Investigation of an abnormality in fatigue crack growth curves-the Marci Effect

Abstract

The threshold stress intensity factor range of materials, {Delta}K{sub th}, is important for the design of engineering structures. The intrinsic component of this parameter can be effectively measured using the K{sub max}-constant test method, where the maximum stress intensity factor, K{sub max}, is kept constant, while the minimum stress intensity factor, K{sub min}, is gradually increased until the crack growth rate reaches the threshold for fatigue crack propagation. In the scientific literature, a perplexing phenomenon has been reported in which several high temperature titanium alloys were found not to exhibit an intrinsic threshold value in the K{sub max}-constant test-a phenomenon that the authors refer to as the Marci Effect. This effect occurs if the K{sub max} value is sufficiently high and the fatigue amplitude, {Delta}K, becomes sufficiently small. Under these conditions abnormal crack growth behavior occurs, wherein cracks propagate due to cyclic amplitudes below {Delta}K{sub T} and, in some instances, the crack growth rate even increases while {Delta}K decreases. So far, this effect has been shown for Ti-6Al-6V-2Sn, IMI 834 and IMI 685, but the reasons for the phenomenon are yet unknown. The issue could be important for practical applications, because in case this effect can occur where structural componentsmore » are exposed to high stress levels combined with vibratory loading, the ultimate utility of an intrinsic threshold value as a design criterion is in question. To study the features of the Marci Effect, two different alloys were examined: the nickel-base super alloy IN100 and the titanium alloy Ti-6Al-2Sn-4Zr-6Mo (Ti-6246). The alloy IN100 was investigated to determine whether a typical high strength nickel-base superalloy would also exhibit the abnormality.« less