Criteria for Unstable Tearing of Elastomers

Abstract

Abstract 1. Unstable, or stick-slip, tearing has been evaluated in a series of unfilled elastomers. It was found that unstable tearing in these materials occurs only in well defined regions of temperature and rate. The behavior of the material in these regions follows a definite time-dependent pattern, suggesting the existence of simple time-temperature correspondence. This has been demonstrated by showing that the same shift factors that superpose other viscoelastic functions also describe the time-dependent nature of unstable tearing. Previous work showed that the magnitude of unstable tearing could be controlled by test-specimen geometry. It is concluded in this work that the mechanistic origin of unstable tearing can be traced to viscoelastic processes of the rubbery network. The combined results of these 2 studies show that unstable tearing is a characteristic feature of the polymer network which may be controlled or eliminated by test-specimen design. 2. For the amorphous elastomers investigated, it has been shown that the boundaries between regions of stable and unstable tearing can be explained in terms of simple extension properties and characteristic viscoelastic transitions of the elastomer. Specifically, the long-time (or high-temperature) boundary between conditions of stable and unstable tearing has been associated with a critical extension ratio, λc, which corresponds to an upturn in tensile stress-strain curves. If temperature and rate conditions are such that the deforming material at the tear tip does not extend beyond λc, unstable tearing does not occur. This is consistent with an anisotropic tear-tip reinforcement model of unstable tearing which has been discussed in detail. The short-time (or low-temperature) boundary between stable and unstable tearing conditions has been related to a characteristic time, ttr, associated with the onset of the rubber-to-glass transition. This characteristic time also corresponds to a change in the simple extension response of the elastomer. At times shorter than ttr, a distinct yield point was observed in constant rate stress-strain curves. It is therefore surmised that unstable tearing does not occur in this region due to the plastic yielding of the material, which interrupts the formation of the anisotropically reinforced structure at the tear tip. 3. Time-dependent tear-energy master curves of elastomers over broad time scales all display the same general features. Near the glassy region, the elastomers were found to approach a glassy tear energy of approximately 60 kJ/m2. At much longer times, in the terminal regions of the mastercurves, the tear energy dropped rapidly. In between these two extremes, a plateau was observed for all the elastomers, with NR displaying the lowest plateau slope.