13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Influence of high mean stresses on lifetime and damage of the martensitic steel X10CrNiMoV12-2-2 in the VHCF-regime Tilmann Beck1,*, Stephan Kovacs1, Lorenz Singheiser1 1 Institute of Energy and Climate Research (IEK-2), Forschungszentrum Jülich, Jülich 52425, Germany * Corresponding author: t.beck@fz-juelich.de Martensitic 9-12% Cr-steels are widely applied in steam turbines. In the low pressure rotor blades, fatigue loadings at several 100 Hz induced by the inhomogeneous flow field behind the vanes are superimposed by high mean stress due to centrifugal forces. Using an ultrasonic fatigue testing machine and in-depth microstructure analyses, the present study investigates the influence of mean stresses at load ratios up to R = 0.7 on the VHCF behavior with an ultimate cycle number of 2·109. From R = -1 (no mean stress) up to R = 0.7 crack initiation changes consistently from the surface to internal inclusions at a cycle number around 4·107. Fractography indicates that fine grained areas around the inclusions only occur for R=-1. With increasing load ratio, fractures are observed at increasing cycle numbers. In spite of this fact and in spite of considerable cyclic creep at load ratios up from R = 0.5, the fracture mechanics approach proposed by Murakami consistently describes the lifetime behavior for load ratios up to R = 0.5 over 4 decades of lifetime. The contribution will give a comprehensive overview of these findings together with detailed microstructure studies of the fracture surfaces. VHCF, Cr steel, turbine blade steel, mean stress, fatigue damage 1. Introduction The research activities in the field of fatigue with very high cycle numbers (VHCF) were massively expanded in the past decade. Overviews of the basic mechanisms of VHCF are given in [1-4]. Generally, materials can be distinguished into two types according to their VHCF behavior. Type I materials are single-phase, inclusion-free materials like pure aluminum or copper. At very low stress amplitudes and high number of cycles, fatigue cracks in this type of materials usually originate at persistent slip bands (PSB), even at stress amplitudes below the PSB threshold, induced by local stress concentration at surface roughnesses. Type II materials on the other hand are multi-phase materials with a characteristic transition in the fracture mechanism between 107 and 108 cycles which commonly results in a multi-stage S-N curve as shown by Mughrabi in [5]. At high stress amplitudes, i.e. at low number of cycles to failure, fractures mainly occur at surface inclusions or at PSBs. At lower stress amplitudes in the VHCF-regime, subsurface cracks in the form of so-called “fish-eye fractures” occur, primarily originating from inclusions. Typically, subsurface fractures are reported in connection with the formation of a small zone around the inclusion called optical dark area (ODA), fine granular facets (FGA) or granular bright facets (GBF). Such a multistage fatigue life behavior for type II metals has been found for rotating bending fatigue of Cr-Mo and high carbon bearing steels [4, 6] and also for a 42CrMo4 tempered steel and some aluminum alloys at axial fatigue loading [3, 4, 7, 8]. The stages are mainly influenced by inclusion size, surface roughness and notch factor. The critical inclusion size for internal fracture is not fixed and depends on many factors e.g. yield stress, and typically ranges from 5 µm to 500 µm. For high-strength steels, the critical inclusion size is in the range of 10 µm [2]. In this context, Murakami proposed a fracture mechanics approach, the √area-concept, to correlate the inclusion sizes with VHCF life [9]. He treated inclusions as preexisting short cracks and, based on this assumption, calculated fatigue strength, fatigue life and stress intensity factor of the crack taking into account the largest inclusion of the specimen. However, especially the influence of superimposed mean stresses on the VHCF behavior of metallic materials is not yet sufficiently investigated. Shiozawa et al. [10] and Sakai et al. [11] investigated
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