An experimental and numerical investigation into the effects of bond line thickness variations on the strength of composite bonded joints

The structural integrity of adhesive joints used in the attachment of laminated composite panels has received considerable attention over the past few decades. Of particular concern and the topic of this paper is the effect that a nonuniform bond line thickness has on the load carrying capacity of a bonded joint. To this end, carbon fiber adherends were joined in a single lap joint configuration using a two-part epoxy adhesive. The joints were initially fabricated and tested with various uniform bond line thickness in a single lap joint configuration to establish a benchmark. Bonded single lap joints with linearly varying bond line thickness were then investigated. A finite element study was performed to predict bond strength of the uniform and linearly varying bond lines using the Critical Zone method. The failure strength of the uniform bond line thickness specimens was found to decrease with an increase in the bond line thickness. In the case of linearly varying bond line thickness specimens, the bond strength decreased as the bond thickness at one end of the bonded joint overlap deviated from its intended value. The finite element model using the Critical Zone method was able to predict the failure strength of the joints in both uniform and linearly varying bond thickness configurations. The reduction in failure strength was attributed to an increase in the stresses in the adherend ply adjacent to the bond line as determined through the finite element stress analysis.