Fast detection of early-stage damage in soft elastomers

Subcritical fracture of materials can take place below the critical failure condition by slow accumulation of internal damage. While subcritical fracture accounts for most of the structural failures in use, it is challenging to understand its microscopic origin or predict/prevent it in time, due to the lack of current non-destructive detection methods with suitable resolution. We use simultaneous space-time-resolved multispeckle diffusing wave spectroscopy (MSDWS) measurements and mechanophore mapping to investigate the fracture of polydimethylsiloxane elastomers.  We identify a fracture precursor in the form of large-scale strain rate acceleration, detected over a ~cm² sample area, long time (up to hundreds of seconds) before macroscopic fracture, which we attribute to the far-field elastic deformation induced by localized molecular damage (over a ~ 0.01 mm² sample area). While the evolution of local damage is currently detectable only by mechanochemistry, the resulting strain rate field is readily observable by MSDWS with ordinary imaging and image processing tools. As tested in different conditions and for various materials, our work shows that MSDWS is a powerful tool to characterize and predict microscopic damage, well before it becomes critical.