Modelling the ultrasonic response from rough defects using efficient finite element modelling techniques

Abstract

The work of this Engineering Doctorate addresses the research and development of efficient Finite Element (FE) modelling techniques for calculating the ultrasonic response from rough defects for Non-Destructive Evaluation (NDE) applications specific to the nuclear power generation industry. The project has been carried out in collaboration with Imperial College London and Rolls-Royce allowing for the transfer of novel academic research into an applied industrial context.

Within the UK nuclear power generation industry, one of the fundamental principles of regulation and operation is a robust safety culture where the highest levels of quality assurance are applied to safety critical components. This principle places a requirement on NDE to deploy reliable and accurate inspections to ensure the structural integrity of the plant and its components.

To achieve this goal, modelling techniques can be used to aid in the design and justification of ultrasonic NDE inspections. For smooth, relatively large defects, analytical methods can provide an accurate scattering solution; however, for more realistic rough defects, the limitations of these methods are only applicable for specialised cases of roughness.

Defects which possess rough surfaces greatly affect ultrasonic wave scattering behaviour. Ultrasonic NDE inspections of safety-critical components rely upon this response for detecting and sizing flaws. Reliable characterisation is crucial, so it is essential to find an accurate means to predict any reductions in signal amplitude. An extension of Kirchhoff theory has formed the basis for many practical applications; however, it is widely recognised that these predictions are pessimistic owing to analytical approximations. As a result, NDE inspections can be overly sensitive, meaning that small and insignificant indications are incorrectly classed as being potentially hazardous defects. This increases the likelihood of making false-calls and incurring unnecessary expenditure to the programme.

A numerical full field modelling approach does not fall victim to such limitations, and therefore, FE modelling techniques have been developed to deliver a non-conservative methodology for the prediction of expected back-scattering from rough defects. This has been achieved in two parts: improved performance of absorbing boundary methods for use with commercial FE codes, and application of domain linking algorithms to NDE inspection problems. This thesis presents the development of these methods and their application to industrial NDE inspections. Ultimately, the findings of this work will aid in establishing more reliable, less conservative, reporting thresholds for the inspection of power plant components, reducing false call rates and therefore any unnecessary expenditure.