Ultrasonic Inspection for Complex Geometry

Phased array ultrasonic testing is a widely used method of sub-surface inspection in NDE, and is able to locate and size defects to a greater degree of confidence and precision than standard ultrasonic testing due to its ability to perform multiple inspections from one location [1]. Data is acquired using Full Matrix Capture (FMC), and an image of the ultrasonic response can be produced using the Total Focussing Method (TFM) [2], providing an indication of flaws in the inspection region. In addition to the direct ultrasonic wave from array to defect, there will be a wave which reflects from a back wall in the inspection material, arriving at the defect later in time. This appears distinct from the direct signal in the FMC data. These reflected waves can be imaged using TFM [3], enabling inspection of the component from multiple angles to increase the rate at which defects are detected, and improve characterisation. Typically, applications of the TFM inspect regions of the component which are either directly below or adjacent to the array [4-7] - in other words, within the array's line-of-sight (LoS).

It is common practice in NDE to prioritise inspection of areas most likely to develop defects. In a complex part such as an aircraft engine, it is likely that these areas will not be LoS. This leads to the requirement for disassembly prior to inspection, increasing cost of the inspection process. This research project aims to develop and validate a method which is able to inspect non-line-of-sight (NLoS) areas by reflecting an ultrasonic wave from the geometry of a component. The initial goal is to evaluate how well a phased array can assess these areas using the simple test case shown in figure 1. Subsequent goals are to target the inspection of increasingly complex cases, including abstract part geometry, anisotropic materials or samples whose geometric or material properties are not precisely known. Ultimately, a method capable of inspecting real, industrially relevant samples such that its deployment in an industrial setting will be justified.

Work so far has focussed on developing a ray-based model to produce FMC data including reflections of the ultrasonic wave from any individual wall in the component geometry rather than just the back wall, as well as the expected sensitivity of the probe to side-drilled holes (SDH) in these NLoS TFM images. The sensitivity of a 32-element 5MHz probe to a SDH with diameter 0.4mm is shown in figure 2, where multi-mode TFMs are focussed from side wall reflections. These results indicate that there are views which have non-negligible sensitivity to an ideal defect in NLoS areas. While this is a useful tool for determining the expected signal from a defect in ideal conditions, it does not account for random and coherent noise expected from a real system. These results were therefore validated against finite-element simulations by comparing the expected signal response from defects in a range of positions within the geometry.

Next steps in the project are investigate how dependent this sensitivity is to the probe location with respect to the geometry, to ensure that defects can be reliably detected, as well as performing validation against experimental results. Following this, the complexity of the models will be increased, as the assumptions of a polygonal isotropic solid are relaxed to include an arbitrary, anisotropic geometry whose dimensions may not be exactly known. Ultimately, it will be a requirement to validate the tools developed against real, industrially relevant samples.

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.