Flow stress of an aluminium alloy in the warm working range

The effect of temperature and processing history on the flow stress of a two-phase aluminium (Al-6% Cu) alloy was investigated. The flow stress was determined from the changes in dimensions of flat, ring-shaped specimens using Avitzur’s upper bound theorem. The results indicate that the flow stress increases with the degree of deformation irrespective of the specimen history considered and this tendency is in general more pronounced at large deformations. The flow stress of this alloy at large deformation is considerably lower at 573 K than at the lower temperatures tested. The morphology and distribution of the second phase particles of this alloy changed considerably depending upon the processing history, temperature and deformation.     

High temperature cross-weld characterisation of steel weldments by microtensile testing

Abstract

Study of local material properties and damage mechanisms are undertaken in order to characterise weldments that show significant variation of properties across weldments. One of the methods to characterise the local variation of properties is microtensile (MT) testing of specimens machined out of specific narrow zones of weldments. The literature data, though limited, on microtensile specimen testing are reported at their low temperature behaviour. On the other hand, systematic study of crossweld local material properties at high service temperatures have not yet been reported. In the present study, MT tests are conducted across similar welds of P22 and P91 steels at 550 and 600°C, respectively. In order to study deformation mechanisms and the role of surface condition on properties, specimens with different surface conditions (i.e. machined, polished and electropolished surfaces) are tested. Two different loading rates of 0.2mm/min and 0.5mm/min are used to study the effect of loading rate on deformation and mechanical properties. Variations of material properties yield strength (R_p0.2) and ultimate tensile strength (R_m), for the weldments are presented as a function of surface conditions of specimens and loading speeds. Higher loading rates yield higher values of R_p0.2 and R_m, and specimens with machined and polished surfaces show consistent and higher values of R_p0.2 and R_m compared to specimens with an electropolished surface finish. Deformation behaviour is studied on the side surfaces of tested microtensile specimens using an SEM. Deformation is correlated to microstructural constituent that is observed on specimen side surfaces. The metallographic information is used to interpret the variation of mechanical properties determined in tension at high temperatures. The MT data are compared with standard tensile data obtained on specimens with simulated microstructures. The prospects of using MT tests for characterising the material at high temperatures and feasibility of use of data for assessment of components under service loading conditions are reported.     

一种改进的活塞温度存储测试系统

介绍一种改进的活塞温度存储测试系统,通过增加实时时钟和数据压缩算法,很好地解决传统活塞存储测试实时性不足及测试时间受限的问题.测试系统硬件电路以ATmega168微处理器为核心,以K型热电偶为传感器,包含温度开关、实时时钟模块、多通道温度测试模块、信号放大及调理模块、冷端补偿模块、存储器模块等.系统上位机是以LabVIEW为开发工具设计的软件,用于对数据进行分析、处理和显示.实验结果表明:系统的相关改进准确有效,且能满足对活塞长时间测温的要求,特别是数据压缩算法大大减少存储器的数量,缩小电路体积,为小型内燃机的活塞温度测试提供便利.     

An alternative determination of unmodified temperature T1 for eliminating thermal strain during load testing of engineering structures

A method of determining the unmodified temperature, T₁, for elimination of thermal strain from the load testing is presented. It is based on the fact that the intersection point of two T–ε lines for a test point shows zero stress (or strain) and coincides with the known zero stress at the neutral axis of the structure, if the test point is set at the axis. The abscissa of this intersection point indicates the unmodified temperature T₁. The method has been applied to the load test on a suspect beam in a 4-storey building. The conclusion was reached that although the suspect beam was weaker than normal, it could take the assigned load without any remedial measures. This conclusion was verified by the practical evidence of normal service for more than five years.
The method is applicable to certain types of structures at the moment.     

玩具检测用温升测试系统的设计

该文针对电玩具的温升测试,详细介绍8通道温度测量系统的硬件和软件设计。热电偶信号经AD595芯片放大调理和A/D转换,通过单片机进行处理,再连接到PC机进行显示和操作。     

Finite element analysis of small-scale hot compression testing

This paper models hot compression testing using a dilatometer in loading mode.These small-scale tests provide a high throughput at low cost,but are susceptible to inhomogeneity due to friction and temper-ature gradients.A novel method is presented for correcting the true stress-strain constitutive response over the full range of temperatures,strain-rates and strain.The nominal response from the tests is used to predict the offset in the stress-strain curves due to inhomogeneity,and this stress offset Δσ is applied piecewise to the data,correcting the constitutive response in one iteration.A key new feature is the smoothing and fitting of the flow stress data as a function of temperature and strain-rate,at multiple discrete strains.The corrected model then provides quantitative prediction of the spatial and tempo-ral variation in strain-rate and strain throughout the sample,needed to correlate the local deformation conditions with the microstructure and texture evolution.The study uses a detailed series of 144 hot compression tests of a Zr-Nb alloy.While this is an important wrought nuclear alloy in its own right,it also serves here as a test case for modelling the dilatometer for hot testing of high temperature alloys,particularly those with dual α-β phase microstructures(such as titanium alloys).     

High strain rate out‐of‐plane compression of birch plywood from ambient to cryogenic temperatures

An extensive series of compression tests with birch plywood specimens is conducted in the out‐of‐plane direction. Various experimental conditions are tested with different temperatures and strain rates. To this end, a specially designed Split Hopkinson pressure bar protocol is defined, and the specimen dimensions are validated from quasi‐static EN789 standards experiments. Temperatures are ranging from ambient down to −170°C , while strain rates are increased from 0.004/s up to 700/s . The analysis of the experimental results focuses on the hardening behaviour and the following densification regime at large strains. Evolution with temperature and strain rate of the four coefficients of the stress versus strain curve interpolation is discussed, and models are proposed.     

基于静态压缩试验的缓冲包装材料性能对比分析

聚苯乙烯和聚丙烯缓冲包装材料都是粘弹性材料.通过对这2种材料在不同约束(不同加压速度,不同压缩次数)条件下进行静态压缩试验,研究这2种材料的静态特性.试验得到的数据运用Matlab软件,采用最小二乘法进行曲线拟合,得出材料的应力-应变曲线和缓冲系数-最大静应力曲线.最后分析这2种材料的静态压缩特性,并比较它们的异同点.     

High strain rate compression response of carbon/epoxy laminate composites

Composite materials exhibit excellent mechanical properties over metallic materials and hence are increasingly considered for high technology applications. In many practical situations, the structures are subjected to loading at very high strain rates. Material and structural response vary significantly under such loading as compared to static loading. A structure that is expected to perform under dynamic loading conditions, if designed with the static properties, might be too conservative. Hence, it is necessary to characterize the advanced composites under high strain rate loading. In the current investigations, the response of carbon/epoxy laminated composites under high strain rate compression loading is considered using a modified split Hopkinson Pressure Bar (SHPB) setup at three different strain rates of 82, 164 and 817 s⁻¹. The laminates were fabricated using 32 plies of a DA 4518 unidirectional carbon/epoxy prepreg system. Both unidirectional and cross-ply laminates were considered for the study. In the case of cross-ply laminates, the samples were tested in the thickness as well as in the in-plane direction. The unidirectional laminate samples were subjected to loading along 0° and 90° directions. Dynamic stress–strain plot was obtained for each sample and compared with the static compression test result. The results of the study indicate that the dynamic strength (with the exception of through the thickness loading of cross-ply laminates) and stiffness exhibit considerable increase as compared to the static values within the tested range of strain rates.     

磁流变弹性体压缩模式动态力学性能测试

磁流变弹性体是一种新型的磁流变材料,已被成功地应用于变刚度器件设计中。为了评价磁流变弹性体材料性能,迫切需要建立一套磁流变弹性体性能测试系统。利用电磁振动台的线性扫频功能,采用激光位移传感器同步获取测试系统在不同磁感应强度下测试系统的激励与响应信号,通过系统的运动力学模型和相关理论计算,实现压缩模式下的磁流变弹性体刚度与阻尼性能的测试。该测试系统的建立为研制高性能的磁流变弹性体提供了动态压缩模式下的评价手段。     

Effect of texture and grain size on the mechanical properties of warm-worked cadmium,zinc and zinc-0·35% aluminium alloy

Changes in the grain size and crystallographic texture during warm working and their influence on the room temperature mechanical properties are investigated on Cd, Zn and a Zn-Al alloy. The yield strength increase in the early stages of working in extruded cadmium is accounted for based on the development of a basal texture while in rolled zinc and zinc alloy, the properties are affected more by the grain size. Cadmium exhibits ductile fracture at all extrusion ratios whereas the fracture mode in zinc and the alloy changes from cleavage at small rolling strains to ductile at higher deformation strains.     

主换热器热端温差过大的原因和处理

针对某6500m3/h空分设备主换热器热端温差偏大现象进行了分析,分析了故障原因,并提出了具体的解决方法。     

Stress corrosion and corrosion fatigue of metals in high temperature aqueous environments

The dissolved oxygen content of water influences the susceptibility of austenitic stainless steels and reactor pressure vessel steels to environment assisted cracking by raising the electrochemical potential. The relevance of this fact to the integrity of welded stainless steel constructions, deaerator vessels and reactor pressure vessels steels is discussed. In particulal, the use of slow strain rate test procedures involving applied electrochemical potential control is discussed as a means of assessing the susceptibility to cracking of materials in a range of aqueous environments.     

6063铝合金压缩实验摩擦修正及高温流动应力模型

本构关系是影响模拟结果准确性的重要因素之一,本文采用一种数值模拟和压缩实验相结合的方法鼓度法,即由鼓度θ估算摩擦因子,然后通过主应力法公式去除压缩实验中摩擦因素的影响,得到6063铝合金的真实应力-应变曲线,并回归出本构方程.     

空气源热泵热水机高温工况运行研究与实验

介绍空气源热泵热水机高温工况运行时的两种常规控制方法，分析各自的优缺点，对两种控制方法提出使用建议。最后简要介绍一种适用于单风机机组和小型机组高温控制的根据压力调节蒸发面积的方法。     

不同温度环境中沥青混凝土动态抗压性能试验研究

为研究沥青混凝土在不同温度环境中的动态力学特性,该研究在-20～30 ℃和10-5～10-2 s-1条件下对其进行了动态抗压试验研究.试验结果表明:温度和应变速率对沥青混凝土的力学性能有显著影响,降低温度或增加应变速率导致抗压强度和弹性模量增加,峰值应变减小;当温度大于20 ℃或小于-10 ℃时,应变速率由10-5 s...     

高温ITO薄膜应变计制备及压阻性能EI北大核心CSCD

高温薄膜应变计被广泛应用于极端条件热端构件的应变测量。ITO薄膜应变计通常能够应用于1000℃以上的应变测量,为了研究ITO薄膜的显微结构、XPS光谱、阻温特性及压阻响应,采用磁控溅射在陶瓷基底上制备了ITO薄膜应变计,并在高温纯N2中热处理ITO薄膜。结果表明,其电阻温度系数稳定在-750×10-6℃-1,在1200℃下测试其应变特性,测得电阻漂移率为0.0018 h-1,应变因子为16。ITO薄膜在高温下具有稳定的电阻温度系数和低漂移率,为高温端部件应变的测量提供了可能。     

Measurements of the microhardness and compression testing of ferroelectric lead nitrate phosphate single crystals

Ferroelectric crystals of lead nitrate phosphate (LNP) have been grown employing the controlled reaction between lead nitrate and orthophosphoric acid. The microhardness of LNP crystals has been determined. The effect of annealing and quenching on the mechanical properties of these crystals has been studied.     

主换热器热端温差过大的原因分析和处理

分析 6 0 0 0m3 /h空分装置检修后开车过程中出现的主换热器热端温差过大的原因 ,并介绍了处理办法     

Development of through-thickness texture gradient and persistence of shear-type textures during annealing of commercial purity aluminium sheet processed by accumulative roll-bonding

Ultrafine-grained commercial-purity aluminum (AA1070) sheets produced by four cycles of accumulative roll-bonding (ARB) without lubrication are subjected to annealing treatments in the temperature range from 250 °C to 400 °C. Microstructures and microtextures in the surface and center regions of the ARBed and annealed sheets are measured by electron backscatter diffraction. The results show that annealing treatments at 325 °C or above lead to a reduction in the microstructure gradient but a significant through-thickness texture gradient different from that in the as-deformed state. The center region is featured by the development of a strong cube texture at the expense of rolling components. In the surface region, shear-type components are either enhanced or largely retained, showing a high persistency upon annealing. While the grain structures are restored predominantly by continuous recrystallization in the surface region, a mixture of continuous and discontinuous recrystallization is envisaged for the center region.