在循环高温荷载作用下陶瓷管道的应力与变形

分析了烟囱与陶瓷管道特殊构件受明火燃烧及循环高温荷载条件下的应力与变形.参照德国工业标准DIN 18160中的温度-时间曲线,在奥地利的燃烧中心进行了70余根陶瓷耐火管道的高温明火燃烧实验.测试了管道的温度-时间曲线,管道在高温荷载下的变形和陶瓷材料高温后的强度与弹性模量,并初步分析计算了陶瓷管道在高温下的变形和温度应力.研究表明:整套明火高温实验方案具有可行性,适合研究管道在高温荷载下的受热变形性能,管道径向变形的计算结果与实验结果相吻合.     

Thermal stress–strain of self‐compacting concrete in compression

Self‐compacting concrete or self‐consolidation concrete (SCC) is being used in underground and other industrial structures that may be subjected to high temperatures during operation or in case of an accidental fire. The proper understanding of the effects of elevated temperatures on the stress–strain relationship of SCC is necessary in the assessment of structural safety. This paper presents the high temperature behavior from an experimental study carried out on SCC subjected to high temperatures. The effects of temperature, strength grade, and polypropylene (PP) fibers on the initial elastic modulus, strain at peak stress, and stress–strain curves of SCC are studied, which offered a test basis for estimating the deformation of SCC under high temperature. An empirical constitutive formula for the thermal stress–strain of SCC is developed on the basis of the deformation characteristics of PP fiber‐modified SCC. Copyright © 2012 John Wiley & Sons, Ltd.     

Postfire mechanical properties of Galfan-coated steel cables

Galfan-coated steel cables are widely used in prestressed structures due to their excellent mechanical properties and corrosion resistance. Their postfire mechanical properties are important to evaluate the residual load-bearing capability of the structures after fire. However, the research on the postfire mechanical properties of Galfan-coated steel cables is stillscarce, especially the cable in stress state. Hence, a research based on experimental study was carried out to investigate the deterioration of mechanical properties of Galfan-coated steel cables in stress state after experiencing elevated temperatures in this paper. Eighteen tensile tests were conducted on specimens exposed to elevated temperatures varying from 100°C to 500°C and then cooled down to ambient temperature in air. Both twisting characteristic and constant stress level of Galfan-coated steel cables were considered in this study. The residual nominal yield strength, ultimate strength, elastic modulus, fracture strain, and stress-strain curves of Galfan-coated steel cables after experiencing elevated temperatures were obtained and compared with the existing researches. The results show that the postfire mechanical properties are obviously decreased when the fire temperature exceeds 300°C. Equations for the residual nominal yield strength, elastic modulus, ultimate strength, and fracture strain of the Galfan-coated steel cables were proposed. Furthermore, a modified two-stage Ramberg-Osgood model for Galfan-coated steel cables after experiencing elevated temperatures was established, which can provide reference for the safety assessment and repairment of prestressed structures after fire.     

热载荷对冷拔20/316L复合管结合强度影响的数值模拟及试验研究

为了探讨热载荷对冷拔20/316L双金属复合管内、外管间结合强度即残余接触压力的影响,运用有限元分析软件ABAQUS6.5模拟研究了复合管的拉拔成型过程,并通过重启动分析模拟了热加载、卸载时复合管应力状态及残余接触压力的变化,分析了不同温度对冷拔20/316L复合管残余接触压力的影响。结果表明,300℃是衬管发生弹性与塑性变形的临界温度,350℃是基管与衬管发生脱离的起始温度。最后通过复合管拉拔试样的热载荷试验,验证了数值模拟的正确性。     

铝塑复合管受力状态讨论及优化结构分析

对液体压力作用下的铝塑复合管的受力状态在弹性变形范围内进行了力学分析,得到了在真实应力状态下的管材各承力层的应力分布和管材的优化结构。     

压扁阻断下聚乙烯管道力学响应的实验研究

采用实验方法系统研究了压扁阻断工艺参数,包括压管速度、松管速度、压缩率、环境温度和管径对聚乙烯（PE）管道力学响应的影响。结果表明,压管过程中PE管道载荷随着压管速度的增大、压缩率的增大、环境温度的降低或管径的增大而增大;卸载过程中PE管道的残余变形随松管速度的增大而增大;维持阶段PE管道的载荷衰减随着压管速度的增大、压缩率的降低、管径的减小或环境温度的降低而增大。     

Effects of stress during heating on strength and stiffness of concrete at elevated temperature

Concrete in structures exposed to high temperatures is practically always heated under stress. Yet, there are few experimental studies in which the concrete was heated under stress and then loaded to the peak, and most of these were performed under uniaxial compression. This paper reports on an experimental study of the effects of different heat–load regimes on the stress–strain behaviour of partially sealed concrete under multiaxial compression, at elevated temperature. The specimens were first heated (stressed/unstressed), then loaded to the peak in multiaxial compression. In contrast with previous experimental research, the results show that concrete heated under relatively low compressive stress has lower strength and stiffness than concrete heated without load. The results suggest that the presence of stress during first heating produces a specific damage, which could be the cause for a major component of the load induced thermal strain (LITS) in concrete.     

支架沉降对大口径煤气管道结构安全性的影响

针对大口径煤气管道由于支架地基严重沉降而引起的自内力和强度安全问题,进行有限元模拟、强度评定和沉降规律研究。计算实际支架相对沉降导致的管道变形和应力分布,基于塑性失效等安全裕度准则,对应力进行分类强度评定。通过试探加载法,估算出中间支架单个鞍座的下拉载荷约等于管道自重的三倍。绘出管道底部典型位置的最大当量应力随中间支架相对沉降量的变化曲线,结果显示各处当量应力对支架沉降的响应基本呈线性规律,表明支架沉降引起的管道自内力相当于比例加载。同时发现,应力越高的位置对支架沉降越敏感,即随相对沉降量的增加会变得更加危险(?)据此提出垫高鞍座和局部补强的改造措施并获得良好效果。     

热塑性交联聚乙烯（TPEXa）管材的制备及性能研究

选取耐热聚乙烯（PERT）进行微交联改性制备出一种热塑性交联聚乙烯（TPEXa）管材,使其在保留PERT管热塑性的基础上提高耐热性和抗蠕变性,研究了交联剂含量对TPEXa材料熔体质量流动速率和交联度的影响,并对TPEXa管材的静液压性能、热熔回收性能及回收料的力学性能进行了测试。结果表明,当交联剂的含量为0.08 %时,TPEXa材料的熔体质量流动速率为5.6 g/10 min（190 ℃,21.6 kg）,交联度为2.3 %,负荷变形温度可以达到78 ℃（Tf0.45）,耐温性能较好。制备的管材可以通过95 ℃下22 h、165 h和1 000 h静液压测试,测试所用环应力超过PERT II管接近PEX管材环应力的规定,具有较好的耐温性能和抗蠕变性能,并且可以进行热熔回收,回收料具有较好的力学性能。     

双排玻璃钢夹砂管涵洞室内试验变形特征分析

大孔径玻璃钢夹砂管经常由于管材生产、运输以及公路涵洞标高等问题,在公路涵洞中的应用受到限制。为探讨用小孔径双排管代替大孔径玻璃钢夹砂管的可行性,通过室内模拟试验,分析双排管在单个管涵受荷载作用条件下各自变形的物理力学特征和变化规律。试验结果表明,在单个管涵受荷载作用条件下,双排管的变形之间存在相互作用关系,管涵变形具有不均衡性,且双排管的受力薄弱区域出现在加载管涵的管顶及两个管涵相邻位置,其结果可以作为双排管安全性的控制点和实际工程的设计依据。     

Fire structural modelling and testing of welded aluminium plate

The structural response of welded aluminium in fire is computationally and experimentally analysed. A finite element (FE) model is developed to compute the deformation and failure of gas metal arc welded (GMAW) aluminium plate under combined loading and one‐sided unsteady‐state heating representative of fire. The FE model predicts the deformation of the weld, heat‐affected zone and parent plate based on the combined effects of elastic softening, plastic softening and creep. The effects of residual stresses in the weld and thermal expansion on the deformation response are also analysed. The numerical accuracy of the model is rigorously evaluated using a large amount of deformation and failure stress data obtained from fire structural tests performed with welded AA5083–AA5083, AA5083–AA6061 and AA6061–AA6061 plates. Good agreement is found between results computed with the FE model and experimental testing. The results reveal that GMAW welds do not reduce the structural performance of aluminium in fire unless the maximum temperature remains below the recrystallisation temperature. Copyright © 2014 John Wiley & Sons, Ltd.     

湿度对水化硅酸钙力学性能影响的分子动力学模拟

水化硅酸钙(C-S-H)是水泥水化产物中最重要的组成成分,是水泥基材料的主要胶凝相。C-S-H层间水对其纳米结构和力学性能会产生显著影响。利用分子动力学研究了不同湿度C-S-H在结构和力学性能方面的差异。通过原子径向分布函数和浓度分布、弹性常数以及应力应变关系分析了湿度对C-S-H结构和弹性性质以及拉伸、压缩、剪切力学性能和变形性能的影响。结果表明:湿度增加会导致C-S-H中Si、Ca原子近程范围内的O原子集聚增多,还会导致C-S-H层间距离增大,分层更加明显,同时会降低C-S-H的弹性性质;湿度的增加会降低C-S-H拉伸、压缩、剪切力学性能和变形性能;湿度对抗拉与抗剪强度影响较大,对抗压强度影响较小,对拉伸时的变形性能影响最大,对压缩时的变形性能影响最小。     

The effect of a polypropylene skin on the structure and properties of PE/PP dual layer pressure pipe

A new and growing family of polyethylene (PE)‐based pressure pipes have a polypropylene (PP) skin. The effect of the PP skin on the structure and properties of the core PE pipe was investigated by comparing the skinned pipe with an uncoated pipe made from the same PE material and with the same dimensions. The annealing effect introduced by the skin changed the PE core pipe density profile across the wall thickness, increasing density in the PE core pipe near to its outer surface. The density at the bore of the coated and the uncoated pipe was similar. The melting temperature and enthalpy of melting data from DSC agreed with the density profile results. The melting temperature of PE core pipe material close to the PP skin increased with increasing skin thickness. Residual stress assessment indicated that, as the PP skin thickness increased, the PE core pipe had a lower level of overall residual stress in the hoop direction. Long‐term hydrostatic strength (LTHS) tests were carried out and showed a higher strength for the coated pipe than the uncoated one. The observed structural changes have been used to explain the relative strength of these two PE pipes. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Effect of temperature on mechanical properties of ultra-high performance concrete

High temperature mechanical property data are needed for evaluating fire resistance of structural members. Being a relatively new construction material, there is a lack of temperature-dependent mechanical property data on ultra-high performance concrete (UHPC). To address this knowledge gap, this paper presents results from an experimental study on the effect of temperature on mechanical properties of UHPC. Specimens made of two UHPC mixes: one with only steel fibers (UHPC-S) and the other with hybrid fibers, that is, both steel and polypropylene (UHPC-H), were tested under different heating conditions in 20 to 750°C temperature range. Compressive strength, tensile strength, stress-strain response, and elastic modulus of UHPC were evaluated at various temperatures. Results generated from these property tests on UHPC were compared with property relations specified in design codes for conventional normal strength concrete (NSC) and high strength concrete (HSC). The comparisons show that UHPC experiences faster degradation in compressive strength and elastic modulus as compared to conventional concrete. However, UHPC exhibits slower degradation in tensile strength and ductility at elevated temperatures due to the presence of steel fibers. Data generated from these property tests were utilized to propose relations for expressing the mechanical properties of UHPC as a function of temperature and these relations can be used as input to numerical models for evaluating fire resistance of structures made of UHPC.     

Fracture prediction in tough polyethylene pipes using measured craze strength

In this study, an empirical model is developed that predicts the time to failure for PE pipes under combined pressure and deflection loads. The time‐dependent craze strength of different PE materials is measured using the circumferentially deep‐notched tensile (CDNT) test. In agreement with previous research, results indicate that bimodal materials with comonomer side‐chain densities biased toward high‐molecular‐weight PE molecules exhibit significantly higher long‐term craze strengths. A comparison of currently available PE materials with CDNT samples taken from a PE pipe that failed by slow crack growth in service clearly indicates the superior performance of new‐generation materials. Using measured craze strength data from the CDNT test, a simplified model for predicting failure in buried PE pipes is developed. Extending previous research, the reference stress concept is used to calculate an equivalent craze stress for a pipe subjected to combined internal pressure and deflection loads. Good agreement is obtained between the model predictions and observed failure times in an experimental test‐bed study of pipes under in‐service loading conditions. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Effect of annular expansion pipe extruder head on mechanical properties of pipes

With polymer pipes being used more commonly, performance requirements are increasing. Studies on the enhancement of mechanical properties of polymer pipes are particularly important. In this study, a self-designed annular expansion pipe extruder head was used to enhance the mechanical properties of HDPE pipes. Different morphologies of the HDPE pipes were produced under different processing conditions. When the extrusion angle was 30° (P30), the best mechanical properties were obtained. The hoop tensile strength and axial tensile strength were 14.5% and 41.0% higher, respectively, compared with the specimen without expansion (P0). This improvement of mechanical properties can be attributed to several reasons. First, the processing parameters of P30 reached the threshold shear rate and strain for shish-kebab formation, as shown by scanning electron microscopy. Second, P30 has the highest orientation parameter and crystallinity of 0.679 and 67.27%, respectively, from 2D wide-angle diffraction (WAXD). Polarized FTIR shows the same trend as 2D-WAXD. Third, the outer bamboo-like self-reinforced structure is formed inside the pipe at 30° expansion angle while the core layer has a well-formed crystal structure; the special structure improves the overall performance of HDPE pipe. This method can be utilized in large-scale industrial production.     

高温后混凝土气体渗透性及力学性能研究

火灾高温不仅会降低混凝土结构的力学性能,还会对混凝土的耐久性产生重大的影响.通过高温电阻炉模拟火灾试验,研究高温下混凝土的气体渗透性、弹性模量和轴心抗压强度的变化特征,以及气体渗透性和力学性能的相互关系.试验结果表明:混凝土气体渗透性随火灾温度的升高而逐渐增大,且在500℃时气体渗透性增长72.6倍;在200 ℃高温下,混凝土的弹性模量和轴心抗压强度略有提高,而在350 ℃及以上高温下,其力学性能大幅降低.     

Mechanical behaviour of adhesively repaired pipes subject to internal pressure

Pipes can crack over time, particularly in areas with pipefittings and joints subject to high pressure and unsteady temperatures. Repair of these cracks requires labour, time, and expense and the cracked pipes are currently repaired with two methods. The first method is cutting out the damaged section of the pipe and adding an additional joint, which requires much time and labour. The second method is replacing the damaged pipe, which requires expensive materials. The aim of this study is to propose an alternative method that reduces or eliminates the use of labour, time, and materials, in order to quickly re-activate pipelines. For this purpose, the cracked steel pipes were repaired by using an adhesive, and the mechanical behaviours of the repaired pipes were investigated experimentally and numerically. In the first step of the study, artificial cracks were created on the pipes and the cracked pipes were repaired using adhesive and galvanized steel patches with different overlap lengths, overlap angles and thicknesses. Then, the repaired pipes were subjected to internal pressure in order to evaluate the effects of patch thickness, overlap angle and overlap length on the joint strength. Finally, the numerical analyses and experimental results show that the variation of the patch thickness, overlap length and overlap angle will change the stress distributions and strength of the adhesively repaired pipes.     

Strain rate and curing condition effects on the stress–strain behaviour of epoxy adhesive materials

Adhesive materials evolve properties that change significantly with the preparation procedures and curing conditions. In this study the effects of curing conditions (curing time and temperature), and strain rate on the stress–strain behaviour of the commercially available Lapox epoxy adhesive materials have been evaluated experimentally. The rectangular test specimens have been prepared with different curing temperatures and times. After preparation, the specimens have been tested in small scale tensile testing machine to investigate the stress–strain behaviour at room temperature. It has been observed that as the curing time or curing temperature is increased, the ultimate tensile strength and the elastic modulus of the material also increase. A four parameter hyperbolic tangent model has been fitted to the experimental data and the model constants have been evaluated for different curing conditions and strain rates. Furthermore, for a fixed curing time and strain rate, empirical equations have been developed for modelling the dependence of curing temperature on the stress–strain curves. Finally, the developed equations have been implemented into the finite element analysis of a lap joint to investigate the stress and strain distributions of the adhesive layer for different curing conditions (curing time and temperature).     

Effect of temperature and cooling regime on mechanical properties of high‐strength low‐alloy steel

This paper presents results from experimental studies on the effect of temperature on mechanical properties of high‐strength low‐alloy ASTM A572 steel commonly used in structural members in bridges. A set of high‐temperature tensile strength tests and post‐temperature exposed residual strength tests is carried out on ASTM A572 steel coupons in 20–1000 °C temperature range. The residual strength tests on high‐temperature exposed steel coupons are carried out after subjecting the coupons to two methods of cooling, namely, air cooling and water quenching. Results from these tests indicate that temperature‐dependent strength and stiffness degradation in A572 steel follow the same trend as that of carbon steel but with some variations. A572 steel recovers almost 100% of its room temperature yield strength when heated to temperature up to 600 °C, regardless of the method of cooling, while the extent of strength degradation in coupons subjected to heating beyond 600 °C is dependent on heated temperature and method of cooling. Data generated in these tests are utilized to generate high‐temperature stress–strain and residual stress–strain response of A572 steel. These results are also utilized to propose temperature‐dependent strength, elastic modules, and residual strength reduction factors of A572 steel, which can be used in evaluating residual response of fire‐exposed steel structures. Copyright © 2015 John Wiley & Sons, Ltd.