Joint durability problems

The application of these principles to an actual adhesively bonded joint is anything but straightforward. One problem is the lack of pertinent information on the performance of adhesive joints. Most test data generated by adhesive producers are only useful for comparison purposes and are of limited use to the design engineer. Also, there is only limited information available on the performance of bonded joints exposed to service environments, while subjected to static or dynamic stresses. Adequate adhesive characterization and prediction of joint durability remain goals for the future. 

The primary challenge facing adhesive bonding of metals is to obtain sufficient durability of a bonded structure. Initial bond strength in metal-polymer adhesive joints is almost invariably excellent. Challenging the application of adhesives in polymer-polymer joining, however, is the problem of obtaining a joint that is... 

Surface cleaning/etches. As with aluminum and titanium, the most critical test for bonded steel joints is durability in hostile (i.e., humid) environments. The fact that the problem is a serious one for steel was illustrated in a study [117] that compared solvent cleaned (smooth) 1010 cold-rolled steel surfaces with FPL aluminum (microrough) substrates. Although the dry lap-shear strengths were not markedly different, stressed lap-shear joints of steel adherends that were exposed to a humid environment failed in less than 30 days, whereas the aluminum joints lasted for more than 3000 days. 

The development of inhibited polymer materials for antifrictional purposes has been a new approach to the problem of raising performance and durability of friction joints operating in hostile environments. As experience has shown, protective inhibited lubricants are not always efficient for movable joints where the lubricant is squeezed with time from the friction zone. During long-term operation of a friction joint it is necessary that Cl be continuously supplied to the friction zone and the inhibited antifrictional plastics have proved to be most suitable for this purpose [37]. 

A Quality Assurance Program should be conducted to assure satisfactory end-product strength and durability. This should (1) establish limits on bonding factors to ensure acceptable joint and end-product (2) monitor the production processes and quality of bond in joint and end-product and (3) enable detection of unacceptable joint and end-product, detenmning the cause, and correcting the problem [3]. 

For durability, the polysulfides are designed to last for 10 to 20 years in outdoor environments. The polyurethanes should last 20 years, and silicones should have a life longer than 20 years. One of the problems with polysulfides is that they have the largest compression set of the three, causing joint failure, especially in winter months. With the trend toward narrower and prettier joints, the polysulfides (65 to 75 percent recovery) are being outperformed by the polyurethanes (85-95 percent recovery) and the silicones (90-100 percent recovery). 

The more common environmental problem with metal joints is attack by water. In addition to alloy composition, pre-treatment and primer, durability is affected by the adhesive used (see Pre-treatment of metals prior to bonding. Pre-treatment of polymers. Primers for adhesive bonding). Sell has ranked adhesives in the order of durability they provide with aluminium substrates ... 

Other investigations [2], with aging times of 2l6. and 3 years under different artificial and natural conditions, show that on the PP side of bonded joints (in that case, PP steel joints) no remarkable changes in the adhesional area occur. So it can be stated that the durability of adhesive bonding of PP is no problem when the surface of the PP is treated as mentioned earlier. 

In the discussion of factors affecting joint strength and durability, it was seen that the presence of water was responsible for many of the problems. Most bonded structures, if exposed to water or high humidity, will lose strength over a period of time and in extreme cases will fail completely. 

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