Impact resistance and fractography in ultra high molecular weight polyethylenes

The Texas A&M University Hypervelocity Impact Laboratory: A modern aeroballistic range facility

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.

A novel hip joint prosthesis with uni-directional articulations for reduced wear

A novel polymer-on-metal hip joint prosthesis design that makes use of uni-directional articulations was developed and tested in this work. The new implant was tested using two polymer variants, virgin ultra-high molecular weight polyethylene (UHMWPE), and Vitamin E-infused highly crosslinked polyethylene (VEHXPE). The degrees of freedom of the ball-and-socket are reproduced by three cylindrical orthogonally-aligned articulations. This unconventional design leverages on the molecular orientation hardening mechanisms of the polyethylene and increased contact area to minimize wear. An experimental hip joint simulator was used to compare the gravimetric wear of the conventional ball-on-socket and the new implant. The new prosthesis including UHMWPE components produced a 78% reduction in wear, whereas the new prosthesis with VEHXPE components produced a 100% reduction in wear, as no measurable wear was detected. Machining marks on the acetabular cups of the new prosthesis were retained for both polyethylene variants, further demonstrating the low levels of wear exhibited by the new implants. Both polyethylene materials produced particles in the range of 0.1-1.0 μm, which are the most biologically active. Nonetheless, the extremely low wear rates are likely to induce minimal osteolysis effects. Furthermore, the novel design also offers an increase of more than 24% in the range of motion in flexion/extension when compared to a dual-mobility hip implant. A prototype of the prosthesis was implanted into a Thiel-embalmed human cadaver during a mock-surgery, which demonstrated high resistance to dislocation and the possibility of performing a figure of four position.

Structures and impact strength variation of chemically crosslinked high-density polyethylene: effect of crosslinking density

Impact strength of high-density polyethylene (HDPE), especially at low temperature, is crucial for its applications outdoors because of its poor impact strength. In order to improve the impact strength of HDPE, crosslinked HDPE was prepared by the addition of a peroxide crosslink agent, bis(tert-butyldioxyisopropyl)benzenehexane, and the effect of the crosslinking density on the microstructures and mechanical properties, especially impact strength between −60 °C and 23 °C, were investigated. The results show that the crosslinking density is controlled by varying the content of the crosslinking agent. It is found that, at room temperature, with increase in the content of crosslink agent from 0% to 0.5–0.7%, the impact strength increases from 4 kJ m⁻² to about 80 kJ m⁻² and the elongation at break increases from 20% to about 550%. With further increase in the content of crosslink agent to 1.5%, the impact strength and the elongation at break reduce to 64 kJ m⁻² and 360% respectively. With increase in crosslink agent, the flexural modulus, yield strength, crystallinity, mean lamellar thickness, crystal size and spherulitic size and the brittle–ductile transition temperature (BDTT) decrease, and the gel content, impact strength of the HDPE at low temperature, intensity of β transition increase significantly. In considering both the room temperature mechanical properties and low temperature impact strength, the optimized content of the crosslink agent is about 0.7%. Overall, crosslinking significantly improves the toughness and impact strength of HDPE and extends its application, especially at low temperature.

Crosslinking significantly improves the toughness and impact strength of HDPE and extends its application, especially at low temperature.