Lightweight Garage #11 Vault Structured Sheet Metal in Mercedes SLK

The Vehicle

Behind this garage door is a vehicle that probably everyone has seen before. It is the Mercedes-Benz SLK, a roadster introduced by Mercedes-Benz in 1996. The vehicle is related to the C-Class and is known for its foldable steel roof. However, there is also a hidden innovative solution which will be presented here. This is the rear wall, which has a special vaulted structure and has been ensuring less weight in the SLK since 2004.

©Steve Coulter, Mercedes Benz SLK, CC BY-NC-ND 2.0

The Technology

Vault structuring is a forming process used to introduce squared or honeycomb structures into sheet metal. For production, the sheet is supported at certain points and subjected to external pressure. This creates a self-organized 3-D structure. It differs from conventional methods of sheet metal forming (e.g. stamping, beading, hydroforming) by an energy-minimized self-organization process. Since the material automatically assumes a shape with minimal energy, the energy required to create the vaulted structure is low and the surface finish is fully preserved.

The introduced structures represent a form of instability failure, which is why such a sheet is automatically in the post-buckling range when in use. The reason for the improved properties is the work hardening that occurs during buckling. Compressive and bending stresses with slight plastic deformations occur in the depressions and tensile stresses in the folds, which stabilize the structure.

©Dr. Mirtsch GmbH Germany, Vaulted sheet metals, CC BY-SA 3.0

Vault structures are originally known from nature and are therefore a good example of a bionic application. For example, the honeycomb-like patterns occur in turtle shells. They have been optimized evolutionarily by nature for millions of years and thus form a perfect carapace for the animal. Technical applications cleverly mimic this structure in order to benefit from the positive properties.

©Punch, Turtle with vault structured carapace, CC BY-SA 2.0

The Lightweighting Effect

The weight advantage of the vault-structured rear wall of the SLK results from the increase in dimensional stiffness, which allows a reduction in wall thickness. Similar to the introduction of beads in sheet metal, the area moment of inertia is increased by a distance to the neutral fibre, thus reducing the load. This results in high stiffness with low weight, reduced need for design space and good acoustic properties. Due to the special manufacturing process, however, vaulted structures have other positive properties. They are more resistant, more thermostable, more aerodynamic and have less glare than beaded sheets.

All these positive properties mean that vault-structured components can be made up to 30% lighter compared to conventional sheet metal parts.

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