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

Abstract

Large assemblies of blocks are common in many civil engineering structures such as dams, costal defences and ancient buildings. Such assemblies are also found in graphite cored nuclear reactors. These consist of large graphite bricks, which are used to moderate the reaction and are assembled in arrays up to 10m in height and 10m in diameter. In this paper, the bricks considered are 200mm x 200mm x 800mm. The seismic response of such a structure is dependent on both the properties of the bulk material and the nature of the interface between individual bricks. The bricks are in dry contact with those above and below, and it is this contacting interface that governs the rocking behaviour, which is therefore an important factor in assessing the response of the columns to vibration or a seismic event.

This paper describes an investigation into the effect of microscopic and macroscopic surface features on the rocking behaviour of a single graphite brick. Typical real surfaces exhibit some form of macroscopic waviness in addition to the microscopic roughness and therefore only a small fraction of the surfaces are in contact. A number of analytical models are used to compare the effects of specific surface features with experimental data from real bricks. These models are compared with experimental results to assess their importance in explaining the rocking behaviour observed experimentally. Factors such as surface curvature, contact size and position are shown to be particularly important to the rocking stiffness of a brick. In most cases, the reduction in the rocking stiffness compared to perfectly flat surfaces is due to both the reduced moment arm of the contact forces and the increased strain at the contact region due to the high stress caused by the small area of contact.