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。     

钢材高温材性模型对火灾下钢框架结构倒塌模拟的影响研究

“9·11”事件发生后,火灾下钢结构倒塌研究得到了世界各国学者的广泛关注。钢材的物理、力学性能在高温下会显著降低,从而钢结构极易发生倒塌。因此,该文基于平面钢框架在单柱受火条件下的倒塌试验,应用大型商业有限元软件ABAQUS的显式动力分析模块,并采用梁单元对试验进行数值模拟,重点研究了钢材高温材性模型对火灾下钢框架结构倒塌模拟的影响,提出了适用于火灾下钢框架结构倒塌模拟的钢材高温应力-应变本构模型和热膨胀系数模型的建议。该文列举了常用的三种高温应力-应变本构模型,同时考虑应变率效应的影响,通过模拟结果与试验结果的差异对比,分析了各个模型应用于火灾下钢框架结构倒塌模拟的有效性。理想高温应力-应变本构模型,会导致临界温度与失效模式模拟不准确,应用存在一定局限性;EC3高温应力-应变本构模型可较为准确地模拟准静态失效模式,但对于动力失效模式的模拟存在一定局限性;因此,对于准静态失效模式倒塌模拟,建议采用EC3或改进的EC3高温应力-应变本构模型;对于动力失效模式的倒塌模拟,建议采用改进的EC3高温应力-应变本构模型,并采用应变率效应增大系数（DIF）的方法以考虑应变率效应的影响。此外,通过四种钢材热膨胀系数模型的对比,进一步研究了钢材“相位变换”现象对火灾下钢框架结构倒塌模拟的影响。结果表明,虽然欧洲规范EC3给出了精细的热膨胀系数模型,但采用GB 51249取值的计算结果与试验符合更好,建议采用中国规范GB 51249的相关规定。     

预应力钢绞线高温力学性能试验研究

该文采用非接触式应变视频测量系统,开展了冷拉1860级钢绞线高温力学性能试验研究。基于试验测试的钢绞线高温应力-应变全过程曲线,建议了预应力钢结构用钢绞线的比例极限、弹性模量、名义屈服强度、断裂强度的高温折减系数以及高温应力-应变函数关系。试验结果表明,高强冷拉钢绞线高温下应力-应变全过程具有显著的应力强化阶段和颈缩阶段,1.25%应变下的高温名义屈服强度适用于高强冷拉钢绞线,钢绞线在高温下的捻度松弛效应对其高温力学性能存在影响。该研究成果进一步完善了预应力张拉钢结构用冷拉高强钢绞线高温下基本力学性能指标体系。     

基于Eshelby等效夹杂理论的沥青混合料有效粘弹性质分析

利用Eshelby 等效夹杂理论研究了沥青混合料的单轴压缩蠕变行为。通过时间域内的Laplace 变换将问题线性化, 得到了沥青混合料的蠕变本构关系。开展了不同温度、应力水平条件下沥青砂的单轴压缩蠕变实验, 根据数据拟合了沥青砂四参量流变模型的模型参数。在此基础上, 预测了沥青混合料在不同温度、应力水平下的蠕变曲线, 分析了温度、应力水平对沥青混合料蠕变行为的影响。结果表明:在相同的应力水平下, 沥青混合料的应变和应变率都随温度的升高而增大, 并且在沥青软化点附近发生明显突变;在相同的温度下, 沥青混合料的应变和应变率都随加载应力的增加而增大。     

1.25Cr0.5Mo钢的高温疲劳-蠕变交互作用行为

通过1.25Cr0.5Mo钢高温环境应力控制的疲劳-蠕变交互作用试验,揭示和分析了1.25Cr0.5Mo钢高温疲劳-蠕变交互作用下应力循环特性以及平均应变、非弹性应变范围等参数随温度、加载历史、加载水平的变化规律,在此基础上,利用扫描电镜对试样断口进行了分析。研究表明:高温环境下,材料的应力循环特性依赖于加载水平和加载历史,平均应变、非弹性应变范围等参数依赖于温度、加载水平和加载历史,材料的断裂从以解理断裂为代表的脆性断裂过渡到以韧窝型剪切断裂为代表的延性断裂,为混合型断裂。     

新型耐火耐候钢材高温力学性能与本构模型研究

采用标准试验方法,对首钢集团生产的楼承板用SQ410FRW耐火耐候钢冷轧钢带进行稳态拉伸试验,以测定其典型高温力学性能指标,包括钢材高温弹性模量、规定塑性延伸强度、抗拉强度、断后伸长率和断面收缩率。通过非线性回归方法得出相应的高温折减系数表达式,包括弹性模量折减系数、规定塑性延伸强度折减系数和抗拉强度折减系数。根据试验应力-应变关系曲线,回归得出基于Ramberg-Osgood模型(R-O模型)的应力-应变本构模型,以用于后续有限元参数化建模过程中对结构构件的温度场分析和顺序热力耦合分析。试验结果表明：绝大多数拉伸试样均在平行段内或标距段内发生断裂,且断后伸长率随温度升高呈现总体增大趋势;高温弹性模量、高温规定塑性延伸强度和抗拉强度在600℃及以下时降低较少,均保持在常温名义值的60%以上,基本满足耐火钢的力学性能指标要求;基于R-O模型的应力-应变本构关系表达式的拟合优度均在90%以上,与试验应力-应变曲线吻合良好,故提出的本构模型可用于相关钢结构或组合结构构件的有限元抗火分析。     

升、降温火灾下混凝土瞬态热应变的计算

该文首先借助等效热膨胀系数将混凝土瞬态热应变转化为自由热膨胀应变;接着基于已有一维瞬态热应变模型和通用有限元程序的开放接口,自编程序实现了考虑升、降温火灾条件和三维应力状态下混凝土瞬态热应变的计算;最后利用相关试验数据对方法验证。结果表明考虑瞬态热应变,结构在升、降温阶段的变形会增大,且计算结果更接近试验值;进一步表明了采用等效热膨胀系数的方法考虑瞬态热应变的合理性。     

多聚磷酸/SBS复合改性沥青的高温流变特性

为了研究多聚磷酸(PPA)/SBS复合改性沥青的高温流变特性,利用动态剪切流变仪(DSR)对四种不同掺量的PPA/SBS复合改性沥青及SBS改性沥青进行重复蠕变恢复试验(RCRT)。结果表明:PPA/SBS复合改性沥青变形恢复能力不受加载循环次数与应力水平的影响,但受温度的影响,其在蠕变阶段主要发生黏性流动变形,且卸载后黏性变形不可恢复;PPA/SBS复合改性沥青的累积应变随温度、应力水平和荷载作用次数的增加显著增大,特别是温度和应力水平对改性沥青累积应变的影响较大;在温度和应力水平相同的条件下,复合改性沥青中PPA 1%+SBS 3%的改性沥青蠕变劲度的黏性部分G_v值最大,具有最好的高温抗变形能力。     

重组竹顺纹受压蠕变性能及预测模型

为研究重组竹顺纹受压蠕变性能,对12个重组竹受压试件进行了蠕变试验,通过分析蠕变增长率和Burgers模型中各参数,得到重组竹受压试件受压蠕变变化规律。结果表明,重组竹在持续受压荷载作用下,应力水平较低时,蠕变变形发展稳定,不会发生承载力破坏;应力水平较大时(达到60%极限载荷水平),将会直接导致重组竹受压屈曲破坏,蠕变变形呈非稳定性增长;重组竹受压蠕变应变增长率随时间增加而逐渐增大,其增长速率则呈减速状态,受压蠕变的应变增长率随应力比增加而增大。基于试验结果,分析了重组竹受压蠕变试验的弹性变形、黏弹性变形和黏性变形的变化规律,对不同荷载水平的Burgers模型参数进行确定,并进一步推导了Burgers模型参数随应力水平的变化规律。基于此,建议考虑荷载水平的重组竹受压蠕变预测模型,模型预测结果与试验曲线一致。      

GH3536合金的蠕变性能及蠕变行为

对GH3536合金在815～900℃不同应力水平下进行了高温蠕变试验,绘制了蠕变曲线并讨论了试验温度和试验应力水平对GH3536合金蠕变性能的影响规律,通过最小二乘法拟合得到了稳态蠕变速率的应力指数和Monkan-Grant关系式的参数,采用L-M参数法建立了外推公式,并对GH3536合金的蠕变断裂强度进行了外推。结果表明:在试验温度和试验应力水平下,GH3536合金的高温蠕变机制主要为位错的滑移和攀移,蠕变过程中有第二相析出,在晶界以及第二相颗粒处形成R型和W型裂纹;蠕变断裂由颈缩引起,为韧性断裂,断裂过程中发生了微孔的聚集和长大;根据外推公式GH3536合金在815℃下蠕变断裂时间2 000h所对应的蠕变断裂强度为58MPa。     

静不定结构的热应力分析

分析了受约束机构内材料热应力存在的危害,给出静不定结构内热应力的定量描述方式:弹性变形状态时,热应力仅与线膨胀系数、弹性模量及温度变量成正比关系;当热应力达到材料在某温度下的屈服强度时,其热膨胀发生弹塑性转变,热应力为当时温度下的屈服强度。采用热模拟试验机测试了材料的温度与热应力关系,分析结果与数值模型基本吻合,测出刚性约束下45钢在升温至620℃时热应力等于其屈服强度约200 MPa。     

Creep behavior of SiC fiber-reinforced hot-pressed Si3N4 composites

The creep response of SiC fiber-reinforced Si₃N₄ composites has been measured using four-point flexural loading at temperatures of 1200–1450°C and stress levels ranging from 250 to 350 MPa. Parameters characterizing the stress and temperature dependence of flexural creep strain rates were determined. A numerical analysis was also performed to estimate the power-law creep parameters for tensile and compressive creep from the bend test data. The incorpoporation of SiC fiber into Si₃N₄ resulted in substantial improvements in creep resistance even at very high stresses. The steady-state creep deformation mechanism, determined to be subcritical crack growth in the unreinforced matrix, changed to a mechanism in the composites of repeated matrix stress relaxation-fiber rupture-load dispersion by the matrix. Multiple fiber fracture rather than multiple matrix cracking resulted. The tertiary creep in the composite resulted from the rapid growth of the microcracks which initiated from the fiber rupture sites. Fiber strength, matrix cracking stress and interfacial shear strength have been identified as the key microstructural parameters controlling the creep behavior of the composite.     

沥青混合料低温临界开裂温度的确定

为了同时考虑沥青混合料在降温过程中温度应力的累积和松弛作用,确定临界开裂温度,对试件进行了线收缩系数试验,并利用间接拉伸试验确定其抗拉强度和蠕变柔量,由蠕变柔量和松弛模量的关系得到松弛模量的Prony系列表达式;由Boltzmann叠加原理,得到温度应力公式,计算出不同降温速率下产生的温度应力,根据低温抗拉强度曲线,确定出沥青混合料的临界开裂温度,并对结果予以验证。结果表明:该方法考虑了应力累积和松弛二者的综合作用,能够较好地反映沥青混合料的低温开裂特性,其计算结果与约束试件温度应力试验结果相近;该方法不仅适用于恒定降温速率,还适用于现场连续变速降温工况;随温度的降低或降温速率的增加沥青混合料内部温度应力累积速度加快,临界开裂温度随降温速率增加而升高。      

Behaviour of S 355JO steel subjected to uniaxial stress at lowered and elevated temperatures and creep

This paper considers main mechanical properties of structural-high strength low alloy (HSLA) S 355JO (ASTM A709 Gr50) steel subjected to uniaxial tensile tests at lowered and elevated temperatures. The engineering stress vs strain diagrams as well as curve’s dependence of ultimate and yield strengths vs both lowered and elevated temperatures are presented. The focus is also on specimen elongations vs temperature at elevated temperatures. Short-time creep tests for selected constant stresses at selected temperatures were curried out. Uniaxial creep behaviour for selected creep test was modeled by the rheological model. The creep curve determined by modeling procedure was compared with experimentally obtained one. Also, notch impact energy test, using Charpy pendulum impact machine was performed and according to the proposed formula, fracture toughness is calculated. All of experimental tests were performed using modern computer directed experimental systems.     

Creep and Aging Evaluation of Phenol–Formaldehyde Carbon Fiber Composites in Overhead Transmission Lines

The use of reinforced polymers as cores of transmission cables can provide significant advantages compared to traditional steel cores, such as high tensile strength, low thermal expansion coefficients, and low sag between towers. This work evaluates the applicability of pultruded rods consisting of phenol–formaldehyde resin reinforced with carbon fiber as cores of transmission cables. In this work, the samples were divided into three groups: samples without aging, and samples UV and thermally aged. At first, a dynamic mechanical analysis was performed on samples without aging in order to determine the viscoelastic properties of the material based on the application to see if it would be compatible. In addition to this test, tensile strength and Young's modulus were determined for the three groups. Since the composite cores are susceptible to creep in high temperatures, the applicability must be below the glass transition temperature. Regarding creep behavior, results showed that at a reference temperature of 100 °C, the stress level necessary to cause failure after 50 years was 89% of the ultimate strength. The results of tensile tests were favorable for application of the pultruded system as transmission cables cores and the accelerated aging affected positively in these composites.

     

Small punch and uniaxial creep fracture behaviours of modified SUS304H steel at various temperatures

Commercial austenitic stainless steel SUS304H with small amount of vanadium addition was used in this study. Small punch (SP) creep and uniaxial tensile creep tests were conducted at 650, 700, and 750 °C to measure creep lives and the minimum displacement rates or the minimum creep strain rates. The measured parameters were compared between the two test methods, seeking empirical relationships among the parameters using Larson-Miller Parameter and Monkman-Grant relation. Magnitude of the applied stress (MPa) in the uniaxial tensile creep test was approximately equal to the applied load value (N) in the SP creep test at all test temperatures. It was shown that during the creep deformation of the SP creep specimen, crack initiation and accompanying crack growth occur simultaneously. Competing failure mechanisms of creep deformation and crack growth may affect the SP creep life and consequently determine the proportionality function, α, in the relation between the SP load and the tensile creep rupture stress in creep tests.     

Short-term tensile creep and shrinkage of ultra-high performance concrete

The tensile creep and free shrinkage deformations of ultra-high performance concrete (UHPC) were examined through short-term testing to assess the influences of stress/strength ratio, steel fiber reinforcement, and thermal treatment. The use of fibers and the application of thermal treatment decreased 14-day drying shrinkage by more than 57% and by 82%, respectively. Increasing the stress-to-strength ratio from 40% to 60% increased the tensile creep coefficient by 44% and the specific creep by 11%, at 14 days of loading. Incorporating short steel fibers at 2% by volume decreased the tensile creep coefficient by 10% and the specific creep by 40%, at 14 days. Also, subjecting UHPC to a 48-h thermal treatment at 90 °C, after initial curing, decreased its tensile creep coefficient by 73% and the specific creep by 77% at 7 days, as compared to ordinarily cured companion mixes. Comparison of tensile creep behavior to published reports on compressive creep in UHPC reveal that these phenomena differ fundamentally and that further evaluation is necessary to better understand the underlying mechanisms of tensile creep in UHPC. Results from this study also showed that the effects of both thermal treatment and fiber reinforcement were more pronounced in tensile creep behavior than tensile strength results of different UHPC mixes. This emphasizes the importance of conducting tensile creep testing to predict long-term tensile performance.     

高强钢TRB高温下热流变特性

为了准确仿真高强钢板热冲压成形过程,获得高强钢高温下的材料本构关系模型,利用Gleeble3500热模拟试验机在不同温度和应变速率下对不同厚度的高强钢B1500HS钢板进行了单向拉伸试验,获得各种工艺条件下的应力-应变曲线,并基于变形抗力数学模型,引入板材厚度参数,通过最小二乘法进行数据拟合获得高强钢TRB高温下的材料本构关系.利用试验结果对本构关系模型进行的拟合验证表明,拟合程度较好,说明建立的材料本构关系能很好地描述高强钢TRB在高温下的应力-应变关系.