Mild steel/epoxy joint

FIGURE 10.3 Effect of silane adhesion promoter on the durability of a mild steel joint bonded with epoxy adhesives.16... 

FIGURE 15.21 Shear strength of mild steel joints, (a) two-part epoxy, (b) two-part acrylic, (c) anaerobic acrylic, (d) cyanoacrylate, (e) PVC plastisol, (/) one-part heat cured epoxy. ... 

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... 

Fig. 16 shows these three equations applied to a single lap joint, with 1.6-mm-thick adherends, 25 mm wide, at various overlap lengths. The three adherends are hard steel, gauge steel and mild steel, which have initial yield points at 1800 MPa, 430 MPa, and 270 MPa, respectively. The adhesive is a modem epoxy AVI 19 by... 

The second stage involves a loss of integrity of the interfacial regions. In the particular case of the epoxy/mild-steel Joints, this arises from the rupture of interfacial secondary bonds. In other adhesive systems, however, any of the mechanisms discussed above could be envisaged. 

Kinloch and Shaw [106] have investigated the effect of joint width by employing a TDCB specimen consisting of mild-steel substrates bonded with a simple brittle epoxy or a rubber-toughened epoxy adhesive. For the former, low toughness adhesive no effect was observed. However, for the tough adhesive the relationships between Gic and adhesive layer thickness, /la,... 

Figure 8.6 Fracture stress of epoxy/mild steel (abraded solvent-cleaned pretreatment) butt joints as a function of the time exposed to various environments all fracture tests conducted at room temperature [30].

Figure 8.12 Durability of epoxy/mild steel joints employing various silane-based primers [49].

Figure 8.23 Rate of interfacial debonding of epoxy/mild steel joints versus reciprocal of the water immersion temperature [30].

To use the above model to predict the durability of the epoxy/mild steel joints (see Fig. 8.6) first requires a knowledge of the rate of diffusion of water into the adhesive layer in the joint. The diffusion and solubility coefficient of water into the bulk adhesive was measured and the diffusion process was also found to be of the concentration independent Fickian type [147]. Further, work [147,149,150] has also shown that the diffusion of water into the adhesive layer may often be modelled by assuming Fickian diffusion. Support for this comes, for example, from studies which have employed tritiated water and so enabled the water concentration in the joint to be directly measured good agreement with the calculated water concentration profile was obtained. 

Figure 8.29 The fracture stress, o-f, of butt joints at 20 °C of epoxy/mild steel joints versus time in environment. Points are experimental measurements and the solid curves are theoretical predictions from the model described in the text [47,123].

Similar experiments have been carried out by Higuchi et al. (2002) to validate the finite element analysis of a single-lap joint. The specimens used in this work were mild steel plates bonded with an epoxy adhesive strain gages have been applied (in longitudinal direction) on the mid thickness of the adherends to measure the local strain during the impact (see O Fig. 21.21) the impact velocity was about 1 m/s. The goal of this work is to assess the stress distribution on the adherend-adhesive interface, which is obtained from finite element results, since it is not possible to measure it directly (the model is validated by comparing its results in points at mid-thickness of the adherends, where experimental measurements have been... 

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