This paper deals with the debonding behavior of paper-based friction materials under out-of-plane tensile loading. Double cantilever beam tests were carried out for four kinds of friction materials in order to evaluate the debonding fracture toughness and clarify the influence of constituents on fracture toughness. The constituents of the friction materials are aramid, cellulose, carbon fiber, filler and phenolic resin. From the double cantilever beam tests, the following results were obtained: (1) The debonding fracture toughness of the friction materials can be estimated by using double cantilever beam test for interlaminar fracture toughness of laminated composites. (2) The debonding fracture resistance of the friction materials increases with crack extension, and becomes constant after the crack extension reaches 50mm. (3) The debonding fracture toughness is enhanced by aramid fibers and decreased by carbon fibers. The influence of the constituents on debonding fracture toughness agrees with that on in-plane fracture toughness. (4) SEM micrographs of fracture surface imply that fibrillation of aramid fibers increases the debonding fracture toughness.
