Abstract
Chromium-doped α-alumina is naturally photo-luminescent with spectral properties that are characterized by R-lines with two distinct peaks known as R1 and R2. When the material is subjected to stress, shifts in the R-lines occur, which is known as the piezospectroscopic (PS) effect. Recent work has shown that improved sensitivity of the technique can be achieved through a configuration of nanoparticles within a polymer matrix, which can be applied to a structure as a stress-sensing coating. This study demonstrates the capability of PS coatings in mechanical tests and investigates the effect of nanoparticle volume fraction on sensing performance. Here, measurements of spectral shifts that capture variation in stress of the coating during mechanical testing and in the region of substrate damage showed that stress contours are more noticeable on a soft laminate than hard laminate. It was found that the 20 % volume fraction PS coating showed the most distinct features of all the coatings tested with the highest signal-to-noise ratio and volume fraction of α-alumina. Post failure assessment of the PS coatings verified that the coatings were intact and peak shifts observed during mechanical testing were due to the stress in the substrate. The results suggest the ability to design and tailor the “sensing” capability of these nanoparticles and correlate the measured stress variations with the presence of stress and damage in underlying structures. This study is relevant to nondestructive evaluation in the aerospace industry, where monitoring signs of damage is of significance for testing of new materials, quality control in manufacturing and inspections during maintenance.
Highlights
The aerospace industry relies on nondestructive evaluation (NDE) methods to determine mechanical properties of new materials through standardized tests and quality inspection during manufacturing and maintenance.1 One of the significant concerns is monitoring the health of aerospace structures
Notable differences in capturing the stress variations that correlate with the presence of crack initiation and propagation in each open-hole tension (OHT) carbon fiber reinforced polymer (CFRP) specimen were observed in the PS maps
Comparing the PS maps for the hard laminates and the soft laminate, the stress contours are more clearly observable for the PS coating on soft laminate with lower applied loads since it has more ±45○ fibers than the hard laminate
Summary
The aerospace industry relies on nondestructive evaluation (NDE) methods to determine mechanical properties of new materials through standardized tests and quality inspection during manufacturing and maintenance. One of the significant concerns is monitoring the health of aerospace structures. When implementing α-alumina nanoparticles as piezospectroscopic coatings for the study of aerospace materials, the ability to sense stress variations when the materials are subjected to loading has been successfully demonstrated.. Based on a study by Stevenson et al it is expected that the coating’s sensitivity to changes in stress, which correlates with the PS coefficient (Equation 1), increases as the particle volume fraction increases. To validate this trend, the PS coatings tested were assessed for their sensitivity based on the SNR and luminosity, and the peak shift contour plots showing the qualitative stress distribution of the loaded specimens. The damaged OHT CFRP test specimens were assessed for post failure investigation using the piezospectroscopic coating
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