Epoxy adhesives steel joints

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. 

To predict the service life of adhesive joints exposed to a hostile environment requires a knowledge of the mechanisms and kinetics of attack. In the relatively simple epoxy/abraded steel joints exposed to water the work described in previous sections has identified the mechanism as being the rupture of interfacial secondary forces, as predicted from a consideration of the thermodynamics of the system, and the kinetics as being governed by the diffusion of water through the adhesive to the interface. Thus, in this system one might hope to describe a model to predict quantitatively the service life, and Kinloch and co-workers [47,123] have proposed the following model. 

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. 

Allen and Alsalim 22 compared the effect of various pretreatments of stainless steel (martensitic structure) on the torsional shear strength of napkin ring joints formed with an epoxy adhesive (Redux 319 (Bonded Structures Ltd.)). They concluded that... 

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

Weldbonded aluminum joints (1-mm aluminum alloy 2036-T4) where compared with spot welded aluminum (1-mm aluminum alloy 2036-T4) and spot welded steel (1-mm steel 1010) joints.45,46 Weldbonded 2036-T4 joints made with a vinyl plastisol adhesive and with a one-part modified epoxy had a fatigue strength about twice that of the spot welded aluminum alloy and approximately the same as that of the spot welded steel. The fatigue strength of the weldbonded aluminum joints with polysufide-epoxy adhesive or with high-peel-strength epoxy adhesive is higher than that of steel spot welds alone. 

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... 

Polyamide (nylon) Acetone, methyl ethyl ketone 1. Abrasion. Grit or vapor blast or abrade with 100-grit emery cloth followed by solvent degreasing. 2. Prime with a spreading dough based on the type of rubber to be bonded in an admixture with isocyanate. 3. Prime with resorcinol formaldehyde adhesives. Sand or steel shot is suitable abrasive Suitable for bonding polyamide textiles to natural and synthetic rubbers Good adhesion to primer coat with epoxy adhesives in metal-to-plastic joints... 

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. 

Impurities in the water have a great effect on the water resistance of the joints. The maximal strength decrease of glass-reinforced plastics is caused by distilled water. The strength of steel joints cemented with adhesive based on the Epoxy-1000 resin decreased by 44% when subjected for 60 days to rainwater, by 21% with river water, and by 18% with sea water. Adhesives based on PN polyester resin form comparatively water-resistant joints with glass-reinforced plastics [211], while steel joints cemented by these adhesives are not water-resistant. When subjected to warm (60°C) water for 400 h and to boiling water for 120 h, the strength of steel joints bonded by polyester adhesives decreased 3—4 times. 

A close look at the stress state indueed within this joint indicates clearly a complex interaction of strain, and there are many useful commentaries on the limitations of this test(4, 5. 25, 31). If the adherend and adhesive moduli are very different (e.g. the ratio of epoxy to steel moduli, E E = 40), the axial strains in the adhesive will be about 40 times greater than those in the adherend with a similar ratio for the lateral (Poisson s) strains. Where the two materials join, this conflict is resolved by generating large interfacial radial shear stresses (Fig. 4.11(c)). Joint strength inereases with a decrease in adhesive thickness, and in a thin bondline affected completely by adherend restraint a eomplex stress arises. The ratio of the applied stress to the strain across the adhesive is then defined as the apparent or constrained Young s modulus, a(5)-... 

Although the term was originated to describe the failure of organic coatings, it is clear that cathodic disbondment can lead to the accelerated failure of adhesive joints as well. The work of Davis and Watts illustrates this phenomenon for steel substrates bonded with an epoxy adhesive and shows how a combination of XPS and Secondary Ion mass spectroscopy (SIMS) are able to provide a definitive picture of the locus of failure. Such a combined approach enables a detailed mechanism of failure to be postulated. 

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... 

We will consider a torsional load on a co-axial joint, with an overlap of 25 nun using a single part hot setting epoxy adhesive. The inner tube external diameter is 25 mm and both tubes are 1.5 mm thick steel. A torsional load of 100 Nm and a glue line of 0.05 mm gives a stress distribution as shown in Fig. 30. 

In lightweight metal constructions, tubes and hollow profiles of any cross section can be designed and bonded as socket joints. Door and window frames are made from steel and aluminum profiles with angles bonded in place (epoxy adhesives), a heat-insulating intermediate layer being applied by bonding or made by casting with an adhesive (polyurethane, epoxy adhesives). 

A full, finite-element analysis of an adhesively bonded butt joint with rigid adherends has been performed to investigate the range of applicability of the asymptotic solutions. The failure load and epoxy properties used in these calculations are the same as those used in the small-scale yielding calculations for steel-epoxy adhesively bonded butt joints presented above (i.e. E = 3.5 GPa,... 

Fig, 5. Atomic force microscopy (AFM) images of the fracture surfaces of an electrogalvanised (EG) steel joint bonded using an epoxy-paste adhesive [19], (a) Adhesive side, (b) Metal side. 

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... 

Japanese researchers [18] described a stress analysis of butt joints of steel to aluminium in which joints were assembled with epoxy adhesives and subjected to cleavage loads. They found that the normal and shear stresses were maximised at the edge of the interface on the load application side between the substrates and the adhesive bond. However, both stresses were greater at the edge of the interface between the higher-modulus substrate (steel) and the bond. 

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