Home > Notre information

A general design rule is a relation of the type: "Material Properties > Function (geometry, loads)"

The engineering work will consist of having a good model to evaluate the "Function". For most adhesive joints, this can be achieved using fracture mechanics. Concepts such as the stress concentration factor K and the energy release rate G can be used to predict failure. In such models, the behavior 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.

Modes of failureMode I is an opening or tensile mode where the loadings are normal to the crack.
Mode II is a sliding or in-plane shear mode where the crack surfaces slide over one another in direction perpendicular to the leading edge of the crack. This is typically the mode for which the adhesive exhibits the higher resistance to fracture.
Mode III is a tearing or antiplane shear mode.
As the loads are usually fixed, an acceptable design will result from combination of a material selection procedure and geometry modifications, if possible. In adhesively bonded structures, the global geometry and loads are fixed by structural considerations and the design procedure focuses on the “material properties” of the adhesive (i.e. select a "good" adhesive) and on local changes on the geometry.

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

The bonded zone is large
It is mainly loaded in mode II
Stable crack propagation will follow the appearance of a local failure.