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TAPPI T 808 Flat crush






Flat crush test of corrugated board
(flexible beam method)


The TAPPI T 808 Flat crush test is a measure of the resistance of the flutes in corrugated board to a crushing force applied perpendicular to the surface of the board under prescribed conditions. The test is satisfactory for single-faced or single wall (double-faced) corrugated board, but not for double-wall or triple-wall corrugated board or other complex corrugated structures, because of lateral motion of the central facing or facings.


Flat crush is a measure of the flute rigidity of corrugated board. A high flat crush value indicates a combination of good flute formation and at least adequate strength medium. Low flat crush can indicate a number of conditions. Among these are low strength medium, leaning flutes, crushed flutes, or poorly formed flutes.

This method generally does not simulate the reaction to crushing forces exerted on a container during actual use. Therefore, the correlation may be low between flat crush and the ability of a box to perform in the field. However, this method does provide a measure of medium strength available for keeping the two liner facings apart.

Boxes made from crushed corrugated board may have low flat crush and may not perform well. Board with leaning flutes may have low flat crush, but this may not affect box performance. For leaning flutes to cause low flat crush, there must be lateral movement between the two liners during the tests. After the board is formed into a box, lateral movement between the two liners is restricted by the geometry of the box, and the low flat crush cannot become operative. Also, this test method does not take into account the effect of linerboard stiffness in resisting crushing forces.


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