High Acoustic Emission Activity Research Articles

The sensitivity of Acoustic Emission (AE) for monitoring the effect of SAPs in fresh concrete

Fresh concrete is characterized by numerous processes taking place simultaneously such as settlement, hydration, early-age cracking and shrinkage. Those processes have a strong impact on the strength and durability, hence monitoring the material at an early stage is essential to ensure the required performance. Acoustic Emission (AE) is a non-destructive, highly sensitive technique that records elastic waves propagating through the medium and has proven to be adequate to monitor the complex processes taking place at an early age. In this study, the method of AE is applied on fresh concrete cubes with or without the addition of SuperAbsorbent Polymers (SAPs). SAPs are particles that have been recently used for internal curing or self-healing of cementitious materials. The water-releasing action of SAPs is accompanied by high AE activity. The purpose of this study is to investigate the possibility to control the internal curing of concrete based on real-time AE data, as well as to make projections towards the final mechanical properties while still at a young age, in order to ensure the desired concrete performance.

Identification of damage mechanisms of polymer-concrete in direct shearing tests by acoustic emission

• The damage characteristics of the polymer-concrete interface were recorded by the AE technique. • AE ringing counts and amplitude can effectively detect the first occurrence of cracks. • Generation of new cracks in the interface of polymer and concrete causes high AE activity. • The statistical damage constitutive models using AE parameters were established. As a new type of trenchless material, non-water reacting grouting polymer has been widely used in the reinforcement and trenchless rehabilitation of concrete infrastructure construction. Understanding the interfacial properties of the polymer-concrete composite structure is of great significance for assessing the serviceability and stability of concrete structures. In this paper, the acoustic emission (AE) technique was applied to monitor the interface damage and reveal the interface failure mechanism of polymer-concrete composite specimens. Direct shear tests were performed on polymer-concrete specimens with different polymer densities, and the AE technique was used to monitor the damage process. Various AE statistical parameters, such as AE ringing counts, energy, amplitude distribution, frequency and b-value, were analyzed. With the Weibull random distribution assumption, statistical damage constitutive models using AE parameters were established. The results reveal that AE events existed throughout the whole failure testing process and were the most significant in the yield stage. The interfacial damage of polymer-concrete specimens could be qualitatively predicted by the sharp increase in the accumulated AE ringing count and energy; the overall b-value of the polymer-concrete specimens with different polymer densities was the same and had a similar trend of a rapid decrease in the yield stage. In addition, the damage variable based on AE parameters can be used to better characterize the interfacial cracking process. The present study indicates that the AE technique is an effective technique in capturing crack development characteristics of polymer-concrete composite structures.

In situ characterization of the functional degradation of a [formula omitted] orientated Fe–Mn–Al–Ni single crystal under compression using acoustic emission measurements

Acoustic emission (AE) measurements were conducted in situ during cyclic compressive loading on an [001¯] orientated single crystal of Fe-Mn-Al-Ni shape memory alloy to study functional degradation of its superelastic response. The acoustic investigations were corroborated by optical microscopy, employing video imaging, and transmission electron microscopy. The analysis of acoustic emissions recorded during repeated loading and unloading sessions revealed two categories of AE signals that are differed by their characteristics in time and frequency domains. These two distinct types of AE signals were related to two underlying mechanisms: (i) the nucleation and reverse transformation of stress-induced (twinned) martensite, and (ii) the lateral growth and shrinkage of one dominant martensite variant and related dislocation activities, respectively. In addition, an asymmetry in the AE activity during forward and reverse transformation during mechanical loading and unloading was detected. In particular, an unexpected high AE activity was observed during the superelastic unloading of martensitic microstructure from the point of maximum load/strain. This effect was attributed to the reverse transformation of small, tiny areas of martensite as well as to unpinning and annihilation effects related to dislocations.

Применение метода акустической эмиссии для мониторинга ультразвуковых усталостных испытаний

The ultrasonic fatigue testing (USFT) is an effective method for rapid determination of the fatigue properties of structural materials under high cycle (≥106 cycles) loading. However, the occurrence and accumulation of fatigue damage with this test method remain uncertain due to the limitations of the existing measurement methods. Currently used monitoring methods allow detecting the fatigue cracks, but only in the late stages of failure. Despite the superior sensitivity to localized processes in materials, the use of the acoustic emission (AE) method in ultrasonic testing is extremely difficult due to the presence of resonant noise. This work aimed to suppress resonant noise and extract the signal for early detection of fatigue damage. The authors tested the samples of the AlSi9Cu3 aluminum alloy under the asymmetric cyclic loading (R=0.1) at a resonant frequency of 19.5 kHz with a non-threshold AE registration. The fracture surfaces were analyzed by electron and optical microscopy. The authors processed AE by two different methods: (1) the digital filtering method consisted of detecting resonant noise and removing it from the spectrum; (2) the φ-function method consisted of differentiating the spectrogram by time. The processed spectrograms were integrated by the frequency with further extraction of the AE events using the threshold method. The digital filtering method revealed a correlation between AE signals and fatigue damage, whereas the undamaged control sample showed no signals. The φ-function technique demonstrated ambiguous results, showing high AE activity on the control sample.

Application of the shear-tensile source model to acoustic emissions in Westerly granite

Traditionally in seismology and in acoustic emission (AE), full moment tensor (MT) is applied as a default model of the mechanism. We present an alternative - an application of shear tensile crack (STC) source model to AEs generated by uniaxial compression loading of Westerly granite. The advantages of STC over the conventional MT are as follows: (i) contrary to the MT, the STC is physical source since it describes straight and simple fracture modes anticipated inside a loaded sample, namely the shear-slip and both of opening and closing tensile cracks; and (ii) the STC is simpler because it is described by fewer parameters (five instead of six required for an unconstrained MT), an essential feature for stabilizing the inverse problem.Better suitability of STC over MT is demonstrated by three exemplary AEs (tensile, shear, and combined). The obtained results were confirmed using a statistical analysis of 1630 reliably determined source mechanisms. The STC, as compared to the MT, provides smaller confidence regions for orientation and even smaller regions for decomposition parameters. Thus, the STC solution appeared to be substantially more useful than the MT, namely for mechanisms with a high content of non-double-couple (non-DC) component, as it allowed better distinction between tensile and shear AEs.Grain scale cracks of tension and combined source type, localized within the middle-height circumferential portion of the specimen, dominated fracturing. Azimuthal distribution of fault planes was found to be approximately uniform for all three source types. The fault dip increased with increasing content of the non-DC component. The average values were 16°, 21°, and 26° for the tensile, combined and shear source types, respectively. The specimen failed by flaking in areas of a high AE activity. AE locations and failure mechanisms indicate a perfect confinement end-boundary conditions between the tested specimen and the loading platens.

Acoustic Emission and Numerical Analysis of Pine Beams Retrofitted with FRP and Poplar Wood.

Acoustic emission (AE) released by pine beams retrofitted with fiber reinforced plastic (FRP) and poplar planks in bending is analyzed. Basalt fabric (FB), carbon fabric (FC), and carbon pultruded laminate (LC) have been used as FRP. Experimental results and AE behavior are discussed based on an elastoplastic finite-element numerical model. The model demonstrates a strong strain concentration at the end of poplar planks, which causes high AE activity in these areas and acts as a precursor of the delamination of the poplar plank. Based on the experimental results, some AE criteria for predicting the onset of the delamination are tentatively proposed.

Reaction-induced embrittlement of the lower continental crust

Field observations and geophysical data reveal a causal link between brittle seismic failure and eclogitization of the lower continental crust. We present results from experimental deformation of plagioclase-rich samples at eclogite-facies conditions and quantify the link between rock rheology and the kinetics of the eclogitization reactions. The deformation was ductile both in the absence of reaction and when the progress of eclogitization was fast compared to the imposed strain rate. However, when the reaction rate was relatively slow, the breakdown of plagioclase into nanocrystalline reaction products triggered embrittlement, highlighted by a high acoustic emission activity. Fluid-induced plagioclase breakdown under eclogite-facies conditions is an exothermic reaction accompanied by a negative change in solid volume. This is similar to other mineral transformations that are known to trigger transformational faulting. We demonstrate that mineral reactions lead to brittle deformation in situations where reaction rates are slow compared to the deformation rate.

Review on In Situ Acoustic Emission Monitoring in the Context of Structural Health Monitoring in Mines

A major task in mines and even more in underground repositories for nuclear waste is to investigate crack formation for evaluation of rock mass integrity of the host rock. Therefore, in situ acoustic emission (AE) monitoring are carried out in mines as part of geomechanical investigations regarding the stability of underground cavities and the integrity of the rock mass. In this work, the capability of in situ AE monitoring in the context of structural health monitoring (SHM) in mines and in various geological settings will be reported. SHM pointed out, that the AE network is able to monitoring AE activity in rock with a volume up to 10 6 cubicmeter and distances up to 200 m (e.g., 100 m × 100 m × 100 m) in the frequency range of 1 kHz to 150 kHz. Very small AE events with source size in approximately centimeter to millimeter scale are detected. The results show that AE activity monitors rock deformation in geological boundaries due to convergence of the rock. In addition, high AE activity occurs in zones of dilatancy stress in homogenous rock. In conclusion in situ AE monitoring is a useful tool to observe instabilities in rock long before any damage becomes visible.

AE monitoring of corrosion process in cyclic wet–dry test

Recently, a number of deterioration of reinforced concrete (RC) structures due to salt attack have been reported. After corrosion of reinforcing steel-bar (rebar) is nucleated, expansion of corrosion products results in corrosion-induced cracks in RC. Thus, development of non-destructive evaluation (NDE) techniques is important for inspection of corrosion damage. It is reported that acoustic emission (AE) could identify the onset of corrosion in rebar and the nucleation of concrete cracking due to expansion of corrosion products in the corrosion process. In this study, AE techniques are applied to a cyclic wet–dry test of RC beams. It is confirmed that both the onset of corrosion and the nucleation of concrete cracking are clearly observed as two periods of high AE activity. Kinematics of micro-cracks are identified by SiGMA (Simplified Green’s functions for moment tensor analysis) analysis of AE. To compare with findings, cross-sections of rebars are observed by SEM (Scanning Electron Micrograph). From these results, a great promise for AE techniques to monitor the corrosion process in RC structures is clarified.

Phenomenological Model of Corrosion Process in Reinforced Concrete Identified by Acoustic Emission

The concrete reinforcing steel bar (reinforcing bar) corrosion process is phenomenologically modeled as three phase corrosion loss. Reinforcing bar has corrosion initiated at the first phase. Under aerobic conditions at the second phase, there is a decreased rate of corrosion loss. Due to corrosion products, reinforcing bar expansion nucleates concrete cracking at the third phase of anaerobic corrosion. For such a corrosion process to be identified as reinforced concrete corrosion loss, continuous acoustic emission (AE) monitoring was conducted in a cyclic wet and dry test and an accelerated corrosion test. There was characteristic observation of two high AE activity periods. The slope gradient of AE amplitude distribution and AE waveform parameters were investigated to elucidate these two activities. Cracking mechanisms were classified as the other-type cracks than a tensile crack and AE events were found to be of small amplitudes at the first period of high activity. That ferrous ions vanished at some surface regions was demonstrated by scanning electron microscope examination of the reinforcing bar surface, although no corrosion products were visually observed on the reinforcing bar surface. The corrosion's onset in the reinforcing bar results in the first high AE activity is suggested by this, corresponding to the phenomenological model's first phase. Corrosion products were visually observed on the surface of the reinforcing bar following the second high AE activity period. It suggested that in concrete around the reinforcing bar, there was nucleation of cracks. Reasonably, AE event amplitudes were found to be fairly large and AE sources were classified as tensile cracks. Concrete cracking is associated with these AE events due to corrosion product expansion that could be observed at the model's third phase. Thus, the corrosion loss phenomenological model agrees remarkably with reinforced concrete AE generating behaviors. Reinforced concrete corrosion time could be identified, by AE activity monitoring, at reinforcing bar corrosion initiation and also at the nucleation of cracks in concrete. There is earlier detection of even the latter AE activity than through other nondestructive evaluation techniques.

Influence of equal-channel angular pressing on the acoustic emission behaviour of magnesium alloy AZ31 under compression

To investigate the effect of severe plastic deformation on the acoustic emission behaviour of the magnesium alloy AZ31, equal channel angular pressing (ECAP) was carried out at 200 °C up to four passes following route B C. Acoustic emission was measured during compression tests at various temperatures from room temperature up to 300 °C. The compression tests at room temperature of undeformed specimens revealed a significantly higher activity of acoustic emission as compared to that of material that had been pressed four times through the ECAP die. At elevated temperatures the acoustic emission vanished for ECAP processed specimens, as opposed to the case of the undeformed material. In the case of undeformed specimens, the high acoustic emission activity can be explained in terms of collective dislocation motion and mechanical twinning. ECAP processed AZ31 exhibits a recrystallized microstructure with a grain size of approximately 800 nm. It was found that during compression tests, the concomitant acoustic emission is significantly reduced.

In situ Acoustic Emission for wear life detection of DLC coatings during slip-rolling friction

Different diamond-like carbon (DLC) coatings on a steel substrate (100Cr6) were tested under slip-rolling friction conditions against uncoated counter bodies of the same steel. The initial maximum Hertzian pressure was varied in a range of P 0 = 1.5–2.3 GPa. The friction tests were carried out under dry conditions and with an unadditivated paraffin oil as lubricant. It could be shown that the thickness of the coatings affects the respective wear life. Further, a very important factor for the wear life of a coating under lubricated slip-rolling conditions is the roughness of the surface of the respective counterbody. The wear life tests were monitored by recording in situ the Acoustic Emission (AE) signals. Some causes for a high AE activity could be identified.

Study of the Performance of TBC under Thermal Cycling Conditions using an Acoustic Emission Rig

An experimental rig based on the use of infrared quartz lamps has been developed to monitor the degradation mechanisms causing failure of thermal barrier coatings (TBC) under thermal-cycling conditions. An acoustic emission (AE) technique monitored these degradation mechanisms, and advanced signals processing identified the key parameters that classify the AE signals according to the long-term behavior of the TBC. The AE technique enabled the localization of degradation sources inside the TBC with a linear resolution of ∼5 mm by the use of two transducers fixed at both ends of the sample. Furthermore, sample zones of high AE activity showed typical vertical cracks at the surface and delaminations at the interface between the ceramic and the bond-coat layer. Vertical cracks were induced preferentially during the heating period of the thermal cycles when the ceramic coating was in a tensile-stress state, while delaminations were induced during the cooling period when the TBC was in a compressive-stress state.

Influence of micro structure on acoustic emission behavior during stage 2 fatigue crack growth in solution annealed, thermally aged and weld specimens of AISI type 316 stainless steel

Acoustic emission (AE) behavior during fatigue crack growth (FCG) in solution annealed, thermally aged and weld specimens of AISI type 316 austenitic stainless steel is reported in this article. The presence of two substages 2(a) and 2(b) during stage 2 FCG could be distinguished by a change in the AE behavior corresponding to a sharp change in the crack growth rate ( da/dn). The transition point in the AE parameter vs. number of cycles ( n) plot matches well with that of the ( da/dn) vs. ‘ n’ plot and is found to occur at a da/dn ≈ 3 × 10 −7 m cycle −1 which is in agreement with the reported value. The high AE activity observed during the substage 2(a) is attributed to the extensive cyclic plasticity within the cyclic plastic zone (CPZ) and the increasing size of the CPZ with Δ K under plane strain conditions prevailing during stage 2(a). The low AE activity observed during stage 2(b) is attributed to a reduction in the mean free path for dislocation movement and a decrease in the size of the CPZ under plane stress condition prevailing during stage 2(b). The AE during stage 2(a) is found to have a strong influence on the microstructure. The presence of carbide precipitates in thermally aged specimens reduces the AE activity. The high AE activity in weld specimen is attributed to the combined influence of cyclic plasticity, residual stress induced micro cracking and roughness induced crack closure phenomena.

Acoustic emission for identification of stage IIa to IIb transition during stage II fatigue crack growth in parent and weld metal in AISI 316 austenitic stainless steel

The acoustic emission behaviour during stage II Paris fatigue crack growth in parent and weld specimens of AISI type 316 stainless steel has been reported. Substages IIa and IIb, occurring during stage II fatigue crack growth, could be distinguished by a change in the acoustic emission activity, corresponding to a sharp change in the crack growth rate. The transition point in the plot of acoustic emission cumulative ring down count v. number of cycles agrees well with that of the crack growth rate ν number of cycles. The transition from stage IIa to IIb is found to occur at a crack growth rate da/dn=3 × 10−7 m cycle−1, which is in reasonable agreement with the values reported in the literature. The high acoustic emission activity during stage IIa is attributed to the extensive cyclic plastic deformation and the increasing size of the cyclic plastic zone with ∆K (the cyclic stress intensity factor) under the plane strain conditions prevailing during stage IIa. The low acoustic emission activity during stage IIb is attributed to the reduction in the mean free path for dislocation movement and the decrease in the size of the cyclic plastic zone under the plane stress condition prevailing during stage IIb. The higher acoustic emission activity in weld specimens compared with the parent metal is attributed to the combined influence of cyclic plasticity, residual stress induced microcracking, and roughness induced crack closure phenomena.MST/3201

Effect of transient stress on acoustic emission behaviour during firing of dental porcelain

Acoustic emission activity was measured in the glass transition range of dental porcelain during firing. Transient and residual stresses in porcelain during cooling from a temperature higher than porcelain sag point and during re-heating of the tempered porcelain were calculated by computer simulation using a viscoelastic stress analysis. The detected acoustic emission event was discussed with the relative rules of the simulated transient stresses. High acoustic emission activity was detected at the temperature where the internal stress faded away for heating and build up for cooling. The low-level acoustic emission pulses were only detected in the following conditions: (1) in the temperature range where porcelain behaved like an elastic solid; (2) at temperatures higher than the deformation point of porcelain; (3) with a re-heating process of the porcelain without tempered stress. From these results, it was concluded that elastic energy is released related to transient stress in porcelain during viscoelastic deformation and can be detected by the acoustic emission method. The acoustic emission method is considered to be helpful in non-destructive testing in order to understand transient stress due to viscoelastic deformation of glassy materials in heat treatment.

Transverse strength and acoustic emission characteristics of commercial denture base resins

Transverse strength and acoustic emission (AE) characteristics were measured by the transverse test in deionized water at 37 degrees C on commercial denture base resins (five heat-cured type resins and one polysulfone). Difference in flexural property of five heat-cured denture base resins was not shown from the transverse deflection according to JIS, but high toughness of polysulfone was recognized in transverse deflection, flexural strength, flexural modulus, flexural rigidity, flexural proof stress, and fracture energy. The five heat-cured denture base resins showed a low AE activity, but the polysulfone resin high AE activity. Significant rates of AE for polysulfone were detected at a kgf of approximately 50-60% the maximum load. The presence of Kaiser effect in its cycle transverse test was confirmed.

AE load-cycle dependence applied to monitoring fatigue crack growth under complex loading conditions

Acoustic emission (AE) has been used to continuously monitor fatigue critical zones in a Mirage aircraft undergoing full-scale fatigue testing, in order to establish the feasibility of detecting AE due to crack growth during flight. The major problem was to distinguish crack growth from the many spurious AE sources. Thus, to discriminate between AE due to different sources, the activity in each zone was analysed in terms of its load-cycle dependence. Regions on the load cycle where AE due to fatigue crack growth was most likely to occur were identified, and a correlation was found between crack growth and relatively high AE activity in this region. Active cracks deeper than 0.3–0.6 mm were detected, although the size of the AE indication was not proportional to the cracking activity and zoning calibration studies had to be taken into account. No significant cracks were missed.

An evaluation of acoustic emission from fibre-reinforced composites

An analysis of acoustic emission(AE) from epoxy matrices of different amounts of hardener and model composites containing a glass bead, carbon and glass fibres has been carried out to identify the sources of emission. A few AE events generated by microcracking were observed for epoxy matrix near the final fracture strain. From microscopic and emission observations it was found that the emission was generated by interfacial debonding at the pole for the model composite containing a single particle of the glass bead, and that the source of AE bursts for a continuous single carbon fibre/epoxy composite was succeeding fibre fractures along fibre length. The high AE activity due to fibre fracture was observed for a composite consisting of a bundle of glass fibres. The total of AE events was in agreement with the number of fibre fracture counted with the aid of a microscope in a carbon/epoxy composite. The shear strength at the carbon/epoxy interface was evaluated by a critical length of the fractured fibres using the AE results.

Hypocenter distribution and focal mechanism of AE events during two stress stage creep in Yugawara andesite

More than a thousand acoustic-emission (AE) hypocenters were determined in a cylindrical andesite specimen under two-stage uniaxial creep at stresses of 204 and 214 MPa. Strains were monitored for 6 peripheral points at the middle part of the cylindrical specimen's wall. The strain data indicate gradual increase of nonuniform deformation during steady creep and strong intensification of the nonuniformity during acceleration creep and, therefore, biased stress distribution within the specimen. The correlation between dilatant strain and AE hypocenters was investigated for whether or not tensile cracks emit AE. The region with high AE activity shows only a small dilatant strain. This negative correlation between AE and the dilatant region may eliminate tensile cracks as possible AE sources. A composite focal-mechanism solution of local AE events, covering a wide solid angle of the focal hemisphere, indicates that shear fractures emit AE waves. The direction of the compressional axis in this solution shows a significant deviation from that inferred from the applied external force, suggesting that the local stress field is governed by preexisting weak zones that are, presumably, produced by tensile cracks within the specimen. AE hypocenters tended to form clusters during steady creep under the constant compressional stress. During acceleration creep caused by a small step increase of the external stress, the preceding clusters disappeared while a new cluster appeared in an incipient fault plane. This suggests that changes in seismicity pattern such as migrations or quiescences of swarm—important clues for earthquake predictions—may be caused by an instantaneous change in the tectonic-stress levels.