By 1st Lt. Michael Sherburne, M.SAME, USAF, 1st Lt. Candice Mueller, M.SAME, USAF, Thomas Weber, Ph.D., M.SAME, Maj. John Brewer, Ph.D., M.SAME, USAF, and Hengky Chandrahalim, Ph.D., M.SAME
Imagine a situation in the future where vehicular assets undergoing years of continuous use need to be quickly checked for structural integrity in order to help ensure their safe operation. Currently, this would require a laborious inventory. But a new nanomaterial-based technology has the potential to produce real-time 2D surface maps of an asset’s structural health. A team of researchers from the Air Force Institute of Technology and Los Alamos National Laboratory has been studying the strain-sensing properties of a polymer paint impregnated with a nanomaterial—colloidal quantum dots (CQDs)—that are the same materials used in QLED television displays. To adapt CQDs to this technological use case, light is used to excite the dots, which then emit a specific wavelength of color based on the diameter of the dots themselves. They are described as “quantum dots” because their emission wavelength is quantized.
In the Air Force/Los Alamos study, the research team used an adhesion-enhancing interfacial layer between the paint and the surface to be measured. This allows the paint to be used in realistic environments outside the laboratory. Some of the applications could be aircraft, ships, ground vehicles, buildings, and bridges. In addition, all the materials employed to bring this technology into reality are already commercially available.
The motivation behind the development of the paint came from a need to help simplify the non-destructive evaluation process from an Air Force maintainer. Researchers had been investigating using CQDs for fast x-ray detection and took note of the possibility for quantum dots to potentially be used to measure strain at a macro scale. After further investigation, it was determined that the dots do change their wavelength of emission with an applied strain.
A preliminary study was conducted to see if the dots could be used to sense strain at a macro scale. A primary concern was that the signal-to-noise ratio would be too low to be able to detect any changes using basic equipment. Fortunately, the study was successful, and it motivated additional efforts into applying this technology for the warfighter.
Since the quantum dots are applied within a polymer-based paint, they could be used on any surface. The paint with its interfacial layer is no more than 20-µm thick. This has an advantage toward other non-destructive evaluation technologies that require certain materials to work well. In addition, the thin coating makes it viable for use on platforms that have concerns with weight. Since the paint measures deformation locally, it could be utilized with a rastering, or deliberate scanning method, camera or another method to make a real-time 2D strain surface map. This is projected to be around 10 times less expensive than other 2D strain surface map technologies such as digital imaging correlation. Moreover, digital imaging correlation requires a painted speckled surface, the use of two cameras that have to be carefully calibrated, and it cannot do measurements in real time.
Spatial resolution of the CQD-loaded paint has been demonstrated with the basic setup to be at most 1.34-µm. However, the research team believes the spatial resolution can be improved due to the quantum dots being approximately 10-nm or less in diameter. This means nanometer spatial resolution is possible, although improvements would need to be made to both the excitation and measurement technology.
In addition, it has been postulated that this nanomaterial-based strain-sensing paint could be used to detect crack formations by measuring the strain fields occurring around cracks. Furthering this capability would require additional research and development. Along with the 2D strain surface map, a camera that can take spatial measurements could be utilized to place a 2D surface map onto a 3D model of the object. This would have ample use when comparing data over consecutive measurements. In addition, specialists could remotely view deformation of a vehicle or structure from the data collected from their technicians.
There are quite a few ways to utilize this optical non-destructive evaluation. Since the paint is an epoxy, it could potentially be integrated within composite materials along with fiber optics to both excite and collect the emission of the paint layer. The paint also could be applied on the other side of a material’s surface that is optically transparent to its wavelength of emission and optically transparent to the wavelength used to excite it. This would allow the paint to be utilized to measure strain within materials or in hard-to-access areas.
TRANSITIONING THE TECHNOLOGY
Looking ahead to implementing the technology across a fleet, there are many logistical considerations to process. Generally, once the paint is applied to a structure of interest, a baseline scan would have to be collected. Then, after a regular maintenance interval or when damage is expected, the structure would be scanned again. Any permanent deformation in the structure should demonstrate a measurable change from the baseline.
Researchers expect that a crack could be detected by varying the load from the baseline configuration and measuring stress field variations. If a component was repaired or replaced, a new coating of CQD-loaded paint would be applied and a new baseline would need to be collected.
While operational uses of this paint are still years away, the technology recently won Top 2 in the inaugural Air Force Material Command’s Spark Tank, a competition of new or proven ideas. From there, the idea placed in the Top 15 out of over 300 submissions in the overall 2021 Air Force’s Spark Tank. Senior leaders valued that the paint could revolutionize non-destructive evaluation across the Department of Defense. The researchers are presently looking to secure funding and industrial partners to complete their next phase of research that will allow them to transition the technology to the warfighters.
1st Lt. Michael Sherburne, M.SAME, USAF, is Developmental Engineering Officer, Kirtland AFB, N.M.; firstname.lastname@example.org.
1st Lt. Candice Mueller, M.SAME, USAF, is Student Pilot, Laughlin AFB, Texas; email@example.com.
Thomas Weber, Ph.D., M.SAME, is Scientist, Los Alamos National Laboratory; firstname.lastname@example.org.
Maj. John Brewer, Ph.D., M.SAME, USAF, is Assistant Professor, and Hengky Chandrahalim, Ph.D., M.SAME, is Assistant Professor, Air Force Institute of Technology. They can be reached at email@example.com; and firstname.lastname@example.org.
The views expressed are those of the authors and do not reflect the official policy or position of the U.S. Air Force, Department of Defense, Department of Energy, or the U.S. government.
[This article first published in the July-August 2021 issue of The Military Engineer.]