Biology produces a range of composite structures that evolve to resist a wide range of loading conditions from their environments. The mechanical function of these biological composites is expected to be governed by the properties of the interfaces between distinct hard and soft constituents at different length scales. However, difficulties exist in applying composite theories to biological structures since the interfaces present between the nanoscale biological constituents are typically below standard measurement length scales. Hierarchical biological composites found in nacre and arthropod exoskeleton are distinct examples of structures potentially optimized to resist dynamic loading conditions. Understanding the deformation and failure of such biological composites thus require evaluations beyond quasi-static conditions. Knowing the dynamic mechanical properties of the biological interfaces at small scales would make a better understanding how nature designs different biological structures to serve their specific mechanical functions and potentially provide better guidance for synthesizing bio-inspired composites.
Therefore, the aim of this PhD project is to examine the rate-dependent mechanical properties of the interfaces in different biological composites using a novel mechanical testing technique incorporating scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and atomic force microscopy (AFM) to understand their interfacial mechanical behaviour under different loading conditions and establish a relationship between their interfacial mechanics and their physiological loading conditions.
As most biological composites are physiologically in hydrated condition, it is therefore critical to justify the applied experimental methodology capable of mechanically testing biological samples in hydrated condition effectively. Elastic modulus of nacre fabricated using FIB at the microscale were shown to be similar for both dry and hydrated samples under SEM vacuum and ambient air conditions, validating our methodology of mechanically testing hydrated biological samples under SEM vacuum condition at the sub-microscale. Nacre was then studied by performing the AFM nanoscale interfacial shear test under loading rates with the range of two orders of magnitude and a shear strength decrease of around 10% was found. General interfacial mechanical behaviour within biological composites was further explored by comparing interfacial mechanical behaviour from nacre and arthropod exoskeleton to the interfacial shear behaviour of the NCP-MCF interface in antler bone. All the three biological composites exhibited a weakened interface with increasing loading rates, but the biological interface with less confinement showed a shear strength more sensitive to varying loading rates and appeared to adapt to less dynamic physiological loading conditions. Finally, this work evaluated mechanically graded tendon-to-bone interfaces, highlighting the flexibility of the experimental approach used. Microscale beams of tendon-to-bone attachment fabricated using FIB were successfully tensile tested using in situ AFM. An analytical model based on a simple rule of mixtures was used to predict the elastic moduli of the tendon-to-bone beams by consideration the spatial compositional variations within the larger interfacial regions, again providing a more complex-structure function relationship in a biological composite. Therefore, this PhD work highlights the use of mechanical testing using AFM and SEM to investigate the rate-dependent mechanical behaviour of small scale interfaces in a variety of structural biological composites
