Prediction of J–R curves of thin-walled fuel pin specimens in a PLT setup

Recently, a Pin-Loading-Tension setup has evolved for evaluation of fracture behavior of thin-walled tubular specimens which are machined from nuclear reactor fuel pins. In this work, the geometric functions required for estimation of plastic component of J-integral from experimental load–load-line displacement data has been derived by carrying out a detailed 3D finite element analysis of the Pin-Loading-Tension test setup using the concept of limit load. The fracture resistance behavior (in terms of J–R curve) of the fuel pin specimens have been derived using: (a) multiple-specimen and (b) single specimen load-normalization technique.     

Surface acoustic wave measurements of surface cracks in ceramics

An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.     

Observation of crack microstructure in oxides and its correlation to oxidation and hydrogen-uptake by 3D FIB Tomography – case of Zr-ZrO2 in reactor

Abstract

Zirconium based alloys are used as fuel claddings in Light Water Reactors due to their good resistance to degradation and low neutron absorption cross section. However, life limiting processes occur during the service of the cladding such as oxidation and hydrogen-uptake. During the oxidation of the material, hydrogen enters the metal and it precipitates as brittle hydrides. In this study the 3D microstructure of a high burn-up and a low-burnup LK3/L Zircaloy-2 cladding is characterized and compared using FIB Tomography. 3D reconstruction of the oxides of the claddings shows that the crack volume fraction increases with the number of cycles in the reactor, reducing its protectiveness against further corrosion and H-uptake. The visualization of the metal-oxide interface revealed that the oxidation of the hydrides in the metal could induce crack formation in the oxide and therefore it could be one of the causes of the increasing oxidation and H-uptake in this material.     

Effect of thermomechanical process on microstructural evolution,mechanical and corrosion properties of zircaloy-4 tubes of mock-up dissolver vessel

This paper presents the methodology used for manufacturing the Zircaloy-4 tubes required for the Zircaloy-4 mock-up dissolver assembly. The evolution of the microstructure at different stages of production of Zircaloy-4 tubes was characterized using scanning electron microscopy (SEM). Microstructural evolution follows the sequence as dendritic structure (as-cast ingot) → Widmanstätten structure (β-quenched) → bimodal grain size (hot extruded) → heterogeneous deformed structure (pilgered) → partially recrystallized structure (final annealed). High strength and low ductility were obtained for the Zircaloy-4 tubes at pilgered condition due to grain size refinement and work hardening during the pilgering process. Compared with the pilgering stage, the Zircaloy-4 tubes in the final annealed condition exhibited moderate strength and high ductility due to the partially recrystallized microstructure. Autoclaving was carried out to improve the corrosion properties of the pilgered Zircaloy-4 tubes in boiling 11.5?M nitric acid. When exposed to boiling 11.5?M nitric acid for 1000?h, a lowest corrosion rate of 0.003 mpy was obtained for autoclaved Zircaloy-4 tubes. Laser Raman Spectroscopy (LRS) analysis confirmed that the origin of passivity of pilgered and autoclaved Zircaloy-4 tubes was due to the presence of a protective passive film composed of ZrO₂.     

The Effect of Hydrogen on Fatigue Properties of Metals used for Fuel Cell System

The effect of hydrogen on the fatigue properties of alloys which are used in fuel cell (FC) systems has been investigated. In a typical FC system, various alloys are used in hydrogen environments and are subjected to cyclic loading due to pressurization, mechanical vibrations, etc. The materials investigated were three austenitic stainless steels (SUS304, SUS316 and SUS316L), one ferritic stainless steel (SUS405), one martensitic stainless steel (0.7C-13Cr), a Cr-Mo martensitic steel (SCM435) and two annealed medium-carbon steels (0.47 and 0.45%C). In order to simulate the pick-up of hydrogen in service, the specimens were charged with hydrogen. The fatigue crack growth behaviour of charged specimens of SUS304, SUS316, SUS316L and SUS405 was compared with that of specimens which had not been hydrogen-charged. The comparison showed that there was a degradation in fatigue crack growth resistance due to hydrogen in the case of SUS304 and SUS316 austenitic stainless steels. However, SUS316L and SUS405 showed little degradation due to hydrogen. A marked increase in the amount of martensitic transformation occurred in the hydrogen-charged SUS304 specimens compared to specimens without hydrogen charge. In case of SUS316L, little martensitic transformation occurred in either specimens with and without hydrogen charge. The results of S-N testing showed that in the case of the 0.7C–13Cr stainless steel and the Cr–Mo steel a marked decrease in fatigue resistance due to hydrogen occurred. In the case of the medium carbon steels hydrogen did not cause a reduction in fatigue behaviour. Examination of the slip band characteristics of a number of the alloys showed that slip was more localized in the case of hydrogen-charged specimens. Thus, it is presumed that a synergetic effect of hydrogen and martensitic structure enhances degradation of fatigue crack resistance.     

Constraint effects at brittle fracture initiation in a cast ferritic steel

The fracture resistance of a cast low carbon manganese ferritic steel intended for containers for spent nuclear fuel has been analysed by combining several approaches. Based on data from three-point bend specimens with shallow and deep cracks the effect of crack tip constraint at brittle fracture initiation has been followed. Q-parameter was used for the constraint quantification. The crack length effect on the fracture toughness–temperature diagram has been analysed and peculiarities of fracture behaviour in the lower shelf region have been explained. The role of cleavage fracture stress in brittle fracture initiation under the influence of crack tip constraint has been analysed.     

Stress intensity factors for cracks in some fracture mechanics test specimens under displacement control

The stress intensity factors for cracks of varying length in some common fracture mechanics test specimens have been calculated when the displacement at the loading pins is held constant. For centre and edge cracked plates with a given crack length the stress intensity factor decreases with the decrearing separation of the loading pins. For a three-point bend specimen of span to width of 8 a plateau is observed in the stress intensity factory over a range 0.3< a/w <0.6. For CKS-type and T-type wedge-open-loaded specimens the stress intensity factor decreases monotonically with increasing crack length for a/w > 0.3. The implications of these results to fracture mechanics testing are discussed.     

Cold neutron tomography of annular coolant flow in a double subchannel model of a boiling water reactor

Dryout of the liquid coolant film on fuel pins at the top of boiling water reactor (BWR) cores constitutes the type of heat transfer crisis relevant for the conditions of high void fractions. It is a limiting factor in the thermal power, and therefore the economy, of BWRs. Ongoing research on multiphase annular flow, specifically the liquid film thickness, is fundamental not only to nuclear reactor safety and operation but also to that of evaporators, condensers, and pipelines in a general industrial context. We have performed cold neutron tomography of adiabatic air water annular flow in a scaled up model of the subchannel geometry found in BWR fuel assemblies today. All imaging has been performed at the ICON beamline at the neutron spallation source SINQ at the Paul Scherrer Institut in Switzerland. Neutron tomography is shown to excel in investigating the interactions of air water two phase flows with spacer vanes of different geometry. The high resolution, high contrast measurements provide spatial distributions of the coolant on top of the surfaces of the spacer, including the vanes, and in the subchannel downstream of the spacers.     

On The Problem of an Axial Semi-Elliptical Crack in a Hollow Ductile Cylinder

We revisit the problem of an axial crack in a hollow cylinder and attempt to show that the wall thickness is as a governing length scale for the design of such structures. In particular, yield based design should be performed for thin-walled tubes leading to a critical pressure. Large thick-walled tubes should on the other hand, be designed using the fracture mechanics criterion based on a material dependent fracture toughness.     

Microstructure-fracture toughness correlation in weld joints of Cr-Mo steel

The strength-toughness-microstructure relationship in relation to the micromechanics of a fracture process has been investigated in the weld joints of two alloys: 0.5 Mo and 2.25 Cr-1 Mo steels. These alloys are extensively used to fabricate super-heater tubes, boilers, piping, gas lines, etc., by welding. The applications require high temperature and pressure to be maintained during service. The crack initiation toughness and tearing resistance were evaluated using crack tip opening displacement/J-integral parameters at different temperatures. Quantitative analysis of micro-structure and fracture surfaces was used to study the micromechanics of fracture process in the heat-affected zone (HAZ) of the alloys. Molybdenum steel exhibited a higher percentage of ferrite and lower martensite content, while the other steel showed aligned carbide as the major constituent. The higher hardness and strength values in the HAZ and welding zone (WZ) of Cr-Mo steel, compared to molybdenum steel, may be attributed to the higher amount of martensite phase in the alloy. The higher initiation toughness at 200° C in both the alloys was reflected in the larger dimple size, compared to the size observed at room temperature. A tendency for void sheet formation was noticed in both alloys. Acicular ferrite and martensite appeared to be the most influential constituents affecting tearing resistance and initiation toughness.     

事故容错燃料包壳候选材料的研究现状及展望

2011年福岛核电站事故中,反应堆堆芯燃料中的锆合金包壳在事故工况下与高温水蒸汽发生剧烈氧化反应继而产生大量的氢气和热量,最终导致反应堆堆芯熔化和氢气爆炸,对社会和环境造成极大负面影响。自此之后,国内外纷纷展开对事故容错燃料的研究开发。相较于传统的UO2-Zr合金燃料体系,事故容错燃料能够在反应堆正常运行工况下维持或提高燃料性能,并在事故发生后相当长的一段时间内维持堆芯完整性,提供足够的时间裕量来采取事故应对措施。反应堆堆芯环境非常极端,包壳长期处于高温高压腐蚀介质中,同时还受到中子辐照的影响,因此新型包壳材料需要较好的耐腐蚀性和辐照稳定性。经不同研究者的研究评估,目前能够替代Zr合金的事故容错燃料包壳材料可分为陶瓷材料和金属材料两类:陶瓷材料主要以SiC/SiC复合材料为代表;金属材料主要有以FeCrAl为代表的Fe基合金和以Mo为代表的难熔金属及其合金。上述三种替代Zr包壳的材料各有其利弊,均未达到工程应用水平,并且都存在待解决的关键性问题。其中,FeCrAl合金的研发进展最快,目前在热学性能、力学性能、抗腐蚀性能、抗辐照性能等方面表现较好,但在工业加工和焊接等方面仍有待进一步改善。就SiC/SiC复合材料而言,由于SiC自身的高脆性而导致力学强度不足,不同的研究者提出了不同的结构设计思路试图降低包壳管失效概率,但包壳最终的结构设计仍未确定,而辐照引起的热导率急剧降低及连接密封和加工制造等方面还在不断研究中。Mo及Mo合金的力学性能和抗辐照性能较好,但自身抗腐蚀性较差,解决思路主要集中在提高钼纯度、调整合金的元素成分、进行表面涂层等方面。目前,对后两种材料包壳管的加工能力均未达到薄壁长管的工业制造水平。对于这几种候选包壳材料,需要建立属性数据库和一套完善的标准来衡量材料的质量。此外,还需开发相应的程序来评估包壳在堆内的行为。本文主要综述了SiC/SiC复合材料、FeCrAl合金、Mo及Mo合金三种候选包壳材料的研究进展,包括候选包壳材料的物理性质、耐腐蚀性能、力学性能、抗辐照性能、芯块-包壳力学与化学相互作用、在事故工况下的行为和工程应用等,综合分析了事故容错燃料包壳材料当前的研究现状,指出了各事故容错燃料包壳未来需集中解决的关键性问题。     

Uniform corrosion behavior of FeCrAl alloys in borated and lithiated high temperature water

The uniform corrosion behavior of model FeCrAl alloys as well as commercial Zircaloy-4 as reference material were investigated in 360℃borated and lithiated water.The results reveal that FeCrAl alloys exhibited better corrosion resistance than Zircaloy-4.It is found that the oxide films formed on the FeCrAl alloys are composed of outer Fe3O4 layer,inner layer consisting of compact spinel layer and porous spinel layer,and transition layer containing Al-Cr-rich oxides and matrix enriched with Fe.The spinel oxides in the inner layer are FeFe2-x-y-zCrxAlyMozO4.The corrosion mechanism of FeCrAl alloys in high temperature water is discussed.     

Towards Prediction of Failure in Ti‐6AI‐4V Aerospace Materials by Surface Observations

Abstract: This paper deals with the development of a methodology for the prediction of material failure in metallic aerospace alloys by evaluating changes in surface characteristics directly prior to unstable fatigue crack propagation. The study is based on in situ nondestructive characterisation of the depression zone ahead of the crack tip of fatigue‐pre‐cracked titanium alloy specimens subjected to static loading. A relationship between the surface characteristics of the deformation zone ahead of the crack and the stress intensity factor of the material was obtained. This relationship was common to a variety of microstructural conditions such as mill‐annealed and β‐annealed microstructures. Based on the analysis, prediction of the impending fracture in cracked samples of the material was enabled. The outcome of this study can be used for optimising the service life of structural components.     

Locking-free triangular plate element using polynomial incompatible approximation for analysis of cracked thick–thin plates

This work describes a new filament deposition in fused deposition modeling process through criterion based on mechanical stress. This criterion requires that the filaments’ directions to follow the principal directions of the stress in the sample. The article also presents several Crack-test specimens that have been printed with and without respect to this criterion. The fracture behavior of these specimens has been investigated. The results show that criterion leads to an improvement of 30% in the fracture toughness. Digital image correlation has been extensively used to study the local strain field in the specimens. The strain cartographies reveal a drastic change in fracture behavior. The modification of filament direction leads to “ductile-like behavior” in crack extension which is characterized by a large deformation zone associated with a slow crack growth rate during the crack propagation.     

CRACK INITIATION AND SMALL FATIGUE CRACK GROWTH BEHAVIOUR OF SQUEEZE-CAST Al-Si ALUMINIUM ALLOYS

Abstract— Fatigue strength, crack initiation and small crack growth behaviour in two kinds of squeeze-cast aluminium alloys, AC8A-T6 and AC4C-T6 were investigated using smooth specimens subjected to rotatary-bending fatigue at room temperature. Fatigue resistance of these alloys was almost the same as that of the wrought aluminium alloys because of their fine microstructure and of the decrease in defect size due to squeeze-casting. Fatigue crack initiation sites were at the eutectic silicon particles on the surface of specimens or at internal microporosity in the specimens. Crack initiation life, defined as a crack length of 50 μm on the specimen surface, was successfully estimated from an evaluation of initiation sites using fracture mechanics and the statistics of extrema. Small fatigue crack growth in the two kinds of alloys obeys the relation proposed by Nisitani et al. , namely that d(2c)/d N = C (σ_a/σ_B)ⁿ· (2 c ), where C is a constant and σ_B is the ultimate tensile strength. It is pointed out that an improvement in fatigue strength of cast aluminium alloys can be expected by refining the eutectic silicon rather than by an increase in static strength.     

Influence of Impact on the Mechanical Behaviour of the Gamma‐Based TiAl Alloy TNBV3B

The quasi‐static and fatigue behavior after impact of the TiAl alloy TNBV3B produced via three different processing routes—cast, forged and extruded—has been studied on flat and airfoil‐like shaped specimens making use of ballistic impact experiments. For impacts resulting in cracks the behavior can be described using a linear‐elastic fracture mechanics approach. The residual strength is described on the basis of the fracture toughness. The residual fatigue strength of impact‐cracked specimens is estimated on the basis of the threshold for crack growth of the TNBV3B alloys. However, when there is no visible crack or when the crack length is below the size of the deformed impact area, residual stresses and micro‐damage play a dominating role making the linear‐elastic fracture mechanics approach invalid. The deformation hardened zone in TiAl has been studied making use of micro‐hardness tests showing their extension and the degrees of deformation for different impact energies.     

Manifestation of Hydrogen and Low-Temperature Brittleness in Near-Threshold Cyclic Crack Resistance of Materials

We experimentally show that the realization of conditions of plane deformation at the tip of a fatigue crack is not sufficient for guaranteeing the unique dependence of the crack growth rate on the range of the stress intensity factor, which is explained by the effect of crack closure. We describe advantages and disadvantages of the effective range of the stress intensity factor as a parameter that determines the mechanical conditions for the propagation of a fatigue crack. We analyze the phenomenon of positive influence of strengthening factors (a decrease in the temperature of testing and hydrogenation) on the cyclic crack resistance of materials in a low-amplitude range of loading determined with regard for the effect of crack closure. The decrease in the crack growth rate and the increase in fatigue thresholds are intensified as the level of loading decreases and the ductility of materials increases. Differences in the influence of strengthening factors in low- and high-amplitude ranges of loading are explained by different mechanisms of fracture controlled by the shearing strength and the tensile strength, respectively. We give several examples of the mechanical behavior of materials that show the inversion of the influence of hydrogen on the resistance to fracture: fatigue fracture of smooth steel specimens in gaseous hydrogen, high-temperature corrosion fatigue of preliminary hydrogenated titanium alloys, and the influence of hydrogenation on the wear resistance of structural steels in the process of friction and cavitation and on the parameters of cutting of a tool steel.     

Embrittlement of Zircaloy-2 on exposure to ThO<Subscript>2</Subscript>–UO<Subscript>2</Subscript> and simulated high burn-up fuel (SIMFUEL) powders at 1200 °C

In order to assess the changes in mechanical properties of Zircaloy cladding at extreme conditions, tensile testing and hardness measurements were carried out on Zircaloy-2 specimens that were annealed with ThO₂–3.45% UO₂ at 1200 °C for 15 min. Similar studies were repeated with simulated high burn-up fuel powders based on ThO₂–3.45% UO₂. The above treatment resulted in increase in hardness and reduction in elongation in Zircaloy-2. This is attributed to the oxygen embrittlement of Zircaloy-2.     

Fracture behavior of notched specimens of TiAl alloys

Fracture behavior of a two-phase TiAl alloy was investigated using notched specimens. Fracture surfaces and metallographic sections of surviving notch in double notched specimens are observed. The fracture process of notched specimens of TiAl alloys was described as that several inter-lamellar cracks initiate and extend directly from the notch root and propagate preferentially along the interfaces between lamellae and stop at various obstacles. With increasing applied load, cracks connect with each other and propagate further by translamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, could be reducing the driving force for crack propagation. The higher toughness of near fully lamellar microstructure than that of finer duplex microstructure is attributed to the path of crack propagation. On the fracture surfaces of the finer duplex microstructure, more low-energy-spending interlamellar fracture facets are observed, which means that it is easier for crack to bypass a fine duplex lamellar grain with lamellae perpendicular to the main crack and to take a interlamellar path.     

Studies on fracture toughness and fractographic features in Fe---Mn base alloys

Fractographic examinations of fracture surfaces of single edge crack plate tension fracture toughness test specimens of some new Fe---Mn base maraging alloys have been conducted. The interrelations between the fractographic features, fracture toughness and other mechanical properties of these alloys have been studied. It is observed that the width of the stretched zone between fatigue and rapid fracture is related to K/σ_ys of the material where K is either K_IC, K_Q or the stress intensity for onset of microscopic slow crack growth. The stretched zone width is approximately equal to the average dimple size. Also it is of the order of the process zone size (calculated by modified Krafft's model) and the critical crack opening displacement in plane strain condition. Hahn and Rosenfield's model to estimate K_Ic was found to show much higher values in those cases where the fracture mode was predominantly cleavage, quasicleavage or intergranular.