Krzysztof Lalik

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.

Preliminary Research on Applying Self-Excited System for Bridge Stress Measurement

Autodyne effect is an improvement to radio signal amplification using the light bulb type amplifier. By adjusting the tuning of this tuned circuit, Armstrong was able to dramatically increase the gain of the amplifier. Further increase in tuning resulted in amplifier reaching self-oscillation.

Piezoelectric Self-Excited System in Mining Roof Anchor Stress Change Measurement

The paper presents the application of a new type of a measurement system called Self-excited Acoustical System (SAS). It was applied for stress change monitoring of anchors which are used to secure roofs and walls in mines and in hollowed tunnels. The knowledge about the state of anchors can indirectly indicate the state of rock masses which is crucial for mining safety. One of the main problems so far has been reliable sensor mounting, especially in difficult mining conditions. The SAS system overcomes this problem because it can be mounted easily at the anchors end or even on the anchors flange. The laboratory tests were conducted on a hydraulic tensile testing machine. The mining roof anchor was placed in the machine and then stretched. The changes of the resonance frequency caused by the increase of the tensile strength were observed during the research.

Harmonic Analysis of Self-Excited Acoustical System for Stress Changes Measurement in Compressed Steel Structural Section

This paper presents the application of the Self-exited Acoustical System (SAS) for monitoring stress changes in steel. The change of the speed of wave propagation, which is associated with the change of the resonance frequency in the system is caused by the deformation of the examined material. The SAS system is based on this phenomenon and can be used for an indirectly measurement of the stresses changes in the steel. Preliminary calculations such as static analysis and dynamic simulation of the system were carried out. The results of calculations were compared with the experimental data of the steel beam.

Analysis of Dependence between Stress Change and Resonance Frequency for Self-Excited Acoustical System

Continuous supervision of mechanical endurance is very important in the time of the advance constructions. The Self-excited Acoustical System (SAS) is a kind of a self-oscillating system, which uses the auto-resonance phenomena for measurement of construction stress changes. The advantage of this solution is the ability to apply it to a wide spectrum of materials from elastoplasts to brittle ones where the conventional techniques cannot be used. The Self-excited Acoustical System is characterized by its great sensitivity in comparison with the traditional techniques. This paper presents the results of the research carried out on a metal bar expanded by variable force. The dependence between the relative elongation of the sample and the transition time of the wave was elaborated. The delay occurring during a wave transition has a direct impact on the value of self-oscillation frequency. The influence of the coupling force between a metal sample and the wave receiver was also analysed.