13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- A Ramberg-Osgood formulation was adopted to simulate cyclic stress strain relationship, and a time hardening creep constitutive model was used to characterize creep behavior. The detailed material properties adopted are given in Table 1. In this creep fatigue damage assessment, the LMM was used to evaluate a steady-state cyclic behavior and to construct a saturated hysteresis loop. Then obtained total strain range during the cycle was used to assess fatigue damage combining R66 fatigue endurance curves [1]. The evaluated creep strain and stress relaxation data were adopted to evaluate creep damage considering time fraction rule and using the experimental creep rupture data. The final lifetime of the cruciform weldment was then obtained based on the calculated fatigue and creep damage under creep-fatigue interaction conditions. Table 1. Material constants in Ramberg-Osgood model (Eq. 9) and creep time hardening model (Eq. 13) Zone E(MPa)B(MPa) β σy(MPa) A(MPa -n/h) n m Parent 1600001741.96 0.2996 270.7 6.604e-19 5.769 -0.55 Weld 122000 578.99 0.1016 307.9 6.597e-23 7.596 -0.5 HAZ 1540001632.31 0.2530 338.7 6.6e-21 6.683 -0.525 3.2. Numerical results and verifications Figure 4. Results of creep-fatigue assessment in application to Type 2 cruciform weldment with experimental comparisons [11, 12] The creep fatigue lifetime of Type 2 cruciform weldment assessed by the LMM and its experimental comparisons [11, 12] are presented in Fig. 4. Visual comparison of the observed and predicted number of cycles to failure in Fig. 4 for 3 variants of dwell period Δt (Δt=0 for pure fatigue) shows that 9 of the 11 simulations accurately predict the experimental results. Therefore, it can be used for the formulation of an analytic assessment model suitable for the fast estimation of lifetime for a variety of loading conditions. The low computational effort required by the LMM compared to other computational techniques makes it possible and relatively easy to extrapolate numerical predictions for loading conditions not captured by the available experiments. The contour plots of LMM solutions including total strain range, creep strain, creep stresses at the beginning and end of dwell period, and the outputs of creep fatigue assessment procedure including
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