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TAPPI T 836 Stiffness






Bending stiffness, four point method


The TAPPI T 836 Stiffness test specifies the method of determining the bending stiffness, also called flexural rigidity, in the machine and cross directions, of corrugated board using four-point loading. The procedure may also be used for solid boards and paperboard. The method is applicable to boards with a bending stiffness of 0.5 – 200 Nm (4.4 – 1770 lbf • in.).


A rectangular test specimen is clamped at each end with a clamping pressure P over the length s, leaving a free span l in the center. Each clamp can be turned around a rotation axis r. A force F is applied at each clamp at a distance h from the rotation axis. The bending moments F • h cause the test specimen to bend, and maximum deflection from the plane through the rotation axes is measured.  The bending stiffness is calculated from the bending moment, the width of the test specimen, the free span and the deflection. The time from the application of the force to the measurement is 10 ± 3 s.


It has been shown that the top-to-bottom compression strength of corrugated boxes is partly dependent on the load-carrying ability of the central panel areas. The ability of these central areas to resist bending under load will increase the stacking strength of the box. The difference in box compression strengths between boxes of identical size made from the same components but of different flute sizes (if the load is applied parallel to the flutes) is primarily due to differences in the bending stiffness of the box panels. Bending stiffness measurements of the combined board when used with the edge crush test and box dimensions, can accurately predict the top-to-bottom compression strength of a box.


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