Fracture Initiation Sites Research Articles

Fracture behaviour and fracture toughness of M2 high speed steel

The fracture behaviour of M2 high speed steel was examined at both ambient and elevated temperatures using four-point bend specimens oriented both longitudinally and transversely to the inherent carbide banding. For the former orientation it was shown that the sites of fracture initiation frequently included the cross section of a carbide cluster as well as cavities. The fracture behaviour was found to be well described by the concepts of linear elastic fracture mechanics. In the transversely oriented specimens the fracture path basically followed the banding network and hence to a large extent the fracture surfaces were covered by areas of high carbide density. No clear sites of fracture initiation were found on these fracture surfaces.

Characterization of self-curing acrylic bone cements.

AbstractFour self‐curing acrylic bone cements were surveyed by infrared, solubility, viscometry, quantitative metallography, microscopy, and physical testing techniques: CMW, Palacos R, Sulfix‐6, and Surgical Simplex P. Results show that these bone cements were primarily composed of poly(methyl methacrylate) and that no cross‐linking was evident. Solubility analysis confirmed this latter observation, as the bone cements dissolved completely except for a small insoluble fraction, which was identified as the radiopaque filler. For each bone cement, the viscosity‐average molecular weights of both the powdered phase and the cured two‐phase product remained unchanged, varying overall from 1 to 5 × 105. Using standard quantitative metallography, porosity ranged from 1 to 8% and the dispersed powder phase decreased 11–46%. Microscopy revealed the nature of the porosity, radiopaque fillers, the powder size and shape, and the fracture morphology. From tensile and fracture toughness tests, five physical properties were determined at ambient conditions and at 37°C after conditioning in distilled water at 37°C for 10 months: the modulus of elasticity, the ultimate tensile strength, the elongation at break, the fracture energy, and the mean inherent flaw size. At ambient conditions, the ultimate tensile strength decreased 33–55% when compared with commercial unmodified poly(methyl methacrylate), Plexiglas G. While the fracture energy remained rather invariant, the mean inherent flaw size increased fivefold over the commercial acrylic tested. This marked increase in the mean inherent flaw size could lower the fatigue resistance of a material, since more and/or larger fracture initiation sites are available. When tested at 37°C after protracted conditioning, the deleterious trends observed at ambient temperature continued. To some degree, porosity, particle‐matrix interfaces, residual stresses, low molecular weight products, inorganic and/or other organic additions, and water contributed to the inherent flaw size at the expense of the working stress. Several modifications are suggested by which the importance of these factors might be minimized.

The fracture behaviour of ABS polymers

The fracture behaviour of three different grades of ABS has been investigated over a range of temperature and strain-rates. A two-stage fracture process has been observed consisting of a slow crack propagation region followed by catastrophic failure. The surface features in these two regions have been examined in detail using scanning electron microscopy. In all cases, the fracture initiation site was located at either a surface defect or at an internal inclusion much larger than the average rubber particle size.

Hydrogen transport by dislocations

A kinetic model is presented for the transport of hydrogen, as Cottrell atmospheres on dislocation, at a rate appreciably in excess of that for lattice diffusion. The particular destinations for the hydrogen which are modeled here are ductile fracture initiation sites such as inclusions and microvoids. The functional predictions of the mechanism are shown to be consistent with available experimental evidence on ductile fracture behavior in the presence of hydrogen.

Model of dislocation sweep-in of hydrogen during fatigue crack growth

Electroplating of aerospace parts generates hydrogen embrittlement, which is controlled by 200-h sustained-load tests performed on cylindrical notched samples. Those tests are costly and offer only a pass-or-fail assessment, with limited interpretability. In this article, we propose a new fractographic method to obtain a quantitative indication of the embrittlement severity of electroplated samples that failed the sustained-load test. On the fracture surfaces of failed samples, intergranular (IG) zones were observable at fracture initiation sites, surrounded by ductile zones, with intermediate regions containing mixed (ductile+IG) features. Within the framework of the recently proposed Hydrogen-Enhanced Plasticity-Mediated DEcohesion mechanism (HELP-mediated HEDE), we argue that there is a correlation between the intergranular appearance of the fracture surfaces and the subcritical nature of the crack propagation. Using this argument, we computed the stress intensity factor K for each crack and loading condition, with the input of intergranular zone sizes and finite element analysis. Minimum and maximum K values were identified and statistics of the extremes was used to calculate the minimum expected K values for various embrittlement conditions. It is proposed that this value is an estimate of the threshold for embrittlement, Kth. Values of Kth between 9 MPa m0.5 and 30 MPa m0.5 were measured for various plating conditions. Those estimations are more conservative than the thresholds reported in the literature, which were measured using more complex experimental procedures. Moreover, the fracture toughness estimated with the maximum K values was possibly affected by high initial hydrogen concentrations as per the HELP+HEDE embrittlement mechanism.