The secret to strong, reliable adhesive bonding is to start with good clean surfaces.
Lawrence W. Fisher
Manager, Mechanical Systems Dept.
Measurement Analysis Corp.
Adhesives are unlike other fastening systems: It's difficult to determine whether the bonded joint has the required strength and integrity without destroying it. It's easy to judge the quality of other fastening methods, such as threaded bolts with torque wrenches. The direct measurement of applied torque confirms the joint preload while joint-qualification tests provide a rational basis for inferring the bolted joint's strength and integrity.
In contrast, adhesive joints don't allow for this type of direct measurement of strength and integrity. It's difficult to inspect or evaluate the integrity of bonded assemblies without simultaneously destroying them. Trained assemblers can inspect a joint for obvious flaws as well as subtle ones. The latter include lack of bond line uniformity or a soft, improperly cured adhesive. Although this information is useful, it by no means guarantees an effective adhesive bond. Designers rely on five key factors to help ensure adhesive-bonded assemblies have strong reliable joints:
- Strength/stiffness of the substrates to be bonded
- Adhesive bond strength to the substrates
- Adhesive strength/modulus
- Geometry of bonded area relative to applied load
- Adhesive thickness
These important joint properties are best considered early in the design process so they can be appropriately included in the joint design. Another key element in achieving maximum joint strength is good preparation of the substrate surface. Designers must include this preparation process when selecting the substrate and adhesive that provide desired joint properties. Poor surface preparation often results in reduced joint strength. This region on the bond depends on the substrate's chemical composition and processing methods that may produce undesirable surface conditions. For example, metal oxides often form on metallic substrates during processing or storage. Plastics can have impurities on their surface related to processing or as a result of the bonding process.