Études en cours

Understanding and modelling mechanisms and geometry effects in ductile to brittle transition

Aboubakr AMZIL


Brittle rupture in the transition region is preceded by significant ductile tearing. These rupture mechanisms are very different and require specific models to precisely describe both phases of crack propagation. It is in particular very important to precisely describe the stress field ahead of the propagating ductile crack as these fields are used as input data to evaluate brittle crack initiation. In order to develop and validate the use of these models, it is necessary to confront them to experimental results obtained at various temperature (to check to ability of the models to predict the change in rupture mechanism) on different geometries (to check the transferability between specimens and to a structure and to validate the understanding of the role of loading on fracture). The proposed methodology is based on two main research axes:

  • Develop a strategy to model rupture in the transition region by combining a ductile damage model to a model for brittle fracture
  • Conduct an experimental work program on a structural steel close to the steels used in french nuclear plants to validate the proposed modeling strategy. The program will include tests at various temperatures and on different specimen geometries

Non destructive testing of turbine blade by X-ray diffraction

Alexiane ARNAUD

Safran Aircraft Engine (SAE) is the Safran Group’s company specialized in the design and manufacture of turbojet engine for civil and military aircraft. This PhD study is performed with Safran Tech research center for the needs of SAE. The field of study concerns high pressure turbine blades.
The high pressure turbine is the second part of the turbojet hot section, behind the combustion chamber. The turbine consists of several compression levels made of monocrystalline nickel superalloy blades. The monocrystalline structure is obtained by directional solidification casting, by lost-wax process. During the blade manufacturing process, crystalline defects can appear leading to multi grain structured blade rather than a monocrystal piece.
The current non-destructive testing involves manual detection of external grains followed by crystal orientation measurement by back-reflection method of Laue diffraction. Because of the complexity of the blade geometries with several walls and cavities, a new X-ray system is developed, based on transmission method of Laue diffraction. Using a transmission method allows to go through all the cavities of the blade to probe each walls.
A blade diffraction pattern generated by a polychromatic X-ray beam is given to this new system. The diffraction pattern is used to identify the blade crystalline orientation and, track defects by detecting a second orientation.

Multiscale experimental and numerical investigation of the behavior of a cast aluminum alloy



There is currently significant interest in understanding precipitation reactions in casting alloys which are used for cylinder heads. In this work, the precipitation of β-Mg2Si particles in an A356+0.5 Cu cast aluminum alloy is addressed. Consideration of its simultaneous precipitation alongside θ-Al2Cu phase is incorporated.
The theoretical approach taken enables the simultaneous treatment of nucleation, growth and coarsening. The model is based on the framework of the KWN method and uses an implicit finite difference scheme. The continuity equation is discretized in time and space in order to obtain a matrix form. The KWN model is directly coupled to the CALPHAD software (Thermo-Calc) which provides data on the driving force for precipitation and the evolution of local equilibrium value for the solute concentration at the matrix/precipitate interface, taking into account the Gibbs-Thomson effect. Simulations with software can be used to provide quantitative predictions on the impact of the
cooling rate and age-hardening heat treatments on the size distribution of particles. The results of the model have been validated by TEM measurements.
A micromechanics model then takes in size distributions of particles to provide a prediction of yield strength of the material.
This kind of multiscale approach provides new perspective on microstructure evolution for highly loaded components such as cylinder heads, i.e. it enables a more accurate prediction of the microstructure and its evolution as a function of thermal history.

Hydrogen trapping by irradiation-induced defects in a 316L stainless steel

Anne-Cécile BACH


The irradiation-induced defects in stainless steel internal components of pressurized water reactors combined with hydrogen uptake during the oxidation process could be a key parameter in the mechanism for Irradiation-Assisted Stress Corrosion Cracking (IASCC). The aim of this study is to characterize the effects of irradiation defects on hydrogen uptake during the oxidation of an austenitic stainless steel (SS) in primary water. The focus was made on the interactions between hydrogen and these defects. A heat-treated 316L SS containing a low amount of defects is compared with ion implanted samples. After the characterization of each ones by Transmission Electron Microscopy (TEM), hydrogen uptake of the samples is promoted either by room temperature cathodic charging or during corrosion experiments in PWR primary conditions. Then the trapping of hydrogen is studied by different characterization techniques such as GD-OES (Glow discharge Optical Emission Spectroscopy), SIMS (Secondary Ions Mass Spectrometry) or TDS (Thermal Desorption Spectroscopy) in association with a kinetic model to simulate diffusion and trapping of hydrogen in stainless steel.

Modelisation of ductile-brittle transition on plastified PVC skins, in high speed choc (firing airbag)


Context and issues: The materials involved in the opening of the airbag for the automotive industry require in addition to functional and aesthetic properties, good breaking behavior at the time of deployment at a speed of about 50 m / s and temperatures ranging from -30°C to 85°C. This last property requires a good control of the breaking mechanisms of plasticized PVC skin : not too fragile so that the structure is not divided into several pieces projected on the passenger; not too ductile for the break to be controlled. The objective of the industrial partner is to :

  • To equip with a real size experimental device - thus instrumented - to fire airbags to understand the mechanisms of deformation and rupture in dynamics of the skin in order to lead to a realistic modeling of the opening;
  • Evaluate the "material" effect by varying the plasticizer content as well as the porosity and other parameters in order to analyze the influence on the modeling (behavior and rupture) of the firing air bag.

Contribution to the prediction of the effective elastic behaviour of unidirectional polymer matrix composites thanks to the modelling of local morphological changes and matrix confined areas

Jennifer BLONDEL

One of the factors that make composite materials so attractive to many industrial sectors is their microstructural and multi-scale modularity. This specificity offers engineers the means to adapt them to a wide range of performance criteria. But it also makes it difficult to predict part efficiency. To cope with this complexity, engineers have for many years developed a robust approach requiring numerous physical structural tests. Today, a wider use of composite materials requires the development of cost-effective solutions including “virtual testing”. One of the current trends is to develop multiscale simulations addressing the industry’s requirements and able to accurately predict composites effective properties from the properties of their constituents and morphologies. In this context, coupled numerical and analytical micromechanical modelling approaches are an efficient way to move forward.
In this thesis with Michelin as industrial partner, the transverse elastic behaviour of transversely isotropic multi-phased materials is studied. In contrast to longitudinal properties, transversal properties are much more sensitive to the local morphology of the material and the transverse direction constitutes a weakness of the material which must be well apprehended. As an application example, the case of UD with "trapped" matrix regions is investigated. For this purpose, a “n-phase” Generalized Self-Consistent Scheme (GSCS) coupled with a Morphologically Representative Pattern (MRP) approach has been developed. Analytical solutions, depending on two parameters “m” and “c”, are provided to predict the transverse shear modulus and the transverse bulk modulus. Moreover, the proposed model, written in a transversely isotropic formalism, is valid for a wide range of inclusion volume fractions and specifically for the highest volume fractions we can find in high performance composites. Morphological descriptors are used in order to address the effect of “trapped” matrix on the transverse elastic behaviour of UD composites.
A microstructural Finite Element Model (FEM) is here used to handle the complex morphologies approaching as closely as possible real microstructures. Those numerical simulations help supplying the analytical implementation for a better description of interactions between the constituents and calibrate the “m” and “c” parameters. Then, we will have to find a convenient image analysis procedure on a real microstructure that will provide the desired settings using suitable morphological descriptors. Furthermore, estimations of the local fields such as displacement, stress or strain fields will be calculated either on an analytical way or on a numerical way thanks to a finite element modelling improved by using the p-version of the Finite Element Method.
Finally, mechanical testing would be performed to assess the relevance of the proposed approach.

Phase transformations and high-temperature mechanical behavior of a zirconium alloy (M5): effect of thermal history



The safety of light-water nuclear reactors rests heavily on the good behavior of the fuel cladding tubes, made of zirconium alloy. In hypothetical accidental situations, those tubes might be exposed to complex thermomechanical loading. In particular, their temperature may vary non-monotonically, up to more than 800°C, which may cause several partial or full phase transformations, and strong evolution of its microstructure and mechanical properties. The purpose of this work is to study and model phase transformations, resulting microstructural evolution and mechanical behavior of M5® zirconium alloy as a function of thermal history.
To this aim, phase transformation kinetics and mechanisms will be characterized under isothermal and anisothermal conditions, particularly after a first high-temperature treatment. This first part of the project will deepen our understanding of phase transformation mechanisms as a function of thermal history.
Based on this, samples with various model microstructures will then be subjected to creep tests to improve our knowledge of the relation between chemical composition, microstructure and resistance to viscoplastic deformation.
Experimental findings from this work and previous works will be capitalized into analytical and numerical models describing the phase transformations and resulting creep behavior all along the thermal transient.

Microstructure – Properties relationships for AA-2024 & AA-5086 (aluminum alloys) after long term and high-temperature aging

Louise BRIEZ


Aluminum alloys possess low weight and cost and have a high strength which makes them one of the most used materials in aircraft industry. For example, AA-2024 (Al-Cu-Mg) is used for the wings or fuselage and AA-5086 (Al-Mg) is used for pipes and both are supposed to operate in the temperature range from – 55 to + 85°C.
Nevertheless, different parts of aircrafts can be exposed accidentally (fire, overheating during processing) at temperature significantly higher than 85 °C. The impact of such accidents on microstructure and, especially, mechanical properties of these alloys is not well-known since the data are lacking in the literature.
Therefore, the aim of this study is to characterize the microstructure and mechanical properties evolution of AA-2024 after different times of exposure at temperatures above 85°C.
2024 T3 and T8 and 5086 H111 alloys are artificially aged at different temperatures in the range from 85 to 250°C for different durations varying from 1 to 10000 h. Then, tensile and hardness tests are carried out to investigate the influence of high temperature ageing on mechanical properties. The combined use of SEM, TEM and DSC allows to characterize the microstructure at different scales. The evolution of precipitation is correlated with the change of mechanical properties at different temperatures. Using the time-temperature equivalence, the prediction for microstructure would be done for the temperature range from 100 to 175°C and for durations longer than 10000h to approach service conditions.

Anisothermal behavior and failure of zircaloy-4 fuel cladding: application to RIA



The Reactivity Initiated Accident (RIA) is a hypothetical accident scenario affecting the Pressurized Water Reactors' hearts. During this accident, the zirconium alloy clad is subjected to a rapid thermo-mechanical transient that could potentially lead to its rupture.
It's a fast transient (a few ms) during which the tube is simultaneously subjected to a temperature ramp up to 1000°C/s and a strain ramp up to 5/s. Furthermore, the mechanical load during this transient is not uniaxial: the clad experiences a time-dependent non-uniform biaxial mechanical load, making the overall thermo-mechanical history quite complex.
The anisothermal character of the RIA scenario remains a major axis of study because the development of laboratory tests coupling rapid and simultaneous transients of strain and temperature is quite difficult.
A thorough understanding of the effect of a temperature transient on the mechanical response of the clad will enable the consolidation of behavior laws and failure criteria, previously established under isothermal conditions.
The aim of this work is to study of the influence of transient temperature on the mechanical behavior and failure of Zircaloy-4 thanks to the interpretation of these new tests.

Multiscale approach of the mechanical behaviour of hot-dip zn-al-mg coatings on a steel sheet

Houssem Eddine CHAIEB


Arcelor-Mittal is carrying out a research program aiming at determining the forming and mechanical properties of Zn-Al-Mg coatings on hot-dip galvanized steel sheets, depending on the solidification microstructures. The research strategy is developed along 5 lines: material selection, advanced characterization techniques, macroscopic mechanical properties of coatings, mesoscale modelling of microstructures and identification of deformation and interand transgranular damage mechanisms.
The proposed PhD project aims at applying advanced characterization and modeling techniques for a better understanding of the mechanical properties of Zn-Al-Mg coating along the previous lines. The objective is to draw from these observations new guidelines for the microstructure optimization.

Physico-mechanical approach for non-linear modeling of filled elastomers


Filled elastomers’ mechanical properties are very interesting in a large temperature and frequency range. Those are of prior importance for vibration dampers applications in automotive or even value-added application in aeronautic. Mechanical prediction of those materials is compulsory for Hutchinson for industrial designing.
In fact, reinforcements – especially nano-silica which are very compatible with silicone – drastically improve mechanical properties (stiffness, ultimate strength, abrasion, absorption…). In return, some dependencies and non-linearities (invisible in pure matrix) occur in the mechanical behavior which make the finite element analysis very complex.
In this context, we aim at building a robust physico-mechanical model consistent with experiments. Two objectives for this thesis :

  • Helping formulation of new materials by multi-scale simulation of micro-structures.
  • Quantitative finite element modeling and prediction of non-linear mechanical properties of filled elastomers in order to design new vibration damper structures in aeronautic.

Study of the embrittlement of a stainless martensitic steel under extreme cooling conditions

Constance CHANH

Among the various criteria used to evaluate a new steel grade intended for the automotive industry, spot weld strength is crucial as Resistance Spot Welding (RSW) is the most frequently used joining technique for car structures. Although they present many advantages, Martensitic Stainless Steels tend to form brittle spot welds, since RSW involves very fast cooling sequences. The work aims at finely understanding embrittlement induced by RSW, by analyzing the effect of the welding cycle parameters on the final microstructure and mechanical properties of the weld, for a given steel chemistry. This involves determination of cross tensile strength for spot welds issued from different cycles, of fracture paths and microstructural characteristics. The results are to be capitalized into a model correlating weld performance with spot weld characteristics, the interconnected characteristics being:

  • The mechanical state of the specimen during cross tension test, which involves the thermo-mechanical simulation of the different weld region, and the creation of a FE model,
  • The final microstructure of the specimen and its transformation history, which are to be determined by using local chemical analysis, EBSD and numerical simulation of local thermal history.

Modeling of ductile fracture using local approach : reliable simulation of crack extension

Youbin CHEN


Predicting fracture resistance is important to ensure the safety of industrial facilities under hypothetical accident loads. To achieve this goal, numerous damage models taking into account physical degradation mechanisms have been developed to numerically simulate damage extension. However, the application of these models in structural calculations still remains problematic. The major issues are: mesh dependency and numerical volume locking related to large plastic deformation. These two problems have been identified and one model has been proposed to solve these problems during the thesis work of ZHANG (2016). However, with his model, the reliability of numerical calculations remains poor due to the lack of an appropriate technique to manage crack extension. In these simulations, a Gauss point is considered to be broken once its void volume fraction exceeds a given value. Stress and stiffness at these points are both zero. In that case, the finite elements including these Gauss points can become extremely distorted. This problem appears especially in the damaged
zone and can induce the divergence of simulations. Therefore, the main objective of the thesis, is to find out solutions to prevent this spurious behavior.
The first step of the thesis is to make the current model more robust. Three aspects are or will be addressed :

  • Resolution algorithm at integration point level: an improved Newton algorithm, developed by Eric Lorentz, is used. Based on the classical Newton method and the bisection method, this algorithm improves the convergence rate of classical Newton algorithm
  • Numerical schemes for solving the internal damage variable: both implicit scheme and explicit scheme are used to update the damage variable
  • Void coalescence criterion: Instead of using a fixed value to initiate void coalescence, the plastic limit load model of Thomason will be used. This model indicates that fracture is not only related to void volume fraction, but also to void spacing and stress triaxiality.

The second step is to propose some solutions to prevent excessive element distortions: Use of viscosity to stabilize elements containing broken gauss points: The main idea of this method is to increase the strain rate of the soften element, which gives an additional stiffness to the damaged points. An overlay model (a model which is independent of the inviscid material) is proposed.
The third step is to study some properties of non-local locking-free GTN model using the small-scale yielding model.
The last step is to realize the parameter fitting of GTN model based on the database of the ATLAS+ project. These parameters will be used to realize crack extension in CT3D, SENT3D specimens and in an industrial pipeline. This step is crucial to prove the robustness of the current damage model.

Microstructural evolutions and their consequences on the mechanical properties of a niobium-stabilized austenitic stainless steel

Nicolas CLICHE

The French Alternative Energies and Atomic Energy Commission (CEA), Valduc center, in collaboration with École Nationale Supérieure des Mines de Paris and partners (Aubert & Duval, Naval Group and DGA), have launched a PhD project about the study of the microstructural evolutions and of their consequences on the mechanical properties of a niobium-stabilized austenitic stainless steel, namely, the 316 Nb steel. This steel is used in the manufacturing of components which undergo thermomechanical loading (temperature and pressure) during very long time.
Previous work showed the effect of some hot forming parameters (thermomechanical loading and annealing) on the final microstructure of the material. The goal of this PhD project is, on the one hand, to assess the impact of the remelting process of the steel and, on the other hand, to get deeper understanding of the effects of the microstructural changes brought by each processing step on the final mechanical properties of the material. A numerical modelling of theses microstructural changes will be developed.
The previous PhD work (A. Hermant, MINES ParisTech, 2016) focused on a 316 Nb steel elaborated under air atmosphere, then electro-slag remelted (ESR). It revealed several links between the thermomechanical path and microstructural evolution (in particular, recovery and recrystallization phenomena), through hot torsion tests. A first link with the hot forming parameters (temperature, strain and strain rate, multi-pass character, cooling rate) was established.
The first step of the present PhD project was to perform similar torsion tests in order to measure the impact of a change in the solidification process of the steel, and in particular the use of vacuum arc
remelting (VAR) which leads to a slight change in N and Mn contents that could influence the competition between recovery and recrystallization phenomena.
In the sequel, the doctoral student focused on the relationships between the microstructural evolutions found during the manufacturing process and the final mechanical properties. To do so, several thermomechanical paths on blanks obtained from the two different remelting processes have been applied e.g. pancake forging. These tests enable to get closer to the actual parts, both concerning the size and the strain heterogeneities and to get enough hot deformed material to extract mechanical testing specimens from, in order to access local mechanical properties (such as uniaxial tensile behavior and impact toughness).
In order to assess the evolutions of microstructure, an extensive use of EBSD technique on a FEG-SEM is being done. It enables a quantification of recrystallization, grain size and other parameters of interest such as texture.
Up to now, the partners have no numerical models to assess links between the processing parameters, the local thermomechanical path, the microstructure and the mechanical properties in a continuous manner. The above presented experimental work, along with the already available data, provides a substantial experimental database on which the doctoral student is working to be able to develop, test and even optimize a metallurgical post-treatment model in order to take the specificities of the 316 Nb steel into account.

Numerical modelling of crack initiating, growth and arrest in ductile materials

Valentin DAVAZE

When a car crash occurs, parts made with steel or aluminium sheets can tear up. Rupture of ductile materials is not reliably predicted yet, and so, many parts and tools have to be re-designed, which increases the production cost. Ductile material behaviour was studied in many scientific works, in which comparisons between numerical simulation and experiment are made. Unfortunately, numerical methods used for making such comparisons are not adapted to an industrial context. In particular, computation time is excessive for the development of new cars. The aim of this work is to identify and develop a numerical method reliable and efficient which will enable the prediction, using the Finite Element Method, of the ductile tearing occurring on parts made with steel and aluminium sheets. The objectives are defined by two important issues: predicting the crack initiating and growth, which correspond to local phenomena occurring in a small scale (< mm), and keep the computation time compatible with dynamic simulations on car structures in a large scale (> mm). Regarding the behaviour till the crack initiating, softening constitutive models for damage in large strain require the use of a damage regularisation method to avoid pathological mesh dependency. Concerning the crack growth, element deletion usually used is not reliable enough. The XFEM methods are not able to gasp local phenomena enough, required for estimation of the crack initiating criterion and growth without prior remeshing. For this work, the idea is to use a recent and innovative remeshing method to gasp these local phenomena and, in the same time, to catch the crack front and lips with an optimum compromise precision/cost.

Oxide/Oxide composite in extreme environment



Regarding the demanding technical characteristics relating to a high thrust-to-weight ratio, an increase in service temperatures is needed for high performance aero-engines. This is one of the reasons why advanced ceramics materials such as oxide/oxide composite are currently developed. This kind of materials is now considered in the design of the new generation of engines. The mechanical behavior at room temperature was modeled. However, designer still need some information about their mechanical behavior at high temperature to correctly size the different parts. Consequently, it would be interesting to have a better knowledge of the degradation of their properties at higher temperatures.
In this context, considering the oxide/oxide composites developed at ONERA, a better understanding of their mechanical behavior and even of the degradation mechanisms at high temperature is required. For this objective, composites will be manufactured and mechanically characterized. Tensile and compressive tests in several directions and creep tests will be performed up to 1300°C under laboratory air, inert gas and water vapor. So, the influence of the temperature and the influence of the oxidant species on the degradation mechanisms could be dissociated. After these characterizations, a fine microstructural analysis will allow to understand the physical and chemical mechanisms associated to the damage of the materials.
In parallel, data from mechanical tests will be analyzed and used to develop a model for predicting, versus temperature and atmosphere, mechanical behavior and damage.

Intergranular corrosion of austenitic stainless steels in oxidizing nitric media

Adrien EMERY

In France, the recycling of nuclear fuel takes place in Areva’s industrial centre of La Hague, using the PUREX process. The used nuclear fuel is dissolved in concentrated nitric acid kept at boiling temperature. Austenitic stainless steels, such as AISI 304L or 316L, are used as structural materials for equipments handling this very acidic media, due to their excellent behavior against corrosion due to the formation of a stable passive layer. During the dissolution of the fuel, oxidizing ions are released in the solution. It causes the corrosion potential of the steel to shift toward the transpassive domain, which leads to the apparition of a new form of corrosion, localized at grain boundaries. Only a special stainless steel (US1N), with 4% wt. of silicon, can resist this intergranular corrosion (IC).
Therefore the main purpose of this thesis is to determine the origin and mechanism of IC, mainly from a metallurgical point of view using different industrial stainless steels.
The first step is to study the influence of impurities (N, S, P, B) on IC. The ambivalent role of silicon, which can either enhance IC or protect the steel against intergranular corrosion, is more particularly studied
First, it requires to do some oxidation tests on some austenitic stainless steels, including US1N, in oxidizing nitric media, to confirm their behavior against IC.
Analyses with TEM-EDX will be conducted at grain boundaries to

confirm or infirm the intergranular chemical segregation of impurities. Some model steel, with critical behavior againt IC, can be used as well to verify different hypothesis.

Stability of hardening phases for future nickel-base superalloys

Laurane FINET

Polycristalline nickel-base superalloys present remarkable mechanical properties at high temperatures thanks to their specific γ-γ’ microstructure. However it appears more and more difficult to keep going the race for higher turbine inlet temperature by optimizing the composition and microstructure of γ-γ’ nickel base superalloys.
An accurate survey of patents on new compositions of nickel-base superalloys for turbine discs published in the last ten years coupled with a bibliography analysis suggest the intention to replace γ’ precipitates by other phases (more stable at temperature higher than 800 °C) to provide precipitation hardening. However, there is still a lack of information about these phases (like η or δ) in terms of crystallographic structure, precipitate morphology, composition and thermodynamic stability. For instance, η and δ phases may often be confused. Nevertheless, the different properties of hardening phases should be well established to be able to ensure an optimal hardening effect through appropriate heat treatment resulting in required precipitate distribution.
Therefore, the present study aims at determining the temperature stability of phases possessing potential hardening effect, like δ-Ni3Nb, δ-Ni3Ta, η-Ni3Ti and η-Ni3(Al,Nb), for different compositions of nickel-base superalloys intended to be used for turbine disc application.
Nickel-base superalloys of different compositions susceptible to form δ-Ni3Nb, δ-Ni3Ta, η-Ni3Ti and η-Ni3(Al,Nb) phases are investigated. The key idea is to simplify the chemical composition of existing alloys (available in patents and scientific papers) to avoid the formation of the phases considered as irrelevant in this study, like TCP phases or carbides and borides, which might overburden the microstructures. Such a composition simplification is based on thermodynamic calculations carried out using Thermo-Calc software coupled with TCNI7 database.

Mechanical characterization of Titanium Linear Friction Welds

Juan-Manuel GARCIA

Titanium (Ti) alloys are widely used in the aerospace industry. Its excellent corrosion resistance, mechanical resistance and low density give them increasing importance. A need to improve the buy-to-fly ratio is raising, e.g. by the use of novel joining techniques. LFW is a recent solid state joining process that works as follows: A cantilever work piece is in contact with another following a linear oscillatory motion. After a few seconds of friction, a forge pressure is applied in order to achieve a target axial shortening. Impurities in the contact surface and mass are extruded to the surroundings. This process is very quick, auto-cleaning and presents few defects. High levels of plastic deformation and localized heat are generated around the weld interface. Thus micro structural changes, unknown mechanical properties and considerable levels of residual stresses are found in the surroundings of the weld line.
This study relies on the mechanical characterization of the joints and the detection of potential defects caused by the process. The first step consists in the non destructive X-Ray observation of the weld interface. The possible presence of cavities is studied. Secondly, parent materials and LFW joints welds tensile tests results are compared. Thirdly, the plausible contamination caused by the block machining is analyzed. A study of the local strain field of the specimen surface compared with a microhardness analysis have been performed to get a better understanding of the elasto-plastic behavior of the weld. Finally the fracture behavior under high cycle fatigue is studied.

Fatigue damage characterization and modelisation on thermoplastic composite reinforced by continuous fibres

Stéphane GILLET

Mass reduction for ground vehicle is a major goal of automotive industry in order to decrease C02 emission. Thermoplastic composite is an interesting solution in order to obtain compromise between mass reduction, mechanical performance and production rate.
Thermoplastic composites with long or short fibres are currently used in structural application like frame for engine mount. Their durability behaviour is becoming know but numerical models are not deployed for industrial application. An improvement is to use continuous fibres in order to increase mechanical properties and the possibility of mass reduction.
It is necessary for industrial actors to have model in order to predict the durability of technical parts. For this purpose, they need predictive mechanical models as well as monotonic than fatigue solicitation. Concerning thermoplastic composites reinforced with short and continuous fibres we must investigate before mechanical behaviour and damage taking account environment parameters (temperature and humidity) and micro-structure induced by process.
This work associate experimental and numerical approach in order to identify and simulate damage mechanism at different scale.
The first step of this thesis was to analyze the service condition part in order to limit the thesis perimeter (Stress ratio, stress triaxiality level, water uptake, tests procedures…). The second step was to obtain representative sample by designing and building an injection mold. Currently we analyse micro-structure of samples by MEB and micro-tomography. The next step is the experimental campaign with the purpose to understand mechanical behavior and damage mechanism. For this we develop a new X-ray tomography in-situ testing machine. Final step will concern numerical approach, with micro-mechanic model in order to represent damage at microscopic scale, and the determination of fatigue criteria at industrial part scale level.

Effect of microstructure on low temperature ductility of hot stamping martensitic stainless steels for automotive applications

Hélène GODIN

Growing demand for passenger protection and emission reduction leads to the continuous development of high strength steels used in automotive industry. Hot forming of martensitic steels is a very attractive process to reach very high mechanical resistance in formed parts, avoiding difficulties arising from cold forming of multiphase high strength steels. Coming after a decade of industrial experience with 22MnB5, martensitic stainless steels optimised for hot stamping constitute a major step forward, combining process simplification (suppression of AlSi coating, ultrafast heating, air-hardening, multi-step forming) and improved material performance (bending, fatigue, corrosion). Among key properties required for automotive structural parts, impact toughness is one component controlling crash worthiness. This project focusses on the brittle fracture resistance of laboratory steels from this family, for various chemistries and processing conditions. To identify quantitative fracture criteria, the values of critical cleavage fracture stress have been determined for each condition by combining low temperature tensile tests on notched specimens and mechanical analysis by the finite element method. On the other hand, the effects on the characteristic microstructural lengths, carbon distribution and retained austenite stabilization have been investigated to explain the variations in brittle fracture resistance.

Detection and simulation of viscoplastic instabilities in rotating disks (rotor integrity)



During the design of turboshaft engines, regulation rules impose on manufacturers to demonstrate the integrity of rotating parts with over-speed experiments: parts should not burst under mechanical and thermal loads below the rotation speed imposed by the regulation. This requirement guarantees a minimal safety margin depending on the operating conditions.
The goal of the Ph.D is to determine failure criterion by viscoplastic instabilities on rotating disks. Elastoviscoplasticity properties of super-alloys have to be taken into account under temperatures that range from 20 to 500°C. These laws have to be integrated into finite element simulations of plastic deformations under extreme rotational speed conditions.
The approach consists on, first, complete the experimental basis in order to identify the material parameters for INCO718; second, define a local fracture criterion which leads to the fracture of three super-alloys (INCO718 and Udimet 720) under temperature and triaxial loading similar to those on rotors, and finally, establish a verification procedure of the criterion than can be directly applied in production control.

Modelling stress corrosion cracking initiation of cold-worked austenitic stainless steels



Low carbon austenitic stainless steels, such as 316L steel, have been selected as materials in contact with the primary environment of pressurized water reactors (PWRs) in nuclear power plants because of their good resistance to uniform corrosion at high temperatures. However, cases of intergranular stress corrosion cracking (SCC) on cold-worked stainless steels in PWRs were reported.
To better understand the link between corrosion, microstructure map, local mechanical fields and cracking network and to identify a failure criterion for oxidized grain boundaries by taking advantage of this link, SCC tests are performed in a primary water environment on cross-shaped specimens. This geometry is used to apply a loading path change, considered more severe in terms of cracking than a monotonous loading.
The microstructural fields and local deformation fields are followed throughout the various loading steps and correlated to the cracking network obtained at the end of the test. As both stress concentration and intergranular oxidation are supposed to play a key role in SCC initiation, the mechanical stress fields at uncracked and cracked grain boundaries are computed by crystal plasticity finite element (FE) simulation of polycrystalline aggregates generated from EBSD (electron backscattered diffraction) analyses, the experimental displacement fields being used as boundary conditions. In addition, the intergranular oxidation depths for uncracked grain boundaries and crack depths for cracked boundaries are expected by focused ion beam (FIB) cross-sectioning and imaging.

Study of rhenium containing nickel-base superalloys for single-crystal turbine blade applications

Vincent HULEUX

The properties of nickel-base superalloys have always been evolving with the aim of improving their mechanical resistance, microstructural stability, oxidation and corrosion resistance, as well as their microstructural compatibility with thermal barrier coatings.
These evolutions have led to the introduction of elements such as rhenium and ruthenium into nickel-base superalloys.
It is now widely admitted that the addition of rhenium in superalloy significantly improves creep properties at high temperature. However, the creep strengthening mechanism, brought by rhenium, still remains doubtful and needs to be clarified.
In this work, five commercial superalloys, different in their rhenium content (various generations, from 1st to 3rd), are investigated on the basis of multiscale microstructure analysis and creep tests.
The creep test temperatures are chosen to cover a wide range of applications for these alloys as well as to be able to investigate different deformation mechanisms occurring at low (760 °C), intermediate (950 °C) and high temperatures (1050 and 1150 °C). A number of those tests was stopped at different stages of the creep deformation in order to analyse the relevant microstructures. The microstructure evolution, particularly the rafting of γ' precipitates, and the formation of Topologically Closed Packed (TCP) phases regarding temperature and stress are studied using SEM. Additional chemical analyses of γ and γ’ phases and the fine analysis of dislocation microstructures is carried out by observations performed by TEM.
Correlations between microstructural deformation mechanisms and rhenium content are attempted. Substantial microstructural transformations (γ' rafting and TCP precipitation) are also studied by TEM and considered in the interpretation of the effect of rhenium on creep strength.
Moreover, optimised heat treatments were performed to study the defect effect on creep lifetime and microstructure evolutions. This will lead to conclude about the interest of optimising heat treatments, particularly the solution heat treatment, by removing all the eutectic aggregates and decrease segregation.

From constituent properties to the mechanical behavior of composite structures

Faisal ISLAM

The aim of the project is to advance the state-of-the-art composite failure models which are strongly overdesigned and develop advanced and industry-friendly characterization techniques for measuring the required input data. The three key data are: fibre strength distribution, matrix properties and interfacial properties. The previous work of the project partners has identified clear drawbacks in the conventional single fibre tests for measuring the fibre strength distribution. The objective now is to develop and compare methods for efficiently measuring fibre strength distributions at both long and short gauge lengths and to achieve a breakthrough in the reliability of methods for measuring input data and spread the gained knowledge to institutes all over the world.
In order to extract useful and reliable information about the fibre behavior from the obtained experimental results, some statistical analysis would be done using both classical frequentist approach and recent Bayesian approaches. This would lead to development of useful statistical tools for experimental data analysis.
Effect of matrix and interfacial properties on the composite would also be analysed. Interfacial adhesion strength would be quantified using improved versions of present models using data obtained from single fibre fragmentation tests.

Mechanical design of a cylinder block by continuous modeling from process to lifetime


The cylinder block has many functions that result in a geometric complexity requiring a die casting process with prior insertion of cast iron sleeves in the mold. Most of the time, cylinder block’s failure means the ruin of the powertrain and thus, a special attention is paid to the control of the manufacturing process and the mechanical design protocol. The project aims to provide a robust and continuous modeling of the aluminum alloy behavior during the die casting process and later, under conditions of use.
The first step will be to analyze loadings applied to the structure as well as to characterize the metallurgical state of the material. The second phase of the work will then focus on the development of a mechanical characterization strategy to characterize the behavior during and after quenching. These "extreme" test conditions with respect to the melting temperature of aluminum will require the development of original experimental protocols.
Furthermore, a constitutive model allowing to predict the mechanical response of the structure will be developed on the basis of collected experimental data. The required model must take into account the stress relaxation phenomena over a very wide range of temperature and strain rate. A predictive representation of the deformed shape resulting of manufacturing and evolution during service life must indeed be obtained. Finally, the model will be integrated into the PSA company calculation tools and will be validated by comparison with the measurements made on a real instrumented component.

Effect of filler metal on the fracture behavior of welded joints of high strength steel



High-strength steels are of great importance in modern structural parts. These materials possess an excellent combination of high strength and toughness, suitable for many applications such as pressure vessels. The use of these steels in welded structures requires developing consumable electrodes which can be used to deposit weld metals with similar properties as the base metal. In particular, the fracture resistance of the weld metal should be at least that of the base metal. The weld metal microstructure, especially in a multipass weldment, is complex due to the inheritance from reheating and tempering of weld beads. This research project aims at improving the current understanding of the impact toughness of weldments made of high-strength steel.
Starting from an experimental campaign, the chosen approach relies on the quantification of microstructural parameters relevant to the cleavage fracture resistance of selected welds. This will further enable the improvement of existing knowledge on the link between the microstructure of the weld metal and its resistance to cleavage fracture. The knowledge acquired during this project will help in the development of new filler materials.

Ductile damage study for complex loading paths and low stress triaxiality via 3D synchrotron imaging and FE simulations

Xiang KONG


In the pursuit of lighter materials and optimized thin-walled components for transportation, knowledge about the characteristic ductile damage mechanisms in metal sheets is key. As the study of damage in materials has, up to now, been focused on proportional loading, the strain-damage interaction is not understood for the highly application-relevant bi-axial loading with load path changes.
Thus, our proposal aims at three-dimensional (3D) imaging of the microstructure inside flat sheet specimens evolving during material testing under such loads. For this, we extend the capability of in situ synchrotron laminography to overcome inherent shortcomings of other 3D techniques for such kind of samples. It shall be developed into a unique multiscale imaging approach from a few hundred micrometres resolution down to the nanometre scale to determine the ductile damage nucleation and growth kinetics. Strain in the material bulk will be measured using digital volume correlation. Such hierarchical 3D data will then serve as valuable input for microscopic simulations and the formulation and validation of continuum damage models suited to predict engineering-relevant mechanical properties.

Study of the multi-axial fatigue damage mechanisms for a glass fiber reinforced thermoplastics (PA66GF30)

Fang LU

Thermoplastics reinforced with fibers which have a good compromise of density and performances are increasingly used to replace metals in both aeronautical and automotive applications. In particular, the use of these materials under the hood allows weight reduction in vehicles and as a consequence a reduction of energy consummation and CO2 emissions. In order to be able to predict the lifetimes of such structures under their condition of service (multi-axial dynamic loads, temperatures up to 100°C, humidity), it is necessary to bring a better understanding of the damage mechanisms which can arise and the constitutive equation which describe them. Then, this study aims to predict the behavior of a given structure under cyclic loading by establishing the damage criteria related to the evolution of the microstructure:

  • at macroscopic scale : biaxial tensile fatigue tests were realized in order to predict the fatigue life and to quantify the parameters of a behavior law based on a visco-elasto-plastic damageable rheology model, as well as to take into account the influence of temperature/humidity/solicitation frequency and the influence of internal variables of the material which are responsible for its damage.
  • at microscopic scale : in order to improve the understanding of the damage mechanisms of discontinuously charged polymer composites and to bridge micro-scale damage and macro-scale behavior of composite, fatigue crack was interrupted and was imaged post-mortem by synchrotron X-ray tomography.

Influence of dissolved oxygen on the oxidation and stress corrosion cracking of cold-worked austenitic stainless steels in the primary water of pressurized water reactors


As of 2017, Pressurized Water Reactors (PWR) represent 69% of operating nuclear power plants worldwide. Experience on those reactors shows cases of Stress Corrosion Cracking (SCC) affecting cold-worked stainless steel components in the primary circuit. SCC is a degradation phenomenon caused by the interaction of the environment and the applied stress on the material.
In this context, oxygenated transients are reactor operating steps during which dissolved oxygen is present in PWR primary water. These oxygenated transients are considered as a possible detrimental factor of the SCC phenomenon. In nominal conditions, on the contrary, PWR primary water does not contain any dissolved oxygen.
The global aim of this work is to study the influence of oxygenated transients on the SCC susceptibility in PWR primary water of a cold-worked 316L stainless steel, with a special attention paid to the coupling between strain localization and oxidation.
For this purpose, slow strain rate tensile (SSRT) tests are performed on tensile pre-strained specimens in nominal or oxygenated PWR primary water. The resulting cracking networks are correlated to the microstructural and local strain fields obtained by electron backscatter diffraction (EBSD) and digital image correlation (DIC), respectively.
In parallel, the effect of oxygen on oxidation is studied on non-strained and cold-worked specimens, for both water chemistries. To this aim, the kinetics, structure and composition of the oxide films and oxide penetrations (grain boundaries, slip bands) are characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).

Creep behaviour at high temperature of a stabilized austenitic steel for gen-iv nuclear power plants


ASTRID is a fast-reactor prototype for the 4th generation of nuclear power plants. The material used for fuel cladding is a cold-worked austenitic stainless steel called AIM1. In case of incidental situations, the cladding might rapidly reach higher temperatures (700-950°C) where its stability might be affected.
Isothermal creep tests up to 1000°C under a wide range of stress levels enable to study viscoplastic flow, microstructural evolution under stress and damage/failure processes. In order to evaluate the effect of loading, microstructural characterizations (precipitation, recovery, and recrystallization states) on stress-free thermally-aged samples were also performed and compared with post-mortem examinations of creep specimens.
Up to 750°C, AIM1 shows better creep strength than previous generations of 15-15Ti grades. Beyond 750°C, dislocation mobility increases which promotes recovery and recrystallization processes. As a consequence, competition between work hardening due to viscoplastic deformation and softening due to dynamic recovery takes place. At 950°C, viscoplastic flow is strongly affected by recrystallization during creep test, especially in the tertiary stage. Softening due to recrystallization leads to longer tertiary stages and higher ductility during tests under lower stress levels (about 40 MPa applied stress). Ductile fracture predominates at any temperature.
Characterizations on thin foils and carbon extractive replicas showed a large variety of precipitates, such as Cr-rich borides, phosphides, and Cr- and Ti-rich carbides.

Inclusions and Ti+Nb microalloying element precipitation in Press Hardened Steels: towards improvement of in-use properties



The automotive industry is facing two major challenges. On one side, vehicles must reduce their CO2-emission and, on the other side, standards for crash resistance are becoming more and more severe. The use of very high strength steels with improved ductility appears to be a suitable solution for structural lightweighting and improved crashworthiness behavior. One of the most commonly used steel families are the so-called PHS (Press Hardened Steels). These steels are delivered coated with low mechanical properties at hot stampers to be hardened by die-quenching before being used by carmakers. The hardening treatment is obtained by a first austenitizing treatment at 900°C followed by a rapid quench which transform the austenite into martensite. Formation of 0.1-0.3%C martensite microstructure results in the production of a high strength steel solution (between 1000 and 2000 MPa). These steels are now being intensively used in the body-in-white structures, in particular the Usibor® and Ductibor® families developed by ArcelorMittal. The development of these widely marketed materials is subject to a strong international concurrency.
One way currently investigated to improve their mechanical strength consists in introducing microalloying elements such as titanium which is able to protect boron against nitride formation and consequently, to retain its ability to refine austenitic grain size. Furthermore, the addition of microalloying elements is also used for grain size refinement either by solute drag or by grain boundary pinning by precipitation of fine (Ti,Nb)C. These additions come, however, together with a
coarser precipitation formed at the beginning of processing mostly during the solidification. This precipitation might affect their resistance to crack initiation, during tests such as bending tests which are used as a reference to evaluate their energy absorption through the calculation of fracture strain. The two interesting aspects in our study on crash ductility will be the energy absorption and the anti-intrusion resistance. The limited link established between the amount of these chemical elements, the surrounding martensitic microstructure, the form of microalloying elements (precipitates and solid solution) and ductile cracking resistance during bending is thus of prime importance. More generally, the project aims at improving the crashworthiness of these steels by providing process guidelines on the industrially used quantities of these elements.
The major industrial objective of this PhD project is to optimize the chemical composition of PHS with respect to their population of inclusions in order to improve their in-use properties, and then set up tools to enhance our comprehension of the mechanicals phenomena involved. The associated research work combines metallurgy, mechanics and modelling, aiming at a better understanding of :

1/ The effects of microalloying on inclusion formation and precipitation (amount, size and spatial distribution) as a function of carbon and possibly of nitrogen.
2/ The consequences of the addition of these microalloying elements on the other microstructural constituents.
3/ The link with in-use properties after the hot stamping step.
4/ The fracture mechanisms, in order to establish a fracture criterion taking chemistry-induced microstructural effects into account.

Characterization and modelling of welded short fiber reinforced polymer mechanical strength


In automotive industry, more and more pieces traditionally made from metal are now made with short fiber reinforced polymers in order to diminish weight and cost. However some pieces have hollow and complex shapes which prevent them to be injected in one operation. Those pieces must be injected in several parts that are then joined together by welding. Although, welding operations are efficient for pristine polymers, it is not the case for glass fiber reinforced polymers. In fact, when fibers are added to the polymer, failure of the pieces generally occurs in the welded zone. Thus, the aim of this PhD project is to better understand the mechanisms governing the weld strength of these materials in order to reduce safety factors in the development of new products.
First part of this work consists in studying the microstructure of several materials samples welded with different parameters. For this purpose, microscopic and X-Ray tomography analyzes will be performed on samples to determine if fibers reorientation or cavities growth in the matrix occur during the welding operation. The evolution of matrix crystallinity in the thermally affected zone of the weld will also be studied. This can be done by DSC and FTIR measurement or SAXS/WAXS analysis. Then, the mechanical strength of welded samples in monotonic and creep tensile tests will be performed in order to link microstructure observations with mechanical performances. The aim is to understand why, as read in literature, it is not possible to achieve very high strength value with reinforced polymer while pristine polymer can be welded without any loss of mechanical properties. Finally, after having determined the most influencing factors, modelling of the mechanical strength of welded materials will be proposed.

Fatigue failure in fibre-dominated CFRP composite laminates

Fabrizio PAGANO

Carbon fibres reinforced plastic (CFRP) composite materials are being introduced in primary load-bearing structures. Reducing design margins for static loadings increases the stress state into the laminate. Under repeated loadings, these severe stresses may affect the lifetime of composite materials, inducing fatigue degradation and consequent premature failures.
For most multidirectional composite laminates, the longitudinal plies govern the fatigue life performance of the laminate. The fibres give almost the entire contribution of the stiffness and strength of the ply. Thus, the failure of the 0° plies, which are essentially driven by the fibres behaviour, causes the final failure of the laminate.
The objective of this work is to study the fatigue evolution of fibre breaks inside the longitudinal plies of unidirectional and multidirectional laminates subjected to cyclic tension load, and the consequent fatigue failure of the laminate. Tension-tension low cycle fatigue tests are realized on different stacking sequences of a CFRP. The damages evolution is then analysed in both on-axis and off-axis plies and their interaction in terms of stress transfer mechanisms are followed.

Characterization of deformation mechanisms in polymer and composite materials via three dimensional X-ray imaging and diffraction


PEKK (Poly-Ether-Ketone-Ketone) is a high-performance polymer material developed by Arkema. This material is destined to be used as the polymer matrix in future composite materials in highly demanding areas of application such as aerospace and offshore energy, industries requiring replacements for metallic materials and structures. Such composite materials promise weight savings while maintaining excellent mechanical and industrial properties (lightweight, strength at elevated temperature, formability and recyclability). The material is used in a semi-crystalline condition and the crystalline part is crucial for keeping mechanical strength with increasing temperature.
Polymer materials experience multiple damage processes. In situ testing combined with tomography allows the study of porosity and cavitation once pores reach a sufficient size, but this corresponds to a very advanced state of damage. The aim of this subject is to explore also, in the same experiment, prior microstructural changes revealed by diffraction with a spatially-resolved approach. Several key parameters such as degree of crystallinity, polymer chain orientation, crystallite fragmentation... are determinant for mechanical properties and lifetime but are not visible by absorption tomography. The understanding of the evolution of these parameters during deformation is fundamental for better lifetime predictions and the design of polymers and polymer-based composites.

Understanding and modeling of fracture toughness tests with pop-in: application to 6061-T6 aluminum and neutron irradiation influence



This study investigates the so-called ‘‘pop-in’’ phenomenon, i.e. unstable but limited crack propagation associated with a sudden decrease in load. Its origin is linked to an interaction between material tearing modulus and machine stiffness. A small scale yielding criterion from the literature is applied to compact tension (CT) samples. To validate this criterion experimentally, firstly tests are performed on aluminum alloy with various tearing moduli. Secondly the effect of mechanical loading on pop-in behavior is studied by varying machine stiffness using an innovative setup to vary system compliance. Pop-ins are shown to be mechanical instabilities due to interaction between material tearing modulus and system stiffness.

Development of advanced multidirectional composites


Fiber reinforced polymer composites are finding increasing uses during the last four decades in high technology as well as conventional applications. Selection of the reinforcement materials for a typical application depends on various factors such as specific stiffness, specific strength, toughness, dimensional stability, corrosion resistance, weight and cost. A single reinforcement system in general does not fulfill all the favorable criterion for a typical application. Hybrid fiber reinforcement composites hence are the best solution for such applications. Using hybrid reinforcements is not a new topic among researchers, but in the current scenario where the applications of composites have been vastly widened, newer combinations of fibers and their fabric manufacturing processes are gaining lot of interests. The most important driving factors for most studies today including this PhD study are: limiting overdesigning of composites, increasing scope and area of utilization of composite materials, tailoring the design of composites for specific applications and possible saving in cost and weight.
The present thesis hence will include following novel studies :

  • Studies on process of fiber spreading for glass rovings and comingled carbon/glass rovings which so far have not been used in the spreading process.
  • Manufacturing and optimizing of non-crimp fabrics (NCF’s) using hybrid reinforcements. Introduction of thin plies in hybrid composites is a very new concept.
  • Analytical modeling of hybrid multidirectional composites to predict the stiffness of the composites
  • Comparative study on type of hybridization; intra-layer, inter-layer and intermingled hybrids for the selected fiber combination and characterizing their mechanical properties those including tensile, flexural and impact behavior.
  • Cost performance analysis for different fiber hybrids and their manufacturing processes.

Study of processing/structure/properties relationships on aramid fiber at filament scale

Clotilde RICHARD


Technical fibers, and particularly the aromatic polyamide (commonly known as aramid), are frequently used as reinforcements in demanding applications. The processing conditions of aramid fiber strongly influence its response mechanisms once integrated into the final object. During twisting and thermal treatment steps, the fiber is subject to various thermomechanical stresses that modifies its properties.
The aim of this thesis is to understand how the fiber structure and its mechanical properties evolve during processing steps at filament scale. Mechanical characterization at the single fiber scale is challenging especially when the diameter is as small as 15 μm; but it’s essential to be able to optimize the performance of technical fibers in final object.
Fibers mechanical properties are linked to their highly oriented structure, therefor they are strongly anisotropic. Two major experiments will be used to study them: longitudinal tensile tests and single fiber transverse compression test. The physico-chemical characteristics of the fiber will be also investigated through various ways, e.g. DRX, TGA, DSC…

Modelling the long-term behaviour of carbon fibre composites


The starting point of the thesis is a multiscale modelling approach for tensile failure of unidirectional fibre-reinforced composites, developed by Alain Thionnet and Anthony Bunsell. At the base of the modelling stands the realization that composites are time-dependent materials. To account for it, stochastic character of fibre strength and matrix viscosity are taken into account. This allows a successful representation of phenomena observed in real-life composite structures, such as filament-wound composite pressure vessels.
The objective of the thesis is to work on phenomena that have not been accounted for so far by the existing model. For instance, large porosity ratios present in some pressure vessels can have an important influence on mechanical properties. Tomography allows a characterization of voids present in real-life structures and together with numerical modelling provides a deeper understanding of the influence they have on damage processes taking place in the composite.
Another important factor is the environmental conditions. Composites are often used in parts of the world where extreme temperatures or high humidity are common. Taking this into account is necessary to correctly assess the safe operating conditions of such structures. This is particularly important in light of the rapidly increasing demand of the transport industry for light-weight, composite-based structures.
This research is done within the framework of the FiBreMoD project and has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement N° 722626.

Linking the microstructure to the ductile-to-brittle transition temperature of high strength quenched and tempered steel


High strength quenched and tempered steel are largely used for manufacturing of off-shore drilling parts and some components of automobile gear boxes. Because of their mixte martensite-bainite microstructure, this steel has a good trade-off balance of mechanical properties. Consequently, they are used for structural applications that require high strength, high wear resistance and relative good ductility.
The aim of this study is to find the link between microstructure, mechanical and fracture properties of the studied steel. An order aspect of the study is the development of the predictive tool of the impact toughness for this grade of steel. Thus, this study has microstructural, experimental and numerical aspects. From the as quenched and tempered cylindrical bar, we have realized two tempering at different temperatures in order to modify the microstructure, by improving the precipitation of carbides while varying the contrast between matrix and carbides. All the microstructures obtained are being analyzed (microstructures observations and mechanical tests). At this time, the results about the microstructure, carbides characterization in particular, have given good informations about the link between upper self-energy (on Charpy impact test) and the microstructure. The results of mechanical characterization are also very interesting in term of the link between tempering temperature and mechanical behavior of the different microstructures.
Finally, numerical calculations are performed, based on the experimental results of mechanical tests. These calculations will allow the identification of the constitutive equations of the steel and the development of ductile fracture criterion for these steels.

Crack growth under multiaxial fatigue loading leading to large scale yielding


For very high temperature conditions and high mechanical loading, as found for instance for combustion chambers in engine aircraft, cracks could initiate and propagate near perforated zones. In particular, we are interested in the evaluation of the superalloy HAYNES® 188 and its degradation by fatigue crack growth. For such component and material, damage is the result of a complex set of phenomena: multiaxial fatigue loading, thermal gradients and large scale yielding under oxidizing environment. Fatigue crack growth has been experimentally analyzed and modeled for high temperature and large scale yielding for this kind of
material but rather limited to uniaxial macroscopic loading. This study consists in experimentally characterizing and modeling crack propagation under 2D in plane macroscopic loading representative of in-service loading conditions, including high temperature and large scale yielding. The design of such an experiment is a challenging issue to control both thermal loading and strain/stress within the gage length. We first focused on the design of such an experiment using cruciform notched specimen heated by a new induction coil processed by laser beam melting. Additive manufacturing has allowed optimizing the geometry of the coil enabling the control of thermal field in both homogeneity and control of positioning (centering and parallelism to the gage length). Specific LVDT has been used to control cyclic displacement applied to the gauge length. Subsequent thermal and displacement fields have been measured using respectively Infrared thermography and digital image correlation. These fields are needed to prescribe relevant thermal and displacement boundary condition for a 3D FEA. Optical microscopy was also used to measure in situ the crack tip location. Typical loading was achieved by testing fatigue crack growth successively under equibiaxial and macroscopic shear loading for either force or displacement control. The obtained fatigue crack growth database tests the influence of large scale yielding under biaxial loading on fatigue crack growth mechanisms, fatigue crack growth rate as well as the mechanical condition of crack bifurcation. As a first attempt, previous model identified on uniaxial loading is tested with neither modification of the model nor modification of its constitutive parameters. This validation was successfully performed by post-processing of elastic and plastic energy obtained by 3D FEA in the vicinity of the notch loading by non-local approach. This post-processor does not take into account explicit meshing of the crack growth. A second attempt is proposed, using conform remeshing and a new version of the FCGR model, also based on the plastic and elastic strain energy partitioning, and again using a non-local approach for a 2D analysis. Experimental results and assessment of FCGR will be finally discussed so as to determine driving parameters for fatigue crack growth under large scale yielding condition for biaxial fatigue loading. The limitation of the proposed models will be also discussed.

Development of new damage-tolerant titanium alloys


TRIP/TWIP (transformation-induced plasticity and twinning-induced plasticity) titanium alloys are amongst most advanced and promising structural materials because of their exceptional stability against strain localization, due to the unusual combination of mechanical properties (high fracture elongation around 40 % and exceptional strain hardening ability, TS/YS > 2).
This enables opening potential applications of titanium alloys to new kinds of requirements, in order to address the high industrial demand in lightweight components (40 % lower density than austenitic stainless steels). Moreover, in contrast to the wide family of titanium alloys, these new alloys additionally possess a high damage resistance (Charpy impact toughness of 194 J.cm-2 and fracture toughness (CT specimens) of 145 MPa.m1/2). The first stages of the
Tensile direction
35 μm
alloy design have been successfully achieved and these alloys have been patented for aircraft applications in collaboration with TIMET and SAFRAN companies.
This PhD project deals with highly-hardenable and ductile β titanium alloys, which deformation mechanisms and microstructure evolution remain unclear. Indeed, the deformation occurs by dislocation slip, twinning and phase transformation, and a synergy between them seems to be existing.
Considering this, the resistance to crack propagation is investigated after optimizing the grain size by hot rolling and heat treatments on the raw material. Various mechanical tests, such as uniaxial tensile tests, fracture toughness tests and fatigue tests, are processed in order to study the crack propagation in this constantly-evolving complex microstructure and microstructural evolutions due to strain. The effect of loading conditions on microstructural evolutions during tests and on corresponding mechanical response is investigated by SEM.

Accumulation of Fibre Breaks under Time-Dependent Loads of CFRP Pressure Vessels

Martinus Putra WIDJAJA

The understanding of damage mechanics on carbon fibre composite is critical in order to determine the reliability of pressure vessels. It has been found that the failure process starts with randomly distributed fibre breaks and as the loading continues, it will coalesce together into a cluster of fibre breaks. Sooner or later those clusters will cause a total failure. On the other hand, there are some interesting findings of new pressure vessels that have an altered reliability compare with the aged vessels. It is believed that the viscoelastic properties of the matrix is the reason behind this phenomenon. However, it is still being under investigation.
For the moment, there is no existing method to quantify the time-dependent effect on carbon fibre pressure vessels or composite pressure vessels (CPV) in general. Whether the existing model can be used or using another measurement method to gain more data is still in question. Therefore, it is required to find a suitable method to explain such phenomenon and even develop furthermore to give a strength criterion for certain types of pressure vessels. Finally, a hint for the scatter of such criterion would also become an added value to this research. In the end, it might be used for evaluating different composite materials, such as: glass, aramid, hybrid, etc.


Études en cours - MINES ParisTech


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