Showing papers on "Acoustic emission published in 1997"

Distributed damage, faulting, and friction

Abstract: We present a formulation for mechanical modeling of geological processes in the seismogenic crust using damage rheology. The seismogenic layer is treated as an elastic medium where distributed damage, modifying the elastic stiffness, evolves as a function of the deformation history. The model damage rheology is based on thermodynamic principles and fundamental observations of rock deformation. The theoretical analysis leads to a kinetic equation for damage evolution having two principal coefficients. The first is a criterion for the transition between strength degradation and recovering (healing), and is related to friction. The second is a rate coefficient of damage evolution which can have different values or functional forms for positive (degradation) and negative (healing) evolution. We constrain these coefficients by fitting model predictions to laboratory data, including coefficient of friction in sawcut setting, intact strength in fracture experiments, first yielding in faulting experiments under three-dimensional strain, onset and evolution of acoustic emission, and dynamic instability. The model damage rheology accounts for many realistic features of three-dimensional deformation fields associated with an earthquake cycle. These include aseismic deformation, gradual strength degradation, development of process zones and branching faults around high-damage areas, strain localization, brittle failure, and state dependent friction. Some properties of the model damage rheology (e.g., cyclic stick-slip behavior with possible accompanying creep) are illustrated with simplified analytical results. The developments of the paper provide an internally consistent framework for simulating long histories of crustal deformation, and studying the coupled evolution of regional earthquakes and faults. This is done in a follow up work.

Statistical Properties of Fracture Precursors

Abstract: We present the data of a mode-I fracture experiment. The samples are broken under imposed pressure. The acoustic emission of microfractures before the breakup of the sample is registered. From the acoustic signals, the position of microfractures and the energy released are calculated. A measure of the clustering of microfractures yields information about the critical load. The statistics from energy measurements strongly suggest that the fracture can be viewed as a critical phenomenon; energy events are distributed in magnitude as a power law, and a critical exponent is found for the energy near fracture. [S0031-9007(97)04346-9] PACS numbers: 62.20.Mk, 46.30.Nz Fracture is a problem which has recently received a lot of attention in the physics community [1–3]. It is troublesome to calculate the force needed to break a heterogeneous material. Instead, it is customary to resort to tests involving the destruction of the sample. Therefore it is interesting to provide additional knowledge about cracks by studying the events that occur prior to the fracture. Besides , despite great experimental and numerical efforts [1– 6], many aspects still remain unclear about the fracture process itself. Conceptually simple models, such as per-colation [6] and self-organized criticality [7], are attractive but often fail to convey the complex phenomenology observed. The main motivation of this work is to understand if these models can reproduce the main features of crack formation. We report here some experimental results that may help to gain valuable information in that direction. Our main tool is the monitoring of the microfractures, which occur before the final breakup, by recording their acoustic emissions (AE). Because of its ability to pinpoint the emission source, this technique has been widely used in seismography and to map the nucleation of fractures [8]. From these signals, we have also obtained the acoustic energy of each microfracture, which is a fraction of the total energy released. The behavior of the energy just before fracture is a good parameter to compare with the above mentioned models. In order to avoid noise, we have designed a setup in which there are no moving parts, the force being exerted by pressurized air (see Fig. 1). A circular sample having a diameter of 22 cm and a thickness of 5 mm is placed between two chambers between which a pressure difference P ෇ P 2 2 P 1 is imposed. The deformation of the plate at the center is bigger than its thickness, then the load is mainly radial [9,10]. Therefore, the experience can be thought of as a mode-I test with circular symmetry. The pressure difference P supported by the sample is slowly increased and it is monitored by a differential transducer. This measure has a stability of 0.002 atm. The fracture pressure for the different tested materials ranges from 0.7 to 2 atm. We regulate P by means of a feedback loop and an electronically controlled valve which connects one of the two chambers to a pressurized air reservoir. The time taken to correct pressure variations (about 0.1 s) is smaller than the characteristic time of the strain rate. An inductive displacement sensor gives the deformation at the center of the plate with a precision of about 10 mm (the deformation just before fracture is of the order of 1 cm). The apparatus is placed inside a copper box covered with a thick foam layer to avoid both electrical and acoustical noise. Four wide-band piezoelectric microphones are placed on the side of the sample (see Fig. 1). The signal is amplified, low-pass filtered at 70 kHz, and sent to a digitizing oscilloscope and to an electronic device which measures the acoustic energy detected by the microphones. The signal captured by the oscilloscope is sent to a computer where a program automatically detects the arrival time of the AE at each microphone. Afterwards, a calculation yields the position of the source inside the sample. A fraction of the detected events is rejected, either as a result of a large uncertainty of the location, or because they are regarded as noise. The mean standard error for the calculated positions is about 6 mm, which results mainly from the uncertainty of the arrival time. The electronic device that measures the energy performs the square of the AE amplitude and then integrates it over a time window of 30 ms, which is the maximum duration of one acoustic event. The output signal is proportional to the energy of the events [11], and

Plasticity and avalanche behaviour in microfracturing phenomena

Abstract: Inhomogeneous materials, such as plaster or concrete, subjected to an external elastic stress display sudden movements owing to the formation and propagation of microfractures. Studies of acoustic emission from these systems reveal power-law behaviour1. Similar behaviour in damage propagation has also been seen in acoustic emission resulting from volcanic activity2 and hydrogen precipitation in niobium3. It has been suggested that the underlying fracture dynamics in these systems might display self-organized criticality4, implying that long-ranged correlations between fracture events lead to a scale-free cascade of ‘avalanches’. A hierarchy of avalanche events is also observed in a wide range of other systems, such as the dynamics of random magnets5 and high-temperature superconductors6 in magnetic fields, lung inflation7 and seismic behaviour characterized by the Gutenberg–Richter law8. The applicability of self-organized criticality to microfracturing has been questioned9,10, however, as power laws alone are not unequivocal evidence for it. Here we present a scalar model of microfracturing which generates power-law behaviour in properties related to acoustic emission, and a scale-free hierarchy of avalanches characteristic of self-organized criticality. The geometric structure of the fracture surfaces agrees with that seen experimentally. We find that the critical steady state exhibits plastic macroscopic behaviour, which is commonly observed in real materials.

Listening to the sound of materials: Acoustic emission for the analysis of material behaviour

Abstract: Applying a mechanical load to a composite material can result in many types of damage: debonding between the reinforcement and the matrix material, matrix cracking, failure of the reinforcement and delamination in the case of layered composites. To create a better understanding of the initiation, growth and interaction of the different types of damage, damage monitoring during mechanical loading is very important. Acoustic emission is the only nondestructive test technique capable of detecting all of the above mentioned damage types in composites. The possibilities of this technique to detect certain types of damage can only be proven by using other complementary techniques (replica technique, radiography, ultrasonics, optical and scanning electron microscopy). The research work on damage detection with the acoustic emission technique for different types of composite materials carried out at the Department of Metallurgy and Materials Engineering is presented.

Localization and classification of fracture types in concrete with quantitative acoustic emission measurement techniques

Abstract: The interactive forces between concrete and the steel cross-sections of the reinforcement can be investigated using the acoustic emission (AE) technique. A quantitative AE technique was developed for a comprehensive analysis of the AE waveforms including time domain and frequency. A 3D-localization as well as a classification of the events was implemented. The characterization of the fracture mechanisms is obtained by the newly developed relative moment tensor inversion method using a cluster analysis technique. This potentially improves the conclusions of AE measurements and will lead to a better way of assessing the damage of concrete.

Acoustic emission in single crystals of ice

Abstract: The dislocation dynamics during the creep deformation of single crystals of ice Ih was studied using acoustic emission (AE) measurements. The AE activity was recorded during uniaxial compression and torsion creep tests. The results were interpreted in terms of dislocation dynamics with the help of an AE source model relating the amplitude of an acoustic event to the number of dislocations involved in the event and to their velocity. This model was first validated by a comparison between the global AE activity and the global strain rate. Then, it was possible to evaluate the density of moving dislocations during creep deformation. Two regimes were revealed. Without significant polygonization, the density of mobile dislocations, deduced from AE, was proportional to the stress, but increased much faster after polygonization, in agreement with theoretical arguments. Finally, the power law distributions observed for AE amplitudes, the slow driving process, the very large number of interacting dislocations invo...

Molecular dynamics investigation of dynamic crack stability

Abstract: A series of molecular-dynamics simulations has been performed in order to evaluate the effects of several physical factors on dynamic crack stability These factors are the crystalline structure and the interatomic interaction modeled by various empirical potentials For brittle crack propagation at low temperature we find that steady-state crack velocities are limited to a band of accessible values Increasing the overload beyond ${\mathrm{K}}_{\mathrm{Ic}}$, the crack can propagate with a steady-state velocity, which quickly reaches the terminal velocity of about 04 of the Rayleigh wave speed The magnitude of the terminal velocity can be related to the nonlinearity of the interatomic interaction Further increasing the overload does not change the steady-state velocity dramatically, but significantly increases the amplitude of acoustic emission from the crack tip Loading the crack even further leads to instabilities which take the form of cleavage steps, dislocation emission, or branching The instability is closely related to the buildup of a localized coherent, phononlike field generated by the bond-breaking events The form of the instability depends critically on crystal structure and on the crystallographic orientation of the crack system but can also be correlated with the relative ease of dislocation generation (and motion)

Monitoring of Particle Fracture by Acoustic Emission during Charge and Discharge of Li / MnO2 Cells

Abstract: Charge and discharge of Li/MnO 2 cells were examined with monitoring of particle fracture of manganese dioxide by acoustic emission. Manganese dioxide used was electrolytic manganese dioxide heat-treated at 400°C for 24 h in air [HEMD(400)]. The acoustic-emission technique worked well to monitor events that occurred inside a cell. During the first discharge to prepare a deep-discharge product, a closely packed series of acoustic events was observed, especially in the latter half of the discharge process, which contained most of the acoustic events. During cycling, acoustic events were concentrated at the end of discharge while no event was observed during charge, indicating that particle fracture took place during lithium-ion insertion into a solid matrix. Rate-capability tests showed that the rate of acoustic events was a function of current drain, i.e., a higher discharging current accelerated particle fracture. From these results we discuss the important role of mechanical properties of materials upon the lithium-insertion scheme. We also discuss the ideal considerations regarding insertion materials for advanced batteries.

Fracture mechanics applications to drilling and blasting

Abstract: — This paper gives a brief review of research in rock fracture mechanics as conducted at the Fracture and Photo-Mechanics Laboratory (FPML) at Vienna University of Technology. The mechanisms pertaining to percussion drilling and blasting are investigated, with specific reference to the application of fracture mechanics. In order to gain an improved understanding of the mechanisms controlling rock fragmentation, a multidisciplinary approach is followed which includes laboratory experiments conducted in plexiglass and rock, in-situ field experiments and analytical/numerical modelling techniques. Field experiments revealed that percussively drilled holes exhibit a very shallow region of damaged rock. An analytical model to simulate damage accumulation and crack initiation due to elastic waves generated by impacting drill bits was developed. This model, based on damage and fracture mechanics, was incorporated into a numerical finite difference code. Fracture and damage mechanics parameters are related to the moment tensor which is determined experimentally by means of acoustic emission. Small scale model blasts were used to investigate the blast-induced fractures in the near-borehole zone as well as in the far field. Analytical and numerical investigations give insight into stress wave and gas driven fracturing. The applicability of the dynamic finite difference program SWIFD to the interaction between stress waves and cracks is illustrated.

Characteristics of damage and fracture process of carbon fiber reinforced plastic under loading-unloading test by using AE method

Abstract: The purpose of present paper is to estimate the damage of carbon fiber reinforced plastic (CFRP) by using a new factor of acoustic emission (AE) and secant modulus during loading-unloading test. Acoustic emission signal and fracture process were monitored by AE measurement system and video microscope in real time during the test. In the investigation of fracture process with AE amplitude distributions, high amplitude signal, middle amplitude signal and low amplitude signal correspond to fiber breaking, debonding and matrix cracking, respectively. Felicity ratio and secant modulus were studied during the sequential loading-unloading test. It is concluded that this factor is to be a good indicator to predict the damage condition and final fracture time.

Inertial cavitation and associated acoustic emission produced during electrohydraulic shock wave lithotripsy

Abstract: The inertial cavitation and associated acoustic emission generated during electrohydraulic shock wave lithotripsy were studied using high-speed photography and acoustic pressure measurements. The dynamics of cavitation bubble clusters, induced in vitro by an experimental laboratory lithotripter, were recorded using a high-speed rotating drum camera at 20 000 frames/s. The acoustic emission, generated by the rapid initial expansion and subsequent violent collapse of the cavitation bubbles, was measured simultaneously using a 1-MHz focused hydrophone, The expansion duration of the cavitation bubble cluster was found to correlate closely with the time delay between the first two groups of pressure spikes in the acoustic emission signal. This correlation provides an essential physical basis to assess the inertial cavitation produced by a clinical Dornier HM-3 shock wave lithotripter, both in water and in renal parenchyma of a swine model. In the clinical output voltage range (16–24 kV), the expansion duration...

Cracking resistance of thin hard coatings estimated by four-point bending

Abstract: In this work, a fast and easily performed four-point bending test for evaluation of cracking resistance of thin hard coatings is presented. A bending device, small enough to be put in an SEM and observed in situ, has been designed. By crack formation studies, a measure of coating cracking resistance is obtained. Two methods of crack detection are utilised: detection of acoustic emission and direct observation in the SEM. The two methods yield virtually identical values of the cracking resistance. However, the acoustic evaluation is much faster and easier to perform and is therefore to be preferred in long test series. SEM observations, on the other hand, allow a more straightforward interpretation. In this paper the test is used to determine the coating strain corresponding to crack initiation in TiN and CrN coatings on high speed steel. The test yields values of cracking resistance for the coatings in the range 0.1% (TiN) to 0.7% (CrN). In the SEM studies, cracks were found to nucleate predominantly at defect sites in the coating and propagate, highly aligned, perpendicular to the length of the beam. The cracks usually terminated when reaching the substrate. However, if hitting carbide at the substrate-coating interface, the crack also continues through the carbide, and terminates when reaching the substrate matrix.

Detection of incipient cavitation in pumps using acoustic emission

Abstract: This work concerns the detection of incipient cavitation in pumps using acoustic emission (AE). Three activities have been pursued in this context: (a) the construction of a small-scale rig for the investigation of cavitation detection using AE sensors; (b) the acquisition of data on a 75 kW single-stage centrifugal pump in an industrial test loop under normal running and cavitation conditions; (c) the determination of parameters that could be used for the early diagnosis of cavitation within pumps.In the laboratory-scale apparatus water was pumped around a short loop by a 3 kW centrifugal pump. The flow loop contained a section specifically designed to induce cavitation by means of reducing the pressure level to that of the vapour pressure of the fluid. This apparatus was used to produce a variety of well-controlled cavitation conditions which were useful in determining the suitability of AE for the detection of cavitation.The industrial-scale tests consisted of progressively reducing the net pos...

Reversible Crystal Plasticity

Abstract: Partial Contents: 1. Transformation of Dislocations. Dislocation Description of a Phase Transformation Front. 2. Dislocation Theory of Elastic Twinning. Twinning of Crystals: Principal Definitions. 3. Statics and Dynamics of Elastic Twinning. Discovery of Elastic Twinning. Verification of the Validity of the Static Theory in a Description of the Macroscopic Behavior of an Elastic Twin. 4. Thermoelastic Martensitic Transformation. Martensitic Transformation: a Diffusionless Process of Rebuilding the Crystal Lattice. 5. Superelasticity and the Shape Memory Effect. Main Characteristics of Superelasticity and Shape Memory Effects. 6. Reversible Plasticity of Ferroelastics. Ferroelastics: Main Definitions. 7. Investigation of Reversible Plasticity of Crystals by the Acoustic Emission Method. Emission of Sound by Moving Dislocations andTheir Pileups. Methods Used in Experimental Investigations of the Acoustic Emission Generated by a SingleTwin. Acoustic Emission Associated with Elastic Twinning. 8. Influence of Reversible Plasticity of Superconductors on Their Physical Properties. Reversible Changes in the Parameters of Traditional Superconductors under the Action of Elastic Stresses.

Damage and Failure Analysis of Brittle Materials by Acoustic Emission

Abstract: Application of stress or changes in environmental conditions can cause a material such as concrete or rock to become damaged, thereby affecting the performance of the structure. Because damage processes produce microseismic events called acoustic emission (AE), the growth of damage and the onset of failure can be identified by monitoring AE. In particular, a probability density function of acoustic emission events was used to describe the development of damage. It was observed that more porous or cracked materials displayed a higher AE rate than less porous or cracked materials at the same percentage of the maximum stress prior to failure. This suggests that the AE technique may be applied to diagnose the level of damage in a brittle material and may be useful to select materials for certain applications. Also, the experimental results indicated that the average root-mean-square value from a number of AE signals could be used as a real-time monitoring tool to predict the onset of failure.

Acoustic Emission Source Location Using Lamb Wave Modes

Abstract: This paper describes the development of a technique based on acoustic emission (AE) technology that can be used for global nondestructive evaluation (NDE) of steel structures. This will also be useful for the subsequent local inspection used to further monitor cracking, defective connections, retrofit measures, and so forth. The dispersive nature of Lamb wave propagation in plates makes it possible to perform a linear source location of AE events based on the arrival of different frequency components (Lamb wave modes) of an AE event at a single transducer. Experiments on steel beams and plates in the laboratory explored the theory of wave propagation and digital signal processing needed to pursue this objective. Subsequent field investigation on a real bridge substantiated the applicability of this technique. This new technique could allow AE inspection to locate structural problems better, and with a reduced number of transducers.

A theoretical model for Kaiser effect in rock

Abstract: —The mechanism of Kaiser effect was studied with the aid of a damage model for rock. Recognizing that the AE counts are transient elastic waves due to local damage of the rock, the quantitative relation between AE counts and statistical distribution of the local strength of the rock has been established. Subsequently, according to Damage Theory, an expression for Kaiser Effect under uniaxial stress state was derived from the model. This is found to be in good agreement with the experimental results.

Acoustic harmonic generation due to thermal embrittlement of inconel 718

Abstract: This paper describes an attempt to characterize the deterioration of a structural material's mechanical properties by nonlinear acoustics. In this particular case, the damage was caused by “thermal embrittlement” during which the material, here the nickel-based alloy Inconel 718, loses a significant fraction of its fracture toughness. Harmonic generation was the experimental method used to characterize the microstructural changes in the material as a function of exposure time at elevated temperatures. Tests were performed on two heats of Inconel 718 with slightly different chemistries, with one heat showing particular sensitivity of the fracture toughness to the elevated temperature exposure with corresponding higher changes in the nonlinearity parameter. As a mechanical measure of the fracture toughness deterioration, a small specimen punch test was used in which the ductility of a thin slice of material is determined. A clear difference between the two heats was noted in the metallographic examination, which is reflected in the harmonic generation as well as the punch test data. An explanation for the changes of the harmonic generation during the embrittlement process is speculative at the present time.

Process analysis and monitoring in abrasive water jet machining of alumina ceramics

Abstract: This paper first attempts to develop an analytical model for material removal in abrasive water jet machining (AWJM) of brittle material. The size of fracture that takes place on the backside of the workpiece as the jet passes through the workpiece is then predicted. The proposed model is experimentally evaluated. In addition, acoustic emission (AE) generated from AWJM of alumina ceramics is evaluated in view of establishing an experimental relationship with the parameters of material removal process. The experimental results indicate that a strong correlation between the RMS AE signal and the characteristics of AWJM exists and, therefore, suggest that AE can be implemented in practice for monitoring purpose.

Acoustic emission monitoring to study sodium sulphate crystallization in monumental porous carbonate stones

Abstract: Soluble salts inside porous stone can induce stresses Under these circumstances, elastic waves (acoustic emission/microseismic activity, AE/MS) can be generated spontaneously In the present study, acoustic emission monitoring of salt crystallization tests performed on monumental stones was carried out in order to determine the deterioration processes occurring during the test The studied materials were porous carbonate stones from Spanish monuments with different porosity and pore opening radius distribution Sodium sulphate was the selected salt AE/MS recorded during the different cycle stages (immersion, drying and cooling) was related to sodium sulphate characteristics and pore space characteristics of the stones

Transient oscillation of cavitation bubbles near stone surface during electrohydraulic lithotripsy.

Abstract: Using high-speed photography and acoustic emission measurements, we studied the dynamics of a transient cavitation bubble near a stone surface and the concomitant shockwaves generated during electrohydraulic lithotripsy (EHL). At each spark discharge, a vapor plasma and subsequently a cavitation bubble oscillating around the tip of an EHL probe are produced. Simultaneously, three distinctive shockwave pulses are generated. The first shockwave is produced by the rapid expansion of the vapor plasma, while the second and third waves are produced by rebounds of the cavitation bubble. Depending on the proximity of the probe to the stone surface, the collapse of the cavitation bubble may be symmetric, resulting in a strong shockwave emission; or asymmetric, leading to the formation of a liquid jet. For the Nortech AUTOLITH lithotripter with a 1.9F probe that was used in this study, maximum shockwave emission is produced when the probe is about 1 mm from the stone surface, whereas the maximum jet velocity is produced when the probe tip is at distance equivalent to the maximum bubble radius of about 3 mm. These findings are consistent with clinical experience, which suggests that for optimal treatment results, the EHL probe should be placed close to the stone surface.

Transducer for measuring acoustic emission events

Abstract: An apparatus for detecting and measuring cracks in plate-like Structures using acoustic emission technique is disclosed. A transducer coupled with the structure is equally sensitive to in-plane and out-of-plane acoustic emission signals. A splitter receives the output signal of the transducer and divides the acoustic emission signal into a high-frequency signal and a low-frequency signal. A circuit generates a high-frequency peak amplitude and a low-frequency amplitude from there high-frequency and low-frequency signals. A computer computes the ratio of the high-frequency peak amplitude to the low-frequency peak amplitude.

Modal analysis of acoustic emission signals

Abstract: This report shows the results of a continuing study to determine the type of waves present in plates of different thickness created by breaking pencil leads in-plane (IP) and out-of-plane (OOP) on the plates and by detecting the resulting stress waves with a high fidelity acoustic-emission transducer mounted on the edge and surface of the plates. The results show that the extensional velocity over the range of plate thickness from 3.13 to 12.5 mm does not change, and the flexure wave is dispersive in accordance with plate wave theory. The procedure in this work split the signal into two frequency ranges, a high frequency range with a 100 kHz high-pass filter and a low frequency range with a 20-70 kHz bandpass filter. This frequency split provided an easy method of measuring the velocity of the low frequency flexure wave. A bulk shear wave, not predicted by plate wave theory was observed along with the extensional wave in the high frequency channel. In order to confirm the presence of the shear wave, two shear plates, one with particle motion vertical to the plane of the plate and the other parallel to the plane of the plate were attached to the end of one of the bars. These transducers were used as receivers for IP and OOP pencil-lead breaks, and also as transmitters excited by a 150-V spike pulse. Both the Sv and Sh waves, created with pulsing, could be detected with a transducer mounted on the surface with petroleum jelly. The Sh wave had a much higher amplitude than the Sv wave. The results show that mode conversion of the Sv wave to a flexure wave occurs, as it propagates down the bar, which partially accounts for its loss of amplitude. From this study, it is suggested that the crack growth signal detected by most AE tests on large thin-wall structures utilizing 100 kHz high-pass filtering, resonant transducers, and large transducer spacing, is the Sh wave created by crack propagation.