Study on detectable size and depth of defects in noncontact acoustic inspection method

Abstract

We study a noncontact inspection method for large-scale structures such as tunnels and bridges. This method involves the use of a high-powered sound source and a scanning laser Doppler vibrometer (SLDV). In our previous study, we proposed a tone burst wave method to improve the signal-to-noise ratio (SNR) of the measured result. Using this method, a defect that was difficult to detect using our previous method was detected. In this study, we examined the detectable size and depth of the defect by using a model wall with circular defects. The distance between the sound source and the concrete test piece was 5 m, and the output sound pressure was about 100 dB near the surface of the concrete test piece. As the transmitted wave, tone burst waves with different center frequencies from 500 to 7000 Hz were used. A conventional investigation by the hammer method was also simultaneously carried out for comparison and almost identical performance was confirmed. From the experimental result, we confirmed that the bending resonance frequency detected was proportional to the depth of the circular defect, and was in inverse proportion to the plane size (area) coincident to the analytical result for a circular plate. We also found that the vibration energy of the defect shows a strong dependency on its depth. Therefore, the possibility of defect depth estimation using the resonance frequency and the vibration energy ratio is expected. In the future, a practical investigation system that will replace the hammer method might be developed.

Detection of Internal Defects in Concrete and Evaluation of a Healthy Part of Concrete by Noncontact Acoustic Inspection Using Normalized Spectral Entropy and Normalized SSE

“…Figure 1 shows the fundamental experimental setup of the noncontact acoustic inspection method [ 7 , 8 , 9 ]. As a sound source, a long-range acoustic device (LRAD; Genasys Inc., San Diego, CA, USA; LRAD-300X) is used.…”

“…In this method, vibration energy is radiated to the entire surface by sound waves, and the vibration velocity distribution of the target surface is measured by a scanning laser Doppler vibrometer (SLDV). It has been clarified that cracks or cavity defects existing in the shallow layer of concrete can be detected even by the vibration of sound waves, because flexural resonance is likely to occur [ 9 ]. On the other hand, if the return light of the SLDV cannot be sufficiently obtained depending on the surface condition, an abnormal measurement point is observed and it is a problem at the time of measurement.…”

“…On the other hand, if the return light of the SLDV cannot be sufficiently obtained depending on the surface condition, an abnormal measurement point is observed and it is a problem at the time of measurement. However, since the vibration velocity spectrum measured in that case can be regarded as white noise, the defect detection algorithm [ 10 ] and the healthy part extraction algorithm [ 11 ] using the vibrational energy ratio [ 9 ] and the spectral entropy were devised. Further, since a scanning laser Doppler vibrometer (SLDV) for measuring the target surface two-dimensionally has high sensitivity, there is the issue that it is affected by strong reflected sound waves and causes a resonance phenomenon in the internal mechanism such as a Galvano mirror of a laser head.…”

Non-Destructive Inspection of Concrete Structures by Noncontact Acoustic Inspection Technology Using Acoustic Irradiation Induced Vibration

“…(STNB : Single ToNe Burst)波，時間周波数ゲート (13) ，振 動エネルギー比 (14) 及びスペクトルエントロピーを用いた欠 陥検出アルゴリズム (15) 及び高速計測を可能にするマルチ トーンバースト(MTNB : Multi ToNe Burst)波 (16) などが考 案された．実際に，トンネル，高架橋 (17) 及び地下空洞 (18) と いった実コンクリート構造物における探査実験，タイル外壁 供試体を用いた飛行中のドローンからの音波照射加振実 験 (19), (20) などが実施されている．更に面的な振動速度分布を 利用して内部欠陥に起因する共振現象と LDV 自体の共振現 象を識別することができる空間スペクトルエントロピー (SSE : Spatial Spectral Entropy) (21), (22) や送信した信号により 加振されたかどうか判定する共振判定処理 (23) 波で一つの周波数を使用する STNB 波 (13) ，1 回の送波で複 数の周波数を使用する MTNB 波 (16) 15), (17) 図 4 共振判定処理における受信波形分割の概念図 (23) 図 5 NCAI 法の基本セットアップ図 (26) テスト結果(検査者全員の結果が一致しない場合は△) ，下 段は NCAI 法による欠陥中心部における共振周波数の探査 結果例である(模擬欠陥の共振ピークは確認できるものの， 図 (15), (17) 図 13 N2U-Bridge における実験結果例 (15), (17) 163 図 14 高架橋での実験セットアップ図 (17) 図 15 高架橋床版のスキャンエリア図 (17) 図 16 NCAI 法による振動エネルギー比分布 (17) 図 17 叩き点検による検証実験 (17) 図 タイル外壁供試体の模擬欠陥配置図 (19), (20) 図 20 タイル外壁点検実験のセットアップ図 (20) 図 振動エネルギー比分布 (20) 5.5 民間マンションでの外壁点検実施例…”

“…に示すようIEICE Fundamentals Review Vol 16,. No.トーンバースト波の模式図(16) (a)シングルトーンバースト(STNB)波，(b)マルチトーンバースト(MTNB)波． 表 1 健全部・欠陥部・計測不良点の識別 17)レーザを用いた SLDV の場合には約 3.6 dB 以下はほぼ健全 部とみなしてよいことが判明している(14) ．スペクトルエン トロピーのしきい値は，健全部としてみなされた領域の最小 値若しくは標準偏差 σ と平均値 μ を利用した μ−2σ 及び μ2012 年当時の探査結果例を表 2 に示す(SLDV としては PSV-400-H4(Polytec GmbH)を使用) ．表は 2 段になって おり上段は岩検ハンマーを用いた叩き点検によるブラインド IEICE Fundamentals Review Vol.16, No.二つの音響特徴量を用いた欠陥検出アルゴリズム (…”