Amplitude-Based Error Magnitude Assessment Using SAFT

The evaluation of defect size and the determination of the detection limit play a central role in ultrasonic testing: They ensure that critical defects can be detected and addressed from a fracture mechanics perspective.

In SAFT, the size of defects has traditionally been determined by evaluating the spatial extent of the indication in the reconstruction result—that is, the number of voxels over which an indication extends. However, this method can only evaluate defect indications that are larger than or equal to the wavelength. For small defects, this method fails, meaning size evaluation is not possible. Furthermore, this method provides no information about the method’s detection limit. A major advantage of SAFT over conventional ultrasonic testing, however, is precisely the improvement in the detection limit, since SAFT increases the SNR relative to structural noise and reduces statistical noise. To take advantage of (and quantify) this key benefit, a method is needed for SAFT to determine the detection limit and to assess the size of indications of small imperfections. In addition to their spatial extent, indications in the SAFT result can also be characterized by their amplitude if the probe characteristics and the geometric conditions during the scan are taken into account. More specifically, by the amplitude sum, which—in addition to the echo amplitude—also incorporates the angle-dependent scattering characteristics.

This article demonstrates that the amplitude sum is suitable for determining detection limits and assessing the size of small defects. The relationship between the amplitude sum and the size of defects is presented, and it is shown that the indication position and the shape of the test object can also play a role. On this basis, a method for determining the size of small indications in the SAFT result is developed. This allows the SAFT result to be represented as a substitute defect size, just as is done in conventional ultrasonic testing, e.g., using AVG diagrams. With the addition of size evaluation, SAFT is no longer merely an imaging technique but can instead serve as a fully-fledged measurement method.