All-climate mechanochromic Al₂O₃/Epoxy composites as compression stress sensors

Mechanochromic materials, which exhibit visible color changes in response to mechanical stress, offer significant potential for real-time damage detection and structural health monitoring. However, many existing systems suffer from high activation thresholds and limited operational temperature ranges, which hinder their practical application in complex environments. In this study, we present the development of a Rhodamine-functionalized epoxy composite (Rh-MC) reinforced with nano-Al₂O₃, demonstrating excellent mechanochromic performance across a wide temperature range (−60 °C∼140 °C). Under compressive stresses as low as 150 MPa, the composite undergoes progressive color changes and fluorescence enhancement due to the ring-opening of Rhodamine units, exhibiting a strong linear correlation between applied stress and fluorescence intensity ratio. The incorporation of nano-Al₂O₃ significantly enhances the mechanical properties of the composite, promoting uniform stress transfer at the bulk scale while simultaneously amplifying local stresses near mechanophores, thereby enabling more efficient mechanochromic activation. Reversibility tests demonstrate that the mechanochromic signal can be thermally erased, confirming the recoverable nature of Rhodamine activation and the thermal stability provided by the nano-filler. To the best of our knowledge, this is the first report of a mechanochromic composite exhibiting reliable, reversible, and quantifiable stress responses across such an extensive temperature range. This enables the potential application of Rh-MC as a robust all-climate, multifunctional material platform for stress visualization, damage pre-warning, and intelligent sensing in harsh service environments.