Visualizing Stress Localization in Fiber Reinforced Polymer Composites Using Mechanophores

Polymers matrix composites exhibit tunable mechanical properties, critical for numerous industrial applications, due to their complex structure-property relationships. Predicting material behavior under mechanical stress requires precise control of these structure-property relationships. Mechanophores, stimuli-responsive molecules that undergo optomechanical changes under stress, offer a pathway to visualize molecular-level deformation mechanisms in real-time. Despite extensive studies in polymer mechanochemistry, their integration within fiber-reinforced polymer composites remains challenging due to the complexities of interfacial stress transfer and crack propagation. In this work, we optimized mechanophore-functionalized systems to map stress distributions in elastomeric matrices, scaling these observations to polymer composites by modulating fiber-matrix interfacial strength. We developed a model composite consisting of a continuous single fiber embedded in spiropyran (SPN) mechanophore functionalized poly(dimethylsiloxane) (PDMS) elastomer and subjected it to uniaxial tension. Using in situ confocal microscopy, fluorescence intensity was captured during deformation, providing real-time stress visualization. We established a scaling relationship between fluorescence intensity and localized stress fields, quantitatively validated through finite element analysis (FEA). The results demonstrate a power-law scaling of fluorescence intensity with stress concentration near the fiber-matrix interface, allowing for the precise quantification of stress distribution. Furthermore, this calibration was extended to different fiber end geometries, enabling stress mapping in short fiber-reinforced polymer composites. The method offers a robust, scalable approach for quantifying stress distribution in heterogeneous composites, providing new insights into failure mechanisms critical for advanced polymer composite design.