK- enzie, and J. Campbell. 2010. Ballistic impact damage observations in a Physics of glass failure during rod penetration. Pp. 23-32 in Advances in hot-pressed boron carbide. Pp. 45-55 in Advances in Ceramic Armor V. J.J. Ceramic Armor IV: Ceramic Engineering and Science Proceedings, Volume Swab, D. Singh, and J. Salem, eds. New York, N.Y.: John Wiley & Sons. 29, Issue 6. L.P. Franks, ed. Hoboken, N.J.: John Wiley & Sons.

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28 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS b a FIGURE 3-5 A 200 × 200 × 75 mm3 monolithic soda lime glass target (confined on all sides with polymethyl methacrylate plates) partially penetrated by a 31.75 × 6.35-mm-diameter heminosed steel rod impacting at 300 m/s (a); a surface of section through the shot line showing damage around the projectile cavity (b). SOURCE: Shockey, D., J. Simons, and D. Curran. 2010. The damage mechanism route to better armor materials. International Journal of Applied Ceramic Technology 7(5): 566-573. Finding 3-2. An examination of the mechanics of penetra- croscopy to observe cracking details and the size and shapes of fragments (Figure 3-5b). tion in brittle materials reveals four important characteristics Glass and ceramic targets show the radial cracks, ring of microstructures that are key for improved body armor cracks, cone cracks, and lateral cracks typical of a rod or materials. The structures must (1) resist deformation and particle impact. Target fragments about one to three pro- macro (cone and lateral) cracking; (2) be more difficult to jectile radii in diameter form a cylindrical zone (tunnel) comminute; (3) break into fragment geometries that are more resistant to flow; and (4) form more dilatant fragment beds.10 surrounding the embedded projectile. An uplifted “lip” of material is often produced at the impact surface. The size and shape of MZ fragments can be determined and quantified by PENETRATION MECHANISMS IN POLYMERIC examining petrographic sections through the tunnel debris. MATERIALS The MZ of highly comminuted material at the leading edge of the penetrator shown in Figure 3-5b is smaller than what Polymers such as polycarbonate are often used in armor would be expected during penetration because the stresses systems as backing plates (spall shields), as intermediate at the tip of an arresting penetrator are smaller than those in layers in a laminated glass or ceramic system, as a scratch- advance of a moving penetrator. tolerant front plate, or as a matrix to embed strong fibers. The fragmentation and cracking patterns suggest that Because the material failure mechanisms are sensitive to material ahead of the projectile is loaded, damaged, and boundary conditions, they are somewhat determined by the d isplaced in three successive steps under consecutive application. Real-time observation with high-speed cameras tensile-, shear-, and compression-dominated stress states shows that the penetration of polycarbonate plates by cylin- (Figure 3-6). A material element in the path of an advancing drical projectiles occurs by elastic dishing, petalling, cone cracking, and plugging.11 The projectile initially indents the penetrator initially experiences tension and develops closely spaced cone cracks running at acute angles to the penetration surface of the plate, causing the distal plate surface to bulge direction. Subsequent lateral cracks break up the material and shear yielding around the impact site. As the penetrator between adjacent cone cracks. As the projectile moves closer, advances, cracks form ahead of it. Depending on the projec- a local volume (about the size of the projectile nose) of the cracked material is overrun by a low-confinement field of 10Shockey, D.A., J.W. Simons, and D.R. Curran. 2010. The damage high shear and is comminuted into fine fragments. Third, mechanism route to better armor materials. International Journal of Applied Ceramic Technology 7(5): 566-573. the projectile imposes high pressure and extrudes the com- 11Wright, S.C., N.A. Fleck, and W.J. Stronge. 1993. Ballistic impact minuted material into the cracked and coarsely fragmented of polycarbonate: An experimental investigation. International Journal of tunnel and to the sides of the projectile nose. Impact Engineering 13(1): 1-20.

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29 MECHANISMS OF PENETRATION IN PROTECTIVE MATERIALS voids and kinks and was assisted by the residual stresses that arose during processing.14 While the details of the tensile failure mechanism are not well known, high magnification shows that the fibrils in the fibers are stretched, suggesting tensile failure analogous to that seen in tensile tests of metals. Fiber material very likely undergoes homogeneous plastic deformation and localized plastic deformation in much the same way as metals; failure may also occur by the nucleation of voids, cracks, and shear bands. It is not understood how the material microstructure at this level (the nano level) influences the deformation, localization, and failure behavior of the material. Failure initiators are thought to originate in material defects such as tiny voids, foreign particles, and chain entanglements (shown FIGURE 3-6 Three material processing zones and three stress in Figure 3-10) resulting from chemical inhomogeneities or states experienced by a material element in the path of an advanc- ing penetrator. SOURCE: Shockey, D., J. Simons, and D. Curran. processing procedures. 2010. The damage mechanism route to better armor materials. In- Fiber failure modes other than tensile failure are also ob- ternational Journal of Applied Ceramic Technology 7(5): 566-573. served. For example, a projectile’s impact on fabric backed with a stiff plate of ceramic compresses the fabric against the backing and causes transverse loads on the yarns and fibers that can result in deformation and failure. When compressed tile nose shape, plate perforation occurs by petalling or by fibers are examined by SEM, they and the fibrils show flat- plugging—that is, by pushing a cylinder of material ahead tening, kinking, and buckling. of the projectile through the distal plate surface. Evidence Finding 3-3. The influence of the nano- and microstructure of melting has been observed. Material failure mechanisms may include tensile failure by nucleation, growth, and of polymeric materials on the deformation, localization, coalescence of planar cracks, spherelike voids, and shear and failure behavior of the materials is not well understood, instabilities. In glassy polymers, crazing, or the formation of especially at high strain rates and high pressures. oriented fibrils and intervening voids, is a common precursor Finding 3-4. Closing the large gap between the currently at- to crack formation and tensile failure. Polymer fibers are used in ballistic materials and as tainable and the theoretical strengths of fibers would benefit reinforcing elements in composite materials. A careful and greatly from studies of ballistically (and quasi-statically) detailed study of nanoscale failure phenomenology would failed fibers at the nano- and micro levels to determine the be most useful in developing fibers with better ballistic mechanism(s) of material failure and identify the nano - performance. Figure 3-7 shows a fabric after it has been im- structural features initiating the failure process or otherwise pacted by a platelike projectile.12 The failure mechanisms of assisting it. polymer fibers can be determined by examining the severed fiber ends with a scanning electron microscope (SEM).13 FAILURE MECHANISMS IN CELLULAR-SANDWICH For example, the internal structure of a 20-μ-diameter poly- MATERIALS DUE TO BLASTS p-phenylene benzobisthiazole (PBZT) fiber consists of large length-to-width, ribbonlike fibrils typically 1 μ wide, which A cellular material sandwiched between two faceplates in turn are made up of microfibrils of similar geometry but provides mass-efficient protection against blast loads. The only a few nanometers wide (Figure 3-8). Figure 3-9 indi- structure absorbs energy and reduces the transmitted force cates that tensile fracture first occurred at defects such as as the walls of each cell deform and fail. Thus, to improve or tailor the response of cellular structures to blast loads, cell failure mechanisms must be understood. Cellular materials include polymers, ceramics, and met- als and metal foams; cell geometries include honeycomb and other lattices as well as stochastically random geometries. 12Shockey, D.A., D.C. Erlich, and J.W. Simons. 2004. Lightweight Bal - Several material properties contribute to the effective absorp- listic Protection of Flight-Critical Components on Commercial Aircraft, Part 2: Large-Scale Ballistic Impact Tests and Computational Simulations, DOT/ FAA/AR-04/45,P2. Available online at http://www.tc.faa.gov/its/worldpac/ 14Allen, S.R., A.G. Filippov, R.J. Farris, and E.L. Thomas. 1981. Macro - techrpt/ar04-45p2.pdf. Last accessed April 15, 2011. 13Hearle, J.W.S., B. Lomas, and W.D. Cooke. 1998. Atlas of Fibre Frac - structure and mechanical behavior of fibers of poly-p-phenylene benzobis - ture and Damage to Textiles. Boca Raton, Fla.: CRC Press. thiazole. Journal of Applied Polymer Science 26(1): 291-301.

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30 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE 3-7 Post-test observation of fabric damage from a platelike projectile showing yarn breakage characteristics (left); the projectile size is shown with the fabric flap in its original position (right). SOURCE: Shockey, D.A., D.C. Erlich, and J. W. Simons. 2004. Lightweight Ballistic Protection of Flight-Critical Components on Commercial Aircraft, Part 2: Large-Scale Ballistic Impact Tests and Computational Simulations, DOT/FAA/AR-04/45,P2. Available online at http://www.tc.faa.gov/its/worldpac/techrpt/ar04-45p2.pdf. Last accessed April 15, 2011. tion of the energies of blast loads. These properties include the elastic stiffness, the yield strength, strain hardening, and the level of plateau stress at which a material compresses plastically. In foams of ductile aluminum, discrete bands of col- lapsed cells establish the onset of yielding, the hardening, and the level of the plateau stress (Figure 3-10).15 When the load is applied slowly, the bands form progressively and independently; at higher loading rates the cellular structure deforms by the advancement of a crush front from the impact surface.16 Figure 3-11 shows three specific failure mechanisms operating sequentially in a ductile aluminum cell under a quasi-static load.17 First, localized plastic straining occurs at a cell node. Next, the cell membrane plastically buckles. Finally, the cell collapses. Blast-loaded aluminum foam may fail in 15Bastawros, A.-F., H. Bart-Smith, and A.G. Evans. 2000. Experimental analysis of deformation mechanisms in a closed-cell aluminum alloy foam. Journal of the Mechanics and Physics of Solids 48(2): 301-322. 16Tan, P.J., S.R. Reid, J.J. Harrigan, Z. Zou, and S. Li. 2005. Dynamic compressive strength properties of aluminum foams Part I: Experimental FIGURE 3-8 SEM micrograph revealing fibrillar microstructure data and observations. Journal of the Mechanics and Physics of Solids in an as-spun PBZT fiber. SOURCE: Allen, S.R. 1983. Mechanical 53(10): 2174-2205. and morphological correlations in poly-(p-phenylene benzobis - 17Bastawros, A.-F., H. Bart-Smith, and A.G. Evans. 2000. Experimental thiazole) fibers. Ph.D. Dissertation. Amherst, Mass.: University of analysis of deformation mechanisms in a closed-cell aluminum alloy foam. Journal of the Mechanics and Physics of Solids 48(2): 301-322. Massachusetts.

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31 MECHANISMS OF PENETRATION IN PROTECTIVE MATERIALS FIGURE 3-9 SEM side views (A,B) and end- on views (C,D) of matching fracture ends of a tensile-fractured PBZT fiber. SOURCE: Allen, S .R. 1983. Mechanical and morphological correlations in poly-(p-phenylene benzobis - thiazole) fibers. Ph.D. Dissertation. Amherst, Mass.: University of Massachusetts. FIGURE 3-10 Sequence of computerized axial tomography scan images showing macro deformation bands in quasi-static compression- loaded ductile aluminum foam. (Dominant bands are identified with arrows, and the cells most visibly subjected to distortion are circled.) Note the buckling deformations exhibited by one of the membranes in each cell. SOURCE: Bastawros, A.-F., H. Bart-Smith, and A.G. Evans. 2000. Experimental analysis of deformation mechanisms in a closed-cell aluminum alloy foam. Journal of the Mechanics and Physics of Solids 48(2): 301-322.

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32 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS FIGURE 3-12 Stress-strain curve for a brittle aluminum foam subjected to quasi-static compression (top). Bands of fractured cells after imposed quasi-static engineering compressive strains of 0, 5.6 percent, 11.7 percent, 33.3 percent, and 60 percent, respectively (lower four images). SOURCE: Song, H-W., Q-J. He, J-J. Xie, and FIGURE 3-11 Sequential mechanisms responsible for cell col- A. Tobota. 2008. Fracture mechanisms and size effects of brittle lapse in ductile aluminum foam under quasi-static load. SOURCE: metallic foams: In situ compression tests inside SEM. Composites Bastawros, A.-F., H. Bart-Smith, and A.G. Evans. 2000. Experimen- Science and Technology 68(12): 2441-2450. tal analysis of deformation mechanisms in a closed-cell aluminum alloy foam. Journal of the Mechanics and Physics of Solids 48(2): 301-322. sion, and shear and by friction and shear between fractured tension18 owing to a reflected tensile stress wave or recoil cells (Figure 3-13).19 after compaction. Brittle metallic foams fail at the macroscopic level by Finding 3-5. Cellular materials absorb blast energy by de- forming fracture bands similar to the deformation bands in formation and failure of cell walls. ductile aluminum (Figure 3-12). Compression, tension, and shear cause cell walls to crack; friction and shear operate CONCLUSIONS between fractured cells. At the cell membrane level, failure is by the brittle cracking of cell walls under compression, ten- This brief survey of how materials undergo penetration shows that • Penetration occurs by material failure; 18Langdon, G.S., D. Karagiozova, M.D. Theobald, G.N. Nurick, G. 19Song, H-W., Q-J. He, J-J. Xie, and A. Tobota. 2008. Fracture mecha - Lu, and R.P. Merrett. Fracture of aluminum foam core sacrificial cladding subjected to air-blast loading. International Journal of Impact Engineering nisms and size effects of brittle metallic foams: In situ compression tests 37(6): 638-651. inside SEM. Composites Science and Technology 68(12): 2441-2450.

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33 MECHANISMS OF PENETRATION IN PROTECTIVE MATERIALS • Developing innovative laboratory tests that invoke the pertinent damage mechanisms; • Designing tests so as to favor only one mechanism and so avoid the complication of several mechanisms operating simultaneously and influencing each other. • Designing other tests that invoke two or more mecha- nisms, to investigate synergistic effects. Such tests must be conducted under well-controlled and moni- tored conditions of load, rate, and temperature and need to measure the governing (nonconventional) material failure properties; • Performing other tests in which the load application is stopped at various percentages of the maximum load and the specimens sectioned and examined microscopically to observe the damage at increas- ing stages of development and the interaction of the damage with microstructural features; • Prescribing microstructures that repress or interfere with failure mechanisms and interacting with pro- FIGURE 3-13 SEM images of failed cells in brittle aluminum foam cessing engineers to innovate ways to achieve these showing failure modes (a) under compression, (b) tension and shear, microstructures; (c) face cracking, and (d) friction and shear between fractured cells. • Choosing and implementing chemistries and process- SOURCE: Song, H-W., Q-J. He, J-J. Xie, and A. Tobota. 2008. ing routes; Fracture mechanisms and size effects of brittle metallic foams: In • Performing laboratory tests and ballistic tests, noting situ compression tests inside SEM. Composites Science and Tech- results, adjusting initial thoughts, and exploring a nology 68(12): 2441-2450. second generation of chemistries and processing; and • Continuing iterations with the goal of achieving ever better protective materials. • Material failure occurs at the microstructural level, The time and expense of developing a superior armor including the chemical phase and phase composition, material can be greatly reduced if the response of a chosen and the nanostructural level; microstructure to ballistic attack can be at least approxi- • Failure can occur in five modes—adiabatic shear mately predicted by means of a computational simulation. bands, cracks, voids, plastic deformation, and A material deformation and failure model based on observa- phase changes—more than one of which can occur tions of penetrated targets and quantified by parameters de- simultaneously; rived from laboratory tests can be expected to provide more • Material failure is a kinetic process, involving reliable results than current models. To assist in construct- nucleation, growth, and coalescence of these failure ing a computational model of damage evolution, damage modes; and features observed in the interrupted tests should be counted • Cellular materials absorb blast energy by deforma- and measured as a function of location in the specimen (by tion and failure of cell walls. determining a nucleation rate). These data can be correlated with a stress and strain history obtained by computational Recommendation 3-1. Organizations and individuals en- simulations of the tests in order to develop equations that gaged in developing protection materials should seek to describe damage development. These equations would con- maximize penetration resistance. A comprehensive method- stitute the computational materials damage model and could ology for this should entail the following: be used in finite element codes to compute damage evolution during target penetration. • Using the understanding of how armor materials fail Thus, achievement of improved protective materials ne- to suggest possible microstructures—that is, those cessitates that damage development be hindered, most likely microstructures discovered through experimental/ by specifying microstructures that resist the nucleation, computational investigations as well as those identi- growth, and coalescence of cracks, voids, and shear bands fied during manufacturing trials that may oppose and by specifying as well chemistries and processing routes damage development; to achieve those microstructures. • Designing microstructures that inhibit, disrupt, or avoid altogether the failure mechanisms operating in armor during projectile attack;

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34 OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS Recommendation 3-2. Organizations and individuals en- • Formulate mathematical models of damage evolu- tion and use these in computational simulations of gaged in developing protection materials should ballistic penetration scenarios to assist and expedite the design of improved armor materials and systems. • Choose materials based on their ability to inhibit or avoid material failure mechanisms, as opposed to choosing materials based on their bulk properties.