Which combination increases the likelihood of brittle fracture?

Which combination increases the likelihood of brittle fracture?

• A. Small grain size and thin materials
• B. Rapid heating and cooling
• C. Large grain size and thicker materials
• D. Low temperature service

Answer: (C)

Brittle fracture becomes more likely when the material has a coarse microstructure and is highly constrained by its geometry. Large grain size means fewer grain boundaries to interrupt and deflect a advancing crack, so cracks can propagate more easily along the grains—the material is less tough and less able to undergo plastic deformation before fracturing. When the section is thicker, there’s greater constraint at the crack tip and less ability for local plastic flow to blunt or arrest the crack, which raises the stress intensity that drives rapid, brittle crack growth. Put together, large grains in a thick section create conditions where fracture can occur with little warning, especially under stresses or temperatures that favor brittleness. Small grain size and thin sections, by contrast, provide more grain boundary barriers and less constraint, enhancing toughness. Rapid heating and cooling can cause thermal stresses, but they don’t inherently combine the microstructural and geometric factors that make brittle fracture more likely. Low-temperature service is a known risk for brittleness, but without the coarse grains and high constraint from thickness, the likelihood is not as high as the combination of large grain size with a thick cross-section.

Brittle fracture becomes more likely when the material has a coarse microstructure and is highly constrained by its geometry. Large grain size means fewer grain boundaries to interrupt and deflect a advancing crack, so cracks can propagate more easily along the grains—the material is less tough and less able to undergo plastic deformation before fracturing. When the section is thicker, there’s greater constraint at the crack tip and less ability for local plastic flow to blunt or arrest the crack, which raises the stress intensity that drives rapid, brittle crack growth. Put together, large grains in a thick section create conditions where fracture can occur with little warning, especially under stresses or temperatures that favor brittleness.

Small grain size and thin sections, by contrast, provide more grain boundary barriers and less constraint, enhancing toughness. Rapid heating and cooling can cause thermal stresses, but they don’t inherently combine the microstructural and geometric factors that make brittle fracture more likely. Low-temperature service is a known risk for brittleness, but without the coarse grains and high constraint from thickness, the likelihood is not as high as the combination of large grain size with a thick cross-section.