• cumulative fatigue damage theories
• transient behavior
• fatigue life evaluation theories
• fatigue life
• strain softening
• ratcheting

The thesis aims at studying and comparing different methods for fatigue life evaluation with particular attention on random loading case.
This work is part of a project regarding the newly developed cold forming process, the linear flow splitting process. This process allows the bifurcation of thin metal sheets by severe plastic deformation. The material used is ZStE340 (European equivalents: HC340LA), a microalloyed steel with high yield strength for cold forming.
A large number of experimental tests (uniaxial, stress and strain controlled cyclic loading under constant and variable amplitudes) has been performed. The experimental part of the work has provided experimental data for the subsequent validation of fatigue life evaluation methods. Moreover, it has allowed to investigate cyclic properties of ZStE340, both in as-received state and after the linear flow splitting process.
Regarding cyclic properties, strain softening and ratcheting features have been clearly observed for the material ZStE340 under uniaxial cyclic tests at room temperature. Ratcheting is defined as the accumulation of secondary deformation proceeding cycle by cycle under stress-controlled conditions. In particular, papers on ratcheting state this phenomenon occurs only with non zero mean stress. In this work, a full description of ratcheting has been provided for ZStE340. Perfomed tests have allowed to study its relation to the applied mean stress, stress amplitude and stress ratio. In particular, compressive ratcheting for negative mean stress and unexpected ratcheting with zero mean stress have been observed. A similar behavior has been reported only by Yaguchi for 9Cr-1Mo steel, by Park for Inconel 718 and by Lim for Elbrodur-NIB, though at high temperatures. A tension-compression asymmetry is believed to cause ratcheting under zero mean stress. Further studies are expected to clarify this phenomenon.
Regarding fatigue life evaluation methods, the most representative ones have been considered, following Fatemi's classication: Smith-Watson-Topper parameter; Vormwald model, based on fracture mechanics of short cracks; Chaboche Non-Linear-Continuous-Damage (NLCD) model, supported by Continuum Damage Mechanics; Ellyin method as representative for energy-based damage theories. Moreover, a graphical user interface (GUI) using MATLAB has been built. This procedure allows fatigue life evaluation, by taking as input the same file required by the test machine and the user can select any method among those above-mentioned. By comparing experimental data with results from considered methods, Chaboche model seems to be the most suitable in the evaluation of fatigue life for ZStE340.
In all these methods, an important input has been found to be the material stress-strain response, especially considering these strong strain softening and ratcheting features. Analyzed methods consider a material stress-strain response which do not reflect satisfactorily the actual material behavior. For this purpose, a semi-empirical new method has been formulated. The new method is based on the framework of non-linear kinematic hardening theory and it should improve the fatigue life method accuracy. Further studies will be carried out to validate its effectiveness.
In conclusion, a better comprehension of cyclic behavior for ZStE340 has been reached with emphasis on its ratcheting feature. A rare phenomenon, ratcheting with zero mean stress, has been observed. The main fatigue life evatuation methods have been studied and compared with experimental data, both in constant and variable amplitude. Chaboche Non-Linear-Continuous-Damage (NLCD) model shows the best agreement with experimental data for ZStE340. A new method has been formulated aiming at improving fatigue life method accuracy.