Net-section based approach for fatigue crack growth characterization using compact tension specimen: Physical correlation of mean stress or stress ratio effects

Abstract

A new approach is demonstrated here for the correlation of fatigue crack growth (FCG) data, generated using compact tension (CT) specimens, for various mean stress levels or stress ratios (R) of testing. The approach uses the change in the cyclic strain energy of the net-section as a parameter to correlate fatigue crack growth rates. The derivation of the change in net-section strain energy, for the CT specimen, is demonstrated here on the basis of energy principles of solid mechanics. The total change in net-section strain energy is determined as the sum of the changes in the tension and the bending strain energy components, arising from the tensile stress and the bending moment acting on the net-section of the CT specimen. This leads to a compact physical equation for the change in cyclic strain energy, which is expressed as a direct function of applied cyclic load level, crack length, stress ratio, and specimen dimensions. Extensive experimental evaluations, using the data for steels, copper and titanium alloys, were conducted and are presented here to show that the change in net-section strain energy can successfully correlate the fatigue crack growth for various stress ratios or mean stress levels. The present work provides a new direction for physically-based characterizations of fatigue crack growth data generated using CT specimens.