Mechanical Testing of Epoxy Adhesives for Naval Applications

With advances in composite technology and an understanding of composite application, requirements to develop new design approaches exist. With composite structures today, a basic understanding that both material architecture and joining process, affect the strength of the structure. These design requirements force old techniques of joining, such as bolting and riveting to be re-evaluated. Advantages of adhesives over mechanical means of fastening include higher stiffnesses, more uniform load distribution, parts consolidation, no holes drilled in adherends (with resulting stress concentrations), and, generally, less labor. Adhesives have proven to be a good solution for joining when composites are utilized, but this necessitates the next step in engineering, which is to quantify adhesive properties. By quantifying bulk properties for adhesives at varying conditions, application for adhesives is promoted. The following study attempts to implement testing techniques for qualification of the use of adhesives in hybrid connections on naval hulls. E-glasdvinyl ester composite specimens adhesively bonded to aluminum specimens were tested. The three varieties of specimens tested were: single lap tensile shear specimens, double lap tensile shear specimens, and single flexure specimens. These geometries where chosen because they closely resemble applications currently explored in the AHFID and MACH projects. Instrumentation was used to collect displacement and load data. Some samples where exposed to environmental conditions to determine the performance of the adhesive when exposed to moisture. Increased residual stresses due to moisture absorption are ignored in this study. The data was then used to characterize the performance of the adhesive for varying bondline thickness and varying surface preparations. The results indicate that the grit blasted surface preparation technique had a marked effect on the strength of the bond. The bondline thickness markedly affected the ultimate load capacity of the joint. Modes of failures where characterized in an attempt to determine cause of failure.