Microstress cracking

As well as producing these broadly beneficial effects, thermal microstresses can also degrade the strength of composites. The tensile components of stress can help in crack initiation. In a composite with a uniform distribution of particles, the tensile components act only over distances comparable with... 

The lamina macrostresses o , involved in Eq. (6), are obtained by averaging the microstresses, Eqs. (II), across the length of the representative segment as explicit functions of the relative crack density O "" and relative delamination area Djf... 

Microstresses arise inside the material as a result of the unequal thermal expansion coefficients of the glass and of the crystals. This phenomenon is characteristic of polyphase ceramics. In porcelains, the stresses arise in particular at the boundary between large quartz crystals and glass. Their formation is also contributed to by the modification inversion of quartz. The stresses sometimes bring about formation of microscopically visible cracks inside the quartz grains or around them. The structure of porcelain thus contains weak points which enhance the development of fracture on loading. 

The differences in thermal expansion coefficients of the individual phases and also their anisotropies result in non-uniform shrinkage on cooling. Tf this non-uniform shrinkage cannot be met by deformation, stresses arise restricted to short distances (microstresse.s). This phenomenon is characteristic for ceramics and influences their mechanical properties. High tensile stresses may even result in the formation of ciacks visible under the microscope, for example iji fireclay or porcelain. These cracks arc usually situated at phase boundaries. 

In view of the particle cracking observed in most composite specimens, we focus attention on the microstress the PSZ phase, and suppose that fracture occurs when a latent... 

---