Identification of Plastic Behaviour and Formability Limits of Aluminium Alloys at High Temperature
In order to simulate accurately the stamping process of sheet metals, their constitutive behaviour, as well as their formability limits, must be accurately evaluated. In this work, tensile and Nakazima tests are conducted at high temperature on aluminium
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Identification of Plastic Behaviour and Formability Limits of Aluminium Alloys at High Temperature G. Chiappini, L.M. Mattucci, M. El Mehtedi, and M. Sasso Abstract In order to simulate accurately the stamping process of sheet metals, their constitutive behaviour, as well as their formability limits, must be accurately evaluated. In this work, tensile and Nakazima tests are conducted at high temperature on aluminium alloys of the 5000 series. The tensile tests are mainly used to determine the flow stress curve of the material, while the Nakazima tests, are used to determine the admissible elongation before failure at different strain ratios. The raw load-displacement curves and the deformation of the samples, measured by the optical grid method, are included in an inverse FEM procedure to best identify the real elasto-plastic law of the material and its formability limits. Keywords Aluminium alloys • Grid method • Inverse method • Plastic behaviour • Formability
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Introduction
The hot-forming process has reached a preminent position in most of the mechanical fields, like automotive or aerospatial industry; as obvious, the trend is to obtain, even in mass production, finished products with the best quality characteristics as well a reduction of the waste due to breaking or surface defects or overcoming of the geometrical tolerance limits. In this scenario, the necessity to product semi-finished or finished parts with a low order of wasted material involved by process errors or surface defects, has determinated a concentration of sperimental, theoretical and numerical studies about the sheet metal forming. For a correct process simulation [1, 2] it is necessary to obtain the mechanical and formability characteristics of the material, in ordered to simulate even the most complex products, most of all for the barely workable materials. The deformation behaviour of aluminium alloys has been frequently object of researches to determinate the principal mechanisms involved. The easiest test to characterize the deformation behaviour of a material is the tensile one, however it is just valid in uni-axial tensile cases. To investigate more complex deformation processes, like the hot forming ones, it is necessary to lead a test where all the tensile directions, or at least two of them, are involved like the bi-axial ones. Among them, one of the most diffuse is the Nakazima test, which allows to analyze the material in different deformation conditions, through its own specimen having different width. In this job, a semi-automatical optical technique, has been used for the measure of the deformation, based on the measure of a grid previously printed on the specimen surface: the calculation of the spatial position of the grid intersections obtained by a stereoscopic couple of camera, three-dimensionally calibrated according to the Heikkila algorithm, allowed to reconstruct the 3D surface of the specimen. Then the right Cauchy-Green tensor has been created that allowed to obtain the logarithmic strain field. The results of t
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