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Hierarchically enhanced impact resistance of bioinspired composites, Gu, Takaffoli, Buehler, Advanced Materials, 2017

Novelty/impact/significance:

A methodology combining simulation, additive manufacturing of biomimetic structures, and mechanical testing is developed, and the function of structural hierarchy is explicated. The findings provide insightful guidance for designing future protective materials and devices.

Results reveal that the multilevel of cross-lamellae (structural hierarchy) can enhance impact resistance by 70% and 85% compared to the single-level structure and the stiff constituent, respectively.

Scientific question:

What is the function of structural hierarchy in controlling material properties and how?

Key of how:

Through an integrated approach combining manufacturing, experimental, and simulation, hierarchical structure generates multiple pathways for crack deviation and enhances impact resistance.

In hierarchical, crisscrossed lamellar structure (inclined stiff phase with soft interface), crack could pass through the stiff sheets, pass through and/or along the intersheet-, interlamella-, and interlayer- soft phase; this creates an exponential increase in the number of possible cracking paths and forces the crack to change directions constantly during crack propagation. This dissipates energy, distributes damage, and arrests/stops crack from reaching the third layer, thus protecting the composite from fracturing (my understanding from the work).

Major points:

1. Human protective gears need novel material designs as effective body armors. Many biological materials consisting of hard (mineral) and soft (organic) constituents with a hierarchical architecture show preferably high strength and high toughness, ideal for armor designs.

2. Conch shells exhibit exceptional toughness via complicating the crack propagation through its three-tier hierarchical, crisscross lamellar structure (nano (CaCO3 sheets separated by organic interface) to micro); this shows great potential for biomimetic designs.

3. The difficulty to fully capture the complex structural hierarchy and the lack of understanding of the hierarchy-impact relationship can be addressed by an integrated studying approach of multimaterial 3D printing of conch shell prototype, 3D finite element simulation, analytical model, and drop tower testing.

4. For 3D printed synthetic conch shell, Hier-2: (1) construct the multilayer unit cell, 3rd order stiff sheets with soft phase constitute the first-level lamella (stiff sheets in each lamella are in crisscross with those in next one), then these lamellae with soft phase constitute the second-level lamella forming a layer; three layers at top, middle, and bottom with soft phase as interlayer are successively rotated 90o; (2) repeat this multilayer unit cell in the in-plane direction to obtain the overall laminate. Control group, Hier-1: three layers with soft phase as interlayer, with only single level, stiff lamellae in each layer being in crisscross with those in neighboring layers.

Both are fabricated by Stratasys printer. Volume fractions are approximately soft 25% : stiff 75%, and the smallest dimension is the interlayer soft phase, 0.25 mm (lamellar dimensions and layer thickness are less than 4 mm).

5. Drop tower testing results show that Hier-2 is best in resisting impact, next Hier-1 and then the bulk stiff material, in terms of preventing perforation, stiffness, critical impact energy, and damage patterns. Hier-1 shows severe damage and catastrophic failure, while Hier-2 shows no deep crack penetration and maintains integrity.

6. In Hier-2, crack could pass through the stiff sheets, pass through and/or along the intersheet-, interlamella-, interlayer- soft phase, which creates an exponential increase in the number of possible cracking paths and forces the crack to change directions constantly during crack propagation. Such diverse cracking path results from the hierarchical structure (sheets, lamellae, layers) and the complex angled alignment of the constituents (cracks have to propagate tortuously zig-zag) and leads to multiple crack deflections/kinking. This dissipates energy, distributes damage, and arrests/stops crack from reaching the third layer, thus protecting the composite from fracturing.

7. Finite element analysis, in agreement with experiments, show that hierarchical composite can stop the projectile and avoid complete perforation; cracks initiate in the soft phases and are distributed, and the complex hierarchical structure and the right-angle alignment force crack to propagate along intricate paths, dissipating more energy, thus not sufficient for complete fracture.

8. Hier-2 composite shows significantly improved impact resistance than Hier-1, initiation of distributed cracks v.s. complete penetration and a perforated hole, 85% increase in threshold impact energy v.s. 10% increase compared with that of the bulk stiff material.

9. An analytical model for the crack deflection and penetration behaviors at the interface based on linear elastic fracture mechanics predicts a threshold incident angle (50o), below which crack deflection occurs; the weaker the interface, the larger the threshold incident angle. Striking a balance between the two materials is important for desired overall stiffness and allowing crack deflection, as exemplified by the biomimetic design (45o incline of sheets).

10. Results also suggests that the hierarchical, crisscrossed lamellar structure can toughen brittle materials through introducing soft phase as an architectured interface, being applicable to other materials systems.

The study is interesting and insightful, and convincingly demonstrates the role of structural hierarchy and crisscrossed lamellar structure in enhancing the impact resistance. The findings of general structure design of hierarchy and complex lamellae, and specific details such as incline angles, as well as the methodology, are applicable and favorable to develop various, high-performance composite materials.

Here is the link of the fulltext: https://onlinelibrary.wiley.com/doi/full/10.1002/adma.201700060