Thermosets fracture energy

There is considerable evidence in the thermoset literature that the fracture energy decreases with increasing crosslink density, consistent with the intuitive result that crosslinking inhibits flow. In the limit of very high crosslink density, where for example we approach the structure of diamond, fracture can occur on a single crystal plane such that... 

The fracture energy cannot be related to the failure of chemical bonds which may contribute only with a few Jm-2. Furthermore, the possibility of crazing is not allowed in thermosets because fibrils cannot exist due to the high crosslink density. So, in the case of high-Tg cross-linked materials the main source of energy absorption before failure is the yielding of the network. This assumption is obviously valid only above the ductile-brittle transition temperature (Fig. 12.5), where yielding is temperature-dependent. ... 

The work of King and Andrews and Swetlin has shown that the rubbery fracture energies of epoxy thermosets are time-temperature superposable and sensitive to network structure. These studies incorporated different amine/DGEBA... 

To the authors knowledge, there have been no reports in the literature quantifying an dependence of the glassy fracture energy of thermosets. In fact, a number of studies indicate that such a simple dependence does not exist For example, epoxy networks with nearly equivalent M, have... 

Fig. 13.41 The temperature dependence of the critical fracture energy Gic in DGEBA epoxy-resin thermosets, modified either by rubber particles or by debonding glass spheres, either in tests of conventional extension rates or in Izod impact tests, compared with the generally flat behavior of unmodified epoxy resin (Kinloch (1985) courtesy of Springer).

The formation of such grafts must modify the interfacial tension in a similar way to block copolymers, as discussed in chapter 6. The mechanical effect of grafting has been studied by Norton et al. (1995), who looked at the effect on fracture energy of grafting polystyrene with a carboxy end-group at an interface between polystyrene and a thermosetting epoxy resin. The unmodified... 

In contrast to polyethylene and EVAs, Epoxide adhesives are thermosets and are much stiffer and less ductile. The adhesion fracture energy (Gc, see Fracture mechanics), between both unmodified and rubber-toughened epoxies (see Toughened adhesives) and several metals has generally been found to be much higher when microfibrous surfaces were involved (Table 2) ... 

However, this theory has several drawbacks. It can only explain the modest increase in toughness and cannot explain the dramatic increase in fracture energy reported in many rubber-toughened epoxy systems. There are many discrepancies between reported data for improvement in toughness for thermoset resins due to the sensitivity of the toughening effect to the curing condition and inherent matrix ductility [119, 120]. The effect of G on test temperature and strain rate cannot be explained using... 

Another method used to improve the fracture energy of BMI resins consists in mixing the thermosetting material with linear thermoplastic polymers. This can be illustrated by the behaviour of mixtures containing Compimide 796 and TM 123 BMI resins with GE Ultem 1000 , poly(ether-imide) 26 (Fig. 13) [70]. The critical stress intensity factor Kic of the linear polymer is six times higher than that of the BMI matrix and does follow the mixture law for aU BMI/Ultem combinations. The linear polyimide can also be added as 20- 0 [im spherical particles to the BMI resin before it is polymerised. In another example, particles of a soluble... 

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