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High-pressure gas (HPG) blasting is safe and environmentally friendly, and has replaced traditional explosive blasting in some applications. The characteristics of HPG impact load and the propagation law of the stress wave in layered rock mass (LRM) under HPG blasting were investigated experimentally and numerically. Through experimental tests of hole wall pressure, a segmented exponential model of the hole wall pressure under the impact of HPG was developed; it accurately accounted for the time history characteristics of the hole wall load. Experimental tests of stress wave propagation were conducted; the soft rock–hard rock interface was found to have a significant effect on stress wave propagation in the LRM. Based on the experimental results, a specific strain model for 10 MPa HPG impact was fitted for a single-hole LRM. A polynomial exponential model and a specific strain model (20 MPa HPG impact) were found for the double-hole LRM. The stress wave attenuation laws for single-hole and double-hole LRM were characterised well. The propagation characteristics of stress waves in LRM under the impact of HPG were analysed and a numerical model of stress wave propagation was established. The numerical model adopted the Riedel–Hiermater–Thoma (RHT) material model for the test parameters. The proposed segmented exponential models of hole wall pressure were applied to the lower, middle and upper parts of the blasthole. With the feasibility of the numerical model analysed, the stress wave propagation characteristics were studied by numerical simulation of an underground pipe gallery. This study provides theoretical guidance and practical value for improving rock-breaking theory and optimising HPG blasting in LRM. [ABSTRACT FROM AUTHOR]

Due to the lack of reliable dynamic shear test technology, there are few studies on dynamic pure shear performance of concrete materials. Based on the one-dimensional stress wave propagation theory, a large-scale torsional split Hopkinson Tube (LTSHT) with an outer diameter of 110 mm was developed. The LTSHT comprises a bearing system, an axial driving system, a four-lug symmetrical impact loading system, an incident tube, and a transmission tube. Numerical simulations showed that the LTSHT produced a well-defined torsional wave, and the bending wave was very small when the lug-error was not more than 0.05 mm. Furthermore, a data measurement and processing of the LTSHT were proposed. Dynamic pure shear tests with shear strain rate ∼14 s−1 on three thin-walled mortar specimens were conducted. The failure process was recorded. The dynamic shear strength was ∼8 MPa and shear stress–strain curves of the three thin-walled mortar specimens showed great repeatability. [ABSTRACT FROM AUTHOR]

Acoustoelasticity is an inexpensive and reliable technique for measuring stresses in mechanical structures, provided the influencing factors are controlled. However, ultrasonic waves propagating inside solid bodies are affected by temperature, roughness, differences in microstructure, defects, and others. Non-uniformity is one of the most influential characteristics as it is difficult to evaluate. In this study, experiments were performed to assess the time-of-flight (TOF) of critically refracted longitudinal (Lcr) waves traveling in different regions of a unidirectional carbon-epoxy sample with allowable manufacturing non-uniformities using transducers of 1 MHz and 3.5 MHz to vary the depth of propagation. A phased array system (PAS) with transducers of 5 and 10 MHz was used to identify the regions with non-uniformities in the structure through the signal-to-noise ratio (SNR) and B-Scan images. The material under test is a unidirectional composite bar made of carbon fibers (HexTow® AS4) pre-impregnated with epoxy matrix (HexPly® 8552). The investigation shows that the TOF of the Lcr wave and the SNR results can be linearly related, for an optimized configuration. Therefore, the influence of non-uniformities can be removed from TOF using SNR values. Based on the results, a coupled PAS-Lcr ultrasonic system is proposed to measure stress in composites, including those with non-uniformities. [ABSTRACT FROM AUTHOR]

Excavation disturbance on the dynamic variation of the three-dimensional stress field is the main cause for the dynamic disasters of the surrounding rock mass of the roof. The stress condition in the surrounding rock mass of the roof during entry excavation and its impact on entry stability are systemically studied in this study. It is found that the surrounding rock mass of the roof is mainly influenced by the combined effect of the stress unloading and stress transference induced by entry excavation. A servo-controlled true triaxial material testing system is used to conduct the true triaxial loading and unloading experiments of rocks under different stress paths. The influence of different stress paths, especially the variation of the principal stress direction, on the mechanical characteristics and fracture characteristics of rocks is investigated. The results indicate that the variation of the principal stress direction has a significant impact on the macroscopic fracture characteristics of the rock. The main macroscopic fracture plane of the rock highly depends on the intermediate principal stress. The fracture evolution of the roof rock mass during entry excavation is analyzed. The results show that the change of the three-dimensional stress field induces the formation of complex fracture networks in the surrounding rock mass of the roof. The roof is likely to dislocate horizontally and collapse. The corners of the entry are seriously damaged. Based on the above findings, a support scheme is proposed to maintain the stability of a gob-side entry. The field experience suggests that the support scheme can achieve good results. [ABSTRACT FROM AUTHOR]

Prediction/detection of exit burrs is critical in manufacturing automation. In this research, an intelligent burr sensing/monitoring scheme is proposed. Acoustic emission (AE) was selected to detect burr formation during drilling. For effective extraction of information contained in the collected AE signals, wavelet transform (WT) was adopted and the selected features through WT were fed into a back-propagation artificial neural net (ANN) as input vectors. To validate the in-process AE monitoring system, both WT-based ANN and cutting condition-based ANN outputs (cutting speed, feed, drill diameter, etc.) were compared with experimental data. The results show that the proposed scheme is not only efficient with fewer inputs, but more reliable in predicting drilling burr types over cutting condition-based ANN. [ABSTRACT FROM AUTHOR]

Acoustic emission patterns were monitored during electric-field-induced antiferroelectric to ferroelectric phase transitions for fine- and coarse-grained antiferroelectric lead lanthanum zirconate stannate titanate ceramics [Pb0.97La0.02(Zr0.77Sn0.14Ti0.09)O3]. For the initial phase-transition cycle, acoustic emissions were detected just before and right after the major strain changes. During subsequent cycles, the strain response becomes jumplike and associated with additional acoustic emissions. Acoustic emissions before strain changes are assigned to domain nucleation and those at high fields to domain jumps, nucleation-annihilation processes, or microcracking. The intermediate ones are associated with defect charge rearrangement within the material. Backswitching generates acoustic emissions in the fine-grained material but only for certain drive conditions in the coarse-grained ceramic. Implications for domain nucleation processes, strain, and the role of defects in antiferroelectrics are discussed. [ABSTRACT FROM AUTHOR]

A method using an acoustic emission (AE) technique to monitor automatically a tool wear was examined. A flank wear is calculated using a statistical model which uses RMS value of AE signal and cutting speed as variables. The investigation reveals that the AE signal is influenced by tool vibration, specially during chatter. This contradicts the theory that the AE signal is not susceptible to mechanical vibration. The paper discusses the mechanism that explains the effect of the tool vibration on the AE signals. [ABSTRACT FROM AUTHOR]

The concept of CIM and FMS has created a whole new area of what is called 'tool management system' (TSM). For efficient operation of TMS, a real-time monitoring of tool condition is described. Tool health monitoring is critical in today's automated production and is likely to become crucial as we progress towards the unmanned factory of the future. The paper describes the development of a microcomputer-based acoustic emission (AE) system for monitoring progressive wear of a cutting tool. It also includes results of experiments conducted with the system. The state-of-the-art in AE technique and its comparison with other techniques for tool condition monitoring are also presented. [ABSTRACT FROM AUTHOR]

Acoustic Emission (AE) events are often generated when a material undergoes irreversible changes to its internal structure and thus provides a basis for the detection of damage in structural components. A method for AE localisation featuring a single laser source coupled to three optical fibre Bragg grating sensors is described and experimentally demonstrated on a thin aluminium panel. The localisation algorithm analyses the time domain waveform and the corresponding frequency spectrum to determine the time of arrival and velocity of an AE signal from which its epicentre is estimated using a grid search. Experimental results are shown to demonstrate the accuracy of the method. [ABSTRACT FROM AUTHOR]

Current design and construction codes and standards for bamboo do not contain strength grading procedures beyond cursory visual inspections. This deficiency arguably limits the safe and economic use of the material. This paper presents findings from an international research project which seeks to develop a strength grading system for bamboo culms. Over 200 four-point bending tests were carried out on Guadua angustifolia Kunth culms for which numerous mechanical and physical properties were measured. Correlations between flexural strength, static modulus of elasticity, dynamic modulus of elasticity from stress waves and density provided mediocre results, with R2 ranging from 0·27 to 0·47. However, properties such as flexural stiffness, flexural capacity and mass per unit length, which are less dependent on geometric properties, provided much stronger correlations, with R2 ranging from 0·86 to 0·92. The quality of these correlations indicates that these could be used as indicating properties for flexural capacity in a simple yet reliable strength grading procedure for bamboo. The findings of this paper invite a reconsideration of the extant stress-based approach for bamboo design, and instead a move to a capacity-based approach, as is often used with engineered timber products. [ABSTRACT FROM AUTHOR]

• A new method to solve the signal ringing of intermediate strain rate material test was developed. • A signal restoration algorithm was developed based on vibration theory and numerical experiment. • The dynamic oscillation of the designed loading sensor was fully considered through modal analysis and dynamic simulation. • With dynamic material tests, this method was proved easy to conduct and provides a high strain rate up-limit to 1000/s. Loading measurement is one of the critical components of mechanical tests. Different technologies are applied for loading measurement at different spans of time and space. In the past century, the quasi-static hypothesis has been widely applied for low-speed loading conditions (strain rate < 1/s), and the stress wave theory has been applied for high-speed loading conditions (strain rate > 1000/s). However, so far, the feasibility of the quasi-static and stress wave approaches for loading measurement is questionable under intermediate strain rate loading conditions (1 000/ s > strain rate > 1/s). For quasi-static approach, the signal ringing (caused by dynamic oscillation) is the main problem that hinders the accuracy of the loading measurement with the increment of strain rate. In this present study, in order to tackle the ringing signal obtained from the intermediate strain rate material test, a signal recognition algorithm was introduced, named the semi-period perturbation algorithm (SPPA). As SPPA is based on the first-order spring-mass oscillatory model, the corresponding test method for intermediate strain rate test was called single-mode method (SMM). To satisfy the basic hypothesis of SPPA, the corresponding load cell and material specimen was designed. Through modal analysis and dynamic simulations, SPPA and the load cell-specimen assembly were proved effective in restoring the loading signal from the ringing signal. Based on the high-speed tests of 6061 aluminum alloy and Q235 steel, and compared with the preponderant material test approach based on SEP 1230/ISO 26,203–2 sample, the SMM was proved effective in the loading measurement of intermediate strain rate tests. What's more, based on the accessibility of the test design and the simplicity of the data processing procedure of SMM, this method exhibited great application potential for similar mechanical scenarios of impact tests. [ABSTRACT FROM AUTHOR]

The Midland Links motorway viaducts, carrying the M5 and M6 around Birmingham, UK have been subject to a regular programme of assessment, repair and strengthening since 1989, after corrosion problems, primarily due to chloride contamination, were first detected in 1979. A number of the longer spans comprise multiple steel and concrete composite box girders and some of these superstructures are supported by steel box girder cross-beams. Assessment showed areas of very high overstress in the web plate at the locations of internal bracings. These overstresses affected ultimate, serviceability and fatigue limit states and arose because of poor detailing of the cross-braces and stiffeners. Acoustic emission sensors were installed to determine if the predicted fatigue activity was actually occurring and elastic shell finite-element (FE) modelling was undertaken for an improved determination of stresses. Plastic redistribution was considered to demonstrate adequate ultimate limit state (ULS) performance and an analysis was undertaken with potential fatigue damage modelled to prove the structure would not collapse with such damage at the ULS. In situ strain monitoring was undertaken to calibrate the FE results, improve the predicted fatigue life and allow preparation of a long-term strategy for managing and monitoring fatigue activity using a damage tolerant approach. [ABSTRACT FROM AUTHOR]

Acoustic emissions generated by a structure under stressed condition provide an insight in to the dynamic behaviour of flaws in the structure for characterization of failure modes. Fiber failure mechanism in T-700 carbon epoxy composites is characterized by testing unidirectional specimens in longitudinal mode. Acoustic emission parameters like amplitude, energy, duration, and signal strength have been recorded and studied with respect to the applied load to assess the fiber failure characteristics. The AE data is analyzed with different correlation plots for visual pattern recognition. Significant fiber breakage is observed at above 70% of the load. Bi-linear trend of the cumulative amplitude distribution curve indicates distinctively matrix and fiber failures. Matrix cracking failure mechanism dominated the entire loading cycle and is represented by AE hits of up to 85 to 90 dB amplitude and the peak amplitude distribution is 58 to 75 dB. The wave forms of matrix cracking hits with less than 90 dB and 100 units of energy are having up to 273 kHz frequency with a peak around 100 kHz. The wave forms of fiber breakage hits with more than 90 dB and 100 units of energy have up to 448 kHz frequency and with a peak from 168 to 437 kHz. From the low amplitude filtering technique the border line for fiber breakage is observed from 89 to 92 dB. [ABSTRACT FROM AUTHOR]

• Pulse duration and confinement affect the amplitude and profile of laser-generated stress waves. • At low fluence without confinement, stress profiles are conformal to that of the laser pulses. • Under high fluence with confinement, pulse duration makes little difference in measured stress. • Without confinement, shorter pulses produce higher stress and strain rates. • With confinement, pulse duration only affects stress generation under low laser fluence. Laser-induced stress waves have been a powerful technique for high strain-rate material characterization. The stress amplitude and duration can be strongly influenced by both laser characteristics and sample configuration. In this study, stress histories of silicon substrates loaded by pico- and nanoseconds laser pulses, with and without confinement of the interaction volume, were investigated. Shorter pulse duration and confinement of the absorbing medium have been found to increase the amplitude of laser-induced stress waves, but they may also alter the stress evolution. Without confinement or at low energy densities the stress wave profile is similar to that of the laser pulse. At high energy densities with confinement shock loading with stretched unloading is observed. [ABSTRACT FROM AUTHOR]

A new methodological approach to the location of noise-like sources of continuous acoustic emissions with a weak signal-to-noise ratio is suggested based on spectral classification of signals and emphasis on the specific features in them that are intrinsic for a particular source. The validity of the procedure was experimentally proven for hydraulic testing of pressure vessels for integrity. [ABSTRACT FROM AUTHOR]

Passive acoustic mapping (PAM) has been recently demonstrated as amethod of monitoring focused ultrasound therapy by reconstructing the emissions created by inertially cavitating bubbles (Jensen et al 2012 Radiology 262 252-61). The published method sums energy emitted by cavitation from the focal region within the tissue and uses a threshold to determine when sufficient energy has been delivered for ablation. The present work builds on this approach to provide a high-intensity focused ultrasound (HIFU) treatment monitoring software that displays both real-time temperature maps and a prediction of the ablated tissue region. This is achieved by determining heat deposition from two sources: (i) acoustic absorption of the primary HIFU beam which is calculated via a nonlinear model, and (ii) absorption of energy from bubble acoustic emissions which is estimated from measurements. The two sources of heat are used as inputs to the bioheat equation that gives an estimate of the temperature of the tissue as well as estimates of tissue ablation. The method has been applied to ex vivo ox liver samples and the estimated temperature is compared to the measured temperature and shows good agreement, capturing the effect of cavitation-enhanced heating on temperature evolution. In conclusion, it is demonstrated that by using PAM and predictions of heating it is possible to produce an evolving estimate of cell death during exposure in order to guide treatment for monitoring ablative HIFU therapy. [ABSTRACT FROM AUTHOR]

Um Bauwerke bis zu ihrem wirklichen Lebensende zu nutzen, ist es von großer Bedeutung, den aktuellen Grad der Schädigung einer Struktur bestimmen zu können. Mithilfe der zurzeit gültigen Nachweis- und Bemessungskonzepte (z.B. CEB-FIP Model Code 2010) ist dies nicht möglich. Ein gangbarer Weg, den Schädigungsgrad einer Betonstruktur vor Ort zu bestimmen, ist der Einsatz von zerstörungsfreien Prüfmethoden (Monitoring). Dieser Bereich ist allerdings noch nicht zur Gänze erforscht. Das konstante Monitoring von der Entstehung eines Bauwerks bis hin zu dessen Lebensende wird als eine vielversprechende Möglichkeit der Lebenszeitabschätzung gesehen. In diesem Beitrag werden Ermüdungsversuche an Betonprüfkörpern, begleitet mit konstantem Monitoring durch Ultraschall- und Körperschall-Sensoren, näher betrachtet und ein möglicher Weg zur Bestimmung des Schädigungsgrades und der Lebenszeitabschätzung aufgezeigt. Monitoring based lifetime assessment of concrete structures - Research Project MOSES In order to use structures up to their real end of lifetime it is of great importance to know the degree of damage of the structure. By using the actual Codes and Specifications (e.g. CEB-FIP Model Code 2010) it is not possible to define the real degree of deterioration. A practicable way of determining the degree on-site is the employment of non-destructive testing methods (monitoring). This field until now is not finally explored. The constant monitoring from the erection of a structure up to the end of its lifetime is seen as a very promising possibility to assess the residual lifetime. In this article fatigue tests on concrete specimens, accompanied with ultrasonic and acoustic emission measurements will be investigated closer and a possible way for the determination of the degree of damage and lifetime assessment will be proposed. [ABSTRACT FROM AUTHOR]

Acoustic emission by a modulated two-dimensional electron gas (2DEG) is studied analytically, numerically, and experimentally. Optical reflectance measurements provide the full acoustic spectrum emitted by a modulated GaN-based 2DEG, including various surface acoustic wave (SAW) modes that propagate with different polarizations and penetration depths within the epitaxial layer structure. A 2DEG is shown to provide a source of SAWs that is several orders of magnitude more intense than an interdigital transducer (IDT) of the same chip area. Furthermore, a 2DEG SAW source is shown to have the ability to switch between different acoustic modes based on applied DC bias conditions. [ABSTRACT FROM AUTHOR]

The resonance-free frequency response of the thermo-acoustic emission is demonstrated under a full digital drive. The device is composed of a thin-film heater electrode, a nano-porous silicon layer, and a single-crystalline silicon wafer. When sequential electrical pulse trains converted by the density modulation of an analog signal are introduced into the heater electrode, a significant sound pressure is reproduced with a sufficiently low distortion. The characteristic output behaviour in the audible ultrasonic band is clarified in either open- or closed-space. The advantageous features of thermally induced sound emission and its underlying physics have been made clear. [ABSTRACT FROM AUTHOR]

The propagation of a phase front during a thermally induced martensitic transition is discontinuous due to pinning at various defects, an effect which results in acoustic emission. Here we analyze the consequences of an applied compressive stress exemplarily on a Ni50.4Mn27.9Ga21.7 single crystal. Our experiments show that the distribution of the energies of the acoustic emission events follows a power law for more than three decades. This indicates that the transition exhibits avalanche criticality. The exponent characterizing the distribution of energies depends on the applied stress, and decreases from 1.9 ± 0.1 at zero stress to 1.5 ± 0.2 at stress above 3 MPa. This decrease could be attributed to the reduced multiplicity of variants possible under uniaxial compression. [ABSTRACT FROM AUTHOR]

Results are provided for experimental study of the deformation and failure mechanism for polymer materials with specimen uniaxial tension. An acoustic emission method is used for quantitative evaluation of the test process. Comparative analysis is provided for volumetric structural changes in specimens and corresponding acoustic emission elastic wave parameters for brittle and ductile materials. [ABSTRACT FROM AUTHOR]

Safety of suspension or stay cable bridges depends on the durability of their cables; stay cable bridges may present a healthy aspect on observable lengths but may be damaged in the nonvisible parts (anchorages). It is important to be able to detect as early as possible the defects affecting them. In this study, the acoustic emission (AE) technique is employed to detect the presence and location of broken wires in anchorage. Allowing a cable to vibrate can induce AE from interwire fretting. The study of the behavior of a broken wire during bending contributed to the understanding of the origin of the AE, that is, interwire fretting and the definition of the most suited bending conditions. For the identification of interwire friction, the main parameters of the acoustic signals are the number of events, their energy, and frequency distribution. These parameters are very dependent of roughness, lubrication, contact strength between wires, and recovery length. Several cables were studied with various surface conditions including corroded, dry, and oiled. [ABSTRACT FROM AUTHOR]

This paper discusses monitoring of bonded posttensioned (PT) concrete elements using the acoustic emission technique. In particular, a statistical pattern recognition technique based on a multivariate outlier analysis is presented to identify initial yielding and the onset of failure. Experimental tests on large-scale single-tendon bonded PT concrete beams, subjected to multiple load cycles, will be presented to validate the proposed monitoring system. [ABSTRACT FROM AUTHOR]

Modern day aerospace, automotive, marine, mechanical and civil structures rely on the advanced composites for their added benefits over conventional metallic structures. The complex damage process in composites involves various failure modes such as matrix cracking, fibre-matrix debonding, fibre fracture and delamination. This paper presents an acoustic emission technique for the failure characterisation of top-hat stiffener specimens using conventional piezoelectric acoustic emission sensors and modern fibre Bragg Gratings. Strain insensitive fibre Bragg Grating sensors are used in this experimental investigation to compare the performance of piezoelectric sensors for composite structures. This work has contributed to the development of an acoustic emission sensing system based on fibre Bragg Gratings. Main objectives of this work are to compare the sensing characteristics simultaneously in a composite structure with a surface-bonded acoustic emission–fibre Bragg Grating sensor and acoustic emission–piezoelectric sensor. The feasibility of the system is demonstrated in typical applications of in-situ structural health monitoring based on acoustic emission techniques. [ABSTRACT FROM PUBLISHER]

A new acoustic emission source location method is presented based on near-field beamforming, that is, the acoustic emission source location method is established based the beamforming process of acoustic emission signals measured by a few sensors installed in small region. First, the propagation characteristics' differences between the acoustic signals and acoustic emission signals by using plate wave theory, and on the basis of these results the influencing factors in acoustic emission beamforming location is analyzed. Then, the present method is verified by artificial acoustic emission which is created by breaking a mechanical pencil lead on the surface of the plate. Finally, the present method is used to locate the rotor-stator rub fault occurring on the experimental device of rub fault, and identify rub position efficiently. The results show that it is an optional method for locating AE source. [ABSTRACT FROM AUTHOR]

Implications of infusion of small (compared to the pore space) amounts of water for the dynamics of fracturing are studied in the laboratory experiments on the models of a heterogeneous geological medium. Variations in the acoustic emission that precede macrofracture are analyzed, including as candidate precursors. In all experiments, the macrofracture had occurred not immediately after the mechanical loading or water infusion but with a time-delay after a period of acoustic quiescence and subsequent activation. The enhancement in acoustic activity preceding macrofracture is well reproduced by the exponential law; the correlation between the actual number of events (or the released energy) and the exponential approximation exceeds the 95% confidence level. The power law is slightly worse although also a confident approximation of the acoustic emission process. The facts of subsequent occurrence of quiescence and activation suggest that, in principle, this phenomenon can be used as a precursory signature in the prediction of macrofracturing. [ABSTRACT FROM AUTHOR]

The various failure mechanisms in bidirectional glass/epoxy laminates loaded in tension are identified using acoustic emission (AE) analysis. AE data recorded during the tensile testing of a single layer specimen are used to identify matrix cracking and fiber failure, while delamination signals are characterized using a two-layer specimen with a pre-induced defect. Parametric studies using AE count rate and cumulative counts allowed damage discrimination at different levels of loading and Fuzzy C-means clustering associated with principal component analysis were used to discriminate between failure mechanisms. The two above methods led to AE waveform selection: On selected waveforms, Fast Fourier Transform (FFT) enabled calculating the frequency content of each damage mechanism. Continuous wavelet transform allowed identifying frequency range and time history for failure modes, whilst noise content associated with the different failure modes was calculated and removed by discrete wavelet transform. Short Time FFT finally highlighted the possible failure mechanism associated with each signal. [ABSTRACT FROM AUTHOR]

The unreliability of AE signal generation within the wall of a steel object in any Lamb wave mode and the unrepresentative nature of its properties are demonstrated. Results are presented for experiments on whose basis it is concluded that under hydraulic testing conditions for objects absence of AE signals at a receiving point does not mean absence of their emission by developing defects, and for a number of reasons it is impossible to provide reliable calibration of AE equipment within an object filled with water. [ABSTRACT FROM AUTHOR]

Some of the most significant architectural works are monumental masonry constructions. Among these, the Cathedral of Syracuse can be viewed as a fundamental element in the cultural heritage of Europe. For the preservation of these monuments, it is necessary to assess their durability by taking into account cumulative damage and cracking conditions in the structures. The paper describes the methods used by the authors to determine the conditions of the materials and the crack patterns in the stonework structures of the Cathedral. In particular, the acoustic emission (AE) technique is used to evaluate the time dependence of damage and the onset of critical conditions in a pillar, which is part of the vertical load-bearing structures. The authors show that the damage evolution in the stonework structures, experimentally investigated in situ by the AE technique, can be described by a power law characterised by a non-integer exponent, β. In this way, the time dependence of damage is evaluated by working out the β exponent and making a prediction of the stability conditions of the structure. Furthermore, the achievement of the critical condition is characterised through another synthetic parameter, the b-value of the Gutenberg-Richter law. The b-value systematically changes during the different stages of the failure process and tends to 1.0 as the structure reaches the final collapse. In the present study, this behaviour is documented by several AE tests carried out on specimens of different dimensions extracted from the pillar. In addition, these results are compared with the AE data obtained from the in situ-monitored pillar. [ABSTRACT FROM AUTHOR]

Investigations on the effect of strain rate on tensile properties of two materials, namely, aluminum alloy 7075 T651 and IS 2062 mild steel, are presented. Experimental studies were carried out on tensile split Hopkinson pressure bar (SHPB) apparatus in the strain rate range of 54-164/s. Uncertainty analysis for the experimental results is presented. Johnson-Cook material constitutive model was applied to predict the tensile yield strength of the tested materials at different strain rates. It is observed that the tensile yield strength is enhanced compared with that at quasi-static loading. During tensile SHPB testing of the specimens, it was observed that the peak force obtained from the strain gauge mounted on the transmitter bar is lower than the peak force obtained from the strain gauge mounted on the incident bar. An explanation to this is provided based on the increase in dislocation density and necking in the tested specimens during high strain rate loading and the consequent stress wave attenuation as it propagates within the specimen. The fracture behavior and effect of high strain rate testing on microstructure changes are examined. The peak force obtained based on strain gauge mounted on the transmitter bar is lower than the peak force obtained based on strain gauge mounted on the incident bar. There is an increase in tensile yield strength at high strain rate loading compared with that at quasi-static loading for both materials. The enhancement is more for IS 2062 mild steel than that for aluminum alloy 7075 T651. In the range of parameters considered, the strength enhancement factor was up to 1.3 for aluminum alloy 7075 T651 and it was up to 1.8 for IS 2062 mild steel. Generally, there was a good match between the experimental values and the Johnson-Cook model predictions. [ABSTRACT FROM AUTHOR]

This paper presents results of acoustic emission (AE) testing carried out during hydrotesting of cold and hot exchange towers of a heavy water plant. Point type defect indications were observed in certain locations during ultrasonic inspection of the exchange towers. Acoustic emission monitoring during hydrotesting was carried out to determine the nature of these defects, ie whether these defects are growing or not. The cold and the hot towers were pressurised to a maximum pressure of 26.5 kg/cm2 and 27.8 kg/cm2, respectively, with holds at different pressures. Acoustic emissions were picked up using sensors placed selectively at areas of interest where ultrasonic testing showed defect-like indications and at some critical areas. AE events were observed during the first pressurisation of both the towers and were attributed to micro yielding. No significant acoustic emissions were detected from either tower during the hold periods or the repressurisation. This confirmed that there is no extraneous noise and there are no severe defects in the regions monitored by the sensor arrays and nearby regions monitored by individual sensors. The combined results from AE and ultrasonic techniques confirmed the structural integrity of different regions of the towers and supported returning the towers to service for continued operation. [ABSTRACT FROM AUTHOR]

Authors of the paper present investigation results concerning properties of ADP, ADC and ADNC descriptors which have been obtained during measurements made within three oil transformers and then tested also by other methods (electric and dissolved gas analysis ones). Methodology which makes easier an analysis and identification of acoustic emission signals generated by partial discharges is presented. Results obtained by acoustic emission method have been referred to results coming from other measuring methods. [ABSTRACT FROM AUTHOR]

Some of the most interesting questions in geosciences are whether results from laboratory experiments can be applied to processes in the earth crust and whether in situ studies with high spatio-temporal resolution can bridge the gap between laboratory work and seismology. In this study, acoustic emission (AE) activity caused by stress changes due to the backfilling of a cavity in an abandoned salt mine is studied to answer questions regarding (1) the dependence of AE event rates, event distribution and b-value on the stress state, (2) the stress memory effect of rock (Kaiser effect), (3) the possibility to detect significant changes in the system like the initiation of macrocracks and (4) the possibility to estimate future activity from previous AE records. The large number of events studied allows a spatial resolution of the order of 1 m and a temporal one on the order of 1 hr. Stress changes are created due to the thermal expansion and contraction of the rock mass in response to the temperature changes caused by the backfilling. A roughly 20 × 50 × 50 m section of the mining complex just above the backfilled cavity is well covered by a network of 24 piezo-electric receivers and poses an optimal volume for the study. Results of a 2-D finite element thermoelastic stress model are in agreement with the spatio-temporal AE event distribution. In addition to the initial upward migration of the AE event front, which correlates with the calculated stress field, the rock salt exhibits a pronounced Kaiser effect for the first few thermal loading cycles throughout the whole study region. The deviation from the Kaiser effect during later loading cycles seems to be caused by the initiation of a planar macroscopic crack, which is subsequently reactivated. AE activity tends to concentrate along this macrocrack. Calculated b-values decrease before and increase after the supposed initiation of the macrocrack supporting this explanation. In intact rock volumes not subjected to macrocracking a linear relation between the maximum event rate and the calculated absolute Coulomb stress increase is observed. This indicates that future maximum AE event rates can be estimated from expected loading. AE activity during stress loading cycles is most prominent in regions with Coulomb stress maxima indicating possible shear cracking that has the potential to create macrocracks. Strong bursts of AE activity observed during thermal unloading phases are concentrated in regions for which the minimum principal stress becomes tensile. These regions exhibit significantly higher b-values than those active during thermal loading. We interpret these weak events with tensile microcracking. [ABSTRACT FROM AUTHOR]

Complex researches of light-emitting structures based on A3B5 compounds have been carried out. It has been shown that at current loading exceeding the acoustic-emission threshold, there arises a change in the electroluminescence intensity, fluctuation of current and light. It has been ascertained that natural ageing leads to a general improvement of some practically important parameters of light-emitting structures, in particular to increase of maximal admissible currents and reliability. [ABSTRACT FROM AUTHOR]