Fragmentation of Shell Casings and the Theory of Rupture in Metals

Abstract

In para. 2 the Gri.th theory of rupture in brittle materials is reviewed; in paras. 3 and 4 the information available about the stress strain curves of metals at high rates of strain is reviewed, and it is suggested that the increase in the yield point of steel of a given composition is independent of the degree of cold work. A correlation is also suggested between the variations of yield point with temperature and with rate of strain. In para. 5 the evidence is discussed for the hypothesis that metals in a given state of plastic strain will fracture when the maximum principal stress reaches a given value. It is shown that the fracture stress of steel is either independent of strain or increases slowly with it. The fracture stress appears to increase with rate of strain, by about the same amount as the yield point. In para. 6 it is pointed out that two well-defined types of rupture occur, brittle and shear; the condition for the occurrence of either is discussed. Para. 7 applies the conclusions reached to certain phenomena observed in the rupture of bomb and shell casings. Para. 8 discusses the evidence that the rupture stress increases with rate of strain. Para. 9 discusses the scatter observed in the values of the reduction in area obtained in tensile tests; this scatter was assumed in a previous report on fragmentation to be responsible for the average size of fragments. It is shown that a numerical estimate of the order of magnitude of this scatter, and of its dependence on the properties of the steel, can be made using very general assumptions about the mechanism of rupture. It appears probable that the magnitudes of the scatter will be the same for shear rupture as for brittle rupture.