EFFECTS OF STRAIN GRADIENTS ON THE GROSS STRAIN CRACK TOLERANCE OF A 533-B STEEL.

Abstract

To satisfy the need for a measure of fracture toughness at stress levels above yield, a quantity called gross strain crack tolerance has been proposed. It is the total elastic-plus-plastic strain measured at maximum load in a laboratory test of a specimen containing a surface crack and is, of course, a function of crack size and temperature. The object has been to develop a test to produce results that can be used to estimate the tolerance of material in full-scale hardware for strain in the presence of a crack. Previous work utilized precracked tension specimens of A533-B steel to study the effects of specimen geometry and crack size as a function of temperature. Now, the effects of strain gradients have been studied by loading the specimen in bending or by placing the crack in the bottom of a groove. Analysis of the displacement measurements showed that proper characterization of all the strain gradients tested required consideration of the strain values at two locations. The first, defined as the gross strain crack tolerance (GSCT), was located in the net section normal to the crack plane at mid-crack depth, the approximate location along the crack border where fracture originated. The second strain value was the gross strain on the back face in the gross section above and below the crack. At temperatures within the transition region, GSCT increased in direct proportion to the degree of tensile strain on the back face of the gross section. Tests in bending, which put the neutral axis between the crack tip and the back face, produced the lowest value of GSCT, while those in which the crack propagated into regions of increasing strain yielded the highest GSCT. These effects were attributed to the influence of back face strain on the development of the plastic zone and consequent relief of constraint.