Viscoplastic crack initiation and propagation in crosslinked UHMWPE from clinically relevant notches up to 0.5 mm radius

Abstract

Highly crosslinked UHMWPE is now the material of choice for hard-on-soft bearing couples in total joint replacements. However, the fracture resistance of the polymer remains a design concern for increased longevity of the components in vivo. Fracture research utilizing the traditional linear elastic fracture mechanics (LEFM) or elastic plastic fracture mechanics (EPFM) approach has not yielded a definite failure criterion for UHMWPE. Therefore, an advanced viscous fracture model has been applied to various notched compact tension specimen geometries to estimate the fracture resistance of the polymer. Two generic crosslinked UHMWPE formulations (remelted 65kGy and remelted 100kGy) were analyzed in this study using notched test specimens with three different notch radii under static loading conditions. The results suggest that the viscous fracture model can be applied to crosslinked UHMWPE and a single value of critical energy governs crack initiation and propagation in the material. To our knowledge, this is one of the first studies to implement a mechanistic approach to study crack initiation and propagation in UHMWPE for a range of clinically relevant stress-concentration geometries. It is believed that a combination of structural analysis of components and material parameter quantification is a path to effective failure prediction in UHMWPE total joint replacement components, though additional testing is needed to verify the rigor of this approach.

Crack initiation from a clinically relevant notch in a highly-crosslinked UHMWPE subjected to static and cyclic loading

“…In a previous study by the authors, a power law relation was obtained between crack initiation time and applied static load for crosslinked UHMWPE [35]. A similar relationship was postulated in this study between cyclic crack initiation time (obtained through video observation of the notch surface) and test frequency (eq.…”

“…The findings for crosslinked UHMWPE [27], as well as findings for non-crosslinked UHMWPE [33] support that static creep loading is a dominant mechanism for crack propagation in crosslinked UHMWPE. As a result of this finding, previous studies by the authors [34,35] focused on the role of static creep loading on determining crack initiation time and crack propagation velocity from a notch in crosslinked UHMWPE. It was found that the Williams' viscous fracture model [36] was able to predict both crack initiation and propagation velocity from a notch in two crosslinked UHMWPE formulations.…”

“…Both the cyclic and the static tests were conducted under mode-I loading conditions. The maximum load of P max = 800 N was selected based on the results from a previous study [35] where crack initiation from notches of variable radii under static loading conditions were evaluated, utilizing loads varying from 800N to 1000N. In that study, finite element analysis was conducted to assess the initial J-Integral condition, J 0 , at the notch as well as the critical condition, J C , for crack initiation.…”

“…For the fatigue tests, a fan was directed towards the specimen to help mitigate any potential temperature rise at the notch during load cycling. For all tests (fatigue and creep), the notch surface was observed using a video microscope as in a previous study by the authors [35](Figure 1). The notch surface was also examined during testing to observe multilayer vs single-layer crack initiation as reported in a previous study by the authors [34].…”

Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) as a Promising Polymer Material for Biomedical Applications: A Concise Review

“…Crosslinking of UHMWPE significantly improves wear performance [37][38][39][40], which can be achieved through the use of a silane [41,42], or chemical methods using peroxides [38,43] and irradiation [44][45][46]. The free radicals produced by these methods create the inter-chain covalent bonds, leading to the formation of crosslinking.…”