Low frequency laser ultrasonics for nondestructive testing of large structures

(1)激光特性分析比较了两种激光器：长脉冲激光(脉冲宽度50μ，能量2.5J，峰值功率50kW)和调Q激光(脉冲宽度10 ns，能量0.1J，峰值功率10 mW)。研究发现，调Q激光对物体表面有损伤作用，但会激发低强度振动，而长脉冲激光不会对物体表面造成损伤，但会引起较大幅度的振动。(2)大型结构的激光快速扫描检测方法。当以兰姆波的相速度扫描长脉冲激光时，产生定向兰姆波。我们发现，每个频率分量相对于扫描方向的传播角度不是恒定的，而是依赖于频率。这种与频率相关的方向性对于识别未知散射体的方向是有用的。我们开发了一种有效的方法来检测和定位250X250 mm…区域内的缺陷(3)低频共振法评估大型结构物我们发展了一种方法，通过扫描激励的激光光束来实现对奇形物和/或粗糙物的强健成像。我们成功地获得了600×300×60 mm混凝土平板试件的共振振动模式的清晰图像，试件表面覆盖有瓷砖，缝隙宽度为3 mm，深度为10-40 mm。当激发的YAG激光照射在亚表面狭缝上方时，振动幅度显著增加，我们从共振频率的降低精确地估计了狭缝的深度。(4)粗糙表面的可视化为了缩短测试大范围所需的时间，我们开发了一种利用光折变晶体BSO的振动成像方法。采用相位调制参考光的光学锁模方法，获得了粗糙金属表面在1~500 kHz频率范围内的振动模式。对结构瓷砖也进行了研究。较少

(1 ) Analysis of laser propertiesWe compared two types of lasers, a long pulse YAG laser (pulse width 50μ s , energy 2.5J, peak power50 kW) and a Q-switched YAG laser (pulse width 10ns, energy 0.1J, peak power 10MW). We found that the Q-switched laser can damage the object surface but excite low intensity vibration, whereas the long pulse laser does not damage the object surface but excite large amplitude vibration.(2) Evaluation of large structures by fast scanning of laser beamWe developed a testing method of large metal plate structures. When the long pulse laser is scanned at the phase velocity of Lamb waves, directive Lamb waves are generated. We found that the propagation angle of each frequency component with respect to the scanning direction is not constant but dependent on the frequency. This frequency-dependent directivity is useful for identifying direction of unknown scatterer. We developed an effective method for detecting and locating defects within a area of 250 X 250 mm … More in one excitation by using the frequency dependent directivity.(3) Evaluation of large structures by low frequency resbnance methodWe developed a method by scanning the excitation YAG laser beam to realize a robust imaging of odd-shaped and/or rough objects. We succeeded to obtain clear images of resonance vibration modes in 600 x 300 x 60 mm concrete plate specimen covered by ceramic tiles on the surface, with a slit of 3 mm width and 10 - 40 mm depth. The vibration amplitude is significantly enhanced when the excitation YAG laser beam irradiates the position just above the subsurface slit, and we precisely evaluated the depth of the slits, from a decrease in the resonance frequency.(4) Visualization of rough surfacesTo shorten the time required for testing wide area, we developed a vibration imaging method by using a photorefractive crystal BSO. When we introduce the optical lock-in method using phase modulated reference beam, vibration modes of rough metal surface in the frequency range from 1 to 500 KHz were visualized. Structural ceramic tile was also studied. Less