Papers by Author: Robert J. Moon

Abstract: The wear behaviour of Ca a-sialon ceramics of two distinct microstructures, fine equiaxed grains (EQ) and large elongated grains (EL), with the same chemical composition was investigated as a function of apparent contact pressure and sliding speed, using ball-on-disc type tribometers at room temperature and at 600°C. For room temperature tests, the EL microstructure exhibited a lower wear rate than EQ in the severe wear regime due to a greater resistance to large crack-induced material removal. As the apparent contact pressure decreased, mild wear appeared for both microstructures. The mechanism that dominated the material removal in EQ was grain pullout. In contrast, the controlling mechanism for EL was transgranular fracture. Therefore, EL had a lower wear rate than EQ in the mild wear regime. For wear tests at 600°C, crack-induced severe wear occurs in both EQ and EL samples for all contact pressures. EL had a slightly lower wear rate than EQ. Wear particles were generated on the wear track, but no tribofilm was observed and no oxidation products were detected. Wear models revealed that the grain aspect ratio plays a more important role than grain diameter in influencing the crack propagation during severe wear and grain pull-out during mild wear.

Abstract: The propagation of cracks in graded materials under monotonic and cyclic loading was investigated via experiment and simulation. Graded alumina/epoxy composite specimens exhibiting a variation in composition from 5% to 65% epoxy, representing a twenty-fold variation in Young’s modulus, across a region of width between 6 and 20 mm, were produced by a multistep infiltration technique. Crack initiation and propagation under monotonic and cyclic four-point bend loading was monitored and crack trajectories and growth rates were measured. Initial crack deflection was observed, in agreement with theoretical and computational predictions in the literature. Cracks exhibited further deviation as they traversed the graded region. Higher deflection angles were observed for specimens with steeper gradients, and for those with cracks initially located closer to the compliant side of the gradient. Homogeneous specimens in the composition range 5% to 55% epoxy were also produced to investigate the composition dependence of mechanical, fracture and fatigue properties for aluminaepoxy composites. Crack propagation resistance appeared to differ between monotonic and cyclic loading, though an increase with crack extension was observed in both cases. The significant variation in measured crack-propagation resistance, for cracks in graded specimens, was accordingly interpreted as a combination of crack-extension effect and spatial variation of both intrinsic and extrinsic crack-growth resistance. A finite element model has been developed to simulate the propagation process, with particular attention paid to crack propagation and deflection criteria. Results from homogeneous specimens were utilised for estimating spatial property distribution and crack-extension effects in the graded specimens. Experimental results for crack path and crack-growth resistance profile show good agreement with modeling predictions.