PSI - Issue 38

Kimiya Hemmesi et al. / Procedia Structural Integrity 38 (2022) 401–410 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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two analysis approaches, in the following referred to as FKM Guidelines (2020) and (2019). The former one is applicable under a precondition of the elastic stress analysis, whereas the latter one takes account of plasticity effects and, thus, requires elastic-plastic stress and strain calculations. The influence of moderate overloads on the fatigue strength can be treated in both Guidelines by means of the linear damage accumulation rule. Additionally, if plastic deformations at overloads occur, residual stresses can also affect the fatigue strength and be taken into account via a mean stress correction factor. However, no sufficient experimental validation is available for the respective assessment methods if the overload level approaches the component strength. Moreover, residual stresses are usually unknown or can hardly be measured or calculated for component ’s conditions . In such cases, FKM (2020) provides a pragmatic guidance as to the consideration of overloads. Accordingly, overload levels up to 75 % of the component strength are regarded as admissible and can be accounted for by the linear damage accumulation rule. More severe overloads are not allowed or may not be treated by the method of FKM (2020). Within the present study, experimental and numerical analyses are performed to quantify the effect of overloads on the fatigue behaviour of the quenched and tempered steel 42CrMoS4 and the wrought aluminium alloy EN AW-6082 T6, and to validate the allowable overload limit currently considered in FKM (2020). overload level 1, stress amplitude of the initial 5 overload cycles overload level 2, stress amplitude of the initial 5 overload cycles damage sum, ratio of applied load cycles to endurable load cycles mean stress sensitivity factor a stress amplitude a,OL stress amplitude in overload cycles a,10 6 a,10 6 ,OL median value of fatigue strength at 10 6 load cycles for specimens subjected to overloads m mean stress RS residual stress in the load (longitudinal) direction 2. Material tests 2.1. Specimen geometries Experimental testing was performed on specimens fabricated from quenched and tempered cylindrical billets of the steel 42CrMoS4 and from solution heat-treated and artificially aged (T6) cylindrical work pieces of the wrought aluminium alloy EN AW-6082. No further heat treatment or machining was applied. median value of fatigue strength at 10 6 load cycles determined in tests with no overloads Nomenclature CAL constant amplitude loading digital image correlation DIC OL OL 1 OL 2 overload

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Fig. 1. Geometries of the notched specimens: a) 42CrMoS4, b) EN AW-6082.