Classical molecular dynamics is exploited to examine mechanical characteristics (ultimate tensile strength, and tensile strain-to-failure) of graphene sheets containing highdensity ensembles of 5-8-5 defects. Each such a defect represents a divacancy associated with "pentagon-octagon-pentagon" atomic configuration in hexagonal crystal lattice of graphene. We revealed that the ultimate tensile strength of graphene sheets significantly degrades (by tens of percent) due to the presence of 5-8-5 defects in graphene, as compared to the tensile strength of ideal (defect-free) graphene. Also, results of our computer model indicate that both the ultimate tensile strength and the tensile strain-to-failure of graphene sheets containing high-density ensembles of 5-8-5 defects are sensitive to temperature. In particular, when temperature increases, the ultimate strength decreases in the almost linear way, and the tensile strain-to-failure decreases in the way similar to linear one.