Fracture toughness is a property of materials that measures their ability to resist crack propagation when subjected to stress. It quantifies the material's resistance to fracture in the presence of pre-existing flaws such as cracks or voids. Fracture toughness is expressed as a critical stress intensity factor (K_c), which combines the effects of the applied stress, the size of the crack, and the material's properties.
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Fracture toughness measures resistance to crack advance under specified loading and environmental conditions. In linear elasticity, is the critical stress intensity factor. For opening-mode loading, a valid plane strain measurement is denoted , with units , commonly . It differs from a tensile yield strength: a crack changes the near-tip stress concentration.
For a fixed geometry with , the threshold givesFor example, , and give in this idealized geometry. The crack-tip plastic zone must remain small for the elastic intensity description to apply; extensive plasticity in continuum mechanics requires a different characterization.[ref]