Adhesive bond good joint design

Many adhesive bonded assemblies are designed to take advantage of the generally good resistance adhesive joints have to shear stresses. 

The best approach to achieving good performance in bonded joints is to select the proper adhesive, design the joint properly for the specific application, and maintain rigid process control. One disadvantage in the use of adhesives is the absence of good non-destructive tests to determine the strength of adhesive-bonded joints in a wide variety of ord applications... 

The basic requirements for a good bond to plastic substrates are surface cleanliness, wetting of the surface by the adhesive, solidification of the adhesive, and proper selection of adhesive and joint design. These requirements are briefly defined here and then more thoroughly discussed in following sections. 

The basic requirements for a good adhesive bond are cleanhness, wetting, sohdification, and proper selection of adhesive and joint design. 

Thus, again, we return to the idea of needing to consider the adhesive system when (a) formulating and designing for good joint durability, and (b) predicting the service-life of bonded components and structures. 

This is one of the most important tests because adhesive bonds show a good resistance to tensile shear stresses and it is always recommended to load the bonded parts in tensile shear mode in order to get the highest resistance (refer to the chapters Design and calculation of bonded joints in Volume 2 and Bonding metals in Volume 5). 

As already stated, the durability or permanence of a bonded assembly is dependent on the intended use and service conditions to which the bond will be exposed. However, the joint design, choice of substrates, adhesive selection, substrate preparation, and primer selection, where appropriate, plus the method of application and assembly all have significant impact on the service life of adhesively bonded materials. Most or all of these considerations are interdependent, for example, the joint design and substrates chosen will limit the range of suitable adhesives that can be employed. In a similar way, the durability of a sealed joint is only as good as the adhesion of the sealant (and primer) to the surfaces forming the joint. Primers and/or sealants will adhere to surfaces only if those surfaces are properly prepared. A very large proportion of all sealant joint failures result from poor or inadequate surface preparation. 

It is quite difficult to check visually the adequateness of adhesion bonding after the joining process. Therefore, design of bonded joints, process optimization, and its control are very important. In this chapter, some important aspects are discussed to obtain practically good adhesion in the fabrication lines of products having adhesively bonded joints. The quality control in adhesives production is not treated here. This aspect is, for example, treated in Petrie (2007). 

The strength of a bonded joint will be determined by the strength of its weakest component, whieh is generally designed to be the adhesive. It follows that the requirements for satisfactory joint performance eu-e (a) good contact between the adhesive and substrate (b) absenee of weak layers in the joint (e) that the adhesive should possess appropriate mechanical properties. These basic requirements are implieit in the essential elements of bonding, and are related to many faetors which affect the performance of bonded assemblies as summarised in Table 4.2. 

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