PSI - Issue 60

340 P.K. Sharma et al. / Procedia Structural Integrity 60 (2024) 335–344 P.K. Sharma/ StructuralIntegrity Procedia 00 (2023) 000 – 000 −1 = 1.75 − 1.5( ) −1 = 0.75 − 1.5( )+1.5(1− )/ −1 (9) Hence, the load-displacement and crack growth data Vs applied displacement can be used for evaluation of the J integrals. Crack initiation toughness can be obtained using 0.2 mm offset blunting line expression as discussed in subsequent section. 2.4. Evaluation of crack initiation toughness using intersection of 0.2 mm offset blunting line with J-R curve During initial loading of the specimen, blunting of the initial sharp crack obtained through fatigue pre-crack occurs followed by ductile crack initiation. The magnitude of blunting can be observed in the unloading compliance as pseudo crack growth. The radius of blunting zone is taken approximately as 0.2 mm in the ASTM standard [19]. Before crack initiation, the energy release rate vary with pseudo crack growth following Eq. (10) = 2 .∆ (10) where is the yield stress of the material. For evaluating the crack initiation toughness, a 0.2 mm offset line parallel to the initial blunting line is drawn and the intersecting point of 0.2 mm offset line with J-R curve gives us the corresponding crack initiation toughness (J i,0.2 ). However, if we consider strain hardening of the material during plastic deformation, this equation gets updated to Eq. (11) as shown by cornec et al. 1986. ∆ =0.4 ∗ ( ) (11) where ∆ is the apparent crack growth due to blunting of crack-tip, E is Young's modulus of elasticity of the material, J is value of J-integral which is dependent upon the applied load, ∗ is blunting coefficient which depends upon both yield stress and harde ning exponent ‘n’. The blunting coefficient ∗ is evaluated in terms of effective stress and a factor as follows. ∗ = [( ) −1 ] (12) = 0.787 + 1.554 − 2.45 2 + 16.952 3 − 38.206 4 + 33.13 5 (13) The effective stress is given as = 10 (14) where =[ +1 ] log( 0 ) (15) The representative strain 0 is obtained from Eq. (16). 0 = +0.002 (16) Using the 0.2 mm offset line parallel to Eq. (10), the value of J i,0.2 is calculated from intersection of the above line with J-R curve. 3. Load-displacement behavior and progress of ductile crack growth in SENT specimen at different temperature Load Vs displacement data was obtained from the experiments on SENT specimens at different temperatures. Crack growth is obtained through unloading compliance method. 3.1. Load-displacement response at different temperatures It can be observed from Fig. 3 that the maximum load carrying capacity of Alloy 690 material is around 13 kN at room temperature. This gets reduced to 6.3 kN when the temperature is increased to 700°C and from 6.3 kN to 5.8 kN 6 (8)