scholarly journals On the recovery of the stress-free configuration of the human cornea

2020 ◽  
Vol 2 (4) ◽  
pp. 11-33
Author(s):  
Anna Pandolfi ◽  
Andrea Montanino
Keyword(s):  
Numerical Simulations ◽  
Inverse Analysis ◽  
Material Model ◽  
Free State ◽  
Human Cornea ◽  
Material Models ◽  
Material Parameters ◽  
Optimal Values ◽  
Stress Free State ◽  
Actual Stress

Purpose: The geometries used to conduct numerical simulations of the biomechanics of the human cornea are reconstructed from images of the physiological configuration of the system, which is not in a stress-free state because of the interaction with the surrounding tissues. If the goal of the simulation is a realistic estimation of the mechanical engagement of the system, it is mandatory to obtain a stress-free configuration to which the external actions can be applied. Methods: Starting from a unique physiological image, the search of the stress-free configuration must be based on methods of inverse analysis. Inverse analysis assumes the knowledge of one or more geometrical configurations and, chosen a material model, obtains the optimal values of the material parameters that provide the numerical configurations closest to the physiological images. Given the multiplicity of available material models, the solution is not unique. Results: Three exemplary material models are used in this study to demonstrate that the obtained, non-unique, stress-free configuration is indeed strongly dependent on both material model and on material parameters. Conclusion: The likeliness of recovering the actual stress-free configuration of the human cornea can be improved by using and comparing two or more imaged configurations of the same cornea.

A New Approach to Improve Material Models

1995 ◽  
Vol 117 (1) ◽  
pp. 14-19 ◽  
Author(s):  
H. Braasch ◽  
H. Duddeck ◽  
H. Ahrens
Keyword(s):  
Material Model ◽  
Original Material ◽  
Inelastic Behavior ◽  
Shape Functions ◽  
Simple Shape ◽  
Material Models ◽  
New Approach ◽  
Material Parameters ◽  
Material Functions ◽  
Optimal Material

The inelastic behavior of materials is described most efficiently by unified models when their material functions are determined so that flow, hardening, creep etc. will be covered correctly. In this paper, the adaptation of a model is not confined to finding the optimal material parameters but is extended to the identification of the optimal shape of the material functions itself. Material functions given by series of simple shape functions defined in discrete sections which merge smoothly together lead to the best adaptation to experimental results. Furthermore, any remaining shortcomings of the model reveal deficiencies in the modelling of the microphysics of the material. Then by careful interpretation of the uncovered physical properties the original material model has to be amended leading to the derivation of even entirely new models. Thus, a powerful tool is presented here by which a unified model can be checked and improved.

scholarly journals Identification of thermal material parameters for thermo-mechanically coupled material models

Meccanica ◽  
2021 ◽  
Vol 56 (2) ◽  
pp. 393-416
Author(s):  
L. Rose ◽  
A. Menzel
Keyword(s):  
Evolution Equations ◽  
Constitutive Relations ◽  
Material Model ◽  
Basic Material ◽  
Dissipated Energy ◽  
Model Formulation ◽  
Material Models ◽  
Material Parameters ◽  
Simple Tension ◽  
The Impact

AbstractThe possibility of accurately identifying thermal material parameters on the basis of a simple tension test is presented, using a parameter identification framework for thermo-mechanically coupled material models on the basis of full field displacement and temperature field measurements. Main objective is to show the impact of the material model formulation on the results of such an identification with respect to accuracy and uniqueness of the result. To do so, and as a proof of concept, the data of two different experiments is used. One experiment including cooling of the specimen, due to ambient temperature, and one without specimen cooling. The main constitutive relations of two basic material models are summarised (associated and non-associated plasticity), whereas both models are extended so as to introduce an additional material parameter for the thermodynamically consistent scaling of dissipated energy. The chosen models are subjected to two parameter identifications each, using the data of either experiment and focusing on the determination of thermal material parameters. The influence of the predicted dissipated energy of the models on the identification process is investigated showing that a specific material model formulation must be chosen carefully. The material model with associated evolution equations used within this work does neither allow a unique identification result, nor is any of the solutions for the underlying material parameters close to literature values. In contrast to that, a stable, that is locally unique, re-identification of the literature values is possible for the boundary problem at hand if the model with non-associated evolution equation is used and if cooling is included in the experimental data.

Using Nonlinear Kinematic Hardening Material Models for Elastic-Plastic Ratcheting Analysis

2015 ◽  
Author(s):  
Tim Gilman ◽  
Bill Weitze ◽  
Jürgen Rudolph ◽  
Adrian Willuweit ◽  
Arturs Kalnins
Keyword(s):  
Material Model ◽  
Pressure Vessels ◽  
Growth Strain ◽  
Temperature Dependent ◽  
Material Models ◽  
Hardening Material ◽  
Material Parameters ◽  
Elastic Plastic ◽  
Material Data ◽  
Inelastic Analysis

Applicable design codes for power plant components and pressure vessels demand for a design check against progressive plastic deformation. In the simplest case, this demand is satisfied by compliance with shakedown rules in connection with elastic analyses. The possible non-compliance implicates the requirement of ratcheting analyses on elastic-plastic basis. In this case, criteria are specified on maximum allowable accumulated growth strain without clear guidance on what material models for cyclic plasticity are to be used. This is a considerable gap and a challenge for the practicing CAE (Computer Aided Engineering) engineer. As a follow-up to two independent previous papers PVP2013-98150 ASME [1] and PVP2014-28772 [2] it is the aim of this paper to close this gap by giving further detailed recommendation on the appropriate application of the nonlinear kinematic material model of Chaboche on an engineering scale and based on implementations already available within commercial finite element codes such as ANSYS® and ABAQUS®. Consistency of temperature-dependent runs in ANSYS® and ABAQUS® is to be checked. All three papers together constitute a comprehensive guideline for elasto-plastic ratcheting analysis. The following issues are examined and/or referenced: • Application of monotonic or cyclic material data for ratcheting analysis based on the Chaboche material model • Discussion of using monotonic and cyclic data for assessment of the (non-stabilized) cyclic deformation behavior • Number of backstress terms to be applied for consistent ratcheting results • Consideration of the temperature dependency of the relevant material parameters • Consistency of temperature-dependent runs in ANSYS® and ABAQUS® • Identification of material parameters dependent on the number of backstress terms • Identification of material data for different types of material (carbon steel, austenitic stainless steel) including the appropriate determination of the elastic limit • Quantification of conservatism of simple elastic-perfectly plastic behavior • Application of engineering versus true stress-strain data • Visual checks of data input consistency • Appropriate type of allowable accumulated growth strain. This way, a more accurate inelastic analysis methodology for direct practical application to real world examples in the framework of the design code conforming elasto-plastic ratcheting check is proposed.

scholarly journals Selection of material model of chosen photocurable resin for application in finite element analyses

2020 ◽  
Author(s):  
Danuta Miedzińska
Keyword(s):  
Finite Element ◽  
Additive Manufacturing ◽  
Numerical Simulations ◽  
Main Idea ◽  
Material Model ◽  
Numerical Analyses ◽  
Finite Element Analyses ◽  
Material Models ◽  
Manufacturing Technique ◽  
Selection Of

The paper deals with numerical simulations of materials printed with SLA technology. SLA (stereolitography) is an additive manufacturing technique, which main idea is printing with the use of a photocurable resin, e.g. epoxy or acrylic. The crosslinking is carried out under UV exposition. The chosen resin was experimentally tested and then numerical analyses were carried out using material models available in LS Dyna library (MAT_24, MAT_168, MAT_081). The results were compared. The best convergence was achieved for MAT_168.

Using Nonlinear Kinematic Hardening Material Models for Elastic–Plastic Ratcheting Analysis

2016 ◽  
Vol 138 (5) ◽  
Author(s):  
Jürgen Rudolph ◽  
Tim Gilman ◽  
Bill Weitze ◽  
Adrian Willuweit ◽  
Arturs Kalnins
Keyword(s):  
Kinematic Hardening ◽  
Material Model ◽  
Growth Strain ◽  
Temperature Dependent ◽  
Material Models ◽  
Hardening Material ◽  
Material Parameters ◽  
Elastic Plastic ◽  
Material Data ◽  
Asme Paper

Applicable design codes for power plant components and pressure vessels demand for a design check against progressive plastic deformation. In the simplest case, this demand is satisfied by compliance with shakedown rules in connection with elastic analyses. The possible noncompliance implicates the requirement of ratcheting analyses on elastic–plastic basis. In this case, criteria are specified on maximum allowable accumulated growth strain without clear guidance on what material models for cyclic plasticity are to be used. This is a considerable gap and a challenge for the practicing computer-aided engineering engineer. As a follow-up to two independent previous papers PVP2013-98150 ASME (Kalnins et al., 2013, “Using the Nonlinear Kinematic Hardening Material Model of Chaboche for Elastic-Plastic Ratcheting Analysis,” ASME Paper No. PVP2013-98150.) and PVP2014-28772 (Weitze and Gilman, 2014, “Additional Guidance for Inelastic Ratcheting Analysis Using the Chaboche Model,” ASME Paper No. PVP2014-28772.), it is the aim of this paper to close this gap by giving further detailed recommendation on the appropriate application of the nonlinear kinematic material model of Chaboche on an engineering scale and based on implementations already available within commercial finite element codes such as ANSYS® and ABAQUS®. Consistency of temperature-dependent runs in ANSYS® and ABAQUS® is to be checked. All three papers together constitute a comprehensive guideline for elastoplastic ratcheting analysis. The following issues are examined and/or referenced: (1) application of monotonic or cyclic material data for ratcheting analysis based on the Chaboche material model, (2) discussion of using monotonic and cyclic data for assessment of the (nonstabilized) cyclic deformation behavior, (3) number of backstress terms to be applied for consistent ratcheting results, (4) consideration of the temperature dependency (TD) of the relevant material parameters, (5) consistency of temperature-dependent runs in ANSYS® and ABAQUS®, (6) identification of material parameters dependent on the number of backstress terms, (7) identification of material data for different types of material (carbon steel, austenitic stainless steel) including the appropriate determination of the elastic limit, (8) quantification of conservatism of simple elastic-perfectly plastic (EPP) behavior, (9) application of engineering versus true stress–strain data, (10) visual checks of data input consistency, and (11) appropriate type of allowable accumulated growth strain. This way, a more accurate inelastic analysis methodology for direct practical application to real world examples in the framework of the design code conforming elastoplastic ratcheting check is proposed.

Development of Reactive Flow Models for Energetic Materials

2009 ◽  
Author(s):  
David L. Littlefield ◽  
Kenneth C. Walls ◽  
David E. Lambert
Keyword(s):  
Energetic Materials ◽  
Software Package ◽  
Model Development ◽  
Material Model ◽  
Reactive Flow ◽  
Material Models ◽  
Flow Models ◽  
Cylinder Test ◽  
Optimal Values ◽  
Verification Test

In order to make the process of material model development more systematic for energetic materials, we have developed a procedure that makes use of nonlinear optimization to derive optimal values of parameters in the material models. A framework has been developed that makes use of the optimization software package DAKOTA driving the hydrocode CTH (both CTH and DAKOTA are developed and maintained at Sandia National Laboratories). CTH was used to model a set of characterization experiments that were used as the basis for the calibration of a particular model; for example, the cylinder test was used to calibrate the JWL equation of state, and the wedge test for the HVRB reactive burn model. As a verification test of the framework, we have determined material model constants for TNT and compared them to values published in the literature.

Lifetime Prediction of Tires with Regard to Oxidative Aging5

2008 ◽  
Vol 36 (1) ◽  
pp. 63-79 ◽  
Author(s):  
L. Nasdala ◽  
Y. Wei ◽  
H. Rothert ◽  
M. Kaliske
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Temperature Distribution ◽  
Superposition Principle ◽  
Accelerated Aging ◽  
Material Model ◽  
Aging Behavior ◽  
Element Analysis ◽  
Material Parameters ◽  
Reaction Processes

Abstract It is a challenging task in the design of automobile tires to predict lifetime and performance on the basis of numerical simulations. Several factors have to be taken into account to correctly estimate the aging behavior. This paper focuses on oxygen reaction processes which, apart from mechanical and thermal aspects, effect the tire durability. The material parameters needed to describe the temperature-dependent oxygen diffusion and reaction processes are derived by means of the time–temperature–superposition principle from modulus profiling tests. These experiments are designed to examine the diffusion-limited oxidation (DLO) effect which occurs when accelerated aging tests are performed. For the cord-reinforced rubber composites, homogenization techniques are adopted to obtain effective material parameters (diffusivities and reaction constants). The selection and arrangement of rubber components influence the temperature distribution and the oxygen penetration depth which impact tire durability. The goal of this paper is to establish a finite element analysis based criterion to predict lifetime with respect to oxidative aging. The finite element analysis is carried out in three stages. First the heat generation rate distribution is calculated using a viscoelastic material model. Then the temperature distribution can be determined. In the third step we evaluate the oxygen distribution or rather the oxygen consumption rate, which is a measure for the tire lifetime. Thus, the aging behavior of different kinds of tires can be compared. Numerical examples show how diffusivities, reaction coefficients, and temperature influence the durability of different tire parts. It is found that due to the DLO effect, some interior parts may age slower even if the temperature is increased.

scholarly journals Variation of Passive Biomechanical Properties of the Small Intestine along Its Length: Microstructure-Based Characterization

2021 ◽  
Vol 8 (3) ◽  
pp. 32
Author(s):  
Dimitrios P. Sokolis
Keyword(s):  
Small Intestine ◽  
Orientation Angle ◽  
Axial Direction ◽  
Material Model ◽  
Biomechanical Properties ◽  
Intestinal Wall ◽  
Model Parameters ◽  
Material Models ◽  
Hyper Elastic ◽  
Rat Tissue

Multiaxial testing of the small intestinal wall is critical for understanding its biomechanical properties and defining material models, but limited data and material models are available. The aim of the present study was to develop a microstructure-based material model for the small intestine and test whether there was a significant variation in the passive biomechanical properties along the length of the organ. Rat tissue was cut into eight segments that underwent inflation/extension testing, and their nonlinearly hyper-elastic and anisotropic response was characterized by a fiber-reinforced model. Extensive parametric analysis showed a non-significant contribution to the model of the isotropic matrix and circumferential-fiber family, leading also to severe over-parameterization. Such issues were not apparent with the reduced neo-Hookean and (axial and diagonal)-fiber family model, that provided equally accurate fitting results. Absence from the model of either the axial or diagonal-fiber families led to ill representations of the force- and pressure-diameter data, respectively. The primary direction of anisotropy, designated by the estimated orientation angle of diagonal-fiber families, was about 35° to the axial direction, corroborating prior microscopic observations of submucosal collagen-fiber orientation. The estimated model parameters varied across and within the duodenum, jejunum, and ileum, corroborating histologically assessed segmental differences in layer thicknesses.

scholarly journals Stress-free state of the red blood cell membrane and the deformation of its skeleton

2012 ◽  
Vol 17 (2) ◽  
Author(s):  
Tjaša Švelc ◽  
Saša Svetina
Keyword(s):  
Blood Cell ◽  
Red Blood Cell ◽  
Variational Calculation ◽  
Lateral Distribution ◽  
Free State ◽  
Initial State ◽  
Proposed Model ◽  
Deformed State ◽  
Concentric Circles ◽  
Stress Free State

AbstractThe response of a red blood cell (RBC) to deformation depends on its membrane, a composite of a lipid bilayer and a skeleton, which is a closed, twodimensional network of spectrin tetramers as its bonds. The deformation of the skeleton and its lateral redistribution are studied in terms of the RBC resting state for a fixed geometry of the RBC, partially aspirated into a micropipette. The geometry of the RBC skeleton in its initial state is taken to be either two concentric circles, a references biconcave shape or a sphere. It is assumed that in its initial state the skeleton is distributed laterally in a homogeneous manner with its bonds either unstressed, presenting its stress-free state, or prestressed. The lateral distribution was calculated using a variational calculation. It was assumed that the spectrin tetramer bonds exhibit a linear elasticity. The results showed a significant effect of the initial skeleton geometry on its lateral distribution in the deformed state. The proposed model is used to analyze the measurements of skeleton extension ratios by the method of applying two modes of RBC micropipette aspiration.

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