分形维数和弹性模量衰减表征2D-C/SiC的拉伸蠕变损伤

真空条件对2D—C／SiC复合材料在1300℃和1500℃进行了高温拉伸蠕变试验，蠕变进行到0、0．5h、2h、10h、25h、50h中断试验，用SEM观察表面形貌，用盒维数法计算试样表面裂纹的分形维数；同时测量试样的弹性模量。结果表明，由于2D—C／SiC特有的蠕变损伤形式，所形成的损伤尺度都较短，其分形维数介于0～1之间。用分形维数和弹性模量衰减都可表征2D—C／SiC的蠕变损伤，两种损伤参量所描述的蠕变损伤总的发展趋势基本一致，即蠕变开始阶段损伤发展较快，随后进入缓慢发展的第二阶段。在第二阶段中，分形维数表征的损伤持续单调增加；而用弹性模量衰减表征的损伤在该阶段出现先下降随后升高的现象。以基体裂纹为主要损伤形式的条件下。分形维数主要反映蠕变试样局部的损伤，而弹性模量衰减反映的是蠕变试样整体的损伤。     

镍基单晶合金蠕变第一阶段性能晶体取向相关性研究

根据镍基单晶合金材料细观结构的定向特点，提出了分析蠕变第一阶段性能晶体取向相关性的细观单元模型。对由晶格错配度和热不协调性所产生的内应力进行了模型分析，用镍基单晶合金ＤＤ３单晶平板８５０和９５０℃实验测量了内应力的宏观响应，并在推导一解析解的基础上，确定模型参数，给合（００１）、（０１１）、（１１１）取向圆棒拉伸试样试验和晶体滑移有限元蠕变分析表明，蠕变第一阶段的晶体取向相关性可以归于以下三个因素     

高温蠕变裂纹扩展参量Q*(t)及其应用

针对C^*参数的不足，本研究修正Q^*并得到了一个新的蠕变裂纹扩展速率(CCGR)控制参数Q^*(t)。该参数综合了温度、应力场、激活能等影响因素，很好地表征了在不同应力或温度条件下蠕变裂纹扩展的特性。该参数能完整关联整个蠕变裂纹扩展的不同阶段，并从材料蠕变断裂的机制上反映了蠕变裂纹扩展的本质。     

沥青砂混合料粘弹塑力学特性研究

在0.1MPa、0.15MPa、0.2MPa、0.25MPa和0.3MPa下进行了沥青砂试样单轴压缩和蠕变实验,分析了其压缩和蠕变性质,根据变形机理提出了粘弹塑本构模型可由粘弹性和粘塑性的两个子模型串联构成,通过对粘塑性子模型中粘性系数进行改进,理论推导了模型蠕变本构方程,确定了模型参数,并求得模型参数与加载应力函数关系。进行模型预测与实验结果对比,结果表明:该模型能够描述沥青砂试样在不同应力下蠕变变形的3个阶段,反映了沥青砂混合料粘弹塑变形特点。     

K417合金返回料蠕变三阶段性能研究

本文测定了K417合金返回料的蠕变断裂全寿命曲线,着重研究了返回料在不同重熔次数和过滤与否对蠕变第三阶段性能的影响。结果表明:K417合金返回料经多次重熔后,有的出现蠕变三阶段特征,有的不具备蠕变三阶段特征,而经过滤净化处理的返回料,均具有蠕变三阶段特征。     

Al—8.5Fe—1.3V—1.7Si合金的压缩蠕变行为

以采用粉末冶金法制备的Ａｌ－８．５Ｆｅ－１．３Ｖ－１．７Ｓｉ耐热铝合金在３００～４５０℃范围内的压缩蠕变研究表明，该合金的蠕变应力指数相当大，蠕变表现激活能超大超过自扩展激活能，这种反常蠕变行为可以用建立在位错／粒子相互吸收并最后分离的ＲＡ模型加以描述，得到松弛因子的平均值为０．９３，表征这种相互作用具有中等强调弱强度，基于新为方程的预测值与实验值吻合很好。     

Udimet500镍基高温合金长期蠕变行为

系统分析 U dim et5 0 0合金经由 80 0℃和 90 0℃不同应力条件下的蠕变行为规律 ,并结合显微组织分析以揭示合金的蠕变变形行为。结果显示 ,合金很快达到稳态蠕变阶段 ,随后马上进入蠕变第三阶段 ,即蠕变过程主要由加速蠕变控制。组织分析表明 ,高温蠕变过程中γ′强化相粗化 ,晶界相的析出及加宽 ,σ相的析出可能是导致蠕变加速的主要原因     

岩石蠕变损伤模型研究

本文基于第三阶段蠕变响应法,建议了一种岩石介质的蠕变损伤模型。在蠕变模型的基础上,进一步讨论了一般材料和一般加载情况下的损伤定义形式。     

单晶镍基合金高温烃期间的组织结构及演化

通过测定［００１］取向单晶镍基合金的蠕变曲线，结合ＳＥＭ、ＴＥＭ观察表明，合金中组织结构的变化对蠕变抗力有明显影响。蠕变Ⅰ、Ⅱ阶段，蠕变的微观机制是位移的攀移；蠕变第三阶段，位错大量切入筏状γ＇相中，降低了合金的蠕变抗力，发现交替滑移使筏状γ＇相扭曲，致使γ＇／γ两相界面产生空穴或微裂纹，是蠕变断裂的直接原因。     

单晶镍基合金高温蠕变期间的组织结构及演化

通过测定［００１］取向单晶镍基合金的蠕变曲线，结合ＳＥＭ、ＴＥＭ观察表明，合金中组织结构的变化对蠕变抗力有明显影响。蠕变Ⅰ、Ⅱ阶段，蠕变的微观机制是位错的攀移；蠕变第三阶段，位错大量切入筏状γ相中，降低了合金的蠕变抗力，发现交替滑移使筏状γ相扭曲，致使γ／γ两相界面产生空穴或微裂纹，是蠕变断裂的直接原因。     

Constitutive description of creep behavior of M-4Al-1Ca alloy

Creep behavior of an advanced magnesium alloy AX41 (4 wt.% Al, 1 wt.% Ca, Mg balanced) was investigated in temperature interval from 343 to 673 K and stresses from 2 to 200 MPa. Compressive creep experiments with stepwise loading were used in order to obtain stress dependence of the creep rate in interval from 10⁻⁹ to 10⁻³ s⁻¹ for a given temperature. All stress dependences can be well described by the Garofalo sinh relationship with natural exponent n = 5. An analysis of the parameters of this relationship has shown that lattice diffusion controls creep at all experimental conditions. While climb-controlled creep mechanism is decisive at lower stresses and higher temperatures, glide-controlled mechanisms act at higher stresses and lower temperatures. A typical power-law breakdown is observed at intermediate stresses and temperatures. __________ Translated from Problemy Prochnosti, No. 1, pp. 36–39, January–February, 2008.     

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

Primary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).     

金属蠕变律及蠕变行为研究

蠕变过程中 ,材料内部状态的不断演化 ,使得材料的蠕变行为发生改变。本文提出考虑损伤和硬化影响的蠕变律。利用该蠕变律讨论了 12 Cr1Mo V钢蠕变行为。分析结果表明 ,ε·c在蠕变过程中始终变化 ,第二阶段仅仅是ε·c相对稳定的阶段 ,其相对稳定程度和持续范围与载荷大小有关。在相同寿命分数下 ,不同应力水平引起的硬化状态也不相同     

Influence of bonding on the tensile creep behavior of paper in a cyclic humidity environment

It has been shown, as paper structure is improved through increased bonding (by increasing relative bonded area or specific bond strength), a fully efficient loaded structure can be achieved. Once fully efficient, further improvements in bonding become redundant and have no effect on some paper deformation behaviors; deformation is dictated only by the characteristics of the fibers. Although previous work had shown this was true for elastic modulus and short time duration stress-strain behavior, it has only recently been shown to be true for constant humidity tensile creep behavior. In this study, the goal was to ascertain if cyclic humidity tensile creep behavior (known as accelerated creep) would follow the same trend. To accomplish this, sheets were made at differing levels of relative bonded area and specific bond strength. This was done by applying two different wet pressing levels (to alter relative bonded area) and using bonder and debonder (to change specific bond strength). It was found that accelerated creep behavior of paper sheets is no different than constant humidity creep behavior; changing bonding does not influence accelerated creep if the sheet has a fully efficient loaded structure. If the sheet structure is inefficiently loaded (there is no redundancy in bonding), accelerated creep will be affected by bonding. However, it is proposed that the only reason accelerated creep is influenced by bonding when inefficiently loaded is because constant humidity creep behavior determines the accelerated behavior and it is influenced, in this case, by bonding.     

Proposal of a new concept on creep fracture remnant life for a precracked specimen

Abstract

The creep life time of a smooth specimen can be predicted using existing laws for creep deformation and steady state creep rate. When crack growth behaviour is involved, it is necessary to construct a law of creep crack growth rate to predict creep fracture life. Creep fracture life can be measured by integrating the law of creep crack growth rate. One example is the creep crack growth rate, represented by the parameter Q*. In this study, we investigated the applicability of this prediction method to creep fracture remnant life for a cracked specimen. The Ω criterion is proposed to predict creep fracture remnant life for a smooth specimen for creep ductile materials. In this study, the correlation between Q*_L derived from the paremeters Q* and Ω is investigated. The correlation between Q_L* and Ω provided a unified theoretical prediction law of creep fracture remnant life for high-temperature creep-ductile materials in the range from smooth to precracked specimens.     

A three dimensional view of high temperature creep damage

Abstract

A three dimensional view of creep voids in a hydrogen reformer tube is presented. By separating and reconstructing various microstructural features present, the proper representation of creep voids in 3D can be fully studied. Useful measurements of parameters such as void volume, void-to-void distance, and grain boundary angles were obtained. The data presented here represent the initial collection of creep void information for use in various creep void nucleation and growth models. Additional data are currently being collected for material subjected to different service conditions.     

Measurements of Creep Internal Stress Based on Constant Strain Rate and Its Application to Engineering

This research is carried out on the basis of Constant Strain Rate （CSR） to measure creep internal stress. Measurements of creep internal stress are conducted on the material test machine by using the CSR method. A mathematical model of creep internal stress is also proposed and its application is presented in this paper.     

Primary and Anelastic Creep of a Near α-Ti Alloy and Their Dependencies on Stress and Temperature

The primary creep behaviour of a high temperature near -Ti alloy Ti6242Si has been investigated in the temperature range from 500 to 625°C, and the stress range from 80 to 450 MPa. The results are analysed in terms of the dependencies of stress on strain (strain hardening) and on strain rate (strain rate sensitivity). Furthermore, full unloading experiments were conducted in order to gain additional information as to the nature of primary creep. It is shown that primary creep can be described by an athermal component, strain hardening, with a mean strain hardening coefficient of 0.37, and a thermally activated component, strain rate sensitivity, with a strain rate sensitivity coefficient suggesting a mechanism based on climb controlled recovery. This is confirmed by the activation energy of 259 kJ/mol determined at different stresses, which is similar to the activation energy of Ti self diffusion in -Ti. The anelastic strain obtained on full unloading was analysed in its fast stage in a similar way. The kinetics of anelastic creep and its activation energy are in many aspects very similar to those of primary creep. It is thought that, in the stress and temperature range investigated, primary creep is to a relatively high extent anelastic in nature, and is controlled by the climb controlled bow out of pinned dislocation segments, particularly dislocations pinned at lath boundaries.