Physical Basis of Fracture Mechanics

In this chapter, the classical theories of failure are summarized first, and their inadequacy in accounting for the failure (fracture) of bodies that contain crack(s) is highlighted. The basic development of fracture mechanics, following the concept first formulated by A. A. Griffith, is introduced. The concepts of strain energy release rate and stress intensity factor, and their identification as the driving force for crack growth are introduced. The experimental determinations of these factors are discussed. Fracture behavior of engineering materials is described, and the importance of fracture mechanics in the design and sustainment of engineered systems is considered.Classical Theories of FailureClassical theories of failure are based on concepts of maximum stress, strain, or strain energy and assume that the material is homogeneous and free from defects. Stresses, strains, and strain energies are typically obtained through elastic analyses.Maximum Principal Stress (or Tresca) CriterionThe maximum principal stress criterion for failure simply states that failure (by yielding or by fracture) would occur when the maximum principal stress reaches a critical value (i.e., the material's yield strength, σYS, or fracture strength, σf, or tensile strength, σUTS).