建筑结构用高强度钢材力学性能研究进展

随着钢材生产工艺的提高,新型高强度、高性能结构钢材出现;由于目前钢结构设计规范和钢材标准对结构钢材的力学性能指标有着严格的限值要求,特别是屈强比,高强度钢材钢结构的发展和应用受到了制约。全面介绍国内外建筑结构用高强度钢材的生产工艺和牌号种类,总结包括欧洲、北美、澳大利亚、日本和中国在内的国内外规范标准对结构钢材力学性能的要求,并对几个重要的力学性能指标,如屈强比、断后伸长率等进行对比分析,整理大量国内外高强度钢材材性试验数据和研究进展,分析并得到其力学性能特点,提出适用于数值计算的材料本构模型。研究结果表明,新型高强度结构钢材在具有高韧性的同时也具有较高的屈强比和较低的断后伸长率,且往往超过大多数规范的限值规定,这大大限制了此类钢材钢结构的工程应用。因此,需要进一步讨论和定量研究钢材力学性能与结构安全性的关系,研究是否可以适当放松规范对结构钢材力学性能的限值规定。该研究有利于更合理全面地认识高强度结构钢材的力学性能特点和相应的规范限值规定,促进高强度钢材钢结构的相关研究工作,也有利于高强度钢材钢结构的工程应用和规范修订。     

PVDF膜材在自然老化和人工加速老化下力学性能变化的相关性研究

对6组自然老化下的PVDF膜材和1组人工加速老化试验下的Ferrari 1202T膜材的主要力学性能数据进行了汇总,包括单轴抗拉强度、断裂延伸率、撕裂强度以及单轴和双轴杨氏模量,在此基础上对经历了自然老化的第6组膜材补充了人工加速老化试验并测得其抗拉强度.通过对比第1,2组自然老化膜材的试验结果,研究了环境因素对PVDF膜材老化性能的影响;研究和对比了PVDF膜材在自然老化与人工加速老化试验下力学性能的变化规律,基于性能保持率的等效时间为表征指标,验证了互易定律的失效性.采用两种经典拟合方法估算PVDF膜材的使用寿命,结果表明PVDF膜材的寿命以等效时间表示为1860h,换算成自然老化时间后,第1,2,3,4,6组膜材的寿命分别为18年、20年、17年、19年和19年,最终为膜结构的理论分析、设计计算和寿命预测模型的建立积累了重要数据.     

Experimental investigation of post-fire mechanical properties of cold-formed steels

Cold-formed steel members are widely used in residential, industrial and commercial buildings as primary load-bearing elements. During fire events, they will be exposed to elevated temperatures. If the general appearance of the structure is satisfactory after a fire event then the question that has to be answered is how the load bearing capacity of cold-formed steel members in these buildings has been affected. Hence after such fire events there is a need to evaluate the residual strength of these members. However, the post-fire behaviour of cold-formed steel members has not been investigated in the past. This means conservative decisions are likely to be made in relation to fire exposed cold-formed steel buildings. Therefore an experimental study was undertaken to investigate the post-fire mechanical properties of cold-formed steels. Tensile coupons taken from cold-formed steel sheets of three different steel grades and thicknesses were exposed to different elevated temperatures up to 800 °C, and were then allowed to cool down to ambient temperature before they were tested to failure. Tensile coupon tests were conducted to obtain their post-fire stress–strain curves and associated mechanical properties (yield stress, Young׳s modulus, ultimate strength and ductility). It was found that the post-fire mechanical properties of cold-formed steels are reduced below the original ambient temperature mechanical properties if they had been exposed to temperatures exceeding 300 °C. Hence a new set of equations is proposed to predict the post-fire mechanical properties of cold-formed steels. Such post-fire mechanical property assessments allow structural and fire engineers to make an accurate prediction of the safety of fire exposed cold-formed steel buildings. This paper presents the details of this experimental study and the results of post-fire mechanical properties of cold-formed steels. It also includes the results of a post-fire evaluation of cold-formed steel walls.     

钢板桩围堰力学性能验算

以某铁路特大桥梁施工过程中采用的钢板桩围堰为例,对钢板桩围堰的力学性能进行了验算。验算的内容包括:钢板桩的验算、围囹的验算、支撑的验算、支撑稳定性的验算等,验算的结果表明,钢板桩围堰的力学性能满足要求,研究成果对相同类型钢板桩围堰的设计及验算具有一定的参考作用。     

A comparison of the mechanical properties of fire-resistant and S275 structural steels

The mechanical properties of two fire-resistant (FR) structural steels and an S275 steel have been investigated from room temperature up to 700°C, with the aim of assessing the viability of FR steels. The major focus has been on the tensile and creep properties of the steels at high temperature and the effect of these properties on the fire resistance of model steel beams. It has been found that the FR steels performed significantly better than the S275 at high temperature, retaining 50% of their room temperature strength at a temperature of approximately 650°C. The S275 steel retained 50% strength to a temperature of 550°C, this being consistent with literature values. In creep tests carried out at 600°C, the FR steels also exhibited lower maximum strains and strain rates than the S275. The improved strength reduction factors led to a significant increase in fire resistance for a model steel beam with various floor slab configurations, showing that FR steels can become a practical alternative to conventional structural steels.     

Water absorption of wood polypropylene composite sheet piles and its influence on mechanical properties

The water absorption behavior and durability of extruded wood polypropylene composite (WPC) material used in Z-section sheet piles was investigated. An experimental technique to determine percentage water absorption and its effect on mechanical properties was implemented. Water absorption tests were carried out on specimens cut from flanges and webs of the Z-section by immersion in tap water at 21, 45 and 70 °C. Freeze–thaw resistance of saturated composites was studied by cycling temperatures in the range of 21 to ?29 °C. The absorption behavior followed the kinetics of a Fickian diffusion process. This behavior was modeled using an analytical solution based on Fick’s second law of diffusion. Quasi-static bending tests were conducted periodically on specimens immersed in water to assess degradation in flexural properties. A significant decrease in mechanical properties of the water-saturated specimens compared to the dry control was observed. Degradation of flexural modulus was found to be a strong function of water absorption with initial decrement as percentage water absorption increased and finally stabilized as the absorption process approached saturation. However, freeze–thaw cycling did not have any significant effect on mechanical properties of saturated composites. A phenomenological model for moisture intrusion and its effect on mechanical properties was implemented and applied to explain the experimental findings.     

Experimental study of the mechanical properties of light gauge cold-formed steels at elevated temperatures

In recent times, light gauge cold-formed steel sections have been used extensively as primary load-bearing structural members in many applications in the building industry. Fire safety design of structures using such sections has, therefore, become more important. Deterioration of mechanical properties of yield stress and elasticity modulus is considered the most important factor affecting the performance of steel structures in fires. Hence, there is a need to fully understand the mechanical properties of light gauge cold-formed steels at elevated temperatures. A research project based on experimental studies was, therefore, undertaken to investigate the deterioration of mechanical properties of light gauge cold-formed steels. Tensile coupon tests were undertaken to determine the mechanical properties of these steels made of both low- and high-strength steels and thicknesses of 0.60, 0.80 and 0.95 mm at temperatures ranging from 20 to 800 °C. Test results showed that the currently available reduction factors are unsafe to use in the fire safety design of cold-formed steel structures. Therefore, new predictive equations were developed for the mechanical properties of yield strength and elasticity modulus at elevated temperatures. This paper presents the details of the experimental study, and the results including the developed equations. It also includes details of a stress–strain model for light gauge cold-formed steels at elevated temperatures.     

The effect of alkali reactivity on the mechanical properties of concrete

The effects of highly reactive aggregate, fused silica, and slowly reactive aggregate, Thames Valley sand, on the mechanical properties of concrete were investigated in this paper. The mechanical properties studied over a period of 12 months were the compressive strength, direct tensile strength, tensile splitting strength, flexural tensile strength (modulus of rupture), static modulus of elasticity and water absorption. The effects of both reactive aggregates on the mechanical properties of concrete were compared with that of sound concrete mix. The results presented in this investigation show that the effect of the reactive aggregates on the mechanical properties varies, depending on the type of reactive aggregate. From this study it appears that the direct tensile and static modulus of elasticity were the best indicators of reactivity.     

卸荷速率和孔隙水压力对砂岩卸荷特性影响研究

为了研究卸荷速率和孔隙水压力对砂岩卸荷力学特性的影响,设计进行了不同卸荷速率(0.005,0.02,0.05,0.1 MPa/s)和不同孔隙水压力(0,0.3,0.6,0.9,1.2 MPa)下的三轴卸荷试验。研究结果表明:(1)在加载阶段,随着孔隙水压力的增大,岩样的应力–应变曲线斜率逐渐降低;(2)在围压卸载阶段,卸荷速率越大,卸载阶段的应变围压柔量越小,岩样破坏时的围压越小,岩样强度相对较高,但破碎程度更严重,而且,在相同的卸荷速率情况下,孔隙水压力越大,岩样侧向扩容现象越明显,岩样越容易破坏;(3)在围压卸载阶段,岩样的变形模量出现了先缓后陡的劣化趋势,而且,卸荷速率越小、孔隙水压力越大,变形模量劣化幅度越大;(4)卸载过程中,卸荷速率越大,岩样脆性破坏特征越明显;孔隙水压力越大,岩样破坏时的近轴向的张性裂纹越多和追踪次生裂纹越多,孔隙水压力在岩样内部裂纹、裂隙尖端的应力集中是导致岩石变形破坏的主要原因。     

氧化石墨烯对粉煤灰基地质聚合物力学性能的影响

通过改进Hummers法制备了一系列不同尺寸的氧化石墨烯（GO）,研究了GO对粉煤灰地质聚合物流变性能、力学性能和微观结构的影响,结果表明当粉煤灰地聚物净浆流动度随GO的尺寸和掺量增大而降低;此外GO的掺入,会阻碍粉煤灰地聚物早期强度发展,但是会提高后期强度的发展速度。通过扫描电镜观察粉煤灰地质聚合物微观结构,结果表明GO的掺入会导致粉煤灰地质聚物中出现大量的针棒状结晶体。     

碳纤维混凝土(粗骨料)中纤维含量对其力学、压敏性能的影响

目前碳纤维水泥砂浆的研究很多，而对于碳纤维混凝土的研究较为少见。作为结构材料必须加入粗骨料。粗骨料的引入使碳纤维混凝土力学和压敏性能较碳纤维水泥砂浆有较大的不同。本文介绍了碳纤维混凝土的试验室配制工艺及其电阻测量方法，并在此基础上研究了碳纤维混凝土的力学、压敏特性。通过综合比较相同条件下不同纤维掺量的混凝土其力学和压敏性能，确定了较为合理的碳纤维掺量。     

Flowability of fibre-reinforced concrete and its effect on the mechanical properties of the material

Fibre distribution is an important factor, affecting both the fresh- and hardened-state properties of concrete. The paper describes a study in which the fibre distribution and orientation were investigated by means of a suitable technique, by using a translucent fluid model with a yield stress (Carbopol 940). The observation confirmed the ability of the developed method to provide data on the orientation and distribution of steel fibres within concrete. It was showed that orientation and distribution are dependent on the yield stress of the fluid material. Concrete specimens, containing 0.5% of 35 mm long hooked steel fibres, were also cast and tested under four-point loading, in order to study the correlation between rheology, fibre distribution and the mechanical properties of steel fiber-reinforced concrete. The results confirm that the rheology of concrete has an important influence on the orientation of the fibres. The flexural strength depends on the fibre distribution and orientation and is significantly improved when the fibres are oriented in the direction of the tensile stresses fresh (concrete with good workability). On the contrary, for concrete with poor workability, an inadequate orientation of fibres occurred, leading to a poor contribution of the fibres to the flexural behaviour of the tested specimens, despite the relatively higher compression strength of the tested concrete material.     

加载速率对不同温度状态石灰岩力学性能影响的试验研究

借助美国MTS810电液伺服材料试验机和高温炉,对常温和600℃两种温度状态下石灰岩试件进行不同加载速率下的单轴压缩试验,得到石灰岩力学性能随加载速率的变化规律。结果表明:常温时,石灰岩岩样在3×10-4³×10-3 mm/s的低应变率范围内,加载速率对峰值应力和弹性模量影响不大,在加载速率为3×10-33×10-3 mm/s的低应变率范围内,加载速率对峰值应力和弹性模量影响不大,在加载速率为3×10-3³×10-1 mm/s的区段内,峰值应力和弹性模量均呈明显上升趋势;600℃时,峰值应力和弹性模量随加载速率增加变化不大。常温时,不同加载速率下石灰岩岩样均为竖向劈裂破坏,且在3×10-33×10-1 mm/s的区段内,峰值应力和弹性模量均呈明显上升趋势;600℃时,峰值应力和弹性模量随加载速率增加变化不大。常温时,不同加载速率下石灰岩岩样均为竖向劈裂破坏,且在3×10-3³×10-1 mm/s的加载速率区段中,随加载速率的增加,劈裂面逐渐增多;600℃时,石灰岩岩样在不同加载速率下均为剪切破坏。     

复合保温砌块力学性能与节能效果试验研究

以城市废泥浆、河道淤泥、建筑废料为主要原料制成空心砌块,并在砌块的孔洞中充填保温材料,降低导热系数和传热系数,制成烧结复合自保温砌块,该砌块具有更好的保温节能效果.通过对砌块的物理力学性能和保温性能的研究和对比,为复合保温砌块的生产和应用提供参考依据.     

The effect of pectin on the physicochemical and mechanical properties of cement containing boron

The effects of varying concentrations of pectin (0.1–1 wt% of cement) on the properties of ordinary portland cement (PC) and cement containing colemanite ore waste (CW) were investigated. Variation in setting time, compressive strength, and volume expansion of hydrated specimens were determined. It was found that pectin acts as an accelerator for PC cement and as a retarder for CW cement at a level below 0.1%. The inclusion of pectin in PC resulted in a reduction in the early strength of the mortar. However, pectin had positive contribution to the early compressive strength of the CW mortar.     

Assessment of relationships between drilling rate index and mechanical properties of rocks

This paper examines the relationships between drilling rate index (DRI) and some mechanical properties of rocks in order to evaluate the effect of strength and indexes of rock on rock drillability. For this purpose, some index properties (in situ Schmidt rebound hardness (SRH), Shore scleroscope hardness (SSH), and point load strength (PLS)) and strength properties (uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS)) values of 32 sedimentary, igneous and metamorphic rock samples were determined. The relationships between DRI and both strength and indexes properties were evaluated using regression analysis and statistical methods. As a result, decreasing linear relationships were found between DRI and uniaxial compressive strength, Schmidt rebound hardness, Shore scleroscope hardness, diametral and axial point load strength.     

Combination form analysis and experimental study of mechanical properties on steel sheet glass fiber reinforced polymer composite bar

The concept of steel sheet glass fiber reinforced polymer (GFRP) composite bar (SSGCB) was put forward. An optimization plan was proposed in the combined form of SSGCB. The composite principle, material selection, and SSGCB preparation technology have been described in detail. Three-dimensional finite element analysis was adopted to perform the combination form optimization of different steel core structures and different steel core contents based on the mechanical properties. Mechanical tests such as uniaxial tensile, shear, and compressive tests were carried out on SSGCB. Parametric analysis was conducted to investigate the influence of steel content on the mechanical properties of SSGCB. The results revealed that the elastic modulus of SSGCB had improvements and increased with the rise of steel content. Shear strength was also increased with the addition of steel content. Furthermore, the yield state of SSGCB was similar to the steel bar, both of which indicated a multi-stage yield phenomenon. The compressive strength of SSGCB was lower than that of GFRP bars and increased with the increase of the steel core content. Stress-strain curves of SSGCB demonstrated that the nonlinear-stage characteristics of SSGCB-8 were much more obvious than other bars.     

Effect of welding on local mechanical properties of stainless steels for concrete structures using universal hardness tests

This study analyses the effect of welding on mechanical properties of ribbed bars of two cold-deformed austenitic stainless steels (new, low-nickel AISI 204Cu and traditional 304L types) and two duplex stainless steels (SAF 2205 type, cold and hot-worked). Welds have been carried out using shielded metal arc welding. The effect of welding on local mechanical properties has been studied using a universal hardness equipment, measuring universal and Vickers hardness, elastic and plastic energies and Young modulus. Measured values are correlated with the microstructure of base materials and heat-affected zone. Results show that heat during welding promotes recrystallisation of the microstructure, effect that depends on the degree of deformation. These phenomena can be easily studied with universal hardness. Results suggest that hot-worked 2205 duplex stainless steel can be welded in construction without meaningful modification of its properties. Cold-deformed grades will comply ductility requirements of Eurocode 2 after welding, although reductions of yield strength can be found in 2205 and 304.