13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- In order to optimize the full-use of the already acquired database, the P91 samples investigated in this work where submitted to the same heat treatment as follows: austenitising at 1050°C for 10 min with air cooling, tempering at 750°C for 70 min and PWHT at 760°C for 2 h as it is often performed for components with welds. The microstructure of P91 consists of tempered martensitic, see Fig. 1. During creep, fatigue or creep-fatigue interactions, the microstructure of such steels is instable. A drop in the dislocation density, an augmentation of the subgrain size leading to an equiaxial microstructure and a growth in the size of precipitates such as carbides M23C6 (Cr23C6) and carbonitrides MX (VN, VC, NbC) in the matrix are influencing the global deformation behaviour. Figure 1. Microstructure of P91-steel, light optical images 2.2. Mechanical testing For the crack growth tests side-grooved Compact Tension specimens C(T)-25 were used. The starter notch was realised by fatigue pre-cracking. All samples have an a0/W-ratio in the range of 0.52 to 0.60. Creep Crack Growth tests (CCG) were performed according to ASTM E 1457-07 [3]. Creep-Fatigue Crack Growth tests (CFCG) were performed with a load ratio R = Fmin/Fmax of 0.1 and with hold periods at maximum load of 6 and 60 min according to ASTM E 2760-10 [4]. Fatigue Crack Growth tests (FCG) were performed with a load ratio R = 0.1 and frequency of 0.5 Hz. All tests were carried out at 580°C and 600°C. During the tests, the load line displacement was measured by means of capacitive high temperature strain gauges. The crack propagation was monitored online using the Alternating Current Potential Drop (ACPD) technique. At the end of each experiment, the potential drop signal was calibrated with the final crack length measured on the fractured specimens. For this, the specimen was broken up under liquid nitrogen after the test and the crack length was measured according to [3]. 3. Description of crack initiation and crack propagation under creep and fatigue conditions Crack initiation and crack propagation under creep conditions can be described by the usual fracture mechanics parameter C* and the stress intensity factor KI. - In linear-elastic fracture mechanics the stress intensity factor KI is generally used for components with predominantly linear-elastic behaviour. Only a small plastic or creep zone at the crack tip can be assessed using this parameter. For side grooved C(T)-specimens the stress intensity factor is calculated as: ⋅ ⋅ + ⋅ ⋅ = W a f W a 2 B B W F K N I (1)
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