An easy-to-implement method based on mode-converted waves was proposed for quantitatively detecting shallow subsurface defects using the ultrasonic time-of-flight diffraction (TOFD) technique. First, an isotropic model with a shallow subsurface defect was established to analyze the ray path of the mode-converted wave in TOFD B-scan image. The mode-converted wave with the shortest travel time and highest amplitude was identified to calculate the flaw depth via Snell's law. Subsequently, the quantitative expression of flaw depth was corrected by introducing the travel time of mode-converted wave, improving the measurement accuracy of the flaw height. Simulated results showed that the depth of the dead zone in carbon steel was reduced from 5.5 to 2.4 mm. The measurement error of shallow subsurface cracks was no more than 0.02 mm when the crack height was greater than or equal to 0.7 mm. Finally, three shallow subsurface defects were experimentally detected with the mode-converted waves in B-scan image. The quantitative errors of the measured heights were within 8.5% of the actual values, verifying the validity of the proposed method. This method is appropriate for the quantitative detection of shallow subsurface defects in thick-walled components.

中文翻译：

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利用飞行时间衍射技术中的模式转换波定量检测浅层次表面缺陷

提出了一种基于模式转换波的易于实施的方法，用于使用超声飞行时间衍射（TOFD）技术定量检测浅层次表面缺陷。首先，建立了具有浅次表面缺陷的各向同性模型，以分析TOFD B扫描图像中模式转换波的射线路径。识别出走时最短、振幅最高的模式转换波，通过斯涅尔定律计算缺陷深度。随后，通过引入模态转换波的传播时间对缺陷深度的定量表达式进行修正，提高了缺陷高度的测量精度。模拟结果表明，碳钢的死区深度从5.5 毫米减少到2.4 毫米。浅层次表层裂纹测量误差不大于0。当裂纹高度大于或等于 0.7 mm 时为 02 mm。最后，用 B 扫描图像中的模式转换波通过实验检测了三个浅层次表面缺陷。实测高度的定量误差在实际值的8.5%以内，验证了所提方法的有效性。该方法适用于厚壁构件中浅层次表面缺陷的定量检测。