Construction Helmet Response Under Severe Impact

Construction caps composed of fiberglass and an army helmet mounted on a thin-walled aluminum spherical shell purporting to model a human head have been subjected to severe impact by impingement of wooden boards at speeds of about 50 fps (15 m/s). All caps failed catastrophically by both fracture of the cover and partial or total loss of the suspension system; the army helmet exhibited a noticeable dent. Force histories of the transients propagating through the target to the support, measured by strain gages, were found to be nonreproducible with amplitudes ranging from 1,700 lb to 33,600 lb (7,400 N to 150,000 N). This variability is attributed to small changes in impact geometry in successive tests that profoundly affect the contact phenomenon. However, the duration of these transients ranged from 40 ?sec to 90 ?sec, an interval considerably less than the natural period of transducers currently employed in acceptance tests. Such instruments cannot respond to pulses of this type.