Pseudopotentials and Loading Surfaces for an Endochronic Plasticity Theory with Isotropic Damage

The endochronic theory, developed in the early 70s, allows the plastic behavior of materials to be represented by introducing the notion of intrinsic time. With different viewpoints, several authors discussed the relationship between this theory and the classical theory of plasticity. Two major differences are the presence of plastic strains during unloading phases and the absence of an elastic domain. Later, the endochronic plasticity theory was modified in order to introduce the effect of damage. In the present paper, a basic endochronic model with isotropic damage is formulated starting from the postulate of strain equivalence. Unlike the previous similar analyses, in this presentation the formal tools chosen to formulate the model are those of convex analysis, often used in classical plasticity: namely pseudopotentials, indicator functions, subdifferentials, etc. As a result, the notion of loading surface for an endochronic model of plasticity with damage is investigated and an insightful comparison with classical models is made possible. A damage pseudopotential definition allowing a very general damage evolution is given.     

Cyclic behaviour of partly plastic pinned circular tubes: II. Testing and verification of the model

This paper is a verification of an analytical model for the elastic-partly plastic behaviour of circular hollow steel struts subjected to static cyclic axial loading previously presented. The experimental procedure and apparatus are described. The results of the model are compared with 5 experimental observations for 4 cycles. The comparison shows a good agreement. The sensitivity of the analysis to variations of the model parameters is discussed.     

Application of Exponential-Based Methods in Integrating the Constitutive Equations with Multicomponent Nonlinear Kinematic Hardening

The von-Mises plasticity model, in the small strain regime, along with a class of multicomponent nonlinear kinematic hardening rules is considered. The material is assumed to be stabilized after several load cycles and therefore, isotropic hardening will not be accounted for. Application of exponential-based methods in integrating plasticity equations is provided, which is based on defining an augmented stress vector and using exponential maps to solve a system of quasi-linear differential equations. The solutions obtained by this new technique give very accurate updated stress values that are consistent with the yield surface. The classical forward Euler method is reformulated in details and applied to the multicomponent form of the nonlinear kinematic hardening in order to provide a comparison for the suggested technique. Moreover, a consistent tangent operator for the exponential-based integration strategy and also for the classical forward Euler algorithm is presented. In order to show the robustness and performance of the proposed formulation, an extensive numerical investigation is carried out.     

Modeling and Characterization of Grain Scale Strain Distribution in Polycrystalline Tantalum

A common sample geometry used to study shear localization is the "tophat": an axi-symmetric sample with an upper "hat" portion and a lower "brim" portion. The gage section lies between the hat and brim. The gage section length is on the order of 0.9 mm with deformation imposed through a Split-Hopkinson Pressure Bar system at maximum top-to-bottom velocity in the range of 10-25 m/sec. Detailed metallographic analysis has been performed on sections of the samples to quantify the topology and deformation state of the material after large deformation shear. These experiments performed with polycrystalline tantalum have been modeled using a multi-scale polycrystal plasticity approach. A Voronoi tessellation based microstructural model and a coupled thermo-mechanical elasto-viscoplastic crystal plasticity model was used. The crystal plasticity model allowed for slip to occur on the twelve {110}<111> and twelve {112}<111> slip systems. Three numerical models were produced using three different realizations of initial crystallographic texture distribution within the same morphological microstructure and the results presented. The detailed metallographic analysis of the deformed sample shear zone produced an estimate for the strain profile within that region and these results are compared directly to the three numerical simulation results. Although the models predict a stress response which is greater than that observed experimentally, the local strain response compares very well with the results of the metallographic analysis.     

Thermo-Mechanics and Microstructure Evolution in Manufacturing Simulations

Thermal stresses and deformations are present and important for many manufacturing processes. Their effect depends strongly on the material behavior. The finite element method has been applied successfully for manufacturing simulations. There are numerical challenges in some cases due to large deformations, strong non-linearities etc. However, the most challenging aspect is the modeling of the material behavior. This requires in many cases coupled constitutive and microstructure models.     

On yield criteria in plasticity coupled with damage

The coupling of elasto-plasticity and damage is still the subject of many papers, namely in the construction of evolution laws for internal parameters. In this paper we propose to partly overcome some difficulties by using the only basic fact that a plastic-damaged body is made up of two physical constituents, namely the matrix material and the micro-defects. The elasto-plastic matrix material is the resisting skeleton connecting the elements of the body and the micro-defects bring their own contributions to reversible and irreversible strains. Naturally strong couplings exist between the stress states of the body and the matrix material. These considerations will lead to generalisations and new formulations of the so-called equivalence principles and a new equivalence principle will be proposed. Finally, concerning the irreversible strains due to both plasticity of the matrix and growing of micro-defects, we will prove that yield conditions must be used simultaneously on the body as well as on the matrix, leading to some non-smooth resultant yield-damage surface. The modified Gurson model for porous material is then analysed in order to illustrate this last point. In this paper, large strains are considered, but time-dependant plasticity and thermal effects are excluded.     

屈曲约束支撑结构性能的数值模型

介绍一种耗能屈曲约束支撑循环结构性能的数值模型,该支撑常用来替代传统的轴向支撑作为框架结构和其他结构中的抗震构件。这些支撑通常由1根细长的插入短粗套管中的钢芯组成,套管是预先设置的,以防止其受压时屈曲。套管由砂浆或钢构成,在钢芯和套管间设置滑移面以阻止剪应力过度传递。钢芯的性能、砂浆套管的性能和界面的滑移性能分别通过塑性破坏模型、各向同性的破坏模型和罚函数模型来描述。按照给出的公式通过ABAQUS软件完成了这3个模型的建模。在许多典型但简单的工况下,该算法被证实可以模拟屈曲约束支撑的循环性能。模型模拟结果与试验结果吻合较好,证实了其正确性。对初步得到的结论进行讨论并指出进一步研究的方向。     

Ultimate Uplift Capacity of Multiplate Helical Type Anchors in Clay

In recent years, the use of helical anchors has expanded beyond their traditional use in the electrical power industry. The advantages of rapid installation and immediate loading capability have resulted in their being used in more traditional civil engineering infrastructure applications. Unfortunately, our current understanding of these anchors is unsatisfactory, and the underlying theoretical framework adopted by engineers has proven to be largely inappropriate and inadequate. A better understanding of helical anchor behavior will lead to increased confidence in design, a wider acceptance as a foundation alternative, and more economic and safer designs. The primary aim of this research is to use numerical modeling techniques to better understand multiplate circular anchor foundation behavior in clay soils. A practical design framework for multiplate anchor foundations will be established to replace existing semiempirical design methods that are inadequate and have been found to be excessively under- or overconservative. This framework can then be used by design engineers to confidently estimate the pullout capacity of multiplate anchors under tension loading.