Causes of mixed fatigue-tensilecrack grovvth and significance of microscopic crack behaviour

Abstract

Alternating fatigue- and tension-crack growth is often observed in both laboratory and service failures. This phenomenon is described for some aluminium-alloy plate fatigue specimens where it has been found that for conditions where σ _max remains substantially constant each tensionjump occurs at a constant value of √a/1, where a is maximum depth of crack and 1 is crack-front length. The √a/1 value for the ‘pop in’ crack front also remains constant, but at a lower value. Clearly plate thickness, σ _max , and K _IC will affect the value of √a/1, and the general equation can be written:

√a/1=K_IC/Cl_oσ_max√π

where l_o+plate thickness=shortest possible crack front, and C is the appropriate factor, excluding any correction for crack shape. The fatigue period causes a reduction of curvature of the ‘pop in’ crack front, thereby shortening it until the critical jump value for √a/1 is reached. Straightening of the crack front also occurs on a microscopic scale, starting from the fragmented condition of the tensile crack and progressing to an increasingly more coherent and continuous state. The use of the concept of a critical √a/1 in practical failures analysis is described in the paper, and examples are given of the effects of crack-front length.