Acoustic Emission Energy Distribution Research Articles

Pre-peak acoustic emission characteristics of tight sandstone failure under true triaxial stress

In this study, experimental investigations were conducted on low-porosity sandstone specimens to reveal their pre-peak cracking characteristics under constant mean stress (σm, 100 MPa) and varying stress Lode angles (θσ, −30° to +30°). The acoustic emission (AE) characteristics 50 s before specimen failure, as well as the strengths and fracturing surface networks, are presented and discussed. The results show that the values of the three principal stresses when the specimen failure occurs change linearly with increasing θσ, in which the maximum principal stress linearly decreases. The AE energy distribution exhibits three ranges, which can be speculated as the different stages of rock cracking propagation. The AE peak frequency is distributed unevenly in seven bands, in which over 90% of the AE signals are located in the 75–95 kHz and 96–130 kHz ranges, which correspond to the tensile and shear cracks, respectively. The RA-AF dynamic evolution curves reveal that the ultimate failure is driven by shear fractures under 3D stress states (for all θσ). The pattern of macrocrack networks shows the “double-X″ through-going mode, and clear tensile and shear areas were directly observed on the fracturing surfaces. These findings are deemed essential for determining the failure mechanism of deep engineering rock masses.

Experimental Analysis of Sandstone Under Uniaxial Cyclic Loading Through Acoustic Emission Statistics

The aim of this study is to assess the evolution of acoustic emission (AE) energy distribution and acoustic emission waiting time of sandstone under uniaxial cyclic loading. Experimental study was conducted to collect AE data for this purpose. The uniaxial cyclic loading test results were statistically analyzed to determine the change in the AE energy distribution as the number of loading cycles increases. The outcomes of this study show that the relationship between energy distributions of acoustic emission signals, P(E), and the absolute energy level can be expressed by a power law where its exponent, e, changes as the number of cycles increases. Close to the failure stage, e decreases sharply as the number of cycles increases in accordance with the mean field theory. Furthermore, the existence of temporal correlations was studied by the waiting time and it reveals the existence of acoustic emission signals clustering also in power law (characterized by τ). The evolutions of e and τ identify a way to signal cyclic loading collapse for brittle material before the sudden disaster failure.

Experimental investigation of correlation between attrition wear and features of acoustic emission signals in single-grit grinding

The cutting performance of grit is controlled by its shape and geometric parameters. However, the geometry changes with the progression of attrition wear, thereby increasing grinding temperature, grinding force and decreasing grinding efficiency. It is important to monitor the cutting condition of grit to confirm its state of wear. Acoustic emission (AE) signals have been widely used in monitoring cutting tool conditions and have been proven effective in detecting tool wear and failure. To identify the correlation between attrition wear and AE features, comparison tests were carried out with a pair of sharp and blunt grits scratching on hardened AISI4340 workpiece and the AE signals during the scratching process were obtained. The signals were analyzed in the time, frequency, and time-frequency domain The numbers of AE counts in the time domain differed for the sharp and blunt grits. It is also found that the proportions of AE energy in the frequency band of 0–90 kHz and 90–250 kHz differed was attributed to differences in strength of the various AE sources. The AE energy was further analyzed in time-frequency domain using discredit wavelet transform (DWT). The AE energy at certain decomposition level was extracted. A validation test was carried out with a worn grit. It was suggested that features associated with AE energy are appropriate for detecting attrition wear of grit, but the number of AE counts fluctuated owing to the complex friction behavior of the worn surfaces. The results of AE energy distribution in the time-frequency domain reveal that the amplitude of the AE energy deteriorates more rapidly when the grit is worn.

Impact fracture of ZnSe ceramics

Structurally different ZnSe ceramics prepared by various techniques were subjected to fallingweight impact fracture. Mechanoluminescence (ML) pulses generated during the motion and multiplication of dislocations were detected, as well as acoustic emission (AE) pulses produced predominantly during the growth of macroscopic (on the specimen scale) cracks. The luminescence began immediately at the moment of contact of a striker with the surface of the specimen, whereas the emission of sound occurred within 50–100 μs after the impact. The emission maxima in the ML and AE time series coincided with each other. The signal series were used to construct energy distributions upon the emission of light and the generation of sound. It was established that the ML amplitude (the number of emitted photons) is proportional to the energy released due to dislocation rearrangements, and the intensity (the square of the amplitude) of AE pulses is proportional to the energy released due to discontinuities of the material. It was found that the ML energy distribution follows a power law, which indicates the self-organization of an ensemble of dislocations during rapid plastic deformation. The AE energy distribution, on the contrary, was found to be random, i.e., typical of the growth of non-interacting cracks. It was shown that the efficiency of the interaction of dislocations depends, to a certain extent, on the technological prehistory of ZnSe ceramics.

Impact fracture of granite at temperatures from 20 to 500 C

Earthquake nucleation and fracture propagation in deformed rocks generate elastic waves, within acoustic frequencies. Strain-induced acoustic waves appear both in field tectonic structures and in laboratory samples, thus making laboratory acoustic emission (AE) data from load tests suitable to interpret natural seismic processes. However, laboratory tests are commonly run at room temperature, while the natural rocks at the earthquake origin depths are as hot as hundreds of degrees centigrade. We report AE data for thermally and mechanically loaded granites subjected to impact fracture at different temperatures. The energy distribution in the time series of acoustic signals emitted from fine-grained granite fits a power law of the type of the Gutenberg-Richter relationship at temperatures from 20 to 500C. Medium- and coarse-grained samples behave in this way only within 300C but show a Poissonian (random) AE energy distribution above 300C.

Strain localization peculiarities and distribution of acoustic emission sources in rock samples tested by uniaxial compression and exposed to electric pulses

Strain localization peculiarities and distribution of acoustic emission sources in rock samples tested by uniaxial compression and exposed to electric pulses

Deviations from the Gutenberg-Richter law

The energy distributions of acoustic emission (AE) signals have been analyzed on two scaling levels corresponding to (i) deformation of granite samples and (ii) processes on a commercial mining enterprise. It is established that, in cases of localized fracture, the AE energy distribution has a commonly accepted shape described by a power law, while a dispersed fracture is characterized by an exponential energy distribution of the AE signals. Analysis of the functional form of the energy distribution performed at the early stage of loading allows one to recognize a spatial region in the sample where localization of the defect formation will subsequently take place.

Effect of Ti and Al on the pulverization resistance of MgNi-based metal hydride electrodes evaluated by acoustic emission

The pulverization of amorphous MgNi, Mg0.9Ti0.1Ni and Mg0.9Ti0.1NiAl0.05 materials prepared by mechanical alloying was evaluated by acoustic emission (AE) measurements during their electrochemical hydriding. It was confirmed that the pulverization of MgNi-based electrode occurs mostly at the end of the charge when the hydrogen evolution reaction is initiated. On the basis of the AE activity measurements, the positive influence of the Ti substitution and Al addition on the MgNi-based alloy pulverization resistance is demonstrated. This is mainly attributed to the lower porosity of these materials, limiting the accumulation of H2 bubbles in the agglomerate pores. Combined with a better corrosion resistance, this characteristic produces an improvement of the cycle life of the MH electrodes. It was also shown that the AE energy distribution can be described by the Gutenberg–Richter relationship. The AE energy is higher for cracking related to the volume expansion than for the pulverization associated with the formation of H2 bubbles in the agglomerate pores. Ti substitution and Al addition also induce a larger proportion of high-energy AE signals.

Monitoring the initiation and growth of delamination in composite materials using acoustic emission under quasi-static three-point bending test

Control and optimization of machining processes are important issues affecting the development of productivity. Monitoring systems have become indispensable in the evaluation of materials during machining. In this paper, the path toward the delamination-free drilling of glass/epoxy composite material is established using some novel methods based on acoustic emission features. Acoustic emission monitoring with three techniques, sentry function, acoustic emission energy distribution, and acoustic emission count distribution, are developed to detect and realize the critical force at the onset of delamination process on glass/epoxy composite materials. The three-point bending test simulates thrust force, the most effective factor in delamination, throughout the process of drilling without backup plate. Sentry function is regarded as a new method based on the combination of AE information and mechanical behavior of composite materials. The sentry function was used to study the initiation and growth of delamination process. Two types of specimen with different layups, woven [0, 90]s and unidirectional [0]s, leading to different levels of damage evolution, were studied. Results show that AE parameters and sentry function method are useful tools for the examination of initiation and the growth of delamination during drilling process and can help to avoid delamination damage while drilling.

Effect of tetragonality ratio on photo-induced domain switching in poled (Pb1-xLax)TiO3 ferroelectric ceramics

The effect of the tetragonality (c/a) ratio on photo-induced domain switching in (Pb1−xLax)TiO3 ferroelectric ceramics preliminarily poled at 1.5kVmm−1 was investigated by measuring photovoltaic current and acoustic emission (AE) signals. Results of total increment of photovoltaic current (Jpv), total AE event count and AE energy distribution showed that non-steady state behavior of Jpv was dominantly dependent on the activity of 90° domain switching during illumination. The activity of photo-induced domain switching was explained in terms of the coercive field and the space charge field. It was confirmed that the magnitudes of the space charge field and coercive field were found to be optimal for the photo-induced 90° domain switching at a c/a of 1.021.

Study on Fractal Characteristics of Acoustic Emission and Crack Propagation in Rocks.

In this paper, the fractal nature of crack propagation in rocks is demonstrated by means of acoustic emission (AE) method. AE behavior is monitored during fracture toughness tests conducted on compact tension specimens of three types of rocks. It is shown that the microcrack fracture process zone proceeds the macroscopic crack extension, resulting in nonlinear fracture behavior. Iidate granite, Tohoku marble and Akiyoshi limestone were used. In Iidate granite, the fracture process zone grows in narrow region. Tohoku marble shows that the fracture process zone consists of distributed microcracks surrounding the marcroscopic crack. In Akiyoshi limestone which is preheat treated, extensive crack branching is observed. The AE behavior is characterized by a fractal dimension b which is the slope of the log-log frequency and AE energy distribution. The trend in variation of b-value is shown to correspond to the crack propagation bahavior. In addition, it is found that the fluctuation of b-value can also be characterized by fractal. This observation implies that the self-similarity lies in the rock fracture process.