The crush resistance of paper under compressive stress is measured as the short span compression strength. To prevent the sample kinking, the free span length must be very small (typically 0.7 mm). The resistance measured according to DIN 54 518 gives the ultimate crushing load per unit width.
The flat crush resistance (ISO 3035, 1982) and the edge crush resistance (ISO 3037, 1996) are used to characterize the strength of corrugated board. In the for¬mer test, a circular sample of corrugated board is exposed to increasing pressure between two parallel planar plates until the corrugation collapses, and the max¬imum load is measured. In the second test, the load is applied in the direction of corrugation starting from the edge of the sample. The edge crush resistance is used in DIN 55 468 Part 1 for the classification of corrugated boards.
The folding strength is an important property of both packaging and graphic papers. It can be determined either as described in ISO 5626 (1993) as the Schop¬per double fold value, or as described in ISO 5626 (1993) as the Köhler-Molin folding endurance value. The sample is subjected to specified tension in both cases. The Brecht-Wesp pressure folder functions without tension. A sharp fold inthe sample is produced between two rolls and is repeated up to fifty times in the same position. The ratio of the tensile strength of the folded sample to that of the unfolded sample is the folding strength.