Showing papers on "Acoustic emission published in 2008"

A critical analysis of effectiveness of acoustic emission signals to detect tool and workpiece malfunctions in milling operations

Abstract: The industrial demands for automated machining systems to increase process productivity and quality in milling of aerospace critical safety components requires advanced investigations of the monitoring techniques. This is focussed on the detection and prediction of the occurrence of process malfunctions at both of tool (e.g. wear/chipping of cutting edges) and workpiece surface integrity (e.g. material drags, laps, pluckings) levels. Acoustic emission (AE) has been employed predominantly for tool condition monitoring of continuous machining operations (e.g. turning, drilling), but relatively little attention has been paid to monitor interrupted processes such as milling and especially to detect the occurrence of possible surface anomalies. This paper reports for the first time on the possibility of using AE sensory measures for monitoring both tool and workpiece surface integrity to enable milling of “damage-free” surfaces. The research focussed on identifying advanced monitoring techniques to enable the calculation of comprehensive AE sensory measures that can be applied independently and/or in conjunction with other sensory signals (e.g. force) to respond to the following technical requirements: (i) to identify time domain patterns that are independent from the tool path; (ii) ability to “calibrate” AE sensory measures against the gradual increase of tool wear/force signals; (iii) capability to detect workpiece surface defects (anomalies) as result of high energy transfer to the machined surfaces when abusive milling is applied. Although some drawbacks exist due to the amount of data manipulation, the results show good evidence that the proposed AE sensory measures have a great potential to be used in flexible and easily implementable solutions for monitoring tool and/or workpiece surface anomalies in milling operations.

Phenomenological Model of Corrosion Process in Reinforced Concrete Identified by Acoustic Emission

Abstract: The concrete reinforcing steel bar (reinforcing bar) corrosion process is phenomenologically modeled as three phase corrosion loss. Reinforcing bar has corrosion initiated at the first phase. Under aerobic conditions at the second phase, there is a decreased rate of corrosion loss. Due to corrosion products, reinforcing bar expansion nucleates concrete cracking at the third phase of anaerobic corrosion. For such a corrosion process to be identified as reinforced concrete corrosion loss, continuous acoustic emission (AE) monitoring was conducted in a cyclic wet and dry test and an accelerated corrosion test. There was characteristic observation of two high AE activity periods. The slope gradient of AE amplitude distribution and AE waveform parameters were investigated to elucidate these two activities. Cracking mechanisms were classified as the other-type cracks than a tensile crack and AE events were found to be of small amplitudes at the first period of high activity. That ferrous ions vanished at some surface regions was demonstrated by scanning electron microscope examination of the reinforcing bar surface, although no corrosion products were visually observed on the reinforcing bar surface. The corrosion's onset in the reinforcing bar results in the first high AE activity is suggested by this, corresponding to the phenomenological model's first phase. Corrosion products were visually observed on the surface of the reinforcing bar following the second high AE activity period. It suggested that in concrete around the reinforcing bar, there was nucleation of cracks. Reasonably, AE event amplitudes were found to be fairly large and AE sources were classified as tensile cracks. Concrete cracking is associated with these AE events due to corrosion product expansion that could be observed at the model's third phase. Thus, the corrosion loss phenomenological model agrees remarkably with reinforced concrete AE generating behaviors. Reinforced concrete corrosion time could be identified, by AE activity monitoring, at reinforcing bar corrosion initiation and also at the nucleation of cracks in concrete. There is earlier detection of even the latter AE activity than through other nondestructive evaluation techniques.

Application of AE and cutting force signals in tool condition monitoring in micro-milling

Abstract: The paper presents acoustic emission and cutting force signals application in tool condition monitoring in micro-milling of cold-work tool steel. The results obtained revealed strong influence of tool wear on acoustic emission signal, providing acceptable results even while used separately. The signal was easy to register, and showed a very short reaction time to the tool–workpiece contact. As excitation frequency, equal to edge passing frequency, was much higher than in conventional milling, cutting forces, usually the best for this purpose, were strongly disturbed by resonance vibration of the table dynamometer. Despite these disturbances, the signals still show dependence on tool wear, making them useful for tool condition monitoring.

Critical defect size distributions in concrete structures detected by the acoustic emission technique

Abstract: Extensive research and studies on con- crete fracture and failure have shown that concrete should be viewed as a quasi-brittle material having a size-dependent behavior. Numerous experimental techniques have been employed to evaluate fracture processes, and a number of modeling approaches have been developed to predict fracture behavior. A non- destructive method based on the Acoustic Emission (AE) technique has proved to be highly effective, es- pecially to assess and measure the damage phenomena taking place inside a structure subjected to mechani- cal loading. In this paper, comparing AE frequency- magnitude statistics in solids subjected to damage processes with defect size distributions for disordered materials, critical parameters defining instability con- ditions for monitored structures are found. In addition, an experimental investigation conducted on concrete and RC structures by means of the AE technique is described. Experimental results confirm the described theories.

Evaluating residual properties of thermally damaged concrete

Abstract: Detailed observations are presented on the evaluation of the residual properties of high strength concrete specimens after exposure to high temperatures. The variables considered were fiber reinforcement consisting of steel or polypropylene microfibers and two different specimen sizes. Both heat treated and undamaged (no heat treatment) specimens were tested by conducting three-point bending experiments at ambient conditions approximately one month after exposure to the high temperature. Residual strength and post-peak response were monitored using a closed-loop load frame, and the damage zone was observed with optical and acoustic techniques. Electronic speckle interferometry provided high resolution measurements of the displacement field due to the development of fracture. The size and shape of the localized damage zone were identified through the acoustic emission due to pre-peak microcracking, one of the significant factors influencing the strength of quasi-brittle materials.

Realistic numerical simulations of solar convection and oscillations in magnetic regions

Abstract: We have used 3D, compressible, nonlinear radiative magnetohydrodynamics simulation to study the influence of magnetic fields of various strengths on convective cells and on the excitation mechanisms of acoustic oscillations by calculating the spectral properties of the convective motions and oscillations. The results reveal substantial changes of the granulation structure with increased magnetic field and a frequency-dependent reduction in the oscillation power in a good agreement with solar observations. These simulations suggest that the enhanced high-frequency acoustic emission at the boundaries of active regions (acoustic halo phenomenon) is caused by changes of the spatial-temporal spectrum of turbulent convection in a magnetic field, resulting in turbulent motions of smaller scales and higher frequencies than in quiet-Sun regions.

Imaging compaction band propagation in Diemelstadt sandstone using acoustic emission locations

Abstract: We report results from a conventional triaxial test performed on a specimen of Diemelstadt sandstone under an effective confining pressure of 110 MPa; a value sufficient to induce compaction bands. The maximum principal stress was applied normal to the visible bedding so that compaction bands propagated parallel to bedding. The spatio-temporal distribution of acoustic emission events greater than 40 dB in amplitude, and associated with the propagation of the first compaction band, were located in 3D, to within +/- 2 mm, using a Hyperion Giga-RAM recorder. Event magnitudes were used to calculate the seismic b- value at intervals during band growth. Results show that compaction bands nucleate at the specimen edge and propagate across the sample at approximately 0.08 mm s(-1). The seismic b-value does not vary significantly during deformation, suggesting that compaction band growth is characterized by small scale cracking that does not change significantly in scale.

High temperature characterisation of cordierite-mullite refractory by ultrasonic means

Abstract: Refractory materials containing cordierite (2MgO·2Al 2 O 3 ·5SiO 2 ) and mullite (3Al 2 O 3 ·2SiO 2 ) are used as support in furnaces, because of their low thermal expansion properties (coefficient of thermal expansion (CTE) ≈3–4 × 10 −6 K −1 ) which confer them a very good ability to thermal shock resistance. Composed of two phases presenting very different CTE (1.5–3 × 10 −6 for cordierite and 4–6 × 10 −6 K −1 for mullite), these materials can develop damage during thermal cycling due to internal stresses. This paper is devoted to the characterisation of the damage generated by this CTE mismatch, thanks to the application of ultrasonic techniques like long bar mode echography and acoustic emission (AE). The combination of these two techniques allows, during the applied thermal cycle (20–1200 °C), to continuously follow both the evolution of the elastic properties (Young's modulus) and the acoustic emission activity generated within the material. The analysis of these two characteristics, which are closely related to the damage evolution, makes it possible to propose a chronology of the mechanisms (damage, expansion) acting during the heating and the cooling stages.

Real-time acoustic emission testing based on wavelet transform for the failure process of thermal barrier coatings

Abstract: The fracture type differentiation, quantification, and source identification are desirable and yet intractable in the acoustic emission (AE) testing of a complex coating system. In this letter, a technique combining wavelet transform and conventional AE parameter analysis was developed to study the tensile failure process of thermal barrier coatings in real time. It is demonstrated that the failure of thermal barrier coatings originates from surface vertical cracking and follows interface cracking, and that the AE count increases with tensile load following a power law. The cracking source identified from AE signals agrees well with that observed by optical microscopy. This technique provides a powerful tool for the study of failure processes of a wide range of coatings and thin films.

Study of polycrystalline Al2O3 machining cracks using discrete element method

Abstract: Discrete element method (DEM) was introduced to simulate crack initiation and propagation of polycrystalline alumina during the brittle model machining process. A bonded particle model (BPM) was employed in the DEM simulations procedure to generate a particle assembly system similar to the micro-structure of the polycrystalline alumina. Particle and parallel bond properties, which were calibrated through a series of numerical tests, were subsequently used in the simulations of polycrystalline alumina cutting process and scratching tests. It is found that the cracks initiated right under or in front of the machining tool. There were many micro-cracks remained on the machined surface, some of them propagated downwards to form macro-cracks or forwards to lead material removal. Both DEM simulations and acoustic emission measure experiments have found that the fracture became acute when the normal and the tangential force changed suddenly, causing the crack number to increase. In 3D DEM scratching simulation, the surface cracks length and subsurface cracks depth linearly increased with the scratching depth, the value agreed well with the experimental results, and the surface-damage width decreased gradually with the depth to the surface, looking like half of a coin.

Acoustic Emission and Its Relationship with Friction and Wear for Sliding Contact

Abstract: Further investigation of the relationships between friction and wear properties and the characteristics of acoustic emission was conducted in the case of dry and grease-lubricated sliding contact using a ball-on-cylinder testing apparatus. The effect of contamination simulated by the inclusion of glass bead particles was also explored. Experiments were performed at sliding speeds ranging from 0.09 m/s to 1.47 m/s, while maintaining a fixed load and duration. As a first observation and contrary to what could be expected, the higher speed did not contribute to the decrease in friction interpreted by a worsening of the starved regime that had a consequence of increasing wear. However, the results revealed a good correlation between the friction coefficient and acoustic emission (AE) rms voltage for dry sliding. Such a relationship may allow the prediction of a reasonable friction coefficient μ from an AE signal. It was also determined that the friction work correlated well with the corresponding integrated A...

Acoustic emission source location methods using mode and frequency analysis

Abstract: Traditional acoustic source location techniques are greatly affected by the threshold value of the acoustic signals. Based on the theory of modal acoustic emission, acoustic signals have characteristics indicative of multi-mode, broad band and dispersion. One-dimensional acoustic source location theory is developed to incorporate these characteristics. The arrival times and wave velocities needed for source location are influenced by mode and frequency. Based on the number of sensors used, two universal source location techniques have been investigated. To obtain the arrival time of one mode at certain frequency, the Gabor wavelet transform is applied to the analysis of acoustic signals. One-dimensional acoustic source location experiments in a thin plate have been performed using a lead break as acoustic source. The acoustic source location techniques have been greatly improved and the accuracy of these acoustic source location methods has been verified. Copyright © 2007 John Wiley & Sons, Ltd.

Using Acoustic Emission to monitor fatigue damage and healing in Asphalt Concrete

Abstract: This paper presents an Acoustic Emission (AE) characterization of damage accumulation and strength recovery in Asphalt Concrete (AC) mixture. A series of uniaxial tensile cyclic tests with and without rest period has been conducted on a 19 mm nominal maximum size of aggregate (NMSA) AC at 20°C. During the fatigue tests, key AE parameters including emission counts were acquired from two piezo-type sensors attached to the middle of a 150 mm high, 75 mm diameter cylindrical specimen. Test results indicate that accumulative AE energy and AE count may be used not only to assess the initiation and propagation of fatigue damage, but also to quantify the beneficial effect of rest period on the performance of AC. The frequency-amplitude analysis shows that crack formations coincide with the peaks of maximum AE amplitude (Amax) with higher frequencies, while healing is best described by Amax with lower frequencies during the rest periods. In addition, it is demonstrated that the Kaiser effect, the stress dependence of AE generation, does not hold for fatigue in AC.