Fracture Mechanics of Adhesive Joints

Essentially, continuum fracture mechanics is the study of the strength of a structure which contains a flaw, usually considered as an elliptical crack. The theories were originally developed for the cohesive fracture of materials, and several reviews [1 ] have been published on this subject, but a large body of work has also been reported on extending the principles of fracture mechanics 

It has become evident that failure criteria such as maximum shear stress are inadequate since failure can also occur through other modes, such as tensile stresses in adhesive joints. Also, the uncertainty in the stress mode is magnified because properties of and stresses in adhesive joints are highly dependent on joint geometry. Therefore, complexities arise when deciding the proper criterion applicable to designing structural adhesive joints. 

Adhesive materials, as well as all polymers, inherently contain flaws such as porosity, microcracks, or other voids. Typically, these voids are incorporated during the use of these materials in various applications. The realization that these voids actually govern the performance of the material has led to the study of fracture mechanics. 

Fracture mechanics studies the effect of stress concentrations that occur when a load is applied to a body containing a void, independent of the geometry or material of the body. By this definition it would seem logical that the fracture toughness of a material, as determined by specific fracture mechanics techniques, would be an appropriate design criterion. 

As discussed in Chapter 1, fracture mechanics relies on the assumption that the materials tested behave as linearly elastic solids. Structural adhesive bonds fail by crack initiation and propagation in the adhesive layer. Assuming that the structural adhesive is brittle, then it would seem appropriate 

The following discussion will begin with a review of the basic theories of fracture mechanics, focusing on both the energy approach and the stress intensity approach. The application of fracture mechanics to adhesive joints will include the pioneering work in the area and test techniques which can be employed. Finally, consideration will be given to the effect of temperature, loading rate, adhesive bond thickness, and environment on fracture properties. 

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The last severe environment on our list is external stress. External stresses (14,33) especially affect the water uptake or saltwater penetration of adhesive bonds. However, some adhesives may appear insensitive to stresses. Presumably, the critical stress level (34) is so high that no environmental attacks can take place below that level. Thus, it is difficult to discuss the effect of external stresses without considering the fracture mechanics of adhesive joints. In general, external stresses accelerate bond degradation when the joint is exposed to severe environments. 

FRACTURE MECHANICS OF ADHESIVE JOINTS 7.6 EFFECT OF RATE AND TEMPERATURE 7.6.1 Introduction... 

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