George Irwin, the father of fracture mechanics (FM), is well known for his contributions to linear elastic fracture mechanics, especially the development of the crack-tip field approach and the use of the crack tip stress intensity factor, K. During the early years of FM he was well aware that the limits on the use of K restricted its application to high-strength and low-toughness materials. He encouraged the development of new concepts that would be able to extend the use of FM to the more usual engineering materials, materials that might reach yield stress before fracture toughness on a typical structural component. Concern with that area of FM later led to the development of the technology called elastic-plastic fracture mechanics (EPFM). Besides the encouragement he gave, there are several areas where he contributed technical ideas that helped with the development of the EPFM technology. These ideas include use of crack tip field parameters, the energy and crack tip parameter equivalence, the R-curve approach to fracture toughness evaluation and the concern about constraint effects on fracture behavior.
This paper takes a historical look at these developments in elastic-plastic fracture mechanics and the role that George Irwin played. An important observation that will be made is that the approach originally proposed by Irwin gave the correct direction in each case considered in this paper. The original ideas merely needed further development to become mature technical concepts in the EPFM approach.