cross section crash boxes

Figure 3.29. (cont.)
3.5. Motivations
(b)
Foam filling of tubular structures becomes efficient after a critical foam density
and/or plateau stress (Figure 3.17). At low foam plateau stresses the specific energy
absorption of filled tube is below that of empty tube because of the ineffective foam
strength over total weight ratio. Filling with higher foam densities at present is however
limited by an upper density, 0.4 g cm -3 , of the commercially available Al foams. In
addition, the use of high density Al foams will increase the total cost of the crash boxes.
On the other hand, the use of multi cell tubular structures was shown to improve SEA
values. Similar to the use of the high strength foam filing, the use of the multiple cell
tubular structures will increase the unit cost of the crash boxes. In partially foam filling
in which the foam filler length is smaller than that of the column, a certain improvement
in SEA was achieved (Altenhof, et al. 2002, Chen and Nardhi 2000, Wang, et al. 2005,
Wood, et al. 2006). The efficiencies of foam filling depend on, besides the density of
the foam filler, the geometry and the base material of the structure and the length of the
foam filler. A complete analysis of the effects of tube wall thickness and base material
63