13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- crack initiation behaviour. The ductility in this case is represented comfortably by the material’s creep rupture strain. The more deformation within the region of the crack tip is attainable, the later the crack initiation will start. Therefore it is sensible to set up a strain based criterion to describe crack initiation. With this, the influence of ductility is directly implemented. To gain a concept which is valid for either high or low ductility, materials differing in their ductility must be analysed to validate the concept. Describing crack initiation applying a stress formulated concept - such as the commonly used assessment procedures- will be too conservative. The influence of ductility on crack initiation have been analysed theoretically or have been approximated, [10], [11], [12], [13]. Concerning the TKD, a modification based on the creep rupture strain has been proposed in [13]. This was done mainly for the crack-tip-damage region. For the ligament-damage mode a dependency of ductility could not yet be established. Figure 1. Creep rupture strains of a 10Cr-steel at T = 600 °C, different heats The specimens investigated will have Double-Egde-Notched-Tension (DENT) in different sizes or compact-tension (CT) geometry. The loading times for the numerically treated specimens are up to operational relevant times of 200.000 h. The aim is the determination of initial crack sizes as a function of the creep rupture strain for different specimen sizes which account for Eq. 1. Below this limit of initial crack size alimit, a fracture mechanics assessment is not necessary. The creep data used to set up the material law is available up to 140.000 h for uniaxial creep tests. The fracture mechanics experiments are available up to 20.000 h. 2. Investigated materials To account for the influence of the material’s ductility, one 10Cr-steel and two 1Cr-steels have been considered. The creep rupture strain of the 10Cr-steel is shown in Figure 2. X12CrMoWVNbN10-1-1 Figure 2. Creep rupture strains of a 10Cr-steel at T = 600 °C, different heats Crack initiation No crack initiation Crack-tip-damage Mixed-Mode-Damage Ligamentdamage 2W a a W=50mm npl 10mm 10 100 1000 10000 100000 0 5 10 15 20 25 30 35 40 45 50 X12CrMoWVNbN10-1-1 / 1A X12CrMoWVNbN10-1-1 / AXN T = 600°C Creep rupture strain Au / % Time to failure tF / h
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