title: Additive Manufacturing, Experimental Characterization, and Constitutive Modeling of Functionally Graded Polymeric Structures
Creators: name: Valizadeh, Iman
Publisher: name: UNSPECIFIED
Year: 2024

description: Vat Photopolymerization-based additive manufacturing methods, such as stereolithography and digital light processing, typically use a single material to fabricate structures, presenting a limitation of this printing method. A potential solution to this constraint involves the utilization of grayscale masked stereolithography (gMSLA) for the purpose of 3D printing functionally graded materials using vat photopolymerization. Adjusting the grayscale values of the masks allows precise control over light intensity throughout the material, affecting crosslink density and photopolymer solidification. This control significantly influences the mechanical properties and dimensions of the printed components. In vat photopolymerization, besides grayscale values of the mask, it is crucial to recognize that the resin's curing process is also influenced along the thickness and on curing plane by two other key controllable parameters: layer thickness and exposure time.
In this dissertation, the relationship between gMSLA process parameters and material properties is examined, which results in the development of parametric constitutive models that are dependent on process parameters. A unified parameter, exposure intensity, that combines process parameters to enhance gMSLA for parametric constitutive models is introduced. Depending on the specific application of interest, parametric hyperelastic, viscohyperelastic, and elasto-visco-plastic constitutive models, all in terms of exposure intensity, are developed and validated through experiments. Subsequently, the study systematically investigates the influence of user-controllable process parameters on geometrical deviations due to overcuring and undercuring. The constitutive parameters and geometrical deviations are represented using hyperbolic tangent functions in terms of exposure intensity. The optimized choice of process parameters enables the engineering design of parts with controllable and graded mechanical behaviors, as well as reliable geometric dimensions using gMSLA. Remarkably, selecting an appropriate parameter set significantly reduces the print time while maintaining identical mechanical behavior. Furthermore, the developed constitutive models are used to analyze energy dissipation in graded shell lattice structures at various strain rates, including the establishment of complementary functions for approximating viscoelastic material dissipation. The observations of material behavior regarding applications in energy absorption show that the tough photosensitive resins suffer from limitation of small failure strain and low plastic deformation. To overcome this limitation the tough resin is blended with a flexible resin to enhance the structure's flexibility for energy absorption applications significantly, and these systematic studies establish essential relationships for determining constitutive parameters in resin mixtures.
The framework proposed in this dissertation extends the capabilities of photopolymerization-based additive manufacturing, allowing for the creation of intricate structures with customized material behaviors, including mechanical properties, flexibility, and precise geometric dimensions. During the development of this framework, a remarkable consistency is observed between experimental findings and numerical models, suggesting that the obtained results can be extrapolated to similar material systems and 3D printing technologies.