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): while elastic elastomers maintain a nearly constant CTOD across the velocity jump, viscoelastic elastomers show a pronounced reduction. CTOD reflects the extent of shape recovery in the unloaded region behind the crack tip from the pre-stretched state. In viscoelastic elastomers, this recovery requires a characteristic relaxation time (τ_R). When crack propagation accelerates at Γ∗, the observation time for CTOD (t_obs) at the specimen center becomes much shorter than τ_R, leading to incomplete recovery and, consequently, reduced CTOD. Independent creep-recovery tests demonstrate that τ_R corresponds closely to the critical observation time (t_obs∗) at which CTOD begins to decrease from its fully opened state. These findings reveal a unique strain-retention effect during rapid crack propagation in viscoelastic solids, providing new insights into their fracture dynamics.