Fatigue of notched case-hardened specimens of steel SAE 5120 in the VHCF regime and application of the weakest-link concept

Notched specimens machined from steel SAE 5120 (DIN EN 20MnCr5) in two cleanliness conditions were case hardened in two differing ways. The first material is used for high performance components and is nearly free of oxides. The second material is standard steel that is used for common requests in the field of drive engineering containing non-metallic inclusions in a dimension that is state of the art in steel making. VHCF tests were performed under two stress ratios up to 200 million cycles. Fatigue failure is possible at all applied numbers of cycles which was not known before. The weakest-link concept was adapted to the measured lifetimes considering the local stresses and the local strength of the material. Different fatigue criteria were used to calculate local survival probabilities at the specimen surface and in the specimen volume. By this, a prediction of crack-initiation sites is possible.