Infrared themographic image of a nanoengineered composite heated via electrical probes (clips can be seen at bottom of image). The scalebar of colors is degrees Celsius. The MIT logo has been machined into the composite, and the hot and cool spots around the logo are caused by the thermal-electrical interactions of the resistive heating and the logo “damage” to the composite. The enhanced thermographic sensing described in the paper works in the same way.
Image: Roberto Guzmán de Villoria, MIT
MIT Researchers have devised a new way to detect internal damage, using a simple handheld device and heat-sensitive camera. Their approach also requires engineering the composite materials to include carbon nanotubes, which generate the heat necessary for the test.
Their approach, described in the March 22 online edition of the journal Nanotechnology, could allow airlines to inspect their planes much more quickly, Wardle says. This project is part of a multiyear, aerospace-industry-funded effort to improve the mechanical properties of existing advanced aerospace-grade composites. The U.S. Air Force and Navy are also interested in the technology, and Wardle is working with them to develop it for use in their aircraft and vessels.
Advanced composite materials are commonly found not only in aircraft, but also cars, bridges and wind-turbine blades, Wardle says.
One method that inspectors now use to reveal damage in advanced composite materials is infrared thermography, which detects infrared radiation emitted when the surface is heated. In an advanced composite material, any cracks or delamination (separation of the layers that form the composite material) will redirect the flow of heat. That abnormal flow pattern can be seen with a heat-sensitive (thermographic) camera.
This is effective but cumbersome because it requires large heaters to be placed next to the surface, Wardle says. With his new approach, carbon nanotubes are incorporated into the composite material. When a small electric current is applied to the surface, the nanotubes heat up, which eliminates the need for any external heat source. The inspector can see the damage with a thermographic camera or goggles.
“It’s a very clever way to utilize the properties of carbon nanotubes to deliver that thermal energy, from the inside out,” says Douglas Adams, associate professor of mechanical engineering at Purdue University. Adams, who was not involved in the research, notes that two fundamental challenges remain: developing a practical way to manufacture large quantities of the new material, and ensuring that the addition of nanotubes does not detract from the material’s primary function of withstanding heavy loads.
The new carbon nanotube hybrid materials that Wardle is developing have so far shown better mechanical properties, such as strength and toughness, than existing advanced composites.
Non-destructive evaluation techniques can offer viable diagnostic and prognostic routes to mitigating failures in engineered structures such as bridges, buildings and vehicles. However, existing techniques have significant drawbacks, including poor spatial resolution and limited in situ capabilities. We report here a novel approach where structural advanced composites containing electrically conductive aligned carbon nanotubes (CNTs) are ohmically heated via simple electrical contacts, and damage is visualized via thermographic imaging. Damage, in the form of cracks and other discontinuities, usefully increases resistance to both electrical and thermal transport in these materials, which enables tomographic full-field damage assessment in many cases. Characteristics of the technique include the ability for real-time measurement of the damage state during loading, low-power operation (e.g. 15 °C rise at 1 W), and beyond state-of-the-art spatial resolution for sensing damage in composites. The enhanced thermographic technique is a novel and practical approach for in situ monitoring to ascertain structural health and to prevent structural failures in engineered structures such as aerospace and automotive vehicles and wind turbine blades, among others.
We have presented a new technique to evaluate the condition of advanced composite structures using resistive local heating in combination with thermographic imaging. The Joule heating effect of CNT networks embedded within structural composites offers a new and simple way to detect both internal and surface damage in situ using thermography when the composite is beyond the electrical percolation threshold. Aligned CNTs provide the conductive network that allows low power resistive heating, and damage visualization is enabled through change in both electrical and thermal transport around the damage. The local and low-power heating in the NET-NDE technique allow facile and low-power operation, including the opportunity for tomographic-like imaging by varying the spatial positioning of the electrical heating probes in real time. The technique eliminates the need for large external heaters thereby enabling in situ sensing for many structural inspection situations. This technique benefits from low power operation and simple electrical contacts, including hand applied electrical probes convenient for structural inspection. A series of tests have confirmed that this new technique can sense progressive damage, small cracks (very high resolution damage sensing), and internal damage not visible on the surface. Applications of this new NDE technique include transportation vehicles and infrastructure, including wind turbine blades, concrete-based structures repaired with composite overwrapping etc.. The technique is not limited to nano-engineered composites, and can be applied for other building materials such as concrete and foam where CNTs may be incorporated directly. The thermal nanoengineered NDE technique demonstrated here can provide a new and effective inspection route for monitoring future generations of safer vehicles and infrastructure. Additional capabilities related to those demonstrated here include higher resolution thermography, temporal information using highrate thermographic acquisition, and potentially damage mode detection. In addition to this advantage for applications using Joule heating, CNTs significantly enhance the toughness and strength of the composite materials, demonstrating a structural material that is mechanically superior and therefore truly multifunctional when it is used as a sensor (as in NET-NDE).