A high-speed camera system is adopted to measure the instantaneous deformation of a free–free beam struck by a projectile at anycross-section along its span. The material in the experiment is made of hard aluminum (LY12R) which is less sensitive to the strain rate. A theoretical model is based on an elastic– plastic linear hardening material and the formulation is derived from the minimum principle in dynamics of elastic–plastic continua at finite deformation in association with a numerical technique. Some experimental results and theoretical predictions are given for the beam impacted at mid-span and 1/4 span, and attention is focused on the dynamic behavior of the beam in the early, transient stage of the response. r 2002 Elsevier Science Ltd. All rights reserved.