Influence of a damage zone on high temperature crack growth in brittle materials

High temperature crack growth in ceramics often occurs by the nucleation, growth and coalescence of cavities in a region ahead of the crack tip known as the damage zone. Models describing this type of behaviour generally assume that the presence of cavities does not affect the stress distribution ahead of the crack tip. In this study, a crack growth simulation has been developed which incorporates the effects of cavity nucleation, growth and coalescence on the stress field ahead of the crack. Cavity growth dominated both by grain boundary diffusion and by surface diffusion has been modelled. The models follow both the transient effects which occur following initial loading and the development of a steady-state regime under conditions of constant applied stress intensity factor KI. In general, the wedging action due to cavity growth reduces the stress field near the crack tip. However, this is largely compensated for by an increase in the damage zone size, as a result of load transfer from the crack tip to the end of the damage zone. We have therefore demonstrated that the established analytical models which do not account for stress redistribution give a much better description of steady-state crack growth behavior than one would expect.