Finite Element Modeling of Textiles in Abaqus™ CAE

Izabela Ciesielska-Wrobel

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781032091310
• eBook ISBN: 9780429531606
• Topic: Physical Sciences
• Subtopic: Mechanics

The aim of the book is to provide engineers with a practical guide to Finite Element Modelling (FEM) in Abaqus CAE software. The guide is in the form of step-by-step procedures concerning yarns, woven fabric and knitted fabrics modelling, as well as their contact with skin so that the simulation of haptic perception between textiles and skin can be

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Introduction to Creation of Geometry and Modeling with Abaqus CAE

1.1 Introduction

The aim of the book is to provide textile engineers with a practical guide to finite element modeling (FEM) in Abaqus CAE (Complete Abaqus Environment) software. The guide is in the form of step-by-step procedures concerning fiber, yarn, knitted fabric modeling, as well as their contact with skin so that the simulation of haptic perception between textiles and skin can be provided. The specific modeling procedure will be preceded by a theoretical background concerning the mechanical characteristics of modeled elements or phenomena (e.g., in the case of the modeling of haptic perception of textiles, a detailed model of the soft tissue (human skin) and modeled textile objects will be given). Models will be validated and discussed. In addition, virtual object test results will be presented and compared with the outcome of the modeling process. The discussion of the convergences and divergences between real case studies and their models will be analyzed.

The scope of the book is as follows:

1. Theoretical background concerning mechanical characteristics and structure of basic textile products.
2. FEM of selected objects, namely single fiber, three-ply yarn, plain stitch knitted fabric, single-layer skin model, three-layer skin model (skin compliance), and contact of the skin with fabric (skin compliance, friction).
3. The results of the physical tests on textiles will be provided to verify the proposed modeling approach.

The process of modeling is time consuming, especially for beginners in this field or for those who have knowledge of the subject but are not familiar with the software that allows modeling. Normally, one needs to follow the basic instructions provided by the developer of the software (Abaqus CAE User’s Manual). To create a complex, original model, one needs to collect information from many existing sources and apply a method of trial and error when using the collected information. In many cases, instead of modeling existing phenomena, one creates an animation of phenomena without applying careful attention to the simulation of phenomena. This book will guide users of finite element software (not only Abaqus CAE users, as the modeling rules are common for most of the existing FEM software) from the stage of complex graphic creation to material selection, mesh, contact properties, and boundary conditions up to analysis of the results (Dassault Systèmes, 2016).

In everyday life, one needs to perform plenty of physical tests in the lab to make sure that the specific fabric complies with required standards or simply to verify the quality of materials. In the case of scientific studies, the cost of operator-tested samples is high. That is why it is sometimes easier to perform modeling on the computer instead of carrying out laboratory tests. The other great example of the application of FEM using software is the fact that very complex and expensive samples (e.g., carbon composite materials) can only be tested a few times. The introduction of modifications after each test is extremely costly. This is the moment when FEM and Abaqus CAE come into play. This method certainly contributes to a reduction of tests and development cycling times. Eventually, it does induce quicker turnaround times and saves money.

1.1.1 Textiles—Why Model Them?

Textiles are unique products; they can be made of different raw materials, starting from natural and ending with highly modified synthetics. Their composition, structure, and abilities determine their final application. In order to predict whether a specific raw material or product can be successfully applied to a complex structure—for example, carbon fabric composite application used in car hulls—one usually conducts a tedious time-, energy-, and financial resources-consuming process. This process is aimed at verifying whether this specific compound of materials with a specific architecture meets the requirements, or whether it fails before achieving the desired requirement— for example, a high value of breaking force. If this specific compound fails, one needs to perform another verification test, and then another, modifying the compound to achieve the goal, which in this case could be meeting the mechanical criteria—for example, achieving a tensile strength of 110 MPa.

Nowadays, to avoid a trial-and-error method becoming a very costly approach, one utilizes a high-performance computational software that is able to provide the user with an extremely precise and fast solution. Unfortunately, the word “fast” is a key keyword here. Why? Well, to perform any verification or modeling of the objects using this type of software, one needs to gain an understanding of the phenomena (e.g., breaking force), objects (e.g., fiber, yarns), and the tool itself (software, e.g., Abaqus). These are key issues related to the FEM of all modeled objects and their materials, not only textiles (although textiles are the most important subjects in this book), which all Abaqus users need to deal with.

Most textiles can be described as uneven, which makes them truly unique objects, but unfortunately in many cases, this special feature is omitted, simplifying the model objects.

In the context of this book, the process of modeling the textiles is understood as creating the geometry of the textile object, ascribing to it the necessary material parameters, combining them into one assembly, preparing a step-by-step scenario for the assembled objects, selecting the parameters to be calculated by the software (the final values, the outcome of the modeled phenomenon), establishing the forces, loads working on the object in the system and boundary conditions of the system or its elements, performing the analytical part, assessing the results, and finally reporting.

This kind of procedure will be performed for several textile objects, and at the end, the examples of the real study utilizing the modeling of textiles will be presented.

The modeling of textiles is not only conducted to save time and financial resources, which are usually spent when performing tests on textiles in a traditional manner in the metrology laboratories, but also to gain a better understanding of the phenomena taking place in the textiles assembled or for educational purposes when one can observe interactions taking place between twisted yarns.

1.1.2 Finite Element Analysis and Finite Element Models

The finite element analysis (FEA) method was introduced by M.J. Turner et al. (1956) in the article titled “Stiffness and Deflection Analysis of Complex Structures,” where a method for yielding accurate structural data allowing further dynamic analysis was introduced. The concept of FEA has been widely applied in different fields of engineering and bioengineering. It is usually defined as a discretization technique in mechanics, specifically in structural mechanics. As the term suggests, the technique requires dividing the model (object) into elements that have less complicated geometry than the geometry of the whole initial model. This simplification of the geometry allows elements to be obtained (fragments of the initial model). These are called finite elements. Any analytical attempt to consider the whole model, without it being subdivided, could be extremely difficult or even impossible to perform. The division, called discretization, helps to investigate the model through its elements. The mathematical answer regarding the whole model can be considered as an estimate of all the elements taken into consideration collectively. The idea is to gain a better understanding of all the elements separately so that one can understand the whole object and predict its behavior. The purpose of this is to predict the response of a model to some form of external loading, stress, external elevated temperature or to some nonequilibrium initial conditions.

The finite element method in mathematics is a specific mathematical technique (approach) utilized to approximate some solutions to problems presented in the form of differential equations. In many cases, finite element method refers to finite element models; however, in these cases, it is specifically mentioned that one is referring to the modeled objects, not the method that was applied to provide the solution.

1.1.3 Abaqus CAE and Other Software as Tools for Modeling

Commercially available FEA software gained great popularity among engineers, especially constructional engineers in industry, architecture, tectonics, and among researchers representing different fields (e.g., bioengineers in academia). Despite the fact that this book is meant for textile engineers and all the examples presented in the book refer to textiles, the methodology of creating a model, no matter what the object of the modeling is, remains the same and can be easily applied in different fields.

The idea of applying FEA software to model an object or phenomena existing in real life or only hypothetically, appeared when it was found that performing a trial-and-error testing method became too expensive and too robust to continue. Why prepare the object, test it and analyze the results only to find out that it is a failure when the same action can be taken using a powerful computational technique to obtain approximate testing results or solutions to a variety of engineering problems without leaving the desk and the computer?

The idea of modeling in Abaqus CAE or any other FEA software listed in this book is to recreate an environment (load, boundary conditions) that surrounds an object with a specific geometry (model) and to imitate the real-life circumstances happening to this model (torque or external pressure).

Imagine working on a textile sample in the lab. Your textile object is prepared and then destroyed in the course of a test (e.g., a tearing test). There is at least one thing you know and at least one thing you do not know about this sample. You definitely know that your sample was damaged in the course of the test when you applied a specific force to tear the sample apart. What you do not know is exactly in which place the sample was destroyed and how did it started and ended, in which spots on the sample. In order to have a better insight into a sample, a microscopic imaging can be applied. This may answer some of your questions, but it will probably lead you back to the lab and you will be forced to perform more tests. This is not an issue if your samples are not expensive in terms of raw materials and the methodology used in preparing the sample for the test. Now, imagine a model with the same geometry as your sample. This model was created in the FEA software, where you initiate tearing of the sample using a relatively simple combination of the conditions in the software (steps, contact, load, boundary conditions). As soon as the model is created, you can initiate the destruction of samples, investigating the selected moment and places on the sample. You are also free to change the applied forces to verify whether different loading exerts the same or different effect on the tested (modeled) sample. In fact, you may decompose the whole model, observing only selected elements in the model. This ability of FEA software that allows discretizing the whole model and observing selected elements or points (nodes) makes the software a useful e-tool for solutions to engineering problems.

Released in 1978, Abaqus CAE is an environment that has graphical applications. In 2005, it was purchased by Dassault Systèmes and has been cocreating the SIMULIA brand. This allows interactive operating with the designed objects. It also allows the rapid and relatively easy creation of models of objects that have a simple geometry. In the case of a complex model, the software allows the geometry to be imported; and, by producing or importing the geometry of the structure, and decomposing it into smaller fragments, it can be easily analyzed. This operation is called meshing. If the model is checked and submitted, Abaqus CAE can monitor and control the process of analysis (so-called “job”). The visualization module can then be used to interpret the results.

There is a number of Abaqus platforms that can be utilized depending on the needs of the user. The first one, and the most common, is Abaqus/Standard, which is a general-purpose analysis product that can provide solutions to a great variety of linear and nonlinear problems involving the static, dynamic, thermal, and electrical response of modeled elements. The examples presented in this guide refer to this specific platform. Abaqus/Standard solves a system of equations implicitly at each solution “increment.” On the other pole, one may find that Abaqus/Explicit can provide a solution over time in small time increments without solving a coupled system of equations at each increment. This is suitable for modeling brief, transient, dynamic events, such as impact and blast problems, or the change of the temperature on one surface related to the heating of another surface in close vicinity to the first one. Abaqus/Explicit is not utilized in this guide.

Thermal convective flow and incompressible flow problems may be solved by computational fluid dynamics (CFD) tool, Abaqus/CFD; however, this does not feature in this guide.

Abaqus CAE software has its direct and indirect competitors, which means that some of them can provide the same platform for the solution of specific problems, while some of them have different profiles and are more focused on specific areas (e.g., biomechanics).

The existing list of software allowing FEM of objects and phenomena is very long, and the list presented here contains only some selected multipurpose software, the most popular in the opinion of the author and/or useful in terms of their application in the area of skin and textile modeling. It does not mean that other existing software is worse. It just means that the author was not aware that they exist or that they are excellent but are meant for applications that are irrelevant in here (e.g., sets of PLAXIS 2D and PLAXIS 3D developed and marketed by Plaxis BV), which are specially designed for soil and rock mechanics with the emphasis on the analysis, design, and simulation of underground constructions and the soil-structure interaction (http://plaxis.nl/); Bridge, Lusas Bridge LT, and Lusas Bridge Plus by LUSAS is a set of software graded from a basic version to a sophisticated one, where one may perform an FEA of all types of bridge structures. Bridge LT is suitable for linear static analysis of structural frames and grillages. Bridge is suitable for everyday linear static and linear dynamic analysis using beams, shells, solids, and joints. Bridge Plus includes an extended advanced high-performance element library which allows for more advanced analyses to be undertaken. ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication Page
• Disclaimer
• Table of Contents
• Preface
• Acknowledgments
• Author
• 1. Introduction to Creation of Geometry and Modeling with Abaqus CAE
• 2. Basics of Abaqus CAE Software
• 3. Modeling of a Fiber
• 4. Haptic Perception of Objects
• 5. Geometry of a Knit Structure
• Index