Date of Award

1-1-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Civil and Environmental Engineering

Department

Civil and Environmental Engineering

First Advisor

Sumanta Das

Abstract

Over the past decades, composite materials have become an essential part of real-life structures because of their exceptional mechanical performance and significantly low weight to load-carrying capacity ratio compared to metals. However, many composite structures suffer from early failures under various loading conditions. These failures may vary depending on the type of material used and its corresponding material properties such as concrete (brittleness), sandwich composites (delamination and core failure) and polymer composites (lower strength and energy absorption). Over the years, the demand for stronger materials has increased dramatically as the new designs are getting more complex that require better materials to build. Therefore, in order to meet the increasing demands, the next-generation composite materials must be designed to offer exceptional mechanical responses while minimizing the current drawbacks. Thus, this dissertation aims to address some of the drawbacks of the two most widely used composite materials, concrete, and polymer-based composite materials. Concrete is one of the most widely used structural materials; however, it is inherently brittle in nature under static and dynamic loading conditions. Concrete structures such as buildings, bridges, and dams are routinely exposed to dynamic loads. In addition, human-made high strain rate events in recent years pose a severe threat to the security of the common life all over the world. One of the motivations of this dissertation emanated from addressing the aforementioned concern of poor dynamic performance of Portland cement concrete towards utilization of waste materials and other natural fibers such as wollastonite in concrete to develop a material that provides improved performance. The outcomes from this study indicates that the fibers/particulates cam significantly enhance material performance for multiple application scenarios. In addition to the cementitious composites, this dissertation also focuses on performance-enhancement of polymer composites, in particular, sandwich structure with lattice core and hybrid polymers for additive manufacturing. Manufacturing of various complex shapes for lattice structures have become a reality with the aid of the layer-based additive manufacturing method, which is otherwise challenging with conventional manufacturing methods such as drilling, milling, lathing etc. These structures show promising high energy absorption and higher stiffness with significant weight reduction. Thus, lattice structures are great candidates as a core structure for sandwich-based composites. These sandwich-based composite structures are designed in such a way to maximize their energy absorption while keeping their weight as minimum as possible. Hence, such a combination of lattice structure and sandwich composite brought tremendous advantages in terms of various multifunctional applications. Out of all lattice structures, this dissertation focused on the gyroid structure owing to its intersection-free surface shape. This intersection-free surface enables smooth transition inside the structure where loads are distributed evenly without any stress concentrations due to the zero-mean curvature. While the promising advantages of gyroid under static loading have been investigated in previously published articles, this dissertation study focused on the performance-based of novel gyroid structure as core in sandwich composites under dynamic loading conditions using shock-tube experiment. The findings from this study shows that gyroid structure has significant energy absorption capacity. In addition, a finite element-based prediction tool has also been developed that is validated with the experimental data. While the previous section focused on enhancing the energy absorption by incorporating an optimized lattice structure, this dissertation also explores another performance enhancement route modifying the polymer filaments itself. With the advancement of additive manufacturing (3D printing) to create complex structures using polymer filament, these 3D printed structures have evolved dramatically in the last decade both in terms of improving printing quality and a significant cost reduction. Most additive manufacturing methods such as the Fused Deposition Method (FDM), Stereolithography (SLA) use molten or liquid-based polymers to create desired parts. Common printing materials include thermoplastic polymers such as PLA, ABS, PETG, TPU, PC etc. While each of these polymers has their own advantages and disadvantages in terms of performance, manufacturing hybrid polymers can provide various efficient means to leverage the advantages of two or more polymers. Furthermore, additives such as carbon nanotubes have been shown to enhance the performance of these hybrid polymers even more. However, such additive enhancers are expensive when used in a large-scale real structure. Therefore, this dissertation attempts to find a cost-effective route for performance enhancement by incorporating waste iron powder (or short fibers) into thermoplastic polymers. This is motivated from the previous study mentioned in the cementitious composites above, where the waste iron powder significantly enhanced the fracture toughness. In this dissertation study, waste iron powders are blended into the ABS-TPU hybrid to evaluate the enhancement in their mechanical performance. The findings showed that the iron particle improved the performance of the polymer blend in terms of elastic modules, hardness, as well as fracture toughness. Overall, this dissertation highlights the advantages of tuning the material structures at different length scales so as to attain significant performance-enhancement in cementitious as well as polymer composites under static and dynamic loading conditions.

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