Real-Time Early Detection of Crack Propagation Precursors in Delayed Fracture of Soft Elastomers

The fracture of materials can take place below the critical failure condition via the slow accumulation of internal damage followed by fast crack propagation. While failure due to subcritical fracture accounts for most of the structural failures in use, it is theoretically challenging to bridge the gap between molecular damage and fracture mechanics, not to mention predicting the occurrence of sudden fracture, due to the lack of current nondestructive detection methods with suitable resolution. Here, we investigate the fracture of elastomers by using simultaneously space- and time-resolved multispeckle diffusing wave spectroscopy (MSDWS) and molecular damage mapping by mechanophore. We identify a fracture precursor that accelerates the strain-rate field over a large area (cm2 scale), at considerably long times (up to thousands of seconds) before macroscopic fracture occurs. By combining deformation or damage mapping and finiteelement simulations of the crack-tip strain field, we unambiguously attribute the macroscopic response in elastic deformation to highly localized molecular damage that occurs over a sample area of about 0.01 mm2. By unveiling this mechanism of interaction between the microscopic molecular damage and the minute but long-ranged elastic deformation field, we are able to develop MSDWS as a flexible, wellcontrolled tool to characterize and predict microscopic damage well before it becomes critical. Tested using ordinary imaging and simple image processing, MSDWS predictions are proven applicable for unlabeled and even opaque samples under different fracture conditions.