Strengthening crack deflection

In the platelet reinforced glass composite referred to above, the thermal expansion coefficient of the inclusions (alumina, a,- = 8 10 /°C) is higher than that of the borosilicate matrix (a = 3.3 10 /°C), resulting in a hoop compression stress in the matrix around the platelets [128], Such a stress field will deflect the crack and cause it to travel around the platelet. These residual compressive stresses in the matrix will also contribute to the strengthening of the material (see Figure 6) since they will reduce the effect of any externally applied tensile stresses. 

We saw in section 7.2.1 that a crack can be deflected by adding particles that the crack cannot penetrate. One example for a ceramic strengthened this way is sintered silicon nitride (SSN, Si3N4) which contains elongated, rod-shaped crystallites with a large aspect ratio. Due to the sintering process, they are... 

---