合金钢的高温单轴棘轮效应实验研究1.25Cr0.5Mo

利用MTS810电液伺服试验机在300℃温度下对1.25Cr0.5Mo合金钢进行了单轴棘轮效应实验研究,探讨了温度、应力幅值、应力峰值、平均应力以及加载历史对材料棘轮效应的影响.以0.1%/s及0.2%/s两种不同的应变加载速率分别对材料进行单轴拉伸实验,结果表明1.25Cr0.5Mo合金钢在所研究的温度范围内的变形行为是率无关的;在0.4%应变幅值下进行了单轴应变循环实验,响应应力幅值随循环次数几乎没有变化,可视为循环稳定材料;单轴棘轮效应实验表明棘轮应变速率随平均应力、应力幅值及温度的升高而增大;棘轮变形依赖于加载历史,先前较大的循环应力对后继较小的循环应力下的棘轮变形起抑制作用.     

Low cycle fatigue behavior of zirconium–titanium–steel composite plate

The low cycle fatigue behavior of zirconium–titanium–steel composite plate under symmetrical and asymmetric stress control was studied. The effects of mean stress and stress amplitude on cyclic deformation, ratcheting effect and damage mechanism were discussed in detail. The results show that under symmetric stress control, the forward ratcheting deformation is observed. Under asymmetric stress control, the ratcheting strain increases rapidly with mean stress and stress amplitude increasing. Under high stress amplitude, the influence of mean stress is more significant. In addition, by studying the variation of strain energy density, it is found that the stress amplitude mainly promotes the fatigue damage, while the mean stress leads to the ratcheting damage. In addition, fracto- graphic observation shows that the crack initiates in the brittle metal compound at the interface, and the steel has higher resistance to crack propagation. Finally, the accuracy of life prediction model considering ratcheting effect is discussed in detail, and a high-precision life prediction model directly based on mean stress and stress amplitude is proposed.     

SiCP/ 6061Al 合金复合材料的高温单轴棘轮行为及其时间相关特性

对T6 热处理后的SiC_P / 6061Al 合金复合材料的高温(300 ℃) 单轴应变循环特性和棘轮行为进行了实验研究, 讨论了具有两种颗粒体积分数的复合材料在高温下不同加载条件时的循环软/ 硬化特性和棘轮行为特征。实验研究表明: 颗粒增强金属基复合材料宏观上表现出与金属材料相类似的应变循环特性和棘轮变形规律, 即复合材料在非对称应力循环下也将产生一定的棘轮变形, 并随应力幅值和平均应力的增加而增加; 颗粒的引入使复合材料抵抗棘轮变形的能力增强, 棘轮变形随颗粒体积分数的升高而下降; 在高温下棘轮行为体现出明显的时间相关特性, 即棘轮应变值明显依赖于加载率和峰值保持时间, 并具有明显的蠕变-棘轮交互作用。在对该类复合材料的棘轮行为进行本构描述时必须考虑复合材料的微结构特征、加载条件以及时间效应等的影响。      

An integrated thermal and mechanical investigation of molten-salt thermocline energy storage

Thermal ratcheting is a critical phenomenon associated with the cyclic operation of dual-medium thermocline tanks in solar energy applications. Although thermal ratcheting poses a serious impediment to thermocline operation, this failure mode in dual-medium thermocline tanks is not yet well understood. To study the potential for the occurrence of ratcheting, a comprehensive model of a thermocline tank that includes both the heterogeneous filler region as well as the composite tank wall is formulated. The filler region consists of a rock bed with interstitial molten salt, while the tank wall is composed of a steel shell with two layers of insulation (firebrick and ceramic). The model accounts separately for the rock and molten-salt regions in view of their different thermal properties. Various heat loss conditions are applied at the external tank surface to evaluate the effect of energy losses to the surroundings. Hoop stresses, which are governed by the magnitude of temperature fluctuations, are determined through both a detailed finite-element analysis and simple strain relations. The two methods are found to yield almost identical results. Temperature fluctuations are damped by heat losses to the surroundings, leading to a reduction in hoop stresses with increased heat losses. Failure is prevented when the peak hoop stress is less than the material yield strength of the steel shell. To avoid ratcheting without incurring excessive energy loss, insulation between the steel shell and the filler region should be maximized.     

25 Cr2 Ni2 MoV钢焊接接头循环变形行为的有限元模拟与分析

以25Cr2Ni2MoV汽轮机转子钢V形平板焊接接头为研究对象,考虑不同热影响区材料与母材的弹性模量比值、坡口角度及载荷特性（包括不同应力比、平均应力和应力幅值）等因素的影响,采用大型有限元软件ABAQUS和Chaboche循环本构模型对接头的循环变形行为、局部塑性变形分布及其随循环周次的演化特性进行了有限元模拟与分析。结果表明,随着热影响区材料与母材的弹性模量比值的增大,焊接接头整体抗拉和抗棘轮变形能力增强,发生局部破坏的可能性也随之增大;随着坡口角度的增加,焊接接头的抗拉性能和抗棘轮变形能力降低;应力比、平均应力和应力幅值越大,焊接接头越容易发生破坏。研究成果实现了对平板焊接接头结构及性能的初步优化,为提高该类焊接接头的设计水平提供了参考。     

Ratcheting and fatigue-led wear in rail–wheel contact

A computer model, which simulates the ratcheting wear of a ductile material subject to repeated loading, is presented and discussed in detail. Variation of material properties is a feature of the model, failure by ductility exhaustion occurring at isolated points or extending regions of failure. Such regions form crack‐like features. Mechanisms for removal of weakened material from the surface as wear debris are described. The wear process causes a degree of surface roughness. The simplicity of the model enables simulation of millions of load cycles in only a few hours' computer time. The computer model is used to study the effect of partial slip on wear rate. When creepage is relatively low, the wear rate increases sharply with creepage. When creepage is relatively high, the wear rate is largely insensitive to the creepage.     

Adhesive Layer Shrinkage in Bonds Subjected to Thermal Cycling

Residual stress states within certain classes of polymeric adhesives andcoatings subjected to cyclic temperature conditions can becomeincreasingly tensile with exposure to cyclic temperature conditions.Under some conditions, these tensile stresses are believed to beresponsible for initiating debonds which may eventually propagatethroughout the bond. In certain potting-type applications in which thepolymer is constrained in a particular fashion, these debonded layerscan shrink extensively. In an industrial application, this shrinkage hasreached as much as 15%, nearly two orders of magnitude larger thanexpected based on simple thermal contraction. This paper provides athermal racheting explanation for this dramatic shrinkage. Numericalstudies have confirmed the explanation, and provide additional insightsinto the effects of temperature range, coefficient of friction betweenthe adhesive and substrates, and the viscoelastic nature of theadhesive. Based on the hysteretic nature of the normal forces whichaffect the frictional forces, the proposed mechanism usesfriction hysteresis under cyclic thermal conditions to explain observedshrinkage.     

ETFE薄膜单轴循环拉伸力学性能

对6组250μm厚乙烯-四氟乙烯(ethylene-tetrafluoroethylene,ETFE)薄膜进行了不同应力幅值单轴循环拉伸试验.利用自编MATLAB程序分析了试验所得应力-应变曲线,得到了循环弹性模量、屈服应力、棘轮应变以及滞回环面积等力学性能参数;分别建立了循环弹性模量、棘轮应变和滞回环面积与循环次数的关系式.试验和分析结果表明:随着循环次数的增加,循环弹性模量、棘轮应变和滞回环面积的变化率逐渐减小,分别近似稳定于13,14,14次循环.     

Lateral earth pressure behind an integral abutment

Integral abutment bridges have gained increasing attention in the past few decades. They provide a cost-effective solution to the high maintenance expenses associated with the joints and bearings found in conventional bridges. This paper describes the observed behaviour of granular soil backfill retained behind an integral abutment subjected to cyclic loading. Significant pressure build-up was observed in the soil behind the abutment in most locations. The pressure build-up is attributed to several mechanisms such as sand particle flow and densification due to cyclic loading, and the shearing of dense sand during bridge expansion. Therefore, the applicability of using a linear soil pressure distribution assumed by the classical theories in designing the integral abutment system is discussed. Furthermore, the vertical and lateral distribution of the soil pressure behind the abutment has also been analysed. Results from the data measured show that bridge skew resulted in bigger soil pressures at the obtuse side of the abutment compared to the acute. The conclusions of this paper highlight several new design aspects, which are usually overlooked by the common design methodologies of integral abutments, that more accurately predict the vertical and lateral variation in the soil pressure behind abutments.