Microbond Method for Determination of the Shear Strength of a Fiber/Resin Interface: Evaluation of Experimental Parameters

We have examined the influence of various experimental parameters on fiber~resin interfacial shear strengths determined for four fiber/resin systems ( aramid, glass and carbon fibers with an epoxy resin)by the microbond pullout method introduced in Part I. The initial tension on the fiber before shearing begins has no effect on the shear strength; neither do the small variations in loading rate caused by fiber length variations at constant extension rate. Because the applied shearing force in any pull-out experiment decreases with distance from the point of application, measured shear strength decreases if the embedment area is increased several-fold. Shearing of a microdroplet by a debonded microdroplet results in the same shear strength value as shearing by the microvise; this supports our contention that the force exerted on the interface in the microbond experiment is predominantly a shearing force. Finally, the large numbers of individual measurements for a given system possible with the microbond technique provide distributions of shear strengths that reflect real differences in bond strength attributed to fiber surface heterogeneity.