Beetle Elytra Inspired Cellular Cores for Sandwich Structures

Abstract

Beetle Elytron Plates (BEPs) are new types of biomimetic sandwich plates inspired by the internal
architecture of the beetle elytra. Beetle Elytron Plates have remarkable configurations and properties
and have raised awareness among scientists and engineers due to their lightweight characteristics,
high strength, and excellent energy absorption capacity. However, BEP core topologies haven’t been
so far examined in sufficient detail. Moreover, the majority of the research on BEP cores has focused
on the performance of theses core structures within the nonlinear and plastic regime, because of
significant potential in energy absorption applications. In-depth studies of their elastic engineering
constants have not been reported yet; the knowledge of those constants would allow a more complete
comparison with other cellular structures and accelerate their use when designing applications to
other engineering fields. The conventional BEP core topologies contain straight cylinders, whereas
wavy/corrugated cylinders are also commonly employed in broader engineering applications. Novel
designs of BEP cores would also represent an important aspect to potentially increase the mechanical
properties of the BEP configurations. Most research works related to Beetle Elytra Plates are related
to integrated BEPs, in which the structures are 3D-printed with integrated face sheets and a core.
Adhesive bonding between core and face skins represents the state-of-the-art for producing sandwich
panels, but there is only a handful of papers available in open literature about adhesively bonded BEPs
sandwich structures. The potential debonding behaviour, shear and flexural properties are a critical
research issue for adhesively bonded sandwich panels.
In this work, the in-plane engineering elastic constants of BEP cores are firstly investigated using
numerical and experimental methods with parametric analysis. The simulations are carried out on the
Representative Volume Element (RVE) models followed by an RVE-based FE homogenization method
using Periodic Boundary Conditions (PBCs). Each RVE FE models are simulated using two types of
configurations for validation. The simulations are also conducted using full-scale FE models for
comparison and cross-validation. Experimental tests are carried out using uniaxial and 45-degree
uniaxial tensile tests to benchmark the numerical results. All the results are compared to those from
conventional honeycomb core structures. The results show that the in-plane nondimensional tensile
modulus of the EBEPs raises significantly as the size of the cylinder increases, while the modulus of the
MBEP cores slowly drops. Although both MBEP and EBEP cores have hollow cylinders in their unit cells,
the cylinders in the EBEP cores are located at the joints of cell walls; their torsional and tensile
mechanical performance can be fully realised because the joints are critical points for load bearing. In
the case of the MBEP configurations, their main load-bearing points are the same of those of the
hexagonal honeycombs. Moreover, their cylinders add extra material, compared to the original
hexagonal lattice. Next, the out-of-plane elasticity of BEP core structures are also investigated using
numerical techniques, with the benchmark provided by experimental results. The parametric analysis
has been also carried out using RVE models with PBCs applied and calculation based on the FE
volumetric homogenization method to explore the influences of geometric parameters on the
mechanical properties of the structure. The simulations are also performed on the full-scale FE models
under the flatwise compression, transverse pure shear and simple shear loading conditions to crossvalidate the numerical results from RVE models. Experimental tests including flatwise compression and
transverse pure shear are also conducted to benchmark the numerical results. Experimental and
numerical results are also compared to the conventional honeycomb core structures. The results show
that the out-of-plane Young’s modulus of all types of configurations increases when the sizes of the
cylinders or ribs grow. The modulus of the MBEP configuration has a steeper increase than the one of
the EBEP configurations. Both types of BEP architectures possess larger modulus than the hexagonal
honeycombs under the same cylindrical size or rib thickness.
Novel BEP cores with wavy cylinders have been also designed, instead of the conventional straight
cylinders in classical BEPs. The in-plane and out-of-plane elastic engineering constants of these novel
cores have been evaluated. The investigations have been carried out using a numerical parametric
analysis based on RVE FE models with PBCs, followed by volumetric homogenization method. The
results are compared to the straight cylinder BEP cores and hexagonal honeycomb tessellations. The
results show that the EBEP patterns with waved cylinders have larger in-plane tensile and shear
modulus compared to topologies with straight cylinders, especially for configurations with a single
half-wave. The nondimensional in-plane tensile and shear moduli of the EBEP topologies are larger
than those of configurations without waves. The opposite however happens for the MBEP
configurations, in which the nondimensional moduli are smaller than those of topologies without
waves. The height of the core positively contributes to in-plane stiffness of the two cellular
configurations, with and without wave-shaped cylinders. The in-plane Poisson’s ratio of EBEPs is more
sensitive to the geometric parameters than in the case of the MBEP cores. The out-of-plane
nondimensional tensile and shear moduli of both EBEP and MBEP topologies decrease due to the
increase of the relative density.
Finally, the adhesively bonded BEP entire panels are designed and produced by adhesively connecting
the novel BEP core with face sheets. These BEPs are tested under four-point bending test and
simulated under the same loading conditions using full-scale FE models to validate the experimental
results. The flexural behaviour of these BEPs is evaluated following a homogenised method. The core
shear modulus is then calculated and compared to the modulus obtained from the previous core
studies. All results are compared to the BEPs with conventional straight cylinder cores and the
hexagonal honeycomb sandwich panels. The results show that all the sandwich panels have material
failure earlier than adhesive bond failure. BEPs with wavy cylinders have larger peak loads compared
to those with straight cylinders, and all BEPs show higher peak loads than those of hexagonal
honeycomb sandwich panels. Furthermore, all the BEPs exhibit better ductility than the hexagonal
honeycomb sandwich panels. Compared to EBEPs, the MBEPs exhibit longer post-peak load curves
with an ascent during the plastic stage, indicating better ductility. The EBEP cores exhibit similar peak
shear stresses compared to hexagonal honeycomb cores, whereas the MBEP topologies have larger
peak values due to the increased number of cylinders. However, BEPs show decreased values of
normalised shear stress values compared to the hexagonal honeycomb panel due to the larger mass
contribution provided by the cylinders.

Date of Award	11 Oct 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Fabrizio Scarpa (Supervisor) & Mark Schenk (Supervisor)