Crack opening and closure in fatigue

Experimental and analytical crack opening and closure levels are determined for cracks growing in notched members. Replica techniques and the scanning electron microscope were used in the experiments. The analytical procedure involves the determination of the reversed plastic zone size, the crack opening and closure stresses and is based on Budiansky-Hutchinson's theoretical study of closure. Results indicate consistently that crack opening levels are higher than crack closure levels. Small cracks in bluntly notched members under R = − 1 loading exhibited lower crack opening (closure) levels than longer cracks. However, for R = 0 loading the crack opening (closure) levels remained relatively unchanged with increasing crack length. The implications of crack opening and closure results on the behavior of small cracks are discussed. An ad hoc approach for determining crack closure levels is presented in the Appendix. The suggested method gives good estimates of crack closure levels in the absence of plasticity and short crack effects and may be useful to engineers and designers.