13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- A strain based criterion for creep crack initiation A. Hobt1,*, A. Klenk1, K. Maile1 1 MPA University of Stuttgart, Pfaffenwaldring 32, 7569 Stuttgart Germany * Corresponding author: alexander.hobt@mpa.uni-stuttgart.de Abstract Commonly, for components under creep loading, the maximum tolerable inelastic strain is limited to 1 %. The strains are calculated for components which are free from defects, using appropriate material models. Within the fabrication process internal defects such as crack or cavities cannot be avoided, especially for large thick-walled components. For these internal defects fracture mechanic concepts must be applied for safety assessment. Existing fracture mechanics concepts such as the Two-Criteria-Diagram or the Time-Dependent-Failure-Assessment-Diagram do not include the effect of the material’s ductility which has a clear effect on crack initiation process. Under creep load, the strains within the crack tip region dominate local material separation. Therefore a concept is introduced, which formulates a deformation exhaustion rule. With this procedure a better material utilisation is achieved. In a next step, crack initiation is calculated using an internal damage variable, which can be used as a life time parameter. Within this approach, the influence of a multiaxial state of stress on the damage evolution must be implemented. This procedure simplifies crack initiation assessment, replacing fracture mechanic methods by the evaluation of the damage variable. Keywords Creep, crack initiation, failure criterion, creep strain 1. Introduction For components under creep loading, the maximum tolerable accumulated inelastic strain must be limited, [1]. This limitation in strain is well-founded on evaluations of the remaining life-span with reference on microstructural damage, [2]. If an inelastic strain of 1 % is accumulated, monitoring arrangements must be established. This does not account for internal defects or cracks. For a safe operation of the component, the assessment must include the description of crack initiation and crack growth. If now the period to crack initiation is longer, than the period to accumulate the limit strain, the assessment procedure could be simplified. This can be described by the relation of the time to crack initiation tA to the time, say for 1 % inelastic strain, t1%: . (1) The aim of this work was to identify crack sizes for which Eq. (1) is met. Therefore the time to crack initiation tA must be identified, which requires an appropriate assessment concept. Commonly used concepts, such as the Two-Criteria-Diagram, [3], (TKD, see Figure 1) or the Time-Dependent-Failure-Assessment-Diagram, [4], [5], are based on stress relations. The TKD differs between three damage modes. Within the crack-tip-damage region the specimens or components have large cracks with a low mean stress. Within the ligament-damage region there are small cracks with a high mean stress and a high accumulated strain within the far-field region of the specimen. In between there is the mixed-mode damage region. Ligament damage is defined by accumulation of 1 % strain in the farfield of the specimen, [3], [4]. The analysed specimens within this work are aimed to have an R = 0.75 ÷ 1 and RK ≈ 0,5 and lie on the edge of the ligament-damage field. Over the past decade, there have been analysed various steels used in power plant operation generating a wide database for describing the material’s behaviour and crack initiation, [5], [6], [7], [8], [9]. Modern high chromium steels however, exhibit a high creep ductility influencing strongly the A 1% t t
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