Читать книгу Monument Future - Siegfried Siegesmund - Страница 284

Cracking, crack growth and friction of fracture planes in brittle materials like marble generate short pulses of acoustic waves. These acoustic emissions (AE) can be detected by piezoelectric sensors on the surface of the material. The analysis of AE signal features can give a deeper insight into deterioration mechanisms (Tschegg, 2016).

Figure 3 shows the experimental setup that was used to study the decay mechanism of thermally treated marbles. All parts are glued together with silicone, which is a good acoustic couplant between marble and sensors. The cylindrical marble samples have a diameter of 20 mm and a height of 50 mm.

In each experiment four marble samples and one reference sample of stainless steel were tested simultaneously in a climate chamber. In each cycle the temperature changed from 20 °C to 90 °C and back to 20 °C with a rate of 1 °K/min. The temperature was measured on the inner side of the acrylic glass cylinder with a thermocouple. Every temperature level was held for at least four hours. Ultrasonic velocity was measured automatically at 20 °C at the end of each cycle. For wet cycles the acrylic glass cylinders were filled with demineralized water before heating.

Figure 3: Experimental setup of the acoustic emission testing. Four specimens were tested at the same time in a climate chamber.

An AMSY5 AE-system with 10 AE-sensors type VS150MS was used to detect, process and to store the AE-data and to measure the ultrasonic velocity by active pulsing of the sensors. The bandpass filter was set to 25–850 kHz and the evaluation threshold was set to 36 dB_AE. For noise filtering the signals detected by the two sensors of each specimen were grouped to events by special time criteria. This type of filter is very effective and as expected, after filtering only a negligible number of events were detected on the reference specimen of stainless steel. In addition, the ultrasonic velocity measured on the reference specimen was constant throughout all tests.