PSI - Issue 13

A. Prokhorov et al. / Procedia Structural Integrity 13 (2018) 1521–1526 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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The dissimilar welding was carried out Laser Welding. Welded sample dimension is 125mmX 120mmX 2mm. Laser used for welding is Nd:YAG solid-state laser with o perating wavelength of 1.06μm. A 2kW continuous average power Nd:YAG laser (Lumonics MW 2000) was used in the experiments. The temperature monitoring during the tensile test was conducted by the infrared camera CEDIP 420 with resolution 320 x 256 pixels and temperature sensitivity 25 mK on temperature 30 °C. The temperature evolution during the tensile test of P91 SS316LN specimen is presented in figure 2. As it can be seen from the figure 2, initial localization of a heat source is on the 316LN part of the specimen. During the test, the thermal wave is propagating and the heat source moves to the P91 part. The fracture of the specimen is observed on the P91 part and not in the welded joint as it has been expected. To simulate and explain this phenomenon, the mathematical model has been proposed.

Fig. 2. Evolution of a surface temperature (K) during the quasi-static loading of P91-SS316LN specimen. Experimental results (upper part of the specimen is SS316 LN steel)

3. Numeric Simulation

Numerical simulation of a tensile test was performed in the finite-element package Comsol Multiphysics under plane stress conditions. It is assumed that SS316LN and P91 materials are joined in the middle of the specimen. The model was meshed by quadratic elements with maximum and minimum mesh sizes of 0.00136 m and 2.72E-6 m respectively. Each material was considered as elasto-plastic with von Mises yield criterion and nonlinear isotropic hardening. Figure1(b) shows strain hardening curves for P91 steel and SS316LN steel in plastic strain – stress coor dinates. Simulation was carried out for two cases: under small strain assumption and taking into account large strains. 3.1. Small strains Quasistatic tension process obeys equilibrium equation (1):   σ 0 , (1) where σ - Cauchy stress tensor. In case of small strains, total strain tensor ε is defined as:   1 2 T    ε u u , (2) where u - displacement vector,  - nabla operator. Cauchy stress tensor is defined according to the isotropic Hook’s law with two elastic constants  and  :   1 2 I     e e σ ε E ε , (3) where E - unit tensor, 1 I - the first invariant (trace) of the tensor, e ε - elastic strain tensor. Under an assumption of