Constructive Design of Bonded Joints

In conclusion, the following principles apply to the constructive design of bonded joints ... 

This is an extremely complicated subject and will oidy be touched upon here. The ultimate objective is to develop a design method for bonded construction based on the principles of mechanics and rational engineering design so that joint behavior can be predicted. 

Bonded joints are extensively employed in the construction of composite structures in aerospace applications, maritime structures, lifting equipment, wind mills as well as automotive industries [3, 4, 5], Unlike the bolt hole in mechanical fastening that causes a stress concentration in the composite joint plates, adhesively bonded joints minimize the potential for stress concentration within the joint. Besides, applications where lower structural weight, improved damage tolerance design philosophy are required, adhesively bonded joints provides a potential solution. Bonded joints are an efficent fastening solution also for hybrid structures, i.e., structures where composite parts are connected to metal parts. 

In lightweight metal constructions, tubes and hollow profiles of any cross section can be designed and bonded as socket joints. Door and window frames are made from steel and aluminum profiles with angles bonded in place (epoxy adhesives), a heat-insulating intermediate layer being applied by bonding or made by casting with an adhesive (polyurethane, epoxy adhesives). 

The primary desired method of load transfer through an adhesive bonded joint is by shear. This means that the design mnst avoid peel, cleavage, and normal tensile stresses. One practical apphcation of a method avoiding other than shear stresses is the nse of a rivet bonded construction (Fig. 4.35). The direction of the fibers in the onter ply of the composite (against the adhesive) should not be 90° to the expected load path. 

Certain specifications and standards provide excellent tutorials on adhesives and sealants. For example, MIL-HDBK-691 offers a complete handbook on adhesive bonding, and MIL-HDBK-725 provides a guide to the properties and uses of adhesives. ASTM C 962 provides an excellent source of information regarding sealant joint design and the types of sealants that are appropriate for various substrates. Although this specification is primarily for construction sealants, much of the information that it contains is generally useful for other sealant applications. 

Performance of primers has been studied both in general construction and water-retaining structures.Further information may be found in articles on Selection of joint sealants. Sealant joint design, Paint primers and Primers for adhesive bonding. 

The most successful application example of this technology is the Fokker F 27/F50 Friendship , in which about 70% of the structure (ca. 550 parts) are bonded. The proven durability (> 1000 aircraft in service for up to 30 years) of this design is also due to the superior fatigue properties compared to a riveted construction. Additionally the chromic acid anodizing (CAA) of the aluminum surftices before bonding resulted in corrosion-resistant joints. 

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