Advanced Acoustic Emission Research Articles

Acoustic Emission Research on Thermal Shock Behaviors of Three-Dimensional C/SiC Composites in Different Environments

Three-dimensional (3D) carbon fiber reinforced silicon carbide matrix composites (C/SiC) were prepared by a low-pressure chemical vapor infiltration method. The thermal shock behaviors of the composites in different environments were researched using an advanced acoustic emission (AE) system. Damage initiation and propagation were easily detected and evaluated by AE. The thermal shock damage to C/SiC composites mainly occurred at the process of cooling and was limited at argon but unlimited at wet oxygen atmosphere. Also correlations have been established between the different damage mechanisms and the characteristics of acoustic emission signals obtained during thermal shock tests. In this way, the paper contributes to the development of the acoustic emission technique for monitoring of damage development in ceramic-matrix composites.

Acoustic emission analysis and experiments with physical model systems reveal a peculiar nature of the xylem tension

Advanced acoustic emission analysis, special microscopic examinations and experiments with physical model systems give reasons for the assumption that the tension in the water conducting system of vascular plants is caused by countless minute gas bubbles strongly adhering to the hydrophobic lignin domains of the xylem vessel walls. We ascertained these bubbles for several species of temperate deciduous trees and conifers. It is our hypothesis that the coherent bubble system of the xylem conduits operates as a force-transmitting medium that is capable of transporting water in traveling peristaltic waves. By virtue of the high elasticity of the gas bubbles, the hydro-pneumatic bubble system is capable of cyclic storing and releasing of energy. We consider the abrupt regrouping of the wall adherent bubble system to be the origin of acoustic emissions from plants. For Ulmus glabra, we recorded violent acoustic activity during both transpiration and re-hydration. The frequency spectrum and the waveforms of the detected acoustic emissions contradict traditional assumptions according to which acoustic emissions are caused by cavitation disruption of the stressed water column. We consider negative pressure in terms of the cohesion theory to be mimicked by the tension of the wall adherent bubble system.

Time-Frequency Visualization of AE Signal using Wavelet Transform for Precise Measurement and Estimation

AE (Acoustic Emission) techniques are universally used for investigating and watching the deformation and crack of various materials because they enable the non-destructive and continuous measurement. However, it is not always easy to clarify the main cause or mechanism of AE signal generation by its analysis and an AE sensor is generally vulnerable to the noise, and therefore advanced measurement and estimation techniques of AE signal are needed. In this paper, the authors present time-frequency visualization of AE signal using the wavelet transform. The proposed method enables especially precise measurement of the frequency band and generation time of AE signal and gives a clue in advanced AE techniques.

Cycle-based analysis of damage and failure in advanced composites under fatigue: 1. Experimental observation of damage development within loading cycles

Damage development within loading cycles and the possibility of cycle-based modeling of fatigue damage evolution and failure in advanced composites are addressed in this work. In Part 1 of the work, the fatigue damage development in an unnotched composite laminate is studied by an advanced acoustic emission (AE) technique. The overall time history of the acoustic emission is recorded and separated into the emission from loading and unloading phases of the loading cycles. The source location histories and distributions of the AE events over the fatigue stress range are extracted and analyzed. Qualitatively different time histories and stress distributions for the emission generated during loading and unloading are observed and analyzed, for the first time. It is concluded that most of the emission from the unloading phase is due to internal friction between the crack faces and most of the damage is developed during the loading phase of the cycles. The location distribution of the fatigue damage is found to be fairly random to failure, indicating that the dominating fatigue fracture process in an unnotched laminate is scattered damage nucleation and accumulation. The time history of the overall damage evolution is found to exhibit the classical initial, steady, and final failure development stages. Within the loading cycles, the damage is developed over the entire stress range, with the substantial amount developed at stresses below the maximum fatigue stress. The latter observation sheds light on the reported effects of the loading cycle shape on the fatigue behavior and life of composites. The results of this study provide an insight into fatigue damage development in composites and constitute a fundamental basis for the development of a cycle-based model in Part 2 of this work.

Studies on the relationship between acoustic emission characteristics and fractu re toughness of materials

The corelation between acoustic emission characteristics and their fracture toughness has been investigated in this paper based on the study of high-strength al uminum alloy cracking process by using advanced acoustic emission experiment sys tem and smooth tensile specimens as well as DCB specimens with premade cracks. E xperimental results showed that the specimens emitted strong signals with an amp litude above 35 dB when a stable crack extension started at K1C. Stro nger and more complicated acoustic emission signals were found during unstable crack extension, and the cumulative energy of acoustic emission events were found to b e linearly related with the macroscopic fracture energy.

Acoustic emission monitoring of fatigue crack propagation

Abstract The monitoring of fatigue crack propagation in steel and welded steel compact tension and T-section girder test specimens, using an advanced acoustic emission system with accurate source location, is described. The compact tension test specimens were subjected to load ratios of 0.1, 0.3, 0.5 and 0.7 while the T-section girders were subjected to a load ratio of 0.3. Located acoustic emission events were filtered for a narrow band containing the fatigue crack, and separated for different regions of the applied load range. The test results indicate that acoustic emission count rates, for small percentages of the applied load range close to the peak load, show reasonable correlation with crack propagation rates. Based on these correlations it may be possible to predict the remaining service life of fatigue damaged structures from the results of short term acoustic emission monitoring.

On the Possibility of Discrimination of Mixed Mode Fatigue Fracture Mechanisms in Adhesive Composite Joints by Advanced Acoustic Emission Analysis

Acoustic emission (AE) from pure and mixed mode fatigue fracture of adhesive composite joints was recorded and analyzed. Transient AE signal classification was attempted by a computational pattern recognition analysis. Good separation was achieved for pure mode AE signals from the double cantilever beam (mode I) and end notch fracture (mode II) tests. AE signals from the mixed mode lap joint test were found to be closer to the mode II fracture signals. The results may provide insight into the fracture mechanisms in joints and can be used for the development of mechanism-based predictive models of fracture and life.

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

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.

Integrity evaluation of glass-fiber composites with varied fiber/matrix interfacial strength using acoustic emission

An advanced acoustic emission analysis method was used to study the dynamics and sequence of microfractures in uni-directional glass-fiber reinforced plastics (UD-GFRPs). UD-GFRPs (60 wt% fiber) with different interfacial qualities were prepared by adding a small amount (4 or 8 wt%) of paraffin wax into vinylester matrix without affecting much the mechanical properties of the matrix. The orientation dependence of both the P-wave velocity and attenuation were accurately measured by the laser ultrasonic method, and incorporated into the acoustic emission analysis of visco-elastic media. The analysis iteratively compares the acoustic emission signals (measured as out-of-plane displacements) with theoretical waveforms calculated assuming the time history and type of an acoustic emission source. The waveform simulation allows us to estimate the fracture dynamics and the sequence of the Mode-I fiber fracture (Type 1), Mode-I debonding (Type 2) and Mode-II disbonding (Type 3). The fiber fracture in UD-GFRPs without wax started at 0.1% strain. The initiation strain increased to 0.25 and 0.3% in the specimens with wax. The specimens with lower interfacial strength showed rapid succession of microfractures at higher stresses (above 100–200 MPa). Fracture dynamics showed distinct differences by wax addition. Using the present analysis method, we can determine quantitatively effects of interfacial quality on the microfracture processes and assess the integrity of UD-GFRPs.

アコースティック・エミッションによる非破壊検査の現状

アコースティック・エミッションによる非破壊検査の現状

Effect of rate on the fracture mechanism of TiAl

This study investigated the effect of microfracture mechanisms on the fracture toughness of both fully lamellar and duplex TiAl. It is known that the static fracture toughness of TiAl is improved by a lamellar microstructure and also depends on the grain size. The effect of loading rate on the fracture toughness is very important. In this paper, we measured the static fracture toughness of TiAl with different grain sizes of lamellae and various volume fractions of γ phase, and also estimated the effect of loading rate on the fracture toughness. The static fracture toughness increased with an increase in grain size of the lamellae and with a decrease in volume fraction of γ phase. Many acoustic emission (AE) signals were detected before the final fracture, and microcracking was observed. The toughening due to microcracking and sequential shear ligaments was demonstrated to be a major mechanism for improvement of the toughness, which was confirmed by our toughening model. Our advanced AE technique also demonstrated that it took several microseconds to generate microfracture. The dynamic fracture toughness decreased with an increase in grain size of the lamellae and with a decrease in volume fraction of γ phase. It was concluded that the toughening mechanism under static conditions could not occur under dynamic conditions and that another toughening mechanism operates under dynamic conditions.

Estimation of acoustic properties and fracture dynamics of polycrystalline graphites by AE signal processing

This study aims to measure the acoustic properties and to investigate the fracture mechanism and dynamics of artificial polycrystalline graphites with different densities. Advanced acoustic emission (EA) signal processing was conducted to achieve this purpose. Phase velocities of elastic waves estimated by FFT phase spectrum analysis were found to be constant in the AE frequency range. A new displacement-sensitive AE monitoring system detected a number of AE signals produced by Mode-I cleavage fracture during tensile testing of chevron notched graphites. Source inversion processing of the monitored displacement with the analytical Green's function revealed that the average size of cleavage cracks ( 8×10 −2 mm 2 ) in low density graphites was larger than that ( 2×10 −2 mm 2 ) in high density ones, but their propagation rates were almost the same. Both computer simulation of surface displacements and SEM observations indicated that the crack size estimated by the source inversion processing coincided with the characteristic cleavage cracks observed on the fracture surface.

FRACTURE STRENGTH OF LASER FUSION PRODUCED COATINGS BY AN ELASTIC WAVE CHARACTERIZATION

Abstract An advanced Elastic Wave(EW) or Acoustic Emission(AE) characterization system was developed to determine the fracture strength and to elucidate the microdynamics of fracture in fusion produced coatings. The system makes it possible to measure the minute surface displacement and to do the processing of obtained signals. Fracture dynamics was estimated as a “forward problem” where the computed surface displacement at sensor position was compared with the detected displacement. It was demonstrated that the strength and the microdynamics of fracture under four point bending could quantitatively be determined for laser clad titanium and laser produced titanium aluminide.

Determination of crack location, type and orientation ina concrete structures by acoustic emission

Elastic waves emitted by microfracturing are called acoustic emission (AE). Recently AE techniques have been extensively applied to concrete engineering as a non–destructive testing (NDT) method. A key aspect of AE is that the nucleation of internal cracks is directly detected by a surface observation. In conventional AE techniques, such AE parameters as event count (activity), amplitude, energy and spectra are correlated with the failure process of materials. Thus, AE characteristics are utilized for NDT. The AE source (crack) is also located by employing a multichannel (more than 5) AE detecting and recording system. In this Paper an advanced AE analysis procedure is proposed. In addition to crack locations, crack types and crack orientations are determined from AE relative amplitudes of the first motions. The procedure classifies cracks into tensile cracks and shear cracks. Based on information of the crack type, the crack orientation is determined; this is the direction of crack opening in the case of tensile cracks, and the direction of sliding motion in the case of shear cracks. The proposed procedure is applied to a pull-out test of an anchor–bolt from a concrete block and a cylinder–tensile test. In the pull out test the opening directions of the tensile cracks is perpendicular to the failure surface, while the directions of sliding motion of the shear cracks are parallel to the failure surface. In the cylinder–tensile test the opening directions of the tensile cracks are perpendicular to the loading direction and all sources are located near the final plane. The proposed procedure is therefore able to determine the microcrack kinematics generated in concrete.

微小変形によるモーメントテンソルの理論的解析法とその微視割れへの適用

An infinitesimal deformation in a material can be described by a moment tensor in the “eigenstrain method” of micromechanics. In the present treatment, a microcracked region was modeled as a dislocation source with moment tensor components. The advanced acoustic emission (AE) technique was developed to evaluate AE signals quantitatively and to characterize AE sources. A method to determine moment tensor components from the signals recorded by six multi-transducers was developed. This method was applied to analyze quasicleavage facets of an embrittled Cr-Mo-V steel (A470) during the fracture toughness testing. A dynamic response function of the compact tension specimen for each AE source location was calculated by a finite difference method. An iterative deconvolution algorithm was developed and moment tensor components were determined. Deconvolution results showed that mixed mode microcracks with about 60 μm across were generated at the crack tip, and these microcracks which nucleated prior to macroscopic crack extension were identified to be responsible for the embrittlement of the steel.

Acoustic emission surveillance for improving availability of nuclear power plants

An experimental study of advanced acoustic emission monitoring of BWR components is being performed at Philadelphia Electric Company's Peach Bottom Atomic Power Station. The study addresses the feasibility of using continuous acoustic emission monitoring for increasing availability of LWR power units. Here we present a summary of a “Valve Surveillance” program and we describe the installation and performance of AE instrumentation for the “Pipe Surveillance” program.