Evaluation of overload effects on fatigue crack growth and closure

Abstract

Fatigue crack propagation tests with single tensile peak overloads have been performed in 6082-T6 aluminium alloy at several baseline DK levels and stress ratios of 0.05 and 0.25. The tests were carried out at constant DK conditions. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. The observed transient post-overload behaviour is discussed in terms of overload ratio, baseline DK level and stress ratio. The crack closure parameter U was obtained and compared with the crack growth transients. Experimental support is given for the hypothesis that plasticity-induced closure is the main cause of overload retardation for plane stress conditions. Predictions based on crack closure measurements show good correlation with the observed crack growth rates for all the post-overload transients when discontinuous closure is properly taken into account.

Fatigue crack growth in thin aluminium alloy sheets under loading sequences with periodic overloads

“…As reported for single overloads in the analysed alloy [6,7] the observed crack growth trends under periodic overloads are consistent with the plasticity-induced crack closure phenomenon. Each overload produces higher monotonic plastic zone than the baseline loading.…”

“…6(a) (DKZ7.5 MPa m 1/2 , da/dNZ1.4!10 K5 mm/cycle) corresponds to the stable crack growth rate stage following the first overload. This figure shows a marking line after each overload cycle similar to that observed following single peak overloads [7]. As expected, the spacing between these markings increases with crack length because the crack growth rate increases with DK.…”

“…In recent work, the authors observed that the plasticity-induced crack closure phenomenon could generally explain the crack growth behaviour following single peak overloads under both load control mode [6] and constant DK conditions [7]. The present work intends to analyse the fatigue crack growth due to several loading sequences containing periodic single overloads and to clarify if the observed behaviour can also be correlated with the crack closure phenomenon.…”

“…The majority of the work carried out in this field has been on the effects of single peak tensile overloads [3][4][5][6][7] simply because this type of loading can lead to significant load interaction effects. In contrast, other variable amplitude loading sequences have not yet been exhaustively investigated.…”

Effects of Shot-Peening and Stress Ratio on the Fatigue Crack Propagation of AL 7475-T7351 Specimens

“…Overloads can lead to significant interaction effects on crack propagation, as have been reported in many [11][12][13][14][15][16][17][18][19][20][21][22]. Several mechanisms have been proposed to explain crack growth retardation; including models based on residual stresses, crack closure, crack tip blunting, strain hardening, crack branching and reversed yielding.…”