Finite element modeling of superplastic sheet forming processes. Identification of rheological and tribological parameters by inverse method

International audienceSuperplastic forming is a thermoforming-like process commonly applied to titanium and aluminum alloys at high temperature and in specific conditions. This paper presents the application of an inverse analysis technique to the identification of theological and tribological parameters. The method consists of two steps. First, two different kinds of forming tests have been carried out for rheological and tribological identification, using specific mold shapes. Accurate instrumentation and measurements have been done in order to feed an experimental database (values of appropriate observables). In a second step, the development of an inverse method has been carried out. It consists of the minimization of an objective function representative of the distance - in a least squares sense - between measured and calculated values of the observables. The algorithm, which is coupled with the finite element model FORGE2(R), is based on a Gauss-Newton method, including a sensitivity matrix calculated by the semi-analytical metho

Analysis of the blank holder force effect on the preforming process using a simple discrete approach

Simulation of the dry reinforcement preforming, first step of the Resin Transfer Moulding process, become necessary to determine the feasibility of the forming process, compute the fiber directions in the final composite component, and optimize process parameters during this step. Contrary to geometrical approaches, based on fishnet algorithms [1, 2], finite element methods can take into account the actual physical parameters, the real boundary conditions and the mechanical behaviour of the textile reinforcement [3, 4]. The fabric can be modelled either as continuum media with specific material behaviour [5, 6], or using discrete structural elements to describe the textile structure at the mesoscopic scale [7, 8]. A semi-discrete approach, which is a compromise between the above continuous and discrete approaches [9, 10], is also used for simulation. A discrete approach for the simulation of the preforming of dry woven reinforcement has been proposed and presented in a previous paper [11]. This model is based on a “unit cell” formulated with elastic isotropic shells coupled to axial connectors. The connectors, which replace bars or beams largely studied in other discrete approaches [12], reinforce the structure in the yarn directions and naturally capture the specific anisotropic behaviour of fabric. Shell elements are used to take into account the in-plane shear stiffness and to manage contact phenomena with the punch and die. The linear characteristic of the connectors [11], has been extended to a non linear behaviour in the present paper to better account for fabric undulation. Using this numerical model, we propose, in this work to study the effect of process parameters on the woven fabric deformation during the performing step. The emphasis will be placed on the analysis of the influence of the blank holder pressure on the shear angle distribution.This work has been undertaken within the framework of the Défi composite project. The authors would like to thank Oséo for its financial support, the project leader Airbus-France and other partners (EADS IW and LoireTech) for provided facilities

Cold forming by stretching of aeronautic sheet metal parts

http://dx.doi.org/10.1080/0951192X.2013.800641In this article, the development of an industrial prototype for manufacturing aeronautical fuselage panels is investigated. Deep drawing of large components such as aircraft fuselage panels is not an easy task in terms of dimensional accuracy, reliable material behaviour laws and failure criteria. Hot stretching processes ensure large ductility range of some materials. Nevertheless, when using high-performance aluminium alloys with acceptable low-plastic strain at ambient temperature, cold forming might be employed. A special stretching machine of 40-ton (400 kN) capability was instrumented and piloted in that way. Typical operations involved in the forming of parts are carried out with a die on which the sheet metal is successively stretched and drawn in several steps. Currently, the shape of the forming tool is directly determined from CAD models of the final sheet geometry without taking into account springback or residual effects. To increase the dimensional accuracy of the final components, a methodology to define the die shape and to control the process is proposed, taking into account the parameters influencing the forming operations. A feedback loop based on digitalised physical geometry and numerical simulation is carried out in order to ensure that the final shape of the sheet will be accurately obtained

Experimental study and superplastic rheological characterization of Ti-6Al-4V

The superplastic behaviour of a titanium alloy base Ti-6Al-4V during biaxial test was investigated. A numerical model using a finite element method is proposed to examine the superplastic deformation behaviour of an appropriate axisymmetric shaped part. An experimental procedure based on the measurement of the pressure applied and the height of the dome apex of the formed part, is used to identify the rheological parameters of the material behaviour law. This identification was carried out by comparisons between the results of simulation and experimentation relating to thickness, equivalent strain and stress. Three inflation tests were carried out at different equivalent strain rates (4×10-4, 6×10-4 and 8×10-4 s-1). This procedure is applied successfully to the forming of an industrial part. Indeed, this study henceforth allows one to predict thickness evolution in any point of an industrial part for an optimization of its uniformity

Increased caloric intake soon after exercise in cold water.

During the preforming stage of woven reinforcement, in the first step of the RTM process, frictional phenomenon occurring at the tool/reinforcement interfaces and the reinforcement/reinforcement interfaces is one of the key parameters of the forming process. This behaviour must be correctly taken into account when modelling the process and a better understanding of the contact and friction phenomena occurring during the woven fabric preforming process is necessary for realistic simulation of the preforming process. Although some existing studies concerning friction of reinforcement have been published, the complex frictional behaviour of fabrics is still not completely clear. The experimental characterization of the frictional behaviour of a specific carbon woven reinforcement (G1151) used for aeronautical applications, is the aim of this paper, and three interfaces have been studied (G1151/G1151, G1151/Plexiglas, G1151/Aluminium). The Coulomb coefficients of friction occurring during contact between two layers of fabric and between the fabric and other materials have been determined. The effect of the variation of the normal pressure and the temperature on the frictional behaviour of this reinforcement has also been analysed. Comparisons between several frictional models, described in the literature, are also conducted, associated with these experimental results. This study highlights a significant tribological anisotropy of the G1151 reinforcement and a dependence of the frictional characteristics on the applied pressure and the temperatureThis work has been undertaken within the framework of the Défi composite project. The authors would like to thank Oséo for its financial support, the project leader Airbus-France and other partners (EADS IW and LoireTech) for the facilities provided