沥青混合料黏弹性主曲线模拟及换算

基于时间-温度等效原理将不同温度下沥青混合料的储存模量和蠕变柔量进行平移形成主曲线,采用广义Maxwell和Kelvin模型分别模拟沥青混合料的松弛和蠕变特性,利用Prony级数表达式和Laplace变换实现黏弹性主曲线间的相互换算,并将换算结果与试验结果进行了比较。结果表明,广义Maxwell和Kelvin模型可以较好地模拟沥青混合料黏弹性主曲线,储存模量和蠕变柔量主曲线相互换算结果与试验结果规律基本一致,松弛模量实际结果应介于由储存模量和蠕变柔量换算的松弛模量主曲线之间,这可为沥青混合料黏弹性力学研究及黏弹性参数的获取提供有益参考。     

采用间接拉伸蠕变试验评价沥青混合料低温抗裂性能

为深入评价沥青混合料的低温抗裂性能,对3种(粗、中和细)级配AC-20沥青混合料进行了3个温度(0,-10和-20℃)条件下的间接拉伸低温蠕变试验。根据幂函数模型计算得到了沥青混合料的低温蠕变柔量,并基于时-温等效原理,以-10℃为参考温度建立了幂函数模型蠕变柔量主曲线,分析了沥青混合料的低温蠕变性能和低温抗裂性能。结果表明,温度在0℃时,中间级配混合料的低温抗裂性能最好,而随着温度降低到-20℃时,粗级配混合料的低温抗裂性能变最好;采用幂函数模型能够较好地描述沥青混合料扩宽时间域的低温蠕变柔量主曲线,其参数幂指数m能较好地表示沥青混合料的低温松弛能力,在宽时间域内,细级配混合料的低温松弛性能增大,相应的其低温抗裂性也最好。     

考虑老化的温拌沥青混合料低温性能

为明确温拌沥青混合料中温拌剂和热老化对混合料低温性能的影响,对原样、短期老化、长期老化的沥青混合料,利用UTM-100进行了0℃、-10℃和-20℃下的小梁弯曲试验和小梁弯曲蠕变试验。研究结果表明：可用蠕变试验数据拟合出伯格斯模型参数,计算出温度应变能密度,进而由弯曲和蠕变试验结果预估出温拌沥青混合料的开裂温度,作为评价温拌沥青混合料低温性能的一个综合指标;可以用延时拌和法表征沥青混合料在短期老化过程中达到平均老化程度的热老化方法;沥青混合料在不同温度、不同老化程度下低温性能发生变化是由于拟合参数发生了变化;温拌剂在一定程度下增加了沥青混合料的抗短期老化性能;发现加入有机降黏型RH温拌剂会降低沥青混合料的低温性能,而益路温拌剂对沥青混合料的低温性能有利,故在寒冷地区推荐使用益路温拌剂。     

沥青混合料低温抗裂性能试验方法研究进展

在寒冷和温差较大的地区,低温开裂是沥青路面病害的主要破坏形式.裂缝的产生影响了路面的完整性、连续性及美观,会导致水通过裂缝渗入路面结构,进而削弱了沥青与骨料之间的粘附力,易诱发其他病害,严重缩短了沥青路面的使用寿命.为评价沥青混合料低温抗裂性能,广大科技工作者探索了多种室内试验和评价方法.但由于沥青混合料是一种多相、多组分、多尺度的黏弹性材料,受力极其复杂.因此不同试验方法的试件尺寸、形状、试验温度、加载速率等对混合料的受力状态影响显著,试验结果存在较大的差异性,导致评价指标和评价标准也各有差异,未形成统一标准,故目前仍没有一种简单有效且公认的方法.目前,根据沥青路面开裂的相关判据及理论基础将现有试验分为:连续体试验、断裂力学试验和声发射试验.连续体试验试件完整、无需再处理,评价多为力学指标;断裂力学试验评价多为能量指标,评价更全面;声发射试验为无损检测技术,可结合宏观试验进行裂缝扩展特性研究.本文结合路面低温开裂的机理,系统归纳了沥青混合料低温抗裂性能试验方法的研究进展,对各种评价方法的优缺点进行评述和对比总结,并结合试验评价难度、试验结果变异性与现场实际受力相关性等方面,探讨了进一步深研的方向,以期为确定科学合理、客观真实、系统全面的沥青混合料低温抗裂性能试验与评价方法提供参考.     

沥青低温性能评价方法研究进展

对沥青低温性能的评价方法进行了分析比较，论述了这些方法的优缺点，结合Hajek模型、Haas加拿大机场道面模型和Superpave低温开裂模型的研究成果，分析了路面低温裂缝的性能预测结果，表明沥青材料的劲度模量是预估沥青路面低温开裂的基本参数，它和松弛性能都是评价沥青低温性能的核心指标，因此要求沥青劲度模量S不能过大、松弛性能m不能过小。     

混凝土硬化过程中的应力-应变发展和基于内部湿度的开裂风险预估

建立了一种对水泥混凝土开裂风险进行评估的新方法。模型基于硬化过程中混凝土的应变-约束应力发展, 研究了开裂时间与收缩变形率和混凝土内部湿度降低率的关系。研究结果表明:混凝土开裂风险与收缩变形速率有关, 开裂时间与开裂时的临界收缩率存在唯一对应关系。该研究根据混凝土湿度与变形的关系, 提出了一个基于混凝土内部湿度降低速率的混凝土早期开裂预估方法。该方法避免了工程实际中为确定混凝土开裂风险而需要进行的变形测量及复杂的需要考虑徐变、应力松弛、强度和模量的发展等因素但又未必准确的应力计算过程, 简化了混凝土硬化过程中的开裂风险预测。     

基于损伤理论的沥青混合料抗疲劳性能试验

为系统研究沥青混合料在疲劳破坏过程中的路用性能衰减过程,在不同温度、应变、频率下开展了四点弯曲疲劳试验,基于损伤理论从劲度模量变化情况和耗散能变化情况两方面评价了沥青混合料抗疲劳性能,并通过损伤面积法、灰关联分析法讨论了试验因素对沥青混合料抗疲劳性能的影响程度。研究结果表明：沥青混合料的抗疲劳性能可以采用基于耗散能的损伤因子随加载次数的变化的双对数曲线进行描述,疲劳寿命应由初始损伤和损伤累积速率共同决定;从劲度模量方面分析,沥青混合料疲劳寿命随温度的升高而增大、随应变的增大而减小、随频率的增大而减小;劲度模量和耗散能两方面的分析结果均表明温度对疲劳寿命的影响程度大于应变,劲度模量分析显示频率对疲劳寿命影响最次,耗散能分析结果显示频率与疲劳寿命的相关性不显著。     

细粒式沥青混合料断裂愈合预估模型

为分析沥青混合料断裂愈合预估模型,采用70#基质沥青和SBS改性沥青制备AC-13混合料半圆试件,通过半圆弯曲试验获得荷载-位移曲线,将断裂的半圆试件拼接后放入烘箱中养护,并对养护后的试件进行二次加载.采用愈合后和愈合前两次加载的临界荷载比作为愈合指数(Healing index,HI),并用其评价混合料的断裂愈合性能.根据毛细流动原理,建立沥青混合料愈合预估模型,并根据试验结果验证该模型的适用性,根据HI建立愈合时间和愈合温度的转换关系.结果表明:发生开裂的沥青混合料试件在特定的环境中养护后具备二次承载的能力,虽然HI<1.0,但试件在100℃&12 h环境中愈合后,HI可达到0.7,即愈合后抗拉强度恢复了70％;愈合时间从4～8 h延长至8～12 h时,100℃环境中70#AC-13混合料的HI增长速率分别为0.096/h和0.023/h,SBS AC-13混合料的HI增长速率分别为0.096/h和0.009/h,说明HI随愈合时间的延长而增加,但增长速率先增后减;采用函数HI(t)=(-1+e-Ct)2(C>0)能较好地拟合不同愈合温度下愈合指数随愈合时间的变化,拟合系数R2>0.90;根据HI值建立愈合温度和愈合时间的转换模型t(T)=C3·eC4/T(00.95).     

考虑加载速度影响的沥青混合料疲劳方程

为了建立沥青混合料强度与疲劳行为之间的联系,通过不同加载速率下的沥青混合料直接拉伸强度试验,揭示了强度随加载速率的幂函数变化规律;基于不同加载速率下的强度值,得到了与疲劳加载速率对应的沥青混合料疲劳真实应力比;通过疲劳试验,创建了基于名义应力比和真实应力比的沥青混合料疲劳方程,基于名义应力比的疲劳方程后延后与横坐标的交点远比1 大,不具有后延性,而基于真实应力比的疲劳方程可以后延到疲劳寿命为1 的强度破坏点,统一了强度破坏与疲劳破坏行为;据此推导了沥青路面抗拉强度结构系数计算新方法;研究结果可为我国公路沥青路面设计规范的修订提供理论依据.     

基于VIC-3D技术的沥青混合料界面力学特性实验研究

为研究沥青混合料开裂及裂缝发展规律,通过SBS改性沥青混合料半圆试件三点弯曲实验,基于数字图像相关技术,利用VIC-3D(video image correlate,3D)采集系统对试件从加载到破坏全过程进行图像采集,并利用VIC-3D软件计算得到试件在受弯拉过程中三维位移场、应变场的变化及变化速率,分析并得出裂缝出现及发展规律;利用扫描电子显微镜对开裂试件断口进行观察,结合位移场、应变场及其变化速率揭示了试件开裂及裂缝发展的机理。结果表明,利用水平方向应变随时间变化规律可探究裂缝的发展轨迹;沥青混合料的开裂和裂缝发展沿沥青砂浆与粗集料的界面区发生,且趋向于表面情况相对差的集料的界面区;裂缝发展速率与界面区的厚度有关,可根据界面区厚度的代表值预测裂缝发展速率。     

The influence of hygrothermal conditions on the damage processes in quasi-isotropic carbon/epoxy laminates

The effects of hygrothermal conditions on damage development in quasi-isotropic carbon-fiber/epoxy laminates are described. First, monotonic and loading/unloading tensile tests were conducted on dry and wet specimens at ambient and high temperatures to compare the stress/strain response and damage development. The changes in the Young's modulus and Poisson's ratio were obtained experimentally from the monotonic tensile tests. The critical stresses for transverse cracking and delamination for the above three conditions are compared. The delamination area is measured by using scanning acoustic microscopy (SAM) at various loads to discuss the effects of delamination on the nonlinear stress/strain behavior. Next, the stress distributions under tensile load including hygrothermal residual stresses are computed by a finite-element code and their effects on damage initiation are discussed. Finally, a simple model for the prediction of the Young's modulus of a delaminated specimen is proposed. It is found that moisture increases the critical stresses for transverse cracking and delamination by reducing the residual stresses while high temperature decreases the critical stresses in spite of relaxation of the residual stresses. The results of the finite-element analysis provide some explanations for the onset of transverse cracking and delamination. The Young's modulus predicted by the present model agrees with experimental results better than that predicted by conventional models.     

Time-dependent uniaxial behavior of rolled magnesium alloy AZ31B at 393 K and room temperature

To address the time-dependent properties of rolled AZ31B alloy, we conducted typical tests of the rate jump, creep, and stress relaxation at room temperature and 393 K. In the rate jump tests, the tensile curve exhibited a strong dependence on the strain rate, whereas compression was totally insensitive to the stress rate at both temperatures. For the creep and stress relaxation test, we observed creep strain and decay stress in the compression, which was weaker than the tensile curve. The plastic viscosity increased at 393 K because the dislocation motion was thermally activated. We then applied thermal activation theory for the repeated stress relaxation tests. The activation volume implies that cross-slip and dislocation nucleation are the operating mechanisms for creep and stress relaxation.

     

Characteristics of two-component epoxy modified bitumen

Coal tar bearing emulsions were used in the Netherlands as binder in anti-skid surfaces for runways because of their perfect adhesion and fuel resistance properties. They are however toxic and will not be allowed anymore after 2010. Therefore alternatives need to be developed. As one of the alternatives, two types of two-component epoxy modified bitumen have been investigated by means of direct tensile tests (DTT), relaxation tests (RT) and dynamic shear rheometer (DSR) tests. The effect of the curing temperature on the strength development of the epoxy modified bitumen was tested. The results show that the tensile strength increases with increasing curing time and temperature. DTT and RT results indicate that this new epoxy modified bitumen has a much higher tensile strength, cures faster than a bitumen emulsion as a binder. Furthermore, it shows a good stress relaxation even at lower temperatures. The curing speed and the ultimate tensile strength after full curing can be easily adjusted. The DSR results show that the complex modulus of this epoxy modified binder is less susceptible to changes in temperature. The results also suggest that this epoxy modified bitumen has better anti-crack properties at lower temperature and less permanent deformation than bituminous binders at higher temperatures. All these results shows that this type of two-component epoxy modified bitumen can be promising as a binder in anti-skid layers.     

Analysis of cracking of lithium tantalate (LiTaO3) single crystals due to thermal stress

Quantitative estimation of failure of a LiTaO₃ single crystal due to thermal stress was investigated. Cylindrical test slabs were heated in a silicone oil bath, then subjected to large thermal stress by pouring silicone oil with room temperature. Cracking occurred during cooling. A transient heat conduction analysis was performed to obtain a temperature distribution in a test slab at the time of cracking, using the surface temperatures measured in the test. Then thermal stress was calculated using a temperature profile of the test slab obtained from the heat conduction analysis. It is found from the results of thermal stress analyses and the observation of the cracking in the test slabs that the cracking induced by thermal stress occurs mainly in the cleavage planes due to the stress component normal to the plane. As for a size effect of failure stress, large-sized cylindrical test slabs show lower failure stress than small-sized ones. Four-point bending tests were also performed to examine the relationship between the critical stress for cracking induced by thermal stress and the four-point bending strength. A useful relation was derived for predicting the critical stress for cracking induced by thermal stress from the four-point bending strength.     

超高性能混凝土圆环约束收缩试验研究

以约束水平、环境条件（密闭或干燥）和钢纤维等为参数,开展了超高性能混凝土（UHPC）圆环约束收缩试验。研究了钢环应变随龄期的发展规律;分析了各参数对圆环约束下的残余应力与各关键龄期的力学性能的影响;采用了拉应力水平和应力松弛率来评价UHPC的开裂性能。为配合圆环收缩试验,开展了自由收缩与基本力学性能试验。试验表明,未掺钢纤维的UHPC早期开裂风险大,在14 d前均发生开裂,裂缝平均宽度大于0.25 mm,含钢纤维试件均未开裂。不同约束程度对拉应力水平与应力松弛率的影响均显著,降低约束程度能有效降低开裂风险。与自由收缩测试结果不同,圆环约束UHPC在密闭条件下后期的开裂风险会高于环向干燥条件。建议以密闭条件下14 d的抗裂性能作为控制指标评价圆环约束下UHPC的开裂性能。     

插销法研究自保护焊U75V钢轨的冷裂敏感性

采用插销试验法研究了JDHS-38#自保护药芯焊丝焊接U75V钢轨的冷裂敏感性,评估了预热温度、线能量、焊道数3个因素对U75V钢轨临界断裂应力的影响.结果表明,现场对钢轨施焊时,采取250℃的预热温度,14 kJ/cm左右的线能量,双层焊等措施可以使钢轨接头热影响区少产生甚至不产生焊接冷裂纹,使其临界断裂应力达646 MPa,与抗拉强度相当.若采用其他工艺参数,临界断裂应力较低,且当拘束拉伸应力高于抗拉强度时,于热影响区发生氢致断裂.焊接接头过热区组织主要为马氏体、贝氏体,断口显微形貌主要为河流花样、泥状花样及爪状花样.     

预应力钢绞线温度膨胀及高温蠕变性能试验研究

火灾高温下钢绞线的温度膨胀及高温蠕变性能是影响张拉钢结构抗火能力的重要因素。该文以1860级预应力钢绞线为研究对象,采用非接触式应变视频测量系统,测试了钢绞线在15个温度水平下的热膨胀应变,得到热膨胀应变随温度的变化规律,并拟合出热膨胀系数计算公式。研究发现,大约750℃时钢绞线材料发生相变,导致热膨胀应变突变。通过钢绞线在不同温度及应力水平下的高温蠕变试验,得到蠕变应变与时间的关系曲线,并建议了钢绞线高温蠕变计算公式。高温蠕变试验现象表明,在较高温度及应力水平时,蠕变会导致钢绞线断裂;对经2 h高温蠕变未断裂的钢绞线自然冷却到常温后,进行抗拉强度试验,发现其抗拉强度有一定程度的退化,且经历的蠕变温度越高,应力水平对高温蠕变后试件抗拉强度的退化影响越明显。     

Origins of tensile stress-induced circumferential cracking of waterwall tubes in boilers

Abstract

The leakage and rupture of boiler tubes in power plants is a serious problem that can lead to unscheduled and costly outages. The predominant failure location of current concern is circumferential cracking on the fireside of waterwall tubes in the furnace waterwall section of a boiler. Although there is basic agreement that cracking results from a combination of thermal fatigue and corrosion, a complete explanation of the basic phenomena needed to establish the root causes of this problem is lacking. The purpose of the present study was to analyse the sources of the tensile stress responsible for initiating circumferential cracking and to identify the key parameters controlling this tensile stress. The results of analytical modelling suggested that a combination of increasing tube wall temperature with increasing thickness of internal oxide layers, and temperature spiking due to deslagging events eventually may result in tensile stresses sufficient to crack the fireside oxide and initiate the development of circumferential cracks. This scenario also led to suggestions for reducing tensile stresses in waterwall tubes which, in turn, would be expected to delay/avoid circumferential cracking and improve the reliability of waterwall tubes.     

SCC Behaviour of X-70 Pipeline Steel in Near-Neutral pH Solutions

The stress corrosion cracking (SCC) behaviour of X-70 pipeline steel in near-neutral pH solutions was studied via slow strain rate testing (SSRT). The results showed that the cracking mode of X-70 pipeline steel in near-neutral pH solutions was transgranular at different temperatures and applied potentials with the feature of quasi-cleavage. The pH value of the solution decreased with increasing the addition of CO2, which increased the susceptibility to SCC. SCC susceptibility increased as the applied potential moved towards the cathodic direction, suggesting that hydrogen induced cracking (HIC) dominated the cracking process at cathodic potentials. The slight decrease of pH values with decreasing temperature of the solution increased the susceptibility to SCC, which attributed to the change of solubility of CO2 in the solution at different temperatures. The propagating directions of SCC cracks were different at different potentials. At rather negative cathodic potentials, the cracks were almost perpendi     

Susceptibility to Hot Cracking and Weldment Heat Treatment of Haynes 230 Superalloy

This study investigates the susceptibility of hot cracking and weldment heat treatment of Haynes 230 superalloy. The Varestriant test was conducted to evaluate this susceptibility. Welding was performed by gas tungsten arc welding （GTAW） and plasma arc welding （PAW） with stress relief heat treatment and solid solution heat treatment. A tensile test is then performed to measure the changes in the mechanical properties of the heattreated material. The results indicate that the number of thermal cycles does not affect the susceptibility of Haynes 230 superalloy to hot cracking. However, it does increase the strain. In weldment of heat treatment, stress relief annealing increases the yield strength and tensile strength of the welded parts. The section of the tensile specimens shows fibrous fractures on the welded parts, regardless of whether they are heat-treated.