小冲杆蠕变试验微试样的应变分析

在假设以弹塑性薄膜变形分析所获得的试样中心挠度与非蠕变中心应变之间的关系近似等于蠕变时试样中心挠度与中心蠕变应变之间的关系的基础上 ,使用塑性薄膜伸张变形模型模拟小冲杆试样的蠕变变形过程 ,设计了以小冲杆试验实测中心挠度数据估算试样中心蠕变应变以至材料蠕变性能的方法 ,并具体推导得到了特定小冲杆试验装置和试样尺寸对应的蠕变应变工程估算方程     

塑料土工格栅长期蠕变行为的预测

建立塑料土工格栅蠕变性能测试室,测试了格栅试样的蠕变应变随时间的变化情况。测试在一定温度、湿度条件下进行,并对各试样加载了不同的载荷。从得到的时间-应变关系图中发现,当载荷水平较低时,蠕变发展缓慢;载荷水平较高时,蠕变发展较快且应变较大,并在测试周期内发生断裂。采用时间-温度等效和时间-应力等效相结合的叠加方法,预测了格栅的长期蠕变行为。根据叠加结果得出,格栅在温度为15℃、载荷水平为20％的使用环境下,使用寿命可达100a以上。测试的格栅试样在低于20％载荷水平下使用是安全的,而当载荷水平高于40%时,很可能在短时间内发生断裂。     

塑料土工格栅力学行为研究--Ⅰ.短周期力学行为的分析

测试并记录土工格栅试样的蠕变伸长随时间的变化情况,得到试样应变和时间的关系.应变-时间关系显示载荷较小时,蠕变发展缓慢;载荷较大时,蠕变应变较大且发展较快.由应变-时间关系得到了应变速率-应变关系和等时载荷-应变关系,由此可评价试样蠕变发展的趋势,确定蠕变应变极限.这些工作为进一步对土工格栅长期蠕变行为研究做好了准备.     

R60702锆材小冲孔蠕变特性

小冲孔蠕变试验技术是一种采用微小试样、近乎无损地评定材料高温力学性能的新方法.为了研究R60702锆在200℃下蠕变特性,对R60702锆材试样进行了载荷为454-572 N的小冲孔蠕变试验,得到了R60702锆材试样蠕变挠度-时间曲线.结果表明：R60702锆材在200℃时存在着明显的蠕变现象,而且在载荷水平较高时具有明显的蠕变曲线三阶段的特征;建立了一种新的Norton方程关系式ε^c=A（σ-σcon）^n;R60702锆材在200℃时的蠕变机理为位错蠕变.     

侧限荷载和温度对PE-HD土工格栅蠕变特性的影响

采用能同时研究温度和侧限荷载对土工格栅蠕变特性影响的自制设备,研究了温度和侧限荷载对高密度聚乙烯(PE-HD)土工格栅蠕变特性的影响,并采用相关性分析方法进行了分析。结果表明,在低荷载水平(拉伸强度的20%)条件下,温度和侧限荷载对土工格栅的蠕变特性影响显著。土工格栅的蠕变应变随土工格栅温度的增加而显著增大。当温度为25℃时,在侧限荷载分别为0,50,100 kPa条件下,土工格栅的最终蠕变应变比15℃时分别增大67%,77%,58%;当温度为35℃时,在侧限荷载分别为0,50,100 kPa条件下,土工格栅的最终蠕变应变比25℃时分别增大34%,35%,22%。蠕变应变随土工格栅所受侧限荷载的增加而显著减小。在侧限荷载为50 kPa条件下,土工格栅在15,25,35℃时的最终蠕变应变比无侧限荷载时减小19%,14%,14%;在侧限荷载为100 kPa条件下,土工格栅在15,25,35℃时的最终蠕变应变比侧限荷载为50 kPa时减小9%,18%,26%。对土工格栅蠕变应变与时间对数曲线进行线性拟合,拟合度大于0.9。温度、侧限荷载对土工格栅蠕变应变的相关性分别为0.735,–0.643,土工格栅温度对土工格栅蠕变应变的影响程度大于侧限荷载。     

堇青石-莫来石材料高温弯曲蠕变行为的研究

利用蠕变本构方程和材料力学中的小变形假定导出了三点弯曲下的蠕变关系式，利用两种堇青石－莫来石材料的试验初步拟合上述关系。研究表明：稳态蠕变时，中点挠度的速率与应力呈指数关系。     

输电线路碳纤维复合材料芯加速蠕变试验研究

碳纤维复合材料芯在服役条件下会发生蠕变。该文研究了碳纤维复合芯在不同温度和应力水平下的蠕变行为。通过温度和应力加速蠕变试验,得到了典型温度和应力下碳纤维复合材料芯蠕变应变演化规律以及碳纤维复合材料芯使用工况下的蠕变行为;通过不同加速模型,预测了碳纤维复合材料芯使用工况下的老化寿命并进行了对比分析。     

含卸压孔煤岩蠕变特性数值模拟研究

随着煤炭开采深度的增加，煤岩体赋存条件变得更加复杂，冲击地压事故频发，严重影响了煤炭的安全高效开采。大直径钻孔卸压是治理冲击地压的有效方法之一，为了研究含卸压孔煤岩的蠕变特性，基于FLAC^3D数值模拟软件，采用Burgers蠕变本构模型开展了煤岩蠕变数值模拟研究，对比分析了单轴压缩条件下完整试样与含卸压孔试样在不同应力水平下的蠕变特性，结果表明：蠕变曲线全面反映了数值模型的瞬时弹性变形、减速蠕变和等速蠕变的演化阶段；卸压孔为煤岩体提供了变形补偿空间，减少了巷道的变形量；在合理的卸压孔尺寸范围内，卸压孔直径越大，其可为煤岩提供的变形空间越大，越有利于减小冲击地压。工程应用结果显示，卸压后的巷道变形量为同时期未卸压巷道变形量的70%左右。研究成果可为煤矿巷道的长期稳定和冲击地压解危提供参考。     

碳纳米管填充PTFE复合材料蠕变性能

模压成型烧结制备碳纳米管(CNTs)/聚四氟乙烯(PTFE)复合材料,采用长期压缩蠕变测试方法对复合材料的蠕变性能进行研究,绘制压缩应力应变和应变-时间曲线分析材料蠕变行为。结果表明,该试验条件下观察到蠕变的第一个阶段为普弹形变。试验初期应变随时间的变化与是否填充CNTs无直接关系,应变量的增加是PTFE分子链上各键键角在载荷的作用下平衡位置振动的结果。对于玻璃态的PTFE,载荷大于试样屈服应力时会强迫链段甚至整链发生运动,使试样宏观表现出强迫流动(屈服),CNTs在试样发生强迫流动(屈服)阶段削弱了链段运动。CNTs的填充可提高PTFE的抗蠕变性能,这是通过对PTFE分子链链段运动的约束作用实现的。CNTs填充质量分数的合适值为5%左右,较大的CNTs填充量会因CNTs自身的团聚弱化对链段运动的约束,抗蠕变性能有所降低。     

疲劳与蠕变复合作用下PS的应变与寿命

对PS在疲劳/蠕变复合作用下应变与寿命进行了研究。结果表明：其疲劳/蠕变曲线与纯蠕变曲线十分相似。加载时间周期越短和疲劳载荷变化越频繁。结束普弹应变阶段应变越小，进入延迟弹性变形的平台应变阶段越早。在疲劳/蠕变复合作用下聚苯乙烯存在疲劳和蠕变的交互损伤，其断裂寿命比纯疲劳或纯蠕变的断裂寿命低。断裂寿命减小，疲劳/蠕变的交互损伤程度与温度密切相关，PS在较低温度的疲劳/蠕变交互损伤作用大于较高温度的交互损伤作用。随温度升高，疲劳/蠕变断裂寿命下降是疲劳和蠕变各自的单独损伤增加所致。     

Bending Creep Test to Measure the Viscosity of Porous Materials during Sintering

A bending creep test is proposed for measuring the change in viscosity of a porous material during densification. Equations, based on simple beam deflection theory, were derived to obtain the viscosity from a series of loading experiments using rectangular samples of different densities. By measuring the deflection in the center of the specimen between the spans, the viscosity of a porous material during densification can be measured. Experiments with porous Y₂O₃-stabilized ZrO₂ beams were used to illustrate the bending creep test. Consistent with theory the viscosity increased from 50 to 400 GPa·s as the sample densified from 87% to 98% density. The rapid increase in viscosity was considered to be a result of both densification and grain growth.     

On the prediction of long‐term creep‐failure of GRP pipes in aqueous environment

The aim of this work was to study the long‐term failure of GRP (Glass fiber‐reinforced Polymer) pipes under the influence of moisture absorption. These pipes are used in water transportation, which has an important effect on the mechanical properties of the polymeric matrix. The GRP pipes are usually tested under ring deflection or internal pressure conditions. This study presents and analyzes experimental creep‐rupture data obtained from standard test methods under ring deflection conditions. This loading configuration simulates in laboratory the conditions verified in a subsoil installation. The creep testing was carried out under constant dead weight on unconditioned and preconditioned samples in a submerged condition. The diametrical deflection of samples was measured periodically, and the time to failure of each sample was recorded. The main purpose of this work was to determine the short and long‐term rupture energies of GRP pipes and assess the influence of moisture preconditioning on those values. The observed failure mode was always the same. It was concluded that the energy at failure decreases with time. The influence of the preconditioning on the creep‐rupture of GRP pipes was considered negligible. Different time‐dependent failure models were described and used for long‐term extrapolation of the experimental data. The maximum strain at failure decreased about 12% from 0.1 to 1,000 hr of creep testing. Furthermore, data extrapolation to 50 years predicts a reduction of strength of about 60%, founded on the most conservative time‐dependent failure criterion. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Damage-Enhanced Creep in a Siliconized Silicon Carbide: Mechanics of Deformation

Continuum mechanics methods were employed to analyze creep deformation of a grade of siliconized silicon carbide at elevated temperatures. Three loading modes (tension, compression, and bending) are considered in this analysis. In tension, deformation is accompanied by cavitation at stresses in excess of a temperature-dependent threshold level, resulting in bilinear power-law creep. In compression, greater applied stresses are required to achieve the same rate of strain, and although bilinear creep behavior is also observed, a single power-law creep equation was assumed to simplify the mathematical analysis of the flexure problem. Asymmetrical creep in siliconized silicon carbide leads to a number of unique features in flexural creep. At steady state, a threshold bending moment exists below which no damage occurs. The neutral axis shifts from the geometric center toward the compressive side of the specimen by an amount that depends on the level of applied stress. Cavitation zone shapes, which are predicted to develop in a four-point bend specimen as a function of load, are found to be in qualitative agreement with those obtained experimentally. For transient creep under bending, the time-dependent neutral axes for stress and strain do not coincide, although they do converge toward a single axis at steady state. Quantitative predictions are given for relaxation of tensile stresses at the outer fiber, reverse loading in the midplane region, and the growth of the damage zone toward the compressive side of the flexural specimen. This load redistribution leads to a prolonged transient stage as compared to its counterpart in uniaxial creep.     

Uniaxial compressive creep of polytetrafluoroethylene

A machine to measure the creep deformation of plastics under uniaxial compressive loads is described. The problems associated with accurate creep testing in compression, primarily the application of a uniform stress to the specimen and the measurement of the resultant strain, receive particular attention. For the specimen geometries used, the effect on the measured strain of frictional restraints at the specimen ends is negligible provided the strain measurement is made with an extensometer attached to the specimen. The effect of fabrication techniques on the deformation behavior of polytetrafluoroethylene (PTFE) has been examined. Sintering time and temperature are found to be the most significant variables in the processing of PTFE. A comparison of uniaxial tensile and compressive creep data has shown that the non-linear viscoelastic behavior of the material extends into the low strain region.     

压缩条件下发泡聚苯乙烯蠕变本构关系及有限元分析

为提高应用发泡聚苯乙烯（EPS）工程的长期稳定性，对EPS的蠕变性能进行了试验分析。试验中EPS试件随着密度变小压缩蠕变量的增大幅度逐渐变大，从密度为30．9kg／m^3时的蠕变应变0．8％增加到20．5kg／m^3时的4．9％；随着恒定荷载的增加相同时间所对应的蠕变变形量增大，从荷载40kPa时的应变0．5％增加到80kPa时的3．7％。在试验结果的基础上提出了EPS块体的压缩蠕变本构关系。采用有限元软件对EPS块体的蠕变模型进行分析。两者同现场实测的EPS板蠕变结果进行了比较，三者图形趋势较为一致，说明用该本构模型预测EPS块体的蠕变性能是可行的。     

Calculation of Stresses and Strains in Four-Point Bending Creep Tests

An analysis of strains and stresses in four-point bending creep tests in the limit of small beam deflections resulted in a general equation which relates the load-point deflection, the applied load, the creep exponent ( N ), and the geometrical parameters of the loading system. Measurements of load-point deflection rates, which are experimentally easy to accomplish in ceramic systems, vs the applied load lead to the direct determination of the creep exponent and the creep compliance in a steady-state creep test. The creep compliance is a function of the temperature, grain size, and all other factors except stress. The elastic equation relating the load-point deflection and the outer fiber strain is strictly valid for viscous creep and approximately valid for nonviscous creep (i.e. N >1) if the ratio of the distance between the support points to the distance between the load points is not very large.     

Comparison of High-Temperature Crack Growth in SiC-Whisker-Reinforced Mullite and Si3N4

Crack growth behavior under creep conditions was studied in SiC-whisker-reinforced mullite and silicon nitride. Tests of four-point bend specimens with indentation cracks were periodically interrupted to observe the creep behavior. At each interruption the bulk creep strain of the specimen, the growth of the indentation cracks, and the nucleation and growth of creep-induced cracks were measured. A strong linear correlation was observed in both materials between the crack growth rate and the creep strain rate. For a given strain rate, cracks in the silicon nitride composite propagated at velocities about an order of magnitude greater than those in the mullite composite. On the other hand, for similar nominal stresses, creep rates in the silicon nitride composites were about an order of magnitude less than with the mullite composite.     

Evaluation of the Strength and Creep–Fatigue Behavior of Hot Isostatically Pressed Silicon Nitride

The strength of a commericially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long-term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150°, 1260°, and 1370°C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated by allowing individual creep tests to continue until specimen failure. The applicability of the four-point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150°C, failure was controlled by a slow crack growth mechanism. At 1260° and 1370°C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman–Grant relation.     

Compressive creep testing of refractories at elevated loads—Device,material law and evaluation techniques

In order to cost-effectively characterize the high temperature compressive creep behaviour of refractories a testing device was designed for application at elevated loads. Special measures have been taken necessary to enable an even stress distribution within the specimen. To identify Norton-Bailey strain hardening creep law parameters a general inverse procedure using a Levenberg–Marquardt algorithm was developed. Satisfying experimental results could be received from the creep measurement in a wide range of temperatures and loads for both shaped and unshaped materials. By fitting the strain/time curves the creep law parameters of refractories under various temperatures can be precisely identified. The measurements also reveal that at elevated loads all three creep stages can be observed.