一个用于低周疲劳寿命预测的损伤函数

选择循环塑性应变能作为损伤变量,建立了它的瞬态响应数字模型,采用损伤力学分析方法,导出了计及循环相关的非线性疲劳损伤函数,得到了用于低周疲劳寿命预测的数学公式。利用该损伤函数预测的低周疲劳寿命与试验结果符合较好。     

Modelización numérica del comportamiento estructural cíclico de barras esbeltas de acero con pandeo restringido

This work presents a numerical model of the cyclic structural behavior of dissipative buckling-restrained braces, commonly used as an alternative to classical concentric braces for seismic protection of building frames and other structures. Such devices are usually composed of a slender steel core embedded in a stockiest casing that is intended to prevent its buckling when it is under compression. The casing is made either of mortar or steel, and a sliding interface is interposed between the core and the casing to prevent excessive shear stress transfer. The behavior of the steel core is described by a damage and plasticity model; the behavior of the mortar casing is described by an isotropic damage model and the sliding behavior of the interface is described by a contact penalty model. These 3 models are implemented in the Abaqus software package following an explicit formulation. In a previous article (published in an earthquake engineering journal) the model was briefly described, its ability to reproduce the cyclical behavior of buckling-restrained braces was preliminarily pointed out and their results were satisfactorily compared with those of experimental tests. The aim of this paper is to describe the model thoroughly and to present new judgments about its usefulness.     

Microplane modeling of cyclic behavior of concrete: a gradient plasticity-damage formulation

A microplane model is developed to simulate the behavior of concrete under cyclic loading conditions. Pure damage mechanics or pure plasticity models yield satisfactory results for concrete under monotonic loading but cannot capture correctly the unloading and reloading response. Therefore, coupling damage and plasticity is necessary for accurate constitutive modeling of concrete. The microplane model offers a straightforward approach to simulate induced anisotropy by formulating the material laws on many randomly oriented planes. Distinguishing between compression and tension response using the proper plastic yield function and damage laws is considered. Furthermore, gradient enhancement is employed to handle the pathological mesh sensitivity related to strain softening. The new formulation is implemented within a 3D finite element code and a numerical example is simulated and compared to experiments in order to evaluate the capabilities of the model. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modelling of interfacial damage and delamination of sprayed coatings with cohesive elements

Delamination of thermally sprayed corrosion protection coatings as a consequence of thermo-mechanical fatigue is investigated. This study focusses on the modelling of interfacial damage initiation and evolution under cyclic loading with the help of a cohesive zone model. The presented model features a slight non-linearity at unloading from the exponential “damage locus” as well as cyclic damage accumulation restricted to (re)loading conditions. Additionally, an endurance limit is introduced indicating the maximum sustainable traction for an infinite number of load cycles. The capability of the model is demonstrated. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Meso-Macro Modelling of Coated Forming Tools under Thermal Shock Conditions

In hybrid-forming processes workpieces are heated up before forming in order to reduce the forming forces. They are innovative methods for the production of components with graded properties, particularly with regard to tailored material properties and geometrical shape. During service life the forming tools are subjected to cyclic thermal shock loading conditions which can result into damage and failure. For improvement of the tool durability in the hybrid-forming process coated forming tools with multilayered coating systems are considered to be applied in future. This contribution shows the actual state of work for the development of a twoscale FE model for the simulation of the multilayered coated forming tool. Within this model the three-dimensional model of the forming tool builds the macromodel. On the macrolevel the multilayered coating is discretized with one element over the coating thickness. The mesomodel of the coating considers the actual layer design with metallic and ceramic layers. The macro-meso transition is realized with a Taylor-assumption. As the microscale is not considered in our model, the constitutive equations are formulated on the mesoscale. The meso-macro transition is done using volume averaging procedures. Furthermore, a damage model is included for particular layers. The scalar damage variable is used in a thermo-mechanical coupled model for simulation of a reduced heat transfer through a partially damaged layer. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micromorphic homogenisation and its application to a model of ductile damage

Heuristic gradient extensions of classical damage models are established to overcome their spurious mesh dependency. In contrast, an extension of Gurson's model of ductile damage towards the microdilatational theory is presented through a straight-forward homogenization. FEM simulations prove the regularization capabilities of the model. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The generalized Masing models for deteriorating hysteresis and cyclic plasticity

The Masing model originally proposed for one-dimensional cyclically stabilized hysteretic behavior is generalized for deteriorating hysteresis and for cyclic plasticity, with system degradation and cyclic hardening effects taken into account. The generalization into the multi-dimensional case is based on the concept of a universal stress–strain curve and the associated effective stress and effective strain. Numerical simulations confirm the validity of the generalized models that are conceptually simple and parametrically parsimonious. The generalization method employed also provides a unifying way of extending 1-D hysteretic models to multi-dimensional constitutive models in cyclic plasticity.     

Cyclic and non-cyclic scheduling of 12 h shift nurses by network programming

In this paper, we present a mathematical model for cyclic and non-cyclic scheduling of 12 h shift nurses. The model exploits the fact that a nurse's schedule is made up of an alternating sequence of work-stretch and ‘off-stretch’ patterns. We introduce a concept called a stint, which is a pattern characterized by a start date, a length, a `cost' and the shifts worked. Using the stints as nodes in a network, we construct an acyclic graph on which the nurse's schedules can be defined. The resulting model is essentially a shortest-path problem with side constraints. The model is quite flexible and can accommodate a variety of constraints. With a minor modification, the network is used to define both the cyclic and non-cyclic scheduling problems. The models are illustrated on sample data from a local hospital and solved using CPLEX optimization software on an IBM RISC6000/340 workstation.     

Identification of Viscoelastic Properties and Damaging Effects of Highly Extensible Polyurea

The present work aims particular at the experimental identification of the viscoelastic properties of polyurea as well as on the onset of the damage. For the viscoelastic part, several relaxation experiments are performed. From the measured data a general viscoelastic model is derived where we use two different approaches. At first we identify a general Maxwell model (combining spring and damping elements for finite deformations) to use a prony series with N elements, which requires the identification of 2N + 1 parameters. At second, a model of generalized fractional elements [3] is employed. Both approaches are studied in detail and are compared to data from literature; furthermore a comparison concerning the effort is presented. Damaging effects of Polyurea are investigated using tensile tests with and without cyclic loading. In particular we focus on the the onset of damage by cavitation. To this end the recovered specimens were analyzed using a laser microscope; the surfaces of the ruptured areas are compared in terms of quantity and size of voids. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

反映混凝土单边效应的弹塑性损伤本构模型及应用

为了尽可能有效和准确地描述混凝土材料的非线性力学特性,在研究国内外混凝土损伤本构模型的基础上,基于连续介质损伤力学和不可逆热力学的理论框架,采用统一强度理论作为屈服破坏准则,分别定义拉、压双标量损伤来考虑材料的拉、压迥异特性,同时引入反向加载影响因子以修正拉压交替循环加载时材料的单边效应,以及多轴应力状态下拉、压损伤累积的相互影响,最终采用显式积分算法建立了一种改进的混凝土弹塑性损伤本构模型.不同素混凝土加载试验模拟结果初步验证了建议模型的有效性,而通过对含I型裂缝的混凝土简支梁试验进行数值分析,结果表明,所得的荷载 挠度曲线与试验结果吻合良好,进一步检验了模型应用于结构非线性分析的有效性.     

Deformation and Damage Analysis of a Hybrid-Forming Tool under Thermal Shock Conditions

In the hybrid–forming process for gradient structures [1] inhomogeneous cyclic thermo–mechanical stresses and strains lead to higher risks of failure of the forming tool. The main topic of this paper is the validation of finite element calculations for a tool–like specimen under complex thermo–mechanical loadings in order to predict the material behaviour [3]. To this end thermal shock experiments of tool–like specimens are performed. Optical measuring systems are used for three–dimensional digitalisation of the specimens to get a sufficient amount of data. Results of experimental optical measurings and results of finite element calculations are compared. Additionally, damage analysis using the eddy current method is performed to characterize the surface state of the cyclically thermal shocked specimens. This damage analysis provides data for lifetime prediction models under thermal shock conditions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

ECONOMICALLY OPTIMAL MARINE RESERVES WITHOUT SPATIAL HETEROGENEITY IN A SIMPLE TWO‐PATCH MODEL

Bioeconomic analyses of spatial fishery models have established that marine reserves can be economically optimal (i.e., maximize sustainable profit) when there is some type of spatial heterogeneity in the system. Analyses of spatially continuous models and models with more than two discrete patches have also demonstrated that marine reserves can be economically optimal even when the system is spatially homogeneous. In this note we analyze a spatially homogeneous two‐patch model and show that marine reserves can be economically optimal in this case as well. The model we study includes the possibility that fishing can damage habitat. In this model, marine reserves are necessary to maximize sustainable profit when dispersal between the patches is sufficiently high and habitat is especially vulnerable to damage.     

Multiscale Modeling for the Simulation of Damage Processes at Refractory Materials under Thermal Shock

A brittle damage model based on multiscale considerations and homogenisation procedures is presented. Cell models are developed as RVE including different microstructural features. The material laws themselves are formulated on the continuum level. Local failure occurs if the damage variable reaches a critical value. For simple configurations of the microstructure, the relation between stress, strain und temperature is derived from analytical considerations. In order to properly model the thermo-mechanical coupling, the temperature-dependence of material constants is taken into account. Fracture and damage mechanical approaches are combined using different techniques. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fatigue damage modeling of fibrous soft tissues

Ultimate tendon failure is often caused by fatigue loading. Recent interventions revealed a three-phase progression of histological changes during cyclic loading of the tendon. It starts from localized kinked fiber deformations, continues with additional fiber delaminations and finally leads to fiber angulations and discontinuities [5, 6]. In the present contribution, we propose a physically motivated constitutive model able to describe fatigue evolution in tendon subject to cyclic loading. The damage of the collagen fibers is elucidated by a successive permanent opening of tropocollagen molecules [7], which represent the basic building blocks of collagen fibrils. The fibril strain increase is triggered by a time-force depending rupture of glycosaminoglycan sidechains of adjacent collagen fibrils. The so obtained model is in line with recent experimental findings available in literature. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A three-dimensional progressive damage model for fiber reinforced composites with an implicit-explicit integration scheme

This contribution proposes a fully three dimensional “continuum damage model” (CDM) to describe the interlaminar and intralaminar failure mechanisms of transversely isotropic elastic-brittle materials under static loading. The constitutive model is derived from an energy function with independent damage variables for each damage mode. The evolution law is based on energy dissipation within the damage process, taking into account the critical energy release rate to weaken the effect of mesh dependent outcome. The onset of damage can be predicted with Cuntze's failure mode concept [1] as well as with Hashin's failure criteria. In this model linear stress decreasing is assumed. In addition, an implicit-explicit integration scheme, first proposed by Oliver [3] for isotropic damage models, is adapted to increase the stability and robustness of numerical simulations and to decrease the computational cost of material failure analyses. By comparing the results from implicit-explicit integration schemes and standard implicit integration schemes, a high level of agreement is found. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A constitutive model for thermoplastic materials subjected to high strain rates

Reliable prediction of the behaviour of structures made from polymers is a topic under considerable investigation in engineering practice. Especially, if the structure is subjected to dynamic loading, constitutive models considering the mechanical behaviour properly are not available in commercial finite element codes yet. A constitutive model is derived including important phenomena like necking, strain rate dependency, unloading behaviour and damage. In particular, different yield surfaces in compression and tension and strain rate dependent failure, the latter with damage induced erosion, is taken into account. With the present formulation, standard verification tests can be simulated successfully. Also, an elastic damage model can be used to approximate the unloading behaviour of thermoplastics adequately. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Determination of fatigue life in bending with rotation

The fracture and stress redistribution processes in a specimen stressed in cyclic bending with rotation are considered. The calculations are based on the assumption of linear damage accumulation and a linear dependence of the modulus of elasticity on the damage; the fatigue strength curve for cyclic tension-compression is employed. The calculated damage accumulation and crack penetration times are compared. Specific calculations are made for the case of PMMA fatigue under isothermal conditions.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 875–880, September–October, 1971.     

Evaluación sísmica de los edificios de mampostería típicos de Barcelona aplicando la metodología Risk-UE

In this work, a study about the vulnerability and seismic damage of unreinforced masonry buildings is carried out. Three models of buildings representatives of the Eixample district of Barcelona have been chosen. The seismic vulnerability is evaluated by means of the Risk-UE methodology. The seismic demand is described by elastic project spectrum, in this case, defined by the Eurocode 8. Fragility curves are obtained from a nonlinear analysis, considering the capacity spectra. Expected seismic damage is gotten with the damage probability matrices, which indicate the occurrence probability of a damage state for a seismic demand given. The analysis of the buildings has been performed by TreMuri program by means of a macroelements model, which represents of a whole masonry panel. The buildings, here, analyzed are real and detailed structural drawings and reports have been used to model them. The results shown a considerable vulnerability in this type of buildings, therefore, in spite of the seismic hazard the expected seismic risk is significant.