Experiments were conducted to compare the fragmentation test with the microbond pull-out test for determining the interfacial shear strength between carbon AS4 fibers and a thermoset matrix consisting of a Di-Glycidyl Ether of Bisphenol A (DGEBA) resin cured with a diamine (metaphenylenediamine, m-PDA) curing agent. The results indicate that, for the microbond test, diffusion of the rather volatile m-PDA curing agent at early stages of cure leads to low values of interfacial shear strengths when compared with results obtained for the same system with the fragmentation test.With the microbond test, a distinct relationship between the glass transition temperature of the droplets and their size is noticed. Smaller (<150ym) droplets have very low Tg's and are incompletely cured. While changing to a modified curing cycle and/or using a m-PDA-rich curing environment alleviates the diffusion problem, the interfacial shear strength values are still not in good agreement with the fragmentation test results. Microbond data from another system consisting of DGEBA resin cured with a different, less volatile dimaine curing agent indicates that diffusion of the curing agent becomes less severe as the volatility of the curing agent decreases and the corresponding microbond interfacial shear strengths agree better with fragmentation test results.KEY WORDS Single fiber fragmentation test; microbond pullout test; interfacial shear strength; interface; fiber-matrix adhesion; variable stoichiometry effect on epoxy properties.
Ultra-high molecular weight polyethylene fibers possess exceptional strength and stiffness combined with low density. Their use as reinforcements in polymer matrix composites has, however, been limited by poor fiber adhesion. In the present study, individual fibers of UHMW-PE (Allied-Signal SPECTRA-1000) were modified by direct implantation of N+, Ti+ and Ar+ ions, at energies between 30 and 100 KeV, to doses ranging between 1x1014 and lx1015 ions/cm2. Fiber tensile strength was generally unaffected by these irradiations. Single-fiber droplet pull-off tests using DGEBA Epoxy with m-PDA curing agent have shown that ion beam surface modification of the fibers can give an increase of over 300% in interfacial shear strength (ISS). The improvements were found to vary with dose but were relatively insensitive to implant species. TEM observation of transverse microtomed sections confirmed a substantial improvement in fiber-matrix adhesion for ion beam modified UHMW-PE composites.
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