Fatigue behavior of weldbonded joints

Abstract

The effects of material and process variables on fatigue behavior were determined for a newly developed, lowcost weldbonding process for the assembly of durable aircraft structures. The weldbonding process involves spot-welding components through a previously applied adhesive, and then oven-curing the assembly to achieve a bonded structure. Both low-load and high-load transfer specimen geometries with each of two alloy combinations (7075-T6/7075-T6 and 2024-T3 alclad/7075-T6) were evaluated. Fatigue behavior of weldbonded specimens with different nugget sizes and different manufacturing defects was compared with that of riveted and adhesive-bonded specimens. In low-load transfer fatigue, weldbonding was better than riveting, but not as good as adhesive bonding, while in high-load transfer fatigue, weldbonding was equal to or better than riveting and adhesive bonding. ELDBONDING was developed to reduce the cost of joining aluminum components such as airframe skins to doublers. The process involves spot-welding components together through a previously applied adhesive, and then oven-curing the assembly to achieve a bonded structure. The spot welds replace expensive fixturing which would otherwise be needed for adhesive bonding during the cure cycle, and an economical oven can be used in place of an autoclave. The process also eliminates expensive hole preparation and fastener installation costs associated with mechanical fastening. The weldbond system used for this work was developed under Northrop Independent Research and Development and Air Force Materials Laboratory contracts.13 A summary of the process follows: the parts to be joined are alkaline cleaned, deoxidized, phosphoric acid/sodium dichromate anodized at 1.0-1.5 V, coated with a modified epoxy paste adhesive (B.F. Goodrich A1444B) at the faying surface, spot-welded together, and oven-cured at 121 °C (250°F) for 1 h.