Narrow Results

A systematic investigation of the ac susceptibility of polycrystalline RuSr₂GdCu₂O₈ was carried out with different field amplitudes and various frequencies. The study covered a wide range of frequencies between 0.020 to 12.5 kHz and temperatures from 7 to 50 K. The first derivative of susceptibility curves indicated the presence of a superconducting intragranular transition coupled by weak links distribution of intergranular transitions. The intragranular-, χ_g, and intergranular-, χ_i contribution to susceptibility were estimated and found to be consistent with the prediction of the critical state model. Variation of the peak temperature of the imaginary part with frequency was employed to determine the vortex activation energy. The derived low activation energy values provide clear evidence of the thermally activated flux creep process in the superconducting state. Qualitative analysis of the χ′′ - χ′ parametric relation was carried out.

A sub-grain size finite element modelling approach is presented in this paper to investigate variations in fracture mechanics parameters for irregular crack paths. The results can be used when modelling intergranular and transgranular crack growth where creep and fatigue are the dominant failure mechanisms and their crack paths are irregular. A novel method for sub-grain scale finite element mesh consisting of multiple elements encased in ~50–150 μm-sized grains has been developed and implemented in a compact tension, C(T), mesh structure. The replicated shapes and dimensions were derived from an isotropic metallic grain structure using representative random sized grain shapes repeated in sequence ahead of the crack tip. In this way the effects of crack tip angle ahead of the main crack path can be considered in a more realistic manner. A comprehensive sensitivity analysis has been performed for elastic and elastic-plastic materials using ABAQUS and the stress distributions, the stress intensity factor and the J-integral have been evaluated for irregular crack paths and compared to those of obtained from analytical solutions. To examine the local and macroscopic graph path effects on fracture mechanics parameters, a few extreme cases with various crack-tip angles have been modelled by keeping the macroscopic crack path parallel to the axis of symmetry. The numerical solutions from these granular mesh structures have been found in relatively good agreement with analytical solutions.

A dual phase titanium alloy of the Ti-6Al-4V type was tested at a slow strain rate under uniaxial tension in ambient environment. The fracture was found to be largely transgranular, but the fracture initiation was observed to occur from grain boundary microcracks. A fracture model has been proposed to explain the interesting phenomenon. According to the model, the fracture begins with microcrack initiation at α grain boundaries (especially the triple points) or α/β interfaces. As the applied load increases, sliding occurs in those grain boundaries whose orientation coincides with that of the maximum shear stress. The titanium alloy has a very low work hardening rate; therefore, above a certain stress level, strain localisation occurs to form concentrated shear band at roughly 45 degrees to the loading axis, leading to the final transgranular fracture by shear.