value of the stress-intensity factor at the tip of a crack at which unstable propagation of the crack occurs
NOTE 1 This value is also called the fracture toughness. The parameter KIC is the fracture toughness for plane strain and is an inherent property of the material. For stress conditions other than plane strain, the fracture toughness is denoted KC. In fracture mechanics analyses, failure is assumed to be imminent when the applied stress-intensity factor is equal to or exceeds its critical value, i.e. the fracture toughness. See 3.2.25
NOTE 2 The term fracture toughness is used as a synonymous.