Modification of fiber-matrix bond strength through surface treatments

A model system of Ni-Cr wires in a vinyl ester resin matrix is used to study the effect of surface treatments on shear bond in axisymmetric macrobond tests. A stress-based approach presented by Zhandarov and colleagues is modified to allow tests with different fiber diameters to be compared on the same plot of average shear stress at debond, τd versus embedment length divided by fiber diameter, l_e/d_f. Two parameters are used to fit the theoretical equation to experimental data. One parameter, c, is related to the shear lag parameter, ß, and controls the shape of the curve. This parameter is nearly constant in the range of geometries considered in this work. The second parameter is the ultimate shear stress,τ_uit, which is the value of the fitted curve at l_e/d_f equal to zero. As c is nearly constant for the geometries considered, the same value of c is used to explicitly solve for τ_uit for additional tests where the surfaces of the Ni-Cr wires were modified via baths in sulfuric acid. This treatment resulted in significant increases in τ_uit, albeit with increased scatter in the results. Specific results of surface treatments are discussed. This work establishes a framework for developing micromechanical models for bond strength based on measureable characteristics of the surface with the ultimate goal of developing micromechanical models to predict the effect of surface treatment a priori.