Hydrogen enhanced fatigue in full scale metallic vessel tests – Results from the MATHRYCE project

This study investigates the fatigue life of CrMo pressure vessels for hydrogen storage by hydraulic and hydrogen pressure cycle tests. Two different sized cylinders have been tested; 35 L inner volume and 28 MPa working pressure (WP) and 198 L volume and 41 MPa WP. On the inner surface of the cylinders U-shaped notches of different depths were machined by electro discharge machining technique. The initial notch sizes were designed based on a two stage fatigue predictive model based on fracture mechanics to develop through wall cracks in the deepest notches after about 50,000 hydraulic cycles together with crack propagation of the intermediate notches and crack initiation in the smallest. The cylinders were cycled between the nominal pressure of 2 MPa and the WP until leak before break (LBB). Strain gauges were placed at the external surface of the cylinders in correspondence of the internally machined notches. On the notches which developed through wall, the strain showed a progressive decrease followed by an increase of the hoop strain during the final stage of crack propagation until LBB failure. Hydrogen effect was clearly identified by the reduction in the number of cycles to failure comparing tests in hydrogen and in oil. Subsequent failure analysis at the end of each test revealed a typical trans-granular fatigue crack surface morphology (with fatigue striations) for tests in oil, while quasi cleavage and intergranular fracture appearance were found for hydrogen tests.     

Designer Viscoelastic Materials Tailored to Minimize Probabilistic Failures for Thermal Stress-Induced Dynamic Column Creep Buckling

ABSTRACT

The dynamic response of linear viscoelastic temperature-dependent prismatic columns is investigated under axial compressive loads and due to thermal stresses and moments. Creep buckling instabilities and probabilities of material failures are analyzed to determine column life or survival times. Optimum designer materials are studied to minimize thermal stress and axial load effects while concurrently lowering failure probabilities and extending survival times.     

Damage nucleation in nuclear graphite

Abstract

This paper reports the use of advanced materials characterisation techniques, X-ray microtomography and surface strain mapping by electronic speckle pattern interferometry (ESPI), to study the mechanisms of damage nucleation in polygranular nuclear graphite. It is found that strain localisation occurs owing to the heterogeneous microstructure, giving rise to microcrack nucleation and coalescence before tensile failure. This produces a substantial damage zone at stress concentrations, such as notches and crack tips, which can be observed directly and in situ. Crack propagation occurs by the coalescence of microcracks in the damage zone. The measured R curve is a consequence of both frictional bridging in the crack wake and energy dissipation in the microcrack process zone.     

THERMAL SHOCK FRACTURE IN A W-Cu DIVERTOR PLATE WITH A FUNCTIONALLY GRADED NONHOMOGENEOUS INTERFACE

The plane elasticity solution is presented in this article for the crack problem of a W-Cu divertor plate under thermal shock. The material is made of a graded layer with exponentially varying thermomechanical properties bonded between a homogeneous substrate and a homogeneous coating and is subjected to a cycle of heating and cooling on the coating surface of the material. The surface layer contains an embedded or a surface crack perpendicular to the boundaries. Using superposition the problem is reduced to a perturbation problem in which the crack surface tractions are only external forces. The dimensions, geometry, and loading conditions of the original problem are such that the perturbation problem may be approximated by a plane strain mode I crack problem for an infinite divertor plate. Fourier transforms are used to formulate the crack problem in terms of a singular integral equation. After giving some sample results regarding the distribution of thermal stresses, stress intensity factors for embedded and surface cracks are presented. Also included are the results for a crack/contact problem in a divertor plate that is under compression near and at the surface and tension in the interior region.     

Testing for hydrogen—a-106 steel compatibility

Hydrogen compatibility of A-106 pipeline steel was systematically investigated for various stress states, strain rates and stress ranges using different specimen geometries and loading configurations. Test samples were either exposed to hydrogen during testing or had been precharged by exposure to gaseous hydrogen at pressure to 14 MPa. Ductility reduction, changes in fracture mode, decreases in fracture energy and reductions in low cycle “ratcheting” (fatigue) life were observed although no delayed failures were seen. These adverse effects were associated with the severity of the stress state to which the material was subjected and with geometrical constraints such as machining marks, scratches and notches.     

Niobium-clad 304L stainless steel PEMFC bipolar plate material: Tensile and bend properties

Niobium (Nb)-clad 304L stainless steel (SS) is currently under consideration for use as a bipolar plate material in polymer electrolyte membrane fuel cell (PEMFC) stacks. Because metal bipolar plates will likely be formed by stamping, the sheet-metal properties of this material were characterized in both the as-rolled and an optimized annealed condition via a series of bend and quasi-static tensile tests. Results from tensile testing demonstrate that annealing significantly softens and thereby improves the ductility of the material. Bend test results indicate that springback is nearly independent of the bend direction relative to rolling direction for both the as-rolled and annealed conditions. In the as-rolled condition, springback is also nearly independent of specimen orientation (i.e. whether the cladding layer is on the inside or outside of the bend). However, in the annealed condition, springback does depend on the cladding orientation relative to bending and was found in all cases to be substantially lower than that observed in the as-rolled condition. Microstructural analysis of the specimens indicates that two failure conditions can potentially arise, dependent on the thermomechanical condition of the material. In the as-rolled condition, failure initiates via fracture through the Nb cladding. In the annealed specimens, failure can occur by brittle fracture of an interfacial intermetallic layer that forms during the annealing treatment. This generates a series of crack-induced pores along the interface between the Nb cladding and the SS core, which eventually leads to ductile failure of the Nb cladding via localized necking. However, the conditions required for this phenomenon to take place are fairly extreme and can be readily avoided in practice. In general, the results suggest that to achieve acceptable stamping tolerances, the material should be annealed prior to forming and the bipolar plate flow channel pattern should be designed such that extreme levels of strain at the cladding/core interface are avoided to mitigate the potential for partial delamination within the material.     

基于电测法的水轮发电机组上机架应力特性分析

为了解立式悬垂式水轮发电机组上机架的应力情况,以耿达水电站3G(#3水轮发电机组)为例,在拟定的典型工况下,运用电阻应变测量法(电测法)测试了上机架的力学性能。结果表明,3G上机架靠近中心体的两条对称的支腿腹板的应力变化梯度较大;应力集中主要发生在上机架支腿肋板拐点;3G上机架最大应力为64.606 MPa,安全系数为3.6,符合脆性材料的强度要求。     

Heat transfer from a cylinder in axial turbulent flows

Local convective heat transfer coefficients were measured on a two-diameter long cylinder in axial flows of air at conditions unexplored so far, by using thermochromic liquid crystals (TLC) coated on an electrically heated strip-foil consisting bonded to the external surfaces. The Reynolds numbers (Re) based on the cylinder diameter were between 8.9 × 10⁴ and 6.17 × 10⁵, and the flow in front of the cylinder was modified in some cases by the use of a turbulence generating grid, or by circular disc inserts of two sizes placed upstream of the cylinder. These created a major change in the local convective heat transfer coefficient distribution on the cylinder. Increase of the turbulence intensity from Tu < 0.1% to Tu = 6.7% at the same Re increased the average calculated Nusselt number Nu over the cylinder by 25%, and decreased the Nu non-uniformity over the surface. One of the flow modification inserts also reduced significantly the Nu non-uniformity. The position of flow reattachment was measured using tufts. Our heat transfer data agree well with the small amount if data published of others, when extrapolated to their conditions. Correlations between the Nu and Re in the form Nu = CRe^e were established and presented for the average Nu on the front, middle and rear cylinder surfaces, and the variation of the local exponent e was shown along the cylinder. Introducing a new technique, a TLC-coated heated flat plate mounted in the flow above the cylinder in the meridional plane was demonstrated to help visualize the flow field above the cylinder. A track of maximum convective coefficients on this plate was found similar in position to the stream line dividing the forward and backward flows in a case measured for the separated flow in a past study.     

Thermal Stresses Around a Crack in a Non-Homogeneous Diffusion Layer Between a Coating Plate and an Elastic Half-Plane

ABSTRACT

This article proposes a method for determining the thermal stress field around a crack in a thin non homogeneous layer located between a ceramic plate and a metallic half-plane. For these calculations, the crack surfaces are assumed to be insulated and a uniform heat flux flows perpendicular to the crack. The material properties of the layer are assumed to vary continuously from those of the ceramic plate to those of the half-plane. The Fourier transform technique is employed to transform the problem into a set of integral equations. These equations are solved by expanding the differences in the crack surface temperature and the crack surface displacements in a series of functions that are automatically zero outside the crack. The Schmidt method is then used to determine the unknown coefficients in the series. Using this procedure, the stress intensity factors are calculated numerically for several ceramic plate thickness values.     

Evaluation of fracture mechanics parameters for bimaterial compact tension specimens

Abstract

The elastic–plastic fracture mechanics parameter J and its analogous creep fracture parameter C* are widely used to measure the fracture resistance of a material. The non-linear component of the J and C* parameters can be evaluated experimentally using the η factor. For weldments, the η factor is dependent on the relative properties of the base (parent) and weld materials, particularly the mismatch in their yield strengths. In this work, the η factor has been evaluated using non-linear finite element analyses in a standard compact tension C(T) specimen for a power law material. A range of mismatches in base/weld material properties have been considered. A through thickness strip of weld material, of height 2h, has been modelled, which was positioned at the mid height of the specimen. The η factor has been evaluated for a range of crack lengths and power law hardening exponents under both plane stress and plane strain conditions and the results compared with literature where available. For a given crack length and weld width, the η solutions of the undermatched and overmatched conditions examined show a maximum variation of 12% from the mean value. A relationship has been proposed with respect to crack length for the C(T) specimen to describe the decrease in the η factor with an increase in mismatch ratio.     

THERMAL STRESS IN AN ORTHOTROPIC PLATE CONTAINING A PAIR OF COPLANAR CENTRAL CRACKS

Abstract

The thermal stress problem for a pair of coplanar central cracks contained in an orthotropic plate is investigated using the techniques of Fourier transforms and finite Hilbert transforms. The crack surfaces are subjected to symmetrical thermal loadings. Exact expressions for the temperature field, the crack shapes, and the thermal stresses in the crack plane are obtained for the case of constant temperature. The opening-mode stress intensity factors at the inner and outer crack tips are also obtained in closed forms in terms of the material properties and the distance between the cracks. The properties of the stress intensity factors are shown to be different from the shear-mode stress intensity factors because of the disturbance of a uniform heat flow by a pair of central cracks.     

Loading rate effect on ductile crack resistance of steels using precracked Charpy specimens

While loading rate effects were extensively investigated in the transition regime where fracture occurs in a brittle manner, they are comparatively much less studied in the ductile regime. The main objective of this paper is to provide experimental data on the effect of loading rate on the ductile fracture behavior and examine the relation that might exist between the various material properties. In particular, the crack resistance behavior of two ferritic steels, A533B plate and 20MnMoNi55 forging, at quasi-static and dynamic (impact) loading rates was examined.     

Hydrogen embrittlement and segregation of impurities in type 10MnNi2Mo pressure vessel steel

This paper summarizes the findings of a study in which the brittle fracture resistance and hydrogen embrittlement resistance of a 10MnNi2Mo steel were investigated both in the quenched and tempered state, and after a subsequent heat treatment that simulated the thermal cycling in the heat-affected zone around a weld joining thick-walled components. This simulating treatment included two alternative stress relieving procedures, at 650 and 580°C respectively. The simulating treatment caused marked coarsening of the prior austenitic grains; had no substantial effect on the static and dynamic fracture toughness, nor on the transition temperatures; but reduced the resistance against hydrogen embrittlement, and during the development of hydrogen-induced cracks produced some intercrystalline facettes. A supplementary study of surface segregation, by Auger electron spectroscopy on the free surfaces of the specimens, indicated that this incidence of intercrystalline failure may be at least partly ascribed to the pronounced segregation of phosphorus during stress relieving at both of the temperatures employed. This made an interesting comparison with 20Mn steel, where the simulating treatment profoundly reduced both the fracture and notch toughness levels but led to favourable resistance to hydrogen embrittlement. The fact that no intercrystalline failures were detected in hydrogen-charged 20Mn steel is ascribed in part to the lower observed segregation activity at the stress relieving temperatures, in part to the resultant microstructure of this non-alloyed steel.     

Surface coating and texturing on stainless-steel plates to decrease the contact resistance by using screen printing

Stainless steel is an attractive material for use in bipolar plates of polymer-electrolyte-membrane fuel cells, except for its high interfacial contact resistance (ICR). Inexpensive surface treatment is required to decrease the ICR. A carbonaceous conductive composite was coated on stainless-steel plate surfaces by using a screen-printing technique. A grid-like texture of the same material as the coating was also printed on the coated plate surface. The cross section showed that conductive carbon particles were well dispersed in the coating layer, which favors through-plane electrical conductivity. The coated and textured plates exhibited a much lower ICR than that of bare stainless steel. The ICR of textured plates was lower than that of coated plates under lower compaction pressures. A single cell with coated and textured bipolar plates exhibited higher power densities than that of bare stainless-steel bipolar plates.     

TWO CRACK GROWTHS IN A FUNCTIONALLY GRADED PLATE UNDER THERMAL SHOCK

This article examines the problem of two thermal cracks under a transient temperature field in a ceramic/metal functionally graded plate. When the functionally graded plate is subjected to thermal shock, multiple cracks often occur on the ceramic surface. It is shown that the crack paths are influenced by interaction between multiple cracks and a compositional profile of the functionally graded plate. Transient thermal stresses are treated as a linear quasi-static thermoelastic problem for a plane strain state. The crack paths of two cracks are obtained using the finite element method with mode I and mode II stress intensity factors.     

Fracture behaviour of radiolytically oxidised reactor core graphites: a view

Abstract

This paper provides a view on the fracture behaviour of polygranular graphites, used to moderate gas cooled nuclear reactors. Graphite is often cited as a classic example of a brittle material because failure, in tension, is associated with small strains. However, attempts to characterise the fracture behaviour of graphite by linear elastic fracture mechanics methods have been largely unsuccessful. Observations of graphite fracture show that elastic strain energy may be dissipated by the formation of distributed microcracks, and their formation may be responsible for non-linearity in the rising load–displacement curve. Progressive softening behaviour may also be observed in some specimens after the peak load. This type of load–displacement behaviour is a characteristic of quasi-brittle materials. Radiolytic oxidation increases the proportion of porosity within reactor core graphite so that the microstructure becomes increasingly skeletal. Consideration is given to the fracture of radiolytically oxidised graphite to support an argument for quasi-brittle behaviour.     

Thermoelastic Problems of Thin Circular and Rectangular Plates

In this paper an attempt is made to determine the temperature, displacement and stress functions of a thin circular plate by applying finite Hankel transform and Laplace transform techniques. This plate that is assumed to be in the plane state of stress is subjected to axisymmetric boundary conditions. As a further simplification, special cases of the third kind of boundary condition are used on the two plane surfaces, while zero temperature is maintained on the outer curved surface of the thin circular plate. A particular case of the boundary conditions is discussed in detail, and numerical results are presented graphically. A mathematically similar problem is that of determining temperature distribution, displacement and stress functions on an edge of a thin rectangular plate with the stated boundary conditions. The results are obtained by applying finite Marchi–Fasulo transform and Laplace transform techniques.     

Annealing induced interfacial layers in niobium-clad stainless steel developed as a bipolar plate material for polymer electrolyte membrane fuel cell stacks

Roll-bonded niobium (Nb)-clad 304 L stainless steel (SS) is currently being developed as a metallic bipolar plate material for polymer electrolyte membrane fuel cell stacks. Prior work has shown that post-roll annealing significantly softens the constituent core and cladding materials, enhancing the ductility and formability of each. However under the vacuum annealing condition employed in the previous study (900 s, 982 °C (1800 F)), an interfacial layer was observed to form between the two bonded materials. Subsequent bending and flattening tests indicated brittle failure of this interfacial region under high strain conditions. The present study employs transmission electron microscopy to examine the composition, structure, and thickness of phases generated at the Nb/SS interface as functions of annealing temperature and time. Corresponding selected electron diffraction patterns indicate that above a threshold annealing temperature of ∼950 °C, the formation of (Fe_1−xCr_x)₂Nb appears to control the failure behavior of the Nb/304 L SS material. Annealing treatments conducted below this temperature generally avoid the formation of this intermetallic layer.     

Thermal Emission of a Disc Body of Semitransparent Material

INTRODUCTIONThethermalemissionofseveraJshapesofsemi-transparelltbodieshasbeeninvestigatedinthepastdecades.Forexample,Mcmahonderivedananalyt-icalexPressionforthehemisphericalendssivityofasemitransparentparallelslabl1];Gardonpresentedamethodforevaluatingtheemissivityofaglassslabl21;byimprovingtheGardon'smethod,Isardcalculatedtheendssivitiesofglassspheres,cylindersandtubes,andobtainedanapproximateexpressionwithtwoex-ponentialtermsl3];TuInerandLove,byusingtheMollterCarlomethod,simulatedthe…     

Monitoring of stress corrosion cracking of sensitised 304H stainless steel in nuclear applications by electrochemical methods and acoustic emission

Abstract

A hybrid monitoring technique for stress corrosion cracking (SCC) has been developed that employs simultaneously localised corrosion monitoring, electrochemical noise and acoustic emission (AE) techniques. The application of the hybrid technique for detection of SCC initiation and propagation in sensitised 304H stainless steel in dilute tetrathionate solutions at ambient temperature is reported. Initial result shows that SCC initiation and its early stage propagation can be detected by the localised corrosion monitoring and electrochemical noise methods. The dimensions of the crack can be estimated from the charge values derived from the detected transients. The locations of AE events determined using two sensors are in good agreement with the locations of cracks observed in the specimen. The AE technique is sensitive to rapid crack propagation, but does not appear to be sensitive to SCC initiation and early stage propagation for the present material environment load combination. It is postulated that AE is sensitive to SCC propagation involving a relatively large volume of plastic deformation. On the basis of test results and on information from the literature, it is suggested that in this material environment system SCC cracks initiate via slow anodic dissolution at the chromium depleted grain boundaries. Subsequently, elemental sulphur adsorbed on the surface around the crack tip catalyses the entry of hydrogen atoms produced by the hydrogen reduction reaction into the steel matrix ahead of the crack tip; this hydrogen accumulates gradually over a relatively long period of time and preferentially at carbide/matrix interfaces, eventually causing hydrogen induced brittle fracture along grain boundaries.