where δu is the uniform elongation of the tensile specimen.
Eq. (1) had considered the intrinsic contribution (crack tip plasticity) to fracture toughness. However, the extrinsic contribution caused by crack path tortuosity was neglected. Based on the research by Jiang [29], the extrinsic contribution caused by crack path tortuosity can be considered by multiplying a magnification factor to Eq. (1), as shown in Eq. (3).