Investigation of a mounting bolt failure in an automobile air brake assembly

The premature fatigue failure of a mounting bolt in an air brake assembly was traced to non-adherence to the established design. The heat input from a projection weld coupled with a sharp neck radius of curvature, and the proximity of the projection weld to the bolt neck combined with a higher-than-anticipated carbon content to produce acicular martensite in a region of high stress concentration. This combination of conditions increased the susceptibility to fatigue and caused the failure.