Kazuko Sugimoto

Defect-detection algorithm for noncontact acoustic inspection using spectrum entropy

In recent years, the detachment of concrete from bridges or tunnels and the degradation of concrete structures have become serious social problems. The importance of inspection, repair, and updating is recognized in measures against degradation. We have so far studied the noncontact acoustic inspection method using airborne sound and the laser Doppler vibrometer. In this method, depending on the surface state (reflectance, dirt, etc.), the quantity of the light of the returning laser decreases and optical noise resulting from the leakage of light reception arises. Some influencing factors are the stability of the output of the laser Doppler vibrometer, the low reflective characteristic of the measurement surface, the diffused reflection characteristic, measurement distance, and laser irradiation angle. If defect detection depends only on the vibration energy ratio since the frequency characteristic of the optical noise resembles white noise, the detection of optical noise resulting from the leakage of light reception may indicate a defective part. Therefore, in this work, the combination of the vibrational energy ratio and spectrum entropy is used to judge whether a measured point is healthy or defective or an abnormal measurement point. An algorithm that enables more vivid detection of a defective part is proposed. When our technique was applied in an experiment with real concrete structures, the defective part could be extracted more vividly and the validity of our proposed algorithm was confirmed.

Extraction of healthy part using two acoustic characteristics for defect detection by non-contact acoustic inspection method

We have studied a non-contact nondestructive acoustic inspection method using a long range acoustic device (LRAD) and a laser Doppler vibrometer (SLDV) to detect internal defects of concrete. Specimens or actual concrete structures have been measured from 5 m or more. Internal defects were detected and can be visualized. In order to detect internal defects of real concrete structures, quantitative evaluation of healthy part of concrete is important. The purpose is to detect defective parts by identifying and extracting healthy part of concrete.

The defect detection algorithm that combined spectrum entropy with vibrational energy ratio for acoustic inspection method

We have studied the non-contact and non-destructive acoustic inspection method using airborne sound wave and a laser Doppler vibrometer. Internal defects (crack, peeling etc.) of concrete near the surface are excited by vibrational energy of acoustic radiation. The resonance frequency of flexural vibration of the defect (inside to depth 8-10 cm) of concrete can be measured. By our technique, a non-destructive test is possible from a long distance more than 5 m max to 30 m. The two-dimensional vibration velocity data are measured and processed numerically. An image of the defect is reconstructed by vibrational energy ratio. There was a problem of an optical noise resulting from the leakage of light reception depending on surface state of concrete. To solve it, we introduced “Spectrum Entropy.” The data under influence of an optical noise show a frequency characteristic similar to a white noise. Spectrum entropy is calculated as an information entropy and express white nature of a signal. Therefore, we prop...

High speed non-contact acoustic inspection method using long distance acoustic irradiation-induced vibration

The non-contact acoustic inspection method using air-borne sound can detect the cavity defect and crack of inside concrete near the surface. As a feature of this technique, focusing on the physical phenomenon of flexural vibration of defects, we succeed in improving energy efficiency to vibrate the defect effectively. It is possible to measure all day using a small dynamo. Moreover, using a low-output laser Doppler vibrometer (LDV), high signal to noise (S/N) ratio is achieved by our proposed broadband “Tone Burst Wave” and “Time & Frequency-gate (T-F gate).” Therefore, although a possible distance from a sound source and a LDV to target is dependent on output power of acoustic radiation, it was already confirmed that a measurement of long distance over 30 m away is possible under the condition that a target is excited efficiently. In order to shorten measurement time, we propose the “Multi Tone Burst wave” that fully utilized the principle of T-F gate. After many experiments using a concrete test objects...

Study on the measurement speed and signal to noise ratio of multi tone burst wave for high speed non-contact acoustic inspection method

The non-contact acoustic inspection method using an air-borne sound can detect the cavity defect and crack near the measurement surface by using flexural resonance. By this method which uses the conventional single tone burst wave, since only one frequency was used for one sound wave emission, the length of measurement time had become a problem. Therefore, the multi-tone burst wave was devised for high speed improvement. However, measurement time and signal to noise (S/N) ratio change with the parameters in which a multi-tone burst wave is contained at the time of one sound wave emission, such as the number of frequency, the arranging method, pulse length, and the average number of times. Therefore, the experiment using the concrete test object having the styrofoam which imitated the cavity defect was conducted and a multi-tone burst wave was used, it was investigated how the measurement time and the S/N ratio of this method would change. From the experimental result, it became clear that the improvement ...

High speed non-contact acoustic inspection method using multi tone burst wave

The non-contact acoustic inspection method using an air-borne sound can detect the cavity defect and crack near the measurement surface by using flexural resonance. By this method using a single tone burst wave, the length of the measurement time of a two-dimensional scan was a practical problem. This is because only one frequency was used for one sound wave emission so that a high signal to noise (S/N) ratio could be obtained with the laser Doppler vibrometer (LDV) of a weak laser power (e.g., He-Ne 1mW). However, two or more frequency can be used at the time of one sound wave emission (multi tone burst wave) by taking into consideration the difference of the propagation velocity of a sound wave and laser light. That is, it becomes possible to perform high-speed measurement as compared with the conventional method, using effectively the time & frequency gate for raising a S/N ratio by terminating measurement before the reflective sound wave reaches a laser head from the measurement surface. The experimen...

The non-contact acoustic inspection method for concrete structures using the defect detection algorithm based on the statistic evaluation for a healthy part of concrete

The deterioration of concrete structure becomes a social problem in Japan. Social needs increase as for maintenance, check and renewal of concrete structures such as a tunnel or a bridge. The check of concrete structures has been performed conventionally by a hammering test. Not the inspection depended on a human experience and sense, but the inspection by a quantitative measuring system are demanded. We have developed the technique “non-contact nondestructive acoustic inspection method” to measure the internal defects near the concrete surface to a depth of about 10cm using airborne sound and the laser Doppler vibrometer at a distance 5―10 m far from measurement surface. Depending on the surface state (reflectance, dirt, etc.) of concretes, there is a problem to decrease the quantity of the light of the returning laser and there arise optical noise resulting from the leakage of light reception. To remove an abnormal measurement point, we proposed a defect detection algorithm, in which vibrational energy ...

Effect of wavefront distortion by propagation through inhomogeneous layer to the directivity of ultrasonic transducer

Quantitative analysis of ultrasonic wavefront distortion caused by propagation through biological tissue is important for the development of both imaging and tissue characterization techniques. In this study, using a periodic phantom made of oil jelly, quantitative analysis was performed to investigate the relationship between wavefront distortion and the degradation of directivity caused by it. The effect of wavefront distortion due to biological tissue is also discussed.