Correct Sizing of Reflectors Smaller Than One Wavelength

Vrana, J.; Vrana GmbH; Germany

Seeber, A.; Siemens AG; Germany
Vrana, J.; Vrana GmbH; Germany
Moonshofer, H.; Siemens AG; Germany
Goldammer, M.; Siemens AG; Germany

ID: ECNDT-0177-2018
Download: PDF
Session: Microstructural Scattering - UT 1
Room: J2
Date: 2018-06-13
Time: 09:40 - 10:00

In the field of ultrasonic testing there are two key questions: Which defects can be found and – in the case that indications are found – do they restrict the use of the part? Regarding both questions, the prerequisite is a method for defect sizing.
Over the last decades sizing methods were established like DGS or DAC for defects smaller than the beam profile. Those methods utilize the Kirchhoff approximation, meaning the echo amplitude is proportional to the defect area. However, this approximation is only accurate for defects larger than one wavelength even that experience shows it can be applied for slightly smaller defects.
With the progress of material technology and ultrasonic inspection the need to detect and size smaller defects is growing. Due to higher sound attenuation associated with the increase of ultrasonic frequency is impairing the inspection this means that defects to detect are becoming significantly smaller than the ultrasonic wavelength.
In this publication, we investigate how to correctly size small defects below one wavelength. Utilizing a grid based simulation method we calculate echo signals of cylinder and disk shaped reflectors of various sizes. By properly choosing the simulation method and grid we ensure that all physical wave modes are included in the simulation and that the discretization error is negligible.
We found good correspondence between the simulation and classical defect sizing for defects larger than one wavelength. In the region between one quarter of a wavelength and one wavelength we found resonance effects, which means that classical defect sizing methods give conservative results. In the region below one quarter of a wavelength classical DGS and DAC sizing leads to undersizing. We discuss this in detail and derive a formula for defect sizing, which is applicable to small as well as large defects.