Time and temperature effect on the linear–nonlinear viscoelastic transition threshold of a polymeric system

In the present study, the transition of a polymeric material from the linear to the nonlinear viscoelastic behavior and the determination of the nonlinearity stress threshold variation with time and temperature are investigated. For this purpose a systematic experimental program consisted of thermal and mechanical characterization of certain polymeric material followed by isothermal creep tests at different stress levels and temperatures was conducted. Through isochronal curves that occurred from creep tests, the nonlinearity threshold dependence on both time and temperature is presented. The reported results provide information that is critical for the design and development of polymer structures and components. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Microstructure and creep of eutectic indium/tin on copper and nickel substrates

The behavior during creep in shear of eutectic indium-tin joints on copper and nickel substrates was examined in order to determine the effect of creep deformation on the microstructure of the alloy. Primary creep was absent in all the samples tested. The stress exponent at 20° C was much higher for samples on nickel than for those on copper, indicating a different creep deformation mechanism. Also, the microstructure within the indium-tin joints differed significantly with a change of substrate. Samples with a nickel substrate showed a colony microstructure that was not observed in those with a copper substrate. Thus, eutectic morphology is important in determining deformation mechanism during creep. The microstructure within the joints in tested and untested regions of the samples was effectively the same. This is quite distinct from the behavior of eutectic lead-tin observed in previous work. Lead-tin formed a recrystallized band of material along a region of concentrated shear deformation. While the deformation pattern in the indium-tin samples appeared to be the same as for lead-tin, the microstructural changes did not occur. This implies that the behavior observed in lead-tin is not universal to eutectic alloys.     

Drying of Prone to Twist Boards in a Pre-Twisted Position

A method to minimize twist has been investigated; i.e., drying the boards in a pre-twisted position. It has been found that Norway spruce boards can be deformed toward straightness by this method. The force exerted by a board on its holders during drying in a pre-twisted position has been measured. When the variation in torsional stiffness as a function of temperature and moisture content is accounted for, it is found that a creep deformation of the board occurs both during the drying phase and the cooling phase. In addition to mechano-sorptive creep, a deformation released by a change in temperature also seems to occur.     

体外CFRP筋预应力混凝土箱梁长期受力性能试验研究

碳纤维(CFRP)筋具有优异的物理力学性能,可用于替代传统的预应力钢筋。制作了体外配置碳纤维筋预应力混凝土箱梁模型,对持续均布荷载作用箱梁的截面应力重分布、长期挠曲变形及裂缝发展等规律进行了1001 d的试验观测。基于素混凝土柱体的实测徐变系数,运用双线性法和曲率法分别对试验箱梁的长期挠曲变形进行预测。试验结果表明:受压钢筋应变较初值增长225%～268%,受拉钢筋的应变较初值增长36.2～38.6%,混凝土表面压应变较初值增长164%～224%。按现行设计规范计算长期荷载作用特征裂缝宽度较实测值偏小11.8%～55.5%。跨中长期挠曲变形实测值为初始变形的2.32～2.42倍,较现行设计规范取值偏大18.5%。     

Service Reliability Assessment of Weld Repaired CrMoV Steel Rotors

This research deals with the service evaluation of a retired CrMoV steel rotor that has been repaired using deep groove multi‐pass submerged arc welding (SAW) and gas tungsten arc welding (GTAW). Accelerated creep tests were conducted at several elevated temperatures and extrapolated to the service temperatures. The coalescence of carbides and cavity growth during creep were evaluated. For the CrMoV‐GTAW samples, it was found that the creep life depended on the location of the crossweld in the welded specimen. The service lives of the weldments were extrapolated.     

Effect of Microstructure on the Accelerated Creep of 20CrMoV12‐1 and P 91 Steels

In the initial part the change of microstructure for steel X20 CrMoV 121 is discussed in terms of the distribution of carbide precipitates and its effects on accelerated creep resistance and hardness are presented. In the following, experimental results of microstructure and accelerated creep resistance are presented for the steels X20CrMoV 121 and P91 annealed for up to 8760 hours at 650°C and 750°C before the testing. A similar evolution of the distribution of carbide particles of a size above 102 nm is found for both steels, while the accelerated creep resistance is diminished much stronger for the steel X20CrMoV 121. This difference is due to a greater stability of NbC than that of VC precipitates, both related to the evolution of the chemical composition of complex chromium, molybdenum and iron carbide particles.     

热塑性塑料管材抗蠕变性能评价

描述了热塑性塑料管材的抗蠕变性及其有关的评价方法，介绍了几种标准测试方法。     

CAE技术在汽轮机高温强度计算中的应用

为分析某百万千瓦超超临界汽轮机高压缸旋转部件在高温以及高离心力载荷下的强度状况,建立高压缸第一级动叶片的模型.根据蠕变试验数据,采用最小二乘法拟合幂蠕变方程中的各个参数,并详细介绍各参数的拟合、优化和验算过程;用CFD分析软件计算流体域内的对流传热系数并插值到有限元计算程序中,分析叶根倒角处危险区域的蠕变应力状况.数据验算结果表明,利用拟合数据计算得到的蠕变应力值与试验值比较接近,证明该方法可靠;CAE计算结果表明,叶根倒角处危险区域蠕变应力考核合格,并且有充足的安全裕量.     

铸造镍基高温合金的蠕变阻力

在对颗粒强化理论和位错蠕变理论进行回顾、评价基础上，发展了一个位错蠕变阻力模型，认为蠕变阻力是影响铸造镍基高温合金蠕变机制的重要因素。当施加应力足以使位错切入γ′相时，主要蠕变机制是位错切割γ′相过程，蠕变阻力就是位错切入γ′相的临界门槛应力。在低施加应力区，位错只能借助于热激活攀移过程通过γ′相。蠕变阻力包括两部分：第一项是位错攀移临界门槛应力，与施加应力无关；第二项是与施加应力有关的阻力项，代表了其他强化机制的贡献。位错攀移机制蠕变阻力的上限是切割机制门槛应力。在3种铸造镍基高温合金中（定向凝固DZ17G合金，IN100合金和IN738合金），对上述模型进行了验证，理论计算应用了SL强化理论，与实测值符合较好。     

CREEP BEHAVIORS OF Pt-RE ALLOYS AT HIGH TEMPERATURES

The creep behaviors of Pt-RE alloys have been studied at 1200℃ and 1400℃.The results show that asmall amount of RE elements improves the creep behaviors of platinum greatly.The creep behaviors of PtGd0.5,PtLa0.5 and PtLa0.3 Gd0.2,are best among all the alloys studied.As far as the creep behaviors are concerned,the traditional heat-resistance alloy PtGd10 can be replaced by PtGd0.5.Particularly,the properities of PtGd0.5are near to those of PtRb10.For most of the Pt-RE alloys,long-time,static,super high-temperature treatment inair is of no advantage to the creep rupture life.The mechanisms of the effects of rare-earths on high-temperaturecreep properties of platinum are discussed.