Hydrogen embrittlement of ASTM A 203 D nuclear structural steel

The influence of hydrogen on the mechanical properties of ASTM A 203 D nuclear structural steel has been studied by tension, bend and delayed-failure tests at room temperature. While the tension tests of hydrogen charged unnotched specimens reveal no change in ultimate strength and ductility, the effect of hydrogen is manifested in notched specimens (tensile and bend) as a decrease in ultimate strength (maximum load in bend test) and ductility; the effect increases with increasing hydrogen content. It is observed that for a given hydrogen concentration, the decrease in bend ductility is remarkably large compared to that in tensile ductility.Hydrogen charging does not cause any delayed-failure upto 200 h under an applied tensile stress, 0.85 times the notch tensile strength. However delayed failure occurs in hydrogen charged bend samples in less than 10 h under an applied bending load of about 0.80 times of the uncharged maximum load.Fractographs of hydrogen charged unnotched specimens show ductile dimple fracture, while those of notched tension and bend specimens under hydrogen-charged conditions show a mixture of ductile dimple and quasi-cleavage cracking. The proportion of quasi-cleavage cracking increases with increasing hydrogen content and this fracture mode is more predominant in bend specimens.The changes in tensile properties and fracture modes can reasonably be explained by existing theories of hydrogen embrittlement. An attempt is made to explain the significant difference in the embrittlement susceptibility of bend and tensile specimens in the light of difference in triaxiality and plastic zone size near the notch tip.     

Strength and fracture toughness of heavy concrete with various iron aggregate inclusions

An extensive experimental study of the mechanical properties and fracture properties of heavy concrete used mostly in the construction of radiation shielding structures is presented. The mixtures considered herein are developed according to the one adapted in the Kuosheng nuclear power plant in Taiwan; tests of the basic mechanical characteristics properties conform to the ASTM and the fracture properties are determined by the method proposed by Karihaloo and Nallathambi [RILEM Report 5, Fracture Mechanics Test Methods for Concrete (1991) 1]. A crack analysis using pre-cracked specimens and a dye technique was also conducted to examine the crack fronts and the corresponding residual strengths of heavy concrete. Test results indicated that the elastic modulus of heavy concrete is higher than that of regular mortar and increase with iron ore content. The compressive strength of heavy concrete also increases with iron ore content, while the tensile strength declines. The concrete including 40% metallic aggregate content by volume performs higher compressive strength and fracture toughness in this study.     

Effect of Temperature and Deformation Rate on Fracture Strength of Sintered Uranium Dioxide

The fracture strength of two kinds of UO₂ specimens possessing pores of different maximum sizes (60 and 140 μm) was measured in the range of room temperature ? 1,300°C by means of diametral compression testing. The fracture strength thus obtained proved to be smaller than any of the values reported by previous authors who mainly used bending tests. Finite element analysis showed that the method used in the present study should logically yield results close to the true tensile fracture strength. The descrepancies noted with the results reported from the other studies were attributable to the differences in the methods used for the measurements.

The fracture strength was found to remain almost constant in the relatively low temperature region (R.T.–800°C) beyond which the value increased with temperature (intermediate temperature region of 1,000–1,300°C). Electron-microscopic observations of the fractured surface indicated that the brittle-to-ductile transition temperature (T_c ) was situated between 800 and 1,000°C when the strain was applied slowly. Raising the strain rate proved to affect both fracture strength and T_c . These dependences of temperature and strain rate on the fracture strength are explained from the relation between dislocation velocity and deformation rate. Griffith's theory is cited to describe the relation between the largest pore size and fracture strength.     

Effect of microstructure on the cleavage fracture strength of low carbon Mn-Ni-Mo bainitic steels

The effects of the microstructure on the cleavage fracture strength of low carbon Mn-Ni-Mo bainitic steels were examined. A four-point bend test and double-notched bend specimens were used to measure the cleavage fracture strength of the alloys and identify the cleavage initiating micro-cracks, respectively. The cleavage fracture strength and DBTT of Mn-Ni-Mo bainitic steels were strongly affected by the alloy carbon content. The decrease in the alloy carbon content resulted in a decrease in the inter-lath cementite-crowded layers and higher cleavage fracture strength. Micro-cracks that formed across the inter-lath cementite-crowded layers were observed to initiate cleavage fracture. The width of these inter-lath cementite-crowded layers was accepted as a cleavage initiating micro-crack size in the micro-mechanical modeling of the cleavage fracture, and the measured cleavage strength values of the bainitic Mn-Ni-Mo steels were well represented by the modified Griffith relationship.     

Analysis of tensile deformation and failure in austenitic stainless steels: Part I - Temperature dependence

This paper describes the temperature dependence of deformation and failure behaviors in the austenitic stainless steels (annealed 304, 316, 316LN, and 20% cold-worked 316LN) in terms of equivalent true stress-true strain curves. The true stress-true strain curves up to the final fracture were calculated from tensile test data obtained at −150 to 450 °C using an iterative finite element method. Analysis was largely focused on the necking and fracture: key parameters such as the strain hardening rate, equivalent fracture stress, fracture strain, and tensile fracture energy were evaluated, and their temperature dependencies were investigated. It was shown that a significantly high strain hardening rate was retained during unstable deformation although overall strain hardening rate beyond the onset of necking was lower than that of the uniform deformation. The fracture stress and energy decreased with temperature up to 200 °C and were nearly saturated as the temperature came close to the maximum test temperature 450 °C. The fracture strain had a maximum at −50 to 20 °C before decreasing with temperature. It was explained that these temperature dependencies of fracture properties were associated with a change in the dominant strain hardening mechanism with test temperature. Also, it was seen that the pre-straining of material has little effect on the strain hardening rate during necking deformation and on fracture properties.     

Proposal of failure criterion applicable to finite element analysis results for wall-thinned pipes under bending load

In this work, a failure criterion applicable to large strain Finite Element Analysis (FEA) results was proposed in order to predict both the fracture mode (collapse or cracking) and the limit bending load of wall-thinned straight pipes. This work was motivated from the recent experimental results of Tsuji and Meshii (2011); that is, fracture mode is not always collapse, and the fracture mode affects the limit bending load. The key finding in comparing their test results and a detailed large strain FEA results was that the Mises stress distribution at the limit bending load of a flawed cylinder was similar to that of a flawless cylinder; specifically, in case of collapse, the Mises stress exceeded the true yield stress of a material for the whole “volume” of a cylinder with a nominal wall thickness. Based on this finding, a failure criterion applicable to large strain FEA results of wall-thinned straight pipes under a bending load that can predict both fracture mode and limit bending load was proposed and was named the Domain Collapse Criterion (DCC). DCC predicts the limit bending load as the lower value of either the M_c^FEA, which is the load at which the Mises stress exceeds the true yield strength of a straight pipe for the whole “volume” with a nominal wall thickness (fracture mode: collapse), or the M_c^FEAb, which is the load at which the Mises stress in a section of the flaw ligament exceeds the true tensile stress (fracture mode: cracking). The results showed that the DCC could predict the fracture mode appropriately and the experimental limit bending load fundamentally on the conservative side within a maximum 20% difference regardless of the fracture mode. Another advantage of the DCC is that it uses the true yield and true tensile strength as the critical strength of the material and not the ambiguous flow strength.     

Failure and fracturing analysis of concrete structures

Some aspects of fracture analysis of concrete structures are discussed in this article. In particular it is shown that when localized failure occurs (by macrofracture propagation or localization of strain) structural size effects come into play. Mesh dependent finite element solutions are then observed unless size effects are correctly accounted for.Tensile fracture is examined first. The “classical” discrete and smeared crack approaches are reviewed and their extension to nonlinear fracture models like the fictitious crack model and the crack band model is illustrated. The smeared crack approach coupled first with a tensile strength criterion, second with a linear elastic fracture mechanics criterion is then applied to the failure mode analysis of a PCRV.Plastic fracturing with localization into shear bands, strain softening, mesh dependence and its correction are examined next. The use of plasticity for tensile fracture simulation is also discussed.Finally numerical difficulties inherent to the modeling of softening behavior are investigated.     

应用VASP分析含氧碳化硅的拉伸性能

利用VASP模拟了含氧碳化硅晶体的理想拉伸过程,研究间隙位氧原子和替换位氧原子对碳化硅机械性能的影响。研究发现掺氧碳化硅的应力 应变曲线在峰值之后出现突然下降,说明含氧碳化硅的机械性能发生了变化。同时还分析了不同掺氧方式碳化硅模型的拉伸强度和杨氏模量,结果显示,含氧碳化硅的拉伸强度和杨氏模量均有不同程度的减弱。     

Evaluation of local deformation behavior accompanying fatigue damage in F82H welded joint specimens by using digital image correlation

By using digital image correlation, the deformation behaviors of local domains of F82H joint specimens welded using tungsten inert gas (TIG) and electron beam (EB) welding were evaluated during tensile and fatigue testing. In the tensile test specimens, the tensile strength decreased in the TIG-welded joints, and ductility decreased in both the EB- and TIG-welded joints. Because axial strain increased in the tempered heat-affected zone (HAZ) and led to the fracture of the TIG-welded joint, the strength was considered to have decreased because of welding. In fatigue testing, the number of cycles to fracture for the welded joint decreased to less than 40–60% of that for the base metal. For both fracture specimens, the largest value of shear strain was observed in the region approximately between the fine-grained HAZ and tempered HAZ; this shear strain ultimately led to fracture. Cavities and macrocracks were observed in the fine-grained HAZ and tempered HAZ in the cross sections of the fracture specimens, and geometrical damage possibly resulted in the reduction of fatigue lifetime.     

Effect of iodine partial pressure on the iodine stress corrosion cracking behaviors of Zr–Sn–Nb alloy using the ring tensile test

In order to evaluate the effect of iodine partial pressure on the iodine stress corrosion cracking (I-SCC) behaviors of a Zr–Sn–Nb alloy, ring tensile tests were conducted at 350 °C and in atmosphere without iodine and with iodine partial pressure of 10², 10³, and 10⁴ Pa, respectively. Results show that the maximum load, fracture displacement, tensile strength, and fracture energy of the Zr–Sn–Nb specimens decrease monotonically with the increase of iodine partial pressure. The fracture morphology of specimen with 10² Pa iodine exhibits two different fracture regions. One is the mixture form of ductile and brittle fracture, and the other is only ductile fracture. For the specimen with 10³ Pa, stair-shaped fracture surface is formed as a result of the alternative propagation of transgranular cracks and intergranular cracks. The critical iodine partial pressure of the Zr–Sn–Nb alloys is lower than 10² Pa under the present conditions.     

Analysis of tensile deformation and failure in austenitic stainless steels: Part II - Irradiation dose dependence

Irradiation effects on the stable and unstable deformation and fracture behavior of austenitic stainless steels (SSs) have been studied in detail based on the equivalent true stress versus true strain curves. An iterative finite element simulation technique was used to obtain the equivalent true stress-true strain data from experimental tensile curves. The simulation result showed that the austenitic stainless steels retained high strain hardening rate during unstable deformation even after significant irradiation. The strain hardening rate was independent of irradiation dose up to the initiation of a localized necking. Similarly, the equivalent fracture stress was nearly independent of dose before the damage (embrittlement) mechanism changed. The fracture strain and tensile fracture energy decreased with dose mostly in the low dose range <∼2 dpa and reached nearly saturation values at higher doses. It was also found that the fracture properties for EC316LN SS were less sensitive to irradiation than those for 316 SS, although their uniform tensile properties showed almost the same dose dependencies. It was confirmed that the dose dependence of tensile fracture properties evaluated by the linear approximation model for nominal stress was accurate enough for practical use without elaborate calculations.     

Tensile properties of K-doped W–3%Re

In this work, the tensile properties of K-doped W–3%Re were investigated. This material was fabricated by powder metallurgy and hot rolling on an industrial scale. It is expected that there would be improvement of the high-temperature strength, an increase of the recrystallization temperature, and a decrease in the ductile–brittle transition temperature (DBTT) of pure tungsten due to the dispersion of K bubbles and the addition of 3% Re. In addition, suppression of the formation of irradiation-induced defect clusters is also expected. Tensile tests in the temperature range from room temperature to 1800 °C were conducted. After the tensile tests, fracture surface observations were carried out using a scanning electron microscope (SEM). The tensile strength decreased with increasing test temperature. Elongation of K-doped W–3%Re was observed above 500 °C. The results of fracture surface observation showed that delamination of the layered structure occurred at 500, 700, and 900 °C and cracking along the grain boundaries occurred at 1500 and 1800 °C.     

Hoop strength and ductility evaluation of irradiated fuel cladding

The objective of this study is to evaluate the hoop-directional mechanical properties comprising strength such as yield strength and ultimate tensile strength as well as mechanical ductility such as uniform elongation and total elongation. Therefore, in this paper, the ring tensile tests were performed in order to evaluate the mechanical properties of high burn-up fuel cladding under a hoop loading condition in a hot cell. The tests were performed with Zircaloy-4 nuclear fuel cladding whose burn-up is approximately 65,000 MWd/tU in the temperature range of room temperature to 800 °C. All the experiments were carried out at a constant strain rate of 0.01/s.On the basis of the ring tensile tests for a high burn-up Zircalay-4 cladding, the following conclusions were drawn. Firstly, the mechanical properties are abruptly degraded beyond 600 °C, which corresponds to a design-basis accident condition such as a RIA. Secondly, the un-irradiated fuel cladding showed ductile fracture behaviors such as 45° shear type fracture, cup and cone type fracture, cup and cup type fracture and chisel edge type fracture. While the high burn-up Zircalay-4 cladding showed a brittle fracture behavior even at the high temperatures (e.g. over 600 °C) which are achievable during a RIA. Thirdly, in the case of the high burn-up Zircalay-4 cladding, the strength, ductility and the energy to break are strongly dependent on the material property itself which are degraded by oxidation and hydriding during an operation rather than the temperature. Fourthly, hydride rim formation in the vicinity of metal-oxide interface can play an important role in the degradation of the mechanical properties for high burn-up fuel cladding.     

国产A508-3钢小尺寸拉伸样品的拉伸颈缩行为研究

对不同厚度国产A508-3钢小尺寸拉伸样品进行了室温拉伸试验,分析了拉伸性能及颈缩段参数,并基于有限元逆运算构建了小尺寸拉伸样品拉伸过程的GTN（Gurson-Tvergaard-Needleman）细观损伤模型,研究了厚度对小尺寸拉伸样品拉伸颈缩行为的影响规律与机理。试验结果表明,小尺寸拉伸样品在变形过程中发生了弹性变形、均匀塑性变形和颈缩变形;随着样品厚度由0.75 mm降低至0.30 mm,屈服强度、抗拉强度和均匀延伸率无明显变化,非均匀延伸率及总延伸率逐渐降低,颈缩角逐渐增大,断裂角在厚度降低至0.50 mm后逐渐增大。GTN细观损伤模型中用于表征空洞形核和融合率的参数在0.30 mm样品中明显降低,此结果与小尺寸拉伸样品颈缩行为规律相互印证。     

Lattice strain and damage evolution of 9-12%Cr ferritic/martensitic steel during in situ tensile test by X-ray diffraction and small angle scattering

In situ X-ray diffraction and small angle scattering measurements during tensile tests were performed on 9-12% Cr ferritic/martensitic steels. The lattice strains in both particle and matrix phases, along two principal directions, were directly measured. The load transfer between particle and matrix was calculated based on matrix/particle elastic mismatch, matrix plasticity and interface decohesion. In addition, the void or damage evolution during the test was measured using small angle X-ray scattering. By combining stress and void evolution during deformation, the critical interfacial strength for void nucleation was determined, and compared with pre-existing void nucleation criteria. These comparisons show that models overestimate the measured critical strength, and require a larger particle size than measured to match the X-ray observations.     

Effect of irradiation dose on plastic deformation of vacuum hot pressed ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene is a type of semi-crystalline polymer which is widely used in artificial joints. Degradation of it’s mechanical properties under irradiation is an important aspect for its applications. In this study, molded and ram extruded material were gamma irradiated in vacuum at doses of 50 kGy, 100 kGy and 150 kGy and its uniaxial tensile properties were investigated at room temperature. The results showed that the tensile modulus, true ultimate stress and true ultimate strain were reduced as the dose increased. There was no significant difference of the true yield strain between non-crosslinked and crosslinked material, but the true yield stress of non-crosslinked material was significantly higher than the crosslinked material. The curvature of the true stress-true strain curve after yielding did not exhibit a significant relationship as a function of dose. However, both molded and ram extruded material showed a significant exponent relationship with dose.     

The behaviour of WB 35 piping material under dynamic loading in the high speed tensile test

For the determination of the strength-, deformation- and fracture behaviour of the material 17 MnMoV 6 4 (WB 35) which is used for piping components, tensile tests were carried out at different loading rates (monotonic and impact-type) on smooth and notched pipe strip specimens over a temperature range extending from − 30°C to 250°C.For the conduct of the tests a hydraulic high speed tensile machine having a free motion device was used; the velocity of impact was preset at ca. 7 m/s.With impact-type (dynamically) loaded specimens in general higher strength and deformation values were obtained than with monotonic (statically) loaded ones. In all of the specimens having low deformation values which were investigated microfractographically, ductile portions were found adjacent to the notch on the fracture surface.     

Local response of reinforced concrete to missile impacts

An experimental and computational study was undertaken to determine the response of reinforced concrete walls to impacts from postulated tornado and other missiles. The study included laboratory-scale missile impacts, experiments to characterize concrete, computational model development, and two-dimensional simulations of missile impacts. Impact experiments with rods and pipes on small reinforced concrete walls showed crushing, cratering, spalling, radial cracking, and plug formation. The mechanisms governing this material response appear to be crushing, shearing, and tensile fracture. Static triaxial and dynamic plate impact experiments were used to determine the material properties. Dynamic strengths were higher than static; tensile strengths were ten times as high. A CAP constitutive model developed for concrete described compaction, Mohr-Coulomb yielding, and tensile separation following tensile strain accumulation. Model parameters were derived separately from the dynamic and the static data. Two-dimensional computational simulations were made of a rod impact experiment with threshold cracking using both static and dynamic parameters. The correct locations of fractures were predicted with the static parameters, but penetration and severity of failure were overpredicted. Penetration distance was correctly given with the dynamic parameters, but fracture was underpredicted. A model combining dynamic shaer and compaction properties with intermediate-rate tensile properties may be appropriate.     

添加纳米Y2O3的ODS-HT9钢的显微结构和性能

为了研究纳米Y₂O₃对HT9钢的显微结构和力学性能的影响,采用粉末冶金工艺,制备了纳米Y₂O₃含量为0.1%~0.9%的ODS-HT9钢样品,测定了样品的抗拉强度、伸长率、维氏硬度等力学性能,利用透射电子显微镜（TEM）观察和分析了样品中纳米Y₂O₃颗粒的分布状况、形状和相结构,利用扫描电子显镜（SEM）观察了样品拉伸断口的形貌。研究表明,球磨和热压烧结后,纳米Y₂O₃颗粒能够均匀地分布于基体中,相结构和形状未发生明显变化。弥散分布的纳米Y₂O₃硬质颗粒,具有明显的弥散强化作用,导致ODS-HT9钢的抗拉强度和维氏硬度随Y₂O₃含量的增加而显著增加,伸长率显著降低。Y₂O₃含量低于0.7%时,样品以韧性断裂为主,进一步增加含量,断裂方式将由韧性断裂转变成脆性断裂。纳米Y₂O₃含量为0.3%~0.5%的ODS-HT9钢,抗拉强度达到了913~936 MPa,伸长率为10.7%~11.2%,具有良好的综合力学性能。本文研究结果有助于ODS-HT9钢高温性能的研究及其在反应堆中的实际应用。      

B4C-Al中子吸收材料拉伸性能及断裂机理

为评价粉末冶金法制备的B₄C-Al中子吸收材料的力学性能,采用静态拉伸的试验方法研究板材的室温及高温拉伸性能,并运用扫描电镜进行断口形貌观察,对复合材料的断裂机理进行讨论。研究表明：室温下,B₄C质量分数为30%的B₄C-Al复合板材的屈服强度为200 MPa,抗拉强度为250 MPa,断后伸长率为2.5%;在实验范围内,随着温度的升高,材料的抗拉强度和屈服强度下降,并趋于一致,材料的断后伸长率增加并达到最大值。复合材料的断裂宏观表现为脆性断裂,断裂是由基体内微孔隙成核、聚集和基体与增强相的界面脱粘断裂的共同作用造成的。