Temperature and Stress Dependence of Occurrence of Strain Burst in Cyclic Creep of AI-Mg Alloy

Cyclic creep tests of Al-Mg alloy have been conducted at a wide range of stress and temperature.When Mg content is beyond 0.45%, the strain bursts can be observed in the cyclic creep curves.There exists a critical stress for the occur- rence of the strain burst,which increases with an in- crease of Mg content and is almost 60% of the ul- timate tensile stress.The occurrence of strain burst is proposed to be controlled by the easiness of the cross-slip of dislocations.For Al-Mg alloy,the dynamic strain aging plays an important role in the strain bursts.

Instantaneous Creep in Face-centered Cubic Metals at Ultra- low Strain Rates by a High-resolution Strain Measurement

Instantaneous creep in face-centered cubic metals, 5N Al（99.999%）, 2N Al （99%） and 4N Cu （99.99%） with different grain sizes, was firstly investigated by sudden stress-change experiments at ultra- low strain rates ε ≤10-10 s-1 and temperature T 〈 0.32 Tn. The experimental results indicate that the observed instantaneous creep is strongly dependent on grain size, the concentration of impurity, and stacking fault energy. Creep in high-purity aluminum, 5N Al, with a very large grain size, d 〉 1600μm, shows non-viscous behavior, and is controlled by the recovery of dislocations in the boundary of dislocation cells. On the other hand, for 5N A1 with a small grain size, d=30μm, and low-purity aluminum, 2N A1, with d8= 25μm, creep shows viscous behavior and may be related to ＇low temperature grain boundary sliding＇. For high-purity copper, 4N Cu, with d= 40 grn and lower stacking fault energy, creep shows a non-viscous behavior, and is controlled by the recovery process of dislocations. For all of the samples, creep shows anelastic behavior.

Relations of complete creep processes and triaxial stress-strain curves of rock

Based on the results of triaxial compressive creep tests for five kinds of rock under the different stress loading,unloading and cycle-loading-unloading conditions,the creep deformation is not only a function of stress and time,but also it has the corresponding relations to the triaxial stress-strain curves of rock.The deformation properties of soften-strain,harden-strain and ideal plasticity presented by conventional triaxial compressive test curves under the different stress states were utilized,and the creep characteristics,the creep starting stress and the different entire creep process curves of rock were studied systematically according to creep experiment results,and the relations of the triaxial stress-strain curves to the creeping starting stress,the terminating curve,the different creep processes,and the different creep fracture properties were established.The relations presented in this paper were verified partially by the creep experiment results of five types of rock.

库水作用下滑坡土体渗流与蠕变耦合试验研究

水库滑坡变形具有显著的渗流与蠕变耦合效应。为此,采用自主研发的渗流与蠕变耦合三轴试验仪,分别开展了渗流对蠕变的影响以及变形对渗流的影响试验研究,并改进GDS三轴仪完成了高精度控制和量测的渗流与蠕变耦合试验。结果表明:渗流作用下,土样体积变形随时间先逐渐增大后缓慢减小,主要原因是超孔隙水压力导致围压卸荷,使得土样产生回弹变形,但渗透压力持续作用于土体,使得土样体积持续缓慢减小;获取了渗流过程中土样体积的变化结果,分别建立了渗流开始前试样e-k(孔隙比-渗透系数)关系曲线及渗流稳定后试样e-k关系曲线,结果表明考虑变形影响的e-k关系曲线更能反映土体的流固耦合效应。研究成果可望为水库滑坡变形演化预测提供更加科学严密的理论和方法。

Analysis of axial strain in one-dimensional loading by different models

Different phenomenological equations based on plasticity, primary creep （as a viscoplastic mechanism）, secondary creep （as another viscoplastic mechanism） and different combinations of these equations are presented and used to describe the material inelastic deformation in uniaxial test. Agreement of the models with experimental results and with the theoretical concepts and physical realities is the criterion of choosing the most appropriate formulation for uniaxial test. A model is thus proposed in which plastic deformation, primary creep and secondary creep contribute to the inelastic deformation. However, it is believed that the hardening parameter is composed of plastic and primary creep parts. Accordingly, the axial plastic strain in a uniaxial test may no longer be considered as the hardening parameter. Therefore, a proportionality concept is proposed to calculate the plastic contribution of deformation.

Numerical Simulation on Interfacial Creep Failure of Dissimilar Metal Welded Joint between HR3C and T91 Heat-Resistant Steel

The maximum principal stress, von Mises equivalent stress, equivalent creep strain, stress triaxiality in dissimilar metal welded joints between austenitic（HR3C） and martensitic heat-resistant steel（T91） are simulated by FEM at 873 K and under inner pressure of 42.26 MPa. The results show that the maximum principal stress and von Mises equivalent stress are quite high in the vicinity of weld/T91 interface, creep cavities are easy to form and expand in the weld/T91 interface. There are two peaks of equivalent creep strains in welded joint, and the maximum equivalent creep strain is in the place 27-32 mm away from the weld/T91 interface, and there exists creep constrain region in the vicinity of weld/T91 interface. The high stress triaxiality peak is located exactly at the weld/T91 interface. Accordingly, the weld/T91 interface is the weakest site of welded joint. Therefore, using stress triaxiality to describe creep cavity nucleation and expansion and crack development is reasonable for the dissimilar metal welded joint between austenitic and martensitic steel.

简易机械式蠕变测试结构的开发

设计了一款机械式蠕变测试结构,用于搜集金属和非金属样条的蠕变力学行为。首先,采用盒型有导轨可伸长测试工装,可以保证测试方向不变,减少测试过程中样品的扭曲;其次,通过设计较小的空间和可实时反馈的温控系统,以及使用重锤形成的恒定的拉力,实现了稳定高精度的测试条件;再次,通过将标距设置为100 mm,采集位移的分辨率为0.5 mm,可得到精确到每0.5%的蠕变应变。此外,设计了保护装置,降低潜在的安全风险。所设计的结构为蠕变测试提供了一个简易、低成本的设备方案。

Reliability Evaluation of QFN Devices Soldered Joints with Creep Model

The solder joint reliability of quad flat non-lead （QFN） package,which has become very popular over the past few years,has received intense interest.The finite element method （FEM） is essential to evaluate the reliability of QFN device.In this paper,Garofalo-Arrheninus model was implemented to simulate the deformation of QFN soldered joints.Equivalent creep strain of the soldered joints was calculated by means of finite element analyses,and was used to evaluate the reliability of QFN packages.It is found that the critical soldered joint of QFN is located the package corner while the maximum creep strain is obtained at the top interface of peripheral soldered joint.The creep strain is provided with periodicity and additivity as the thermal cycling.Nonlinear analysis of QFN package with different lead counts was presented as well,in which the phenomenon that the value of induced creep strain arise as the package size decreasing is noted.Moreover,SnPb and two lead-free solders,namely,Sn3.5Ag/Sn3.8Ag0.7Cu,were both taken into consideration.Simulated results indicate that the creep strain value of lead-free soldered joints is lower than that of SnPb soldered joints,which can be attributed to the difference of stiffness and coefficient of thermal expansion among three solders.Garofalo-Arrheninus model is used to calculate the creep strain of the QFN device for the first time in this study.The results provide an important basis for evaluating the reliability of QFN package.

不同控制模式下316L不锈钢的高温疲劳变形行为及寿命预测

在550℃下对核电用316L不锈钢进行应变控制(应变幅在0.3%~1.2%)、应力控制(应力幅在230~300 MPa)低周疲劳试验和应变控制蠕变疲劳试验(3种波形,拉伸保载60,180,600 s,压缩保载60,180 s,拉压对称保载180 s),通过疲劳寿命、循环响应特征和应力-应变滞回曲线分析了不同控制模式下试验钢的疲劳变形行为;构建疲劳寿命预测模型,评估了Manson-Coffin-Basquin模型、SWT模型和能量法模型对不同控制模式下试验钢疲劳寿命的预测能力。结果表明:在不同控制模式的疲劳循环载荷下,316L不锈钢的循环应力响应均包括循环硬化、循环软化和失效断裂3个阶段;在低周疲劳试验中,疲劳寿命随应变幅或应力幅的增大而缩短;在蠕变疲劳试验中,疲劳寿命随拉伸保载时间的延长而缩短,随压缩保载时间的延长而增大,这与动态应变时效和蠕变对疲劳损伤的综合作用有关;在相同保载时间下,压缩保载下的疲劳寿命比拉伸保载下的短,这与不同加载方向引起的氧化层致裂机制有关。能量法模型对316L不锈钢在不同控制模式下的疲劳寿命预测精度最高,预测精度在1.5倍误差带以内,Manson-Coffin-Basquin模型的预测精度最低。

镍基合金FN3蠕变-疲劳交互试验特性研究

超高参数燃煤电站汽轮机工作条件非常复杂,有必要对复杂应力状况下的新材料力学行为进行评价。对进汽温度大于650℃的先进超超临界燃煤电站用镍基高温合金FN3,在725℃下进行了应变控制蠕变-疲劳交互试验,拉伸峰值应变保载时间为0.5 min和3 min。结合应变变程划分(SRP)模型和M-C模型,从半寿命对应的总应变划分出弹性应变和非弹性应变,将非弹性应变划分为拉向塑性与压向塑性应变、拉向蠕变与压向塑性应变、拉向蠕变与压向蠕变应变以及拉向塑性与压向蠕变应变,提出了各应变分量与失效循环次数的幂函数关系,并拟合各分量的关系参数。对试验结果和断口特征进行了比较分析,结果表明,在相同的应变幅下,随着保载时间的增加,FN3合金蠕变损伤量增加,疲劳失效次数降低。由于析出相的强化作用,合金抵抗裂纹扩展的能力增强,总失效时间增加。研究成果可为先进超超临界汽轮机机组高温部件的选材和使用提供参考。

Creep resistance of as-cast Mg-5Al-5Ca-2Sn alloy

In the present study, creep properties of as-cast Mg-5Al-5Ca-2Sn(AXT552) alloy were investigated by means of a GWT304 creep testing machine at temperatures of 175 °C and 200 °C in the stress range of 35-90 MPa. Results show that creep rates increase with applied stress at an identical temperature. Creep strain at 100 hours is 0.0518% and 0.083% at creep conditions of 175°C/75 MPa and 200°C/60 MPa, respectively, which is comparable to MRI230 D and much lower than most of AX series alloys. By the observation and analysis for samples before and after creep tests using a Shimadzu XRD-7000 type X-ray diffractometer(XRD) and a Hitachi S-3400 N type scanning electron microscope(SEM), it was found that Al_2Ca(C15) phase precipitated out of C36 phase or matrix. The cavity formation and connection at the interface of soft matrix and hard intermetallics caused the propagation of cracking along the eutectic phase during creep process and dislocation accommodated grain/phase boundary sliding is expected to be the dominant creep mechanism.

ISOCHRONOUS STRESS-STRAIN CURVES OF LOW ALLOY STEEL CROSS-WELD-SPECIMEN AT HIGH TEMPERATURE

In this work, a parametric approach is presented and utilized to determine the creep properties of weldments; then the model of creep strain for cross weld specimen is given. On the basis of the experimental results, attempt has been made to establish equations of the isochronous stress-strain for weld joint that can predict the function of loading and service time in use of the creep data of base metal and weld metal.

Time-dependent Early-age Behaviors of Concrete under Restrained Condition

To investigate the early-age behaviors of concrete under a restrained condition, a set of apparatus was developed. In this way, the tensile creep and other early-age properties can be investigated in depth. By measuring the modulus of elasticity of concrete, synchronous shrinkage of concrete and steel rings and free shrinkage of concrete, the deformations of concrete ring can be quantified respectively. The experimental results show the tensile stress in concrete is time-dependent, and the stress at cracking is much lower than the tensile strength at that age; the tensile creep plays an important role in relaxing the tensile stress and postponing the cracking of concrete.

Consolidation and Creep Behaviors of Two Typical Marine Clays in China

This paper presents an experimental investigation into the deformation characteristics of two typical marine clays obtained from Dalian and Shanghai, respectively, in China. Three kinds of laboratory tests, i.e. conventional oedometer tests, one-dimensional and triaxial creep tests were carried out. The results obtained from consolidation tests demonstrate linear e?logσv relationships for Shanghai clay at normally consolidated state, while partly or even global non-linear relationships for Dalian clay. The compression index Cc for both clays follows the correlation of Cc = 0.009(wL ?10) where wL is the liquid limit of soil. The relationship between logkv (kv is the hydraulic conductivity of soil) and void ratio e is generally linear and the hydraulic conductivity change index Ckv can be described by their initial void ratio for both clays. The secondary compressibility of Dalian clay lies in medium to high range and is higher than that of Shanghai clay which lies in the range of low to medium. Furthermore, based on drained triaxial creep tests, the stress-strain-time relationships following Mesri’s creep equation have been developed for Dalian and Shanghai clays which can predict the long-term deformation of both clays reasonably well.

Creep rate sensitivities of materials by a depth-sensing indentation technique

The strain rate sensitivity to creep of single crystal Cu（110）, metal tantalum, and 128°Y-X LiNbO3 piezoelectric single crystal were measured at room temperature by MTS Nanoindenter XP. Among the three kinds of materials studied, Cu showed the highest degree of resistance to creep-induced deformation, which is followed by Ta, while the LiNbO3 single crystal deformed more readily than the others. The values of the steady-state strain rate sensitivities determined by the indentation methods are in the range of 0.002-0.006, 0.02-0.06 and 0.02-0.03 for Cu, Ta, and LiNbO3, respectively. The mechanisms for the indentation-induced creeping behavior and the factors that influenced the creeping are discussed.

Creep Modelling of a Material by Non-Linear Modified Schapery’s Viscoelastic Model

This research work aims at modeling the creep behavior of a material by a non-linear schapery’s viscoelastic model. We started with analytical part where three powerful methods of creep modeling have been developed and compared. That is the Heaviside, the Nordin and Varna and lastly our own proposed methods. From this preliminary study, it came out that our method is different to the two others because we took into account the loading time at the creep beginning. Besides we studied several loading programs and retained a five order non-linear polynomial which is the program that gave us satisfactory results. The other loading functions led to divergent results and wasn’t present here as consequence. In the second part of this work, we devoted ourselves to the determination of non-linear parameters in the schapery’s viscoelasticity equation, through a well developed and illustrated methodology. From this study, it is straight forward that non-linear parameters are stress dependent;confirming the results of several authors that preceded us in this studying field.

Strain-induced Microstructural Changes and Effects of Alloying Elements for Fe_3Al-based Alloys

The strain-induced microstructural changes of Fe3Al-based alloys during room temperature deformation and high temperature creep were investigated. The results illustrated the strain-induced disor dering occured during room temperature deformation. Creep strain could induced two opposite processes, which are strain-induced disordering and creep recovery-induced reordering. These two opposite creep induced processes during creep result in reducing the influence of primary microstructure on the rupture life.

Superplasticity-like creep behavior of coarse grained ternary Al alloys

Enhanced tensile ductilities in coarse grained Al-Mg-Zn and Al-Mg-Fe materials were studied.The materials were Al-2Mg-5Zn,Al-3Mg-5Zn,Al-4Mg-5Zn,Al-3Mg-0.11Fe,Al-3Mg-0.27Fe,and Al-3Mg-0.40Fe.Tensile elongation-to-failure tests were conducted at constant cross-head speed and constant temperatures from 300 to 450℃.Strain rate change tests were conducted at a constant temperature from 300 to 450℃and in strain-rate range from 4.31×10-5to 1.97×10-2s-1.Experimental results show that over 100%ductilities are consistently achieved in these materials.This superplasticity-like behavior is rate-controlled by solute-drag creep.Although ternary Zn and Fe additions do not have an adverse effect on solute-drag creep and ductility,they increase stress exponent and its sensitivity to Mg content during solute-drag creep.

Mathematical and numerical modelling of large creep deformations for annular rotating disks

A simulation model is presented for the creep process of the rotating disks under the radial pressure in the presence of body forces. The finite strain theory is applied. The material is described by the Norton-Bailey law generalized for true stresses and logarithmic strains. A mathematical model is formulated in the form of a set of four partial differential equations with respect to the radial coordinate and time. Necessary initial and boundary conditions are also given. To make the model complete, a numerical procedure is proposed. The given example shows the effectiveness of this procedure. The results show that the classical finite element method cannot be used here because both the geometry and the loading （body forces） change with the time in the creep process, and the finite elements need to be redefined at each time step.

Elevated temperature strain softening characteristics of 12Cr1MoV steel friction welding pipe joints

The strain softening characteristics analysis of 12Cr1MoV streel friction welding pipe joints.which were removed from heating apparatus pipes ager over 9 and is yearly of service, was undertaken. It is concluded that the evident softening did not occur in the weld metal and HAZ during serice. whereas it did in the base metal 2mm apart from the center of the weld metal. The difference is due to the restriction of the pipe hoop, which is formed on the joint during friction welding. The bainite. microstructure in the weld metal and HA Z, which is harder than the pearlite, pecrostpucture in the base metal, makes contpibution too. ThIS phenomenon is peculiar to the friction weldings pipes.