Effects of surface roughness on thermo-mechanical fatigue life of a P91 power plant steel

P91 martensitic steel has now been widely used for power plant components such as steam pipe sections and headers. With the shift to renewable sources, traditional fossil power plants are increasingly expected to operate under so called “two shifting” conditions (high frequency start up/shut down cycles from a partial load condition) to match market demands. Such conditions increase the potential for large thermal stresses to be induced in thick walled components, making thermo-mechanical fatigue (TMF) and creep-fatigue interaction a life limiting concern. It is important to investigate the behaviour of P91 power plant steel under cyclic creep-fatigue interaction conditions in order to estimate the component remnant life under various possible operating strategies. Specimens used for TMF testing are commonly hollow (unlike solid specimens used in isothermal tests) to allow for higher cooling rates (with insignificant radial temperature variations) by injecting air. It is difficult to polish the internal surface to the same extent as the external surface of the specimen (with a roughness (Ra) of 0.8μm). Concerns have been expressed as to whether this type of uncontrolled surface roughness could significantly affect the fatigue life of the specimen since most fatigue cracks often initiate at the surface of the material. In this work, the roughness profile of the internal surface of the TMF sample is measured using Alicona optical profilometer. Resultant surface profiles are idealised and used to simulate distributions of stress and plastic strain under fatigue load using multi-axial visco-plasticity model. Concentration of stress and higher plastic stain accumulations are observed at the peak region of the roughness profile and crack initiations are expected to occur at those regions. Using accumulated plastic strain as a failure criterion for the fatigue, shorter fatigue lifetime is expected for specimen with rougher surface relative to the polished specimen. Optical and scanning electron microscopy (SEM) has been used to investigate the nature of the cracks initiating from the internal and external (polished) surfaces of a failed TMF test specimen

Viscoplasticity: A thermodynamic formulation

A thermodynamic foundation using the concept of internal state variables is given for a general theory of viscoplasticity, as it applies to initially isotropic materials. Three fundamental internal state variables are admitted. They are: a tensor valued back stress for kinematic effects, and the scalar valued drag and yield strengths for isotropic effects. All three are considered to phenomenologically evolve according to competitive processes between strain hardening, strain induced dynamic recovery, and time induced static recovery. Within this phenomenological framework, a thermodynamically admissible set of evolution equations is put forth. This theory allows each of the three fundamental internal variables to be composed as a sum of independently evolving constituents

Simulation of the fatigue behaviour of a power plant steel with a damage variable

The fatigue behaviour of a 9Cr power plant steel at a temperature of 600 °C was studied by uniaxial fatigue tests and microstructural analysis using electron microscopy. A continuum damage mechanics apporach was coupled to the constitutive equations of the Chaboche elasto-visco-plastic model to describe the fatigue behaviour of the 9Cr power plant steel at 600 °C. A stress partition method is introduced to understand the fatigue softening behaviour, and used to give an initial estimate of the material constants in the Chaboche model. Further optimisation procedures with plastic strain range dependency of the material constants were introduced in the optimisation procedure in order to accurately predict the material behaviour, especially after damage initiation. An ABAQUS UMAT subroutine was coded to allow the full life cyclic softening behaviour of a power plant component to be accurately predicted in a finite element calculation. The multiaxial capability of the coding is validated against notched bar test data

Thermodynamics of Local State: Overall Aspects and Micromechanics Based Constitutive Relations

The thermodynamics of irreversible processes, based on a set of internal state variables, is revisited, paying attention on two complementary aspects:- The Generalized Standard Models are shown to introduce too stiff constraints, both for kinematic hardening and for damage modellings. A slightly less restrictive approach is then considered, based on several independent  potentials and several independent multipliers;- The micro-macro approach of elastoplasticity is formulated through the Transformation Field Analysis of Dvorak and the use of a correction method. Moreover, based on previous works of Suquet and Nguyen, the approach is generalized with energy considerations, incorporating continuous fields of eigenstrains

On the thermodynamics of stress rate in the evolution of back stress in viscoplasticity

A thermodynamic foundation using the concept of internal state variables is presented for the kinematic description of a viscoplastic material. Three different evolution equations for the back stress are considered. The first is that of classical, nonlinear, kinematic hardening. The other two include a contribution that is linear in stress rate. Choosing an appropriate change in variables can remove this stress rate dependence. As a result, one of these two models is shown to be equivalent to the classical, kinematic hardening model; while the other is a new model, one which seems to have favorable characteristics for representing ratchetting behavior. All three models are thermodynamically admissible

An investigation of the failure mechanisms in high temperature materials subjected to isothermal and anisothermal fatigue and creep conditions

Many engineering components are subjected to conditions which have a detrimental effect on the materials from which they are made. Such components are used, for example, within high temperature regions of aeroengines (e.g. turbine discs) and power plant (e.g. steam pipes) and such conditions can include periods of isothermal and/or thermo-mechanical cyclic loading which may cause fatigue, excessive plasticity and creep. The combination of conditions to which the materials are subjected can have a strong influence on the failure mechanisms induced within the material. This study is concerned with the identification of the failure mechanisms which occur in RR1000 (a Nickel-based superalloy used in aeroengine turbine discs) tested under both isothermal and anisothermal cyclic conditions. The various types of test conditions applied to the specimens (e.g. waveforms which contain high temperature tensile conditions or alternatively low temperature tensile conditions) and the related failure mechanisms (e.g. intergranular, transgranular or mixed cracking), have been identified. Comparisons of the predictions of failure lives with experimental data from tested specimens, subjected to various test conditions, are also presented

Les dimensions culturelles, discursives et territoriales de production et de marchandisation d’un mythe moderne

La globalisation culturelle du sport est‑elle compatible avec le caractère mythique de certains clubs professionnels de football ? Cet article vise à interroger les dimensions culturelles, discursives et territoriales de production et de marchandisation du mythe moderne que constituerait le FC Barcelone (Barça), qui dit être plus qu’un club (Més que un club). S’interrogeant sur l’interaction entre mythe et globalisation sportive, on fait l’hypothèse que le système de mythification du Barça lui permet de transcender l’apparente antinomie entre son nationalisme revendiqué et sa globalisation recherchée. L’examen des éléments sportifs, organisationnels et identitaires de construction de ce mythe moderne combiné à l’analyse des modalités de son exploitation marchande et du rôle catalyseur de la culture barcelonista permet d’éprouver et de valider l’hypothèse initiale.Is the cultural globalisation of sport compatible with the mythical character of some professional football clubs? This article discusses the cultural, discursive and territorial dimensions of the production and commodification of the modern myth that FC Barcelona (Barça) is supposed to represent. Indeed, Barça claims to be « more than a club » (Més que un club). Reflecting on the interaction between myth and the globalisation of sport, our hypothesis is that the system of mythification of Barça allows the club to transcend the apparent antinomy between its proclaimed nationalism and its desired globalisation. The study of the sports, organisational and identity aspects that have helped to build this modern myth combined with the analysis of its marketing methods and the catalysing role played by the barcelonista culture will help us to test out and validate the initial hypothesis

Endochronic theory, non-linear kinematic hardening rule and generalized plasticity: a new interpretation based on generalized normality assumption

A simple way to define the flow rules of plasticity models is the assumption of generalized normality associated with a suitable pseudo-potential function. This approach, however, is not usually employed to formulate endochronic theory and non-linear kinematic (NLK) hardening rules as well as generalized plasticity models. In this paper, generalized normality is used to give a new formulation of these classes of models. As a result, a suited pseudo-potential is introduced for endochronic models and a non-standard description of NLK hardening and generalized plasticity models is also provided. This new formulation allows for an effective investigation of the relationships between these three classes of plasticity models

Finite strain viscoplasticity with nonlinear kinematic hardening: phenomenological modeling and time integration

This article deals with a viscoplastic material model of overstress type. The model is based on a multiplicative decomposition of the deformation gradient into elastic and inelastic part. An additional multiplicative decomposition of inelastic part is used to describe a nonlinear kinematic hardening of Armstrong-Frederick type. Two implicit time-stepping methods are adopted for numerical integration of evolution equations, such that the plastic incompressibility constraint is exactly satisfied. The first method is based on the tensor exponential. The second method is a modified Euler-Backward method. Special numerical tests show that both approaches yield similar results even for finite inelastic increments. The basic features of the material response, predicted by the material model, are illustrated with a series of numerical simulations.Comment: 29 pages, 7 figure

A low cycle fatigue model for low carbon manganese steel including the effect of dynamic strain aging

International audienceCarbon manganese steel A48 (French standards) is used in steam generator pipes of the nuclear power plant where it is subjected to the cyclic thermal load. The Dynamic Strain Aging (DSA) influences the mechanical behavior of the steel in low cycle fatigue (LCF) at favourable temperature and strain rate. The peak stress of A48 steel experiences hardening–softening–hardening (HSH) evolution at 200°C and 0.4% s-1 strain rate in fatigue loading. In this study, isotropic and kinematic hardening rules with DSA effect have been modified. The HSH evolution of cyclic stress associated with cumulative plastic deformation has also been estimated