13th International Conference on Fracture June 16–21, 2013,Beijing, China -10- In the present work several pre-strained tensile specimens were tested under given strain amplitude with the loading ratio 0.1. The results are shown in Fig. 8. From the diagram one sees that the pre-strain reduces LCF fatigue life of the specimen. In the figure the fatigue life is expressed in the form of Basquin model εa = A Nf ( ) m, (17) with A and m as material specific parameters. For the base material it was identified A=7.034 and m=-0.3083. For the pre-strained specimen A=8.322 and m=-0.35. This result implies that the pre-strained specimen loses almost 50% LCF life of the original untreated specimen. The grade of pre-strain seems to affect fatigue damage secondarily. One major for the life reduction is caused by the significant difference in applied stress amplitude. Due to huge strain hardening induced by plastic deformations and martensite phase transformation in pre-straining treatment, the pre-strained specimens are significantly high strengthed than the base material. It follows that the pre-strained specimens were under much higher stresses than the base material specimens, by giving strain amplitudes, as shown in Fig. 9. The stress amplitude in Fig. 9 is the final stabilized stress amplitude in strain-controlled fatigue tests. From the figure one learns that the pre-strained specimens are much stronger than the base material specimens, that is, by giving fatigue life, the pre-strained specimens can bear much higher mechanical loads. Figure 9: The stabilized stress amplitude as a function of fatigue life from strain-controlled fatigue tests. 6. Conclusions In the present paper a systematical work on martensite transformation in stainless steel 304 and its effects to material damage is presented. The first experiments confirm that matensite phase transformation in SS304 can be described by the Santacreu model and shows significant dependence on stress triaxiality. The plasticity model with the martensite transformation is established based on after Santacreu model and can be applied to describe plastic behavior of SS304 with severe plastic deformations. The plasticity model predicts strain hardening under both compression and tension uniformly. Computational predictions agree with experimental results reasonably. Although the fracture strain of SS304 can be characterized by the equivalent plastic strain precisely, fatigue tests display strong influence of the pre-strains to the fatigue life. Whereas the strain-fatigue life curve shows acceleration of fatigue damage in strain-controlled fatigue tests, the stress versus fatigue life curve reveals significantly higher bearing capacity due to pre-strains. This result implies
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