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Adhesively bonded composite joints can help reduce weight in structures and avoid material damage from fastener holes, but stress concentrations formed at the edges of the adhesive bond line are a main cause of failure. Stress concentrations within the adhesive can be reduced by lowering the stiffness at these edges and increasing the stiffness in the center of the joint. This may be achieved using a dual-cure adhesive system, where conventional curing is first used to bond a lap joint, after which high energy radiation is applied to the joint to induce additional crosslinking in specific regions. Anhydride-cured epoxy resins have been formulated to include a radiation sensitizer enabling the desired cure behavior. Tensile testing was performed on cured systems containing varying levels of radiation sensitizer in order to evaluate its effects on young’s modulus as a function of radiation dose. 

Grading adhesive properties across a bondline can lead to more unniform stresses and increased strength without altering the geometry of the adherends. In this research, radiation sensitizers have been added to adhesives to create a secondary cross-linking possibility that is activated with radiation. In this way the adhesive stiffness and strength can be controlled by controlling the exposure to radition. In this paper, a system of grading adhesive properties is introduced and the double cantilver beam test results show that the gradation not only changes stiffness and strength, but also mode I fracture properties. Additionally, specimens were created with graded properties along the bondline and test results will be presented in the final paper. 

Adhesively bonded joints contain stress concentrations at geometric and material discontinuities within the joint, causing the joint to be inefficient. This study investigates a method to grade the material properties of an adhesive across the bondline to have a soft, flexible adhesive near the stress concentration and a stiff, strong adhesive elsewhere. Theoretical studies and a few experiemental studies have shown an that the load is distributed more evenly along the joint and strength is increased. Adhesive gradation is achieved through a secondary crosslinking system in the adhesive which is activated via radiation. After an adhesive is initially cured, the joint can be exposed to varying levels of radiation to grade the properties. Initial results demonstrate the ability to grade stiffness using radiation shielding, and final results will demonstrate the application in an adhesively bonded joint. 

Functionally graded adhesive bondlines are currently being researched to relax stress concentrations at the re-entrant corner of bonded joints and improve the strength of joints. Bi-adhesive joints have been under development for some time, but lately adhesives with continuous gradation have been shown to theoretically enable more stress reductions and greater strength benefits. Several researchers have shown the potential to create a working adhesive gradation system with very promising results, but adhesive stability over long periods of time has proven difficult to realize. Nearly as important as adhesive development are analysis methods for functionally graded adhesive joints, since the gradation must be designed to yield beneficial results. Therefore, this work addresses the potential gains provided by design of functionally graded adhesive joints driven by finite element analysis. A parametric study on a strap joint with homogenous adhesive is conducted to highlight parameters which influence the global strength of an adhesively bonded joint. A statistical approach is used to identify significant correlations between strength and adhesive material parameters. Results from the statistical study are applied to drive strategies to create joints with optimized gradation and validated by failure analysis within the finite element model. A strap joint is analyzed as example of the potential gain of functionally graded joints.