Effect of Surface Finish on Fatigue Properties of Notched Carbon Steel Specimens

Abstract

Abstract. A new approach to fatigue analysis was developed in Japan. Parts of the approach were reviewed and incorporated into the Japan Society of Mechanical Engineers (JSME) Environmental Fatigue Evaluation Method, 2022 Edition. This evaluation method specifies the best-fit fatigue curves and requires that the factors affecting fatigue properties be considered separately. Among these factors, the factor for the effect of surface finish Ksf is also included in this evaluation method. However, since the Ksf was developed based on literature reviews and a limited number of test results, further experimental investigations were conducted. This paper reports on the effects of surface finish on the fatigue strength of carbon steel. First, smooth round bar specimens were prepared, including specimens polished in the parallel portion and those machined to target surface roughness levels of maximum height of profile (Rz) = 6.3, 25, and 50 μm. Strain-controlled fatigue tests were then performed. To quantitatively evaluate the effects of surface finish, Ksf values for each machined specimen were calculated using a method similar to that of the JSME. The calculated Ksf values were confirmed to match those specified by the JSME. Subsequently, considering that fatigue failures in actual components typically occur at structural discontinuities, load-controlled fatigue tests were conducted on two types of round notched specimens: one with the notch root polished, and the other machined to Rz = 25 μm. The test results confirmed that the Ksf equation specified by the JSME, derived from smooth specimens, is also applicable to structural discontinuities.