Symposium 6: Microstructures and Fracture in Advanced Materials

The objective of this work is to model fatigue damage of the aluminium alloy AA2139 at the microscopic scale. It combines an experimental campaign and numerical simulations for a complete modelling of the alloy. Special attention is given to the reproduction of the alloy grain morphology and crystallography. Moreover, intermetallic phases are preferred sites for fatigue crack initiation in this alloy. Therefore, a method for taking into account the alloy microstructural complexity including the presence of intermetallic phases is presented. Finally, a fatigue damage model using a fatigue indicator parameter (FIP) is considered for the introduction of a crack and its propagation in simulations.
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The temperature dependence of the mode I fracture behavior of a rolled Mg AZ31 alloy having near basal texture is studied in this work through four-point bend fracture experiments in the temperature range from 25 to 100 deg Celsius. It is found that the operative fracture mechanism changes from twin-induced quasi-brittle cracking to one mediated by ductile void growth and coalescence as temperature is raised above 65 deg Celsius. A concomitant reduction in tensile twin development near the crack-tip is observed with enhancement in temperature, while at the specimen far-edge it increases, resulting in pronounced texture changes at higher temperature. The reduction in tensile twin evolution with energy release rate and enhancement in micro-void growth rate near the crack-tip over the above temperature range are rationalized through simplified analyses. The change in fracture mechanism from brittle to ductile and higher dissipation due to tensile twinning at the specimen far-edge as temperature increases results in significant enhancement in fracture toughness.
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Crack closure effects play an important role in dominating small crack propagation behaviors, which were rather less well investigated, especially at high temperature in the air. Based on photomicroscopy and digital image correlation, small crack growth behaviors and growth rates are investigated both at 600℃ and RT in air for a Powder Metallurgy superalloy, then the crack displacement fields are measured. Two max. stress levels and two stress ratios are considered in order to understand their effects on small crack growth behaviors. The experimental results reveal the crack growth behaviors ranged from 80 m to ~1000m. With the help of EBSD at the grains of the crack growth path, links of this particular growth behaviors with the microstructure features, such as the orientation, grain boundary, are discussed. Using DIC-measured crack opening displacement with the crack growth, the roles of oxides and roughness induced crack closure in early small crack propagation at high temperature are analyzed. Finally, a crack closure model is proposed including the combined effects of oxide-induced, roughness-induced and plasticity-induced crack closure (OICC, RICC and PICC).
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High nitrogen chromium alloyed PM tool steels display, among other attractive properties, high strength and improved corrosion resistance. Also, powder metallurgy is a manufacturing process that allows the fabrication of near net shape complex geometries and high-quality components in an economical way. Nitrogen acts in an effective way replacing the carbon by the formation of hard carbonitride phases and permits higher amounts of chromium in solid solution. The low carbon and high nitrogen contents, where most of the carbon is replaced by nitrogen, suppresses the formation of metal carbides is in favor of the formation of metal nitrides (MX) and carbonitrides (M2X) allowing a higher nominal amount of chromium in solid solution. As a result of the PM rout well dispersed and fine distribution of nitrides and carbonitrides is created in a martensitic matrix. The microstructure obtained controls the mechanical properties of the steel grade, and a balance of high strength and high toughness is required. In the present study the influence of the nitride and carbonitride distributions on the fatigue properties, and in particular the stress intensity threshold and the early crack growth, was investigated using a 20 kHz ultrasound test equipment.
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We present fracture mechanics-based approaches to characterize interfacial fracture parameters for the tensile and shear behavior of a typical ice/aluminum interface. An experimental framework employing single cantilever beam, direct shear, and push-out shear tests were developed to achieve near mode-I and near mode-II fracture conditions at the interface. Both analytical (beam bending and shear-lag analysis), and numerical (finite element analysis incorporating cohesive zone method) models were used to extract mode-I and II interfacial fracture parameters. The combined experimental and numerical results, as well as surveying published results for the direct shear and push-out shear tests, showed that mode-II interfacial strength and toughness could be significantly affected by the test method due to geometrically induced interfacial residual stress. As a result, the apparent toughness of the zero-angle push-out test could reach an order of magnitude higher than those derived from direct shear tests. Moreover, it was found that the interfacial ice adhesion is fracture mode insensitive and roughness insensitive for tensile and shear modes, for the observed modes of failures in this study.
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