Reliability of adhesive bonds

The author would like to thank the many researchers who responded to the questionnaire for the National Materials Advisory Board Workshop on "Reliability of Adhesive Bonds Under Severe Environments. Some of their names appear in the references. A summary of this paper, presented to the Workshop held on May 3-4, 1984 at the National Academy of Sciences, will become a part of the Committee Report to be published separately. 

Statistical methods are indispensable for ensuring the reliability of adhesively bonded joints. The mathematical background of statistical methods is very briefly explained in this section. See other textbooks (e.g., Montgomery 2005) for more details. 

When the reliability of adhesively bonded joints should be considered, joints too weak in strength are out of use. However, too strong joints are not problems. Therefore, P n-k < X < fi + oc) is more crucial than P( a — krelations between them can be shown as follows ... 

Surface preparations must be carefully controlled for reliable production of adhesive-bonded parts. If a chemical surface treatment is required, the process must be monitored for proper sequence, bath temperature, solution concentration, and contaminants. If sand or grit blasting is employed, the abrasive must be changed regularly. An adequate supply of clean wiping cloths for solvent cleaning is also mandatory. Checks should be made to determine if cleaning cloths or solvent containers have become contaminated. 

For the future the evolution of adhesive bonding as a joining technique in automobile production points in two directions. On the one hand, well-known applications have to be optimized and improved to make them eost-elfeetive but nevertheless reliable and trustful processes enjoying increasing acceptanee for adhesive bonding. On the other hand, there will be new applications with different adhesive requirements, and adhesive suppliers must anticipate these changes and develop eompatible adhesive eompounds to satisfy the new requirements. 

Furthermore, the application of existing European or national test and performance standards for epoxy bonded products are much too penalising, since they merely impose severe conditions that are not verified in service, or are inadequate because they were developed originally for other adhesives, namely for phenolic and aminoplastic adhesives used in very thin bondlines [7, 52-58]. Moreover, current standard proposals developed for gap-filling adhesives focus only on the initial bond quality control [59-63]. The lack of standards in this field impedes the objective evaluation of the reliability of a bonded-in rod connection, causing engineers to avoid this type of approach altogether. 

The second step, after determining the quality of incoming materials, is adherend surface preparation. Surface preparation must be carefully controlled for reliable production of adhesive-bonded parts. 

A major opportunity for growth is the replacement of mechanical fasteners by adhesive bonds. Structural adhesives are the existing products that serve this market segment. The key to success in the fastener applications is acceptance by end users, mainly because of concern about reliability and reparability of adhesive bonds. 

One problem, always present in the use of structural adhesives, is how to determine the reliability of the bonded structure. This may be at the time of manufacture, to ensure that a good joint has been produced, or during service, to monitor behavior under operating conditions. It is often impractical, or impossible, to use the types of test methods employed to determine mechanical properties and interest has centered on nondestructive test methods."" " ... 

With the advantages and disadvantages of these systems it is still going to be difficult to convince customers that the reliability of the bonded products using waterborne adhesives will be as good as for products produced with solvent-based systems. 

Diffusion of moisture or solvent into the adhesive can significantly change the state of the stress field and hence the reliability of the bonded joint. Various researchers modeled the effects of viscoelasticity and diffusion in the analysis of bonded joints.( 9-42) in all these works, either the effects of moisture or the coupling between the viscoelastic stress field and the moisture/solvent concentration were not considered. In general, the diffusion coefficient is a function of the dilatational strain, temperature field, and solvent concentration. Recently,... 

Hart-Smith, L.J., Reliable nondestructive inspection of adhesively bonded metallic structures without using any instruments, McDonnell Douglas Paper MDC 94K009I. In Proceedings of the 40th International SAMPE Symposium and Exhibition, Anaheim, CA. May 8-11, 1995, pp. 1124-1133. 

Nondestructive Testing (NDT, NDE, NDi, NDC) -Techniques used to examine the quality of adhesive bonds without causing damage to the item or bond. The terms NDT (Testing), NDE (Evaluation), NDI (inspection) and NDC (Characterisation) tend to be Interchangeable. Nondestructive inspection systems may be manual or automated to some extent. Whilst effective for finding defects in the bond, no method currently exists which allows a reliable, quantitative measure of bond strength. 

M.G. Perichaud et al.. Reliability Evaluation of Adhesive Bonded SMT Components in Industrial Applications, Microelectronics Reliability, Vol 40, 2000, p 1227-1234... 

Over the past several decades, much study has been focused on surface energetics, wetting, adhesion, etc., and some generalizations have been developed. These can be useful guides to the adhesive selection and surface preparation needed to form reliable, strong adhesive bonds. Only a brief exposure to some of this valuable information can be presented here, but further reading is available in Chapter 3 and other chapters of this Handbook, and in listed references. 

There have, for decades, now, been reliable analytical tools available for the design and analysis of adhesively bonded joints. Even so, there is still far too much reKance on the oversimplified model whereby the bond strength is assumed to be the product of some fictitious uniform adhesive allowable shear stress and the bond area. If more joint strength were needed, all one had to do, according to this procedure, was to increase the bonded area. Bonded joints do not obey such rules. Such a formula is valid today only in the context of short-overlap test coupons in which the goal is to create as closely as possible a uniform state of... 

Many of the loudspeakers assembled in North America make extensive use of adhesive bonding. Adhesives are used to bond the fabric to the cone, attach the cone to the speaker frame and secure the permanent magnet (Fig. 6.3). High production rates and good joint reliability, both easily achieved with automated adhesive dispensing, have made this... 

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