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作者： Rypl, Rostislav Brno University of Technology

Technické textilie hrají v současnosti velmi důležitou roli v materiálovém inženýrství. Používají se pro vyztužování nebo zvyšování houževnatosti ve vlá... 详细信息

Technické textilie hrají v současnosti velmi důležitou roli v materiálovém inženýrství. Používají se pro vyztužování nebo zvyšování houževnatosti ve vláknitých kompozitech, které nachází uplatnění v celé řadě průmyslových odvětví sahajících od aeronautiky až po stavební inženýrství. S rostoucí produkcí vláknitých materiálů roste také potřeba standardizovaných a reprodukovatelných metod řízení jakosti. Tato doktorská práce se zaměřuje na tahovou pevnost vysokomodulových vláknitých svazků z teoretického i praktického úhlu pohledu. V obou těchto aspektech jsou představeny nové přístupy. Co se týče teoretické pevnosti vláknitých svazků, je v této práci formulován model délkově závislé tahové pevnosti, který rozlišuje tři asymptotická chování pevnosti v závislosti na délce. Zdůrazněna je především problematika přechodu z módu svazku nezávislých paralelních vláken (kratší svazky) do módu řetězce nezávislých svazků (delší svazky). Ukazuje se, že tento přechod závisí na kotevní délce vláken ve svazku a je možné jej experimentálně identifikovat pomocí standardních tahových zkoušek svazků na různých délkách. V experimentální části práce se autor zabývá koncentrací napětí v uchycení svazků při tahové zkoušce. Vysokomodulové svazky s křehkými vlákny jsou v průběhu tahové zkoušky velmi náchylné ke koncentracím napětí, což často znemožňuje použití standardních metod pro jejich zkoušení. V rámci této práce byl vyvinut a vyroben adaptér uchycení pro existující zkušební stroj Statimat 4U firmy Textechno GmbH. Byla provedena série komparativních tahových zkoušek na vláknitých svazcích s vyvinutým adaptérem a standardními metodami a výsledky jsou v práci diskutovány. Tyto zkoušky jsou rovněž porovnány s~teoretickou pevností svazků predikovanou Danielsovým statistickým modelem pro svazky.

关键词： vláknité svazky pravděpodobnost a statistika vliv velikosti tahová zkouška vláknitých svazků fibrous yarns probability and statistics size-effect tesile test of fibrous yarns Text

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An exploration of how training set composition bias in machine learning affects identifying rare objects

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ASTRONOMY AND COMPUTING 2022年第0期40卷

作者： Lake, S. E. Tsai, C-W Chinese Acad Sci Natl Astron Observ Beijing 100012 Peoples R China

When training a machine learning classifier on data where one of the classes is intrinsically rare, the classifier will often assign too few sources to the rare class. To address this, it is common to up-weight the examples of the rare class to ensure it is not ignored. It is also a frequent practice to train on restricted data where the balance of source types is closer to equal for the same reason. Here we show that these practices can bias the model toward over-assigning sources to the rare class. We also explore how to detect when training data bias has had a statistically significant impact on the trained model's predictions, and how to reduce the bias's impact. While the magnitude of the impact of the techniques developed here will vary with the details of the application, for most cases it should be modest. They are, however, universally applicable to every time a machine learning classification model is used, making them analogous to Bessel's correction to the sample variance. (C) 2022 Elsevier B.V. All rights reserved.

关键词： Methods : data analysis Methods : statistical Machine learning probability and statistics

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Optimal Stopping Rules for Preventing Overloading of Multicomponent Systems

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MATERIALS 2023年第7期16卷 2817-2817页

作者： Grzybowski, Andrzej Z. Domanski, Zbigniew Derda, Tomasz Czestochowa Tech Univ Dept Math PL-42201 Czestochowa Poland

When random-strength components work as an interconnected parallel system, then its carrying capacity is random as well. In a case where such a multicomponent system is a subject of the stepwise-growing workload, some of its components fail and their loads are taken over by the ones that are intact. When the loading process is continued, the additional loads trigger consecutive failures that degrade the system, eventually leading to a complete failure. If the goal of the system is to carry as much load as possible, then the loading process should be continued, but no longer than until the loading capacity of the whole system is reached. On the other hand, with every additional load step, a failure of the system becomes more probable, as the carrying capacity is random and known solely through its probability distribution. In such cases, the decision on when to cease the loading process is not obvious. We introduce and analyse a minimal model of failure spreading in an array of progressively loaded pillars controlled by a decision-maker who stops the process when a required load is attained. We show how to construct an optimal stopping rule. Under some additional assumptions regarding the adopted loss function, it is argued that the optimal stopping rule is of the threshold type and it significantly depends on the shape of the load-step probability distribution.

关键词： array of pillars failure multicomponent system optimal stopping rule probability and statistics strength

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Resilience assessment of mobile emergency generator-assisted distribution networks: A stochastic geometry approach

Journal of Economy and Technology

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Journal of Economy and Technology 2023年 1卷 48-74页

作者： Chenhao Ren Rong-Peng Liu Wenqian Yin Qinfei Long Yunhe Hou Department of Electrical and Electronic Engineering The University of Hong Kong Hong Kong SAR China Department of Electrical and Computer Engineering McGill University Montreal QC H3A 0G4 Canada

Escalation of extreme weather events represents substantial threat to power system infrastructure. Mobile emergency generators (MEGs) can form part of a flexible restoration strategy against such destructive events. However, with continued expansion of distribution networks, quantification of the impact of MEGs has become increasingly challenging owing to extreme-weather-event-induced uncertainties. In this paper, we propose a stochastic geometry-based method for assessing the impact of MEG deployment on distribution networks affected by extreme weather events through investigation of structural features. First, we propose a distance measure to represent the electrical connection between power grid components. Subsequently, we adopt the point process and Voronoi tessellation to describe the spatial distribution of power grid components and the service coverage provided by MEGs under different scenarios. Then, we propose a set of assessment metrics to evaluate the survivability of power grid components and the resilience of the entire distribution network under extreme weather events. Finally, we derive accurate analytical expressions for the distance distribution and resilience metrics, such as coverage probability and load shedding, enabling us to explore the relationship between MEG deployment decisions, structural features, and power grid resilience. The proposed method enables analytic assessment of the impact of MEG deployment on the resilience of distribution networks, and provides beneficial insights to help formulate efficient measures for enhancing resilience. Case studies demonstrated that the proposed method is accurate and efficient in dealing with network analysis and assessment problems for distribution networks under massive potential failure scenarios.

关键词： Power grid structure Stochastic geometry Poisson process Mobile emergency generator Resilience assessment Power system resilience probability and statistics

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Multiscale modeling of discontinuous dynamic recrystallization during hot working by coupling multilevel cellular automaton and finite element method

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INTERNATIONAL JOURNAL OF PLASTICITY 2021年 145卷 103064-103064页

作者： Chen, Fei Zhu, Huajia Chen, Wen Ou, Hengan Cui, Zhenshan Shanghai Jiao Tong Univ Natl Engn Res Ctr Die & Mold CAD 1954 Huashan Rd Shanghai 200030 Peoples R China UCL Dept Mech Engn Torrington Pl London WC1E 7JE England Altair Engn Inc Shanghai 2803 Capital Sq268 Hengtong Rd Shanghai 200070 Peoples R China Univ Nottingham Dept Mech Mat & Mfg Engn Nottingham NG7 2RD England

Discontinuous dynamic recrystallization (dDRX) is considered an effective way to obtain fine grain microstructures during hot working of materials with low-to-medium stacking fault energy (SFE). However, to date, investigation and modeling of dDRX in complex hot working processes are not appropriately performed, which hinders further control of the microstructure and forming quality of products during hot working. In this study, a multiscale modeling framework, namely the MCAFE-dDRX model, was constructed by coupling the multilevel cellular automaton (MCA) and finite element (FE) method. The data acquired via the FE method was used as an input for MCA simulation by discretizing the increment in FE time to consider the deformation history of materials. Compared to previous studies where only the effects of constant strain rate and temperature on the deformation of materials are analysed, the MCAFE-dDRX model can evaluate the dDRX microstructure evolution at different Zener-Hollomon levels, which has been validated by hot extrusion in this study. The developed simulation framework facilitates the prediction of microstructure evolution during heterogeneous and non-isothermal deformation of materials.

关键词： Microstructures Thermomechanical processes Metallic material probability and statistics Multilevel cellular automaton

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Transient mechanics of slide-ring networks: A continuum model

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JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 2021年 146卷 104212-104212页

作者： Vernerey, Franck J. Lamont, Samuel Univ Colorado Dept Mech Engn Program Mat Sci & Engn Boulder CO 80309 USA

Slide ring gels, developed from molecularly threaded polymer chains, have been getting increased attention for their unique modes of relaxation and toughness in the past ten years. While the methods of synthesis have become more sophisticated and efficient, theory has not kept up with experiments, and there exist many unexplained observations. In this paper, we develop a continuum-level model for slide ring gels based on thermodynamically consistent equations. We begin at the level of a single chain and perform statistical averaging to capture the unique molecular dynamics shown by polyrotaxane, the backbone chain of the network. We discover a unique dependence of relaxation on both level of strain and speed of deformation. We explain these properties quantitatively by assigning internal state variables that represent chain reptation and relative degree of chain-stretch and solve for their evolution during deformation.

关键词： Constitutive behavior Polymeric material Viscoelastic material Energy methods probability and statistics Transient network theory

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Hierarchical scaling model for size effect on tensile strength of polycrystalline rock

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INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES 2023年第1期247卷

作者： Wang, Zhi-Yang Li, Ying-Wei Wu, Zhi-Jun Wuhan Univ Sch Civil Engn Wuhan 430072 Hubei Peoples R China

Here, we proposed an analytical model for quantifying the intriguing size-dependent tensile strength of polycrystalline rock, i.e. first ascending then descending with the increase of specimen size. The meso-structure of polycrystalline rock was firstly degraded to a hierarchical arrangement of regularly cuboid mineral grains. The tensile strength distribution of mineral grain interface (GI) is modeled by Weibull distribution, and the stress state near the failed GI is analytically calculated. Then a probabilistic analysis of the failure evolution in hierarchical mineral grain sets (sets of sets) was performed by considering the meso crack initiation at the GI to their propagation at macro scale, so that the tensile strength distribution and the meso crack accumulation in the rock specimen can be derived. The consistency between the model predictions and experimental results demonstrates the proposed model is valid. In addition, the results revel that the size effect trend, ascending or descending, is determined by the dispersion degree of structure strength distribution.

关键词： Rock Tensile strength Size effect Analytical model probability and statistics

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On Estimation of Functional Causal Models: General Results and Application to the Post-Nonlinear Causal Model

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ACM TRANSACTIONS ON INTELLIGENT SYSTEMS AND TECHNOLOGY 2016年第2期7卷 13-13页

作者： Zhang, Kun Wang, Zhikun Zhang, Jiji Schoelkopf, Bernhard Max Planck Inst Intelligent Syst D-72076 Tubingen Germany Lingnan Univ Dept Philosophy Hong Kong Hong Kong Peoples R China

Compared to constraint-based causal discovery, causal discovery based on functional causal models is able to identify the whole causal model under appropriate assumptions [Shimizu et al. 2006;Hoyer et al. 2009;Zhang and Hyvarinen 2009b]. Functional causal models represent the effect as a function of the direct causes together with an independent noise term. Examples include the linear non-Gaussian acyclic model (LiNGAM), nonlinear additive noisemodel, and post-nonlinear (PNL) model. Currently, there are two ways to estimate the parameters in the models: dependence minimization and maximum likelihood. In this article, we show that for any acyclic functional causal model, minimizing the mutual information between the hypothetical cause and the noise term is equivalent to maximizing the data likelihood with a flexible model for the distribution of the noise term. We then focus on estimation of the PNL causal model and propose to estimate it with the warped Gaussian process with the noise modeled by the mixture of Gaussians. As a Bayesian nonparametric approach, it outperforms the previous one based on mutual information minimization with nonlinear functions represented by multilayer perceptrons;we also show that unlike the ordinary regression, estimation results of the PNL causal model are sensitive to the assumption on the noise distribution. Experimental results on both synthetic and real data support our theoretical claims.

关键词： Systems and Information Theory Learning probability and statistics Causal discovery functional causal model post-nonlinear causal model statistical independence maximum likelihood

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Quantification of particle crushing in consideration of grading evolution of granular soils in biaxial shearing: A probability-based model

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INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS 2018年第3期42卷 488-515页

作者： Cheng, Zhuang Wang, Jianfeng City Univ Hong Kong Dept Architecture & Civil Engn B6409Acad 1Tat Chee Ave Kowloon Hong Kong Peoples R China

A probability-based model is presented to estimate particle crushing and the associated grading evolution in granular soils during isotropic compression and prepeak shearing in biaxial tests. The model is based on probability density functions of interparticle and intraparticle stress (ie, particle normalized maximum shear stress and particle average maximum shear stress) derived from discrete element method simulations of biaxial tests. We find that the probability density functions of normalized maximum shear stress are dependent on the current sample grading, implying coupling effects between particle crushing and sample grading such that the particle crushing is affected by the current sample grading, and the grading change is also dependent on the current particle crushing extent. To incorporate these coupling effects into the model, particle crushing and grading change are calculated for each load increment, in which the crushing probability of a particle during any loading increment is denoted as the corresponding increment of probability of the internal maximum shear stress exceeding its maximum shear strength. The model shows qualitative agreement with published experimental data. The effects of the model parameters, including initial porosity, particle strength, initial grading, and crushing mode, on the calculated results are discussed and compared with previous studies. Finally, the strengths and limitations of the model are discussed.

关键词： biaxial test coupling discrete element method grading evolution particle crushing probability and statistics

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Statistical distribution and size effect of residual strength of quasibrittle materials after a period of constant load

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JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS 2014年第0期64卷 440-454页

作者： Salviato, Marco Kirane, Kedar Bazant, Zdenek P. Northwestern Univ Dept Civil & Environm Engn Evanston IL 60208 USA Northwestern Univ Dept Mech Engn Evanston IL 60208 USA

In preceding studies, the type of cumulative probability distribution functions (cdf) of strength and of static lifetime of quasibrittle structures, including their tails, was mathematically derived from atomistic scale arguments based on nano-scale cracks propagating by many small, activation energy-controlled, random breaks of atomic bonds in the nanostructure. It was shown that a quasibrittle structure (of positive geometry) must be modeled by a finite (rather than infinite) weakest-link model, and that the cdf of structural strength as well as lifetime varies from nearly Gaussian to Weibullian as a function of structure size and shape. Excellent agreement with the observed distributions of structural strength and static lifetime was demonstrated. Based on the same theoretical framework, the present paper formulates the statistics of the residual structural strength, which is the strength after the structure has been subjected to sustained loading. A strength degradation equation is derived based on Evans' law for static crack growth during sustained loading. It is shown that the rate of strength degradation is not constant but continuously increasing. The cdf of residual strength of one RVE is shown to be closely approximated by a graft of Weibull and Gaussian (normal) distributions. In the left tail, the cdf is a three-parameter Weibull distribution consisting of the (n + 1)th power of the residual strength, where n is the exponent of the Evans law and the threshold is a function of the applied load and load duration. The finiteness of the threshold, which is typically very small, is a new feature of quasibrittle residual strength statistics, contrasting with the previously established absence of a threshold for strength and lifetime. Its cause is that there is a non-zero probability that some specimens fail during the static preloading, and thus are excluded from the statistics of the overload. The predictions of the theory are validated by available test data

关键词： Fracture Quasibrittle materials Structures Residual strength probability and statistics

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