Quantifying Localized Stresses in a Single Fiber Reinforced Polymer Composite Utilizing Mechanophores*

Understanding the onset of failure in fiber-reinforced polymer composites is critical for increasing their service life. However, the instantaneous nature of fracture makes it difficult to obtain real-time experimental data on the damaged regions. A compounding challenge is the lack of experimental tools to sense and quantify localized stress and distribution within the matrix during fracture. Visual feedback from mechanoresponsive force probes (mechanophores) employed within the system can directly monitor stress buildup. In this work, we employed spiropyran (SP) mechanophore within polydimethylsiloxane (PDMS) matrix to visualize the stress localization during fracture in a single fiber-reinforced system. The SP mechanophore transitions from a fluorescent inactive state to an active state (merocyanine) via isomerization under force and strain. One single fiber within the matrix was used to simplify the composite model, demonstrating the fundamental failure modes prevalent in traditional fiber-reinforced composites during uniaxial tensile testing along the fiber direction. At the interface, the tensile load translates from matrix to fiber via shear stress. Confocal microscopy was used to visualize mechanophore activation and quantify the fluorescence intensity. The uniaxial tensile test of dogbone samples showed a gradual stress buildup in the matrix from the fiber/matrix interface. The results indicated that these mechanoresponsive molecules could be promising for visualizing real-time stress distribution and designing high-performance composites.