Sideways and stable crack propagation in a silicone elastomer

Abstract

We have discovered a peculiar form of fracture that occurs in a highly stretchable silicone elastomer (Smooth-On Ecoflex 00–30). Under certain conditions, cracks propagate in a direction perpendicular to the initial precut and in the direction of the applied load. In other words, the crack deviates from the standard trajectory and instead propagates perpendicular to that trajectory. The crack arrests stably, and thus the material ahead of the crack front continues to sustain load, thereby enabling enormous stretchabilities. We call this phenomenon “sideways” and stable cracking. To explain this behavior, we first perform finite-element simulations that demonstrate a propensity for sideways cracking, even in an isotropic material. The simulations also highlight the importance of crack-tip blunting on the formation of sideways cracks. Next, we provide a hypothesis on the origin of sideways cracking that relates to microstructural anisotropy (in a nominally isotropic elastomer). To substantiate this hypothesis, we transversely prestretch samples to various extents before fracture testing, as to determine the influence of microstructural arrangement (chain alignment and strain-induced crystallization) on fracture energy. We also perform microstructural characterization that indicates that significant chain alignment and strain-induced crystallization indeed occur in this material upon stretching. We conclude by characterizing how a number of loading conditions, such as sample geometry and strain rate, affect this phenomenon. Overall, this paper provides fundamental mechanical insight into basic phenomena associated with fracture of elastomers.

Simultaneously Toughening and Stiffening Elastomers with Octuple Hydrogen Bonding

“…[ 26,27 ] As the strain further increases, fiber‐like flaws are generated around the crack tip at the edge of the specimen (Figure 5d,e), followed by the delamination (Figure 5f–h). [ 28 ] The specimen does not fracture during the loading because of the limitation on the maximum displacement (Δ d max = 6.7 mm; gauge length ≈ 6 mm, as shown in Figure S20, Supporting Information) of the test device. During the unloading of the specimen (Figure 5i–l), the local damage can clearly be observed at the crack tip (Figure 5i,j).…”

An adaptive quasicontinuum approach for modeling fracture in networked materials: Application to modeling of polymer networks

“…Furthermore, increased disorder may lead to elevated force magnitudes in regions away from the crack tip leading to a more complex fracture pattern and potentially thicker cracks as was observed in Fig.8. Such complex force field distribution in highly anisotropic and disordered networks suggest the possibility of potential crack growth in unusual directions as has been observed recently as sideway cracks in silicone elastomers [36]. The QC methodology may enable systematic discovery of these new patterns.…”

Fatigue Testing of Wearable Sensing Technologies: Issues and Opportunities

“…Further, the area behind the crack-tip behaves elastically due to the offloading of the strain, and ahead of the crack-tip behaves plastically. While crack-tip-blunting is common in viscoelastic polymers, the elastic strain energy is dissipated by the actual propagation of the crack, which results in plastic deformation behind the crack-tip [ 115 , 148 ]. Additional research has shown that cracks in some viscoelastic polymers, such as silicone, can propagate sideways.…”

“…The sideways cracks help dissipate energy and eventually arrest, allowing the crack-tip to sustain the applied load; therefore, the area ahead of the crack-tip remains elastic, allowing for further stretching of the polymer. Sideways cracks were noted to occur in thick samples that underwent tensile testing at low strain rates [ 148 ]. Currently, whether other viscoelastic polymers undergo sideways cracking or if the sideways cracks help to extend the fatigue life of such polymers is unknown.…”