Yoko Norose

Interval of Observation Plane in Visualization of Region near Defects in Billets Using Ultrasonic Computerized Tomography Method

We performed defect detection simulation considering billets with a deep-hole or spherical defect. We conducted defect detection in a billet of duralumin with a deep-hole defect and found no discrepancy between our previous and present research results because the images obtained are similar. We also conducted defect detection in a billet of steel with a spherical defect. We obtained visualization images in multiple measurement planes. We also obtained three-dimensional visualization images by binarizing the pseudo sound velocity. From the images, we found that the three-dimensional visualization of spherical defects is possible and that the scanning pitch in the longitudinal direction is about 10 mm at maximum.

Nondestructive Inspection for Steel Billet Using Phase-Modulated Signal by Gold Sequence for Improving Measurement Speed

We have proposed an ultrasonic computerized tomography method using the time-of-flight (TOF) of a longitudinal wave as a defect detection method for a steel billet. However, it took a long time to measure the TOFs because the transmissions were made one by one from the requirement of independent signal transmission. In this study, to speed up the TOF measurement, we proposed a simultaneous measurement method of TOFs using the phase-modulated signals by Gold sequences, and evaluated the ability of simultaneous measurement by an experiment. The reflected wave from the billet surface had a very adverse effect on the measurement of TOF, so a short signal was required as the transmitted signal. To make the transmitted signal short, a half-sine pulse phase-modulated by a Gold sequence was employed. As a result, five simultaneous transmissions were possible to be used for the inspection of the billet. When five simultaneous transmissions are made, the total measurement time can be decreased to 1/5 of the previous one.

Application of ultrasonic computerized tomography using time-of-flight measured by transmission method to nondestructive inspection for high-attenuation billets

For the nondestructive inspection of steel billets, the pulse echo method is generally employed in their manufacturing process. However, for high-attenuation billets, the pulse echo method is useless owing to the low echo level, while the transmission method is expected to detect defects inside high-attenuation billets. In this study, we consider the effectiveness of ultrasonic computerized tomography (CT) using time-of-flight (TOF) measured by the transmission method compared with the pulse echo method. From the simulation and experimental results, even though it was difficult for the pulse echo method to detect defects inside high-attenuation billets, the defects could be visualized by ultrasonic CT using TOF measured by the transmission method. Therefore, ultrasonic CT using TOF measured by the transmission method is more effective for detecting defects inside high-attenuation billets than the pulse echo method.

Speeding Up of Nondestructive Inspection for Shape-Distorted Billet by Simultaneous Measurement of Time of Flight

We have proposed a nondestructive inspection method for steel billets using ultrasonic computerized tomography by time of flight (TOF). It has been found that the proposed method may detect defects inside high-attenuation materials with the aid of sequential pulse transmission all around the surface. However, it takes a long time to measure the TOF by this method owing to large numbers of scans and transmissions. In this paper, we propose a TOF measurement method to apply our inspection method to simultaneous TOF measurement in a shape-distorted billet to speed up the inspection. In this method, besides a measurement plane and a reference plane, a standard reference plane is used to compensate not only the effect of the characteristics between a transmitter and a receiver but also the effect of the shape distortion in simultaneous measurement. In this paper, the performance of the present method was verified by numerical simulation and experiment. As a result, the defects in a shape-distorted billet were successfully visualized in simultaneous TOF measurement by the present method. Moreover, it was confirmed that the proposed method diminished the artifacts in the computerized tomography (CT) images by compensating the error caused by multiple simultaneous transmissions. It was revealed that the present method is more robust to noise than the previous method. Therefore, it is expected that the TOF measurement in the nondestructive inspection of a shape-distorted billet can be sped up by the proposed method.

Artifact Reduction in Tomographic Images for Nondestructive Testing of Square Billets Using Ultrasonic Computerized Tomography

To reduce the intensity of artifacts in visualized images for detecting defects in square billets by ultrasonic computerized tomography, we propose a method, which utilizes the crosspower-spectrum phase (CSP) function between a measurement signal and a reference signal, to measure the time-of-flight (TOF) difference. The artifacts arise owing to measurement errors of the TOF difference, which are caused by the effects of propagation characteristics and side lobes in correlation waveforms. The effects of propagation characteristics may be reduced using the reference signal, which contains the same characteristics as those of the measurement signal. The side lobes in correlation waveforms may also be decreased using the CSP method. The results obtained by experiment suggest that the intensity of artifacts was reduced because the proposed method reduced the side lobe level by about -14 dB and the number of measurement errors. The visualized images showed that the defects whose diameter was more than 2.0 mm were detected using the input signal whose wavelength was in the range from 2 to 6 mm.

Noise reduction in ultrasonic computerized tomography by preprocessing for projection data

In this study, an ultrasonic computerized tomography (CT) using time-of-flights (TOFs) has been used for the nondestructive inspection of steel billets with high acoustic attenuation. One of the remaining problems of this method is noise in CT images, which makes it difficult to distinguish defects from noise. Conventionally, noise is suppressed by a low-pass filter (LPF) in the process of filtered back projection (FBP). However, it has been found that there is residual noise even after filtering. To cope with this problem, in this study, the noise observed in ultrasonic testing was examined. As a result, it was found that the TOF data used for CT processing contains impulse noise, which remains in the CT image even after filtering, owing to the existence of transducer directivity. To remove impulse noise selectively, we propose a noise reduction technique for ultrasonic CT for steel billet inspection, that is, preprocessing (outlier detection and removal) of TOF data. The performance of the proposed technique was evaluated experimentally. The obtained results suggest that the proposed technique can remove impulse noise selectively and markedly improve the quality of the CT image. Hence, the proposed technique can improve the performance of ultrasonic CT for steel billet inspection.

Simulation-Based Study on the Effect of Scan Pitch for Ultrasonic Non-destructive Inspection in Cast Billet with Computerized Tomography

This paper deals with ultrasonic non-destructive inspection of cast billet using computerized tomographic method, which has been proposed by the authors. This method reconstructs pseudo sound velocity distributions, which reflect defect existence in cast billet as the decrease of the velocity. Although this method enabled to detect the defects whose size was 2.0 mm in our previous work, transducer scanning with small pitch, which resulted in long testing time, was taken. In this paper, we analyze the effect of scan pitch for ultrasonic non-destructive inspection in cast billet with computerized tomography, with aid of numerical simulation of wave propagation, to find the necessary and sufficient scan pitch to detect defects.

Defect Detection in Square Billet Using Phase Information of Longitudinal Waves

Defect detection is a technique that is essential for improving the quality of cast billets that have the possibility of defects because of remaining inclusions and stress during cooling and extrusion. The authors have proposed a method to nondestructively detect defects of approximately 2 mm in diameter using the time-of-flight (TOF) of longitudinal ultrasonic waves. This method detects defects as decreases in the apparent sound velocity, which is reconstructed with the aid of computerized tomography (CT), by measuring the TOF with a transmission method. In this study, the method is improved, especially in the measurement of the TOF. Unlike the previous method, which transmits a short-burst chirp signal and calculates the cross correlation between transmitted and received signals, the new method proposed herein transmits a long-burst single frequency signal and measures the TOF from two receiving signals by comparing two measurement planes (one plane is free of pinholes and the other state is unknown). The method proposed here has the disadvantage that the detectable size in the pinhole has an upper limit, but the advantage is that it uses narrowband transducers and enables cheaper calculation cost. Under the assumptions that there is a single pinhole, whose diameter is 2 mm, in a square billet made of duralumin whose size is 100 × 100 (mm2), the proposed method could detect the pinhole successfully.

Investigation of simultaneous signal transmission in non-destructive inspection of steel billet

A non-destructive method of steel billet, which uses time-of-flight of ultrasonic longitudinal wave and detects defects as decrement of pseudo sound velocity reconstructed by computerized tomography, has proposed. The remaining problem is long measurement time, owing to step-by-step measurement corresponds to each sound path whose number becomes huge. One of the solutions is simultaneous measurement, which can be achieved by transmitted signals simultaneously. To avoid interference among signals, choice of the transmitted signal becomes one of the most important points to be considered. In the case of non-destructive inspection of steel billet, many reflected waves from boundaries may cause adverse influence. Therefore, in this study, the parameters of the signal; modulation scheme, modulated signal, signal length, and signal frequency are investigated. Simulation-based study suggested that the signal is required to be short and independent each other. From those conditions, some experiments about defect ...