Evaluation of fracture limit in automotive aluminium alloy sheet forming

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Abstract

In finite element simulation of auto-body panels forming, an accurate forming limit criterion is very important. To evaluate accurately the formability of automotive aluminium alloy sheet, in this paper, a ductile fracture criterion developed by the author is introduced in finite element simulation. Based on Barlat’s yield function and Hollomon’s hardening equation, the simulations of aluminium alloy sheet forming are carried out. Material constants in the ductile fracture criterion are determined by means of uniaxial tension and hemispherical punch stretching tests. The critical punch strokes of the aluminium alloy sheets X611-T4, 6111-T4 and 5754-O in cylindrical complex forming, which includes deep drawing and stretching modes, are calculated by the ductile fracture criterion. The comparison of the calculated results with the experimental values shows that the critical punch strokes of the three sheets mentioned above are predicted successfully by the ductile fracture criterion.

Introduction

Aluminium alloy sheet is becoming one of the main materials to take the place of steel components to reduce the vehicle weight due to the advantage of low special weight, high strength and corrosion resistance [1], [2], [3]. However, the deformation characteristic of aluminium alloy sheet is different from that of conventional steel sheet in stamping forming. Fracture phenomenon is one of main obstacles affecting the successful forming. Accurate prediction of fracture initiation is very important to stamping working. Due to the defect of low ductility of aluminium alloy sheets, fracture often occurs before necking instability. Therefore, the conventional forming limit diagram based on the tensile instability or the bifurcation theories, for example, M–K theory [4] and S–R theory [5], which can predicted the forming limit of steel sheet well, is not exact to evaluate the formability of aluminium alloy sheet.

Nowadays, finite element simulation of auto-body panels forming becomes popular, and for correct analysis of forming process an appropriate forming limit criterion is required. In all fracture limit criteria, ductile fracture criterion has been proven to be an effective method to predict forming limit of aluminium alloy sheet [6], [7], [8]. When ductile fracture criterion is employed in engineering, the two key problems, including model of ductile fracture criterion and determination method of material constant in this criterion, is very important to engineers, because they directly affects the precision or the correctness of analysis result.

In the present paper, the formability of the automotive aluminium alloy sheets X611-T4, 6111-T4 and 5754-O in the complex forming, which includes deep drawing and stretching modes, is evaluated by means of the ductile fracture criterion developed by the author, and the calculated results are compared with the experimental results.

Section snippets

Materials and forming tests

The test materials are the aluminum alloy sheets X611-T4, 6111-T4, and 5754-O, with thickness of 1.25, 1.25, 1.5 mm, respectively. Table 1 shows the material properties of the test materials obtained from standard uniaxial tension test. The parameters in Table 1 are given with the average values in the directions of 0°, 45°, 90° to the rolling direction.

The cylindrical deep drawing and the hemispherical punch stretching tests are carried out. The dimensions of the tools in deep drawing test

Ductile fracture criterion

In this study, a criterion for ductile fracture, which is developed by the Yang and Yu [9], is employed to evaluate the formability of the automotive aluminium alloy sheets. This criterion is written0ε¯fσmσ¯ε¯pdε¯=C,where σm, σ¯ are the average stress and the equivalent stress, respectively, ε¯,ε¯f are the equivalent strain and the equivalent strain at fracture; and p, C are material constants.

In contrast with the ductile fracture criteria listed in the reference [6], Eq. (1) considers the

Simulation result

The simulations of all forming processes are carried out by means of the commercial finite element software Ls-Dyna. In calculations, the material model is Barlat’s yield function [10] and Hollomon’s hardening equation, the finite element model is Belytschko-Tsay shell element, and the friction coefficient is assumed to be 0.08.

To check the accuracy of numerical simulation, simulation thickness is compared with experimental one. Fig. 3 shows the comparison between the calculated and the

Conlusions

In this paper, the evaluation of fracture limit of automotive aluminium alloy sheet by means of Eq. (1) is carried out, and the comparison between the calculated and the experimental results shows that Eq. (1) can successfully evaluate the formability of automotive aluminium alloy sheet, while conventional Forming limit diagram cannot exactly evaluate it.

Acknowledgment

This work was supported by the National Outstanding Youth Science Foundation of China (No. 50225520) and the R&D Foundation of GM.

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