Measurement of the stiffness of joints in a graphite brick assembly

Abstract

Large, dry-contacting assemblies of graphite blocks are a feature of many nuclear reactors, and the seismic response of these structures is dependent on both the bulk and surface properties of the blocks. The nature of the interblockinterface will influence the load-deflection relationship of the blocks, which is important in understanding the response of the structure to seismic loading. The stiffness of this interface is dependent on surface roughness effects. In this paper, ultrasonic reflection coefficient measurements are used to investigate the stiffness distribution of this interface under various loading conditions. A low-frequency quasi-static spring model of the interface was adopted. This allowed the interfacial stiffness to be determined as a function of applied load, frequency, surface roughness and graphite composition. Cyclical loading of the graphite-graphite interface has also been carried out. This has shown that there is only a slight plastic deformation under normal loading, and also is indicative of a degree of intersurface adhesion at the interface. Direct measurement of the load-deflection relationship of an entire brick was also carried out which, in conjunction with the ultrasonic measurements, suggests that it is the effect of macroscopic surface features rather than microscopic roughness that is the dominant interfacial contribution to brick-tilting stiffness.