This book perfects the theoretical system of fracture mechanics of nonhomogeneous materials through the establishment of the piecewise exponential model and expands the fracture research scope to nonhomogeneous materials containing complex interfaces through proposing the domain-independent interaction integral concept. The piecewise exponential model has overcome the problem of fracture mechanics of nonhomogeneous materials and clarified the doubt of traditional exponential models in recent 30 years. The domain-independent interaction integral method is not affected by material nonhomogeneity and discontinuity, which greatly facilitates its numerical implementation in the investigation of fracture behaviors of nonhomogeneous materials with complex interfaces.