Page 153 - Handbook of Adhesives and Sealants

122 Chapter Three

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Figure 3.27 Movements in lap shear joints. Original sealant dimensions are ⁄2 ⁄2
inches. (a) Displacement of 0.56 inches with sealant only extended 50%, using a 50%
capacity sealant (medium modulus). (b) Displacement of 0.87 inches with sealant only
extended 100%, using sealant with 100% extension capability (low modulus). 14
3.7 Common Sealant Joint Designs
Design of a sealant joint depends to a great deal on the type of sealant
selected. Common joints are the simple butt joint, lap joint, and angle
joint, as shown in Fig. 3.28.
Where a choice is available, the sealant most compatible with the
part configuration should be used. For example, a free flowing liquid
sealant cannot be applied to a vertical surface, such as a wall joint.
In this application, a non-sagging type must be used.

3.7.1 Butt joints
There are many variations of the common sealant joint configurations.
Various versions of the compound butt joint are shown in Fig. 3.29.
If sealant extension is important in a working butt joint, a release
agent should be placed on the bottom inner surface so that the sealant
does not bond to this face. Thus, the sealant is free to stretch when
joint volume increases (Fig. 3.29b, c, and d). Figure 3.30 shows that
the same basic principle applies to corner joint designs. Here a corner
bead is used as a back-up material.

3.7.2 Lap joints
Lap joints can be easily sealed with tape; bead sealed, if the thickness
of the joined sheets is sufficient to support a bead of sealant; and
sandwich sealed. Sandwich sealing is a common method with struc-
tural adhesives. Sealant thickness is more critical when the material
is applied between surfaces. In all joints, production time can be de-
layed considerably by excessively thick applications of the material.