Page 146 - Handbook of Adhesives and Sealants

Joint Design 115

3.5.5.2 Reinforced plastics. Reinforced plastics are often anisotropic
materials. This means their strength properties are directional. Joints
made from anisotropic substrates should be designed to stress both
the adhesive and adherend in the direction of greatest strength. Plas-
tic laminates, for example, should be stressed parallel to the lamina-
tions. Stresses normal to the laminate may cause the substrate to
delaminate. Single and joggle lap joints are more likely to cause de-
lamination than scarf or beveled lap joints. The strap-joint variations
are useful when bending loads occur.

3.5.6 Wood joints
Wood is an anisotropic and dimensionally unstable material. Proper-
ties differ with grain direction. Joints should be made from similar
types and cuts of wood to avoid stress concentrations.
Tensile strength of wood in the line of the grain is approximately
ten times greater than in a direction perpendicular to the grain. Ad-
hesive joints should be designed to take this factor into account.
Wood changes dimensions with moisture absorption. The degree of
dimensional stability is also dependent on grain direction. When wood
is bonded to a much more dimensionally stable material, the adhesive
must be strong enough to withstand the loads caused by dimensional
distortion of the wood.
A plain scarf joint with very flat slopes is the most efficient joint
design for wood, but it requires the application of lateral or transverse
pressure during the time the adhesive hardens. Common wood joints
that are self-aligning and require no pressure are shown in Figs. 3.20
and 3.21.

3.6 Sealant Joint Efficiency
Many factors ultimately affect the performance of a sealant joint, but
the shape and dimensions of the sealant cross-section are considered
of primary importance. The primary dimensions of a sealant joint are
its depth and width as shown in Fig. 3.22b. As with adhesives, the
design of the seal varies with the types of material being used. How-
ever, there are only two main types of sealant joint configurations: butt
joints and lap joints.

3.6.1 Stress distribution in butt joints
In butt joints, as temperature rises, the sealant will go into compres-
sion due to the thermal expansion of the adherends being sealed. As
the temperature declines, the adherends will contract and the joint