【正文】 和作為其指示長度，即33mm。表52 缺陷級別評定缺陷IIIIII級別IVIVIV 由表52可知，板材共存在三處缺陷，等級均為IV，不符合合格級別，應進行焊縫返修。但是，我們也應看到，針對我國當前的實際情況，手動人工超聲波探傷仍是主要的探傷方法，且應用依然相當廣泛。針對這些實際問題，在指導老師的悉心指導下，在前人成果的基礎上，本文對焊接缺陷的超聲波探傷技術進行了詳細介紹，并通過鋼板焊縫的超聲波探傷實驗詳細講述了超聲波探傷的操作步驟、注意事項和等級評定標準。2. 詳細講述了超聲波探傷技術的原理、分類、評定等級和評定標準。焊縫超聲波檢測作為檢驗焊縫質量的一種有效方法，其檢測的可靠性和有效性還待進一步完善。我們堅信，隨著研究工作的進一步深入，此問題將會不斷完善。值此論文完成之際，謹向胡老師及過控系各位老師致以最崇高的敬意和衷心的感謝！很慶幸自己在求學階段遇到這樣好的老師，師恩難忘。衷心感謝我的同學在論文的撰寫和答辯的準備工作中給予我的熱情的幫助。 最后，向審閱本文的老師致以深深的敬意，并再一次真誠的感謝所有關心、幫助過我的老師、同學和朋友們! 在此，謹向你們致以最崇高的敬禮，謝謝！附錄英文文獻 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。? 1 cycle/second= 1Hz? 1000 cycles/second= 1KHz? 1,000,000 cycles/second= 1MHzThe time required to plete a full cycle is the period (T), measured in seconds. The relation between frequency and period in a continuous wave is given in Equation (1).Eqn. 1 f = 1/T of Ultrasound and WavelengthThe velocity of ultrasound (c) in a perfectly elastic material at a given temperature and pressure is constant. The relation between c, f, λ and T is given by Equations (2) and (3):Eqn. 2 λ = c/f Eqn. 3 λ = cTλ = Wavelengthc = Material Sound Velocityf = FrequencyT = Period of timeTable 1 on page 40 lists the longitudinal and shear wave velocities of materials that are monly tested with ultrasonics. Propagation and Particle MotionThe most mon methods of ultrasonic examination utilize either longitudinal waves or shear forms of sound propagation exist,including surface waves and Lamb waves.●The longitudinal wave is a pressional wave in whichthe particle motion is in the same direction as the propagation of the wave.●The shear wave is a wave motion in which the particle motion is perpendicular to the direction of the propagation.●Surface(Rayleigh)waves have an elliptical particle motionand travel across the surface of a velocity isapproximately 90%of the shear wave velocity of the materialand their depth of penetration is approximately equal toone wavelength.●Plate(Lamb)waves have a plex vibration occurring inmaterials where thickness is less than the wavelength ofultrasound introduced into it.Figure(3)provides an illustration of the particle motion versus the direction of wave propagation for longitudinal waves and shear waves. UltrasoundUltrasonic nondestructive testing introduces high frequency sound waves into a test object to obtain information about the object without altering or damaging it in any basic quantities are measured in ultrasonic testing。 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