Stress-Strain Relationship of Mild Steel under Impulsive Tensile Loading

Studies of tensile properttes of steels under high speed impulsive loading-I

Abstract

Steels are subject often to an impact load or to a suddenly increasing load in service conditions or in its manufacturing processes. Study on the behaviours of the materials under such impulsive loading or under high rate of strain, which affects remarkably the strength and the deformation characteristics, is not only theoretically interesting but also important for practical purposes. The present authors designed and installed a new impact tensile testing machine, that is equipped with a large rotary disk, by which the testing speed up to 140m/s could be obtained
In this paper, the impact tensile properties of a mild steel at a room temperature under various test conditions were reported and the form of the observed load-time curve was discussed theoretically. The load-time curve was measured by a strain-gauge attached on a load cell connected at the back end of the specimen opposite to the impact end. In this measurement, as the impact speed is increased or as the specimen becomes longer, the form of the observed load-time curve deviates from that of the conventional stress-strain curve, which is assumed to be similar to the form of the curve obtained in the static test. The propagation theory of the elastic and plastic strain waves was applied to the interpretation of the difference between the form of the observed load-time curve and that of the conventional stress-strain curve. The load-time curve was divided into two portions, that is, the loading and unloading ones, and the strain wave which corresponds to the latter portion was assumed to propagate at a similar velocity to that of the elastic strain. Thus, it was found that the form of the theoretical load-time curve derived from the conventional stress-strain curve under the above-mentioned assumption was in good agreement with that of the curve recorded experimentally.
The impact tensile strength of an annealed mild steel was increased markedly with the impact speed up to 20m/s but the strength was kept unchangedin the test of the larger impact speed than 20m/s. The observed critical impact velocity of the same steel was considerably smaller than the calculated one. This disagreement is due to a disregard of the strain rate effect in the calculation. Finally, the effect of the ferrite grain size of the specimen on the impact tensile properties was reported.