Ballistic Impact Mechanisms of Materials
At low impact velocities (say a few hundred meters per second), the penetration resistance of a material is governed by the dynamic deformation mechanisms within the projectile and target. However, as the impact velocity increases into the hypervelocity regime (several thousand meters per second), hydrodynamic effects dominate and the penetration response becomes controlled by only the density of the impacted material and projectile. Since the resistance of a material to penetration by low velocity projectiles is controlled by dynamic deformation and fracture mechanisms, the materials science community has been greatly interested in their optimization. In general the mechanisms that are activated depend upon the thickness, strength, ductility, toughness, acoustic impedance and density of both the target material and projectile and the velocity of the projectile.
For thin targets that are much softer than the projectile, the panel bends creating tensile hoop stress in the plate. This leads to failure by the growth of several (typically 3-7) radial cracks which leads to a petalling mode of failure. In thicker materials, Figure 1, such projectile/material combinations lead to crater formation on the impact face and a “plug” that is pushed out of the plate. The crater forms as a result of the plastic flow needed to accommodate the volume of the projectile. The plug results from high shear stresses and large (adiabatic) shear band formation in material near the periphery and just ahead of the projectile, Figure 1(b). In this case increasing the materials shear strength and strain and strain rate hardening are beneficial. In thicker targets, the shear plug requires too high a stress to form, and instead the projectile penetrates by ductile hole enlargement which is governed by the dynamic flow stress of the target material. Efforts to increase resistance to penetration have focused upon the development and use of materials of very high yield strength. However, other failure modes can then begin to become important (for example, spallation and fracture).
Figure 1: Example of the projectile penetration mechanisms for a soft material impacted by a hard projectile.
The best way to resist a projectile is to use a material that is harder (and ideally tougher) than the projectile. Hard ceramics such as Al2O3, SiC, B4C and TiC are widely used for this purpose. Since they are brittle, these materials are usually supported by a tough support plate, Figure 2. If the dynamic strength of the ceramic significantly exceeds that of the projectile, the contact pressure in the projectile can exceed its dynamic strength causing it to plastically deform (flatten) or fragment and flow over the surface of the ceramic. However, this begins to be accompanied by plastic flow and microfracture in the ceramic - especially as the impact velocity rises. Eventually a fracture conoid is formed in the ceramic and this ‘pushes” against the support plate with a force that is governed by the deceleration of the projectile. The big problem with ceramics is that a second impact near the fracture conoid is less well resisted by the partially failed material.
Figure 2: The resistance of a material to impact by a projectile is maximized when the dynamic strength of the impacted material is greater than that of the projectile. Hard ceramics are therefore widely used for ballistic applications. However, brittle ceramics are vulnerable to fracture and a second impact near the fracture conoid of a first impact is less well resisted.
One approach to improve the repeated impact response of a material is to sub-divide it into small square, hexagonal or even triangular tiles as shown in Figure 3. It can then be reassembled using a cellular metal or composite as shown in the figure. Our group has developed many such cellular materials and has been exploring their ballistic impact response. As an example, we have developed ways to extrude aluminum alloys with various prismatic core topologies. The empty cells can then be filled with ceramics of various topologies as shown in Figure 4. In collaborations with Frank Zok’s group at UCSB, we have used the UCSB light gas gun to explore the ballistic impact mechanisms and investigated how these are influence by the ceramic topology as shown in Figure 5 below.
Figure 3:Cellular materials provide a means for subdividing ceramics and localizing their region of fracture to improve performance under repeated impact.
Figure 4: Examples of extruded (prismatic) aluminum alloy cross sections with alumina (white) inserted in the core spaces.
Figure 5: A typical gas gun set up for observing and characterizing a ballistic impact event (this one was set up by Frank Zok at the University of California at Santa Barbara)
Cross sectional views of one of the impacted topologies is shown in Figure 6 as the impact velocity is increased systematically to about 1800 m/s. At low velocities, the projectile is deformed and fractured and radially flows over the ceramic surface. This is accompanied by comminution of the ceramic under the impact site. Eventually, the impacted ceramic pushes against the rear face sheet with sufficient force to cause its failure. The velocity at which this happens is a sensitive function of the ceramic size and shape and the mechanical properties of the aluminum alloy encasement. Figure 7 schematically illustrates the processes activated as failure of the rear face is approached. As the projectile impact speed increases, the maximum pressure exerted on the back face rises and the tensile stretching forces that must be sustained increase.
Figure 6: Effect of impact velocity upon the damage processes in a aluminum/alumina hybrid panel.
Figure 7: The impact of a typical tri-layer structure results in development of a pressure pulse at the rear of the sample. This causes the rear face to experience compressive stresses in the region impacted by the fragmented projectile/ceramic. The rear face is deflected and then goes into tension.
One way to improve the resistance to penetration of a bi- or tri-layer structure is to replace the aluminum with a material of much higher tensile strength and elastic stiffness. Figure 8(a) shows some possible materials. All are fibers and so would be used in a composite material. Since one usually wants to improve the penetration resistance without increasing the weight too much, we are really interested in the specific properties (that is a property of a material divided by its density). In the case of strong fibers, it is also important to address their strain to fracture since the product of strength and strain to failure is proportional to the energy absorbed. This strength times strain to failure product (divided by density) is shown on the ordinate of figure 8(b). The square root of the Young’s modulus divided by density (the longitudinal wave speed) is plotted on the abscissa. The best materials are to the upper right, and include ultrahigh molecular weight polyethylene (UHMWPE) such as the fibers in Dyneema or Spectra composites, PBO and PIPD filaments. PBO fibers can be environmentally unstable (if exposed to moisture and UV radiation) and PIPD filaments are not yet in commercial production.
Figure 8: (a) A map of the tensile strength and Young’s elastic modulus for various types of high strength fiber. (b) The specific energy absorbed by stretching a fiber to failure and longitudinal elastic wave speed in fibers of various types. Resistance to ballistic penetration is highest to the upper right.
A multiscale description of a Dyneema composite is shown in Figure 9. The highly crystalline filaments are arranged in a 0/900 lay-up laminate for best performance. The filaments are bound together with about 17 vol% of a polyurethane resin. Our group is collaborating with the makers of this material (DSM) and colleagues at the University of Cambridge (UK) and UCSB, to understand the mechanisms of failure under dynamic loading and to explore ways of improving the performance of the material. These studies are aided by the use of high resolution X-ray tomography. Figure 10 shows a projectile that has been fragmented by part of an armor system and then arrested by a 5 mm thick Dyneema laminate. Figure 10(a) is a schematic illustration of the region examined showing the two planes imaged with the X-ray technique. The images provide compelling evidence that both filament failure in the compressed region of the laminate and global stretching were both activated to arrest this fragmented projectile.
Figure 9: Multiscale description of a typical polymeric composite used for ballistic applications. In this Dyneema composite example, filaments of highly oriented (crystalline) polyethylene of ultra-high molecular weight are laminated to form 0/90o laminates using a polyurethane resin to bid the structure together.
Figure 10: (a) A schematic illustration of an arrested (and fractured) projectile by a sheet of Dyneema composite. (b) Shows a high resolution X-ray tomographic image of the projectile and composite. The plane of the image contains the 0o filament direction. (c) An orthogonal tomogram to that of (b).
Experiment Assessment of the Ballistic Response of Composite Pyramidal Lattice Truss Structures, Christian Yungwirth, Darren Radford, Mark Aronson, Haydn Wadley, Composites B, 39, p. 556-569, 2008.
Dynamic Rupture of Polymer-Metal Bilayer Plates, G.J. McShane, C. Stewart, M.T. Aronson, H.N.G. Wadley, N.A. Fleck, V.S. Deshpande, International Journal of Solids and Strucutres, 45, p. 4407-4426, 2008.
Impact Response of Sandwich Plates with a Pyramidal Lattice Core, Christian J. Yungwirth, Haydn N.G. Wadley, John O'Connor, Alan Zakraysek, Vikram S. Deshpande, International Journal of Impact Enginering, 25th anniversity special issue, 35, p. 920-936, 2008.
Exploration of Hybrid Sandwich Panel Concepts for Projectile Impact Mitigation, C.J. Yungwirth, J. O'Connor, A. Zakraysek, V.S. Deshpande, H.N.G. Wadley, Journal of American Ceramic Society, 94, p. S62-S75, 2011.
Impact Response of Aluminum Corrugated Core Sandwich Panels, H.N.G. Wadley, K.P. Dharmasena, M.R. O'Masta, J.J. Wetzel, International Journal of Impact Engineering, 62, p. 114-128, 2013.
The Effect of Shear Strength on the Ballistic Response of Laminated Composite Plates, K. Karthikeyan, B.P. Russell, N.A. Fleck, H.N.G. Wadley, V.S. Deshpande, European Journal of Mechanics/A Solids, 42, p. 35-53, 2013.
Effect of Core Topology on Projectile Penetration in Hybrid Aluminum/Alumina Sandwich Structures, H.N.G. Wadley, M.R. O'Masta, K.P. Dharmasena, B.G. Compton, E.A. Gamble, F.W. Zok, International Journal of Impact Engineering, 62, 99-113, 2013.