基于概率配点法的岩土材料参数随机场及其响应分析

概率配点法是进行不确定性问题分析的一种有效方法。通过对输入参数场进行Karhunen-Loeve展开,将其表示为一系列独立随机变量在不同权重下的线性组合,再以与之相同的随机变量组合形成混沌多项式展开对输出随机场进行分解,通过某种算法选取随机变量的值,将其作为插值点的组合(配点),在这些配点上,概率方程演化为一个确定性问题方程。由此,可以直接利用现有软件或者确定性问题计算程序进行求解,生成混沌多项式的系数矩阵后,即可得到该随机问题的各阶统计矩,从而实现参数随机场的不确定性分析。本文将该方法引进岩土工程材料参数随机场,将体积模量视为输入随机场,位移视为输出场,分别进行了弹性及塑性变形计算。结果表明该方法极大地降低了随机问题的求解难度,与MC法(Mento Carlo)相比,减少了运算消耗,提高了计算效率,具有明显的优越性。     

Fracture toughness from the standpoint of softening hyperelasticity

Fracture toughness of brittle materials is calibrated in experiments where a sample with a preexisting crack/notch is loaded up to a critical point of the onset of static instability. Experiments with ceramics, for example, exhibit a pronounced dependence of the toughness on the sharpness of the crack/notch: the sharper is the crack the lower is the toughness. These experimental results are not entirely compatible with the original Griffith theory of brittle fracture where the crack sharpness is of minor importance.¹To explain the experimental observations qualitatively we simulate tension of a thin plate with a small crack of a finite and varying sharpness. In simulations, we introduce the average bond energy as a limiter for the stored energy of the Hookean solid. The energy limiter induces softening, indicating material failure. Thus, elasticity with softening allows capturing the critical point of the onset of static instability of the cracked plate, which corresponds to the onset of the failure propagation at the tip of the crack. In numerical simulations we find, in agreement with experiments, that the magnitude of the fracture toughness cannot be determined uniquely because it depends on the sharpness of the crack: the sharper is the crack, the lower is the toughness.Based on the obtained results, we argue that a stable magnitude of the toughness of brittle materials can only be reached when a characteristic size of the crack tip is comparable with a characteristic length of the material microstructure, e.g. grain size, atomic distance, etc. In other words, the toughness can be calibrated only under conditions where the hypothesis of continuum fails.     

A simple framework for full-network hyperelasticity and anisotropic damage

A formulation of a constitutive behaviour law is proposed for hyperelastic materials, such that damage induced anisotropy can be accounted for continuously. The full-network approach with directional damage is adopted as a starting point. The full-network law with elementary strain energy density based on the inverse Langevin is chosen as a reference law which is cast into the proposed framework. This continuum formalism is then rewritten using spherical harmonics to capture damage directionality. The proposed formalism allows for an efficient (and systematic) expansion of complex non-linear anisotropic constitutive laws. A low order truncated expression of the resulting behaviour is shown to reproduce accurately the stress-strain curves of the exact behaviour laws.     

Antiplane elastic wave cloaking using metamaterials,homogenization and hyperelasticity

We consider the problem of how to cloak objects from antiplane elastic waves using two alternative techniques. The first is the use of a layered metamaterial in the spirit of the work of Torrent and Sanchez-Dehesa (2008) who considered acoustic cloaks, motivated by homogenization theories, whilst the second is the use of a hyperelastic cloak in the spirit of the work of Parnell et al. (2012). We extend the hyperelastic cloaking theory to the case of a Mooney–Rivlin material since this is often considered to be a more realistic constitutive model of rubber-like media than the neo-Hookean case studied by Parnell et al. (2012), certainly at the deformations required to produce a significant cloaking effect. Although not perfect, the Mooney–Rivlin material appears to be a reasonable hyperelastic cloak. This is clearly encouraging for applications. We quantify the effectiveness of the various cloaks considered by plotting the scattering cross section as a function of frequency, noting that this would be zero for a perfect cloak.     

A stochastic model for the temporal aspects of flow intermittency in micropillar compression

Systematic experimental investigations have demonstrated that the plastic deformation of micropillar proceeds through a sequence of intermittent bursts, the sizes of which follow power-law statistics. In this study, a stochastic model based on the power-law distribution of burst size is formulated in the framework of crystal plasticity in order to investigate the temporal aspects of flow intermittency in micropillar compression. A Monte Carlo simulation scheme is developed to determine the burst size when a burst activity is captured. This burst size is considered as the displacement boundary condition of burst deformation. Three-dimensional finite element analysis of the model is performed and its predictions are validated by comparison with results from both micro-compression experiments and simulation tests of bulk crystals using the classic crystal plasticity finite element method (CPFEM). The model provides a reasonable prediction of stress–strain responses both at the macroscopic and microscopic scales. Finally, the capability of this model is shown with applications to the intermittent plastic deformation in micropillar compressions, in particular for their burst time durations and burst velocities. The results from such stochastic finite element analysis are shown to be consistent with earlier experimental findings and results of mean-field theory.     

On the structure of the group g1 appearing in hyperelasticity

In the representation theorem for the stored energy function of a hyperelastic material, it is necessary to determine a subgroup g^*₁ of the symmetry group g of the stress tensor. It is the purpose of this note to study the construction of g^*₁ and its relation to the other subgroups ot g, viz. the commutator subgroup g_c and the orthogonal subgroup g₀.     

An illustration of the equivalence of the loss of ellipticity conditions in spatial and material settings of hyperelasticity

The loss of ellipticity indicated through the rank-one-convexity condition is elaborated for the spatial and material motion problem of continuum mechanics. While the spatial motion problem is characterized through the classical equilibrium equations parametrised in terms of the deformation gradient, the material motion problem is driven by the inverse deformation gradient. As such, it deals with material forces of configurational mechanics that are energetically conjugated to variations of material placements at fixed spatial points. The duality between the two problems is highlighted in terms of balance laws, linearizations including the consistent tangent operators, and the acoustic tensors. Issues of rank-one-convexity are discussed in both settings. In particular, it is demonstrated that if the rank-one-convexity condition is violated, the loss of well-posedness of the governing equations occurs simultaneously in the spatial and in the material motion context. Thus, the material motion problem, i.e. the configurational force balance, does not lead to additional requirements to ensure ellipticity. This duality of the spatial and the material motion approach is illustrated for the hyperelastic case in general and exemplified analytically and numerically for a hyperelastic material of Neo-Hookean type. Special emphasis is dedicated to the geometrical representation of the ellipticity condition in both settings.     

Probabilistic norm of operators and resonance theorems

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Stability of stochastic Leipholz column with stochastic loading

Summary The Leipholz column which is having the Young modulus and mass per unit length as stochastic processes and also the distributed tangential follower load behaving stochastically is considered. The non self-adjoint differential equation and boundary conditions are considered to have random field coefficients. The standard perturbation method is employed. The non self-adjoint operators are used within the regularity domain. Full covariance structure of the free vibration eigenvalues and critical loads is derived in terms of second order properties of input random fields characterizing the system parameter fluctuations. The mean value of critical load is calculated using the averaged problem and the corresponding eigenvalue statistics are sought. Through the frequency equation a transformation is done to yield load parameter statistics. A numerical study incorporating commonly observed correlation models is reported which illustrates the full potentials of the derived expressions.

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Stabilität eines Leipholz-Stabes mit stochastischen Eigenschaften und Belastungen

Übersicht Behandelt wird der Leipholz-Stab, dessen Elastizitätsmodul, Masseverteilung und tangential folgende Streckenlast stochastisch sind. Die nicht selbstadjungierte Differentialgleichung und die Randbedingungen werden als solche mit Zufallskoeffizienten betrachtet und die übliche Störungsmethode benutzt. Im Regularitätsbereich werden die nicht selbstadjungierten Operatoren benutzt. Hergeleitet wird die vollständige Kovarianz-Struktur der Eigenwerte der freien Schwingung und kritischen Lasten als Funktionen der Eigenschaften zweiter Ordnung der zufälligen Eingangsgrößen, die die Schwankungen der Systemparameter charakterisieren. Der Mittelwert der kritischen Last wird aus dem gemittelten Problem berechnet und die zugehörige Eigenwert-Statistik wird gesucht. Über die Frequenzgleichung wird eine Transformation vorgenommen, um die Statistik des Lastparameters zu erhalten. Eine numerische Studie illustriert die Leistungsfähigkeit der hergeleiteten Ausdrücke.

     

Rupture of rubber

Summary The theory of the tearing of rubber has been further confirmed by experiments on three test pieces of widely different shapes, for each of which the tearing energy can be calculated. This rupture concept has been used to interpret measurements of out growth under conditions of both static and dynamic loading.

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Zusammenfassung Die Theorie des Reißens von Gummi wird durch Versuche an drei Proben von wesentlich verschiedener Gestalt, für die jeweils eine Reiß-Energie berechnet werden kann, weiter erhärtet. Diese Vorstellung über den Bruchvorgang wird dazu verwendet, Meßergebnisse bezüglich des Einschnittwachstums (Rißvergrößerung) sowohl bei statischer als auch bei dynamischer Versuchsführung zu interpretieren.

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Presented at a meeting on Flow, Fatigue, and Failure held in Leeds on January 8th and 9th, 1959, by the British Society of Rheology.     

Stiffness and damping coefficients of rubber

Summary An analytical-experimental study has been carried out for the estimation of the stiffness and damping coefficients of rubber. A given specimen of a rubber pad (Neoprene GN) in the shape of a solid circular cylinder is subjected to a vibrating force created by an electrodynamic shaker together with a vibration exciter.In the analytical investigation, a mathematical model is developed from which the stiffness and damping coefficients for rubber as well as the damped energy can be evaluated. From the stimulated mathematical model together with the experimental data, the stiffness and damping coefficients could be estimated. Subsequently, the solution for the heat conduction equation describing the temperature field in the rubber speciemen can be formulated. The analytical behaviour of the temperature is found to agree fairly well with the measured temperatures at various locations inside the rubber specimen. This renders the simulating model to be significant with potential for further extension.

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Steiügkeits- und Dämpfungskoeffizienten von Gummi

Übersicht In einer analytisch-experimentellen Arbeit wird das Verhalten einer zylindrischen Probe aus Gummi unter pulsierender Belastung untersucht. Mit Hilfe eines mathematischen Modells und experimenteller Daten können die Steifigkeits- und Dämpfungsparameter von Gummi bestimmt werden. Anschließend wird die Lösung der Wärmeleitungsgleichung für das Temperaturfeld in der Probe angegeben. Die analytische Lösung stimmt mit den Meßwerten gut überein.

     

Probabilistic interval reliability of structural systems

The probabilistic reliability approach is the most widely used method for reliability analysis. The recent research shows that the reliabilities of structural systems strongly depend on the parameters of the probability model. It is possible that the little error in the estimation of the parameters may lead to the remarkable error of the resulting probability. In this study, we introduce the interval approach into the conventional reliability theory. We present a novel approach which allows us to obtain the system failure probability interval from the statistical parameter intervals of the basic variables. This approach is a combination of the two techniques, namely the classical reliability theory and the interval analysis. In the end of this paper, we show the feasibility of the proposed approach through two examples of the truss systems.     

应变疲劳强度概率分布

利用概率论方法建立了应变疲劳强度概率分布与应变疲劳寿命概率分布之间的数学关系式，并根据应变－寿命转换关系，由已知的给定应变水平下的疲劳寿命概率分布求出给定寿命下的应变疲劳强度分布函数，便于进行可靠性分析。     

随机参数超空泡射弹弹道随机特性分析

超空泡射弹的发射装置及射弹在制造,安装过程中存在各种不确定性.本文采用最大熵法来分析这些不确定性对射弹弹道的影响.介绍了超空泡射弹的动力学模型,对射弹的受力特性和随机参数进行了分析.根据给出的理论编制了基于最大熵法的随机参数超空泡射弹弹道随机特性分析的仿真程序,并对一个模型弹仿真计算.计算表明,与发射装置有关的随机参数...     

Applied mechanics of rubber

International Applied Mechanics -