Self-Excited Acoustical System for Stress Measurement in Mass Rocks

The paper is devoted to theoretical end experimental studies of the low-frequency Self-excited Acoustical System (SAS), which allows monitoring stress changes in various elastic media including metals, concrete and mass rocks. The main principle of the SAS system is using a vibration exciter and vibration receiver placed on a sample with a positive feedback, which causes the excitation of the system. Stress changes manifest themselves in small but detectable variations of resonance frequency which can be used to indirectly measure stress changes in the material. In the paper the considerations concerning working frequency of SAS were performed. It was suggested that in the case of stress variation in mass rock monitoring, the low frequency (even infrasound) band should be selected, in contrast to the stress monitoring in columns of marble or concrete, where frequencies from an acoustic band should be used. Computer simulations conducted in the MATLAB-Simulink environment were based on the research performed in the laboratories. They focused on finding a relationship between the compressing force and velocity of sound in a specimen made of concrete. Results of the simulations allowed to state that the frequency of self-excited oscillations of simulated SAS change linearly with the pressing force. In the next step the laboratory experiments were carried out. The impact on stress measurement parameters such as: the position of sensors, actuator, and the influence of geometrical shape and dimensions of the sample. A sample of sandstone compressed in a frame by a hydraulic press was used in the study. The results proved the applicability of the design system. Additionally, the new possible applications of SAS were suggested, such as monitoring stress variations of stresses in mass rock, particularly in the active seismic zones.