Retroreflective Sheeting Materials for Use as Passive Strain Sensors

<br> Retroreflective sheeting materials (RRSM) are used for various applications in engineering, but primarily for traffic signs and pavement markings. There are several ASTM standard types of RRSM that have required values of retroreflection to ensure safe usage. Retroreflectivity (RR) is the portion of light returned to the light source measured in candelas per lux per square meter, as measured using a retroreflectometer. It is theorized that as load is applied to RRSM, the retroreflection will change and have a reasonably linear relationship to the material’s strain (ε), thus, opening the possibility for using RRSM material as a passive strain sensor for structural health monitoring that is low cost, practical, and innovative. A total of ten materials, manufactured by Avery Dennison and 3M, were loaded in tension and their strain and retroreflectivity were measured at 4.45 and 8.90 Newton (one and two pound) load intervals. Specimens were subjected to three loading and unloading cycles, and their sensitivity (retroreflectivity divided by strain) was calculated to determine their feasibility for use as passive strain sensors. Results show that the retroreflectivity of RRSM do change when subjected to load, and that certain materials demonstrate a reasonably linear relation to strain. Furthermore, others do not return to their baseline retroreflectivity but degrade through the course of repeated loading. The ten materials were also subjected to strength tests to determine their failure stresses and strain to failure. Four of the materials have the highest retroreflective sensitivity to strain and have high failure stresses and strains and are the most likely to perform well as passive strain sensors. These four materials were subjected to microscopic evaluation of their reflecting layer to determine why retroreflectivity changes with applied load. Additionally, initial environmental exposure tests were performed on one material type to determine best methods of adhering RRSM to common substrates and to evaluate how retroreflectivity changes with extended exposure. Future research will include continued testing of RRSMs to better understand their potential use as passive strain sensors.