Long-term memory and delayed shear localisation in soft glassy materials

We study theoretically the dynamics of soft glassy materials during the process of stress relaxation following the rapid imposition of a shear strain. By detailed numerical simulations of a mesoscopic soft glassy rheology model and three different simplified continuum fluidity models, we show that a dramatic shear localisation instability arises, in which the strain field suddenly becomes heterogeneous within the sample, accompanied by a precipitous drop in the stress. Remarkably, this instability can arise at extremely long delay times after the strain was applied, due to the long-term memory inherent to glassy systems. The finding that a catastrophic mechanical instability can arise long after any deformation could have far reaching consequences for material processing and performance, and potentially also for delayed geophysical phenomena.