Before a joint is designed, a number of basic questions are required to be answered. These are:
- What kind of materials are to be bonded?
- What are the mechanical properties of the substrates?
- Which mechanical or chemical surface preparation of the substrates is required and/or possible?
- What are the expected stresses (continuous and peak) to which the joint will be exposed?
- What surface finish or final appearance of the joint is required?
- What resistance to chemical exposure is required from the adhesive joint?
- What resistance to UV exposure or to thermal exposure is required from the joint?
Sika's System Engineering and Technical Service groups can provide FEM computations for the use of marine architects and engineers to assist in calculating the dimensions of adhesive joints. Where appropriate, some chapters also contain basic information regarding adhesive joint dimensioning.
In service, adhesive joints can be subjected to a multitude of forces:
Shear (sliding forces)
Tension (pulling forces)
Compression (squashing forces)
Torsion (torsional forces)
Peel (tearing forces)
The strength of a joint is basically determined by the area of the bond, the inherent strength of the adhesive or the substrate and the stress distribution within the joint. A poorly designed joint can lead to high-stress concentrations in the joint itself and/or in the substrates connected, which in turn can lead to premature failure. Good joint design, which takes into account the practicalities of application as well as the geometry of the joint, is essential for a long service life in a demanding Marine environment.
Peel forces are the most difficult to counter and must be avoided at all costs.
Examples of Joints
||Best suited to operate in shear
||Best suited to operate in compression or tension in one plane (no peel)
||To increase strength for both compression and tension, the cross sectional area should be increased
||Scarf joints are another way of increasing the cross sectional area, however, this method is not well suited for elastic adhesives
||This anti-peel method provides a rigid spine which prevents the bonded area from curling
||Using this anti-peel method, the stepped component extends a flange under the joint preventing any lifting. The other component has its edge contained by the step which prevents it catching. Both surfaces have an extended bond cross sectional area that can be as large as necessary