Detecting defects to improve the sustainability of composite manufacture

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Gary Atkinson, Associate Professor in Engineering at UWE Bristol and part of Lyndon Smith’s team in the Centre for Machine Vision, is developing the “Polarisation Vision for Composite Part Inspection” – a system to automatically detect defects in raw and processed composite materials and so reduce waste and huge expenses for composite industries.

“I realised this project was a real opportunity to make a difference,” said Gary, highlighting the project’s potential impact on industrial sustainability, where testing and subsequent discarding of materials is a big issue. Digital Engineering Technology Innovation (DETI) funding has enabled him to optimise the technology over the past two years.

The Weakest Link

Composite material strength lies in the uniform pattern of either unidirectional or interwoven carbon fibres lying undisturbed in resin. But any composite component is only as strong as its weakest link, and weaknesses caused by fibre misalignment or gaps is a common problem in composite manufacture.

The dark, shiny nature of composite materials makes defects tricky to spot using visualisation techniques based on traditional camera technology. The current industry standard is to perform time consuming and eye straining manual inspections to ensure the safety of components, particularly those destined for applications in the aerospace industry.

Gary is utilising a different type of visualisation technique to identify the problem areas.

Finding the flaws

Deep in the Machine Vision laboratories at UWE Bristol is a specialised camera (see photo), which detects when light becomes linearly polarised – with its electric field confined to a single plane – on reflection from a surface. Instead of blocking glare from reflected light like polarised sunglasses, the camera detects the reflected polarised light and, with specially developed software, produces a “photo” of composite fibres – with each orientation of fibre shown in a different colour – see images of composite fibre orientation below.

Polarised Vision setup imaging a composite sheet

The images produced by the software highlight tiny, localised defects, but individual fibre orientation can also be computed to give an overall report on a material’s quality. In the example histogram (below), the majority of fibres orientate in two directions, a uniform 90° from one another.

Statistical analysis of fibre distribution
G. A. Atkinson, T. J. Thornton, D. I. C. Peynado and J. D. Ernst. High-precision polarization measurements and analysis for machine vision applications. In Proc. European Workshop on Visual Information Processing. Tampere, Finland, 2018. © IEEE

“There’s no way you’d consistently detect localised defects like that using standard manual inspection or traditional methods from machine vision,” said Gary, explaining how his instrumentation could provide rapid and detailed defect detection for composite manufacture.

In recent months, Gary and his team have painstakingly explored what type of defects his “Polarisation Vision” can detect. They’ve taken a huge range of lab-based scenarios, as well as composites in realistic factory conditions, and developed the detection algorithms to automatically find component flaws using the polarised images.

Now they’re ready to start real-world trials, collaborating with Airbus to feed more and more data into the machine learning algorithm, training it further in defect auto-detection.

An alternative view

But the machine vision team haven’t stopped there. Digging even deeper into possibilities for composite fibre visualisation, they’ve invented another system for picking up flaws. The image below shows the moving infrared light scanner or “Photometric Stereo” setup. This type of system has previously been used for shape estimation, but in Gary’s setup here it provides composite fibre detail.

Photometric Stereo setup

The team are curious as to whether this alternative system will highlight a different set of flaws to those visible with Polarisation Vision, and whether the two could be used as complimentary techniques to aid industrial scale composite manufacture.

If you would like to know more about the research happening within the Faculty of Environment and Technology at UWE Bristol please read more on the Engineering our Future website.

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