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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - MULTILAT (Multi-phase Lattice Materials)

Teaser

Summary

Multi-phase lattice materials are a new class of engineering material with wide application to lightweight transport, security, sustainable buildings and electronic packaging. For example, the increasing demand for energy in transport poses a global threat: there is a finite supply of energy (oil, coal, gas, nuclear) and sustainable solutions are insufficient to meet demand. There is a major driver to reduce energy consumption by lightweighting in all transport sectors (aerospace, automotive and marine). Micro-architectured materials comprise periodic or stochastic lattices and foam made from a single material, and can be made lightweight. There has been much recent activity in the development of new lightweight and multifunctional solids, based on foams and lattices, but this has been incremental. Can we do better?

There is an opportunity to engineer a new class of multi-phase, multi-scale lattices that combine multiple interpenetrating topologies, multiple materials and even multiple length scales to provide an enormous range of new combinations of properties. For example, they can deliver high stiffness, strength and damage tolerance, but possess very low weight. The systematic procedure towards developing the new class of multi-phase lattices and lattice coatings involves (i) computational capabilities that permit accurate simulation and prediction of properties based on its constituents, (ii) additive manufacturing processes with precise control of material morphology and topology, and (iii) experimental characterization methods with resolution capable of measuring the microstructural features as they deform under applied loads.

The main objectives of the MULTILAT project are as follows:
(i) To invent multi-phase multi-scale lattices that combine structural hierarchy, with freedom to select the topology, constituent material and length scale of each lattice.
(ii) To determine the effective stiffness, strength and resistance to crack growth (or fracture) as a function of lattice topology, length scale and material combination, for multi-phase lattices.
(iii) To develop a systematic method of filtering the enormous range of candidate constituents in order to develop the optimal multi-phase lattice materials for a given application based on the fundamental micromechanics.
(iv) To scope out the manufacture and use of the newly invented lattices in representative applications: a hard but porous coating, a functionally graded compliant coating and a lightweight structural panel with high energy absorption and high toughness.
(v) To measure and to predict the extreme properties of a coating made from a nano-scale lattice.
The outcomes of the proposed project will open up new horizons for the underpinning science and technology of new applications and devices.

Work performed

A combination of novel manufacturing methods, experimental techniques, and numerical simulations were used to invent, manufacture and test new classes of lattice material, and thereby to achieve a number of objectives inline with the MULTILAT project. The main results of the project are as follows:

(i) Scaling laws have been developed for the stiffness, strength, and resistance to crack growth of single-phase and of two-phase metallic and polymeric lattices. These scaling laws were obtained in terms of the relevant properties of the constituent material and lattice topology.
(ii) Manufacturing processes introduce imperfections in the lattice. A strong sensitivity on stiffness and strength due to the dispersion in material strength at a strut level was found. The geometric imperfections have only a minor effect. Fabricating novel lattices topologies by water-jet cutting of metal sheets serves as a first step towards developing lattices with optimized mechanical responses. Two-phase lattices have been made by infiltration of a liquid into an open-cell foam. As a first step, the problem of rising damp has been addressed: how does water migrate through a porous medium? Experiments and predictive models reveal that water rise through a cellulose foam by both capillary and diffusive action. This has immediate implication to transport phenomena in cellular solids such as paper, wood and in concrete, and to the melt infiltration of cellular solids.
(iii) Design concepts for enhancing the resistance to crack growth in single-phase and multi-phase lattices were explored: the use of 2 networks of varying length scale and properties shows immense potential in developing tear-resistant fabrics (akin to rip-stop nylon) and crack-resistant foams. This knowledge enables tailoring of lightweight composite materials based on specific applications. A printing technique has been used to manufacture a two-phase Thermo-Plastic Elastomer (TPE) lattice. The stiff phase is a hexagonal lattice made from a relatively stiff co-polymer, whereas the infill has a much lower modulus. This 2D interpenetrating lattice has a high damage tolerance.
(iv) A strategy for the design of protective structures such as helmets is to place a top stiff and strong layer on top of a foam core. A stiff face sheet when attached to a foam enhances the indentation strength of the bi-layer by membrane stretching of the face sheet. The collapse of foam sandwich beams is explained in terms of a collapse mechanism that is new to the literature. The analytical models developed herein allow for an optimisation to be performed in terms of geometry and material choice.
(v) Nanolattices and nanofoams have been invented. These include development of a model for nanofoaming via inflation of cell walls and a model for stabilization of gold nanowires to promote nanolattice formation via self-assembly using organic ligand shells. The potential of structural hierarchy is explored in the fabrication of open-cell nanoscale graphitic foams. Nickel nano gyroid lattice coatings were fabricated via a fast and easy self-assembly process. Experiments and numerical simulations reveal that the nano gyroid topologies attain compressive strengths higher than those reported for micron scale lattices materials made via 3D printing.

Final results

MULTILAT has revealed that it is possible to develop new combinations of lattice materials in order to generate new combinations of properties such as enhanced toughness and ductility. Multi-scale lattices have been developed such that the macroscopic properties (such as stiffness or strength) are due to multiplicative scaling from one length scale to the next. This emphasises the Achilles heel of the hierarchical approach: a knock-down at any length scale leads to a knock-down in overall, macroscopic properties.
There has been particular success in the development of infilled lattices: the infiltration of a lattice by a liquid which then hardens. The interaction between the infill and the parent lattice can lead to high stiffness, strength and damage tolerance.
The project reveals that it is possible to engineer lattices that can accommodate a large internal swelling without fracture. This has direct application to the invention of new types of solid state lithium ion battery.