Mechanisms of Dynamic Deformation and Failure in Ultra-High Molecular Weight Polyethylene Fiber-Polymer Matrix Composites

Ultra-high molecular weight polyethylene (UHMWPE) molecules, with molecular weights approaching 107 Da and lengths approaching 10 μm, can be gel spun and drawn into highly crystalline fibers with more than 95% of the molecules oriented in the fiber direction. The very high tensile strength (approaching 4 GPa) and elastic modulus (200 GPa) combined with a very low density (970 kg m-3) result in a fiber with very high specific strength and modulus. While the strength per unit mass of the materials in the fiber direction is ~25 times greater than that of conventional steels, weak (van der Waals) bonds between molecules leads to strengths transverse to the fibers of only a thousandth that in the fiber direction. This weak intermolecular strength also leads to creep deformation under prolonged loading at ambient temperatures, and complete failure of the polymer when the intermolecular bonds “melt” at 155°C. These materials are therefore used in weight sensitive applications, where a high uniaxial stress must be supported for relatively short periods of time. Examples include mooring cables, the sails of racing ships and ballistic impact protection panels. For ballistic applications, the 10-20 μm diameter fibers are combined with compliant thermoplastic polymer matrices to form thin (typically 50 μm thick) unidirectional plies containing ~85% by weight fibers. These plies are then layered to form a cross-ply ([0°/90°]n) structure, and pressed (at 127°C) to create a composite panel. This dissertation investigates the structure, mechanical properties and dynamic deformation and failure mechanisms during the ballistic impact of these UHMWPE reinforced [0°/90°] polymer matrix composites by a model projectile.

Six UHMWPE [0°/90°] polymer composite systems were investigated in the study. The laminates had measured tensile strengths (a fiber dominated property) in the range of 800 – 1100 MPa, which was 500-5,000 times higher than the laminates’ measured interlaminar shear strengths (a matrix dominated property). Digital image correlation techniques have been used to show that the Poisson expansion of a ply under compressive loading was also highly anisotropic, with a Poisson’s ratio of ν23 = 0.5 transverse to fiber direction, and ν13 = 0 in fiber direction. During uniform out of plane (through thickness) compressive loading of [0°/90°] composites, this anisotropic Poisson expansion of adjacent 90° plies has been shown to cause fiber tension in the 0° ply by a shear lag mechanism. Failure of the compressed sample occurs when the tension induced stress in the fibers reaches the plies failure strength (in excess of 1 GPa), and agreed well with experimental data collected on thick laminates with lateral dimensions substantially larger than the shear lag length.
The out of plane compressive strength of the [0°/90°] composites was discovered to be dependent upon the laminate thickness; as the laminate thickness was decreased the strength of the laminates decreased to 60%-70% of the indirect tension strength prediction. Using a combination of optical and ultrasonic C-scan imaging techniques in conjunction with micro-X-ray tomography, two classes of defects have been identified in the [0°/90°] composites. One defect type consisted of tunnel cracks that were parallel to the fibers in a ply and approximately equally spaced in the transverse direction. These are shown to form as a result of anisotropic thermal strains within the laminates during cooling after consolidation processing. The second void-like defect results from missing groups of fibers within each ply. Like the tunnel cracks, this defect extended many centimeters in a ply’s fiber direction. While tunnel cracks were healed during ambient temperature out of plane compression, and therefore had little effect on a laminates out of plane compressive strength, the missing fiber defects significantly degraded the compressive strength of thin laminates. Compression tests using pressure sensitive film and acoustic emission monitoring reveal that regions containing missing fiber defects in thin laminates are shielded from load by defect free regions, which then fail at lower sample pressure during loading. A simple statistical model was developed that successfully predicted the contrast observed in optical and ultrasonic images, and the effect of missing fiber defects upon the out of plane compressive strength.

The dissertation also investigated the mechanisms of projectile penetration during impact of UHMWPE fiber-reinforced composites with a spherical projectile using model targets designed to dynamically load the laminates in different ways. The response of the samples were studied using a combination of synchronized high speed photography with three cameras, and 3D digital image correlation together with post-test characterization via X-ray tomography and optical microscopy. It was found that a rear supported laminate, which was prevented from deflecting, was progressively penetrated by the projectile. Since the projectile applied only a compressive pressure to the laminate, it is argued that penetration occurred by the indirect tension mechanism. Edge clamped laminates that are allowed to freely deflect have an improved impact resistance, especially if the projectile is fragmented before impacting the laminate, or the laminate is given an out of plane velocity prior to direct impact by the projectile. The results are used to propose a projectile penetration process model that incorporates both the activation of indirect tension and membrane stretching. It predicts that suppression of high compressive stress in the [0°/90°] laminate forces the laminate to respond in a bi-axial membrane stretching mode where the kinetic energy of the projectile is expended in the very significant work needed to stretch the laminate. This hypothesis was tested with a model impact target that spatially distributed the load to the laminate and was found to substantially increase the resistance of the laminate to penetration and failure.