Time-of-flight diffraction method for joint with linear misalignment

The time-of-flight diffraction (TOFD) method is known as one of the most accurate flaw sizing methods among the various ultrasonic testing techniques. However, the standard TOFD method cannot be applied to weld joints with linear misalignment because of its basic assumptions. In this study, a geometric model of the TOFD method for weld joints with linear misalignment is introduced and an exact solution for calculating the flaw tip depth is derived. Since the exact solution is extremely complex, a simple approximate solution is also derived assuming that the misalignment is sufficiently small relative to the probe spacing and the flaw tip depth. The error in the approximate solution is confirmed to be negligible if the assumptions are satisfied. Numerical simulations are conducted to assess the flaw sizing accuracy of both the exact and approximate solutions considering the constraint of the probe spacing and the influence of the excess metal shape. Finally, experiments are conducted to prove the applicability of the proposed method. As a result, the proposed method is proven to enable accurate flaw sizing of weld joints with linear misalignment.

Study of the ultrasonic creeping wave propagation over a concave metal surface

Creeping ultrasonic waves are used in echo flaw detection of near-surface and near-bottom zones of metal products of plane-parallel or cylindrical shape. The creeping (lateral) waves are also used in testing products by the time-of-flight diffraction method as the earliest (reference) signal, followed by the useful signals of waves diffracted on metal discontinuities. The purpose of this study is to evaluate the ability of the creeping wave to propagate over a concave metal surface. We have studied experimentally the attenuation of the amplitude of a creeping wave with the distance upon wave propagation over concave metal surfaces of different radii. The velocity of propagation of the creeping wave does not depend on the radius of curvature and equals to the velocity of the bulk longitudinal wave. The results obtained provide the possibility of using the time-of-flight diffraction method in control of the objects with concave surfaces, in particular, for in-tube testing.

Towards an Alternative to Time of Flight Diffraction Using Instantaneous Phase Coherence Imaging for Characterization of Crack-Like Defects

Time of flight diffraction (TOFD) is considered a reliable non-destructive testing method for the inspection of welds using a pair of single-element probes. On the other hand, ultrasonic phased array imaging has been continuously developed over the last couple of decades, and now features powerful algorithms, such as the total focusing method (TFM) and its multi-view approach to rendering detailed images of inspected parts. This article focuses on a different implementation of the TFM algorithm, relying on the coherent summation of the instantaneous signal phase. This approach presents a wide range of benefits, such as removing the need for calibration, and is highly sensitive to defect tips. This study compares the sizing and localization capabilities of the proposed method with the well-known TOFD. Both instantaneous phase algorithm and TOFD do not take advantage of the signal amplitude. Experimental tests were performed on a ¾″-thick steel sample with crack-like defects at different angles. Phase-based imaging techniques showed similar characterization capabilities as the standard TOFD method. However, the proposed method adds the benefit of generating an easy-to-interpret image that can help in localizing the defect. These results pave the way for a new characterization approach, especially in the field of automated ultrasonic testing (AUT).