PhD Oral Exam - Arash Khodaei, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Thermoplastic composites excel in fatigue and impact resistance over thermoset composites. In particular, they possess the unique possibility for in-situ consolidation using the Automated Fiber Placement (AFP) which reduce manufacturing costs and energy. However, challenges remain in achieving high-quality AFP-made laminates especially in addressing lower bond strength compared to autoclave method. Comprehensive understanding is crucial to overcome these challenges.

Microscopic observation reveals that thermoplastic composite consolidation involves two main stages: intimate contact between two surfaces in which the interfacial contacted area increases by spreading the softened surface, and healing, which refers to intermolecular diffusion across the interfaces of two layers which are in intimate contact. Therefore, this study initially focuses on understanding these consolidation stages thoroughly. Reviewing literature identifies two significant gaps in the consolidation process.

Regarding intimate contact, numerous theoretical models have been proposed to study its development across various manufacturing processes. However, there are very few experimental methods available to measure the degree of intimate contact. To fill this gap, the first part of this study proposes a new experimental technique called Bearing Area Curve (BAC) based on the topology of the tape surface for measuring intimate contact development. This method evaluates the effects of process parameters like placement speed and temperature on intimate contact development, showing promise in accurately measuring intimate contact.

Regarding healing, despite numerous experimental techniques and theoretical models proposed to optimize the process, the healing obtained from AFP is still unacceptable. To address this, the second part of the study focuses on developing a new experimental technique called Vibration-Assisted Thermal Bonding (VATB) to effectively improve the degree of healing. Further to this, a new healing model is developed to predict the degree of healing for the VATB process.

The final segment of this investigation extends the developed methodologies, specifically evaluating the effectiveness of vibration-assisted thermal bonding in repairing thermoplastic composites with an integrated amorphous polymer layer such as Poly-Ether-Imide (PEI) between CF/PEEK layers. The results showed that using vibration-assisted thermal repairing (VATR) rather than thermal repairing increases the bonding strength by up to 22% and decreases the void content by up to 35%.