Competition of strength and stress disorder in creep rupture

Based on a fiber bundle model of subcritical fracture with localized load sharing, we show that the interplay of threshold disorder and the inhomogeneous stress field gives rise to a rich dynamics with intriguing aspects. In the model, fibers fail either due to immediate breaking or to a slow damage process. When the disorder is strong, a large amount of damage occurs, which is randomly diffused over the system; however, for weak disorder, a single growing crack is formed, which proceeds in a large number of localized bursts. The microstructure of cracks is characterized by a power-law size distribution, which is analogous to percolation in the regime of diffusive damage; however, it becomes significantly steeper when a single crack dominates. Simulations showed that the size distribution of breaking bursts and of the waiting times in between have a power-law functional form with a load-dependent cutoff. The burst size exponent proved to be independent of the damage process; however, it strongly depends on the external load with a minimum value of 1.75. The waiting time distribution is sensitive to the details of the damage process with an exponent decreasing from 2.0 to 1.4 as bursts get more and more localized to an advancing crack front.