A new approach to model strain change of gelled waxy crude oil under constant stress

Deformation of gelled waxy crude oil with loaded stress is worthy of research for the flow assurance of pipelining system. A dispersion parameter was introduced to characterize the disruption degree of wax crystal structure in crude oil with shear action. Based on fractional calculus theory, a rheological model incorporating dispersion parameter was proposed to describe creep of gelled waxy crude. A discrete and numerical algorithm was proposed to solve the model. Combining with the experimental results of five kinds of waxy crude oil, the model parameters were regressed and found to change monotonously with test temperature. Multiple creep curves of gelled waxy crude oil at a certain temperature can be described with this model.     

胶凝原油蠕变的时间-温度-应力等效性

黏弹性蠕变是胶凝原油重要的流变特性,要描述胶凝原油的黏弹性蠕变行为,必须同时考虑 应力、时间和温度等参数. 于是对胶凝原油进行了不同应力和不同温度下的蠕变实验,利用 时间-温度-应力等效原理分析了胶凝原油的非线性蠕变行为,得到了参考应力和参考温度下 的蠕变主曲线,并用黏弹性流变模型拟合得到了蠕变主曲线的表达式. 从而可以分别通过较 高温度和应力水平下的短期蠕变行为来预测较低温度和应力水平下的长期蠕变行为.     

Cyclic multiaxial and shear finite deformation response of OFHC: Part I,experimental results

A series of experiments has been conducted on oxygen free high conductivity (OFHC) copper hollow cylinders under cyclic free-end torsion and biaxial tension–torsion at large strains. In addition, equations are developed to account for the finite rotation and strains in electrical resistance strain gages. In free-end cyclic torsion experiments with shear strain range equal to 23%, a significant strain in the axial direction is observed and it accumulates with a constant rate cycle by cycle. In the biaxial tension–torsion (multiaxial ratchetting) experiments, in which the primary (constant) axial stress is larger than the initial yield stress of the material, the loading conditions are varied to determine the influence of primary axial stress, cyclic shear strain range, pre-cyclic hardening and loading sequence on multiaxial ratchetting. Some important experimental features are high-lighted and recommended to help modeling efforts later.     

Experimental investigation and modeling of non-monotonic creep behavior in polymers

The deformation behavior of two unfilled engineering thermoplastics, ultra high molecular weight polyethylene (UHMWPE) and polycarbonate (PC), has been investigated in creep test conditions. It has been found that a loading history (prior to the creep test) comprising of loading to a maximum stress or strain value followed by partial unloading to arrive at the target stress value can greatly modify the strain-time behavior. Under such a test protocol, while the expected increase in strain during creep (constant tensile load) is observed, at relatively low creep stresses specimens have also demonstrated a monotonic decrease in strain. In an intermediate stress range, specimens have demonstrated time dependent behavior comprising of a transition from decreasing to increasing strain during creep in tension. This paper presents experimental results to delineate these findings and explore the effect of prior strain rate on the qualitative and quantitative changes in the output (strain-time) behavior. Furthermore, modification of the viscoplasticity theory based on overstress (VBO) model into a double element configuration is introduced. These changes confer upon the model the ability to yield non-monotonic behavior in creep, and supporting simulation results have been included. These changes, therefore, allow the model to simulate strain rate sensitivity, creep, relaxation, and recovery behavior, but more importantly address the issue of non-monotonic changes in creep and relaxation when a loading history involves some degree of unloading.     

Effect of Varying Test Parameters on Elastic–plastic Properties Extracted by Nanoindentation Tests

A systematic experiment was performed in an effort to investigate how the levels of certain test parameters affect the values of elastic modulus, hardness, yield stress, and strain hardening constant obtained using nanoindentation test. Maximum applied load, loading (unloading) rate, and hold time at maximum load were varied at three levels. The effects of these testing parameters were investigated through a three-level, full factorial design of experiment. The experiments were conducted on ultrafine Al-Mg specimens that were mechanically extruded. Both longitudinal and transverse extrusion directions were examined to investigate effects of anisotropy on mechanical properties and evaluate the persistence of observed variations due to test parameters on different materials orientations. An indentation size effect (ISE) was observed demonstrating that maximum load—and thereby maximum indentation depth—can have a significant effect on values of hardness and elastic modulus. Hardness values decreased with higher loading rates, and higher rates of unloading resulted in higher values of elastic modulus (5–10 GPa increases). Strain-hardening exponent showed a decreasing trend with increasing loading rate while yield stress exhibited a consistent correlation to hardness across all studied parameters. The material exhibited very little creep during the hold period, and values of the calculated properties were not significantly altered by varying the length of the hold time. Anisotropy effect was observed, particularly in the values of yield strength. This is attributed to the preferred grain orientation due to extrusion.     

泥岩三轴蠕变实验研究

采用MTS伺服刚性压力实验机对高家梁煤矿泥岩进行了三轴蠕变实验研究,获得了泥岩在不同应力条件下的蠕变变形规律。实验表明围压对泥岩微观缺陷的发展有一定的限制作用。随着围压的增加,泥岩的三轴抗压强度和弹性模量均有所增加。在围压一定时:衰减蠕变量随轴压的增大而增大;衰减蠕变率随偏应力的增加而增大;稳态蠕变率随偏应力的增大而增大。在偏应力一定时,稳态蠕变率随围压的增大而减小。当围压大于4MPa后,围压的限制作用将明显减小。随着围压继续增大,稳态蠕变率变化并不明显。根据获得的实验数据,用回归法求出了H-K模型蠕变方程的参数。结果表明H-K模型能较好地模拟实验结果。在实际工程中可通过支护增加围压以提高围岩屈服强度,也可根据实际工程中泥岩蠕变的变化趋势确定合理的二次支护时间,避免流变破坏的发生。     

An experimental study of uniaxial creep,cyclic creep and relaxation of aisi type 304 stainless steel at room temperature

Following previous work (Krempl, 1979), a servocontrolled testing machine and strain measurement at the gage length were used to study the uniaxial rate(time)-dependent behavior of AISI Type 304 stainless steel at room temperature. The test results show that the creep strain accumulated in a given period of time depends strongly on the stress-rate preceding the creep test. In constant stress-rate zero-to-tension loading the creep strain accumulated in a fixed time-period at a given stress level is always higher during loading than during unloading. Continued cycling causes an exhaustion of creep ratchetting which depends on the stress-rate. Periods of creep and relaxation introduced during completely reversed plastic cycling show that the curved portions of the hysteresis loop exhibit most of the inelasticity. In the straight portions, creep and relaxation are small and there exists a region commencing after unloading where the behavior is similar to that at the origin for virgin materials. This region does not extend to zero stress.The results are at variance with creep theory and with viscoplasticity theories which assume that the yield surface expands with the stress. They support the theory of viscoplasticity based on total strain and overstress.     

Effects of cooling regime on the formation of voids in statically cooled waxy crude oil

Waxy crude oil solidifies when exposed to certain range of low seabed temperature. In this condition, predicting an accurate restart pressure in the pipeline has remained to be a challenge. Recent researches highlighted that the presence of gas voids in the gel has an impact on the gelled crude oil morphology and consequently on the restarting pressure. The gelling due to cooling of the oil is complex and not well understood. This study is aimed at analyzing the effects of cooling regimes on the formation of gas voids along and across the pipeline using a 3 T Magnetic Resonance Imaging (3 T-MRI) system. A flow loop rig simulating offshore oil transportation was designed and developed to produce the gel. It was found that voids were formed within gelled crude oil by 11.3% in volume. The results from the cooling regime under constant start temperature showed that the widest cooling regime resulted in a maximum voids volume in all regions in the pipe. However, reducing the cooling regime did not guarantee a decrease in volume of voids, for which lower cooling regimes occasionally resulted in a higher percentage of voids volume. For the cooling regimes under constant end temperature, in general, an increase in cooling regime resulted in a higher percentage of voids volume in the gel at lower end temperature. On the contrary, the intra-gel voids formation at higher end temperature was found to be different and a higher cooling regime did not result in a higher voids volume near pipe wall. Indeed, a higher cooling regime formed a higher percentage of voids volume around pipe core.     

Rheology and flow-birefringence from viscoelastic polymer-clay solutions

 The shear orientation of viscoelastic clay-polymer solutions was investigated by means of rheology and flow birefringence (Δn). The polymer chains are in dynamic adsorption/desorption equilibrium with the clay particles to form a “network”. The elastic behavior of the network was characterized by constant stress, oscillatory shear, and stress relaxation experiments. Constant stress experiments indicated a yield stress upon which shear flow started and no strain recovery could be observed. Oscillatory shear experiments showed a broad elastic region followed by flow when a critical strain was reached. Stress relaxation experiments showed several relaxation times when the same critical strain was reached. Experiments under steady flow characterized the transient behavior of the network. With increasing steady shear rate a pronounced minimum in birefringence was observed at a critical shear rate. The shear rate dependent viscosity showed near power law behavior and no corresponding critical feature. While birefringence detects orientational effects on a microscopic length scale, rheology averages over macroscopic changes in the sample. The same degree of orientation could be achieved under constant shear rate or constant stress conditions. Received: 25 January 2001 Accepted: 22 May 2001     

侧向受拉脆性岩石压缩本构及蠕变失效研究

轴向压缩作用下,脆性岩石侧向应力严重影响岩石力学特性。侧向压应力影响下的轴向压缩岩石力学行为已经得到广泛研究,然而侧向拉应力对轴向压缩岩石力学行为影响研究很少。本文基于脆性岩石翼型裂纹扩展模型中,初始裂纹面法向应力与剪切应力的正负方向为判断依据,研究了侧向拉应力对轴向压缩力学行为的影响。发现恒定的侧向拉应力作用下,轴向压缩应力渐进变化过程中,脆性岩石内部细观初始裂纹面的法向应力只能为压缩应力,不存在拉应力情况。分析了从侧向压应力到拉应力转化过程中,脆性岩石轴向压应力与细观裂纹扩展长度关系、轴向压应力与轴向应变关系、岩石峰值强度、裂纹启裂应力及初始弹性模量的变化规律。并分析了侧向拉应力对岩石蠕变裂纹长度、裂纹速率、轴向应变及应变率演化曲线,以及对蠕变失效时间及稳态蠕变应变率的影响。讨论了侧向拉压应力突变转化以及侧向拉应力分级增大对轴向压缩岩石蠕变演化行为影响。该研究为深部地下工程围岩稳定性评价提供了一定理论依据。     

Rheology of concentrated microgel solutions

Viscosity, modulus, and yield stress for 0–6 wt% aqueous solutions of Carbopol 941 were investigated using constant shear rate, constant shear stress, and dynamic oscillatory experiments. The microgel character of the polymer was evident from the solid-like behavior of the solutions above 1 wt%. Yield stress increased with concentration, but yield occurred at a critical shear strain of 40%, independent of concentration. The static stress-strain relationship became non-linear at ~ 25% strain, in fair agreement with the onset of non-linear response in the storage modulus at ~ 10% strain. Small strain moduli from static and low frequency measurements agreed rather well; modulus values obtained from the recoverable strain after yielding were 30–40% smaller. Solutions flowed at near-constant stress in the low shear rate regime; at higher rates the stress increases with shear rate more rapidly. The viscosity did not obey the Cox-Merz rule. Steady-state viscosity scaled with polymer concentration to the 3/4 power. Results were interpreted using a cellular, deformable sphere model for the polymer, in analogy to emulsions and foams.     

Rate (time)-dependent deformation behavior: an overview of some properties of metals and solid polymers

The inelastic deformation behaviors of metals and polymers are discussed with the aim of finding a common base that would simplify academic and engineering analyses. Only monotonic loading conditions at room temperature are considered. For loading at different rates, nonlinear relations between loading rate and stress level, creep stress level and creep strain, and relaxation rate and stress were common to both type of materials. There are, of course, significant differences in elastic properties, strength levels and the strains involved. Special properties such as relaxation behaviors and creep anomalies can be qualitatively and quantitatively reproduced by the state variable model VBO (viscoplasticity theory based on overstress). Since experimental investigations typically concentrate on one particular aspect of inelastic deformation behavior such as creep or strain-rate dependence, it is often difficult to gather a comprehensive data set for a given material. In spite of this, considerable similitude in the deformation behavior of metals and polymers in various test conditions has nevertheless been established.     

Rheokinetics of urea-formaldehyde resins gelation

Rheokinetics of gelation of aqueous suspensions of urea-formaldehyde oligomers was studied. Experiments were carried out with a rotational viscometry method using two regimes of deformation: constant stress or constant shear rate. It was shown that a maximum on a viscosity vs. time curve is observed and it reflects sedimentation of rather large gelled particles. Then, gelation of a whole system (determined as the point of the unlimited growth of viscosity) takes place. Experiments carried out at different temperatures gave us an apparent activation energy which depends on the oligomer content in a composition. Characteristic points of a rheokinetic curve do not depend on the concentration of a curing agent. The problem of the temperature increase due to heat dissipation is also discussed. It was demonstrated that shear stress is a kinetic factor influencing the rate of a reaction. A master curve for a rheokinetic curve was constructed, time scale being reduced by the gel time.     

纳米颗粒增强镍基MEMS器件材料的蠕变性能研究

利用同步辐射LIGA微铸复合工艺,将纳米氧化物增强颗粒复合到微电子机械系统(MEMS)结构材料中。制作了专用夹具,采用微力材料试验机测量了纳米Al2O3颗粒增强镍基复合材料的强度为1GPa;将恒加载速率/载荷法和恒载荷法相结合,利用纳米压痕仪测量了该材料的室温蠕变速率敏感指数m。结果表明,LIGA复合技术得到的纳米颗粒增强镍基复合材料具有较高的强度;MEMS器件材料在室温下会发生蠕变;材料在相同压深下最大载荷不随加载速率而改变,加载段粘弹性和粘塑性变形极少;主要由局部高应力导致压痕蠕变;材料的蠕变速率敏感指数m值为0.004,说明纳米Al2O3颗粒可有效增强基体材料的抗蠕变能力;且不同恒.P/P下获得的m值基本相同,表示此种材料对加载速率不敏感。     

Constitutive relation for rheological processes: Properties of theoretic creep curves and simulation of memory decay

We study the nonlinear stress-strain constitutive relation proposed earlier for describing one-dimensional isothermal rheological processes in the case of monotonous variation of the strain (in particular, viscoplasticity, creep, relaxation, plasticity, and superplasticity). This relation contains integral time operators of the strain and strain rate, which are the norms in the Lebesgue and Sobolev spaces equipped with special weight factors, one material function, and nine material parameters determined by the results of tests of the material for relaxation, creep, long-term strength, and constant-rate strain.We analytically inverse the constitutive relation and study the properties of the inverse operator. We derive the equation of creep curves corresponding to an arbitrary law of loading at the stage of passing from the zero stress to a given constant level. We study their dependence on the material parameters and the loading stage characteristics and find restrictions on the material parameters which ensure that the asymptotic behavior of the creep curves for large times is independent of the length of the loading stage and the specific law of stress variation during this stage, i.e., we find the conditions of the model memory decay in creep. Thus we have proved that the constitutive relation proposed above can adequately model both creep and the effect of the material memory decay.     

PBX-1炸药的力学性能和本构关系

PBX的力学行为对其安全性有重要影响。为研究PBX-1的力学性能,以PBX-1为研究对象,进行准静态力学实验和SHPB(分离式霍普金森压杆)实验研究。结果表明,准静态压缩实验中,试样的裂纹出现在与加载方向大约成45°的最大剪应力方向。SHPB实验中,在应变率100~1 500 s^-1范围内,随着应变率的提高,PBX-1炸药的动态屈服强度、动态压缩强度和破坏应变不断提高。动态屈服强度逐渐从静态的2.77 MPa增加至16.1 MPa;压缩强度从7.46 MPa增加至16.1 MPa,破坏应变从6.23%增加到26.4%。同时,基于Z-W-T模型,建立了一种含损伤的动态黏弹性本构模型,在330~1 500 s^-1应变率范围内具有较高的精度,可以较好地描述PBX-1炸药在达到破坏前的动态力学行为。     

高强混凝土动态压缩试验分析

准确测量混凝土动态压缩性能及其应变率强化效应一直是冲击动力学研究领域的重点和难点之一。针对混凝土大口径SHPB实验,分析探讨了其中几个主要问题:应力均匀性问题、恒应变率问题和端面接触问题。研究表明:对于此次试验中混凝土试件而言,应力均匀性假设限制试验最大应变率小于166 s^-1;杆和试件端面接触不平和接触不良使得测算出的杨氏模量和屈服强度明显小于实际值;在此基础上,给出了五步测试法和预应力法;利用复合整形技术实现了近似恒应变率加载。利用以上所发展和改进的技术得到了C110混凝土动静态应力应变曲线,结果显示,在试验范围内混凝土杨氏模量并没有应变率效应,其单轴压缩屈服强度与应变率对数呈线性正比关系,其唯象应变率强化因子为0.10。理论分析表明,大口径SHPB试验所得混凝土应变率效应是一种唯象效应,对于混凝土类压力敏感屈服材料而言,应该根据其屈服面方程对其进行校正,从而得到其本构方程中材料的应变率强化因子,分别利用Tresca屈服准则和K&C本构中屈服面方程对其进行校正,得到C110材料的真实应变率强化因子分别为0.015和0.038。     

薄膜涂层材料界面纯剪破坏标准试验法的开发

界面强度是薄膜涂层材料的最重要的性能指标之一，目前尚缺乏有效的测量方法。测量界面强度的主要困难在于寻求一种便于试验的试件形状和加载方式，使得界面上能够产生不同的应力状态，即在不同的剥离应力和剪应力比的状态下发生破坏。本文采用有限元数值分析（ABAQUS），研究了几种具有简单几何形状的试件的界面应力，并基于大量的数值试验，设计了剪应力为破坏支配因素的试件形状和加载方式，并且给出了便于应用的最大界面剪应力的经验估算公式。该经验公式可以适用于各种材料组合和薄膜涂层的厚度。研究结果表明，通过对薄膜涂层材料试件的基体引入缺口以产生应力集中，进行普通的四点弯曲试验，可以进行剪应力占支配地位的界面破坏试验。利用本文提出的试件形状和相应的最大界面剪应力经验公式，可以通过破坏试验简单地得到界面的剪切强度。     

Gelation of waxy crude oils by ultrasonic and dynamic mechanical analysis

A novel technique, based on low intensity ultrasonic wave propagation, has been applied to investigate the gelation of a waxy crude oil, caused by the crystallization of paraffin fractions as the temperature reduces below a threshold value, called WAT (wax appearance temperature). Because this phenomenon significantly affects the rheological behavior of crude oils, the knowledge of the conditions under which it occurs, during oil storage and/or transportation, is a topical issue in the oil industry. In this work, an ultrasonic equipment has been set up, able to propagate longitudinal waves in the MHz range and to display in real time the behavior of ultrasonic velocity and attenuation when the crude oil sample is subjected to heating and cooling cycles. When the ultrasonic probes alternatively rotate as parallel plates of a conventional rheometer, low intensity longitudinal waves (in the megahertz range), and shear oscillations (in the hertz range) are simultaneously applied on the sample, thus widely broadening the frequency range of investigation. On cooling, the crystallization of paraffin fractions and the consequent formation of a network structure in the oil matrix are responsible of the development and growth of the crude oil elastic response, which becomes dominant over the viscous response. This process can be reliably detected by dynamic mechanical analysis and by ultrasonic analysis through the increase of the storage modulus G′ and longitudinal velocity, respectively. The growth and further association between wax crystals causes a dissipation of acoustic energy, which is indicated by the increase of the wave attenuation. The combination of rheological and ultrasonic methods has provided a better insight both on the gel transition of crude oils and the viscoelastic behavior of gelled samples. The ultrasonic wave propagation has demonstrated to be a powerful tool for monitoring the sol–gel transition in waxy crude oils. Finally, the effect of ultrasonic waves with different intensity on the gel build-up has been also evaluated. A reduction of the gel strength with increasing wave intensity has been observed and the recovery of elastic response after removing ultrasonic irradiation has been monitored. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.