Virtual strength testing of crankshafts taking into account residual stresses from the surface hardening process

In real engine operation, crankshafts are subjected to a combined - temporally uncorrelated - bending and torsional load, which leads to multi-axial and non-proportional stress states in the notches.

Although these local stresses can be simulated with sufficient accuracy, it is very difficult to perform an equivalent test outside the engine, so a simplified procedure is used. Here, separate fatigue strength tests are carried, whereby the moments to be endured are derived from the real engine load.

The aim of the project was the virtual strength testing of crankshafts and the deepening of knowledge of the strength-increasing influencing variables like residual stresses from hardening. The residual stress distribution was simulated by means of an imposed strain, whereby the residual stress state at the surface was verified by measurements.

Subsequently, fatigue strength analyses were carried out. In the analyses including surface hardening, two effects were considered: on the one hand the increase in strength of the material (change in microstructure) derived from the hardness distribution, and on the other hand the residual stress distribution. The increase in strength achieved in the test can be attributed to a lesser extent to the material and to a greater extent to the residual stresses.