## 4 Experimental Results  
Uniaxial compression tests were carried out as described in the previous paragraphs. Compression-decompression cycles were repeated six times at four different temperatures (20 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C). The maximum amount of compression at each cycle was 1 mm of total packed bed height (which corresponds to about 0.2%-0.25% strain), since higher amounts of compression resulted in material failure of individual particles. The recorded stress/strain relations during compression and decompression of the packed bed at cycle one, three and six are displayed in Figure 6.  
The first observation to be made is the difference in compression and decompression stresses. The compression stresses at each cycle are consistently higher compared to the following decompression, which results in a hysteresis-like curve progression. This effect is explained due to energy dissipation processes in the form of inner friction and plastic deformation of the packed bed during the compression phase. There is also an obvious non-linearity in the stress-strain relation. Both of these effects are typical and well-observed phenomena of granular soil mechanics [11]. The measured stresses tend to be lower for the first compression cycle, whereas the curve progression of the following cycles is nearly identical, but at a higher level than the first cycle. The reason for this is a less densely packed bed configuration of the pebbles at the initial state before the first compression. Irreversible particle displacements lead to denser configurations, which result in a significant increase in maximum stress, up to 40 % after the first cycle as displayed in Figure 7a. For the measurements at 20 \({}^{\circ}\)C, this behavior is less pronounced, which means the particles were already densely packed before the first compression. This is a consequence of the manual filling process and general stochastical variance of particle configurations in a packed bed. Another observation is the increased stress during compression for higher temperatures. While this is less notable for the 200 \({}^{\circ}\)C data, the maximum exerted stress raises from 0.79 MPa at 20 \({}^{\circ}\)C to 0.9 MPa at 400 \({}^{\circ}\)C and 1.06 MPa at 600 \({}^{\circ}\)C. This increase is a result of the likewise increasing Young's modulus of the ceramic material at elevated temperatures. Another result is the change of the packing factor during testing. Here, the packing factor (also called packing fraction or packing density) is defined  
Figure 5: (**a**): UCT simulation sequence; (**b**): visualization of a packed bed simulation in LIGGGHTS. as the rate of the accumulated solid volume of all pebbles in relation to the total packed bed volume, including the void volume. It is a measure of the densification level of the packed bed and thus is linked to its stress/strain response. Figure 7b displays the change in packing factor after each cycle in relation to the packing factor after the initial first cycle. The initial packing factor is similar for all four packed bed configurations at around 59 %. There is only a minor increase in packing factor (between 0.02% and 0.1 %) for subsequent cycles, most notably after the first compression cycle. This means the initial configuration is already  
Figure 6: Stress-strain relations of ceramic packed bed at different temperatures: (**a**) 20 \({}^{\circ}\)C; (**b**) 200 \({}^{\circ}\)C; (**c**) 400 \({}^{\circ}\)C; (**d**) 600 \({}^{\circ}\)C.  
Figure 7: (**a**) Maximum stresses for each cycle and temperature; (**b**) increase in packing factor based on the initial packing factor. quite dense and close to the final configuration, which again, is a result of the manual filling process of the packed bed into the pressure cell. Another conclusion is that the exerted stresses from the packed bed are quite sensitive to the packing factor, since even small changes in packing factor result in significant changes of stress.