Wave propagation effects in dynamic loading

Abstract 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. In fact, the sodium-fuel reaction, the event judged today to be the most probable for the DBA, can be generated in any subassembly. Moreover, rupture of the thermal shields at one point inside the vessel could provoke the concentration of a pressure pulse in a restricted area of the vessel. This would lead to a asymmetric loading on the vessel which might be very dangerous insofar as it has not been foreseen in the calculations of pressurized vessels. In addition, this pulse, localized in a restricted area of the vessel, will propagate elastoplastic waves along the wall until they collide at a point symmetrically opposite to the one at which the pulse was generated. These reflections can therefore give rise to pressure peaks in the material which are capable of becoming considerably higher than the original ones. Yet another complication is caused by the presence of penetrations, nozzles, etc. which create stress concentration points and give rise to further reflections. We 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 10 3 sec −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 trans-mission of deformation waves in the vessel, and of their reflections. We are therefore preparing some experiments on wave propagation in long test-pieces which will also contribute to determine the constitutive equations of materials.