Noncontact Strain Mapping Using Laser-Induced Fluorescence from Nanotube-Based Smart Skin

Stress fields around structural discontinuities such as cracks usually cause complex but distinct strain contours/maps when structures are subjected to load. Hence, mechanical strain on structural surfaces can provide useful information on the condition of materials, including damage location and severity. The phenomena of stress concentration or strain concentration around discontinuities (such as holes and cracks) can be used to perform nondestructive evaluation (NDE) of structural components. Among analytical computation, numerical simulation, and experimental studies to investigate the stress/strain field around a structural discontinuity, experiments are the most accurate in revealing the actual complex strain conditions. In this paper, a strain-sensing smart skin (S4) film sensor was used to study the strain distribution in metallic plates near different structural discontinuities. S4 is a newly developed, noncontact, full-field strain technology that utilizes the strain-sensitive photoluminescent properties of single-walled carbon nanotubes (SWCNTs). Aluminum bars in tension were studied in two cases—with a central hole and with an edge notch. In both cases, S4 film sensors measured the residual strain contours near structural discontinuities under large axial loading at room temperature. Linear elastic fracture mechanics (LEFM) was used to compute the closed-form solution of strain fields. Finite-element elastoplastic nonlinear models were also constructed and validated by using strain gauge data from the experiments. The FE analysis results were found to match the strain distribution obtained from S4 measurements. S4 technology can be usefully applied in the realms of nondestructive evaluation, experimental mechanics, and structural health monitoring.