PhD Oral Exam - Sepanta Mandegarian, 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

This thesis explores the development, optimization, and impact behavior of hybrid thermoplastic composite laminates and sandwich panels, focusing on environmentally sustainable materials and advanced reinforcement techniques. Double-belt and compression molding lamination methods were optimized to fabricate full thermoplastic composite sandwich panels with 100% recycled PET foam cores. Analyzing the fabrication parameters and final products’ skin-to-core adhesion revealed the critical role of PET foam density and lamination approach in achieving proper bonding. Even though panels made with compression molding outperformed under flexural loads, the continuous nature of the double-belt method offers a reliable and cost-effective alternative production approach.

In the current study, a two-step compression molding method was used to impregnate the metallic mesh layers with PP resin and form a proper connection between the composite layers. Furthermore, the effect of hybridization using stainless-steel mesh layers to reinforce composite laminates and sandwich panels was investigated. Variations in mesh wire size, orientation, and stacking sequence were analyzed to evaluate the energy absorption and damage propagation mechanism under different ranges of impact energies. While composite plates show improved performance when the reinforcing metallic layer was positioned at the midplane, it is preferred to reinforce the collision-facing side of sandwich panels for enhanced impact resistance. This strategic placement of the hybrid layer effectively increased the perforation threshold under Low-Velocity Impact (LVI) loading conditions.

In addition, Shape Memory Alloy (SMA) wires were introduced as reinforcing agents of the thermoplastic composite laminates not only to improve the impact performance, but also to take advantage of their specific healing properties to recover the post-impact deformations. Repeated LVI tests demonstrated that the SMA-assisted heating recovery process can restore over 50% of the after-impact deformations, further enhancing the durability and resilience of the composite materials.