Khari Harrison's research while affiliated with Texas A&M University and other places

Lightning damage to composite aircraft structures results from a concurrent and sequential interaction between arc discharge multi-physics and the anisotropic electrical and thermal composite properties. In this study, the impact of nominal 50 and 125 kA lightning strikes on damage formation in a carbon-epoxy Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) was investigated. A combination of visual inspection, ultrasonic phased array testing, destructive sectioning, and various microscopy techniques was employed to characterize the damage. Localized damage, including severe matrix decomposition, melting of polyester warp-knitting yarns, fiber splitting, and large-scale delamination, was confined to the immediate vicinity of the lightning attachment point and was accompanied by more diffuse surface damage (i.e., widespread small-scale split fiber tufts and surface primer scorching). The severity and extent of internal damage increased with higher lightning current intensities but were limited to the outermost nine-ply warp-knitted skin stack. Electrically non-conductive through-thickness Vectran™ stitches and polyester warp-knitting yarns had a profound effect in mitigating lightning damage formation. These results suggest that such through-thickness reinforcement can dramatically enhance the lightning damage resistance and tolerance of composite aircraft structures.

A foldcore is a novel core made from a flat sheet of any material folded into a desired pattern. A foldcore sandwich composite (FSC) provides highly tailorable structural performance over conventional sandwich composites made with honeycomb or synthetic polymer foam cores. Foldcore design can be optimized to accommodate complex shapes and unit cell geometries suitable for protective shielding structures. This work aims to characterize hypervelocity impact (> 2000 m/s, HVI) response and corresponding damage morphologies of carbon fiber reinforced polymer (CFRP) FSCs. A series of normal (0° impact angle) and oblique (45° impact angle) HVI (~3km/s nominal projectile velocity) impact tests were performed on CFRP FSC targets to understand the effects of projectile impact on redirected debris formation, and variable debris cloud expansion. HVI damage in FSC targets were assessed using visual inspection and high-speed imaging analysis. The results from the present study indicate that debris cloud propagation and expansion are strongly influenced by foldcore impact location/angle and open-channel direction. This work serves as a baseline study to understand HVI response of FSC targets and to identify critical FSC design parameters to optimize HVI mitigation performance.

Novel engineering materials and structures are increasingly designed for use in severe environments involving extreme transient variations in temperature and loading rates, chemically reactive flows, and other conditions. The Texas A&M University Hypervelocity Impact Laboratory (HVIL) enables unique ultrahigh-rate materials characterization, testing, and modeling capabilities by tightly integrating expertise in high-rate materials behavior, computational and polymer chemistry, and multi-physics multiscale numerical algorithm development, validation, and implementation. The HVIL provides a high-throughput test bed for development and tailoring of novel materials and structures to mitigate hypervelocity impacts (HVIs). A conventional, 12.7 mm, smooth bore, two-stage light gas gun (2SLGG) is being used as the aeroballistic range launcher to accelerate single and simultaneously launched projectiles to velocities in the range 1.5–7.0 km/s. The aeroballistic range is combined with conventional and innovative experimental, diagnostic, and modeling capabilities to create a unique HVI and hypersonic test bed. Ultrahigh-speed imaging (10M fps), ultrahigh-speed schlieren imaging, multi-angle imaging, digital particle tracking, flash x-ray radiography, nondestructive/destructive inspection, optical and scanning electron microscopy, and other techniques are being used to characterize HVIs and study interactions between hypersonic projectiles and suspended aerosolized particles. Additionally, an overview of 65 2SLGG facilities operational worldwide since 1990 is provided, which is the most comprehensive survey published to date. The HVIL aims to ( i) couple recent theoretical developments in shock physics with advances in numerical methods to perform HVI risk assessments of materials and structures, ( ii) characterize environmental effects (water, ice, dust, etc.) on hypersonic vehicles, and ( iii) address key high-rate materials and hypersonics research problems.

When developing layered or architected protective structures to mitigate hypervelocity impacts (HVIs), understanding and characterizing the ultra-high rate response and energy dissipation of each constituent is critical. Incorporation of lightweight polymeric materials as intermediate or inner-most structural layers could optimize HVI damage resistance and tolerance without compromising cost or weight. One key challenge is developing a fundamental understanding of the effects of molecular architecture on the macroscopic dynamic material response and damage formation. In this work, two common and affordable thermoplastics, namely ultra-high molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE), were assessed. Flat square targets of two distinct sizes were subjected to a series of normal HVIs with 10 mm diameter 1050 aluminum spheres at velocities in the range 2–6.5 km/s. The debris cloud velocity, mass loss, and perforation radius were found to be functions of impact velocity for both materials. High-speed images show HVIs to UHMWPE resulted in quasi-brittle responses while HVIs to HDPE resulted in apparent bulk-melting of the material and large-scale plastic deformation. When subjected to HVIs in the tested range, UHMWPE plates exhibited greater mass loss than similar HDPE plates despite the perforation radii being larger in HDPE. This suggests that the momentum and kinetic energy of the debris clouds for UHMWPE targets were greater than that for HDPE targets subjected to identical impacts. These HVI experimental results combined with polymer material characterization data indicate that differences in the polymeric molecular structure and chemistry of the polyethylenes affect their macroscopic HVI performance.

In-flight lightning strike damage to aircraft composite structures may compromise aircraft airworthiness. Hence, it is crucial to incorporate adequate protection systems to mitigate the lightning current. Most lightning strike protection (LSP) techniques involve bonding a metallic conductive layer to the cured laminate exterior. In this study, a novel LSP integration technique was used to develop unitized panels with three different protection layers: pitch carbon fiber paper (PCFP), graphene paper (GP), and copper mesh (CM). Each LSP layer was overlaid on through-the-thickness VectranHT stitched warp-knit multiaxial dry carbon fabric stacks, resin-infused, and oven-cured. A series of lightning strike tests to protected and unprotected stitched carbon-epoxy laminates were conducted at a nominal peak current of 150 kA. Visual inspection was used to investigate lightning damage resistance, understand the damage mechanisms, and evaluate the surface morphology at the strike locations. The size and severity of the damaged area depended on several factors: the outermost ply fiber direction, the strike location relative to Vectran HT and polyester knitting treads, the lightning peak current, and the conductivity of the protection layers. The CM and GP protection layers effectively dissipated the lightning current in-plane and showed no damage to the underlying composite. The degree of lightning damage on an unprotected laminate was significantly lower than for a similar panel with PCFP protection. The presence of Vectran HT structural stitches and polyester warp-knitting threads profoundly reduced the size and severity of lightning damage. These threads appeared to promote close contact between adjacent carbon fiber tows, resulting in better in-plane and through-thickness electrical/thermal conductivities and reduced lightning damage.

In this study, hypervelocity impact experiments were performed on both unstitched and through-thickness Vectran™-stitched laminates. Both laminate types were fabricated from DMS-2436 class-72 warp-knit multiaxial carbon fabric, infused with API-1078 resin using a Controlled Atmospheric Pressure Resin Infusion (CAPRI) process. The laminates were impacted by 4 mm diameter, spherical, Nylon 6/6 projectiles at nominal velocities of 4 km/s using a two-stage light gas gun. The primary measures of the performance of the composite at protecting against impact were in plane hole damage areal comparisons and the comparison of the target back-face debris cloud (BFDC) velocities relative to the incoming projectile velocities. Additional post-shot forensics include characterization of damage morphology and analysis of high-speed videos. Initial inferences about the damage produced in the laminate indicate that the Vectran™ stitching can effectively arrest in-plane damage propagation; impacts at or near a stitchline resulted in no damage propagation across the stitchline boundaries.

When developing protective structures to defend against hypervelocity impacts (HVIs), optimizing specific energy dissipation is critical. Incorporation of lightweight materials, such as polymers, into novel layered shielding concepts could improve HVI performance in space and military applications without compromising cost or weight. One key challenge is developing a fundamental understanding of the effects of molecular architecture on the macroscopic dynamic material response and damage formation. In this work, two common and affordable thermoplastics, namely ultra-high molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE), were assessed. Flat rectangular targets of fixed geometries were subjected to a series of normal HVIs with spherical 10 mm diameter aluminum 1050 projectiles at velocities in the range 2-6.5 km/s. The back face debris cloud velocity, mass loss, and perforation radius were found to be functions of impact velocity for both materials. Additional post-impact forensic analyses were performed to provide insight into the distinct failure modes of the two polyethylenes.