Mechanochemistry, in which mechanical forces induce chemical changes, can allow for targeted damage detection by way of embedded mechanophore units, which emit a measurable signal change correlating to an applied force. In this work, we successfully created stress-sensing, functional composites by employing microparticles of the mechanophore dimeric 9-anthracene carboxylic acid in a thermoset polyurethane matrix. The goal being to study the application of the particles as universal stress-sensing fillers in network polymer matrix composites, after previously evaluating the particles in an epoxy matrix. Under a compressive force, there is bond breakage in the mechanically weak cyclooctane photodimers, such that there is reversion to the fluorescent anthracene-type monomers. This fluorescent emission was then correlated to the applied strain, and the precursors to damage were detected with a noticeable signal change at a strain of only 2%, which was attributed to increased interactions between the matrix and the particles, with possible surface grafting occurring. This early damage detection was additionally possible at very low particle loadings of 2.5 and 5 wt%, with the 5 wt% loading showing enhanced material properties, due to particle reinforcement. Overall, the stress-sensitive particle filler allows for facile addition of advanced functionality to these ubiquitous thermoset composites. POLYM. (C) 2016 Society of Plastics Engineers