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Reducing Internal Stresses

SpecialChem / Nov 3, 2004

Internal stresses within the adhesive or sealant joint can significantly reduce the inherent adhesive strength. Internal stresses can occur on setting or curing, but they may also develop due to the joint's aging in service. For example, internal stresses could occur by bending forces or by dimensional change of the substrate due to absorption of moisture or aging. This article will focus on internal stresses that occur during the setting process. These stresses are difficult to foresee and avoid. They are common to all adhesive and sealant joints and detract from the ultimate strength of the joint even before it is placed into service. Figure 1 qualitatively illustrates the various forces that are at work to yield the measured bond strength. Internal stresses are a significant factor in why the theoretical (maximum) adhesion values are never realized in practice. In fact, the bond strength measured on well prepared lap-shear specimens is basically the inherent bulk strength of the adhesive material minus the internal stresses.

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