Keywords: failure, crash analysis, finite element modelling, aluminium extrusions.

A correct representation of the plastic deformation and failure of individual components and parts is essential to obtain accurate crashworthiness simulation results. However, several phenomena contribute to the failure process of aluminum alloys. In thin-walled materials at least three different failure-related mechanisms must be properly represented and characterized: thinning instability, ductile damage and through-thickness shear instability. Failure of a given component during a crash event might occur as an interaction between these different phenomena, i.e. it might initiate due to one phenomenon and propagate due to a combination of the other two. Recent activities have shown that proper representation of instabilities and damage evolution in non-linear finite element (FE) analyses of forming and crash of automotive structures requires a relatively sophisticated modeling approach.

One such approach has been proposed by Hooputra et al. [1] where a procedure to predict the local instability has been shown, together with a phenomenological model for ductile and shear fracture. The different parameters can be obtained by tensile and shear testing as well as bulge tests, but in general, three different tests are needed for each fracture curve to calibrate the model for isotropic materials. If anisotropy is assumed, the model requires nine tests (three tests in three directions) for ductile fracture and three tests for shear fracture. If both static and dynamic fracture is included a total of twelve static and twelve dynamic tests are needed to calibrate the model.

Another approach is explored in the present study. It is assumed that a correct prediction of thinning instability requires an accurate representation of the material anisotropy, which in the models is represented by an anisotropic yield criterion [2]. Failure in the material is determined based on the Cockcroft-Latham criterion [3], or a shear criterion [4]. The calibration of the yield criteria is based on uniaxial tension and compression tests, whereas the fracture parameter in the Cockcroft-Latham criterion is based on a simple shear test. The shear criterion is here calibrated through inverse modelling of a plane strain test.

The main objective of the present project has been to assess this approach by predicting the response of laboratory tests on aluminium extrusions subjected to an axial deformation mode under impact loading conditions.

Quasi-static and dynamic axial crushing tests were carried out on a two-chamber extrusion in AA7108 temper T6. The quasi-static axial crushing tests gave a deformation mode with nine regular lobes. Although there was some fracture of the extrusion, this did not significantly influence the deformation pattern. However, the majority of the dynamic tests failed due to severe fractures where the extrusion side walls "peeled" off. Several material tests of the extrusion material were carried out in order to calibrate the model. The component tests were analyzed with LS-DYNA, and reasonable results were obtained with the calibrated parameters.

1

H. Hooputra, H. Gese, H. Dell, H. Werner, "A comprehensive failure model for crashworthiness simulation of aluminium extrusions", International Journal of Crashworthiness, 9, 5, 449-464, 2004. doi:10.1533/ijcr.2004.0289

2

A. Reyes, O.S. Hopperstad, O.-G. Lademo, M. Langseth, "Modeling of textured aluminum alloys used in a bumper system: Material tests and characterization", Computational Materials Science, 37, 3, 246-268, 2006. doi:10.1016/j.commatsci.2005.07.001

3

M.G. Cockcroft, D.J. Latham, "Ductility and the Workability of Metals", Journal of the institute of metals, 96, 33-39, 1968.

4

J.D. Bressan, J.A. Williams, "The use of a shear instability criterion to predict local necking in sheet metal deformation", International Journal of Mechanical Sciences, 25, 3, 155-168, 1983. doi:10.1016/0020-7403(83)90089-9

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £145 +P&P)