Читать книгу Monument Future - Siegfried Siegesmund - Страница 272

Generally, the thermal expansion behavior of rock is affected by the temperature history. To avoid the influence of such a history, termed the Kaiser effect, the peak amplitude of the AE (mV) within the large temperature change that the specimens were first exposed to is shown in Figure 2. The air and rock surface temperatures shown in Figure 2 are data obtained by thinning out every 10 s from the data recorded at each time interval. Regarding 182the AE signal, the integrated peak amplitude for 10 s, excluding the peak amplitude < 100 mV from the measured data, is shown.

Figure 2: The AE amplitude and the rock and air temperatures versus time evolutions of the rock samples. A: granite, B: marble, C: sandstone.

There is a difference in the size of the amplitude and the appearance frequency over time of the AE amplitude depending on the rock types; however, it can be seen that the AE signal occurred in all specimens when the temperature increased and decreased. Following the test, no apparent damage such as cracks was found in the specimens. However, the P-wave velocity decreased by 25 % for the granite (3,476 m/s), 7 % for the marble (4,090 m/s), and 0.1 % for the sandstone (3,014 m/s). Therefore, the AE signal is considered to correspond to stress waves when microcracks form at grain boundaries.

The range of the strain due to the temperature change is the largest for the strains of granite (the range of axial strain = 380) followed by that of the marble (351) and sandstone (262). The occurrence of the AE amplitude corresponds to this amount of strain, and the maximum peak amplitude is greatest in the granite (4,140 mV), followed by that of the marble (1,597 mV) and sandstone (1,000 mV). Excluding the effects of crack opening and closing due to temperature changes and the hysteresis effect, the amount of strain and generation of AE signals are closely related. The porosity is lowest for the granite and highest for the sandstone. In other words, microcracks are more likely to occur with the temperature change at the grain boundaries of the granite where the minerals are in closer contact. Because the sandstone has a higher porosity than that of the granite and marble and is not dense, the microcrack occurrence at grain boundaries is considered to be the smallest. Very heterogeneous textures such as sandstone are thought to be less responsive to thermal changes.