JP3796570B2 - Eddy current flaw detection method and flaw detection probe - Google Patents
Eddy current flaw detection method and flaw detection probe Download PDFInfo
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- JP3796570B2 JP3796570B2 JP2002071307A JP2002071307A JP3796570B2 JP 3796570 B2 JP3796570 B2 JP 3796570B2 JP 2002071307 A JP2002071307 A JP 2002071307A JP 2002071307 A JP2002071307 A JP 2002071307A JP 3796570 B2 JP3796570 B2 JP 3796570B2
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Description
【0001】
【発明の属する技術分野】
本発明は、渦電流探傷によって薄板はもとより厚板金属材の表面から、表面、内部及び裏面のキズ検査を行うことができる渦電流探傷法及び探傷プローブに関する。本明細書で記載するキズは、き裂、空孔、窪み、非金属介在物等の渦流探傷により検査できる金属材の欠陥を意味し、これらの欠陥の全てを対象とする。
【0002】
【従来の技術】
一般に、電磁応用非破壊検査の一つである渦電流探傷試験は、1)表面感度が高い、2)高速かつ非接触な探傷が可能、3)センサの構造が簡単で設計が容易、4)信号が直接電気信号として得られるため信号処理や記録が容易、5)自動探傷・遠隔操作に適している、6)材質や形状の変化などの、雑音因子の影響を受け易い、といった特徴を有している。
渦電流探傷試験はその原理上表面感度は良いものの、表皮効果により板厚方向に渦電流が減衰するため、厚板の裏面を探傷するには不向きであるため、従来は表面探傷もしくは薄板の探傷に限定されて用いられて来た。
【0003】
渦電流探傷試験の適用例の一つに加圧水型軽水炉発電プラントの蒸気発生器伝熱管の供用中検査が挙げられる。とりわけこの蒸気発生器伝熱管の探傷に関し、渦電流探傷は過去十数年間で数値解析技術や新型プローブの開発において著しい進展を遂げている。
現在、板厚1.27mmの伝熱管の探傷では、裏面20%き裂を検出し、逆問題解析によりき裂形状を再構築することが可能となっている。今後、これらの技術を他の検査対象に応用することが期待される。
原子力プラントや航空機などの構造物では、欠陥許容基準、あるいは損傷許容設計に基づいて非破壊検査を行うのが望ましいという事情があり、高い欠陥検出能力と欠陥の形状評価が必要とされる。
【0004】
他方、これらの構造物には蒸気発生器の伝熱管のような薄板ではなく、厚肉材で構成されている部分が数多く存在する。
こうした箇所には、オーステナイト系ステンレス鋼の溶接部など、従来用いられている超音波探傷法では探傷困難なき裂もあり、電磁応用非破壊検査の適用も検討されている。よって、厚肉材へ渦電流探傷試験を適用し、その特長を検査に反映させ高度化を図ることが要求される。
【0005】
厚肉材への渦電流探傷試験の適用に当たり問題となるのは、探傷面とは反対の裏面キズの検出である。一般に表皮効果による渦電流の減衰に阻まれ裏面の探傷は難しく、この問題を克服する必要がある。
なお、従来渦電流探傷試験法は、板厚1〜1.5mmの被検査体に適用されてきたが、それを超える厚さ、特に板厚6mm以上の厚さでは、渦電流によりキズを検査することが事実上困難であった。
【0006】
【発明が解決しようとする課題】
本発明は、原子力プラントや航空機などの構造物の、厚板のキズ検査が可能であり、表面のみならず内部及び裏面(内面)すなわち、探傷面とは反対の裏面キズの検出ができる渦電流探傷法及びそれに最適な探傷プローブを提供することを課題とする。
【0007】
【課題を解決するための手段】
上記の課題を解決するために、本発明者らは鋭意研究を行った結果、渦電流探傷プローブの構造を改良することにより、被検査体である材料表面のみならず、内部及び、特に探傷面と反対の裏面キズの検出が、より正確に測定できるとの知見を得た。なお、本発明の探傷方法は厚板材料のキズ検査に特に有効であるが、薄板材にも適用できることは言うまでもない。
本発明はこの知見に基づき、
1.それぞれの軸の方向が対象物の検査面に対して垂直に配置した無コア型励磁コイル対に互いに逆向きの電流を流し、該励磁コイル対の間の領域において、それぞれのコイルによる渦電流を重ね合わせ、該励磁コイル対の間に配置した検出コイルにより探傷することを特徴とする渦電流探傷法
2.検出コイルが差動形コイルであることを特徴とする上記1記載の渦電流探傷法
3.検出コイルの直下の裏面渦電流密度と表面渦電流密度の比(裏面渦電流密度/表面渦電流密度)が0.5以上となるように、励磁コイル間の距離を設定することを特徴とする上記1又は2記載の渦電流探傷法
4.励磁コイルの内半径rと高さhの比を、r/h=0.5〜1.5の範囲とすることを特徴とする上記1〜3のいずれかに記載の渦電流探傷法
5.板の表面及び板の表面から板の内部及び裏面のキズ検査を行うことを特徴とする上記1〜4のいずれかに記載の渦電流探傷法
6.板厚6mm以上の厚板のキズ検査を行うことを特徴とする上記1〜5のいずれかに記載の渦電流探傷法
7.逆問題解析手法により、き裂形状を再構成することを特徴とする上記1〜6のいずれかに記載の渦電流探傷法、を提供する。
【0008】
本発明は、また
8.それぞれの軸の方向が対象物の検査面に対して垂直に配置され、かつ互いに逆向きの電流を流す無コア型励磁コイル対と、該励磁コイル対の間に配置した検出コイルからなることを特徴とする渦電流探傷用プローブ
9.検出コイルが差動形コイルであることを特徴とする上記8記載の渦電流探傷用プローブ
10.検出コイルの直下の裏面渦電流密度と表面渦電流密度の比(裏面渦電流密度/表面渦電流密度)が0.5以上となる離間位置に励磁コイルを設置したことを特徴とする上記8又は9記載の渦電流探傷用プローブ
11.励磁コイルの内半径rと高さhの比が、r/h=0.5〜1.5の範囲であることを特徴とする上記8〜10のいずれかに記載の渦電流探傷用プローブ
12.板の表面及び板の表面から板の内部及び裏面のキズ検査を行うことを特徴とする上記8〜11のいずれかに記載の渦電流探傷用プローブ
13.板厚6mm以上の厚板のキズ検査を行うことを特徴とする上記8〜12のいずれかに記載の渦電流探傷用プローブ、を提供する。
【0009】
【発明の実施の形態】
本発明では、渦電流探傷試験の応用分野の拡大を目指し、厚肉材の裏面キズを検出できる渦電流探傷プローブを提案する。
このために、変形磁気ベクトルポテンシャル法(A.Kameari, Solution of Asymmetric Conductor with a Hole by FEM Using Edge-element, COMPEL, 9, (1999), pp.230-232.)と辺要素有限要素法(A.Kameari, Three Dimensional Eddy Current Calculation Using Edge Element for Magnetic Vector Potential, Applied Electromagnetics in Materials, pp.225-236, (1988).)による渦電流解析にもとづいてプローブの各種パラメータを設定し、設計・試作を実施した。
実験により開発したプローブの性能を検証し、得られた結果を用いて逆問題解析を行い、き裂の定量的な形状評価を行った。以下に、プローブの設計及びプローブの実験結果について説明する。また、実験結果に対する逆問題解析結果を示し、同様にその結果を以下に説明する。
【0010】
厚板に渦電流探傷試験を適用しようとする際、問題となるのは渦電流の表皮効果である。具体的には下記の事項によって裏面キズの探傷が阻害されると考えられる。すなわち、1)表皮効果による渦電流の板厚方向の減衰により、裏面キズを検出するための強大な渦電流が裏面では得難いこと、2)表皮効果の影響により、表面の渦電流が裏面の渦電流に比べ著しく大きくなり、裏面の欠陥信号が表面形状あるいは材質の変化によるノイズに影響を受ける可能性があること、の2点である。
【0011】
上記の問題は、薄板であれば周波数を適切な値に設定することによって解決することができるが、厚板では2点を同時に解決することが難しくなる。
周波数を小さく設定し表皮深さを増大させれば、渦電流が微弱なものになってしまう。欠陥検出信号の強度は欠陥が存在する箇所に誘導された渦電流の強度に大きな影響を受けるので、それに伴い信号も微弱になってしまう。
逆に、周波数を大きく設定し渦電流の強度を増大させれば、表皮効果による渦電流の減衰が著しくなり裏面の欠陥信号が表面のノイズに大きな影響を受けてしまう。とりわけ励磁コイル近傍においてこの傾向は顕著に現れる。
したがって、厚肉材の探傷には、周波数の調整のみならず、強力な渦電流を板厚方向に平坦に浸透させることのできるプローブの形状の探索が要求される。
【0012】
強力な渦電流を裏面に発生させるため、本発明においては、2個の励磁コイルに互いに逆向きの電流を流し、励磁コイル間の領域においてそれぞれのコイルによる渦電流を重ね合わせた。この励磁方式には、過度な励磁コイルの巨大化やそれに伴うリフトオフの増大を軽減できるという著しい利点がある。この点は、本発明の大きな特徴の一つである。
個々の励磁コイルの寸法は、単体でも裏面に強力な渦電流を発生させることができるよう3次元渦電流解析を用いて選定した。
【0013】
一方、裏面の欠陥信号が表面のノイズに強く影響されないようにするため、強大な渦電流を平坦に浸透させることのできる位置を探し、そこで信号を検知するようにした。
検出コイルは励磁コイル間の領域に配置しなければならないので、この問題は励磁コイル間の距離を調節して渦電流分布を変化させることに帰着する。ここでも3次元渦電流解析を利用することにより、励磁コイル間距離を選定した。
上記の2点を設計方針として開発した渦電流探傷プローブの概念図を、図1に示す。図1において、符号1は励磁コイル、符号2は検出コイル(2個の差動式検出コイル)、符号3は電流、符号4は被検査体10の表面、符号5は被検査体10の裏面、符号6は磁束を示す。
【0014】
(数値解析によるプローブの寸法の決定)
本発明では、プローブの開発において3次元渦電流解析を用いて寸法値を決定した。この解析手法では、変形磁気ベクトルポテンシャル法による辺要素有限要素法を採用したものである。
辺要素を用いることにより、支配方程式から電気スカラーポテンシャルを削除し、渦電流の発散に関する方程式を連立する必要がなくなる。また、変形磁気ベクトルポテンシャル法を用いることにより、励磁コイルを導体やそれを取り巻く空間とは独立に扱うことができる。このため、数値解析に要する記憶容量を大幅に低減できる効果が得られる。
この他、この解析手法にデータベースを適用した高速解法があるが、これを後述する実験結果と解析結果の比較対照に用いた。これは、渦電流探傷信号の計算において、データベースを作成して解析領域を、き裂があると予測される領域に限定することにより、計算の高速化を実現したものである。
【0015】
以下に、本発明の具体例を示すが、試験材(被検査体)として板厚7mmのインコネル(INCONEL)を用いて厚板用渦電流探傷プローブを設計した。なお、リフトオフは0.2mmである。
周波数は、5kHzの時表皮深さが約7mmとなるが、信号強度を考慮して10kHzに設定し計算を行った。また、励磁コイルの電流密度は1.0×106[A/m2]に固定した。
以下、これらの値は本具体例における全ての解析に共通する。
【0016】
励磁コイルに関しては多数のパラメータが存在するが、ここではコイルの内径(具体的には内半径を使用)、巻幅、高さに着目した。
既存のプローブの励磁コイル(内径1mm、巻幅0.5mm、高さ0.5mm)を等倍で拡大し、拡大後のコイルに関し内部の半径、巻幅、高さを対象としてパラメータ・サーベイを行った。図2に励磁コイル1の寸法形状を示す。図2において、符号7は高さ、符号8は巻幅、符号9は内半径を示す。符号10は被検査体である。解析によって得られた各パラメータの変化と裏面の渦電流密度の関係から、巻幅が裏面渦電流に対して大きな影響を与える。本具体例では、空間的な制約を考慮し、巻幅を6mmに設定した。但し、この巻幅については、検査対象である材料の種類や大きさ(厚さ)等及び渦電流探傷プローブの設計に応じて任意に変えることができる。
【0017】
巻幅の設計値を選定した後、内径、高さを同時に変化させた時の裏面渦電流の変化を調べた。その解析結果を図3に示す。
同図において(内径)/(高さ)=1の直線に沿って、裏面渦電流が比較的急勾配で変化しているのが分かる。概ね、この直線に沿って更に励磁コイルを拡大して行くのが望ましいが、その好適な範囲は(内径)/(高さ)=0.5-1.5である。これらの解析結果を考慮して、本具体例では、最終的に励磁コイルの寸法を内径10mm、巻幅6mm、高さ12mmとした。なお、この具体的な寸法は検査対象である材料の種類や大きさ(厚さ)等及び渦電流探傷プローブの設計に応じて任意に変えることができる。
巻数に関しては、巻線の断面積から1995ターンとした。実施の過程において、上記のプロセスを盛り込むことにより、裏面に強力な渦電流を容易に誘導することのできる励磁コイルの作製が可能となった。
【0018】
励磁コイルの寸法決定後、2個の励磁コイル間の距離を調節して渦電流密度分布を変化させる。励磁コイル間距離を変化させた時の渦電流の変化を3次元渦電流解析によって調べた。
励磁コイル間距離と表面と裏面の渦電流密度及びこれらの比(裏面渦電流密度/表面渦電流密度)の関係を図4に示す。図4より、2個の励磁コイルを近接させると表面と裏面の渦電流密度に大きな差異が生じることが分かる。
一方、励磁コイル間距離が、例えば10mm以上、特に12mm以上になると、(裏面渦電流密度)/(表面渦電流密度)である渦電流密度比が、微小な増加を続けては行くものの、勾配が緩やかになるのが分かる。裏面渦電流密度は漸近的な減少を続けて行く。裏面渦電流の強度と表皮深さの両者を勘案し、本具体例においては、励磁コイル間距離を12mmとした。また、本具体例では、(裏面渦電流密度)/(表面渦電流密度)が0.7となっているが、通常0.5以上で裏面き裂検出の良好な感度が得られる。
【0019】
励磁コイルの各種設計値を設定後、検出コイルの設計値を選定する。励磁コイルの設計に際しては、検出コイルの直下の検出強度が高く、かつ裏面渦電流密度と表面渦電流密度の比(裏面渦電流密度/表面渦電流密度)が0.5以上となるように、かつ励磁コイルの内半径rと高さhの比がr/h=0.5〜1.5の範囲となるように、励磁コイルの内半径、高さ、幅のそれぞれの寸法、及び励磁コイル間の距離を設定する。
一般に、検出コイルに関しては、その巻数が多いほどき裂に対する感度は向上する。検出コイルに関しては、励磁コイル間の領域を充填しその後高さ方向に拡大する方針で設計した。本具体例においては、最終的に検出コイル高さを6mmとし、ターン数は1300ターンとした。
以上の数値解析結果で決定したプローブによる渦電流分布から、ローブの対称面を切断面とした時の面内における渦電流の様相を表しているが、被試験体に誘導された渦電流の実数部、振幅に関して、(裏面渦電流密度)/(表面渦電流密度)がそれぞれ0.75、0.72と高い値を示した。
虚数部は0.35であるが、実数部の方がより支配的であるため、探傷に大きな支障を与えることはないと考えられる。これにより、仮に表面と裏面の渦電流がほぼ同位相であったとしても、新たに設計したプローブでは表面のノイズに裏面の検出信号が大きな阻害を受けることはないものと予測できる。
【0020】
(渦電流探傷システム)
本発明の具体例で用いた渦電流探傷システムでは、試験片を載せた二次元電動ステージを、GPIBボードを介してパソコンで制御することにより実施した。
プローブは定位置に固定する。プローブの信号は探傷器で処理され、実数部と虚数部に分けてA/Dボードによりパソコンに取り込まれる。探傷器はアスワン電子製のASSORT-PC2を用いた。
離散的なデータを取るためステージ制御用パソコンでステージを移動しながら、探傷信号や測定点の座標といったデータを同時に取り込むことによって行った。
【0021】
(試験及びその結果)
本具体例において用いた試験片は、純国産のH-IIAロケットの配管を同じ材料(材質はINCONEL718)及び同じ板厚(7mm)の平板で模擬したものを使用した。試験片中央に長手方向の溶接線が存在し、溶接線は余盛を除去した。
この厚さ7mmの試験片に対し、溶接線と母材の境界上に3個の半楕円形人工き裂が存在する。いずれもき裂の長さは10mm、幅は0.2mmである。楕円の短半径をき裂の深さとした時、それぞれのき裂の深さは1.00mm(14.3%)、0.50mm(7.1%)、0.25mm(3.6%)であった。
これらのき裂を、き裂が開口している面から、あるいは開口していない面から探傷することにより、内面き裂(ID)、外面き裂(OD)の探傷とする。
実験における探傷器の設定は、試験周波数を10kHz、ゲインを79dBとした。ステージ制御の設定は、スキャンピッチをX、Y方向共に0.5mmとし、プローブと試験片のリフトオフは0.2mmとした。
【0022】
(比較−絶対値型コイル)
比較のため、絶対値型コイルで実験を行った。このプローブの欠陥検出能力は、板厚1.25mmのINCONEL600板においてID20%、OD60%である。試験周波数は10kHz以外に5kHz、1kHzを適用し、深さ1mmの外面き裂の探傷を行った。
10kHz、5kHz、1kHzのいずれの場合も、き裂が検出できないことが確認できた。この結果、絶対値型コイルは厚肉材の探傷に不向きであると言える。さらにこの試験片において溶接部と母材とでは、導電率、透磁率といった物性値の変化が微小であることが分かる。
【0023】
(本発明のプローブを用いた検出結果)
本発明の具体例であるプローブを用いて、本プローブによる深さ0.25mmの半楕円形内面き裂の実験を行った。検出の条件としては、周波数:10kHz、リフトオフ:0.2mm、位相:283.0度とした。
2次元走査して得た Vy 信号の2次元表示を行った結果、1個のき裂に対し4個のピークが存在する。これは2個の検出コイルの差動を検出信号としているためであり、さらにプローブが左右対称で自己差動特性を備えているからである。
き裂は4個のピークの中心に存在するが、き裂を挟んでX方向に現れるピークの組は検出コイルの差動特性によるものである。
一方、Y方向に現れるピークの組は自己差動特性によるものである。本実験ではき裂も左右対称なため、自己差動特性により信号の絶対値はき裂の中心で最小となる。なお、探傷信号がy方向に顕著に表れるよう設定しており、x方向ではノイズを含む信号が得られた。
同様に外面き裂に関しても、同様に4個のピークが確認できる。深さ0.5mmの外面き裂の実験結果を図5に示す。また、同図に示される線に沿った B スキャン信号を抽出し、図6及び図7に示す。ここで2個の B スキャン信号それぞれにおいて正負2個のピークが確認できる。これにより、開発したプローブによって深さ0.5mmの外面き裂を検出できたことが分かる。
【0024】
(数値解析結果と実験結果の比較)
実験と同条件で、データベースを用いた辺要素変形磁気ベクトルポテンシャル法による順問題解析で渦電流探傷信号を計算し、実験信号と比較した。ここで、人工き裂の幅は0.2mmと分かっているので、き裂の弁別にはき裂方向の1次元信号を用いれば十分である。
き裂信号は、2個の検出コイルの片方がき裂の延長線上にある時最大となるので、き裂方向のき裂中心を0mmとして、片方の検出コイルの中心点が+17.5mmから−17.5mmまで移動する時の1mm間隔(一部間隔2mm)の1次元データ22点の信号を以下の比較に用いる。
【0025】
探傷器出力では励磁電流、フィルタによる位相差などが不明であるため、計算信号との絶対値及び位相の比較は不可能である。このため、まず実験結果から計算結果で得られる電圧値への換算を行う必要がある。先の22点の信号に対し、実験結果と計算結果のピークの振幅及び位相が合うように次式で回転拡大する。
S’=αejθS (1)
ここで、S及びS’は換算前後の実験信号であり、αは拡大係数、θは回転角度である。深さ1mmの半楕円形内面き裂の実験信号が解析信号と一致するよう係数を下記の通りとした。
α=2.15 (2)
θ=−52.1[degree]
【0026】
(1)式及び(2)式の係数を使って、それぞれの実験結果を変換した。対応する計算結果と換算した実験結果を比較して図8(a)-(d)に示す。図8(a)-(d)に示されるように、内面き裂に関しては実験結果と計算結果が良好な一致を見せていることが分かる。
裏面き裂に関しては振幅や位相に多少の誤差が見られるが、逆問題解析に支障を来たすほど大きな違いは見られない。実験結果よりき裂形状の再構築を行うことが可能であると考えることが出来る。
【0027】
(逆問題解析手法)
き裂形状の定量的評価に用いた逆問題解析の計算手順を以下に示す。
1)解析モデルにき裂形状を与え、先の高速順問題解析で渦電流探傷信号を求める。
2)実験信号と解析信号を比較し、整合しないようなら最急降下法により形状を修正し、再度信号の計算を行う。
3)実験と解析の信号の誤差が所定の値よりも小さくなるか、もしくはその変化が小さくなるまでこの手続きを行う。
この解析では、データベースを用いた高速順問題解析を利用しているため、逆問題解析自体も高速であるという特徴がある。
【0028】
(き裂形状の再構成)
前記において変換した信号を用いて、き裂形状の推定を行う。き裂の再構成は、変換された入力信号を目標に、酷似した信号が得られるまで反復計算を行うことにより実現される。
き裂の幅は0.2mmに固定し、18mm×0.2mm×7mmの矩形をき裂が存在する領域であるSuspect Regionとする。き裂の形状を長方形の並びで近似表現し、1mm毎(一部2mm)に16個のパラメータを設定する。計測点は、前記のように検出コイルの中心線上を通る方向の22点ある。
Huangらによって開発された逆問題解析手法(H.Huang, T.Takagi, H.Fukutomi and J.Tani, Forward and Inverse Analysis of ECT Signals Based on Reduced Vector Potential Method Using A Database, Electromagnetic Nondestructive Evaluation(II), IOS press, (2001), pp.313-321)を利用し、き裂形状を推定した。この推定結果を、図9(a)-(d)に示す。
【0029】
内面き裂に関しては、き裂長さが多少ずれてしまうものがあるものの、き裂深さは実際のき裂と良好な一致を見せた。き裂長さの方向の誤差が最大18%であるのに対して、き裂深さの誤差は最大1.1%と極めて小さい。
一方、外面き裂の推定に関しては、検出可能であった2個のき裂に対し、き裂形状を精度よく再構成することができた。深さの誤差は最大4.1%、長さの誤差は9%である。これらのき裂形状の再構築により、開発したプローブは厚肉材の探傷において、裏面のき裂に対する逆問題解析にも適していると言える。
【0030】
【発明の効果】
本発明は、3次元渦電流解析を利用して薄板はもとより、厚肉平板の表面から、該板の表面、内部及び裏面に存在するき裂等の欠陥を精度良く検出できる渦電流探傷法及びそのための渦電流プローブを提供するものである。
渦電流探傷法の特徴は、2個の励磁コイルに互いに逆向きの電流を流すことにより、強力かつ板厚方向に平坦に浸透する渦電流を発生させるものであり、この結果、裏面にも強力な渦電流を発生させ、かつ裏面の欠陥信号が表面のノイズに影響されない渦電流探傷プローブを提供できる。検出コイルの位置では、本プローブの場合(裏面渦電流密度)/(表面渦電流密度)は0.5以上、さらには0.7以上を示し、強大な渦電流密度を保った状態で高い値を示す。
本プローブを用いて実験を行った結果、厚さ7mmのINCONEL試験片における検出能力は、内面き裂は深さ0.25mm、外面き裂は0.5mmであるというレベルに達することができる。さらに、実験結果と解析結果を比較し、逆問題解析が可能であり、この逆問題解析を行うことによって、外面き裂に対しても良好にき裂形状を復元できるという優れた利点がある。
以上から、本発明のプローブは厚肉材の探傷において、高い欠陥検出能力を有し、欠陥の寸法評価にも優れ、欠陥検出に適している著しい効果を有する。
【図面の簡単な説明】
【図1】本発明の渦電流探傷プローブの概観を示す図である。
【図2】励磁コイルの寸法形状の説明図である
【図3】内径、高さを同時に変化させた時の裏面渦電流の変化を調べた解析結果を示す図である。
【図4】励磁コイル間距離と表面と裏面の渦電流密度及びこれらの比の関係を示す図である。
【図5】本発明のプローブを用いて、半楕円形内面(裏面)き裂を2次元走査(Cスキャン)して得たVy信号の画像を示す図である。
【図6】図5に示される線に沿ったBスキャン(走査線1の)信号を抽出したVy信号のグラフである。
【図7】図5に示される線に沿ったBスキャン(走査線2の)信号を抽出したVy信号のグラフである。
【図8】数値解析結果と実験結果の比較を示す図である。
【図9】逆問題解析による欠陥形状の再構成の結果を示す図である。
【符号の説明】
1 励磁コイル
2 検出コイル
3 電流
4 被検査体の表面
5 被検査体の裏面
6 磁束
7 高さ
8 幅
9 内半径
10 被検査体[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an eddy current flaw detection method and a flaw detection probe capable of performing a flaw inspection on a front surface, an inside, and a back surface from the surface of a thick plate metal material as well as a thin plate by eddy current flaw detection. The flaw described in this specification means a defect of a metal material that can be inspected by eddy current flaw detection such as a crack, a hole, a dent, and a non-metallic inclusion, and covers all these defects.
[0002]
[Prior art]
In general, eddy current testing, which is one of electromagnetic non-destructive testing, has 1) high surface sensitivity, 2) high-speed and non-contact testing, 3) simple sensor structure and easy design 4) Signal processing and recording are easy because the signal can be obtained directly as an electrical signal. 5) Suitable for automatic flaw detection and remote operation. 6) Easy to be affected by noise factors such as changes in material and shape. is doing.
The surface sensitivity of the eddy current test is good in principle, but the eddy current attenuates in the plate thickness direction due to the skin effect, so it is not suitable for flaw detection on the back side of the plate. It has been used limited to.
[0003]
One of the application examples of the eddy current flaw detection test is an in-service inspection of a steam generator heat transfer tube of a pressurized water reactor. In particular, with regard to the flaw detection of this steam generator heat transfer tube, eddy current flaw detection has made remarkable progress in the development of numerical analysis techniques and new probes over the past decades.
At present, in flaw detection of heat transfer tubes with a thickness of 1.27 mm, it is possible to detect a 20% crack on the back surface and reconstruct the crack shape by inverse problem analysis. It is expected that these technologies will be applied to other inspection objects in the future.
In a structure such as a nuclear power plant or an aircraft, there is a situation that it is desirable to perform nondestructive inspection based on defect tolerance standards or damage tolerance design, and high defect detection capability and defect shape evaluation are required.
[0004]
On the other hand, these structures are not thin plates such as heat transfer tubes of steam generators, but have many portions made of thick materials.
In these places, there are cracks that are difficult to detect by conventional ultrasonic inspection methods, such as welded parts of austenitic stainless steel, and application of non-destructive inspection using electromagnetics is also being considered. Therefore, it is required to apply an eddy current flaw detection test to a thick material and reflect its features in the inspection to make it more sophisticated.
[0005]
The problem in applying the eddy current flaw detection test to thick materials is the detection of the back surface flaw opposite to the flaw detection surface. In general, it is difficult to detect flaws on the back side due to eddy current attenuation due to the skin effect, and it is necessary to overcome this problem.
In addition, the conventional eddy current flaw detection test method has been applied to an object to be inspected with a plate thickness of 1 to 1.5 mm. It was practically difficult.
[0006]
[Problems to be solved by the invention]
The present invention is capable of inspecting scratches on a thick plate of a structure such as a nuclear power plant or an aircraft, and can detect not only the surface but also the inside and back surface (inner surface), that is, the back surface scratch opposite to the flaw detection surface. It is an object of the present invention to provide a flaw detection method and a flaw detection probe optimal for the flaw detection method.
[0007]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors have conducted intensive research. As a result, by improving the structure of the eddy current flaw detection probe, not only the surface of the material to be inspected but also the inside and particularly the flaw detection surface. It was found that the detection of scratches on the opposite side of the surface can be measured more accurately. The flaw detection method of the present invention is particularly effective for scratch inspection of thick plate materials, but it goes without saying that it can also be applied to thin plate materials.
The present invention is based on this finding,
1. Currents opposite to each other are passed through a pair of coreless excitation coils in which the directions of the respective axes are arranged perpendicular to the inspection surface of the object , and eddy currents generated by the respective coils are generated in the region between the excitation coil pairs. 1. An eddy current flaw detection method characterized in that flaw detection is performed using a detection coil arranged between the exciting coil pairs . 2. The eddy current flaw detection method according to 1 above, wherein the detection coil is a differential coil. The distance between the excitation coils is set so that the ratio of the back surface eddy current density directly below the detection coil to the surface eddy current density (back surface eddy current density / surface eddy current density) is 0.5 or more. 3. Eddy current flaw detection method according to 1 or 2 above 4. The eddy current flaw detection method according to any one of the
[0008]
The present invention also relates to 8. Each axis direction is arranged perpendicular to the inspection surface of the object, and includes a coreless type excitation coil pair for passing currents in opposite directions , and a detection coil arranged between the excitation coil pair. Features of eddy current
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, an eddy current flaw detection probe capable of detecting a back surface flaw of a thick material is proposed with the aim of expanding the application field of the eddy current flaw detection test.
For this purpose, the modified magnetic vector potential method (A.Kameari, Solution of Asymmetric Conductor with a Hole by FEM Using Edge-element, COMPEL, 9, (1999), pp.230-232.) And the edge element finite element method ( A.Kameari, Three Dimensional Eddy Current Calculation Using Edge Element for Magnetic Vector Potential, Applied Electromagnetics in Materials, pp.225-236, (1988).) A prototype was implemented.
The performance of the probe developed through experiments was verified, and the inverse results were analyzed using the results obtained, and the shape of the crack was evaluated quantitatively. The probe design and the experimental results of the probe will be described below. Moreover, the inverse problem analysis result with respect to an experimental result is shown, and the result is demonstrated below similarly.
[0010]
When applying eddy current testing to thick plates, the skin effect of eddy currents becomes a problem. Specifically, flaw detection on the back surface is considered to be hindered by the following matters. That is, 1) It is difficult to obtain a strong eddy current on the back surface due to the skin effect due to the attenuation of the eddy current in the plate thickness direction. Two points are that it is significantly larger than the current, and the defect signal on the back surface may be affected by noise due to changes in the surface shape or material.
[0011]
The above problem can be solved by setting the frequency to an appropriate value in the case of a thin plate, but it is difficult to solve two points simultaneously in the case of a thick plate.
If the frequency is set small and the skin depth is increased, the eddy current becomes weak. Since the intensity of the defect detection signal is greatly affected by the intensity of the eddy current induced in the place where the defect exists, the signal becomes weak accordingly.
On the other hand, if the frequency is set large and the eddy current intensity is increased, the eddy current attenuation due to the skin effect becomes significant, and the defect signal on the back surface is greatly affected by the noise on the surface. This tendency is particularly noticeable in the vicinity of the exciting coil.
Therefore, flaw detection of a thick material requires not only adjustment of the frequency but also searching for a probe shape capable of allowing a strong eddy current to penetrate flatly in the plate thickness direction.
[0012]
In order to generate a strong eddy current on the back surface, in the present invention, currents in opposite directions are passed through the two exciting coils, and the eddy currents of the respective coils are overlapped in the region between the exciting coils. This excitation method has a significant advantage that an excessive increase in the excitation coil and the associated increase in lift-off can be reduced. This is one of the major features of the present invention.
The dimensions of the individual exciting coils were selected using a three-dimensional eddy current analysis so that a strong eddy current can be generated on the back surface of the single exciting coil.
[0013]
On the other hand, in order to prevent the defect signal on the back surface from being strongly influenced by the noise on the front surface, a position where a strong eddy current can be permeated flat is searched for and the signal is detected there.
This problem results in changing the eddy current distribution by adjusting the distance between the excitation coils, since the detection coils must be arranged in the region between the excitation coils. Again, the distance between the exciting coils was selected by using a three-dimensional eddy current analysis.
A conceptual diagram of an eddy current flaw detection probe developed with the above two points as a design policy is shown in FIG. In FIG. 1,
[0014]
(Determination of probe dimensions by numerical analysis)
In the present invention, the dimension value is determined using three-dimensional eddy current analysis in the development of the probe. This analysis method employs the side element finite element method based on the modified magnetic vector potential method.
By using the edge element, it is not necessary to remove the electric scalar potential from the governing equation and to connect the equations relating to the eddy current divergence. Further, by using the modified magnetic vector potential method, the exciting coil can be handled independently of the conductor and the space surrounding it. For this reason, the effect that the storage capacity required for the numerical analysis can be greatly reduced is obtained.
In addition, there is a high-speed solution that applies a database to this analysis method, and this was used as a comparison between the experimental results and analysis results described later. In the calculation of the eddy current flaw detection signal, the calculation speed is increased by creating a database and limiting the analysis region to a region where a crack is predicted to exist.
[0015]
Hereinafter, specific examples of the present invention will be described. An eddy current flaw detection probe for a thick plate was designed using INCONEL having a thickness of 7 mm as a test material (inspected object). The lift-off is 0.2 mm.
When the frequency is 5 kHz, the skin depth is about 7 mm, but the calculation was performed by setting the frequency to 10 kHz in consideration of the signal strength. The current density of the exciting coil was fixed at 1.0 × 10 6 [A / m 2 ].
Hereinafter, these values are common to all analyzes in this example.
[0016]
There are many parameters for the exciting coil, but here we focused on the inner diameter of the coil (specifically, using the inner radius), winding width, and height.
The excitation coil (inner diameter 1mm, winding width 0.5mm, height 0.5mm) of the existing probe was expanded at the same magnification, and a parameter survey was conducted on the inner coil, winding width, and height of the expanded coil. . FIG. 2 shows the dimensional shape of the
[0017]
After selecting the design value of the winding width, changes in the back surface eddy current when the inner diameter and height were changed simultaneously were investigated. The analysis result is shown in FIG.
In the figure, it can be seen that the back surface eddy current changes relatively steeply along a straight line of (inner diameter) / (height) = 1. In general, it is desirable to further expand the exciting coil along this straight line, but the preferred range is (inner diameter) / (height) = 0.5-1.5. In consideration of these analysis results, in this specific example, the final dimensions of the exciting coil were an inner diameter of 10 mm, a winding width of 6 mm, and a height of 12 mm. The specific dimensions can be arbitrarily changed according to the type and size (thickness) of the material to be inspected and the design of the eddy current flaw detection probe.
The number of turns was set to 1995 turns based on the cross-sectional area of the winding. By incorporating the above process in the course of implementation, it became possible to produce an exciting coil that can easily induce a strong eddy current on the back surface.
[0018]
After determining the dimensions of the exciting coil, the eddy current density distribution is changed by adjusting the distance between the two exciting coils. The change of eddy current when the distance between exciting coils was changed was investigated by three-dimensional eddy current analysis.
FIG. 4 shows the relationship between the distance between the exciting coils, the eddy current density on the front surface and the back surface, and the ratio thereof (back surface eddy current density / surface eddy current density). From FIG. 4, it can be seen that there is a large difference in the eddy current density between the front surface and the back surface when the two exciting coils are brought close to each other.
On the other hand, when the distance between the exciting coils is, for example, 10 mm or more, particularly 12 mm or more, the eddy current density ratio (back surface eddy current density) / (surface eddy current density) continues to increase slightly, but the gradient Can be seen to be gradual. The backside eddy current density continues to decrease asymptotically. Considering both the strength of the back surface eddy current and the skin depth, in this specific example, the distance between the exciting coils was set to 12 mm. Further, in this specific example, (back surface eddy current density) / (surface eddy current density) is 0.7, but a good sensitivity for detecting back surface cracks is usually obtained at 0.5 or more.
[0019]
After setting various excitation coil design values, select the detection coil design value. When designing the excitation coil, the detection intensity directly under the detection coil is high, and the ratio of the back surface eddy current density to the surface eddy current density (back surface eddy current density / surface eddy current density) is 0.5 or more. And the dimensions of the inner radius, height and width of the exciting coil, and the exciting coil so that the ratio of the inner radius r and the height h of the exciting coil is in the range of r / h = 0.5 to 1.5. Set the distance between.
Generally, with respect to the detection coil, the sensitivity to cracks increases as the number of turns increases. The detection coil was designed with the policy of filling the area between the excitation coils and then expanding in the height direction. In this example, the detection coil height was finally 6 mm and the number of turns was 1300 turns.
The eddy current distribution by the probe determined from the above numerical analysis results shows the aspect of eddy current in the plane when the symmetric plane of the lobe is the cut surface, but the real number of eddy currents induced in the DUT As for the part and amplitude, (back surface eddy current density) / (surface eddy current density) showed high values of 0.75 and 0.72, respectively.
Although the imaginary part is 0.35, the real part is more dominant, so it is considered that it will not cause any serious trouble in flaw detection. Thus, even if the eddy currents on the front surface and the back surface are substantially in phase, it can be predicted that the detection signal on the back surface will not be significantly hindered by the noise on the front surface with the newly designed probe.
[0020]
(Eddy current flaw detection system)
In the eddy current flaw detection system used in the specific example of the present invention, the two-dimensional electric stage on which the test piece was placed was controlled by a personal computer via the GPIB board.
The probe is fixed in place. The probe signal is processed by a flaw detector, and is divided into a real part and an imaginary part and is taken into a personal computer by an A / D board. The ASSORT-PC2 manufactured by Aswan Electronics was used as the flaw detector.
In order to obtain discrete data, the stage was moved by a personal computer for stage control, and data such as flaw detection signals and measurement point coordinates were simultaneously acquired.
[0021]
(Test and results)
The test piece used in this example was a model of a purely domestic H-IIA rocket pipe that was simulated with the same material (material is INCONEL718) and a flat plate with the same thickness (7 mm). A weld line in the longitudinal direction is present at the center of the test piece, and the weld line has been removed from the excess.
For this 7 mm thick test piece, there are three semi-elliptical artificial cracks on the boundary between the weld line and the base metal. In both cases, the crack length is 10 mm and the width is 0.2 mm. When the short radius of the ellipse was taken as the crack depth, the crack depths were 1.00 mm (14.3%), 0.50 mm (7.1%), and 0.25 mm (3.6%).
By detecting these cracks from the surface where the cracks are open or from the surface where the cracks are not open, the internal cracks (ID) and the external cracks (OD) are detected.
In the experiment, the flaw detector was set to a test frequency of 10 kHz and a gain of 79 dB. For the stage control settings, the scan pitch was 0.5 mm in both the X and Y directions, and the lift-off between the probe and the test piece was 0.2 mm.
[0022]
(Comparison-Absolute value type coil)
For comparison, an experiment was performed using an absolute value type coil. The defect detection capability of this probe is ID20% and OD60% for an INCONEL600 plate with a plate thickness of 1.25 mm. In addition to 10 kHz, the test frequency was 5 kHz and 1 kHz, and an external crack with a depth of 1 mm was detected.
It was confirmed that cracks could not be detected in any of 10 kHz, 5 kHz, and 1 kHz. As a result, it can be said that the absolute value type coil is not suitable for the flaw detection of thick materials. Furthermore, in this test piece, it can be seen that changes in physical properties such as conductivity and magnetic permeability are minute between the weld and the base material.
[0023]
(Detection results using the probe of the present invention)
Using a probe which is a specific example of the present invention, a semi-elliptical inner surface crack having a depth of 0.25 mm was tested using this probe. The detection conditions were: frequency: 10 kHz, lift-off: 0.2 mm, phase: 283.0 degrees.
As a result of two-dimensional display of the Vy signal obtained by two-dimensional scanning, there are four peaks for one crack. This is because the differential of the two detection coils is used as a detection signal, and the probe is symmetrical and has a self-differential characteristic.
The crack exists in the center of the four peaks, but the set of peaks appearing in the X direction across the crack is due to the differential characteristics of the detection coil.
On the other hand, the set of peaks appearing in the Y direction is due to self-differential characteristics. In this experiment, the crack is also symmetrical, so the absolute value of the signal is minimized at the center of the crack due to self-differential characteristics. Note that the flaw detection signal was set so that it appeared remarkably in the y direction, and a signal containing noise was obtained in the x direction.
Similarly, four peaks can be confirmed for the external crack. FIG. 5 shows the experimental results of an external crack with a depth of 0.5 mm. Also, B scan signals along the lines shown in the figure are extracted and shown in FIGS. Here, two positive and negative peaks can be confirmed in each of the two B scan signals. This shows that the developed probe was able to detect an external crack with a depth of 0.5 mm.
[0024]
(Comparison of numerical analysis results and experimental results)
Under the same conditions as the experiment, the eddy current flaw detection signal was calculated by forward problem analysis using the side element deformation magnetic vector potential method using a database, and compared with the experimental signal. Here, since the width of the artificial crack is known to be 0.2 mm, it is sufficient to use a one-dimensional signal in the crack direction for discrimination of the crack.
Since the crack signal is maximum when one of the two detection coils is on the extension line of the crack, the crack center in the crack direction is assumed to be 0 mm, and the center point of one detection coil is +17.5 mm to -17.5 The signal of 22 points of 1D data with 1mm interval (partial interval 2mm) when moving to mm is used for the following comparison.
[0025]
In the flaw detector output, since the excitation current, the phase difference due to the filter, etc. are unknown, it is impossible to compare the absolute value and the phase with the calculation signal. For this reason, it is necessary to first convert the experimental result into a voltage value obtained from the calculation result. The above 22 signals are rotated and enlarged by the following equation so that the peak amplitude and phase of the experimental result and the calculated result match.
S ′ = αe jθ S (1)
Here, S and S ′ are experimental signals before and after conversion, α is an enlargement factor, and θ is a rotation angle. The coefficients were set as follows so that the experimental signal of a semi-elliptical inner surface crack with a depth of 1 mm matched the analysis signal.
α = 2.15 (2)
θ = −52.1 [degree]
[0026]
Each experimental result was converted using the coefficients of equations (1) and (2). The corresponding calculation results and the converted experimental results are compared and shown in FIGS. As shown in FIGS. 8 (a)-(d), it can be seen that the experimental results and the calculated results are in good agreement with respect to the internal crack.
Although there are some errors in the amplitude and phase of the backside crack, there are no significant differences that would interfere with the inverse problem analysis. From the experimental results, it can be considered that the crack shape can be reconstructed.
[0027]
(Inverse problem analysis method)
The calculation procedure of inverse problem analysis used for quantitative evaluation of crack shape is shown below.
1) Give a crack shape to the analysis model and obtain the eddy current flaw detection signal by the previous high-speed forward problem analysis.
2) Compare the experimental signal and the analysis signal. If they do not match, correct the shape by the steepest descent method and calculate the signal again.
3) This procedure is performed until the error between the experiment and analysis signals becomes smaller than a predetermined value or the change becomes small.
Since this analysis uses high-speed forward problem analysis using a database, the inverse problem analysis itself is characterized by high speed.
[0028]
(Reconstruction of crack shape)
The crack shape is estimated using the converted signal. Crack reconstruction is achieved by iterative calculations with the transformed input signal as a target until a very similar signal is obtained.
The width of the crack is fixed at 0.2 mm, and a rectangle of 18 mm x 0.2 mm x 7 mm is defined as the Suspect Region where the crack exists. Approximate the shape of the crack as a rectangle, and set 16 parameters for every 1mm (2mm in part). There are 22 measurement points in the direction passing through the center line of the detection coil as described above.
Inverse analysis method developed by Huang et al. (H. Huang, T. Takagi, H. Fukutomi and J. Tani, Forward and Inverse Analysis of ECT Signals Based on Reduced Vector Potential Method Using A Database, Electromagnetic Nondestructive Evaluation (II) , IOS press, (2001), pp.313-321). The estimation results are shown in FIGS. 9 (a)-(d).
[0029]
As for the internal crack, although the crack length might be slightly different, the crack depth was in good agreement with the actual crack. The error in the crack length direction is 18% at maximum, whereas the error in crack depth is 1.1% at maximum.
On the other hand, regarding the estimation of the external crack, the crack shape could be accurately reconstructed for the two cracks that could be detected. The depth error is up to 4.1% and the length error is 9%. By reconstructing these crack shapes, it can be said that the developed probe is also suitable for inverse problem analysis for cracks on the back side in flaw detection of thick materials.
[0030]
【The invention's effect】
The present invention provides an eddy current flaw detection method capable of accurately detecting defects such as cracks existing on the surface, inside and back surface of a thick plate as well as a thin plate using three-dimensional eddy current analysis, and An eddy current probe for this purpose is provided.
The feature of the eddy current flaw detection method is to generate strong and eddy current that penetrates flatly in the plate thickness direction by flowing currents in opposite directions to the two exciting coils. Therefore, it is possible to provide an eddy current flaw detection probe that generates a simple eddy current and the defect signal on the back surface is not affected by noise on the front surface. At the position of the detection coil, in the case of this probe, (back surface eddy current density) / (surface eddy current density) is 0.5 or more, further 0.7 or more, and it is a high value while maintaining a strong eddy current density. Indicates.
As a result of experiments using this probe, the detection capability of an INCONEL specimen with a thickness of 7 mm can reach the level that the inner surface crack is 0.25 mm deep and the outer surface crack is 0.5 mm. Further, the inverse problem analysis is possible by comparing the experimental result and the analysis result, and by performing the inverse problem analysis, there is an excellent advantage that the crack shape can be well restored even for the outer surface crack.
From the above, the probe of the present invention has a high defect detection capability, excellent defect dimension evaluation, and has a remarkable effect suitable for defect detection in the flaw detection of thick materials.
[Brief description of the drawings]
FIG. 1 is a diagram showing an overview of an eddy current flaw detection probe according to the present invention.
FIG. 2 is an explanatory diagram of the size and shape of an exciting coil. FIG. 3 is a diagram showing an analysis result of examining changes in back surface eddy current when the inner diameter and height are changed simultaneously.
FIG. 4 is a diagram showing the relationship between the distance between exciting coils, the eddy current density on the front surface and the back surface, and the ratio thereof.
FIG. 5 is a diagram showing an image of a Vy signal obtained by two-dimensional scanning (C scan) of a semi-elliptical inner surface (back surface) crack using the probe of the present invention.
6 is a graph of a Vy signal obtained by extracting a B scan (scanning line 1) signal along the line shown in FIG.
7 is a graph of a Vy signal obtained by extracting a B-scan (scanning line 2) signal along the line shown in FIG.
FIG. 8 is a diagram showing a comparison between numerical analysis results and experimental results.
FIG. 9 is a diagram illustrating a result of defect shape reconstruction by inverse problem analysis;
[Explanation of symbols]
DESCRIPTION OF
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KR101254300B1 (en) * | 2010-08-12 | 2013-04-12 | 한국원자력연구원 | Apparatus for detecting thickness of the conductor using dual core |
JP6634629B2 (en) * | 2015-07-01 | 2020-01-22 | 日鉄テクノロジー株式会社 | Eddy current flaw detector |
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