Volume 18, Issue 6 (November 1990)
Characterization of the Effects of Large Unloading Cycles on the Ductile Tearing Toughness of HSLA Steel
This paper describes part of an ongoing project to study the effects of interactions between intense fatigue cycling and ductile tearing on the fatigue crack growth rate demonstrated by structural steels. Earlier work has utilized a cyclic J integral approach to characterize the cyclic crack growth rate and a relatively standard J-resistance curve approach to characterize the material ductile tearing toughness. This work has shown that a relatively simple superposition can be used to predict specimen life when a combination of tensile fatigue cycling and ductile tearing is applied. The results are far from perfect but seem to offer a scheme which is conservative and hence much more desirable than an LEFM approach which invariably gives nonconservative life estimates.
This new work looks at the effect of complete unloadings and of complete crack opening displacement reversals on the subsequent ductile tearing toughness and fatigue crack growth resistance. Initial data show that the effect of a complete unloading cycle can be predicted using the baseline cycle J crack growth rate data of the test material. A complete COD reversal, however, produces a much greater crack growth step than would be predicted using an extrapolation of the standard cyclic J method. To account for this effect a cyclic J resistance curve is developed and compared with the measured data. Fractographic SEM work is also used to support the unloading compliance measurements and the use of a ductile tearing procedure to predict the crack growth during these intense compression cycles.