PSI - Issue 13

Marijana Milković et al. / Procedia Structural Integrity 13 (2018) 1861 – 1866 Marijana Milkovi ć / Structural Integrity Procedia 00 (2018) 000–000

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It is possible to see that graver’s line are not straight after 658 122 cycles and more slip lines in loading direction with deeper gap occurred on surface of specimen treated by higher number of cycles. However, the line distance, shown in Fig. 4.b) are not significantly changed. Main change in grain separation on surface of specimen occurred. It is obvious that residual stresses are decreased with number of cycles, as is shown in Fig. 3.a) and 3.b). Since the roughness of surface of specimen after 6 000 cycles and 658 122 cycles are similar, in spite of fact that surface degradation process occurred, it is interesting to find if the correlation between residual stress and roughness exists. Figure 6 shows dependence of residual stress vs. roughness (Ra) parameter. One can conclude that after initial fatigue stage (after 100 000 cycles) the linear correlation between residual stress and roughness parameter Ra occurred. The both average values are decreasing. 4. Conclusion Electropolishing changes influence of the surface treatment. On the electropolished part we get real residual stresses which have influence on fatigue behavior of the material. Due to fatigue, the degradation on surface of material is taking places without significant changes of line distance but mainly edge of line and micro-grain separation process occur. Fatigue has the effect on changes in local plastification and separation of some grains and it leads to the change of residual stresses and roughness. After fatigue the roughness is reduced, as well as the residual stresses. Results of investigation show correlation between residual stresses and roughness on the surface of specimen. It is possible to monitor the degradation of the material during fatigue by measuring change of residual stresses and under certain condition by roughness, too. Acknowledgements Financial support by ARRS and by the Austrian Federal Government and the Styrian Provincial Government under the frame of the Austrian COMET Competence Center Program (Project A4.20-WP1) are gratefully acknowledged. References Milković M., Đonlagić D., Gubeljak N., 2017., Correlation between the surface roughness and mean strain values of Al7075-T6 plate exposed to fatigue loading in ICSID 2017, Zagreb, pp.223 Papadopolous I.V., 1999., Multiaxial Fatigue Limit Criterion of Metals, In: Van K.D., Papadopolous I.V. (editors) High- Cycle Metal Fatigue, International Centre for Mechanical Sciences (Courses and Lectures), vol. 382, Springer, Vienna, McMillan A., Jones R., Peng D., Chechkin A.G., 2018., A computational study of the influence of surface roughness on material strength, Meccanica 53, pp. 2411-2436 Fitzpatrick M.E., Fry A.T., Holdway P., Kandil F.A., Shackleton J., Suominen L., 2005, Measurement Good Practice Guide No. 52, Determination of Residual Stresses By X-Ray Diffraction, Issue 2, Teddington, Middlesex, United Kingdom, NPL-National Physical Laboratory Withers P.J. and Bhadeshia H.K.D.H., 2001, Materials Science and Technology, vol.17, pp.355-365 Damjanović D., Kozak D., Marsoner S., Gubeljak N., 2017, Residual stress state in pipe cut ring specimens for fracture toughness testing, Material testing: Vol. 59, No.6, pp. 530-535 Todt J., Keckes J., Winter G., Staron P., Hohenwarter A., 2018., Gradient residual strain and stress distributions in a high-pressure torsion deformed iron disk revealed by high energy X-ray diffraction, Scripta Materialia 146, pp. 178-181 Trško L., Guagliano M., Bokůvka O., Nový F., Jambor M., Florková Z., Influence of Severe Shot Peening on the Surface State and Ultra-High Cycle Fatigue Behavior of an AW 7075 Aluminium Alloy, 2017, Journal of Materials Engineering and Performance, Vol.26, Issue 6, pp. 2784 2797