Issues regarding TOFD inspection of fillet welds

The last edit to this post was made by Zhongyuanren on 2016-12-3 at 20:32. The applicable thickness range for TOFD is ≥13 mm. Principle of TOFD: TOFD technology utilizes two broadband narrow-pulse probes, one for transmission and one for reception, with these probes arranged symmetrically with respect to the weld centerline. The transmitting probe generates unfocused longitudinal wave beams that strike the workpiece under inspection at a certain angle; some of these beams propagate along the near surface and are detected by the receiving probe, while other beams are reflected off the bottom surface before being detected by the probe. The receiving probe determines the location and height of the defect by detecting the diffraction signals from the tip of the defect as well as their time differences. Advantages: a) A single scan can cover almost the entire weld area (except for the blind zones on the upper and lower surfaces), enabling a very high detection speed ； b) It should have good reliability, with a high detection rate for defects in the middle of welds ； c) Capable of detecting various types of defects, insensitive to the direction of the defects ； d) Defects extending toward the surface can be identified ； e) D-scanning imaging is used, making defect assessment more intuitive ； f) The quantification and positioning in the direction perpendicular to the defect are highly accurate, with a precision error of less than 1 mm ； g) The detection performance is better when combined with pulse reflection method, with a coverage rate of 100% ； Limitations of TOFD technology: a) There is a blind zone near the surface, resulting in insufficient reliability in detecting defects in that area; b) It is difficult to determine the nature of defects; c) Interpreting the images requires extensive experience; d) Detecting transverse defects is challenging; e) It is difficult to detect defects in materials with coarse grains; f) It is hard to measure workpieces with complex geometries. Detection method: TOFD ultrasonic imaging system. 1) TOFD technology offers good reliability. Since it primarily uses diffraction waves for detection, and the diffraction signals are not affected by the sound beam, defects in any direction can be detected effectively, giving this technology a very high defect detection rate. The evaluations derived from tests on defect detection rates at foreign research institutions are as follows: for manual UT, 50-70% ； TOFD, 70-90% ； Mechanical scanning UT+TOFD, 80-95%. It can be seen that the TOFD inspection technique offers much higher reliability compared to conventional manual UT inspection. 2) The TOFD technique has high quantitative accuracy. The use of diffraction time difference technology for defect quantification provides a precision that is significantly higher than that of conventional manual ultrasonic testing. It is generally believed that for linear or area defects, the TOFD quantitative error is less than 1 mm. The measurement error for the height of cracks and lack of fusion defects is usually only a few tenths of a millimeter. 3) TOFD testing is simple and fast; the most commonly used non-parallel scanning method can be carried out by just one person, as the probe only needs to be moved along both sides of the weld. There is no need for zigzag scanning, resulting in high testing efficiency and low operational costs. 4) The TOFD testing system is equipped with automatic or semi-automatic scanning devices that enable determination of the relative position of defects with respect to the probe, and the signals can be converted into TOFD images through processing. The amount of information displayed in the image is much greater than that in A-scan display; in the A-type display, only one A-scan signal can be shown on the screen, whereas a TOFD image displays a collection of numerous A-scan signals related to weld inspection. Compared to the waveform display of Type A signals, TOFD images, which contain rich information, are more conducive to the identification and analysis of defects. 5) The TOFD detection systems in use today are all high-performance digital instruments that completely overcome the poor signal recording capabilities of analog ultrasonic flaw detectors and simple digital ultrasonic flaw detectors. They can not only record signals throughout the entire process and store data for a long time, but also perform large-scale signal processing at high speeds. 6) In addition to being used for detection, TOFD technology can also be employed for monitoring the progression of defects; it is one of the effective methods for accurately measuring crack growth. 7) TOFD enables precise determination of the depth position of defects, as well as quantitative assessment of their height. 8) Since the defect diffraction signal is independent of angle, the reliability and accuracy of detection are not affected by angle. 9) The position of the diffraction spots is determined based on the propagation time difference of the diffraction signals; the quantitative localization of defects does not rely on the signal amplitude.

In principle, it is not possible. TOFD is essentially an extension of ultrasonic testing that uses multiple probes and chips, and it is particularly effective for inspecting butt joints.