Dynamic Axial Crush Tests of Roll Wrapped Composite Tubes: Plug vs. Non-Plug Crush Initiators

This paper discusses the effects of the method of crush initiation on the dynamic axial crush response of roll wrapped composite tubes. This constitutes a portion of a successful fundamental study conducted at GM R&D of the dynamic axial crush of automotive rail-sized composite tubes reinforced variously with carbon fiber, Kevlar® and hybrid combinations of the two, and manufactured using roll wrapping techniques suitable for the low cost high volume needs of the automotive industry. All tubes were manufactured using roll wrapping from multiple layers of uni-directional thermoset prepreg with the uni-directional fabric plies being oriented at ± 15° with respect to the longitudinal axis of the tube. A total of 21 dynamic axial crush tests were conducted using a free flight drop tower facility. Tests spanned a range of tube geometries — circular and square with different wall and cross section dimensions with cored and uncored walls and differing numbers of plies — and a range of drop heights/impact velocities and drop masses. A general finding was that stable and progressive crush occurred at acceptable load levels in all of the roll-wrapped tubes that were tested for all methods of crush initiation that were considered — a 45° lead end bevel either alone, with a zero radius plug-type crush initiator, or with a 12.7 mm radius plug. The method of crush initiation was, however, found to be capable of producing major differences in the crush initiation force Fp, the displacement average crush force Fav(D), the specific energy absorption SEA, and the crush morphology. As examples, both the displacement average value of dynamic axial crush force and the value of SEA were changed by as much as a factor of eight by the method of crush initiation.