On the evaluation of overload effects on the fatigue strength of metallic materials

In this paper, experimental and numerical studies are performed to quantify the influence of overloads on the fatigue behaviour of the quenched and tempered steel 42CrMoS4 and the wrought aluminium alloy EN AW-6082 T6. The main objective is to incorporate the influence of overloads in the fatigue assessment. Three series of fatigue tests per material were performed, each under load control conditions, at R ═-1:i) constant amplitude loading (CAL) tests with no overloads; ii) CAL tests after five overload cycles of the magnitude OL 1; iii) CAL tests after five overload cycles of the magnitude OL 2. The respective overload levels are selected to achieve 75 % of the static strength of the test samples either estimated according to the FKM Guideline (2020) for OL 1 or determined directly by specimen testing for OL 2. During the overload cycles, the local notch strains were monitored by means of the digital image correlation (DIC) technique. X-ray diffraction was applied to quantify the residual stresses in the specimens due to manufacturing, after applying overload cycles, as well as after completing tests on runouts. Fatigue test results demonstrate that, depending on the material and the overload level, the fatigue strength can either decrease or increase or, in some cases, be only slightly affected by overloads. To explain the experimental findings, residual stresses determined from both experimental measurements and elastic-plastic finite-element analyses are included in the calculations of the notch stresses and strains and a fatigue damage parameter. The latter is then involved in the damage calculations to quantify the effect of overloads in terms of the magnitude and number of cycles to failure.