Fracture mechanics rigid adhesives

In the following section, fracture mechanics methods will be discussed for adhesive bonds consisting of rigid adherends, since these are most common in structural adhesive applications. 

The analytical methods of fracture mechanics (both cohesive and adhesive) are described in a number of references [21-24] and will not be repeated here. However, a brief outline of one simple approach provides some insight into the concepts, principles, and methodologies involved for the reader who is not familiar with fracture mechanics. In the previous discussion of peel tests, it was noted that Gent and Hamed [ 18-20] had performed some extremely informative fracture mechanics tests using peel specimens. We consider a simplified fracture mechanic analysis of the 90° peel test shown in Fig. 17. Here we assume that the substrate is rigid and the peel adherend is very flexible and perfectly elastic. The stress distribution in the vicinity of the 90° bend is complex and difficult to determine. If the material is perfectly elastic, however, this stress distribution is... 

The interphase provided by the adhesion promoter may be hard or soft and could affect the mechanical properties. A soft interphase, for example, can significantly improve fatigue and other properties. A soft interphase will reduce stress concentrations. A rigid interphase improves stress transfer of resin to the filler or adherend and improves interfacial shear strength. Adhesion promoters generally increase adhesion between the resin matrix and substrate, thus raising the fracture energy required to initiate a crack. 

The propensity for image collapse depends on the width and proximity of the features, its aspect ratio, the substrate surface, and the mechanical properties of the imaged resist. Two characteristic failure modes have been identified deformation, where fracture occurs within the resist structure, and adhesion loss, where the resist feature separates from the substrate at their interface. Low surface-tension rinse liquids [in the extreme, supercritical fluids (101)] and the use of rigid and highly adhesive resists reduce the frequency of image collapse. 

As stated above, peel tests provide practical adhesion values that include polymer and substrate mechanical properties, stored stresses, plastic deformation, and other parameters. It has been demonstrated that an analysis of the peel test mechanics allows to extract the fundamental adhesion from the experimental data [71]. A method to calculate the interfacial fracture energy of a polymer bonded to a rigid substrate by using peel tests has also been presented [72]. 

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