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Time of Flight Diffraction

By Kevin Chan 09 December 2019

Time of Flight Diffraction is a reliable method of non-destructive ultrasonic testing used to look for flaws in welds. In Time of Flight Diffraction (TOFD) systems, a pair of ultrasonic probes reside on opposite sides of a weld-joint or area of interest. A transmitter probe emits an ultrasonic pulse, which is captured by the receiver probe on the opposite side. In an undamaged part, the signals picked up by the receiver probe are from two waves: one that travels along the surface (lateral wave) and one that reflects off the far wall (back-wall reflection). When a discontinuity such as a crack is present, there is a diffraction of the ultrasonic sound wave from the top and bottom tips of the crack. Using the measured time of flight of the pulse, the depth of the crack tips can be calculated automatically by trigonometry. This method is more reliable than traditional radiographic, pulse echo manual UT (Ultrasonic Testing) and automated UT weld testing methods.

The accurate and reliable inspection of anisotropic welds is a hot topic in Non-Destructive Evaluation (NDE) as it is common to a wide range of safety critical components across industry. Their heterogeneous nature makes conventional delay-and-sum imaging algorithms much less effective for reliable defect detection and imaging.

AnisotropicFigure 1: Inspection of anisotropic weld geometry in OnScale


The welding process deposits anisotropic material in a variety of orientations, which has a direct effect on both ultrasonic wave velocity and direction. This renders some of the more standard modelling techniques in NDE, such as ray-tracing, unusable in a practical sense. This is where a Finite Element Method (FEM) based solution has considerable merit. Through Electron Back Scatter Diffraction (EBSD) of a sectioned weld, it is possible to quantify the orientations of the grain structure. The information can be grouped into dominant orientations and displayed in a color map where each unique color represents a different orientation of the material.


TOFD was initially developed as a method of accurate monitoring and sizing of through-wall height of in-service discontinuities in the nuclear industry. It has now been independently validated as one of the most effective techniques for locating and sizing discontinuities in ferritic welds.

Time of Flight DiffractionFigure 2: Time domain wave propagation in weld


With OnScale’s highly efficient solvers and our scalable cloud platform, full 3D weld structures and test scenarios can be simulated in practical time frames, allowing you to develop a better understanding of how anisotropic structures can be evaluated using ultrasound.

Kevin Chan
Kevin Chan

Kevin is a Senior Application Engineer at OnScale. He tests and helps with the development of OnScale. His background and experience with the solvers has allowed him to work on a wide range of projects with a big focus on MEMS & RF. Kevin holds a MEng in Electronic and Electrical Engineering at the University of Strathclyde.

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