On the Failure of Crankshafts in Thermoelectric Power Plants: Multiaxial Fatigue Analysis and a Comparative Survey on Crack Growth Threshold ΔKth

Despite being designed considering infinite fatigue-life, failures of motor crankshafts forged from DIN 34CrNiMo6 steels have been reported in Brazilian power plants. As such, the present work aims to discuss the failure of a crankshaft within this context, with the purpose of verifying whether the stresses developed in critical locations of the component were in accordance with the steel's fatigue limits, as well as if the material exhibits an adequate resistance to crack propagation. Taking into consideration a set of critical-plane stress-based multiaxial fatigue criteria, namely Findley, Matake, McDiarmid and Susmel and Lazzarin, the fatigue behaviour of the material is analysed and discussed. Furthermore, da/dN versus ΔK experiments were carried out with the purpose of determining the DIN 34CrNiMo6 steel's crack growth threshold ΔKth and comparing it to the ΔKth of three other commercially available steels (DIN 42CrMo4, SAE 4140 and SAE 4340). The selected multiaxial fatigue criteria indicated that the stresses developed throughout the component were not sufficient to drive the crankshaft to failure, thus indicating safety. On the other hand, the DIN 34CrNiMo6 steel presented the lowest ΔKth (6.6 MPa m1/2) among all the considered steels (10.86, 12.38 and 7.22 MPa m1/2 for the DIN 42CrMo4, SAE 4140 and SAE 4340, respectively), therefore being susceptible to shorter fatigue lives in comparison to the other materials.