Determining Fatigue Limit at Case Depth for Crankshaft

Crankshaft is usually hardened on its main and pin journal surfaces and fillets. The subsurface area right underneath the hardened zone is therefore the most vulnerable area (if the case depth is not deep enough). During the bending tests fatigue failures have been occurred on the subsurface sites of the pin journal fillets. Finite element method can be employed to simulate the bending tests. Using the stress data measured on pin fillet surface, the fatigue behavior of its subsurface can be characterized. The results of this research can be used to predict fatigue failure of crankshaft subsurface under engine operating condition. In this research, the finite element model is validated by comparing the stresses on pin surface with the bending tests. Excellent correlation has been achieved between finite element modeling and the test results. Then, the stresses along the case depth are calculated and the curve of stress vs. case depth is plotted. Using this curve, the fatigue limit of the subsurface can be determined at any given case depth location. The results have been applied for qualifying crankshaft durability under engine operating condition, and good predictions have been achieved.