In paper testing, folding endurance is defined as the logarithm (to the base of ten) of the number of double folds

Double fold (paper testing)

A double fold is when a paper sample is folded first backwards and then forwards about the same line, i.e. one complete oscillation. The number of double folds that is required to make a test piece break is used to determine the material's folding endurance and fold number.The total folding angle ...

 that are required to make a test piece break under standardized conditions:

F = log₁₀ d,

where F is the folding endurance and d the number of double folds.

Folding endurance is especially applicable for papers used for maps, bank notes, archival documents etc. The direction of the grain in relation to the folding line, the type of fibres used, the fibre contents, the calliper of the test piece etc., as well as which type of folding tester that is used affect how many double folds a test piece can take.

Folding endurance must not be confused with the related term fold number

Fold number

Fold number refers to how many double folds that are required to cause rupture of a paper test piece under standardized conditions. Fold number is definied in ISO 5626:1993 as the antilogarithm of the mean folding endurance:...

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Standards on folding endurance

• ISO 5626: Paper – Determination of folding endurance
• TAPPI Test Method T 551: Folding endurance of paper (MIT tester)
• TAPPI Test Method T 423: Folding endurance of paper (Schopper type tester)