A functionally graded adhesive (FGA) is a specific class of functionally graded material whose properties can be locally modulated, enabling a more uniform stress distribution along a bonded joint. Motivated by the need to develop FGAs with well-defined and stable gradients in properties, a new approach is proposed involving the use of thermal/ionizing radiation dual-cure adhesives. To generate adhesives with dual-cure characteristics, a series of epoxy resin formulations containing a reactive liquid rubber-based additive that is rich in unsaturation have been developed. Networks are thermally cured and then subjected to γ irradiation, and their capacity to undergo secondary crosslinking reactions is examined via chemical, thermal, and mechanical characterization. While the addition of large concentrations of liquid rubber additives can result in an initial reduction in properties such as modulus and glass transition temperature, their addition can significantly boost radiation sensitivity, enabling the realization of large, well-controlled variations in properties as a function of radiation dose. A facile method is then utilized to create materials with gradients in hardness, confirming the potential of such formulations to generate FGAs. Additionally, the behavior of these radiation sensitive resins is successfully described by a model proposed by Shibayama. This model provides a useful means to predict the glass transition temperatures of dual-cure epoxy resins following radiation-induced crosslinking and provides additional insights into molecular-level behavior of these networks. This effort highlights the promise of this generalized approach for the formation of FGAs based on the careful design and processing of systems sensitive to both thermal and radiation curing.

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.

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