The fatigue fracture of mounting bracket: A microstructure characterization

We report the fatigue fracture in a mounting bracket, which is attached to a front bumper. We conducted the macro- and micro-scale fracture morphology analyses and base materials characterization. We used a bracket made of QSTE 420 TM high-strength steel and did not find any metallurgical defects. Three cracks were formed on the bracket, and the cracks were initiated from the outer edges of the screws. From the scanning electron microscopy (SEM) images, we observed ratchet marks among the three cracks at their initiation positions with a similar distance between each other. We also observed fatigue striation in the propagation zones. We deduce from the macro- and micro-scale analyses that the three cracks were initiated and then expanded spontaneously with a similar growth rate. The stress concentration, assembly operation and joining effects cause the bidirectional bending fatigue fracture in the three locations. Optimized tightening forces and new gaskets are recommended to extend the fatigue life of the mounting bracket.