"Quantifying Localized Stresses in the Matrix of a Fiber-Reinforced Composite via Mechanophores"

A profound understanding of the factors triggering damage in fiber-reinforced polymers is pivotal in enhancing their durability. Incorporating mechanoresponsive molecular force probes, known as mechanophores, offers a promising avenue for directly monitoring stress concentration. Our investigation focuses on the utilization of spiropyran (SP) mechanophores (MPs) embedded within a polydimethylsiloxane (PDMS) matrix to unveil stress localization during fracture within a singular fiber-reinforced polymer. The SP mechanophore undergoes a transformative shift from a non-fluorescent state to an active state, referred to as merocyanine, in response to mechanical forces. Our experimental approach involves uniaxial tensile testing of a single fiber-reinforced polymer, effectively replicating the primary failure modes seen in traditional fiber-reinforced composites. By subjecting samples to uniaxial tensile loading along the fiber's orientation, we observe the concentration of stresses through the activation of mechanophores. Evidently, these stresses tend to amass in the matrix, primarily in the vicinity of the fiber, gradually decreasing away from the fiber's surface. To gain deeper insights, we employ confocal microscopy, allowing us to visually track mechanophore activation and conduct a quantitative evaluation of fluorescence intensity. In parallel, our methodology also incorporates finite element modeling to construct a calibration mechanism for quantifying stress levels based on the observed fluorescence intensity. This approach provides a dynamic framework to understand and measure the stresses encountered in real-time, which can significantly contribute to the formulation of high-performance composites.