考虑箍筋约束效应的快速轴压加载下钢筋混凝土短柱性能数值分析

为了研究在地震作用下应变率效应对约束钢筋混凝土轴压短柱力学性能的影响,该文建议了同时考虑应变率效应和箍筋约束效应的混凝土塑性模型等效单轴受压本构曲线,建立了分析约束钢筋混凝土轴压短柱在快速加载下动力行为的有限元模型。通过模拟结果与文献中试验研究结果的比较,表明该模型可有效描述约束钢筋混凝土短柱在地震作用下考虑混凝土材料应变率敏感性时的力学性能,建议的等效单轴受压本构曲线是合理的。利用该有限元模型,分析了配置箍筋构形、箍筋间距和纵筋配筋率这三个可影响约束效应的参数对约束钢筋混凝土短柱在考虑率效应时的力学性能的影响。结果表明随应变率的提高,轴压短柱的承载力明显提高,但延性降低,力-轴向变形曲线下降段变陡。箍筋构形、间距以及纵筋配筋率对约束钢筋混凝土轴压短柱的动力力学性能具有重要的影响。     

钢筋混凝土长柱快速轴心受压试验与模拟研究

为了研究钢筋混凝土长柱在快速轴心受压加载下应变率和惯性效应对其性能的影响,开展了钢筋混凝土长柱快速轴心受压加载试验和动力有限元模拟研究。设计了两组共6根构件,长细比分别为12和22,快速加载试验中实测混凝土材料应变率达到10-3/s数量级。试验结果表明,与准静载相比,快速轴心受压下构件承载力及对应的轴向变形均有不同程度的提高,并且动力增大系数随长细比增大而增大。将CEB-FIB模式规范的混凝土材料动态应力-应变关系引入混凝土塑性损伤模型,在ABAQUS中建立了钢筋混凝土长柱快速轴心受压下力学性能分析的显式动力有限元模型,数值模拟结果与试验结果吻合良好。最后,运用所建立的有限元模型分析快速加载下构件承载力动力效应的形成机制。分析结果表明,构件长细比较小时其惯性效应可以忽略,动力增大系数可以主要由应变率效应来描述,随长细比增大,惯性效应对动力增大系数的贡献越来越明显,而应变率效应仍不可忽略。     

Micromechanical Analysis of Interphase Damage for Fiber Reinforced Composite Laminates

In the present study, the initiation and evolution of the interphase damage and their influences on the global stress-strain relation of composite laminates are predicted by finite element analysis on a micromechanical unit cell model. A thin layer of interphase elements is introduced and its stress-strain relation is derived based on a cohesive law which describes both normal and tangential separations at the interface between the fiber and matrix. In addition, a viscous term is added to the cohesive law to overcome the convergence difficulty induced by the so-called snap-back instability in the numerical analysis. The matrix behavior is described by a recently developed nonlinear viscoelastic constitutive model. As application examples, glass fiber/epoxy unidirectional laminates under off-axis loadings are analyzed. One-quarter of the unit cell is used in the analysis accounting for the geometrical symmetry of the model, and the corresponding periodic boundary conditions for combined global shear and normal loading are derived. Results show that the initiation and evolution of the interphase damage can be well simulated and the predicted global stress-strain responses are in good agreement with the experimental results.     

应变率突增对混凝土动态拉伸破坏影响的细观模拟

实际工程中混凝土结构往往遭遇多次动态荷载作用或在承受一定初始损伤荷载基础上再承受不同应变率的动态荷载.建立了哑铃型混凝土三维细观数值模型,模拟不同名义应变率单独作用下混凝土材料的单轴动态拉伸破坏行为,又分别对混凝土单轴拉伸应力应变曲线上升段和软化段的应变率突增行为开展了细观模拟,初步分析了应变率突增行为对动态拉伸破坏强...     

CRACK GROWTH BEHAVIOUR IN A THERMAL FATIGUE TEST. EXPERIMENTS AND CALCULATIONS

Abstract— This study is concerned with the results of experiments in which thermal cycles have been repeatedly applied through the wall thickness of axisymmetrically cracked tubular specimens. The investigated material is a Cr–Mo steel used for the moulds when fabricating centrifugally cast iron pipes. Crack growth rates have been measured by using the interrupted tests technique. A methodology is proposed to model the crack growth rates under such thermal fatigue loadings. The elastic and plastic stress-strain fields are calculated on the uncracked specimen by means of a finite element code. Special attention was paid to reach a mechanical steady state regime. Fatigue crack growth rates data were obtained, both under isothermal and anisothermal conditions, on CT and SEN specimens. The latter specimens were tested under large-scale yielding in order to obtain the data appropriate to the cyclic stress-strain field calculated in the thermal fatigue specimens. An effective stress intensity factor, which takes into account both plastic strains and crack closure effect, was used to correlate the results of isothermal tests on CT and SEN specimens and to calculate the thermal fatigue crack growth rates in tubular specimens. It is shown that the use of the effective stress intensity factor gives a satisfactory agreement between the observed and the calculated crack growth rates.     

A simple analytical crack tip opening displacement approximation under random variable loadings

A simple analytical approximation is proposed in this paper to calculate the crack tip opening displacement under general random variable amplitude loadings. This approximation is based on a modified Dugdale model for cyclic loadings. The discussion is first given under constant amplitude loading and is extended to several simple cases under variable amplitude loadings. Following this, a general algorithm is proposed under general random variable loadings. Numerical examples are verified with finite element simulations. Following this, hardening effect is included by including a hardening correction function. The proposed analytical approximation is very efficient compared to the direct finite element simulation. The solution can be used for detailed fatigue crack growth analysis under random variable amplitude loadings.     

具有局部分层的对称铺设层合板在非保守力作用下的屈曲分析

利用弹性非保守系统自激振动的拟固有频率变分原理,推导出复合材料矩形板受非保守随从力作用的变分方程,进而导出复合材料层合板含分层损伤的有限元基本方程及求解临界荷载力和固有频率的特征方程。用载荷增量法计算了不同边长比的复合材料矩形板在面内受随从力作用且含有分层的临界载荷,分析了不同角铺设方向对临界载荷的影响。分析表明,分层对临界载荷有一定影响,复合材料层合板的角铺设方向对临界载荷有较大影响。     

A temperature dependent creep damage model for polycrystalline ice

We present a continuum damage model for the temperature dependent creep response of polycrystalline ice under a multiaxial state of stress, suited for ice in polar regions. The proposed model is based on a thermo-viscoelastic constitutive law for ice creep and a local orthotropic damage accumulation law for tension, compression and shear loadings. Orthotropic damage is represented by a symmetric second-order damage tensor and its effect on creep is incorporated through the effective stress concept. The unknown model parameters are first calibrated using published experimental data from constant uniaxial stress tests and then predictions are made for constant strain rate and multiaxial loadings. The predicted results are in good agreement with both experimental and numerical results in the literature illustrating the viability of the proposed model. The model is mainly intended for studying the failure mechanisms of polar ice at low deformation rates with depth varying temperature profiles.     

Design of ribbed shells subject to physically nonlinear deformation

The method of successive loadings is used together with special discontinuous functions to study the stress-strain state of ribbed shells subject to physically nonlinear deformation. In the solution of differential equations with variable and singular coefficients, the surface of the shell is divided into elements and the variable coefficients are averaged within each element. The solutions of the equations are constructed in the form of a combination of functions of regular and special discontinuous functions, which makes it possible to convey the irregular nature of the stress state and account for the physical nonlinearity of the material.Translated from Problemy Prochnosti, No. 4, pp. 91–98, April, 1996.     

2.5D C/SiC复合材料连续损伤本构模型

基于连续损伤力学建立了一种包含拉伸与剪切损伤变量的2.5D C/SiC复合材料连续损伤本构模型。分别开展了拉伸和剪切试验,获得应力-应变曲线,并通过拟合试验曲线获得各损伤变量的演化参数。采用子程序技术将本构模型嵌入商用有限元软件ANSYS,应用有限元法计算了材料的应力-应变曲线。考虑了拉剪损伤耦合效应,计算了偏轴拉伸情况下的应力-应变曲线。结果表明:沿经纱拉伸、沿纬纱拉伸以及面内剪切的应力-应变曲线与试验结果吻合,最大偏差依次为4.30%、3.09%及3.73%;偏轴拉伸计算与试验应力-应变曲线也吻合较好。      

土与结构接触面弹塑性损伤模型用于单桩与地基相互作用分析

基于粗粒土与结构接触面弹塑性损伤静动力统一模型(称作EPDI模型)建立了可用于有限元分析的弹塑性损伤接触面单元。对接触面试验进行了模拟,采用不同的接触面本构模型及参数对单调和循环荷载作用下的单桩基础的侧摩阻力和桩顶位移进行了有限元分析。结果表明:包括剪应力应变关系和剪胀特性在内的接触面力学特性对桩土相互作用分析有重要影响,需要合理地加以描述。基于试验结果建立的弹塑性损伤接触面单元能够有效地用于土体与结构物相互作用分析,并能够合理地反映土与结构接触面的包括体应变及其与剪应变耦合特性在内的接触面主要静动力学特性。     

Can micro-compression testing provide stress-strain data for thin films?: A comparative study using Cu, VN, TiN and W coatings

Micro-compression testing using an instrumented micro- or nanoindenter equipped with a flat punch is a promising approach to measure the stress-strain response of miniaturized materials and to complement hardness and modulus measurements gained by nanoindendation. Focussed ion beam milling is employed to fabricate micron-sized compression pillars from 1 µm thick single crystal Cu(001), TiN(001), and VN(001) films grown on MgO(001), and from a 6.7 µm thick polycrystalline W coating deposited on Si(001). In situ micro-compression tests in a scanning electron microscope reveal reproducible stress-strain curves for W, a considerable statistical scatter in the flow stress for Cu and VN, and failure of TiN pillars by cleavage. A linear work-hardening rate of 2.7 ± 1.2 GPa is determined for the polycrystalline W coating. The results are critically discussed taking into account material defects and the stiffness of the film-substrate system.     

A hybrid crystal plasticity and phase transformation model for high carbon steel

Presence of retained austenite in two-phase steels can have a significant effect on the response of the material. The current state-of-the-art in multi-phase modeling of polycrystalline plasticity has limits on scaling, explicit phase modeling, and tracking multiple plasticity types. To address these issues, a hybrid material model is developed for high-carbon steels. The model can account for crystal plasticity of martensite and stress-assisted transformation from austenite to martensite in dual-phase high-carbon steel. The parameters of the model are calibrated to experimental stress-strain and volumetric transformation strain data. The combination of scalability and multiple phase representation gives a useful tool for the investigation of two-phase high-carbon steel behavior.     

针刺C/SiC复合材料应力-应变模型及试验验证

研究了室温下针刺C/SiC复合材料的静拉伸应力-应变行为。基于显微CT技术重构的微观型貌,选取恰当的代表体积单元,建立了针刺C/SiC复合材料应力-应变性能预测的单胞模型。基于可实现任意加卸载下单向纤维增强C/SiC复合材料应力-应变计算的界面摩擦模型,由材料的细观组分性能计算出单向纤维束层的应力-应变响应,然后将单向纤维束层的应力-应变响应代入到单胞模型中,通过有限元法计算得到针刺C/SiC复合材料的整体应力-应变响应。进行了针刺C/SiC复合材料静拉伸试验,测得材料的应力-应变响应,计算结果与试验吻合较好。      

A procedure for determining the true stress-strain curve over a large range of strains using digital image correlation and finite element analysis

A procedure for determining the stress-strain curve including post-necking strain is proposed. Hourglass type specimens were used for tensile tests, and the stress-strain curves were identified through an iteration process using finite element analysis. The strain at the position of minimum diameter was measured by digital image correlation. This procedure was applied to carbon steel of various degrees of cold work. The radius of the minimum section of the hourglass type specimen was changed in order to investigate the effect of stress triaxiality on the failure strain. The procedure could derive the stress-strain curve including the post-necking strain. From the obtained curve, it was shown that the stress-strain curves for different degrees of cold work were almost identical when the plastic strain by the cold working was added to the strain. Furthermore, it was revealed that the true stress-strain curve could be approximated well by the power law equation and the curve could be estimated by using the stress-strain relation for before-necking strain.     

材料模型对1Cr18Ni9Ti管材拉伸有限元仿真的影响

为研究材料模型对有限元模拟1Cr18Ni9Ti管拉伸的影响,将管材单向拉伸试验获取的真实应力应变曲线分别拟合成线性硬化和指数硬化材料模型,并用于有限元模拟。经对比分析认为,采用真实应力应变模型的分析结果与实验结果吻合良好,并能正确显示出颈缩发生时刻和颈缩形状;采用指数硬化模型的有限元模拟结果接近真实应力应变模型,颈缩区应力应变分布略显分散;采用线性硬化模型的有限元模拟结果未能显示实际管拉伸后期的局部颈缩形状。     

Creep model of ice for monotonically increasing stress

A numerical integration method of predicting the strain path corresponding to a given stress history, and hence the stress-strain relation, has been developed for polycrystalline materials like ice at high homologous temperatures. The method is based on a generalized creep equation incorporating the grain size effect and predicts strain in terms of elastic, delayed-elastic, and viscous or permanent strains. The theory has been tested successfully during strength tests of ice in the temperature range of ?5 to ?30°C. The monotonous increase in strain rate observed during testing under conditions of constant cross-head displacement rate can now be explained analytically.     

含线性阻尼的正切型包装系统跌落冲击响应分析的NHB法

目的 研究温湿度对蜂窝纸板力学性能的影响规律,提高蜂窝纸制品包装在高温高湿环境下的承载性能和使用寿命。方法 制作不同温湿度参数下处理的蜂窝纸板试样,进行静态压缩试验,得到其应力-应变曲线;建立一定温湿度环境下的蜂窝纸板有限元模型,进行有限元仿真分析,得到其位移云图和应力-应变曲线,并与试验结果进行对比分析。结果 蜂窝纸板静态压缩试验结果和有限元仿真结果相似,包括弹性、屈服和平台等阶段,试验和有限元分析所得的应力-应变曲线吻合。蜂窝纸板的等效弹性模量和屈服应力与温湿度有关,且随着相对湿度的增大而减少,随着温度的升高而增加。结论 通过试验研究和有限元分析,发现湿度对蜂窝纸板力学性能影响显著,研究结果对蜂窝纸制品包装的承载应用有重要参考价值。     

钨合金动态力学性能的三维数值模拟研究

采用数值模拟的方法对钨合金在冲击载荷作用下的动态力学响应进行了研究。运用有限元动力分析程序建立了具有典型微观结构钨合金三维有限元单胞模型,对钨合金在拉伸载荷作用下的动态力学性能进行了数值模拟研究,分析了应变率对其力学性能的影响,给出了不同应变率条件下单胞模型的应力和应变分布云图,在与实验结果对比的基础上验证了该有限元模型的可靠性。