Methods
Methods of AE measurements
The LESMS has created a whole complex of technical and instrumental infrastructure, and also the software, to study the influence of external energy actions on the processes of destruction of geomedium.
Rock samples (Westerly and Tabylga granites; granodiorite; halite; marble; gabbro) as well as artificial materials such as: ceramics (zirconium dioxide), concrete with granite chips and pyrophyllite inclusions are tested for creep under constant compression by means of spring and gravitational presses (UDI with maximal compression load of 100 ton, UDI-L with maximal compression load of 35 ton and rheological add-in device with maximal compression load of 20 ton).
Additional impacts of physical fields are exerted during measuring sessions under constant and quasi-stationary level of compressive load. For vibration effects we use a small-sized vibrator of membrane type (buzzer) which is attached to the side surface of a rock sample and connected to the generator of sinusoidal voltage Г3-112. Electric pulses come into action during application of voltage to graphite electrodes held close to the sample surfaces from the generator of rectangular pulses Г5-54 with amplitude up to 50V, the generator of sinusoidal voltage Г3-112 or from the generator of triangular pulses ГИ-1 with amplitude up to 300V. A short term influence of strong electrical discharges up to 800V is exerted with the help of condenser discharge device. Magnetic field is excited by a coil located near the other side surface so that the direction of magnetic field is perpendicular to that of electrical field. The coil is powered by the generator Г3-33.
AE signals are recorded by piezoelectric sensors produced by DECI Company in a wide frequency band from 80 kHz to 2 MHz and are then amplified by units of preliminary and measuring amplifiers. The equipment is in a standby mode and is launched when the signal exceeds the specific threshold value at the output of the measuring AE channel. The signals are digitized by the fast analogue-digital converters working in CAMAC standard, or by the analogue-digital converter unit of USB 3000, and are recorded to the hard disk of PC in automated mode. Fig.1 shows the basic diagram of AE measurements.
Fig. 1.Basic diagram of AE measurements. Д – AE sensor, АЭ, ЭЛ1, ЭЛ2 – electrodes, ИВ – impact source, ПУ – preamplifier, БС – unit of synchronization, ИУ – measuring amplifier, АД – amplitude discriminator, ПК – personal computer. Arrows show the longitudinal compression load.
The programs of AE registration allow watching the registration of accelerograms in a real-time mode. Usually, minimum 10 thousand AE events are recorded during one experiment.
Together with AE measurements, the changes of stress-deformation are controlled.
The role of sensors which record deformation (lateral and longitudinal) and load is played by the linear differential transformers LVDT designed to measure linear displacement. These sensors have a high accuracy, linearity, sensitivity and resolution. They also provide friction-free performance with high rigidity. The Laboratory uses two types of LVDT sensors produced by the Lucas Shaevitz Company: MHR 010 и MHR 050.
Then, the signals from LVDT sensors enter to the unit of conversion and amplification of tensometric parameters, and then the converted and amplified signals enter to the analogue-digital converter of ЛА-И24USB recorder. Then the digitized values are passed to the computer for processing by the software supplied with ЛА-И24USB recorder.
Basic diagram of tensometric measurements is shown in Fig.2.
Fig. 2. Basic diagram of tensometric channel
Since 2001, we have carried out a lot of experiments on rock samples and artificial materials with different types of energy actions.
