Multi-mechanism energy absorption in extension-twist coupled composite tubes

Analytical and experimental study of a compact energy absorbing device, exploiting three sequential failure mechanisms is presented. The device is comprised of two concentric cylindrical composite tubes with opposite angles of fibers. The tubes are stitched together, contain a light weight foam filling inside the inner tube and the annular space between the two tubes is filled with sandwich core. The tubes exhibit opposite extension-twist coupling. Upon being loaded axially, the tubes twist in opposite directions. Stitches and sandwich core prevent relative rotation of the tubes prior to their failure. At a certain peak load, the sandwich core begins to fracture and stitches begin to rip, dissipating the elastic energy stored inside them. In addition to this, crush foam filling within the inner tube eventually gets crushed and dissipates energy. The device exhibits variations in force-displacement characteristics with changes in the parameters involved, and provides wide range of available strokes and peak loads. It was found that the device exhibits 10-15 times higher specific energy absorption (SEA) and 2-3 times higher volumetric energy absorption (VEA) than currently used devices (such as wire benders etc.) for similar purposes. Analytical model for the crush part of the device demonstrated a close agreement with the experimental results.