Effect of interlayer adherend inclusions on strength of composite bonded joints

Foreign objects such as backing tapes may get embedded between the plies of a laminated polymer composite during its fabrication which can compromise the composite panel or the adjacent bond joint. This paper investigates the effect of such inter-layer inclusion defects in laminated carbon fiber composite adherends, on the strength of an adjacent single lap bonded joints, using both experimental and computational methods. Composite laminates composed of 0 unidirectional lamina were fabricated with Polytetrafluoroethylene (PTFE) strips embedded between the first two plies closest to the bond line. Three different inclusion positions were investigated and the location of the inclusion defect yielding the lowest strength was identified. The inclusion location in the most compromised joint was then used to study the effect of this type of defect in adherends having different ply orientations. Finite Element (FE) stress analysis was performed to understand the variation in the shear and peel stresses due to the presence of the inclusion. FE analysis revealed higher shear and peel stresses in bonded joints adjacent to inclusion defects.