The ability to sense applied damage and correlate it with a measurable signal is extremely desirable in any material application to prevent catastrophic failure and the possible loss of use of the material or human injury. Mechanochemistry, in which mechanical forces induce chemical changes, can allow for targeted damage detection by way of embedded mechanophore units, with certain mechanophore chemistries emitting a fluorescent signal in response an applied force. In this work, we successfully employed microparticles of the mechanophore dimeric 9-anthracene carboxylic acid (Di-AC) in a thermoset polyurethane matrix to study their application as universal stress-sensing fillers in network polymer matrix composites. Under a compressive force, there is bond breakage in the mechanically weak cyclooctane photodimers of Di-AC, 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 fluorescent signal change at an applied strain of only 2%. 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 compared to the 2.5 wt%, due to particle reinforcement in the composite. Overall, the synthesis of Di-AC as a stress-sensitive particle filler allows for facile addition of advanced functionality to these ubiquitous thermoset composites.