Cyclic deformation and fatigue behavior of case-hardened steels

The goals of this research were to experimentally, analytically and numerically investigate the monotonic and cyclic deformation as well as fatigue behavior of case-hardened materials. Fatigue life predictions were made for both constant and variable amplitude conditions. Experimental evaluations were made for constant amplitude loading tests, two-level load tests, periodical overload tests, and service load tests, and the experimental results were compared with the prediction results.; FE models for the case-hardened specimens were constructed and multiaxial stresses were found from the FE models and residual stress relaxation was monitored in the FE models. Crack nucleation sites of the case-hardened specimens were predicted by the comparison of the "controlling damage parameter" and the "local fatigue damage resistance", and it was found that the predictions matched well with the experimental results.; Multi-layer FE models were used to obtain monotonic and cyclic deformation behaviors of the case-hardened specimens. Prediction results with two-layer and four/six-layer models were close to each other. The predicted monotonic stress-strain curves were close to the experimental curves, but the predicted cyclic stress-strain curves were higher than the experimental curves.; The "strongest link" method, which is based on the longest life of all the layers, was used and found to give good life predictions for the constant amplitude, two-level load and the fully reversed periodic overload tests. The "weakest link" method was found to give good life predictions for the periodic overload tests with constant maximum strain and service load testing.; Both standard constant amplitude and effective strain-life curves based on periodic overload tests were used in the life prediction for variable amplitude tests. The Linear Damage Rule (LDR) was applied in the calculations for all variable amplitude tests. The Damage Curve Approach (DCA), which is a nonlinear damage rule, was applied in calculating the fatigue life of periodic overload tests and two-level load tests. Life predictions with Linear Damage Rule (LDR) and Damage Curve Approach (DCA) were similar for the case-hardened specimens under periodic overload testing. For the two-level load tests, DCA was found to result in better predictions than LDR.