STRENGTH OF MATERIALS IN CASE OF MECHANIC AL IMPACT

The resistance of materials to mechanical impact was investigated in t he context of safety of guards used in machine teals. Such guards are meant to protect persons from injuries caused by parts which, in the c ase of technical failure, are projected at high speeds out of the work zone of the machine. Impact tests with blunt, cylindrical projectiles were carried out on steel and aluminium sheets and on polycarbonate a nd polymethylmetacrylate shields. The measure of impact strength was t he ultimate projectile energy which could be applied to the specimen w ithout causing perforation. The impact strength of new 8-mm-polycarbon ate shields turned out to be approximately comparable to that of 3-mm- steel sheets St 12.03. Polycarbonate, however, when it is exposed to c ooling lubricants as they are used in machining processes, suffers fro m a strong reduction of its impact strength. In the tests, there was g ood proportionality between the impact strength of the materials and t heir thickness, ultimate tensile strength and fracture elongation. In addition, the impact strength increased as the projectile diameter got bigger.