Low frequency laser ultrasonics for nondestructive testing of large structures

（1）激光特性分析比较了长脉冲YAG激光器（脉宽50μ s，能量2.5J，峰值功率50 kW）和调Q YAG激光器（脉宽10 ns，能量0.1J，峰值功率10 MW）。结果表明，调Q激光能损伤物体表面，但能激发出低强度的振动，而长脉冲激光对物体表面无损伤，但能激发出大振幅的振动。(2)利用激光束的快速扫描对大型结构进行检测我们提出了一种大型金属板结构的检测方法。当长脉冲激光以兰姆波的相速度扫描时，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不是恒定的，而是依赖于频率。这种频率依赖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，用于检测和定位250 × 250 mm范围内的缺陷 关于我们 在一个激励中通过使用频率相关的方向性。(3)低频共振法评价大型结构我们提出了一种通过扫描激发YAG激光束来实现奇异形状和/或粗糙物体的鲁棒成像的方法。我们成功地获得了清晰的图像的共振模式在600 × 300 × 60毫米的混凝土板试件覆盖瓷砖的表面上，一个狭缝的3毫米宽，10 - 40毫米深。当激发YAG激光束照射在表面下狭缝的正上方位置时，振动振幅显著增强，并且我们从谐振频率的降低中精确地评估了狭缝的深度。(4)粗糙表面的可视化为了缩短大面积测试所需的时间，我们开发了一种使用光折变晶体BSO的振动成像方法。当我们引入光学锁定方法使用相位调制参考光束，振动模式的粗糙金属表面的频率范围从1到500千赫可视化。结构瓷砖也进行了研究。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