The samples of anthracite and bituminous coal are studied in cyclic freeze–thaw at different water contents. In the freeze–thaw cycles, the samples were subjected to continuous ultrasonic sounding. It is found that different contents of water have an essential influence on the spectrum of recorded signals. Regarding the water-unsaturated check samples, the spectrum change is reversible, and the freeze–thaw treatment results in no failure. An increase in the water content of the samples fosters the irreversible change in the spectra of the signals against the background of macro-cracking along stratification planes. Anthracite exhibits higher persistence to freeze–thaw damage than bituminous coal.

The thermal and mechanical tests of different porosity limestone show that an increase in the axial load results in the higher velocities of elastic waves while elevation of temperature decreases them. Higher temperatures act to raise velocities of P- and S-waves with increasing mechanical load, which enhances acoustic strain-sensitivity of rock. The spectral analysis of the recorded signals shows that higher temperature shifts spectrum maxima to lower frequency region. It is found that size of pores has influence on attenuation frequency of ultrasonic signals. The authors describe new approaches to acoustic strain-sensitivity control in rocks and to stress measurement reliability enhancement toward stability of underground structures.

The behavior of acoustic emission in uniaxial cyclic loading of rock salt samples from the Kaliningrad deposit is determined. The samples were tested under varied temperatures and ratios of maximal stresses in sequential loading cycles. The experimental curves of acoustic emission activity and maximal stress and temperature of the previous cycle are obtained. Stress memory in acoustic emission manifests itself equally stably under constant higher and lower temperatures. Memory of the maximal stress of the previous cycle persists under higher temperature in the next cycle and vanishes under lower temperature in the next cycle. In case of the same maximal stresses and constant or higher temperatures in the successive cycles, the stress memory effect is vague: the stress estimated on this base is lower than the maximal stress of the previous cycle.

Acoustic emission activity and longitudinal and volumetric deformations in rock salt samples subjected to uniaxial mechanical loading with a constant strain rate and thermal stress are measured. The features of acoustic emission during deformation under various thermobaric experimental conditions are analyzed It is shown that, in contrast to the deformation parameters, the change in the activity of acoustic emission at the boundaries of the indicated stages is nonmonotonic in nature, as well as features that make it possible to accurately determine each stage and estimate the elastic and strength properties of the rock salt

The effects of acoustic emission (AE) in soils during freezing and thawing under cyclic thermal and quasi-static mechanical loading have been studied. The composition and characteristics of the developed instrumentation set are presented. The informative AE parameters and primary data processing approaches were justified. The acoustic emission patterns of soil material behavior under the variable thermobaric conditions were obtained. The comparative tests were performed using static probing and ultrasonic scanning.

Effects of cyclic freezing and thawing (FTC) on coals properties, such as technological, structural and sorption, has recently attracted wide interest. This article is dedicated to studying of FTC on behavior of coals of different types at low- and high-temperature oxidation. Scanning electron microscopy revealed that coals particles morphology after FTC alters similarly for coals of different types. These changes relate to formation of partially oxidized coal matter films. Thermogravimetric studies allowed to observe that, after FTC, the combustion interval for bituminous coals was widened by additional high-temperature stage (750-900 degrees C). The rate of combustion and its activation energy were decreased significantly. As for lignite, FTC does not lead to any pronounced alterations of the combustion pattern. Isothermal calorimetry (at 40 degrees C) allowed revealing that FTC at the lignite led to increase of heat generation during low-temperature oxidation. But no significant changes were observed for bituminous coals. The obtained results on changes of coals structure and properties after FTC are similar to given in the literature regarding the behavior of coals after mild oxidation. This allows to presume that FTC of the studied coals led to their mild oxidation. The interrelation was found between the rate of deactivation of the active sites of the first type and the combustion rate constant. It was shown that this relation determines the decrease of the combustion rate and its activation energy for coals after FTC. Active sites of the second type presumably determine the intensity of the processes of coals low temperature oxidation.

The article presents the experimental studies into the stress memory in acoustic emission (Kaiser effect) in anthracite samples subjected to different number of freeze-thaw cycles. All in all, 6 groups of relatively uniform samples were tested; one group (0) was a reference set unexposed to cryo-thermal effects; the number of a group conformed with the number of treatment cycles. After the cryo-thermal action, all samples were subjected by two cycles of mechanical loading up to maximum stresses of 10 and 14 MPa, respectively. During a loading cycle, acoustic emission activity of a sample was measured, and the Kaiser effect in the second cycle of loading was assessed by two characteristics. These characteristics were the factor of retention FR of the stress memory, or the ratio of the stress of emission initiation in the second cycle to the maximal stress of the first cycle, and the index Δ, or the ratio of the averaged acoustic emission activities before and after the moment of the Kaiser effect. The values of FR and Δ are obtained and analyzed as functions of the number of anthracite freeze-thaw cycles. The value of FR drops but Δ grows with increasing cryogenic disintegration of coal. The maximal change in FR and Δ, and, accordingly, the highest dynamics of damage in anthracite takes place in the first cycles of cryo-thermal treatment.

Cyclic freezing–thawing can lead to fracture development in coal, affecting its mechanical and consumer properties. To study crack formations in coal, an ultrasonic sounding method using shear polarized waves was proposed. Samples of three coal types (anthracite, lignite and hard coal) were tested. The research results show that, in contrast to the shear wave velocity, the shear wave amplitude is extremely sensitive to the formation of new cracks at the early stages of cyclic freezing–thawing. Tests also show an inverse correlation between coal compressive strength and its tendency to form cracks under temperature impacts; shear wave attenuation increases more sharply in high-rank coals after the first freezing cycle. Spectral analysis of the received signals also confirmed significant crack formation in anthracite after the first freeze–thaw cycle. The initial anisotropy was determined, and its decrease with an increase in the number of freeze–thaw cycles was shown. The data obtained forms an experimental basis for the development of new approaches to preserve coal consumer properties during storage and transportation under severe natural and climatic conditions.

Synchronized acoustic emission and strain measurements were carried out in rock salt samples subjected simultaneously to different levels of uniaxial mechanical and incrementally increasing temperature effects. Methodological and hardware support of such measurements is described. Experimental dependences are obtained, which reflect changes in shear strains and acoustic emission activity of samples as functions of time and temperature for different axial stresses. As the stresses increase, rock salt transits to the stage of progressive creep at lower temperatures. The transition to each subsequent stage of the temperature effect is accompanied by an increase in the steepness of shear strains and activity-average acoustic emission. The patterns of changes in these parameters at the stages of steady and progressive creep of rock salt are analyzed. The advantages of using acoustic emission measurements to predict rock salt failure due to progressive creep, as well as their importance for solving the problem on estimating salt rocks properties in real thermobaric conditions for the construction and operation of underground gas storages are noted.

The subject of the research is to establish the fundamental laws of acoustic emission in frozen soils, which allow to create ways to control (monitor) their stability under the influence of variable temperature fields and quasistatic mechanical stress from engineering objects located on these grounds for various purposes. The applied importance of such methods is to increase the speed and reduce the complexity of engineering geological surveys in the northern regions of Russia, carried out with the aim of predicting the loss of stability of the bases of buildings and structures to ensure their safe operation. The study was performed on the original instrumental complex. Its description and characteristics are given. With the use of this complex, thermoacoustic emission effects arising from the repeated alternation of freezing and thawing cycles of the soil during the development of its deformed state, starting from the normal compaction phase and up to the final stage of destruction (the bulging phase), have been studied. It is shown that on the basis of such informative parameters as thermally stimulated activity and duration of acoustic emission pulses, an indicator can be obtained that quantitatively characterizes the stages of the stress-strain state of soils. An experimental dependence of the field of values of this indicator as a function of the mechanical stress and the fractional composition of the test soil is given. The qualitative convergence of this dependence with the classical soil deformation diagram obtained by N.M.Hersevanov is shown, where the stages of compaction, loss of stability (shifts) and destruction are highlighted. Possible physical mechanisms and features of the formation of an acoustic emission response at each of these stages are considered and substantiated. It is noted that the approaches to receiving, processing and interpreting acoustic emission measurement information, which are grounded within the framework of the study, allow to control and monitoring of the carrying capacity and stress-strain state of soils directly in the field.

The article is devoted to the investigation of the spectral characteristics of acoustic emission signals that appear under various schemes of composite materials loading. The tests involved samples of composites reinforced with layers sheets of carbon fiber fabric and dispersed carbon fibers. Based on the results of laboratory tests, a comparison is made between the traditional parameters of acoustic emission and the complete spectrograms of the acoustic emission response developed with the use of a special algorithm. The relationship between the emission activity and the change in the spectral composition of emission hits is shown. For example, for some composites, the acoustic emission memory effect (Kaiser effect) manifests itself not only in the time domain but also in the spectral domain in a form of a sharp change in the amplitudes in the frequency range 130/150 kHz. Also, when the samples were loaded according to the Brazilian scheme, the presence of the so-called "inverse" Kaiser effect is observed, in which the memory carrier "remembers" the previously experienced level of tensile stresses and reproduces this information during subsequent unloading. Such effect manifests itself in the form of a sharp change in the amplitudes in the low-frequency region of the spectrum.

Acoustic emission response of fossil coals being at different stages of metamorphism to cyclic variation of effective thermal stresses is experimentally investigated. The equipment and procedure used in the experiments are described. The features of the response are revealed and analyzed in the samples of anthracite, lignite and bituminous coal with different damage extent governed by the preliminary cyclic freezing and thawing, as well as by water saturation. It is shown that the signature of such features is a thermal analog of the Felicity effect which appears in each cycle of temperature action. The regularities of this effect are found, and their physical explanation is given based on the analysis of defect formation in coals at different stages of thermal treatment. The methodical approaches are proposed and substantiated, which allow structural damage, thermal resistance, oxidation and proneness to frost weathering of coal to be estimated by the Felicity effect in the acoustic emission response of coal to cyclic thermal forces. Possibility of using the found features to predict structural changes in coal products which are in long-term storage under specific climatic conditions, as well as for forecasting risk of self-heating and spontaneous combustion of coal products is discussed.

Stress memory in consolidating composites in acoustic emission is studied experimentally to understand feasibility of its application in stress state control in rock mass. The tests show that, owing to uniformity and comparatively high responsiveness of acoustic emission behavior under straining, composite materials, when placed in a geomedium, allow highly accurate identification of tensor of actual stresses in it.

Traditional methods of coal thermal resistance characterization are informative but considerably time-consuming and require utilization of a complex and expensive equipment. This limits the effectiveness of their application. In this paper, authors experimentally investigated potential application of thermally stimulated acoustic emission method for developing of relatively simple and rapid coals thermal resistance assessment method. Features of thermally stimulated acoustic emission of anthracite, lignites and bituminous coal samples subject to cyclic thermal loading have been experimentally investigated. For the first time, it has been shown that there exists a relationship of such patterns with structural parameters and properties of the coal samples, as well as their thermal resistance. The results indicate the possibility of applying the method of thermally stimulated acoustic emission to control processes of cryogenic disintegration and thermal resistance of fossil coals. The description of the equipment and methodological support needed for the implementation of this method have been provided.

The article describes the acoustic method developed by the authors to determine the contact conditions between coal seam and host rocks on the basis of boundary inverse problem solution. Justification of the applicability of the method is made by means of physical modeling, in which samples from the model material were subjected to step-by-step uniaxial loading with simultaneous ultrasonic sounding using a multichannel scheme. In this case, various friction conditions were set for different samples at the “sample-plate of the press” interface. Based on the results of sounding, the distribution of velocities in the illuminated domain of the sample was reconstructed using tomographic processing. The array of data obtained as a result of acoustic tomography was used as input data to solve the inverse problem. As a result of the solution of the inverse problem, it was shown that the appearance of slip regions at the boundaries of the sample generates tensile stresses, the magnitude of which reaches 0.8 from the axial load. The developed method makes it possible, on the basis of acoustic measurements, to identify areas with a reduced coefficient of friction between the coal seam and the host rocks, the presence of which may be the reason of dangerous dynamic phenomena.