Why can a brittle material fail suddenly near a notch or crack?

Brittle materials are particularly susceptible to sudden failure near notches or cracks due to the phenomenon of stress concentration.

When a material contains a geometric discontinuity—such as a notch, crack, or sharp reentrant corner—the local stress at that site can become significantly higher than the nominal stress applied to the structure. As the radius of curvature at the tip of a notch or crack decreases, the concentration of stress increases dramatically.1

In brittle solids, these sites serve as points from which a crack can nucleate. Once a crack begins to propagate, spontaneous failure can occur when the energy released from the elastic field surrounding the crack balances the work required to create new fracture surfaces within the material. Because brittle materials lack the ability to undergo significant plastic deformation to redistribute these high localized stresses, the crack can propagate rapidly and catastrophically. This behavior necessitates careful engineering design to account for potential stress concentration sites, as they represent critical points of structural vulnerability.12