A numerical model for induction heating processes coupling electromagnetism and thermomechanics

This paper presents a mathematical and numerical model developed for coupling the various physical phenomena (electromagnetic, thermal and mechanical) taking place in axisymmetrical induction heating processes. All three electromagnetic, thermal and mechanical models are time dependent and take full account of the electromagnetic and thermal non‐linear effect especially with magnetic materials. The electromagnetic problem is discretized and solved in the workpiece, air and inductors. The heat transfer equation and the mechanical equilibrium equations are solved in the workpiece only, both using a finite element method. The mechanical model can take into account thermoelastic–plastic behaviour for the part. The model has been successfully applied to several cases of induction heating. Comparisons between numerical and experimental results show an excellent agreement. Copyright © 2003 John Wiley & Sons, Ltd.     

Parametric study of riveted joints

As one of the most reliable fasteners, solid riveted joints are widely utilized in many industrial areas. In the present work, the authors recalled some results on the riveting process and the strength of one kind of riveted joints obtained by simulation and experimental investigations in a previous paper. The numerical results were in very good agreement with the experimental results, allowing us to validate our simulation approach and its use for further studies. We selected several engineering parameters for the riveted joint: initial assembly, friction coefficient, rivet’s geometry and sheets’ geometry, in order to carry out a parametric study and determine their relative importance. These were conducted in FEA software. The results showed the impact on riveting process and the strength of the riveted joint by varying each parameter which was interesting for the industry.     

Experimental and numerical studies on failure modes of riveted joints under tensile load

Though engineers are not advised to utilize riveted joints in tension, rivets will inevitably bear tensile load in realities. Hence, it is interesting to investigate the failure modes of riveted joints when they are under tensile load. Following previous studies (Chen et al., 2011), in this work, the authors selected three sizes of riveted joints to conduct the riveting and tension processes experimentally and numerically. Three kinds of failure modes including pull through, shank breaking, and head breaking were observed. Simulations are able to give almost the same results as those from experiments. Furthermore, three formulas were proposed to calculate the maximum tensile strength of riveted joints. Though the values calculated from these three formulas are approximate, they have the same order of magnitude as real ones. Moreover, they could be utilized to estimate which kind of failure mode may take place when riveted joints were under tensile load.     

Semianalytical models to predict the crystallization kinetics of thermoplastic fibrous composites

The growing interest for continuous fiber‐reinforced polymer composites leads to the development of new processes such as resin transfer molding for thermoplastics (RTM‐TP) or tape placement. In the aim of optimization, their simulations are required and have to include all involved physical phenomena and the associated couplings. During the consolidation step, the crystallization of the semicrystalline matrix occurs between the fibers of the multiscale reinforcement. A tricky task is to provide a realistic model able to describe the crystallization kinetics, which includes the effect of fibers on the polymer phase change and avoiding large computation time. In 2004, Haudin and Chenot proposed a generalization of the Avrami model, written in a differential form to compute the evolution of the crystallization of a neat thermoplastic in an infinite volume. In the present article, new extensions are proposed to predict the crystallization in long‐fiber thermoplastic composites, without or in the presence of transcrystallinity on fiber surfaces. In both cases, they are compared to three‐dimensional numerical simulations using a previously validated numerical method. All the numerical and analytical results are consistent. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44508.     

An implicit incrementally objective formulation for the solution of elastoplastic problems at finite strain by the F.E.M.

The mechanical formulation presented in this paper is based on an incremental updated Lagrange procedure using the principle of virtual work at the end of each load increment and an implicit incremental flow rule obtained by an approximate time integration of the objective rate constitutive equations. The approximate time integration is carried out along a particular path in the deformation and rotation space. This path ensures the incremental objectivity and minimizes the equivalent strain over the increment among all the possible paths, consequently avoiding an artificial increase of the plastic equivalent strain during the interpolation. The mechanical formulation presented leads to a fixed set of nonlinear equations, whose unknowns are the nodal displacements of the structure. A numerical algorithm based on a quasi Newton-Raphson method is then proposed to solve this system. The separation of the mechanical formulation from the resolution algorithm ensures the path independence. Numerical tests are carried out for a material obeying an isotropic with work-hardening von Mises criterion and associated flow rule. Single element tests show that this approach gives a very accurate solution even when the strain increment reaches twenty times the elastic strain up to yield. A structural test on a beam measures the influence of the incremental objectivity on the displacements, the equivalent plastic strain and the stresses.     

A plane-strain elastoplastic finite-element model for cold rolling of thin strip

In cold rolling of thin strip, elastic roll deformation is a prominent phenomenon which may indeed govern the whole process. Analysis of the literature suggests a number of methods to solve this coupled problem; for the most severe operations, the coupling technique is more important than the precision of the computation of stress and strain. To perform as general an analysis as possible, a completely coupled finite element model is formulated, meshing a global strip-roll system with internal interface with sliding and friction. The model is two-dimensional and only analyzes roll flattening. The basic equations and numerical formulation are described. Application to several kinds of rolling passes is examined (temper rolling, thin foil rolling) with special emphasis on roll deformed shape and behaviour of metal in the roll gap (sliding/sticking zones, elastic/plastic zones).     

Nanoscale investigation of AlGaN/GaN-on-Si high electron mobility transistors

AlGaN/GaN HEMTs are devices which are strongly influenced by surface properties such as donor states, roughness or any kind of inhomogeneity. The electron gas is only a few nanometers away from the surface and the transistor forward and reverse currents are considerably affected by any variation of surface property within the atomic scale. Consequently, we have used the technique known as conductive AFM (CAFM) to perform electrical characterization at the nanoscale. The AlGaN/GaN HEMT ohmic (drain and source) and Schottky (gate) contacts were investigated by the CAFM technique. The estimated area of these highly conductive pillars (each of them of approximately 20-50?nm radius) represents around 5% of the total contact area. Analogously, the reverse leakage of the gate Schottky contact at the nanoscale seems to correlate somehow with the topography of the narrow AlGaN barrier regions producing larger currents.     

Numerical and experimental studies of the riveting process

As one of important mechanical joining methods, rivet joints are widely used in buildings, bridges, aircrafts and automotives and in many other fields. Many variables have influences on the response of the rivet joints, such as the geometry of the joints, the material parameters of the parts, the clearance of the assembly, etc. In this paper, a finite element numerical model is developed for the analysis and optimization of the riveting process. Our approach is three-dimensional in order to be able to model non axisymmetric situations of riveting and testing of the joint strength. Four different sizes of solid rivets are considered for simulation and experimental study of the process. The comparison of the results between the numerical simulations and the experiments allows us to validate our approach.     

A numerical application of the slip line field method to extrusion through conical dies

The industrial importance of cold extrusion is now well known. Unfortunately, the productivity of the process is restricted by the manifestation of a specific defect: the central burst, which is closely linked to ductility and to a depressive stress state in the core of material. An accurate knowledge of these facts is therefore necessary to the extension of the process. In this paper, we develop a slip line field model in order to calculate exactly the hydrostatic pressure on the axis of the system. Its peculiarity is that it can be associated with a mechanical test of material ductility. This method is developed for axisymmetric extrusion through conical dies, with a perfectly plastic material, obeying Tresca's yield criterion with its associated flow rule and with the Haar-Kàrmàn hypothesis. We assume that the pressure has a linear distribution along the die, which makes it possible to construct the slip line field. We then derive the velocity field, and determine by trial and error the pressure gradient along the die so that the velocity field satisfies all the boundary conditions. We apply this method to several extrusion ratios and die-angles; we compute the stress field in the deformed regions, obtaining an upper-bound for the extrusion pressure and a value for the hydrostatic pressure on the axis. Comparisons with experimental results show quite a good agreement between theoretical and experimental values of the extrusion pressure. The method will enable us in a future work to introduce friction along the die, metal work hardening and different profiles of the die.