The phenomenon of long-term strength of brittle heterogeneous materials characterized by a decrease of their strength in time under loading is explained based on the process of redistribution of stresses between elastic-creep media and elastic inclusion. The rules governing a crack's behavior when it is not able to pass the elastic inclusions without fracturing them are considered. For the model of this process, crack propagation in elastic-creep media with elastic inclusion, simulated by an elastic connection near the crack tip, is considered.