Stress-Induced Crystallization around a Crack Tip in Natural Rubber

Abstract

Stress-induced crystallization, orientation, and crystallinity have been measured by X-ray diffraction around cracks in a cis-1,4-polyisoprene sample drawn at low ratio, λ < 3.5. A zone of maximum crystallinity and a transition zone of varying crystallinity are observed; the dimensions of these semicrystalline zones surrounding the crack tip are measured as a function of the draw ratio, crack length, and cross-link density. An isocrystallinity map is established; this permits one to measure the local draw ratio and then the local stress around the crack tip. The stress distribution around the crack tip is compared with the scaling laws predicted by the linear or nonlinear elasticity theories. Finally, one shows the existence of a relaxed zone between the crack tip, the crack surface, and the lateral sides of the sample. The surface of this zone is comparable to the area of the semicrystalline zone around the crack tip and to the crack opening surface.