Looking beneath the skin

US researchers have developed a way of looking below the surface of composite parts to find microscopic defects. If it can make the leap from the laboratory into the commercial sphere, it could pave the way for real time monitoring of composite materials in an array of industries. The technique relies on a new generation of chemical additive – carbon nanotubes. These particles, around 1,000 times smaller than the traditional fibres used in composites, are added to the composite part when it is made. The electrically conducting nanotubes, when distributed evenly throughout the part, form a conductive network. The nanotubes account for around 0.5% by weight of the final part. “We load materials using a mechanical testing machine and measure the electrical response,” says Erik Thostenson, assistant professor at the University of Delaware's Department of Mechanical Engineering and Center for Composite Materials. “As damage is initiated in the polymer matrix, the cracks break the conductive pathways and change the electrical response.” Cracks usually form in composites at the interface between the polymer matrix and the embedded fibre, such as glass or Kevlar. Thostenson says that the ability to detect these cracks as they form could have wide application across industry – especially in areas such as aerospace and automotive, which are using increasing amounts of composites. “In future, it could be used for in-service health monitoring of composite structures,” says Thostenson. “We can monitor the changes in electrical resistivity in real time during testing.” Because the nanotubes have such a high aspect ratio (length/diameter), only a small amount is needed to create the conductive network. This has an added benefit: the tiny amount of material has a negligible effect on the polymer matrix to which it has been added. Thostenson says that the technique can monitor both the nature and the extent of damage in a composite part – adding that detecting microcracks in materials is currently very difficult to achieve using conventional techniques. A paper published in Advanced Materials shows that specific types of composite failure – such as interlaminar fracture and fibre breakage – can be identified using this method. Despite its accuracy in detecting defects, the experimental technique is not complex or expensive to carry out. In their experiment, Thostenson and his co-researcher, Professor Tsu-Wei Chu, compounded the nanotubes with an epoxy resin. This was then laid up with unidirectional glass fibre mats. Once cured, the mats were machined into shape for testing. Electrodes were attached to the composites using conductive silver paint and the specimen was tensed and flexed using a screw-driven load frame. Changes in resistance were measured with a sensitive voltage-current meter. “We apply a few volts to the specimen and measure the electrical current as we deform the composite,” says Thostenson. “Here, we are measuring distributed damage – but with multiple inputs it may be possible to detect more localised damage and map this graphically.” A key part of the technique is regular dispersion of the nanotubes throughout the part. Thostenson and Chu have developed a way of processing the polymer so that the nanotubes are highly dispersed. Using a three-roll mill, in which the rolls spin at progressively higher speeds, high shear forces can be applied to the material to ensure thorough mixing. “This is potentially scalable for industrial applications,” says Thostenson. Ongoing research will look at factors such as varying the concentrations of nanotubes in the polymer mix. “While the technology is at an early stage, we have a lot of ongoing research in this area,” he says. “We believe there is the potential to do innovative work in design, processing and characterisation. Pointers * Distributing carbon nanotubes through a composite matrix allows cracks to be detected by monitoring electrical resistance * Around 0.5% by weight of the composite will be nanotubes, which will not affect its physical properties * In future, it could be used for in-service health monitoring of composite structures Under strain The University’s ‘crack detection’ technology is some distance from commercialisation. Something already in the commercial sphere is a technology from measurement instrument provider HBM, which embeds strain gauges into fibre composite components during manufacture. This allows the gauges to be fitted in areas that are inaccessible once the components are complete, according to the company. The strain gauge has contact pins attached vertically, allowing it to be embedded within the composite material. It also means measurement wires do not need to be run inside the material, which simplifies component manufacture, HBM says. The 15mm contact pins function as electrical contact points and also fix the gauge during curing. HBM engineer Sebastian Klein says: “This is the first commercial way of embedding a strain gauge in [composite] material.” He says there are two integration methods. The gauge can be embedded within layers of a material, to a maximum depth of 13mm. The pins then pierce layers of material stacked on top of the gauge. Alternatively, with “inverse” application, the pins are pushed into the already stacked material. Wires are soldered to the pins after manufacture. Applications include the monitoring of repaired structures, where the gauge is embedded into the repair patch. The gauges can also be used on the surface of an aerodynamic structure, where a normal gauge would change the properties of the structure due to its wiring and structure. Klein says: “By embedding the gauge you will not change the properties. It can be surface-mounted, but the pins would be inside the structure, so the outside is not influenced.” Other applications include hybrid structures and measurement in bonding areas where there is no access to the application area after manufacture. Klein says the gauges can be embedded into a number of materials including epoxy resins, carbon and glass fibre, dry and pre-peg materials. However it cannot be used above 200ºC.