Epoxy adhesives fracture energy

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) ... 

A further important parameter is the adhesive thickness within a joint. In certain adhesive systems such as rubber-toughened epoxies, there is an optimum thickness of adhesive within which energy-dissipation processes can take place. Above and below this critical size, the adhesive fracture energy will be lower. 

Figure 7.16 The effect of joint width upon the maximum adhesive fracture energy, Clem, for a rubber-toughened epoxy adhesive bonding steel substrates using a TDCB geometry [106].

Figure 8.4 Applied adhesive fracture energy, Gic, as a function of the corresponding modulus, Ea, for the epoxy adhesive whose static fatigue behaviour is shown in Fig. 8.3 [20].

Figure 8.26 Adhesive fracture energy, Gic, versus crack velocity, a, in water for epoxy-TEPA/aluminium alloy joints [36,136].

Novolac resins have been widely used in many applications such as adhesives, coatings, construction, and composite matrices in the aerospace and electronic industries. This is due to their high strength, excellent flameretardant, low cost, good chemical and corrosion resistance and good dimensional stability [19-21]. Elastomeric and thermoplastic modifiers have been successfully incorporated into epoxy networks to improve their fracture energies and impact strengths [22],... 

By measuring water uptake, the diffusion coefficient and equilibrium concentration of water for the bulk adhesive were obtained at different temperatures. A value of 37 kJ/mol was also calculated for the activation energy of diffusion. A value for the plane-strain stress intensity factor, Kic, for the bulk adhesive was obtained using compact tension specimens. Tensile butt joints were prepared from mild steel blocks bonded with the epoxy adhesive and the fracture stress determined as a function of time of exposure to water at the different temperatures. An activation energy of 32 kJ/mol was calculated for joint failure, in close agreement with that obtained for the diffusion of water. This supports the view that the processes responsible for loss of joint strength are controlled by water diffusion. It was found that joints exposed to 20°C/55% RH showed no reduction in strength, even... 

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