Ninshu Ma

Predicting welding deformation in thin plate panel structure by means of inherent strain and interface element

Abstract In this study, welding distortion in a large thin plate panel structure was predicted by means of elastic finite element method based on inherent strain theory and interface element formulation. The welding distortions in the thin plate model computed by large deformation theory and small deformation theory were compared. The comparison suggests that the geometrical non-linearity should be carefully considered when welding distortion in a thin plate structure is predicted. In addition, the influences of welding procedure and assembly sequence on the final distortion were examined numerically. Simulation results indicate that both welding procedure and assembly sequence significantly affect the final deformation.

Iterative substructure method employing concept of inherent strain for large-scale welding problems

When structures such as ships, automobiles, and bridges are assembled by welding, distortion and residual stress are produced as unavoidable consequence of local shrinkage due to welding. The dimensional error deteriorates the performance of the structures and becomes an obstacle to achieve smooth manufacturing if the error exceeds the tolerable limit. On the other hand, residual stress plays an important role in crack initiation and fatigue life. Thus, it is necessary to predict the welding distortion and stress beforehand, so that effective measure and control can be taken. Since the welding is a highly nonlinear problem, it is difficult to predict the distortion quantitatively. For accurate prediction, finite element analysis (FEA) can be a powerful tool. In this research, an enhanced FEA scheme namely i-ISM is developed based on the inherent strain concept and iterative substructure method (ISM). Its capability of solving large-scale practical problems is demonstrated through typical models.

Microstructural Characteristics and Mechanical Properties of Friction Stir Welded Thick 5083 Aluminum Alloy

Joining thick sections of aluminum alloys by friction stir welding (FSW) in a single pass needs to overcome many challenges before it comes to full-scale industrial use. Important parameters controlling the structure-properties relationships both across weld cross-section and through thickness direction were investigated through mechanical testing, electron backscatter diffraction technique, transmission electron microscopy, and occurrence of serrated plastic flow. The evolution of the properties in the weld cross-section shows that the presence of undissolved and fragmented Al

Simulation of cracking phenomena during laser brazing of ceramics and cemented carbide

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Simulation Procedures for Welding Heat Conduction, Welding Deformation, and Residual Stresses Using the FEM Programs Provided on the Companion Website

6MnFe particles cause discrepancies in establishing the Hall-Petch relationship, and derive the strengthening from the Orowan strengthening mechanism. A ‘stop action’ friction stir weld has been prepared to understand the role of geometrical features of the tool probe in the development of the final microstructure after complete weld. Sectioning through the ‘stop action’ weld with the probe in situ displays the individual effect of thread and flat on the grain structure formation. The material at the thread surface experiences more severe deformation than the material at flat surface. Both the high-angle boundaries and mean grain size are found to be higher at the thread surface. The strain hardening capacity, stress serration amplitude, and frequency are observed to be higher in the stir zone than other weld regions.

Strategic Simulation Analyses for Manufacturing Problems Related to Welding

This study aims to ensure the safety of nuclear power plants. The accidents involving leaks from the welded zones at the pipe penetration part of a reactor vessel or at a coolant pipe are reported at home and abroad. One of the main causes is the welding residual stress. So, it is important to know the welding residual stress for maintaining high safety of the plants, the estimation of plants life cycle and the plan of maintenance. The welded joints of the nuclear power plants have complex shapes, and the welding residual stresses also have complex distributions three-dimensionally. In this study, the inherent strain method combined with finite element method is used to measure the welding residual stresses accurately. The mock-up is idealized for the welded joint at the pipe penetration part of the actual reactor vessel. The inherent strain method is applied to measure the residual stresses. In this method, the inherent strains are unknowns. When the residual stresses are distributed complexly in a three-dimensional stress-state, the number of unknowns becomes very large. So, the inherent strains are expressed with some functions to decrease the number largely. The theory, the experiment process and the analysed results are explained. The characteristics of the distributions of residual stresses and their production mechanisms are discussed. The inherent strain method gives the most probable values and the deviations of the residual stresses. The deviations are small enough for the most probable values. It assures the high reliability of the estimated results.

Q&A for FEM Programs

Abstract To investigate the mechanism of the occurrence of the microcracks produced in laser brazing of silicon carbide and cemented carbide using a silver–copper–titanium alloy as the filler metal, a numerical simulation for thermal cycle and thermal stress was performed using the finite element method. The laser-irradiated area of the cemented carbide was heated selectively, and almost uniform temperature distribution in the filler metal was obtained. The tensile thermal stress in the silicon carbide appeared near the interface between the silicon carbide and the filler metal, and the maximum principle stress at about 400 K during the cooling process reached the fracture strength of the silicon carbide. This resulted in the formation of microcracks in the silicon carbide which was observed in the experiment.

Introduction to Welding Mechanics

A computational approach based on the thermal elastic plastic finite element method was developed for predicting welding residual stress in low carbon alloyed steel welds by taking into account the effect of the solid-state phase transformations. The kinetics of phase transformations was described by Johnson Mehl Avrami Kolmogrov (JMAK) equation for bainitic transition and by Koistinen-Marburger (K-M) relationship for martensitic transition. Moreover, an additive rule depending on volumetric phase fraction was adopted to represent the material property changes during heating and cooling. Consequently, the residual welding stresses in a 2.25Cr1Mo steel TIG welded plate were computed. Early calculation results suggest that the bainitic and martensitic transformations took place in the weld the heat-affected zone drastically reduce the residual longitudinal tensile stress in the region.