Observations on the Tearing of Elastomers

Abstract

Abstract 1. Frequency distributions of stick-slip tear arrest and initiation energies have different standard deviations; initiation values show greater variance. Tear arrest frequency distributions are symmetrical and have been described by a normal distribution. The tear-initiation distribution is skewed and has been modeled using both the extreme value and Weibull distributions. As a result, it is concluded that two separate mechanisms are responsible for the creation of the force minima and maxima observed in stick-slip tearing. Consequently, data reduction methods which average these values to estimate tear energy are inappropriate. 2. The direction of skewness of the tear-initiation energies indicates a distribution of the largest extreme. This has been contrasted with constant-rate tensile-rupture data which are distributed according to the smallest extreme. As a result, it is concluded that the process responsible for tear initiation is not a weakest link mechanism, but, rather, depends on a process which structurally inhibits tear propagation. A mechanism of anisotropic tear-tip reinforcement caused by large regional deformation has been discussed as a possible explanation. 3. Tear energies calculated from tear-initiation forces have been shown to be dependent upon the energy stored in simple tensile extension and the volume of material available to be extended. This type of behavior shows that the tear-initiation energy is perhaps a reasonable indicator of strength but does not meet the criterion for an inherent material property. Tear-arrest values have been shown to be independent of these features and are considered unbiased estimators of inherent material tear energies. 4. It has been shown that for experimental purposes, stick-slip tearing can be eliminated and tear-tip diameter can be held constant using a constrained trouser-tear test. This is in agreement with earlier work by Gent and Henry. It has also been shown that the constrained tear energy thus measured is an inherent feature of stick-slip tearing results, requiring only additional data reduction to extract. Consequently, the constrained tear energy can be estimated from stick-slip tearing, and conversely, the magnitude of stick-slip tearing is predictable from a constrained tear energy and simple tensile extension data.