Kaita Ito

Locating Point of Impact on an Anisotropic Cylindrical Surface Using Acoustic Beamforming Technique

A beamforming array technique with four sensors is applied to a cylindrical geometry for detecting point of impact. A linear array of acoustic sensors attached to the plate record the waveforms of Lamb waves generated at the impact point with individual time delay. A beamforming technique in conjunction with an optimization scheme that incorporates the direction dependent guided Lamb wave speed in cylindrical plates is developed. The optimization is carried out using the experimentally obtained wave speed as a function of propagation direction. The maximum value in the beamforming plot corresponds to the predicted point of impact. The proposed technique is experimentally verified by comparing the predicted points with the exact points of impact on a cylindrical aluminum plate and a cylindrical composite shell. For randomly chosen points of impact the beamforming technique successfully predicts the location of the acoustic source.

Detection of segmentation cracks in top coat of thermal barrier coatings during plasma spraying by non-contact acoustic emission method

Abstract Numerous cracks can be observed in the top coat of thermal barrier coatings (TBCs) deposited by the atmospheric plasma spraying (APS) method. These cracks can be classified into vertical and horizontal ones and they have opposite impact on the properties of TBCs. Vertical cracks reduce the residual stress in the top coat and provide strain tolerance. On the contrary, horizontal cracks trigger delamination of the top coat. However, monitoring methods of cracks generation during APS are rare even though they are strongly desired. Therefore, an in situ, non-contact and non-destructive evaluation method for this objective was developed in this study with the laser acoustic emission (AE) technique by using laser interferometers as a sensor. More AE events could be detected by introducing an improved noise reduction filter and AE event detection procedures with multiple thresholds. Generation of vertical cracks was successfully separated from horizontal cracks by a newly introduced scanning pattern of a plasma torch. Thus, generation of vertical cracks was detected with certainty by this monitoring method because AE events were detected only during spraying and a positive correlation was observed between the development degree of vertical cracks and the total AE energy in one experiment.

Cold Cracking Susceptibility Of Austenitic And Martensitic Weld Metals

Cold cracking susceptibility of the weld metal made with a 16 %Cr-7 %Ni wire was investigated using the y groove weld cracking test. This wire was made on an experimental basis to be applied to 980 MPa grade steel without preheating. The welding was conducted at the condition of 260 A-26 V-23 cm/min with no preheating. The test results show that no cracking was observed when the diffusible hydrogen was 1.5 ml/100 g, while some cracks were found when the diffusible hydrogen was 4 ml/100 g. And the acoustic emission monitoring showed that the cracks occurred in the weld metal after the temperature decreased below 100 °C. Since the strength of the weld metal made with this prototype wire is over 980 MPa, this wire can be a candidate for the application of 980 MPa grade steel without preheating.

A Numerical Study on Generation Mechanism of Vertical Cracks in Top Coat of TBCs During APS Deposition

Clarification of crack generation mechanism in the top coat of the thermal barrier coatings (TBCs) during atmospheric plasma spray process is important to improve the reliability of TBC. In this study, finite element analyses of stress and strain during the deposition process were conducted with layer-by-layer method to understand the cracking behaviors. Stress relaxation by generation of vertical cracks was expressed as an elasto-plastic behavior of the coating. The effects of pre-heating temperature of the substrate and plasma power on crack development were analyzed by changing of the initial and atmospheric temperatures in simulation, respectively. The simulation results of radial strain explained the experimental results of crack monitoring by non-contact laser acoustic emission method.

Evaluation of Generation Mechanism of Vertical Cracks in Top Coat of TBCs During APS Deposition by Laser AE Method

Vertical cracks can be generated in the top coat of thermal barrier coatings (TBCs) deposited by atmospheric plasma spraying (APS). Since they are known to improve the durability of TBCs such as in the case of dense vertically cracked TBC, clarification of the mechanism and the criteria of cracking are very important. In this study, generation of such vertical cracks was monitored during APS process by laser acoustic emission (AE) method, which is an in situ, non-contact, and non-destructive technique. Temperature was also monitored inside and on the surface of a specimen during APS process for estimation of the temperature field in the top coat. Results of the AE and temperature monitoring were combined to evaluate the relationship between cracking and thermal stress in the top coat. Most of the AE events due to the generation of vertical cracks were detected during rapid heating of the surface of the top coat by the heat flux from the torch. It showed that the vertical cracks were induced due to the tensile stress caused by the temperature difference in the top coat from the rapid heating. Furthermore, the estimated critical thermal stress for vertical cracking from the monitoring results was consistent with a previously reported strength of YSZ coating deposited by thermal spray.

Impact localization on a cylindrical plate by near-field beamforming analysis

A beamforming array technique with 4 sensors is applied to a cylindrical plate for detecting point of impact. Linear array of acoustic sensors attached to the plate record the waveforms of Lamb waves generated at the impact point with individual time delay. An optimization technique with an objective function is incorporated into the beamforming technique in order to deal with the direction dependent Lamb wave speeds in a cylindrical geometry. The optimization is carried out using the experimentally obtained wave speed as a function of propagation direction. The maximum point in beamforming plot with minimized objective function corresponds to the localized point of impact. The proposed technique is experimentally verified by comparing the predicted points with the exact points of impact on a cylindrical aluminum plate. For randomly chosen points of impact the beamforming technique successfully predicts location of the exact acoustic source.

In-Situ Observation and Acoustic Emission Analysis for Corrosion Pitting of MgCl2 Droplet in SUS304 Stainless Steel

Acoustic emission with near-field detection and optical video microscope monitoring was proposed to investigate the pitting corrosion caused by the small magnesium chloride droplet. For the corrosion testing of work-hardened SUS304 stainless steel, four types of the detected waveforms were characterized by the wavelet analysis. After the high activity AE signals, the corrosion product sheet was extended without or with low activity AE signals. With the cross-section observations, the detected AE signals were mainly attributed to the following three sources, the longitudinal and transverse cracking around the covered pitting, cracking of the oxidation products, and the early stage of the corrosion potential fluctuation.

IN SITU MONITORING OF PLASMA SPRAYING PROCESS BY LASER ACOUSTIC EMISSION METHOD

Estimation of microfractures in ceramic coating layer during plasma spraying process is critical for its reliability. Acoustic emission (AE) method enables in-process monitoring of such microfractures. Laser AE method was adopted to realize the monitoring of plasma spraying process by non-contact detection of AE with laser interferometer. Also a high performance method for noise reduction of laser AE waveform was investigated. In this new method, laser AE signal was continuously sampled and transformed into spectrogram by time-frequency analysis to cut out noise component effectively. After this noise reduction process, inverse transform was applied to obtain a clear AE signals in time domain. Whole these processes can be done in real time. The effectiveness of this method was confirmed by a detection test of simulated AE and successfully applied to the monitoring of plasma spraying process. Two types of AE events with different duration time range were found and the sources of these AE were presumed.

Evaluation of cracking due to dynamic temperature fluctuation during plasma spraying process by laser AE method

Evaluation of cracks in the ceramic coatings is very important to insure the reliability of the coating itself and the coated materials, parts and whole systems. Laser acoustic emission (AE) method was applied to enable a non-contact and in-situ monitoring of cracks in the ceramic coating immediately after plasma spraying. In this study, originally developed Continuous Wave Memory (CWM) was used as an AE measurement system to enable the continuous waveform recording during plasma spraying process. AE signals due to cracks could be successfully distinguished from large noise components such as mechanical vibration and laser scattering during the process. The AE events during plasma spraying revealed correlation with the dynamic temperature fluctuation on the surface of the specimen which was generated by the scanning of the plasma torch. Furthermore, the larger temperature fluctuation brought higher active AE signals.

EVALUATION OF ULTRASONIC FATIGUE TESTING PROCESS OF HIGH STRENGTH STEEL BY NON‐LINEAR ULTRASONIC ANALYSIS

Non‐linear ultrasonic and acoustic emission (AE) signals during ultrasonic fatigue testing were analyzed by using Laser Doppler Vibrometer (LDV) and continuous AE waveform analysis system (1 MHz/12 bit). Notched specimens of a high strength low alloy steel were prepared for the ultrasonic fatigue test with exciting vibration frequency of 20 kHz. The detected surface velocity was longitudinal direction on the end of specimen with frequency range from 100 Hz to 500 kHz. During the waveform monitoring of the fatigue test, a distorted exciting waveform was observed in final stage of the test. Then the burst type noise mixed with the distorted exciting waveform was obtained just before the final failure. AE signal and upper harmonics of exciting frequency were analyzed by the FFT method. As the result, after the intensity of 2nd and 3rd harmonics increased rapidly, AE events were detected continuously. It can be concluded that non‐linear ultrasonic waveform analysis both of harmonics and AE signal were effecti...