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01/02/2006
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Roger Bishop reports on some pioneering work being undertaken to incorporate novel energy absorbing elements into armatures for front and rear bumper
A hydroformed bumper beam concept with an integrated crash box has been developed by steel industry engineers to meet industry weight and performance targets, writes Roger Bishop. They now hope to persuade vehicle manufacturers to take it forward to future new vehicle platforms.
The latest work by the team at Corus in the UK and the Netherlands includes a patented two-stage energy-absorbing element. The solution is compatible with the pedestrian impact targets now being considered by the industry and meets both European and North American standards.
The latest design builds on an earlier concept that used Hyfo laser-welded tube formed from sheet materials of different thicknesses and strengths which was then hydroformed into armatures for front and rear bumpers incorporating crashbox elements. The design was highly successful in that it resulted in a single component replacing, typically, assemblies of up to eight parts and met most of the industry’s criteria.
Included in these targets were that the system mass – armature, crashboxes and fixings – should not exceed 8kg for a C segment car and be comparable (or lower) in cost than benchmark systems (€15 to €20). Very importantly, it had to be sufficiently robust to remain attached to the main structure at the non-struck side of a vehicle in a 64km/h offset barrier crash test. In addition, it had to meet Allianz Insurance, IIHS corner strike and Pendulum tests while accommodating style cues and pedestrian safety.
Called CIB I, this initial design (see illustration) met the cost targets, enabling it to compete with hot-stamped beams while remaining competitive against rolled section components. Its performance was as good as or better than the targets set and the Corus team came “tantalisingly close” to achieving the 8kg mass requirement, a reduction of 20%.
The vehicle OEMs were encouraged enough for one of them – a European car-maker – to provide a donor vehicle that allowed Corus to develop a prototype at vehicle level rather than system level.
“Despite this, the risk perception of the OEMs was that there were too many new technologies,” says Maarten Kelder, Corus product development manager. “The laser-welded tailored tube with its two material thickness grades was seen as being a step too far.”
So the team went back to basics to bring the mass below the 8kg target and to reduce the perceived risk by removing a technology step change. The result is CIB II, which uses a dual-phase steel, a new crash-box element and a materials performance analysis process that Corus calls Forming to Crash, or F2C. The collision energy absorbing element has been patented by the group.
F2C is based on the knowledge that the hydroforming process changes material strength (through hardening) and thickness, both of which influence component crash performance. The unique aspect of Corus’s work is the group’s development of techniques that enable these changed properties to be included in computer crash analysis models. The graph shows the superior results achievable when hydroforming properties are included in the crash model. All industry standard modelling techniques can be used. F2C analysis can also be used in the engineering design of components to be manufactured using sheet material processes.
The patented energy absorbing element results from a combination of geometry, materials and dimensions that provide for better energy absorption over a shorter distance. The wider section collapses first, stabilising the impact area and ensuring that the crushing mechanism is predictable and repeatable. The narrower section absorbs most of the crash energy. Thanks to F2C, it may be possible to tune the response to meet pedestrian impact targets.
The hydroforming processes envisaged for components based on the techniques developed for CIB II are: pre-bending of the tube; pre-forming within the press; and high pressure hydro-forming with axial feed. Axial feed improves beam ends by strengthening and hardening the material where the beam is attached to the longitudinals. This ensures that the structure can remain attached on the non-struck side in an offset barrier test.
The steel envisaged is Dual Phase 800 (800MPa tensile strength), an alloy of softer ferrite and hard and brittle martensite materials which offers the combination of ductility and strength required, tunable by wall thickness and F2C techniques. Mild steel has a typical strength of 250 to 300MPa.
While Corus’s latest solution meets the targets originally set by the vehicle-makers on cost, weight and performance, further benefits could be obtainable should an OEM choose to return to the original concept of using a tailored blank tube material but using the new design of energy absorbing element combined with the F2C techniques and knowledge developed by the Corus team.
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Author
Roger Bishop
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