A group of scientists from Rice University in the United States has manufactured a smart skin in paint format that detects incipient internal damage in buildings and other large structures. Artificial skin uses carbon nanotubes and more specifically their fluorescent properties to report that a surface is dangerously deforming due to structural stress.
The smart skin is in the form of a paint, a multi-layer coating that can be applied to large structures such as buildings, ships, bridges and airplanes , places where higher than normal structural stress can lead to a catastrophe, and all this without giving pre-alarm signal.
As the researchers point out in their study published in Scientific Reports , many cases of structural failures in buildings, bridges, ships, and airplanes are closely related to stress concentration . The measurement of strain is a direct indicator of the existence of stresses, so it is crucial when you want to know the structural health of a certain element.
The new smart skin is the main component of S4 , an optical monitoring system that does not require physical contact. Until now, structural stresses have been measured using two technologies, one with contact and one without.
In contact detection, piezoresistive and fiber Bragg grating (FBG) sensors are used, which are placed on the very structures to be measured. Both piezoresistive and FBG sensors measure strain in only one direction, which makes it more costly and a worse result when you want to map the strain of the entire field. Yes, they are useful when you already have prior knowledge of the areas in which the tensions are concentrated, because that way they are placed in the exact place.
Non-contact deformation detection techniques have, for their part, two fundamental advantages. The first is that no electrical or fiber optic connections are required between the sensors and the measurement device. The second is that they show the strain distribution in a two-dimensional region of interest, which is important for damage detection and fracture behavior studies. In non-contact detection, the DIC or digital image correlation system is mostly used. This system is used to compare photos that have been taken over time of surfaces with embedded “specks”. The researchers claim that the S4 system may be even better than the DIC but that the two can work perfectly together.
What the scientists at Rice University have done is to incorporate the S4 technique and the DIC into a single system, the smart skin. Artificial skin has three layers , which are configured according to the surface to be covered. In general, an opaque layer containing the DIC specks is given first. A second layer is then applied, which is a transparent polyurethane that isolates the base of the nanotubes. The third layer, which is the nanotube detection layer, is made of toluene. This component evaporates, making the detection layer less than a micron thick. To make the leather last for years, a final protective layer can be applied.
The stress detection system also needs a reading device, a small laser that stimulates the nanotubes. A portable spectrometer is also necessary to see if they are strained or not.
Referencia: Meng, W., Pal, A., Bachilo, S.M. et al. 2022. Next-generation 2D optical strain mapping with strain-sensing smart skin compared to digital image correlation. Scientific Reports. DOI: https://doi.org/10.1038/s41598-022-15332-1
