Identification of Material Damage Model Parameters: an Inverse Approach Using Digital Image Processing

A reliable prediction of ductile failure in metals is still a wide-open matter of research. Several models are available in the literature, ranging from empirical criteria, porosity-based models and continuum damage mechanics (CDM). One major issue is the accurate identification of parameters which describe material behavior. For some damage models, parameter identification is more or less straightforward, being possible to perform experiments for their evaluation. For the others, direct calibration from laboratory tests is not possible, so that the approach of inverse methods is required for a proper identification. In material model calibration, the inverse approach consists in a non-linear iterative fitting of a parameter-dependent load–displacement curve (coming from a FEM simulation) on the experimental specimen response. The test is usually a tensile test on a round-notched cylindrical bar. The present paper shows a novel inverse procedure aimed to estimate the material parameters of the Gurson–Tvergaard–Needleman (GTN) porosity-based plastic damage model by means of experimental data collected using image analysis. The use of digital image processing allows to substitute the load–displacement curve with other global quantities resulting from the measuring of specimen profile during loading. The advantage of this analysis is that more data are available for calibration thus allowing a greater level of confidence and accuracy in model parameter evaluation.     

Relationship Between the Microstructure/Pore Structure of Oil-Well Cement and Hydrostatic Pressure

In cementing operations, hydrostatic pressure reduction in cement slurries is a serious threat to operation safety and cementing quality. This study combines X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, low-field nuclear magnetic resonance, and nano-computed tomography to investigate the mechanism of hydrostatic pressure reduction in cement slurries. The experimental results reveal that hydrostatic pressure transmission in a fresh cement slurry follows Pascal’s law. However, in the slurry, some cement particles undergo sedimentation with an increase in the hydration time, which reduces a part of the hydrostatic pressure. Further, another part of the hydrostatic pressure, which is maintained by the free water in the slurry pores, is reduced during the hardening stage. When the hydration reaction of the slurry is accelerated, the pore water in the slurry is supersaturated and hydration products start to rapidly nucleate and grow between the cement particles. These hydration products are porous gel structures and can change the pore structure of the cement slurry; a macro-pore is divided into many micropores, such as capillary pores and gel pores. Because these pores are filled by water in the slurry, during this process, free water in the macro-pores is changed to capillary water and gel water. However, gel water and capillary water cannot transmit hydrostatic pressure in the cement slurry. Meanwhile, in the fresh cement slurry, many pores containing free water are connected and some hydration products rapidly grow in the macro-pores and fill them, which may reduce the column height of the free water in the pores and lead to hydrostatic pressure reduction in the slurry.     

基于支持向量机的结构损伤分步识别研究

支持向量机是一种基于统计学习理论的机器学习算法,能够较好地解决小样本的学习问题。本文介绍了支持向量机分类和回归算法,提出了基于支持向量机的结构损伤分步识别方法:以模态频率作为损伤特征,首先根据支持向量机分类算法的概率估计确定可能的损伤位置,重新构造训练样本,然后利用支持向量机回归算法计算损伤位置;最后估计损伤程度。以梁的损伤识别为例进行了验证,结果表明该方法可以提高损伤识别的精度。     

以u—U形式方程为基础的结构—饱和土壤相互作用的固结有限元分析

提出用u-U形式方程对饱和土固结问题进行分析,对于结构则采取多重子结构技术实现向饱和土交界面上的凝聚,从而建立了结构与饱和土相互作用的固结控制方程。文中给出了方程的求解方案。文末给出了数值算例。本文的工作已在DIASS程序系统中实现。     

Interaction of a Traveling Pressure Perturbation with a Boundary Layer

The interaction between a traveling pressure perturbation and a laminar compressible boundary layer is investigated for a perturbation level higher than that needed to initiate steady-state flow separation. It is found that if the velocity of the pressure perturbation is fairly high the flow may remain unseparated and its direction of motion determines the nature of the perturbation propagation in the boundary layer. It is shown that even in the linear approximation the perturbations are mainly induced by the linear wall layer and not by the critical layer, which always remains nonlinear. It is also found that in the unsteady case shortwave perturbation oscillations are damped with time while the longwave ones grow and that the growth of the perturbations with time amplifies their damping along the streamwise coordinate while damping reduces it.     

以DIASS程序为基础的饱和土与防护结构相互作用动力分析研究

介绍了新开发的防护结构动力有限元分析程序DIASS系统。该系统装有空爆冲击波荷载数值自动生成器,并可将基础土壤模拟为饱和多孔介质模型,采取多重子结构技术进行结构与土体相互作用的动力分析。文中利用该程序系统进行数值试验,给出若干初步结论。     

Identification of Mechanical Material Behavior Through Inverse Modeling and DIC

Inverse methods offer a powerful tool for the identification of the elasto-plastic material parameters. One of the advantages with respect to classical material testing is the fact that those inverse methods are able to deal with heterogeneous deformation fields. The basic principle of the inverse method that is presented in this paper, is the comparison between experimentally measured strain fields and those computed by the finite element (FE) method. The unknown material parameters in the FE model are iteratively tuned so as to match the experimentally measured and the numerically computed strain fields as closely as possible. This paper describes the application of an inverse method for the identification of the hardening behavior and the yield locus of DC06 steel, based on a biaxial tensile test on a perforated cruciform specimen. The hardening behavior is described by a Swift type hardening law and the yield locus is modeled with a Hill 1948 yield surface.     

Interaction of Spontaneous Vortex Structure with a Flame Front

Spontaneous vortex formation in combustion products near a flame front propagating in a combustible mixture down a vertical tube has been studied experimentally. It is shown that under certain conditions, free convection suppresses or stimulates vortex perturbations at the flame front. It is found that periodic changes in vortex intensity depend on oscillations of the flame front. This dependence is controlled by the rate of heat exchange among the flame front, combustion products, and pipe walls. The vortex flow structure depends on the shape of the closed trajectory along which the leading point of the flame moves in the coordinate system attached to the vortex.     

激波诱导气流与液幕、液柱相互作用的实验研究

利用激波管对激波诱导气流与液幕、液柱的相互作用进行了实验研究。通过比较发现,这种相互作用下的液体块变形破碎过程与以往对于液滴进行的研究结果很不相同。当激波与液幕相互作用时,阴影照片和直接照相都表明,液幕的变形破碎行为有很强的三维性,较之液滴的变形破坏机理更为复杂,并且在局部区域,初始时刻液幕破碎抛撒的速度相较激波诱导气流速度为快,本文应用一维变截面激波管理论对这一现象进行了理论分析。     

Poromechanical Modelling and Inverse Approach of Drying Tests on Weakly Permeable Porous Rocks

The present paper deals with the determination of permeability in partially saturated conditions for weakly permeable porous rocks such as argillites or deep clayey formations. The level of permeability can be obtained via the measurements of transient weight loss of a sample submitted to a decrease in relative humidity imposed by saline solution in a hermetic chamber. An identification method based on simplified uncoupled 1D-linear and 1D-non-linear modelling was presented in a previous paper (Giraud et al. Trans Porous Media 69(2):259–280, 2006). The present paper takes into account generalized mass transfer phenomena such as Darcean advective transport of liquid and gas mixtures and Fickean diffusive transport of the vapour specie inside a gas mixture. Poromechanical coupling as well as 3D effects due to the geometry and finite dimensions of the tested samples are also covered by this approach. The coupled THM finite element computer code Code_Aster is then used to model the forward problem. The parameter identification procedure is based upon the solution of an inverse problem. The Levenberg–Marquardt algorithm was used for the problem of minimization. Comparisons between previous simplified 1D modelling and 2D-axisymmetrical coupled modelling show that the former method efficiently provides the correct order of magnitude of the level of permeability or the equivalent storage coefficients. Due to the boundary condition, the real 2D-axisymmetrical geometry of the sample must not be neglected if we are to obtain accurate results.     

刃型位错和圆弧形刚性线夹杂的干涉效应

位错和夹杂的干涉效应对于理解材料的强化和韧化机理具有十分重要的意义。文中研究了晶体材料中刃型位错和多条共圆弧刚性线夹杂的干涉作用。利用Riemann—Schwarz反照原理和复势函数的奇性主部分析技术，得到了问题的一般解答；对于只含一条刚性线夹杂的情况，给出了复势函数的封闭形式解。由Peach-Koehler公式求出了作用在刃型位错上的位错力，并讨论了圆弧形刚性线夹杂对位错力的影响规律，发现弧形刚性线对刃型位错有很强的排斥作用。本文解答不但可作为格林函数获得任意分布位错的相应解答，而且可以用于研究刚性线夹杂和任意形状裂纹的干涉效应问题。     

孤立波与多孔介质结构物的非线性相互作用

基于精确至Ｏ（εμ＾２,μ＾４）的多孔介质无压渗流模型方程和均匀流体质波动的Ｂｏｕｓｓｉｎｅｓｑ方程,本文对孤立波与多孔介质结构物的相互作用了较系统的数值实验。控制方程采用基于有限差分方程离散,在时域上采用了预估－校正方法进行了时间积分。在求解演化方程的过程中,引入“内迭代”过程实现流体域和多孔介质交界面的连接条件。结果表明孤立波在多孔介质上的反射与在不可渗透的界面上的反射类似,形成反向的孤立波但     

自然超空泡与尾喷流相互作用的数值模拟

认识带尾喷流和自然超空泡的水下高速航行体流体动力特性并发展其预报与控制方法仍是水动力学领域极具挑战性的课题．本研究采用CFD方法对尾喷流和自然超空泡之间的相互作用进行了数值研究．针对发动机欠膨胀超音速喷流,采用现有实验结果验证了基于两方程湍流模型的二维轴对称流动数值模型的可靠性．尾喷流在空气和蒸汽环境中流动的数值计算结果表明,由于蒸汽环境中背压较低,欠膨胀尾喷流无法及时形成压缩波,使得蒸汽环境中尾喷流的过膨胀区和气相扩散区的体积比空气中大;尾喷流很难形成规则的激波格栅,波系结构相对简单．针对携尾喷流的平头航行体超空泡流状态的数值模拟结果表明,尾喷流注入超空泡后迅速充满航行体周围的空腔区域;尾喷流与超空泡尾迹区域形成的回射流相互作用最终导致超空泡断裂,断裂过程中伴随着燃气泡的下泄现象;受空泡壁面约束,尾喷流难以在狭窄的超空泡空腔内完全膨胀,尾喷流的激波波系结构有显著的变化：在喷嘴附近受到压缩,在远离喷嘴区域受到超空泡水汽掺混的破坏;空泡内压强基本维持在饱和蒸汽压附近,没有显著增加．     

Inverse Approach in Ordinary Differential Equations: Applications to Lagrangian and Hamiltonian Mechanics

This paper is on the so called inverse problem of ordinary differential equations, i.e. the problem of determining the differential system satisfying a set of given properties. More precisely we characterize under very general assumptions the ordinary differential equations in \(\mathbb {R}^N\) which have a given set of either \(M\) partial integrals, or \(M first integral, or \(M partial and first integrals. Moreover, for such systems we determine the necessary and sufficient conditions for the existence of \(N-1\) independent first integrals. We give two relevant applications of the solutions of these inverse problem to constrained Lagrangian and Hamiltonian systems respectively. Additionally we provide the general solution of the inverse problem in dynamics.     

爆炸载荷作用下地下圆形结构动态分析

应用有限元方法分析了在爆炸载荷作用下地下圆形结构与土介质的动力相互作用问题。结构与土介质均假定为弹塑性，并用Ｍｉｓｅｓ屈服准则描述，而且考虑其几何非线性的影响。文中使用了人为阻尼边界和模型几何尺寸限制条件，从而大大提高了计算精度。文中还对几种解进行了比较和讨论     

基于精细积分方法的多自由度结构动态荷载辨识研究

荷载的辨识问题研究在动力分析和应用中具有重要意义。本文利用Gauss三点积分公式对载荷辨识公式进行推导,结合精细时程积分法和线性载荷假设对载荷辨识问题进行时域分析。概念明确、算法清晰简单的直接算法由此提出。经仿真检验该方法具有有效性和实用性。     

六面体单元柔性桁架结构模型与逆模型的辨识

介绍了用自适应LMS滤波器离线辨识六面体单元柔性结构模型和逆模型的方法。文中论述了自适应滤波理论和自适应逆控制之间的关系，讨论了LMS算法，并以实验法测得的六面体单元柔性结构随机振动数据为基础对其模型和逆模型进行了辨识，为进一步开展该结构的自适应逆控制研究奠定了基础。     

Inverse Estimation of Cohesive Fracture Properties of Asphalt Mixtures Using an Optimization Approach

Tensile cracking in asphalt pavements due to vehicular and thermal loads has become an experimental and numerical research focus in the asphalt materials community. Previous studies have used the discrete element method (DEM) to study asphalt concrete fracture. These studies used trial-and-error to obtain local fracture properties such that the DEM models approximate the experimental load-crack mouth opening displacement response. In the current study, we identify the cohesive fracture properties of asphalt mixtures via a nonlinear optimization method. The method encompasses a comparative investigation of displacement fields obtained using both digital image correlation (DIC) and heterogeneous DEM fracture simulations. The proposed method is applied to two standard fracture test geometries: the single-edge notched beam test, SE(B), under three-point bending, and the disk-shaped compact tension test, DC(T). For each test, the Subset Splitting DIC algorithm is used to determine the displacement field in a predefined region near the notch tip. Then, a given number of DEM simulations are performed on the same specimen. The DEM is used to simulate the fracture of asphalt concrete with a linear softening cohesive contact model, where fracture-related properties (e.g., maximum tensile force and maximum crack opening) are varied within a predefined range. The difference between DIC and DEM displacement fields for each set of fracture parameters is then computed and converted to a continuous function via multivariate Lagrange interpolation. Finally, we use a Newton-like optimization technique to minimize Lagrange multinomials, yielding a set of fracture parameters that minimizes the difference between the DEM and DIC displacement fields. The optimized set of fracture parameters from this nonlinear optimization procedure led to DEM results which are consistent with the experimental results for both SE(B) and DC(T) geometries.