Fatigue Crack Growth Threshold of Ni Based Castalloys Using Ultrasonic Fatigue Tests

The purpose of this study is to evaluate fatigue crack growth (FCG) characteristics and its threshold of Ni based castalloys, 246 and 100. Results of FCG tests by 4-points bending show that the FCG path is highly deflected when the FCG rate is low, while it is macroscopically straight when FCG rate is high. The former and the latter behaviors are denoted mixed mode and mode I crack growths, respectively. The effective stress intensity factor range, ΔKeff, in mode I obtained by displacement field around a crack tip measured by digital image correlation (DIC) is dominant on FCG rate even under the mixed mode crack growth. To determine the threshold value and to evaluate FCG rate under extremely slow region, maximum stress intensity factor, Kmax, -constant test using ultrasonic fatigue testing machine is performed. As the result, fracture surface roughness and FCG rates are correlated and the role of crystal orientation on the fracture surface formation is clarified. The threshold condition for FCG is constant independently of the crystal orientation near crack tips and threshold stress intensity factor range, ΔKth, is about 1 MPa√m.