Optical Measurement of Residual Stress at the Deep-Rolled Crankshaft Fillet

Crankshaft fillet is subjected to a cyclic bending stress during operation. Fatigue cracks are observed at the fillet during the fatigue test. Compressive stresses are generated by deep-rolling process in order to increase the surface hardness and improve the fatigue strength. To examine the deep-rolling effect, the residual stresses at the fillet need to be investigated.

Incremental hole drilling and ISSR (interferometric strain/slope rosette) method is applied to measure the residual stresses at the bottom of the fillet. Incremental hole drilling process is to gradually remove material and mill a hole on the specimen surface in order to relax stress. The ISSR is composed of three micro-indentations, which are indented near the hole and would generate interferometric fringe patterns upon incident laser beam. With incremental drilling, stress relaxation causes the relieved strains, which in turn cause the shifts of interferometric patterns. The relieved strains can be calculated from the recorded shifts of fringe patterns. To determine residual stresses, the strain-stress response coefficients need to be calculated using the finite element method.

Because the fillet is an annular groove with small semicircle cross-section, the miniature ISSR shows its advantages over the common resistance strain rosette. It is challenging to precisely model the fillet and simulate the drilling process.