Smart paint detects microscopic cracks

A paint that can detect microscopic faults in bridges, mines, and the foundations of wind turbines has been developed by researchers at Strathclyde University in Scotland. The paint uses nanotechnology to sense movement in large structures. Highly aligned carbon nanotubes that can carry an electrical current are combined with fly ash, an inexpensive recycled waste product, and mixed into paint. When the carbon nanotubes begin to bend, their conductivity changes. These changes in conductivity are detected by electrodes incorporated within the structure that is coated with the paint. Any change in the flow of electrical current above a predetermined threshold can be interpreted as a sign of structural defect. The electrodes are attached to a battery, and wireless transmitters are placed throughout the structure. A master transmitter tracks changes in conductivity from all the structure's electrodes, which can be monitored remotely. The wireless communication nodes are powered in part by a battery, but they are also expected to rely on energy-harvesting methods where possible. Most methods for checking large structures for defects use either a visual inspection (which requires an engineer to take a trip to the site) or time-consuming, complex, and expensive instrumentation. The paint can be sprayed on to any surface. With electrodes attached, it can detect structural damage long before failures occur. When mixed, the paint is hard and durable, like cement. This makes it especially useful for coating structures that must withstand harsh environments, as well as those located where severe weather can make safety monitoring by human inspectors particularly difficult. Since fly ash is very inexpensive, the paint is expected to cost very little. The process of monitoring involves, in effect, a wireless sensor network," Dr. Mohamed Saafi of the university's department of civil engineering said. "The paint is interfaced with wireless communication nodes with power harvesting and warning capability to remotely detect any unseen damage, such as micro-cracks in a wind turbine concrete foundation." The research team consists of Dr. Saafi and David McGahon. McGahon says energy harvesting methods might include using the vibrations of cars or trains going through a tunnel. "The idea is to make it more sustainable so you're not running out to your bridge or structure to change the battery all the time." The research team is also examining the possibility of incorporating electrical impedance tomography technology into a structure to help locate cracks. This technology creates a conductivity map. If a change in conductivity indicates a crack, a finite element model will show the exact location within the structure. The team has developed a prototype, and the paint/electrode combination is undergoing testing. The researchers have been trying to discover the exact percentage of carbon nanotubes needed to make the product cost-effective. They have also conducted bending tests using strain sensors. Further tests will be carried out in the next couple of months.

A couple of years ago, a technology that enables real-time diagnostics and on-site repair was developed. Researchers have developed a simple new technique for identifying and repairing small, potentially dangerous cracks in high-performance aircraft wings and many other structures made from polymer composites. By infusing a polymer with electrically conductive carbon nanotubes, and then monitoring the structure�s electrical resistance, the researchers were able to pinpoint the location and length of a stress-induced crack in a composite structure. Once a crack is located, engineers can then send a short electrical charge to the area in order to heat up the carbon nanotubes and in turn melt an embedded healing agent that will flow into and seal the crack with a 70% recovery in strength. Real-time detection and repair of fatigue-induced damage will greatly enhance the performance, reliability, and safety of structural components in a variety of engineering systems, according to principal investigator Nikhil A. Koratkar, an associate professor in Rensselaer�s Department of Mechanical, Aerospace and Nuclear Engineering. Graduate students Wei Zhang and Varun Sakalkar were co-authors of the paper. The majority of failures in any engineered structure are generally due to fatigue-induced micro cracks that spread to dangerous proportions and eventually jeopardize the structure�s integrity. His research is looking to solve this problem with an elegant solution that allows for real-time diagnostics and no additional or expensive equipment. The team made a structure from common epoxy, the kind used to make everything from the lightweight frames of fighter jet wings to countless devices and components used in manufacturing and industry, but added enough multi-walled carbon nanotubes to comprise 1% of the structure�s total weight. The team mechanically mixed the liquid epoxy to ensure the carbon nanotubes were properly dispersed throughout the structure as it dried in a mold. The researchers also introduced into the structure a series of wires in the form of a grid, which can be used to measure electrical resistance and also apply control voltages to the structure. By sending a small amount of electricity through the carbon nanotubes, the research team was able to measure the electrical resistance between any two points on the wire grid. They then created a tiny crack in the structure, and measured the electrical resistance between the two nearest grid points. Because the electrical current now had to travel around the crack to get from one point to another, the electrical resistance - the difficulty electricity faces when moving from one place to the next - increased. The longer the crack grew, the higher the electrical resistance between the two points increased. �The beauty of this method is that the carbon nanotubes are everywhere. The sensors are actually an integral part of the structure, which allows you to monitor any part of the structure,� Koratkar said. �We�ve shown that nanoscale science, if applied creatively, can really make a difference in large-scale engineering and structures.� The new crack detection method should eventually be more cost effective and more convenient than ultrasonic sensors commonly used today. His sensor system can also be used in real time as a device or component is in use, whereas the sonic sensors are external units that require a great deal of time to scan the entire surface area of a stationary structure. The system features a built-in repair kit. When a crack is detected, the voltage going through the carbon nanotubes at a particular point in the grid can be increased. This extra voltage creates heat, which in turn melts a commercially available healing agent that was mixed into the epoxy. The melted healing agent flows into the crack and cools down, effectively curing the crack. These mended structures are about 70% as strong as the original, uncracked structure - strong enough to prevent a complete, or catastrophic, structural failure. This method is an effective way to combat both micro cracks, as well as a less-common form of structural damage called delimitation. The system should help increase the lifetime, safety, and cost effectiveness of polymer structures, which are commonly used in place of metal when weight is a factor.

A team of researchers from MIT�s Department of Aeronautics and Astronautics has developed a way to identify cracks inside composite materials. As composite materials become more common in the construction of everything from aircraft to bridges to wind-turbine blades, it is becoming ever more of an imperative to be able to locate these internal cracks. This is because composite materials do not necessarily show damage on the surface, like traditional metals do. A common method for locating these cracks until now has been to use infrared thermography (heat-sensitive cameras) to detect where heat is being redirected. This is cumbersome (and no-doubt uncomfortable), because it requires the use of large heaters. The MIT team's ingenious solution is to build the detection mechanism into the material. They have incorporated electrically conductive aligned carbon nanotubes into the design of the composite material, so that when a small electric current is applied to it, any areas of increased resistance indicate signs of potential damage. The entire investigator needs to do, is to wear a pair of thermo graphic goggles, or use a thermo graphic camera, and any signs of greater heat provide evidence of damage to the nanotubes, and thus the actual structure.