Performance of a bio-mimetic 3D printed conch-like structure under quasi-static loading

Abstract

The conch shell is known for its excellent ability to initiate, deflect and bridge cracks to maintain its strength whilst enhancing its toughness. Impressively, it is mainly composed of aragonite, a brittle ceramic, but boasts a high fracture toughness. Understanding and mimicking the unique mechanisms of the structure of conch shells can toughen lightweight materials. However, due to the complexity of the hierarchical architecture of conch, studies have focused on mimicking it as a bi-material composite. In this research, 3D printing is employed to develop a proof-of-concept single edge notched panel that mimics the composite structure of conch. The instructions (G-code) of the dual extrusion 3D printer were programmed to produce a multi-layer composite architecture. The conch-like panel was tested under tension, and a numerical model was developed and validated using experimental observations. Parametric studies were conducted to improve the performance of the conch-like structure. The conch-like panel was benchmarked against a bulk panel and showed noticeable improvements in strength and toughness. Several key parameters were identified, which can guide the future design of lightweight materials for applications requiring high energy dissipation. This preliminary proof-of-concept study can thereby guide the development of more complex bio-mimetic structures for protective applications.

Introduction

A conch (Strombus gigas) shell has a hierarchical structure from the nano to the macro scale [1], [2], [3], [4], [5], [6]. This structure is composed of inner, middle and outer macro layers (Fig. 1a, b). These layers are composed of three-order lamellae, which are arranged in a similar structure to that of cross-plywood (Fig. 1b–d). The unique toughening characteristic of a conch shell structure is its ability to deflect and arrest cracks through its cross-plywood-like structure, which has been shown to enhance its fracture toughness [1], [7], [8], [9], [49]. This behavior has been correlated with the orientations of the first-order, second-order and third-order lamellae [3], [4]. The first order lamellae (Fig. 1b) are composed of several sheets that are 5–60 μm thick and hundreds of micrometers wide [3], [4], [7]. Each sheet (first-order lamellae) is composed of a number of rectangular beams that are 5–30 μm thick, 5–60 μm wide and up to several hundred micrometers in length [3], [4]. A rectangular beam (Fig. 1c) in a second-order lamella is assembled from thousands of parallel third-order lamellae [3], [4]. The third-order lamellae (Fig. 1d) are the building blocks of conch. They are assembled from numerous nano-twinned aragonite platelets, which are rectangular plank-like structures with a width of 150–200 nm, thickness of 75–100 nm and length of tens of micrometers. The third-order lamellae are stacked in parallel, interlocked due to their surface roughness and surrounded by a thin organic matrix [2], [3], [4], [7]. The organic matrix (approximately 0.1% wt. of the shell), works together with the stiffer mineral phase (accounts for 99.9% wt. of the shell) to provide superior strength and toughness to the conch shell [7].

Researchers have attempted to establish the influence of the structural components of a conch shell on its mechanical properties through experiments: compression [8], [10], [11], bending [2], [7], [8], [10] and indentation [3], [4], [9], [12]. The cross-lamellar structure was found to provide high energy dissipation via well-known fracture mechanisms, namely crack deflection and bridging, with the help of the oblique orientations of its second to third order lamellae [1], [2], [7], [8], [9]. The orientation of the first order lamellae, which is almost perpendicular between adjacent macro-layers, creates a weak direction for the crack to propagate, namely through the organic matrix layer between the lamellae, or directly through the second-order lamella [4]. Interlocking from the rough surfaces between lamellae increases their adhesion strength, thereby dissipating more energy. Shin et al. [3] showed that the alignment angles of the crossed-lamellar structure have a strong influence on the hardness of the shell. They showed that an angle of ±45° provides prominently higher penetration resistance and a higher peak load prior to failure compared to that of 0°/90°.

Other researchers developed conch-like structures using advanced manufacturing techniques [49]. Chen et al. [13] and Gu et al. [14] demonstrated the benefits of a conch-like panel over an equivalent monolithic panel under three-point bending and impact, respectively. In both studies, the conch-like panel showed a prominent increase in fracture toughness compared to the monolithic panel. Recently, Jia et al. [15] fabricated conch-like (cross-lamellar) and nacre-like (staggered brick and mortar) structures using 3D printing and assess their fracture resilience under quasi-static loading. They observed that the work of fracture of the cross-lamellar structure is two times smaller than that of the nacre-inspired structure. The authors explained that the low fracture toughness was attributed to the confinement of cracks to the interface between the stiff and soft materials, rather than penetrating through the sample.

Several researchers have developed computational models to understand the toughening mechanisms of conch structures under various loading conditions. DiPette et al. [16] developed a 2D finite element model to capture the toughening mechanisms of conch under four point bending, which they validated using experimental results reported by Kuhn-Spearing et al. [2]. They observed that the high energy dissipation of the conch shell was attributed to the orientation and thickness of the cross-lamellar layer. Shin et al. [3] carried out a numerical study to simulate a single edge notched fracture test on a conch-like specimen and calculated the crack-tip J-integral. They found that the nanotwin microstructure in the third order lamellae effectively enhanced the fracture toughness of the overall structure by deflecting, arresting and delocalizing cracks, which reinforces their experimental observations of the deformation mechanisms of conch. Gu et al. [14] developed a 3D bi-material finite element model to simulate the behaviour of a conch-like panel under low velocity impact from a drop tower. They found that the conch-like panel, which was composed of strong brittle planks bonded by a soft elastic material, outperformed an equivalent bulk panel by arresting and deflecting cracks through the soft interface.

Table 1 compares the mechanical properties of a conch shell with other biological structures, natural stones and engineering ceramics. The conch shell and nacre are all made from aragonite (a brittle mineral and a form of calcium carbonate, CaCO₃). However, the arrangement of the aragonite in conch shells provides a work of fracture that is approximately ten times higher than that of nacre. Although the main constituent of conch shells and natural stones (marble and limestone) is calcium carbonate (CaCO₃), the work of fracture of conch significantly outweighs that of natural stones. Similarly, the work of fracture of conch shells is two orders of magnitude higher than that of engineering ceramics such as Alumina and Silicon Carbide. Nevertheless, to compensate for its high work of fracture, conch possesses a lower strength than nacre, natural stones or engineering ceramics. Mimicking the architecture of conch by manipulating the anisotropy of a material can lead to an increase in fracture toughness without a significant reduction in strength.

In this paper, the cross-lamellar structure of the conch shell is mimicked via finite element modeling and 3D printing techniques to enhance the toughness of a monolithic panel without increasing its mass. Firstly, the extended finite element method (XFEM) with a bilinear softening cohesive law was first employed to establish a link between the material orientation and fracture toughness. The model was then validated using the stress intensity approach in linear elastic fracture mechanics. Next, a conch-like panel was 3D printed to verify its toughening mechanisms experimentally. The numerical model was then modified to capture the observed crack patterns in the experiment. Due to the limitations of the manufacturing process, a user subroutine was subsequently developed to generate more complex conch-like designs. To this effect, parametric studies were carried out to investigate the effects of different material parameters on the toughness of the conch-inspired structure.