PSI - Issue 2_A

Dong-Jun Kim et al. / Procedia Structural Integrity 2 (2016) 825–831 Dong-Jun Kim et al. / Structural Integrity Procedia 00 (2016) 000–000

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2

Nomenclature a

crack length

α , β material constants in MDF p A i , n i , m i coefficients in creep constitute model ( i =1-3) C(T) compact tension k MDF multi-axial ductility factor ε strain σ stress W specimen width ω , Δω accumulated damage and incremental damage

factor to incorporate the element size effect on plastic damage

A number of studies have been reported in the literature up to present on numerical modelling of the creep fracture. Numerical techniques to simulate the creep failure using a stress-based damage model have been developed by Hayhurst D.R. (1972). Yatomi M. (2003) presented a technique to simulate creep crack growth using strain-based damage models and ductility exhaustion concept to develop a damage model. For all models mentioned, only creep damage was considered to simulate creep crack growth. The crack-tip plasticity may play a role in the crack initiation and growth. This suggests that a creep damage model needs to include the contribution of a plastic damage on the creep crack growth. This paper proposes a combined plastic and creep damage model to simulate creep fracture, extending the previous model whereby only creep damage is considered. Predictions using the proposed model are compared with the experimental data and those using the previous model. 2. Experimental test data 2.1. Tensile and creep tensile test In the present work the material considered is P91 steel. All experimental test in this paper was conducted at Korea Atomic Energy Research Institute (KAERI). Tensile test was performed using plate bar. The thickness and width of bar were 2 mm and 6.25 mm, respectively with gauge length of 25 mm. True stress-strain curve in Fig. 1(a). Uniaxial creep tensile tests of P91 were conducted at 600 ºC using smooth bars under constant stresses from 140 MPa to 180 MPa. The diameter of smooth bars was 6 mm with gauge length of 30 mm. The results of creep tensile tests are shown in Fig. 1(b). These tensile data and creep tensile data will be used as input to elastic-plastic creep FE damage analysis. More detailed information on testing can be found in Kim W.G. et al. (2013). 2.2. Fracture toughness test Fracture toughness test was performed at 550 ºC using 1-T compact tension specimen. The specimen have W =50 mm, B =25.4 mm and a/W =0.5. The specimen was side-grooved to 20 percent of the specimen thickness. The tests were conducted in accordance with the ASTM E 1820-05 standard test method (2005). Resulting J-resistance curve is shown in Fig. 2 and will be used to determine plastic damage criteria. 2.3. Creep crack growth test Creep crack growth test at 600 ºC were performed using a compact tension specimen. The width of specimen was W =25.4 mm, and the thickness is half of the width. Specimens were side grooved to a total thickness reduction of 20%. Initial crack ratio ( a/W ) was about 0.45, and pre-cracking length was about 2.0 mm. Various loads ranging from 3.8 kN to 5.2 kN were applied and load line displacements and crack growths were measured as described in ASTM E 1457-00 (2001). Fig. 3(a) and 3(b) show the results of experimental load line-displacement and crack growth curve. More detailed information on testing can be found in Kim W.G. et al. (2013).