Failure investigation of the crankshaft of diesel engine

In this work the failure, stress and modal analysis of the crankshaft of diesel engine was performed. Visual examination of damaged part showed that the fatigue beach marks were observed on the fracture. Results of additional investigation using the scanning electron microscope revealed the presence of micro-cracks in crack origin area. In next part of experimental investigations the specimens were cut from damaged shaft. Results of tension test showed that mechanical properties of the steel used for the crankshaft manufacturing is in the range defined by the standard. In order to explain the reason of premature crankshaft damage, the finite element method was utilized. In first step the numerical model of crankshaft with the connecting rods was prepared. The boundary conditions were next defined on bearing journal surfaces. The complex load cases were also defined in order to model the real engine loadings. Results of nonlinear stress analysis performed for the crankshaft model showed that during work of engine with a maximum power the high stress area was located in another zone than the crack origin. This result was a reason for extension of investigation on the dynamic problems. In last part of the study the numerical modal analysis was performed for the crankshaft. In this analysis both the frequencies and modes of free vibration were obtained. Results of modal analysis showed that during second mode of free vibration the high stress area was located in the crack origin zone. Based on results of performed investigations it was concluded that the main reasons of premature failure are resonant vibrations of the crankshaft.