How tip geometry controls fracture in ductile polymer glasses and brittle elastomers

Based on spatially-temporally resolved polarized optical microscopy (str-POM) measurements, we studied the fracture behavior of ductile and brittle glassy polymers as well as highly crosslinked rubbers to draw the following conclusions: (1) there is no tip plasticity below a threshold load in ductile plastics such as polyethylene terephthalate. (2) In ductile polymer glasses, before tip yielding at a common tip stress, the remote load scales with notch length a as a^-1/2, in agreement with the Inglis solution. (3) A finite stress saturation zone is observed in elastomers at loading levels even well below fatigue threshold due to significant crack tip blunting. (4) When the thickness is small enough for the plane stress condition to prevail at crack tip, in double-edge notch tension (DENT) for both ductile glassy polymers and rubbers that is characterized by ligament length l, nominal strain in the ligament is defined by ε_lig = X/l, where X is tensile displacement; tensile force F increases linearly with X independent of l; tip stress increases linearly with the far-field σ_lig (∼ε_lig). By demonstrating stress concentration at the crack tip in DENT in elastic materials and characterizing crack propagation in ductile polymers, the present study fills the missing gap in our understanding of fracture behavior in a wider range of polymeric materials. The acquired knowledge may be useful to guide specific design for packaging materials.