Wave propagation effects in dynamic loads

The problem of the maximum credible accident (MCA), and more recently that of the design basis accident (DMA) for LMFBR, has up to now been studied under conditions of symmetry with respect to the source of the dynamic loading. Calculation codes have been developed to take into account the events generated in the centre of the core, with respect to which the vessel is in position of symmetry. Thus, even experiments on models have been carried out taking similar loading conditions into account. Under these conditions, no waves propagate in the vessel inasmuch as it is deformed symmetrically with respect to its central axis. In reality, however, asymmetries are found. The authors have performed dynamic tensile tests using the Hopkinson bar system on short test-pieces of AISI 304L and AISI 316L and various carbon steels, both at room and working temperatures, and for strain rates of between 10-2 and 103sec-1. Mild steels present, as expected, a sharp increase of stress-strain curve and a reduction of elongation to rupture with increasing strain rate, and a sharp peak for yield stress. Austenitic stainless steels show a rise of stress-strain curve and a reduction of elongation to rupture at increasing strain rate; at strain rates higher than 800 sec-1 the flow stress tends to oscillate. These instabilities would lead one to expect a variation in the propagation velocity of the plastic waves during the deformation. If this hypothesis is proved to be correct, it would add yet another complication to the laws of the transmission of deformation waves in the vessel, and of their reflections. (Auth.)