Academic literature on the topic 'Multiaxial dissipation'

Dissipation energy in viscoelastic solids under multiaxial loads On the basis of the three-dimensional constitutive equations for strains resulting from the Kelvin-Voigt's model and modified Hooke's law for multiaxial stress in viscoelastic solids, the formulae for the energy dissipated in a given time per unit volume have been derived. It is shown that after application or removal of triaxial static load there is no difference in the time functions governing the dissipation of strain energy of volume change and the dissipation of strain energy of distortion. Harmonic in-phase stress and harmonic out-of-phase stress as well as multiaxial periodic stress are also considered. It is demonstrated that in the process of energy dissipation due to normal and shear stress components the role of the latter is dominant.

Specific forms for both the Gibb’s and the complementary dissipation potentials were chosen such that a complete potential based multiaxial, isothermal, viscoplastic model was obtained. This model, in general, possesses three internal state variables (two scalars associated with dislocation density and one tensor associated with dislocation motion) both thermal and dynamic recovery mechanisms, and nonlinear kinematic hardening. This general model, although possessing associated flow and evolutionary laws, is shown to emulate three distinct classes of theories found in the literature, by modification of the driving threshold function F. A parametric study was performed on a specialized nondimensional multiaxial form containing only a single tensorial internal state variable (i.e., internal stress). The study was conducted with the idea of examining the impact of including a strain-induced recovery mechanism and the compliance operator, derived from the Gibb’s potential, on the uniaxial and multiaxial response. One important finding was that inclusion of strain-induced recovery provided the needed flexibility in modeling stress-strain and creep response of metals at low homologous temperatures, without adversely affecting the high temperature response. Furthermore, for nonproportional loading paths, the inclusion of the compliance operator had a significant influence on the multiaxial response, but had no influence on either uniaxial or proportional load histories.

Many materials under multiaxial periodic loading exhibit rate-independent internal dissipation per cycle. Constitutive modelling for such dissipation under spatially variable triaxial stresses is needed for calculating modal damping of solid bodies using computational packages. Towards a micromechanically motivated model for such dissipation, this paper begins with a frictional microcrack in a linearly elastic solid under far-field time-periodic tractions. The material is assumed to contain many such non-interacting microcracks. Single-crack simulations, in two and three dimensions, are conducted using ABAQUS. The net cyclic single-crack dissipation under arbitrary triaxial stresses is found to match, up to one fitted constant, a formula based on a pseudostatic spring-block model. That formula is used to average the energy dissipation from many randomly oriented microcracks using Monte Carlo averaging. A multivariate polynomial is fitted to the Monte Carlo results. The polynomial is used in finite-element simulation of a solid object, wherein modal analysis is followed by computation of the net cyclic energy dissipation via elementwise integration. The net dissipation yields an equivalent modal damping. In summary, starting from a known formula for a single crack, this paper develops and implements a method for computationally modelling the modal damping of arbitrarily shaped solid bodies.

The work reports the observed fatigue damage of C45 steel specimens tested in a previous work under multiaxial loading conditions and its relationship with the thermal energy dissipation which has been used in the last decades to estimate the uniaxial fatigue behavior of metals. For this purpose, fatigue data relevant to thin-walled samples made of quenched and tempered C45 steel tested under completely reversed combined axial and torsional cyclic loadings with different biaxiality ratios and phase-shift angles have been analysed. The analyses of crack paths at the initiation point of failure were performed after a 50% stiffness loss that corresponded to a crack size ranging from 7 to 15 mm; afterwards, the characteristic crack paths of each loading condition were analysed by using a digital microscope to identify the orientation of the crack initiation plane. After having broken all fatigue tested specimens under static tensile loading, the fracture surfaces were inspected close to the crack initiation point using a digital microscope. Despite the stress states and fatigue damage mechanisms dependent on the load condition, the Q parameter applied to the present experimental results proved to correlate all multiaxial fatigue test results in a single fatigue scatter band.

Fiber reinforced plastics are increasingly being used in the construction of primary structures in the aerospace and energy industries. While their elastic behavior and fatigue response have been the subject of considerable research, less is known about the performance of continuous fiber composites following initial damage. Several competing models for the post-damage response of orthotropic composite materials are explored in this thesis. Each of these models includes only the in-plane loads experienced by the material and characterizes damage based on the local state of strain. Starting with previous work performed at the Naval Research Laboratory and at MSU, the energy dissipated in multiaxially loaded coupons was used to optimize an empirical function that relates the three in-plane strains to the local dissipated energy density. This function was used to approximate a three dimensional damage initiation envelope as well as to quantify the severity of damage following first ply failure in a fiberglass laminate. Carbon fiber reinforced epoxy was characterized using an assumed bilinear constitutive response. The elastic properties of the material were first optimized to minimize deviation from experimental data and then the necessary coefficients for a per-axis strain softening response were found using a similar optimization. This model provides detailed insight into the residual strength of significantly damaged material, as well as dissipated energy as a direct consequence. To facilitate the need of these models for diverse local in-plane loading configurations, the MSU In-Plane Loader (IPL) was utilized. The tests performed in the IPL for this thesis were instrumental in validating a new image-correlation-based displacement monitoring system.

Les nanomatériaux sont actuellement largement utilisés dans le domaine bio-médical et jouent un rôle crucial dans les stratégies modernes pour remédier aux dysfonctionnements des tissus souples naturels tels que les tendons, les ligaments et les disques intervertébraux. Par ailleurs, les progrès de la biomécanique sont étroitement liés à l'élaboration de nouveaux biomatériaux tout en répondant à certains besoins spécifiques. Au cours des dernières décennies, une attention toute particulière a été portée à la combinaison de la nanotechnologie à d'autres domaines scientifiques dans le but d'obtenir de nouveaux biomatériaux avancés. Les matériaux souples renforcés par des nanoparticules inorganiques sont un exemple d'une telle combinaison entre la nanotechnologie et la science des biomatériaux. Ces biomatériaux peuvent imiter les propriétés chimiques, mécaniques, électriques et biologiques des tissus naturels. La présente thèse aborde le problème de la représentation constitutive multi-échelle du comportement inélastique multiaxial des matériaux souples renforcés par des nanoparticules inorganiques. La principale réalisation de cette thèse concerne le développement d'un modèle entièrement tridimensionnel, dans le cadre d'un traitement micromécanique, pour analyser la rupture, la capacité d'auto-guérison et les mécanismes de renforcement des nanoparticules tout en tenant compte des effets environnementaux. La représentation constitutive du système matériau est traitée à l'aide d'une cellule unitaire cubique contenant neuf nanoparticules ; une nanoparticule centrale relie huit autres placées aux sommets du cube via un certain nombre de chaînes polymères afin de tenir compte du rôle effectif des nanoparticules sur le comportement macroscopique non linéaire en grandes transformations. Les interactions directes en champ proche entre les nanoparticules et le réseau de chaînes sont physiquement décrites à l'aide d'une transition d'échelle micro-macro dans le cadre de la théorie de l'inclusion d'Eshelby. Le modèle considère explicitement le réseau de chaînes ainsi que les mécanismes réversibles de détachement / ré-attachement des liaisons dynamiques pour décrire de manière cohérente l'extensibilité extrême dépendante de la vitesse de sollicitation et certaines caractéristiques inélastiques, notamment la forte hystérésis lors des phases d'étirement-rétraction et la relaxation continue. Une évaluation quantitative de notre modèle est présentée à l'aide de comparaisons à des données expérimentales disponibles pour une variété de systèmes matériaux nanocomposites contenant une large gamme de concentrations de nanoparticules et pour différents modes de déformation lors de séquences de chargement monotones et cycliques. Le modèle s'avère capable de reproduire avec succès les différentes caractéristiques de la réponse multiaxiale macroscopique. Il est enfin utilisé pour mettre en évidence certaines informations clés sur les mécanismes de renforcement des nanoparticules et leur rôle sur la dissipation multiaxiale, la rupture multiaxiale et la capacité d'auto-guérison à température ambiante tout en tenant compte des effets de gonflement
Nanomaterials are currently widely used in bio-applications and play a crucial role in modern strategies to remedy malfunctions of natural soft tissues such as tendons, ligaments and intervertebral discs. Besides, progress in biomechanics is closely related to the elaboration of new biomaterials tailored to suit certain specifications. The combination of nanotechnology with other fields of science has attracted increasing attention during the past decades to get improved biomaterials. Soft materials reinforced by inorganic nanoparticles are an example of such a combination between nanotechnology and biomaterial science. These biomaterials can mimic the chemical, mechanical, electrical, and biological properties of native tissues. The present PhD dissertation addresses the problem of the multiscale constitutive representation of the multiaxial inelastic behavior of soft materials reinforced by inorganic nanoparticles. The main achievement of this PhD concerns the development of a fully three-dimensional model within a micromechanical treatment to analyze the failure, the self-healing facility and the nanofiller reinforcement mechanisms considering the environmental effects. The material system is representatively regarded as a cubic unit cell containing nine nanoparticles; a central nanoparticle connects eight nanoparticles placed at the cube vertices via a number of polymer chains to account for the effective role of nanoparticles on the nonlinear and finite-strain macro-behavior. The near-field direct interactions between the nanoparticles and the chains network are physically described using a micro-macro scale transition within the Eshelby inclusion theory. The model explicitly considers the chains network with dynamic reversible detachable/re-attachable mechanisms of bonds to coherently capture the rate-dependent extreme stretchability and some inelastic features including strong hysteresis upon stretching-retraction and continuous relaxation. A quantitative evaluation of our model is presented by comparisons to available experimental data of a variety of nanocomposite material systems over a wide range of nanoparticle concentrations for different modes of deformation upon monotonic and cyclic loading sequences. The model is found being able to successfully reproduce the significant features of the multiaxial macro-response. It is finally used to highlight some important insights on the nanoparticle reinforcement mechanisms and their role on the multiaxial dissipation, multiaxial failure and room temperature self-healing facility considering the swelling effects

Les travaux de recherche discutés dans ce manuscrit concernent la conception des générateurs de puissance électrique pour l'aéronautique. L’augmentation de la puissance massique de ces équipements passe par une augmentation des vitesses de rotation, donc une augmentation des contraintes. Un premier point est de s'assurer de la bonne tenue mécanique des matériaux. Un deuxième point est de pouvoir prendre en compte les modifications du comportement magnétique (et donc in fine du couple) lorsqu'ils sont soumis à un état de contraintes multiaxial. L’étude présentée vise en particulier à illustrer l’influence d'états de contraintes biaxiaux sur le comportement magnétique des matériaux constitutifs du rotor. Le défi repose sur la mise en place de méthodes de caractérisation du comportement magnéto-mécanique dissipatif uniaxial et multiaxial des nuances développées par Aperam et utilisées par Thales Avionics pour leurs applications aéronautiques (en FeCo-2V et Fe-3%Si à grains non orientés). Des essais non conventionnels seront effectués sur des échantillons en forme de croix de manière à s'approcher des contraintes réellement subies par le rotor. Les essais sont effectués sur la machine d'essai triaxiale Astrée du LMT-Cachan. L'état de contraintes est estimé par corrélation d'images et par diffraction des rayons X. Des mesures magnétiques anhystérétiques et de pertes d'énergie sous contraintes sont reportées. D'autre part, un modèle multi-échelle multiaxial, décrivant le comportement d’un VER à partir de l'équilibre énergétique à l'échelle microscopique sera présenté. L’approche est fondée sur la comparaison des énergies libres de chaque domaine. Une comparaison probabiliste est faite pour déterminer les variables internes que sont les fractions volumiques des domaines. Différentes stratégies envisageables pour modéliser la dissipation statique seront discutées. Puis nous présentons l’approche magnéto-élastique que nous avons retenue visant à une meilleure considération de l’effet de la contrainte sur le comportement des matériaux ferromagnétiques
The research presented in this thesis is motivated by the design of rotors for high speed rotating machines. The increased power density of these devices requires a higher rotation speed, leading to higher levels of centrifugal forces and stress in the rotor. A first point is to ensure good mechanical strength of the materials. A second point is to take into account changes in the magnetic behavior (and ultimately torque) when they are subjected to a multiaxial stress state. The present study aims at exploring the influence of biaxial stress states on the magnetic behavior of the materials of the rotor. The challenge lies in the development of methods for the characterization of the magneto-mechanical dissipative uniaxial and multiaxial behavior of metal sheets developed by Aperam Alloy and used by Thales Avionics for their aeronautical applications (in FeCo-2V and non-oriented Fe-3%Si). Non conventional experiments are performed on cross-shaped samples in order to apply biaxial stress representative of the loadings experienced by rotors of rotating machines. These experiments are performed on a multiaxial testing machine, Astrée. Stress level is estimated thanks to digital image correlation and X-ray diffraction Both anhysteretic and dissipative magnetic responses to magneto-mechanical loadings have been recorded. On the other hand, a multi-scale multiaxial model describing the behavior of a RVE from the energy balance at the microscopic scale is presented. The approach is based on a comparison of the free energy of each domain. A probabilistic comparison is made to determine the volume fraction of domains used as internal variables. Different strategies for modeling the static dissipation are discussed. Then we present the chosen magneto-elastic approach, improving the description of the effect of stress on ferromagnetic materials behavior

L'objet de ce travail est de proposer une approche multi-échelle de la fatigue fondée sur l'énergie, et susceptible d'estimer les durées de vie associées à des chargements multidimensionnels variables. Le fondement de la démarche consiste à supposer que l'énergie dissipée à petite échelle régit le comportement à la fatigue. À chaque point matériel, est associée une distribution stochastique de points faibles qui sont susceptibles de plastifier et de contribuer à la dissipation d'énergie sans affecter des contraintes macroscopiques globales. Ceci revient à adopter le paradigme de Dang Van en fatigue polycyclique. La structure est supposée élastique (ou adaptée) à l'échelle macroscopique. De plus, on adopte à l'échelle mésoscopique un comportement élastoplastique avec une dépendance de la fonction de charge plastique non seulement de la partie déviatorique des contraintes, mais aussi de la partie hydrostatique. On considère également un écrouissage cinématique linéaire sous l'hypothèse d'une plasticité associée. Au lieu d'utiliser le nombre de cycles comme variable incrémentale, le concept d'évolution temporelle du chargement est adopté pour un suivi précis de l'historique du chargement réel. L'effet de la contrainte moyenne est pris en compte dans la fonction de charge mésoscopique ; une loi de cumul non linéaire de dommage est également considérée dans le modèle. La durée de vie à la fatigue est ensuite déterminée à l'aide d'une loi de phénoménologique fondée sur la dissipation d'énergie mésoscopique issue du cycle d'accommodation plastique. La première partie du travail a porté sur une proposition d'un modèle de fatigie à gradient de mise en oeuvre plus simple que les précédents modèles
The aim of this work is to propose a multi-scale approach to energy-based fatigue, which can estimate lifetimes associated with variable multidimensional loading. The foundation of the approach is to assume that the energy dissipated on a small scale governs the fatigue behavior. Each material point is associated to a stochastic distribution of weak points that are likely to plasticize and contribute to the dissipation of energy without affecting global macroscopic stresses. This amounts to adopting Dang Van's paradigm of high cycle fatigue. The structure is supposed to be elastic (or adapted) on a macroscopic scale. In addition, we adopt on the mesoscopic scale an elastoplastic behavior with a dependence of the plastic load function not only of the deviatoric part of the stresses, but also of the hydrostatic part. Linear kinematic hardening is also considered under the assumption of an associated plasticity. Instead of using the number of cycles as an incremental variable, the concept of temporal evolution of the load is adopted for a precise follow-up of the history of the actual loading. The effect of mean stress is taken into account in the mesoscopic yield function; a law of nonlinear accumulation of damage is also considered in the model. Fatigue life is then determined using a phenomenological law based on mesoscopic energy dissipation from the plastic accommodative cycle. The first part of the work focused on a proposal for a fatigue model with a simpler implementation gradient than the previous models

Les élastomères sont utilisés dans les systèmes mécaniques pour assumer des tâches de suspension ou de liaison, comme dans le cas des pneumatiques. Les élastomères d’intérêt sont renforcés par des particules de noir de carbone. L’ajout de charges améliore certaines propriétés mécaniques telles que la raideur ou la résistance à l’abrasion, mais induit aussi un fort adoucissement de ces matériaux, dit effet Mullins, lors de l’étirement initial. Certaines applications soumettent des élastomères à des sollicitations extrêmes, pouvant provoquer une fissuration critique.Ce travail étudie l’impact de l’adoucissement par effet Mullins sur la propagation de fissures lors de chargements monotones.Des résultats expérimentaux préliminaires font ressortir des difficultés à caractériser la fissuration dans les matériaux étudiés. Une méthode d’analyse locale est développée, estimant les champs de déformation fortement hétérogènes auxquelles sont soumises les éprouvettes pré-entaillées. Ces observations valident des hypothèses permettant le calcul du taux de restitution d’énergie, qui caractérise la fissuration. Une campagne expérimentale est menée pour évaluer l’impact de précharges variées sur la propagation de fissures dans un élastomère chargé au noir de carbone. Afin d’expliquer certains résultats obtenus, l’équilibre énergétique théorique inhérent à la propagation de fissure est réévalué pour prendre en compte le caractère dissipatif de l’adoucissement. Enfin, les mesures expérimentales de la déformation locale sont ensuite exploitées pour compléter le bilan énergétique, en caractérisant la dissipation énergétique localisée due à l’effet Mullins
Rubber-like materials are currently used in machine design for suspension or connection functions, such as pneumatic tyres. The elastomers of interest are reinforced by carbon-black particles. The addition of these particles improves mechanical properties such as stiffness and abrasion resistance. However, it also leads to undesired strong softening of these materials, commonly known as Mullins effect, when first stretched. Elastomers can be submitted to extreme loading conditions according to the applications, generating critical crack propagation.This works studies the impact of softening caused by Mullins effect on crack propagation in filled rubbers submitted to monotonic loading.Some early experimental results point out the difficulties to characterize a crack propagation criterion. A local analysis is developed, allowing to study the highly heterogeneous strain fields witnessed when loading notched specimens. These observations lead to the validation of assumptions, which enable to calculate the strain energy release rate that characterizes the crack propagation. An experimental campaign was then performed to evaluate the impact of various preloads on crack propagation in a carbon-black filled rubber. In order to explain some of the results obtained, the theoretical global energy balance when the crack propagation occurs was revisited in order to take into account the dissipation caused by the Mullins softening. Finally, the experimental measures of local strain were used to complete the energy balance and characterize the localized energy dissipation due to Mullins effect

Les élastomères sont utilisés dans les systèmes mécaniques pour assumer des tâches de suspension ou de liaison, comme dans le cas des pneumatiques. Les élastomères d’intérêt sont renforcés par des particules de noir de carbone. L’ajout de charges améliore certaines propriétés mécaniques telles que la raideur ou la résistance à l’abrasion, mais induit aussi un fort adoucissement de ces matériaux, dit effet Mullins, lors de l’étirement initial. Certaines applications soumettent des élastomères à des sollicitations extrêmes, pouvant provoquer une fissuration critique.Ce travail étudie l’impact de l’adoucissement par effet Mullins sur la propagation de fissures lors de chargements monotones.Des résultats expérimentaux préliminaires font ressortir des difficultés à caractériser la fissuration dans les matériaux étudiés. Une méthode d’analyse locale est développée, estimant les champs de déformation fortement hétérogènes auxquelles sont soumises les éprouvettes pré-entaillées. Ces observations valident des hypothèses permettant le calcul du taux de restitution d’énergie, qui caractérise la fissuration. Une campagne expérimentale est menée pour évaluer l’impact de précharges variées sur la propagation de fissures dans un élastomère chargé au noir de carbone. Afin d’expliquer certains résultats obtenus, l’équilibre énergétique théorique inhérent à la propagation de fissure est réévalué pour prendre en compte le caractère dissipatif de l’adoucissement. Enfin, les mesures expérimentales de la déformation locale sont ensuite exploitées pour compléter le bilan énergétique, en caractérisant la dissipation énergétique localisée due à l’effet Mullins
Rubber-like materials are currently used in machine design for suspension or connection functions, such as pneumatic tyres. The elastomers of interest are reinforced by carbon-black particles. The addition of these particles improves mechanical properties such as stiffness and abrasion resistance. However, it also leads to undesired strong softening of these materials, commonly known as Mullins effect, when first stretched. Elastomers can be submitted to extreme loading conditions according to the applications, generating critical crack propagation.This works studies the impact of softening caused by Mullins effect on crack propagation in filled rubbers submitted to monotonic loading.Some early experimental results point out the difficulties to characterize a crack propagation criterion. A local analysis is developed, allowing to study the highly heterogeneous strain fields witnessed when loading notched specimens. These observations lead to the validation of assumptions, which enable to calculate the strain energy release rate that characterizes the crack propagation. An experimental campaign was then performed to evaluate the impact of various preloads on crack propagation in a carbon-black filled rubber. In order to explain some of the results obtained, the theoretical global energy balance when the crack propagation occurs was revisited in order to take into account the dissipation caused by the Mullins softening. Finally, the experimental measures of local strain were used to complete the energy balance and characterize the localized energy dissipation due to Mullins effect

Les lignes d’arbres de transmission navale sont soumises à des chargements cycliques complexes. Afin de caractériser en fatigue ces lignes d’arbres, deux objectifs complémentaires sont achevés. Dans un premier temps, les chargements des lignes d’arbres sont caractérisés afin d’identifier des cycles de fatigue pertinents. Lors de cette étape, une méthode paramétrique originale de dimensionnement en fatigue est mise en place. Cette méthode repose sur une modélisation des chargements avec une prise en compte de leur variabilité, de la contrainte moyenne et de la non-proportionnalité. Cette méthode montre qu’il existe deux modes d’endommagement avec un mode associé au régime cyclique établi de la flexion rotative et un mode associé aux manoeuvres du navire. Dans un second temps, une méthode de caractérisation rapide est mise en place afin d’étudier le comportement des aciers des lignes d’arbres en fatigue pour un grand nombre de configurations de chargements. Un modèle permettant d’identifier le comportement en fatigue à grand nombre de cycles, à partir des mesures d’autoéchauffements, est employé. Les aciers de l’étude sont caractérisés en traction-torsion pour diverses configurations de contraintes moyennes et de non-proportionnalité du chargement. Dans ce cadre, la notion de surface d’iso-auto-échauffement est introduite. Elle permet, pour une éprouvette, de modéliser le comportement élastoplastique et dissipatif du matériau dans l’espace dédié des contraintes. Leur utilisation permet de définir un critère de fatigue multiéchelle basé sur les invariants du tenseur des contraintes. Dans l’étude, une modélisation du comportement du matériau est proposée pour les chargements de très faible amplitude (fatigue à très grand nombre de cycles VHCF) montrant une différence forte de dissipation par rapport au régime d’amplitudes plus importantes (i.e. le domaine de la fatigue à grand nombre de cycles HCF)
Marine shaft lines undergo complex cyclic loadings. In order to characterize these structures in fatigue, two complementary objectives are achieved. Firstly, the marine shaft’s loads are characterized in order to identify relevant fatigue cycles. During this step, an original parametric fatigue design method is implemented. This method is based on the definition of an equivalent load considering multiaxiality, variability, non-proportionality and mean stress. This method allows to distinguish two damage modes with a mode associated with the established cyclic regime of rotary bending and a mode associated with the ship’s maneuvers. Secondly, a rapid characterization method is implemented to characterize in fatigue the marine shafts’ steels for a large number of loading configurations. The method is based on a model which enables fatigue identification behaviors from self-heating measurements. The steels of the study are characterized in tension-torsion for various configurations of mean stress and nonproportionality of the loading. In this context, the notion of iso-self-heating surfaces is introduced. It allows, for a specimen, to model the elastoplastic and dissipative behavior of the material in the dedicated stress space. Their use makes it possible to define a multiscale fatigue criterion based on the invariants of the stress tensor. In the study, a modeling of the behavior of the material is proposed for very low amplitude loads (VHCF) exhibiting a strong difference in dissipation compared to the regime of higher amplitudes (i.e. HCF domain)

Fatigue life assessment is the key task of the design of mechanical components subjected to service loads for avoiding failure occurring in the form of incipient cracks which may cause damages to the entire mechanical system or even worse to people. The nowadays industrial applications increasingly require mechanical components having complex geometry subjected to complex loading conditions. Considering the guidelines of the fatigue design of welded joints as an example, the standards report several stress-based fatigue design curves each one related to the most common welded structural details under a given loading direction. For this reason, by adopting the nominal approach, if the welded detail is different from those reported in the standards, the choice of the proper design fatigue curve might not be done for certain. This dissertation deals with fatigue assessment of metallic material and components by adopting local energy-based parameter which can quantify the local damage due to stress gradient caused by notches as well as defects. More precisely, the extension of the applicability of three energy-based approaches to several factors that influence the fatigue strength of material and components in addition to those already covered is the aim of the present dissertation. The first energy-based method, the so-called Peak Stress Method deals with the fatigue assessment of welded joints by means of a numerical FE-oriented application of the Notch- Stress Intensity Factors (N-SIFs). The equivalent peak stress (the fatigue damage parameter used for assessing the fatigue strength of welded joints) can be obtained by invoking the averaged Strain Energy Density (SED) criterion. The second one deals with the fatigue characterization of metallic component by assuming the specific heat loss per cycles as a fatigue damage parameter which can be evaluated in a standard constant amplitude fatigue test by adopting an easy experimental technique based on temperature measurements of material surface. The third one deals with the fatigue characterization of metallic materials produced by additive manufacturing, one of the most attractive and studied technology nowadays. Since these materials are affected by the presence of irregular defects, energy-related fracture mechanics approaches seem to be the most suitable for fatigue life assessment. In the first chapter, the state of the art of energy-based methods for fatigue and fracture mechanics characterization are described along with their theoretical frameworks. In the second chapter, the theoretical framework for extending the applicability of Peak Stress Method to the fatigue strength assessment of welded joints subjected to multiaxial loading conditions has been presented. Then, several multiaxial fatigue data taken from the literature relevant to both aluminum and steel welded joints were analysed by using the PSM for validating the theoretical prediction. The equivalent peak stress has shown to correlate with good approximation about all the experimental data. The third chapter deals with the analysis of the thermal energy dissipated during fatigue tests on severely notched AISI 304L stainless steel specimens. For the first time, the specific heat loss per cycle (Q parameter) was evaluated experimentally on 4-mm-thick, hot-rolled AISI 304L stainless steel specimens, characterized by 3, 1 and 0.5 mm notch tip radii by means of a FLIR SC7600 infrared camera during fully reversed axial fatigue tests. The new fatigue test results were successfully included in the existing heat energy-based scatter band previously calibrated on plain and bluntly notched specimens. Finally, an analysis of the heat energy dissipated around the notch tip has been presented and discussed with the aim of proposing a semi-automatic procedure to evaluate the thermal energy dissipated distribution. The fourth and fifth chapters deal with the analysis of the thermal energy dissipation on both AISI 304L and C45 steels specimens subjected to multiaxial loads. The specific heat loss per cycle was measured during constant amplitude multiaxial fatigue tests adopting two different phase shift angles of the applied loads and two biaxiality ratios. All the fatigue test results on both materials are in good agreement with the relevant scatter band previously calibrated except for the out of phase multiaxial fatigue results relevant to the AISI 304L steel. These results seem to be justified by the strain-induced martensitic transformation in metastable austenitic stainless steel, significantly present in out of phase cyclic loading condition. In the sixth chapter, the influence of the defect on fatigue behaviour of maraging steel specimens has been investigated. Axial fatigue tests were carried out on three batches of AMed maraging steel specimens produced by two different AM systems. Furthermore, axial fatigue tests were carried out on wrought maraging steel specimens both in annealed and in aged condition. After failure, the √area of the killer defects was examined by SEM observations of the fracture surfaces. A stress intensity factor-based design curve for all the test series was obtained taking into account the short crack effect by means of the El-Haddad-Smith-Topper model. Due to the lack of expensive experimental data to determine the relevant material length parameter a0, a novel rapid method to approximately evaluate a0 has been proposed. In particular, it consists in matching El-Haddad-Smith-Topper model with Murakami’s expression of the threshold range of mechanically short cracks. The advantage of the adopted engineering approach is that only Vickers hardness of the material is necessary. Theoretically, this rapid method can be also adopted to estimate the size of the control volume of the averaged SED approach due to the analogy of the latter to the material length parameter a0. In the end, the stress intensity factor-based design curve was adopted to estimate the fatigue strength of sharp V-shaped notches characterized by a reduced notch opening angle.

Cette thèse s’inscrit dans une démarche originale pour les matériaux composites stratifiés (verre/époxy) consistant à développer des protocoles expérimentaux adaptés aux essais de fatigue classique et aux essais d’auto-échauffement multiaxiaux pour trois types de sollicitations: Traction, cisaillement pur et traction-cisaillement combinés via un montage Arcan. Ces protocoles permettent de traiter des bilans locaux d’énergie et s’appuient sur des techniques d’imagerie qualitative et quantitative, en combinant deux techniques d’imagerie: la première technique concerne la thermographie infrarouge par La méthode de lissage 2D permettant d’étudier séparément les effets dissipatifs, et les effets thermoplastiques. Les effets dissipatifs sont les responsable de l’endommagement par fatigue de la structure. La corrélation d’images digitales a permis d’accéder aux champs cinématiques pour pouvoir estimer l’énergie de déformation localement mise en jeu sur un cycle de chargement, et de la comparer à l’énergie dissipée. Une étude qualitative a été faite sous tomographe pour caractériser les mécanismes de d’endommagement suite à un essai de fatigue pour les trois types de sollicitation
In this thesis the original approach for laminated composite materials (glass / epoxy) of developing experimental protocols adapted to test conventional fatigue and self-heating tests in multiaxial for three types of loading: traction, pure shear and combined tensile shear using Arcan mounting. This method has been adapted to rapid determination of damage threshold of laminate composites. These protocols allow to deal with local energy balance and are based on qualitative and quantitative image processing techniques. It combines two image processing techniques: Infrared thermography method and digital image correlation. The Infrared thermography method through 2D smoothing allowsstudying separately dissipative effects which are related to for fatigue damage of the structure and thermoelastic effects accompanying self-heating tests. While the digital image correlation gives access to kinematic fields and estimation of the strain energy locally into play on a loading cycle, and compare it to the energy dissipated. A qualitative study was made under tomography to characterize the mechanisms of damage during fatigue testing for the three types of stress

Abstract Guidance for the formulation of robust, multiaxial, constitutive models for advanced materials is provided by addressing theoretical and experimental issues using micromechanics. The multiaxial response of metal matrix composites, depicted in terms of macro flow/damage surfaces, is predicted at room and elevated temperatures using an analytical micromechanical model that includes viscoplastic matrix response as well as fiber-matrix debonding. Macro flow/damage surfaces (i.e., debonding envelopes, matrix threshold surfaces, macro “yield” surfaces, surfaces of constant inelastic strain rate, and surfaces of constant dissipation rate) are determined for silicon carbide/titanium in three stress spaces. Residual stresses are shown to offset the centers of the flow/damage surfaces from the origin and their shape is significantly altered by debonding. The results indicate which type of flow/damage surfaces should be characterized and what loadings applied to provide the most meaningful experimental data for guiding theoretical model development and verification.

Abstract The advent of additive manufacturing (AM) has enabled the prototyping of periodic and non-periodic metamaterials (a.k.a. lattice or cellular structures) that could be deployed in a variety of engineering applications where certain combinations of performance features are desirable. For example, these structures could be used in a variety of naval engineering applications where light-weight, large surface area, energy absorption, heat dissipation, and acoustic bandgaps are critical. Furthermore, combining the multifunctional design optimization of these structures with progressive degradation due to cyclic fatigue would create attritable systems with tailorable performances not yet in reach by current conventional systems. Nevertheless, in order to deploy these complex geometry structures their multiphysics response has to be well understood and characterized. The objective of the current effort is to describe an initial approach for designing a uniaxial fatigue specimen as the first step toward the design of a multiaxial fatigue test coupon. In order to compare bending- and stretching-dominated structures, two strut-based lattices made of Ti-6Al-4V alloy consisting of the octet and tetrakaidecahedron (or Kelvin) cells are examined. The specimens are designed to fail in the central gauge area where edge effects are minimized. Finite element results of the relevant structural mechanics are used to compare the performance of the four geometries and to evaluate the effect of relative density on fatigue life.