PSI - Issue 14

Bimal Das et al. / Procedia Structural Integrity 14 (2019) 619–626 Das et al./ Structural Integrity Procedia 00 (2018) 000 – 000

625

7

5. Finite element simulation results

Finite element simulation of the fatigue specimen of P91 steel has been performed using ABAQUS-6.17 software package. The above constitutive material model is implemented in the finite element software through user material subroutine (UMAT). The kinematic hardening rules are discretized using backward difference method. Radial return mapping algorithm is employed to integrate the cyclic plastic material model based on the work of Koboyashi et al. [14]. The maximum and minimum strain in the asymmetric strain cycling during the computational analysis is controlled by a user amplitude subroutine (UAMP). The materials parameters for the present models calibrated from the stabilized loop of the symmetric strain controlled test at strain amplitude of 0.3% as described by Bari et al. [15] are presented in Table 4.

Table 4. Material constants of P91 steel for Ohno-Wang model. Category Material Constant P91 steel

C 1 -C 12 = 202375, 133147, 371268, 83764, 399858, 121658, 115937, 50962, 47148, 45222, 41998, 47982 γ 1 - γ 12 = 2, 29, 20, 6, 30, 18, 25, 14, 17, 20, 24, 11

The capability of the material model to predict the influence of mean strain on the cyclic plastic deformation is assessed by the response of the mean stress with progress in cycles. The results of cyclic mean stress relaxation for both experiment and simulation are presented in Fig. 5. The nature of the mean stress response predicted is similar in nature. However the material model over predicted the mean stress in the initial cycles, but after 20 cycles the predictions are reasonably well.

Fig. 5. Mean stress variation with number of cycles for mean strain of 0.1% and strain amplitude of 0.3%.

6. Conclusions

The symmetric and asymmetric strain controlled behavior of P91 steel are evaluated with both experiment and cyclic plastic material modeling. Cyclic plastic deformations are studied by the analysis of hysteresis loops, softening curve and mean stress relaxation. Finite element analysis is performed to predict the mean stress relaxation