Dynamic Humidity Fracture Mechanics in Crosslinked Polyurethane Laminates
Cyclic relative humidity drives interfacial shear cracks in polyurethane-laminated paperboard through hygro-expansion mismatch and localized network plasticization.
Strain Energy Release
Mechanical Physics
Dissipation of accumulated elastic deformation energy per unit crack extension quantifies the driving potential for fracture along substrate and adhesive interfaces. In high-speed packaging conversion, strain energy release measures the capacity of stored mechanical strain within deformed paperboard or composite webs to propagate interface separation. The mechanical framework balances stored elastic energy against the thermodynamic work of surface formation.
Applicability ceases when excessive yielding across ductile polyolefin layers obscures the crack tip boundary, demanding nonlinear plastic dissipation analysis.
Kinematic Driving Forces
External tension applied during web slitting, die cutting and creasing stores recoverable strain energy throughout the elastic bulk of solid bleached boxboard. When an initiated crack or edge nick advances, this stored strain energy relieves locally, converting elastic potential into crack propagation energy. Machine-direction orientation increases stored strain energy due to higher elastic stiffness, driving rapid tear propagation along fiber alignment paths.
Multi-ply cardboards relieve stored lamination strain through delamination of the weakest inner mechanical pulp layer. Dynamic crack speed accelerates when the available strain energy release exceeds the intrinsic critical fracture toughness of the fiber network.
Lamination Integrity
Flexible barrier pouches filled under pneumatic pressure develop high hoop stresses that feed strain energy directly into the sealed flange seams. In heat-sealed packaging, sudden pressure discharges liberate sufficient strain energy to drive peel fronts across bonded perimeter tracks. Laminated pouch structures combining polyester and foil mitigate this failure by diffusing local stresses through ductile adhesive layers before the critical threshold is reached.
Creasing scores in heavy cartonboard generate intense local bending strain, which must not exceed the cohesive peel threshold of the underlying ply boundaries. Structural carton failure emerges when high residual strain energy accumulated during platen die cutting vents through spontaneous edge delamination.