Showing papers by "Brian Cotterell published in 1994"

Fatigue of sintered steel

Abstract: A cloud of microcracks opens plastically around the tip of a long fatigue crack in sintered steel as it propagates. Because of the dilatation caused by the plastic opening of the microcracks, a negative stress intensity factor is induced at the crack tip which reduces the maximum crack tip stress intensity factor. At low values of the stress ratio the minimum crack tip stress intensity factor can be less than zero causing the crack to close and thus reducing the effective crack tip stress intensity factor range and retarding crack growth. Short cracks growing from pores do not have a systematic shield of microcracks and so propagate faster than long cracks at low stress ratios.

Microstructure and Growth Mechanism of Al2O3/Al Ceramic Matrix Composite Made by Directed Oxidation of Al Alloys

Abstract: Since the Lanxide process was advanced for forming of Al2O3 ceramic composite by directed oxidation of Al alloys, much work has been done with various mechanisms being proposed. The mechanisms have claimed that only certain dopants are essential to the growth process. Nevertheless, no united consensus has yet been reached. In the present work, Al alloy containing 5% Mg was oxidised in air for 12 hours at 1150°C with or without surface dopants of MgO or Pd. The resultant composites showed very different microstructures. Without any surface doping, the alloy did not develop any portion of composite as the initial intimate oxide film stops further oxidation. This intimate oxide film can either be broken off by mechanical means or penetrated by reaction with surface dopants, so that the composite can grow and develop. The results show that the previously reported incubation time is not only related to reaction processes but also to the initial mechanical disturbances. Doping with Pd made the composite darker in colour as the grains of the alumina ceramic matrix and inclusions of Al metal are finer. This shows that Pd may make the top oxide layer less intimate, and more nucleation sites are therefore available for oxidation. A new model is presented for oxide sustained growth based on the existence of oxygen active top surface layer and the capillary flow of molten metal around ceramic phase.