Investigation of Fiber Bridging in Double Cantilever Beam Specimens

Abstract

An investigation was conducted to (1) see if fiber bridging could be eliminated or at least reduced and (2) evaluate an alternative approach for determination of in-situ Mode I fracture toughness values of composite matrix materials. Toward this end, double cantilever beam (DCB) specimens were made using unidirectional layups of T6C/Hx205 composite material in which the delaminating halves were placed at angles of 0°, 1.5°, and 3° to each other. The small angles between the delaminating plies were used to avoid fiber nesting without significantly affecting Mode I behavior. A starter delamination was introduced by using a thin Teflon® insert. DCB specimens were also fabricated and tested with a 0.0254-mm thick bondline of Hx205 between aluminum adherends. This study resulted in the following conclusions: the extent that fiber bridging and interlaminar toughness increase with crack length can be reduced by slight cross ply at the delamination plane to reduce fiber nesting; some fiber bridging may occur even in the absence of fiber nesting; the first values of toughness measured ahead of the thin Teflon® insert were observed to be very close to the toughness of the matrix material with no fiber bridging; and thin (0.0254-mm) adhesive bondline of matrix material appears to give toughness values equal to the interlaminar toughness of the composite matrix without fiber bridging.