Effect of interfacial adhesion and statistical fiber strength on tensile strength of unidirectional glass fiber/epoxy composites. Part I: experiment results

The effects of fiber surface treatment on ultimate tensile strength (UTS) of unidirectional (UD) epoxy resin matrix composites are examined experimentally. The interfacial shear strength (IFSS) and statistical fiber strength are significantly altered by five different kinds of surface treatments, which are: (a) unsized and untreated; (b) gamma-glycidoxypropyltrimethoxysilane (gamma-GPS); (c) gamma-methacryloxypropyltrimethoxysilane (gamma-MPS); (d) mixture of gamma-aminoxypropyltrimethoxysilane (gamma-APS), film former (urethane) and lubricant (paraffin); and (e) urethane-sized. The maximum UTS is obtained for the relatively strong interfacial adhesion (glass/gamma-MPS/epoxy) but not for the strongest interfacial adhesion (glass/gamma-GPS/epoxy). The governing micro-damage mode around a broken fiber and the interface region is matrix cracking for gamma-GPS treated fibers, and a combination of interfacial debonding and matrix cracking for gamma-MPS treated fibers. The microdamage mode related to the interfacial adhesion strongly affects the fracture process, and thus the UTS of UD composites. The results also indicate that the interfacial adhesion can be optimized for effective utilization of fiber strength for fiber composites. A parameter called "efficiency ratio" of fiber strength in UD composites is proposed to examine and distinguish different effects of IFSS and fiber strength on the UTS of UD composites. The experimental results show that improved UTS of UD composites due to surface treatments mainly result from the increase in fiber strength but not from the modified interface. (C) 2000 Elsevier Science Ltd. All rights reserved.