Lattice structures are good candidates for either aerospace parts (weight saving), or biomedical applications (osseointegration). They have very interesting properties, one of them being their large variety of strength-to-weight ratio  depending on the chosen material, topology, density,... Additive manufacturing such as powder-bed-fusion methods seems the most appropriate type of process to produce them. In this work EBM was used to manufacture lattice structures and to find ways to integrate them in critical aircraft parts (in terms of fatigue resistance) in order to save some weight. However, great challenges are still to be overcome. One of them comes froms the process inherited defects that are quite detrimental for the fatigue lifes (especially notch-like defects). As machining is impossible for lattice structures, appropriate post-treatments were found, such as chemical etching and in a lesser extent ultrasonic shot-peening. Work on single struts was performed prior to this work, hence the focus on lattice structures. To reproduce the tensile-tensile fatigue tests realized on single struts, a stretching dominated unit-cell was used : the octet-truss.

In this work, a simple lattice structure was tested. It consists in an elongated structure made of 4 octet-trusses. This made easier X-ray tomography characterization on the whole lattice structure, as well as experimentally possible to follow which strut fails during fatigue tests. The idea was to optimize the fatigue life of this structure, but also to be able to predict it. To that end, post-treatments such as chemical etching and ultrasonic shot-peening were used, and a numerical method using Timoshenko beam elements was developed to predict the failure. This method used a damage accumulation law called the Miner's rule, and has the benefit to be easily tested with different sets of parameters (radius size distributions, S-N curve distributions).

The project was funded by the "Agence Nationale de la Recherche" (see the project web page here)

Densification from firn to ice is an essential phenomenon to understand for the interpretation of the climate record. A good knowledge of this mechanism is necessary for the precise dating of the air embedded in the ice. Pore closure (or close-off) is the stage at which air becomes entrapped in the ice. Typically, the close-off arises approximately 50 - 120m under the ice sheet surface. Because of gas flow in the firn column above close-off, the ice is older than the entrapped air. The difference between ice and gas is defined as Δage and may reach several millennia in certain sites. The precise determination of the Δage is mandatory to link temperature changes (recorded in the ice) and greenhouse gas concentrations (recorded in the gas phase). This issue may be addressed through the modelling of the firn densification processes that lead to pore-closure.

Firn densification consists of grain rearrangements, sintering and viscoplastic deformation. Although the viscoplastic behaviour of the ice crystal is strongly anisotropic, densification models do not take into account this anisotropy. Firn also bears some granular characteristics that may affect its densification. The interactions between pore closure and microstructural mechanisms in the firn are still misunderstood.

The aim of this PhD thesis is to incorporate both the granular aspect of firn and its anisotropy into an innovating approach of firn densification modelling. The mutual indentation of viscoplastic monocrystalline ice cylinders was experimentally carried out to propose a contact law that is based on indentation theory and that takes into account the preferential viscoplastic deformation on the basal plane. We have integrated this contact law into a DEM (Discrete Element Method) code for the prediction of firn densification.

3D X-ray micro-tomography was performed on polar firn at different stages of the densification (ρ= 0.55 - 0.88 g/cm3) and depths (23 - 130m). A thorough investigation of pore closure and of different morphological and physical parameters was achieved for the Dome C and the newly drilled Lock In polar sites. In addition to these ex situ analyses, in situ X-ray micro-mechanical experiments were carried out on firn extracted from Dome C in order to model its densification. Ex situ and in situ microstructural observations indicate significant differences that can be explained by the relatively large strain rates imposed to the firn during in situ tests. These large strain rates allow for a decoupling of the effects of diffusion kinetics and of viscoplastic deformation. Their relative weights on the morphology of pores and on their closure are discussed. To measure pore closure, we propose a connectivity index, which is the ratio of the largest pore volume over the total pore volume. We show that this index is better suited for X-ray tomography analysis than the classical closed porosity ratio to predict the close-off density.

As there should be no barriers in science, all presented work (in conferences) or publications are available for download at the bottom of the page.

As I defended my phD at the end of 2017, here is the final manuscript.

The tensile specimens consists in 4 superimposed octet-trusses : Octet-trusses near dense parts are made with fillets and conical struts so that failure is localized in a central gauge made of two octet-trussed lacking fillets. This structure can be modelled by Timoshenko beam elements with different radius size. The applied stress on the lattice structure is here defined by the applied force over the section shown in the figure below. Such model, using beam theory, enables to determine easily the stress distribution within the lattice structure.