Experimental failure analysis of adhesively bonded steel/CFRP joints under quasi-static and cyclic tensile-shear and peel loading

Abstract Epoxy resin adhesives are widely used in lightweight and structural applications and are suitable for mixed structures with steel and CFRP. The design of composite structures, especially under cyclic loading, is often carried out without consideration of interphase-specific effects. In this study, shear and peel test specimens made of steel and CFRP are tested under static load. The influences of material combinations and fiber orientation on strength and failure are evaluated. Cyclic tests are carried out on shear and peel test specimens to describe the evolution of damage and to determine crack initiation. For this purpose, a method is presented with which crack initiation can be determined based on the dynamic compliance. The main experimental results are as follows. The shear strength in mixed structures is reduced due to asymmetric shear stress distribution, which is intensified by the reduction of CFRP stiffness due to varying fiber angles. For specimens under peel load, the fillet size is an essential factor, since it can lead to high stress peaks and thus to delamination. Mixed structures show a reduced fatigue strength under cyclic loading due to earlier crack initiation. Peel loads lead to an accelerated crack growth and reduce the fatigue strength significantly.