Additive Manufacturing of Engineered Polymeric Flexible Structures
Type of DegreePhD Dissertation
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Polymer-based additive manufacturing (AM) processes allow the flexibility, rapid, and low-cost fabrication of complex geometries. In recent years, fused filament fabrication (FFF), also known as fused deposition modeling (FDM) has attracted the attention of researchers to fabricate flexible fabric structures. Several researchers attempted to additively manufacture fabric structures, but the mechanical property evaluation was not performed due to the complexity of the geometry and the loading difficulty on the testing fixture. In addition, poly(lactic) acid (PLA) is mostly explored. It is brittle in nature and has elongation at break in the range of 2.5 – 6.0 %, which limits its application in fabrics and fashion industries. In this context, this dissertation deals with the design, additive manufacturing, and mechanical properties characterization of fabric structures. In addition, it discusses the manufacture and characterization of novel composite filaments to use in FDM. The creep behavior of 3D printed composite filaments is also modeled. In chapter 3, the design, 3D printing and analysis of mechanical and microstructural behavior of interlaced fibrous structures were assessed. It was found that the mechanical strength is higher in warp direction as compared to that of in weft direction for 2/1 twill weave fabrics. The flexural strength of weft knitted fabrics is lower in course direction. The cylindrical braids are capable of holding compressive displacement of more than 25 mm. The printed samples contained voids which decreased the mechanical strength. In chapter 4, the effect of heat treatment on 2/1 twill weave fabrics and dog-bone samples were studied. They were compared in terms of mechanical properties and crystallinity. The crystallinity of both samples improved significantly when they were heat treated at temperatures above the glass transition temperature (T_g) of the material. But an improvement in mechanical properties of twill weave fabric was not observed whereas they were improved for dog-bone samples. In chapter 5, a multi-physics computational model is utilized to study a coupled thermo-mechanical-viscoelastic properties of 3D printed plain weave fabrics at and above T_g. The temperatures considered for this study are 60 ℃, 65 ℃, and 70 ℃, and the viscoelastic properties is represented by a Prony series. In this study, unit cells were 3D printed, and then using dynamic mechanical analysis (DMA), the tensile and compression tests were conducted in a closed thermal environment at temperatures mentioned above. The computational analysis was performed in ABAQUS simulation software, and the results were compared with the experimental results. The relative error percentages in the peak forces at each temperature were 23.60% at 60 ℃, −8.85% at 65 ℃, and – 6.25% at 70 ℃. A better agreement in peak forces was seen for unit cells above T_g. The computational model developed for unit cells was used to predict the thermo-mechanical-viscoelastic response of large additively manufactured fabric structures which is difficult to evaluate experimentally. In chapter 6, composite filaments using PLA and TPU were manufactured using twin-screw and single-screw extruders. Different material characterization methods were utilized to study the material properties of the composite filaments. The tensile stress and Young’s modulus of the filaments decreased whereas the elongation increased by more than 500 %. The crystallinity of the materials were increased which were studied using differential scanning calorimetry (DSC) and polarized optical microscope (POM). The analysis also showed a partial miscibility of the polymer constituents. In chapter 7, the effect of a plasticizer on mechanical, thermal, chemical, and morphological properties were studied. Poly(ethylene) glycol (PEG) was used as a plasticizer in PLA/TPU composition. It was found that although the yield stress decreased, the ultimate tensile stress of the filaments did not show a drop in their values. This might have happened due to the plasticizing effect of PEG. Finally, the composite filament was used to 3D print a plain weave fabric structure which demonstrates its feasibility in 3D printing applications. The last chapter of this dissertation discusses the creep behavior and computational modeling of 3D printed composite filaments. The composite filaments were used to 3D print creep samples. The creep test was performed under a constant tensile load of 100 N. A computational model is developed using the Generalized Voigt-Kelvin solid model and three terms in the Prony series. The experimental results and computational results were compared and found that the maximum error is approximately 6 %. This proved the reliability of the model developed and can be used to predict creep response of similar polymers.