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香蕉app免費視頻在線觀看: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. 振動及波的相關物理量
物體(ti)沿直線或(huo)曲(qu)線在(zai)某(mou)一(yi)(yi)平(ping)衡位(wei)置附(fu)近做周期性往復運動(dong)(dong)，稱(cheng)為(wei)(wei)機械振(zhen)(zhen)(zhen)動(dong)(dong)。日常(chang)生活(huo)中(zhong)隨處(chu)可見振(zhen)(zhen)(zhen)動(dong)(dong)現象，如鐘擺的往復運動(dong)(dong)、旗幟的擺動(dong)(dong)、聲帶的振(zhen)(zhen)(zhen)動(dong)(dong)等。常(chang)見的表(biao)征振(zhen)(zhen)(zhen)動(dong)(dong)的參(can)數有：波長(chang)，任(ren)一(yi)(yi)質(zhi)點(dian)經歷一(yi)(yi)次(ci)完整(zheng)振(zhen)(zhen)(zhen)動(dong)(dong)時(shi)波所(suo)傳(chuan)播(bo)的距(ju)離，通(tong)(tong)常(chang)用符號λ表(biao)示(shi)，單(dan)(dan)位(wei)為(wei)(wei)m（米）；波速(su)，波在(zai)單(dan)(dan)位(wei)時(shi)間內(nei)所(suo)傳(chuan)播(bo)的距(ju)離，通(tong)(tong)常(chang)用符號c表(biao)示(shi)，單(dan)(dan)位(wei)為(wei)(wei)m/s（米/秒）；頻率，質(zhi)點(dian)在(zai)1s內(nei)進行全振(zhen)(zhen)(zhen)動(dong)(dong)的次(ci)數，通(tong)(tong)常(chang)用f表(biao)示(shi)，單(dan)(dan)位(wei)為(wei)(wei)Hz（赫茲）；周期，質(zhi)點(dian)次(ci)全振(zhen)(zhen)(zhen)動(dong)(dong)所(suo)需的時(shi)間，通(tong)(tong)常(chang)用T表(biao)示(shi)，單(dan)(dan)位(wei)為(wei)(wei)s（秒）；振(zhen)(zhen)(zhen)幅，質(zhi)點(dian)離開(kai)平(ping)位(wei)置的最大距(ju)離，通(tong)(tong)常(chang)用A表(biao)示(shi)。
上述波(bo)長(chang)、波(bo)速(su)、頻率三者(zhe)之間關系為：λ= c/f 由上式(shi)可知(zhi)，頻率一(yi)(yi)定時(shi)，波(bo)速(su)越大，波(bo)長(chang)就越長(chang)；而波(bo)速(su)一(yi)(yi)定時(shi)，率越低，波(bo)長(chang)就越長(chang)。周期(qi)和頻率描述的(de)是振動的(de)快(kuai)慢，兩者(zhe)大小與周期(qi)互為倒數。振幅(fu)描述的(de)是振動的(de)強弱。
2. 機械(xie)波
振動(dong)在(zai)介(jie)質或空(kong)間(jian)中(zhong)的(de)(de)(de)傳(chuan)播過程叫作波(bo)(bo)(bo)動(dong)，簡稱(cheng)(cheng)波(bo)(bo)(bo)，可分為(wei)機(ji)(ji)械(xie)波(bo)(bo)(bo)與電磁波(bo)(bo)(bo)兩種(zhong)。本書中(zhong)僅(jin)涉及機(ji)(ji)械(xie)波(bo)(bo)(bo)。振動(dong)在(zai)彈性(xing)介(jie)質中(zhong)的(de)(de)(de)傳(chuan)播稱(cheng)(cheng)為(wei)機(ji)(ji)械(xie)波(bo)(bo)(bo)，如固體(ti)、液體(ti)、氣體(ti)均可視為(wei)彈性(xing)介(jie)質。機(ji)(ji)械(xie)波(bo)(bo)(bo)的(de)(de)(de)頻(pin)率范圍很(hen)寬，從(cong)10-4Hz到(dao)102Hz，跨(kua)越16個(ge)數(shu)量(liang)級其中(zhong)人類聽覺所能感知的(de)(de)(de)范圍在(zai)20~20000Hz。人們把能牽動(dong)聽覺神(shen)經的(de)(de)(de)機(ji)(ji)械(xie)波(bo)(bo)(bo)稱(cheng)(cheng)為(wei)聲波(bo)(bo)(bo)，當(dang)聲波(bo)(bo)(bo)頻(pin)率低(di)于20Hz時，稱(cheng)(cheng)為(wei)次聲波(bo)(bo)(bo)；當(dang)聲波(bo)(bo)(bo)頻(pin)率于20000Hz時，稱(cheng)(cheng)為(wei)超(chao)聲波(bo)(bo)(bo)。
為(wei)簡(jian)單(dan)闡述機械(xie)(xie)波是如何(he)產(chan)生和傳播的(de)(de)(de)(de)，建立如圖所彈(dan)性(xing)介(jie)質(zhi)模型。圖2-1中質(zhi)點(dian)(dian)(dian)間以小彈(dan)簧連接在(zai)一(yi)(yi)起(qi)，這(zhe)種質(zhi)點(dian)(dian)(dian)間以彈(dan)性(xing)力(li)(li)(li)連接在(zai)起(qi)的(de)(de)(de)(de)介(jie)質(zhi)稱為(wei)彈(dan)性(xing)介(jie)質(zhi)。外力(li)(li)(li)F首先對質(zhi)點(dian)(dian)(dian)A產(chan)生作用(yong)(yong)，A偏離均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)，此時附近(jin)質(zhi)點(dian)(dian)(dian)將對A產(chan)生力(li)(li)(li)的(de)(de)(de)(de)作用(yong)(yong)，迫(po)使(shi)(shi)其回(hui)到(dao)(dao)均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)。而當A回(hui)到(dao)(dao)均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)時，其他(ta)質(zhi)點(dian)(dian)(dian)對其不(bu)產(chan)生作用(yong)(yong)，但是由于慣性(xing)的(de)(de)(de)(de)存在(zai)，A具有一(yi)(yi)定的(de)(de)(de)(de)速度，不(bu)會(hui)停(ting)留在(zai)均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)，反(fan)而繼續向前方(fang)運(yun)動，并沿相反(fan)方(fang)向離開均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)。一(yi)(yi)旦(dan)離開均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)，A又會(hui)受到(dao)(dao)附近(jin)質(zhi)點(dian)(dian)(dian)對它相反(fan)的(de)(de)(de)(de)彈(dan)性(xing)力(li)(li)(li)，使(shi)(shi)A再次回(hui)到(dao)(dao)均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)。導致質(zhi)點(dian)(dian)(dian)A在(zai)均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)來回(hui)往復產(chan)生振動。與此同時，A附近(jin)的(de)(de)(de)(de)質(zhi)點(dian)(dian)(dian)也會(hui)受到(dao)(dao)同樣力(li)(li)(li)的(de)(de)(de)(de)作用(yong)(yong)使(shi)(shi)它們(men)離開均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)，并在(zai)各自的(de)(de)(de)(de)均(jun)(jun)(jun)(jun)衡(heng)點(dian)(dian)(dian)附近(jin)運(yun)動。這(zhe)樣彈(dan)性(xing)介(jie)質(zhi)中單(dan)一(yi)(yi)質(zhi)點(dian)(dian)(dian)的(de)(de)(de)(de)振動引起(qi)鄰近(jin)質(zhi)點(dian)(dian)(dian)的(de)(de)(de)(de)振動，鄰近(jin)質(zhi)點(dian)(dian)(dian)的(de)(de)(de)(de)振動又會(hui)引起(qi)更遠(yuan)質(zhi)點(dian)(dian)(dian)的(de)(de)(de)(de)振動最(zui)終由近(jin)及(ji)遠(yuan)地向各個方(fang)向傳播，從而形成機械(xie)(xie)波。
超(chao)聲波是一種以波動(dong)(dong)(dong)形式在介(jie)質(zhi)(zhi)中(zhong)傳(chuan)播的(de)(de)(de)(de)機(ji)(ji)械(xie)振(zhen)(zhen)動(dong)(dong)(dong)，提到了機(ji)(ji)械(xie)波傳(chuan)播所必需的(de)(de)(de)(de)兩(liang)個條(tiao)件：做機(ji)(ji)械(xie)振(zhen)(zhen)動(dong)(dong)(dong)的(de)(de)(de)(de)波源和能夠傳(chuan)播機(ji)(ji)械(xie)振(zhen)(zhen)動(dong)(dong)(dong)的(de)(de)(de)(de)介(jie)質(zhi)(zhi)。振(zhen)(zhen)動(dong)(dong)(dong)與波動(dong)(dong)(dong)是相互關聯的(de)(de)(de)(de)，振(zhen)(zhen)動(dong)(dong)(dong)產生(sheng)波動(dong)(dong)(dong)，而波動(dong)(dong)(dong)是振(zhen)(zhen)動(dong)(dong)(dong)的(de)(de)(de)(de)傳(chuan)播。波動(dong)(dong)(dong)時介(jie)質(zhi)(zhi)中(zhong)的(de)(de)(de)(de)質(zhi)(zhi)點(dian)會在各自(zi)的(de)(de)(de)(de)平衡位(wei)置(zhi)附近做往復運動(dong)(dong)(dong)，并不隨(sui)波前(qian)進，在波動(dong)(dong)(dong)中(zhong)由(you)各質(zhi)(zhi)點(dian)的(de)(de)(de)(de)位(wei)移連續變化將振(zhen)(zhen)動(dong)(dong)(dong)能量傳(chuan)播出去。
