Restoration of charpy toughness for “in service” temper embrittled steels

This paper describes a detailed examination which could be conducted during a planned outage. It is concerned with assessing reverse temper embrittlement of CrMoV steel turbine bolts after 120 000 h of service. A small section of material was removed from a non-critical location of all the 92 IP and HP bolts. From this section, the chemical composition, average hardness and average prior austenite grain size were measured. The toughness of the bolts was measured by Charpy impact testing and/or Auger electron spectroscopy. From the various parameters investigated, it was established that grain size and phosphorus level were the only factors which consistently identified whether a bolt was embrittled or non-embrittled. Indeed, at a phosphorus level of 0.01%, bolts with grain sizes less than 20 μm were not embrittled while those with larger grain sizes suffered reverse temper embrittlement during service. An embrittlement estimative diagram was established by plotting grain size versus phosphorus level (%P). This portrayed two distinct regimes, an embrittled and non-embrittled regime which were separated by a critical embrittled-non-embrittled interface which could be described by d × (%P) = 0.18 where d is the grain size in μm. Such an embrittlement estimate diagram represent a cogent and practical route in the identification of in-service embrittled bolts.     

Stresses and strains in a race track strap with a radial clearance

In certain situations it is desirable to transfer only tensile loads between two points in a structure whilst minimising any stress concentrations. In such circumstances a so-called race track strap can be utilised. The strap consists of two semi-circular elements that transfer the load, from bolts at each end, to two parallel flexible elements joining the whole together. The strap is loaded in tension by means of these bolts. This paper considers the situation where there is clearance between the bolts and the semi-circular elements of the strap. To develop an analytical model, it is assumed that engineering beam theory is applicable, that the influence of the membrane strains can be ignored and that the clearance is small as compared with the bolt and strap radii. It was found that the simple analytical model compared well with both finite element calculations and experiments.     

A CW ultrasonic bolt-strain monitor

There exists a need for a relatively inexpensive system for measuring strain in bolts. The torque wrench is one technique for straining bolts which has been widely applied. Unfortunately, friction in the bolt threads and between the nut and the work tend to make such a simple system inaccurate. In practice, a torque wrench is unacceptable for many situations where strain is critical. In this article, an ultrasonic technique is described which can indicate changes in bolt strain to better than one part in 10⁴. The technique is based on the one-dimensional propagating-ultrasonic-wave model and uses a new ultrasonic instrument called a Reflection Oscillator Ultrasonic Spectrometer which is a closed-loop feedback marginal-oscillator system that frequency locks the device to the peak of a mechanical resonance in the bolt. The instrument indicates a shift in the bolt resonance frequency due to elongation and changes in velocity of sound due to strain. Data are presented comparing a standard torque wrench to the ultrasonic monitor for different measured stresses on the bolt as well as for different bolt conditions. The strain instrument can be used to monitor changing stresses, to measure material properties and may be applied as a strain gage or load cell.     

Stress concentrations in bolt-thread roots

The apparent geometric origin of fatigue fractures in oversize shank bolts is considered in this report. Bolt-nut interaction between the bolt and both a standard nut and a special nut are investigated. The “frozen stress” method of three-dimensional stress analysis was used to determine the magnitudes and locations of points of stress concentration in plastic models of both the oversize shank bolt and a standard bolt in combinations with standard and special nuts. The results indicate that, for installations in which the special nut is used, bolt geometry may be the controlling factor in determining the fatigue life of the boltnut combination, because the stress-concentration factors due to the nut loading are of the same order of magnitude as the stress-concentration factors due solely to bolt geometry. On the other hand, for installations in which standard nuts are used, the nut-induced stress-concentration factors are much larger than those due to bolt geometry; so, bolt geometry will have less effect on the fatigue performance of the combination. If the nut is threaded onto the bolt so far that the first loaded thread of the nut is in the thread-runout area of the bolt, fatigue performance of the installation will be greatly reduced due to the combination of stress-concentration factors from thread loading and section change and, again, bolt geometry may be a prominent causative factor.     

动载荷下螺栓连接的优化设计

把螺栓法兰密封连接结构简化成-质量、弹簧、阻尼系统，其中阻尼包括黏性阻尼和库仑阻尼两个部分。用此模型对法兰在指数衰减脉冲激励下的响应进行了数值仿真，研究了螺栓中预应力、螺栓个数、法兰螺栓间的阻尼对法兰结构响应的影响，螺栓中预应力和螺栓个数对螺栓变形峰值有较大影响，阻尼对螺栓峰值的影响较小。     

球形爆炸容器法兰联接螺栓的应变增长现象

本文在球形爆炸容器法兰连接螺栓的动态响应实验研究中,发现了法兰螺栓的应变增长现象。以实验数据为基础,通过数值模拟对螺栓发生应变增长的原因进行分析。研究结果表明:受到开口结构的影响,端盖螺栓受到的压力载荷强度较容器内壁大幅增强。螺栓的应变增长是被增强压力载荷的前2个周期与结构响应形成共振时引发的,通过改变端盖质量、预紧力大小可以避免螺栓发生应变增长。     

螺栓扭矩系数影响因素的试验研究

在螺栓连接结构中,为保证连接的可靠性,必须为螺栓提供合适的预紧力。工程中通常采用扭矩法装配螺栓连接结构,因此需要确定预紧力与扭矩之间的关系(即扭矩系数),以施加合适的装配扭矩。本文对输电塔中常用的M16镀锌螺栓进行预紧力试验研究,测试分析了螺栓强度等级、有无垫圈和有无润滑对螺栓连接副扭矩系数的影响。研究结果表明,高强度螺栓比普通螺栓的扭矩系数小,数据离散性也小;无垫圈的螺栓比有垫圈的螺栓扭矩系数小,但数据离散性大;有润滑的螺栓比无润滑的螺栓扭矩系数小,数据离散性也小。本文同时探讨了如何控制、调整扭矩系数,以及螺栓连接中要注意的问题,为设计和施工单位提供参考。     

Evaluation of fracture behavior of iron aluminides

Comparative fracture tests of three Fe-28%Al iron aluminides showed that alloys with Zr and C addition (FA-187) or with B, Zr, and C addition (FA-189) are extrinsically more susceptible to environmental embrittlement than the base ternary alloy (FA-186) under constant tensile loading condition. This may be caused by the variations of grain boundary morphology (i.e. changes of grain size and grain boundary cohesive strength) caused by the alloy addition. The effect of grain boundary size and cohesive strength are further investigated with reference to the susceptibility of hydrogen embrittlement. Finite element simulation of initial intergranular fracture of two iron aluminides (FA-186 and FA-189) are made. The computational scheme involves coupling the stress and mass diffusion analyses to determine crack-tip stress state and the crack tip hydrogen diffusion. Maximum strain failure criteria was adopted to simulate intergranular fracture. The numerical modeling results correlated well with the experimental data. The result further confirmed that the grain boundary morphology is important as it appears to control the intrinsic and extrinsic fracture behavior of iron aluminides.     

螺栓连接件微动疲劳特性分析

工程中钢结构构件经常采用螺栓连接,被连接部位多处于复杂受力状态,单轴疲劳理论已无法满足该 形式下构件的寿命评估需求．针对现有理论不足,本文建立了螺栓连接件有限元模型,研究了不同工况下被连 接件微动疲劳裂纹萌生位置,并基于临界平面的多轴疲劳理论,比较了四种常用模型的适用性以及预测了连接 件的疲劳寿命．结果表明：(1) 裂纹萌生位置位于受拉端螺栓孔附近的滑移粘着区,在相同螺栓预紧力下,该 位置与施加的疲劳载荷大小无关;(2) 基于临界平面方法的四种模型均可以较好判断裂纹萌生位置,其中SWT (Smith-Watson-Topper)模型对不同载荷水平下的螺栓连接件微动疲劳寿命预测效果较好,大部分预测结果位于± 2倍分散带之内,预测结果优于其他三种模型;(3) 在规范规定的螺栓预紧力范围内,被连接件裂纹萌生区域距 孔边的距离与预紧力大小无关,可能是由于预紧力变化范围内的粘着滑移区未发生明显变化所致,并且随着预 紧力减小,被连接件的寿命预测值反而增大     

锚固正交各向异性岩体的本构关系和破坏准则

从理论上对由系统锚杆加固的正交各向异性岩体,取出包含锚杆的表征单元.根据“等效材料”的概念,在原岩体的本构关系和破坏准则中计人锚杆刚度和强度的“贡献”,从而建立了相应锚固岩体的木构关系和破坏准则.然后列举算例,考察了表征单元的应力-应变关系和破坏强度随锚杆安置角度变化的各向异性表现.最后将计算与一个简单试验的结果作了对比,看到二者的吻合程度较好,因而初步地验证了所提力学模型的可靠性.     

材料三维微结构表征及其晶体塑性有限元模拟

材料的力学性能，尤其是在有限变形下所呈现的宏观各向异性，是材料结构设计和服役寿命考虑的关键因素。由于宏观模型不能较好地反映材料微观结构（晶粒的形貌和取向等）对宏观塑性各向异性的影响，因此，本文建立了能实际反映晶粒形貌的三维Voronoi模型，并基于晶体塑性理论对铝合金在有限变形下的响应进行计算。首先，建立反映材料微结构的代表性体积单元RVE模型进行计算，并与实验结果进行对比验证。然后，以单向拉伸为例，分析了有限变形过程中试件的晶粒形貌和取向分布等微观因素对宏观各向异性演化的影响，并从材料和结构两个层面讨论了微观结构对宏观力学性能的影响。结果表明，本文模型能够反映微观结构对宏观力学性能的影响，为实际生产制造领域构件的力学性能提供可靠的预测。     

Local stress distribution and yielding of steel sheets due to concentrated transverse loads

In this paper, a theoretical investigation on local stress distribution and yielding of steel sheets subjected to concentrated transverse loads, caused by bolts, is presented. The stress state produced by the above loading leads to the final extraction of the bolt. For both elastic and elastoplastic cases, it is assumed that only a circular area of the metal sheet is affected by the load of the bolt and hence, one can study a representative circular plate of radius a and thickness t, fixed at its ends with a hole of radius b at its center. In addition, it is assumed that the force acting on the metal sheet by the bolt is applied at the edge of the hole at r =?b.     

面向增材制造的熔池凝固组织演变的相场研究

激光增材制造(laser additive manufacturing, LAM)技术极适合复杂整体构件的近净成形和高附值损伤件的快速修复. 然而, 激光增材制造熔池内部复杂的动态凝固过程显著影响成形件的终态组织, 进而制约其服役性能. 本文针对激光直接能量沉积(direct energy deposition by laser, DED-L) Inconel 718过程, 构建宏观传热传质与多相场耦合的多尺度数学模型, 解决了熔池宏?微观温度场的直接耦合, 并基于MPI并行程序设计实现了熔池二维的全域定量模拟, 研究了凝固过程中的晶粒演变过程. 结果表明, 模拟的熔池尺寸、凝固界面与实验结果吻合较好. 熔池凝固界面形态和晶体择优取向是影响晶粒演变的重要因素. 在熔池横截面上, 凝固过程主要受温度梯度方向的驱使, 取向与温度梯度方向夹角越小的晶粒占优生长. 在纵截面上, 晶粒的生长表现出弯曲生长以及“上三角”的晶粒特征, 温度梯度方向的渐变导致了晶粒弯曲, 相邻晶粒的竞争行为决定了晶粒形貌. 本文阐明了金属激光增材制造晶粒演变的机理, 有助于厘清增材制造热物理、化学、冶金过程, 为凝固组织的预测和调控提供理论指导. 此外, 该多尺度数学模型也适用于其他金属材料的激光增材制造过程.      

轴向交变载荷作用下螺栓联接结构的松动试验研究

在设计、试制螺栓加载装置的基础上,对螺栓联接结构进行了轴向振动的疲劳试验,获取了螺栓夹紧力的时变曲线,并对试验后的螺栓接触表面进行损伤分析,研究螺栓联接结构的松动机理.结果表明:在轴向振动条件下,螺栓联接结构的夹紧力下降较为明显,但拧出力矩相对于预紧力矩变化不大.螺栓的松动过程可分为以下两个阶段:试验初期由于螺栓接触面的塑性变形,夹紧力迅速下降;然后由于接触面之间的微动磨损,夹紧力缓慢下降.螺纹接触面之间的损伤机制主要为黏着磨损、磨粒磨损和剥层.     

The hydrodynamic wall effect for a disperse system

The flow of a highly dilute suspension of spheres (radius α) between two parallel ridid planes (distance L) in slow shearing motion is studied. Even for the limiting situation, (α/L) small but finite, there is a layer-one sphere diameter thick—immediately adjacent to the wall in which bulk quantities are so complicated functionals of the parameters of the microstructure that evaluating them seems out of the question. Nevertheless, it is still simple to obtain average bulk quantifies (e.g. apparent viscosity) and even the evaluation of local bulk quantities far away from the wall poses no problem. The reason being that the customary continuum constitutive equation for the bulk stress can be utilized, although a slip velocity has to supplement it. This applies to any disperse system and can be applied to different flows, too. For the spherical suspension at hand an explicit expression for this slip velocity is obtained.     

Modelling of plasticity and damage in a polycrystalline microstructure

The mesomechanical behavior of a polycrystalline microstructure subjected to monotonic and cyclic loadings is investigated. The analysis is based on a Voronoi polygonization strategy for generation of grains embedded in a contiguous matrix. The main emphasis is to investigate the interaction between the microconstituents and the failure processes along grain boundaries. A rational interface theory based on damage development coupled to inelastic slip and dilatation is developed. The theory uses the interface width as a constitutive parameter, which regularizes the theory of LEMAITRE [1992], that is restricted to perfect bond between grain and matrix. In a series of FE-analyses parameter variations were performed: The unit cell size (as compared to the average grain diameter), the grain-matrix area ratio, the interface width and constitutive parameters. It appears that the composite behavior can be designed as brittle or ductile solely depending on the strength and rate of damage development in the interfaces. A localization band was detected, and its orientation is in the range that is predicted within continuum theory.     

Effects of lattice misorientations on strain heterogeneities in FCC polycrystals

It is well documented that the highly heterogeneous deformation behaviour and lattice rotation typically observed within grains in a polycrystal are attributed to microstructural features such as grain structure, topology, size, etc. In this work, the effects of low- and high-angle grain boundaries on the mechanical behaviour of FCC polycrystals are investigated using a micro-mechanical model based on crystal plasticity theory. The constitutive framework relies on dislocation mechanics concepts to describe the plastic deformation behaviour of FCC metallic crystals and is validated by comparing the measured and predicted local and macroscopic deformation behaviour in a thin Al-0.5% Mg polycrystal tensile specimen containing a relatively small number of surface grains. Comparisons at the microscopic (e.g. local slip distribution) and macroscopic (e.g. average stress-strain response) levels elucidate the role of low-angle grain boundaries, which are found to have a profound effect on both the local and average deformation behaviour of FCC polycrystals with a small number of grains. However, this effect diminishes when the number of grains increases and becomes negligible in bulk polycrystals. In light of the widely accepted view that high-angle grain boundaries strongly influence the mechanical behaviour of very fine-grained metals, this work has shown that low-angle grain boundaries can also play an equally important role in the deformation behaviour of polycrystals with a relatively small number of grains.     

The scale effect on the yield strength of nanocrystalline materials

The microstructure of the nanocrystalline can be divided generally into two parts: grain and interface. When the grain size is about or less than 10 nm, the interface can be divided into grain boundary and triple junctions. The mechanical performance of nanocrystalline materials with complicated microstructures is greatly different from that of the coarse grain materials. In this paper, the nanocrystalline material is considered as a composite with three phases: the grain core, the grain boundaries, and the triple junction. The model analysis for nanocrystalline material deformation is established and the relationship between yield strength and grain size is obtained. The obtained result explains the inverse Hall–Petch relation.     

Microslip and macroslip in bolted joints

The dynamic forces which occur in structures whose jointed parts may slide during dynamic loading are associated with energy dissipation. The high-strength friction grip bolted joint (HSFGBJ) is a structural element which under certain circumstances may dissipate a large amount of vibrational energy. This energy is an increasing function of the frictional force and of the magnitude of slip of the joint. However, in a joint with a relatively large number of bolts, the amount of slip cannot be large, as the bolt holes are not much larger than the bolt diameter. Thus, some bolts of the joint may be sheared at the beginning of the full-slip stage of loading. In this paper, mathematical equations for the frictional force and slip in both stages of loading—partial slip and full slip—are established, and the dissipation of energy is evaluated. It is shown that if the joint is designed so that the magnitude of slip is at the border partial slip/full slip, the joint may dissipate a large amount of vibrational energy.     

Ultrasonic measurement of the bending of a bolt in a shear joint

An ultrasonic pulse/echo technique is used to measure preload in bolts used in structural joints. In this paper, the same instrument is used in a different way to measure change in the ultrasonic measurements due to bending in the bolts. A theory that explains the ultrasonic measurements is developed. The bending loads result in a rotation and a translation of the ultrasonic pulse reflecting face. It also creates a stress gradient in the bolt. This results in a phase variation (or gradient) in the received ultrasonic beam across the face of the transducer. It also results in a physical shift in the received beam relative to the ultrasonic transducer. The phase gradient and the shift in the beam results in change in the pulse travel time. A number of experiments were performed on the bolt to study the effect of the bending on the ultrasonic measurements. The experiments and the theory validate a sensitive new method for measuring the bending loads in the bolts.