Quantitative procedures for the assessment of the fracture resistance of medium and low strength steels are currently being developed, an ~ttracrive approach [1-3] being the measurement of the critical ยข~rack opening displacement CCOD) for fracture. A simple method is to use a double r notched specimen, with fatigue cracks at each notch, loaded symme~rically so that the unfTactured notch is in a state immediately prior ~o fracture initiation; the opening of the unfractured crack tip is then measured. Such a measurement is made when the specimen is unloaded whereas, in applying results to practical situations, on-load values are required.
This problem highlights the desirability of determining the difference r between on-and off-load measurements of plastic deformation at the root of a crack, and accordingly a theoretical investigation is being undertaken, this report's purpose being tO indicate the progress to date. The model consists of a body of finite width containing a sharp crack which is normal to one surface, and loaded so that the deformation mode is anti-plane strai~. Plasticity at the crack tip is confined to an infinitesimally thin-zone coplanar with the crack, the shear stress within the zone being representatiye of the material's yield stress. The ratio of the difference between the onand off-load opening displacements at the cr~ck tip to the on-load displacement has been determined as a function of the length of plastic zone, by extending the approach [4,5] employed for the model in which the body has an infinite w~dth. The calculations show that the ratio is 0.5 for short plastic zones, decreasing to 0.4 at general yield for a situation where the crack depth to specimen width ratio is 1/3. A~cordingly there may be an appreciable difference between the on-and off-load measurements, unless fracture occurs well beyond general yield. For this situation, if it is assumed that the shear yield strength and specimen width have representative values of respectively 15,000 psi and 0.5 in, the zecoverable displacement is less than 0.003 in, irrespective of the crack depth, a value which is in accord with experimental values [6].
In conclusion, therefore, the analysis shows that off-load measurements of COD are adequate when fracture occurs well after general yield ~ in the test specimen, and that the recoverable displacement may be disregarded for CON values greater than about 0.@20 in; in such situations, of course, a material is so ductile that the critical crack length will be very large. However, the analysis supports the viewpoint that caution must be exercised for ~ measurements obtained with materials having an intermediate ductility such that fracture precedes general yield, but where the plas~tic zone is not sa short that conventional fracture mechanics procedures may be used. It is precisely this region where crack opening displacement procedures have a major role to play in the assessment of the fracture characteristics of materials.
This report is published by permission of the Uniled Kingdom Atomic Energy Authority.