Study of rubber‐modified brittle epoxy systems. Part II: Toughening mechanisms under mode‐I fracture

Abstract

The toughening mechanisms in grafted‐rubber concentrate (GRC), dispersed acrylic rubber (DAR), and Proteus rubber‐modified brittle epoxy (i.e., highly crosslinked) systems are examined using scanning electron microscopy, optical microscopy and transmission electron microscopy techniques. The toughening of the GRC‐modified brittle epoxy system is found to be due to cavitation of the GRC rubber particles, followed by formation of limited shear yielding when the crack propagates. Crack bifurcation and crack deflection are also observed in this system. Only crack bifurcation, crack deflection, and possibly crack/particle bridging mechanisms are operative in the DAR‐modified brittle epoxy system. In the case of the Proteus rubber‐modified system, the rubber appears to be rigid (Tg ≈ 28°C). As a result, the crack/particle bridging mechanism is not observed. Only crack deflection and crack pinning mechanisms are found. These observations are in agreement with the toughness measurement results (see Part I), which indicate that the GRC rubber provides the most effective toughening, followed by the DAR rubber, and then by the Proteus rubber. An approach for toughening brittle epoxies is also discussed.

Fracture behaviour of model toughened composites under Mode I and Mode II delaminations

“…This indicates that the matrix surrounding the highly elongated rubber particles also plastically deformed to a similar degree ( ~ 100% plastic strain). This phenomenon is found to be comparable to that of the CSRmodified neat epoxy system previously investigated [32]. Thus, the dominant toughening mechanisms in this toughened composite system are found to be cavitation of the CSR particles, which relieves the constraint in front of the crack tip, followed by shear yielding of the matrix.…”

“…7). This is a substantially smaller cavitation zone than that observed in the CSR-modified neat resin case, where a rubber particle cavitation zone of ~ 25 gm is observed [32]. This implies, as has been pointed out by others [3,20], that the fibres hinder the growth of the rubber cavitation zone, which, in turn, may limit the [3,20,32,33], (ii) the additional constraint imposed by the neighbouring fibres which induces a higher maximum principle stress, component around the crack-tip region produces a longer crack-tip plastic zone [33], and (iii) the fibre bridging mechanism takes place.…”

“…Reflected Nomarski OM was utilized to study the sub-surface damage zone prepared parallel to the fibre direction and perpendicular to the fracture surface from the mid-plane core region. The fracture behaviour of this toughened composite system is found to be similar to that of the Proteus particle-modified neat epoxy system [32]. Under Mode I testing, the crack appears to grow unperturbed through the particles.…”

The mechanisms and mechanics of the toughening of epoxy polymers modified with silica nanoparticles

“…This method has been previously employed very successfully by Sue et al [35,36] and Pearson et al [37]. In this test, two near-identical natural cracks are produced by tapping a razor blade into each machined-notch.…”

Structure and property relationships in model diglycidyl ether of bisphenol-a and diglycidyl ether of tetramethyl bisphenol-a epoxy systems. I. Mechanical property characterizations

“…1 Since then, many endeavors have been made to gain an understanding on how the rubber particle size and type, 2,8,9 crosslink density of epoxy, 3,7,9 curing schedule, 4 and curing agents 4 influence the toughening effect of epoxies and other thermosets. While progress was made in the understanding of physics and mechanism(s) in rubber toughening, compromises brought about by the rubber toughening approach, such as reduction in modulus, thermal stability, dimensional stability, and processability, were found to be too big a compromise for many structural and electronic packaging applications.…”