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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Table 1. Creep constants in Eqn. (6) for P91 at 600ºC. A 1 2.1E-31 A 2

1.2E-18

2.7E-12

A 3 n 3 m 3

9.0

4.1 0.0

5.4 3.0

n 1

n 2

-2.6

m 1

m 2

(a)

(b)

Fig. 4. Typical FE meshes to simulate (a) fracture toughness and (b) creep crack growth test.

4.2. FE models The simulations for fracture toughness test were conducted to determine material constants used in the plastic damage criteria. The first order solid elements were used with an element size of about 100 μm. The FE mesh is shown in Fig. 4(a). The determined constants, α and β are 2.35 and 0.015, respectively. Simulated results of J-R curves are compared with experimental data in Fig. 5. Fig. 5 shows the comparison of experimental J-R curve with simulation results of various critical damage factor. It is shown in Fig. 5 that the FE results with element size of 0.1 mm has critical damage factor of 1.3. Creep crack growth simulations were performed using only creep damage FE analysis and elastic-plastic creep FE analysis. Three-dimensional (3-D) FE analysis were performed using eight node brick elements with full integrations (element type C3D8). Noting that the average grain sizes of P91 are in the range of 50μm to 100μm. Hence, the element size of 100 μm is chosen. Considering symmetry conditions, a quarter model was used. Side grooves in each side were explicitly modelled. Typical FE meshes for C(T) specimen is shown in Fig. 4(b). In this work the 5.0kN applied load cases were compared. In FE analyses, true stress-strain data for plastic properties, shown in Fig. 1, were directly given in the FE analysis. For creep, Eqn. (6) was implemented using the CREEP user subroutine with the strain hardening rule. The large geometry change option was used. 4.3. Simulation Results Simulated load line displacement and crack growth with time are compared with experimental results in Fig. 6. For simulations, two results are shown; one using the only creep damage model and the other using the plastic-creep combined damage model. Simulated results with combined damage model are overall close to experimental load line displacement and crack growth data. Conversely, simulations with only creep damage model give quite conservative results. In the creep damage model the damage was calculated from only creep strain. However, in the combined plastic-creep damage model damages was calculated from both plastic strain and creep strain. Therefore the combined damage model gives similar analysis results with the experimental data. To determine the plastic