Reimann eta/94’96,110 obtained typical load displace­ment data for the ceramic pebble beds introduced above. The typical setup used is given in Figure 19.1 The pebble beds, contained in cylindrical cavities, are compressed by a piston and the pressure (equal to the uniaxial stress s) and the bed deformation (strain e) are measured, and the modulus E of the pebble bed is derived. These uniaxial compression tests (UCTs) should not be performed with bed height H to diameter D ratios larger than 1 so as to avoid a significant influence of friction effects at the cylindrical wall.

Figure 20110 shows a typical result in terms of stress and strain. Key features of the pebble-bed deformation under monotonous isothermal mechan­ical loading are as follows:

• Nonlinear elasticity: the pebble-bed stiffens with higher degrees of deformation.

• Irreversible deformation is observed after initial unloading due to pebble relocation and plastic deformation at pebble-pebble and pebble-wall contacts (which disappears after multiple loading­unloading cycles).

• Thermal creep at constant load with further irreversible deformation.

• Further stiffening observed at loading and unload­ing cycles.

• In addition, friction forces may provide some hysteresis at any loading-unloading cycle.