Key Engineering Materials Vols. 353-358

Abstract: Owing to localized heating by the welding process and subsequent rapid cooling, the residual stresses can arise in the weld itself and in the base metals. The prediction procedures of the residual stresses in welding process were presented by using finite element techniques. The bilinear elastic-plastic material model based on Von Mises yield criterion was developed. The material non-linearity of weldment and welding fluid was dealt with using an incremental technique. Inside each step, the Newton-Raphson iteration method was utilized. A fully coupled thermo-mechanical two-dimensional analysis was performed with finite element method. The model applied in this study adopts the technique of element birth and death to simulate the weld filler variation with time in multi-pass welded joints. The effects of welding speed on residual stresses are discussed.

Abstract: This paper investigates the mechanical performance deterioration of aluminum alloy welded joints under thermal cycling condition. The real-time temperature, load and strain of the specimens are monitored during the test. Results show that a long time thermal cycling process can cause a performance decrease in its plasticity and fracture strength.

Abstract: The stainless steel SUS316 grain growth process in heat affected zone (HAZ) of gas tungsten arc welding (GTAW) process is studied with Monte Carlo (MC) simulation. The heat transfer and fluid flow model provides the thermal history and thermal distribution of the weldments for MC grain growth simulation. The grain growth evolution is simulated both in isothermal and in HAZ environment. The simulating results show clearly the “thermal pinning” effect on the grain growth evolution in HAZ compared with the isothermal results.

Abstract: The leather is cut by hand or by hand with some help of semiautomatic machine in general leather cutting technology. In order to enhance the cutting efficiency and quality, the machine tool to realize high speed automatic leather cutting was developed. The final goal of the design is to cut a piece of hide on a working table with cutting velocity 2 meters per second, the dimension of the machine tool is big. It is very difficult to design the frame of this machine tool to meet the performance index for high speed cutting, so the static and dynamic force analysis to the body of it was implemented. First, the digital model of the body of it was built with AutoCAD software, then, the digital model in IGES format was transmitted to the Ansys software and the finite element analysis to it were carried out. Because the static force and vibration from driving system have great influence on the deformation of this machine tool and this deformation will affect the cutting precision and the whole performance of it. So the static force analysis and modal analysis were carried out. Finally, the results of finite element analysis were discussed, corresponding modification to the design was proposed. The analysis results indicate the mechanical property of the body of the developed machine can meet the requirements for high speed cutting.

Abstract: A direct metal RP (rapid prototyping) process based on micro-plasma arc welding (MPAW) is presented. The impact of R (ratio of width to height of the deposited track’s cross-section) on part quality is investigated. Taguchi method is adopted to analyze the effect of each process parameter on R, and the optimized process parameters are obtained. The results show that the quality of the parts with larger R is better than that with smaller R, and the peak current, duty cycle of pulse duration, wire-feeding speed, scanning speed and plasma gas flow rate all exert significant effects on R. The overlapped surface smoothness, tensile strength and elongation of the parts fabricated with optimized parameters are measured to show obviously better performances than those of the parts fabricated with ordinary process parameters.

Abstract: Thermal barrier coatings (TBCs), that reduce the temperature in the underlying substrate material, are an essential requirement for the hot section components of industrial gas turbines. Recently, in order to take full advantage of the potential of the TBC systems, experimental and analytical investigations in TBC systems have been performed. However there is a little information on the deformation behavior of the top coating. In addition, the effects of the thermal exposure and the process parameters on the mechanical properties of the top coating have never been clarified. From these backgrounds, the effects of the process variables in APS and the thermal exposure on the mechanical properties were investigated in order to optimize the APS process of top coatings. The experimental results indicated that the mechanical properties of the APS-TBC, i.e. the tensile strength and the elastic modulus, were significantly changed by the process variables and the long term thermal exposure. The microstructural investigation was also carried out and the relationship between the mechanical properties and the porosity was discussed.

Abstract: Distributions of residual stress in diffusion bonding of dissimilar materials intermetallics TiAl to steel 40Cr were simulated by FEM calculation. Results showed that destructive residual stresses presented in the minute area adjacent to bond-line of the base material with the smaller thermal expansion coefficient. Reducing bonding temperature and diminishing bonding time are in favor of the mollification of interface tresses. Concepts of the residual stress factor Rf and the thickness factor Tf were propounded. As selecting the interlayer, the interlayer with smaller |Rf| and Tf should be given precedence, and with a prerequisite of sufficient physical contact small thickness of the interlayer are finer.

Abstract: An interfacial resistance measuring method was proposed to evaluate the quality of diffusion bonded joints. To study the feasibility of the method, a finite element (FE) model with periodically distributed interfacial microdefects was presented. The influences of bonded area fraction, width and length of microdefects on increments of resistance were analyzed by applying the electromagnetic module in ANSYS software. Results indicated that the evolution of resistance increment along with the interfacial bonded area fraction could be described by a hyperbolic function. Based on the FE results, a modified theoretical solution was developed referred to Lodge’s work to accommodate the size effects of interfacial microdefect and thus could lead to a more precise quality evaluation for diffusion bonded joints.

Abstract: Load response and failure modes of three-dimensional (3-d) four-directional braided composite lugs were studied analytically and experimentally. The objective of the study was to get information on the stiffness, strength and failure mode of the lug, as well as on the applicability of the analysis method used to predict lug load response and failure. The test lugs were manufactured with the RTM (Resin Transfer Molding) technique. The test specimens were loaded parallel to the lug centerline. Two types of specimens were tested to failure. Three of them were instrumented with 18 strain gages in each type of lug. There are three basic failure modes in braided composite joints: net-tension, shear-out, and bearing. Net-tension failure is associated with matrix and fiber tension failure due to stress concentrations. Shear-out and bearing failures result primarily from the shear and compression failures of fiber and matrix. The analyses were performed using finite element method. Shell elements were used. A steel pin was modeled to apply the loading. The loading was applied with a constant force distribution through the center of the pin. A contact was defined between the pin and the surrounding lug surface. The measured strains showed fairly good correlation with the analysis results. The strain response was almost linear. It can be concluded that with correct material properties the FE approach used in the analyses can provide a reasonable estimate for the load response and failure of 3-d braided composite lugs

Abstract: For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.