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 interblock interface will in¯uence the load±de¯ection relationship of the blocks, which is important in understanding the response of the structure to seismic loading. The stiVness of this interface is dependent on surface roughness eVects.In this paper, ultrasonic re¯ection coeYcient measurements are used to investigate the stiVness distribution of this interface under various loading conditions. A low-frequency quasi-static spring model of the interface was adopted. This allowed the interfacial stiVness 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±de¯ection relationship of an entire brick was also carried out which, in conjunction with the ultrasonic measurements, suggests that it is the eVect of macroscopic surface features rather than microscopic roughness that is the dominant interfacial contribution to brick-tilting stiVness. NOTATION d half-depth of the brick face (m) E Young's modulus (Pa) F force (N) h height of the brick (m) I second moment of area (m 4 ) K interfacial stiVness per area (Pa/m) l half-width of the brick face (m) p local contact pressure (Pa) p nom nominal contact pressure (Pa) R ultrasonic re¯ection coeYcient R a centre-line average roughness (m) u average interfacial separation (m) W total weight of the brick (N) x distance along the brick face from the centre of the edge (m) x 0 position along the brick face at which contact with the substrate is lost (m) z acoustic impedance (kg/m 2 s) ¬ ratio of interfacial stiVness per area to root of pressure (Pa 1=2 /m) ¡ torque applied to the brick (N m) brick de¯ection (m) angle of tilt of the brick (rad) ! ultrasonic angular frequency (rad/s) IMPORTANCE OF GRAPHITE±GRAPHITE INTERFACESThe cores of many commercial nuclear reactors contain large assemblies of graphite blocks which are used to moderate the fast neutrons in the reaction. Fuel and control rods are positioned in holes in this assembly. These blocks are typically around 1 m high and are arranged in arrays ten or more layers high. The bricks cannot be physically constrained into a ®xed matrix owing to the extreme irradiation and heat-induced deformation they undergo, but instead ®t together withThe MS was

The influence of interface effects on the rocking behaviour of graphite blocks

“…3 that bulk effects account for only a small proportion of the measured deflections and this difference increases as the brick begins to tip. This was also noted by Robinson et al [9] and Herbet-Jackson [6]. It is postulated that this extra deflection was caused by the macroscopic and microscopic profile of the surface of the brick in contact with the foundation.…”

“…Robinson et al [9] measured the interfacial stiffness of graphite-graphite and graphite-metal joints due to surface roughness with ultrasonic reflection coefficient measurements. The calculated stiffness values were used to model the effect of surface roughness on the rocking behaviour of a single graphite brick.…”

On the Use of Ultrasound Waves to Monitor the Local Dynamics of Friction Joints

“…Some initial tests were carried out to evaluate the capabilities of this technique in a static configuration. These tests aimed at verifying that changes of contact pressure led to variations in the reflected/transmitted ultrasound signal as shown in previous studies [21,[31][32][33][34][35][36][37][38]. The specimens' heads with the shear plates attached were mounted on an Instron tensile/compressive machine via a specially designed flange (see Fig.…”

“…However, the basic spring model has been found to be satisfactory for most of the engineering applications, where the ultrasound frequency is not extremely high (few MHz), and for this reason it will be used here. Several studies investigated the tribological contacts under static conditions [31][32][33][34][35][36][37][38]. Drinkwater et al [31] have carried out an extensive study for a partially contacting aluminium interface under different loads.…”

The Use of Ultrasound to Measure Contact Stiffness and Pressure in Large Contacting Interfaces

“…Robinson et al [2] measured the interfacial stiffness of graphite-graphite joints due to surface roughness with ultrasonic reflection coefficient measurements [3], where reflection coefficient is defined as the proportion of the incident signal reflected. The calculated stiffness values were used to model the effect of surface roughness on the rocking behavior of a single graphite brick.…”