Design of Hierarchical Crash Cans

Tesla

This project was a continuation of the Tesla 2017 project by a second team.

 Goal:

To explore the design and optimize the performance of hierarchichal crash cans, which are cans that include cells of different sizes, so as to maximize the amount of energy per unit mass absorbed by the structure upon a frontal collision.

Problem:

The performance of crash cans crucially relies on the onset of progressive buckling when impacted with a high-enough impulse, since deformations of this type absorb a large amount of energy as plastic deformation. However, the crash can can also deform by buckling in the first mode, as any column would do, or switch to buckling in the first mode after a short length of progressive buckling. The problem is to define the geometry of the crash can so that it reliably deforms through progressive buckling upon impact.

The team was tasked to explore the feasibility of designing hierarchical crash cans, and if feasible, to find an optimal design for each design concept. The motivation behind considering these designs was to evaluate whether the smaller-scale features, even if built with thinner materials, could broaden the set of designs that would deform through progressive buckling.

What did the team do?

The team decided to first explore a number of the designs shown in the first figure by applying a modification of the "Super Folding Element Theory" to this type of cells. When they accounted for manufacturing constraints such as minimum thicknesses, as well as for the maximum average force the structure of the car would withstand, they identified the crush can in the second figure as a promising one to explore in detail. In the process, the team created a workflow to automatically vary geometric parameters, generate the CAD information, mesh it, perform the LS-Dyna simulation and then post-process it. The team also needed to look into the finer details of the Super Folding Element Theory to build a heuristic extension to the types of geometries considered here, and performed some validation studies by comparing with LS-Dyna results, with rather mixed results. The extensive parametric studies they performed highlighted how the pattern of buckling folds that appear changes as the value of C1/Width does, see the second figure, and then they were able to fix the geometry and change the thickness and width of the structure while mantaining a constant average force of ~154 kN, to identify the design with the largest specific energy absorption value. 

The optimal design promises to have 6% larger specific energy absorption value than the optimal five-cell design of the 2017 project.