Exploring the mechanical properties of hard botanical structures of two tropical plants

Two hard botanical structures have been characterised in terms of mechanics and structure: the pseudofruit of Coix lacryma-jobi, a native plant of Southeast Asia, and the kernel of the drupe fruit of Thevetia peruviana, a native plant of tropical America. Two indentation techniques at different scales (micro and nano) were employed. The reference point indentation technique was used both in the cross-section and on the external side of the samples, while the nanoindentation technique was used to monitor the external layers. The manner in which natural structures such as those present in Coix lacryma-jobi and Thevetia peruviana are organised are not fully understood. A more complete understanding of the mechanical properties of the hard structures would provide more information for qualifying characteristics as well as an inspiration for designing novel materials. This study aimed to provide information regarding the organisational composition of these natural structures and thus shed light on how natural structures achieve several physical properties, such as hardness. Although the samples have a similar organic and inorganic composition, the results showed differences in their external mechanical resistance. The obtained differences in the external mechanical resistance are to be interpreted in light of the complex hierarchical structure.