压痕法确定纤维复合材料界面影响区蠕变特性的数值模拟法

研究由平头压痕蠕变试验来确定纤维复合材料界面影响区蠕变性能参数的可行性。利用有限元蠕变分析确定在定压痕应力下的压痕蠕变率，重点放在稳定压痕蠕变率和复合材料材料界面影响区的蠕变性能参数的关系上。计算结果表明，界面影响区的剪应力沿纤维轴向在蠕变的主要过程中均匀分布，并且保持不变；详细地研究了压头大小对压痕蠕变响应的影响：提出两种方法由压痕蠕变试验来确定界面影响区蠕变性能参数，并给出了算例。这些结果也有利于准确认识平头压痕蠕变试验从而拓宽其应用范围。     

小冲杆蠕变试样中心挠度-蠕变应变关系的有限元分析

利用有限元数值方法分析了小冲杆试样中心挠度δ与中心蠕变应变ε之间的关系 ,指出该δ ε关系与蠕变时间、载荷大小和材料性能等条件均无关。计算结果还表明 ,在薄膜变形分析的基础上 ,非蠕变条件下试样中心挠度 中心弹塑性应变函数关系和蠕变状态下试样中心挠度 中心蠕变应变函数关系等价     

碳对HP—Nb合金显微组织和蠕变断裂强度的影响

利用光学金相技术和蠕变断裂试验研究了碳对ＨＰ－Ｎｂ合金显微组织和蠕变断裂强度的影响。在Ｎｂ含量一定情况下，随着含碳量的增加共晶Ｍ７Ｃ３增加，共晶ＮｂＣ基本不变，晶内二次碳化物沉淀数量增多，沉淀和粗化速度加快，含碳量与蠕变断裂强度的关系表现为具有极大值特征，含碳在０．４５％～０．５５％（ｍ／ｍ）时蠕变断裂强度最高。     

碳对Hp－Nb合金显微组织和蠕变断裂强度的影响

利用光学金相技术和蠕变断裂试验研究了碳对ＨＰ－Ｎｂ合金显微组织和蠕变断裂强度的影响。在Ｎｂ含量一定情况下，随着含碳量的增加共晶Ｍ＿７Ｃ＿３增加、共晶ＮｂＣ基本不变，晶内二次碳化物沉淀数量增多，沉淀和粗化速度加快。含碳量与蠕变断裂强度的关系表现为具有极大值特征，含碳在０．４５％～０．５５％（ｍ／ｍ）时蠕变断裂强度最高。     

耐火材料的蠕变和显微结构

本文论述了蠕变行为和相应粘结相的显微结构特征之间的关系，并分析了蠕变机理，同时列出各种耐火材料的蠕变活化能。     

PAN基炭纤维在石墨化过程中的蠕变效应

探讨了PANCF在石墨化过程中的蠕变效应，实验结果表明：在张力一定的条件下影响蠕变的主要因素是温度和时间。通过适当的蠕变可以得到高模高强炭纤维。揭示了蠕变伸长率和蠕变速度与抗拉模量和抗拉强度的关系。     

HTPB固体推进剂蠕变损伤模型研究

为研究HTPB推进剂的蠕变特性，开展了0.15～0.35MPa应力下推进剂的单轴拉伸蠕变试验；结合线性黏弹性理论和连续损伤力学理论，建立了HTPB推进剂蠕变损伤模型；根据试验数据，确定了不同应力水平下蠕变损伤模型的参数，并获取了应力对各参数的影响规律，最后利用蠕变应力0.2MPa下的试验数据对模型进行了验证。结果表明，HTPB推进剂存在明显的衰减蠕变、稳定蠕变和加速蠕变三阶段过程；建立的蠕变损伤模型结合Burgers模型的特征和连续损伤力学理论，串联含损伤的黏壶元件，克服了Burgers模型无法反映蠕变破坏阶段特性的不足。提出的蠕变损伤模型与试验值拟合度高，误差在3%以内，可以准确地描述推进剂在不同应力水平下的全过程蠕变特性。     

胶凝原油非定常参量蠕变模型研究

胶凝原油的蠕变特性对保障管道安全流动具有重要意义。现有蠕变模型在不同剪切应力下的拟合参数显著不同,不便工程应用。为此,研究从流变参量的非定常化角度出发,对经典Maxwell模型进行扩展,利用求解微分型本构关系的方法建立非定常参量蠕变模型。利用三种胶凝原油对蠕变模型进行验证,结果表明:一个蠕变方程即可较好地描述油品在多个应力下的蠕变过程,拟合精度高,模型未知参数少,且拟合参数不随剪切应力变化。油品温度降低,胶凝原油黏弹性增强,模型拟合参数呈现单调变化趋势。借助拟合确定的模型参数对胶凝原油其它应力下的蠕变响应进行预测,预测结果与试验结果较为一致,表明模型具有良好的工程应用价值。     

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.     

聚碳酸酯高温蠕变过程中的延迟弹性应力与分子取向

采用广角X射线在线研究了140℃下聚碳酸酯(PC)蠕变过程中的取向度变化。利用经典Burgers模型对蠕变结果进行了拟合,获得了延迟弹性变形阶段的推迟时间。根据聚合物非线性变形的分子理论,假设了延迟弹性应力与取向的关系,并利用模型推导出延迟弹性应力与时间的关系式。关系式对不同热压压力和外载荷条件的PC制品进行了取向计算,结果与广角测得的Hermans取向相比较。讨论了关系式中两个参数的物理意义,以及自由体积与外载荷在分子取向发展过程中的作用。利用延迟弹性应力合理解释了蠕变过程中的分子取向变化,为高温下利用宏观蠕变数据推测微观分子取向变化提供了参考。     

Accelerated characterization of a chopped fiber composite using a strain energy failure criterion

The creep and creep rupture response of a chopped fiber composite material (SMC-R50) were investigated experimentally and analytically. The goal of this research was to use the short time laboratory data to predict long time creep and creep rupture behavior. The creep response data up to 200 min duration were obtained at various constant temperature and stress levels. The short time creep data were then modeled using a modified power law equation. The modified power law equation contains the parameters of the so-called accelerated characterization procedure. Using this power law equation, the short time creep response at the elevated temperatures were able to successfully predict the long time creep response at a lower temperature and stress level. To predict the creep rupture behavior, the modified power law equation was then coupled with a strain energy based failure criterion. It was found that the same parameters that were used in the prediction of the long-time creep response can also be used to predict the creep rupture. At a given temperature level, the strain energy density related to creep rupture was found to be a constant. Furthermore, this strain energy density was found to increase with an increase in temperature. With a limited amount of data, it was found that the strain energy based failure criterion coupled with the modified power law equation can be used to predict long time creep rupture behavior.     

Semi-analytical solution for electro-thermo-mechanical non-stationary creep behaviour of rotating disk made of nonlinear piezoelectric polymer

Electro-thermo-mechanical non-stationary creep response of a rotating disk made of nonlinear polymeric piezoelectric material has been investigated. The viscoelastic properties of the material are time, stress and temperature dependent which vary along radius. The long-term creep constitutive equation is the Burgers viscoelastic model. A non-homogeneous differential equation with variable coefficients is derived using stress-displacement relations, equilibrium equation, charge equation of electrostatics and the Maxwell equation. Time-dependent creep strains are involved in the non-homogeneous term of the differential equation. A semi-analytical solution has been developed to obtain displacement, stresses, strains and electric potential in terms of creep strains. Then, Prandtl–Reuss relations and the creep constitutive model are employed in a novel numerical procedure based on the Mendelson method to obtain history of displacement, stresses, electric potential and strains. It has been concluded that the displacement is increasing with time while effective stresses are decreasing. The results are validated by finite element methods modelling using ABAQUS software. A very good agreements between the results can be observed.     

Temperature-dependent mechanical properties and constitutive equation of cellulose nitrate

The strain-time response under tensile loading (creep tests) and the stress strain response under constant tensile stress rate (proportional loading tests) have been evaluated at 4 temperatures 20, 45, 55, and 65°C, for samples of cellulose nitrate. A time-dependent constitutive equation (or stress-strain relation) for the nonlinear visco-elastic material is deduced from invariant theory with a hypothesis of a creep potential. The procedure for determining the seven material constants involved in the deduced constitutive equation is described for the creep and proportional loading tests and the variation of these constants with temperature is presented. The deduced constitutive equation gives good agreement with the actual observations for the creep and proportional loading tests, independent of the values of temperature, creep stress, or stress rate.     

基于有限元计算方法的制氢转化炉管蠕变寿命分析

以制氢转化炉为研究对象,通过炉管材料高温蠕变实验,确定了蠕变方程中相关系数。采用有限元计算方法,建立了制氢转化炉整体数值计算模型。考虑高温蠕变效应,对其服役6 a后各管系的蠕变应力进行了计算与强度评定。结合拉森-米勒尔曲线,通过对其不同操作周期寿命分数的计算,最终得到了该转化炉剩余寿命为6.24 a。     

Separating stress and time dependent effects in the temperature dependent creep of polyethylene nanocomposites

The nonlinear time dependent creep of linear‐low density polyethylene (LLDPE) reinforced with montmorillonite layered silicate was investigated. A previous study related the time/stress dependence of creep compliance of the material at room temperature using the Burger and Kohlrausch‐Williams‐Watts models. Using both the creep and recovery compliance curves, we employ the Schapery formulation to study the relationship between deformation, time, stress, and temperature of LLDPE nanocomposites. Smooth mastercurves are constructed using time–temperature–stress superposition principles. The stress and temperature‐related creep constants and shift factors were determined for the material using the Schapery nonlinear viscoelastic equation. The prediction results confirm the enhanced creep resistance of nanofillers even at extended time scales and low temperatures. POLYM. ENG. SCI., 50:1646–1657, 2010. © 2010 Society of Plastics Engineers     

The flexural creep of refractory materials

Conclusions An equation was derived for determining the maximum stresses in a rectangular-section specimen subjected to creep in pure bending when the rates of creep of the material in compression and tension are unequal. It was shown that the redistribution of the stresses in this case may result in a significant decrease of the maximum tension stresses in the specimen.Translated from Ogneupory, No. 5, pp. 42–45, May, 1977.     

High-Temperature Creep of NaCl-KCl Solid-Solution Alloys

The high-temperature creep of polycrystalline NaCl-KCl solid solutions and pure KCI was studied under constant compression stress at 600°C. The creep rate of the ionic solid-solution alloys appeared to obey approximately Weertman's microcreep equation, = KD_S, where D , is taken to be the diffusion coefficient of the cation and G the shear modulus of the material. Transient creep in the ionic alloys was similar to that in metallic solid-solution alloys which depends on the third power of stress. These results support the view that creep in many polycrystalline metallic and nonmetallic alloys is controlled by the same mechanisms. The creep rate of pure KCI depended on stress to the 5.0 power, a dependence similar to that of most pure metals and to that reported for pure NaCl.     

粘塑性统一本构理论在单晶镍基合金中的应用

借助于传统的粘塑性统一本构模型构建方法的基础上,基于不可逆热力学定律,应用包含屈服面信息的非弹性乘子代替塑性乘子表现材料的塑性性质,推导出含有运动硬化、等向硬化的流动方程以及内变量演化方程,构建金属材料的粘塑性统一本构模型。将该模型应用到单晶镍基合金中,模型能准确的模拟单晶镍基合金的拉伸性能、蠕变性能。     

Creep Parameters of Alumina Containing a Glass Phase Determined in Bending Creep Tests

An alumina ceramic containing a glass phase is investigated in bending creep tests. It is shown how the constitutive equation describing the material behavior can be derived from outer fiber strain measurements.     

A model for nonlinear creep and physical aging in poly(vinyl chloride)

The creep behavior of polymers depends on the physical age of the material at the time of stress application. Creep curves shift to longer times for more highly aged material and, in poly(vinyl chloride), (PVC), this can be modeled by an empirical equation in which the magnitude of an effective mean retardation time parameter is dependent upon, and increases with, the age of the polymer. Creep compliances for PVC also depend on the magnitude of the applied stress when this exceeds about 4 MPa. This nonlinear behavior is caused, at short creep times, by a reduction in the value for the retardation time parameter on application of the creep stress. Specimens appear therefore to be initially de-aged by elevated stresses. Subsequently, this parameter increases with creep time implying that physical aging has been reactivated, but the rate of increase also depends on the stress level. These influences of elevated stresses can be described by an extension of the creep model, and parametric expressions have been derived which relate creep compliance values to time, stress, and the age of the polymer. It is shown how the parameters can be determined from a short series of creep experiments and thus how creep deformations can be calculated over wide ranges of time, stress, and age.