Fatigue Parameter Based on the Assessment of Stress Components in all Material Planes
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FATIGUE PARAMETER BASED ON THE ASSESSMENT OF STRESS COMPONENTS IN ALL MATERIAL PLANES C. Lu,1,2 J. Melendez,1 and J. M. Martínez–Esnaola1 A new fatigue parameter is proposed, which provides a new way of thinking to assess fatigue damage problems. The complete stress state at a certain material point, i.e., taking into account any material plane at that point, is included in the method. The influence of tension and compression state and also the mean stress are also included. Some experiments with different materials and loading conditions are used to validate the capabilities of the proposed method. The results show that the method provides good predictions for axial cyclic and/or torsion cyclic conditions with zero or nonzero mean stresses, in-phase and out-of-phase, different shapes of the specimen, loading waveform and loading path. Keywords: fatigue parameter, mean-stress effect, fatigue damage curve, complex loading, material planes.
Introduction Fatigue is one of the most common damage mechanisms in engineering components. The methods used for the evaluation of fatigue damage can be split into three main categories according to the mechanical magnitudes used in the definition of various criteria, i.e. stress-based, strain-based, and energy-based methods. As a general rule, the stress-based methods are used for high-cycle fatigue, the strain-based methods are used in low-cycle fatigue and can also be used for high-cycle fatigue, and the energy-based methods can be used for both high- and low-cycle fatigue because they contain contributions of both stress and strain magnitudes. Basically, in the stress/strain-based methods, the maximum normal or shear stress/strain is used in the case of tension fatigue or torsion fatigue and then some simple modifications, such as amplitude, mean value, separation between the elastic/plastic components are used. The stress invariants, like J 2 , hydrostatic pressure σ H , or some equivalent stress/strain states, such as Mises, Tresca, etc. are also used to predict fatigue damage. More complex formulations assume that fatigue damage should take into account the stress or strain components (typically, normal and shear components) that can be active in different planes at a given material point. Thus, a fatigue parameter (FP) given by Δγ max /2 + SΔε n , where Δγ max is the range of maximum shear strains, Δε n is the range of normal strains in the plane of maximum range of shear strains, and S is a material constant, was proposed in [1]. It is therefore assumed that both shear and normal strains can affect fatigue damage, the shear strain is the predominant factor in the course of crack formation and in the first stages of crack development, while the normal strain is predominant during macrocrack propagation. In [2], the authors modified the Kandil method as follows: Δγ max /2 + (1 – σ n /2σ y )Δε n , where σ n is a normal stress in the plane of maximum shear-strain range and σ y is the yield limit. In this modification, the influence of normal stresses 1 2
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