New extruded multi-cell aluminum profile for maximum crash energy absorption and weight efficiency
Introduction
An aluminum space frame is considered as a very promising type of car body structure, gaining increasing popularity for its high weight efficiency [1]. Another merit of the aluminum space frame is that almost every arbitrary cross-section can be produced by the extrusion process. In this regard extruded members are superior to the thin-walled structures made by unitizing sheet metal by spot-welding or bonding. For example, Honda uses complex hexagonal extruded cross-section members for the front side rail of its new hybrid passenger car, Insight [2].
The front longitudinal rail is the most important crash energy member in case of the frontal collision [3]. The forward part of the rail structure is generally straight to induce axial progressive collapse, which is the most efficient mode of collapse for crash energy absorption. The crushing behavior of front end structure is very important because it affects the overall profile of the crash pulse, directly related to the passenger injury criteria [4]. So far, square or rectangular cross-section or multi-cell profiles such as double cell and triple cell have been used for the front end part of the space frame structure. A useful feature of the extrusion process is that arbitrary cross-section members can be made easily. Improved cross-sectional profiles are sought in this study for the front end part based on the theory of plastic collapse of the thin-walled structure. The conventional profiles have not been shape-optimized in terms of crash energy absorption and weight efficiency. The effect of the cross-sectional shape on the crash resistance under bending-dominant collapse was extensively studied by Kim and Wierzbicki [5]. For the three-dimensional ‘S’ shaped frame, the crash energy absorption was shown to increase more than 200% without losing weight efficiency by re-designing the cross-sectional shape with a diaphragm. This idea has been extended to the ‘S’ frame with hat-type profile [6]. Another way of improving the energy absorption proposed by Lee and Wierzbicki [8] was to introduce a stepwise thickness of square extruded member.
In this study, innovative new multi-cell profiles are proposed for higher crash energy absorption and weight efficiency. Also, the stability for progressive collapse is addressed. Various design aspects of the new multi-cell members are investigated, and the optimization is carried out as an exemplary design guide. Commercially available nonlinear dynamic explicit finite element code PAM-CRASH is used throughout this study.
Section snippets
Formulation of the problem and finite element modeling
The structure considered in this study is the thin-walled prismatic column with square cross-section. The dimension of the cross-section is 80×80 mm, and the length of the column is 400 mm. The geometry is determined from typical geometric dimensions of the front side rail of a passenger car. The loading condition is the collision to the rigid wall at an initial speed of 30 mph (=48.27 kph). The added mass of 600 kg is attached to the bottom end of the column. The configuration of the model is
Effect of the ‘complex’ trigger
The trigger or initiator is commonly used in the design practice of energy absorbing structures subject to axial compressive collapse. The trigger lowers the peak crushing load and induces progressive folding avoiding the global bending mode. For the square or rectangular cross-section members, generally the imperfection types corresponding to simplest collapsing mode are used (Fig. 3). The trigger is based on the first elastic buckling mode shape [Fig. 3 (a)], or the shape of plastic
Design of new multi-cell profile
It is known from extensive study of the crushing of a prismatic thin-walled column that the energy is dissipated by membrane deformation and bending deformation along the bending hinge line. Generally, it is observed that the severe deformation of combined bending and membrane deformation takes place near the corners of the column. This observation was well utilized in the study by Lee and Wierzbicki [8] for maximum crash energy absorption, where the effect of the material distribution on the
Optimization of design parameters
For the maximum crash energy absorption and weight efficiency, an optimization process is employed for the Case 2 model. A summary of the optimization process is shown below.
Target: Maximize specific energy absorption
Constraints:
(1) The stable progressive collapse of square corner element
(2) Non-interference between the folds of square corner elements
Design variables: C [0–40] and t [0.7–3.0] (see Fig. 11)
In the optimization process, the analytical expression of the mean crushing force is used.
Relative merits over the conventional profiles
Double cell, triple cell or quadruple cell profiles are used as the conventional multi-cell members for crash applications. In this section, the performance of each cross-section is compared in terms of mean crushing force. The mean crushing force and the material area of the cross-section A of the various single or multi-cell profiles are calculated as follows.
Conclusion
New types of trigger and multi-cell profile are proposed and investigated in this study. The new trigger is based on the second elastic buckling mode shape of the cross-section. With the modified trigger, the complex mode of crushing is induced for only the first plastic fold, and the crushing mode changes into the regular crushing mode over the subsequent second and third folds.
The new multi-cell profile is based on the idea of adding a square element to the corner part of a cross-section for
Acknowledgements
This research was sponsored by joint MIT/Industry Consortium on the Ultralight Metal Body Structure. Thanks are due to Altair Computing and Engineering System International for providing free academic licenses for the programs HYPERMESH and PAM-CRASH. The author also wishes to express deep gratitude to Professor T. Wierzbicki for valuable discussions and revision of the manuscript.
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