Analysis of Acoustic Emissions from a Steel Bridge Hanger

A steel bridge hanger with three fatigue cracks was monitored for acoustic emission (AE) using combined source location, strain gauge monitoring, and waveform analysis. AE activities from all three cracks were clearly identified and classified as crack growth or noise signals using location, strain magnitude, position on strain cycle, and uniqueness of waveforms as the primary criteria. A vast majority of AE from the cracks was found to be due to crack face rubbing and the crushing of corrosion products between the crack faces while limited crack growth emissions were detected. Results from laboratory tests on A588 compact tension specimens under variable-amplitude tension-tension fatigue loading were used to aid in interpreting AE data from the hanger. Crack growth AEs from these tests were detected only on overload cycles mostly above 92% of the maximum load while AE due to crack face rubbing occurred throughout the load cycle.     

Source kinematics of acoustic emission based on a moment tensor

A source-inversion procedure for determining crack types and orientations (source kinematics) has recently been developed for acoustic emission (AE). Quantitative waveform analysis of AE is introduced on the basis of the moment tensor representation. To determine source kinematics, the eigenvalue analysis of the moment tensor is investigated. This leads to the separation of eigenvalues into the shear component, compensated linear vector dipole (CLVD) component and mean component. Based on ratios of these components, AE sources are classified into a shear or a tensile crack. Since the orientations of a shear crack and a tensile crack can be determined using eigenvectors, source kinematics of crack types and orientations are completely identified from AE waveforms. The procedure is applied to AE waveforms monitored during a pull-out test of an anchor bolt from a concrete block. The detected AE waveforms are analysed by the procedure and crack locations, types and orientations are determined. The source kinematics obtained agree with experimental results.     

Quantitative acoustic emission source characterization of microcrackings in steel

Acoustic emission (AE) source wave analysis is a new NDE technique for the investigation of dynamic fracture process. We applied this technique to the quantitative characterization of crack sources in ductile fracture. Using two samples of ASTM A533B steel with different sulfur content, acoustic emissions during fracture toughness tests were detected, located, and analyzed. The detected AE signals were classified into two types according to the analyzed source waveforms. One was a signal due to microcracking at the MnS inclusion, and the other was a signal due to coalescence of the voids. The results of the source wave analysis showed that microcracking at the inclusions was due to Mode I type tension crack with sizes of 10–30 µm, and the coalescence of the voids was due to tension shear mixed cracks with sizes of 60–100 µm. It was confirmed that this technique is very effective for the quantitative evaluation of microcrackings and for the detection of the nucleation and growth of cracks.     

Freeze-thaw crack determination in cementitious materials using 3D X-ray computed tomography and acoustic emission

As concrete freezes and thaws cracks may develop. These cracks can provide a path for water and ionic species to penetrate the concrete. This may reduce the service-life of the concrete element. In this study, X-ray computed tomography (CT) was used as a non-destructive technique to characterize the microstructure of mortar samples that were exposed to different levels of freeze-thaw damage by varying degree of saturation in the samples (75, 90, 95, and 100% degrees of saturation). Acoustic emission (AE) experiments were performed during freezing and thawing to investigate sample cracking behavior. The volume of cracks present within the mortar samples after freezing and thawing were determined using X-ray CT and compared to passive acoustic emission data. The location/source of cracks was also determined using X-ray CT. The crack sources (i.e., void, aggregate, interfacial transition zone, or paste) were determined using X-ray CT and were related to AE activities during cracking. Crack volumes were found to increase with increased levels of saturation, and visual observations of cracking were found to correlate with AE signatures of various crack sources.     

Damage analysis of concrete members containing expansive agent by mechanical and acoustic methods

Shrinkage compensating concrete (SCC) and Self-stressing concrete (SSC) technique have been employed for reducing early-age cracking and leakage while the addition of expansive agent would have a negative impact on mechanical properties and durability. The objective of the current research was to quantitatively assess the damage development in cementitious materials with expansive agent by both the strength tests and nondestructive acoustic tests including ultrasonic measurements and acoustic emission (AE) tests. The damage degree was defined based on strength as well as ultrasonic properties and a significant linear relationship was observed between the damage degree and autogenous strains. AE parameters such as AE amplitude, AE counts and AE energy were related to AE activity of the cement-expansive agent system. Crack mode identification was performed based on the relationship between average frequency and RA value (rise time/amplitude). A decreasing ratio of tensile cracks and an increasing ratio of shear cracks were observed which could be an indication of aggravated damage inside the materials.     

Damage Mode Identification for the Clustering Analysis of AE Signals in Thermoplastic Composites

An objective analytical procedure for the investigation of damage mechanisms in the thermoplastic self-reinforced polyethylene (UHMWPE/PE) composites under quasi-static tensile load has been established, using Unsupervised Pattern Recognition (UPR) technique for the clustering task of Acoustic Emission (AE) signals. Focus is on the correlating between the obtained classes and their specific damage mechanisms. This was carried out by waveform visualization and Fast Fourier Transform analysis. Pure resin and fiber bundles were tested to collect typical waveforms of matrix cracking and fiber fracture respectively, in order to label the signal classes in the composites. The evolution process of various damage mechanisms in the composites revealed that the correlating method was effective. The AE characteristics of different damage modes found out in this study can be used as the reference for identifying unknown AE signals in the UHMWPE/PE composites. The established procedure is also potential in the investigation of failure mechanisms for composite materials with UPR technique.     

Acoustic emission source location in composite materials using Delta T Mapping

The location capability of the acoustic emission (AE) technique is often considered its most powerful attribute. However, assumptions made in the calculation of location by current algorithms can be limiting in complex geometries and materials. This work forms a detailed study into the use of a novel mapping technique for AE source location in fibre reinforced composite materials. Both the performance and the robustness of the approach are assessed using artificial and real AE sources. Furthermore a large fatigue specimen was used to demonstrate detection and location of damage onset and development, where findings were validated using a thermo-elastic stress analysis (TSA) system. Substantial improvements in location accuracy were observed and early detection of damage onset was seen to outperform TSA.     

Localization of a ``Synthetic' Acoustic Emission Source on the Surface of a Fatigue Specimen

The finite geometry of a laboratory specimen influences a measured acoustic emission waveform because of reflections, transmission, and mode conversion at the interface and boundaries of the specimen, thus making it difficult to determine the location of an acoustic emission (AE) source. The objective of this investigation is to develop a model experiment to identifiy the exact source location on the surface using ``synthetic' AE signals. The AE event is generated by a short local thermal expansion. This expansion is produced by the absorption of a short laser pulse which provides a noncontact and broad-band generation of elastic waves. The signals are detected by a noncontact, broad-band, and high-fidelity sensor: a laser interferometer. The triangulation with several detectors is replaced by a single probe laser interferometer located at different coordinates under reproducible conditions. The recorded signals are analyzed by wavelet transform in order to determine the arrival times of waves for several frequency levels. These arrival times are used to quantify the location of the AE source in the surface as well as the velocity of the most dominant feature, the Rayleigh wave, and the time lag between the instant of the AE and the recording of the signal. The accuracy of the method is demonstrated by comparing the identified source location with the exact one.     

Effects of loading and sample geometry on acoustic emission generation during fatigue crack growth: Implications for structural health monitoring

The reliability of traditional non-destructive methods for crack detection is well understood and characterised using Probability of Detection (POD) curves. Structural Health Monitoring (SHM) techniques in contrast remain largely unquantified. The performance of the Acoustic Emission (AE) technique for damage detection and location in potential SHM applications is underpinned by the intensity of AE signal generation from the damage site. In this paper, factors influencing the rates of emission of Acoustic Emission (AE) signals from propagating fatigue cracks were investigated. Fatigue cracks were grown in specimens made from 2014 T6 aluminium sheet while observing the effects of changes in crack length, loading spectrum and sample geometry on rates of acoustic emission. Significant variation was found in the rates of AE signal generation during crack progression from initiation to final failure with a number of distinct phases identified in that progression implying different failure mechanisms operating at particular stages in the failure process. A new ‘probability of hit’ method for quantifying crack detecting capability using AE is also presented.     

Lamb wave evaluation and localization of transverse cracks in cross-ply laminates

This paper investigates the effect of transverse cracks on the S ₀ mode velocity in GFRP and CFRP cross-ply laminates, and proposes a new AE source location method that considers the change in the S ₀ mode velocity due to the transverse cracks. We found experimentally that the stiffness and the velocity decreased as the transverse crack density increased. Analytical predictions deduced from the combination of the complete parabolic shear-lag analysis, the classical plate theory and the laminated plate theory are in good agreement with the experimental results. Utilizing this relationship between the velocity and the mechanical damage, we located AE sources of transverse cracks in cross-ply laminates with the calculated in situ velocity. We were able to show that highly accurate source location requires the reduction of the in situ value of the velocity. The present method is simple but quantitative and useful in health-monitoring for detecting and localizing the damage in composite structures.     

Acoustic emission source localization technique based on least squares support vector machine by using FBG sensors

An intelligent acoustic emission (AE) source localization technique by using fiber Bragg grating (FBG) sensors was investigated. Four FBGs sensing network was established for detecting the AE signal. And power intensity demodulation method was initialized employing narrow-band tunable laser. The intelligent AE source localization method was proposed based on wavelet transform, cross-correlation analysis, and least squares support vector machines (LS-SVM). LS-SVM modal’s input is signal time difference and output is AE source position. The location experiments were carried out on a 500 mm × 500 mm × 2 mm aluminum alloy plate. The results showed that the AE source position abscissa and ordinate localization errors are all less than 10 mm. The maximum and average localization errors are 8.65 and 6.78 mm, respectively. The research results provided a novel method for AE source localization by using FBG sensors.     

Acoustic Emission Study of Fatigue Cracks in Materials Used for AVLB

The Armored Vehicle Launch Bridge (AVLB) is subjected to cyclic loading during launching as well as during tank crossings. The cyclic loading causes cracks to initiate in critical bridge components, and then to propagate. Unless these cracks are detected and repaired before they rapidly grow to reach their critical stage of propagation, the failure of bridge components can occur. Three AVLB components, the splice doubler angle, the splice plate, and the bottom chord, are susceptible to fatigue damage. In the present study, laboratory fatigue tests on the materials used for the components, aluminum 2014-T6, aluminum 7050-T765, and ASTM A36 steel, were conducted using the acoustic emission (AE) fatigue crack monitoring technique. A total of fourteen compact-tension specimens were prepared in this study: six aluminum 2014-T6, four aluminum 7050-T76511, and four ASTM A36 steel specimens. The characteristics of AE signals associated with the stress intensity factor, K, were obtained to understand AE behavior corresponding to the fatigue crack growth in the materials. Several AE parameters, such as AE counts, energy, and hits, have been shown to be useful tools for detecting cracks, providing early warnings, and preventing failure of the AVLB structures. A major jump in AEcount rate as well as AE hit rate occurred when K_max reached a value of about 30~MPam (27 ksiin.) for aluminum 2014-T6 specimens and about 50 MPam (46 ksiin.) for aluminum 7050-T76511 specimens. Also, AE source location techniques were able to successfully locate the path ofcrack propagation.     

冻融作用对自密实轻骨料混凝土声发射特性影响EI北大核心CSCD

为研究冻融后自密实轻骨料混凝土单轴压缩下的声发射(AE)特性,对未掺引气剂的自密实轻骨料混凝土试件进行0次、50次、100次快速冻融试验。结果表明:随着冻融次数增加,试件轴压应力-应变曲线趋于完整,峰值应力有明显降低;声发射峰值频率主要位于15 kHz~45 kHz,85 kHz~105 kHz,235 kHz~255 kHz和285 kHz~320 kHz这4个“优势频段”区间,对应于混凝土内部预存裂纹或孔隙压密,骨料/砂浆界面增强层的开裂,粗骨料断裂破坏以及砂浆的开裂。受冻后的试件在轴压作用经历拉伸裂缝与剪切裂缝之间的交替转化,最终形成主裂缝导致破坏。冻融作用以及应力水平变化对AE信号源分布有较大影响,随着应力水平的增加,AE信号源趋于活跃,且在试样断裂面有聚集趋势。     

Analysis of Acoustic Emission Propagation in Metal-to-Metal Adhesively Bonded Joints

Acoustic emission (AE) monitoring shows promise as one of the most effective methods for condition monitoring of adhesively-bonded joints. Previous research has demonstrated its ability to detect, locate and classify adhesive joint failure, though in these studies little attention appears to have been paid to the differences in AE wave propagation through the bonded and un-bonded sections of the specimens tested, or to the effects of the wave modes excited or the propagation distances. This paper details an experimental study conducted on large aluminium sheet specimens to identify the effects of the presence of an adhesive layer on AE wave propagation. Three specimens are considered; a single aluminium sheet, two aluminium sheets placed together without adhesive, and an adhesively-bonded specimen. A pencil lead break (PLB) is used as a simulated AE source, and is applied to the three specimens at varying propagation distances and orientations. The acquired signals are processed using wavelet-transforms to explore time-frequency features, and compared with modified group-velocity curves based on the Rayleigh–Lamb equations to allow identification of wave-modes and edge-reflections. The effects of propagation distance and source orientation are investigated while comparison is made between the three specimens. It is concluded that while the wave propagation modes can be approximated as being constant throughout all three specimens, there is a significant change in the received waveforms due to the attenuation of high-frequency components exhibited by the bonded specimen. These findings may be utilised to provide a deeper understanding of acquired AE data, improving the current abilities to identify, locate and characterise damage mechanisms occurring within adhesive joints, ultimately improving safety in the use of adhesive bonding for critical applications.     

Tensile Performance and Damage Evolution of a 2.5-D C/SiC Composite Characterized by Acoustic Emission

The tensile behavior of a unique 2.5 D C/SiC composite and associated damage evolution was characterized by means of acoustic emission (AE) technique. The results show that the stress–strain curves exhibit mostly nonlinear behavior, which corresponds well with the AE activities. All the evolutions of damage characteristics indicate the phenomenon of matrix cracking saturation which is in agreement with AE results. Microstructural observations reveal extensive matrix cracks and the fracture of yarns always occurred in yarn crossover areas due to the unique weave architecture.     

Determination of the damage threshold in woven-ply thermoplastic laminates at T > Tg: Acoustic emission and microscopic damage analysis

An original in situ measurement of acoustic emission (AE) was applied to monitor damage progress in discrete steps during gradual load/unload tensile tests on [±45°]₇ C/PPS laminates at temperatures T > Tg, when matrix ductility is enhanced. In order to understand the specific damage behavior of such materials under severe environmental conditions, AE analysis was accompanied by microscopic observations to detect the damage initiation threshold as well as the damage mechanisms within the composite material. Once the AE source mechanisms have been separated into classes thanks to the pattern recognition software Noesis, they have been identified to match physical phenomena. Earliest cracks events occur at the crimps where the rotation of warp/weft fibres takes place, followed by the intra-bundles splitting on free surface. It is observed that the onset of intralaminar cracking and debonding is affected by the presence of matrix-rich regions between the plies, because of an extensive plasticization of the PPS matrix. The study of the specific acoustic activity of neat PPS resin specimens confirms that the local plastic deformation in matrix-rich areas contributes to delay the initiation of damage, and subsequent AE signals. Finally, AE proved to be a relevant technique to investigate damage mechanisms and to determine accurately the damage threshold in TP-based composites to be used in aeronautical applications at T > Tg.     

Detection of segmentation cracks in top coat of thermal barrier coatings during plasma spraying by non-contact acoustic emission method

Numerous cracks can be observed in the top coat of thermal barrier coatings (TBCs) deposited by the atmospheric plasma spraying (APS) method. These cracks can be classified into vertical and horizontal ones and they have opposite impact on the properties of TBCs. Vertical cracks reduce the residual stress in the top coat and provide strain tolerance. On the contrary, horizontal cracks trigger delamination of the top coat. However, monitoring methods of cracks generation during APS are rare even though they are strongly desired. Therefore, an in situ, non-contact and non-destructive evaluation method for this objective was developed in this study with the laser acoustic emission (AE) technique by using laser interferometers as a sensor. More AE events could be detected by introducing an improved noise reduction filter and AE event detection procedures with multiple thresholds. Generation of vertical cracks was successfully separated from horizontal cracks by a newly introduced scanning pattern of a plasma torch. Thus, generation of vertical cracks was detected with certainty by this monitoring method because AE events were detected only during spraying and a positive correlation was observed between the development degree of vertical cracks and the total AE energy in one experiment.     

Evaluation of the repair on multiple leaf stone masonry by acoustic emission

Aim of this research was to evaluate the possible application of the AE technique to study the response of multiple leaf masonry repaired with different techniques. The results of shear tests carried out on three specimens repaired after failure are presented on an explorative basis; on two of them the acoustic emission (AE) technique was also used. Through a number of sensors and by triangulation, this technique makes it possible to identify the location of the damage, unknown initially, and subsequently to assess the stability of its evolution. Using the AE technique the area of the material where the cracks propagated during shear tests was identified. These analyses made it possible to evaluate the effectiveness of repair interventions through parameters other than stress and strain, and contributed to the identification of the most suitable methodology for their optimisation.     

Estimation of cracking and damage mechanisms of rock specimens with precut holes by moment tensor analysis of acoustic emission

This work presents an experiment on the acoustic emission (AE) of coarse grain granites with two square-shaped precut holes under uniaxial loading. Studies were carried out on the temporal–spatial evolution behavior of micro-cracks by AE mechanisms with the use of the simplex location method and the moment tensor theory, with further analysis in comparing the numerical simulations using the software RFPA \(^\mathrm{2D}\) (Rock Failure Process Analysis). The results show that during the loading process, from beginning to rock failure, shear-mode micro-cracks are prominent, constituting more than 60 % of the total events; next most common are tensile-mode micro-cracks a less than 35 % of the total events. Variations of micro-cracks of the three modes during the loading process have the same increase tendency, i.e. fewer were generated in the initial loading stage, with a rapid increase when the stress values are between 40 and 60 % of the peak stress, and a rate dimunition before rock failure. It is observed that the tensile stress concentration is prone to appear at the tops of the two holes in the form of tensile type cracks, while the shear stress concentration usually appears at the bottom in the middle region of the specimen in the form of shear type cracks. The findings of the present work may serve as guidance for the prevention of roof and floor collapse in the stope exploration of mines.     

基于核主成分分析及支持向量机的水轮机叶片裂纹源定位

结合核主成分分析(KPCA)以及支持向量机对水轮机转轮叶片裂纹源的声发射信号进行定位。结果表明,利用核主成分分析提取的特征参数进行定位的精度高于原始参数的定位精度,即输入9个特征参数时,支持向量机在叶片区域的识别率为100%,在裂纹源对焊缝距离的支持向量回归分析中的最大误差为20cm。因而结合KPCA和支持向量机对复杂的大尺寸结构进行定位是一种较好的方法,既减少了输入信号的维数,又提高了定位精度。