Composites under the microscope

When it comes to investigating the characteristics of composite materials and how they fail, microscopes are an essential piece of kit for materials scientists.

“If a picture paints a thousand words, then a good quality microscopy image can tell you chapter and verse about a composite material,” begins Dr Geraint Havard, composites manager at R-TECH Materials.

Widely regarded as one of the fastest growing test laboratories for composite materials in the UK, R-TECH Materials is used by manufacturers in the aerospace, marine and automotive industries worldwide.

According to Dr Havard, microscopy helps to understand a number of features of a composite material, which ultimately determine a composite component’s quality. This might include inspection of resin rich areas, fibre alignment, ply layup, ply thickness, void content, fibre volume fraction and how the fibre and matrix interact with each other.

Characterising the material in this way can be especially valuable for first part qualification testing in the aerospace industry, where there is a need to fully understand the quality of the composite laminate before production.

Getting up close

Based in South Wales, R-TECH Materials has developed a wealth of microscopy expertise over 10 years as part of its materials consultancy business. It’s now applying this same expertise to composites. Unlike isotropic materials such as metals, which have uniform properties in all directions, composites are anisotropic in nature, which means their properties are different when measured in different directions. This can have a critical impact on performance and microscopy, along with mechanical testing, can help to establish an understanding of a material’s behaviour in any given orientation.

R-TECH’s facilities include a top-of-the-range Hirox digital microscope. This produces high quality optical images, which can be captured and analysed by software, to not just visualise features, but measure them accurately as well. The Hirox is used on plane samples, and can work at magnifications up to X5,000. Beyond this, R-Tech’s Zeiss EVP 6 Scanning Electron Microscope takes over, with EDX microanalysis, and magnifications up to X20,000. The SEM images in 3D, so that features such as fibre pull-out and matrix delamination can be clearly identified.

“All of our microscopes can be linked to image analysis software for the measurement of image features, such as dimensional measurement or area fraction of different phases,” explains Dr Havard. “And we also have equipment to prepare material samples for microscopic examination, including precision cutting, mounting, grinding, polishing and etching. Indeed, we’ve recently invested in an automatic polisher which means we are able to polish a composite material down to a superior finish, which provides excellent microscopy images.”

The SEM’s magnification and depth of field make it the ideal tool for the examination of features relating to material failures, such as fracture surfaces, internal damage and surface condition. R-TECH’s experts can use this information to determine the causes of material failure, which manufacturers can use for improving component design, manufacturing processes, and product safety.

R-TECH has a wide range of experience in failure analysis of polymers and composites. From stress corrosion cracking of polycarbonate plate covers used on a cruise ship to CFRP overwrap for an oil refinery pipeline and carbon fibre vanes on an aluminium rotor. There’s not much the company hasn’t seen under the lens of a microscope! It’s even used its microscopy skills to characterise the resin interface with capacitors in electrical components.

R-TECH Materials’ microscopy and characterisation expertise complements its established polymer and composite test laboratories. Accredited to perform testing in tension, compression, flexure, shear and hardness to ISO and ASTM standards under controlled temperature and humidity conditions, R-TECH’s reputation for the accuracy and quality of its mechanical testing has been recognised by UKAS, which has awarded it accreditations for 24 test standards. The company is also preparing to secure Nadcap approval later this year – for which excellence in microscopy is a prerequisite.

Dr Havard is quick to extol the virtues of why mechanical testing and microscopy are ideal bed fellows.

“We’ve always believed that as a test house, we should carry out more than just the mechanical testing and chemical analysis of composites. That’s why we’ve spent significant time developing our microscopy skills and investing in state-of-the-art kit.”

Back to the future

Apart from using microscopy to characterise materials or investigate failures, manufacturers are increasingly turning to test laboratories like R-TECH for reverse- or so-called back-engineering.

Dr Havard, who recently reverse-engineered an aircraft interiors panel for a leading Japanese chemical company, explains the process: “Using a combination of microscopy, fourier transform infrared spectroscopy and resin burn off/acid digestion techniques, we were able to determine the number of layers that the panel was made up of, the resin type used, the weave of each different layer of reinforcement and the composition of the core material used.”

Managing director Dr Tony Franks concludes: “The properties of composite materials are controlled at the microscopic level. Our task as materials scientists in putting composites under the microscope is to connect what we see to the performance of the materials in service.”

Clearly, there’s nothing small about the role microscopes play in material characterisation, failure analysis and even reverse engineering. When it comes to composites, microscopy is definitely a big player complementing both mechanical testing and chemical analysis.

www.rtech-materials.co.uk