针对表面裂纹深度和取向的无损表征难题，提出一种基于临界折射纵波(L_CR)的检测方法。以碳钢试块表面垂直裂纹(深度为1～8 mm)、倾斜裂纹(长度为10 mm，倾角为15°～90°)为检测对象进行试验，分析了裂纹深度、取向对L_CR波信号的影响规律，通过信号幅值和B扫描成像表征裂纹。试验结果表明，L_CR波在传播过程中会受到表面裂纹的遮挡作用，并在裂纹尖端产生衍射。随裂纹深度增加，遮挡作用增强，透射L_CR波幅值下降且到达时刻延后，对裂纹取向不敏感；B扫描图像中L_CR波及对应尖端衍射波的位置、方向与裂纹的深度、取向具有良好的一致性。该检测方法为表面裂纹的有效表征提供了新思路。

Aiming at the characterization of the depth and orientation of a surface crack, a nondestructive testing method using critically refracted longitudinal (L_CR) wave was proposed. The carbon steel test block containing vertical cracks (1~8 mm depth) and oblique cracks (10 mm length, 15°~90° inclination angle) were tested. The effects of depth and orientation were analyzed. The amplitude of L_CR wave and the B-scan imaging were used to determine the depth and orientation. Results showed that L_CR wave was blocked and diffracted wave from the crack tips was also observed. As the crack depth increased, the blocking effect of L_CR waves increased, leading to a decrease in the amplitude of transmitted L_CR waves. However, it was not sensitive to the crack orientation. At the same time, the arrival time of L_CR wave was delayed. B-scan images were reconstructed by the change of the diffracted wave with both crack depth and orientation, which provided a new idea for the qualitative assessment of surface cracks.