工件几何形状不规则或材料性质非均匀时，易发生局部应力突变，导致工件产生裂纹。利用传统的超声检测技术（如超声投捕法）对这种早期裂纹进行检测时，由于裂纹尺寸很小以及工件表面其他的散射物（如表面突起、凹槽和腐蚀坑等）对声波的影响，从而难以有效分辨微裂纹。搭建了基于激光超声投捕法的表面缺陷光学检测系统，超声信号由激光线源激发，光偏转差分法探测。对比了传统投捕法、扫描激光源法与扫描加热源法对表面含有突起和微裂纹的样品进行扫描检测的结果。投捕法对信号信噪比要求较高、可靠性较差。扫描激光源法能检测两种缺陷但不能将微裂纹识别出来。扫描加热源法利用微裂纹受热膨胀易发生局部闭合的特性，可以将其与样品表面的突起加以区分。采用有限元方法，分析了裂纹尺寸变化对透射声表面波的影响。

Due to the geometry irregularities of specimens and the inhomogeneous of materials, micro-crack is usually generated by abrupt change of local stress. Traditional detecting techniques using ultrasound, such as pitch-catch method, are difficult to distinguish the micro-crack effectively, because the size of the cracks are so small that they are often hidden from ultrasonic detection by a stronger scattering from the very same structural imperfection, such as surface protuberance, surface grooves, corrosion pits, etc. The laser ultrasonic detecting system is built to detect the surface irregularities based on one-sided pitch-catch method. The laser line source is used to excite ultrasound and the optical deflection beam method is used to detect ultrasound. The traditional pitch-catch method, scanning laser source method and the scanning heating laser source method are used to detect the surface irregularities of specimen which contains a surface protuberance and a surface breaking micro-crack. The pitch-catch method requires high signal to noise ratio of signals and the reliability is poor. The scanning laser source method can detect both one but can not distinguish the crack from them. Based on the characteristics that the micro-crack can get closed partially after illuminated by heating laser, the scanning heating laser method can recognize it efficiently. The finite element method (FEM) is used to calculate the transmitted surface acoustic waves (SAW) which are influenced by different sizes of surface breaking cracks.