Sensitivity assessment of the time-of-flight diffraction method of ultrasonic testing

This paper examines a case study illustrating the ability of ultrasonic testing to detect defects smaller than a wavelength. A single point defect was detected in a welded joint of steel parts produced by flash-butt welding using a time-of-flight diffraction (TOFD) method using a longitudinal wave at a frequency of 5 MHz, i.e., a wavelength of approximately 1.2 mm. Its presence in the weld was subsequently confirmed using phased arrays. Based on the results of metallographic analysis, the defect type was determined to be a rounded slag inclusion with smooth edges and dimensions of approximately 1 mm. It was also shown that the welded joint also contained localized zones with coarse grains and uniformly distributed small non-metallic inclusions with an average size of up to 0.1 mm in the fusion zone. However, during ultrasonic testing, these structures created only background noise and did not interfere with the detection of the slag inclusion. The obtained data confirm the achievable sensitivity of approximately 1 mm in detecting of single volumetric defects using the TOFD method. However, the actual defect sizes are not determined.
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