JP2004037436A

JP2004037436A - Method of measuring sound elastic stress by surface sh wave and measuring sensor

Info

Publication number: JP2004037436A
Application number: JP2002229288A
Authority: JP
Inventors: Hiromi Toda; 戸田　裕己; Hisahiro Go; 呉　尚弘; Yorinobu Murata; 村田　頼信
Original assignee: Sakai Iron Works Co Ltd
Current assignee: Sakai Iron Works Co Ltd
Priority date: 2002-07-02
Filing date: 2002-07-02
Publication date: 2004-02-05
Anticipated expiration: 2022-07-02
Also published as: JP4022589B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of measuring sound elastic stress for removing an influence of a contact medium existing between a probe for sound elastic stress measuring probe and a subject substance to be measured by an SH wave, and to provide a measuring sensor. <P>SOLUTION: Four ultra sonic receiving elements are aligned on a receiver wedge member in a line, the wedge member for the receiver elements is provided with 2 bottom members being in contact with the subject to be measured via a contact medium. To the center parts of the bottom parts each one pair of transmitters are provided with nearly equal to a critical incident angles θ of the ultrasonic wave toward insides. Each pair of the transmitter elements are aligned on the two bottom members with a distance while keeping the same distance as the distance between the two bottom members. An ultrasonic pulse is emitted and a surface SH wave is made to enter the surface of the subject, the surface SH wave is received with two receiving elements from the ultrasonic transmitters, the time difference of the two receiver element receives the surface SH waves, the transmittance time of the surface SH wave is measured. The sound elastic stress measuring method by the surface SH wave in the subject substance and the sensors used in this method are provided. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、表面ＳＨ波による音弾性応力測定方法及び表面ＳＨ波による応力測定に用いられる、超音波の送信子と受信子を対設した応力測定用センサに関するものである。
【０００２】
【従来の技術】
固体中を伝播する振動には、固体を構成する粒子が音の進行方向に動く縦波と、音の進行方向に対して垂直方向に振動する横波があり、更にこの横波には、固体表面に対して垂直な振動成分を有するＳＶ波と、固体表面に対して平行に振動するＳＨ波とがある。表面に沿って進行するＳＨ波を表面ＳＨ波という。
【０００３】
構造物の内部に存在する応力には、外部から加えられた負荷応力と、外部負荷を取り去ってもなお存在する残留応力とがある。残留応力は、製造工程における圧延や塑性変形、溶接、及び供用中の不等沈下や熱応力等により発生する。この残留応力と負荷応力が重畳することにより、設計以上の応力が加わり、変形や応力腐食割れ、疲労強度の低下や脆性破壊の原因となる。このため、残留応力の値を正しく評価することは、構造物の安全上非常に重要である。
【０００４】
橋梁、鉄管、鉄塔、タンク等の鋼構造物の応力を測定する方法として、鋼材内部の超音波の伝播速度が応力により変化する現象を利用して、鋼材表面に密着した超音波送信子から放射したパルス状の超音波を受信子により受信して、超音波の伝播時間を測定することにより応力を測定する音弾性測定法は、非破壊測定法であり、装置が比較的簡易であるため有効である。ところが音弾性測定では非常に高い音速測定精度が要求され、材料の製造工程で生ずる組織異方性を無視できない。そのため音弾性により応力を評価する場合、応力異方性と組織異方性の分離が問題となる。
【０００５】
この両異方性の分離を行う方法として、表面ＳＨ波を用いる方法が提案されている。この方法は原理的に、材料の組織異方性の影響を全く受けず、正確に残留応力が測定できるが、接触媒質や表面粗さ等の影響により、音速測定が困難であり、測定精度に問題がある。
【０００６】
固体内を伝播する超音波の音速が媒質の応力によって変化する現象を音弾性と言うが、表面ＳＨ波を互いに垂直な方向に伝播させて音響異方性を求めるのが表面ＳＨ波音弾性である。図１に示すように、表面ＳＨ波音弾性は、

で表される。ここでΦ_Ｓは音響異方性、Ｖ_Ｓ１と、７_Ｓ２は偏向方向がそれぞれＸ_１、Ｘ_２方向の主応力、μは剛性率、σ_１とσ_２はそれぞれＸ_１、Ｘ_２方向の主応力、σ_１−σ_２は主応力差である。
【０００７】
この音響異方性には、組織異方性が全く関係せず、応力異方性と一対一の関係にある。このため、残留応力を測定するための最も有効な手段である。なお、この測定で得られる応力は、送受信子間隔によって多少変化するが、略適用する超音波の波長程度の深さまでの表面近傍の平均主応力差である。
【０００８】
従来の表面ＳＨ波法による応力測定に用いられるセンサは、図２及び図３に示すように超音波を発信し鋼材内に放射する送信子と、鋼材内を伝播した超音波を受信する受信子を一対にして、両者を互に一定の距離を隔てて、互に斜め内側に向き合うように振動子を対設、固定したものが用いられる。送信子及び受信子の振動子としては、それぞれ水晶をＹカットした横波用圧電素子、ＰＺＴ圧電素子その他各種セラミックスよりなる圧電素子が用いられる。
【０００９】
図２及び図３に示すように、表面ＳＨ波用の送信子４及び受信子５の圧電素子１、８は、それぞれアクリル樹脂やポリスチレン樹脂等の合成樹脂よりなる楔２、９と合成樹脂等よりなる背面材３の間に挟着して、楔２及び楔９の底部を除き送信子４及び受信子５の背部の略全体を連結部材３_Ａに埋設して、送信子４又は受信子５を構成する。送信子４の圧電素子１の底面７に対する角度、即ち試験体６への超音波の入射角θは、鋼材に入射する超音波の屈折角が丁度９０度となる、臨界角に略等しい角度とし、又受信子５の圧電素子８の角度も同じ角度とするのが送・受信効率を上げるために必要である。
【００１０】
図４に示すように、送信子４の圧電素子１からアクリル樹脂やポリスチレン樹脂の楔２を介して、試験体６表面に対して平行で進行方向に垂直に振動する横波を入射すると、試験体６に入射された横波は表面ＳＨ波となり、圧電素子１の高さによるが、試験体６表面から約２０°の範囲の広がりをもって進行する横波として伝播する。
【００１１】
試験体６の表面近傍を伝播した表面ＳＨ波の一部は、受信子５の楔９の底面で屈折して、圧電素子８の方向に屈折伝播し、圧電素子８を振動させ、検出電圧を出力する。
【００１２】
上記従来の表面ＳＨ波法による応力測定において、測定の精度を上げるためには、送信子４と受信子５の中心間距離ｄを常に一定に保ち、且つ送信子４及び受信子５の底面７をそれぞれ試験体６表面に密着させる必要がある。このため送信子４及び受信子５を図２及び図３に示すように連結部材３_Ａに埋設して、送信子と受信子５の間隔を常に一定不変に保ち、且つ送信子４と受信子５の底面７を完全に平面状に研磨する。
【００１３】
このようにし一体に構成した送信子４及び受信子５の底面に、粘稠液よりなる接触媒質（音響結合剤）を薄く塗布して試験体６表面に密着させ、その接触媒質の薄層を介して超音波をできるだけ効率よく伝達させる。その際に送信子４及び受信子５の底面７と試験体６表面の間の面の平行精度が数μ以内になるように、試験体６表面を平面に研磨し、大きな力で送信子４及び受信子５よりなるセンサを試験体６表面に押し付ける必要がある。
【００１４】
普通鋼の設計許容応力は１２０ＭＰａ程度であるが、鋼構造物の応力測定には１０ＭＰａ程度の測定精度が必要である。しかし試験体６の表面と送信子４及び受信子５の底面７の平面度がよくないと、送信子４及び受信子５を３ｋｇｆ以上の力で試験体６表面に押し付けても、応力の測定精度は３０〜４０ＭＰａより悪くなってしまう。試験体６表面を平面に研磨するのは極めて困難であり、又センサ全体を試験体６表面に強い力で密着させるのも容易ではない。
【００１５】
そこで本願発明者らは、試験体６表面の研磨精度をあまり上げる必要がなく、比較的弱い押圧力でセンサを試験体６表面に押し付けるだけで、送信子４と受信子５の間の距離を変えることなく、センサの送信子４及び受信子５の底面７を試験体６の表面に密着させ、比較的高精度で応力測定をすることができるセンサとして、送信子４と受信子５を例えば断面凹字状の剛性の比較的大なる弾性板よりなる連結板１０の両側面にそれぞれ固着し、送信子４及び受信子５の底面７に応力を加えることにより、両底面のなす平面が僅かに撓むことができるようにすることにより、完全な平面に研磨されていない試験体６表面にでも、センサの送信子４及び受信子５の底面７を略完全に密着させることができ、その結果応力の測定精度を容易に向上させることができることを見出し、特願平５−９８８１０（特開平６−３１３７３９、特許第２５５５５２５号）に、図５に示すような、一対の圧電素子をそれぞれ合成樹脂等の楔上に接合し、その背面にそれぞれ背面部材を接合してなる送信子及び受信子を、互に一定の距離を隔てて、該圧電素子が互いに斜め内側に向くように対向して配設した表面ＳＨ波による音弾性応力測定用センサにおいて、該送信子及び受信子を適度の弾性を有する連結板１０により連結して、該送信子及び受信子の該楔の底面を試験体表面に押圧したときに、該連結板１０の僅かな変形により、送信子と受信子の間の距離を変えることなく、該底面が試験体表面に沿って密着するようにした、表面ＳＨ波による音弾性応力測定用センサを提案した。
【００１６】
しかし、上記弾性を有する連結板１０で連結したセンサを用いても、センサと試験体６表面の間に介在させる粘度の高い接触媒質の厚み等の塗布状態が異なると音弾性測定の測定精度を上げることができない。
【００１７】
【発明が解決しようとする課題】
従って本発明は表面ＳＨ波による音弾性応力測定用センサと試験体６の間に介在する接触媒質の影響を取り除くことができる表面ＳＨ波による音弾性応力測定方法及び測定用センサを提供することを目的とする。
【００１８】
【課題を解決するための手段】
上記目的を達成すべく、本発明らが鋭意研究を重ねた結果、２つの超音波送信子と４つの超音波受信子よりなるセンサを用い、４つの受信子を２個の受信子を互に対向して内向きに傾けて配設したもの２組を一体の合成樹脂製等の受信子用楔部材上に一列に配設し、その受信子用楔部材と分離して、超音波送信子を備えた送信子用楔部材２つ、受信子用楔部材の両側に、且つ２つの送信子と４つの受信子が同一直線上に位置するように配設し、２つの送信子からそれぞれ発射した超音波パルスをそれぞれ２つずつの受信子で受信することより、接触媒質の影響を取り除くことができることを見出し、本発明を完成するに到った。
【００１９】
すなわち、本発明は４個の超音波受信子を同一受信子用楔部材上に直線上に配設すると共に、該受信子用楔部材は、平面図上で該直線と同一直線上で互に一定距離を隔てて２個所に、試験体に接触媒質を介して密着する超音波受信用底面を有し、各該超音波受信用底面の略中央に対し超音波の臨界入射角θに略等しい角度になるように内側に向けて互いに対向する位置にそれぞれ一対の該送信子を該各底面の中央から同一距離の位置に配設し、該４個の受信子と平面図上で同一直線上で、且つ該受信子用楔部材の両側に、送信子用楔部材に固着した超音波送信子を臨界入射角θに略等しい角度で互に内側に向けて配設すると共に、該送信子用楔部材及び受信子用楔部材を試験体表面に接触媒質を介して密着させ、一方の該超音波送信子から超音波パルスを発射して試験体表面に表面ＳＨ波を侵入させ、該表面ＳＨ波を２個の受信子で受信して２個の受信子に該超音波パルスが到達するまでの時間差を求め、同様に他方の該超音波送信子から超音波パルスを発射して同様に２個の受信子に超音波パルスが到達するまでの時間差を求め、両該時間差を平均することにより、該受信子用楔部材の該底面と試験体の間に介在する接触媒質の影響を除去しつつ、試験体中の表面ＳＨ波の伝播時間を測定する、表面ＳＨ波による音弾性応力測定方法を要旨とする。
【００２０】
他の本発明は同一直線上に配設した４個の受信子と、平面図上で該直線と同一直線上で互に一定距離を隔てて下面の２個所に配設され、試験体に接触媒質を介して密着するさせるための超音波受信用底面を有する受信子用楔部材と、該受信子用楔部材の両側に配設するための、超音波送信子を有する２個の送信子用楔部材とからなり、該受信子は各該超音波受信用底面の略中央に対し超音波の入射角θに略等しい角度になるように内側に向けて互いに対向する位置にそれぞれ一対ずつ、該各底面の中央から同一距離の位置に配設され、該送信子は該送信子用楔部材の底面に対し入射角θに略等しい角度で配設した表面ＳＨ波による音弾性応力測定用センサ要旨とする。
【００２１】
【発明の実施の形態】
次に本発明の表面ＳＨ波による音弾性応力測定方法及び測定用センサを図面により詳細に説明する。図６は本発明の表面ＳＨ波による音弾性応力測定用センサの一例の正面図である。１１ａ、１１ｂは送信子、１２ａ１、１２ａ２、１２ｂ１、１２ｂ２はそれぞれ受信子であり、受信子用楔部材１３上に一列に配設されている。受信子用楔部材１３の超音波受信用の底面１４ａ、１４ｂの中心Ａ、Ｂから底面１４ａ、１４ｂに垂直に立てた垂線Ｉに対し、受信子１２ａ１と１２ｂ１及び受信子１２ａ２と１２ｂ２はそれぞれ、互いに内向き所定の入射角θだけ傾けて、対向して面対称形に配設されており、受信子１２ａ１、１２ａ２、１２ｂ１、１２ｂ２の中心から受信子の面に垂直に立てた中心線Ｊはそれぞれ底面１４ａ、１４ｂの中心Ａ、Ｂを通る位置に配設される。
【００２２】
送信子１１ａ、１１ｂはそれぞれ送信子用楔部材１５ａ、１５ｂ上に入射角θとなるように固定され、送信子用楔部材１５ａ、１５ｂは受信子用楔部材１３の両側に一定距離を隔てて配設され、送信子１１ａ、１１ｂ及び受信子１２ａ１、１２ａ２、１２ｂ１、１２ｂ２は平面図上で一直線上に並ぶように配置される。入射角θは試験体６や楔部材の材質により異なるが、鋼材の試験体６に対し、ポリスチレン樹脂の楔部材の場合は、鋼材に入射する超音波の屈折角が丁度９０度となる臨界角、２０．７°に一致させるのが好ましい。
【００２３】
送信子用楔部材１５ａ、１５ｂ及ひ受信子用楔部材１３の材質は特に制限はないが、例えばポリスチレン樹脂、アクリル樹脂等の合成樹脂製のものが好ましい。送信子１１ａ、１１ｂ及び受信子１２ａ１、１２ａ２、１２ｂ１、１２ｂ２には、例えばＰＺＴ横波用超音波送受信素子が用いられる。
【００２４】
送信子１１ａから発射されて、送信子用楔部材１５ａの底面１７ａから試験体６に入り、その表面近傍に沿って伝播した超音波の一部は受信子用楔部材１３の底面１４ａから受信子用楔部材１３に入り、受信子１２ａ１で受信され、その超音波の一部は底面１４ｂから受信子用楔部材１３に入り、受信子１２ａ２で受信される。一方送信子１１ｂから発射されて送信子用楔部材１５ｂの底面１７ｂから試験体に入射された超音波の一部は受信子用楔部材１３の底面１４ｂから受信子用楔部材１３に入り、受信子１２ｂ２で受信され、その超音波の一部は底面１４ａから受信子用楔部材１３に入り、受信子１２ｂ１で受信される。
【００２５】
受信子用楔部材１３の底面１４ａ、１４ｂと試験体６の間に介在する接触媒質１６中の伝播時間をそれぞれα、β、試験体６中のＡＢ間の伝播時間をΔＴとする。受信子用楔部材１３中の伝播時間を、Ａ点から受信子１２ａ１、１２ｂ１までの伝播時間をｔ_ａ１、ｔ_ｂ１、Ｂ点から受信子１２ａ２、１２ｂ２までの伝播時間をｔ_ａ２、ｔ_ｂ２とする。
【００２６】
送信子１１ａから発射した超音波を受信子１２ａ１、１２ａ２で受信するまでの試験体６のＡ、Ｂ間の伝播時間をΔＴ_ａとすると、
ΔＴ_ａ＝ｔ_ａ２＋β−（ｔ_ａ１＋α）　　　　　　　　　　　　　（３）
となる。
【００２７】
同様に、送信子１１ｂから発射した超音波を受信子１２ｂ１、１２ｂ２で受信するまでの試験体６のＡ、Ｂ間の伝播時間をΔＴ_ｂとすると、
ΔＴ_ｂ＝ｔ_ｂ１＋α−（ｔ_ｂ２＋β）　　　　　　　　　　　　　（４）
となる。
【００２８】
式（３）と式（４）の平均をとると、
ΔＴ＝（ΔＴ_ａ＋ΔＴ_ｂ）／２
＝（ｔ_ａ２−ｔ_ａ１＋ｔ_ｂ１−ｔ_ｂ２）／２　　　　　　　　（５）
となり、接触媒質１６中の伝播時間α、βを消去することができる。又、送信子用楔部材１５ａ、１５ｂと受信子用楔部材１３の間の距離も測定に影響を与えない。
【００２９】
上記の表面ＳＨ波による音弾性応力測定用センサにより、超音波の伝播時間△Ｔを正確に測定するには、超音波の送受信及び音速測定を公知のシングアラウンド法で行う。シングアラウンド法は、超音波の送信、受信を極めて短いサイクルで極めて多数回繰り返し、積算された伝播時間から平均伝播時間を求める方法である。例えば１０ＭＨｚのクロックカウンターを用いて１０^４回繰り返し測定を行うことにより、測定時間の分解能を１０ｐｓ〜１ｎｓ程度にまで高めることができる。図７にシングアラウンド装置の一例のブロック線図を示す。
【００３０】
【実施例１】
試験体６として軟鋼ＳＳ４００から切り出した、寸法５００×６０×１０ｍｍの短軸引張り試験片を用いた。試験片表面は研磨仕上げし、送受信子と試験片の接触状態を良くした。接触媒質１６としてソニー株式会社製のソニーコートＨＮ−３０を使用した。
【００３１】
図６に示す本発明のセンサとして、受信子間距離１２ｍｍのセンサを用いて、無応力状態で音速の測定精度を評価した。最初に１分間隔で１０分間測定し、時間経過による音速の変化を調べた。測定方向は圧延方向で、シングアラウンド周期が安定してから測定を開始した。この結果を図８に示す。測定精度は３２５８．９５±０．０４ｍ／ｓとなり、時間経過による音速の変化は極めて小さかった。
【００３２】
次に音速測定の再現性を調べた。測定方法は、センサを少し動かして接触状態を変化させた後、同じ位置に戻して測定した。この作業を１０回繰り返し行った。その結果を図９及び図１０に示す。圧延方向とこれに垂直方向の音速はほぼ同じであり、表面ＳＨ波音弾性では、組織異方性の影響を受けないことが確認された。また、接触媒質の影響が残っている状態で測定した時に、式（３）、式（４）で得られた伝播時間を用いた。その精度は、それぞれ３２５５．４７±０．３７ｍ／ｓ、３２６２．１７±０．４６ｍ／ｓとなり、式（５）を用いることにより、測定精度が大幅に向上することが分かる。このことから、接触媒質の影響をほぼ取り除くことができることを確認した。
【００３３】
【実施例２】
試験片に応力を負荷した状態で応力測定を行った。応力は０から１２０ＭＰａまで変化させ、荷重方向及びこれに垂直方向の測定を行った。この結果を図１１及び図１２に示す。圧延方向及び応力方向において、両方向共、音速と応力とが線形関係にあることが確認された。
【００３４】
【発明の効果】
本発明の表面ＳＨ波による音弾性応力測定用センサ、及びこれを用いる音弾性測定法によれば、センサと試験体の間に介在させる必要のある接触媒質の影響をほぼ完全に取り除くことができる。その結果、音速は約±０．１ｍ／ｓ以上の精度なり、受信子間距離が約１０ｍｍの小さいセンサで主応力差を５ＭＰａ以下の誤差範囲内で測定可能となり、従来法に比較して測定精度が大幅に改善される。
【図面の簡単な説明】
【図１】表面ＳＨ波による音弾性応力測定の概念図である。
【図２】従来の表面ＳＨ波による音弾性応力測定用センサの平面図である。
【図３】従来の表面ＳＨ波による音弾性応力測定用センサの正面図である。
【図４】送信子から放射された超音波の表面ＳＨ波の伝播を示す止面断面図である。
【図５】弾性板よりなるＵ字状連結板により送受信子を連結した従来の表面ＳＨ波による音弾性応力測定用センサの正面図。
【図６】本発明の表面ＳＨ波による音弾性応力測定用センサの一例の正面図である。
【図７】シングアラウンド装置の一例のブロック線図である。
【図８】本発明の表面ＳＨ波による音弾性応力測定方法における、時間経過による音速の変化を示すグラフである。
【図９】本発明の表面ＳＨ波による音弾性応力測定方法及により測定した、試験片の圧延方向の音速測定の再現性を示すグラフである。
【図１０】本発明の表面ＳＨ波による音弾性応力測定方法及により測定した、試験片の圧延方向に対し垂直方向の音速測定の再現性を示すグラフである。
【図１１】本発明の表面ＳＨ波による音弾性応力測定方法及により測定した、試験片の荷重方向の応力に対する音速の変化を示すグラフである。
【図１２】本発明の表面ＳＨ波による音弾性応力測定方法及により測定した、試験片の荷重方向に対し垂直方向の応力に対する音速の変化を示すグラフである。
【符号の説明】
１、８　　圧電素子
２、９　　楔
３　　背面部材
３_Ａ　連結部材
４　　送信子
５　　受信子
６　　試験体
７　　底面
１０　　連結板
１１ａ、１１ｂ　　送信子
１２ａ１、１２ａ２、１２ｂ１、１２ｂ２　　受信子
１３　　受信子用楔部材
１４ａ、１４ｂ　　底面
１５ａ、１５ｂ　　送信子用楔部材
１６　　接触媒質
１７ａ、１７ｂ　　底面[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for measuring elasto-elastic stress using surface SH waves and a stress measurement sensor having a transmitter and a receiver for ultrasonic waves used for stress measurement using surface SH waves.
[0002]
[Prior art]
Vibrations propagating in a solid include a longitudinal wave in which particles constituting the solid move in the traveling direction of the sound, and a transverse wave that oscillates in the direction perpendicular to the traveling direction of the sound. There are an SV wave having a vibration component perpendicular to the surface and an SH wave vibrating in parallel with the solid surface. SH waves traveling along the surface are called surface SH waves.
[0003]
The stress existing inside the structure includes a load stress applied from the outside and a residual stress that is still present even after the external load is removed. Residual stress is generated by rolling, plastic deformation, welding, uneven settlement, thermal stress, and the like during the production process. When the residual stress and the applied stress are superimposed, a stress higher than the design is applied, which causes deformation, stress corrosion cracking, reduction in fatigue strength, and brittle fracture. For this reason, correctly evaluating the value of the residual stress is very important for the safety of the structure.
[0004]
As a method of measuring the stress of steel structures such as bridges, iron pipes, steel towers, tanks, etc., a phenomenon in which the propagation speed of ultrasonic waves inside steel materials changes due to stress is used to radiate from ultrasonic transmitters that are in close contact with the steel surface. Is a non-destructive measurement method, which measures the stress by receiving the pulsed ultrasonic waves received by a receiver and measuring the propagation time of the ultrasonic waves. It is. However, very high acoustic velocity measurement accuracy is required in the acoustic elasticity measurement, and the structural anisotropy generated in the material manufacturing process cannot be ignored. Therefore, when stress is evaluated by acoustic elasticity, separation of stress anisotropy and tissue anisotropy becomes a problem.
[0005]
As a method for separating the two anisotropies, a method using a surface SH wave has been proposed. In principle, this method is completely unaffected by the structural anisotropy of the material and can accurately measure residual stress.However, due to the influence of the couplant and surface roughness, etc., it is difficult to measure the sound velocity, and the measurement accuracy is low. There's a problem.
[0006]
A phenomenon in which the speed of sound of an ultrasonic wave propagating in a solid changes due to the stress of a medium is called sound elasticity. Surface SH wave sound elasticity is a method in which surface SH waves are propagated in directions perpendicular to each other to determine acoustic anisotropy. . As shown in FIG. 1, the surface SH wave acoustic elasticity is

Is represented by Here, Φ _S is the acoustic anisotropy, V _S1 and 7 _S2 are the principal stresses in the deflection directions of X ₁ and X ₂ respectively, μ is the rigidity, and σ ₁ and σ ₂ are the X ₁ and X ₂ directions, respectively. The main stress, σ ₁ −σ _2, is the main stress difference.
[0007]
This acoustic anisotropy has no relation to the structural anisotropy at all, and has a one-to-one relationship with the stress anisotropy. Therefore, it is the most effective means for measuring the residual stress. Note that the stress obtained by this measurement is an average principal stress difference in the vicinity of the surface up to a depth of about the wavelength of the ultrasonic wave to be applied, although the stress slightly varies depending on the interval between the transmitter and the receiver.
[0008]
As shown in FIGS. 2 and 3, a sensor used for measuring stress by the conventional surface SH wave method includes a transmitter that emits ultrasonic waves and radiates it into steel, and a receiver that receives ultrasonic waves propagated through steel. And a pair of vibrators fixed to each other at a fixed distance from each other and facing each other diagonally inward is used. As the transducers of the transmitter and the receiver, a piezoelectric element for shear wave, a PZT piezoelectric element, and other ceramics made of various ceramics, each of which has a Y-cut crystal, are used.
[0009]
As shown in FIGS. 2 and 3, the

piezoelectric elements

1 and 8 of the transmitter 4 and the receiver 5 for the surface SH wave are respectively composed of

wedges

2 and 9 made of a synthetic resin such as an acrylic resin or a polystyrene resin and a synthetic resin or the like. and sandwiched between the backing member 3 to become more, by embedding substantially the entire back of the transmitters 4 and the receivers 5 except the bottom of the wedge 2 and the wedge 9 to the connecting member 3 _a, the transmitters 4 or the receivers 5 is constituted. The angle of the transmitter 4 with respect to the bottom surface 7 of the piezoelectric element 1, that is, the incident angle θ of the ultrasonic wave to the test piece 6 is an angle substantially equal to the critical angle at which the refraction angle of the ultrasonic wave incident on the steel material is just 90 degrees. In addition, it is necessary that the angle of the piezoelectric element 8 of the receiver 5 be the same as that of the piezoelectric element 8 in order to increase the transmission / reception efficiency.
[0010]
As shown in FIG. 4, when a transverse wave vibrating parallel to the surface of the test piece 6 and vertically oscillating in the traveling direction is incident on the surface of the test piece 6 from the piezoelectric element 1 of the transmitter 4 via the wedge 2 of an acrylic resin or a polystyrene resin, The transverse wave incident on 6 becomes a surface SH wave, and propagates as a transverse wave traveling from the surface of the test piece 6 with a spread of about 20 ° depending on the height of the piezoelectric element 1.
[0011]
A part of the surface SH wave that has propagated near the surface of the test body 6 is refracted at the bottom surface of the wedge 9 of the receiver 5 and refracted and propagates in the direction of the piezoelectric element 8, causing the piezoelectric element 8 to vibrate and reduce the detection voltage. Output.
[0012]
In the stress measurement by the conventional surface SH wave method, the distance d between the centers of the transmitter 4 and the receiver 5 is always kept constant, and the bottom surface 7 of the transmitter 4 and the receiver 5 is increased in order to improve the measurement accuracy. Must be brought into close contact with the surface of the test piece 6, respectively. Thus the the transmitters 4 and the receivers 5 are embedded in the connecting member 3 _A as shown in FIGS. 2 and 3, always keeping constant unchanged spacing the transmitters and the receivers 5, and the transmitters 4 and the receivers The bottom surface 7 of 5 is polished completely flat.
[0013]
A couplant (acoustic binder) made of a viscous liquid is thinly applied to the bottom surfaces of the transmitter 4 and the receiver 5 integrally formed in this manner, and closely adhered to the surface of the test piece 6, and a thin layer of the couplant is formed. To transmit the ultrasonic waves as efficiently as possible. At this time, the surface of the test piece 6 is polished to a flat surface so that the parallel accuracy of the plane between the bottom surface 7 of the transmitter 4 and the receiver 5 and the surface of the test piece 6 is within several μm. In addition, it is necessary to press the sensor including the sensor 5 and the receiver 5 against the surface of the test body 6.
[0014]
The allowable design stress of ordinary steel is about 120 MPa, but the stress measurement of a steel structure requires a measurement accuracy of about 10 MPa. However, if the flatness between the surface of the test piece 6 and the bottom surface 7 of the transmitter 4 and the receiver 5 is not good, even if the transmitter 4 and the receiver 5 are pressed against the surface of the test piece 6 with a force of 3 kgf or more, the stress is measured. Accuracy is worse than 30-40 MPa. It is extremely difficult to grind the surface of the test piece 6 to a flat surface, and it is also not easy to bring the entire sensor into close contact with the surface of the test piece 6 with a strong force.
[0015]
Therefore, the inventors of the present application do not need to increase the polishing accuracy of the surface of the test body 6 so much, and simply press the sensor against the surface of the test body 6 with a relatively weak pressing force, thereby reducing the distance between the transmitter 4 and the receiver 5. The transmitter 4 and the receiver 5 are, for example, a sensor that can closely measure the stress of the transmitter 4 and the receiver 5 with the bottom surface 7 of the sensor 4 and the surface of the test piece 6 without any change. A flat plate formed by the two bottom surfaces is slightly fixed by applying a stress to the bottom surfaces 7 of the transmitter 4 and the receiver 5 by fixing them to both side surfaces of a connecting plate 10 made of an elastic plate having a relatively large rigidity and having a concave cross section. By allowing the sensor to bend, the bottom surface 7 of the transmitter 4 and the receiver 5 of the sensor can be almost completely adhered even to the surface of the specimen 6 that has not been polished to a perfect plane. Resulting stress measurement accuracy is easily improved In Japanese Patent Application No. 5-98810 (Japanese Patent Application Laid-Open No. 6-313739, Japanese Patent No. 25555525), a pair of piezoelectric elements as shown in FIG. A transmitter and a receiver, each having a back member joined to its back surface, are spaced apart from each other by a fixed distance, and are arranged so as to face each other such that the piezoelectric elements face obliquely inward. In the stress measuring sensor, when the transmitter and the receiver are connected by a connecting plate 10 having a moderate elasticity and the bottom surfaces of the wedges of the transmitter and the receiver are pressed against the surface of the test piece, the connecting plate is A sensor for measuring acousto-elastic stress by surface SH wave was proposed in which the bottom surface was closely adhered along the surface of the test piece without changing the distance between the transmitter and the receiver by a slight deformation of No. 10.
[0016]
However, even when the sensor connected by the elastic connecting plate 10 is used, if the application state such as the thickness of the high-viscosity couplant interposed between the sensor and the surface of the test piece 6 is different, the measurement accuracy of the sound elasticity measurement may be reduced. I can't raise it.
[0017]
[Problems to be solved by the invention]
Accordingly, the present invention provides a method and a sensor for measuring a sound elastic stress by a surface SH wave, which can eliminate the influence of a couplant interposed between the sensor for measuring the sound elastic stress by the surface SH wave and the test piece 6. Aim.
[0018]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors have conducted intensive studies and as a result, using a sensor composed of two ultrasonic transmitters and four ultrasonic receivers, connecting four receivers to two receivers. Two sets, which are opposed and inclined inwardly, are arranged in a row on an integral receiver wedge member made of synthetic resin or the like, separated from the receiver wedge member, and separated from the ultrasonic transmitter. And two transmitters and four receivers are disposed on both sides of the transmitter wedge member and the receiver wedge member, and each of the two transmitters is fired from the two transmitters. The inventor has found that the influence of the couplant can be removed by receiving each of the two ultrasonic pulses with two receivers, and completed the present invention.
[0019]
That is, in the present invention, the four ultrasonic receivers are arranged on the same receiver wedge member in a straight line, and the receiver wedge members are mutually aligned on the same straight line in a plan view. At two places with a fixed distance, the ultrasonic receiving bottom surfaces are in close contact with the test piece via a couplant, and are substantially equal to the critical incident angle θ of ultrasonic waves with respect to the approximate center of each ultrasonic receiving bottom surface. A pair of the transmitters are disposed at the same distance from the center of each bottom surface at a position facing each other inward so as to form an angle, and are aligned with the four receivers on the same straight line in a plan view. And, on both sides of the receiver wedge member, ultrasonic transmitters fixed to the transmitter wedge member are arranged inwardly toward each other at an angle substantially equal to the critical incident angle θ, and The wedge member and the wedge member for the receiver are brought into close contact with the surface of the test piece via a couplant, and the ultrasonic wave is transmitted from one of the ultrasonic transmitters. A wave pulse is emitted to cause a surface SH wave to penetrate into the surface of the test specimen, the surface SH wave is received by two receivers, and a time difference until the ultrasonic pulse reaches the two receivers is determined. Similarly, an ultrasonic pulse is emitted from the other ultrasonic transmitter, a time difference until the ultrasonic pulse arrives at the two receivers is similarly obtained, and the time differences are averaged to obtain a time difference between the two ultrasonic waves. The gist of the present invention is a method for measuring acoustic elastic stress using surface SH waves, which measures the propagation time of surface SH waves in a specimen while eliminating the influence of a couplant interposed between the bottom surface of the wedge member and the specimen.
[0020]
According to another aspect of the present invention, four receivers are arranged on the same straight line, and two receivers are arranged on the lower surface of the lower surface of the same straight line and at a fixed distance from each other in a plan view so as to come into contact with the test piece. A wedge member for a receiver having a bottom surface for receiving ultrasonic waves for adhering through a medium, and two transmitters having an ultrasonic transmitter for disposing on both sides of the wedge members for the receiver. And a pair of wedge members, each pair of the receivers being inwardly facing each other so as to have an angle substantially equal to the incident angle θ of the ultrasonic wave with respect to substantially the center of each of the ultrasonic wave receiving bottom surfaces. A sensor for measuring acoustoelastic stress by surface SH waves disposed at the same distance from the center of each bottom surface and the transmitter disposed at an angle substantially equal to the incident angle θ with respect to the bottom surface of the transmitter wedge member. And
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, a method for measuring a viscoelastic stress using a surface SH wave and a sensor for measurement according to the present invention will be described in detail with reference to the drawings. FIG. 6 is a front view of an example of the sensor for measuring a viscoelastic stress by a surface SH wave according to the present invention. 11a and 11b are transmitters, and 12a1, 12a2, 12b1 and 12b2 are receivers, which are arranged in a row on the receiver wedge member 13. The receivers 12a1 and 12b1 and the receivers 12a2 and 12b2 are perpendicular to the perpendiculars I perpendicular to the bottoms 14a and 14b from the centers A and B of the ultrasonic receiving bottoms 14a and 14b of the receiver wedge member 13, respectively. They are arranged in a plane symmetrical shape facing each other, inwardly inclined by a predetermined angle of incidence θ, and a center line J perpendicular to the plane of the receiver from the center of the receivers 12a1, 12a2, 12b1, 12b2 is They are arranged at positions passing through the centers A and B of the bottom surfaces 14a and 14b, respectively.
[0022]
The transmitters 11a and 11b are fixed on the transmitter wedge members 15a and 15b, respectively, so as to have an incident angle θ, and the transmitter wedge members 15a and 15b are separated from the wedge member 13 by a predetermined distance on both sides thereof. The transmitters 11a, 11b and the receivers 12a1, 12a2, 12b1, 12b2 are arranged so as to be aligned on a plan view. The incident angle θ differs depending on the material of the test piece 6 and the wedge member. However, in the case of the steel test piece 6, in the case of a wedge member made of polystyrene resin, the critical angle at which the refraction angle of the ultrasonic wave incident on the steel material becomes just 90 degrees. , Preferably 20.7 °.
[0023]
The materials of the transmitter wedge members 15a and 15b and the receiver wedge member 13 are not particularly limited, but are preferably made of synthetic resin such as polystyrene resin and acrylic resin. As the transmitters 11a, 11b and the receivers 12a1, 12a2, 12b1, 12b2, for example, PZT transverse wave ultrasonic transmitting / receiving elements are used.
[0024]
A part of the ultrasonic wave emitted from the transmitter 11a, enters the test piece 6 from the bottom surface 17a of the transmitter wedge member 15a, and propagates along the vicinity of the surface thereof. The ultrasonic wave enters the wedge member 13 for reception, is received by the receiver 12a1, and a part of the ultrasonic wave enters the wedge member 13 for receiver from the bottom surface 14b and is received by the receiver 12a2. On the other hand, a part of the ultrasonic wave emitted from the transmitter 11b and incident on the test body from the bottom surface 17b of the transmitter wedge member 15b enters the receiver wedge member 13 from the bottom surface 14b of the receiver wedge member 13 and is received. A part of the ultrasonic wave is received by the receiver 12b2, enters the receiver wedge member 13 from the bottom surface 14a, and is received by the receiver 12b1.
[0025]
The propagation times in the couplant 16 interposed between the bottom surfaces 14a, 14b of the receiver wedge member 13 and the test piece 6 are α and β, respectively, and the propagation time between AB in the test piece 6 is ΔT. Propagation times in the receiver wedge member 13 are represented by t _a1 and t _b1 from the point A to the receivers 12a1 and 12b1, and propagation times from the point B to the receivers 12a2 and 12b2 are represented by t _a2 and t _b2 . I do.
[0026]
Assuming that the propagation time between A and B of the test body 6 until the ultrasonic waves emitted from the transmitter 11a are received by the receivers 12a1 and 12a2 is ΔT _a ,
ΔT _a = t _a2 + β− (t _a1 + α) (3)
It becomes.
[0027]
Similarly, A test body 6 of ultrasonic waves emitted from the transmitters 11b until reception at the receivers 12b1 and 12b2, the propagation time between B When [Delta] T _b,
ΔT _b = t _b1 + α− (t _b2 + β) (4)
It becomes.
[0028]
Taking the average of equations (3) and (4) gives
ΔT = (ΔT _a + ΔT _b ) / 2
= (T _a2 −t _a1 + t _b1 −t _b2 ) / 2 (5)
Thus, the propagation times α and β in the couplant 16 can be eliminated. Also, the distance between the transmitter wedge members 15a and 15b and the receiver wedge member 13 does not affect the measurement.
[0029]
In order to accurately measure the propagation time ΔT of the ultrasonic wave by the sensor for measuring the acoustic elastic stress by the surface SH wave, transmission and reception of the ultrasonic wave and measurement of the sound speed are performed by a known sing-around method. The sing-around method is a method in which transmission and reception of ultrasonic waves are repeated extremely many times in a very short cycle, and an average propagation time is obtained from the accumulated propagation times. For example by carrying out the 10 repeated ^four times measured using the 10MHz clock counter, the resolution of the measurement time can be increased to about 10Ps～1ns. FIG. 7 shows a block diagram of an example of the sing-around apparatus.
[0030]
Embodiment 1
A short-axis tensile test piece having a size of 500 × 60 × 10 mm cut out from mild steel SS400 was used as the test body 6. The surface of the test piece was polished to improve the contact between the transceiver and the test piece. Sony Coat HN-30 manufactured by Sony Corporation was used as the couplant 16.
[0031]
Using a sensor having a distance between receivers of 12 mm as the sensor of the present invention shown in FIG. 6, the measurement accuracy of the sound velocity was evaluated in a stress-free state. First, measurements were taken at one-minute intervals for 10 minutes, and the change in sound speed over time was examined. The measurement direction was the rolling direction, and the measurement was started after the sing-around cycle was stabilized. The result is shown in FIG. The measurement accuracy was 3258.95 ± 0.04 m / s, and the change in sound speed over time was extremely small.
[0032]
Next, the reproducibility of the sound velocity measurement was examined. The measurement method was such that the sensor was slightly moved to change the contact state, and then returned to the same position for measurement. This operation was repeated 10 times. The results are shown in FIGS. The sound velocity in the rolling direction and the sound velocity in the direction perpendicular thereto were almost the same, and it was confirmed that the surface SH wave acoustic elasticity was not affected by the structure anisotropy. When the measurement was performed in a state where the influence of the couplant remained, the propagation time obtained by the equations (3) and (4) was used. The accuracies are 3255.47 ± 0.37 m / s and 3262.17 ± 0.46 m / s, respectively, and it can be seen that the measurement accuracy is greatly improved by using equation (5). From this, it was confirmed that the influence of the couplant could be almost eliminated.
[0033]
Embodiment 2
The stress was measured while applying stress to the test piece. The stress was varied from 0 to 120 MPa, and measurements were made in the load direction and in the direction perpendicular thereto. The results are shown in FIGS. In the rolling direction and the stress direction, it was confirmed that the sound velocity and the stress had a linear relationship in both directions.
[0034]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the sensor for measuring a sound elasticity stress by a surface SH wave of the present invention and a sound elasticity measuring method using the same, it is possible to almost completely eliminate the influence of the couplant which needs to be interposed between the sensor and the test piece. . As a result, the speed of sound has an accuracy of about ± 0.1 m / s or more, and the main stress difference can be measured within an error range of 5 MPa or less with a small sensor having a distance between receivers of about 10 mm. Accuracy is greatly improved.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of a measurement of a sonoelastic stress by a surface SH wave.
FIG. 2 is a plan view of a conventional sensor for measuring acoustic elastic stress using surface SH waves.
FIG. 3 is a front view of a conventional sensor for measuring acoustic elastic stress using surface SH waves.
FIG. 4 is a sectional view showing a propagation of surface SH waves of ultrasonic waves radiated from a transmitter.
FIG. 5 is a front view of a conventional sensor for measuring sonic elastic stress by surface SH waves in which a transmitter and a receiver are connected by a U-shaped connecting plate made of an elastic plate.
FIG. 6 is a front view of an example of the sensor for measuring elasto-elastic stress by surface SH wave according to the present invention.
FIG. 7 is a block diagram of an example of a sing-around apparatus.
FIG. 8 is a graph showing a change in sound speed with the elapse of time in the method of measuring a sonic elastic stress by a surface SH wave according to the present invention.
FIG. 9 is a graph showing the reproducibility of the measurement of the sound velocity in the rolling direction of the test piece, measured by the method of measuring the acoustic elastic stress by the surface SH wave of the present invention.
FIG. 10 is a graph showing the reproducibility of sound velocity measurement in a direction perpendicular to the rolling direction of a test piece, measured by the method of measuring a sonic elastic stress by a surface SH wave according to the present invention.
FIG. 11 is a graph showing a change in sound speed with respect to a stress in a load direction of a test piece, measured by the method for measuring a sonic elastic stress by a surface SH wave according to the present invention.
FIG. 12 is a graph showing a change in sound speed with respect to a stress in a direction perpendicular to a load direction of a test piece, measured by the method for measuring a sonic elastic stress by a surface SH wave according to the present invention.
[Explanation of symbols]
1, 8

Piezoelectric element

2, 9 Wedge 3 Back member 3 _A connecting member 4 Transmitter 5 Receiver 6 Specimen 7 Bottom 10 Connecting plate 11a, 11b Transmitters 12a1, 12a2, 12b1, 12b2 Receiver 13 Wedge member for receiver 14a, 14b Bottom surface 15a, 15b Transmitter wedge member 16 Coupling material 17a, 17b Bottom surface

Claims

The four ultrasonic receivers are arranged linearly on the same receiver wedge member, and the receiver wedge members are spaced apart from each other on the same straight line as the top view in a plan view. At two points, the ultrasonic wave receiving bottom surface is in close contact with the test piece via the couplant, and the angle is substantially equal to the critical incident angle θ of the ultrasonic wave with respect to the substantially center of each ultrasonic wave receiving bottom surface. A pair of the transmitters are respectively disposed at positions facing away from each other at the same distance from the center of each bottom surface, and the transmitters are arranged on the same straight line as the four receivers in a plan view. On both sides of the transmitter wedge member, ultrasonic transmitters fixed to the transmitter wedge member are arranged inwardly toward each other at an angle substantially equal to the critical incident angle θ, and the transmitter wedge member and the receiver The wedge member is brought into close contact with the surface of the test piece via a couplant, and an ultrasonic pulse is emitted from one of the ultrasonic transmitters. The surface SH wave is made to penetrate the surface of the test object, the surface SH wave is received by two receivers, and the time difference until the ultrasonic pulse reaches the two receivers is determined. Similarly, the time difference until the ultrasonic pulse arrives at the two receivers by emitting the ultrasonic pulse from the sound wave transmitter is determined, and by averaging both the time differences, the bottom surface of the wedge member for the receiver is A method for measuring acoustoelastic stress using surface SH waves, which measures the propagation time of surface SH waves in a test object while eliminating the influence of a couplant interposed between the test objects.

2. The method according to claim 1, wherein the transmission and reception of the ultrasonic pulse are repeated very many times by a sing-around method.

Four receivers are arranged on the same straight line, and are arranged at two places on the lower surface of the receiver at a fixed distance from each other on the same straight line in a plan view. A wedge member for a receiver having an ultrasonic receiving bottom surface, and two wedge members for a transmitter having an ultrasonic transmitter for disposing on both sides of the wedge member for the receiver. A pair of the receivers are positioned at a position facing each other inward so as to have an angle substantially equal to the incident angle θ of the ultrasonic wave with respect to a substantially center of each of the ultrasonic receiving bottom surfaces, from the center of each of the bottom surfaces. A sensor for measuring acoustoelastic stress based on surface SH waves, which is disposed at the same distance, and wherein the transmitter is disposed at an angle substantially equal to the incident angle θ with respect to the bottom surface of the transmitter wedge member.

4. The sensor according to claim 3, wherein the incident angle θ is an incident angle substantially equal to a critical angle at which a refraction angle of the incident ultrasonic wave is 90 ° according to the material of the test sample.