Numerical Analysis and Verification of Residual Stress in T Joint of S355 Steel

T joint is a widely used welding form. The welding deformation and residual stress produced during the welding process will affect the integrity and reliability of the structure. In this study, S355 low alloy steel was used as test material, and the thermal-mechanical coupling characteristics of multi-layer welding were combined with SYSWELD finite element software to calculate the residual stress of T joint after welding. The residual stress of multi-layer welding T joint with tangent tube and sheet after welding was measured by X-ray diffraction method. The results showed that the numerical simulation agreed well with the experimental results. For the transverse residual stress, the maximum residual stress appeared near the weld toe, and the transverse stress perpendicular to the weld direction presented tensile stress at the weld center and presented stress at the tube surface far from the weld. For the longitudinal residual stress, the maximum residual stress also appeared near the weld toe, and the value was the largest at the center of the weld and decreased along the direction perpendicular to the weld. The research results can provide a reference for actual welding design

Junction Temperature Consistency Analysis of MMC Submodule

Although modular multilevel converter（MMC）is currently widely used in the field of DC power transmission due to its excellent topology performance, the natural DC bias characteristics inevitably cause thermal imbalance of internal devices. Too high temperature at the junction of power devices is one of the major causes of damage. Therefore, it is necessary to further investigate the factors that affect the device junction temperature. This paper calculated the power loss and junction temperature by combined the thermal impedance model and compared junction temperature under two typical modulation strategies

Structure modal optimization of a strapdown inertial navigation system for an electric helicopter using an adaptive surrogate model

The purpose of this research is to prove the eventuality of using a novel adaptive surrogate model for optimization problems. The adaptive surrogate model is based on iteration sampling and extended radial basis function (ERBF). This method improves the precision by a means that new sample points is placed in the blank area and all the sample points is uniformly distributed in the design region. The precision of the surrogate model is checked using standard error measure to determine whether updating the surrogate model or not. Since the prediction of modal frequencies require structure modal simulations. In order to decrease the number of computer simulations, a Multi-Island GA approach is combined with the adaptive surrogate model to find the optimum modal frequencies of a strapdown inertial navigation system for electric helicopters. The strapdown inertial navigation system is comprised of damping material, counterweight material and inertial navigation sensor. This is a multi-objective functions optimization problem since the modal frequencies are considered from mode 1 to mode 6 in this paper. Several weights of multi-objective functions are utilized to research the modal frequencies. The whole number of 15 sampling points is picked to build the primary surrogate model using Latin hypercube sampling (LHS). The results of adaptive surrogate model show that two new sampling points are needed to reform the precision of the surrogate model under the condition of 2 % confidence bounds. The structure modal optimization results show that the choice of the weights for the multi-objective functions has a major effect on the final optimum modal frequencies. Time- and frequency-domain analysis indicated that the optimum modal frequencies are far away from the excitation frequencies to avoid strapdown inertial navigation system resonance as far as possible