Acoustic emission

About: Acoustic emission is a research topic. Over the lifetime, 16293 publications have been published within this topic receiving 211456 citations.

A New Fault Location Approach for Acoustic Emission Techniques in Wind Turbines

Abstract: The renewable energy industry is undergoing continuous improvement and development worldwide, wind energy being one of the most relevant renewable energies. This industry requires high levels of reliability, availability, maintainability and safety (RAMS) for wind turbines. The blades are critical components in wind turbines. The objective of this research work is focused on the fault detection and diagnosis (FDD) of the wind turbine blades. The FDD approach is composed of a robust condition monitoring system (CMS) and a novel signal processing method. CMS collects and analyses the data from different non-destructive tests based on acoustic emission. The acoustic emission signals are collected applying macro-fiber composite (MFC) sensors to detect and locate cracks on the surface of the blades. Three MFC sensors are set in a section of a wind turbine blade. The acoustic emission signals are generated by breaking a pencil lead in the blade surface. This method is used to simulate the acoustic emission due to a breakdown of the composite fibers. The breakdown generates a set of mechanical waves that are collected by the MFC sensors. A graphical method is employed to obtain a system of non-linear equations that will be used for locating the emission source. This work demonstrates that a fiber breakage in the wind turbine blade can be detected and located by using only three low cost sensors. It allows the detection of potential failures at an early stages, and it can also reduce corrective maintenance tasks and downtimes and increase the RAMS of the wind turbine.

Acoustic emission localization in plates with dispersion and reverberations using sparse PZT sensors in passive mode

Abstract: A strategy for the localization of acoustic emissions (AE) in plates with dispersion and reverberation is proposed. The procedure exploits signals received in passive mode by sparse conventional piezoelectric transducers and a three-step processing framework. The first step consists in a signal dispersion compensation procedure, which is achieved by means of the warped frequency transform. The second step concerns the estimation of the differences in arrival time (TDOA) of the acoustic emission at the sensors. Complexities related to reflections and plate resonances are overcome via a wavelet decomposition of cross-correlating signals where the mother function is designed by a synthetic warped cross-signal. The magnitude of the wavelet coefficients in the warped distance?frequency domain, in fact, precisely reveals the TDOA of an acoustic emission at two sensors. Finally, in the last step the TDOA data are exploited to locate the acoustic emission source through hyperbolic positioning. The proposed procedure is tested with a passive network of three/four piezo-sensors located symmetrically and asymmetrically with respect to the plate edges. The experimentally estimated AE locations are close to those theoretically predicted by the Cram?r?Rao lower bound.

Dislocation modeling and acoustic emission observation of alternating ductile/brittle events in Fe-3wt%Si crystals

Abstract: It has been established that hydrogen assisted subcritical crack growth in Fe-3wt%Si single crystals is discontinuous while accompanied by substantial plasticity. Prop osed micromechanisms of this process are addressed via observed fine-scale {100} cleavage features, acoustic emission tracking and computer simulation analysis. The near crack tip stress distribution in an elastic-plastic analysis enabled insight into how dislocation shielding led to mirocrack nucleation. Such discretized computational models include: the stress due to the dislocation self field, the stress from crack-dislocation interactions, the stress from the crack-external stress interaction and the external applied stress. Stress distributions in both macroscopic and microscopic scales of interest were also examined, appropriate to different slip systems. It was found that the stress at the crack tip became slightly compressive while the position of the maximum stress, approaching the theoretical strength shifts to about 2–30 nm from the crack tip. The high stress region may be correlated with the observed 1 μm discontinuous crack instabilities as detected by acoustic emission. The mutual feedback from experimental findings related to acoustic emission and crystallographical habits vis-a-vis theoretical aspects are analyzed. This approach results in a micromechanical model which has implications to both hydrogen embrittlement thresholds and the ductile-brittle transition.

Hydraulic fracture energy budget: Insights from the laboratory

Abstract: In this paper we present results from a series of laboratory hydraulic fracture experiments designed to investigate various components of the energy budget. The experiments involved a cylindrical sample of Westerly granite being deformed under various triaxial stress states and fractured with distilled water, which was injected at a range of constant rates. Acoustic emission sensors were absolutely calibrated, and the radiated seismic energy was estimated. The seismic energy was found to range from 7.02E−8% to 1.24E−4% of the injection energy which is consistent with a range of values for induced seismicity from field-scale hydraulic fracture operations. The deformation energy (crack opening) of the sample during hydraulic fracture propagation was measured using displacement sensors and ranged from 18% to 94% of the injection energy. Our results support the conclusion that aseismic deformation is a significant term in the hydraulic fracture energy budget.

Application of a modified Griffith criterion to the evolution of fractal damage during compressional rock failure

Abstract: A modified Griffith criterion for a fractal ensemble of cracks is applied to the interpretation of Acoustic Emission (AE) statistics during the compressional deformation of intact and artificially pre-cut rock specimens in the laboratory. A mean energy release rate per unit crack surface area [G] is recovered from the observed AE event rate N and the seismic b value, by calculating an inferred mean crack length [c] and measuring the differential stress sigma for a range of experimental conditions. Temporal variations in [G] under compressive deformation show very similar trends to those predicted by a synoptic model determined by direct extrapolation from observations of subcritical crack growth under tension. (In the tensile case, deformation is centred on a dominant macrocrack and the stress intensity K, which scales as the square root of G, is the relevant measured variable.)The three independent variables measured during the tests (sigma, N, b) are reduced to points that map out a path through a 3-D phase space ([G], N, b), which depends on the material type and the experimental conditions. 2-D slices through this phase space [([G], N), ([G], b)] are compared with results from the tensile tests [(K, N), (K, b)]. The event rate N is found to scale with square-root [G] according to a power law, with an exponent n' which is smaller than that for tensile fracture, reflecting the greater stability of compressional rock fracture in its early stages. The effective subcritical crack growth index n' is correlated with the material type and degree of apparent 'ductility' on a macroscopic scale, with more brittle behaviour corresponding to higher n'. The value of n' is similar on unloading of the stress after dynamic faulting as on the loading portion, though the curve is systematically offset, most probably due to the material weakening associated with faulting. The model does not apply near the period of dynamic failure, where strong local interactions are dominant. The seismic b value is also found to scale negatively with square root [G], in a manner similar to experiments where K can be measured independently. The acceleration of the mean seismogenic crack length [c] = f(t) has a similar power-law form to that predicted from Charles' law for a single tensile macrocrack, with an implied subcritical crack growth index n smaller than that for fracture in compression. The extra dimension introduced by the time dependence of [c] allows an independent check on the validity of the theory used to calculate [G]. In particular n' from the diagram ([c], t) is found to be similar in magnitude to the exponent obtained from the event rate dependence ([G], N), a phenomenon first discovered by empirical observation of tensile subcritical crack growth.