Graphical abstract Abstract This study explores the crack opening dynamics in Mode-I fracture of pre-stretched viscoelastic elastomer sheets under varying strain energy release rate (Γ). The degree of viscoelasticity was widely tuned by altering the mixing ratio of telechelic precursor chains and a tri-functional cross-linker, producing elastomers with different amounts of relaxation components such as dangling chains and finite clusters not incorporated into infinite networks. When Γ exceeds a critical threshold Γ∗, the crack velocity exhibits a sharp, order-of-magnitude increase—independent of viscoelasticity—similar to that observed in conventional elastic elastomers. A key difference, however, emerges in the crack-tip opening displacement (CTOD):...