259 research outputs found
Energy based non-local plasticity models for deformation patterning, localization and fracture
No full textThis paper analyses the effect of the form of the plastic energy potential on the (heterogeneous) distribution of the deformation field in a simple setting where the key physical aspects of the phenomenon could easily be extracted. This phenomenon is addressed through two different (rate-dependent and rate-independent) non-local plasticity models, by numerically solving two distinct one-dimensional problems, where the plastic energy potential has different non-convex contributions leading to patterning of the deformation field in a shear problem, and localization, resulting ultimately in fracture, in a tensile problem. Analytical and numerical solutions provided by the two models are analysed, and they are compared with experimental observations for certain cases
The finite element implementation of 3D fractional viscoelastic constitutive models
No full textThe aim of this paper is to present the implementation of 3D fractional viscoelastic constitutive theory presented in Alotta et al., 2016 [1]. Fractional viscoelastic models exactly reproduce the time dependent behaviour of real viscoelastic materials which exhibit a long “fading memory”. From an implementation point of view, this feature implies storing the stress/strain history throughout the simulations which may require a large amount of memory. We propose here a number of strategies to effectively limit the memory required. The form of the constitutive equations are summarized and the finite element implementation in a Newton-Raphson integration scheme is described in detail. The expressions that are needed to be coded in user-defined material subroutines for quasi static and dynamic implicit and explicit analysis (UMAT and VUMAT) in the commercial finite element software ABAQUS are readily provided. In order to demonstrate the accuracy of the numerical implementation we report a number of benchmark problems validated against analytical results. We have also analysed the behaviour of a viscoelastic plate with a hole in order to show the efficiency of these types of models. The source codes for the UMAT and VUMAT are provided as online supplements to this paper
Cocks on Dunghills-Wollstonecraft and gouges on the women's revolution
No full textWhile many historians and philosophers have sought to understand the 'failure' of the French Revolution to thrive and to avoid senseless violence, very few have referred to the works of two women philosophers who diagnosed the problems as they were happening. This essay looks at how Mary Wollstonecraft and Olympe de Gouges theorised the new tyranny that grew out of the French Revolution, that of 'petty tyrants' who found themselves like 'cocks on a dunghill' able to wield a new power over those less fortunate than themselves. Both offer diagnoses and prognoses that revolve around education. Wollstonecraft argues that a revolution that is not backed by a previous education of the people is bound to result in chaos and violence. Such education, however, must be slow, and it necessitates the reform of the institutions that most shape the public's character. A revolution, perforce, is fast, and it often takes several years, or even generations before the spirit of the reforms finds itself implemented into new institutions. Olympe de Gouges shares Wollstonecraft's worry and she observes that the men who were once dominated quickly become tyrants themselves unless their moral character is already virtuous. But the state of being dominated leaves little room for virtue; hence, newly minted citizens need to be educated in order not to replicate the reign of tyranny onto other. Gouges suggests that the answer to the difficulty she and Wollstonecraft highlighted was to educate the people where they could be found: on the streets, or, where they could easily and willingly be gathered: in theatres. By helping organise revolutionary festivals, highlighting the ways in which citizens could be virtuous, and writing plays to awaken their virtue, and proposing a reform of the theatre, so that the production of such plays would be possible, Gouges offered a plan for the civic education of French citizens in the immediate aftermaths of the Revolution. Unfortunately, the chaos she and Wollstonecraft had sought to remedy, led by the cocks or petty tyrants, ensured that they were unable to see through their plans, with Wollstonecraft having to leave Paris and Gouges being sent to the guillotine. © 2022 the author(s), published by De Gruyter, Berlin/Boston
On the evaluation of the Bauschinger effect in an austenitic stainless steel—The role of multi-scale residual stresses
No full textIn this work, a physically based self-consistent model is developed and employed to examine the microscopic lattice response of pre-strained Type 316H polycrystalline austenitic stainless steel subjected to uniaxial tensile and compressive loading. The model is also used to explain the Bauschinger effect observed at the macroscopic length-scale. Formulated in a crystal based plasticity framework, the model incorporates detailed strengthening effects associated with different microstructural elements such as forest dislocation junctions, solute atoms and precipitates on individual crystallographic slip planes of each individual grain within the polycrystal. The elastoplastic response of the bulk polycrystal is obtained by homogenizing the response of all the constituent grains using a self-consistent approach. Micro-plasticity mechanisms and how these influence the Bauschinger effect are illustrated in terms of the role of residual stresses at different length-scales. Overall, predictions are in good agreement with experimental data of the Bauschinger effect and the corresponding meso-scale lattice response of the material, with the latter measured by neutron diffraction. The results demonstrate that transient softening of the material is related to residual stresses at different length scales. In addition, the (Type III) residual stress at the micro-scale slip system level extends the strain range over which the tensile and compressive reloading curves of the pre-strained material merge
The effect of suction during die fill on a rotary tablet press.
No full textDie fill on a rotary tablet press involves complex powder flow phenomena. Conventional techniques for measuring flowability do not normally provide information that is directly relevant to the design of powder feed systems or to the selection of press parameters for the die filling process. Sinka et al. [I.C. Sinka, L.C.R. Schneider, A.C.F. Cocks, Measurement of the flow properties of powders with special reference to die fill, in: International Journal of Pharmaceutics 280 (1-2) (2004) 27-38] used an experimental shoe-die system to characterise the flow behaviour of pharmaceutical powders. A rigorous data analysis procedure was developed by Schneider et al. [L.C.R. Schneider, I.C. Sinka, A.C.F. Cocks, Characterisation of the flow behaviour of pharmaceutical powders using a model die-shoe filling system, in: Powder Technology (in press)] to evaluate the experimental results, however, when scaling the results to a rotary tablet press, the die fill efficiency was underpredicted by a factor of approximately 2, because the experimental system did not capture major features of the rotary press flow process. The suction effect, whereby the lower punch is moved downwards while the top of the die is exposed to powder in the feed system, is a key element of the process. In this note we describe the development of a model shoe-die system that allows the effect of suction to be investigated. The results demonstrate the improvement offered by suction and illustrate how a fundamental understanding of die fill phenomena could assist the selection of process parameters to maximise the operational speed of a rotary press
Modelling microstructure evolution, creep deformation and damage in Type 316H stainless steel
No full textThe current Advanced Gas-cooled Reactors (AGR) in the UK are approaching their design life. The reactor operator, EDF Energy, is seeking to extend their life due to ecological and financial reasons. One challenge to plant life-extension is the accurate prediction of high-temperature deformation and failure of Type 316H stainless steel, used in components of the reactor boiler section. Under the operating conditions of these components (470-650°C; 10-300 MPa), creep and cyclic plasticity alter the material’s microstructure and failure response. Empirical models are used in industry to predict the deformation and damage response of Type 316H. Predictions can be in poor agreement with plant-recorded/experimental deformation response under complex loading histories. Furthermore, the models are deficient in capturing the sensitivity of the creep damage process to the microstructural state of the material. The overall aim of this research is to improve the understanding of high-temperature deformation and failure response of Type 316H under common plant conditions in order to make realistic life predictions. This is achieved through development of micromechanical models for deformation and damage, employed within polycrystalline modelling frameworks.
An existing physically-based self-consistent model (SCM) for inelastic deformation, developed at Oxford University, captures accurately the global and grain-scale deformation response of Type 316H during short-term plasticity and creep. The model was found deficient in predicting cyclic deformation and stress relaxation. This study enhances the model by incorporating the fundamental physics of these processes. Predictions by the enhanced SCM provide insights into the evolution of deformation, microstructure and residual stress state of Type 316H under plant-relevant loading histories. The insights could inform assessment procedures in industry to more accurately account for the accumulation of inelastic strains and damage. Although the enhanced SCM predicts the global response of the material, localization effects, which are of importance to creep damage processes, are not captured. To address this, a crystal plasticity finite element (CPFE) model was developed. It employs the same micromechanical model as the SCM and predictions of global material response by the two frameworks are in agreement. The CPFE scheme captures stress and strain localization near grain boundaries and it was further extended to describe the grain interface response. A combined CPFE-interface element framework was developed and initially used to study the effects of grain-boundary sliding on creep deformation. Results suggest that the effects of grain-boundary sliding on the macroscopic deformation of Type 316H are limited. Features of the local stress and strain fields at grain boundaries, which could affect intergranular damage response, are captured by the CPFE-interface element scheme.
A common creep damage mode in Type 316H under the operating range of interest is intergranular cavitation. Review of the literature confirmed that cavitation in Type 316H is controlled by cavity nucleation, which is not fully understood. In order to provide further insights into the physics of this process, existing strain-based empirical and stress-based (classical nucleation theory) nucleation models were modified in this study by considering experimentally-observed features of cavity nucleation in Type 316H. The models were employed locally within the developed CPFE-interface element framework. Modelling results suggest that the strain-based model as a function of local inelastic strain rate does not explain the physical nature of the nucleation process. By contrast, the modified classical nucleation theory captures both the macroscopic failure response and trends in distribution of cavities and failure in the microstructure. These findings outline key aspects of the nucleation process, which need to be examined experimentally. A number of missing features are identified in the mechanistic model, which need to be incorporated in future unified cavity nucleation theories
On the behavior of a three-dimensional fractional viscoelastic constitutive model
No full textIn this paper a three-dimensional isotropic fractional viscoelastic model is examined. It is shown that if different time scales for the volumetric and deviatoric components are assumed, the Poisson ratio is time varying function; in particular viscoelastic Poisson ratio may be obtained both increasing and decreasing with time. Moreover, it is shown that, from a theoretical point of view, one-dimensional fractional constitutive laws for normal stress and strain components are not correct to fit uniaxial experimental test, unless the time scale of deviatoric and volumetric are equal. Finally, the model is proved to satisfy correspondence principles also for the viscoelastic Poisson’s ratio and some issues about thermodynamic consistency of the model are addressed
Multiscale modelling of sintering in thermal barrier coatings
No full textMultiscale (analytical and computational) models have been developed based on a thermodynamic variational principle (TVP) to model sintering and eventual mudcracking in thermal barrier coatings (TBCs) made using the electron beam physical vapour deposition (EB-PVD) process. It is assumed that the sintering occurs by interfacial diffusion at local contacts between columns and driven by changes in interface free energy and elastic stored energy of the coating. The models link diffusional processes at the scale of contacting feathery columns with the macroscopic deformation and sintering response. In service, the columns can come into contact and sinter together. As sintering progresses there is a build up of strain energy in the system which reduces the driving force for sintering and leads to either complete or incomplete sintering of the TBC depending on the magnitude of effective modulus (E) of the coating. By seeding the coating with initial imperfections, different types of behaviour are observed depending on the value of E and the spacing between imperfections. For compliant coatings, the response is insensitive to the presence of imperfections and the coating fully sinters. At higher values of E, strain energy is released by the development of intercolumnar cracks in the coating, which can propagate to the interface with the TGO (thermally grown oxide), deflect into the interface and propagate, leading to spallation of regions of the coating and loss of thermal protection. It is observed that cracks develop at initial imperfections in the structure. The greater the spacing between imperfections the faster the development of cracks at these locations. If a TBC contains a distribution of imperfections there is progressive formation of cracks, with the average spacing decreasing with time, after an initial incubation period. The crack density eventually saturates to a constant value, which depends on the mechanical properties of the TBC.Initially, a crack spacing, CS, in the range 1.5H ≤ CS ≤ 3H has been predicted based on trapezoidal contact models. Here H is the thickness of the coating. Crack spacing predicted using this model is consistent in the lower range of experimentally observed crack spacing. However, axisymmetric contact models predict a crack spacing, CS, in the range 4H ≤ CS ≤ 8H, which is in good agreement with experimentally observed crack spacing range 3H ≤ CS ≤ 10H reported in the literature. Compared to the trapezoidal contact models, axisymmetric contact models more accurately predict the sintering response
A theoretical study of creep deformation mechanisms of Type 316H stainless steel at elevated temperatures
No full textThe currently operating Generation II Advanced Gas-Cooled Reactors (AGR) in the nuclear power stations in the UK, mainly built in the 1960s and 1970s, are approaching their designed life. Besides the development of the new generation of reactors, the government is also seeking to extend the life of some AGRs. Creep and failure properties of Type 316H austenitic stainless steels used in some components of AGR at elevated temperature are under investigation in EDF Energy Ltd. However, the current empirical creep models used and examined in EDF Energy have deficiency and demonstrate poor agreement with the experimental data in the operational complex thermal/mechanical conditions. The overall objective of the present research is to improve our general understanding of the creep behaviour of Type 316H stainless steels under various conditions by undertaking theoretical studies and developing a physically based multiscale state variable model taking into account the evolution of different microstructural elements and a range of different internal mechanisms in order to make realistic life prediction. A detailed review shows that different microstructural elements are responsible for the internal deformation mechanisms for engineering alloys such as 316H stainless steels. These include the strengthening effects, associated with forest dislocation junctions, solute atoms and precipitates, and softening effects, associated with recovery of dislocation structure and coarsening of precipitates. All the mechanisms involve interactions between dislocations and different types of obstacles. Thus change in the microstructural state will lead to the change in materials' internal state and influence the mechanical/creep property. Based on these understandings, a multiscale self-consistent model for a polycrystalline material is established, consisting of continuum, crystal plasticity framework and dislocation link length model that allows the detailed dislocation distribution structure and its evolution during deformation to be incorporated. The model captures the interaction between individual slip planes (self- and latent hardening) and between individual grains and the surrounding matrix (plastic mismatch, leading to the residual stress). The state variables associated with all the microstructure elements are identified as the mean spacing between each type of obstacles. The evolution of these state variables are described in a number of physical processes, including the dislocation multiplication and climb-controlled network coarsening and the phase transformation (nucleation, growth and coarsening of different phases). The enhancements to the deformation kinetics at elevated temperature are also presented. Further, several simulations are carried out to validate the established model and further evaluate and interpret various available data measured for 316H stainless steels. Specimens are divided into two groups, respectively ex-service plus laboratory aged (EXLA) with a considerable population of precipitates and solution treated (ST) where precipitates are not present. For the EXLA specimens, the model is used to evaluate the microscopic lattice response, either parallel or perpendicular to the loading direction, subjected to uniaxial tensile and/or compressive loading at ambient temperature, and macroscopic Bauschinger effect, taking into account the effect of pre-loading and pre-crept history. For the ST specimens, the model is used to evaluate the phase transformation in the specimen head volume subjected to pure thermal ageing, and multiple secondary stages observed during uniaxial tensile creep in the specimen gauge volume at various temperatures and stresses. The results and analysis in this thesis improve the fundamental understanding of the relationship between the evolution of microstructure and the creep behaviour of the material. They are also beneficial to the assessment of materials' internal state and further investigation of deformation mechanism for a broader range of temperature and stress
Les Bayadères : valses pour piano en deux suite
No full textOn Stone by R.P.C.Bildbeschriftung: "LES BAYADÈRES VALSES POUR PIANO EN DEUX SUITE.", "DÉDIÉES AUX ÈLÉVES DE MISS HIND. PAR GATIEN MARCAILHOU. SUITE.", "also by the same Author 'Indiana Valse' 2/ Lucrezia Valse, Bouton de Rose, and La Bayaderes Valse Each 3/", "PREMIÉRE SUITE. DEUXIÉME SUITE. Noemi La Joyseuse Pied de Biche Le Papillon bleu Fleur du matin Le Colibri", "Entd. Sta. Hall", "Price 3/-".Herstellungsangaben: "On Stone by R.P.C.“, "LONDON, Published only by R, COCKS & Co 6, New Burlington Street, MUSIC SELLERS TO HER MAJESTY, And in Paris au Bureau Central de Musique
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