Experimental study on interfacial shear transfer in partially-debonded aluminum/epoxy joint

Abstract A six-step phase-shifting method is applied to calculate the whole-field shear stress of an adhesively bonded aluminum alloy-to-epoxy joint containing an initially debonded interfacial crack for studying shear transfer behavior. For a well-bonded interface, the isochromatic fringe order and the interfacial shear stress (ISS) distribute continuously and increase with compression. The epoxy corner formed at the right-angle edges of the bonded interface during specimen preparation exhibits more than eight orders of dense fringes and a maximum shear stress of 1.4 MPa under a load of 3.0 kg. The load is transferred by a shear band that connects the bonding interface to the support. A denser isochromatic pattern occurs at the crack-tip than at the right-angle edge from the partially debonded interface. The crack-tip displays about seven fringe orders, 1.3 MPa of ISS under a load of 3.0 kg, and an increase of about 3 mm in the interfacial crack length. The shear stress decreases rapidly at the debonded interface but takes 26% of the shear force of the entire interface, indicating that the debonded interface obstructs and decreases the load transfer capability from the bonding interface to the support. As the curing temperature decreases to 20 °C, a thermal residual shear stress appears on the interface because of the discrepancy in the coefficients of thermal expansion between the aluminum alloy and the epoxy. The residual shear stress redistributes on the bonded and debonded interfaces due to the formation of the initial crack induced by an external load. The calculated effective stress intensity factors (SIFs) of the interface crack are identical to theoretical prediction.