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The fiber-matrix interface in many composites has a profound influence on composite performance. The objective of this study is to understand the influence of composition and concentration of coupling agent on interface strength by coating E-glass fibers with solutions containing a mixture of hydro-lyzed propyl trimethoxysilane (PTMS) and g-aminopropyl trimethoxysilane (APS). The failure behavior and strength of the fiber-matrix interface were assessed by the single-fiber fragmentation test (SFFT), while the structure of silane coupling agent was studied in terms of its thickness by ellipsometry, its morphology by atomic force microscopy, its chemical composition by diffuse reflectance infrared Fourier transform (DRIFT), and its wettability by contact angle measurement. Deposition of 4.5 1073 mol=L solution of coupling agent in water resulted in a heterogeneous surface with irregular morphology. The SFFT results suggest that the amount of adhesion between the glass fiber and epoxy is dependent not only on the type of coupling agent but also on the composition of the coupling agent mixture. As the concentration of APS in the mixture increased, the extent of interfacial bonding between the fiber and matrix increased and the mode of failure changed. For the APS coated glass epoxy system, matrix cracks were formed perpendicular to the fiber axis in addition to a sheath of debonded interface region along the fiber axis.

The demand for fiber-reinforced polymeric composites in aircraft, automobiles, ships, and housing is increasing. Approximately 95% ofcomposites used today are fabricated from glass fibers, with epoxy resin being the preferred polymeric matrix because of the relatively good price-to-performance ratio, high availability, ease of processing, and dimensional stability. One of the major technical challenges to the use of composites in structural applications is the reliable prediction of long-term performance (e.g., failure behavior, fatigue behavior, durability, stiffness). When composites are manufactured, a small region (< 1 mm) known as the fiber-matrix interphase forms between the fiber and the matrix [1]. This region exhibits properties distinguishably different from the properties of the bulk matrix [2]. Since the fiber-matrix interphase transfers stress between the fiber and matrix, the efficiency of this stress-transfer process and a composite’s strength and durability are controlled by this region’s properties. The interphase stiffness, fiber topography, and fiber-matrix chemical bonding are critically important to the stress-transfer process and composite performance. The efficiency of this process is determined indirectly by micromechanics tests and quantified by a value termed the fiber-matrix interfacial shear strength (IFSS). In addition, micro-mechanics tests are used to probe a compo-site’s strength, durability, and failure behavior.