Our design procedures are based on established analytical methods which predict actual (or potential) adhesive strength, rather than the more common average shear stress approach which may, under some circumstances, be unconservative. Bond integrity is assured by demonstration that the adhesive has the capacity to carry maximum design loads with an adequate margin of safety. Bond overlap length is designed to ensure that the adhesive can achieve the required strength. Elastic-plastic modelling of the adhesive over the service temperature envelope is used to calculate adhesive bond strength. Multi-step bonded joints are analysed based on industry standard software A4EI. A specially developed user interface Entry-I greatly facilitates the analytical process.
The objective of our design philosophy is to ensure that the adhesive bond will never be the locus of failure. The bond must always be stronger than the surrounding structure.
It is relatively easy to produce strong bonds with minimal processing. Such bonds can meet current certification tests and also meet quality assurance testing during production. However, these tests are no guarantee that the bond can sustain the required strength throughout the service life of the component.
Our procedures for selection, validation and specification of adhesive bonding processes are derived from a fundamental understanding of the mechanisms that provide strength for an adhesive bond. These mechanisms also provide the longer term durability of adhesive bonds. We rely on accelerated test methods which have a history of discriminating between bonds that maintain adequate strength in service and those which produce short term strength, but experience bond degradation in later service.
Current procedures for certification of bonded structures rely on a “building block” approach, whereby extensive testing is undertaken to establish design allowable stresses on which an average shear stress design is based. Further testing must be conducted for every design element, to demonstrate that adequate strength can be obtained for the specific combination of materials and geometrical configurations encountered in the design. A lesser number of tests, which include structural details around the joint, follow to demonstrate adequate strength. A smaller number of sub-component tests precede full scale component tests.
The objective of this large test program is to gradually build confidence that the adhesive bond can provide adequate strength. Adhesion Associates use a better, less expensive and more rigorous method.
Firstly, the need for copious lap-shear tests to generate a “design allowable stress” is eliminated by measuring actual design properties for the adhesive. These tests are conducted over the temperature range experienced in-service and are used to predict actual bond strength. This data is independent of the materials being bonded. The analytical methods used by Adhesion Associates take care of properties for different materials being joined. Therefore, the requirement to undertake element testing is substantially reduced.
If structures are designed using the philosophy of ensuring that the bond is always stronger than the surrounding structure, then every failure in elements, details, sub-components and full scale component tests will always occur in the surrounding structure, not through the bond. All that these tests will measure is the strength of the metal being joined. Hence, the number of tests is significantly reduced, to demonstrate that the design philosophy is being met, viz. that the structure breaks first.