To minimize stress on bonds, part joints can be designed to optimize adhesive use and performance. This is largely a matter of common sense and experience. Two basic factors should be the guidelines:
1. Structural joints should be designed so that all of the bonded area equally shares the load.
2. Joint configuration should be designed so that the basic stress is primarily shear, tensile, or compressive with cleavage and peel minimized.
Figure 1 - Joint Design
In moving from left to right in figure 1 above, each joint offers increasingly preferable stress conditions on the bond line. Left is most unfavorable with cleavage stress on the bond. The center design adds a shear block to the joint to strengthen the bond. At right, the bond line is in compression for the most favorable design of the three.
Note the following considerations:
1. Peel stress leads to near line-form, high stress on the adhesive in the bonded joint. This is often many times higher than the final strength of most adhesives.
2. Being a planar joining technique, bonds must be designed to withstand shear stress. The substrates are pulled apart parallel to the bonding surface, meaning that the entire bonding area is subject to stress. In general, the bonding area of bonded joints can be increased such that the stresses can be distributed over an area so large that the point collapse load is not exceeded.
3. Tensile stresses act vertically to the plane of the joint. From a design point of view they are only considered in exceptional cases because increasing the bonding area to increase the mechanical loads than can be withstood, which is relatively simple for bonds that are subjected to shear stress, is often not possible.
4. Bonded joints are not sensitive to compressive stress.
5. The behavior of bonded joints under torsional stress is similar to their behavior under lap shear stress.