A metrological approach to the time of flight diffraction method (ToFD)

Introduction: Non-Destructive Testing (NDT) is widely used in industry, especially in the evaluation of mechanical parts and structures for identifying defects, corrosion, flaws or imperfections. Time of flight diffraction (ToFD) is a method to assess discontinuities high within a workpiece or material, what is not easy to do with conventional pulse-echo NDT method. Additionally, ToFD could be used to determine the thickness of the material, as well as the pulse-eco method. The objective of this paper is to report original findings and an experimental comparison between pulse-echo and ToFD for some most relevant parameters in an NDT inspection using ultrasound. Method: A specially developed software was used to perform all data collection and ToFD parameters calculation. A regular flaw detection equipment and pulse-echo method was used as benchmarking. The comparison statistics was normalized error. The testing body was an acrylic parallelepiped block designed and constructed with a circular 1 mm wire inside simulating discontinuities at distinct depth. The measurements were done in seventeen points along the testing body, each one with different discontinuity depth. Results: It is observed that the pulse-echo method is more effective in detecting discontinuities compared to ToFD, succeeding in the detection of eleven points out of seventeen. ToFD performance regarding this evaluation depends mostly in the separation of the probes. Regarding the block thickness, both ToFD and pulse-echo methods are effective and equivalent in its quantification. Finally, concerning the height of the defect, only ToFD method was able to determine it properly. Conclusions: Probe Center Separation (PCS) is the utmost parameter for a proper use of Time of Flight Diffraction (ToFD) method. Within the work developed and reported herein, practical and immediately useable aspects were studied and has scrutinized ToFD with regard to uncertainty and correspondence to another traditional ultrasonic method: pulse-echo. This paper offers to the non-destructive test community the opportunity to realize and confirm an important outcome for any measurement, namely the estimation of uncertainty.

Automated summary

This paper compares the applications of pulse-echo and ToFD methods in NDT, finding that the pulse-echo method is more effective in detecting discontinuities, while ToFD is more accurate in determining defect height.