The strain energy density criterion developed at the Institute of Fracture and Solid Mechanics of Lehigh University [1-3] has been extensively used during the last decade in the solution of a host of problems of engineering importance.
These problems include the determination of the critical conditions for failure by fracture and/or yielding of twoand three-dimensional bodies with or without initial defects and materials that may behave linearly (reversibly) or non-linearly (irreversibly) [4][5]. The processes of initiation, stable crack growth, and final catastrophic fracture under monotonic, fatigue, or sustained loads can be treated in a simple and unifying manner that makes the strain energy density criterion a versatile and powerful method in failure analysis.
More recently, the strain energy density criterion has been incorporated into a more general theory of material damage [6] such that nonlinear stress and failure analyses can be performed simultaneously. C the uniaxial true stress-true strain diagram.
This implies that two separate conditions are imposed on the same material element, one for the determination of its position, and another for the determination of its failure.