Characterization of Bondline Thickness Effects in Adhesive Joints

Abstract

Growing application of composite materials in airframe structures tends to make significant use of bonded constructions. Despite the large work that has been done on adhesive characterization over previous years, various certification-related issues arise in these applications. The available test methods for determining the in situ properties of an adhesive joint for use in the design must be clarified for thick bondlines, where there is a lack of readily available data. Results from three test methods indicated a decrease in the apparent shear strength as the bondline thickness was increased. The apparent shear strength given by the test methods investigated was found to be highly dependent on the adherend bending stiffness, which directly effects the peel stress distributions in the adhesive layer. In the adhesive strength comparisons as a function of test method, it was found that the thin-adherend tests gave different comparative results than the thick-adherend tests, which were primarily a function of the high peel stresses in the thin-adherend joints. This combined state of stress does not give a true view of the apparent shear stress of the adhesive, but rather an indication of the adhesive behavior under this type of combined loading. It is recommended that thick adherends be used when comparing different adhesive systems for apparent shear strength, and that thin adherends should be used for qualitative tests only.