【正文】 向和形狀時(shí)，常使用這種掃查方法。但當(dāng)探頭靠近凸起的焊道發(fā)現(xiàn)缺陷波時(shí)，用此法估判缺陷形狀是困難的。使用兩種方法也可估判缺陷的形狀?？梢园l(fā)現(xiàn)焊縫或熱影響區(qū)的橫向缺陷(如裂紋等)。(3) 雙探頭掃查雙探頭掃查是用兩個(gè)探頭進(jìn)行探傷，其中一個(gè)用于發(fā)射超聲波，另一個(gè)用于接收回波，根據(jù)兩個(gè)探頭不同位置可分為以下幾種：探頭分別置于焊縫的兩側(cè)或一側(cè)，以便發(fā)現(xiàn)焊縫的橫向或縱向缺陷。它是利用頻率超過(guò)20KHz的高頻聲束在試件中與試件內(nèi)部缺陷（如裂縫、氣孔、夾渣等）中傳播的特性，來(lái)判定是否存在缺陷及其尺度的一種無(wú)損檢測(cè)技術(shù)。當(dāng)發(fā)現(xiàn)焊縫中存在缺陷之后，根據(jù)探頭在試件上的位置以及缺陷回波在顯示屏上的高度，就可確定出焊縫的缺陷位置和大小。探頭折射角的選擇應(yīng)使聲束能掃查到焊縫的整個(gè)截面，能使聲束中心線盡可能與主要危險(xiǎn)性缺陷面垂直。探測(cè)面的修整寬度B應(yīng)根據(jù)板厚t和探頭的斜率K計(jì)算確定。前后移動(dòng)探頭，使所獲得的反射回波最高。K值與入射點(diǎn)等參數(shù)的準(zhǔn)確性對(duì)缺陷定位精度影響很大，其標(biāo)稱值也因制造、磨損等原因與實(shí)際值往往存在差異，因此需在使用前和使用中經(jīng)常測(cè)定。掃描速度的調(diào)整則是與零點(diǎn)校正同時(shí)進(jìn)行的，可使定位更為直接。根據(jù)測(cè)量范圍的要求，使某兩個(gè)回波分別對(duì)準(zhǔn)熒光屏上各自的相應(yīng)刻度，則熒光屏上多標(biāo)尺零點(diǎn)即對(duì)應(yīng)于探頭入射點(diǎn)。由于水平距離L與聲程s的關(guān)系是：, (β=arctanK)故可利用CSKZB試塊上R50和R100兩個(gè)圓弧面的反射進(jìn)行調(diào)整，此時(shí)：將斜探頭對(duì)準(zhǔn)R50、R100，調(diào)整儀器使其回波BB2分別對(duì)準(zhǔn)基線刻度、即可。(6) 距離一波幅(DAC)曲線的繪制由于相同大小的缺陷因聲程不同，回波幅度也不相同。2)依據(jù)工件厚度和曲率選擇合適的對(duì)比試塊，在試塊上所有孔深小于等于探測(cè)深度的孔深中，選取能產(chǎn)生最大反射波幅的橫孔為第一基準(zhǔn)孔。圖43 距離一波幅(DAC)曲線的范圍5)依據(jù)表41規(guī)定的各線靈敏度，在基準(zhǔn)線下分別繪出判廢線、定量線、評(píng)定線，并標(biāo)記波幅的分區(qū)(如圖44)。探傷靈敏度不應(yīng)低于評(píng)定線靈敏度。探頭前后移動(dòng)的范圍應(yīng)保證掃查到全部焊縫截面及熱影響區(qū)。通過(guò)左右掃查測(cè)定缺陷指示長(zhǎng)度；通過(guò)前后掃查并結(jié)合左右掃查找出缺陷的最高回波；通過(guò)定點(diǎn)轉(zhuǎn)動(dòng)和環(huán)繞運(yùn)動(dòng)推斷缺陷的形狀和缺陷性質(zhì)。缺陷指示長(zhǎng)度△l的測(cè)定采用1/2波高法。最大反射波幅位于Ⅱ區(qū)的缺陷，其指示長(zhǎng)度小于10mm時(shí)按5mm計(jì)。最大反射波幅不超過(guò)評(píng)定線的缺陷，均評(píng)為Ⅰ級(jí)；最大反射波幅超過(guò)評(píng)定線的缺陷，檢驗(yàn)者判定為裂紋、未焊透等危險(xiǎn)性缺陷時(shí)，無(wú)論其波幅和長(zhǎng)度如何，均評(píng)定為Ⅳ級(jí)；反射波幅位于I區(qū)的非裂紋性缺陷，均評(píng)為I級(jí)；反射波幅位于Ⅲ區(qū)的缺陷，無(wú)論其指示長(zhǎng)度如何，均評(píng)定為Ⅳ級(jí)。同一條焊縫一般允許連續(xù)返修補(bǔ)焊2次。前者包括當(dāng)量試塊直接比較法、底波高度百分比法、當(dāng)量計(jì)算法和AVG曲線法。由于入射角的不同，使折射后的橫波聲場(chǎng)發(fā)生崎變的程度亦不同。此時(shí)橫波聲場(chǎng)的面積和距入射點(diǎn)的距離都將發(fā)生相應(yīng)的變化。與原有聲源至入射點(diǎn)的距離L之間的關(guān)系有兩個(gè)因素需要考慮：1)隨入射角增大，假想聲源的短軸直徑縮短，有效面積減小，導(dǎo)致折射后橫波聲場(chǎng)近場(chǎng)長(zhǎng)度的縮短，因而假想聲源至入射點(diǎn)距離L39。綜上所述，假想聲源至入射點(diǎn)的距離L39。DS作為歸一化后的橫坐標(biāo)A，取作為歸一化后的當(dāng)量曲線的參數(shù)G。根據(jù)繞射理論，當(dāng)反射體的直徑φ與波長(zhǎng)λt較接近時(shí)，由于既有繞射，又有反射，回波高度的變化將是非單調(diào)性的，但實(shí)際檢測(cè)時(shí)又看不到回波的波動(dòng)情況，這是由于所用探頭具有相對(duì)較寬的頗譜范圍，它足以同時(shí)覆蓋這種波動(dòng)的最大值和最小值。對(duì)于本文，我們介紹最大lx（相對(duì)靈敏度）法中的半波高度法（lx=6分貝）。注意在儀器的使用過(guò)程中若發(fā)現(xiàn)儀器工作不穩(wěn)定，則應(yīng)在現(xiàn)場(chǎng)工作1~2小時(shí)后，一定要重新調(diào)整時(shí)間掃描線和靈敏度，尤其是在確定缺陷大小時(shí)。 2）按通道|設(shè)置進(jìn)入如下對(duì)話框：1. 探頭類型：斜探頭2. 探頭頻率：3. 晶片尺寸：13*134. 折射角：25. 工件聲速：3230 m/s6. 探頭前沿：輸入探頭原始參數(shù)(如上)按確認(rèn)生效。測(cè)零點(diǎn)聲速完成。制作過(guò)程如圖56a、66b、66c、66d所示圖55 RB2制作DAC曲線示意圖(a)(b)(c)(d)圖 56 制作DAC曲線波形圖(4) 其他參數(shù)輸入 按設(shè)置鍵，將所測(cè)前沿值輸入，按確認(rèn)鍵。2）缺陷II缺陷II處存在兩處缺陷反射波如圖59所示圖 59 缺陷II反射波缺陷II定位：由探傷儀可讀取缺陷1水平位置35mm，；缺陷2水平位置45mm。4) 沿缺陷方向左右移動(dòng)探頭，缺陷波降至基準(zhǔn)波高時(shí)，探頭之間的距離即為缺陷的指示長(zhǎng)度。 2）相鄰兩缺陷在一直線上，其間距小于其中較小的缺陷長(zhǎng)度時(shí)，應(yīng)作為一條缺陷處理，以兩缺陷長(zhǎng)度之和作為其指示長(zhǎng)度。表52 缺陷級(jí)別評(píng)定缺陷IIIIII級(jí)別IVIVIV 由表52可知，板材共存在三處缺陷，等級(jí)均為IV，不符合合格級(jí)別，應(yīng)進(jìn)行焊縫返修。針對(duì)這些實(shí)際問(wèn)題，在指導(dǎo)老師的悉心指導(dǎo)下，在前人成果的基礎(chǔ)上，本文對(duì)焊接缺陷的超聲波探傷技術(shù)進(jìn)行了詳細(xì)介紹，并通過(guò)鋼板焊縫的超聲波探傷實(shí)驗(yàn)詳細(xì)講述了超聲波探傷的操作步驟、注意事項(xiàng)和等級(jí)評(píng)定標(biāo)準(zhǔn)。焊縫超聲波檢測(cè)作為檢驗(yàn)焊縫質(zhì)量的一種有效方法，其檢測(cè)的可靠性和有效性還待進(jìn)一步完善。值此論文完成之際，謹(jǐn)向胡老師及過(guò)控系各位老師致以最崇高的敬意和衷心的感謝！很慶幸自己在求學(xué)階段遇到這樣好的老師，師恩難忘。 最后，向?qū)忛啽疚牡睦蠋熤乱陨钌畹木匆猓⒃僖淮握嬲\(chéng)的感謝所有關(guān)心、幫助過(guò)我的老師、同學(xué)和朋友們! 在此，謹(jǐn)向你們致以最崇高的敬禮，謝謝！附錄英文文獻(xiàn) ULTRASONIC PRINCIPLES is Ultrasound?Sound generated above the human hearing range (typically 20KHz) is called ultrasound. However, the frequency range normally employed in ultrasonic nondestructive testing and thickness gaging is 100KHz to 50MHz. Although ultrasound behaves in a similar manner to audible sound, it has a much shorter wavelength. This means it can be reflected off very small surfaces such as defects inside materials. It is this property that makes ultrasound useful for nondestructive testing of materials.The Acoustic Spectrum in Figure (1) breaks down sound into 3 ranges of frequencies. The Ultrasonic Range is then broken down further into 3 sub sections.b. Frequency, Period and WavelengthUltrasonic vibrations travel in the form of a wave, similar to the way light travels. However, unlike light waves, which can travel in a vacuum (empty space), ultrasound requires an elastic medium such as a liquid or a solid. Shown in Figure (2) are the basic parameters of a continuous wave (cw). These parameters include the wavelength (235。 dB=20log10(A1/A2)dB=DecibelsA1=Amplitude of signal 1A2=Amplitude of signal 2 and Resolution●Sensitivity is the ability of an ultrasonic system to detect reflectors (or defects)at a given depth in a test greater the signa that is received from thesereflectors,the more sensitive the transducer system.●Axial resolution is the ability of an ultrasonic system to produce simultaneous and distinct indications from reflectors Iocated at nearly the same position with respect to the sound beam.●Near surface resolution is the ability of the ultrasonic system to detect reflectors located close to the surface of the test piece. DEFINITIONS AND FORMULAS waveform and spectrumTransducer waveform and spectrum analysis is done according to testconditions and definitions of ASTM units are MHz for frequency analysis,microseconds for waveform analysis,and dB down from peak (4)illustrates waveform duration at the 14dB level or 20%amplitude of waveform duration corresponds to 1%amplitude of (5)illustrates peak frequency,upper and lower6dB frequencies and MHz bandwidth relation between MHz bandwidth and waveform duration is shown in Figure(6).The scatter is wider at40dB because the 1%trailing end of the waveform contains very little energy and so has very little effect on the analysis of of the scatter it is most appropriate to specify waveforms in the time domain (microseconds)and spectrums in the frequency domain.The approximate relations shown in Figure(6)can be used to assist in transducer example,if a14dB waveform duration of one microsecond is needed,what frequency transducer should be selected?From the graph,a bandwidth of approximately 1 to corresponds to approximately 1 microsecond14dB waveform a nominal 50%fractional bandwidth transducer, this calculates to a nominal center frequency of 2 to ,a transducer of or may be applicable. Impedance,Reflectivity, and AttenuationThe acoustic impedance of a material is the opposition to displacement of its particles by sound and occurs in many impedance is calculated as follows: Z=ρcZ=Acoustic Impedancec=Material Sound Velocityρ=Material DensityThe boundary betweeen two materials of different acoustic impedances is called an acoustic sound strikes an acoustic interface at normal incidence,some amount of sound energy is reflected and some amount is transmitted acro