Studies on Fracture Initiation (The first report)

Abstract

At present it is a controversial problem how to formulate the mechanical condition for the onset of brittle crack propagation especially when it is accompanied by “large scale yielding”. A proper engineering and macroscopic fracture criterion is essential for any effective fracture-safe design.
Crack opening displacement (COD) concept seems to be promising as one of the possible fracture criteria for yielding materials. But satisfactory experimental checks on its validity and the limitation of its applicability have not been available.
As the first step in a series of investigations intended to establish a comprehensive engineering fracture criterion, the authors carried out an experimental study to clarify fundamental characteristics of critical COD value as the fracture-controlling material parameter. Several kinds of fracture tests were performed, using “double notched specimen” and microscope technique for the determination of critical COD values.
It was found that critical COD values were affected by various parameters such as temperature, specimen size, stress system, notch depth and plastic constraint. Some of these parameters were related to critical COD in terms of the effect of strain rate at the notch root.