A Non-Intrusive Fluorescent Pattern for Internal Microscale Strain Measurements Using Digital Image Correlation

A non-intrusive internal fluorescent pattern is designed, developed, and tested using digital image correlation (DIC) to measure deformation at internal planes of a polymer matrix material. This new patterning technique method applies internal patterns without introducing physical particles to the polymer specimen hence preventing significant changes to the mechanical properties of the material. The feasibility of the internal fluorescent pattern for DIC measurement was established and quantified through a sequence of assessments including noise-floor, rigid body motion, and uniaxial tension tests. The working principle relies on a small amount of a photoactivatable dye, spirolactam of Rhodamine B, which is covalently bound into an epoxy network and patterned through the entire sample volume. A lithographic chrome contact mask, etched with transparent semi-randomly spaced circular features, is used on top of the polymer substrate while the dye is activated with ultraviolet light. The resulting microscale fluorescent pattern, collimated through more than 400 µm of the sample thickness, can be observed using a 514 nm excitation wavelength with a confocal microscope. The assessments demonstrated that this new non-intrusive and non-disruptive method of DIC patterning can measure strain fields on sub-surface planes in a transparent polymer matrix without bias from material deformation above that plane. To the best of our knowledge, this is the first demonstration of performing DIC using sub-surface or internal patterns without adding physical particles internally and opens the possibility of tracking material deformation in three dimensions without using the internal structure or adding particles.