Page 136 - Handbook of Adhesives and Sealants

Joint Design 105

adhesive, the more uniform the stress distribution in the joint and
the higher the bond strength.
9. The higher the Et (modulus x thickness) of the adherend, the less
likely the deformation during load, and the stronger the joint.
10. Within reasonable limits, the adhesive bond-line thickness is not
a strong influence on the strength of the joint. More important
characteristics are a uniform joint thickness and void free adhe-
sive layer.
The remainder of this chapter will provide practical applications of
these rules.

3.5 Common Adhesive Joint Designs
The ideal bonded joint is one in which under all practical loading con-
ditions the adhesive is stressed in the direction in which it most resists
failure. A favorable stress can be applied to the bond by using proper
joint design.
Some joint designs may be impractical, expensive to make, or hard
to align. The design engineer will often have to weigh these factors
against optimum adhesive performance. Common joint designs are
shown in the next several sections for flat and cylindrical adherends,
stiffeners, angle and corner joints, plastic and elastomer joints, and
wood.

3.5.1 Joints for flat adherends
3.5.1.1 Butt joints. The simplest joint to make is the plain butt joint.
However, butt joints cannot withstand bending forces because the ad-
hesive would experience cleavage stress. If the adherends are too thick
to design as simple overlap-type joints, the butt joint can be improved
in a number of ways, as shown in Fig. 3.9. All the modified butt joints
reduce the cleavage effect caused by side loading. Tongue-and-groove
joints have an advantage in that they are self-aligning and act as a
reservoir for the adhesive. The scarf joint keeps the axis of loading in-
line with the joint and does not require a major machining operation.

3.5.1.2 Lap joints. Lap joints are the most commonly used adhesive
joint because they are simple to fabricate, applicable to thin adher-
ends, and stress the adhesive in shear. However, the adherends in the
simple lap joint are offset, and the shear forces are not in-line, as was
illustrated in Fig. 3.7. This twisting of the lap shear specimen results