Composite Crash Box: Roll Wrap Fabrication and Dynamic Axial Crush Performance

Abstract

A feature of many vehicles is a bolt-on, replaceable front end clip designed to protect the remaining structure in 0° to 30° frontal and offset crashes up to 15 km/hr (ECE-R42, European Danner (AZT), Allianze, VDS, or Thatcham Tests). The principle energy absorbing elements in such front clips are called crash or crush boxes. These are hollow cross section often tubular structures located between the bumper and the front ends of the lower rails. Previous studies of the dynamic axial crush response of carbon fiber reinforced composite tubes suggested that both the mass of the crash box and the amount of overhang of the front end clip could be reduced by switching from a metal to a carbon fiber reinforced composite crash box. The axial dimension of the crash box could theoretically be reduced because of the 20% reduction in stack-up exhibited by composite compared to metal tubular structures. The mass could theoretically be reduced because of the higher energy dissipation capability per unit mass of the carbon fiber composite and the shorter length that would be required. The initiative summarized in this paper was the roll wrapping portion of a one year program intended to prove out these benefits. Specifically, it encompassed the design, roll wrapping fabrication, and dynamic axial crush testing of a carbon fiber composite version of the crash box for a mid-size vehicle. All project goals were met. As first steps crush performance of the baseline Al crash box was determined and requirements were established for the geometry and crush force of the composite crash box, the needed crush force being 70 kN. To achieve the desired crush force levels while reducing mass by 20% compared to Al requires an SEA (specific energy absorption) on the order of 45. Crash box specimens manufactured with the roll wrapping process spanned a wide range of fiber architectures which were chosen based on findings of earlier crush tests of composite tubular specimens. Dynamic axial crush tests were then conducted on these specimens. Through these tests we were successful in identifying combinations of fiber type, fiber architecture, and tube wall thickness that simultaneously satisfied the multiple targets of high crush force, low stack-up (high crush efficiency), and reduced mass.