Comparison of the Effectiveness between Code Modulation and Frequency Modulation Pulse Compression Technique for Ultrasonic Time-of-Flight Diffraction Method

Ultrasonic flaw sizing is an important issue for insuring structural reliability. The time-of-f ight diffraction (TOFD) method [1] is recognized as one of the most accurate flaw sizing methods. However, the method is not applicable to high attenuation thick materials due to the low signal to noise ratios (SN ratios) of a lateral wave and flaw tip diffraction wave. In this study, in order to enhance the SN ratios of the flaw tip diffraction wave and lateral wave, pulse compression techniques are applied to the TOFD method. The several code modulation type and frequency modulation type pulse compression techniques are applied for pulse compression and the effectiveness of the parameters for each pulse compression technique, such as duration length of the input wave, type of code modulation, carrier wave frequency and window function for the input wave, are investigated. The SN ratio and flaw sizing accuracy for each method and condition are investigated experimentally using an austenitic stainless steel specimen with an artificial penetration slit.

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