Bonded joints relative requirements

The physical and chemical properties of both the solidified adhesive and the plastic substrate affect the quality of the bonded joint. Major elements of concern are the thermal expansion coefficient, modulus, and glass transition temperature of the substrate relative to the adhesive. Special consideration is also required of polymeric surfaces that can change during normal aging or on exposure to operating environments. 

The design analysis of a scarf may be considered similar to a single lap bonded joint, detailed analysis of which can be found in MIL-HDBK 17-3E 3. The analysis of a bonded joint is made complex, however, by the modulus difference of the adhesive compared to the adherends and the relative thicknesses of both which causes a non-linear distribution of the shear forces in a lap joint with peak stresses at the ends [1]. Scarf repairs provide a more uniform stress distribution however, to achieve this an adequate scarf angle is required [24] shown in Eigure 14.6. 

Appropriate test methods for the control of fabrication procedures and non-destructive testing (NDT) are basic requirements for the formation of structural adhesive joints. Such methods should be based upon relatively easily measurable parameters that have a close identity with the properties of the bonded assembly that need to be controlled. However the quality of bonded joints depends upon many factors, requiring a range of very different procedures. 

Elastic adhesives also act as sealants, and hence they offer a relatively simple but effective way to protect a joint against the ingress of gas or water. Adhesive-bonded joints can also be made resistant to chemical attack. Many mechanical fastening techniques also require sealing of the joint, which implies additional work and expense. 

Application requires a relatively high degree of care to ensure nonwrinkling and removal of separator sheets. Films are often supported on scrim that distributes stress in the cured joint and ensures a uniform bond line thickness throughout the bonded area. 

The bonded area should be large enough to resist the greatest force that the joint will be subjected to in service. The calculation of stress in the adhesive joint is not a reliable way of determining the exact dimensions required. It is relatively difficult to decide on an allowable stress. The strength of the bond is affected by environmental conditions, age, temperature of cure, composition and size of adherends, and the thickness of the adhesive layer. 

The fibre orientation on the bond surface should be parallel to the primary loading direction. Otherwise it is likely that the Joint will fail due to adherend failure at a relatively low loading level. Owing to a great variety of possible laminate structures and applications, it is unrealistic to require that the fibre orientation on the bond surface and the primary loading direction should be parallel in all connections. This requirement is essential for primary structural connections (see 5.1.1 for definition) and for other connections it is a recommendation. The requirement allows the use of fabrics, woven rovings and uni directional reinforcements on bond surfaces when at least half of the fibres are parallel to the loading direction. In primary structural connections mats are not allowed to be used on bond surfaces. 

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