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

Comparison of dynamic mechanical properties of dentin between deciduous and permanent teeth

Purpose: Even though differences between deciduous and permanent dentin have been widely studied, their dynamic mechanical behavior has never been compared. The objective of the present study was to quantify the differences between deciduous and permanent dentin under cyclic mechanical loading, which is similar to masticatory stress.Materials and Methods: Deciduous and permanent teeth, respectively from children (9 ~ 12 years old) and young people (18 ~ 25 years old), were wet-sectioned perpendicular to the longitudinal axis and the central specimens of coronal dentin were evaluated by nanoscopic dynamic mechanical analysis (nanoDMA).Results: The average storage, loss, and complex moduli, as well as the hardness of deciduous dentin were significantly (p < 0.05) lower than those of permanent dentin. Moreover, the tan δ value of permanent dentin was significantly (p < 0.05) lower than that of deciduous dentin across the loading frequency range, indicating that viscoelastic behavior and loss of elastic energy were significantly reduced in the stiffer permanent dentin. All the nanoDMA responses showed a significant influence of the dynamic loading frequency (p < 0.05): Both deciduous and permanent dentin showed reduced viscoelasticty with increased loading frequencies.Conclusions: Compared with deciduous dentin, permanent dentin exhibits higher stiffness with reduced energy loss during deformation, and therefore superior mechanical characteristics for the mastication process.

Structure, processing and performance of ultra-high molecular weight polyethylene (IUPAC Technical Report). Part 4: sporadic fatigue crack propagation

Fatigue tests were carried out on compression mouldings supplied by a leading polymer manufacturer. They were made from three batches of ultra-high molecular weight polyethylene (UHMWPE) with weight-average relative molar masses, M ¯ W ${\overline{M}}_{\mathrm{W}}$ , of about 0.6 × 106, 5 × 106 and 9 × 106. In 10 mm thick compact tension specimens, crack propagation was so erratic that it was impossible to follow standard procedure, where crack-tip stress intensity amplitude, ΔK, is raised incrementally, and the resulting crack propagation rate, da/dN, increases, following the Paris equation, where a is crack length and N is number of cycles. Instead, most of the tests were conducted at fixed high values of ΔK. Typically, da/dN then started at a high level, but decreased irregularly during the test. Micrographs of fracture surfaces showed that crack propagation was sporadic in these specimens. In one test, at ΔK = 2.3 MPa m0.5, there were crack-arrest marks at intervals Δa of about 2 μm, while the number of cycles between individual growth steps increased from 1 to more than 1000 and the fracture surface showed increasing evidence of plastic deformation. It is concluded that sporadic crack propagation was caused by energy-dissipating crazing, which was initiated close to the crack tip under plane strain conditions in mouldings that were not fully consolidated. By contrast, fatigue crack propagation in 4 mm thick specimens followed the Paris equation approximately. The results from all four reports on this project are reviewed, and the possibility of using fatigue testing as a quality assurance procedure for melt-processed UHMWPE is discussed.

Biaxial fatigue crack propagation behavior of ultrahigh molecular weight polyethylene reinforced by carbon nanofibers and hydroxyapatite

Ultrahigh molecular weight polyethylene (UHMWPE) artificial joint has remained the preferred polymer component in total joint replacement surgery. However, more and more concerns have been raised about the failure of UHMWPE components due to the initiation and propagation of cracks at the notches with fixed functions. For this reason, biaxial fatigue crack growth (FCG) experiments of UHMWPE reinforced by carbon nanofibers (CNF) and hydroxyapatite (HA) were carried out using elastic-plastic fracture mechanics theory. The FCG resistance of UHMWPE, UHMWPE/CNF, and UHMWPE/HA was compared, and the effects of stress ratio (R) value and phase difference on FCG rate were investigated. At the same time, the influence of loading path was considered, and the corresponding crack path was analyzed. Results suggest that UHMWPE/CNF has better FCG resistance and the FCG rate increases with the increase of R value and the existence of 180° phase difference. In addition, crack bifurcation behavior is not observed under nonproportional loading conditions. The findings in this study will provide experimental validation and data support for better clinical application of UHMWPE-modified materials.