Mechanism of factory-roof formation

Factory-roof (F-R) patterns produced by cyclic torsion loading of V-notched cylindrical steel specimens were investigated by means of experimental and theoretical methods. A three-dimensional model of F-R patterns was constructed by means of stereophotogrammetry in the scanning electron microscope and the basic geometrical rules of F-R formation were identified. The theoretical analysis revealed that the F-R initiates by elementary mode I branches of semi-elliptical surface cracks growing under mixed-mode II + III. The exact positions of such branches were analytically determined in terms of the maximum synergy of mode II and mode III loadings. An increasing density of the semi-elliptical surface cracks results in the refinement and the size reduction of F-R patterns. This effect, along with strong wear damage, explains the fact that the F-R patterns are usually not observed in the low-cycle fatigue region.