Fatigue Resistance Changes of Structural Steels at Different Load Spectra

Fatigue strength of widely used engineering structural steels was studied at various loading frequencies according to the cantilever bending scheme of rotating cylindrical samples. The inclination angle tangent of the fatigue curve to the axis of durability was taken as the fatigue resistance index. It has been established that 40 and 45 grade steels (UNS G10400/G10420 and G1044/G1045) belong to the group of materials in which a decrease in the loading frequency leads to cyclic softening and a decrease in the fatigue resistance, which is numerically expressed by the increase in the slope of the fatigue curve. Test samples made of 40Cr grade steel (UNS G51400) have shown that the increase in the frequency of loading cycles leads to a noticeable decrease in the fatigue curve slope parameter, i.e., leading to an increase in the fatigue resistance. The decrease in the fatigue resistance parameter is associated with the enhanced hardening of the material of the samples’ surface layers, which reduces the fatigue damage to the surface itself. The tangent dependence of the fatigue curve slope is shown on the surface damage rate when the loading cycles change frequency. Regardless of the frequency, the damage rate of the material’s surface layers increases along the fatigue curve slope. For each of these groups, mathematical relations have been defined. A correlation coefficient that shows the degree of convergence of experimental results and the constructed fatigue curve is adopted as a criterion of the cyclic behavior stability of steels. An increase in the behavior stability of 40Cr steel is observed with the increasing cyclic deformation rate. 45 grade steel tests have shown that a decrease in the cyclic strength with an increasing loading frequency does not affect the material’s fatigue stability. Experimental results showed increased dispersion for 40 grade steel at a low loading frequency despite high values of the cyclic strength at a given loading frequency. Based on the experiments, the behavior dynamics of real machine parts and structures subjected to cyclic loads within the studied loading spectrum has been established.