A local criterion for cleavage fracture of a nuclear pressure vessel steel

Experiments were performed on three heats of A508 class 3 steel in order to determine the mechanical conditions for cleavage fracture. These tests were carried out on various geometries including 4-point bend specimens and axisymmetric notched tensile bars with different notch radii which have been modelized using the finite element method. In one heat, the temperature range investigated was from 77 K to 233 K. It is shown that the cleavage resistance is increased by tensile straining. Moreover, the probability of fracture obeys the Weibull statistical distribution. All the results can be accounted for in terms of a local criterion based on Weibull theory and which takes into account the effect of plastic strain. In this criterion, the parameters which were experimentally determined are found to be temperature independent over the range 77 K to 170 K. The applicability of the approach proposed for cleavage fracture at the crack tip is also examined. It is shown that the experimental results published in the literature giving the variation of fracture toughness with temperature can be explained by the proposed criterion which predicts reasonably well both the scatter in the experimental results and theK _ICtemperature dependence.     

Investigations to determine the maximum permissible cooling velocity for ceramic components made of alumina

The presented paper discusses the problems of heat stress arising as a result of instationary temperature fields (thermally induced stresses) in ceramic components and includes the results of both theoretical and experimental investigations to determine the critical cooling velocity for cylindrical bodies made of alumina. The theoretical and experimental results are compared to show that calculating the stresses using the linearized elasticity theory is sufficiently accurate. The fracture condition is determined so that the fracture stress ascertained in a 4-point-bending experiment is defined as the maximum bearable tensile stress. The statistical theory according to Weibull is applied to transform this fracture stress onto the type of stress occurring during the cooling experiments. In addition to a short derivation of the equations for thermally induced stresses with temperature-dependent material values, this paper also describes the determination of the relevant material values and, in particular, that of the temperature dependencies necessary for the calculation. The experimentally determined results show, in addition to the influence of the thermal diffusivity of the material, a quadratical dependence of the critical cooling velocity on the size of the body, as is also to be expected from an analytical solution to the stress equation.     

Mechanical properties distributions of PM manganese steels analysed by Gaussian and Weibull statistics

Abstract

Powder metallurgy (PM) parts acceptance is determined by compositional and processing parameters and their controls. Statistical procedures are used for assessment; normal distribution and 'six sigma' appear to predominate. For fatigue of metallic materials and strengths of ceramics, fibres and composites, however, Weibull probability of survival analyses are widely used. The original analysis considers a threshold stress at which the probability of failure is zero. This stress is frequently taken to be zero, simplifying the analysis to two parameters. The yield stress has been suggested for the threshold stress, probably not sufficiently conservative for less ductile PM materials. A new three-parameter Weibull analysis, in which it is taken to be the fracture strength minus six standard deviations, is presented. Powder metallurgy manganese steels are under commercial consideration and this approach is applied to 12 variants of such laboratory processed specimens. It is compared with the two-parameter Weibull and Gaussian, the least conservative, analyses.     

全层TiAl基合金室温断裂机制的研究

通过拉伸、压缩、弯曲实验分析研究了全层(FL)组织TiAl基合金的断裂机制。研究发现:拉伸和压缩时材料抵抗裂纹的扩展能力不同,抗压强度远高于抗拉强度,这是由于两者的变形及断裂机制不同。TiAl基合金拉伸断裂机制为脆性解理断裂,压缩变形断裂是剪应力和正应力共同作用下的断裂,是准解理断裂。TiAl基合金的缺口弯曲断裂方式也为解理断裂,其断裂过程是先在缺口处产生微裂纹,一旦裂纹在缺口根部产生,由于材料已积累足够的能量使得材料快速失稳解理断裂。     

Analysis of fracture toughness in the transition-temperature region of an Mn-Mo-Ni low-alloy steel

This study is concerned with the analysis of fracture toughness in the transition region of an Mn-Mo-Ni low-alloy steel, in accordance with the ASTM E1921 standard test method. Elastic-plastic cleavage fracture toughness (K _Jc ) was determined by three-point bend tests, using precracked Charpy V-notch (PCVN) specimens, and relationships between K _Jc , the critical component of J (J _c ), critical distance (X _c ), stretch-zone width (SZW), local fracture stress, and plane-strain fracture toughness (K _Ic were discussed on the basis of the cleavage fracture behavior in the transition region. The master curve and the 95 pct confidence curves well explained the variation in the measured K _Jc , and the Weibull slope measured on the Weibull plots was consistent with the theoretical slope of 4. Fractographic observation indicated that X _c linearly increased with increasing J _c , and that the SZW had a good correlation with K _Jc , irrespective of the test temperature. In addition, the local fracture stress was independent of the test temperature, because the tempered bainitic steel used in this study showed a propagation-controlled cleavage fracture behavior.     

A general approach for predicting the drawing fracture load and limit drawing ratio of an axisymmetric drawing process

Based on Hill’s theory of plasticity and the Swift diffuse instability criterion, new theoretical models are proposed for predicting the drawing fracture load and limit drawing ratio (LDR) of an axisymmetric cup drawing. These models take into account the influence of triaxial stress state, anisotropy, strain hardening, bending, and tool geometry. By introducing both conventional and modified Hollomon’s equations, the influences of these variables on the constitutive relation of sheet steels are also analyzed. It is shown that the theoretical predictions of the drawing fracture load are in good agreement with experimental results for a wide range of sheet steels currently used in the automotive industry. Specific tool geometries are found to decrease the drawing fracture load and the LDR, because of increased triaxial stress states and bending effects at the critical section of the workpiece. The optimum punch-profile radius is found to be between 5.0 and 7.0 times the thickness of the sheet. Additionally, the role of both the anisotropy and strain-hardening properties of the sheet steels in determining the drawing fracture load and the LDR are, subsequently, discussed.     

Fatigue and fracture behavior of bulk metallic glass

Tensile, compressive, cyclic tension-tension, and cyclic compression-compression tests at room temperature were systematically applied to a Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti₅ bulk metallic glass for comprehensive understanding of its damage and fracture mechanisms. Under tensile loading, the metallic glass only displays elastic deformation followed by brittle shear fracture. Under compressive loading, after elastic deformation, obvious plasticity (0.5 to 0.8 pct) can be observed before the final shear fracture. The fracture strength under compression is slightly higher than that under tension. The shear fracture under compression and tension does not occur along the maximum shear stress plane. This indicates that the fracture behavior of the metallic glass does not follow the Tresca criterion. The fracture surfaces show remarkably different features, i.e., a uniform vein structure (compressive fracture) and round cores coexisting with the radiating veins (tensile fracture). Under cyclic tension-tension loading, fatigue cracks are first initiated along localized shear bands on the specimen surface, then propagated along a plane basically perpendicular to the stress axis. A surface damage layer exists under cyclic compression-compression loading. However, the final failure also exhibits a pure shear fracture feature as under uniaxial compression. The cyclic compression-compression fatigue life of the metallic glass is about a factor of 10 higher than the cyclic tension-tension fatigue life at the same stress ratio. Based on these results, the damage and fracture mechanisms of the metallic glass induced by uniaxial and cyclic loading are elucidated.     

100Cr6轴承钢的超高周疲劳性能研究

采用旋转弯曲疲劳、SEM+EDS、面扫描等方法,研究了试验用100Cr6轴承钢的高周及超高周疲劳性能,以及传统疲劳极限附近的失效概率分布,并统计了夹杂物的成分、尺寸等信息。结果表明试验钢100Cr6的传统疲劳极限为967 MPa,在其之下的960 MPa应力幅值条件下,部分试样通过107循环周次后仍出现疲劳断裂失效,非无限寿命。在传统疲劳极限附近,相比于正态分布,疲劳寿命数据更符合二参数威布尔分布。相比于金相检验和面扫描,疲劳+EDS方法更能发现尺寸较大且为刚性的D类和Ds类夹杂物。     

Through-thickness fracture of a Ti-V-N plate steel

The effects of microstructure on through-thickness fracture properties of a Ti-V-N plate steel have been determined directly by through-thickness tensile tests and indirectly by studying delamination fractures in longitudinal tensile and Charpy tests. The initiation of ductile fracture is primarily controlled by inclusions, but overall ductility is influenced by microstructure such that the tensile fracture strain is higher for ferrite-pearlite microstructures than for ferrite-bainite or ferrite-martensite. The cleavage fracture stress is lower for steels which have been rolled belowAr ₃ and contain deformed ferrite than for steels finish rolled aboveAr ₃. Measurements of true stress and true strain for fracture initiation qualitatively fit a model which assumes cleavage fracture occurs at a critical stress, ductile rupture at a critical strain, and a transition fracture mode comprising ductile initiation followed by cleavage. Formerly Visiting Research Fellow, Metals Technology Laboratories, CANMET, Ottawa, ON, Canada Formerly Research Scientist, Metals Technology Laboratories, CANMET     

Through-thickness fracture of a Ti-V-N plate steel

The effects of microstructure on through-thickness fracture properties of a Ti-V-N plate steel have been determined directly by through-thickness tensile tests and indirectly by studying delamination fractures in longitudinal tensile and Charpy tests. The initiation of ductile fracture is primarily controlled by inclusions, but overall ductility is influenced by microstructure such that the tensile fracture strain is higher for ferrite-pearlite microstructures than for ferrite-bainite or ferrite-martensite. The cleavage fracture stress is lower for steels which have been rolled belowAr ₃ and contain deformed ferrite than for steels finish rolled aboveAr ₃. Measurements of true stress and true strain for fracture initiation qualitatively fit a model which assumes cleavage fracture occurs at a critical stress, ductile rupture at a critical strain, and a transition fracture mode comprising ductile initiation followed by cleavage. Formerly Visiting Research Fellow, Metals Technology Laboratories, CANMET, Ottawa, ON, Canada Formerly Research Scientist, Metals Technology Laboratories, CANMET     

Room-temperature mechanical properties of cold-rolled thin foils of binary,stoichiometric Ni3Al

In our previous works, thin foils of boron-free stoichiometric Ni₃Al, with thicknesses ranging from 57 to 315 μm, were fabricated by cold rolling of single-crystalline sheets which were sectioned from directionally solidified ingots. In this article, the room-temperature mechanical properties of the 83 and 95 pct cold-rolled foils were examined. Depending on the initial rolling direction, the foils exhibited two types of deformation microstructures: a banded structure with dual {110} textures and a band-free structure with a single {110} texture. The 83 pct cold-rolled foils showed very high Vickers hardness numbers: 649 and 604 for the banded and band-free structures, respectively. The foils possessed very high tensile fracture stress (1.7 to 2.0 GPa), with no appreciable plastic elongation along the rolling direction. The fracture stress of the 95 pct cold-rolled foils was slightly higher than that of the 83 pct cold-rolled foils. The banded-structure foils showed slightly higher fracture stress than the band-free-structure foils at the 83 pct reduction, but there was no difference between both the structures at the 95 pct reduction. Although there was no appreciable tensile elongation, slip traces were clearly observed on the surfaces of the foil specimens after the tensile test, indicating traces of some plastic deformation. The 95 pct cold-rolled foils possessed bending ductility, which was estimated as about 12 pct elongation on the tension-side surface of the bent specimen.     

Mapping the Strain Distributions in Deformed Bulk Metallic Glasses Using Hard X-Ray Diffraction

The deformation behavior of Cu₄₅ Zr_46.5 Al₇Ti_1.5 bulk metallic glass (BMG) under bending was investigated in-situ using high-energy X-ray synchrotron diffraction. Samples were bent using two different benders with radii of 10 and 20 mm. The components of the strain tensor were determined from the change of positions of the first maximum of the diffracted intensity in reciprocal space. The procedure of data treatment was improved by the introduction of direct beam off-center correction. Comparing results for the two different bending radii, we found that the zero stress region does not necessarily lay within the central part of the specimen. Bending with smaller radius resulted in symmetric strain distribution, whereas a larger bending radius revealed strong asymmetry. Furthermore, bending with a smaller radius (10 mm) shows steeper strain gradients as compared with the situation in which the larger bending radius (20 mm) was used. Using a smaller bending radius implies reaching higher tensile/compressive stresses and reveals the signs of the plastic deformation, which are demonstrated as a saturation of elastic strains.     

Notch fracture inγ-titanium aluminides

The notch fracture behavior of twoγ-titanium aluminide alloys, having duplex and fully lamellar microstructures, has been investigated as a function of notch geometry and test temperature. The unnotched tensile properties and notch fracture loads are used to perform finite element analysis (FEA) to determine triaxial tensile stresses and effective plastic strains in the vicinity of notch roots. These results, together with fractographic examinations of notch failures, indicate that a crack nucleates in the triaxial tensile field when the effective von Mises stress just exceeds the uniaxial tensile yield stress. The high tensile stress component then propagates the nucleated microcrack to failure with local stress intensity reaching the toughness of the material. Thus, both plasticity and high tensile stress are required to cause notch failure.     

Nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in Fe-C-X alloys

The nucleation kinetics of proeutectoid ferrite allotriomorphs at austenite grain boundaries in Fe-0.5 at. Pct C-3 at. Pct X alloys, where X is successively Mn, Ni, Co, and Si and in an Fe-0.8 at. Pct C-2.5 at. Pct Mo alloy have been measured using previously developed experimental techniques. The results were analyzed in terms of the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries. Classical nucleation theory was employed in conjunction with the pillbox model of the critical nucleus applied during the predecessor study of ferrite nucleation kinetics at grain boundaries in Fe-C alloys. The free energy change associated with nucleation was evaluated from both the Hillert-Staffanson and the Central Atoms Models of interstitial-substitutional solid solutions. The grain boundary concentrations of X determined with a Scanning Auger Microprobe were utilized to calculate the reduction in the austenite grain boundary energy produced by the segregation of alloying elements. Analysis of these data in terms of nucleation theory indicates that much of the influence of X upon ferrite nucleation rate derives from effects upon the volume-free energy change,i.e., upon alterations in the path of theγ/(α + γ) phase boundary. Additional effects arise from reductions in austenite grain boundary energy, with austenite-forming alloying elements being more effective in this regard than ferrite-formers. By difference, the remaining influence of the alloy elements studied evidently results from their ability to diminish the energies of the austenite: ferrite boundaries enclosing the critical nucleus. The role of nucleation kinetics in the formation of a bay in the TTT diagram of Fe-C-Mo alloys is also considered. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University     

Creep deformation and damage evaluation of service exposed reformer tube

Abstract

This paper aims at studying the creep deformation behaviour and quantifying creep damage of ～11 years service exposed primary hydrogen reformer tube made of HP40 grade of steel in a petrochemical industry, in terms of Kachanov’s continuum damage mechanics model (K model) and Bogdanoff model (B model) based on Markov process. Hot tensile, conventional creep deformation under identical test conditions, optical microscopy and fractography were extensively carried out. The as received tubes did not possess any sign of degradation including voids or creep cavitations and decarburisation. There was indeed loss of tensile strength from room temperature to 870°C for the bottom portion of the tube due to aging and overheating. Accumulation of damage due to creep has been quantified through microstructural studies in terms of two damage parameters A and A*. Experimental scatter observed in creep deformation and creep strain rate curves of the material at 870°C and at various stress conditions, is probably due to scatter in creep cavitations/voids and also due to variation in the mode of fracture in top as well as bottom portion of the tube. From statistical point of view, Weibull distribution pattern for analysing probability of rupture due to void area, shifts with increase in true strain towards the higher population of void. The estimation of average time to reach a specific damage state from K model and B model is in close agreement with that of experimental data and can describe the sudden changes of the creep damage in the tertiary region as well.     

A kinetic model for ductile-brittle fracture mode transition behavior

A kinetic model for ductile-brittle fracture mode transition has been developed. In the ductile-brittle transition temperature range, brittle and ductile fracture are characterized in terms of thermally activated growth processes of tensile model I and shear mode II cracks, respectively. The thermally activated bond breaking in tension and shear are assumed to proceed under the condition of local stress relaxation due to dislocation nucleation at the crack tip. The disclocation nucleation rate is considered to be proportional to the strain rate. The tensile and shear fracture probabilities are defined by the product of the jump probability from the stretching to the breaking state of bonds at the crack tip, the active volume, and the density of microcracks or microvoids. The ductile-brittle transition temperature can be determined provided both the tensile and shear fracture probabilities are the same. The ductile-brittle transition temperature is given as a function of the activation energies for bond breaking in tension and shear, and dislocation nucleation, as well as the microstructural and mechanical parameters. This kinetic model provides a rationale for an experimentally determined linear relationship of the ductile-brittle transition temperature measured by small punch tests to that by Charpy V-notch tests on temper embrittled steels.     

砂岩Ⅰ型断裂的尺寸效应试验研究

通过对尺寸成等比例的砂岩试样进行三点弯曲试验,研究了I型断裂问题中试样尺寸对砂岩断裂参数的影响.结果表明:随着试样尺寸的增大,名义张拉强度减小,名义断裂韧度和破裂过程区长度增大,三者均为试样尺寸的函数.基于试验结果,引入有效破裂过程区长度cf,建立等效线弹性断裂力学尺寸效应模型,通过理论分析,发现其尺寸效应曲线介于理想脆性和理想塑性材料之间,体现出准脆性材料的尺寸效应特点,并计算得到了不同尺寸试样的破裂过程区长度等断裂参数.为验证该尺寸效应模型在砂岩中的正确性,采用数字图像匹配技术(DIC)测定相应尺寸砂岩试样的破裂过程区长度,测定结果和理论模型计算值相符.      

表面缺陷对单晶高温合金高周疲劳性能的影响

在定向凝固炉中采用螺旋选晶法制备了一种单晶高温合金试棒,标准热处理后加工成旋转弯曲高周疲劳试样,试样中间位置用电火花加工成不同尺寸的孔洞以模拟叶片的表面缺陷,在980 ℃、应力分别为400 MPa和500 MPa条件下,研究表面孔洞对合金高周疲劳性能的影响,用扫描电镜分析了疲劳试样的断口形貌.结果表明,与标准试样相比,带有孔洞合金的高周疲劳寿命都有不同程度的降低,随着表面孔洞尺寸增大,合金的疲劳寿命逐渐减小.在合金试样的高周疲劳断口上可见疲劳源区、裂纹扩展区和瞬断区.相对于标准试样,带有孔洞试样疲劳源除了试样表面,还有表面孔洞,所有试样都为多源疲劳断裂.与高温下拉伸持久的断裂机制不同,高温下旋转弯曲高周疲劳为类解理断裂.