SynopsisThe fracture toughness of epoxy used in the bulk and adhesive form was measured by a previously developed technique. The uniform double cantilever-beam specimen, which was described earlier, was modified to a tapered beam, which simplified the experimental procedure and calculations for obtaining toughness measurements. By varying the ratio of hardener to resin and post-cure temperature on a single epoxy system (DER 332-TEPA), it was found that the toughness of the epoxy used in either bulk or bond form varied by a factor of approximately five. A particular combination of composition and post-curing temperature generally yielded higher toughness in the bulk than in the bond form. This was not always the case, however. At high posbcure temperatures, where the bonds were very tough, their toughness exceeded that of the bulk material. Hence, it does not appear possible to predict joint toughness from bulk toughness measurements. The toughness of joints was found to be a single-valued function of tensile modulus. For the bulk material, on the other hand, the toughness obtained on the epoxy having a specific modulus depended on the combination of composition and post-cure temperature. Joint toughness for any combination of composition and post-cure temperature depended only on the cracking rate. If the epoxy was the type that caused cracks to jump rapidly, the epoxy was tough and vice versa. For a particular epoxy system, toughness was increased by driving the crack a t an increasing rate.

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The dependence of crack velocity, ˙a, on driving force for an adhesive system was shown to be described by a curve having the shape of an inverted L. That is, at high values of ˙a, large increases in G caused small increases in ˙a while at low velocities, small increases in G cause large increases in ˙a. Since the data in the literature on polymethylmethacrylate (PMMA) and mild steel can also be represented by such a curve shape, the inverted L may represent the general dependence of ˙a on driving force. The high velocity branch appears to be associated with rough fracture surfaces and the low velocity branch, with smooth fracture surfaces.

Fracture mechanics is a relatively new topic that makes it possible to define toughness of material in the presence of flaws. The effect of joint geometry, section size, strain rate, and fracture mode on fracture toughness are discussed. The overriding factor in establishing the toughness of any joint is the speed with which the crack moves along the glueline. Slow-moving cracks result in high toughness, while cracks that abruptly jump along the glueline result in low toughness. Changing the fracturing mode from tensile to forward shear results in an order of magnitude increase in toughness.

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