Analysis of acoustic emission waveforms from fatigue cracks

Acoustic Emission (AE) monitoring is an effective tool for prioritising more conventional non-destructive inspections of above ground tanks, either process or storage vessels. Results obtained from AE monitoring may be assessed against an existing data bank permitting plant management to prioritise maintenance to tanks with high active corrosion rates or leaks. AE results may be used to extend inspection periods of tanks without leaks or active corrosion. This paper covers the theory of AE leak detection and corrosion monitoring including some background research into the difference between AE from corrosion and leaks. It looks at the equipment used and provides a summary of tank test results conducted by ATTAR and other overseas organisations as well as reviewing AE tank inspection economics.

Acoustic emission (AE) is a widely used non-destructive technique for real time monitoring. Over the past decades it has been used as an effective technique for the detection, characterization and location of fatigue cracks. The basic principle is based on the elastic energy emitted from crack propagation incidents under stress. The emitted elastic wave propagates through the continuous media and it is detected by sensors attached on the surface of the material. Fatigue cracks are one of the important damage phenomena in metallic structures in aerospace applications. Fatigue life under cyclic loading consists of three stages: stage I (crack nucleation), stage II (crack propagation) and stage III (failure). In this study, the fatigue crack propagation of Ti6Al- 4V four-point bending test specimens is studied during stage II. Two test specimens are used for this study, one corner notched sample and one unnotched sample. In order to monitor the crack propagation, two AE piezoelectric sensors are placed on one side of the specimen and crack propagation gauges are placed on the damage location areas as verification. The rate and the location of the recorded events is in good correlation to the crack growth rate and the position of the crack showing the potential of using AE for fatigue monitoring of this kind of test. Other AE parameters are also studied with respect to their efficiency for characterizing crack propagation, though care should always be taken for reflections and dispersion which inevitably distort the acoustic signals. doi: 10.12783/SHM2015/186

Acoustic emission (AE) was used to monitor the tensile test of composites with adhesive specimens. The mechanical response behavior, damage and failure characteristics, and the corresponding AE characteristics of the composites have been investigated. The results show that the load of the join with defect in the adhesive layer is lower than the join with no defect. The higher AE relative energy and the AE amplitude were obtained in the adhesive specimen with defect in the adhesive layer whereas the variation of the AE relative energy is different from the adhesive specimen with on defect. The characteristics such as AE amplitude distribution, relative energy and cumulative hits are connected with the tensile damage and failure of the adhesively bonded single-lap joints of composite laminate. In the actual AE monitoring, these feature parameters should be considered entirely assess the damage and failure of the composites structures.

Acoustic Emission (AE) is a widely used non-destructive evaluation technique mainly during periodic testing to insure structural integrity of process plant equipment. AE can also be used for continuous monitoring of damage (initiation and propagation) due to localized aqueous corrosion. For instance, AE is the only NDE technique that allows the detection and localization of propagating stress corrosion cracks (SCC). Moreover, SCC initiation is detected by AE, in the early stages compared to other techniques. The correlation between AE parameters and SCC damage is very useful for understanding and modelling the crack propagation process. For wear or abrasion corrosion, the AE signals measurements provide a valuable tool to determine the rate of mechanical abrasion related to the kinetic energy flow of particles. It gives complementary information to other techniques, such as continuous or transient electrochemical recording, more devoted to the monitoring of purely corrosive conditions. AE also appears to be a very promising tool for studying other localized corrosion modes such as pitting or crevice attack.

Acoustic emission (AE) from quartz during heating was measured with a high resolution decrepitometer on more than 350 samples from various conditions of formation and geological settings. The emitted acoustic signals can accumulate in very sharp peaks or extend over a wide temperature range. Different types and conditions of quartz formation can be distinguished from total counts and individual AE-patterns. Correlation of the determined AE peaks with microthermometric investigations, optical and scanning electron microscopical (SEM) studies of polished and etched thin sections showed that the sound, generated during heating, is caused by several distinguishable mechanisms. The main cause is the thermal expansion mismatch and thermal anisotropy of the quartz. In all cases the sound is emitted from opening and propagating microcracks. At temperatures below 380° C the main sources of AE are the rupture of grain boundaries, transgranular fracturing and decrepitation of large fluid inclusions. In the temperature range of 350–550° C, intragranular fracturing, sometimes enhanced by Brasil twinning, and reopening of healed fissures, often decorated with large numbers of small secondary fluid inclusions, cause distinct peaks of acoustic emission. At the α-β inversion temperature (≈ 573° C) massive emission of sound occurs if the quartz is twinned according to the Dauphine law. Measurement of AE can help to determine critical temperatures of material failure and reveal information on the residual strain in rocks. Different generations of quartz veins can be distinguished, even if covered by soil and the extent of alteration zones can be determined by the varying AE patterns.

Acoustic Emission (AE) has been adopted widely in structural health monitoring of buildings, walls, bridges, and has shown great potentials in incipient rotating machine fault detection [1, 2]. However, one inevitable problem that hinders AE of being practically applied in rotating machine fault detection is the transmission of AE signal inside the machine, as AE signal generally comprises high frequency components of typically 100kHz to 1MHz which will require transmission system to have extremely high data-rate for real-time monitoring. Traditional digital wireless system, for instance, Wi-Fi or Bluetooth, cannot satisfy the requirement. This paper addresses the issue of wireless AE transmission for rotating machine, with a modified analogue AE wireless transmission system. Piezoelectric wafer active sensor (PWAS), a lab-crafted miniature sensor was adopted to fulfill the need of small system.

Acoustic Emission (AE) has proven to be a useful tool in the monitoring of fiberglass pressure vessels. The ASME Piping and Pressure Vessel Code (subsection X) implements AE as an approved method for quality control and in service testing of fiberglass pressure vessels, making it an accepted technique for the evaluation of fiber composite structures. This article presents the result of AE monitoring of fiber reinforced plastic (FRP) pipes after impact damage has incurred. The results from these tests indicate that AE can be used as a tool to predict the level of damage incurred in the pipes after impact and during pressurization. The use of the Felicity Ratio as the benchmark for evaluation is presented as the key evaluating value.

Identification and prediction of fatigue crack growth under different stress ratios using acoustic emission data

Acoustic Emission (AE) technique is non-destructive Structural Health Monitoring (SHM) technique which has the potential to detect internal damages in real time. Literature reveals that researchers have used three-dimensional (3d) Finite Element Method (FEM) to predict AE waveforms in isotropic and single-layer anisotropic plates. However, the study to predict acoustic emission waveforms using FEM in layered composites and sandwich plates is limited in literature. Modelling of such layered plates is challenging as material properties may vary from one layer to another across the plate thickness. In this study, an effort has been made for two-dimensional finite element modelling for predicting AE waveforms in layered composites and sandwich plates using a generalised refined plate theory modelling. Numerical examples are first solved for isotropic and anisotropic plates and validated with the results obtained from 3d FEM. New results are presented for multilayered composites and sandwich plates.

Acoustic emission (AE) technique is often employed by condition monitoring personnel to detect abnormalities in moving components of machines. Due to its low signal-to-noise ratio at high-frequency range, it often requires certain signal processing technique to extract valuable information from machine parts. This study intends to obtain the characteristic frequency segments of AE signals that correspond to the physical movement of valves in reciprocating compressor. It involves acquisition of AE signals at various simulated valve conditions and rotating speeds, decomposition of these signals through wavelet packet transform (WPT), and computation of crest factor (CF) of WPT coefficients at a specific crank angle. The characteristic frequency segments that indicate valve problems are often accompanied by an extra high or low CF value. By relating the CF value to valve events, the condition of valve can be predicted. It is hoped that this study can provide a methodology to obtain valve information effectively and efficiently while reducing unwanted and overloading information of AE signals.

Acoustic emission (AE) has previously been shown to be a useful technique in monitoring the state of wear of cutting tools. The piezo-electric transducers conventionally used for AE detection are contacting devices with a limited bandwidth. This paper describes the use of a robust fiber optic interferometer for the in-process measurement of AE during the face milling of annealed En24 steel to provide tool wear information via analysis of the rms AE signal. The interferometer displayed an improved diagnostic capability over a conventional piezoelectric sensor due to its advantages of being non-contacting, broadband with a flat frequency response to displacement, and providing absolute calibration.

Acoustic emission (AE) of fiber break is the sound waves reflecting the stress waves of material during rupturing. It can be employed to evaluate the tensile properties of single fiber during the fiber-bundle tensile test. The key to this technique is to identify AEs of fibers at the break point accurately. However, the severe interference introduced by the background noise and the inter-fiber friction is inevitable. In this paper, a filtering method based on the wavelet de-noising is used to remove the background noise. In order to identify the fiber-broken AEs, a combined parameter that characterizes the amplitude and the density fluctuation along the time axis is proposed. Multiple test results indicate that this parameter can be used to identify wool fiber-broken AEs with accuracy rates higher than 95%.

Acoustic Emission (AE) and hysteresis parameters were studied from PZT-5A (soft) and PZT-8 (hard) ceramics during the application of ac fields in the frequency range of 0.2 Hz to 5 Hz. AE was found to occur mainly due to domain switching in these ceramics during the application of ac fields. In PZT-5A, a threshold field was observed for the AE activity to begin in both the direction of the applied field and in general, it was found to decrease with increasing frequency. Apart from this, AE activity was found to decrease with increasing number of applied ac cycles, and is attributed to domain pinning. In the case of PZT-8, AE was found to occur even at zero field at higher frequencies and AE activity was found to increase with increasing number of field cycles. These are explained on the basis of the defect dipole — domain interaction in these ceramics.

Acoustic emission (AE) has been chosen as a means of monitoring cutting system changes from remote sensor locations. The determination of a processing strategy which can remove signal transmission path variability has been explored. This strategy, termed homomorphic processing, is developed and tested on both simulated and experimentally collected data. The recovery properties of this strategy are analyzed in terms of both AE source characterization and signal path transmission effects. These properties are found to be influenced by the choice of homomorphic lifter cutoff time. Guidelines are given for the selection of this parameter. Spectral representations of the recovered AE source show that signatures collected at remote sensor locations can be successfully used to identify process and structural changes in the cutting system.

Monitoring concrete reinforced with bonded surface plates by the acoustic emission method