Interface solicitation of a bonded joint by Thick Adherend Shear Testing
Résumé
In the aeronautical field, mass reduction of structures is a major objective. Among the wide range of possible solutions, structural bonding can be used. Hybrid bonded joints involving a thin titanium cover sheet on a lighter composite bulk appear to be a good alternative to reduce structure’s weight without losing mechanical properties. In this particular type of bonded parts, titanium alloys adhesion properties are usually enhanced by chemical surface treatment and adherence promoters.
Some studies have shown that in the case of bi-phased (α + β) titanium Ti-6Al-4V alloy, an alkaline etching treatment leads to a selective dissolution of α-phase [1] allowing good bonding properties. As for adhesive bonding, phenolic primer and epoxy adhesive seem to be standard practice. These are the solutions used in the present study.
To characterize the joint properties in nominally pure shear, Thick Adherend Shear Testing (TAST) is used. However, the test geometry involves normal stresses at the edges of the joint which implies a mixed load mode in these areas. TAST can be therefore used to initiate the failure close to the titanium-adhesive interface, in order to exacerbate sensitivity to surface microstructure modifications or to surface treatment parameters.
Thus, this study focuses on the impact of surface treatment parameters upon adhesion properties. Microstructures of titanium alloy and environmental impact were also investigated. Surfaces were characterized before and after etching, and after failure, by scanning electron microscopy and X-ray Photoelectron spectroscopy.