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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.

Force‐responsive molecules that produce fluorescent moieties under stress provide a means for stress‐sensing and material damage assessment. In this work, we report a mechanophore based on Diels‐Alder adductTAD‐Anof 4,4′‐(4,4′‐diphenylmethylene)‐bis‐(1,2,4‐triazoline‐3,5‐dione) and initiator‐substituted anthracene that can undergo retro‐Diels‐Alder (rDA) reaction by pulsed ultrasonication and compressive activation in bulk materials. The influence of having C−N versus C−C bonds at the sites of bond scission is elucidated by comparing the relative mechanical strength ofTAD‐Anto another Diels‐Alder adductMAL‐Anobtained from maleimide and anthracene. The susceptibility to undergo rDa reaction correlates well with bond energy, such that C−N bond containingTAD‐Andegrades faster C−C bond containingMAL‐Anbecause C−N bond is weaker than C−C bond. Specifically, the results from polymer degradation kinetics under pulsed ultrasonication shows that polymer containingTAD‐Anhas a rate constant of 1.59×10⁻⁵ min⁻¹, whileMAL‐An(C−C bond) has a rate constant of 1.40×10⁻⁵ min⁻¹. Incorporation ofTAD‐Anin a crosslinked polymer network demonstrates the feasibility to utilizeTAD‐Anas an alternative force‐responsive probe to visualize mechanical damage where fluorescence can be “turned‐on” due to force‐accelerated retro‐Diels‐Alder reaction.

Stress concentrations in polymer matrix composites occur due to non-uniform loadings which develop near the interface between the matrix and reinforcement in a stressed composite. Methods to better understand the evolution of this stress concentration are required for the development of advanced composites. Mechanophores, which are stress responsive molecules, can be embedded into the polymer matrix and used to quantify the local stresses in a loaded composite. In this work, single particle model composites were fabricated by combining functionalized glass particles embedded into a silicone/mechanophore matrix. Confocal microscopy was then used to measure the mechanophore activationin situduring mechanical loading. The fluorescence intensity was correlated to maximum principal stress values obtained from a finite element analysis (FEA) model of the system utilizing an Ogden hyperelastic model to represent the elastomer. By calibrating stress to fluorescence intensity spatially, quantitative stress measurements can be obtained directly from fluorescent images. To validate this technique, calibrated stress values for a two-particle composite system were compared to a FEA model and good agreement was found. Further experiments were performed on silicone matrix composites containing short cylindrical particles oriented with their major axis parallel or perpendicular to the stretching direction. To demonstrate the versatility of the single particle intensity/stress calibration approach, maximum principal stress values were mapped on the fluorescence images of the cylindrical experiments. This technique has potential to quantify stress concentrations quickly and accurately in new composite designs without the use of FEA models or differential image correlation.