The paper reports results of a study of failures in axially crushed aluminum alloy (Al-6061-T6) tubes used for impact mitigation in the automotive and other industries. Concertina folding leads to tensile failures and tearing. At the same time, unexpected cleft-like features were also observed on the compressed sides of folds. Careful microscopic examination of these zones revealed that bending-induced compression leads first to surface wrinkling. Additional bending increases the amplitude of the wrinkles, and with further bending the surface wrinkles evolve into folds, creases, and sharp discontinuities. In any attempt to reverse-bend such zones, the sharp clefts and creases result in catastrophic fracture of the section. In the case of local dents and concave bends in large structures, unloading due to springback may be sufficient to lead to local fracture.Metallographic examination of axial sections of axisymmetric concertina folds in circular Al-6061-T6 tubes revealed that the surface wrinkles encompass several grains, which deform and conform to the imposed local geometric changes. This implies that this surface instability is mainly governed by continuum level kinematics. The grain deformation conforms to the growth of the wrinkles, to the subsequent folds and to the early stages of creasing that follows. As the creases deepen and develop into clefts, the highly deformed microstructure appears to interact more strongly with these surface features.Although some variation of these features was observed around the circumference of concertina folds, the surface instabilities are fairly close to being axisymmetric. Consequently, in a first attempt at simulating the surface instabilities using continuum mechanics, the tube crushing was idealized as axisymmetric and was discretized with finite elements. The elastic-plastic behavior of the Al-alloy is represented using a non-quadratic anisotropic constitutive model shown in previous studies to capture well the onset and evolution of localization type instabilities. The model successfully simulates the crushing of a tube used in the experiments into several folds. The analysis enabled monitoring of the evolution of surface instabilities on the intrados of the second fold. This part of the model was discretized with a sufficiently fine mesh. Small surface imperfections were introduced on the compressed side to prime the surface instabilities – based on measured surface roughness. As the local strain reaches a level of about 50%, surface wrinkles develop and as the local bending increases their amplitude grows. With additional bending, the wrinkles evolve into local folds and creases that resemble surface features observed in the experiments. The presentation will use details of the experiments and the microscopic evaluation of the surface wrinkles, clefts, and creases to motivate the numerical modeling performed, and discuss the sensitivity of the surface instabilities on model parameters.ReferenceHaley, J.A., Kyriakides, S., “Bending Induced Wrinkling in Axially Crushed Aluminum Tubes.” Int’l J. Solids Structures 185-186, 1-13, 2020.

Wrinkling and Creasing in Concertina Folds of Axially Crushed Aluminum Tubes

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

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