Resistance of bonded composite joints to out-of-plane forces

Abstract

An analytical and experimental study was conducted to examine the mechanical behavior of double-lapped adhesive-bonded joints subjected to bending loads. Adherends consisted of a U-shaped high-strength steel hub and a unidirectional composite of either B/Al or B/epoxy. Adhesives with a range of moduli and peel strengths were evaluated with and without bolts in regions of high stress concentration. Finite element analysis results indicated high shear and normal stress concentrations at the end of the steel hub legs. These concentrations are a direct function of the effective hub stiffness - increases in hub thickness or Young's modulus result in higher adhesive stresses. Stress concentrations in the adhesive layer may be reduced by decreasing adhesive modulus, composite transverse modulus, or composite in-plane shear modulus. Experimental testing of sample joints revealed that failure modes are controlled by the composite adherend material. Joints incorporating a B/Al adherend failed by adhesive peel in the region of high stress concentration. Failure occurred at a load level which stressed the B/Al composite to 75 percent of its 200 ksi ultimate strength. Delamination of the B/epoxy composite was the predominant failure mode of joints utilizing a B/epoxy adherend. Failure by delamination was suppressed by the insertion of high-strengthmore » bolts into the stress concentration region. These bonded and bolted joints reached 68 percent of the composite bending strength (220 ksi, 1517 MPa).« less