Damage evolution during fracture by correlative microscopy with hyperspectral electron microscopy and laboratory-based microtomography.

Damage evolution during fracture of metals is a critical factor in determining the reliability and integrity of the infrastructure that the society relies upon. However, experimental techniques for directly observing these phenomena have remained challenged. We have addressed this gap by developing a correlative microscopy framework combining high-resolution hyperspectral electron microscopy with laboratory x-ray microtomography (XMT) and applied it to study fracture mechanisms in a steel inclusion system. We observed damage nucleation and growth to be inhomogeneous and anisotropic. Fracture resistance was observed to be controlled by inclusion distribution and the size scale of an inclusion-depleted zone. Furthermore, our studies demonstrate that laboratory XMT can characterize damage to the micrometer scale with a large field of view in dense metals like steel, offering a more accessible alternative to synchrotron-based tomography. The framework presented provides a means to broadly adopt correlative microscopy for studies of degradation phenomena and help accelerate discovery of new materials solutions.

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

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.

Peak stress intensity factor governs crack propagation velocity in crosslinked ultrahigh-molecular-weight polyethylene.

Ultrahigh-molecular-weight polyethylene (UHMWPE) has been successfully used as a bearing material in total joint replacement components. However, these bearing materials can fail as a result of in vivo static and cyclic loads. Crack propagation behavior in this material has been considered using the Paris relationship which relates fatigue crack growth rate, da/dN (mm/cycle) versus the stress intensity factor range, ΔK (Kmax - Kmin , MPa√m). However, recent work suggests that the crack propagation velocity of conventional UHMWPE is driven by the peak stress intensity (Kmax ), not ΔK. The hypothesis of this study is that the crack propagation velocity of highly crosslinked and remelted UHMWPE is also driven by the peak stress intensity, Kmax , during cyclic loading. To test this hypothesis, two highly crosslinked (65 kGy and 100 kGy) and remelted UHMWPE materials were examined. Frequency, waveform, and R-ratio were varied between test conditions to determine the governing factor for fatigue crack propagation. It was found that the crack propagation velocity in crosslinked UHMWPE is also driven by Kmax and not ΔK, and is dependent on loading waveform and frequency in a predictable quasistatic manner. This study supports that crack growth in crosslinked UHMWPE materials, even under cyclic loading conditions, can be described by a relationship between the velocity of crack growth, da/dt and the peak stress intensity, Kmax . The findings suggest that stable crack propagation can occur as a result of static loading only and this should be taken into consideration in design of UHMWPE total joint replacement components.

Application of viscoelastic fracture model and non-uniform crack initiation at clinically relevant notches in crosslinked UHMWPE.

The mechanism of crack initiation from a clinically relevant notch is not well-understood for crosslinked ultra high molecular weight polyethylene (UHMWPE) used in total joint replacement components. Static mode driving forces, rather than the cyclic mode conditions typically associated with fatigue processes, have been shown to drive crack propagation in this material. Thus, in this study, crack initiation in a notched specimen under a static load was investigated. A video microscope was used to monitor the notch surface of the specimen and crack initiation time was measured from the video by identifying the onset of crack initiation at the notch. Crack initiation was considered using a viscoelastic fracture theory. It was found that the mechanism of crack initiation involved both single layer and a distributed multi-layer phenomenon and that multi-layer crack initiation delayed the crack initiation time for all loading conditions examined. The findings of this study support that the viscoelastic fracture theory governs fracture mechanics in crosslinked UHMWPE. The findings also support that crack initiation from a notch in UHMWPE is a more complex phenomenon than treated by traditional fracture theories for polymers.

Tradeoffs amongst fatigue, wear, and oxidation resistance of cross-linked ultra-high molecular weight polyethylene.

This study evaluated the tradeoffs amongst fatigue crack propagation resistance, wear resistance, and oxidative stability in a wide variety of clinically-relevant cross-linked ultra-high molecular weight polyethylene. Highly cross-linked re-melted materials showed good oxidation and wear performance, but diminished fatigue crack propagation resistance. Highly cross-linked annealed materials showed good wear and fatigue performance, but poor oxidation resistance. Moderately cross-linked re-melted materials showed good oxidation resistance, but moderate wear and fatigue resistance. Increasing radiation dose increased wear resistance but decreased fatigue crack propagation resistance. Annealing reduced fatigue resistance less than re-melting, but left materials susceptible to oxidation. This appears to occur because annealing below the melting temperature after cross-linking increased the volume fraction and size of lamellae, but failed to neutralize all free radicals. Alternately, re-melting after cross-linking appeared to eliminate free radicals, but, restricted by the network of cross-links, the re-formed lamellae were fewer and smaller in size which resulted in poor fatigue crack propagation resistance. This is the first study to simultaneously evaluate fatigue crack propagation, wear, oxidation, and microstructure in a wide variety of clinically-relevant ultra-high. The tradeoff we have shown in fatigue, wear, and oxidation performance is critical to the material's long-term success in total joint replacements.

Near-terminal creep damage does not substantially influence fatigue life under physiological loading.

Cortical bone specimens were damaged using repeated blocks of tensile creep loading until a near-terminal amount of creep damage was generated (corresponding to a reduction in elastic modulus of 15%). One group of cortical bone specimens was submitted to the near-terminal damage protocol and subsequently underwent fatigue loading in tension with a maximum strain of 2000 µÎµ (Damage Fatigue, n=5). A second group was submitted to cyclic fatigue loading but was not pre-damaged (Control Fatigue, n=5). All but one specimen (a damaged specimen) reached run-out (10 million cycles, 7.7 days). No significant differences in microscopic cracks or other tissue damage were observed between the two groups or between either group and additional, completely unloaded specimens. Our results suggest that damage in cortical bone allograft that is not obvious or associated with a stress riser may not substantially affect its fatigue life under physiologic loading.

Crack propagation resistance is similar under static and cyclic loading in crosslinked UHMWPE: a pilot study.

BACKGROUND: Recent work suggests crack phenomena (eg, crack initiation and propagation) in UHMWPE do not depend on cyclic damage mechanisms. Materials for which crack phenomena occur in static (noncyclic) mode should exhibit similar crack propagation behavior under static and cyclic loading conditions. QUESTIONS/PURPOSES: Do cracks in UHMWPE stably propagate from acute notches under static loading with a velocity dependent on crosslink density? Are material-ranking evaluations for crack propagation resistance similar under static and cyclic loading conditions? Does time to failure for a notched specimen under static loads yield the same material rankings as crack propagation data? METHODS: Notched compact tension specimens were machined from UHMWPE gamma-irradiated with a 0-, 50-, 75-, or 100-kGy dose and subsequently remelted. Static loads were applied until failure occurred or 2 weeks had elapsed. Crack propagation rates and time to failure were recorded and compared to data from cyclic experiments. RESULTS: Static and cyclic loading both produced stable crack propagation and similar performance rankings of material groups, except in the case of the unirradiated material, which did not fail under static loads. Normalized measures of crack propagation velocities generally showed quantitative agreement between the two methods. Normalized time to failure under static loading also agreed well with crack propagation velocity. CONCLUSIONS: Crack propagation under static loading produced qualitatively and quantitatively similar performance results as those under cyclic loading. Time to failure under static loads corresponded closely with the crack propagation velocity and may itself be a robust metric of crack propagation resistance. CLINICAL RELEVANCE: Total joint arthroplasties may experience superficial cracking in the UHMWPE bearing surface or catastrophic fracture. Quantifying resistance to crack phenomena in UHWMPE is important to design engineers and to clinicians using crosslinked UHMWPE materials under challenging mechanical conditions.

Crack initiation in retrieved cross-linked highly cross-linked ultrahigh-molecular-weight polyethylene acetabular liners: an investigation of 9 cases.

Nine cross-linked highly cross-linked ultrahigh-molecular weight polyethylene acetabular liners were retrieved at revision surgery. Eight of the liners were fully intact and functional at retrieval. Six cases contained shallow initiated cracks at the root of rim notches; 1 crack had propagated several millimeters. Optical and electron microscopic inspection of the crack surfaces revealed clam shell markings, which are characteristic of fatigue crack initiation. Crack initiation at notches has been identified in reports of catastrophic cross-linked liner failures, with crack initiation sites exhibiting similar morphology and clam shell markings. Thus, we believe that the shallow cracks identified in this case series are precursors to catastrophic rim fracture. The results of this study recommend further investigations to clarify the etiology and prevalence of crack initiation in cross-linked acetabular liners.

Clinical fracture of cross-linked UHMWPE acetabular liners.

Highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is increasingly used as a bearing material in total hip replacements. Cross-linking of UHMWPE has been shown to increase wear resistance but decrease its fracture resistance. We analyzed the clinical fracture failure of four cross-linked UHMWPE total hip replacement components of four different designs via microscopic observation of the fracture surfaces, and found that in all cases fractures initiated at stress concentrations in an unsupported region of the component (termed the elevated rim). Finite element analyses (FEA) of each individual implant design were then conducted. Results from this analysis demonstrated that the predicted magnitude and orientation of maximum principal stress due to mechanical loading of the elevated rim was sufficient to propagate initiated fatigue cracks in each case. FEA also predicted that cracks may arrest after some amount of growth due to a steep stress gradient near the initiation site. Further, while anatomical positioning of the implant and material properties affect the risk of fracture, we examined whether these failures are strongly related to the notched elevated rim design feature that is common to the four failed cases presented here. We believe that cross-linked UHMWPE remains an excellent bearing material for total hip replacements but that designs employing this material should mitigate stress concentrations or other design features that increase the risk of fracture.