Characteristics of Torsional Fatigue in Shafts: Part II - Fracture Orientation Under Combined or Multi-Axial Loading Conditions

Failure analysts use the terms “flat” and “slant” to describe fracture surface orientation as a function of loading, but we can be more precise. High cycle fatigue fractures are brittle on the macroscale and during Stage II propagate on the plane of maximum normal (tensile) stress, which is perpendicular (90°) to the axis of a shaft under pure axial and/or bending loads and at a 45° angle under pure torsional loading. Under combined axial and/or bending loads plus torsional loading, fracture would occur at a different angle (neither 90° nor 45°). Combined or multi-axial loading conditions can also have cyclic and noncyclic components. A misaligned rotating shaft would experience a combination of cyclic bending and noncyclic torsional loading conditions. This presentation will examine and predict the fracture surface orientation of a misaligned rotating shaft using the Modified Goodman Diagram as a guide or criterion for identifying the fracture plane on which the combined cyclic normal stress and mean normal stress are critical and fatigue cracks are most likely to propagate. This approach will be compared to the Findley Critical Plane Criterion used to predict the plane where fatigue cracks are most likely to initiate.