Themo-elasto-viscoplastic modelling of friction stir welding

Abstract

A coupled two-dimensional Eulerian thermo-elasto-viscoplastic model has been developed for modelling the friction stir welding process. First, a coupled thermo-viscoplastic analysis is performed to determine the temperature distribution in the full domain and the incompressible material flow around the spinning tool. Next, an elasto-viscoplastic analysis is performed outside the viscoplastic region to compute the residual stress. Both frictional heat and plastic deformation heat generation are considered in the model. Furthermore, this is the only known model computing residual stress accounting for plasticity caused by both thermal expansion and mechanical deformation due to material spinning. The computed residual stress is verified by comparing to experimentally measured data.     

New technique to control welding buckling distortion and residual stress with non-contact electromagnetic impact

Abstract

A new technique using non-contact electromagnetic forces has been proposed for controlling welding buckling distortion and residual stresses in welded thin plates. The experimental results show that the method can successfully eliminate the buckling distortion and reduce the residual stresses. Three-dimensional finite element modelling has been developed to study the evolution of the stress and strain throughout the welding and electromagnetic impacts. The predicted welding distortion and residual stresses are in good agreement with the experimental results. The numerical analyses show that the reduction in distortion and stress is a result of the change of the plastic strain field in the weld region: electromagnetic impacts reduce longitudinal compressive plastic strain in the local region near the weld, and even produce the tensile plastic strain. Moreover, it is found that the residual stress can promote the changes of the longitudinal plastic strain state under electromagnetic impact.     

Residual stress and thermal cycling of planar solid oxide fuel cells

Abstract

The published literature relating to damage to planar solid oxide fuel cells caused by thermally induced stresses and thermal cycling is reviewed. This covers reported studies of thermal cycling performance and stresses induced by temperature gradients and differences in thermal expansion coefficients in typical planar SOFC configurations, namely electrolyte supported; anode supported and inert substrate supported cells. Generally good agreement is found between electrolyte residual stresses measured by X-ray diffraction or cell curvature and stresses calculated from simple thermo-elastic analysis. Finite element modelling of temperature distributions in cells and stacks in steady state operation are well advanced and capable of being extended to compute stress distributions. Failure criteria are then discussed for laminated cell structures based on critical energy release rate fracture mechanics models developed originally for coatings. However, in most cases the data required to apply the models quantitatively (such as elastic moduli of actual laminated material and fracture energies of materials and interfaces) are not available. Where data are available there are inconsistencies that require resolution. Seals are critical components in many planar solid oxide fuel cell configurations, but again there are discrepancies in experimental mechanical properties and the role of internal stresses in their fracture. In addition, there is as yet no firm evidence that thermal cycling damage involves any true materials fatigue process.     

Self-similarity phenomena of discharge jets in conventional slab mould

Abstract

Fluid flow in slab moulds is characterised through numerical simulations using the Reynolds stress model of turbulence. Fluid flow under steady state conditions indicates that the jet, from the submerged entry nozzle (SEN), is Reynolds number independent while the flow structure in the meniscus is Reynolds number dependent. These findings lead to the conclusion that there are strong self-similarity phenomena in the jet behaviour. Plots of dimensionless velocity of the jet core against dimensionless jet radius confirmed the authors' expectations. On the other hand, flow structure at the meniscus is conformed by zones with large velocity and vorticity gradients which are probably responsible for flux entrapment and generation of slivers in the slab. These simulations are helping to establish a general comprehensive theory of the turbulent flow in continuous casting moulds.     

Generation of compressive residual stresses by high speed water quenching

Abstract

According to the literature, rapid water quenching can create compressive residual stresses (RSs) near the surface and thereby produce a significant increase in the fatigue limit. This technique is called 'intensive quenching'. In the present paper, some results from a research project will be presented. The project was initiated by the technical committee 'quenching' of the German Heat Treatment Association to deal critically with this issue.

The focus of this paper is on the influence of the martensite transformation on RS generation. A process window was determined, within which one needs to quench fast enough, to get the surface temperature below the M_S temperature, before the maximum temperature gradient in a cylinder is reached, in order to get compressive RSs on the surface.     

Accumulation of stress in constrained assemblies: novel Satoh test configuration

Abstract

A common test used to study the response of a transforming material to external constraint is due to Satoh and involves the cooling of a rigidly constrained tensile specimen while monitoring the stress that accumulates. Such tests are currently common in the invention of welding alloys which on phase transformation lead to a reduction in residual stresses in the final assembly. The test suffers from the fact that the whole of the tensile specimen is not maintained at a uniform temperature, making it difficult to interpret the data. To eliminate this problem, the authors report here a novel Satoh test in which the material investigated is a part of a composite sample. It is demonstrated that this helps avoid some of the complications of the conventional tests and gives results which are consistent with independent tests.     

A novel mathematical procedure to evaluate the effects of surface topography on the interfacial state of stress. Part II. Verification of the method for scarf interfaces

A mathematical procedure was developed to utilize the complementary energy method, by minimization, in order to obtain an approximate analytical solution to the 3D stress distributions in bonded interfaces of dissimilar materials. The stress solutions obtained predict the stress jumps at the interfaces, which cannot be captured by the current FEA methods. As a novel method, the penalty function is used to enforce the displacement boundary conditions at the interfaces. Furthermore, the mathematical procedure developed enables the integration of different interfacial topographies into the solution procedure. In order to incorporate the effects of surface topography, the interface is expressed as a general surface in Cartesian coordinates, i.e. F(x, y, z) = 0. In this paper, the scarf interface problem, i.e. y = x/2 surface is considered for verification of the method by comparison with finite element analysis (FEA) results. Comparison of the results reveals our new mathematical procedure to be a promising and efficient method for optimizing interface topographies.     

Novel capacitance sensor design for measuring coating debonding: the effects of thermal-hygroscopic cycling

The effects of combined thermal and hygroscopic cycling on the adhesion performance of an epoxy coating were measured using a novel electrode sensor. The sensor is uniquely designed, consisting of a series of independent interdigitated electrode traces which are arranged parallel to the sensor edges. Coupled with single-frequency capacitance measurements, the sensor detects changes in capacitance in the adhered coating–sensor interfacial region as a function of the distance from the edge of the sensor, x. Recently, this sensor was utilized by O'Brien and co-workers to measure interfacial diffusion and the concentration profile of fluid in an adhesive joint (Int. J. Adhesion Adhesives 23, 335–338 (2003)). In the present work, large capacitance changes due to debonding and displacement of the coating by fluids at the sensor surface were used to monitor coating delamination. The apparent debond growth rate and number of cycles until failure were determined as a function of coating thickness, fluid environment and sensor surface chemistry. The results show that the coating becomes more durable as the thickness is reduced; and also that thermal and hygroscopic cycling of coatings produces different results than conventional continuous adhesion tests. This study suggests that this novel sensor or a similar design is applicable for the study of adhesion loss and interfacial diffusion processes, and could be extended to other coatings or adhesives in a variety of environments. General trends about coating durability are also discussed.     

Thermal residual stresses in an adhesively-bonded functionally graded single-lap joint

This study investigates three-dimensional thermal residual stresses occurring in an adhesively-bonded functionally graded single-lap joint subjected to a uniform cooling. The adherends are composed of a through-the-thickness functionally graded region between Al₂O₃ ceramic and Ni metal layers. Their mechanical properties were calculated using a power law for the volume fraction of the metal phase and a 3D layered finite element was implemented. In a free single-lap joint the normal stress σ_xx was dominant through the overlap region of the upper and lower adherends and along the adhesive free edges, whereas the transverse shear stress σ_xy concentrations appeared only along the free edges. The peel stress σ_yy and the transverse shear stress σ_xy became dominant along the free edges of the adhesive layer. In addition, the von Mises stress decreased uniformly through the adherend thickness from compressive in the top ceramic-rich layer to tensile in the bottom metal-rich layer. In addition, the layer number had only a minor effect on the through-the-thickness stress profiles after a layer number of 50, except for the peak stress values in the ceramic layer. In a single-lap joint fixed at two edges both adherends underwent considerable normal stress σ_xx concentrations varying from compressive in the top ceramic-rich layer to tensile in the bottom metal-rich layer along the free edges of both adherend–adhesive interfaces, whereas the peel stress σ_yy and transverse shear stress σ_xy reached peak levels along the left and right free edges of the adhesive layer. The layer number and the compositional gradient exponent had only minor effects on the through-the-thickness von Mises stress profiles but considerably affected the peak stress levels. The free edges of adhesive–adherend interfaces and the corresponding adherend regions are the most critical regions, and the adherend edge conditions play more important role in the critical adherend and adhesive stresses. Therefore, the first initiation of the joint failure can be expected along the left and right free edges of the upper and lower adherend–adhesive interfaces.