Ⅱ型模态控制下的GFRP-混凝土界面断裂韧度测定

介绍了一种适用于评价GFRP-混凝土界面断裂性能的理论分析模型,并结合端部开口试件的四点弯曲试件(4ENF)试验,测得了该类界面在II型模态控制下的断裂韧度.该模型考虑了试件各子梁中的横向剪切变形对能量释放率的影响,引入一阶剪切变形梁理论对界面断裂过程进行模拟;同时由于混凝土在受剪及受拉区域容易发生破坏,故试件设计过程中引入钢筋进行加强,有效减少了实验数据的离散性;进一步通过与有限元仿真进行对比发现结果吻合较好,这说明本文方法能够有效测定II型模态荷载控制下复合材料-混凝土界面的断裂韧度,评价复合材料增强混凝土结构的界面断裂性能,预测界面的起裂、裂纹扩展和失效容差.     

平面应变断裂韧度测试的有效性研究

针对平面应变断裂韧度测试厚度存在的问题,研究了几种材料测断裂韧度所需的实际厚度,拟合了平面应力到平面应变过渡状态下σz应力的分布,分析了过渡状态下的裂端应力场.通过分析不同材料特性和应力状态对断裂韧度KIC的影响,结合金属材料断裂的细观机理,认为平面应变断裂韧度KIC反映的是材料正断的性能,仅代表材料发生正断的指标.结果表明:对于某些高韧性材料,即使满足平面应变条件厚度要求,断裂以剪断为主,很难测出KIC;而对于偏脆性材料,即使不满足厚度要求,只要发生正断,也可测出KIC.结合已有试验结果,为平面应变断裂韧度KIC测定的厚度选择,提出一种合理的预测方法.借助三片模型的研究,给出平面应变断裂韧度的正断指标.     

采用压缩单裂纹圆孔板确定岩石动态起裂、扩展和止裂韧度

爆炸、冲击、地震等人为或自然灾害不可避免,经常造成大量土木工程设施的破坏,因此岩石在动态载荷作用下的行为受到特别关注.岩石动态断裂韧度是评价岩石抵抗裂纹动态起裂、扩展和止裂性能的材料参数,开展岩石动态断裂韧度测试方法的研究对相关理论基础和实验技术的要求较高.岩石动态断裂韧度分为动态起裂、动态扩展、动态止裂三种,虽然关于动态起裂和动态扩展的研究已有一些成果,对岩石动态止裂的研究仍是一个难题,至今几乎无人问津.研究表明,在分离式霍普金森压杆撞击压缩单裂纹圆孔板岩石试样的I型动态断裂试验中,动态起裂、扩展、止裂的全过程可以由黏贴在压缩单裂纹圆孔板试样上的裂纹扩展计监测,岩石的动态起裂、扩展、止裂韧度可以用实验-数值-解析法确定.特别值得一提的是首次测出了岩石的动态止裂韧度.裂纹扩展计信号还显示,压缩单裂纹圆孔板在止裂后,停止的裂纹还会再次动态起裂、扩展并超出裂纹扩展计的检测范围.从能量的角度分析了动态止裂的过程,指出测试动态止裂韧度时要注意的一些问题.结果显示,岩石动态起裂韧度和动态扩展韧度分别随动态加载率和裂纹扩展速度的增大而增大,岩石动态起裂韧度略大于动态止裂韧度.     

金属材料三维断裂韧度厚度效应的有限元模拟

随着金属材料大壁厚结构件在工程中的广泛应用，对其断裂韧度的厚度效应研究具有重要的科学意义和工程价值。本研究基于有限元和实验相结合的方法，对金属材料断裂韧度的厚度效应进行预测。首先，通过一组薄壁厚金属材料标准三点弯曲试验得到试样失效时的临界载荷值，并利用内聚力模型与基于虚拟裂纹闭合技术的裂纹扩展模拟方法得到裂纹扩展时的单元临界能量释放率。随后，以此临界能量释放率作为裂纹扩展的启裂准则门槛值，通过有限元计算得到不同试样厚度下裂纹启裂时的裂尖断裂参数随着厚度的变化规律。最后，为了验证有限元模拟结果的准确性，本研究进行了另外两组不同厚度下三点弯曲试样的断裂韧度试验，并将试验结果与有限元结果进行了对比，验证了有限元所模拟的断裂韧度厚度效应的准确性。本研究旨在，通过薄壁厚三点弯曲试样的实验结果结合有限元模拟工作，即可实现金属材料断裂韧度的整个厚度效应曲线，为任意厚度下金属材料断裂韧度预测提供一种可靠的研究方法，有益于缩减试验成本，为大壁厚工程结构件的失效预测提供依据。     

基于线法的U形切口混凝土梁断裂参数研究

混凝土的断裂韧度是重要的材料参数，本文利用含U形切口的三点弯曲梁试验结合临界距离线法来获得混凝土的断裂韧度．理论推导了含断裂韧度与不同根部半径的断裂失效方程，采用有限元法计算了切口应力集中系数．根据断裂失效方程和试验中的材料几何参数，利用最小二乘法拟合计算得到混凝土材料的断裂韧度和抗拉强度．对小切口半径情形，采用双K断裂准则分析计算了混凝土起裂韧度和失稳断裂韧度，分析结果表明：临界距离线法得到的材料断裂韧度与双K断裂准则中的失稳断裂韧度吻合，同时获得相应临界距离值．     

混凝土三点弯曲梁裂缝断裂全过程数值模拟研究

考虑裂缝黏聚力的作用,基于Paris位移公式推导出混凝土三点弯曲梁裂缝扩展过程中断裂过程区上的裂缝张开位移的解析表达式.采用起裂韧度作为裂缝起裂及扩展的判断标准,提出了荷载作用下混凝土裂缝起裂、扩展及失稳破坏全过程的数值模拟方法,并分别与国内外断裂试验实测值及有限元计算值进行了比较.结果表明,本文提出的数值模拟方法形式简单且精度较好.     

表征沥青及沥青混合料高低温蠕变性能的流变学模型

本文根据由高、低温蠕变实验所得到的沥青混合料的变形特性,在分析现有流变学模型性能的基础上,提出了一种能够精确地表征沥青混合料变形特性的流变学模型,导出了其蠕变方程,并根据蠕变实验结果确定了材料的模型参数.这一模型的建立,为采用流变学理论和粘弹性理论研究沥青混合料及沥青路面奠定了基础.     

冲击加载下巷道内裂纹的扩展特性及破坏行为

为了开展含预制裂纹的巷道模型试样在冲击载荷下的动态断裂响应实验，选用青砂岩作为模型材料制作巷道模型试样，以可调速落锤冲击实验机作为冲击加载装置进行试样的动态断裂实验，分析冲击载荷作用下的巷道内裂纹的扩展规律。采用裂纹扩展计及应变片测试系统监测裂纹的起裂时间、扩展速度及止裂时间，并借助于AUTODYN、ABAQUS有限元数值分析软件对实验模型进行数值模拟，计算裂纹的动态起裂韧度、动态扩展韧度、动态止裂韧度等断裂参数。结果表明:巷道内裂纹在扩展路径过程中存在着明显的止裂现象；采用实验-数值方法能够较好地得出裂纹的起裂韧度、扩展韧度和止裂韧度等参数。另外，对止裂现象进行了初步的分析，讨论了试样内应力反射波与透射波对止裂问题的影响。     

Ⅰ型裂纹中低速冲击荷载下起裂韧度测试新方法

为了探寻更加合理的构型试件来研究纯Ⅰ型裂纹在冲击荷载下的起裂及扩展行为, 提出一种新构型试件, 即双倾斜底边中心裂纹试件(double inclined bottom central cracked, DIBCC)。借助于中低速落锤式冲击实验装置进行冲击实验, 通过应力波来使试件内预制裂纹起裂并扩展, 同时利用应变片测试系统监测裂纹起裂时刻, 并采用AUTODYN有限差分软件对实验过程进行数值模拟, 最后计算裂纹的动态应力强度因子, 利用实验测得的起裂时刻, 确定试件的起裂韧度。结果表明:(1)在反射拉伸波作用下, 预制裂纹两侧会产生垂直于裂纹面向外的位移, 使预制裂纹扩张, 从而使裂纹起裂。(2)数值模拟结果与实验结果在裂纹扩展路径上具有一致性, 说明本文中提出的DIBCC构型试件有效, 可以用来测试裂纹在冲击载荷下的断裂韧度。     

准静态条件下金属材料的临界断裂准则研究

本文针对9种金属材料完成了具有不同约束程度的10类试样的延性断裂试验, 获得了发生拉、压、扭和裂尖断裂等破坏形式构型试样的载荷-位移试验关系; 基于圆棒漏斗试样拉伸试验所得直至破坏的载荷-位移曲线, 采用有限元辅助试验(finite-element-analysis aided testing, FAT)方法得到了9种材料直至破坏的全程等效应力-应变曲线, 以此作为材料本构关系通过有限元分析获得了各类试样直至临界破坏的载荷-位移关系模拟. 载荷-位移关系模拟结果与试验结果有较好的一致性, 表明用于解决试样颈缩问题的FAT方法所获得的全程材料本构关系针对各向同性材料具有真实性和普适性. 对应9种材料、10类试样的36 个载荷-位移临界断裂点, 通过有限元分析获得了对应的材料临界断裂应力、应变与临界应力三轴度, 研究表明, 第一主应力在延性变形过程中为主控断裂的主导参量; 通过研究光滑、缺口、裂纹等构型试样的断裂状态, 提出了$-1$至3范围的应力三轴度下由第一主应力主控的统一塑性临界断裂准则.      

Optical response and yield behavior of a polyester model material

A mixture of flexible and rigid polyester resins is one of the materials that has been used in the past to model elasto-plastic prototype behavior. As a result of recent curtailments in plastic production, one of the constituents of the mixture is no longer available. A different flexible resin of the same family is available, however, and has been shown in this program to be suitable for optical-model studies involving deformations into the plastic range of material response. In this paper, complete mechanical and optical properties for the new model material are presented for both uniaxial tensile and compressive loadings. Results from a series of thin-walled cylinder tests under internal-pressure loadings are also presented which provide some information on the optical and yield characteristics of the polyester model material under a biaxial state of stress. Results of the study indicate that stress-strain curves for the material can be modified significantly by changing the mixture ratio or the test temperature. Optical data from the study indicate that the fringe order in the material is a function of the instantaneous principal-strain difference. Data from the uniaxial tension and compression tests, together with limited data from the cylinder tests, indicate that the polyester material may follow a modified von Mises yield criterion which accommodates differences in tensile and compressive yield strengths of a material.     

On some connections between equivalent single material and mixture theory models for fiber reinforced hyperelastic materials

There are two approaches that can be used to model the large strain mechanical response of material systems in which elastic fibers are embedded in an elastic matrix. In the first approach, a fiber reinforced material undergoing large deformation is homogenized in the sense that it is assumed to act as an equivalent single material that is transversely isotropic and hyperelastic. Both constituents then share a common reference configuration, which is typically assumed to be a natural or stress-free configuration for the equivalent single material. The stress depends on a single deformation gradient defined with respect to the natural configuration.In the second approach, the fiber/matrix system is treated as a mixture, with the matrix and the fibrous constituents having their own reference configurations and material symmetries. The total stress depends on the deformation gradients and material symmetries for both constituents, defined with respect to their reference configurations.Under appropriate assumptions, the constitutive theory developed using mixture theory can coincide with the constitutive theory assuming an equivalent single material that is transversely isotropic and hyperelastic. This paper explores the connection between the two approaches by considering the various reference configurations and material symmetries.     

材料的率相关与梯度二、四阶耦合模型的内尺度律与稳定性研究

在率相关与梯度塑性二阶耦合本构模型的基础上,提出了二、四阶率相关与梯度塑性耦合模型。采用简谐波的分析方法对材料的应变局部化及材料的稳定性进行了研究,得到了二、四阶耦合模型在一维情况下的内尺度律的变化及材料稳定性的关系,得到了波长变化的上下界及材料稳定性的条件;并对其进行了对比性研究,得到材料稳定点移动的规律。     

Phase proportioning in CuAlBe shape memory alloys during thermomechanical loadings using electric resistance variation

The microstructure of shape memory alloys changes with the thermomechanical history of the material. During thermomechanical loading, austenite, thermally-induced martensite or stress-induced martensite can be simultaneously present in the material. In applications integrating SMA parts, utilization conditions seriously affect the microstructure and can generate macroscopic strain or stress. Consequently, during thermomechanical loadings, it is important to be able to proportion the different phases and consequently to understand the kinetic transformation. This is very useful in the development of constitutive equations. This study shows, by a series of tests, that the proposed experimental method, based on the measurement of the variation of electric resistance of CuAlBe wires, permits to determine the volume fraction of the different phases present in the material (i.e., austenite, stress-induced martensite and thermally-induced martensite). The proposed method is applied to the most common thermomechanical behavior met in engineering applications of shape memory alloys: pseudoelasticity, pseudoplasticity, recovery-stress and stress-assisted two-way shape memory effect. The proportioning method based on a mixture law integrating the resistivity of pure phases present in the SMA is first performed on different two-phase mixture cases and then applied to a three phase mixture case.     

A linear dynamic model for a saturated porous medium

A linear isothermal dynamic model for a porous medium saturated by a Newtonian fluid is developed in the paper. In contrast to the mixture theory, the assumption of phase separation is avoided by introducing a single constitutive energy function for the porous medium. An important advantage of the proposed model is it can account for the couplings between the solid skeleton and the pore fluid. The mass and momentum balance equations are obtained according to the generalized mixture theory. Constitutive relations for the stress, the pore pressure are derived from the total free energy accounting for inter-phase interaction. In order to describe the momentum interaction between the fluid and the solid, a frequency independent Biot-type drag force model is introduced. A temporal variable porosity model with relaxation accounting for additional attenuation is introduced for the first time. The details of parameter estimation are discussed in the paper. It is demonstrated that all the material parameters in our model can be estimated from directly measurable phenomenological parameters. In terms of the equations of motion in the frequency domain, the wave velocities and the attenuations for the two P waves and one S wave are calculated. The influences of the porosity relaxation coefficient on the velocities and attenuation coefficients of the three waves of the porous medium are discussed in a numerical example.     

Linear dynamic model for porous media saturated by two immiscible fluids

A linear isothermal dynamic model for a porous medium saturated by two immiscible fluids is developed in the paper. In contrast to the mixture theory, phase separation is avoided by introducing one energy for the porous medium. It is an important advantage of the model based on one energy approach that it can account for the couplings between the phases. The volume fraction of each phase is characterized by the porosity of the porous medium and the saturation of the wetting phase. The mass and momentum balance equations are constructed according to the generalized mixture theory. Constitutive relations for the stress, pore pressure are derived from the free energy function. A capillary pressure relaxation model characterizing one attenuation mechanism of the two-fluid saturated porous medium is introduced under the constraint of the entropy inequality. In order to describe the momentum interaction between the fluids and the solid, a frequency independent drag force model is introduced. The details of parameter estimation are discussed in the paper. It is demonstrated that all the material parameters in our model can be calculated by the phenomenological parameters, which are measurable. The equations of motion in the frequency domain are obtained in terms of the Fourier transformation. In terms of the equations of motion in the frequency domain, the wave velocities and the attenuations for three P waves and one S wave are calculated. The influences of the capillary pressure relaxation coefficient and the saturation of the wetting phase on the velocities and attenuation coefficients for the four wave modes are discussed in the numerical examples.     

两相混合流体的非线性本构理论

本文从连续介质力学的基本原理出发,建立了微极流体与经典流体两相流动的非线性扩散理论。给出了混合流体本构方程的一般形式。对单相流体、单相微极流体及稀悬浮体三种特殊情形,得到了具体形式的二阶非线性本构方程,并同已有的理论进行了比较。     

新型混凝土桥面铺装材料的冲击力学性能

利用大尺寸Hopkinson压杆对新型混凝土桥面铺装材料钢纤维增强聚合物改性混凝土（steel fiber reinforced and polymer modified concrete,SFRPMC）进行了冲击实验,并且在相同基准配合比下,与普通混凝土、钢纤维混凝土的冲击性能进行了对比。观察了不同打击速度下三种材料的破坏形态,得到了在不同应变率下的应力应变关系,比较了三种材料的应变率敏感性,最后从机理上分析了掺加钢纤维和聚合物对混凝土材料冲击力学性能的影响。结果表明,钢纤维增强聚合物改性混凝土材料具有良好的冲击韧性,是一种理想的混凝土桥面铺装材料。     

A numerical method to simulate turbulent cavitating flows

The objective of this paper is to develop a numerical method for simulating multiphase cavitating flows on unstructured grids. The multiphase medium is represented using a homogeneous mixture model that assumes thermal equilibrium between the liquid and vapor phases. We develop a predictor–corrector approach to solve the governing Navier–Stokes equations for the liquid/vapor mixture, together with the transport equation for the vapor mass fraction. While a non-dissipative and symmetric scheme is used in the predictor step, a novel characteristic-based filtering scheme with a second order TVD filter is developed for the corrector step to handle shocks and material discontinuities in non-ideal gases and mixtures. Additionally, a sensor based on vapor volume fraction is proposed to localize dissipation to the vicinity of discontinuities. The scheme is first validated for simple one dimensional canonical problems to verify its accuracy in predicting jump conditions across material discontinuities and shocks. It is then applied to two turbulent cavitating flow problems – over a hydrofoil using RANS and over a wedge using LES. Our results show that the simulations are in good agreement with experimental data for the above tested cases, and that the scheme can be successfully applied to both RANS and LES methodologies.     

The influence of different loads on the remodeling process of a bone and bioresorbable material mixture with voids

A model of a mixture of bone tissue and bioresorbable material with voids was used to numerically analyze the physiological balance between the processes of bone growth and resorption and artificial material resorption in a plate-like sample. The adopted model was derived from a theory for the behavior of porous solids in which the matrix material is linearly elastic and the interstices are void of material. The specimen—constituted by a region of bone living tissue and one of bioresorbable material—was acted by different in-plane loading conditions, namely pure bending and shear. Ranges of load magnitudes were identified within which physiological states become possible. Furthermore, the consequences of applying different loading conditions are examined at the end of the remodeling process. In particular, maximum value of bone and material mass densities, and extensions of the zones where bone is reconstructed were identified and compared in the two different load conditions. From the practical view point, during surgery planning and later rehabilitation, some choice of the following parameters is given: porosity of the graft, material characteristics of the graft, and adjustment of initial mixture tissue/bioresorbable material and later, during healing and remodeling, optimal loading conditions.