Experimental and analytical evaluation of fiber-matrix interface adhesion

New fibers and matrix materials are being developed constantly to meet the demands of new applications, rapid molding and curing processes and cost reduction targets.New interface improvements,through surface treatments and sizing materials, are also constantly developed to improve adhesion between fiber and matrix (and ultimately the composite properties). Consequently, there is a need to understand the fiber-matrix interface adhesion better so that an optimum choice of the combination can be made. This paper discusses an effective and efficient test procedure to measure the force required to pull fiber from matrix. The experiment involves pulling 24K, 50K carbon fiber tows bonded with the matrix only in the mid overlap region, thus ensuring that the load is carried only through the fiber-matrix interface. The test results are used to determine fiber-matrix adhesion peak traction separation and the mode II fracture toughness GIIc as required in micromechanics FEA models, where the fiber-matrix interface is represented by cohesive zone elements. This approach clearly distinguishes the effects of surface treatment and sizing on the unidirectional properties. The study also shows the effects of non- circular cross sections of some commercial grade carbon fibers on the fiber-matrix adhesion.. This paper presents the results of 3 commercial carbon fibers (each with their proprietary surface treatments) with a rapid curing epoxy matrix.