May 26, 2011
Heating pinpoints internal cracks in nanoengineered composites
Internal cracking, damage and defects can develop in a material over time and result in failure of the structure. Although there are several non-destructive evaluation techniques for detecting damage in structures, to date such techniques have been difficult and expensive to implement during the service life of a structure. Composite materials, such as those used in planes, bridges and buildings, provide opportunities for overcoming these problems. Composites provide room for engineering new functionality into a material, such as an inner "nervous system" that could relay damage information.
Recently, engineers at Massachusetts Institute of Technology (MIT), US, embedded such a sensing network made of carbon nanotubes (CNTs) into a traditional fibre-reinforced laminate architecture, by placing aligned CNTs throughout the architecture to create a hierarchical nanoengineered material.
When electrical current flows through the CNT network, the temperature of the material increases from Joule heating. If damage that disrupts this conductive network exists somewhere in the material, a localized change in electrical current results, which, in turn, manifests as a localized change in temperature.
A simple low-cost thermal camera or even night-vision goggles enable real-time visualization of damage in the composite. Reporting their results in the journal Nanotechnology, the researchers show that several kinds of damage can be detected in nanoengineered composites using this non-destructive in situ inspection technique including barely visible damage, internal damage produced by impact and other forms of otherwise hidden damage.
The structure is the sensor
This method has numerous advantages over existing non-destructive evaluation techniques. First, the structure itself is the sensor – no wiring, sensor grids or external heaters are necessary. In addition, this sensing technique is truly multifunctional – the nanoengineered composites are stronger and tougher mechanically, with the sensing benefit coming as a secondary effect. Finally, high-resolution inspection is easily obtained and, in the case of nanoengineered composites, with only a tiny amount of power.
The team believes that the strategy employed in the study can therefore serve as the basis for new, improved inspection techniques for monitoring future generations of safer vehicles and infrastructure.
About the author
Roberto Guzman de Villoria is a researcher based in the Department of Aeronautics and Astronautics at MIT, US.