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The Small Punch (SP) test with constant deflection rate is a miniature technique that can provide estimates on the material tensile properties. Linear correlations are usually used for relating the maximum force and displacement at maximum force, recorded during the SP test, to the ultimate tensile strength. Fitting coefficients used in the correlations are calibrated on data from flat SP specimens. SP test requires only a small amount of testing material which represents a clear benefit when irradiated samples have to be tested. Therefore, there is a considerable interest in using SP for testing fuel cladding material properties. In this study we show that the same correlation equations, albeit with adjusted fitting coefficients, can be used to estimate the ultimate tensile strength from tube SP specimens made out of P91 ferritic/martensitic and 15-15Ti austenitic stainless steel. The calculated fitting coefficients lead to reasonable estimates of the ultimate tensile strength at temperatures of up to 650 °C although the coefficients themselves have been computed at room temperature. The coefficients are more suited for assessing ductile materials as the models used for computing the coefficients do not take into account damage (degradation of the material stiffness) or crack initiation and propagation, observed during the SP tests of brittle material. Finally, using the calculated ratios of maximum forces and displacements at maximum forces, one can map the two values of a given curved SP test to the equivalent flat SP values. [ABSTRACT FROM AUTHOR]

The Small Punch (SP) technique is a miniature test used for characterizing irradiated materials or when a testing material is available only in small quantities. In this work Finite Element (FE) models are developed to support the parametric analysis of SP fuel cladding tube specimens in comparison to standard flat ones. FE analysis shows that there are practically no differences between circular and rectangular flat specimens. The tube specimen results in only slightly higher maximal force (F m ). However, F m is attained at significantly lower displacements. This is attributed to the curvature of the specimen. The friction linearly increases the F m and to a lesser extent the displacement at F m . FE analysis also shows that the yield stress and different hardening, while keeping ultimate tensile strength constant, practically do not affect the F m . Significant specimen deformation can be expected at already small (0.1 mm) puncher deflection, which could limit the applicability of the small punch test to ductile materials. Varying degree of clamping in the experimental procedure can cause large scatter in the yield stress estimates. In all cases good agreement between the simulation and experimental results was obtained, best with a friction coefficient of 0.2. [ABSTRACT FROM AUTHOR]

A finite element analysis is performed on the heat transfer process across the tank walls to determine the temperature distributions of hydrogen storage tanks during fast filling. The accuracy of the numerical model is shown by comparison between the experimental measurements and the computed results. A sensitivity analysis of the tank wall thermal conductivity, specific heat capacity, density and heat transfer coefficient between the tank's external surface and the ambient air is carried out and the resulting effects are described. The properties of the tank's composite layer have a larger effect on the temperature history on the tank external surface than the properties of the plastic liner. The heat transfer coefficient between the tank's external surface and the environment has a negligible effect during the filling but a significant impact during the holding time. Increasing the liner thickness significantly decreases the temperature in the composite layer. [ABSTRACT FROM AUTHOR]

Understanding and controlling early damage initiation and evolution are amongst the most important challenges in nuclear power plants, occurring in ferritic, austenitic steels and nickel based alloys. In this work a meso-scale approach to modeling initiation and evolution of early intergranular cracking is presented. This damage mechanism is present in a number of nuclear power plant components and depends on the material (e.g. composition, heat treatment, microstructure), environment and load. Finite element modeling is used to explicitly model the microstructure – both the grains and the grain boundaries. Spatial Voronoi tessellation is used to obtain the grain topology. In addition, measured topology of a 0.4 mm stainless steel wire is used. Anisotropic elasticity and crystal plasticity are used as constitutive laws for the grains. Grain boundaries are modeled using the cohesive zone approach. Different modeling assumptions/parameters are evaluated against the numerical stability criteria. The biggest positive contribution to numerical stability is the use of cohesive-type contact instead of cohesive elements. A small amount of viscous regularization should be also used along with the addition of a small amount of viscous forces to the global equilibrium equations. Two cases of grain boundary damage initiation are explored: (1) initiation due to normal separation and (2) initiation due to a combination of normal and shear separation. The second criterion significantly decreases the ductility of an aggregate and slightly improves the numerical stability. [ABSTRACT FROM AUTHOR]

Highlights: [•] The paper demonstrates the initiation and evolution of the intergranular cracking using cohesive zone in a 3D model. [•] Analytical expression on viscous regularization effect in cohesive element presented, recommendations on usage provided. [•] Statistical evaluation of normal grain boundary stress to applied load ratio for large polycrystalline aggregates. [•] The effect of increased grain boundary stiffness evaluated, resulting in improved stability. [ABSTRACT FROM AUTHOR]

Abstract: In this work a model, based on a X-ray diffraction contrast tomography data of a stainless steel wire with a diameter of 0.4mm is presented. As measured 3D grain geometry and crystallographic orientation of individual grains are directly transferred into a finite element model. Anisotropic elasticity and crystal plasticity constitutive laws are used for the bulk grain material while the grain boundaries are explicitly modeled using the cohesive zone approach. A parametric study on the effects of the grain boundary strength and other cohesive zone parameters on the macroscopic response and damaged grain boundary area of a polycrystalline aggregate is presented. Recommendations for the cohesive zone parameters values aimed at achieving low damaged grain boundary area during numerical tensile tests are given while at the same time taking into account the numerical stability of the simulations. [Copyright &y& Elsevier]

Abstract: Development of advanced models at the grain size scales has so far been mostly limited to simulated geometry structures such as for example 3D Voronoi tessellations. The difficulty came from a lack of non-destructive techniques for measuring the microstructures. In this work a novel grain-size scale approach for modelling intergranular stress corrosion cracking based on as-measured 3D grain structure of a 400μm stainless steel wire is presented. Grain topologies and crystallographic orientations are obtained using a diffraction contrast tomography, reconstructed within a detailed finite element model and coupled with advanced constitutive models for grains and grain boundaries. The wire is composed of 362 grains and over 1600 grain boundaries. Grain boundary damage initialization and early development is then explored for a number of cases, ranging from isotropic elasticity up to crystal plasticity constitutive laws for the bulk grain material. In all cases the grain boundaries are modeled using the cohesive zone approach. The feasibility of the approach is explored. [Copyright &y& Elsevier]

Abstract: An approach that combines computational fluid dynamics (CFD) and structural mechanics simulation is used for evaluating stresses in a divertor cooling finger. Local distributions of heat transfer coefficient (HTC) between the helium and inner surface of the thimble are used as a boundary condition for the structural mechanics analysis. Stresses calculated with a realistic non-homogeneous HTC distribution are compared with the results where averaged HTC values in three representative zones of inner thimble surface are used. The obtained maximum stresses are considerably higher in the case of realistic non-homogeneous HTC boundary condition. The highest thermal stresses in the tile–thimble assembly are obtained on the thimble inner surface, in the region where the highest thermal gradients due to jet cooling can be observed. The results also show that tetrahedral based finite element model underestimate the computed stresses, especially when a coarse mesh is used. [Copyright &y& Elsevier]

Abstract: A novel computational approach for simulation of intergranular cracks in a polycrystalline aggregate is proposed in this paper. The computational model includes a topological model of the experimentally determined microstructure of a 400μm diameter stainless steel wire and automatic finite element discretization of the grains and grain boundaries. The microstructure was spatially characterized by X-ray diffraction contrast tomography and contains 362 grains and some 1600 grain boundaries. Available constitutive models currently include isotropic elasticity for the grain interior and cohesive behavior with damage for the grain boundaries. The experimentally determined lattice orientations are employed to distinguish between resistant low energy and susceptible high energy grain boundaries in the model. The feasibility and performance of the proposed computational approach is demonstrated by simulating the onset and propagation of intergranular cracking. The preliminary numerical results are outlined and discussed. [Copyright &y& Elsevier]

Abstract: Stratified flows may form in pipelines under certain conditions and could lead to increased fatigue loading that was only marginally accounted for during the design phase of the second generation of nuclear power plants designed in accordance with the ASME Boiler and Pressure Vessel Code. Extension of operational license would require explicit account for fatigue loads imposed by stratified flows. This typically involves rather complex state-of-the-art computational technology, which may in some cases be combined with measurements of the temperatures at the outside surfaces of pipes, which comprise pressure boundary of the reactor coolant. A parametric study using detailed finite element analysis of the entire span of the pipe has been performed to quantify the possible range of fatigue loads and fatigue usage factors. The example taken was a typical pressurized water reactor pressurizer surge line containing stratified flow of cold and hot water. The investigated parameters include the film coefficients governing the heat transfer from the both fluids to the pipe wall and the velocity of the interface between then cold and hot water. The main results include the expected ranges of fatigue loading and usage factors given the range of investigated parameters. It is clearly shown that the choice of the film coefficients is essential to arrive at reliable fatigue estimate. Additionally, predictions of readings provided by hypothetical thermocouples at the pipe outer surface are provided. Some of their limitations are identified and discussed. [Copyright &y& Elsevier]

Abstract: The influence of the microstructural inhomogenities in materials on the life-time of safety significant components of nuclear power plants is still not fully characterized. One of the most important sources of inhomogenities in metals is their grain structure. Differences in crystallographic orientations of grains lead to different responses under the applied loads and increased stresses at the grain boundaries. Engineering tools which are used to assess a response of a structural component to the applied loads are typically based upon the mechanics of continuum and are not able to account for these effects. Dedicated polycrystalline aggregate models are therefore being developed to study the effects of the microstructure on the load carrying capabilities of materials. However, a limited number of finite element types can be used due to the geometrical complexity of such models. A procedure for testing the behaviour of different finite elements in simulations involving explicit models of randomly shaped and oriented grains described by crystalline elasticity and crystal plasticity is therefore proposed. Cumulative distributions of the stress/strain tensors in all integration points in the model are compared, enabling comparison of the “average” (macroscopic) as well as “extreme” (local) behaviour of different meshes. Such an approach provides an easy to use probabilistic measure of the quality of results obtained using different element types, formulations and mesh densities. [Copyright &y& Elsevier]

Abstract: The divertor concept for DEMO fusion reactor is based on modular design cooled by multiple impinging jets. Such divertor should be able to withstand a surface heat flux of at least 10MW/m2 at an acceptable pumping power. To reduce the thermal loads the plasma-facing side of the divertor is build up of numerous small cooling fingers. Each cooling finger is cooled by an array of jets blowing through the holes on the steel cartridge. The size, number and arrangement of jets on the cartridge influences the heat transfer and pressure drop characteristics of the divertor. Five different cartridge designs are analyzed in the paper. The most critical parameters, such as structure temperature, heat removal ability, pressure drop, cooling efficiency and thermal stress loadings in the cooling finger are predicted for each cartridge design. A combined computational fluid dynamics and structural model was used to perform the necessary numerical analyses. The results have shown that the cartridge design with the best heat transfer and pressure drop characteristics is not also the most favorable choice from the point of view of minimum stress peaks. [Copyright &y& Elsevier]

Abstract: Recent developments in experimental techniques are enabling researchers to non-destructively characterize complex spatial structures with multiple constituents, e.g. polycrystalline aggregates. However, a combination of the high level of detail of the experimental data and often extreme geometry complexity make the building of models of such structures highly difficult and demanding. Finite element (FE) pre-processor tools can often be inadequate, especially when the structure contains multiple constituents and when the model building process has to be automatized. This paper proposes a novel framework for automatic and parallelized generation of FE models from discrete spatial data (voxels) procured from experimental techniques, e.g. 3D X-ray diffraction microscopy or X-ray diffraction contrast tomography (DCT). The technique can also be applied to analytical spatial geometries. The framework consists of reconstructing the surfaces of different constituents from the experimental data, generating FE meshes of these surfaces, followed by volume meshing of the constituents interior while enforcing the already generated surfaces meshes. This approach assures a conformal mesh between the adjoining surfaces and at the same time enables a fully independent and parallel meshing of the constituents. The conformal mesh allows for a variety of connectivity models between the constituents, including layers of cohesive elements for simulating the grain boundaries. The applicability of the approach is demonstrated first by creating a FE model of a 400μm diameter stainless steel wire characterized in 3D by DCT. FE model generation of spatial Voronoi tessellations, representing models of polycrystalline aggregates with up to 5000 grains, is then demonstrated. Here, anisotropic elasticity and crystal plasticity constitutive laws are used to estimate the scatter of the macroscopic responses due the random nature of the grains’ crystallographic orientations. At 2000 grains this influence is shown to be very small. [Copyright &y& Elsevier]

Abstract: A divertor component of the forthcoming DEMO fusion reactor should be able to withstand heat flux loads larger than 10MW/m2. Successful design should withstand high flux loads for a number of load cycles since initially the DEMO reactor is expected to operate in a non-steady-state mode. Computations for evaluating the structural response of the divertor published so far have, however, been based on the stationary approach. A combined computational fluid dynamics and structural model for evaluating the structural response of a divertor under the non-stationary load conditions is therefore developed in this work. Heat transfer coefficients between the helium and inner surface of the thimble are calculated first by solving the helium steady-state flow equations. Spatially distributed heat transfer coefficients are then used as a boundary condition in a non-stationary thermo-mechanical analysis of the divertor. This analysis is performed for a number of load cycles under different surface heat flux levels. The model is validated against the EFREMOV test experimental conditions, designed to be close to reactor operation conditions. Good agreement of the highest temperatures on the tile’s top surface with the experimental data is obtained. The results suggest that there are three critical regions in the design where damage could initialize: (a) the thimble’s inner surface with the highest thermal gradients, (b) the tile’s outer surface and (c) the filler layer of the brazed tile-thimble joint where the temperature is higher than permissible. Post-examination data of experimental specimen confirm these conclusions as cracks were observed in the above mentioned areas (a) and (b), while melting of the layer (c) was also observed. [Copyright &y& Elsevier]

The dominant processes in the initialization and propagation of microstructurally short cracks include microstructural features such as crystallographic orientations of grains, grain boundaries, inclusions, voids, material phases, etc. The influence of the microstructural features is expected to vanish with distance from the crack tip. Also, the influence of the nearby microstructural features is expected to be smaller for a long than for a small crack Finally, a crack of sufficient length can be modeled using classical fracture mechanic methods. In this paper the approach to estimate the crack length with vanishing influence from the microstructural feature is proposed. To achieve this, a model containing a large number of randomly sized, shaped, and oriented grains is employed. The random grain structure is modeled using a Voronoi tessellation. A series of cracks of lengths from about Ito 7 grain lengths is inserted into the model, extending from a grain at the surface toward the interior of the model. The crack tip opening displacements are estimated and statistically analyzed for a series of random crystallographic orientation sets assigned to the grains adjacent to the crack. Anisotropic elasticity and crystal plasticity constitutive models are employed at the grain size scale. It is shown that the standard deviation of the crack tip opening displacement decreases from about 20% for a short surface crack embedded within a single grain to about 7% for a surface crack extending through seven grains. From the engineering point of view, a crack extending through less than about ten grain sizes is therefore considered to strongly depend on the neighboring microstructural features. [ABSTRACT FROM AUTHOR]

Abstract: A model of microstructurally short cracks that accounts for random grain geometry and crystallographic orientations is coupled with crystal plasticity constitutive model. A short crack is then inserted in the slip plane in one of the grains at the model top boundary and extended into one of the available slip planes of the neighboring grain at monotonic remote load of 0.96R p0.2. Crack tip opening (CTOD) and sliding (CTSD) displacements are then calculated for several different crystallographic orientations and crack lengths. As the crack is contained in a single grain the crystallographic orientation of the neighboring grain can change the crack tip displacements by up to 26%, however, the displacements change by up to a factor of 10, once the crack is extended beyond the grain boundary into the next grain. Significant CTSD values were observed in all the analyzed cases pointing to mixed mode loading. Another important observation is that the random crystallographic orientations of grains beyond the first two crack-containing grains affect the CTOD by a factor of up to 4.4. This effect decreases slightly with increased crack length. [Copyright &y& Elsevier]

Abstract: The paper presents an analysis of the effects of grain orientations on a short, kinked surface crack in a 316L stainless steel. The kinking of the crack is assumed to take place at the boundary between two neighbouring grains. The analysis is based on a plane-strain finite element crystal plasticity model. The model consists of 212 randomly shaped, sized and oriented grains, loaded monotonically in uniaxial tension to a maximum load of 0.96R p0.2 (240MPa). The influence that a random grain structure imposes on a Stage I crack is assessed by calculating the crack tip opening (CTOD) displacements for bicrystal as well as for polycrystal models, considering different crystallographic orientations. Since a Stage I crack is assumed, the crack is always placed in a slip plane. Results from a bicrystal case show that the maximal CTODs are directly related to the stiffness of the grain containing the crack extension. Anisotropic elasticity and crystal plasticity both contribute to this grain stiffness, resulting in maximal CTOD when Schmid factors are the highest on two slip planes. Such crystallographic orientation results in a soft elasto-plastic response. Anisotropic elasticity can additionally increase the softness of a grain at certain crystallographic orientations. Minimal anisotropic elasticity at the crystallographic orientations with the highest Schmid factors causes the CTOD to be maximized. Presuming that the crack will preferably follow the slip plane where the crack tip opening displacement is highest, we show that the crystallographic orientation can affect the CTOD values by a factor of up to 7.7. For a given grain orientation the maximum CTOD is attained when the crack extension deflection into a second grain is between −75.141° and 34°. For the polycrystal case we show that grains beyond the first two crack-containing grains change the CTOD by a factor of up to 3.3 and that the largest CTODs are obtained when placing the crack into a slip plane with crack extension that results in a crack extension being more perpendicular to the external load. [Copyright &y& Elsevier]

This paper deals with the correlation length estimated from a mesoscopic model of a polycrystalline material. The correlation length can be used in some macroscopic material models as a material parameter that describes the internal length. It can be estimated directly from the strain and stress fields calculated from a finite-element model, which explicitly accounts for the selected mesoscopic features such as the random orientation, shape and size of the grains. A crystal plasticity material model was applied in the finite-element analysis.Different correlation lengths were obtained depending on whether the strain or the stress field was used. The correlation lengths also changed with the macroscopic load. If the load is below the yield strength the correlation lengths are constant, and are of the order of the average grain size. The correlation length can therefore be considered as an indicator of first plastic deformations in the material. Increasing the load above the yield strength creates shear bands that temporarily increase the values of the correlation lengths calculated from the strain fields. With a further load increase the correlation lengths decrease slightly below the average grain size. If displacement boundary conditions are used, the correlation length calculated from strain field is lower then the one calculated from the stress field. The opposite is true, if stress boundary conditions are applied. However, with the exception of the load region where significant shear bands appear, both seem to follow similar qualitative rules. [Copyright &y& Elsevier]

A higher order spectral analysis of the cutting process is presented with the purpose of indicating the chatter. First, the information contents of the cutting force components are compared using a bicoherence estimator. The analysis reveals that all three components are equally informative. Additionally, the magnitude bispectrum''s and the bicoherence''s estimator efficiency in identifying chatter is studied with respect to three different cutting regimes. In two of the regimes, which are denoted as strong and weak chatter, several interactions between spectral components in both the magnitude bispectra and bicoherence estimators are outstanding. This suggests the presence of quadratic-type non-linearities in the dynamics of chatter. In the third chatter-free regime of cutting, the magnitude bispectrum and bicoherence estimators reflect almost insignificant interactions between spectral components. It is shown that both magnitude bispectrum and bicoherence estimators identify this reduction in the quadratic-type non-linearities. However, using the magnitude bispectrum is numerically slightly more efficient when compared to the bicoherence estimator, since calculating the magnitude bispectrum is numerically less demanding. Since only short time series of data are available for on-line characterization, the amount of averaging is often insufficient when calculating estimators. This leads to unreliable values of the estimators. In the present work we try to overcome this problem by utilising a combination of a standard segment and additional inner triangle averaging. This approach decreases the sensitivity of the estimator values to the small changes of input parameters. The presented combination is then applied in the bicoherence analysis of the cutting process. [Copyright &y& Elsevier]

• FE model captures crack initiation and propagation in Small Punch flat and tube specimens. • Good agreement between experimental and calculated force–displacement curves. • Model captures crack initiation in cold-worked materials with different levels of ductility. • Damage in the material affects force–displacement curves in vicinity of maximal force. The Gurson-Tvergaard-Needleman damage model is implemented in Small Punch (SP) finite element model with the purpose of evaluating the influence of crack initialization and propagation on the SP force–displacement (F - v) curves and the maximum force, F m , which is used for estimating the R m . Five materials, significantly different in R m and ductility are investigated. Numerical calculations are performed and compared to the experimental measurements to evaluate the accuracy of the model. It is shown that the model is capable of capturing crack initialization in cold worked 15-15Ti stainless steel samples with different levels of ductility, both in terms of the number of cracks and their positions. For these two materials and the more ductile P91, P92 ferritic/martensitic steels, the simulations indicate significant impact of damage on the F - v curves in the vicinity of the maximal force, F m . This can significantly influence the estimation of R m from the F m and v m points. However, the model tends to predict the crack initialization at a larger displacement compared to the experiments. [ABSTRACT FROM AUTHOR]

Abstract: We propose and demonstrate several tests to estimate the performance of the cohesive elements in ABAQUS for modeling grain boundaries in complex spatial structures such as polycrystalline aggregates. The performance of the cohesive elements is checked by comparing the computed stresses with the theoretically predicted values for a homogeneous material under uniaxial tensile loading. Statistical analyses are performed under different loading conditions for two elasto-plastic models of the grains: isotropic elasticity with isotropic hardening plasticity and anisotropic elasticity with crystal plasticity. Tests are conducted on an analytical finite element model generated from Voronoi tessellation as well as on a realistic finite element model of a stainless steel wire. The results of the analyses highlight several issues related to the computation of normal and shear stresses. The most severe issue is found within the plastic grain response where the computed normal stresses on a particularly oriented cohesive elements are significantly underestimated. Other issues are found to be related to topological constraints in the modeling space and result in the increased scatter of the computed stresses. [Copyright &y& Elsevier]

Abstract: The extension of operational licenses of second generation pressurized water reactor (PWR) nuclear power plants depends to a large extent on the analyses of fatigue usage of the reactor coolant pressure boundary. The reliable estimation of the fatigue usage requires detailed thermal and stress analyses of the affected components. Analyses, based upon the in-service transient loads should be compared to the loads analyzed at the design stage. The thermal and stress transients can be efficiently analyzed using the finite element method. This requires that a 3-D solid model of a given system is discretized with finite elements (FE). The FE mesh density is crucial for both the accuracy and the cost of the analysis. The main goal of the paper is to propose a set of computational tools which assist a user in a deployment of modular spatial FE model of main components of a typical reactor coolant system, e.g., pipes, pressure vessels and pumps. The modularity ensures that the components can be analyzed individually or in a system. Also, individual components can be meshed with different mesh densities, as required by the specifics of the particular transient studied. For optimal accuracy, all components are meshed with hexahedral elements with quadratic interpolation. The performance of the model is demonstrated with simulations performed with a complete two-loop PWR coolant system (RCS). Heat transfer analysis and stress analysis for a complete loading and unloading cycle of the RCS are performed. The main results include expected ranges of fatigue loading for the pipe lines and coolant pump components under the given conditions. [Copyright &y& Elsevier]

This paper presents an analysis of thermal fatigue tests of pipe components with 14 mm wall thickness made of 316L austenitic steel or P91 ferritic-martensitic steels. The tests are done by heating the pipes to 550 °C by induction heating and then cyclic cooling by water cooling resulting in secondary thermal stresses and an axial constant with a constant primary stress between 0 and 100 MPa. The damage evolution and crack depth is measured by X-ray tomography and time-of-flight-diffraction. A very complex cracking is observed for the cases with no axial primary load, and with increasing primary load dominant circumferential cracks are observed. The test are analysed by an engineering approach that included a thermal analysis, a cyclic-plasticity analysis and crack propagation. The formation of a 1 mm crack is estimated by fatigue curves. The propagation from a stipulated 1 mm axial or circumferential crack is done by plasticity corrected stress intensity factors in conjunction with Paris law. The fatigue life predictions are compared with the experimental observations and in general very good agreement is reached, in particular for the cases with a large primary load. The proposed approach can be used in an engineering analysis to predict the thermal fatigue life of a component. [ABSTRACT FROM AUTHOR]

This paper presents the segmented expanding cone-mandrel (SECM) test for thin-walled cladding tubes and quantifies its sensitivity to inherent uncertainty factors such as friction. The SECM test is then applied to study hydride re-orientation and embrittlement of pre-hydrided Zircaloy-2 cladding by cyclic thermo-mechanical loadings to induce radial hydrides followed by a ductility test. The paper shows that the SECM can be used to induce hydride re-orientation but the repeatability of the re-orientation needs to be improved. The loss of ductility from radial hydrides can be well quantified with respect to the radial hydride content. The ductility reduction from radial hydrides is however a three-dimensional problem and accurate quantification requires a three-dimensional model. [ABSTRACT FROM AUTHOR]