Research Topic: Bonded Joints and Sandwich Structures

This work is concerned with understanding mechanisms contributing to the mechanical durability of adhesively bonded joints. To that end, the response of coupons bonded with brittle and toughened adhesives are compared under quasi-static and repeated load environments. Since this phase of the work considers adhesive response (rather than adherend failure), the adherends are made of 2024-T3 aluminum. The testing regime is designed to study the effects of peel (double cantilever beam coupons) and shear (wide area lap shear coupons and scarf joints) stress on bond strength and durability. The coupons are bonded using brittle and toughened adhesives (FM-300, FM73, EA9696) so that effects of adhesive strength and toughness may be independently considered.

The objective of this recently-initiated research investigation is to revisit and revise the ASTM D 3762 metal wedge crack durability test. While considered to be a reliable method for investigating adhesive bond durability, the existing standard provides little guidance regarding the conditions and requirements that constitute an acceptable metal bonded joint. Of particular concern is the reduction in strength of the bonded metal joint over time due to hydration. Thus a need exists to revise the existing test standard such that it provides specific guidance on how to successfully develop criteria for the wedge crack durability test. Possible revisions to the standard include proposed exposure environments, and pass/fail criteria with regards to both crack growth extension and failure modes. This research project will initially focus on reviewing the literature and identifying stakeholders associated with the test method. Test results and proposed additions and revisions to the ASTM D 3762 standard will be communicated regularly to ASTM Committee D14 on adhesives. In addition to proposing revisions to this standardized test method, research results from this investigation will be disseminated through an FAA technical report and journal publications. Expected benefits to aviation include an improved adhesive bond durability test method for use in assessing the reliability of adhesively bonded aircraft structures.

The purpose of this research is to determine the effect of atmospheric pressure plasma treatment on Mode I strain energy release rate (GIC) and failure mode of bonded peel ply prepared carbon fiber reinforced polymer composites. Previous research showed that Toray T800/3900 carbon fiber reinforced epoxy composites prepared with Precision Fabrics Group 52006 nylon peel ply and bonded with MetlBond 1515-3M structural film adhesive failed in adhesion at low fracture energies when tested in the double cantilever beam (DCB) configuration. Other research suggested that plasma treatment could be able to activate these “un-bondable” surfaces and result in good adhesive bonds. Nylon peel ply prepared 177°C cure carbon fiber reinforced epoxy laminates were treated with atmospheric pressure plasma after peel ply removal prior to bonding. Surface characterization methods, including contact angle (CA), Fourier transform infrared (FTIR) spectroscopy, and x-ray photoelectron spectroscopy (XPS) were used to determine how plasma treatment changed nylon peel ply prepared surfaces. CA can be used to measure surface energy of a composite prepared for adhesive bonding. This information can help understand one requirement of adhesion: surface wetting of the adherend by the adhesive. FTIR and XPS can be used to measure composite surface chemistry, which can help understand another requirement of adhesion: the formation of chemical bonds between the adherend and adhesive. FTIR and XPS can also be used for the identification of contaminants, which can inhibit adhesive bonding. DCB specimens were bonded with MetlBond 1515-3M and tested to determine failure mode and GIC. Plasma treated samples had acceptable failure modes and fracture energies that were triple that of peel ply only samples. It was demonstrated that atmospheric pressure plasma was able transform poor bonding surfaces to good bonding surfaces.