超聲波是由機械振動引起、在介質中沿一定方向傳播的一種機械波針對在線亞洲日產一區二區:超聲波自身特性和在各類常見介質中傳播時的特點進行基本把握,有助于我們正確應用超聲探傷技術、解決實際檢測過程中的各種問題。在超聲探傷中,常常涉及部分幾何聲學的反射、折射定律及波型轉換,以及物理聲學中波的疊加、干涉和衍射等規律或概念。本章主要介紹不銹鋼管缺陷探傷中的常用無損探傷技術、超聲波物理特性以及探傷規范相關基礎內容。
1. 振動及波的(de)相關物理(li)量
物體(ti)沿(yan)直線或曲線在某一(yi)平衡位(wei)(wei)置(zhi)附(fu)近做周期(qi)性往復運動(dong)(dong)(dong),稱為(wei)機械振動(dong)(dong)(dong)。日常(chang)生活中隨(sui)處可見振動(dong)(dong)(dong)現(xian)象,如(ru)鐘擺(bai)的(de)(de)(de)往復運動(dong)(dong)(dong)、旗幟的(de)(de)(de)擺(bai)動(dong)(dong)(dong)、聲帶的(de)(de)(de)振動(dong)(dong)(dong)等。常(chang)見的(de)(de)(de)表征振動(dong)(dong)(dong)的(de)(de)(de)參數有:波(bo)(bo)(bo)長,任一(yi)質點(dian)經歷(li)一(yi)次(ci)完整振動(dong)(dong)(dong)時(shi)波(bo)(bo)(bo)所(suo)(suo)傳(chuan)播的(de)(de)(de)距離,通(tong)常(chang)用符號λ表示,單位(wei)(wei)為(wei)m(米);波(bo)(bo)(bo)速,波(bo)(bo)(bo)在單位(wei)(wei)時(shi)間(jian)內所(suo)(suo)傳(chuan)播的(de)(de)(de)距離,通(tong)常(chang)用符號c表示,單位(wei)(wei)為(wei)m/s(米/秒(miao));頻率,質點(dian)在1s內進行全振動(dong)(dong)(dong)的(de)(de)(de)次(ci)數,通(tong)常(chang)用f表示,單位(wei)(wei)為(wei)Hz(赫茲);周期(qi),質點(dian)次(ci)全振動(dong)(dong)(dong)所(suo)(suo)需(xu)的(de)(de)(de)時(shi)間(jian),通(tong)常(chang)用T表示,單位(wei)(wei)為(wei)s(秒(miao));振幅,質點(dian)離開(kai)平位(wei)(wei)置(zhi)的(de)(de)(de)最(zui)大距離,通(tong)常(chang)用A表示。
上(shang)述波(bo)長(chang)(chang)、波(bo)速、頻率三(san)者之間關(guan)系為:λ= c/f 由上(shang)式可知(zhi),頻率一定時(shi),波(bo)速越(yue)(yue)(yue)大(da)(da),波(bo)長(chang)(chang)就越(yue)(yue)(yue)長(chang)(chang);而(er)波(bo)速一定時(shi),率越(yue)(yue)(yue)低,波(bo)長(chang)(chang)就越(yue)(yue)(yue)長(chang)(chang)。周(zhou)期(qi)和頻率描述的(de)(de)(de)是(shi)振動(dong)的(de)(de)(de)快(kuai)慢,兩者大(da)(da)小與周(zhou)期(qi)互(hu)為倒數。振幅描述的(de)(de)(de)是(shi)振動(dong)的(de)(de)(de)強弱。
2. 機械波
振動在介(jie)質(zhi)或空間(jian)中(zhong)(zhong)的(de)(de)(de)傳播過程叫作(zuo)波動,簡稱波,可分(fen)為(wei)(wei)(wei)機械波與(yu)電磁波兩種。本(ben)書中(zhong)(zhong)僅涉(she)及機械波。振動在彈(dan)性(xing)介(jie)質(zhi)中(zhong)(zhong)的(de)(de)(de)傳播稱為(wei)(wei)(wei)機械波,如固體(ti)、液體(ti)、氣體(ti)均可視為(wei)(wei)(wei)彈(dan)性(xing)介(jie)質(zhi)。機械波的(de)(de)(de)頻率范(fan)圍很寬(kuan),從10-4Hz到102Hz,跨(kua)越16個數(shu)量級其(qi)中(zhong)(zhong)人類聽覺所能感知的(de)(de)(de)范(fan)圍在20~20000Hz。人們把能牽動聽覺神(shen)經的(de)(de)(de)機械波稱為(wei)(wei)(wei)聲波,當聲波頻率低于20Hz時,稱為(wei)(wei)(wei)次(ci)聲波;當聲波頻率于20000Hz時,稱為(wei)(wei)(wei)超聲波。
為簡單闡述機械波是如何(he)產(chan)(chan)生(sheng)(sheng)和傳(chuan)(chuan)播的(de)(de)(de),建立如圖所彈(dan)(dan)性(xing)介(jie)質(zhi)(zhi)模型。圖2-1中質(zhi)(zhi)點(dian)(dian)(dian)(dian)間以(yi)小彈(dan)(dan)簧連接在一(yi)起(qi),這種(zhong)質(zhi)(zhi)點(dian)(dian)(dian)(dian)間以(yi)彈(dan)(dan)性(xing)力(li)連接在起(qi)的(de)(de)(de)介(jie)質(zhi)(zhi)稱為彈(dan)(dan)性(xing)介(jie)質(zhi)(zhi)。外力(li)F首先對(dui)質(zhi)(zhi)點(dian)(dian)(dian)(dian)A產(chan)(chan)生(sheng)(sheng)作(zuo)用,A偏(pian)離均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian),此時(shi)附(fu)(fu)近(jin)(jin)(jin)質(zhi)(zhi)點(dian)(dian)(dian)(dian)將(jiang)對(dui)A產(chan)(chan)生(sheng)(sheng)力(li)的(de)(de)(de)作(zuo)用,迫使其回(hui)到均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian)。而(er)當A回(hui)到均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian)時(shi),其他質(zhi)(zhi)點(dian)(dian)(dian)(dian)對(dui)其不(bu)產(chan)(chan)生(sheng)(sheng)作(zuo)用,但是由(you)于慣性(xing)的(de)(de)(de)存在,A具(ju)有一(yi)定(ding)的(de)(de)(de)速度,不(bu)會(hui)(hui)停(ting)留在均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian),反而(er)繼續向(xiang)(xiang)前(qian)方(fang)運動(dong),并(bing)沿(yan)相反方(fang)向(xiang)(xiang)離開(kai)均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian)。一(yi)旦(dan)離開(kai)均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian),A又會(hui)(hui)受(shou)(shou)到附(fu)(fu)近(jin)(jin)(jin)質(zhi)(zhi)點(dian)(dian)(dian)(dian)對(dui)它(ta)相反的(de)(de)(de)彈(dan)(dan)性(xing)力(li),使A再次回(hui)到均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian)。導致質(zhi)(zhi)點(dian)(dian)(dian)(dian)A在均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian)來回(hui)往復產(chan)(chan)生(sheng)(sheng)振(zhen)動(dong)。與此同時(shi),A附(fu)(fu)近(jin)(jin)(jin)的(de)(de)(de)質(zhi)(zhi)點(dian)(dian)(dian)(dian)也會(hui)(hui)受(shou)(shou)到同樣力(li)的(de)(de)(de)作(zuo)用使它(ta)們離開(kai)均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian),并(bing)在各(ge)自的(de)(de)(de)均(jun)(jun)(jun)衡點(dian)(dian)(dian)(dian)附(fu)(fu)近(jin)(jin)(jin)運動(dong)。這樣彈(dan)(dan)性(xing)介(jie)質(zhi)(zhi)中單一(yi)質(zhi)(zhi)點(dian)(dian)(dian)(dian)的(de)(de)(de)振(zhen)動(dong)引(yin)起(qi)鄰(lin)近(jin)(jin)(jin)質(zhi)(zhi)點(dian)(dian)(dian)(dian)的(de)(de)(de)振(zhen)動(dong),鄰(lin)近(jin)(jin)(jin)質(zhi)(zhi)點(dian)(dian)(dian)(dian)的(de)(de)(de)振(zhen)動(dong)又會(hui)(hui)引(yin)起(qi)更遠(yuan)質(zhi)(zhi)點(dian)(dian)(dian)(dian)的(de)(de)(de)振(zhen)動(dong)最終(zhong)由(you)近(jin)(jin)(jin)及(ji)遠(yuan)地向(xiang)(xiang)各(ge)個方(fang)向(xiang)(xiang)傳(chuan)(chuan)播,從(cong)而(er)形成機械波。
超聲波(bo)(bo)(bo)是一種(zhong)以波(bo)(bo)(bo)動(dong)(dong)(dong)(dong)(dong)形(xing)式在(zai)介質中(zhong)傳播(bo)的(de)(de)機(ji)(ji)(ji)械振(zhen)(zhen)動(dong)(dong)(dong)(dong)(dong),提到了機(ji)(ji)(ji)械波(bo)(bo)(bo)傳播(bo)所必需(xu)的(de)(de)兩(liang)個(ge)條件:做(zuo)機(ji)(ji)(ji)械振(zhen)(zhen)動(dong)(dong)(dong)(dong)(dong)的(de)(de)波(bo)(bo)(bo)源和(he)能夠傳播(bo)機(ji)(ji)(ji)械振(zhen)(zhen)動(dong)(dong)(dong)(dong)(dong)的(de)(de)介質。振(zhen)(zhen)動(dong)(dong)(dong)(dong)(dong)與波(bo)(bo)(bo)動(dong)(dong)(dong)(dong)(dong)是相(xiang)互關聯的(de)(de),振(zhen)(zhen)動(dong)(dong)(dong)(dong)(dong)產生(sheng)波(bo)(bo)(bo)動(dong)(dong)(dong)(dong)(dong),而波(bo)(bo)(bo)動(dong)(dong)(dong)(dong)(dong)是振(zhen)(zhen)動(dong)(dong)(dong)(dong)(dong)的(de)(de)傳播(bo)。波(bo)(bo)(bo)動(dong)(dong)(dong)(dong)(dong)時介質中(zhong)的(de)(de)質點(dian)會在(zai)各自(zi)的(de)(de)平衡位(wei)(wei)置附近(jin)做(zuo)往復運動(dong)(dong)(dong)(dong)(dong),并不隨波(bo)(bo)(bo)前進,在(zai)波(bo)(bo)(bo)動(dong)(dong)(dong)(dong)(dong)中(zhong)由(you)各質點(dian)的(de)(de)位(wei)(wei)移(yi)連(lian)續變化將振(zhen)(zhen)動(dong)(dong)(dong)(dong)(dong)能量傳播(bo)出(chu)去。