Andrew J. Moffat

Synchrotron radiation computed laminography for polymer composite failure studies

Failures of laterally extended polymer composite panels are imaged using 3D computed laminography. The experimental parameters and capability of the method are studied.

The mechanisms of long fatigue crack growth behaviour in Al–Si casting alloys at room and elevated temperature

Abstract Pistons are commonly made from multicomponent Al–Si casting alloys, which have complex, interconnecting three-dimensional (3D) networks of secondary phase particles. They are non-serviceable parts and so must be able to withstand high cycle fatigue while operating at temperatures between 30 and 80% of T m. Long fatigue crack growth tests were performed at room temperature (RT) and 350°C to assess the micromechanisms of fatigue. The fracture profiles at low and high da/dN were analysed; at low crack growth rates at both temperatures there is no crack path preferentiality with respect to the microstructure. At high da/dN in the RT sample the crack growth occurs preferentially via hard particles, while at 350°C there is a change in mechanism and the crack appears to avoid hard particles. X-ray tomography has been used to image the crack tips and gain a detailed insight into the mechanisms of fatigue in these complex 3D microstructures.

Crack initiation processes in acrylic bone cement

A major constraint in improving the understanding of the micromechanics of the fatigue failure process and, hence, in optimizing bone cement performance is found in the uncertainties associated with monitoring the evolution of the internal defects that are believed to dominate in vivo failure. The present study aimed to synthesize high resolution imaging with complementary damage monitoring/detection techniques. As a result, evidence of the chronology of failure has been obtained. The earliest stages of crack initiation have been captured and it is proposed that, in the presence of a pore, crack initiation may occur away from the pore due to the combined influence of pore morphology and the presence of defects within regions of stress concentration. Furthermore, experimental evidence shows that large agglomerations of BaSO(4) are subject to microcracking during fatigue, although in the majority of cases, these are not the primary cause of failure. It is proposed that cracks may then remain contained within the agglomerations because of the clamping effect of the matrix during volumetric shrinkage upon curing.

Microstructural Analysis of Fatigue Initiation in Al-Si Casting Alloys

Fatigue initiation behaviour in three multi-component Al-Si casting alloys with varying Si content is compared using a range of microscopy and analytical techniques. A higher proportion of stiffer secondary phases leads to load transfer effects reducing particle cracking and particle/matrix debonding. Si appears stronger than the Al9FeNi phase, which cracks and debonds to form initiation sites preferentially over Si. Reducing Si content results in clusters of intermetallics forming, and increased porosity. The effect of porosity, combined with mesoscopic load transfer effects to the high volume fraction intermetallic regions make these potent crack initiation sites in low silicon alloys.

Microstructural factors affecting fatigue initiation in various Al based bearing alloys

Microscale fatigue damage mechanisms in various Al-Sn-Si based bearing alloys used as linings of plain automotive bearings are reported. Extensive work on previously developed alloys has concluded that secondary phase particles such as Sn and Si are potential fatigue initiation sites with a complex combination of various particle geometry parameters. A newly developed alloy contains a number of complex widely scattered intermetallics with much finer and fewer Sn and Si particles. This alloy system appears to be more resistant to initiate microscale fatigue damage compared to the previous systems.