THE CORRELATION OF INTERFACIAL AND MACROSCOPIC TOUGHNESS IN SIC LAMINATES

Specimens have been manufactured by viscous processing of SiC powder into sheets, which were then laminated with thin carbon interlayers and sintered without pressure to give a > 98% dense product. These have been tested both in four-point bend delamination' of single interlayer sandwich specimens, to give interfacial critical strain energy release rate, G(ic), data, and in three-point bending of multi-layer laminates. The value of G(ic) was found to be approximately 7.5 J m-2, which compares with about 28 J m-2 for G(bc) of the bulk SiC itself produced by this route. During three-point bending of the laminates, crack deflection was consistently observed at the interfaces between laminae. This led to energy dissipation during the process, such that the apparent G(c) value of the laminate was around 6 kJ m-2. A numerical model was set up to describe the crack advance sequence. This is based on through-thickness cracks propagating when a critical stress is reached (using a Monte Carlo method to determine the strength of successive laminae, for a given Weibull modulus) and interfacial cracks then advancing a distance dictated by the available energy. This model allowed accurate prediction of both load/displacement curves and the absorption of energy within the specimen (by interfacial cracking). A distinction is drawn between the area under the load/displacement curve and the energy absorbed in the specimen. These differ if the energy available to drive the interfacial cracks is more than sufficient for them to reach the ends of the specimen. It s shown that there is scope for maximization of the energy absorbed in the specimen (i.e., for optimization of the toughness of the laminate when loaded in bend ng) by control over the G(ic) value and the thickness of individual laminae.