Fully bio-based composite and modular metastructures

Abstract

The reliance on fossil-derived components in the design of metamaterials and metastructures presents sustainability and environmental challenges, prompting the development of alternative solutions. In response, this study proposes a fully bio-based and modular metastructure composed of rods extracted from the giant bamboo (Dendrocalamus asper) and plant-based polymeric joints derived from soybean (Glycine max) and castor oil (Ricinus communis), aiming to offer a sustainable alternative for load-bearing structural components. The research investigates the design, fabrication, and mechanical performance of a unit trussed cell (50 × 50 × 50 mm³) engineered to exhibit auxetic-like chiral rotation and enhanced energy absorption under compressive loading. These cells are assembled into trussed beams (400 × 50 × 50 mm³), and further into sandwich beams with 5 mm thick balsa wood skins. Material properties of the bamboo and polymer components are assessed via physical, chemical, and mechanical characterisation to asses their potential chemical-adhesion compatibility, density, and mechanical performance. Following the fabrication of the proposed structures, further experimental evaluation includes compression of the trussed cell and four-point bending of the beam configurations, while finite element analysis (FEA) is used to simulate elastic behaviour under torsional and cantilever loading. Results demonstrate that the metastructure trussed cell (with a mass of ~ 30 g) supports up to 700 kg in compression, achieving ~ 2 mm displacement, 4° rotation, and absorbing ~ 750 μJ/mm³ of energy; it also exhibits a force-displacement slope of ~ 4,200 N/mm and an equivalent Poisson ratio near zero within the elastic regime (up to ~ 1 mm displacement). The trussed and sandwich beams exhibit equivalent densities of ~ 0.19 and ~ 0.21 g/cm³, respectively, while achieving bending loads of ~ 2000 N and ~ 3600 N, corresponding to maximum bending moments of ~ 103 and ~ 188 kN∙mm, and toughness values of ~ 158 and ~ 193 μJ/mm³, respectively. Simulated torsional response of the trussed cell indicates a torque of ~ 7,300 N∙mm per degree of twist, while FEA results for cantilever loading show a homogenised flexural modulus of the beams of ~ 623 MPa (trussed) and ~ 751 MPa (sandwich). These outcomes underscore a promising direction for developing renewable, high-strength, and lightweight composite structures, with applications ranging from civil construction to aerospace engineering.

Keywords: Bamboo; Beam; Bio-based polymers; Composite structure; Lattice; Metastructure; Sandwich structure; Truss.

Fig. 1

a structural applications of truss-based geometries, b sustainable structures based on eco-friendly materials, c sandwich panels with a lattice core, d metastructure lattice core for sandwich design, and e truss structures made of bamboo (whole or laminated culms)

Fig. 2

Concept of the proposed metastructure cell composed of eco-friendly materials and its application for modular assembly of trussed and sandwich beams

Fig. 3

Flowchart detailing the research activities carried out in this work

Fig. 4

Sustainable composite design: a metastructure trussed cell and b shell that compounds the joints (dimensions in mm)

Fig. 5

a Design details of the trussed beam and b assembly of the sandwich beam

Fig. 6

a 3D models and their constituent parts: b joint shells made of soybean-derived polymer, c joint filling formulated with castor oil-based polymer, d bamboo rod and its surface coating of castor oil polymer, and e balsa wood skin and adhesive layer consisting of castor oil polymer

Fig. 7

Process of obtaining cylindrical rods: a drawing system, b bearing apparatus, c milling tools, and d laser cutting

Fig. 8

Overview of the 3D printing process: a 3D printer alongside the washing and curing machine, b 3D-printed joint shells, and c post-curing stage

Fig. 9

Fabrication sequence of the trussed structures: a manual assembly of the metastructure cell and trussed beam using bamboo rods and the soybean-based shell components, b application of castor oil-derived fluid resin for joint filling and rod coating, c moulding process for the production of the sandwich beam, and d fabricated prototypes

Fig. 10

Insights on the mechanical tests of raw materials: a tensile test of the bamboo rod, b compression test of the bamboo rod, c tensile test of the soybean polymer, d tensile test of the castor oil polymer, e specimen dimensions for polymers testing, f three orientations for the tensile specimens of soybean polymer, and g moulding specimens of castor oil polymer

Fig. 11

Supplementary mechanical tests: a illustration and testing of pullout specimens, b three-point bending of balsa wood sheet, and c preparation and testing of lap-shear specimens

Fig. 12

Insights on the mechanical characterisation of the trussed structures: a metastructure trussed cells attached to aluminium plates, b compression test of the trussed cell, c video gauge, d four-point bending of the trussed beam, and e four-point bending of the sandwich beam

Fig. 13

Insights on FEA analysis: a models for validation through the experimental data, b additional loading conditions evaluated through the numerical simulation, and c mesh of the parts used in the model assembly

Fig. 14

FTIR spectra for the raw materials compounding the metastructure trussed cell

Fig. 15

Stress–strain curves for mechanical assessment of the raw materials: a tensile of the bamboo rods, b compression of the bamboo rods, and c tensile of the soybean and castor oil polymers

Fig. 16

Force–displacement curves for mechanical assessment of the supplementary tests: a pullout for the rod-joints attachment, b three-point bending of the balsa wood, and c lap-shear for balsa and castor oil adhesivity

Fig. 17

Mechanical characteristics of the trussed cell: a force–displacement curves under compression, b typical damage observed in the truss members, and c insights into the rotational behaviour of the metastructure

Fig. 18

FEA insights on the metastructure trussed cell: a validation of the model under compression and evaluation of stress distribution in the rods; and b results on the torsion assessment

Fig. 19

Four-point bending curves for the trussed and sandwich beams: a force–displacement data, and b mean stress–strain data

Fig. 20

Failure analysis of the trussed beam

Fig. 21

Failure analysis of the sandwich beam

Fig. 22

FEA insights on the trussed beam: a validation of the model under four-point bending and evaluation of stress distribution in the rods; and b results on the cantilever bending assessment

Fig. 23

FEA insights on the sandwich beam: a validation of the model under four-point bending and evaluation of stress distribution in the rods; and b results on the cantilever bending assessment

Fig. 24

Conceptual drawing of a drone featuring a structure composed of sandwich beams with balsa wood skins and a trussed core entirely fabricated from bio-based materials