Aluminum Structures in Automotive: Experimental and Numerical Investigation for Advanced Crashworthiness
Authors: Miscia Giuseppe , Rotondella Vincenzo , Baldini Andrea , Bertocchi Enrico , D’Agostino Luca 
Published: March 2016

Ductility of aluminum alloys is highly used in automotive applications where crashworthiness becomes relevant. Due to its physical and mechanical properties, aluminum allows structures to be designed with good capacity to absorb energy, without increasing the overall weight of cars. In particular, high elongation allows for the conversion of a great amount of kinetic energy related to crash events in plastic deformation. If this was not the case, the energy involved during an accident could interest also the occupants, causing serious injuries. During large deformation of structures, chassis components may be subjected to failure, limiting the capacity of energy absorption. Therefore, the capability to predict the behavior of structures under crash loads becomes very important during the cars design process. Under these circumstances, finite element analysis is useful to simulate the response and to validate a project.

In the last few years, prediction of materials behavior has become relevant in order to simulate in the best possible way the reaction of structures under dynamic loads. Contrary to what was expected, aluminum alloy might show anisotropic behavior after manufacturing processes. Extrusion, lamination and forging processes can modify crystallography, grains shape, precipitates and dislocations structures, affecting considerably the plastic properties. Furthermore, the failure limit strictly depends on the stress-strain state in the material during the crash event. Tensile state, shear state, compressive state and mixing states generally return different failure limits. Hence, it is indispensable to arrange a huge experimental campaign to define a thorough characterization of an aluminum alloy. Finite element (FE) codes give the possibility to include all these aspects, but several parameters need to be finely tuned. By limiting the number of tests, the present work aims at obtaining the numerical-experimental correlation of some crash absorbers during an impact. Tensile and shear specimens have been cut from the extruded parts of the chassis in 0°, 45° and 90° with respect to the extrusion direction. It is possible to define a fracture locus curve that identifies the equivalent strain limit of the aluminum alloys studied. For instance, Johnson-Cook and Bao-Wierzbicki criteria for aluminum alloys have been defined starting from a complete experimental campaign. They also give approximated analytical functions to define the entire fracture locus curve depending on the stress state. Uniaxial tensile and shear failure limits are the only ones taken into account in this work. Different hypothesis have been considered to define the rest of the fracture locus. Tuning the function parameters of the chosen criteria, a failure curve for compression, shear, tensile and mixing states have been set according to the experimental tests performed.

The material card obtained has been further refined during the numerical-experimental correlation of both the samples and the crash absorbers: mesh size effects have been taken into account to assess the approximations of stress and strain into shell elements. In this work, fine mesh is only used during the initial correlation of specimens. This allows for considerably reducing the computational time of FE models studied. Acceleration signals and failures have been monitored in the crash absorbers. A high correlation between the experimental and numerical tests have validated the current methodology. Because of the few experimental tests performed on samples, it is not possible to study the exact mesh scaling effects at the beginning. Further refining is needed during the correlation of the whole component to get the right failures. In any case, the error given by the experimental dispersion could compromise the correlation and this is the reason why accuracy is not always necessary during the first phases of the correlation settings.