Functionally Graded Adhesive Joints with Improved Strength and Stability

University Of Massachusetts Lowell, Lowell MA

Investigators

Abstract

With greater reliance on fiber-reinforced composites as lightweight alternatives to metals, structural adhesives are increasingly important for joining materials in high performance applications such as airplanes, space vehicles, wind turbine blades, sporting goods, and motor vehicles. In a typical adhesive bond, the outside edges of the adhesive carry most of the peeling load and are the most likely point of failure. By creating a functionally graded adhesive joint with soft, flexible edges and a stiffer core, the stress may be distributed more evenly, increasing joint strength substantially; indeed, enhancements of up to 60 percent have been reported via this approach in the limited testing conducted to date. While such results are highly promising, no one has identified a means of creating functionally graded adhesive joints whose properties are stable over time. This work seeks to investigate the basic science behind functionally graded adhesives whose properties are controlled by adjusting exposure to radiation. Epoxy resins will be formulated for this effort; their thermal and mechanical properties will be measured, and adhesive joints based on these materials will be graded via exposure to varying doses of gamma rays and tested for strength. In tandem, computer simulations will use the measured properties to predict joint performance, with actual joint performance data used to validate the simulations. The resultant computational design tools will provide information on how best to design functionally graded joints. The efforts will open up an almost entirely unexplored field related to the preparation and study of functionally graded adhesives by removing key barriers that currently prevent such investigations. The success of this work will enable engineers to create stronger, lighter, more reliable adhesive bonds, and make more efficient use of high performance composites. The most immediate impact is likely to be felt in sectors where composites and lightweight metals are heavily used - aerospace, ship-building, ground transportation, and wind energy - with many other areas of application to follow as the implications and benefits of these efforts become apparent. Furthermore, results will be integrated into both undergraduate and graduate courses, work will be carried out to demonstrate applications through undergraduate engineering capstone projects and extracurricular activities (such as Formula SAE Competition team), and graduate students along with REU students will be trained and integrated into the research effort, Structural adhesive joints suffer from stress concentrations at their edges, while sensitivity to manufacturing flaws has made many reluctant to use them. The gradual introduction of load through a functionally graded adhesive (FGA) - soft at the edges, stiff in the middle, for instance - addresses both issues. Recent theoretical work predicts dramatic gains in joint strength using FGAs, a conclusion confirmed by the few experimental efforts reported in literature. The slow progress in this area stems from the difficulties in making consistent, stable, robust FGAs. Coupled with a lack of validated knowledge in the area, this has precluded their application. The overall objective is to understand how to design, create, characterize and predict the properties of FGA joints, with the long-term goal of increased joint performance, stability, and reliability. The central hypothesis of this work is that stable property gradients designed using predictive models will enable stronger, more stable bonded joints. This work will provide a fundamental understanding of the design, creation and behavior of FGAs. Design rules for FGA joints will be identified, general methods to create and characterize FGAs and FGA joints will be realized, and predictions of FGA joint properties will be validated. This effort will expand our ability to model complex joints and make and characterize functionally graded materials in general. The efforts will open up an almost entirely unexplored field related to the generalized preparation and study of FGAs by removing key barriers that currently prevent such investigations. They will also provide industry with new options that promise a significant advance in capabilities as far as bonding is concerned, leading to safer, stronger, lighter structures that make more efficient use of limited resources.