PhD Oral Exam - Ehsan Azad, 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

Ceramics exhibit significant potential for advanced applications due to their high strength and hardness; however, their inherent brittleness and limited energy absorption restrict wider use. Nature offers effective design strategies to address these challenges, as exemplified by biological armors such as nacre and abalone shells, which demonstrate exceptional toughness. Inspired by these natural systems, a programmable laser micromachining platform utilizing ultra-short picosecond pulses was developed to fabricate precise multiscale bioinspired surface architectures on alumina (Al₂O₃) tiles. These laser-engraved tiles were subsequently laminated with Surlyn® to create bioinspired ceramic composites. The static and dynamic mechanical properties of these composites—including energy absorption, stiffness, and strength—were systematically evaluated.

Increasing the Surlyn® content promotes plastic deformation while preserving adhesive failure mechanisms. However, the introduction of micro-patterns, commonly found in natural armors, significantly enhances interfacial performance by promoting mechanical interlocking and modifying stress distribution at the interface. This design shift moves the failure mode from adhesive to cohesive, improving interfacial shear strength by 64% and energy absorption by 107%. Further evaluation under static and cyclic flexural loading demonstrates that macro-patterns, implemented via laser-engraved architectures, and tailored stacking sequences enhance energy dissipation by 85%, primarily through mechanisms such as crack deflection and localized plastic deformation. The combination of hexagonal macro-patterns with diagonal micro-grooves significantly improves both interfacial adhesion and impact resistance, nearly doubling energy absorption and deformation capacity. X-ray radiography confirms a reduction in delamination, with the integrated design exhibiting the smallest damage area.

This novel integration of bioinspired design, advanced materials, and laser-based fabrication methods enables the development of lightweight, durable, flexible, and programmable ceramic composites, whose mechanical behavior can be tuned based on specific performance requirements. The design strategies, material systems, and fabrication techniques developed in this work demonstrate strong potential for applications in personal protective equipment, aerospace structures, and biomedical devices.