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"Microstructural and Mechanical Property Characterization of Ultra-High Molecular Weight Polyethylene and its Composites Subjected to Equal Channel Angular Pressing"

Abstract

Total joint devices that can survive more than 20 years in vivo require a bearing surface that can combine toughness, wear resistance, and chemical stability. Ultrahigh molecular weight polyethylene (UHMWPE) remains the gold standard for these bearings, having been utilized as a liner and bearing surface for over five decades. Well-documented material trade-offs lead to failure modes that suggest that hip liners and knee bearing surfaces without optimal processing may be prone to premature failure in the patient. Because the demographic of those receiving implants is changing and smart materials and devices are seeing increased use in medicine, implants may need to be more functional to allow optimization for the patient of the future.

The present work explores material trade-offs by attempting to alter the microstructure of UHMWPE through the use of a severe plastic deformation technique known as equal channel angular pressing (ECAP). Work was performed to confirm the validity and reliability of dynamic mechanical analysis (DMA) as a rapid means to indicate and assess changes to the microstructure of UHMWPE in the form of entanglement density and chain interactions.

ECAP was applied to both neat and conductive composites of UHMWPE in order to understand the microstructural and mechanical changes due to shearing. Two grades of UHMWPE were used to understand the impact of MW, a variety of temperatures were employed to better optimize shear strain, and consolidation and extrusion hydrostatic pressures were altered to determine if chain mobility and consolidation were hindered under large compressive forces. Preliminary modeling of the different thermal gradients that ECAP and compression molded (CM) controls are exposed to was carried out to aid in and inform future experiments. While ECAP did not reveal changes to the microstructure of neat materials as compared to compression molded (CM) controls, work-to-failure was decreased which is hypothesized to be due to residual stresses. Carbon composites, on the other hand, were shown to have increased work-to-failure after shearing which was coincident with a decline in conductivity as compared to CM controls.

Thesis Committee

• Douglas Van Citters, PhD (Chair)
• Ian Baker, PhD
• Erland Schulson, PhD
• Bharat Patel, PhD (External, DePuy Synthes Joint Reconstruction)

Contact

For more information, contact Theresa Fuller at theresa.d.fuller@dartmouth.edu.