Ultrasonic Investigation of the Stiffness of Graphite- Graphite Interfaces

Robinson, Alice M.; Drinkwater, Bruce W.; Dwyer-Joyce, R. S.

doi:10.1007/978-1-4615-4791-4_192

Cited by 2 publications

(2 citation statements)

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“…This shows that, after the ®rst full loading cycle, the surface asperities have undergone some irreversible deformation, leaving them conforming together to a greater degree and making the interface stiVer. Increasing the average roughness of the surfaces or the peak load in the cycles has been seen to slightly increase the level of plastic deformation [14]. It is believed that the hysteresis loop evident in each cycle of Fig.…”

Section: Cyclical Loadingmentioning

confidence: 97%

Measurement of the stiffness of joints in a graphite brick assembly

Robinson

Drinkwater

Dwyer-Joyce

et al. 2001

Proceedings of the Institution of Mechanical Engineers, Part C:

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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

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