The Effects of Pressure on the Fracture of Composite Materials

ABSTRACTThe use of tensile tests with superimposed hydrostatic pressures enables a variety of combinations of equivalent stress and mean stress to be obtained. In conventional metallic materials the use of tests under superimposed hydrostatic pressure is of value in the study of processes such as ductile fracture which involve dilatent events such as void nucleation and growth. Quantitative models of ductile fracture and fracture mechanism maps have been developed based on the use of tests under hydrostatic pressure (D. Teirlinck, F. Zok, J.D. Embury, M.F. Ashby, Acta Metallurgica 36, 1213–1228 (1988)).We have recently extended the technique to metal matrix composites, Al alloys containing particulate Sic, Al alloys with SiC whiskers and directionally solidified Al-Ni alloys. It was observed that in all the materials the ductility increased with an increase in superimposed pressure.In the Al (2014) Sic particulate materials the increase in ductility was up to 20 times. At low pressures fracture was initiated by particle fracture but at high superimposed pressure particle matrix decohesion also occurred. In the materials containing whiskers or aligned rods of Al3Ni an alternative mode of fracture was observed at higher pressure. Strain localization occurred by a combination of dilatent events such as interface debonding and particle fracture and by strain softening due to fibre rotation. These processes result in shear failure at high pressures.The use of superimposed pressure tests provides valuable information in the area of plastic response, damage accumulation and control of fracture events in metal matrix composites. The paper presented both experimental and theoretical work on the role of hydrostatic stress. A published account of portions of the work will be available in the Proceedings of the Conference on Interfacial Phenomena in Composites held by Brown University in June 1988, and in the Proceedings of the 9th Riso Conference on Composites in September 1988.