Fracture toughness of an epoxy system

Abstract

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.

Mode I fracture in adhesively-bonded joints: A mesh-size independent modelling approach using cohesive elements

“…However, since such joints usually fail by the initiation and propagation of flaws [5], other methodologies have been shown to be better suited to investigate these problems [6][7][8]. Particularly, fracture mechanics has gradually become more popular since the pioneering work of Ripling et al [9] and Mostovoy et al [10].…”

Untitled

“…The adhesively-bonded double-cantilever beam (DCB) and tapered double-cantilever beam (TDCB) are both popular fracture-mechanics test geometries, and have been used extensively to determine the adhesive fracture energy, G C , of bonded joints at relatively slow test rates [1,11,12]. An ASTM standard is available (D3433-93) [13] for slow-rate testing and recently, a new protocol has been drafted by the European Structural Integrity Society (ESIS) for these test geometries [14].…”

Durability of Adhesive Joints

“…at the assumed built-in crack tip [46]. These neglected effects are more important than those associated with the shear deformations in predicting the values of G C for TDCB joints manufactured with metallic substrates [114][115][116].…”