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Why do cracklike defects cause fracture in glass and other nonplastic materials?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

In materials science, cracklike defects cause fracture in glass and other nonplastic (brittle) materials primarily due to their stress-concentrating effect.1

When a material is subjected to tensile stress, these microscopic flaws—such as surface scratches or notches—act as points where the local stress at the crack tip can be 100 to 1,000 times greater than the applied nominal stress. In brittle materials, which lack the ability to undergo significant plastic deformation to redistribute or "blunt" this concentrated stress, the energy released from the elastic field at the crack tip quickly exceeds the energy required to create new surfaces.123

Consequently, once the stress at the tip of a flaw reaches a critical threshold, the crack propagates rapidly, leading to catastrophic failure. While ductile materials can often deform plastically to mitigate these concentrations, nonplastic materials remain susceptible to this mechanism, making them highly sensitive to even minor surface defects.143