Papers by Keyword: Anisotropy

Abstract: Nonwoven fabrics are web structures of randomly-oriented fibres, bonded by means of mechanical, thermal or chemical techniques. This paper focuses on nonwovens manufactured with polymer-based fibres and bonded thermally. During thermal bonding of such fibres, as a hot calender with an engraved pattern contacts the fibre web, bond spots are formed by melting of the polymer material. As a result of this bonding process, a pattern of bond points connected with randomly oriented polymer-based fibres form the nonwoven web. Due to their manufacturing-induced composite microstructure and random orientation of fibres, nonwovens demonstrate a complex mechanical behaviour. Two distinct modelling approaches were introduced to simulate the non-trivial mechanical response of thermally bonded nonwovens based on their planar density. The first modelling approach was developed to simulate the mechanical behaviour of high-density nonwovens, and the respective fabric was modelled with shell elements with thicknesses identical to those of the bond points and the fibre matrix having distinct anisotropic mechanical properties. Random orientation of individual fibres was introduced into the model in terms of the orientation distribution function in order to determine the material’s anisotropy. The second modelling approach was introduced to simulate low-density nonwovens, and it treated the nonwoven media as a structure composed of fibres acting as truss links between bond points.

Abstract: Numerical modeling of bones is necessary for design of efficient surgical cutting tools that can provide low cutting forces, reduce damage and prevent thermal necrosis of bone tissue. Development of realistic numerical models of cortical bone tissue requires deep knowledge of its deformation behaviour. Deformation mechanisms of bones differ from those of metals, polymers and composites since bones consist of a living tissue with hierarchical microstructure. The aim of this study is to analyse deformation characteristics of the cortical bone tissue from both experimental and numerical perspectives. Initially, Vickers hardness tests were conducted at various anatomical positions on a cross-section of a bovine femur bone to observe location-based variation of its mechanical response. Various load magnitudes ranging between 1 kgf and 100 kgf were applied in the Vickers hardness tests to analyse the effect of anisotropy on damage evolution. These tests were simulated using a finite element scheme to reproduce the mechanical behaviour of bones in indentation. Finally, results of the hardness tests were compared with those obtained from finite element simulations.

Abstract: The control of the plastic anisotropy during forming of a metallic sheet requires detailed knowledge on its microstructure and, especially, crystallographic texture. During the thermo-mechanical processing of aluminium sheet products in commercial production lines the material experiences a complex history of temperature, time and strain paths, which result in alternating cycles of deformation and recrystallization with the associated changes in texture and microstructure. Thus, computer-based alloy and process development requires integration of models for simulating the evolution of microstructure, microchemistry and crystallographic texture into process models of the thermo-mechanical production of Al sheet. The present study focuses on recent developments in linking softening modules that simulate the progress of recovery and recrystallization with the following texture changes to deformation and microchemistry models.

Abstract: This article presents a new approach of soft material modeling for robotic manipulation which combines physical modeling and tracking of a deformable object. We discuss the construction of geometrical models using MRI system and how to overcome the problem of variability. The physical model focuses on meat/muscles deformation. We introduce the principal criteria for choosing most appropriate models and discuss two modeling methods which suit our problem: mass spring model and tensor mass model. The introduction of anisotropy in these models allows results to be more realistic but evolves an increasing of computing time.

Abstract: The current work investigates anisotropy and forming characteristics of ZE10 Mg alloy. Anisotropic effects are measured by analysing tensile tests data for the specimens from 0o, 45o and 90o rolling directions of thin sheet of 0.8 mm thickness. The effects of temperature and strain rates are also included in anisotropic analyses. The mechanical properties of ZE10 alloy are affected by the physical conditions like temperature and strain rates. The effect is significant on yield strength, UTS and fracture strain while small variation is observed in case of Young’s modulus. Deep drawing experiments have been done to investigate the effects of temperature on forming characteristics and limit drawing ratio (LDR) of an alloy. Forming investigation shows that it is difficult to produce a good or unfractured part at 100 oC, but it can be produce at 150 oC or higher with different combinations of blank holder force and drawing ratio. Also, LDR and blank holder force also effects required punch force.

Abstract: Uniaxial tension tests for galvanized steel sheets are performed. Fractured surfaces of the specimens are observed by means of the scanning electron microscope (SEM). It is confirmed that the specimens experience ductile fracture. Based on the Gurson meso-damage theory, Hill’48-GTN anisotropic damage model is presented, and used to analyze the uniaxial tension test. The true stress-strain curves are fitted by three kinds of main flow stress models. After comparing the fitting precisions, the Voce model is selected as the flow stress model for the simulation analysis. The simulation results show that the Hill’48-GTN model can be used to accurately predict the whole process of damage occurrence, evolution and fracture in tension, and void volume fraction can be taken as a forming limit parameter of sheet metal forming.

Abstract: The paper presents a predictive cutting force model in drilling of anisotropic materials. Three dimensional chip flow in drilling is interpreted as a piling up of the orthogonal cuttings in the planes containing the cutting velocities and the chip flow velocities. The cutting models in the chip flow are determined to calculate the cutting energy using the orthogonal cutting data. Then, the chip flow direction is determined to minimize the cutting energy. The cutting force can be predicted in the determined chip flow model. The cutting force with anisotropy in the material is modeled as the change in the shear stress on the shear plane. The shear stress changes with the rotation angle of the cutter. The cutting force prediction is verified in drilling of a titanium alloy. The anisotropic parameters are identified to minimize the model error between the measured and the predicted cutting forces. The periodical oscillation of the cutting force is also predicted by anisotropy in the shear stress.

Abstract: Sheet metal formability has been studied for a half century. The sheet formability is mostly described by the Forming Limit Diagram (FLD). A prediction of this FLD is a source of interest for industrial companies. Indeed knowing the FLD of a material allows optimization of the production processes which leads to money saving. Nevertheless, the formability tests (tensile, bulge and Nakazima tests) which give the experimental FLD do not really represent the process that the sheet will undergo in industrial conditions. The paper therefore focuses on a cross stamping test. The material of interest is a Zinc sheet. FLD prediction is reported for a wide variety of metals [1] but literature about Zinc is nearly non existent. The studied Zinc sheets exhibit a highly anisotropic mechanical behaviour due to the hcp lattice structure and the inherited rolling texture. This anisotropic behaviour results in an anisotropic formability. The Zinc sheet FLD is influenced by the orientation of the rolling direction during the process. Experimental cross stamping of this material allows describing the studied material behaviour in a large range of mechanical solicitations from tensile to biaxial tension. The experimental results are compared with the finite element simulation and permit to understand where and why failures appear, which leads to a better understanding of Zinc anisotropic formability.

Abstract: The mechanical behavior of metallic materials during sheet forming processes must be related to their anisotropic properties and their formability aptitude. So, in this paper, channel die compression experiments are proposed to analyze the anisotropy of an AA2024 thick sheet. A modified quadratic Hill criterion based on the variation of the coefficients with the cumulated plastic strain is proposed. To compare and to validate the identified Hill coefficients values, classical tensile tests are also analyzed. The results are validated via finite element simulations of the tensile tests and channel die compression experiments using the commercial code LS-DYNA.

Abstract: Two non-quadratic orthotropic yield functions called Yld2011-18p (containing 18 param-eters) and Yld2011-27p (containing 27 parameters) are proposed. The formulations are based on theestablished concept of linear transformations operating on the stress deviator. Application examplesreveal the capabilities of both yield functions to accurately describe complex plastic anisotropy ofsheet metals.