摘要由于超声波频率高、波长短，因此在对微小缺陷的无损检测中具有十分广泛的应用。激光超声检测技术利用激光（连续激光或脉冲激光）照射样品表面激发超声波，并利用光学方法检测超声波，从而实现对样品的无损检测。本文首先对激光超声技术进行了阐述，并介绍了激光激发超声波的机理，声光调制的原理以及激光超声检测系统的组成。在此基础上，本文介绍了两种非接触式超声波探测的手段：激光测振仪以及光偏转法。     在明确了微裂纹在受热后膨胀，容易发生局部闭合的基础上，ᨀ出了激光辅助加热的激光超声微裂纹检测方法，通过比较在同一位置处用不同加热光功率对裂纹加热与使其自然冷却后所得信号的差值，可获取裂纹的状态信息。 此外，在单频超声/声信号的激光激发基础之上，本文进一步加入了另一束低频幅度调制激光，与激发激光光斑重合，聚焦在材料上，对裂纹进行加热。裂纹受热后将进行呼吸式地张合，从而影响声波在材料中的传播，并显现出非线性混频过程。对混频声波的检测可以反映出材料中裂纹的存在，进一步又证明了裂纹宽度变化对时/频域信号的影响。   43611

毕业论文关键词   激光超声   无损检测  声表面波   微裂纹   

Title      Study of the optical-thermal modulated crack   breathing by laser ultrasonic methods  

  Abstract Ultrasounds are commonly used in non-destructive testing to detect small defects, for which are characterized by its high frequency  and short wavelength. The laser-ultrasonic technique is a method that focuses the laser light (cw or pulsed) on the surface of the material to excite ultrasonic waves, and detect the ultrasonic waves by means of optical ways to realize the nondestructive test for materials. This  paper begins with a demonstration of the laser-ultrasonic technique, followed by the mechanism of the laser-induced ultrasounds, the principle of the acousto-optic modulation and the sections of the laser-ultrasonic testing system. On these bases, this paper introduces two non-contact methods to detect ultrasounds: vibro-meter and the deflection. On the basis of that the micro crack tends to close partially when heated，a micro crack  detecting method based on  the assistance from the heating laser  is presented.  By comparing the  difference of the signal detected under the situation of heated and cooled at the same spot，the state  of the crack can be obtained. On the basis of the laser induced ultrasound/acoustic signal, this paper makes a further effort to add a  laser  beam modulated at low frequency, which coincides  with the  generation  laser  beam and focuses on the material. Affected by the laser heating, the crack will breathe (open and close alternately), accordingly, the transmission of the acoustic waves in the material are affected, at the same time, the nonlinear  mixing  progress is shown. Detecting the mixing acoustic wave will  report  the existence  of  the crack in the material,  and  further proved that the  change of the crack width has effect on the time／frequency signal.   Keywords    laser-ultrasonic    nondestructive testing   surface acoustic wave  micro crack

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1 引言 1

1.2 国内外研究现状 .. 1

1.3 本文的主要工作