Adhesive shrinkage

Curing adhesive Shrinkage Internal stress within joint... 

All the practically acceptable methods of decreasing the internal stresses in adhesive-bonded joints can be divided into three groups decrease of the adhesive shrinkage in the course of cure increase of the relaxation rate of the internal stresses in the adhesive and decrease of the difference between the coefficients of linear thermal expansion of the adhesive and the substrate. 

Let us consider in more detail methods of decreasing the internal stresses that are most acceptable for application in adhesive-bonded joints. These are based on the increase of the relaxation rate at the time of formation of the adhesive layer, on decrease of the adhesive shrinkage, and on the decrease of overstresses at the adhesive-substrate interface. 

The brittleness of specimens under normal compression depends also on the ambient medium and on exposme. Specimens fixed with Sprut-5M under water and in oil are less brittle than specimens fixed in air. Reduction of brittleness mder water and in oil is somewhat characteristic of the specimens glued with VAK also. This may be explained by the plasticizing influence of the hquid medium on the adhesive joint borders, leading to partial elimination of the stress concentration factor [360] that occurs with adhesive shrinkage in the process of polymerization. 

In this calibration, the characteristic length. scale is one-half the bond thickness h in Fig. 2). One particularly convenient choice for the characteristic stress plane stress (i.e. stress directed parallel to the interface) found at the center of the layer in a region remote from the stress-free edge. With this definition for a, the same K calibration relation applies to transverse tensile loading and to uniform adhesive shrinkage. This characteristic stress is related to the nominal, applied transverse (butt tensile) stress d for an adhesive layer with Poisson s ratio v by... 

As commented earlier, the rate of loss of strength of a joint under environmental attack will usually be faster if a tensile or shear stress is present 145,126-130], albeit an externally applied stress or internal stresses induced by adhesive shrinkage (incurred during cure) or by adhesive swelling (due to water uptake) [131,132]. Such stresses render primary and secondary molecular bonds more susceptible to environmental attack and also probably increase the rate of diffusion and the solubility in the adhesive of the diffusing medium [133-135]. 

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