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Combined Effect

The preceding discussion looks at adhesive failure and cohesive failure as separate modes. However, in practice they result from the same joint specimen. One can generalize on the influence of the degree of interfacial surface attachment on the adhesive joint strength and the mode of failure. The degree of interfacial surface attachment may vary due to wetting, boundary layer effects, or other phenomena. The tenacity of surface attachment is related to joint strength and failure mode as shown in Fig. 2.6.

FIGURE 2.6 Schematic of interfacial states encountered in adhesion. Here B repre­sents boundary or adhesion failure, C represents cohesive failure, and B/C represents a mixed failure mode

In the boundary failure region, designated by B in Fig. 2.6. the strength of the bond is controlled by physical and mechanical forces active at the interface. In this region, the strength of interfacial contact is much less than the cohesive strength of the adhesive mate­rial. At a certain level, the degree of surface attachment begins to influence the strength of the joint. This critical degree of surface attachment may be referred to as the threshold value.

In the transition region, the strength of the adhesive joint is very sensitive to the degree of surface attachment. In this region, mixed failure is observed. At a critical degree of sur­face attachment, which can be referred to as a minimal or saturation value, the adhesive joint will rupture, showing total cohesive failure, designated by region C. The attainment of this state of surface attachment would represent the ultimate strength of the joint. Above this so-called saturation degree of surface attachment. the strength of the joint levels off into a cohesive plateau. Under these conditions further increases in the degree of surface attachment do not result in an increase in the mechanical strength of the adhesive joint.

THEORIES OF ADHESION

Several theories attempt to describe the phenomena of adhesion. The most common theo­ries of adhesion are based on:

. Adsorption and wetting

. Diffusion

. Electrostatic interactions

. Simple mechanical interlocking . Chemical bonding

. Weak boundary layers

No single theory explains adhesion in a general, comprehensive way. Some theories are more applicable for certain substrates and applications; other theories are more appropriate for different circumstances. Each theory has been subjected to much study. question, and controversy. However, each contains certain concepts and information that are useful in understanding the basic requirements for a good bond