一类基于故障率估计的优化试验设计方法

以先验信息为基础,在给定的总试验量下,以系统故障率估计的均方误差最小为优化目标,给出解析算法和蒙特卡洛模拟两种计算方法,确定了不同环境下试验量的最优分配方案.最后,将该优化试验设计方法应用于某型号车载速射迫炮在三种温度环境下的优化试验设计.     

Stress-dependent Failure Surface of Granular Materials

Numerical computations of geotechnical problems will become increasingly important because of the growing complexity of geotechnical applications. Therefore, a well-founded prediction of stability statements requires appropriate models, which are able to realistically depict the stress-strain behaviour of non-cohesive-frictional granular materials. On several stress paths, drained triaxial compression experiments on compact dense sand specimen exhibited that the size of the failure surface is not independent from the hydrostatic pressure. The failure surface and, thus, the maximal shear stresses at a specific confining pressure σ₃^exp can be increased by a compression preload at a level higher than σ₃^exp. This means that granular materials own several failure surfaces in dependence of the hydrostatic pressure. Consequently, the failure criteria based on the assumption of a compression stress-path-independent single-failure surface cannot recover the newly detected plastic yielding behaviour of granular materials. An improved approach for modelling the plastic hardening and softening behaviour coupled with the new yield properties at the limit state will be presented. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Validation with Numerical Simulations of a Simplified Model of a Hybrid Rocket Motor

The hybrid propulsion offers some remarkable advantages like high safety and high specific impulse and thus it is considered a promising technology for the next generation launchers and space systems. The purpose of this work is to validate through numerical simulations a design tool for hybrid rocket motors (HRM). (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

复合材料破坏准则的唯象理论

本文研究了复合材料的二阶型和三阶型张量多项式破坏准则的几种主要理论。提出了用非线性优化方法来确定准则中的影响系数,并将优化结果与几种典型的复合材料实验数据及各种理论的计算结果进行了讨论和比较,结果表明系数优化的方法是有效的。     

Deep Drawing Simulations Based on Microstructural Data

For the optimization of process chains in sheet metal forming it is required to accurately describe each partial process of the chain, e.g. rolling, press hardening and deep drawing. The prediction of the thickness distribution and the residual stresses in the blank has to be of high reliability, since the subsequent behavior of the semi-finished product in the following subprocesses strongly depends on the process history. Therefore, high-quality simulations have to be carried out which incorporate real microstructural data [1,2,3]. In this contribution, the ferritic steel DC04 is analyzed. A finite strain crystal plasticity model is used, for the application of which micro pillar compression tests were carried out experimentally and numerically to identify the material parameters of DC04. For the validation of the model, a two-dimensional EBSD data set has been discretized by finite elements and subjected to homogeneous displacement boundary conditions describing a large strain uniaxial tensile test. The results have been compared to experimental measurements of the specimen after the tensile test. Furthermore, a deep drawing process is simulated, which is based on a two-scale Taylor-type model at the integration points of the finite elements. At each integration point, the initial texture data given by the aforementioned EBSD measurements is assigned to the model. By applying this method, we predict the earing profiles of differently textured sheet metals. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical Simulations of Arterial Walls Based on IVUS-Data

Atherosclerosis has become one of the most frequent causes of death in the last decades. So the analysis and improvement of corresponding medical treatments, e.g., balloon-angioplasty or stenting, are significant tasks in the fields of biomechanics and biomedical engineering. One important component of the simulation of arterial walls, especially of atherosclerotically degenerated ones, is the knowledge of their anatomical and physiological composition. The intravascular ultrasound and the interpretation by the Virtual Histology provide cross-sectional images of the diseased arterial walls where the different components are characterized by color-coding. The topic of this contribution is the 3D reconstruction and discretization of the artery using these color-coded images and the biplane angiography. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

基于POD方法随机两种群系统的数值解讨论

主要目的是将特征正交分解(Proper Orthogonal Decomposition,简记为POD)方法应用于随机两种群系统,Euler方法是研究和讨论随机两种群系统数值解最有效的一种方法,但是这种方法存在计算时间久,自由度大等缺点.因此考虑POD方法,使其成为一个具有较低维数和较高精度的有限元格式.并给出了简化的有限元解的误差分析,通过数值算例进一步验证了随机两种群系统的POD FE(Finite Element,简记为FE)方法是可行和有效的.     

湍流度对翼型绕流影响的数值模拟及与实验的对比

研究了NACA0012翼型在低Reynolds数不同湍流度下的空气动力学特性．既运用了有限谱法和QUICK格式相结合的数值计算方法,又通过实验手段,研究了湍流度对流场的影响,并得到了计算与实验相符的结果．计算结果说明了有限谱法的高精度及与其他格式结合后应用的灵活性．对翼型受力和流场的结果分析显示:低湍流度下无明显的失速特征,高湍流度下失速特征明显;湍流度还对边界层后的剪切层有很大影响,高湍流度的受力特征和流场结果与高Reynolds数下的情形相似．     

基于贝叶斯因素-时间序列模型的罪犯地理特征仿真研究

分析了可能影响犯罪地点的一些因素,并结合当前热点问题贝叶斯概率模型建立了贝叶斯因素分析模型.通过实例分析发现这种模型比较适合于较少案例情况下,当案件数小于五种时也有一定的预测效果.之后又建立了一种时间序列分析模型,这种模型可以对两个住所的情况和罪犯改变其犯罪策略的情况进行分析,实验发现当案例数较多时,这种模型表现出了很高的准确性.用蒙特卡洛方法生成的数据来测试模型的相关准确性、效率、可信度等数据,并探讨每个模型的优缺点为综合性地运用与实践提供前瞻性的理论依据.     

爆炸和冲击载荷下金属材料及结构的动态失效仿真

通过数值模拟研究爆炸冲击载荷下金属材料和结构的动态失效规律,对表征爆炸冲击毁伤效应及设计新型抗冲击结构有重要意义.强动载下金属材料失效涉及材料大变形、热力耦合、材料状态变化等多个复杂物理过程,给数值仿真带来了极大挑战,其中包括裂纹、剪切带等复杂失效模式的几何描述、动态失效准则的确定、塑性与损伤耦合演化的描述等问题.针对这些挑战性问题,基于能量变分建立描述金属动态失效过程的热弹塑性相场理论和计算模型,实现了断裂与剪切带失效模式的统一描述,并提出了其显式有限元高效求解策略.进一步将该模型应用于爆炸冲击载荷下金属脆韧失效模式转变、绝热剪切带(ASBs)自组织及冲击波作用下薄壁圆盘失效形式转变三个典型金属动态失效问题,验证了理论模型的准确性及计算模型的稳健性.该工作为后续开展基于仿真的爆炸冲击毁伤评估及防护结构设计研究奠定了基础.     

Multi-scale modeling of beam-like structures: A new boundary condition concept for the RVE

In this work a coupled two-scale beam model using Timoshenko beam elements [1] with finite displacements on the macro scale and fully non-linear 3D brick elements on the micro scale is proposed. The calculation is carried out with the so-called FE² concept. To achieve the coupling between the beam and the brick elements, the algorithm from [2] is adapted. Within the degenerated concept of the Timoshenko beam, the introduction of a pure shear deformation leads to significant problems concerning the equilibrium condition on the micro scale. Applying this deformation mode on the RVE with periodic boundary conditions results in a rigid body rotation. Using linear displacement boundary conditions instead, the wrapping deformation is suppressed on the boundary, leading to a length dependency in the torsional deformation mode. In addition, the shear forces introduce a bending moment, which depends on the length of the RVE and adds spurious normal stresses and a length dependency of the shear stiffness. To overcome these problems, periodic boundary conditions are applied and the displacement assumptions are modified such that the shear deformation is achieved with force pairs on both ends of the RVE. The resulting model leads to length independent results in tension, bending and torsion and a domain which is able to produce a pure shear stress state. Consequently, only this domain of the model should be homogenized which can be accomplished by modifying the variations in the algorithm [2]. The concept is validated by simple linear and non-linear test problems. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Validation of Material Models in Grain Scale Simulation based on EBSD Experimental Data

A crystal plasticity model and a homogenization method are used to analyze the local and global mechanical behavior of a ferritic stainless steel. In the first step the material constants are determined based on tensile tests and used to simulate the local deformation behavior on the grain scale in the second step. For that 2D EBSD data are discretized by finite elements. The computed local grain reorientations of three different BCC slip systems are compared to experimental data at the state of 20% elongation. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Validation of Computational Models Based on Multiple Heterogeneous Knowledge Sources

Theories of organizations have brought together multipleheterogeneous theories in computational models. In addition, inartificial intelligence, there has been an emphasis on the generationof knowledge-based systems that include multiple heterogeneousknowledge bases. As a result, increasingly, theory and modeldevelopers have called for the need to validate these computationalmodels. Unfortunately, there has been only limited attention givento validation of multiple knowledge source programs. The primary focus of this paper is on the identification of conflict between multiple knowledge bases. The existence of conflict is particularly critical in those situations where database evaluations are "averaged". For example, what would it mean to average the assessments of supply and demand economists, or surgeons and chemotherapists? Correlational statistics are used to identify conflict situations. In addition, a new approach, referred to as cutpoints, is developed to determine if probability distributions of multiple agents are in conflict. A case study is used to illustrate the problems of combining expertise in multiple agent systems and to demonstrate the approach.     

Least Squares Model Averaging Based on Generalized Cross Validation

Acta Mathematicae Applicatae Sinica, English Series - Frequentist model averaging has received much attention from econometricians and statisticians in recent years. A key problem with frequentist...     

Geometry-Sensitive Ensemble Mean Based on Wasserstein Barycenters: Proof-of-Concept on Cloud Simulations

An ensemble of forecasts generated by different model simulations provides rich information for meteorologists about impending weather such as precipitating clouds. One major form of forecasts presents cloud images created by multiple ensemble members. Common features identified from these images are often used as the consensus prediction of the entire ensemble, while the variation among the images indicates forecast uncertainty. However, the large number of images and the possibly tremendous extent of dissimilarity between them pose cognitive challenges for decision making. In this article, we develop novel methods for summarizing an ensemble of forecasts represented by cloud images and call them collectively the Geometry-Sensitive Ensemble Mean (GEM) toolkit. Conventional pixel-wise or feature-based averaging either loses interesting geometry information or focuses narrowly on some pre-chosen characteristics of the clouds to be forecasted. In GEM, we represent a cloud simulation by a Gaussian mixture model, which captures cloud shapes effectively without making special assumptions. Furthermore, using a state-of-the-art optimization algorithm, we compute the Wasserstein barycenter for a set of distributional entities, which can be considered as the consensus mean or centroid under the Wasserstein metric. Experimental results on two sets of ensemble simulated images are provided. Supplemental materials for the article are available online.     

随机固定资产模型有限元降维格式的数值解

将特征正交分解(Proper Orthogonal Decomposition,简记为POD)方法应用于固定资产模型,简化其为一个具有较低维数和较高精度的有限元格式,并给出简化的有限元解的误差分析.数值例子表明在简化格式解和通常格式解之间的误差足够小的情况下,简化格式能大大的降低维数,提高计算速度和计算精度,从而验证固定资产模型的简化格式是可行和有效的.     

混凝土轴拉加卸载随机损伤模型的建立与试验验证

为了分析及深入探讨混凝土在受拉加载及卸载情况下的力学特性,基于随机损伤本构关系提出了一种混凝土轴拉加卸载模型,推导出了混凝土加卸载的应力 应变关系表达式.为了印证理论表达式,进行了混凝土轴向拉伸及加卸载的试验研究,测得了混凝土的材料参数及其相应的轴拉加卸载应力-应变曲线.结合模型的计算结果,对混凝土的轴拉加卸载试验结果进行了对比分析,结果表明:混凝土轴拉加卸载模型能够预测混凝土的极限强度,同时能描述混凝土的强度软化、加载过程中的弹模折减及卸载后的塑性变形.     

Simulations of thixotropic liquids

In thixotropic liquids the rheological properties depend on the liquid’s deformation history. Clay suspensions (as encountered in oil sands mining) are a prominent example. Activated clay particles form a network. As a consequence of (ionic) transport limitations, the network is not an instantaneous feature; it takes time to build up, and also to break down, the latter as a result of deformation in the liquid. In this paper a procedure for detailed simulations of flow of viscous thixotropic liquids is outlined. The local viscosity of the liquid relates to the level of integrity of the network. The time-dependence of the liquid’s rheology is due to the finite rate with which the network in the liquid builds up or breaks down. This concept has been incorporated in a lattice-Boltzmann discretization of the flow equations. With this methodology the way thixotropic liquids are mobilized in mixing tanks and the drag force experienced by dense assemblies of coarse particles (e.g. sand) in thixotropic liquids have been studied.