On the correlation between the mode I failure of adhesive joints and laminated composites

Abstract

The effect of the matrix constituent on the Mode I interlaminar fracture of laminated composites was determined directly by comparing test results obtained from composite and adhesive joint specimens fabricated from identical matrices. Resins of diverse mechanical properties were employed, including brittle and thermoplastic (PEEK) types. The brittle adhesives exhibited essentially no bond thickness ( t) effect but the fracture conditions of PEEK heavily depended on t. At bond thicknesses approximating the interlaminar resin layer the adhesive fracture energy of PEEK decreased with bond thickness. This interesting behavior that was observed also for BP-907 [H. Chai, 7th ASTM Symp. on Composite Materials, Testing and Design, 2–4 April 1984], a toughened resin, was attributed to the development of a state of nearly “hydrostatic” tension over the majority of the adhesive layer at the crack tip vicinity. In fact, scanning electron micrographs of the fractured surfaces revealed that failure actually occurred by means of shear-yielding that originated from the vicinity of the metal-matrix interface. The brittle (AS4/3502) and ductile (APC-2) matrix-based composites tested exhibited a “resistance” type fracture curve that helped to explain the large variance or scatter in G IC values reported in the literature for a given composite. Only over the initial phase of the crack growth was the failure truly interlaminar and the fracture energy independent of specimen geometry. This value was considered, therefore, as the true measure of G IC . The composite toughness, thus defined, fully coincided with its respective adhesive toughness, provided the thickness of the adhesive layer was sufficiently small.