The Mechanical Performance of Nacre from Seashells: Superior Toughness Through Microstructural DesignFrancois Barthelat, Ph.D., Northwestern University, 2005.
Major Professor: Dr. Horacio D. Espinosa.
Many structural biological materials such as bone, teeth or seashells exhibit surprisingly high mechanical performances. The case of nacre (mother-of-pearl) is a perfect example, which is attracting a lot of attention in the mechanics and materials science communities. Nacre is mostly made of a brittle ceramic (aragonite), yet it exhibits surprisingly high levels of strength and toughness. This is made possible by a very well designed microstructure organized over several length scales (hierarchical microstructure), and optimized through millions of years of evolution. Extensive research has been pursued on nacre, but the exact microstructural features and mechanisms leading to its remarkable mechanical properties are still unknown.
Nacre has a brick and mortar structure, where polygonal aragonite tablets are bonded together by a small fraction of organic material. The prominent deformation mechanism in tension is the sliding of the tablets on one another, which generates large deformations. Through experiments and modeling, we demonstrated that the waviness of the tablets is the key feature that generates tablet interlocking and strain hardening, which is required to distribute inelastic deformations over large volumes.
This mechanism has a direct impact on the toughness of nacre. Fracture experiments revealed a rising crack resistance curve, which explains nacre's impressive crack arrest capabilities. The toughening of nacre was attributed to inelastic deformations in the wake of the advancing crack, which generate closure stresses on the crack tip. This process, similar to transformation toughening in zirconia, was shown to be the main toughening mechanism for nacre. The waviness of the tablets, by spreading inelastic deformations over large volumes around flaws and defects, maximizes toughening and damage tolerance. We envision that these finding will open pathways to novel synthetic composites with superior properties.