A non-intrusive and non-disruptive internal fluorescent pattern is designed, developed, and tested using digital image correlation (DIC), an effective non-contact optical method, to measure deformation at internal planes of a polymer matrix material. The working principle relies on a small amount of a photoactivatable dye (0.05 % of final mass of epoxy), spirolactam of Rhodamine B, which is covalently bound into an epoxy network and patterned through the entire sample volume. A lithographic chrome mask, etched with transparent semi-randomly spaced circular features, is used as a contact mask on top of the polymer substrate while the dye is activated with ultraviolet light. The resulting microscale fluorescent pattern is observed as a columns through more than 400 µm of the sample thickness. 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.  Results show 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 and opens the possibility of tracking material deformation in three dimensions without adding physical particles internally.

Non-intrusive fluorescent pattern for internal microscale digital image correlation

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