DYNAMIC PROPERTIES OF 40HM STEELS AT HIGH STRAIN RATES

When designing ballistic shields, it is of significance to know how materials behave at high strain rates. The paper presents results of the experimental investigation into the influence of the strain rate on the dynamic yield point of 40HM chromium-molybdenum steel. The behaviour of a material differs depending on impact speed, even if the impact energy remains the same. Dynamic tests were conducted on two specimens using two testing stations: a station equipped with a rotary hammer for testing at a speed reaching 40 m/s and a station accelerating the specimen to a high speed (160 to 270 m/s) and launching it to impact a hard, non-deformable shield. The dynamic yield point was determined by using the Taylor theory. To determine the impact speed, an optical measurement method and a digital high-speed camera were used. The impact speed was determined on the basis of the analysis of recorded images using a specialized software program. The impact speed was adjusted experimentally depending on the behaviour of the specimen material. The basic criterion for selecting the impact speed was that the requirement was fulfilled that the cylinder - as a result of the impact - succumbs to an evident plastic deformation without a visible material integrity infringement. The questions of plastic deformation, temperature influence, strain rate and stress-strain relation require an analysis of the formation and effects of dislocations. The unified constitutive law called Johnson-Cook's law describes the relations between the material's tension and strengthening ratio, and between strain rate and temperature. The tests indicated that a strain rate increase considerably improves strength properties of 40HM steel.