A post yield fracture mechanics analysis of three-point bend specimens and its implications to fracture toughness testing

Abstract

A Post Yield Fracture Mechanics analysis of three point bend specimens of a low strength, high toughness alloy steel, was performed on two series of tests. In the first the specimen dimensions were fixed and the crack length varied. In the second geometrically similar sub-standard specimens were tested. The smallest specimen tested was one-eighth valid size. The results were analysed using three post yield theories; the J-integral, the Bilby, Cottrell and Swinden (BCS) model and the Witt equivalent energy method. In the short crack length series the results were adequately described by the J-integral, the BCS model or by standard linear elastic fracture mechanics within the observed experimental scatter. In the geometrically similar series the Witt equivalent energy method consistently predicted toughness values within the scatter band of the valid tests. The BCS and J-integral methods underestimated the toughness for the two smallest sizes tested. This phenomenon is explained in terms of the relaxation of the plane strain plastic constraints towards the unconstrained plane stress state. The results of other workers on bend specimens are reproduced and retreated.