Date of Award

Spring 2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Committee Director

Oleksandr Kravchenko

Committee Member

Sergii Kravchenko

Committee Member

Zhang Xiaoyu

Committee Member

Tian-Bing Xu

Abstract

Additive manufacturing of thermoplastic composites using fused deposition modeling (FDM) offers unparalleled geometric freedom and the ability to tailor reinforcement through short and continuous fiber architectures. However, the process inherently introduces mesoscopic defects, including 15-20% inter and intra-bead voids, weak interlayer bonding, and microcracking, which degrade mechanical and thermal performance. Overcoming these limitations without sacrificing geometric flexibility or requiring rigid tooling remains a critical challenge. This dissertation presents two novel processing strategies, Vacuum Assisted Hot Isostatic Pressing (VaHIP) and Hot Powder Bed Compaction (HPBC), to enable conformable, near-isostatic consolidation pressure of thermoplastic composites. These approaches are designed to systematically transform void morphology, enhance interlayer fusion, and improve overall structural integrity while preserving complex geometries. A comprehensive process-structure-property framework is established to elucidate the role of melt-state and pressure in governing consolidation mechanisms, including void collapse and improved fusion. VaHIP is an extension of hot isostatic pressing specifically tailored to address defects in FDM-processed composites through partial melting of semicrystalline polymers, while simultaneously enabling a high degree of crystallinity and promoting interlayer diffusion. In contrast, HPBC utilizes a densified powder medium to achieve conformable pressure transmission and controlled viscosity, enabling void restructuring and consolidation with controlled geometric distortion. The processability and scalability of HPBC are demonstrated on complex structures, including topology-optimized rocker components, lattice sandwich structures (LSS), and assembled hinge brackets. HPBC reduced void content from 16.8% to 2.3% and enhanced mechanical properties, including a 30.2% increase in flexural modulus (14.9 GPa), a 20.8% increase in tensile modulus (31.1 GPa), and strength improvements of 36.4% (flexural) and 61.9% (tensile). The SEM fracture analysis showed a transition from fiber pull-out failure in pristine FDM part to a monolithic fracture in the HPBC processed part. Furthermore, it enables substantial gains in anisotropic thermal performance, particularly in through-thickness properties, with thermal conductivity increasing to 0.708 W/m·K (+75%) in the build direction and 0.98 W/m·K (+86%) along the fiber direction. These improvements in material properties were effectively translated into enhanced structural performance in complex geometries, as demonstrated through bending and tensile characterization of LSS and rocker components, respectively. Bending tests on LSS confirmed a 2.2× increase in stiffness and a 154% improvement in strength, accompanied by a transition in failure mode from global instability and distributed yielding to more localized, load-bearing behavior. A similar trend was observed in topology-optimized rocker components, which exhibited increases of 35.3% in stiffness and 24.5% in strength. These performance gains are attributed to the ability of HPBC to apply conformable pressure, thereby improving consolidation even in geometrically intricate regions. It was further validated through cross-sectional microscopy of different regions in both LSS and rocker components, which revealed reduced void content and enhanced interlayer bonding, directly correlating microstructural refinement with improved mechanical response. To complement experimental observations, a microstructure-informed continuum damage mechanics framework was developed, integrating finite element modeling with semi-analytical approaches to capture progressive damage evolution in fiber-reinforced FDM composites. The model accurately predicts stiffness degradation, strength, and crack propagation behavior, correlating well with microscopy, SEM, and digital image correlation analyses. Overall, this work establishes a scalable and tool-independent pathway for enhancing the performance of additively manufactured thermoplastic composites. By linking processing conditions to microstructural evolution and macroscopic properties, the proposed methodologies enable the realization of high-performance, complex-shaped composite structures for advanced structural and thermal applications.

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ISBN

9798197809339

ORCID

0000-0001-7228-811X

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Available for download on Wednesday, July 28, 2027

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