Multiscale structure and damage tolerance of coconut shells.

J Mech Behav Biomed Mater. 2017 May 17. pii: S1751-6161(17)30213-8. doi: 10.1016/j.jmbbm.2017.05.024. [Epub ahead of print] Gludovatz B1, Walsh F2, Zimmermann EA3, Naleway SE4, Ritchie RO5, Kruzic JJ6. Author information 1 School of Mechanical and Manufacturing Engineering, UNSW Sydney, NSW 2052, Australia. Electronic address: b.gludovatz@unsw.edu.au. 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. 3 Research Center, Shriners Hospital for Children-Canada, Montreal, QC, Canada H4A 0A9. 4 Department of Mechanical Engineering, University of Utah, Salt Lake City, UT 84112, USA. 5 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; Department of Materials Science & Engineering, University of California, Berkeley, CA 94720, USA. 6 School of Mechanical and Manufacturing Engineering, UNSW Sydney, NSW 2052, Australia. Electronic address: j.kruzic@unsw.edu.au. Abstract We investigated the endocarp of the fruit of Cocos nucifera (i.e., the inner coconut shell), examining the structure across multiple length scales through advanced characterization techniques and in situ testing of mechanical properties. Like many biological materials, the coconut shell possesses a hierarchical structure with distinct features at different length scales that depend on orientation and age. Aged coconut was found to have a significantly stronger (ultimate tensile strength, UTS = 48.5MPa), stiffer (Young's modulus, E = 1.92GPa), and tougher (fracture resistance (R-curve) peak of KJ = 3.2MPa m1/2) endocarp than the younger fruit for loading in the latitudinal orientation. While the mechanical properties of coconut shell were observed to improve with age, they also become more anisotropic: the young coconut shell had the same strength (17MPa) and modulus (0.64GPa) values and similar R-curves for both longitudinal and latitudinal loading configurations, whereas the old coconut had 82% higher strength for loading in the latitudinal orientation, and >50% higher crack growth toughness for cracking on the latitudinal plane. Structural aspects affecting the mechanical properties across multiple length scales with aging were identified as improved load transfer to the cellulose crystalline nanostructure (identified by synchrotron x-ray diffraction) and sclerification of the endocarp, the latter of which included closing of the cell lumens and lignification of the cell walls. The structural changes gave a denser and mechanically superior micro and nanostructure to the old coconut shell. Additionally, the development of anisotropy was attributed to the formation of an anisotropic open channel structure throughout the shell of the old coconut that affected both crack initiation during uniaxial tensile tests and the toughening mechanisms of crack trapping and deflection during crack propagation. Copyright © 2017 Elsevier Ltd. All rights reserved. KEYWORDS: Coconut; Damage tolerance; Deformation; Fracture; Strength; Structure PMID: 28550972 DOI: 10.1016/j.jmbbm.2017.05.024