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Why can very strong metals fracture suddenly near a crack or notch?

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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.

Very strong metals can fracture suddenly near a crack or notch because these features act as sites of significant stress concentration. When a metal is engineered for high strength, it often sacrifices ductility—the capacity to deform permanently and accommodate local stress concentrations.123

In the presence of a notch or crack, the local stress at the tip of the defect is amplified far beyond the nominal stress levels of the surrounding material. Because high-strength metals lack the ability to yield plastically at these points, they cannot redistribute the load effectively. Instead, the energy stored in the material's elastic field is released as the crack propagates, leading to sudden, brittle failure.412

In engineering, this behavior is quantified by fracture toughness, which measures the energy required to extend an existing crack. Generally, increasing a metal's yield strength reduces its fracture toughness, making the material more susceptible to abrupt failure when defects are present.1