Mechanical response of polymer nanotruss scaffolds – MechNanoTruss

1.Design and Fabrication. The project will start with existing lattice geometries and apply shape optimization procedures in order to obtain effective geometries for particular applications combining models from ETH and EP. The lattices will be manufactured using two-photon polymerization on a Nanoscribe Photonic Professional instrument available at ETH.

2.Characterization of the mechanical properties. The project has access to a wide range of experimental facilities. Quasi-static tension-compression tests and setup for indentation test on the lattice scaffolds are already available at the ETH. The group has equally developed a Dynamic compression experimental technique to study the dynamic behavior of complex soft materials, based on high-speed microscopic imaging and direct measurements of dynamic forces and deformations. Micro- and nanoscale experimental facilities SEM, AFM, TEM, X-Ray Tomography, FIB, etc. involving also in-situ testing of microstructures both at EP and at different partner institutions of the future Paris-Saclay University.

3.Numerical Modeling. Both research groups involved in this project have solved several problems in computational mechanics. The ETH group has experience in nonlinear acoustics and will focus the numerical modeling of the nanolattice as a phononic crystal [31].
The EP group will focus on the cyclic loading of the lattice and on the lifetime predictions for applications of the nanoscaffolds. The computational design method will be based on an initial shakedown analysis of the structure and a subsequent lifetime prediction using fatigue criteria

In summary, we have fabricated and analyzed the linear viscoelastic
properties of polymeric microlattice materials. This analysis is an
important step from previous works that were mainly addressing the
quasi static, i.e. elastic or elasto-plastic, properties of microlattices. We
developed a versatile experimental procedure to study stress relaxation
in microlattices, for time scales up to 400 s. Our experimental study
revealed that the effective Young's modulus of polymeric lattice
structures, with different lattice topologies, can be adjusted by adapting
their effective lattice densities. We found that, in the considered density
range, the loss factor is only slightly scaling with the lattice density. We
also studied the loss factor at elevated strains and found an increased
loss factor and damping figure of merit of up to 3.1. The results show
that the loss factor of microlattice materials is mainly increased by large
strains, which is important for impact mitigation. In structural vibration
absorption applications, the reached strains can be quite small. In these
problems, the use of a more dissipative base material or composite is
beneficial to increase the damping capabilities. Numerical simulations
show that high effective density lattices outperform bulk polymeric
blocks in energy dissipation. This result is counter-intuitive, as it
predicts an increased dissipation with using effectively less material.
To increase the range of covered frequencies, experiments at varying
temperatures could be performed, exploiting the temperature/frequency
equivalence of polymer viscoelasticity.

Current research has two objectives, on the one hand side the development of an effective method of shape optimisation based on level-sets and topological optimization and, on the other hand side, a more precise knowledge of polymer materials by DMA tests.

Bauhofer, A., Krödel, S., Bilal, O., Daraio, C., & Constantinescu, A. (2017). Direct Laser Writing of Single-Material Sheets with Programmable Self-Rolling Capability. Bulletin of the American Physical Society, 62.

Krödel, S., Li, L., Constantinescu, A., & Daraio, C. (2017). Stress relaxation in polymeric microlattice materials. Materials & Design.

Three-dimensional lithography (two-photon polymerization) enables the manufacturing of polymeric materials with an architectured microstructure of tailorable geometry. Starting from elementary 3D arrangements of nanotrusses, in the 100-nanometer size, one can build complex scaffolds, in centimeter scale size, by repeating the elementary arrangement periodically. The fabrication of macroscopic samples of these hierarchical materials opens the way to various applications, from biocompatible scaffolds to ultra-light vibration absorbing layers. Approaches to ensure a robust and reliable design are needed for the assessment of these structures. This project aims at understanding the mechanics of biocompatible polymer nano-lattices and at developing a robust method to design them. The project will analyze experimentally their quasistatic monotonic or cyclic tensile loading and the dynamic compression, and it will predict numerically their behavior and lifetime.