13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- dependent on the stiffness of the system. However, Z2 is influenced by the stress in the CT specimen. 3. Creep of a three bar system We now consider the behavior of states 1 and 2 with the aim of determining how the stress relaxes in the three bar system due to creep. The stress-strain and creep properties of the materials are required and hence the first part of this section presents relevant material properties for a Type 316H stainless steel. The solution for the stress relaxation of states 1 and 2 are given in the second part of this section. 3.1 Creep rupture and creep crack growth properties Tensile, steady state creep rate and creep crack initiation properties of Type 316H Austenitic stainless steel at 550°C are summarized in Table. 1. Tensile properties of Type 316H Austenitic stainless steel were measured at a strain rate of 1.5% per min [6]. The relevant creep data are taken from Douglas [7] and fitted using a simple power law as shown in Fig. 2a. The steady state creep rate is given by m D (11) where is the creep rate in , is the applied stress in and and are material constants. A regression fit to data shown in Fig 2a provided values for the constants and . Values are shown in Table 1 for mean, upper and lower bound fits. The upper (UB) and lower bound (LB) curves correspond to standard deviations on the mean, assuming the slope is constant. Seven creep crack tests using CT specimens were completed with test times ranging from about 60 to 16600hrs [8, 9]. The relationship between the applied reference stresses and time to initiate a crack equal to 0.2mm is shown in Fig. 2b. The initiation time is given by f i ref t C (12) where is the plane stress reference stress in for pre-cracked CT specimens, and and are the corresponding material constants. A regression fit to data shown in Fig. 2b provided values for the constants and . These are shown in Table 1. Table 1. Tensile, steady state creep rate and time corresponding to crack extension of 0.2mm properties of Type 316H Austenitic stainless steel at 550°C [6]. Material properties (MPa) Steady state creep rate properties Creep crack initiation properties Young’s Modulus: 160000 Yield stress: 145 0.2% proof stress: 194 Upper bound (UB) and lower bound (LB) factor on D: 2.72 UTS: 648
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