Sandwich core periodic cell topology effects

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


This research work is part of the VIPER project, a European Joint Doctorate network focused
on research in vibroacoustics of periodic media. It has received funding from the European
Union's Horizon 2020 research and innovation programme under Marie Curie grant
agreement No 675441. The Universities involved are the E.U. academic partners Université
de Franche-Comté (FR.) and University of Bristol (U.K.). Since VIPER is part of an Innovative
Training Network (MSCA-ITN) this research work was also carried out in collaboration with
the non-EU academic partner Georgia Institute of Technology (U.S.A.) and the EU industrial
partner iChrome, based in Bristol (U.K.). The aim of the project is to improve the
vibroacoustic properties of periodic structures which are widely used in the engineering
domain due to their main characteristic, periodicity, which makes them a convenient
solution for manufacturing guidelines aspects. Real life macroscopic examples are railway
tracks, bridges, and airplane fuselages, amongst many others. Honeycomb sandwich panels
are another example and are well known to provide interesting static out of plane
properties because of their high equivalent stiffness whilst containing mass and for this
reason, they are widely used as a ‘building brick’ in the Aerospace, Automotive and Naval
industries. The environment in which these materials operate involve external forces which
excites them in the mid-low frequency range. However, while a high stiffness/mass ratio is
a desirable static property, the vibration frequency domain is usually in the high range and
therefore they become poor mechanical and acoustic insulators within the frequency range
they are usually subject to. The question addressed then is simple: how periodic concepts
can improve the broadband vibroacoustic signatures and performances of those structures?
Most of vibroacoustic solutions are frequency band limited, specific, and usually include the
addition of mass, which for certain engineering segments is disadvantageous. Including
vibroacoustic design rules at early stage of product development is one of the main research
targets to improve their performance and would allow to design tuned structures without
any later intervention or mass increment. This work focuses on investigating existing
sandwich panel core topologies and attempt to create novel improved structures. The
research was carried out trying to maintain the desired structural properties which justifies
the usage of such solution in the first place but also considering its potential use as a
platform for Multiphysics resonating periodic material inserts. Such cellular cores had to be
manufactured using Kirigami, which is a variation of Origami, an ancient Japanese technique
that consists in creating 3D structures by folding a 2D sheet of material. This manufacturing
technique can be used as a systematic way to produce general honeycomb configurations
with off-the-shelf long fibre composites by thermoforming and/or autoclaving, which by
itself, is a novelty in the honeycomb domain as they are often produced using metals or
plastics. The main indicator on which I will focus to evaluate the vibroacoustic performance
of the proposed innovative topologies will be the number and range of stopbands, also
known as a bandgaps, which describe the frequency ranges in which elastic waves are not
transmitted within the structure, in combination with the constituent material and its
damping properties. This manuscript is organised in four chapters. The first consists of an
overview of periodic structures in the various engineering domains followed by an
introduction on elastic wave propagation in periodic media. Sandwich panels and their most
popular manufacturing techniques will also be described. Also, phenomena like Bragg or
resonant bandgaps will be explained as well as the Floquet-Bloch theory applied to macroscale structures such as aeronautical cellular cores. The latter mathematical derivation will
be merged with the Finite Element Analysis approach and implemented as the basis for the
numerical prediction tools specially developed to allow parametric investigations on the
complete sandwich panels or bare cores. The Floquet-Bloch theory allows us to harvest
crucial information on the dynamic behaviour of the whole structure by performing our
analysis only on a small portion of it which will be called now on the ‘unit cell’. In the second
chapter, a preliminary bandgap investigation on a simple beam truss structure, and later,
on kirigami derived tessellations is carried out. The reduced number of cell geometries
obtainable with kirigami motivated me to invent Interlocking, which still respects all
kirigami rules and unleashes the geometrical constraints of the parent technique. It also
adds the possibility of obtaining long fibre composites multi material cores, which
introduces a second level of periodicity, and may represent a novel platform able to
accommodate and quickly interchange foams, viscoelastic patches, or resonators. The
concept, manufacturing technique and the parametric bandgap investigation of some
interlocking examples will be widely discussed in this chapter. The fourth chapter will focus
on another vibration index, which is the out of plane transmissibility of interlocked cellular
structures. Various Flax fibre composite honeycombs with thermoset as well as
thermoplastic matrix were manufactured and tested. Finally, the manuscript will end with
a conclusive paragraph where conclusions are drawn and possible future work as well as
applications proposed. The coded software tools used for the investigations and the
material characterisation used for the kirigami manufacturing will constitute the appendix.
Date of Award29 Sep 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
  • Université Bourgogne Franche-Comté
SponsorsH2020 Marie Skłodowska-Curie Actions
SupervisorFabrizio L Scarpa (Supervisor) & Morvan Ouisse (Supervisor)

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