What does fracture toughness measure?

• A. Hardness
• B. Elastic Modulus
• C. Ability to resist crack propagation under load
• D. Ductility

Answer: (C)

Fracture toughness measures how well a material resists crack propagation when a crack is already present and a load is applied. It captures the material’s ability to stop or slow the growth of a crack under service stresses, often summarized by concepts like the critical stress intensity factor (K_IC) or the energy required to advance a crack. In bridges, higher fracture toughness means cracks are less likely to run under actual loads, temperatures, and environmental conditions, which is vital for protecting fracture-critical members.

This property is distinct from hardness, which gauges resistance to surface indentation; from elastic modulus, which describes stiffness and elastic deformation under stress; and from ductility, which describes how much plastic deformation the material can undergo before failure. Fracture toughness specifically focuses on resisting crack growth under load, given existing flaws and service conditions.