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Internal tearing resistance is the force perpendicular to the plane of the paper sheet required to tear multiple plies through a specified distance after the tear has been started. An Elmendorf-type tearing tester is used and the result is quoted in mN/ply. The Elmendorf-type tester is a pendulum that applies the force and measures a loss in the potential energy of the pendulum equating to the work done in tearing the paper.
Smithers use an L&W single tear tester for papers and a James Heal Elmatear tester for heavier weight boards.
Tearing index = Tearing strength/grammage, quoted in mN.m2/g.
Smithers also measures tear initiation using a Finch tear attachment on a tensile test machine. This edge tear test conforms to Tappi T470.
There are many applicable standards including BS EN 21974, ISO 1974, Tappi T414,DIN 53128, and SCAN P11. Smithers is UKAS, ISO 17025 accredited for the determination of internal tear strength against British standard BS EN 21974 and International Standard ISO 1974.
Internal tear strength testing of paper is included in Smithers' Paper Proficiency Service.