Steels epoxy adhesive joints, temperature

For elevated-temperature cures, however, dicyandiamide and melamine curing reactions have been shown to be beneficial in epoxy-based adhesives when used on copper substrates. These epoxy adhesives perform about as well in copper joints as in aluminum or steel joints. The formulations show significantly increased time to adhesive failure on either bare or alkaline permanganate-treated copper.30... 

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... 

The experimental validation presented later is for two geometrically identical wet lay-up CFRP/steel double-strap joints (refer to Fig. 10.10) with their measnred inner adherend s thickness (fi/steei) equal to 4.85 mm. Adhesive/matrix (i.e. Araldite 420 epoxy resin) is the same for both specimens however, they are fabricated with different CF plies (i.e. NM and UHM) and tested under different exposure temperatures (i.e. 0°C and 20 C), respectively. Figure 10.12 portrays the assumed thicknesses for the... 

Figure 7.25 Effect of temperature on the fracture energy, Gic, for a toughened epoxy adhesive bonding steel substrates using a TDCB specimen [154]. x, Bulk behaviour of adhesive O, adhesive joint behaviour 3, predicted joint behaviour, from Equation 7.62. The plastic zone size, 2r yc, calculated from Equation 7.61 is given in parentheses. 

Fig. 119. Effect of storage at elevated temperatures on room temperature strength of stainless steel lap shear joints with an epoxy-phenolic adhesive (Hidux 1197A, Ciba-Geigy) (adapted from Althof, 1966).

For that reason, the properties of stainless steel joints bonded with epoxy systems are of special interest. According to the literature, the aging behavior of such joints is critical [1, 2]. Mechanical tests reveal that the combined attack of water and temperature causes a strong deterioration in their performance [3]. It is necessary to understand the processes that are going on during aging in order to improve the reliability of structural adhesive bonding. 

Schematic representations of the typical response of cold-cure epoxies are depicted in Fig. 4.6. Naturally the relative importance of specific adhesive property data depends upon the application and the envisaged loading and environmental conditions that the real joint will be subjected to. Many analysts(32-34) advise that the strength of the adhesive equilibrated with the worst-case environment is the key to effective design. This implies laboratory tests conducted at high temperatures on specimens pre-equilibrated with high levels of water vapour or liquid water. For the application of adhesives to steel bridges in the USA. Albrecht era/.(34) selected a test environment of 49 °C and 90% r.h.

Gledhill and co-workers [9,43] have studied the kinetics of environmental failure for epoxy/steel joints where the mechanism of attack had been found to be via the displacement of adhesive on the oxide surface by the ingressing moisture, as described in Section 2.2.1. The rate of dissociation of the interface is shown as a function of the water immersion temperature in the form of an Arrhenius... 

The strengths of stainless steel lap-joints bonded with the epoxy-phenolic system (Fig. 5) remained essentially the same throughout the range of test temperatures and gave the highest values obtained for any type of adhesive tested... 

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