13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 2. Material properties of Type 316H Austenitic stainless steel at 550°C (a) Minimum creep strain rate, illustrating the mean, upper and lower bound regression fits ; (b) Time to crack initiation with respect to the reference stress. 3.2. Determination of stress relaxation in the three bar system In order to simplify the analysis, the following three assumptions were made. a) The misfit and the applied load may be sufficient to cause plasticity in the CT specimen, but the surrounding structure remained elastic. The CT specimen was always at the creep temperature and creep only occurred in the specimen. b) For state 2, the applied force on the assembly was constant during the test period. Finally, c) the forces between the specimen and the elastic elements were always in equilibrium. 3.2.1 Relaxation of residual stress alone Stress relaxation is a reduction of stresses with time due to the conversion of elastic strain to inelastic strain under constant total strain. For a material that follows a power creep law the reference strain rate in CT specimen can be expressed in terms of a reference stress, where n ref ref D (13) In a CT specimen the relationship between force, displacement, reference stress and strain, and stress intensity factor are given by [10] 1 ref n L F BWm ref ref L 1 1 ref n L L EBWm K 1 1 F K 1 1 ( ) K BE f W (14) where 2 2 3 4 5 1 2.163 12.219( ) 20.065( ) ( ) 0.9925( ) 20.609( ) 9.9314( ) 1 W W W f W W W W W (15) and 2 0 0 (1 )(1 ( /)) (1 ( /)) Lm w w 0 0 1 W (16) where is the plane stress reference stress, are the reference strain and length respectively of a CT specimen, is the elastic modulus, δ is the load line displacement; , (a) Stress (MPa) 200 300 400 500 100 Steady-state creep strain rate (1/h) 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 Test data Mean UB LB Initial Reference Stress (MPa) 200 300 400 100 Time for 0.2mm Crack Growth (h) 10 100 1000 10000 Test data Fitted line (b)
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