Creep Mechanics
Primary creep in polymers and paper fibres follows a non-linear temporal decay defined by the andrade power law. This relationship models the initial transient stage of deformation where material resistance increases as internal molecular structures reorient under sustained stress. The law states that strain increases proportional to time raised to a fractional power, typically one third.
This mathematical description allows engineers to predict how packaging substrates like corrugated medium or folding boxboard will deform during long duration storage under heavy compressive loads.
Strain Formulation
Precise calculations require identifying the creep coefficient and the specific time exponent for the substrate in question. The andrade power law assumes that material hardening occurs faster than the flow of viscous elements within the fibre matrix. Deviations from this model frequently occur when moisture levels change or when stress exceeds the proportional limit of the pulp formulation.
Accurate estimation of these parameters enables reliable assessment of stack life for containerboard and high-density paper rolls.
Industry Application
Packaging manufacturers apply the principles of the andrade power law to optimize the geometry of fluting profiles and the selection of linerboard weights for vertical shipping stacks. Performance variations observed during box compression testing often originate from discrepancies in how specific fibre types adhere to this time dependent decay curve. Engineers adjust the initial stiffness requirements to compensate for the anticipated deformation calculated through this power function.
Failure to account for the transient creep phase results in structural collapse of secondary packaging long before the theoretical maximum load is reached.
