Showing papers on "Acoustic emission published in 2001"

Observations on chip formation and acoustic emission in machining Ti–6Al–4V alloy

Abstract: Orthogonal cutting tests were undertaken to investigate the mechanisms of chip formation for a Ti–6Al–4V alloy and to assess the influences of such on acoustic emission (AE). Within the range of conditions employed (cutting speed, v c =0.25–3.0 m/s , feed, f=20–100 μm ), saw-tooth chips were produced. A transition from aperiodic to periodic saw-tooth chip formation occurring with increases in cutting speed and/or feed. Examination of chips formed shortly after the instant of tool engagement, where the undeformed chip thickness is slightly greater than the minimum undeformed chip thickness, revealed a continuous chip characterised by the presence of fine lamellae on its free surface. In agreement with the consensus that shear localisation in machining Ti and its alloys is due to the occurrence of a thermo-plastic instability, the underside of saw-tooth segments formed at relatively high cutting speeds, exhibiting evidence of ductile fracture. Chips formed at lower cutting speeds suggest that cleavage is the mechanism of catastrophic failure, at least within the upper region of the primary shear zone. An additional characteristic of machining Ti–6Al–4V alloy at high cutting speeds is the occurrence of welding between the chip and the tool. Fracture of such welds appears to be the dominant source of AE. The results are discussed with reference to the machining of hardened steels, another class of materials from which saw-tooth chips are produced.

On the sources of work hardening in multiphase steels assisted by transformation-induced plasticity

Abstract: The mechanisms effectively responsible for the enhancement of the work-hardening capabilities of multiphase steels assisted by transformation-induced plasticity are highlighted. Different microstructures, some containing a proportion of retained austenite with various mechanical stabilities, are studied. The dislocation density generated within ferrite by the mechanically induced martensitic transformation of retained austenite is shown to scale with the incremental work-hardening exponent. The acoustic emission generated during tensile straining was also measured. The acoustic emission was revealed to result mainly from dislocation motion, especially from the motion of the additional dislocation density generated in intercritical ferrite by the strain-induced martensitic transformation.

Identification of a specific type of PD from acoustic emission frequency spectra

Abstract: The paper presents an attempt to apply spectral analysis tools in processing acoustic emission (AE) pulses generated by partial discharge (FD). The experimental part of the paper describes spark gaps generating four types of PD and specifies parameters of measured acoustic signals and recalls the system used for measurement and analysis of the frequency spectra. Also, a spectral analysis procedure is presented, and frequency-domain descriptors characterizing AE pulses are defined. The results of the analysis are given both as time plots and amplitude and energy density spectra, related to values of the associated descriptors. The spectral analysis results cover AE pulses generated in systems modeling the following types of pD: point-plane type discharges in oil, surface discharges in oil, gas bubble discharges in oil and discharges in indeterminate-potential particles moving in oil. In conclusion, an ability to identify a specific type of FD on a basis of frequency spectra of AE pulses and their descriptors is presented. The comparative analysis enables one to select those descriptors, which can constitute unique criteria for identification of a specific PD type.

Analysis of plate wave propagation in anisotropic laminates using a wavelet transform

Abstract: A new approach is presented for the analysis of transient waves propagating in anisotropic composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time-frequency analysis of dispersive flexural waves in these plates. It can be shown that the peaks of the magnitude of WT in a time-frequency domain is related to the arrival times of the group velocity. Experiments were performed using a lead break as the simulated acoustic emission source on the surface of unidirectional and quasi-isotropic laminates. A method was developed to obtain the group velocity of the flexural mode as a function of frequency. Theoretical predictions were made using the Mindlin plate theory, which includes the effects of shear deformation and rotatory inertia. Our predictions on the dispersion of the flexural mode showed good agreement with the experimental results.

Acoustic emission monitoring of carbon-fiber-reinforced-polymer bridge stay cables in large-scale testing

Abstract: This paper presents acoustic emission (AE) monitoring of damage initiation and progression in carbon-fiber-reinforced-polymer (CFRP) stay cables subject to largescale laboratory tests. The research is part of the University of California, San Diego (UCSD), larger project on the design and construction of a new cable-stayed bridge made of advanced composites. No previous use of AE on large-size CFRP stay cables appears in the literature. Three types of cables of potential use in the UCSD composite bridge were tested at lengths ranging from 5500 mm to 5870 mm. The AE events were monitored to detect damage and provide a qualitative correlation with the type of structural failure. The tests allowed a comparative characterization of the failure behavior of the three types of cables under investigation. An additional study was performed to characterize acoustic attenuation and dispersion phenomena that are relevant to AE testing of largescale CFRP cables. It is shown that despite their large size, these cables are excellent acoustic wave guides exhibiting very low attenuation. Finally, this study shows promising results for an effective use of in situ AE for health monitoring of these structural components in service.

Non-destructive evaluation of damage and failure of fibre reinforced polymer composites using ultrasonic waves and acoustic emission

Abstract: Applications of reinforced composites and heterogeneous solids are widespread, spanning technological areas of various aerospace and mechanical industries. A real challenge concerning these materials is their life time prediction when subjected to wide variety of environmental and mechanical loading conditions that can initiate damage and lead to failure. Indeed, damage at the smallest scales drives damage accumulation at larger length scales until some critical local damage state is attained that causes macroscopic failure. A key issue in predicting life time is to characterise distributed volumic and localised damage and to understand the mechanisms of its initiation, evolution and criticality and so, the identification of relevant precursors of failure. To answer to these questions, volumic and guided ultrasonic waves and acoustic emission are of particular interest. As a matter of fact volumic ultrasonic wave propagation is sensitive to homogeneously distributed microcracks and represents in that case a good damage indicator. Guided waves as Lamb waves especially when generated from inside the material using an inserted piezoelectric element offer a specific sensitivity to localised damage as cracks or delaminations. Besides, acoustic emission which corresponds to the energy released by the material during the damage processes is directly related to the damage mechanisms and so can give pertinent information about the damage initiation and development. In this paper, our aim is to show in the one hand the ability of volumic ultrasonic waves to characterise volumic damage of glass epoxy composites under hydrothermal ageing and also the ability of Lamb waves to detect and identify localised damage. In the other hand our purpose is to demonstrate the potentiality of acoustic emission in understanding the damage mechanisms that occurs during a tensile test of polymer fibre composites and to discriminate in real time the different types of damage occurring at the microscopic scale.

Post-impact damage characterisation on natural fibre reinforced composites using acoustic emission

Abstract: The present study aims to characterise damage due to low velocity impact on jute fibre reinforced polyester composites. To attain this goal, a number of post-impact mechanical tests have been carried out, including tensile tests, three-point bending and indentation, using either a staircase or a continuous loading programme. On all these tests acoustic emission activity (AE) was monitored. The results, compared with damage observed under an optical microscope, show that AE is able to perform a reliable measurement of the level of damage also, on a natural fibre reinforced laminate. The main limitations of this study are owing to the rather low ultimate stress of the material and to the need to apply a loading to evaluate the damage produced by the impact event.

Turbulence Structures and the Acoustic Far Field of a Mach 1.3 Jet

Abstract: The temporal characteristics of the acoustic far field of a Mach 1.3, high-Reynolds-number, ideally expanded axisymmetric jet and their potential correlation with large-scale turbulence structures within the jet were explored. A dual microphone array, placed approximately 30 deg from the jet axis in the acoustic far field, was used to determine the temporal variations of the acoustic field and the approximate locations of intense noise sources within the jet, as well as the time of noise emission with respect to the acquired planar flow images. Simultaneous double-pulse flow visualizations were used to identify turbulence structures, as well as their development and interaction in the region of intense noise generation. The time history of the acoustic data showed individual large-amplitude noise events, periodic large-amplitude noise events, and long periods of relative quiet without any large-amplitude noise production

Damage Location in Steel Bridges by Acoustic Emission

Abstract: This paper explores the use of acoustic emission (AE) for damage location in steel composite bridges. Damage assessment of a steel-concrete composite bridge is obtained using conventional time-of-arrival location techniques in both global and local monitoring trials. The local monitoring aspect of the field investigation is simulated under laboratory conditions and waveforms acquired during both studies are compared. A Finite Element (FE) study of a component of the bridge is presented and compared with location results from the field study confirming regions of possible crack location. Laboratory studies on a 12 m I-beam are presented to explore the use of Lamb waves as an alternative location method to the current time of arrival (TOA) method. The presence and dispersive behaviour of the flexural and extensional Lamb modes in digitised AE signals is observed. The frequency characteristics of the two modes are compared and modal separation via bandpass frequency filtering is demonstrated. This separation...

Acoustic emission source analysis of plasma sprayed thermal barrier coatings during four-point bend tests

Abstract: Plasma-sprayed thermal barrier coatings (TBCs), as-sprayed and pre-oxidized, were tested under four-point tensile bending conditions, and their acoustic emission (AE) responses were monitored by an advanced AE system. On the basis of an inversion processing of AE signals, the damage sources in the deposits were localized, identified and classified into three main cracking modes. Furthermore, cracking source parameters (i.e. rise time and crack volume) were estimated and used to determine the critical cracking or delamination events among the AE signals. Consequently, the damage progressions in the TBCs were elucidated by correlating the fracture source parameters to the strain curves in time domain. In the bending tests, vertical cracks were induced in the ceramic top layer at a low strain rate and delamination at the bond/top coat interface accounted for the spallation of top coat before failure. Pre-oxidized samples tended to crack early at a low tensile strain, and the AE sources were characterized by predominant shear fracture (Mode II) at the bond/top coat interface, corresponding to a degradation mechanism of microcracking before failure.

Monitoring pitting corrosion of AISI 316L austenitic stainless steel by acoustic emission technique: Choice of representative acoustic parameters

Abstract: This experimental work was aimed at investigating the monitoring of pitting corrosion by the acoustic emission (AE) technique, for pits developed by potentiostatic or galvanostatic polarization on two types of 316L austenitic stainless steels, in a 3% NaCl solution acidified to pH 2. The study of the evolution of AE global activity during the test showed the existence of a time delay before pits became emissive. This time delay and the AE events number rate measured during the propagation step of the pits are closely correlated with the sensitivity of the material towards pitting and with the polarization procedure. Moreover, the evolution of cumulative % of AE signals number versus selected acoustic parameters shows that rise time and counts number of signals appear to be discriminating acoustic parameters for monitoring pitting corrosion of austenitic stainless steels by acoustic emission technique in our experimental conditions, whatever the polarization procedure and the type of tested steel.

Memory Effects in Rock Salt Under Triaxial Stress State and Their Use for Stress Measurement in a Rock Mass

Abstract: Regularities of memory effects in rock salt specimens under triaxial stress state were investigated. Each specimen was subjected to two loading cycles. The first cycle was axisymmetric triaxial compression (σ1 >σ2=σ3). The second cycle was uniaxial compression in the direction of σ1 of the first cycle. Distinct acoustic emission (AE) and deformation memory effects were observed in the second cycle at the stress value equal to a linear combination of the first cycle principal stresses given by σ1− (k + 1) σ3, where k is about 0.5–0.6 for rock salt. Anomalies in deformation curves were found to be more reliable than the AE methods in distinguishing memory symptoms. The necessary pre-requisite for memory formation in the first cycle was that σ1 exceeded the elastic limit, corresponding to the given confining stress σ3. Inflections in uniaxial stress versus axial strain and lateral strain curves, in the second cycle, were observed at equal stress values if in the first cycle σ1 exceeded the elastic limit and memory-forming damage was induced. If there was no memory-forming damage, those inflections were seen at different stress values. This characteristic was used to distinguish between true memory effects and natural characteristic points in deformation curves derived from rock salt testing. A new memory symptom was established, namely a turn point in curve “uniaxial stress versus differential coefficient of lateral strains”. The results form a basis for application of the memory effects for stress measurement in rock salt masses.