Fracture toughness (K_c) is a mechanical property that measures a material’s resistance to fracture.  This parameter characterizes the intensity of stress field in the material local to the crack tip when rapid crack extension takes place.  Similar to other microstructurally sensitive material properties, fracture toughness can vary as a function of temperature and strain rate.  But, unlike the yield strength, K_c will be strongly dependent on the amount of crack tip constraint due to component thickness.  The reason why thickness has to be considered in fracture analysis is due to its influence on the pattern of crack tip plastic deformation.  The two thickness limiting crack tip plastic deformation patterns are shown in Figure 2.2.4.  For “thin” plane stress type components, a 45 degree through the thickness yielding pattern develops; in “thicker” plane strain components of the same material, the hinge-type plastic deformation pattern predominates [Hahn, & Rosenfield, 1965].  Section 4 and 7 discuss the effect of thickness and other factors on fracture toughness.

Figure 2.2.4.  Yield Zone Observed on the Surface and Cross Section of a Cracked Sheet Under Uni-axial Tensile Loading in: A-Plane Stress, 45 degree Shear Type; B-Plane Strain, Hinge Type

The linear elastic fracture mechanics approach can only be expected to characterize fracture when the region in which plastic deformation occurs is contained within the elastic crack tip stress field.  When the crack tip plastic deformation is unrestricted by elastic material around the crack, the engineer must resort to using elasto-plastic techniques to predict the critical crack size at fracture.  Presently, it is not possible to say if these techniques will lead to the same type of single parameter characterization of fracture discussed above.