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

Understanding the stress distribution within fiber-reinforced polymers (FRPs) is critical to extending their operational lifespan. The integration of mechanoresponsive molecular force probes, referred to as mechanophores, presents a potential solution by enabling direct monitoring of stress concentrations. In this study, spiropyran (SP) mechanophores (MPs) are embedded within a polydimethylsiloxane (PDMS) matrix to visualize stress localization during loading within a single fiber-reinforced framework. The SP mechanophore undergoes a transition from a non-fluorescent state to an active state (merocyanine) through isomerization in response to mechanical forces. Using a single fiber mounted axially within the matrix, the fundamental failure modes observed in conventional fiber-reinforced composites are replicated. Samples are strained under uniaxial tensile loading along the fiber direction and the localization of stresses is observed via MP activation. Stresses are concentrated in the matrix near the fiber region that gradually decreases away from the fiber surface. Confocal microscopy is used to visualize mechanophore activation and quantitatively assess fluorescence intensity. Finite element modeling is used to develop a calibration to quantify the stresses based on the observed fluorescence intensity. These outcomes underscore the viability of employing these mechanoresponsive molecules as a potential means to visualize real-time stress distribution, thereby facilitating the design of high-performance composites. Mechanical force sensing molecules (mechanophores, MPs) are incorporated into the matrix of a single fiber composite tensile specimen. When the samples are pulled in tension, fluorescence microscopy is used to observe the stress distribution in the loaded composite as the MPs are "activated" when the polymer network is locally deformed.image