13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- A quarter symmetry model was built for computation of local stresses and strains. Due to material difference between base metal and weldment, strains and stresses can be discontinuous on the fusion boundary. Maximum effective plastic strain at the 1250MPa nominal loading stress appears in the center of the weldment, while the maximum Mises stress on the maximal loading is localized on the fusion boundary. This coincides with the test, in which the plastic deformation occurs on the weldment. A comparison of nominal stress-strain curves between FEM simulation results with the experiment was carried out in Fig. 6. Calculated nominal strain was determined based on the initial gauge length of 12mm. A good correlation was obtained. The elastic moduli from base material, weldment material are nearly the same as the nominal behavior. Due to different yield stress of welding material, the nominal yield tensile stress is lower than base material. Yield will first occur in the weldment. Figure 6. Comparison of tensile stress-strain curves. 4. Fatigue Life Prediction 4.1 Extended Cruse-Meyer model The Cruse-Meyer model [15] includes both cyclic strain range Δε and mean stress σm as variable for predicting fatigue life in an explicit power law form for the uniaxial loading, m B C f N A σ ε10 Δ = , (2) where A, B, C are specific model parameters, σm is the mean stress, Nf is the number of cycles to failure, Δε is the strain amplitude. The Cruse-Meyer model is simple and practical for engineering application due to its explicit expression form. The model can be extended to multiaxial fatigue by combining it with the critical plane concept. This critical plane concept postulates that fatigue cracks initiate and grow on certain planes where the material is mostly damaged [16]. Taking the concept of the Cruse-Meyer model, material failure under stress multiaxial state is described by the maximum normal strain range and the mean normal
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