A new class of materials based on nanocomposites may help to detect sources of failure in an aircraft's primary structures such as cracks and delamination.
A research team in the Mechanics and Aerospace Design Lab at the University of Toronto, is working on the development of these self-monitoring adhesives that will join structural elements. The materials may be one part of the solution for coping with the dramatic rise of composite materials in commercial aircraft structures and the lack of associated industry-wide repair knowledge.
The team, led by Professor Shaker Meguid, is developing multi-functional, nano-tailored composites for use as adhesives. The rise of other composite materials in commercial aircraft primary structures has been accompanied by a parallel rise in the use of structural adhesives for joining them together, and for joining them to structures made of other composite materials or metals.
A new class of self-monitoring adhesives may help to predict failure in structural composites used in aircraft such as the Airbus A350 XWB, shown here in its final assembly line in Toulouse, France. Source: Airbus
For these types of joints, structural adhesives are usually preferred over mechanical fastening or welding for several reasons. Fasteners weigh more and create localized areas of stress where they are joined to the composite or metal. Joints made with welds may be subject to phase transformation, as well as additional stress in the weld region. By contrast, adhesive bonding is more uniform, lighter in weight, lacks stress points, and is performed at lower temperatures.
The research team wants to convert polymeric thermoset adhesive resins into multifunctional materials that can simultaneously perform many structural and non-structural functions. This multifunctionality will be achieved via dispersing very small amounts, such as concentrations of 0.1 to 2 percent, of carbon nanotubes and nanowires.
The carbon nanotubes and nanowires are electrically conductive, giving the new resins improved electro-mechanical properties. Currently, the research team is investigating how uniformly dispersed, aligned, and agglomerated carbon nanotubes affect the electrical conductivity of multifunctional nanocomposites. To determine the current's continuity and its level of critical percolation, the team is using a different approach that involves a novel type of network recognition.
The self-monitoring aspect of this story is what fascinates me the most. I'd like to read more about this topic, especially what other areas something like this is being used in. Ann, do you happen to know?
Thanks, Rob. Yes, the hope here seems to be that since the use of adhesives is increasing massively along with the use of composites, adhesives can help provide an early-warning system for detecting structural problems in aircraft. Reading about nanotechnology and its possible applications is like reading about science fiction, far more so than most other leading-edge technologies. I covered early carbon tube and carbon wire R&D efforts several years ago, so it was heartening to see that it's advanced to the level of possible real-world applications. Although this, of course, is still in R&D.
It would be great if adhesives could help provide a warning for structural problems in an aircraft. I look forward to see the progress of this through research and development.
The idea that some sort of nanotechnology adhesive can help predict a structural failure in a composite airplane wing is definitely science fiction-like. How far away is this technology from being commercialized given that composites are increasingly being deployed in planes?
This should contribute greatly to our understanding of failure mechanisms of composites in real-world applications. After all, failure mechanisms of steels is well understood, but composites are in still comparatively new in many of these applications. This is an important story.
@Ann: Can you walk me through how a sensor like this would work? I understand that Professor Meguid's group is studying how alignment of the nanotubes or nanowires affects the electrical conductivity of the adhesive. Is the idea that the presence of a crack would alter the alignment of the nanotubes or nanowires, and that this could be measured as a change in conductivity?
The use of the term "percolation threshold" seems to indicate that they are using graph theory, which is a good example of how seemingly abstract branches of mathematics can sometimes have extremely practical uses.
I can see how this could indeed work to indicate the start of failure. That part does make sense. But the question comes as to how to reset the detection scheme after the repairs are done. In the same way that embedded fiber optics do detect failures, the change is permanent and nonreverseable. Broken fibers and gaps between the microfibers just do not repair. The fix is a replacement. So while the detection system could work, the repairs would equate to replacements.
'percolation' is a physical phenomenon, referring to topological arrangements within a multi-component solid. Imagine for instance a matrix of substance A with embedded uniform spheres of conductive substance B. As you increase the concentration of B, at some point they will start touching each other on a macroscopic scale, so that the material would become conductive---that would be an example of percolation.
The concept is used in many contexts, for instance to describe flow of oil through pores in a rock matrix.
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