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This paper presents the interaction system within the mechanical soil processing process, consisting of two large elements, the metal of the tool and the soil. Due to the two main forces acting on the chisel knives—friction and impact with the sandy soil—the wear of these chisel knives was determined. To determine the wear, a stand was used which allowed testing chisel-type knives in laboratory conditions by changing their functional parameters: working depth, angle of the knives to work the soil, working speed, humidity and granulation of the test environment. The present paper presents an application of the Archard-type wear law to the contact between a chisel-type knife and sandy soil (wet and dry sand). The theoretical model regarding the Archard wear coefficient considered three forms of surface damage (shake down, ratcheting and micro-cutting). The sand was considered spherical and rigid and the surface of the knife was flat. The experimental model considered real steel knives with different surface hardness and operation under controlled conditions of sand granulation, humidity, attack angle, depth of penetration and speed of sliding. The theoretical and experimental results highlight the wear behavior of chisel knives (Archard coefficient) in wet and dry sand.

To mitigate state exposure threats to long-lived instant messaging sessions, ratcheting was introduced, which is used in practice in protocols like Signal. However, existing ratcheting protocols generally come with a high cost. Recently, Caforio et al. proposed pragmatic constructions, which compose a weakly secure ‘light’ protocol and a strongly secure ‘heavy’ protocol, in order to achieve so-called ratcheting on-demand. The light protocol they proposed has still a high complexity. In this paper, we propose the lightest possible protocol we could imagine, which essentially encrypts and then hashes the secret key. We prove it secure in the standard model by introducing a new security notion, which relates symmetric encryption with key updates by hashing. Our protocol composes well with the generic transformation techniques by Caforio et al. to offer high security and performance at the same time. In a second step, we propose another protocol based on a newly defined integrated primitive, extending standard one-time authenticated encryption with an additional output block used as a secret key for the next message. We instantiate this primitive firstly from any authenticated encryption with associated data, and then we propose an efficient instantiation using advanced encryption standard (AES) encryption to update the key and AES-Galois/Counter mode of operation to encrypt and decrypt messages.

Mechanical engineering structures and structural components are often subjected to cyclic thermomechanical loading which stresses their material beyond its elastic limits well inside the inelastic regime. Depending on the level of loading inelastic strains may lead either to failure, due to low cycle fatigue or ratcheting, or to safety, through elastic shakedown. Thus, it is important to estimate the asymptotic stress state of such structures. This state may be determined by cumbersome incremental time-stepping calculations. Direct methods, alternatively, have big computational advantages as they focus on the characteristics of these states and try to establish them, in a direct way, right from the beginning of the calculations. Among the very few such general-purpose direct methods, a powerful direct method which has been called RSDM has appeared in the literature. The method may directly predict any asymptotic state when the exact time history of the loading is known. The advantage of the method is due to the fact that it addresses the physics of the asymptotic cycle and exploits the cyclic nature of its expected residual stress distribution. Based on RSDM a method for the shakedown analysis of structures, called RSDM-S has also been developed. Despite most direct methods for shakedown, RSDM-S does not need an optimization algorithm for its implementation. Both RSDM and RSDM-S may be implemented in any Finite Element Code. A thorough review of both these methods, together with examples of implementation are presented herein.

Актуальність роботи полягає в тому, що врахування циклічної повзучості є надзвичайно важливим при проектуванні інженерних конструкцій та оцінки їх безпеки під час експлуатації. Це пов’язано з тим, що накопичення деформацій ратчетингу суттєво знижує втомну довговічність матеріалів за рахунок прискореного утворення втомних тріщин або вичерпання резервів пластичності. Крім того, навіть якщо руйнування не спостерігається, ефект ратчетингу може призводити до недопустимої зміни геометрії конструкції, що призводить до втрати її функціональної придатності. Об’єкт дослідження в даній роботі вибрано трубчасті зразки та трубні конструктивні елементи. Метою магістерської дисертації є дослідити різницю поведінки ефекту ратчетингу (циклічної повзучості) при різних умовах навантаження та об’єктах на основі експериментальних досліджень. The relevance of the work lies in the fact that consideration of cyclic creep is extremely important when designing engineering structures and assessing their safety during operation. This is due to the fact that the accumulation of ratcheting deformations significantly reduces the fatigue life of materials due to the accelerated formation of fatigue cracks or the depletion of plasticity reserves. In addition, even if the destruction is not observed, the ratcheting effect can lead to an unacceptable change in the geometry of the structure, which leads to the loss of its functional suitability. The object of research in this work is tubular samples and tubular structural elements. The purpose of the master's thesis is to investigate the difference in the behavior of the ratcheting effect (cyclic creep) under different load conditions and objects based on experimental studies.

Zaključna naloga raziskuje pojav cikličnega lezenja v izbrani aluminijevi zlitini, pri čemer nas je predvsem zanimalo nazadujoče oz. napredujoče ciklično lezenje. Naloga zajema pregled literature na področju lezenja in način izvedbe preizkusov, pri čemer se osredotoča na to, kako preizkuse izvesti glede na omejitve preizkuševališča in kako rešiti s tem povezane probleme, ki lahko nastanejo. Naloga obravnava tudi interpretacijo in primerjavo rezultatov, ki jih je zajel trgalni stroj napram tem, ki jih je zajel video ekstenziometer. The final thesis investigates the phenomenon of cyclic creep in a selected aluminium alloy, where we were mainly interested in the shakedown or ratcheting. The thesis includes a review of the literature on creep and how to perform the tests, focusing on how to perform the tests given the constraints of the test system and how to solve the associated problems that may arise. The thesis also deals with the interpretation and comparison of the results captured by the testing system versus those captured by the video extensiometer.

Prehistoric populations across North America seem to grow exponentially, with some variation between regions. Archaeologists have explored the differences somewhat, but have not explained the differences or the sustained growth with any reference to what may be going on under the surface in a way that is relevant to all regions. I propose that environmental limits on population are shaped by what populations eat and how they acquire food, and that when populations are large enough to feel the scarcity in their environment, they change their way of life in a way that increases those limits. The model I propose is well-established, and is called the Malthus-Boserup ratcheting model. I mathematically describe the Malthus-Boserup ratcheting model using a fixed population growth rate and an array of changes in environmental limit, both in the amount of change and rate of change (or how quickly my imaginary populations change their way of life). I then simplify my descriptions of these curves using exponential curves, as archaeologists might simplify real population growth curves in much the same way. I compare the models with their exponential descriptions to form expectations for what the values in exponential curves might mean regarding the archaeological record. Despite having set a constant population growth rate, the exponential curves grow at different rates, depending primarily on the amount of change in environmental population limits. I hypothesize that a list of regions assembled in order of largest to smallest change in population limits should match a list of the same regions assembled in order of exponential growth rate. I use tools in R statistical software to describe the population growth curves in four regions using both exponential curves and summed logistic curves. I then arrange the list of regions according to both the growth rate of the exponential curves and the change in the limits found in the logistic curves. The lists do not match, which suggests that, for a variety of reasons, the exponential curves do not adequately describe the underlying Malthus-Boserup ratcheting process. I then compare the models using the Bayesian Information Criterion. Bayesian Information Criterion is an indicator that increases with both the unexplained variation in the dependent variable and the number of explanatory variables used. In this sense, when comparing models describing the same data, lower values suggest either that information retained outweighs a model’s complexity, or a model’s simplicity outweighs the amount of information lost. In all four cases, the information retained in the Malthus-Boserup ratcheting model outweighs the model’s complexity. Furthermore, the summed logistic models have parameters that researchers can interpret beyond simple rates of change.

In an experimental study that simulated a fast breeder reactor (FBR) vessel near the coolant surface, it was reported that the long distance travel of temperature distribution causes a new type of thermal ratcheting, even in the absence of primary stress. When the distance of temperature travel is moderate, the accumulation of the plastic strain due to this mechanism is finally saturated. Through the large number of Finite Element Analysis, we have found the strong relationship between hoop-membrane distributions of accumulated plastic strain and residual stress in this saturated case. Focusing on this relationship, we have aimed to predict the saturated distribution of the plastic strain based on the residual stress distribution that is required for the elastic shakedown behavior. In this paper, based on classical shell theory, we formulated the plastic strain distribution that brings uniform hoop-membrane stress in the given region. And then, we compared the formulated strain distribution with the accumulated plastic strain distribution obtained by finite element analyses using an elastic-perfectly plastic material. As a result, we confirmed that the formulated strain distribution can be used as the prediction of the plastic strain distribution for the cases with moderate distance of temperature travel. For the cases with long-distance travel of temperature, the region with plastic strain expanded with the repetition of temperature travel. By considering the effect of this expansion, the formulated strain distribution can be used as conservative prediction of the accumulated plastic strain also for the cases with long distance temperature travel.

The diffusion of brake-shoe system has made it necessary to study brake–wheel–rail interactions. This study aims to elucidate this topic with an innovative small-scale test rig, the ‘4-contact machine’. A test procedure is designed in which an ER7 steel wheel is coupled with a R260 steel rail and two different brake-shoe materials (cast iron and sintered material). The results show that the 4-contact machine reproduces the effects of wheel–rail–brake shoe contact and damage similar to those observed on a railway wheel at the end of its life, highlighting marked differences in behaviour between the two brake materials.

Microalloyed steels have emerged to replace conventional plain-carbon steels to achieve longer wheel life on Chinese railroads. In this work, with the aim of preventing spalling, a mechanism that consists of ratcheting and shakedown theory correlated with steel properties is systematically investigated. Mechanical and ratcheting tests were carried out for microalloyed wheel steel to which vanadium was added in the range of 0–0.15 wt.% and the results were compared with that obtained for conventional plain-carbon wheel steel. The microstructure and precipitation were characterized via microscopy. As a result, the grain size was not obviously refined, and the pearlite lamellar spacing decreased from 148 nm to 131 nm in microalloyed wheel steel. Moreover, an increase in the number of vanadium carbide precipitates was observed, which were mainly dispersed and uneven, and precipitated in the pro-eutectoid ferrite region, in contrast to the observation of lower precipitation in the pearlite. It has been found that vanadium addition can lead to an increase in yield strength by precipitation strengthening, with no reduction or increase in tensile strength, elongation or hardness. The ratcheting strain rate for microalloyed wheel steel was determined to be lower than that for plain-carbon wheel steel via asymmetrical cyclic stressing tests. An increase in the pro-eutectoid ferrite content leads to beneficial wear, which can diminish spalling and surface-initiated RCF.

The ITER machine has been classified as a Basic Nuclear Installation (INB n°174) by the French nuclear regulator, which implies that it will be the first fusion reactor to go through the complete French nuclear licencing process. The combination of mechanic and electromagnetic phenomena with the heat loads caused by neutron streaming requires a multi-physics approach to the damage assessment; this has not yet been implemented in the common nuclear codes and standards. In most cases, structural integrity is justified within a linear elastic behaviour. When this route is not enough to respect the design criteria, several non-linear approaches to the material's mechanical behaviour can be considered, requiring a more elaborated demonstration of the design compliance. Nevertheless, the models proposed in the nuclear model database are sometimes not sufficient to properly describe the experimentally observed cyclic plasticity behaviour and, in particular, the ratcheting and shakedown phenomena. The ratcheting phenomenon is an old topic as many papers discuss this subject from the material characterisation up to the model development; most of the studies were performed by J. L. Chaboche and his collaborators. However, the application of a thermomechanical approach to assess this phenomenon remains rare: that is why we proposed to develop a fully coupled thermomechanical model.In the first step of our work, we carried out a mechanical test campaign, executed on the SS316L(N)-IG which is the ITER Vacuum Vessel (VV) structural material. This campaign was composed of several cyclic tension/compression tests performed with an imposed stress (almost non-existent in the literature) and cyclic torsion tests conducted with an imposed moment. The objective was to observe the material response under low cyclic fatigue, to better understand the shakedown (elastic and/or plastic) and ratcheting.In the second step of our work, we developed the material behaviour model, starting from a Chaboche’s model available in the literature (elasto-(visco)-plastic model, with various types of hardening). We then enriched it to explicitly take into account the influence of the temperature on the mechanical behaviour and, reciprocally, the influence of the mechanical behaviour on the temperature, i.e. the thermomechanical coupling. In addition, based on the observations made on the cyclic tests, we proposed to implement a damage variable on both kinematic and isotropic hardening behaviours.In the final step of our work, we implemented the model in the Abaqus® finite element software and simulated different scenarios including a simplified VV geometry, called the VV “rails”, which was tested under thermomechanical cyclic loading.; Le réacteur thermonucléaire ITER a été classifié Installation Nucléaire de Base (INB n°174) par l’autorité de sureté nucléaire française, ceci impliquant qu’ITER sera le premier Tokamak soumis à la réglementation française en vigueur des équipements nucléaires sous pression. Les chargements mécaniques et électromagnétiques combinés aux chargements thermiques induits par la réaction de fusion nucléaires nécessitent d’avoir une approche multiphysique de l’endommagement, ce qui, à ce jour ne fait pas parti des codes & standards nucléaires. La méthode de prévention de l’endommagement se justifie par la garantie de l’intégrité structurelle d’un composant. Les règles de conception suivent deux approches en particulier: la prévention d’un endommagement dû à un effort mécanique monotone et la prévention d’un endommagement résultant d’un chargement cyclique (thermique et/ou mécanique). Dans la majorité des cas, l’intégrité structurelle est justifiée au travers de méthodes analytiques et/ou éléments finis supposant un comportement élastique du matériau. Lorsque ces types de méthode ne suffisent plus à justifier une structure, d’autres méthodes basées sur des comportements non-linéaires du matériau peuvent être utilisées. Cependant, les modèles de comportement proposés dans les codes nucléaires, ne permettent pas de représenter avec précision des comportements observés expérimentalement, plus particulièrement, dans le cas de notre étude: le phénomène de rochet. Les experts matériaux et calculs d’ITER s’accordent à dire que le développement d’un modèle de comportement thermomécanique adapté au matériau ITER, est nécessaire pour affiner la prédiction d’endommagement dans un environnement nucléaire et multiphysique tel que celui d’un tokamak. Cet article décrit le développement d’un modèle de comportement non linéaire pour le matériau de l’enceinte à vide d’ITER (VV), incluant un couplage thermomécanique fort ainsi qu’une variable d’endommagement. Plus précisément, il s’agit d’un modèle type Chaboche (modèle elasto-(visco)-plastique) comprenant différent types d’écrouissages, qui a été amélioré afin de pouvoir prendre en compte l’effet de la température sur le comportement mécanique, et réciproquement, l’influence de la mécanique sur la température. Des essais mécaniques cycliques ont été réalisés sur des échantillons du matériau constitutif du VV, mettant en avant la déformation progressive, c.à.d., le phénomène de rochet. Le présent modèle a été testé sur un problème homogène, communément appelé analyse “0D”, dont les résultats ont été comparés aux essais uniaxiaux. La dernière partie de ce document décrit l’implémentation numérique dans un code éléments finis, permettant de tester le modèle sur un support soudé de la VV.

International audience; The ITER machine has been classified as a Basic Nuclear Installation French nuclear regulator (INB n°174), which implies that it will be the first fusion reactor to go through complete French nuclear licencing. The combination of mechanic and electromagnetic phenomena with the heat loads caused by neutron streaming requires a multi-physics approach to the damage assessment, which has not yet been implemented in the common nuclear codes and standards. The general damage prevention methodology consists in guaranteeing the structural integrity of a component. The development of design rules has mainly two origins: prevention of damage from monotonic mechanical loads and prevention of damage from repeated application of loads. In most cases, structural integrity is justified within a linear elastic behaviour but when this route is not enough to respect the design criteria, several non-linear approaches to the material's mechanical behaviour can be considered, requiring more elaborated demonstration of the design compliance. Nevertheless, the models proposed in the nuclear model database are sometimes not sufficient to properly describe the experimentally observed cyclic plasticity behaviour and, in particular, the ratcheting and shakedown phenomena. According to ITER community experts in materials and analyses, a thermo-mechanical behaviour model fitting the ITER Tokamak materials data will guarantee the best prediction of the damage considering a nuclear and a multi-physic loading condition. This paper describes the assessment of the non-linear behaviour of Vacuum Vessel (VV) material with a strong thermomechanical coupling and a damage parameter to prevent crack initiation. More precisely, Chaboche's models available in the literature (elasto-(visco)-plasticity models, with various types of hardening) have been enriched in order to explicitly take into account the influence of the temperature on the mechanical behaviour and, reciprocally, the influence of the mechanical behaviour on the temperature. Mechanical cycling tests have been performed on the VV constitutive material, emphasizing on the progressive deformation state up to failure mode, i.e., ratcheting. The proposed models have been tested on a homogeneous problem and the results compared with uniaxial test results; this type of simulation is commonly called "0D"analysis. The last part of this document describes the finite element implementation of the constitutive material model and its application to the ITER VV welded support.

Ratcheting is one common phenomenon under excessive earthquakes. This study focuses on clarifying the ratcheting mechanism of beams subjected to the combination of gravity and seismic loadings. The sinusoidal excitations or their combinations represented the seismic loadings. Gravity acted as load-controlled loading and was applied by the self-weight of the beam as well as the additional mass put at the free end of the beam. A cyclic acceleration was applied at the base of the beam to provide the source of dynamic loading assumed as seismic loading. Equivalent loading conditions were put in experiments as in numerical analysis to validate the nonlinear finite element analyses. The analogy between thermal ratcheting and vibration ratcheting was considered to investigate the characteristics of cyclic accelerations. The frequency effects on the applied alternating accelerations were also investigated in this research. The results show that input waves containing the lower frequency components than the natural frequency are more damageable comparing to that containing higher frequency ones. Besides, the component with lower frequency contributes more to the occurrence of ratcheting. The characteristics of dynamic loading due to base acceleration depend on the loading frequency. The low-frequency acceleration acts like load-controlled loading, while the high-frequency acceleration is close to displacement-controlled loading.

Ratcheting is one of the dominant failure modes under excessive earthquakes and may cause extreme failures of structures (e.g., collapse). We focused on clarifying the ratcheting mechanism of piping under sinusoidal excitations. Both finite element analyses and experiments were conducted on bent solid bars, which represented piping in this study. Seismic ratcheting occurred due to the combined effect of constant external compressive force and cyclic vibrations. The external compressive force acted as a load-controlled load. Vibrations were applied to provide the source of the dynamic load. Characteristics of vibrations between load-controlled and displacement-controlled properties were studied from the viewpoint of the frequency ratio of the forcing frequency to the natural frequency of the piping model. In addition, the influence of supports on the occurrence of ratcheting was also considered. The results showed that the resonance effect was evident in the piping model compared with the beam model due to the limited plastic area in the piping model. The vibration with a lower frequency had load-controlled characteristics. In contrast, the vibration with a higher frequency presented displacement-controlled properties. In terms of the occurrence of ratcheting, providing more supports sometimes increased the possibility of the occurrence of ratcheting under relatively higher forcing frequencies because more supports increased the natural frequency and decreased the frequency ratio.

In the present investigation in-plane anisotropy in tensile and ratcheting behavior of cold rolled and annealed commercially pure titanium plate has been studied. Flat tensile and fatigue test specimen oriented at 0, 45, and 90 degree to the rolling direction from the rolling directiontransverse direction (RDTD) plane of the plate has been machined out. Specimens with loading axis at 0, 45 and 90 degree to RD have been designated as 0T, 45T and 90T for tensile and 0R, 45R and 90R for fatigue. Owing to initial TD split basal texture of as received plate, 0T sample has crystallographic direction aligned with loading axis. It shows lowest yield strength but highest ductility in monotonic tension. Although ultimate tensile strength (UTS) and strain to failure of samples 45T and 90T are similar, the former has significantly lower yield strength than the latter, indicating different strain-hardening behavior due to different slip/twin activity. On the other hand, 0R sample exhibits longer ratcheting life while 90R sample accumulates highest ratcheting strain. This has been attributed to the formation of intersecting multi-variant twins, which increases fatigue crack propagation resistance during cyclic deformation of 0R sample. Viscoplastic self-consistent (VPSC) simulations of one-cycle tension-compression-reload tension indicate alternating activity of pyramidal c+a slip and extension twinning with loading cycle. The detwinning of extension twin during compression cycle induces cross slip activity, which causes rapid accumulation of strain leading to early fatigue failure of 45R and 90R sample.

Variations in the physical and mechanical properties of parts made by laser power bed fusion (L-PBF) could be affected by the choice of processing or post-processing strategies. This work examined the influence of build orientation and post-processing treatments (annealing or hot isostatic pressing) on the fatigue and fracture behaviours of L-PBF stainless steel 316L in the high cycle fatigue region, i.e. 104 – 106 cycles. Experimental results show that both factors introduce significant changes in the plastic deformation properties, which affect fatigue strength via the mechanism of fatigue-ratcheting interaction. Cyclic plasticity is characterised by hardening, which promotes mean stress insensitivity and improved fatigue resistance. Fatigue activities, involving the initiation of crack at defects and microstructural heterogeneities, are of greater relevance to the longer life region where the global deformation mode is elastic. As the simultaneous actions of ratcheting and fatigue generate complex nonlinear interactions between the alternating stress amplitude and mean stress, the fatigue properties could not be effectively predicted using traditional stress-based models. A modification to the Goodman relation was proposed to account for the added effects of cyclic plasticity and was demonstrated to produce good agreement with experimental results for both cyclic hardening and softening materials. EDB (Economic Devt. Board, S’pore) Accepted version

An experimental study of the strain of a rectilinear segment of a pipeline Du 200 made of steel 20 under cyclic loading by internal pressure was carry out. The tests were performed on a pneumohydraulic stand under conditions of flat stress, normal temperatures, and load cycle asymmetry with registration of circular and axial elastic-plastic strain. The aim of the work was to test an experimental technique for studying the effect of ratcheting (unilateral accumulation of plastic deformations) on real structural elements when changing the modes of cyclic loading. According to the results of the study, curves of cyclic deformation (ratcheting) of the pipe in the circular and axial direction on the basis 100 cycles were obtained., Виконано експериментальне дослідження деформування прямолінійного сегменту трубопроводу Ду 200 із сталі 20 за циклічного навантаження внутрішнім тиском. Випробування проведені на пневмогідравлічному стенді за умов плоского напруженого стану, нормальних температур, асиметрії циклу навантаження зреєстрацією колових та осьових пружно пластичних деформацій. Метою роботи було відпрацювання експериментальної методики для дослідження ефекту ратчетингу (одностороннього накопичення пластичних деформацій) на реальних конструктивних елементах при зміні режимів циклічного навантаження. За результатами дослідження отримані діаграми циклічного деформування (ратчетингу) труби в кільцевому і осьовому напрямках на базі 100 циклів.

The paper reports the development of an implicit numerical scheme for plane stress cyclic elasto-plasticity, capable of integrating a wide range of hardening rules, and simulating multi-axial ratcheting in metal structural components. Constitutive relations account for von Mises yielding in combination with mixed hardening. Emphasis is given to the kinematic hardening part, which is described with an advanced multiple back-stress model suitable for multi-axial material ratcheting simulation. The constitutive equations are integrated implicitly, and the accuracy of the algorithm is assessed via iso-error maps. Two main novelties of the algorithm refer to the incremental update of the internal variables through the solution of a single scalar equation, and the explicit formulation of the consistent tangent moduli. The numerical scheme is implemented within the finite element environment as an external material subroutine, and its computational efficiency is demonstrated through the simulation of large-scale experiments on pipe elbows. Using the proposed computational framework, two kinematic hardening rules are employed to simulate the elbow response with emphasis on local strain amplitude and accumulation (“ratcheting”). The good comparison between numerical and experimental results demonstrates the computational efficiency of the numerical scheme and highlights some key issues concerning multi-axial ratcheting simulation.

The analysis of cyclically loaded piles is acquiring ever greater relevance in the field of geotechnical engineering, most recently in relation to the design of offshore monopiles. In this area, predicting the gradual accumulation of pile deflection under prolonged cycling is key to performing relevant serviceability assessments, for which simplified pile–soil interaction models that can be calibrated against common geotechnical data are strongly needed. This study proposes a new cyclic p−y model for piles in sand that takes a step further towards meeting the mentioned requirements. The model is formulated in the framework of memory-enhanced bounding surface plasticity, and extends to cyclic loading conditions the previous monotonic, CPT-based p−y formulation by Suryasentana and Lehane (2016); additionally, detailed modelling of pile–soil gapping is introduced to cope with the presence of unsaturated sand layers or, more generally, of cohesive soil behaviour. After detailed description of all model capabilities, field data from an onshore cyclic pile loading test are simulated using the proposed p−y model, with the most relevant parameters calibrated against available CPT data. Satisfactory agreement is shown between experimental and numerical results, which supports the practical applicability of the model and the need for further studies on a fully CPT-based calibration.

Stainless steels with harmonic-structure design have a great balance of high strength and high ductility. Therefore, it is imperative to investigate their fatigue properties for engineering applications. In the present work, the harmonic-structured SUS316L stainless steels were fabricated by mechanical milling (MM) and subsequent hot isostatic pressing (HIP) process. A series of ratcheting-fatigue tests were performed on the harmonic-structured SUS316L steels under stress-control mode at room temperature. Effects of grain structure and stress-loading conditions on ratcheting behavior and fatigue life were investigated. Results showed that grain size and applied mean stress had a significant influence on ratcheting-strain accumulation and fatigue life. Owing to the ultrafine grained structure, tensile strength of the harmonic-structured SUS316L steels could be enhanced, which restrained the ratcheting-strain accumulation, resulting in a prolonged fatigue life. A higher mean stress caused a faster ratcheting-strain accumulation, which led to the deterioration of fatigue life. Moreover, a modified model based on Smith–Watson–Topper (SWT) criterion predicted the ratcheting-fatigue life of the harmonic-structured SUS316L steels well. Most of the fatigue-life points were located in the 5 times error band.

This work provides an experimental and computational analysis of low cycle fatigue of a tantalum polycrystalline aggregate. The experimental results include strain field and lattice rotation field measurements at the free surface of a tension–compression test sample after 100, 1000, 2000, and 3000 cycles at ±0.2% overall strain. They reveal the development of strong heterogeneites of strain, plastic slip activity, and surface roughness during cycling. Intergranular and transgranular cracks are observed after 5000 cycles. The Crystal Plasticity Finite Element simulation recording more than 1000 cycles confirms the large strain dispersion at the free surface and shows evidence of strong local ratcheting phenomena occurring in particular at some grain boundaries. The amount of ratcheting plastic strain at each cycle is used as the main ingredient of a new local fatigue crack initiation criterion.

Ensuring the integrity of components throughout their lifespan is a major industrial issue, especially for nuclear facilities. The safety assessments of components must cover all possible situations, including very exceptional ones, such as earthquakes. For a monotonic loading, several theoretical frameworks exist to predict crack growths. The first part of this thesis is dedicated to the experimental study of the behavior of cavities, laboratory test specimens and a representative structure of a secondary nuclear power plant piping subjected to cyclic loadings. This experimental study is complemented by in-situ X-ray tomography tests conducted on axisymmetric notched tensile specimens loaded cyclically. The second part of this thesis is devoted to a theoretical study of the influence of cyclic loadings on void growth models. First, the void growth model of Morin is implemented in a finite element computational code to compare its predictions with the results of micromechanical simulations. It is shown that a better consideration of the effect of elasticity is essential to correctly predict the growth of a void subjected to cyclic loadings. Then, through determining the exact equation of porosity rate including elastic strains, an improvement of Morin's model is proposed in order to take into account the effects of elasticity.Finally, in the third part of this thesis, a numerical study of cyclic ductile rupture is conducted through simulations of the behavior of laboratory test specimen and a pipe mock-up. These simulations validate the predictions of the proposed model and its relevance to predict the behavior of structures subjected to cyclic loading of large amplitudes., Garantir l’intégrité des structures tout au long de leurs durées de vie est un sujet industriel majeur, en particulier pour les installations nucléaires. La démonstration de sûreté des équipements doit couvrir l’ensemble des sollicitations possibles, y compris les situations très exceptionnelles, comme les séismes. Pour un chargement monotone, plusieurs cadres théoriques existent pour prédire la propagation des fissures. La première partie de cette thèse est dédiée à l’étude expérimentale du comportement de cavités, d’éprouvettes de laboratoires et d’une structure représentative d’une tuyauterie secondaire de centrale nucléaire soumises à des chargements cycliques. Cette étude est complétée par des essais in-situ de tomographie par rayons X menés sur des éprouvettes axisymétriques entaillées chargées de manière cyclique. La deuxième partie de cette thèse est consacrée à l’étude théorique de l’influence des chargements cycliques sur les modèles de croissance de cavités. Le modèle de croissance de Morin est implémenté dans un code de calcul éléments finis afin de comparer ses prédictions avec les résultats de simulations micromécaniques. Dans un second temps, à partir de l’équation exacte d’évolution du taux de porosité incluant les déformations élastiques, une amélioration du modèle est proposée. Enfin, dans la troisième partie, une étude numérique de la rupture ductile cyclique est menée à travers la simulation du comportement d’éprouvettes de laboratoires et de structures de type tuyauterie. Elles permettent de valider les prédictions du modèle proposé ainsi que sa pertinence pour prédire le comportement de structure soumises à un chargement cyclique de grandes amplitudes.

International audience; In the present study, molecular dynamics simulations have been performed to investigate the creep–ratcheting deformation behavior of nanocrystalline aluminum (NC Al) having an average grain size of ~ 8 nm. The influence of deformation temperature on creep–ratcheting behavior has been studied and associated with underlying mechanisms based on the structural evolution of the material identified. The vacancy concentrations, strains and dislocation densities have been evaluated at the end of each stage of creep–ratcheting process for two ratcheting stress ratios and three different temperatures. In the mean time, the microstructural and defect evolution has been investigated. Accumulation of creep–ratcheting strain is found to increase with the deformation temperature in the range of temperature investigated: 10–467 K. Cyclic hardening dominates in the initial stages of creep–ratcheting, whereas cyclic softening dominates in the final stages at a higher temperature. The creep–ratcheting plots exhibit a primary and steady state regions at room temperature (300 K). In addition, a tertiary region is also perceived at high temperature (467 K). The NC Al specimen is also found to be damaged earlier at a higher temperature (i.e., 467 K) than at 10 K and 300 K. The highest dislocation density is attained for room temperature creep–ratcheting deformation. Finally, it is seen from the dislocation analysis that the Shockley partial and full dislocations are the driving dislocations for the creep–ratcheting deformation process.

Moderne Protokolle zur sicheren Kommunikation, die beispielsweise in Messengern wie WhatsApp und Signal verbaut sind, verwenden Techniken zur Erneuerung des verwendeten Schlüsselmaterials-Ratcheting. In dieser Dissertation werden die ersten methodisch gewonnenen und natürlichen Modelle und Definitionen eingeführt, um diese Ratcheting Techniken wissenschaftlich und strukturiert untersuchen zu können. Im Rahmen dessen wird eine Systematisierung der Literatur zu vorangegangener Modellierung durchgeführt. Des Weiteren werden neue, sicherere Ratcheting Protokolle entwickelt, die als Richtwert für zukünftige Forschung dienen. Zur kryptographischen Einordnung dieser Protokolle werden deren Effizienzmerkmale und entsprechende Beschränkungen auf ihre theoretische Notwendigkeit untersucht. Die entwickelten Ergebnisse (re-)strukturieren den Forschungsstand zu kontinuierlichem Schlüsselaustausch und können durch ihren generischen Charakter erweiterte Anwendung finden., Modern secure communication protocols that are, for example, used by messengers like WhatsApp and Signal implement techniques to update the utilized key material - so-called ratcheting. In this dissertation, we develop the first methodically derived, natural models and definitions that enable us to analyze these ratcheting techniques scientifically in a structured manner. In this context, we systematize the literature on previous modeling approaches. Additionally, we develop new, secure ratcheting protocols that constitute a reference for future research. In order to classify these protocols, we analyze their efficiency and performance limitations with respect to theoretic necessities. The developed solutions (re-)structure the state of research on continuous key exchange and, due to their generic nature, can have extended applicability.

In order to investigate the ratcheting behavior and to determine new design rules, some experimental tests were conducted in many countries in the last decades. In France, some tests were carried out under mechanical or thermal cyclic loading to examine this risk. The first section of the current article is addressed to the state of the art concerning the ratcheting effects. The difference between Local and Global Ratcheting is clarified. The second section is dedicated to the experimental observations of ratcheting. The following section describes the constitutive models which are able to simulate material/structural ratcheting responses. The models presented are Linear Kinematic, Armstrong-Frederick, Chaboche, Ohno-Wang and Chen-Jiao-Kim. Finally, the ratcheting rules in design codes are exposed. Both simple and complex rules are presented.

In this study, we describe the ratchet transport of particles under static asymmetric potential with periodicity. Ratchet transport has garnered considerable attention due to its potential for developing smart transport techniques on a micrometer scale. In previous studies, either particle self-propulsion or time varying potential was introduced to realize unidirectional transport. Without utilizing these two factors, we experimentally demonstrate ratchet transport through particle interactions during crowding. Such ratchet transport induced by particle interaction has not been experimentally demonstrated thus far, although some theoretical studies had suggested that particle crowding enhances ratchet transport. In addition, we constructed a model for such transport in which the potential varies depending on the particle density, which agrees well with our experimental results. This study can accelerate the development of transport techniques on a micrometer scale., Link to supplementary materials and movies: https://takinouelab.github.io/HayakawaTakinoue2019/

Long-term cyclic loading is observed in a wide range of human activities, as well as in nature, such as in the case of ocean waves. Cyclic loading can lead to ratcheting which is defined as progressive accumulation of plastic deformation in a material. Long-term cyclic loading causes a time effect (creep), which is a secondary compression effect. In this article, we conducted 15 triaxial tests on four types of cohesive materials in undrained conditions to evaluate the damage and failure mechanism. To characterize the strain and pore pressure development, we modified the Yanbu resistance concept. On the basis of the static creep tests, we concluded that the stress paths for undrained creep behavior have to take into account the pore pressure developed during long-term cyclic loading. Pore pressure build-up and plastic strain accumulation during long-term cyclic loading are dependent on the number of loading cycles. Finally, we proposed the failure criterion, which was based on the Modified Cam-Clay constitutive model.

In Indonesia, the regulation emphasizes the implementation of a performance-based budgeting system. However, the implementation of this system is questioned as the ratcheting effect tends to occur in the budgeting process in local government. The inefficiency that arises due to the occasion should be reduced by transforming it into the performance-based system, especially when a pandemic occurs. Reallocation of budget produced by incremental-line system should be implemented to minimize the risk of the pandemic on social welfare. The data used in this model are APBD data in 2013-2108 from the Ministry of Finance. Regression models and study literature are used in this study to identify the budget ratcheting that occurred in Indonesia and examine the implementation of performance-based budgeting that can reduce the occurrence of government budget behavior bias in the form of a ratchet effect when a pandemic occurs. The result of this study shows that there is an indication of the existence of asymmetric ratchet. As it may lead to inefficiency, to face a pandemic situation, the government should conduct performance target adjusting and program restructuring by reallocating the budget.

Creep–fatigue interaction occurs in many structural components of high-temperature systems operating under cyclic and steady-state service conditions, such as in nuclear power plants, aerospace, naval, and other industrial applications. Thus, understanding micromechanisms governing high-temperature creep–fatigue behavior is essential for safety and design considerations. In this work, stress-controlled creep–fatigue tests of advanced austenitic stainless steel (Alloy 709) were performed at a 400 MPa stress range and 750 °C with tensile hold times of 0, 60, 600, 1800, and 3600 s, followed by microstructural examinations. The creep–fatigue lifetime of the Alloy 709 was found to decrease with increasing hold time until reaching a saturation level where the number of cycles to failure did not exhibit a significant decrease. Softening behavior was observed at the beginning of the test, possibly due to the recovery of entangled dislocations and de-twining. In addition, hysteresis loops showed ratcheting behavior, although the mean stress was zero during creep–fatigue cycling, which was attributed to activity of partial dislocations. Microstructural examination of the fracture surfaces showed that fatigue failure dominated at small hold times where the cracks initiated at the surface of the sample. Larger creep cracks were found for longer hold times with a lower probability of dimpled cavities, indicating the dominance of creep deformation. The results were compared with other commonly used stainless steels, and plausible reasons for the observed responses were described.

In this paper, elasto-plastic analysis of a thick-walled cylinder made of functionally graded materials (FGMs) subjected to constant internal pressure and cyclic temperature gradient loading is carried out using MATLAB. The material is assumed to be isotropic and independent of temperature with constant Poisson's ratio and the material properties vary radially based on a power law volume function relation. The Von Mises’ yield criterion and the Armstrong-Frederick nonlinear kinematic hardening model were implemented in this investigation. To obtain the incremental plastic strain, return mapping algorithm (RMA) was used. At the end, the Bree's interaction diagram is plotted in terms of non-dimensional pressure and temperature which represents an engineering index for optimum design under thermo-mechanical loading. Keywords: Functionally graded material, Nonlinear kinematic hardening, Thermal/mechanical stress, Return mapping algorithm, Shakedown, Ratcheting, Bree's diagram

The objective of this investigation is to evaluate the cyclic plastic deformation response at the subsurface during rolling contact fatigue (RCF) under the conditions of pure rolling and rolling-sliding. The finite element simulation results indicate that the slip-to-roll ratio (SRR)has an immense effect on the cyclic plastic deformation evolution at the subsurface during RCF. At SRR of 0% (pure rolling), symmetric shear stress cycling is evident at the subsurface, and as a result plastic shakedown takes place. On the other hand, at SRR of 5% (rolling-sliding), asymmetric shear stress cycling is noticed at the subsurface, and consequently ratcheting i.e. progressive accumulation of shear strain is evident. Rolling-sliding results in the shear flow of the subsurface material along the sliding direction. Published version The authors are thankful to the Science & Engineering Research Board (SERB), Department of Science & Technology [File No. IMRC/AISTDF/CRD/2018/000052], Government of India for their financial support.

We introduce a learning framework in which a principal seeks to determine the ability of a strategic agent. The principal assigns a test consisting of a finite sequence of tasks. The test is adaptive: each task that is assigned can depend on the agent's past performance. The probability of success on a task is jointly determined by the agent's privately known ability and an unobserved effort level that he chooses to maximize the probability of passing the test. We identify a simple monotonicity condition under which the principal always employs the most (statistically) informa- tive task in the optimal adaptive test. Conversely, whenever the condition is violated, we show that there are cases in which the principal strictly prefers to use less informative tasks. We discuss the implication of our results for task assignment in organizations with the aim of determining suitable candidates for promotions.

We present a theoretical model to describe the response of a one dimensional mechanical system under cyclic loading. Specifically, the model addresses the non-linear response on loading, hysteretic behaviour on unloading and reloading, and the phenomenon of ratcheting under very many cycles. The methods developed are formulated within the hyperplasticity framework. The model can be expressed in the form of general incremental relationships, can therefore be applied without modification directly to any loading history, and can be readily implemented within a time-stepping numerical code. A rigorous procedure is described to accelerate the ratcheting process, so that the effects of very large numbers of cycles can be analysed through a reduced number of cycles. A generalisation from unidirectional to multidirectional loading is described, together with a tensorial form for application to material modelling. The original motivation was for the application to design of piles under lateral loading, and an example of this application is provided. However, the model is equally applicable to many other problems involving unidirectional or bi-directional cyclic loading in which the system exhibits a similar character of hysteretic behaviour, with ratcheting under large numbers of cycles.

Basic qualitative properties of the creep curves generated by the linear integral constitutive relation of viscoelasticity (with an arbitrary creep compliance) under cyclic piecewise-constant uni-axial loadings (with an arbitrary asymmetry stress ratio) are studied analytically. General formulas and a number of exact two-sided bounds are obtained for maximal, minimal and ratcheting strain values during each cycle, for their sequences limits, for the rate of plastic (non-recoverable) strain accumulation and for cyclic creep curve deviation from the creep curve at constant stress which is equal to the cycle mean stress. Their dependence on loading cycle parameters and creep compliance properties are analyzed. Monotonicity and convexity intervals of cyclic creep curves, sequences of maximal and minimal strain values and ratcheting strain sequence, their evolution with cycle number growth and conditions for their boundedness, monotonicity and convergence are examined. The linear viscoelasticity theory abilities for simulation of ratcheting, creep acceleration, cyclic hardening or softening and cyclic stability under symmetric cyclic loadings are considered. The analysis carried out revealed the importance of convexity restriction imposed on a creep compliance and the governing role of its derivative limit value at infinity. It is proved that the limit value equality to zero is the criterion for non-accumulation of plastic strain, for memory fading and for asymptotic symmetrization of cyclic creep curve deviation from the creep curve at the mean stress. The qualitative features of theoretic cyclic creep curves are compared to basic properties of typical test creep curves of viscoelastoplastic materials under cyclic multi-step uni-axial loadings in order to elucidate the linear theory applicability scope, to reveal its abilities to provide an adequate description of basic rheological phenomena related to cyclic creep and to develop techniques of identification and tuning of the linear constitutive relation. In particular, it is proved that the linear constitutive relation with an arbitrary (increasing convex-up) creep compliance function provides the absence of ratcheting and cyclic softening under symmetric cyclic multi-step loadings and the absence of creep acceleration whenever a symmetric cyclic loading is added to a constant load.

To predict the thermal and structural responses of the thrust chamber wall under cyclic work, a 3-D fluid-structural coupling computational methodology is developed. The thermal and mechanical loads are determined by a validated 3-D finite volume fluid-thermal coupling computational method. With the specified loads, the nonlinear thermal-structural finite element analysis is applied to obtaining the 3-D thermal and structural responses. The Chaboche nonlinear kinematic hardening model calibrated by experimental data is adopted to predict the cyclic plastic behavior of the inner wall. The methodology is further applied to the thrust chamber of LOX/Methane rocket engines. The results show that both the maximum temperature at hot run phase and the maximum circumferential residual strain of the inner wall appear at the convergent part of the chamber. Structural analysis for multiple work cycles reveals that the failure of the inner wall may be controlled by the low-cycle fatigue when the Chaboche model parameter γ 3 = 0, and the damage caused by the thermal-mechanical ratcheting of the inner wall cannot be ignored when γ 3 > 0. The results of sensitivity analysis indicate that mechanical loads have a strong influence on the strains in the inner wall.

International audience; In this article, we propose an approach suitable for modeling isothermal fatigue in amorphous polymers. The theory is formulated in a rate form within continuum mechanics framework without the need to measure damage changes per loading cycles. Using the approach, contribution of ratcheting to fatigue of polycarbonate (PC) was investigated and the results were compared to previous experimental observations. When subjected to uniaxial stress-controlled cyclic loadings, ratcheting deformation apparently occurs and increases with mean stress and amplitude. The development of ratcheting deformation shows an initial growth followed by a decrease to almost a constant growth rate which occupies majority of the total lifetime. Ratcheting behavior under multiaxial stress states was also investigated based on finite element analyses of a dogbone-shaped test specimen. The results show that fatigue damage develops at the sites following closely the localized plastic deformation and increases with ratcheting deformation during cyclic loadings. The results indicate that the ratcheting behavior can be exploited in the evaluation of the entire fatigue lifetime.

The nonlinear Maxwell-type constitutive relation with two arbitrary material functions for viscoelastoplastic multi-modulus materials is studied analytically in uniaxial isothermic case to reveal the model abilities and applicability scope and to develop techniques of its identification, tuning and fitting. The constitutive equation is aimed at adequate modeling of the rheological phenomena set which is typical for reonomic materials exhibiting non-linear hereditary properties, strong strain rate sensitivity, secondary creep, yielding at constant stress, tension compression asymmetry and such temperature effects as increase of material compliance, strain rate sensitivity and rates of dissipation, relaxation, creep and plastic strain accumulation with temperature growth. The model is applicable for simulation of mechanical behaviour of various polymers, their solutions and melts, solid propellants, sand-asphalt concretes, composite materials, titanium and aluminum alloys, ceramics at high temperature and so on. To describe the influence of temperature on material mechanical behavior (under isothermic conditions), two scalar material parameters of the model (viscosity coefficient and “modulus of elasticity”) are considered as a functions of temperature level. The general restrictions on their properties which are necessary and sufficient for adequate qualitative description of the basic thermomechanical phenomena related to typical temperature influence on creep and relaxation curves, creep recovery curves, creep curves under step-wise loading and quasi-static stress-strain curves of viscoelastoplastic materials are obtained. The restrictions are derived using systematic analytical study of general qualitative features of the theoretic creep and relaxation curves, creep curves under step-wise loading, long-term strength curves and stress-strain curves at constant strain or stress rates generated by the constitutive equation (under minimal restrictions on material functions) and their comparison to typical test curves of stable viscoelastoplastic materials. It is proved that the viscosity coefficient and the “modulus of elasticity” of the model and their ratio (i.e. relaxation time of the associated linear Maxwell model) should be decreasing functions of temperature. This requirements are proved to provide an adequate qualitative simulation of a dozen basic phenomena expressing an increase of material compliance (a decrease of tangent modulus and yield stress, in particular), strengthening of strain rate sensitivity and acceleration of dissipation, relaxation, creep and plastic strain accumulation with temperature growth.

Due to their high strength-to-weight-ratio, magnesium alloys are very attractive for use in automotive engineering. For application at elevated temperatures, the alloys must be creep-resistant. Therefore, the influence of the operating temperature on the material properties under quasistatic and cyclic load has to be understood. A previous study investigated tensile-tensile fatigue behavior of the magnesium alloys DieMag422 and AE42 at room temperature (RT). The aim of this study was the comparison of both alloys regarding compression, tensile, and compression-compression fatigue behavior. The quasistatic behavior was determined by means of tensile and compression tests, and the tensile-compression asymmetry was analyzed. In temperature increase fatigue tests (TIFT) and constant amplitude tests (CAT), the temperature influence on the cyclic creeping (ratcheting) behavior was investigated, and mechanisms-relevant test temperatures were determined. Furthermore, characteristic fracture mechanisms were evaluated with investigations of the microstructure and the fracture surfaces. The initial material was analyzed in computed tomographic scans and energy dispersive X-ray (EDX) analyses., Materials;13(3)

Due to their high strength-to-weight-ratio, magnesium alloys are very attractive for use in automotive engineering. For application at elevated temperatures, the alloys must be creep-resistant. Therefore, the influence of the operating temperature on the material properties under quasistatic and cyclic load has to be understood. A previous study investigated tensile-tensile fatigue behavior of the magnesium alloys DieMag422 and AE42 at room temperature (RT). The aim of this study was the comparison of both alloys regarding compression, tensile, and compression-compression fatigue behavior. The quasistatic behavior was determined by means of tensile and compression tests, and the tensile-compression asymmetry was analyzed. In temperature increase fatigue tests (TIFT) and constant amplitude tests (CAT), the temperature influence on the cyclic creeping (ratcheting) behavior was investigated, and mechanisms-relevant test temperatures were determined. Furthermore, characteristic fracture mechanisms were evaluated with investigations of the microstructure and the fracture surfaces. The initial material was analyzed in computed tomographic scans and energy dispersive X-ray (EDX) analyses.

Safety assessment of structures can be obtained employing limit design to overcome uncertainties concerning actual response due to inelastic constitutive behavior and more generally to non-linear structural response and loads&rsquo, random variability. The limit analysis is used for evaluating the safety of the structures, starting directly from load level without any knowledge of the load history. In the paper, the lower bound calculation is proposed where a new strain-based approach is used that allowed describing the residual stress and displacement in terms of permanent strain. The strategy uses the permanent strain as effective parameters of the procedure so that it is possible to assess the ductility requirements for the complete load program developed till collapse or shakedown. The procedure is compared to experimental results obtained on aluminum beams in shakedown.

The pre-concept design of the DEMO Vacuum Vessel is going on in view of the 2020 gate review; moreover the nuclear heat loads on the vessel inner shell were determined and found to be about one order of magnitude higher compared to ITER. A subsequent thermal-structural analysis of the vessel inner shell revealed high thermal stresses and a large temperature gradient through the inner shell thickness. Given the simultaneous occurrence of primary membrane stresses in the entire vessel inboard wall and, in proximity of the vessel ribs, high bending stresses due to the coolant pressure, a survey of all options within the design rules was required to identify the inter-dependencies of the individual stress limits (primary membrane, primary bending, thermal membrane plus bending). In order to face this kind of issues a detailed assessment on the design of the inboard wall of DEMO Vacuum Vessel has been conducted and is presented here. The current work evaluates both P and S type damages for the inboard wall of DEMO Vacuum Vessel in case of high nuclear heat load, vacuum vessel coolant pressure and toroidal field coil fast discharge. The elastic analysis method has been used to check the rules for prevention of both types of damage. The rules applied to prevent the aforementioned damages are compliant to Level A criteria, in case of negligible creep and negligible irradiation. In order to check the structural integrity of the inboard wall of DEMO VV against high thermal and mechanical loads, optimization structural analyses were performed and checked against the rules provided in the applicable design code (RCC MRx).

Due to their high strength-to-weight-ratio, magnesium alloys are very attractive for use in automotive engineering. For application at elevated temperatures, the alloys must be creep-resistant. Therefore, the influence of the operating temperature on the material properties under quasistatic and cyclic load has to be understood. A previous study investigated tensile-tensile fatigue behavior of the magnesium alloys DieMag422 and AE42 at room temperature (RT). The aim of this study was the comparison of both alloys regarding compression, tensile, and compression-compression fatigue behavior. The quasistatic behavior was determined by means of tensile and compression tests, and the tensile-compression asymmetry was analyzed. In temperature increase fatigue tests (TIFT) and constant amplitude tests (CAT), the temperature influence on the cyclic creeping (ratcheting) behavior was investigated, and mechanisms-relevant test temperatures were determined. Furthermore, characteristic fracture mechanisms were evaluated with investigations of the microstructure and the fracture surfaces. The initial material was analyzed in computed tomographic scans and energy dispersive X-ray (EDX) analyses.

This paper presents a modeling framework to describe the driving mechanisms of cyclic failure in brittle and ductile materials, including cyclic plasticity and fatigue crack growth. A variational model is devised using the energetic formulation for rate-independent systems, coupling a phase-field description of fatigue fracture to a cyclic plasticity model that includes multi-surface kinematic hardening, gradient-enhanced isotropic hardening/softening and ratcheting. The coupled model embeds two distinctive fatigue effects. The first captures the characteristic features of low-cycle fatigue, driven by the accumulation of plastic strains, while the second accounts for high-cycle fatigue, driven by free energy accumulation. The interplay between these mechanisms allows to describe a wide range of cyclic responses under both force loading and displacement loading, as shown in several numerical simulations. Moreover, the phase-field approach to fracture accounts for the initiation and propagation of fatigue-induced cracks., Minor errata corrections (pages 17 and 18). 48 pages, 22 figures

This paper presents an analytical methodology for calibration of the Hyperplastic Accelerated Ratcheting Model (HARM, Houlsby et al., 2017 [3]), based on a closed-form expression for the accumulation of ratcheting strain with cyclic history. The proposed method requires the fit of one test response and of a few continuous cyclic tests. The initial motivation for this work is the calibration of models for the design of piles subjected to long-term cyclic lateral loading, and the test results from Abadie et al. (2018, 2015) [1,2] are used for calibration and proofing of the model. Nevertheless, the method is applicable to other problems of similar behaviour.

When adhesively bonded joints are subjected to a cyclic load, they will primarily behave viscoelastically. Under certain loading conditions, they can also experience permanent deformation, or ratcheting. This research considered loading conditions that result in ratcheting for a toughened and standard adhesive in scarf joint coupons. Adhesive strain was measured using edge mounted resistance strain gages that was verified with digital image correlation. At low stress levels both adhesives exhibited a purely viscoelastic response. At high stress levels, the standard adhesive exhibited a viscoelastic response in tension-tension and a ratcheting response under reversed loading. At high stress levels the toughened adhesive exhibited ratcheting during tension-tension and reversed loading with higher ratcheting strain in tests that involved reversed loading. Enhanced ratcheting during reversed loading is consistent with a kinematic hardening response, which these adhesives have been shown to follow.

Pseudoelastic NiTi based shape memory alloys (SMA) wires have a good potential in civil engineering applications and often used as dampers in anti-seismic or cable structures when a rate varied cyclic loadings is applied. In accordance with these applications, an experimental characterization of NiTi wires under strain- and stress-controlled cyclic loadings at various strain rates is presented. Based on these loadings, it is shown that the stress–strain response of the material evolves up to a complete stabilization with the increase of the number of cycles. This evolution is accompanied with the degeneration of pseudoelasticity and transformation ratcheting in the case of strain- and stress-controlled mode, respectively. Also, it was found that the cyclic deformation of pseudoelastic NiTi SMA is significantly rate dependent.

The prevention of excessive deformation by thermal ratcheting is important in the design of high-temperature components of fast breeder reactors (FBR). In an experimental study that simulated a fast breeder reactor vessel near the coolant surface, it was reported that the long distance travel of temperature distribution causes a new type of thermal ratcheting, even in the absence of primary stress. In this paper, we propose a simple screening criterion to prevent continuous accumulation of plastic strain derived from long distance travel of temperature distribution. The major cause of this ratcheting is the lack of residual stress that brings shakedown behavior at the center of yielding area. Because residual stresses are derived from constraint by the neighboring elastic region, we focused on the distance from the center of yielding area to the elastic region. Accordingly, the proposed criterion restricts the axial length of the full-section yield area, which is the double of the above distance. We validated the proposed criterion based on finite element analyses using an elastic-perfectly plastic material. As a result of the validation analyses, we confirmed that the accumulation of plastic strain saturates before second cycles in the cases satisfying the proposed criterion, regardless of the shape of temperature distribution.

The integrity assessment of structures subjected to cyclic loading must be verified with regard to cyclic type damage including time-independent fatigue and progressive deformation or ratcheting. Cyclic damage is verified simulating the material elastic-plastic loop and looking at the accumulated net plastic strain during each cycle at all points of the structure subjected to the complete time history of loadings. This work deals with the development of a numerical model producing the Chaboche hardening parameters starting from stress-strain data produced by testing of materials. Then, the total plastic strain can be simulated using the Chaboche inelastic constitutive model requested for finite element analyses. This is particularly demanding for pressure vessels, pressurised piping, boilers, and mechanical components of nuclear installations made of stainless steels. A design optimisation by iterative analyses is developed to approach the stress-strain test data with the Chaboche model. The parameters treated as design variables are the Chaboche parameters and the objective function to be minimised is a combination of the deviations from test data. The optimiser calls a macroinstruction simulating cyclic loading of a sample for different material temperatures. The numerical model can be used to produce hardening parameters of materials for inelastic finite element verifications of structures with complex joints like elbows subjected to a combination of steady sustained and cyclic loads.

The main aim of the presented paper is to show how heat treatment, in our case the induction hardening, will affect the wear rates as well as the ratcheting evolution process beneath the contact surface in the field of line rolling contact. Used wear model is based on shear band cracking mechanism [1] and non-linear kinematic and isotropic hardening rule of Chaboche and Lemaitre. The entire numerical simulations have been realized in the C# programming language. Results from numerical simulations are subsequently compared with experimental data.