13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- induced by QT (quenching and tempering) and nitriding process. After UNSM treatment the surface stress for all UNSMed samples was improved. However different trends with the increase of strike number can be observed for the UNSMed samples. For the un-treated sample, the surface residual stress increase from -217.4 MPa to -318 MPa with U1 and -395.1 MPa with U2 and it can be found that higher compressive residual stress can be obtained as the increase of UNSMed striking number. For the nitriding samples, the residual stress has a growth from -171.9 ~ -161.2 MPa (N8 and N48) to -925 MPa for N8 samples and -1089 MPa for N48 samples with U1 treatment. However a decrease (to -750 MPa for N8 samples and -778 MPa for N48 samples) was found with U2 treatment. This means that though with the growth of surface hardness higher compressive residual stress can be produced, a decrease was found as the increase striking number. This phenomenon is different from the U1, U2 results and other studies [2]. This may be the reason of the grain refinement of the nitrides in the surface zone inducing a relaxation of the surface stress. Figure 7. The residual stress on the surface of specimens before and after UNSM. Figure 8. S-N curves of rotating-bending fatigue test The fatigue test was employed on a rotating bending test machine and the S-N results are shown in Figure 8. The results show that after UNSM treatment the fatigue strength was enhanced for un-treated and N8 samples, but no improvement for the N48 samples. Compared with un-treated sample, more enhancements were obtained for N8 samples. At the same time the improvements of UNSMed samples have a close connection with the value of surface residual stress. Foe ever un-treated or N8 samples, the fatigue strength has a linear relation with the surface residual stress and with the higher residual stress, higher fatigue strength can be obtained. So we can find that a higher fatigue limit of N8 sample can be obtained after U1 treatment than after U2 treatment. It shows opposite compared with un-treated sample after UNSM treatment. This means that though
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