Gross strain crack tolerance of steels

Abstract

It is the intent of most structural designs that general yielding should always precede fracture, even in the presence of flaws. The objective of the gross strain approach is to make direct measurements of flaw tolerance, in terms of gross strain to maximum load, as a function of flaw size and temperature. Gross strain is defined as the strain at the crack location, normal to the crack plane, that would exist if the crack were not there. Gross strain is measured as the axial strain in a tensile specimen containing a fatigue sharpened part-through surface flaw. Measurements of critical gross strain and critical crack opening displacement have been made for A533-B pressure vessel steel. These values undergo an abrupt increase with temperature, at the temperature at which gross section yielding and fracture coincide. The transition temperature increases with flaw size, as expected. Specimen thickness and width criteria have been developed for ensuring adequate constraint at the crack tip. The gross strain approach shows definite promise of becoming a measure of toughness that can be used in a quantitative procedure for design against brittle fracture when the object is to insure yielding before fracture in the presence of the largest expected flaw.