Acoustic Emission Investigation Research Articles

Classification of acoustic emission signals generated from journal bearing at different lubrication conditions based on wavelet analysis in combination with artificial neural network and genetic algorithm

This paper presents the results of acoustic emission (AE) investigation for monitoring lubrication conditions of a journal bearing under various operating conditions. Hydrodynamic Lubrication (HL), Mixed Lubrication (ML), and Boundary Lubrication (BL), are the basic types of the fluid film lubrication. The aim of this investigation is to identify effective frequencies and most useful features of the AE signals for classification of the lubrication types. Continuous wavelet transform (CWT) and time domain signal analysis methods are used for feature extraction of the recorded AE signals. Then, Genetic Algorithms (GAs) in combination with Artificial Neural Networks (ANNs) are applied to select and classify the extracted features. The experimental results showed that the proposed system using AE signal is effective.

Investigation of the damage mechanisms for mode I delamination growth in foam core sandwich composites using acoustic emission

In this article, acoustic emission monitoring was used to identify different types of damage mechanisms which occur during delamination of sandwich structures. Features of the obtained acoustic emission signals and wavelet-based signal processing technique were utilized to discriminate failure mechanisms during the delamination test. Dominant damage mechanisms were classified based on their power spectral density in distinct frequency ranges and the energy distribution criterion in each component. The scanning electron microscopy was also employed to verify the results which were obtained from the acoustic emission investigation. It was concluded that frequency domain analysis and wavelet analysis are efficient tools for identification and discrimination of different damage mechanisms in sandwich structures.

Experimental investigation of acoustic emissions and their moment tensors in rock during failure

We study acoustic emissions (AEs) associated with shear and tensile failures around a horizontal borehole in a sandstone sample subjected to triaxial stress. The aim is to relate the AE event rate to macroscopic observations of sample deformation and the percentage of isotropic and deviatoric components of the seismic moment tensors to the expected failure mechanisms. The horizontal hole interferes with the applied load and forms a strongly spatially dependent anisotropic stress field, focusing the crack initiation into both shear and tensile failures. The recorded AEs follows reasonably well existing damage models, but the elastic solution of hoop stress does not represent the onset of failure around the borehole. The focal mechanisms correlate with the orientation of macroscopic fractures in the sample. Events close to the borehole show a higher fraction of isotropic percentage in moment tensors compared to events occurring in the macroscopic fracture featuring higher double-couple percentages. The inhomogeneous stress field due to the borehole and the stress induced damage is strongly affecting the axial and radial velocities which in turn affect the waveforms of the recorded AEs and the resulting moment tensors. The VP/VS ratio obtained from the ratio of isotropic to compensated linear vector dipole components of the moment tensors is close to that obtained from ultrasonic velocity measurements.

Spectral and energy characteristics of acoustic emission in the laser radiation breakdown of water

Results from experimental investigations of acoustic emission from the optical breakdown zone in liquid formed by laser radiation at a wavelength of 532 nm are presented. The linear dependence of the acoustic pressure on the laser pulse energy is revealed. It is shown that the effect of an additional source of sound at a frequency of 37 kHz enhances the acoustic emission from the optical breakdown zone.

Investigation of AE Features in Grinding

This paper presents recent investigation of acoustic emission (AE) behaviours in grinding processes. It demonstrated the acoustic emission features characterized in time and frequency domain are influenced by thermal behaviours of materials. By control laser conditions, the temperature elevation under laser irradiation can be similar to that in a grinding process. Therefore, an innovative concept that grinding process can be monitored by using thermal AE signatures from laser irradiation tests has been proposed. Accordingly, an artificial neural network (ANN), built on laser irradiation tests, was applied to monitor grinding thermal performance. The results showed that grinding performance variation due to wheel wear can be identified by using the ANN. This development could bring great benefits by reducing experimental works in the preparation of an ANN for grinding monitoring.

In-situ neutron diffraction and acoustic emission investigation of twinning activity in magnesium

The neutron diffraction and acoustic emission (AE) techniques have been used for in-situ investigation of deformation twinning activity in cast polycrystalline magnesium at room temperature. The combination of these two techniques results in obtaining complementary information about the twinning mechanism during the straining. It is shown that loading mode significantly influences the twin nucleation and the twin size. In tension, the twin nucleation is observed during the entire test, while in compression only at the beginning of the plastic deformation. Nevertheless, the overall twinned volume does not differ. The optical micrographs support the above mentioned conclusions.

Investigation of Acoustic Emission Caused by Filtration of an Air Flow through a Porous Medium

We study characteristics of acoustic emission caused by an air flow filtered through samples of solid porous pumice either partially filled with glycerin or dry. Some peculiarities of the laboratory setup and the performed experiments are described. Samples of different porosity and several values of the air pressure drop are tested. Physical reasons for the difference in acoustical emission spectra of samples of different porosity and in the relaxation times of the acoustic emission level caused by different porosity and fluid saturation are discussed. It is proposed to use the phenomenon of acoustic emission for detecting the fluid trapped in the pores of cores. The existing results based on the borehole field experiments are rough. The frequency range of signals recorded in these experiments is less than 5 kHz (up to 10 kHz). It is shown by our laboratory measurements that the required frequency band should be up to 50 - 60 kHz to reflect the features of the spectral composition of the acoustic emission in predicting the fluid content. The level of acoustic noise under field conditions is much higher, i.e., by 10 - 15 dB, than the noise achieved under laboratory conditions. Since the sensitivity of the recorders and the frequency range are considerably higher in our experiments, they are unique when applied to the solved problem.

Acoustic Emission by Quartz Tuning Forks and Other Oscillating Structures in Cryogenic 4He Fluids

We report on experimental investigations of acoustic emission by quartz tuning forks resonating at frequencies 32 kHz, 38 kHz, 77 kHz and 100 kHz immersed in cold gaseous 4He and its normal and superfluid liquid phases. Frequency dependence of the observed low-drive-linewidth at 350 mK together with the temperature and pressure dependences (1.3 K < T < 4.2 K, 0 < p < 25 bar) of the observed damping of the high frequency (77 and 100 kHz) resonators measured in normal liquid 4He and its superfluid phase provide strong and direct evidence of the importance of sound emission by these tuning forks. Three analytical models of acoustic emission by vibrating tuning forks are developed and compared with the experimental results. We also discuss the importance of sound emission for experiments with the commonly used 32 kHz tuning forks as well as other oscillating structures—spheres, wires, grids and various micromachined sensors. We compare the relative importance of dissipative losses due to laminar viscous/ballistic drag and acoustic emission in liquid and superfluid 4He.

Investigation of acoustic emission accompanying stick-slip movement of rock samples at different stiffnesses of spring–block system

Understanding of friction process, especially, transition from stable to stick-slip motion, is very important in various fields from mechanical engineering to seismology. It seems that analysis of acoustic emission (AE) accompanying the friction process can be a sensitive tool for revealing fine details of the friction process. In the present work, acoustic emission accompanying the stick-slip movement of basalt samples has been investigated in laboratory slider-spring device. Exactly, the influence of spring stiffness on the statistical and dynamical characteristics of stick-slip generated AE has been studied. Different time series compiled from recorded AE wave trains have been analyzed using statistical and dynamical data analysis methods. We found that statistical and dynamical changes in acoustic emission of stick-slip movement depend on the stiffness of spring in spring–block system. The obtained results show that dynamics of stick-slip process undergoes both qualitative and quantitative changes at transition from stick-slip to stable sliding friction caused by increased stiffness of frictional system. Extent of regularity in stick-slip process, assessed by analysis of AE temporal distribution characteristics, increases at stiffer springs.

Damage Behaviour of Ncf Carbon / Epoxy Laminates Under Tension

Composites reinforced by non-crimp fabrics (NCF) reach stiffness level close to the ideal cross-ply laminates with unidirectional plies, but damage behaviour of NCF-composites exhibit significant differences from the behaviour of the laminates with unidirectional plies (non-stitched laminates). The paper presents results of experimental studies of the initiation and development of damage in NCF-composites. Some of the observed phenomena can be explained and predicted using FE modelling of the composite deformation. Others are difficult to explain; they present a challenge for understanding the behaviour of NCF-composites. The materials are carbon/epoxy composites, reinforced by biaxial NCF 0°/90° and ±45°. Two different sets of fabrics (different producers) were used for the reinforcement, and the fibre volume fraction of the plates had two different levels: about 45% and about 55%. The samples were loaded in different directions in tension. Damage initiation and development was studied using acoustic emission and X-ray investigation.

Experimental Evaluation of Laser-Assisted Micromilling in a Slotting Configuration

Micromilling can be difficult to apply to many engineering materials due to a variety of scaling induced factors including low cutting speeds, high relative tool deflections and runout, and increased material strength at smaller size scales. To alleviate these problems, laser-assisted micromilling (LAMM) was evaluated on Ti6Al4V, AISI 422, and AISI 316 using 100 μm diameter endmills in slotting operations. A three-dimensional transient finite-volume based thermal model was used to analytically predict appropriate process parameters on the basis of material removal temperatures. A two-dimensional finite element model was created and used to show the effects of cutting edge radius, uncut chip thickness, and material removal temperature on the cutting force. A thorough experimental investigation of acoustic emissions (AEs) during LAMM was performed. In particular, the effects of depth of cut, tool wear, and material removal temperature on the root-mean-square of AEs were studied. The effects of LAMM on the machined surface finish and edge burrs were also evaluated.

Electrochemical and acoustic emission studies of aluminum pitting corrosion

The potentiodynamic and acoustic emission studies were carried out in a typical three-electrode electrochemical cell. The working electrode was prepared from aluminum alloy 1050A. Cyclic polarization and acoustic emission investigations of pitting corrosion were carried out simultaneously. On the basis of acoustic and potentiodynamic (cyclic polarization) investigations pitting corrosion potential was determined and compared.

S62 Combined In Situ Neutron Diffraction and Acoustic Emission Investigation of the Deformation Mechanisms in Magnesium Alloys

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Acoustic Emission Investigation of the Anelastic Recovery Behavior of Basal Plane Textured AZ31B as a Function of Loading Direction

In order to study the texture effects on the anelastic recovery behavior of magnesium alloy, basal plane textured AZ31B alloy was compressed parallel and vertical to the extrusion direction. AE measurement was applied to characterize the deformation and the anelastic recovery behaviors. A model for the correlation between applied strain and the cumulative AE counts in detwinning was proposed. The effects of loading direction to the deformation and twinning behavior were discussed.

An investigation of acoustic emission to monitoring flat lapping with non-replenished slurry

Lapping is a precision manufacturing process. However, the material removal rate and surface roughness show significant variation between trials for repeated experiments and, thus, the repeatability of the results depends on the machine operator’s skill. Acoustic emission (AE) seems to be capable of monitoring the process. Therefore, an understanding of AE generation during lapping is important to predict the performance of the grains and hence the lapping results. Based on a theoretical analysis and experimental results collected during flat lapping, the AE signal was investigated for the situation when slurry is supplied without replenishment. The experiments were carried out with a wireless rotating AE sensor mounted in the middle of the lapping plate. Three parameters related to the AE curve are proposed to monitor the process. The influence of process parameters (lapping pressure, velocity, average grain size, concentration of grains in lapping compound and the number of conditioning rings) on the characteristics of the AE curve was investigated.

An investigation of acoustic emission in Armco iron and structural steel in the process of recrystallization and phase transitions

An investigation of acoustic emission in Armco iron and structural steel in the process of recrystallization and phase transitions

Response of fiber Bragg gratings to longitudinal ultrasonic waves

In the last years, fiber optic sensors have been widely exploited for several sensing applications, including static and dynamic strain measurements up to acoustic detection. Among these, fiber Bragg grating sensors have been indicated as the ideal candidate for practical structural health monitoring in light of their unique advantages over conventional sensing devices. Although this class of sensors has been successfully tested for static and low-frequency measurements, the identification of sensor performances for high-frequency detection, including acoustic emission and ultrasonic investigations, is required. To this aim, the analysis of feasibilty on the use of fiber Bragg grating sensors as ultrasonic detectors has been carried out. In particular, the response of fiber Bragg gratings subjected to the longitudinal ultrasonic (US) field has been theoretically and numerically investigated. Ultrasonic field interaction has been modeled, taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to the elasto-optic effect. Numerical results, obtained for both uniform and Gaussian-apodized fiber Bragg gratings, show that the grating spectrum is strongly influenced by the US field in terms of shape and central wavelength. In particular, a key parameter affecting the grating response is the ratio between the US wavelength and the grating length. Normal operation characterized by changes in wavelength of undistorted Bragg peak is possible only for US wavelengths longer than the grating length. For US wavelengths approaching the grating length, the wavelength change is accompanied by subpeaks formation and main peak amplitude modulation. This effect can be attributed to the nonuniformity of the US perturbation along the grating length. At very high US frequencies, the grating is not sensitive any longer. The results of this analysis provide useful tools for the design of grating-based ultrasound sensors for meeting specific requirements in terms of field intensity and frequencies.

Kinetics of shear banding in a bulk metallic glass monitored by acoustic emission measurements

Detailed acoustic emission (AE) and surface microscopy investigations of the kinetics of shear banding in bulk Zr52.5Ti5Cu17.9Ni14.6Al10 metallic glass at room temperature are presented. The shear band propagates in a jump-like mode as reflected by numerous AE bursts. The time distribution and cluster statistical analysis of AE time series revealed, firstly, that there are two shear banding processes notably different in their spatial scales and, secondly, that formation of shear bands at large strains can be correlated in time and space. Independence of the AE characteristics on the current stress magnitude implies that shear band propagation could not be interpreted as a shear front motion in a viscous Newtonian-like medium. The AE response to shear banding is to a certain extent similar to that of a moving dislocation pile-up escaping to a free surface. It is emphasized that AE and microscopic features of shear banding in the bulk metallic glass are very nearly the same as those found earlier for melt-spun ribbon glasses, indicating that the change in the quenching rate by about four orders of magnitude does not cause the kinetics of shear band nucleation and propagation to vary considerably.

Investigation of Oil Palm and Pineapple Fiber Reinforced Phenol Formaldehyde Composites by Acoustic Emission Technique

The fracture mechanics response of oil palm and pineapple fiber reinforced phenol formaldehyde composites was investigated. The fracture behaviour was influenced by the fiber content, the length of the fibers and the condition of the fiber-matrix interface. Single edge notched tensile specimens were cut from composite plates and examined. The fracture toughness was determined from force-elongation curves. Tensile tests and acoustic emission investigations were carried out simultaneously. It was found out that the failure mode for untreated and treated materials was quite different. Scanning electron microscopy micrographs were taken from the fracture surfaces.

Acoustic emission investigation of microcrack generation at geological boundaries

Acoustic emission measurements are performed to identify microcracking in the rock. Distinct clusters of acoustic emission are observed, both spatially and with respect to time. The locations and the frequency distributions of magnitudes and spacing of acoustic emission events within a significant cluster are discussed. Drilling showed that this cluster was on the boundary between rock salt and anhydrite. A fracture in a thin clay layer was observed at the location of the cluster. The acoustic emission and geological observations demonstrate that the cluster of microcracks marks the boundary of the exposed fracture plane with a diameter of 8m.