Many scientific challenges accompany or anticipate the development of composite structures in particular in the field of aeronautics. These include the prediction of damage under impact, crashworthiness, structural details under static and cyclic loadings. To date, the models developed by the scientific community are essentially limited to simple loadings and are validated on simply designed specimens. The special character of the composites is that the material does not exist prior to the structure. Thus, the scientific community agrees on the relevance of multi-scale or multi-levels approaches for a detailed description of the behavior. However, academic research has so far invested a lot in the lower scales (micro, meso or coupon) to identify and model the various modes of damage. VERTEX project proposes to develop a methodology for analysis and general validation that is positioned at the scale of composite structures. The samples sizes is of the order of tens of centimeters. The choice of this scale allows a unique positioning and can handle a wide range of fundamental problems. Moreover, the aim of VERTEX is to propose a method of analysis or experimental validation by static tests under complex loading like compression / tension / shear. The methodology will enable a dialogue test / calculation improved and extended, which is a necessary step towards to the Virtual Testing. This is a key issue that will enable considerable economic gains and reduced design loop of aircraft or other composite structure of next generation. Like astronomers or astrophysicists who need telescopes to validate or develop their science projects, the multi-level intrinsic to the nature of composites requires the development of complex test methods. The scientific project VERTEX is therefore the first step in developing an original method of measurement and control (WP2) on a specific test device (WP1). Indeed, for model validation or analysis of the phenomenology, it is necessary to perform measurements and complex loading paths either globally across the structure, either at a more local scale (for example crack tips). These phases of engineering and instrumentation are therefore a fundamental point of the study. To test the concept coupled test / instrumentation, three issues are proposed by the partners (WP3). The first concerns the large cuts under complex loading, the second is related to the calculation of the failures of technological specimen with multi-scale calculation approach and the third one is the validation of damage models by complex fracture path. These three issues will be subject to a validation phase by means of test VERTEX (WP4) . Synthetically, the VERTEX project aims to provide a coupled experimental/measure/calculation scientific methodology to identify the behavior of composite structure. This methodology will also be able to discriminate the relevance of different models of literature. Because of its universal character, this approach will enable the aeronautic industry and others to validate research or technology of composite structures at reasonable costs.

The scientific goal of MCMET is to develop a novel strategy for simulating complex energy systems by building upon recent advances in Monte Carlo (MC) path-sampling methods. Recent advances in statistical physics and computer graphics have paved the way to tackle a long standing issue: solving non linear models with MC methods while preserving its fundamental capacity to scale up with the geometry and physics complexity. The project focuses on one specific type of non linearity: the one related to the collision frequency parameter, which defines the geometry of the (radiative, conductive, electronical...) paths. Three applications are targeted: radiant energy conversion systems (photoreactive and photovoltaic) for solar fuels and electricity production; thermal performance of buildings targeting both energy consumption reduction and thermal comfort of inhabitants; and estimation of the ground solar resource in presence of clouds in climate simulations.The locks in these applications can be formulated in a common framework and are due to non linear dependencies to the models’ collision frequency parameters. In this project , the following questions will be addressed: (1) How to formulate non linear physical models under the path-space formulation? (2) How to conceive scientific computation libraries for sampling these new multi-scale multi-physics path spaces? (3) How to implement algorithms in this framework to meet the application needs? The consortium brings together MC specialists, computer scientists and physicists specialized in energy, climate and buildings, to develop a novel modelling paradigm allowing the gain of orders of magnitude in performance and cost of numerical simulations. Impacts will be immediate for the applicative domains thanks ,to the development of application-driven codes. Longer-term outlooks include the creation of MC-based climate services for the energy sector.

Current civilian and military aircrafts and flying systems (e.g. UAVs) are designed from thin, lightweight, multi-material structures assembled by bolting, gluing or built-on fabrication. Due to their constitution, these structures are vulnerable to explosion phenomena that occur on their surface during a natural aggression such as lightning, or intentional aggression such as Directed Energy Weapons (DEA) or lasers. These aggressions are governed by multiphysical mechanisms in the environment close to the impacted surface (thermal, mechanical, electrical, EM). They generate superficial (top plies cracks, skeins of stripped fibres) or core damage (spalling, perforation). The residual performances of the structures are significantly diminished and the internal equipment (tanks, embedded systems) are exposed. It is necessary to protect these systems to limit their vulnerability. The objectives of the project SUSTAINED21 are in line with the civilian and military needs whose systems are susceptible to these different types of aggression, in order to envisage industrial solutions with high added value that will increase their survivability. The challenges arising from these applications are to have available a method for dimensioning the protection of structures that breaks with the current industrial practices, and contribute to the operational superiority of forces. The first objective of this study is to build an experimental database and a mapping of the damage induced by three different means of energy deposition: the lightning mean (which constitutes the reference test), the pulsed laser and the electron gun. The use of this damage mapping will make it possible to assess the similarities between the damage produced by the two alternative means with respect to the "lightning" reference test, the results of which are already available on CFRP composites. It will also ensure that they are representative, particularly with respect to the electromagnetic environment. By extension, this database can be used to compare damage from impacts at very high speeds. The second objective consists in numerically simulating the behaviour of the materials of interest under attack in order to evaluate the capability to predict the damage caused and to identify the limits of current models, particularly with regard to the application of a multiphysical loading. The achievement of this objective will be based on the adaptation of existing models and on the comparison with the mapping of the database. The third objective is to establish a methodology for using the technological bricks developed in objectives 1 and 2. The influence of the protective layers will be explored here in order to help, in the long term, the emergence of a tool for dimensioning protections. This predictive approach will be transposable to the military field for the dimensioning of future directed-energy weapons according to the layers of protection to be penetrated. This approach will satisfy the challenges for industry to reduce the costs of development studies, the most robust protections being the only ones subjected to certification/qualification lighting tests, the lightning generator being used as the final reference mean. The project is based on a partnership between an academic project leader who has worked on the modelling of damage caused by lightning, an academic partner specialized in the implementation of an experimental laser shock device, and an industrial partner expert in specifying protection layers. The proposed work involves a subcontractor in the SME sector with know-how in the implementation and analysis of laser shocks, and will be supported by DGA-Ta as the expert in lightning tests certification. The work is part of the continuity of collaborations and scientific partnerships with the DGA-Ta in Toulouse and Airbus Operation. Being interested in this field, DGA Missiles Testing SG will be able to offer its support.

Ceramic matrix composites are interesting materials due to their good mechanical properties and their good damage resistance, even at high temperature. Nevertheless, their fabrication requires usually expensive constituents, and processing routes with long duration and expensive too. Geopolymers offer advantageous ways to produce ceramic matrix composites reinforced by long woven fibres: less expensive constituents and processing routes, fast fabrication, and low environmental impact. The objective of this project is to develop and optimize the fabrication methods of a sandwich composite material composed of long oxide-ceramic fibres (basalt, alumina) reinforced-skins with geopolymer matrix and a highly porous geopolymer core, and to study its mechanical behaviour and damage mechanisms. This type of sandwich composite is not classical and not well known, but could be useful to produce, for aeronautic or terrestrial transportation applications, structural lightweight parts with high thickness and good mechanical resistance even under temperatures of several hundreds of Celsius degree. The aim of this project is to develop the formulations of the geopolymer suspensions to produce on one hand the skins with an optimised impregnation of the fibres, on the other hand a highly porous geopolymer foam by an optimization of the processing routes to generate high rate of pores, and to perform the skin/core assembly by the simplest fabrication methods. Preliminary studies for this project have shown that this type of assembly is possible but needs to be optimized. A specific study of microstructures, physical and mechanical properties at room temperature will be done in help to reach these optimisations. The sandwich composite will be studied from a thermomechanical point of view to determine its mechanical behaviour at room and at high temperature, and to analyse the damage mechanisms associated. Bending and tensile tests instrumented with acoustic emission monitoring, digital images correlation and thermographic cartographies will be performed for that purpose. They will allow to follow the multi-cracking of the skins and of the cores according to the level and the type of mechanical loading, but will allow also to determine the interaction mechanisms between skins and cores at the interfaces of these parts of the sandwich. Thermomechanical fatigue tests will be analysed to evaluate the mechanical resistance of the composite under periodic loading, and to determine if additional damage mechanisms are induced by fatigue (for example, if microcracks are created by fatigue in the cores and could propagate progressively inside them and along the skins due to cyclic loading). The monitoring of the tests by acoustic emission and the various imaging techniques, but also the microstructural observations by electronic microscopy and X Ray tomography will allow to identify and analyse the damage mechanisms and to introduce them in finite element models in order to simulate the macroscopic mechanical behaviour from a numerical description of the composite at a mesoscopic scale, and to link these simulations to the macroscopic mechanical behaviour determined experimentally. The design and use of this composite will be done in a sustainable development context. That is the reason why the questions concerning recycling of this composite, especially by reusing ground composites as filler in new geopolymer pastes or suspensions, but also the problems linked to repairability of the composite by foam filling or uses of patches on the skins, will be investigated.

Over the past decades progress in modeling, parallel computing and available computational resources made possible the analysis of complex, large scale models of increasingly detailed structures. On a different front, new methodological developments for reliability analysis and reliability based design allowed significant improvements in the efficiency of these approaches. In particular, recent developments in so called active learning reliability analysis approaches have proved to be very efficient for problems with a moderate number of random variables. Active learning (also known as adaptive sampling) approaches consist in constructing a kriging surrogate model (or metamodel) for calculating the reliability constraints and adaptively enriching the surrogate model based on the kriging uncertainty structure. In spite of these recent developments, reliability analyses involving large scale structural models may still pose computational cost issues because the expensive simulations still need to be carried out many times. To address the computational cost issue of running one expensive simulation, reduced order modeling has been proposed in the past and has recently regained a lot of interest. We will consider here reduced order modeling by projection, also known as reduced basis modeling, which consists in solving the large scale system projected on an appropriately defined basis. Drastic reductions by many orders of magnitude are thus achieved in the size of the problem to be solved. The objective of the proposed project is to develop a new methodology for efficient reliability analysis of large scale structures. The novel approach resides in defining an interaction between adaptive sampling and reduced order modeling by projection, by adaptively enriching the kriging metamodel using a reduced order model tuned to have at each step the appropriate fidelity based on the accuracy requirements of the reliability analysis. These accuracy requirements are derived from the kriging uncertainty structure, thus guiding the level of fidelity of the reduced basis model. Far from the limit state leading to failure a very low fidelity (but very cost efficient) reduced basis model may be sufficient. As one approaches the limit state, the fidelity of the reduced basis model will be automatically increased based on the accuracy requirements precisely at the current sampling point. Such a combined approach can thus be seen as a tunable fidelity approach since it tunes the fidelity of the reduced order model to the requirements of the current step of the adaptive reliability analysis. This new methodology is expected to lead to a reduction by several orders of magnitude in the computational time of reliability analyses for large scale structures compared to current state of the art methods. It thus has the potential to be transformative for industry practice, by allowing to undertake reliability based design where it would have not been practical before. The project will first investigate the most appropriate coupling criterion between adaptive sampling and reduced basis modeling, then, based on this coupling, develop and implement new reliability analysis and reliability based design optimization methodologies. Finally, the developed approaches will be applied to two structural mechanics application problems within the aerospace domain. The first one concerns a classical aeronautical certification test on a composite open-hole laminate. The second problem involves a wingbox structural model, which has all the ingredients of large scale structural models and aims at demonstrating the computational savings potential of the proposed approach.