Joint strength moisture effects

The effects of water and temperature on the adhesive itself are also of utmost importance to the durability of bonded structures. In the presence of moisture, the adhesive can be affected in a number of ways, depending on its chemistry and how rapidly the water permeates through and causes significant property changes [51,86-88]. The potential efficacy of moisture penetration on the locus of failure of bonded joints has been discussed in the previous section. As expected, elevated temperature conditions tend to degrade joint strength at a faster rate. 

In a similar study, Brewis et al observed the same type of reductions in the mechanical properties of the adhesive. They demonstrated that the reduction in strength of single lap joints on exposure to moisture was linearly related to the fractional water content of the joints. On saturation with water the joints were weakened by 44%, whereas films of the adhesive were weakened by as much as 74%. It was pointed out that the lack of correlation between the bulk and adhesive joint strength was due to the different stress distributions in the two situations. They concluded that the principal mechanism of loss of adhesive joint strength was by diffusion-controlled water plasticization of the adhesive. There is an apparent discrepency between this work, in which water has a reversible effect on joint strength, and that of Butt and Cotter, in which the effect is irreversible. This will be discussed further in Section III.B. 

Vibration welding, like ultrasonic welding, produces high-strength joints for materials that can be melted. However, it is much better suited to large parts and irregular joint interfaces. Moisture in materials does not usually have an adverse effect on the weld as it does with ultrasonics. 

Surfactants have been reported to affect adhesive joint strength, and the crosslink density of elastomers has been studied with respect to adhesive strength . The effect of moisture on joint strength was investigated by Schonhorn and Frisch . ... 

Brewis et al. studied the effects of moisture and temperature on the properties of epoxy-aluminum joints by measuring changes in the mechanical strength properties of the soaked adhesive [90]. The Tg of the wet adhesive and relative strengths of wet and dry joints were evaluated for up to 2500 hours. They concluded that the joint weakening effect of water was due to plasticization of the adhesive that, in turn, was dependent on the rate of water diffusion within the adhesive. 

Particular regard should be paid to the highly alkaline nature of concrete and any possible adverse effect on bond strength, especially in the long-term. For new construction, the amount of free moisture associated with pouring wet concrete on to the adhesive-coated steel should be noted. It is also likely that a joint configuration employing permeable adherends such as concrete will be inherently less durable than metal/metal joints. 

Like mechanically fastened metal structures, composites exhibit failure modes in tension, shear and bearing but, because of the complex failure mechanisms of composites, two further modes are possible, namely cleavage and puUout. Environmental degradation of a bolted joint, after exposure to hot, wet environment is most likely to occur in the shear and bearing strength properties. The evidence shows that for fiber reinforced epoxies, temperature has a more significant effect than moisture, but in the presence of both at 127°C, a strength loss of up to 40 percent is possible. 

In a more detailed investigation of the effect of moisture on cyanoacrylate-bonded steel joints. Drain et al. concluded that the loss of strength is due to the formation of an oxide layer at the adhesive-metal interface and possibly to concurrent surface hydrolysis of the polymer. The adhesive appears to be relatively impermeable to water which they suggest most likely enters the bondline by wicking along the interface. Polycarbonate joints exposed to the same aging conditions remained virtually unaffected. 

Operating environments are capable of degrading an adhesive joint in various ways. The adhesive may have to withstand temperature variation, weathering, oxidation, moisture, and other exposure conditions. If more than one of these factors are present in the operating environment, their synergistic effect could cause a rapid dechne in adhesive strength. 

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