Degree

Doctor of Philosophy (PhD)

Department

Department of Mechanical & Industrial Engineering

Document Type

Dissertation

Abstract

Ultrasonic welding (USW) is a rapid and energy-efficient joining method for thermoplastic composites, but the rapid heating, cooling, and mechanical deformation involved in the process can strongly alter the crystalline structure at the welded interface. Since crystallinity influences stiffness, toughness, dimensional stability, chemical resistance, and long-term joint performance, understanding how USW controls interfacial crystallization is critical for optimizing welded thermoplastic composite structures. This dissertation investigates crystallinity development and crystallization kinetics in ultrasonically welded glass fiber-reinforced thermoplastic composites, with emphasis on polypropylene (PP), multiwalled carbon nanotube/PP (MWCNT/PP) energy directors, and polyetheretherketone (PEEK).

The first part of this work examined how welding force, vibration amplitude, and MWCNT concentration affect crystallinity in GF/PP welded interfaces. Differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) showed that higher welding force and amplitude increased crystallinity in PP-based interfaces. SAXS and WAXD further revealed that welding induced anisotropic crystalline structures, indicating that mechanical deformation during USW contributed to strain-induced crystallization.

The second part focused on the time-dependent crystallization behavior of PP during USW. Interrupted welding, embedded thermocouple measurements, DSC, WAXD, Raman spectroscopy, SAXS, and Ozawa kinetic analysis were used to relate welding duration and cooling history to crystalline structure. Short welding times promoted crystalline ordering through rapid nucleation and stress-assisted crystallization, while longer welding durations reduced final crystallinity and crystallite size due to excessive thermal and mechanical disruption. These results show that crystallization during USW depends on a balance between heat generation, cooling rate, chain mobility, and vibration-induced deformation.

The third part investigated the effect of Al₂O₃-coated interfaces on crystallization in PP and PEEK. The coating produced opposite responses in the two polymers. In PP, Al₂O₃-coated interfaces increased crystallinity, particularly under high-force/high-amplitude welding conditions. In PEEK, the coated configuration reduced crystallinity compared with uncoated welded specimens. SEM-EDS and XPS showed localized oxide-related surface chemistry but no uniform coating transfer to the extracted energy director films. These findings demonstrate that ceramic oxide coatings do not act as universal nucleating layers; their effect depends strongly on polymer chemistry, chain mobility, and crystallization kinetics.

This dissertation establishes a process-structure framework for controlling crystallinity at ultrasonically welded thermoplastic composite interfaces. The results show that welding parameters, welding duration, cooling history, nanocomposite energy directors, and interfacial coatings can be used to tailor crystalline morphology and surface chemistry. This work provides fundamental insight for improving weld quality, optimizing processing windows, and designing more reliable thermoplastic composite joints for lightweight structural applications.

Date

7-30-2026

Committee Chair

Genevieve Palardy

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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Available for download on Friday, July 16, 2027

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