WEB Mineralised collagen fibre elasto-plasticity: In situ experiments and modelling of bone’s fundamental building block
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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