ON THE MECHANISMS AND BEHAVIOR OF OVERLOAD RETARDATION IN AISI-304 STAINLESS-STEEL

The overload retardation phenomenon has been investigated in AISI 304 stainless steel under different baseline DELTAKs, overload ratios and R ratios. Both the crack growth and crack closure behaviour were carefully monitored. It was found that the extent and degree of retardation increase with overload ratio, decrease with R ratio, but are not correlated in a simple manner with the baseline DELTAK. For R less-than-or-equal-to 0.6, momentary acceleration followed by well-defined delayed retardation were observed. The crack growth behaviour before maximum retardation can be correctly predicted by Paris' law taking into account the experimentally measured crack closure value. Such a correlation failed beyond the maximum retardation point because of the occurrence of a discontinuous closure mechanism. Artificial introduction of crack closure by vacuum-infiltrating epoxy resin into the crack flanks also produced a similar retardation phenomenon. On the other hand, for R ratio greater-than-or-equal-to 0.65, immediate retardation was observed. Mechanisms that have been proposed to explain the overload retardation phenomenon are reviewed and examined against the current experimental observations. It is concluded that the major mechanism causing overload retardation is plasticity-induced crack closure. Secondary mechanisms, such as crack-tip blunting and residual compressive stress ahead of the crack tip, are significant only at high R ratios when crack closure is suppressed.