A general design regulation is a relation of the type is that the material characteristics are greater than the function required (i.e. geometry, loads, etc.). The engineering work will consist of having a good model to evaluate the function. For the majority adhesive joints, this can be achieved by means of fracture mechanics. Concepts such as the stress concentration factor and the strain energy release rate can be used to forecast failure. In such models, the performance of the adhesive layer itself is neglected and only the adherents are considered.

 

Failure will also very much depend on the opening mode of the joint.

 

•           Mode I is an opening or tensile mode where the loadings are usual to the crack.

 

•           Mode II is a sliding or in-plane shear mode where the crack substances slide over one another in direction perpendicular to the leading edge of the crack. This is characteristically the mode for which the adhesive exhibits the superior resistance to fracture.

 

•           Mode III is a tearing or antiplane shear mode.

 

As the loads are usually fixed, an satisfactory design will result from combination of a material selection procedure and geometry modifications, if possible. In adhesively bonded structures, the international geometry and loads are fixed by structural considerations and the design process focuses on the material characteristics of the adhesive and on local changes on the geometry.

 

 

Increasing the joint resistance is usually obtained by scheming its geometry so that:

•           The glued zone is large

•           It is mainly loaded in mode II

•           Stable fracture propagation will follow the appearance of a local failure.