Nonlinear Ultrasonic Measurement of the Remaining Life of Engineering Structures

One of the grand challenges in non-destructive testing (NDT) is the measurement of the remaining life of a structure. Up to now, most NDT methods aims to detect and characterise relatively large defects which occur at the end of the life of a structure using linear ultrasound.

Recently, potential use of non-linear ultrasound has been demonstrated in the literature to be sensitive to early formation of defects such as microcracks. This occurs because as material ages or become defective, they become increasingly non-linear. Different techniques have been developed, namely second harmonic generation, wave mixing, and diffuse field method. However, signals from non-linearity are relatively small compared to linear ultrasound, and hence can be easily masked by noise or other non-linearities. As a result, these techniques have not been fully exploited yet, due to them not having reached the required sensitivity to image the build-up of material non-linearity.

The project aims to develop the required modelling tool to fully understand the measurement scenario to exploit non-linear techniques, understand its limitations and potentially extract (or image) material non-linearity.

The technique I have been exploiting is the wave mixing technique. In this method, two primary waves (blue and green) are made to interact, and under some specific conditions, they will produce an additional wavefield, termed as the resonant wave (red). This wave can be related to the material non-linearity at the point of interaction, hence having the potential of spatial localisation.

Until now, a 2D numerical model has been successfully developed, and validated against theory for non-linear wave mixing. It has been used for the 1D mapping of non-linearity within material, showing that a correction factor has to be included to the equation from the literature relating the amplitude of the resonant wave to material non-linearity. The next step is to explore the potential of arrays for wave mixing.

The proposed CDT in NDE will deliver impact (Industrial, Individual and Societal) by progressing research, delivering commercial benefit and training highly employable doctoral-level recruits able to work across industry sectors.

Industry will benefit from this CDT resulting in competitive advantage to the industrial partners where our graduates will be placed and ultimately employed. The global NDE market itself has a value of USD15 billion p.a. [Markets and Markets NDE report January 2017] and is growing at 8% per year. Our partners include 49 companies, such as Airbus, Rolls-Royce, EDF, BAE Systems, SKF and Shell, whose ability to compete relies on NDE research. They will benefit through a doctoral-level workforce that can drive forward industrial challenges such as increased efficiency, safer operation, fewer interruptions to production, reduced wastage, and the ability to support new engineering developments. Our 35 supply chain partners who, for example, manufacture instrumentation or provide testing services and are keen to support the proposed CDT will benefit through graduates with skills that enable them to develop innovative new sensing and imaging techniques and instrumentation. To achieve this impact, all CDT research projects will be co-created with industry with an impact plan built-in to the project. Our EngD students will spend a significant amount of their time working in industry and our PhDs will be encouraged to take up shorter secondments. This exposure of our students to industry will lead to more rapid understanding, for both parties, of the barriers involved in making impact so that plans can be formulated to overcome these.

Individual impact will be significant for the cohorts of students. They will be trained in an extremely relevant knowledge-based field which has a significant demand for new highly skilled doctoral employees. These graduates will rejuvenate an ageing workforce as well as filling the doctoral skills and capability gaps identified by industry during the creation of this CDT. Our industrial partners will be involved in training delivery, e.g. entrepreneurial training to equip our graduates with the skills needed to translate new research into marketed products. Many of the partners are existing collaborators, who have been engaged regularly through the UK Research Centre in NDE (RCNDE), an industry-university collaboration. This has enabled the development of a 5,10 & 20 year vision for research needs across a range of market sectors and the CDT training will focus on these new priorities. Over the duration of the CDT we will actively discuss these priorities with our industry partners to ensure that they are still relevant. This impact will be achieved by a combination co-creation and collaboration on research projects, substantive industrial placements and as well as communication and engagement activities between academic partners and industry. Events aimed at fostering collaboration include an Annual CDT conference, technology transfer workshops, networking events as well as university visits by industrialists and vice versa, forming a close bond between research training and industrial impact. This approach will create lasting impact and ensure that the benefits to students, industry and society are maximised.

Society will benefit from this CDT through the research performed by our CDT graduates that will underpin safety and reliability across a wide range of industries, e.g. aerospace, energy, nuclear, automotive, defence and renewables. As NDE is an underpinning technology it feeds into many of the UK Government's Industrial Strategy Challenge Fund Grand Challenges, for example in energy, robotics, manufacturing and space. It is aligned to the EPSRC prosperity outcomes, e.g. the Productive Nation outcome requires NDE during manufacture to ensure quality and the Resilient Nation requires NDE to ensure reliable infrastructure and energy supplies.