Multi-materials 3D printing application of shell biomimetic structure

Abstract

Additive manufacturing, or 3D printing, is a rapid prototyping method to fabricate 3D objects by depositing various materials layer by layer. Despite its industrial origin and significant commercial impact, 3D printing technique has found its great potential applications in engineering and scientific research. Here, we demonstrate its application in remodeling nacre-inspired bricked structure. Nacres have extraordinary mechanical properties, such as high fracture toughness and high strength-weight ratio, rooted from its brick-and-mortar structure, with Calcium carbonate platelets as hard bricks alternating with proteins layer as soft mortar. Although it is well proposed that the platelets and mortar serve jointly via shear-lag mechanism, experimental studies on the mechanical effects of brick geometry and stiffness ratio between brick and mortar have been limited. With the help of 3D printing technology, we are now able to fabricate nacre-inspired structures with two types of UV- curable polymers, where stiff plastics as brick and soft rubber as mortar. The geometry of brick and mortar can be tuned in much wider range inaccessible to natural nacre. This allow us a thorough exploration in the parameter space, leading to an understanding of the naturally optimized values found in nacre samples. In the study, 5 samples with nacre-inspired brick-and-mortar composited structures are prepared by Object Connex350 (Stratasys, Inc.) 3D printer. The mortar thickness is taken as 0.21 mm limited by the precision of the printer, and the length-to-width ratio of the brick ranges from 3 to 15. These samples are subjected to tensile force until fracture and the whole process are recorded with time lapse camera. We see three distinct regimes of climbing, horizontal, and declining, which can be closely related to elastic deformation, end-rubber fracture, and horizontal-rubber fracture. For different length-to-width ratio, three different fracture modes are observed. When the ratio is small, zigzag tearing path appears across the width, leading to largest yielding strain and smallest yielding strength. When the ratio is intermediate, vertically linear fracture path jointed by inclined tearing arise, corresponding to longest flow horizontal plastic regimes. When the ratio is large, only vertically linear fracture path can be seen, corresponding to longest horizontal plastic regimes, resulting in largest yielding strength and smallest fracture strain. A scaling theory is developed to elucidate mechanism of the three regimes. As complementary to these experimental results, finite element simulation are carried out to explore the stress distribution associated with different parameters of the composited structures. The force ratio transferred by the end and horizontal rubber is seen to saturate exponentially to constant value as the brick-to-rubber stiffness ratio increases. We also find that stress singularity near the end rubber, where the initial crack nucleation appears in experiments. Our work presents a case study demonstrating the powerful strength in studying the mechanics of bio-inspired composited materials, which is unavailable otherwise.