Acoustic Emission Bursts Research Articles

AE signals as laser cutting quality indicators

The paper presents results of an analysis of acoustic emission (AE) signals obtained during laser cutting of a steel plate, which is frequently used in the production of automotive body components. The acoustic emission in the plate, which is produced by various physical phenomena occurring during the laser cutting process, was measured with contact PZT sensors. During the laser cutting process, continuous AE signals, which are produced by the action of a cutting gas, were captured. After laser cutting, AE bursts produced by various phenomena during material cooling were captured. The results obtained confirm that both the continuous signals occurring during the cutting process and the AE bursts after laser cutting provide a good insight into the quality of a laser cut performed. Results of a factorial analysis of an influence of laser cutting conditions on the measured continuous AE signals are described. Response surfaces of amplitude average and intensity average and their mutual correlation are established. An influence of cutting parameters on the size of a dross formed at the lower laser-cut edge, which is an important indicator of cut quality, is analysed and a relation with the measured AE signals is established.

Improving Hard Disk Drive Unloading Performance With a Disk Bump

Most hard disk drives use ramp load/unload (L/UL) technology because of its many advantages. However, L/UL does have some unresolved drawbacks, such as slider-disk contact during fast unloading. The main objective of our research is to examine the use of a bump on the outer edge of the disk to prevent contact with the disk by the rebounding slider and to improve ramp unloading performance. In particular, this paper focuses on experimental verification of the design of the disk bump that resulted from our previous research. We formed the disk bump on the disk surface using mechanical pressing with tungsten tips and tested the effect of the bump in unloading experiments. An investigation of the acoustic emission burst signal and its power spectrum confirmed that the optimal disk bump prevented slider-disk contact. This improves the ramp unloading performance.

Monitoring laser cut quality using acoustic emission

The paper presents results of an analysis of acoustic-emission (AE) signals obtained during laser cutting of a steel plates, which are frequently used in the production of automotive body components. In the course of laser cutting, continuous signals, which are related to the quality achieved of the laser cut, are measured. After laser cutting characteristic AE bursts in the specimen material, which are results of a thermal influence, can be detected. The presence of dross is an important indicator of poor quality of laser cutting. The investigation on laser cutting was conducted on an unalloyed steel DC04 sheet and on austenitic stainless steel X5CrNi18-10 sheet of 1.5 mm in thickness. The same physical mechanism in the cut formation at both steels, however, resulted in a considerable different cut quality, which was indicated by the AE signals captured after the cutting process.

Assessment of Laser-Cut Quality on the Basis of Acoustic Emission Signals Captured with a PZT Sensor

The paper describes an analysis of the AE signals captured with PZT sensor during and after laser cutting. In the course of cutting the continuous AE signals are treated in a defined time interval. After cutting, bursts of acoustic emission will occur. They are due to solidification of the molten material and cooling of the laser-cut surface. The results shown refer to cutting of steel plate DC04 and of austenitic stainless steel plate X5CrNi18-10. Both plate types are frequently used in automobile industry, where, because of mass production, optimisation of the cutting process is desired. Numerous laser-cutting conditions assuring different quality levels of the cut made were chosen. It was proved that the laser cutting conditions had a significant influence on the amplitude value and intensity of the continuous signal and AE activity after the termination of laser cutting. Separately it was also confirmed that there was a relation between the measured signal and laser-cut quality, which is shown, to a large extent, by the presence of dross at the lower edge of the cut.

Acoustic Emission in Elementary Processes of Friction and Wear (In-Situ Observation of Friction Surface and AE Signals)

The relationship between acoustic emission (AE) signals and tribological phenomena in the elementary processes of friction and wear is examined. The elementary processes of friction and wear were observed by a frictional surface microscope which installed a friction system into the view of an optical microscope. In this paper, the elementary processes are focused on the plastic deformation of the friction surface and the formation of wear particles between friction surfaces. In-situ experiments with the frictional surface microscope were performed by a pin-on-block type sliding test for iron and zinc. The friction surface and its side face were also observed by an atomic force microscope. The results show that two kinds of AE signals, a continuous AE signal of low amplitude and a burst AE signal of high amplitude, are detected in the elementary processes of friction and wear. The continuous AE signal of low amplitude is detected by the generation of slip-lines and wear elements. The burst AE signal of high amplitude is detected by the formation of a transfer particle.

Mechanisms of plastic deformation in AZ31 magnesium alloy investigated by acoustic emission and transmission electron microscopy

The effect of deformation conditions on plastic deformation and acoustic emission (AE) in hot-rolled magnesium alloy AZ31 has been investigated in the temperature range of 20–200 °C by constant strain rate tensile tests. Two sets of samples differing in the preheating temperature before individual passes of hot rolling have been studied. Both the yield stress and the tensile strength decrease with increasing temperature of deformation. The ductility was found to increase significantly with increasing temperature of deformation in both specimens. Unstable plastic deformation (Portevin–Le Chatelier effect) has been observed for all used strain rates both at room and elevated temperatures. Plastic instabilities were accompanied by a pronounced AE activity. The AE bursts were correlated with the individual regions of plastic instabilities on the deformation curve. Mechanisms controlling plastic instabilities are suggested respecting the microstructure evolution as observed by optical and transmission electron microscopy.

Triggering and synchronization of stick slip: Waiting times and frequency-energy distribution

Triggering and synchronization are encountered in many geophysical phenomena, including geodynamics. Both these effects are generated by the action of additional forcing, which is much smaller than the main driving force. That means that triggering and synchronization are connected with nonlinear interactions of objects, in this case with initiation of instability in systems that are close to the critical state. In seismic process the main component is the tectonic stress and the additional forcing is exerted by various external impacts like tides, reservoir exploitation, big explosions, magnetic storms, etc. In the paper, the results of laboratory and field experiments on the electromagnetic (EM) initiation and synchronization of mechanical instability (slip) are presented. Slip events were recorded as acoustic emission bursts. In the first series of experiments strong EM pulses were applied to the mechanical system driven close to the critical state, namely, to the (dry) rock samples placed on an inclined supporting sample at the slope angle less than, but close to the critical slip angle. It has been found that EM impact initiates slip with probability P ≈ 0.07 at the voltage Δ V = 1.3 kV and with probability P ≈ 0.2 at Δ V ≈ 10 kV if the EM field is applied parallel to the slip surface (first mode). On the other hand, the application of EM pulse hampers the slip considerably if the EM field is directed perpendicularly to the slip surface (second mode): the slip was not observed even at the angle that was larger than the critical one. In the second series of experiments the periodic EM and mechanical forcing were applied to the standard slider-spring system. It was discovered that periodic EM force of frequency f superimposed on the constant driving force excites periodic microslips of rock samples with double frequency 2 f. Combined impact of periodic and constant voltages invokes transition from double frequency synchronization to 1:1 synchronization if the direct component of voltage is larger than the periodic one. Synchronization affects not only waiting times, but also frequency-energy distribution: i. the energy of bursts emitted in synchronized mode have much less scatter than in the absence of the periodic forcing, ii. the sudden decrease of synchronizing forcing is followed by acoustic burst of much larger energy than during forcing. The elementary theory of EM triggering and synchronization is given: the effects are explained by the action of EM ponderomotive (electrostriction) forces, which modify Coulomb stress similar to the well known pore pressure model. The formalism of transition from 1:2 to 1:1 synchronization is considered.

Investigation of thermal and deformation properties of quartzite in a temperature range of polymorphous α-β transition by neutron diffraction and acoustic emission methods

The results of combined application of the neutron diffraction and acoustic emission methods for investigation of the physical properties of synthetic quartz and natural quartzite in a temperature range of α-β transition are given. In experiments, the quartzite sample was exposed to heating and uniaxial compression. Changes of the lattice spacings in quartzite were measured in a temperature range 540–620°C. On the basis of these measurements, the inner inner stresses are evaluated and found to exceed the applied stresses by several times. It is found that after the phase transition is finished, short bursts of acoustic emission (AE) occur which are two orders of magnitude more intense than the acoustic emissions produced by thermal cracking of the sample while the sample is heating up to the transition temperatures. An assumption is made that the anomalous behavior of the physical properties of quartz-containing rocks under relatively low pressures near the transition temperature can cause the formation of strong concentrators of local stresses comparable with the breaking point of the material, thereby initiating microcracking.

Acoustic Emission During Phase Transition in Soft PZT Ceramics Under an Applied Electric Field

In the present paper, it is proposed that a sudden appearance of a burst Acoustic Emission (AE) signal under an applied dc field, during temperature decrease cycle indicates the structural phase transition (PT) from cubic to tetragonal in soft PZT ceramics (PZT-5A, 5J & 5H). In general, the PT temperature determined by this method was found to increase with the increase in applied field. In addition to the AE signal observed at PT, a large number of AE signals—which are essentially different from the AE at PT—were also observed in the ferroelectric phase due to the presence of the field. These signals when subjected to pattern recognition resulted in two clusters indicating two different source mechanisms. It is suggested that “piezoelectric strain” and “strain due to domain alignment” are operative at lower (< 2 kV/cm) and higher (2–10 kV/cm) fields respectively. Communicated by Dr. George W. Taylor

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.

Electromagnetic and mechanical control of slip: laboratory experiments with slider system

Abstract. Field and laboratory data reveal the possibility of a significant coupling of elastic and electromagnetic (EM) fields that affect (hamper or initiate) slip. In this work we try to prove experimentally the possibility of controlling the slip regime by relatively weak mechanical or EM impact, in the way it has been done in nonlinear dynamic experiments on the control of chaos. The experimental setup consisted of a system of two plates of roughly finished basalt, where a constant pulling force was applied to the upper (sliding) plate. In addition, the same plate was subjected to mechanical or electric periodic perturbations, which are much weaker when compared to the pulling force. Quite different regimes of slip were excited depending on the amplitude and the frequency of applied weak perturbations. The observed regimes of slip vary from perfect synchronization of slip events, recorded as acoustic emission bursts with the perturbing periodic mechanical or EM impact, to their complete desynchronization. We consider the obtained results as evidence that it is possible to control slip by the application of weak periodic perturbations. The phenomenon can be explained in terms of nonlinear dynamics and synchronization theory.

Observations of serration characteristics and acoustic emission during serrated flow of an Al–Mg alloy

Study of serrated flow is important for the production of metal alloy parts with smooth surfaces and for fundamental understanding of microscale plastic deformation. In this experimental investigation of a rolled Al–Mg alloy, serrated flow was observed for the entire plastic flow region at three different strain rates. Strain and strain rate dependent serration amplitudes and widths were quantified. In addition, the acoustic emission (AE) signature from the plastic flow region was analyzed in detail and was related to the serrations in the macroscopic stress versus strain results. For all experiments, deformation bands on the surfaces of the specimens were found to be consistent with ‘Type B’ serrations. Video observations of the deformation bands were synchronized with the AE, and the deformation bands were found to coincide with more persistent AE bursts and anomalies in the serrations.

Monitoring the acoustic emission of the blades of the mirror suspension for a gravitational wave interferometer

We monitored the acoustic emission activity of the steel blades to be used for the mirror suspension system of a gravitational wave interferometer. We have collected several sets of events getting evidence of a material memory effect (Kaiser effect) associated to the dislocation motion in the steel. This result is more evident when we apply a standard fractal analysis procedure (box counting method) to the timing series of acoustic emission bursts. We conclude that a significant reduction of the emission rate is obtained by applying a few stress cycles to the elastic blades.

Damage evolution and acoustic emission of wood at tension perpendicular to fiber

This paper presents the tracing of damage evolution in spruce loaded in tension perpendicular to fiber direction by means of acoustic emission (AE) analysis. The 2 dimensional burst source location in cross-sectional slabs of boards utilized full wave form recording of AE signals monitored by six simultaneously triggered multiple resonant longitudinal (p-)wave sensors and fast transient recorder PC cards. The location algorithm was based on the minimization of the run time differences between experimental and theoretical signal travel times. The latter accounted for the global polar material anisotropy leading to considerably variable off-axis wave velocities along the wave propagation paths, assumed to be straight; the employed orthotropic on- and off-axis p-wave velocities were obtained experimentally from ultrasound pulse measurements. Below 50% of the ultimate load generally very few burst events were recorded. At further increased load, burst agglomerations were predominantly associated to local areas of elevated stresses here resulting from the specific configuration of anisotropy and loading. For all specimens a distinct burst localisation was obtained in the range of 80 to 90% of the ultimate load, coinciding well with the theoretically highest stressed areas of the fracture plane at brittle failure. The correlation of AE event rates with global strain allowed a tracing of the damage evolution especially when events in confined areas are regarded. Distinct damage localisation was then denoted by a clear AE rate increase, however, not accompanied by a global stiffness change. The investigations revealed that failure of spruce - and most probably of similar softwoods - in tension perpendicular to grain, although being very brittle from a macroscopic perspective, is preceded by progressive and localizing damage evolution on the micro-level which can be traced accurately in space and time by acoustic emission burst source analysis.

Visualisation and Dimension Reduction of Acoustic Emission Data for Damage Detection

This paper is concerned with the compression or dimensional reduction of a number of acoustic emission (AE) signals generated by damage to a structure, namely, a box girder from a bridge. The object of the exercise is to visualise the data and thereby associate clusters tothe measured AE bursts. These can be used tohelp interpret the signals. Two methods are used to reduce the dimension: principal component analysis and Sammon mapping.

Relation between the Second Acoustic Emission Peak and the Serrated Flow of Aged AlZnMgCu Alloy

The acoustic emission (AE) during plastic flow of AlZnMgCu alloy aged under a wide range of temperature and time was investigated. For as-quenched and short time aged samples, there are strong burst-type AE signals accompanied by serrated flow, its onset strain εc delayed with prolonging aging time. For most aged samples, there is a continuous second AE peak (AEII), the height of AEII in ringdown count rate, NII, is a function of aging time t, and the curve NII-lgt has the same tendency as the curve of ultimate tensile strength σB-lgt for the same aging temperature, with both maxima close to each other. For well over-aged and annealed samples, the serrated flow reappears with small stress amplitude and AE bursts, which takes the place of the AEII gradually with prolonged aging time. The activation energies for NII at the maximum, that of εc of serration onset in short time aged and for well over-aged samples were calculated to be 1·4, 0·64, and ≈2 eV, respectively. The origin of AE in aged AlZnMgCu alloy is discussed in connection with the interaction between precipitates and moving dislocations.

On the Feasibility of Catastrophic Cutting Tool Fracture Prediction Via Acoustic Emission Analysis

The prediction of catastrophic cutting tool fracture is explored through monitoring the acoustic emission (AE) from a cutting process. A prediction parameter is derived which combines the AE signal with a physical model of a cracked tool to form an estimate of the spatial energy release rate. Monitoring the energy release rate is found to be largely dependent on the detection of crack advancement. Experiments were performed with both new and artificially cracked inserts during interrupted cutting. Epoches denoting crack advancement were detected through high time homomorphic analysis of the acquired AE signals. AE bursts prior to and leading up to fracture were analyzed for crack advancement. The calculated energy release rate was found to exponentially increase as fracture was approached. Crack advancement could be feasibly detected approximately six cuts prior to fracture.

Characterization of acoustic emission from an electrolysis cell

Evolution of hydrogen and oxygen from electrodes in an electrolysis cell may be conveniently monitored in a non-intrusive manner via its ultrasonic acoustic emission. The apparatus used in this work was comprised of a nickel anode, a stainless steel cathode, and a saturated calomel reference electrode, all situated in a three chamber cell containing sodium hydroxide solution. The potential necessary for gas bubble evolution was conclusively detected by onset of bursts of acoustic emission. Individual acoustic emission signals, captured using a broadband transducer mounted on the working electrode, contained frequencies from the audible range to as high as 800 kHz. These were correlated with the release of bursts of bubbles from the electrode's surface, both visually and via a chart recorder trace of peak acoustic power vs. time. Trends in several time-domain signal descriptors were observed with increase in the applied voltage. Acoustic power spectra were obtained by averaging spectra from many acoustic signals. Semi-quantitative estimates of rate of emission were made by integration of the peak acoustic level. The effects of applied potential and electrolyte concentration on the multiple bursts of acoustic emission were characterized and are presented as a system response surface. Increasing the applied potential resulted in greater rates of bubble emission, which increased the intensity of acoustic emission, but produced an identical acoustic power spectrum. The extent of acoustic emission at high concentrations and applied potentials was less than expected, which suggested decreased efficiency under these conditions. Electrolysis start-up kinetics were also studied using a digitizer capable of large record lengths.

ACOUSTIC EMISSION FROM REPAIRED REINFORCED CONCRETE BEAMS

The acoustic emission behaviors of repaired reinforced concrete beams under flexure load was studied. Useful correlation was established between the acoustic emission activity and crack growth in the repaired reinforced concrete beams. Three different acoustic emission activities were observed during the flexural loading. Flexural crack initiation, flexural crack propagation and slip between repair material and old concrete were identified by monitoring the acoustic emission. The zone of slip initiation and development can be identified from the acoustic emission event-location plot, showing major acoustic emission concentration areas. Repetitive loading showed that the Kaiser effect was not valid after slip was happened at the connected face between repair material and old concrete. Flexural crack growth acoustic emission burst event signals were found to be of higher amplitude compared with those due to the slip. The accumulated number of burst events of these highamplitude emissions, monitored by the use of a high threshold, showed good correlation with the crack growth in the beam.

Slider-disk interactions during the load-unload process

The issue of slider-disk contact during the loading process is investigated. The effects of initial altitude, pitch, and roll are studied by using the LDV (laser Doppler vibrometer). The relative displacement of the slider with respect to the disk and the change of pitch and roll during the loading process are obtained for the 3380-type slider and the thin-film disk. The effect of the loading location of the disk is studied. LDV and AE (acoustic emission) measurements are carried out to detect possible slider-disk contacts during the loading process. The AE signal shows some bursts which have the slider ringing-frequency components. But by comparing the AE signal with the LDV results before and after lubricating the gap between the pivot and suspension, the sources of the AE bursts are found to be the normal contact and slip between the pivot and suspension. It is therefore concluded that carrying out the load-unload process of the positive pressure slider using a mechanism which actuates the middle part of the suspension is a suitable solution to high stiction and friction problems that can develop in the contact-start-stop process. >