© 1992 Kluwer Academic Publishers. Printed in the Netherlands. LOAD-CMOD DATA ANALYSIS FOR CRACK CLOSURE

A major eason for the failure of crack growth rate versus applied stress intensity factor ange correlation under smoothly varying small scale yielding loadings at a given (negative) stress ratio is dissimilarities in the crack closure stress field. It is essential to assess the relevance and accuracy of crack closure measurement before coming to a conclusion that crack closure phenomenon is unable to explain the trends in growth rate in a given experiment [ 1]. While establishing the presence/absence of crack closure in a fatigue crack growth experiment is relatively straightforward, nogeneral method has yet evolved to measure the effect of crack closure on the crack driving force. Load-crack mouth opening displacement (CMOD) records during fatigue cycling are routinely used for estimating crack closure. A fundamental issue is the relevance of the load level at which crack closure begins (or crack is completely open) in estimating the correct crack driving force. Elber [2] hypothesizes that crack extension does not occur while the crack is closed and considers the stress range over which the crack is fully open as the effective range for crack growth. Stress ratio effec