PSI - Issue 57

Ahmad Qaralleh et al. / Procedia Structural Integrity 57 (2024) 649–657 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The urgent need for climate protection necessitates the prioritization of reliable and lightweight structures. This demand drives the development of new materials with exceptional strength and resilience properties. Among these innovations are bainitic (Wirths, et al., 2015) and air-hardening steels (Stieben, et al., 2016), as well as advancements in conventional steel types like precipitation-hardening ferritic-pearlitic steels (Schade, et al., 2016). In the scope of bainitic steels, overloads can trigger an austenite-martensite phase transformation, leading to increased strength, unlike heat-treatable steels that tend to soften under similar conditions (Elek, et al., 2015). Additionally, bainitic steels generate residual compressive stresses in heavily stressed regions where the phase transformation occurs, which can significantly impede crack propagation. This research paper focuses on the local strain concept for estimating the service life of components. Fatigue damage is calculated for each cycle using hysteresis loops and linear damage summation. The evaluation of the strain life curve is performed by decomposing the total strain amplitude into elastic and plastic strains. Plastic strain amplitude is expressed using the Manson-Coffin-Morrow relation (Coffin, 1954); (Manson, 1965); (Morrow, 1965). Manson-Coffin model represents the low cycle fatigue regime, where plastic deformation is predominant. Whereas the Basquin model (Basquin, 1910) characterizes the fatigue life at lower stresses and high cycles, where elastic strains are predominated. The strain life curve is represented mathematically as shown in Equation (1). ∆ 2 = ∆ 2 + ∆ 2 = ′ (2 ) + ′ (2 ) (1) The cyclic behavior of materials and the cyclic stress-strain curve were systematically analyzed using the Ramberg Osgood equation, as shown in Equation (2). This equation provides a comprehensive understanding of the nonlinear relationship between stress and strain for materials experiencing plastic deformation under cyclic loading (Ramberg & Osgood, 1943). ∆ = ∆ +∆ = +( ′ ) 1 ′ (2) Lifetime estimations were carried out for the steering knuckle, considering both constant amplitude loading (CAL) and variable amplitude loading (VAL) with load ratios R = -1 and R = 0. These estimations were performed following the FKM guideline Nonlinear (Fiedler, et al., 2019), utilizing the damage parameters P RAM , see Equation (3) (Bergmann, et al., 1977) and P RAJ , see Equation (4) (Vormwald, 1989). The FKM guideline provides a framework for evaluating the fatigue strength of mechanical components based on the local strain concept, which considers the nonlinear behavior of the material. For determining the local stresses associated with P RAM , the recommended approach is the notch approximation method according to Neuber (Neuber, 1985). On the other hand, the method suggested for P RAJ is the one proposed by Seeger-Beste (Seeger & Beste, 1977). =√( + . ) ∙ ∙ (3) = ∆ =[1.24 ∙ (∆ ) 2 + 1√.02 ′ ∙ (∆ ) ∙ ((∆ )− ∆ )] (4)