Bioinspired Hierarchical Design of Composite Materials for Toughening and Structural Multifunctionality 

Abstract

Composite materials are keystone to lightweight engineering structures due to combining high specific stiffness, high specific strength, and architectural tailorability. Their wider applications in demanding service environments, however, remain limited by damage sensitivity, weak interlaminar resistance, and performance degradation under thermal, mechanical, and electromagnetic loads. These limitations are especially severe in layered composites, where matrix rich regions and interfaces often govern crack initiation, crack propagation, and loss of structural integrity. At the same time, advanced structures increasingly require load bearing materials to provide additional functions, such as electromagnetic interference shielding, without relying on separate protective subsystems. This creates a design problem that could hardly be solved by constituent selection alone.

Nevertheless, biological materials and structures represent evolution-optimized designs that attain both high mechanical properties and exceptional multi-functionalities. Thus, this thesis endeavors to address the above problem through developing bioinspired hierarchical structure designs across three structural levels. The central logic is that biological materials achieve robust performance through organized architecture, controlled interfaces, and integrated structural functions at the microscale, the mesoscale, and the macroscale, respectively, rather than through exceptional constituents alone. At the microscale, architecture is used to regulate strain rate dependent deformation and energy dissipation. At the mesoscale, interface topology is used to control interlaminar crack evolution. At the macroscale, a protective layered architecture is used to combine mechanical reliability with functional performance under extreme service conditions.

The first part investigates baleen-inspired lamellar tubular microstructures for strain rate sensitive toughening. Finite element analysis and constitutive modelling were used to compare conventional cylinder fiber, lamellar tubular, mineralized lamellar tubular, and hollow mineralized lamellar tubular designs under different strain rates. The mineralized lamellar tubular architecture showed the highest strain rate stiffening and toughening. The improvement arose from microstructure induced stress redistribution and enlarged inelastic deformation regions, rather than from changes in constituent type or reinforcement fraction.

The second part examines pangolin-inspired hierarchical interfaces for interlaminar crack control in CFRP/Ti laminates. A multiscale textured interface was produced by combining mesoscale surface patterns with microscale cavity features. Mode I fracture tests and cohesive zone simulations showed that the structured interface increased both crack initiation resistance and crack growth resistance. The improvement was governed by crack deflection, crack path extension, interlocking, and bridging, showing that the interface can be designed as an active fracture control region.

The third part develops beetle elytra-inspired protective laminate structures for multifunctional performance under extreme service conditions. A FeCoCrNiAlTi high entropy alloy was incorporated into GFRP based fiber metal laminates to combine extreme temperature mechanical stability with X band electromagnetic interference shielding. The laminate retained improved structural performance from −100 °C to 300 °C and achieved an average shielding effectiveness of 61.85 dB. The mechanical improvement was associated with high temperature strength retention, reduced thermal mismatch, and restrained interfacial delamination. The shielding response was reflection dominated and mainly resulted from strong front surface reflection and reduced transmission through the laminate architecture.

Beyond the specific material systems investigated, three generalizable design principles emerge from this thesis. First, rate-dependent stiffness and toughness can be tailored through microstructural organization that redistributes local stress and enlarges the regions of inelastic deformation, rather than through constituent modification alone. Second, interlaminar resistance can be improved by treating the interface as an active crack-control region that redirects, extends, and bridges cracks, rather than simply maximizing interfacial strength. Third, structural multifunctionality is most effectively introduced at the laminate-architecture level, where mechanical reliability, environmental stability, and functional performance are treated as coupled design objectives. Together, these principles define a cross-scale design route in which microstructure governs local deformation and energy dissipation, interface topology governs crack evolution, and laminate architecture governs system-level reliability and functionality. This framework provides a transferable materials-design logic for advanced layered composites operating under demanding service conditions.

Date of Award	10 Jul 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	You Fu LI (Supervisor) & Bin WANG (Supervisor)