Thermal Elastic-plastic Finite Element Research Articles

Characteristics of High Temperature Tensile Properties and Residual Stresses in Welded Joints of the SM570-TMCP Steel

TMCP steels produced by thermo-mechanical control process are now spot lighted due to the excellent combinations of strength, toughness and weldability. Recently, in Korea, high strength SM570-TMCP steel whose tensile strength is 600MPa has been developed and applied to steel structures due to its excellent nature. But, for the application of the TMCP steel to steel structures, it is necessary to elucidate not only the material characteristics but the mechanical characteristics of welded joints. In this study, high temperature tensile properties of the SM570-TMCP steel were investigated through the elevated temperature tensile test and the characteristics of residual stresses in welded joints of the TMCP steel were studied through the three-dimensional (3-D) thermal elastic-plastic finite element (FE) analysis on the basis of mechanical properties at high temperatures obtained from the experiment. The results are then compared with the conventional quenched and tempered high strength SM570 steel.

Prediction of residual stresses in welds of similar and dissimilar steel weldments

The fabrication of structural member using dissimilar steels renders steel structures lighter and more economical. However, it always involves welding process and produces different residual stresses in welds as compared with welding of similar steels. This paper presents the characteristics of residual stresses in welds of similar and dissimilar steel weldments by carrying out three-dimensional (3-D) thermal elastic-plastic finite element (FE) analysis. The materials used in this investigation were SM400, SM490, SM520 and SM570, widely used structural steels in welded structure. Results show that the maximum longitudinal residual stresses in welds of the similar steel weldments increase with increasing yield stress of the steel welded (SM400 < SM490 < SM520 < SM570). In case of the dissimilar steel weldments, the difference between the longitudinal residual stresses in welds increases with increasing yield stress of the steel welded together with SM400 (SM490 < SM520 < SM570).

Numerical simulation of welding distortion in large structures

Welding is a crucial manufacturing process and widely used in industries to assemble various products including ships, automobiles, trains and bridges. Welding distortion often results in problems such as dimensional inaccuracies during the assembly and increased fabrication costs. Therefore, prediction and reduction of welding distortion are critical to improve the quality of welded structures. Welding distortion during the assembly process is affected by not only local shrinkage due to rapid heating and cooling, but also root gap and misalignment between parts to be welded. In this study, based on inherent strain theory, an elastic finite element method is developed to precisely predict welding distortion during the assembly process considering both local shrinkage and root gap. First, thermal elastic–plastic finite element method is employed to estimate inherent deformations for different typical welding joints. Second, the proposed elastic FEM is used to predict welding distortion for large welded structures based on the obtained inherent deformations. Meanwhile, the influence of initial gap on welding distortion is investigated. Finally, experiments are carried out to verify the simulated results. The effectiveness of the proposed elastic FEM is confirmed using experimental results.

Numerical analysis of residual stresses in welds of similar or dissimilar steel weldments under superimposed tensile loads

This paper presented the characteristics of residual stresses in welds of similar or dissimilar steel weldments by carrying out three-dimensional (3-D) thermal elastic–plastic finite element (FE) analyses. Moreover, residual stress behavior in welds of the weldments under superimposed tensile loads was further investigated. The materials used in this investigation were SM400, SM490, SM520 and SM570, widely used structural steels in welded structure. Results show that the maximum longitudinal residual stresses in welds of the similar steel weldments increase with increasing yield stress of the steel welded (SM400 < SM490 < SM520 < SM570). When tensile loads are superimposed to edges of the weldments, the maximum longitudinal residual stresses are increased to a higher value. In case of the dissimilar steel weldments, the difference between the longitudinal residual stresses in welds increases with increasing yield stress of the steel welded together with SM400 (SM490 < SM520 < SM570). When tensile loads are superimposed to edges of the weldments, same pattern is noted for the longitudinal stresses in welds and the difference is almost the same as that between the longitudinal residual stresses in welds.

Determination of welding deformation in fillet-welded joint by means of numerical simulation and comparison with experimental measurements

Fillet weld is the most common weld type used in the fabrication of structural members in shipbuilding, automobile and other industries. Fillet-welded joints usually suffer from various welding deformation patterns, such as longitudinal shrinkage, transverse shrinkage, angular distortion and longitudinal bending. Welding deformation has negative effects on fabrication accuracy, external appearance and various strengths of the welded structures. In this study, experiments are performed to investigate the characteristics of welding deformation in the fillet-welded joint. In order to precisely predict welding deformation by numerical method, a 3-D thermal elastic plastic finite element computational procedure is developed. The simulated results are in a good agreement with the experimental measurements. The influence on welding deformation of the flange thickness is investigated by experiment and numerical simulation. In addition, the generation mechanism of angular distortion is clarified through numerical simulation.

Elastic–plastic modeling of heat-treated bimorph micro-cantilevers

This paper models the residual stress distributions within micro-fabricated bimorph cantilevers of varying thickness. A contact model is introduced to calculate the influence of contact on the residual stress following a heat treatment process. An analytical modeling approach is adopted to characterize bimorph cantilevers composed of thin Au films deposited on thick poly-silicon or silicon-dioxide beams. A thermal elastic–plastic finite element model (FEM) is utilized to calculate the residual stress distribution across the cantilever cross-section and to determine the beam tip deflection following heat treatment. The influences of the beam material and thickness on the residual stress distribution and tip deflections are thoroughly investigated. The numerical results indicate that a larger beam thickness leads to a greater residual stress difference at the interface between the beam and the film. The residual stress established in the poly-silicon cantilever is greater than that induced in the silicon-dioxide cantilever. The results confirm the ability of the developed thermal elastic–plastic finite element contact model to predict the residual stress distributions within micro-fabricated cantilever structures with high accuracy. As such, the proposed model makes a valuable contribution to the development of micro-cantilevers for sensor and actuator applications.

Thermal Contact Residual Stress Analysis of Elastic-Plastic Bilayer Micro-Cantilevers with Platinum Electrodes

This paper studies the residual stress distributions and tip deflections of microfabricated bilayer cantilevers of varying beam thickness and platinum electrode length. The bilayer cantilevers discussed here are composed of low-stress silicon nitride films deposited on silicon beams. Platinum electrodes are deposited and patterned on the low-stress silicon nitride layers. A thermal elastic-plastic finite element model is utilized to calculate the residual stress distribution across the cantilever cross-section and to determine the cantilever tip deflection following heat treatment. A contact model is introduced to simulate the influence of contact on the residual stress distribution. The influences of the beam thickness and the platinum electrode length on the residual stress distribution and tip deflections are thoroughly investigated. The numerical results indicate that a smaller beam thickness leads to a larger compressive residual stress within the platinum electrode and delivers a larger tip deflection. The results also indicate that a larger platinum electrode length delivers a smaller tip deflection.

Effect of Strength Mismatch on Ductile Crack Initiation Behavior from Notch Root under Static Loading

It has been well known that ductile fractures of steels are accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the pre-notch root. In order to evaluate the stress/strain state in the pre-notch root of specimens, a thermal elastic-plastic finite element (FE) analysis has been carried out.

Prediction of welding residual stress in multi-pass butt-welded modified 9Cr–1Mo steel pipe considering phase transformation effects

The objective of this paper is to investigate the influence of solid-state phase transformation on the evolution of residual stress distributions in butt-welded modified 9Cr–1Mo steel pipes. A thermal elastic plastic finite element model taking into account the metallurgical phase transformation was developed. Effects on welding residual stress of the volumetric change and the yield strength change due to austenite–martensite transformation were investigated by means of numerical analysis. The simulated results show that the volumetric change and the yield strength change due to martensite transformation have influences on the welding residual stress. The former not only changes the magnitude of residual stress, but also alters the sign of residual stress in the weld zone; and the later only changes the magnitude of residual stress. In the cases in which the volumetric change due to phase transformation is considered, the simulated results are generally in good agreement with the experimental measurements.

Finite element modeling of laser bending of pre-loaded sheet metals

A new laser bending process of sheet metals with applied pre-stresses is put forward. Two different kinds of loading models are analyzed. The desired pre-stress distribution in the heated zone of sheet metal is obtained. A three-dimensional thermal–mechanical model is established. Using the thermal elastic–plastic finite element method (FEM), the laser bending process is simulated numerically and its forming mechanism is analyzed. The deformation of the sheet depends on the integration of the thermal stress and the pre-stress when the laser beam irradiates the sheet. The forming performance can be improved significantly under pre-stress, and the deformation direction can also be controlled easily by changing the direction and value of the pre-loading. The bending angles of the sheet metals increase remarkably with the increase of the pre-loading, and both are almost in exponential relationship.

溶接継手の継手性能と溶接熱サイクルの関係に関する研究 ２ 建築鉄骨柱梁溶接部の組織および硬さに及ぼす積層手順の影響に関する解析的検討

Numerical simulation of temperature, mictostructure, and hardness is performed in multi-pass welded joints of beam-to-column connections of steel framed structure. The effects of temperature history on microstructure and hardness are investigated in this paper. Three-dimensional thermal elastic-plastic finite element analysis is performed by using the moving heat source and considering phase transformation effect. The relation between cooling rate and Vickers hardness in the weld metal is pigeonholed from the result of the simulation. Elapsed cooling time t8/5 is effective to evaluate the hardness in the weld metal. Vickers hardness in heat-affected zone is also computed and well explained from heat cycles. It is found that tensile strength of the weld metal and the heat-affected zone might be predicted by numerical simulation.

Fatigue crack initiation point in load carrying fillet-welded cruciform joints

Summary Load carrying fillet-welded cruciform joints have two fatigue crack initiation points. They are located in the weld toes and weld roots where high stress concentrations occur. Which of the two serves as the initiation point depends on the weld leg length, plate thickness, and weld penetration, and the mechanism concerned is easily understood. Some experimental results indicate that the fatigue crack initiation point is also affected by the magnitude of the stress range. The reason for this phenomenon remains rather obscure. To establish this reason, the present research study is intended to clarify the relationship between the stress range level and fatigue crack initiation point changes and to quantify the effects of the weld leg length and penetration depth on the fatigue crack initiation point. Fatigue tests of welded joint specimens, thermal elastic-plastic finite element method (FEM) analyses of welding residual stresses and fatigue crack propagation analyses considering the residual stress are performed.

固有ひずみ分布の関数表示による残留応力の推定法と測定法 （第４報） すみ肉溶接における固有ひずみの関数表示とその精度

In this paper, three dimensional residual stresses due to single pass and multipass fillet welding are analysed by thermal elastic-plastic Finite Element Method. Then, inherent strain distributions produced in fillet welds are expressed by series function with good accuracy. Based on the results of series function, a very simple distribution pattern of inherent strains is proposed and described as functions which only has a few unknown parameters. To determine these parameters by experiments, required measurements will be greatly reduced.

耐熱性ファインセラミックスの接合に関する研究 ＩＶ Ｓｉ３Ｎ４と金属接合継手における残留応力に関する検討

Residual stress occurred in Si3N4 to metals joints were analyzed by thermal elastic-plastic finite element method. In order to investigate the effect of the bonded material to Si3N4 on residual stress, numerical analysis were conducted on the condition simulating Si3N4-Si3N4, W, Mo, Ta, Nb or Cr-Mo steel joints with the insert layer. Maximum principal stress occurred in Si3N4 was lowering in the order of Cr-Mo steel, Nb, Ta, Mo, W, Si3N4. Maximum principal stress in Si3N4 of Si3N4 to Cr-Mo steel joints decreased with increasing the insert layer thickess in the range up to about 1.0 mm thick. As diameter of specimen increased, maximum principal stress in Si3N4 increased. Maximum principal stress in Si3N4 of Si3N4 to Cr-Mo steel joints was reduced irrespective of diameter of specimen with an increase in the thickness of W being used as the interlayer.