Engineering Tough Laminated Composites Using Novel Additive Manufacturing Techniques and Tuning Resin Properties
Authors:   , Idrees Mohanad Omer Suliman , Alvarez Nicolas , Palmese Giuseppe 
Published: January 2024

High strength, lightweight, and tough materials are crucial for advanced and structural applications. Laminated composites offer excellent specific strength and stiffness compared to metals and alloys, but their toughness often falls short. This deficiency is attributed to their inherent layered design and traditional composite manufacturing techniques that prioritize maximizing the reinforcement volume fraction to enhance strength and stiffness. While mechanical properties are the highest when the composites are loaded in the reinforcement direction, fibers are discontinuous in the lamination direction, and the relatively weak matrix and the matrix fiber interface control the composite properties. This is further exacerbated by the consolidation pressure applied to increase the fiber volume fraction, which inadvertently minimizes the size of the interlaminar regions leading to limited resin deformation and high susceptibility to delamination. Interleaving, the process of adding a thick resin interlayer between fiber plys, improves interlaminar toughness via unconstrained plastic deformation of the interlayer. However, this approach encounters challenges in conventional composite manufacturing. Furthermore, relating interlayer properties to interleaved composite performance is challenging due to the common occurrence of adhesive failures. In this dissertation, we develop a novel approach for processing engineered interleaved composites to overcome processing difficulties and lay design principles for high performance composites. Vat polymerization additive manufacturing (AM), a less explored technique in composite manufacturing, is used to manufacture composites with engineered architecture, such as resin-rich layers (RRL) with prescribed thickness and resin properties. The AM method was benchmarked by demonstrating a composite fiber volume fraction and performance comparable to composites made via vacuum-assisted resin transfer molding (VARTM). Then, we developed a novel reconfigurable tool attachment for commercial vat polymerization printers that allows for the production of curved composite parts. The versatility of the additive manufacturing techniques developed allowed us to answer important fundamental questions regarding the role of matrix and interlayer properties on the interlaminar toughness of interleaved composites (G_IC and G_IIC). While it has been commonly accepted that the toughness translation primarily depends on the properties of the interleaf or interlayer and its thickness, our results challenge this notion. More specifically, the measured composite G_IIC depends more on the shear properties of the RRL than on its thickness. However, in mode I testing, our results uncover significant findings that (i) G_IC is dependent on the RRL thickness, and (ii) for sufficiently large RRL, G_IC depends on a combination of RRL and matrix resin properties. For example, we show that a large mismatch between the interlayer and fiber matrix toughness promotes adhesive failure and poor toughness translation. More specifically, we show that RRL to fiber matrix G_IC ratio ([phi])[less than or equal to]7.2 is required to see cohesive failure, and full toughness translation. This fact significantly restricts the choice of resins for the fiber matrix and the interlayer. Interestingly, a new strategy was discovered to eliminate the dependence on [phi] via interleaving with a toughened fiber ply layer, which eliminates the high value of [phi] near the crack tip. More specifically, when the brittle resin and tough resin interface, i.e., high [phi], was moved away from the crack tip, the fracture initiation was mitigated. This insight underlines the importance of optimizing the interlayer and matrix properties to achieve improved toughness in interleaved composites. In summary, this study demonstrates the effectiveness of vat photopolymerization additive manufacturing for composites with engineered properties. Furthermore, it provides valuable insights into underlying mechanisms governing toughness translation in interleaved composites. The findings outline important design considerations necessary for achieving enhanced toughness, and thus paving the way for engineering high-performance composite materials.