WEB Mineralised collagen fibre elasto-plasticity: In situ experiments and modelling of bone’s fundamental building blockWednesday (23.09.2020) 09:00 - 09:15 C: Characterization 1 Part of:
Bone is made of mineral nanocrystals and mineralised collagen fibrils, abundantly available ingredients, and grown at mild temperatures. It combines high stiffness, toughness and strength at low specific weight through a unique hierarchical setup. To mimic it in engineering materials a key gap in understanding its fundamental building block, the mineralised collagen fibre, needs to be closed. We present micro- and nanomechanical data as well as nanoscale imaging which we use in a statistical model that explains mineralised collagen fibre elasto-plasticity.
We used ultra-short pulsed laser ablation and focused ion beam milling to machine 14 micropillars (6x12 μm) from individual mineralised collagen fibres. We conducted combined micropillar compression and in situ small angle X-ray scattering/X-ray diffraction at beamline ID13 of the European Synchrotron Radiation Facility to determine fibre, fibril, and mineral strains . Micropillars were compressed until 12% apparent fibre strain and exposed to X-rays every 5 s. We performed phase-contrast CT with 20 nm voxel size at beamline ID16A. From this data, we analysed fibril orientation using an auto-correlation approach , tissue density  and fibril diameter. We integrated the experimental data in a novel statistical mechanical model that describes the micro- and nanomechanical behaviour of mineralised collagen fibril arrays. To calculate load transfer between components, we embedded two classical shear lag models .
We found small strain ratios of 22:5:2 between fibre-fibril-mineral levels with the maximum for fibrils towards apparent strength and for mineral nanocrystals towards apparent yielding outlining the load transfer between organic and inorganic components . This spurious ratio agrees with literature on macroscopic samples . The model allowed us to identify a heterogeneous deformation of fibrils under compression which explained the small experimental strain ratios in our samples as well as in the literature. We saw that a variability of 10-15% in micro- and nanomechanical properties is present in the micropillars and that fibril diameter influences hardening behaviour and strength when we generalise the model towards other fibril-reinforced composites .
Findings from our combined in situ testing and statistical modelling aim to inform the design of future bio-inspired materials to tackle the socio-economic burden of bone-related diseases that affect millions of people worldwide.
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