用于胶凝原油蠕变描述的一种垂直移位因子

描述胶凝原油的屈服破坏过程中的非线性黏弹性蠕变行为,必须同时考虑应力、时间和温度等参数。引入时间-温度-应力等效原理描述胶凝原油蠕变特性,在进行移位叠合的过程中,除考虑水平移位外,同时引入一种胶凝原油垂直移位因子,并给出其表达式。对不同温度下形成结构的胶凝原油进行剪切蠕变实验,并用黏弹性损伤模型对蠕变柔量主曲线进行描述。结果表明:等应力移位过程中的垂直移位可忽略,而等温移位过程中的垂直修正不可忽略,且对蠕变柔量主曲线的获得有很大影响;拟合曲线与实验数据吻合很好。     

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。     

基于非线性黏弹性理论的含缺陷PE管的脆性破坏

现阶段聚乙烯管道脆性破坏失效研究,多采用线弹性断裂力学理论进行分析,鲜有考虑管材非线性黏弹性力学行为.针对这一问题,基于非线性黏弹性理论,将材料黏弹性参数用Prony级数表示,在适当假设简化下推导了变栽荷含缺陷黏弹性体能量释放率的一般表达式.结合含轴向表面裂纹PE管的脆性破坏工程案例,给出其理论模型及相应计算结果,为研究PE管道脆性断裂现象提供理论依据.     

交通载荷作用下埋地聚乙烯燃气管道的有限元分析

运用非线性有限元法,在ANSYS中建立埋地PE燃气管道的有限元模型,对交通载荷作用下的埋地PE管进行瞬态分析,得到管道应力及直径变形量随时间的变化关系,比较不同管顶覆土厚度下的管道力学性状.研究发现,建立的有限元模型,其计算结果能够较为真实地反映PE管的蠕变行为,聚乙烯的黏弹性降低了管道应力梯度,管顶覆土对交通载荷作用下埋地PE管道有减荷效果.交通载荷是引起管道形变的主要因素,适当增加管道埋深刻提高管道寿命.     

PE80燃气管道的应力松弛模型与实验验证

采用实验方法研究聚乙烯(PE)管道在机械载荷条件下的应力松弛行为,通过卷积分解析法和基于Maxwell模型的有限单元数值模拟法对其进行验证。结果表明,在小应变下采用Boltzmann叠加原理推导出的PE黏弹性本构模型可用于预测PE管道的应力松弛行为;基于实验参数,利用有限单元法能有效地对PE的黏弹性行为进行预测。     

AC-13级配钢渣沥青混合料粘弹塑本构模型研究

为了研究钢渣沥青混合料非线性粘弹塑性变形特性,提出Schapery模型与改进Swchartz模型组合的积分型粘弹塑本构模型。采用钢渣替换AC-13级配中粒径2.36 mm以上的石灰石粗骨料,制作得到钢渣沥青混合料试件。设计并开展一系列的单轴压缩蠕变实验,通过应力递增蠕变回复实验,获得不同应力条件下材料的弹性、粘弹性应变和粘塑性应变,进而拟合确定本构模型参数。利用0.4 MPa、1.0 MPa下的蠕变回复实验验证模型有效性。结果表明,模型不仅能准确刻画钢渣沥青混合料蠕变过程中的弹性、粘弹性与粘塑性变形,还可用于预测不同应力水平下钢渣沥青混合料蠕变变形规律。     

一个胶凝原油的蠕变模型

为了研究胶凝原油的启动屈服过程,确定胶凝原油的蠕变模型至关重要。基于不同剪应力下胶凝原油的蠕变特性,建立了具有黏性流的黏弹性固体流变模型来描述胶凝原油的黏弹流变特性,通过试验验证该模型能精确描述胶凝原油的黏弹性蠕变过程;若施加的剪应力高于胶凝原油的塑性屈服应力,胶凝原油的蠕变将从稳定流变阶段很快达到加速流变阶段,表现为典型的黏弹塑性特征,将非线性黏塑性体和具有黏性流的黏弹性固体流变模型串联起来得到一个非线性黏弹塑性剪切流变模型,该模型能充分反映胶凝原油的加速剪切蠕变过程,并与试验结果吻合的较好。     

PE80管材热板熔焊焊缝的慢速裂纹扩展行为

在80℃和2．4MPa恒应力拉伸的试验条件下，参照ASTMF1473－97标准，研究了国产燃气用PE80管材与其热板熔焊焊缝的慢速裂纹扩展（SOG）行为。研究表明二者SCG过程的δ－t关系均为同样的阶梯上升形态，这一形态是裂纹尖端区域材料的蠕变钝化与蠕变损伤积累过程相互转换的宏观表现。就所得试验结果的平均趋势而言，与PE80管材本身性能的比较，其熔焊焊缝的SCG抗力相对较差。     

玻纤增强阻燃PBT长期弯曲蠕变行为预测

采用万能电子拉力机测试了不同应力下玻纤增强阻燃PBT(PBT-RG301)的短期蠕变数据,并采用时间应力等效原理、Burgers模型以及Findley指数定律预测了长期蠕变行为.结果发现:依据时间应力等效原理可预测10000h后体系的蠕变数值,在4000s实验时间内Burgers模型和Findley指数定律均可很好的拟合实验结果,但Burgers模型预测的长期蠕变数据高于Findley指数定律和时间应力等效原理预测数值,实验时间14h的跟踪数据也证实了该结果.     

E-44/EP-1环氧树脂胶黏剂在湿热与室温环境下蠕变试验及数值模拟

环氧树脂胶黏剂在长期载荷作用下产生蠕变变形,尤其是在湿热环境下蠕变现象更加明显,不利于工程实践的推广应用。本文采用CARE多功能拉伸试验机对湿热以及室温环境下的环氧树脂胶黏剂试件进行了5MPa、10MPa、15MPa和17.5MPa定载荷蠕变试验,对比了湿热与室温状态下胶体的蠕变行为。试验结果表明湿热环境对胶体蠕变性能影响显著,并且这种影响随着应力的增大而逐步扩大。同时利用ABAQUS有限元软件对不同状态下环氧树脂胶黏剂的蠕变行为进行了数值模拟,通过对比发现,模拟结果与试验结果吻合,为环氧树脂胶黏剂蠕变特性研究提供了一种有效的分析方式。     

PE100管及其热熔接头的蠕变裂纹扩展行为

聚乙烯(PE)管性能优异,广泛应用于城市水及燃气供应系统。PE管的主要破坏形式是长期静压载荷下的慢速裂纹扩展失效。在蠕变条件下,采用光滑试样和裂纹圆棒试样对PE100管及其热熔接头进行了测试,得到了基于蠕变断裂参数C*的蠕变裂纹扩展动力学关系式。利用扫描电子显微镜(SEM)分析了裂纹圆棒试样的断面形貌,对比分析结果发现,蠕变裂纹扩展失效模式对应的最大应力为15.05 MPa,热熔接头熔合面分布有约11个/mm^2、直径范围为1~5μm的微气孔,热熔接头断裂微纤平均长度比母材约小20%~45%。当热熔对接时,熔合面存在的微气孔以及系带分子的浅渗透是导致PE100热熔接头蠕变裂纹扩展抗力降低的主要原因。     

Tensile creep of a long‐fibre glass mat thermoplastic (GMT) composite. II. Viscoelastic‐viscoplastic constitutive modeling

In Part I of this article, the short‐term tensile creep of a 3‐mm‐thick continuous long‐fibre glass mat thermoplastic composite was characterized and found to be linear viscoelastic up to 20 MPa. Subsequently, a nonlinear viscoelastic model has been developed for stresses up to 60 MPa for relatively short creep durations. The creep response was also compared with the same composite material having twice the thickness for a lower stress range. Here in Part II, the work has been extended to characterize and model longer term creep and recovery in the 3‐mm composite for stresses up to near failure. Long‐term creep tests consisting of 1‐day loading followed by recovery were carried out in the nonlinear viscoelastic stress range of the material, i.e., 20–80 MPa in increments of 10 MPa. The material exhibited tertiary creep at 80 MPa and hence data up‐to 70 MPa has been used for model development. It was found that viscoplastic strains of about 10% of the instantaneous strains were developed under load. Hence, a non‐linear viscoelastic–viscoplastic constitutive model has been developed to represent the considerable plastic strains for the long‐term tests. Findley's model which is the reduced form of the Schapery non‐linear viscoelastic model was found to be sufficient to model the viscoelastic behavior. The viscoplastic strains were modeled using the Zapas and Crissman viscoplastic model. A parameter estimation method which isolates the viscoelastic component from the viscoplastic part of the nonlinear model has been developed. The model predictions were found to be in good agreement with the average experimental curves. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Time dependent response of polycarbonate and microcellular polycarbonate

Microcellular polycarbonate is a novel cellular material with cells on the order of 10 μm in diameter and a cell density on the order of 10⁹ cells per cm³. In this study the room temperature creep response of microcellular polycarbonate is experimentally determined and compared with the creep behavior of polycarbonate. The viscoelastic response of polycarbonate and microcellular polycarbonate is characterized using Schapery's theory of nonlinear viscoelasticity. Polycarbonate exhibited a nonlinear creep response at stress levels above 24.13 MPa, while the nonlinear behavior in microcellular polycarbonate was initiated at lower stress levels. Creep strains of microcellular polycarbonate contain a significantly higher viscoplastic component compared with the unfoamed material.     

The Effects of Molecular Weight on the Single Lap Shear Creep and Constant Strain Rate Behavior of Thermoplastic Polyimidesulfone Adhesive

The bonded shear creep and constant strain rate behavior of zero, one, and three percent end capped Thermoplastic Polyimidesulfone adhesive were examined at room and elevated temperatures. End capping was accomplished by the addition of phthalic anhydrides.

The viscoelastic Chase-Goldsmith and elastic nonlinear relations gave a good fit to the experimental stress strain behavior. Ultimate stress levels and the safe levels for creep stresses were found to decrease as molecular weight was reduced.

The primary objective was to determine the effects of molecular weight on the mechanical properties of the adhesive in the bonded form. Viscoelastic and nonlinear elastic constitutive equations were utilized to model the adhesive. Crochet's relation was used to describe the experimental creep failure data. The effects of molecular weight changes on the above mentioned mechanical behavior were assessed.     

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.     

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si₃N₄ was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si₃N₄. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si₃N₄ did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.     

Some aspects of the mechanical response of BMI 5250‐4 neat resin at 191°C: Experiment and modeling

The inelastic deformation behavior of BMI‐5250‐4 neat resin, a high‐temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress–strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain‐time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi‐step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of Creep Damage on the Tensile Creep Behavior of a Siliconized Silicon Carbide

The tensile creep behavior of a siliconized silicon carbide was investigated in air, under applied stresses of 103 to 172 MPa for the temperature range of 1100° to 1200°C. At 1100°C, the steady-state stress exponent for creep was approximately 4 under applied stresses less than the threshold for creep damage (132 MPa). At applied stresses greater than the threshold stress for creep damage, the stress exponent increased to approximately 10. The activation energy for steady-state creep at 103 MPa was approximately 175 kJ/mol for the temperature range of 1100° to 1200°C. Under applied stresses of 137 and 172 MPa, the activation energy for creep increased to 210 and 350 kJ/mol, respectively, for the same temperature range. Creep deformation in the siliconized silicon carbide below the threshold stress for creep damage was determined to be controlled by dislocation processes in the silicon phase. At applied stresses above the threshold stress for creep damage, creep damage enhanced the rate of deformation, resulting in an increased stress exponent and activation energy for creep. The contribution of creep damage to the deformation process was shown to increase the stress exponent from 4 to 10.