Polymer Film Micro Fracture Mechanisms under High Strain Shear in Folding Boxboard Creases
High strain shear during boxboard creasing causes polymer film micro fractures when local elongation exceeds film yield capacity along the outer bend radius.
Tensile Necking
Plastic Deformation
Polymeric films and extrusion coatings laminated onto paperboard undergo localized cross-sectional reduction under high tensile stress. The plastic deformation phenomenon called tensile necking occurs when applied tension causes uniform elongation to yield to localized thinning across a polymer film strip. Packaging converters monitor this material behavior to prevent film web rupture and uneven barrier coating thickness during high-speed converting processes.
Failure Propagation
Uniaxial tensile testing subjects polymer barrier materials to controlled extension while recording stress-strain curves. During tensile necking, material stress reaches a yield point, initiating localized width and thickness reduction at a weak point within the specimen. Fibril orientation within the necked zone hardens the polymer locally, transferring strain to adjacent unnecked sections until necking propagates across the entire gauge length.
In paperboard laminates, severe necking leads to delamination between the paper substrate and plastic film, destroying seal strength and water vapor barrier continuity. Controlling polymer resin selection, molecular weight distribution, and extrusion coating conditions minimizes necking tendency, ensuring uniform film thickness across wide converting webs.
Strain Boundary
Uncontrolled necking terminates in localized ductile fracture when total plastic strain exceeds ultimate material strain capacity. Excessive thickness reduction across necked regions compromises barrier properties long before physical web rupture occurs.