Abstract

The impact properties of six foam materials used for energy absorption as the liner of children's helmets, reported by Gimbel and Hoshizaki are considered further. In high-energy impacts, almost complete compression of the energy-absorbing material (bottoming out) may occur, and the severity of the impact increases greatly. Too soft a material means bottoming out occurs at low speeds, but if it is too stiff, the material itself is injurious. The fitting of equations to results in ‘no bottoming out’ and ‘bottoming out’ conditions may help assessment of what compromise is appropriate. The equations in this article correspond to peak acceleration being proportional to power functions of impactor speed and mass. 1. When there was no bottoming out, peak acceleration was found to be proportional to m ^∧(c−1).v ^∧(2c), with c being approximately 0.25. 2. For bottoming out, peak acceleration was found to be proportional to m ^∧(p).v ^∧(q), with p and q being approximately 2 and approximately 3. 3. The constants of proportionality were related to material density in a regular way.