The relevance of crack closure to fatigue crack propagation

Abstract

Using the d.c. and a.c. resistance techniques it was established that for predominantly plane strain conditions, fatigue crack closure in various steels occurred only when the stress intensity conditions were reduced to below the minimum stress intensity level ( K M I N) for the previous fatigue cycles. Following zero to tension loading, compressive loads were required to cause closure whereas following tension to tension loading, closure could occur at positive stresses but below K M I N for the previous cycles. These conclusions apply equally to single edge notched, centre cracked and CKS specimens. Edge opening displacement measurements and metallographic examination confirmed that closure does not occur under plane strain conditions. Metallography, however, showed that closure does occur above the K M I N values of the previous fatigue cycles in the regions of plane stress in both thin and thick specimens. At large values of alternating stress intensity in thin specimens where the plane stress regions were extensive, closure was detected by the resistance technique at positive stresses following zero to tension cycling. These observations suggest that much of the published data on crack closure were relevant only to the plane stress regions which were examined. The present observations also explain the observed curvature of fatigue crack propagation fronts. The resistance technique showed that the fatigue crack retardation or arrest (for plane strain conditions) caused by reducing the cyclic loads was not due to crack closure. Furthermore, crack closure concepts cannot explain the increase in fatigue crack growth rate with increase in mean stress observed at low alternating stress intensities in steels.