Experimental and numerical study on shear-punch test of 6060 T6 extruded aluminum profile

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Abstract

In the present work a comprehensive experimental/numerical study on a 6060 T6 bumper profile subjected to combined shear-compression (shear-punch test) load is carried out. A set of key plasticity and fracture parameters to predict correctly the mechanical response of the structural component is required. Fracture initiation tests on lab scale specimens provide the sufficient data needed to calibrate the newly extended non-quadratic yield function Yld2000-3d and the isotropic Modified Mohr-Coulomb (MMC) fracture model. The calibrated plasticity and fracture constitutive model is then validated upon disk-shaped specimen. Moreover, test results are reported on full-scale extruded aluminum bumper subjected to shear-punch load. A detailed FE model of the bumper and the supporting structure was built. The numerical simulation outcomes demonstrate that the calibrated material model predicts correctly not only the global force-displacement response but also the local fracture behavior at multiple initiation points. It was shown that various part of the multi-cell cross-section of the extruded profile underwent different loading histories. Thus, in one single test a wide range of stress triaxialities develop which requires to introduce a very complete plasticity and fracture model.

Introduction

Shear test has been conducted in the labs around the world for decades to characterize plastic properties of metal and other materials. It has been serving for several purposes. On one hand, there was a need to determine strength of structural elements subjected to shear loading. On the other, shear tests were conducted to verify varies hypothesis regarding plastic deformation and flow in metals. According to the von Mises yield condition, the ratio of the yield stress in tension to the yield stress in shear is 3. This hypothesizes has been verified by many investigators, see for example [1]. Such study also summarizes some of the most important experimental works done on shear tests up to 2005. Based on an extensive number of experiments conducted by Atkins [2], [3], [4] and Dodd and Atkins [5], [6], a simple computational shear model for blanking was proposed by Atkins [7]. In his model, an apparent plastic instability was attributed not only to the adiabatic heating but also to the decreasing of shear area defined as the current area of common cross-section. The width of the shear zone was considered as a parameter which was later found from the experimental data. Therefore, this solution was incomplete because the prediction of the force-displacement curve was not based on material properties alone. Zhou and Wierzbicki [8] proposed a complete analytical model for the blanking process. Since 2000 most of work on shear-punch prediction was done by means of finite element simulations.

The automotive industry and sheet metal forming industry look very carefully into the problem of the shear-punch loading and blanking to improve the edge quality of sheets and reduce the amount of so-called “burr” and possible micro cracks formed during that process. Such micro-defects are potential of initiation points of major cracks in sheets during subsequent forming or crash. The hole expansion ratio (HER) is a simple but convenient industrial measure of the edge quality. One of the most recent and complete study of shear- punch process and subsequent prediction of the remaining ductility in thin plates was conducted by Wang et al. [9] and Wang and Wierzbicki [10]. A very fine FE mesh consisting of 160 elements through the thickness was able to capture most of the fracture morphology typically observed in this type of experiments. With increasing tool gap between the piece and press-die tool, the shear- punch process gives way to combine bending-tension loading mode. Luo and Wierzbicki [11] developed a computational tool for predicting fracture during stretch-bending operation. That tool relies on the application of Modified Mohr-Coulomb (MMC) fracture criterion, which should be calibrated from suitable tests. A unique dual-actuator loading frame and an experimental procedure for combined shear-tension tests were developed by Mohr and Henn [12] and Dunand and Mohr [13]. A new butterfly specimen was designed in such a way that fracture never initiate from the edges, but always in the center. There are several generations of the butterfly specimens and the most recent version of the so called “smiley” shear specimen was described by Roth and Mohr [14].

Most of the shear experiments are carried out on small-scale coupons cut from sheets or thin sandwich panels with honeycomb core. Extruded aluminum members do not fall into this category. The wall-thickness of typical extrusion is around 3–4 mm and the cross-section dimension could be an order of magnitude larger. Extruded aluminum multi-cell hollow sections are typical structure elements existing all around us from household appliances through lab equipment to bumpers or entire car bodies. To the best of the author's knowledge no large-scale shear test on extruded aluminum bumper has not been described nor published in the open literature. The automotive industry is routinely conducting crash, frontal crash test into a pole of circular cross-section. Such test and corresponding numerical simulation involve three point bending with only small tension. At the same time shear loading using flat rectangular punch and relatively small tool gap provides far more interesting information about material and cross sectional response of the structure. During such experiments various cross sectional profiles follow different loading history that a certain point could lead to fracture initiation. For example, the webs are initially subjected to compression which gives rise local buckles. Meanwhile, the upper flange undergoes a combined shear-tension and bending load due to a very changing condition generated by the collapse of webs. Fracture may initiate at different locations and for the purpose of crash and weight optimization must be predicted with high accuracy.

The objective of the present work is to perform a bumper shear test at industrial scale and correlate the experimental results of global and local response with finite element simulations. Such simulations are routinely carried out by automotive industry but rarely with the fracture option. Here, we have demonstrated that fracture initiation and propagation, in a large-scale aluminum automotive component, can be predicted with good accuracy using the suitably calibrated MMC model. There are several parameters in this model defining plastic yielding and flow as well as fracture. Those parameters have been determined through a comprehensive testing and calibration program involving flat specimens cut from the flanges of the extruded profile. The current study material is a 6060 T6 aluminum alloy whose measured properties were different from those available in the open literature. The test and calibration procedure is not new and closely follows one developed at the Impact and Crashworthiness lab (ICL) at MIT and describes the material plastic [15] and fracture [16] behavior of an extruded profile of 6260 T6 aluminum alloy. What is new, it is the successful correlation of the global response history and prediction of fracture initiation at different points of the deforming bumper. There were several points in the bumper that underwent a complete different stress and strain histories. In order to tackle such a problem fracture predictive model must cover a very wide range of stress triaxialities including negative (compression), through zero (shear) all the way to tension up to 2/3. We have demonstrated a successful prediction of plastic flow and fracture for all of the above states and just one complex experiment. To achieve this goal a rather complex plasticity model (Barlat Yld2000-3d) and fracture model (MMC) had to be used. This requires a careful and complete calibration procedure and the major part of this study is devoted to that subject. In this way it is believed that any interested reader will be able to reproduce the same type of loading and simulation.

Section snippets

Material and plasticity model

Constellium Automotive Structures (Germany) provided a section of extruded aluminum 6060 T6 bumper for the study, as shown in Fig. 1(a).

Its length along the extrusion direction is 1480 mm. The cross-section dimensions are shown in Fig. 1(b) (all dimensions are given in mm). The material in the cross-section planes became inhomogeneous after the extrusion process and quenching treatment. It is important to determine the amount of material inhomogeneity.

MMC fracture model and calibration

The MMC fracture model originally proposed by Bai and Wierzbicki [18] is fully described in Appendix B for the sake of space. It is noteworthy that there exists a certain inconsistency between the strain hardening exponent (n) from Swift hardening curve , Eq. (B.3), used to describe the local fracture criterion and the Voce exponential type, Eq. (4), employed for predicting plastic behavior. It is believed that such inconsistency does not lead to large errors because the plasticity and fracture

Punch test on circular disks

The punch tests and simulations serve as a validation of the present uncoupled fracture model at the size of lab specimens. The disk specimen with a diameter of 70 mm and thickness of 2 mm was cut from B2 section. It was clamped between a circular die and a circular holder, and then loaded monotonically using a hemispherical punch with a speed of 5 mm/min. The radius of the punch is 22.2 mm and the inner radius of die is 24.5 mm, as illustrated in Fig. 14. Considering the symmetric features of punch

Shear-punch test of bumper in industrial application

The input file for the material model consists in a total of 16 parameters. Those are: 8 coefficients α1, …, α8 of the anisotropy yield condition; 3 coefficients of the Voce exponential type function, 2 coefficients of the Swift power law equation and 3 coefficients of the fracture initiation criterion. The determination of these parameters through tests and calibration was described in the previous section. We provided sufficient details so that interested readers will be able to re-create

Results and discussion

Fig. 20 depicts the comparison of the measured and simulated load-displacement curves of bumper shear test. As a comparison, the plasticity model with von Mises yield condition and MMC fracture criterion were used to simulate such an experiment. The predicted force-displacement curve using a simple J2 plasticity setting is plotted in Fig. 20(a) by a red colored line. It is clear that the von Mises model significantly underpredicts the global experimental behavior. This underpins a necessity of

Conclusion

The plasticity and ductile fracture behavior of an extruded aluminum 6060 T6 bumper was investigated by means of a combined experimental-numerical approach. The established material model considering anisotropic plasticity and fracture criterion were verified at the scale of lab tests and then applied to a scale of industrial process. The key conclusions are drawn as follows:

  • 1.

    The amount of material inhomogeneity of extruded bumper was assessed and the bumper can be approximately taken as

Acknowledgment

The support of this work comes primarily from the MIT Industrial Fracture Consortium. The involvement of Constellium Automotive Structures from Germany is highly appreciated in providing material and experimental data. Thanks are also due to Dr. M. Luo, Dr. C. Roth and members of ICL for their valuable contributions. The first author also acknowledges her partial financial support from the Tsinghua Scholarship for Overseas Graduate Studies as well as the National Natural Science Foundation of

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