Fatigue Performance of Forged Steel and Ductile Cast Iron Crankshafts

Fatigue is the primary cause of failure of crankshafts in internal combustion engines. The cyclic loading conditions and the stress concentrations in the crank pin fillets are unavoidable, and can result in fatigue failure. The objectives of this study were to compare the fatigue behavior of forged steel and ductile iron crankshafts from a one-cylinder engine as well as to determine if the fatigue life of a crankshaft can be accurately estimated using fatigue life predictions. Monotonic tensile tests as well as strain-controlled fatigue tests were conducted using specimens machined from the crankshafts to obtain the monotonic and cyclic deformation behavior and fatigue properties of the two materials. The forged steel had higher tensile strength and better fatigue performance than the ductile cast iron. Charpy v-notch impact tests were also conducted using specimens machined from the crankshafts to obtain and compare the impact toughness of the materials. Forged steel in the L-T and T-L directions had a higher impact toughness than the ductile cast iron regardless of temperature. Load-controlled component fatigue tests were performed using the forged steel and ductile cast iron crankshafts. For a given bending moment amplitude, the forged steel crankshaft had a factor of six (6) longer life than the ductile cast iron crankshaft. A finite element analysis of the crankshafts was conducted using boundary conditions similar to the component test. The fatigue properties from the specimen tests were used in life predictions of the crankshafts. The lives based on the S-N predictions were close to the lives based on the component tests for both forged steel and cast iron crankshafts. The lives from the ε-N predictions were very close to the lives from the component tests for forged steel, but less accurate for the cast iron crankshafts.