Session: 12-12-02: Modeling of the Fracture, Failure, and Fatigue in Solids

Paper Number: 145113

145113 - Nanoscale Mechanical Property-Structure Relationships in Coconut Shells 

The renewable characteristics, biodegradability, and inherent biological structure of coconut endocarp deem it as a sustainable material alternative suitable for various engineering applications. With a hierarchical cellular structure spanning various length scales, the endocarp is intricately organized to enhance mechanical properties such as strength, toughness, as well as lightweight character. It is imperative to understand the interconnected correlation between the structural features across various length levels and the mechanical response of the coconut endocarp. Leveraging the advantages of these natural cellular biopolymers offers a myriad of benefits for creating innovative materials with enhanced properties.

Investigating bio-cellular natural polymers through experimental studies presents challenges owing to their intricate hierarchical structure and inherent complexity. Advanced analytical methods like microscopy, spectroscopy, and rheology are indispensable for understanding the diverse structures and properties of these polymers. However, exploring their applications at the nanoscale poses technical difficulties due to the complexities involved in sample preparation. The coconut endocarp exhibits a hierarchical composition consisting of organic materials such as cellulose, hemicellulose, lignin, proteins, and pectins, forming nanofibrils-matrix composite structures at the nanoscale, layered structures at the sub-micron scale, cell wall layers at the micro-scale, and porous vascular bundles at the macro scale, contributing to its diverse properties and functionalities. To deeply investigate and mimic these biological materials, MD simulation offers a promising method, which allows for precise control over nanoscale features, taking into account the pivotal influence of polymer interfaces on complex three-dimensional structures and mechanical behaviors.

Toward this goal, in this study sandwiched polymer models are constructed concentrating on the primary three polymers, as they constitute the major components of the endocarp. And it is speculated that structures with cellulose-hemicellulose-cellulose or cellulose-hemicellulose-lignin-cellulose interfaces exhibit greater stiffness, strength, and toughness compared to those with cellulose-lignin-cellulose or cellulose-hemicellulose-lignin-cellulose interfaces, attributed to the weaker interface between cellulose and lignin under tension. It is suggested that there may be a critical density threshold leading to remarkable mechanical properties. Furthermore, variations in mechanical response are observed due to the involvement of multiple cellulose bundles. A single modeling approach may not capture all relevant details across these scales, making multiscale modeling essential. Subsequently, we constructed a coarse-grained (CG) model of the endocarp using a single bead per sugar residue through the center of mass, enabling exploration of its deformation behavior at a mesoscale. To maintain uniformity in the chain conformation, arrangement, intermolecular packing, and hydrogen bonding across both the all-atom (AA) and CG models, we obtained the CG force field parameters from the results of the atomistic simulations. Later, we predicted a brick-and-mortar structure inspired by nacre where cellulose microfibrils resemble (minerals) embedded by hemicellulose matrices resembles (proteins) as experimentally no clear evidence of the arrangements of cellulose microfibrils found in the literature. In the AA study, two-step failure mechanisms are observed: initially, failure occurs in the soft matrix material at the short interfaces, followed by shear deformation in the soft matrix along the long edges of microfibrils under external tensile loading. However, in the CG study, the composite model with long cellulose bundle chains and a greater degree of polymerization did not show any soft matrix failure at short interfaces. For both cases, the notch greatly affects the mechanical performance of the endocarp model. However, the failure mechanisms were not affected that much. The presence of moisture can reduce the mechanical properties due to the less formation of hydrogen bonds among polymers. Lastly, the three-dimensional models show better deformation behavior than the plate model because they can resist more crack formation and propagation than the plate model. The models were further studied by incorporating ions, and nanofillers such as carbon nanotube, graphene, and silica particles in composites to test the anisotropic properties.

The findings of this work could potentially provide guidelines for the design of coconut-inspired lightweight cellular materials.

Presenting Author: Ning Zhang Baylor University

Presenting Author Biography: Ning Zhang joined the Department of Mechanical Engineering as a tenure-track Assistant Professor at Baylor University in 2022. She received her B.S. in Engineering Mechanics from Dalian University of Technology, China, her M.S. in Solid Mechanics from Huazhong University of Science & Technology, China, and her Ph.D. in Mechanical Engineering from the University of Florida. Before Baylor, Dr. Zhang worked as an Assistant Professor at the University of Alabama from 2019 to 2022. She was also affiliated with the Colorado School of Mines as a Research Assistant Professor and with Missouri S&T and Rice University as a Postdoc. Now her research group focuses on combining experimental and numerical techniques to predict, quantify, and optimize the relationship between the structure-property-processing of various materials including biomaterials, sustainable polymer composites, and high/medium entropy alloys. Dr. Zhang received the NSF CAREER Award (2022), the ORAU Ralph E. Powe Junior Faculty Enhancement Award (2021), and the TMS FMD Young Leaders Professional Development Award (2019). Dr. Zhang was also the winner of the TMS SMD Young Professional Poster Contest (2019). During her PhD study, she was awarded the University of Florida Outstanding Academy Achievement (2013).

Authors:

Ning Zhang Baylor University
Sharmi Mazumder Baylor University