An exploration of the ballistic resistance of multilayer graphene polymer composites

The response of multilayer graphene/polyvinyl alcohol (MLG/PVA) films were studied under quasi-static (Q.S.) and dynamic edge-clamped transverse loading. The 10 μm thick films, reinforced by ∼35 vol.% MLG and measuring 85 mm square were fabricated by liquid exfoliation of the graphene followed by filtration of the MLG/PVA dispersion. The responses of the MLG/PVA films were compared with those of equal areal mass films of pure PVA and aluminum. The moderately conductive (∼10−2 S cm−1) MLG/PVA films had a Young’s modulus approximately twice that of PVA and a low strain rate (10−3 s−1) peak strength that was about 50% higher. Moreover, while the MLG/PVA films had a tensile strength lower than the Al films, they had a higher load carrying capacity compared to the Al films and were stiffer than the PVA films under Q.S. transverse loading. The ballistic limit of the MLG/PVA films was ∼50% higher than the Al films but the higher ductility of the parent PVA resulted in the pure PVA films having a higher ballistic resistance. The ballistic resistance of the MLG/PVA is well predicted by a membrane stretching analysis and this enables us to present an outlook on the ballistic resistance potential of graphene/PVA composites comprising aligned large flakes.The work was supported by the Office of Naval Research Grant N62909-15-1-N058 (Program manager, Dr. Judah Goldwasser). Dr. B.P. Russell was supported by a Ministry of Defence/Royal Academy of Engineering Research Fellowship (Grant Number RG60007)

The fracture toughness of octet-truss lattices

The only engineering materials with both high strength and toughness, and with densities less than 1000 kg m −3 , are natural materials (woods) and some plastics. Cellular structures such as the octet lattice, when made from periodic arrangements of strong, low-density metallic trusses, are known to have high specific strengths and elastic moduli. However, much less is known of their resistance to fracture. Here we investigate the fracture toughness of a Ti-6Al-4V alloy octet-lattice truss structure manufactured using a ‘snap-fit’ method. The samples had densities between 360 and 855 kg m −3 (relative densities of 8–19%) and free truss lengths between 4 and 15 mm. Their fracture resistance was determined using the J-integral compliance method applied to single-edge notched bend specimens. The toughness is shown to increase linearly with the relative density and with the square root of the cell size, while the strength was confirmed to scale only with relative density and the strength of the solid. A moderate increase in resistance with crack length (an R-curve effect) was seen for the higher relative density and larger cell size samples. With a fracture toughness between 2 and 14 MPa m 1/2 and a compressive strength between 20 and 70 MPa, these structures offer a new lightweight engineering material solution for use at temperatures up to 450 °C.We are grateful for the support of this work by the DARPA MCMA program (Grant no. W91CRB-10-1-005) managed by Dr. Judah Goldwasser

Failure Modes of a Laminated Composite with Complaint Interlayers

Composites comprising a high-volume fraction of stiff reinforcements within a compliant matrix are commonly found in natural materials. The disparate properties of the constituent materials endow resilience to the composite, and here we report an investigation into some of the mechanisms at play. We report experiments and simulations of a prototype laminated composite system comprising silicon layers separated by polymer interlayers, where the only failure mechanism is the tensile fracture of the brittle silicon. Two failure modes are observed for such composites loaded in three-point bending: failure under the central roller in (i) the top ply (in contact with the roller) or (ii) the bottom ply (free surface). The former mode is benign with the beam retaining load carrying capacity, whereas the latter leads to catastrophic beam failure. Finite element (FE) simulations confirm this transition in failure mode and inform the development of a reduced order model. Good agreement is shown between measurements, FE simulations, and reduced order predictions, capturing the effects of material and geometric properties on the flexural rigidity, first ply failure mode, and failure load. A failure mechanism map for this system is reported that can be used to inform the design of such laminated composites

An exploration of the ballistic resistance of multilayer graphene polymer composites

The response of multilayer graphene/polyvinyl alcohol (MLG/PVA) films were studied under quasi-static (Q.S.) and dynamic edge-clamped transverse loading. The 10 μm thick films, reinforced by ∼35 vol.% MLG and measuring 85 mm square were fabricated by liquid exfoliation of the graphene followed by filtration of the MLG/PVA dispersion. The responses of the MLG/PVA films were compared with those of equal areal mass films of pure PVA and aluminum. The moderately conductive (∼10−2 S cm−1) MLG/PVA films had a Young’s modulus approximately twice that of PVA and a low strain rate (10−3 s−1) peak strength that was about 50% higher. Moreover, while the MLG/PVA films had a tensile strength lower than the Al films, they had a higher load carrying capacity compared to the Al films and were stiffer than the PVA films under Q.S. transverse loading. The ballistic limit of the MLG/PVA films was ∼50% higher than the Al films but the higher ductility of the parent PVA resulted in the pure PVA films having a higher ballistic resistance. The ballistic resistance of the MLG/PVA is well predicted by a membrane stretching analysis and this enables us to present an outlook on the ballistic resistance potential of graphene/PVA composites comprising aligned large flakes

Defect controlled transverse compressive strength of polyethylene fiber laminates

Using a combination of optical and ultrasonic imaging in conjunction with micro-X-ray tomography, we have identified the existence of two classes of defects in [0°/90°] cross-ply polymeric composites made from ultrahigh molecular weight polyethylene (UHMWPE) fibers and thermoplastic resins, and investigated their effects upon the transverse compressive strength of laminates of various thicknesses and lateral dimension. One defect type consisted of equal spaced tunnel cracks that resulted from anisotropic thermal contraction of the laminates after processing. The second consisted of void-like defects resulting from missing groups of fibers. Like the tunnel cracks, this defect extended many centimeters in a ply's fiber direction. While tunnel cracks were healed upon out of plane compression, and therefore have little effect on a laminates out of plane compressive strength, the missing fiber defects significantly degraded the compressive strength. However, the degradation was reduced by increasing the laminate thickness. Compression tests using pressure sensitive film and acoustic emission monitoring reveal that regions containing missing fiber defects are shielded from load by defect free regions, which then fail at lower sample pressure during loading. A simple statistical model is used to simulate the distribution of missing fiber defects as local reductions in ply thickness, and reproduced the contrast observed in optical and ultrasonic images, as well as the reduction in out of plane compressive strength observed in experiments

Mechanisms of projectile penetration in Dyneema® encapsulated aluminum structures

Polymer composites comprising ultra-high molecular weight polyethylene (UHWMPE) fibers in a compliant matrix are now widely used in ballistic applications with varying levels of success. This is primarily due to a poor understanding of the mechanics of penetration of these composites in ballistic protection systems. In this study, we report experimental observations of the penetration mechanisms in four model systems impacted by a 12.7 mm diameter spherical steel projectile. The four model targets designed to highlight different penetration mechanisms in Dyneema® UHWMPE composites were: (i) a bare aluminum plate; (ii) the same plate fully encased in a 5.9 mm thick casing of Dyneema®; (iii) the fully encased plate with a portion of the Dyneema® removed from the front face so that the projectile impacts directly the Al plate; and (iv) the fully encased plate with a portion of the Dyneema® removed from the rear face so that the projectile can exit the Al plate without again interacting with the Dyneema®. A combination of synchronized high speed photography with three cameras, together with post-test examination of the targets via X-ray tomography and optical microscopy was used to elucidate the deformation and perforation mechanisms. The measurements show that the ballistic resistance of these targets increases in the order: bare Al plate, rear face cutout target, fully encased target and front face cutout target. These findings are explained based on the following key findings: (a) the ballistic performance of Dyneema® plates supported on a foundation is inferior to Dyneema® plates supported along their edges; (b) the apparent ballistic resistance of Dyneema® plates increases if the plates are given an initial velocity prior to the impact by the projectile, thereby reducing the relative velocity between the Dyneema® plate and projectile; and (c) when the projectile is fragmented prior to impact, the spatially and temporally distributed loading enhances the ballistic resistance of the Dyneema®. The simple model targets designed here have elucidated mechanisms by which Dyneema® functions in multi-material structures. © 2014 Elsevier Ltd

Impact response of aluminum corrugated core sandwich panels

The mechanisms of projectile penetration of extruded 6061T6 aluminum alloy sandwich panels with empty and alumina filled, triangular corrugated cores have been experimentally investigated using zero obliquity, 12.7 mm diameter hard steel projectiles whose diameter was about a half that of the core's unit cell width. We find that low momentum impacts are laterally deflected by interactions with the inclined webs of the empty core. Complete penetration occurred by shear-off within the impacted front face sheet, followed by stretching, bending and tensile fracture of the core webs and finally shear-off within the back face sheet. This combination of mechanisms was less effective at dissipating the projectiles kinetic energy than the shear-off (plugging) mechanism of penetration of the equivalent solid aluminum panel. Inserting ballistic grade alumina prisms in the triangular cross section spaces of the corrugated core significantly increased the panel's ballistic resistance compared to the empty panel. The presence of the hard ceramic led to severe plastic deformation and fragmentation of the projectile and comminution and macroscopic fracture of the ceramic. The Al/Al2O3 hybrid panel ballistic limit was reached when pairs of parallel cracks formed in the rear face sheet at core web-face sheet nodes. The separation distance between these cracks was dependent upon the location of the impact with respect to that of the web-face sheet nodes. Nodal impacts resulted in pairs of fractures that were separated by one cell width and a critical velocity below that of the equivalent solid plate. Impacts mid-way between pairs of nodes resulted in back face sheet crack pairs separated by twice the cell width, and a critical velocity higher than the equivalent solid plate. Using X-ray tomography we show this resulted from the formation of oval (not circular) cross section fracture conoids in the ceramics. The conoid angle was about 60 in the extrusion direction but only 30 in the transverse direction. This observation may have interesting consequences for a panel's resistance to a second, close proximity impact. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved

Indentation of polyethylene laminates by a flat-bottomed cylindrical punch

Cross-ply polymer laminates reinforced by ultra-high molecular weight polyethylene (UHWMPE) fibers and tapes have been subjected to quasi-static indentation by a flat-bottomed, circular cross section punch and their penetration resistance and failure mechanisms investigated. Three fiber- and two tape-reinforced grades progressively failed during indentation via a series of unstable failure events accompanied by substantial load drops. This resulted in a 'saw-tooth' load versus indentation depth profile as the load increased with indentation depth after each failure event. The penetration behavior scaled with the ratio of the thickness of the remaining laminate to the diameter of the punch, and the indentation pressure scaled with the through thickness compressive strength. Failure occurred by ply rupture. The results are consistent with penetration governed by an indirect tension failure mechanism, and with experimental reports that tape-reinforced materials have a similar ballistic resistance to the higher tensile strength fiber-reinforced grades in rear-supported test conditions

Effect of confinement on the static and dynamic indentation response of model ceramic and cermet materials

The effect of confinement on the localized impact response of ceramic and cermet tiles is investigated. A scoping study was first conducted using alumina and TiC/Ni cermet tiles encased in a metal matrix composite (MMC) and impacted by high velocity steel balls. The investigation revealed that increasing the MMC casing thickness reduced the cracking in the ceramic (alumina) tile but had a much smaller effect on the cermet tile. This motivated a detailed experimental investigation of the effect of lateral confining pressure on the static and dynamic indentation response of granite and Corian® tiles that serve as model ceramic and cermet materials, respectively. Quasi-static indentation resulted in comminution under the indenter and the formation of radial cracks in the granite tiles, with the number of radial cracks decreasing with increasing confining pressure. By contrast, the plastic indentation and small shallow radial cracks observed in the Corian® tiles were unaffected by variations in the confining pressure. The loading imposed by the high velocity impact of a steel ball resulted in conical and lateral cracks as well as radial cracks and comminution in the granite tiles. Intriguingly, while the cone and radial cracks were suppressed by confining pressure, the lateral cracks appeared only at the higher confining pressures. By contrast, the strain rate sensitivity of the yield strength of the Corian® reduced the plastic indentation under dynamic loading, but this in turn promoted the formation of radial cracks which decreased in number with increasing confining pressure. No lateral cracks, conical cracks or comminution was observed in the Corian®. The study shows that confining pressure has a less significant effect on cermets compared to ceramics. Since confinement systems add considerable weight to ceramic-based ballistic protection systems, this study suggests that the use of lightly confined cermets could reduce the overall weight of ballistic protection systems

Mechanisms of penetration in polyethylene reinforced cross-ply laminates

The mechanisms of progressive penetration for two ultrahigh molecular weight polyethylene (UHMWPE) reinforced laminates have been investigated. One used an UHMWPE fiber reinforcement while the other utilized molecularly aligned tape. Both materials had similar out of plane compressive strengths, but the fiber system had a 40% higher in plane tensile strength than the tape. Laminated, 6 mm thick plates with a [0°/90°] ply architecture were impacted by a 12.7 mm diameter sphere under conditions that either allowed out of plane plate deflection or eliminated this deflection by rear support of the target. The depth of penetration and the ballistic limit in the rear-supported tests were identical for the two materials, and proceeded by progressive ply failure. However, tests in the edge clamped condition resulted in a substantially higher penetration resistance, especially for the higher tensile strength fiber-reinforced material. Edge clamped testing of a bilayer target, where the front third was composed of the tape material and the remainder comprised fiber reinforced laminate, had the same ballistic limit as a target composed of only the higher ply tensile strength fiber reinforced material. Penetration in both test support conditions was discovered to occur by tensile ply rupture under the projectile, consistent with a recently proposed mechanism for converting out of plane compression to in plane ply tension. Lateral displacement of plies was also observed near the sides of impact craters in both materials, indicating the existence of a second mechanism impeding penetration of the spherical shaped projectile