High Amplitude Acoustic Emission Research Articles

Interlayer Shear Sliding Behaviors during the Fracture Process of Thick Sandstone Roof and Its Mechanism Leading to Coal Mine Tremors

To explore the causes of mine tremors in coal mines with sandstone roofs, a three-point bending loading experiment was designed for composite sandstone layers, and the fracture and interlayer shear slip characteristics of the composite sandstone layers were studied using optical measurement and acoustic emission techniques. The results show that the bending of the rock layers led to interlayer sliding deformation, while the fracturing greatly promoted interlayer sliding. The maximum interlayer slip accelerations during bending deformation and fracturing were 0.6 mm/s2 and 3.8 mm/s2, respectively. During the fracturing of the rock layers, the proportion of acoustic emission shear fracture events increased with the continuous occurrence of long-lasting and high-amplitude acoustic emission events. The mechanism of mine tremors in thick sandstone roofs is as follows: the increase in the area of the goaf causes rock bending deformation and fracturing, accompanied by interlayer shear slip, fracturing of the sandstone layer, and friction dislocation at the cementation surface of the adjacent sandstone layers, which jointly cause vibration of the roof.

Predicting the failure of rock using critical slowing down theory on acoustic emission characteristics

The joint surfaces and size affect the mechanical behavior and failure precursors of shale. Therefore, this paper studies the mechanical behavior, the response characteristics of acoustic emission (AE) and critical slowing down (CSD) characteristics of AE for shale with different joint surfaces and size. The results show that with an increase in the number of joint surfaces and the size of the sample, the crack initiation stress, uniaxial compressive strength (UCS), peak strain, and fractal dimension decrease. During the plastic stage, the occurrence of a sharp rise in AE ringing count, RA, a low-frequency, high-amplitude AE signal, and a rapid drop in AF can be characteristics of impending sample failure. Variance and autocorrelation coefficients can be used to characterize the CSD phenomenon. The optimal window length and lag step are determined to be 400 and 100, respectively. The CSD characteristics of the autocorrelation coefficient and the variance of multi-AE parameters are obtained. The variance-based early warning time provides short-term early warning, while the autocorrelation coefficient-based early warning time provides early warning. The research results can provide theoretical references for the mechanical design and failure prediction of underground rock engineering.

Acoustic Emission Analysis of Fracture and Size Effect in Cementitious Mortars

The size effect is a phenomenon where the strength and the ductility of a material depend on the size of the structure. Investigating size effects and related crack formation in brittle materials requires advanced monitoring methods. The aim of this research is to experimentally investigate the impact of size effect with the acoustic emission (AE) technique. Brazilian splitting tests with AE monitoring were performed on cement-based mortar cylinders of three sizes. It was found that in addition to the size, the boundary condition affects the final strength. When adopting similar boundary conditions in samples with different sizes, the larger samples had the lowest tensile splitting strength. For the larger samples, initially, there were fewer AE activities. However, there was a surge of high-amplitude AE events near the peak load. This indicates that as size increases, there is a lack of micro-cracking before macro-crack propagation, and the material fails in a more brittle manner. The width of the fracture process zone was quantified with AE and increased with sample size. A further analysis of the AE amplitude distribution demonstrated a change in the distribution in the pre-peak phase for the larger samples and for the smaller samples in the post-peak phase, signifying the brittle to ductile failure transition that occurs as size decreases.

Effect of frequency during cyclic hydraulic fracturing and the process of fracture development in laboratory experiments

Cyclic injection in hydraulic fracturing has been reported to reduce the level of induced seismicity compared to the conventional continuous injection. We investigate the evolution of acoustic emission (AE) activity, hydraulic energy, fracture area, and fracture development for cyclic injection schemes with different cycle time durations (td) at a repeated maximum cyclic pressure through laboratory experiments. These test were compared with a case under conventional continuous injection at an injection rate of 50 mm3/s. Pocheon granite samples with an outer diameter of 50 mm, height of 100 mm, and an inner borehole of 8 mm were used. The maximum pressure of 5.5 MPa for each cycle represents approximately 77% of the average breakdown pressure from continuous injection cases. Four td values of 30 s, 60 s, 120 s, and 240 s were selected. The results show an exponential decrease in the number of cycles until failure with increasing td. Similarly, the total number of AEs, the peak AE rate, and the cumulative AE energy also decreased with increasing td. These values contrasted with those for continuous injection which reported the lowest number of AEs, and the highest cumulative AE energy. The td influence was also evident on the AE occurrence, with fewer high amplitude AEs at a high td, and all lower than continuous injection. In general, the hydraulic energy and cumulative AE energy decreased with increased with increasing td, although a slight deviation was related to the total testing time. The fracture areas during cyclic injection reported a positive correlation with the cumulative AE energy. Moreover, detailed analysis of the instantaneous injection rate, and the cumulative AEs suggested a three-stage damage process, which was supported with X-ray computed tomography observations. The second stage was characterized by a steady linear injection rate increase, and its slope correlated with the fatigue life or number of cycles to failure. Finally, based on the injection rate evolution, an interpretation of the damage evolution and fracture onset during cyclic hydraulic fracturing could be made.

Estimation of the Limit State Criterion for Steels C1020 and A 516-55 with V-Notch under Quasistatic Loading Using Acoustic Emission Control

There is a need to determine the stresses value for responsible designs in the oil and gas refining, petrochemical and chemical industries, at which the defect reaches its maximum size. This stress value can be taken as the limit state criterion for strength calculations of vessels and apparatus. The article presents a technique for determining the limit state criterion for carbonaceous and low-alloy steels using the example of steels C1020 and A 516-55 in the state of delivery. As a limit state criterion it is proposed to use the stress value corresponding to the microcrack formation moment. The studies are based on the uniaxial tension tests results at room temperature of flat samples with V-shaped stress concentrators of 2 mm deep, accompanied by an acoustic emission nondestructive testing method. Based on the obtained pulse amplitude and pulse energy dependences on time in the process of testing and fractographic studies, a technique for estimating acoustic emission signals was developed to determine the microcrack formation moment for test steels. Stress values that can be taken as the limit state criterion for steels C1020 and A 516-55 in the delivery state for strength calculations vessels and apparatus are determined using the developed experimentally technique based on the high-amplitude acoustic emission to low-amplitude signals ratio. With the help of specimen fractures fractographic analysis the legitimacy of using the developed technique is justified. Thus, at room temperature for steel C1020, the stress at which a microcrack begins to form is 420 MPa, for steel A 516-55 it is 455 MPa.

The influence of fuel type and partial premixing on the structure and behaviour of turbulent autoigniting flames

This paper explores turbulent autoignition and flame stabilisation for a range of fuels, utilising a jet in a hot coflow burner. Jet fuels including: hydrogen, dimethyl ether and hydrocarbons ranging from CH4 to C4H8 are investigated with the influence of partial premixing, dilution and hot coflow temperature. Simultaneous acoustic emission measurements and high-speed chemiluminescence imaging at 10 kHz are performed; investigating the flame lift-off dynamics and to study the initiation and evolution of autoignition kernels. For all fuels studied, a common trend is found for increasing coflow temperatures; where a transition from high lift-off flames exhibiting an autoignition kernel dominated flame stabilisation mechanism, to lower lift-off flames exhibiting a premixed flame propagation stabilisation mechanism. Three key findings are reported: (i) common to all fuels studied for the high lift-off flames, the lift-off height vs. time follows a sawtooth-like trend. The leading edge of the main flame body (flame base) drifts downstream with near constant velocity, whilst upstream of the flame base autoignition kernels form and grow rapidly merging with the flame base; thereby lowering the tip of the flame base. (ii) High amplitude acoustic emission events correlate well with auto-ignition kernel flame base merging events for high lift-off flames, for the fuels studied. The ethylene flames produced the highest sound levels for a given mean lift-off height. (iii) In the high lift-off height regime, the lift-off height for all fuels scales well with corresponding simple 0-D auto-ignition delay calculations. The good correlation of the lift-off height scaling with the computed autionition delay implies that chemical kinetics, rather than turbulent mixing controls the processes at the base of these flames for higher lift-off height flames, indicating that autoignition is the dominant stabilising mode.

Application of acoustic emission method to study metallic titanium hydrogenation process

The changes occurring in metallic titanium specimens during hydrogenation, that are accompanying by high amplitude acoustic emission signals, have been studied. It was determined that the most probable reason for generation of these signals is crack formation in hydrogenated specimens as a result of internal stresses caused by structural changes in material. Also it was determined that typical sizes of the cracks are comparable with particle sizes of hydrogenated specimens, and the atomic ratio [H]/[Ti] in a solid phase, when crack formation is the most intensive, is generally lower for titanium sponge than for powder made from this sponge, which is explained by a larger specific surface of powder comparatively that of sponge.

Investigations on the effects of tool wear on chip formation mechanism and chip morphology using acoustic emission signal in the microendmilling of aluminum alloy

This work investigates the effects of tool wear on surface roughness (Ra), chip formation mechanisms and chip morphology in the microendmilling of aluminum alloy (AA 1100) using acoustic emission (AE) signals. The acquired AE signals are analysed in the time domain, frequency domain using fast Fourier transformation (FFT) and the discrete wavelet transformation (DWT) technique. The time domain analysis indicates that the root mean square of the AE (AERMS) signals is sensitive to the formation of the buildup edge apart from effective machining. The frequency domain analysis indicates that the dominant frequency of the AE signals lies between 150 and 300 kHz. The AE-specific energies are computed by decomposing the AE signals in different frequency bands, using the DWT technique. The higher and lower orders of AE-specific energies are obtained. The higher order of AE-specific energies indicates chip formation mechanisms such as shearing and microfracture. Chip morphology studies are carried out using the FFT analysis. The FFT indicates that low-frequency and low-amplitude AE lead to tight curl chips, while high-frequency and high-amplitude AE lead to elemental/short comma chips. This work provides new significant inferences on tool wear, chip formation mechanisms and chip morphology in the microendmilling of AA 1100.

Characterization of crack growth resistance under thermal shock on silicon nitride with various microstructure

The crack growth resistance under thermal shock loading on silicon nitride was characterized using Disc-on-Rod test which was developed by the authors. The microdamage during thermal shock fracture was monitored by acoustic emission (AE) technique. Specimens were composed of β-Si3N4 with an acicular structure. Cylinders of silicon nitride with various microstructures were sintered at different temperatures. A pre-crack was introduced using Knoop indentation. In the Disc-on-Rod test, the specimens were uniformly heated to 850 oC, and only the center of circular specimen was quenched by contacting with a copper rod. Within a fraction of a second after contacting, crack propagated in an unstable manner, at the same time, a high amplitude AE signal was detected. Subsequently, stable crack propagation was observed, and several low amplitude AE signals were generated corresponding to crack propagation. The specimen sintered at higher temperature showed lower crack growth resistance. From the results of microstructure observation and the fracture mechanical consideration, it is suggested that larger grains with > 1.5 μm in minor axes contribute to the toughening of materials. Consequently, the results of this study provide fundamental insights for the development of ceramic materials with high resistance to thermal shock fracture.

Acoustic emission signal denoising to improve damage analysis in glass fibre-reinforced composites

In this contribution, continuous wavelet denoising technique is developed to enhance the signal to noise ratio of acoustic emission (AE) signals. The time–frequency characteristics of the recorded noise are first determined by calculating the noise wavelet coefficients. Then, an algorithm able to proceed denoising by extracting noise features from those corresponding to AE signals is developed. The denoising procedure is applied to get meaningful signals when the detection threshold is decreased from 30dB to 25dB. Furthermore, the spectral flatness measure was applied to eliminate signals originating from noise. Experiments were conducted on cross-ply composite specimens [ ± 45°]6S to examine the efficacy of the proposed technique. The denoised signals were analysed using the continuous wavelet transform and were also analysed in terms of the dominant frequency band which was processed by a fast Fourier transform. Results show that denoised low-amplitude AE hits are well correlated with high-amplitude AE hits (A ≥ 30 dB) in terms of damage characterisation.

Low-stabilty states of aluminum crystal lattice and acoustic emission under high-temperature straining

Using aluminum as an example it is experimentally established that strain accumulation under low-stability condition at the temperatures higher than 0.5 melting temperature exhibits a non-monotonic behavior and represents sharp deformation changes within a wide range of scale levels (including macroscale). The effects of this stepwise deformation are accompanied by high-amplitude acoustic emission signals. An analysis of the anomalous metal behavior and acoustic emission shows that the activation volume of an elementary deformation event under this low-stability state is exponentially increased with the deformation temperature, indicating an increased scale of cooperative atomic displacements and manifestation of low stability of the crystal lattice of this atomic configuration. The macroscopic character of this stepwise deformation provides evidence on a correlation between elementary deformation events within a volume larger than one strain band. It is assumed that the amplitude of an acoustic signal can serve a criterion of this correlation.

Analysis of fracture process in single-edge-notched laminated composites based on the high amplitude acoustic emission events

The objective of this study is to analyze the fracture process of single-edge-notched (SEN) laminated composites with different lay-up configurations and different fiber composite systems based on the behavior of high amplitude acoustic emission (AE) signals. The classification of signal type according to the dominant frequency band and its magnitude via FFT, combined with the microscopic observations under reflection and transmission optical microscopy, have been also performed in order to support the effectiveness of the analysis based on the high amplitude AE signals. It has been shown that the behavior of high amplitude event rate describes well the fracture process in the SEN laminated composites with different fiber orientations; the direction of the main crack propagation in SEN laminated composites is decisively affected by the fiber orientation and is irrespective of the initial notch direction. Finally, the AE characteristics for SEN laminated composites have been summarized in association with the individual fracture process.

Acoustic emission and deformation processes in aluminum at high temperatures

Processes of high-temperature deformation of polycrystalline aluminum are investigated. It is found out that at high temperatures monotonous deformation transforms into macroscopic deformation jumps accompanied by single high-amplitude acoustic emission pulses correlated with the strain rate.

Characterisation of acoustic emission signals during fracture and fatigue of SiC fibre reinforced titanium alloy composites

Acoustic emission (AE) signals generated during the fracture and fatigue of continuous SiC fibre reinforced titanium alloy composites (SCS-6/β21S, SCS-6/Ti–6Al–4V, and SM1240/Ti–6Al–4V) have been measured, and AE parameters have been correlated with microfracture processes. The amplitude distributions of AE signals in both fracture and fatigue were found to have distinct peaks at ∼90 dB with reference to 5 μV at the output of the sensor used. These high amplitude AE signals were confirmed to correspond to fibre fracture by direct optical observation, and deduced from sharp crack growth rate excursions measured by an electrical potential difference method. Although the distribution of lower amplitude signals (<70 dB)changed progressively during loading to fracture, it was difficult to distinguish microfracture processes precisely since several potential fracture modes overlap in the amplitude distribution. The ratio of the peak amplitude to the rise time of the AE signals could, however, be classified into several categories, and this method of analysis is most useful in discriminating between fibre/matrix debonding events and fibre sliding events. The present work suggests that the AE technique is useful for damage evaluation in these composites, in general, and in particular it can quantify fibre fracture.MST/3352

Characterisation of acoustic emission signals during fracture and fatigue of SiC fibre reinforced titanium alloy composites

Acoustic emission (AE) signals generated during the fracture and fatigue of continuous SiC fibre reinforced titanium alloy composites (SCS-6/β21S, SCS-6/Ti–6Al–4V, and SM1240/Ti–6Al–4V) have been measured, and AE parameters have been correlated with microfracture processes. The amplitude distributions of AE signals in both fracture and fatigue were found to have distinct peaks at ∼90 dB with reference to 5 μV at the output of the sensor used. These high amplitude AE signals were confirmed to correspond to fibre fracture by direct optical observation, and deduced from sharp crack growth rate excursions measured by an electrical potential difference method. Although the distribution of lower amplitude signals (<70 dB)changed progressively during loading to fracture, it was difficult to distinguish microfracture processes precisely since several potential fracture modes overlap in the amplitude distribution. The ratio of the peak amplitude to the rise time of the AE signals could, however, be classified into several categories, and this method of analysis is most useful in discriminating between fibre/matrix debonding events and fibre sliding events. The present work suggests that the AE technique is useful for damage evaluation in these composites, in general, and in particular it can quantify fibre fracture.MST/3352

Damage Evoluation and Dynamic Mechanical Properties in a SiC Fibre Reinforced Ti Alloy Composite

Damage evolution in a SiC fibre reinforced Ti alloy composite (SCS-6/Ti-6Al-4V) has been assessed by modulus loss, change in internal friction (mechanical damping) and acoustic emission (AE). Damage was introduced into unidirectionally reinforced unnotched specimens by four point bending using constant cyclic amplitude loading. Dynamic Young's modulus and internal friction were measured by a flexural resonant method after a predetermined number of cycles, and AE signals were also monitored during the tests. Damage induced in the composites changed progressively primarily as a function of the applied maximum stress levels (σ max ). Fibre fracture dominated at σ max values greater than 0.5 σB (where σ B is the flexural strength of the composite). High amplitude AE signals associated with fibre fracture were observed, and internal friction measurements increased rapidly just before failure of the specimen. In contrast, matrix cracking dominated at σ max values lower than 0.35 σ B . As matrix cracks bridged by intact fibres) grow in these composites, internal friction measurements increase. Small amplitude AE signals were observed to be associated with matrix crack growth. Also, fibre fracture occurred in the bridged zone at σ max values between 0.35 and 0.5σ B . In all tests, the Young's modulus decreased by only 1% of its initial value until just before failure of the specimen. However, internal friction measurements increased rapidly prior to catastrophic failure of the specimen at all stress levels investigated in this study. This result shows that an increase in internal friction is related to damage accumulation in the composites, and suggests that measurement of changes in internal friction may be applicable to the assessment of damage in such composites.

Characteristic of Acoustic Emission Signal in Fracture Process of SiC/Al Composites.

The tensile test of SiCCVD/Al composites and their component materials was carried out by detection of acoustic emission (AE) during deformation. The characteristic of AE in the fracture process was investigated in connection with the microfractures. The results obtained were as follows: when the fibers of the composite were broken, high-amplitude AE signals were generated and the frequency was about 0.2 MFHz. Both duration and oscillation count of the signal increased with increase of the amplitude. Therefore, both parameters are effective for evaluation of microfracture of the composites. The number of microfractures in composite decreased with increase of the fiber volume fraction to the maximum stress, that is, the fracture became noncumulative with increase of the fiber volume fraction.

Acoustic Emission Study of Failure Mechanisms in Ceramic Matrix Composite under Longitudinal Tensile Loading

Acoustic emission (AE) measurements were correlated with macroscopic stress-strain behavior and direct microscopic observations of damage mechanisms in a unidirectional ceramic matrix composite under longitudinal tensile loading. Matrix crack initiation and the onset of AE events correspond to the proportional limit of the material. Both the crack density and cumulative AE counts vary linearly up to the point of crack saturation. Thereafter follows a "strain hardening" region corresponding mainly to fiber-matrix debonding and manifested by an increase of low to moderate amplitude AE activity and leveling off of high amplitude AE signals. At higher strains, beyond the "strain hardening" region, there are two characteristic parts in the AE output, one related to further damage (possibly debonding and fiber fracture) and the other clearly related to interfacial frictional sliding. Upon unloading further AE output related to sliding is observed. AE activity of low to intermediate amplitude continues in the last stages of loading and is believed to be associated with fiber fractures.

A Model of Tool Fracture Generated Acoustic Emission During Machining

This paper presents details of a study of the fundamental nature of the high amplitude acoustic emission (AE) signal generated during the complete breakage of cutting tools. A quantitative model of the peak AE rms voltage, incorporating the effects of tool material and the area of the fracture surface, is proposed. The model, derived from results of Linear Elastic Fracture Mechanics, and stress wave propagation in solids, is shown to be in good agreement with results of notched carbide insert fracture during turning and face milling operations.

Influence of proof stress, composition, and microstructure on acoustic emission during ductile fracture in medium- and high-strength steels

AbstractThe authors were concerned with the factors which influence high-amplitude acoustic emission during ductile crack extension in quenched and tempered low-alloy steels. A systematic approach was adopted to isolate the effects of compositional variables, microstructural variables, and strength level. Experimental steels were used to avoid the effects of uncontrolled changes in composition and manufacturing process which are inevitable when comparing steels which have been produced commercially. The effect on acoustic emission of the following variables was evaluated: 0·2% proof stress, carbon content, sulphur content, and sulphide morphology. It appears that proof stress is the most important factor governing the amount of high-amplitude acoustic emission generated as the result of ductile cracking in steels; a secondary influence is exerted by the sulphur content in steels which contain elongated manganese sulphide inclusions. The results indicated that very low levels of high-amplitude acoustic emi...