Fatigue crack growth in MAR-M200 single crystals

The effects of crystallographic orientation on the fatigue crack growth behavior of MAR-M200* single crystals were examined. Using compact-tension specimens tested at 20 Hz, fatigue crack growth rates were determined at ambient temperature at minimum stress to maximum stress ratios,R, of 0.1 and 0.5. In most cases, subcritical crack growth occurred either along a single {111} slip plane or a combination of {111} planes. The mode of cracking was generally mixed and contained mode I, II, and III components. Considerable crack deflection and branching were also observed. Some fracture surfaces were found to contain a significant amount of asperities and, in some specimens, black debris. Based on Auger spectroscopic analyses and the fracture surface appearance, it appears that the black debris represented oxides formed due to rubbing of the fracture surfaces. Using stress intensity solutions obtained based on the Boundary-Integral-Equation technique, an effective ΔK was successfully used for correlating the crack growth rate data. The results indicate that the effect of crystallographic orientation on crack growth rate can be explained on the basis of crack deflection, branching, and roughness-induced crack closure. Formerly with Southwest Research Institute