Unusual Pseudo-Brittle Failure of Iron and Mild Steel under Dynamic Loading

Abstract

It was observed that under the dynamic tensile test (1.96 m/sec) fine grained iron and mild steel show only slight elongation less than a few per cent, while coarse grained specimens do large elongation. In the case of fine grained specimens which showed apparent brittle failure, the deformation was concentrated near the necked region, and no propagation along the full gage length of the specimen is allowed. From the theory of strain propagation the quasi-critical impact velocity was deduced for a material with lower yield point and Lüders elongation. The quasi-critical impact velocity is similar to the critical impact velocity given by T. von Karman although there are some modifications. It has been proved that the quasi-critical impact velocity decreases with decreasing grain size. In fine grained specimens, therefore, the impact velocity can easily exceed the quasi-critical impact velocity. This is thought to be the cause that fine grained iron and mild steel exhibit unusual apparent brittle failure under the dynamic test.