Nacre is the iridescent material which forms the inner layer of seashells from gastropods and bivalves. It is comprised of microscopic ceramic tablets densely packed and bonded together by a thin layer of biopolymer (Figure 1). This hierarchical microstructure is the result of millions of years of evolution, and it is so organized that its strength and toughness are far superior to the ceramic material from which the individual tablets are made.

To understand the mechanisms responsible for such outstanding performance, we performed a multiscale experimental study over several length scales using TEM, SEM, AFM, nanoindentation, and microtesters. Our work revealed that the inelastic mechanism responsible for this behavior is sliding of the tablets on one another accompanied by transverse expansion in the direction perpendicular to the tablet planes. This mechanism was found to be the result of tablet waviness and the unique properties of the biopolymer present at the interfaces.

Based on these findings, design guidelines for composites mimicking nacre were proposed and we have developed rapid prototyping and microfabrication strategies to make a synthetic material exhibiting the unique properties of seashells. As described above, we have identified a key waviness-induced tablet interlocking mechanism in natural nacre that is activated during tablet sliding. This mechanism facilitates propagation of damage spreading to improve the energy dissipation characteristics of the material. We translate the sliding-induced tablet interlocking mechanism to the synthetic material through the use of an interlocking dovetailed tablet structure. This produces similar progressive damage spreading during tablet sliding while facilitating facile fabrication.

Figure 1: Left: (a) Inside view of red abalone shell (b) Cross section of the shell. (c) Crack resistance (JR) curves for nacre from two experiments. (d) Prior to crack advance a tablet sliding zone develops ahead of the crack tip. (e) As the crack advances it leaves a wake of inelastically deformed material (d and e: optical images; red arrow shows location of crack tip at the onset of crack propagation and the steady state regime). (f) Schematic showing the J contour used in our modeling. (g) Crack deflection and debonding of tablets along a crack. (h) Tablet sliding on one another (g and h: scanning electron micrographs). (i) 3-D microstructural modeling of mechanical response of nacre.

In future work we will emulate the tablet-and-mortar structure of nacre using individual graphene sheets as nanofillers. Graphene sheets are one the most suitable 2D materials as they exhibit the highest in-plane stiffness and strength found in nature. We will study and design nanocomposite materials based on functionalized graphene sheets using a multiscale experimental-computational method. Our approach is designed to guide the synthesis of a broad class of composite materials through a fundamental understanding of the role of building block strength and modulus, as well as shear modulus and geometry of the interfaces, in optimizing shear transfer efficiency in the composite.

Personnel 

• Horacio Espinosa (PI)
• Xiaoding Wei (postdoc)

Selected Publications 

• H.D. Espinosa, A. Juster, F. Latourte, O. Loh, D. Grégoire, and P. Zavattieri "Tablet-level origin of toughening in abalone shells and translation to synthetic composite materials" Nature Communications, Vol. 2, No. 173, 2011.

• H.D. Espinosa, J.E.Rim, F. Barthelat, and M.J. Buehler, "Merger of Structure and Material in Nacre and Bone - Perspectives on de novo Biomimetic Materials," Progress in Materials Science, Vol. 54, No. 8, p. 1059-1100, 2009.
• F. Barthelat and H.D. Espinosa. "An Experimental Investigation of Deformation and Fracture of Nacre-Mother of Pearl," Experimental Mechanics, Vol. 47, No. 3, p. 311-324, 2007.
• H. Tang, F. Barthelat and H.D. Espinosa. "An Elasto-Viscoplastic Interface Model for Investigating the Constitutive Behavior of Nacre," Journal of the Mechanics and Physics of Solids, Vol. 55, No. 7, p. 1410-1438, 2007.
• F. Barthelat, H. Tang, P.D. Zavattieri, C.M. Li, H.D. Espinosa. "On the mechanics of mother-of-pearl: A key feature in the material hierarchical structure," Journal of the Mechanics and Physics of Solids, Vol. 55, No. 2, p. 306-337, 2007.
• F. Barthelat, C.M. Li, C. Comi, and H.D. Espinosa. "Mechanical properties of nacre constituents and their impact on mechanical performance." Journal of Materials Research, Vol. 21, No. 8, p. 1977-1986, 2006.

Patents 

• H.D. Espinosa and F. Barthelat, "Segmented Layered Composite with Dovetail Shaped Tablets" Inclusions: 'Nacre' Composite, NU Disclosure No. 26164 (provisional patent application filed in 2007).