Adhesive Testing

Adhesive bond strength is usually measured by the simple single lap shear test shown in Fig. 8.8. The lap shear strength is reported as the failure stress in the adhesive, which is calculated by dividing the failing load by the bond area. Since the stress distribution in the adhesive is not uniform over the bond area (it peaks at the edges of the joint as previously shown in Fig. 8.3), the reported shear stress is lower than the true ultimate strength of the adhesive. While this test specimen is relatively easy to fabricate and test, it does not give a true measure of the shear strength, due to adherend bending and induced peel loads. In addition, there is no method of measuring the shear strain and thus calculating the adhesive shear modulus that is required for structural analysis. To measure the shear stress vs. shear strain properties of an adhesive as previously shown in Fig. 8.4, an instrumented thick adherend test can be run where the adherends are so thick that the bending forces are negligible. However, the single lap shear test is an effective screening and process control test for evaluating adhesives, surface preparations, and for in-process control. There are many other tests for characterizing adhesive systems.5

When testing or characterizing adhesive materials, there are several important points that should be considered: (1) all test conditions must be carefully controlled including the surface preparation, the adhesive, and the bonding cycle; (2) tests should be run on the actual joint(s) that will be used in production; and (3) a thorough evaluation of the in-service conditions must be tested, including temperature, moisture, and any solvents or fluids that the adhesive will be exposed to during its service life. The failure modes for all test specimens should be examined. Some acceptable and unacceptable failure modes are shown in Fig. 8.9. For example, if the specimen exhibits an adhesive failure at the adherend-to-adhesive interface, rather than a cohesive failure within the

Before Loading

During Loading ^^ Peel Loads

Due to Bending

Fig. 8.8. Typical Single Lap Shear Test Specimen3

Acceptable Failure Modes

Cohesive Failure of Adhesive

Surface Ply Delaminations (Composite Adherends)

Adherend Failure (Static or Fatigue)

Unacceptable Failure Modes

Adhesion Failure of Adhesive/ Adherend Interface

Adhesive Peel (Excessive Out-of-Plane Loads)

^^ Adherend Yielding

(Induces Excessive Adhesive Strains, Causes Rapid Progressive Failure or "Unzipping"Effect in Adhesive)

Fig. 8.9. Typical Failure Modes of Bonded Joints3

adhesive, it may be an indication of a surface preparation problem that will result in decreased joint durability.

Was this article helpful?

0 0

Post a comment