Joints shimmed

One of the ways to increase the bearing strength of a joint is to use metal inserts as in the shimmed joint of Figure 7-40. Another way is to thicken a section of the composite laminate as in the reinforced-edge joint in Figure 7-40. 

The viscosity of epoxy resins and curing agents can be used to control the bond line thickness within the adhesive joint. But the bond line can also be regulated by the incorporation of fillers, by the use of scrim cloth or woven tapes as internal shims within the adhesive itself, or by the careful regulation of the cure cycle. 

Another option is to design mechanical shims into the joint itself. The parts to be assembled are designed with bps or stops so that the adhesive cannot flow out of the joint area or so that a certain predetermined adhesive thickness is always maintained. 

The Bonding Process. Before the actual assembly operation, the cleanliness of the shop and tools should be verified. The shop atmosphere should be controlled as closely as possible. Temperature in the range of 18 to 32°C and relative humidity from 20 to 65 percent are best for almost all bonding operations. All parts should be fitted together without adhesive or sealant to indicate possible production problems due to fit. The suitability of fit is established by either visual inspection or direct measurement with gauge or shim. It is desirable that the extremes in mechanical tolerances also be noted and that test specimens be made with the worst possible fit to ensure that the bonding process will always provide reliable joints. 

Use mechanical shims or stops, which can be removed after the curing operation. Sometimes it is possible to design stops into the joints. 

It is highly desirable to have a uniformly thin (2- to 10-mil) adhesive bond line. Starved adhesive joints, however, will yield exceptionally poor properties. Three basic methods are used to control adhesive thickness. The first method is to use mechanical shims or stops, which can be removed after the curing operation. Sometimes it is possible to design stops into the joint. 

In films or structural tape, the carrier is usually porous and the adhesive saturates the carrier. Glass, polyester, and nylon fabric are common carriers for supported B-staged adhesive films. With fabric constructions, (1) the carrier provides for a method of applying the adhesive (2) it may also may act as reinforcement for the adhesive and (3) it can provide an internal shim to control the thickness of the adhesive regardless of the amount of pressure applied to the joint during the time of curing. 

A scrim cloth can also be used in adhesive bonding as a automatic means of retaining a certain adhesive thickness. In this process, the scrim is used as a cloth spacer, impregnated with adhesive. The glass fibers in the scrim cloth act as shims to separate the two adherends regardless of the amount of pressure placed on the joint. 

It is possible to set the roller gap independently of the roller pressure by using an adjustable shim or depth stop on the roller wheels. This should be set to less than the combined thickness of the materials and can be set to zero to allow the joint thickness to be fully controlled by the applied pressure. 

Use compatible shims or washers in association with fayed joints and fasteners. This control technique works well where exposure to the electrol5de is again intermittent and surfaces are well-drained. By using a shim or washer that extends 3-6 mm (1/8-1/4 in.) beyond the interface of the cathodic material with the magnesium surface, the electrolyte path is extended to a range where the resistance of typical aqueous saltwater electrol54es is... 

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