Sub-wavelength characterisation of defects in inaccessible regions using guided waves

Lead Research Organisation: University of Bristol
Department Name: Mechanical Engineering

Abstract

In an increasing number of industries, including those in the power generation, petrochemical and nuclear sectors, plant is extended beyond its original design life and there is a need to ascertain the integrity of areas that were not originally anticipated to require inspection. Guided acoustic waves provide the necessary range (tens of metres) for remote inspection and commercially available systems are routinely used for rapid screening of pipework. Recent research by the applicants has lead to the development of guided wave synthetic focusing techniques for pipe inspection, which have been shown to give an order of magnitude improvement in the sensitivity to small defects. However, the applicants have also shown that the quantitative information in the images is still not of sufficient resolution to enable sizing of critical defects to the accuracy required for structural integrity assessment. The reason why accurate sizing cannot be achieved is because, like other imaging systems, the resolvable detail is diffraction-limited by the wavelength of the probing wave. Resolution can be increased by operating at higher frequency but this is achieved at the expense of reduced range due to the increased attenuation and scattering. In many imaging fields so-called super-resolution techniques have been investigated that enable detail below the diffraction limit to be extracted. Although the benefits of super-resolution have been demonstrated in certain applications, its exploitation is highly case-specific. This is because, it must be tailored according to a priori knowledge of the interaction of the probing wave with the features likely to be encountered.The purpose of this project is to develop the use of sub-wavelength (super-resolution) characterisation techniques for the characterisation of otherwise inaccessible defects in safety-critical pipes using guided waves. The principal goal will be to determine the maximum penetration depth of a crack or corrosion patch, and also its orientation or shape. In discussion with the industrial collaborators the following target specifications have been agreed that represent the minimum that must be satisfied in order for guided wave sizing to be practically useful. The target will be to achieve a depth resolution of +/-0.1T (T = pipe wall thickness) for defects that are larger than 3T in lateral extent and deeper than 0.3T. This project will advance the understanding of guided wave scattering from realistic-shaped defects and will develop new super-resolution techniques to enable defects to be characterised using the scattered guided wave field collected by a transducer array. The project therefore involves basic science applied to a highly relevant industrial problem, which makes it appropriate for EPSRC funding.

Publications

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Description The project examined how guided ultrasonic waves could be used to gain quantitative information about the severity of defects in inaccessible locations in safety-critical structures. In discussions with the industrial sponsors, pipes were selected as the most appropriate structure for investigation. A typical scenario might be a defect in a pipe passing through a containment wall in a nuclear power station where there is no direct access to the pipe inside the containment.

In essence a sizing method has been defined that gives an estimate of maximum defect depth in cases where the defect profile does not contain sharp changes; if the defect is large in circumferential and axial extent (>~100 mm) then the depth can be estimated from reflection measurements alone, whereas if it is smaller, reflection and transmission measurements must be used. A method to diagnose the presence of sharp profile changes, and hence to determine whether the sizing method is applicable, has also been developed. The technique is being disseminated to the guided wave inspection community and can be implemented in commercial equipment. It will now have to be tested in the field on multiple defects.
Exploitation Route Techniques have been implemented on commercial guided wave inspection equipment and deployed in the field.
Sectors Aerospace, Defence and Marine,Chemicals,Energy,Transport

 
Description Findings have been implemented in defect-sizing protocols employed in commercial guided wave testing.
Sector Aerospace, Defence and Marine,Chemicals,Energy,Transport
Impact Types Economic