Measurement of the stiffness of joints in a graphite brick assembly

Large, dry-contacting assemblies of graphite blocks are a feature of many n uclear reactors, and the seismic response of these structures is dependent on both the bulk and surface properties of the blocks. The nature of the in terblock interface will influence the load-deflection relationship of the b locks, which is important in understanding the response of the structure to seismic loading. The stiffness of this interface is dependent on surface r oughness effects. In this paper, ultrasonic reflection coefficient measurements are used to i nvestigate the stiffness distribution of this interface under various loadi ng conditions. A low-frequency quasi-static spring model of the interface w as adopted. This allowed the interfacial stiffness to be determined as a fu nction of applied load, frequency, surface roughness and graphite compositi on. Cyclical loading of the graphite-graphite interface has also been carri ed out. This has shown that there is only a slight plastic deformation unde r normal loading, and also is indicative of a degree of intersurface adhesi on at the interface. Direct measurement of the load-deflection relationship of an entire brick was also carried out which, in conjunction with the ult rasonic measurements, suggests that it is the effect of macroscopic surface features rather than microscopic roughness that is the dominant interfacia l contribution to brick-tilting stiffness.