The fundamental question posed in this study is, What crystallographic conditions are satisfied at fatigue crack nucleation sites in polycrystalline iron? To answer this question it is vital to establish an effective technique to determine the crystallographic orientation of each grain under observation. This problem was solved by developing an etch pit method of grain orientation analysis. Stereographic projection geometry was successfully applied in determining the orientation of each grain with great accuracy from the measured angles between ridge lines of sharp-edged etch pit patterns on the film. The critical experiment, then, is to examine the geometrical relationship between the nucleated fine cracks and the orientations of surrounding grains on fatigued specimen surfaces. The final step is to extract such crystallographic conditions as are commonly satisfied at the sites of crack nucleation.
Electrolytically polished commercial base pure iron plate specimens were tested under completely reversed plane bending stress in the high-cycle fatigue range. Tests were interrupted at about half of the fracture life, and the crystallographic configuration of microcracks with respect to the grain orientation was thoroughly examined by making use of the newly developed technique. Cracks were observed to nucleate only along grain boundaries on the specimen surface. And it was found that there are five conditions satisfied at those cracked sites. These conditions concern the favorable configuration of slip systems, Schmid factor, grain size, and the direction of grain boundaries. Three more conditions were added from information pertaining to the crystallographic structure below the surface gained by observation after removal of the surface layer and of cross sections containing a crack. A brief discussion is also given on some statistical aspects of important parameters.