Elastic Boundary
Mechanical stress limits mark the transition point between reversible elastic deformation and permanent plastic deformation in structural materials. In paperboard converting and structural package design, yield strength defines the maximum tension or compression force a sheet withstands before undergoing permanent set. Standard tensile testing applies axial load to paperboard specimens until non-linear force-elongation behavior begins.
Beyond this point, internal fibre bonds rupture and cellulose networks realign irreversibly. High yield values allow packaging panels to absorb handling impacts without suffering permanent denting or panel bowing.
Converting Response
Forming and scoring operations rely on controlled stress distribution to achieve clean permanent folds without tearing outer plies. Substrates with high yield strength require higher scoring nip pressures to induce localized plastic deformation along crease lines. Insufficient scoring pressure results in elastic recovery, causing carton flaps to spring open on automated packaging lines.
Machine direction orientation exhibits higher yield values than cross direction orientation due to aligned fibre structures from wet-end web forming. Selecting paperboard with predictable yielding behavior ensures consistent creasing and structural rigidity in finished cartons.
Material Limit
Force applied beyond the yield point causes progressive structural degradation until ultimate tensile rupture occurs. Plastic deformation past yield alters sheet thickness and destroys original material geometry. Measuring the yield point establishes the safe load limit for stackable paperboard packaging.
