The novel concept of generalized fracture toughness characterization of brittle materials subjected to multiaxial loadings is presented. The theory emphasizes the fracture process as the result of the opening action of the crack surfaces. The generalized fracture toughness values describing the failure events due to combined loading systems lie on a Fracture Envelope characteristic for a given material. The Cartesian equation of the Envelope in the KIK2 plane is specified by the conventional fracture toughness K~c and Poisson's ratio u. A Griffith-type fracture criterion permits the prediction of crack propagation onset and crack growth direction.
Nomenclature
r "fro curve of family F, corresponding to the value 0 of the argument angular coefficients of quadratic form Q(O) Fracture Envelope family of curves in k~, k2 plane symmetric stress intensity factor, ksi (in.) x/2 skew-symmetric stress intensity factor, ksi (in.) ~/2 fracture toughness, ksi (in.) I/2 critical skew-symmetric stress intensity factor, ksi (in.) ~12 critical set of stress intensity factors leading to fracture positive quadratic form in k~, k2 critical value of Q(O) corresponding to crack propagation onset radial distance of volume element from crack tip crack-tip region where stress field (7) is valid elastic potential (per unit volume) argument of volume element within R or parameter of family F crack propagation angle from original crack direction shear modulus Poisson's ratio tangential stresses, ksi radial stresses, ksi shear stresses, ksi