Evaluation of J-initiation fracture toughness of ultra-high-molecular-weight polyethylene used in total joint replacements

Abstract

Fracture of ultra high molecular weight polyethylene (UHMWPE) total joint replacement components is a clinical concern. Thus, it is important to characterize the fracture resistance of UHMWPE. To determine J-initiation fracture toughness (J Q ) for metals and metallic alloys, ASTM E1820 recommends a procedure based on an empirical crack blunting line. This approach has been found to overestimate the initiation toughness of tough polymers like UHMWPE. Therefore, in this study, a novel experimental approach based on crack tip opening displacement (CTOD) was utilized to evaluate J Q of UHMWPE materials. J-initiation fracture toughness was experimentally measured in ambient air and a physiologically-relevant 37°C PBS environment for three different formulations of UHMWPE and compared to the blunting line approach. The CTOD method was found to provide J Q values comparable to the blunting line approach for the UHMWPE materials and environments examined in this study. The CTOD method used in this study is based on experimental observation and, thus, does not rely on an empirical relationship or fracture surface measurements. Therefore, determining J Q using the experimentally based CTOD method proposed in this study may be a more reliable approach for UHMWPE and other tough polymers than the blunting line approach.

Crack Propagation Resistance Is Similar Under Static and Cyclic Loading in Crosslinked UHMWPE: A Pilot Study

“…Thus, while our findings in this pilot study are encouraging, a future study with increased statistical power is warranted to more rigorously evaluate the efficacy of static loading crack propagation protocols. Second, the loss of remaining ligament was used to infer crack growth rate, as tracking the crack tip leads to substantial error, particularly early in a test where the crack tip typically moves in a retrograde manner toward the load line [16]. However, there is also some reduction in the ligament due to gross elastic and inelastic deformation of the specimen.…”

“…The crack propagation direction was oriented along the ram extrusion axis for the GUR1050 specimens. Specimens were also side-grooved with the same cutter used for the notch, with a depth of 1 mm, and included an angle of 60°to inhibit crack tunneling effects by counteracting the drop in stress intensity factor that occurs near the free surface of the specimen [2,16]. The specimen notch was left with the machined radius (ie, not razor sharpened before testing).…”

“…Ligament reduction was used instead of a direct crack growth metric due to gross specimen distortion that can occur during long-duration creep deformation experiments and the ambiguous behavior of the crack tip in the early stages of growth [16].…”

“…Limitations inherent in the use of fatigue crack propagation data include the use of the linear elastic fracture mechanics stress intensity factor K to describe crack tip conditions in a highly nonlinear, viscous solid. While nonlinear fracture mechanics have been used to describe crack growth and fracture conditions in UHMWPE [9,13,16], these do not typically include viscous dynamics, which likely are a strong factor in the behavior of the material in the fracture process zone. Recent work suggests crack propagation in conventional UHMWPE correlates to the maximum of the cyclic waveform (as opposed to the load excursion) [7] and demonstrates frequency independence and predictable waveform effects [11].…”

Static fracture resistance of ultra high molecular weight polyethylene using the single specimen normalization method

“…From the instantaneous values of J and a, the J-R curve can be obtained. For a detailed description of normalization procedure the reader is referred to Varadarajan [20].…”