5.3 Case Studies Using Biomimetic Approach
5.3.1 Fiber-reinforced Bone-inspired Composites
Using a biomimetic approach, we designed a new fiber-reinforced com- posite (FRC) inspired by the microstructure of cortical bone. We devel- oped a new manufacturing technique to realize the material with its innovative pattern. The bone-inspired composite is a synthetic mate- rial made of glass fibers (GF) and carbon fibers (CF) embedded into an epoxy matrix and showing an internal organization aimed at reproducing
Figure 5.4 (a) Bone microstructure. The main structural features are color-highlighted.
(b) Schematic of the internal structure of the bio-inspired composite. Reproduced with permission from [22].
1. Osteon 2. Cement lines 3. Interstitial lamellae 4. Harvesian canal
Unidirectional glass fibres
± 45 carbon fibre sleeve Resin
(a) (b)
2 4 1
3
the bone microstructure with its characteristic features, the osteons [22].
Figure 5.4b shows the bio-inspired design, along with a picture showing bone Haversian structure (Figure 5.4a). The main structural features repli- cated in the biomimetic structure are color-highlighted.
The characteristic elements of the newly designed composite are the osteons (i.e. internal lamellae and cement lines), the interstitial lamellae, and the outer circumferential system.
t The osteons are reproduced by means of unidirectional bundles of glass fibers (UD-GF), providing resistance to axial loadings, and filled up with rowing of UD-GF (Roving 2400tex by 3B fiberglass company), aimed at mimicking the internal concentric lamellae. The osteon outer boundaries, called “cement lines”, are implemented in the composite by means of sleeves made of ±45° CF, Torayaca T300 1k by Toray Carbon Fibers America. Here, the main function of such sleeves is to pack the internal part of each osteon, pre- serving the fiber alignment. However, these tubular struc- tures also aim to provide mechanical bending strength, and enhance fracture toughness by activating bone-like tough- ening mechanisms, such as crack deflection and twisting.
t Interstitial lamellae, which offer compactness and fill up the gaps between osteons, are reproduced by means of longitu- dinal UD-GF impregnated into an epoxy resin.
t The outer circumferential system, which packs the osteon architecture in cortical bone, is replicated in the synthetic material by means of two external layers of non-crimp fab- ric (NCF), made of UD-GF (92145 Finish FK 144 by P-D Interglas). These two skins of NCF, placed at the top and
bottom of the composite, respectively, offer a final flat and uniform surface to the whole composite panel.
The design includes many simplifications:
i. The new material is made of synthetic constituents and could not replicate the living functions, such as remodel- ing, self-healing and bloody flow. Moreover, osteons are not mimicked as hollow, since there is no internal canal with a fluid inside.
ii. The length scale is different: the CF sleeve diameter is one order of magnitude larger than that of the osteon.
iii. The new material is produced as flat plates, and not as cylinders, in order to cut specimens for material characterization.
All these simplifications have been introduced during the manufacturing phase. Several attempts have been made before reaching the material with a good finishing and resin impregnation, the desired fiber ratio and a lim- ited amount of defects.
We also produced another material, from now on called comparative material, made of the same type of fibers and resin in the same ratio, by classic lamination technique. The comparative material has the follow- ing stacking sequence: [GF-(0°)4, CF-(±45°)2, GF-(0°), CF-(±45°)]s. The GF outer layers and the CF layers are both NCF, whereas the internal layers of GF are UD fibers. Both materials are characterized by 54% vol.
fibers (6.5% vol.-CF and 47.5% vol.-GF). Figure 5.5 shows the materials adopted to manufacture the bio-inspired composite and the composite laminate used for comparison.
To allow us a direct comparison and to verify the validity of the bio- inspired design, a complete characterization was performed on both
Figure 5.5 Materials adopted to manufacture the new biomimetic composite:
(a) ±45°—CF sleeve, (b) UD–GF, (c) UD–GF–NCF, and (d) ±45°—CF–NCF.
(b)
(a) (c) (d)
materials. After testing both materials, new solutions to further improve the design were proposed.
The bio-inspired design allowed us to replicate some of the toughening mechanisms occurring in the Haversian structure of cortical bone, such as crack deflection and longitudinal splitting. In particular, crack deflec- tion occurred at osteon–osteon interface as shown by microscopic analyses (see Figure 5.6a). However, the new design did not show an improve- ment in fracture toughness with respect to the comparative laminate (see Figure 5.6b). It showed a different mechanical behavior than the conven- tional laminate, owing to the different internal structure. In particular, the bio-inspired structure was characterized by a noticeable anisotropy, with higher mechanical properties (i.e. stiffness and strength) in the longitudi- nal direction (parallel to the osteon main axis), but it had some limitations in the transversal direction, though maintaining a higher tensile stiffness.
Hence, despite its limitations, mainly due to the lack of cohesion between two adjacent osteons, and the lack of reinforcement in transversal direction, with this bio-inspired design, it was possible to implement the typical bone fracture mechanism of crack deflection.
To improve the mechanical properties, we adopted a combination of biomimetics and engineering design. In particular, based on the initial biomimetic design and on engineering principles, we proposed new solu- tions to further improve the fracture toughness by enhancing the phenom- enon of crack deviation. We suggested a new design with multiple rows of osteons. The presence of more osteons could indeed lead to more crack deviations, enhancing the energy dissipation. To improve the transversal
Figure 5.6 (a) Mechanisms of crack deflection observed in the bio-inspired composite.
Image from SEM—reproduced with permission from [22]. (b) Comparison between the fracture behavior of the bio-inspired composite and that of the comparative laminate in terms of translaminar fracture toughness.
Crack deflection
100 m
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0 1st desig
n
1st laminate 20
KTL (MPa√m) 40
properties, we decided to enhance the osteon–osteon interaction and increase the amount of fibers in transversal direction. We also proposed to further increase the mechanical properties by adding to the resin reinforc- ing nanoparticles, with a platelet shape and a proper characteristic size, aimed at mimicking the HAP platelets.