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A ferritic-pearlitic nodular iron automobile suspension knuckle was fatigue tested in the laboratory using a constant amplitude load level that simulated a severe service condition. It was found that cracks always initiated from surface casting defects and that the fatigue life could be extended significantly by machining away the as-cast surface in the fatigue sensitive locations. Both local strain and fracture mechanics approaches were used successfully to predict the fatigue life of the component.

Fatigue life predictions for notched components are typically based on constant amplitude fully reversed strain-life data derived from smooth specimens. A mean stress parameter is applied to account for the effects of mean stress in the crack initiation location. Fatigue life predictions using this approach are usually accurate for constant amplitude fatigue but are almost always unconservative for variable amplitude service loading. A considerable amount of work has now related the unconservative predictions to reductions in crack closure during the large cycles in the variable amplitude load history which result in lower crack opening stresses for the small cycles than those in the constant amplitude reference tests used in design. This increases the effective strain range and the damage done by the small cycles and results in shorter than predicted fatigue lives.