2D hybrid modeling of defects in an ultrasonic inspection situation

Licentiate thesis, 2021

Advanced methods of nondestructive evaluations (NDE) are widely used for in-service inspection in many industrial applications, e.g. nuclear and aerospace industries. In these applications the components are exposed to different degradation mechanisms (e. g. fatigue, corrosion, stress corrosion cracking). In-service caused defects such as fatigue and stress corrosion cracks can be sized and monitored in order to postpone repairs or replacements.

The reliability of NDE methods and the interaction between applied energy and addressed defect is highly dependent on the equipment adjustment to a specific object and to the expectation of the crack features. The crack feature and morphology vary widely between different crack mechanisms and between material types, in which crack appears.

Complex shaped defects, such as fatigue and stress corrosion cracks (SCC), are in many cases difficult to characterize with ultrasonic NDE methods. SCC has in many cases a heavily branched macroscopic shape with a large number of crack tips. Ultrasonic NDE method is not always reliable in sizing such defects, as the diffraction from the crack tips is commonly used as the basis for such analysis. In this case, thoroughly validated mathematical models could be used to do the parametrical studies that address such interactions.

In the current work, a developed hybrid model is described. This model is based on a combination of a semi-analytical model with a numerical approach. The basic idea is to use the numerical solution for interaction between the wave and the complex shape defect, which could be done by surrounding it with a volume modelled by a finite element scheme. The analytical method is used for describing the wave propagation between the probe and the volume that contains the actual defect.

Using hybrid models for parametrical study, could help to avoid costly and time-consuming experimental work.