scholarly journals Hyperelastic and Viscoelastic Properties of Brain Tissue in Tension

2012 ◽  
Author(s):  
Badar Rashid ◽  
Michel Destrade ◽  
Michael D. Gilchrist
Keyword(s):  
Finite Element ◽  
Brain Tissue ◽  
Brain Injuries ◽  
Diffuse Axonal Injury ◽  
Viscoelastic Behavior ◽  
Tensile Tests ◽  
Uniaxial Tensile ◽  
Strain Rates ◽  
Average Rise

Mechanical characterization of brain tissue at high loading velocities is particularly important for modelling Traumatic Brain Injury (TBI). During severe impact conditions, brain tissue experiences a mixture of compression, tension and shear. Diffuse axonal injury (DAI) occurs in animals and humans when the strains and strain rates exceed 10% and 10/s, respectively. Knowing the mechanical properties of brain tissue at these strains and strain rates is thus of particular importance, as they can be used in finite element simulations to predict the occurrence of brain injuries under different impact conditions. In this research, uniaxial tensile tests at strain rates of 30, 60 and 90/s up to 30% strain and stress relaxation tests in tension at various strain magnitudes (10%–60%) with an average rise time of 24 ms were performed. The brain tissue showed a stiffer response with increasing strain rates, showing that hyperelastic models are not adequate and that viscoelastic models are required. Specifically, the tensile engineering stress at 30% strain was 3.1 ± 0.49 kPa, 4.3 ± 0.86 kPa, 6.5 ± 0.76 kPa (mean ± SD) at strain rates of 30, 60 and 90/s, respectively. The Prony parameters were estimated from the relaxation data. Numerical simulations were performed using a one-term Ogden model to analyze hyperelastic and viscoelastic behavior of brain tissue up to 30% strain. The material parameters obtained in this study will help to develop biofidelic human brain finite element models, which subsequently can be used to predict brain injuries under impact conditions.

Diffuse Axonal Injury and Degradation in Mechanical Characteristics of Brain White Matter

2008 ◽  
Author(s):  
Nabi Abolfathi ◽  
Abhai Naik ◽  
Mahdi Sotudeh ◽  
Ghodrat Karami ◽  
Mariusz Ziejewski
Keyword(s):  
Brain Tissue ◽  
Fibrous Composite ◽  
Axonal Injury ◽  
Diffuse Axonal Injury ◽  
Viscoelastic Behavior ◽  
Severe Disabilities ◽  
Post Injury ◽  
Brain White Matter ◽  
Injured Brain

Diffuse Axonal Injury (DAI) can happen due to sudden motions of head and is one of the major causes of fatality and severe disabilities. To study DAI, any change in material characteristics of brain tissue post injury needs to be well understood. In this study, the focus will be on changes in the viscoelastic material properties of white mater in the brain due to DAI resulting in axonal disconnections. Using a micromechanics fibrous composite modeling for white mater, we have developed an algorithm to analyze the effect of discontinuity due to breakage of axons inside the surrounded matrix. Repeated unit cell (RUC) was assumed to represent the axonal distribution within the extracellular matrix. Relaxation test were conducted for characterization of the viscoelastic behavior. The result of this study provides a modeling technique for characterization of injured brain tissue in white mater and proposes necessity of including the appropriate post injury axonal mechanical properties. These findings can improve the understanding of injury from mechanical perspective and help in predicting vulnerability of any such injured tissue against further injuries.

A Constitutive Material Model With Strain-Rate Dependency for Brain Tissue

2019 ◽  
Author(s):  
Mohammad Hosseini Farid ◽  
Mohammadreza Ramzanpour ◽  
Mariusz Ziejewski ◽  
Ghodrat Karami
Keyword(s):  
Constitutive Model ◽  
Brain Tissue ◽  
Brain Injuries ◽  
Viscoelastic Behavior ◽  
Strain Rates ◽  
Compression Tests ◽  
Rate Dependency ◽  
Rate Dependent ◽  
Constitutive Material Model ◽  
The Brain

Abstract In almost all scenarios of traumatic brain injuries (TBIs), the brain tissue goes under mechanical loading at high strain rates. In experimental works, it has also shown that brain tissue behavior is highly rate-dependent. We are presenting here the results of a study on mechanical properties of bovine brain tissue under unconfined compression tests at different rates. The tissue specimens are compressed with deformation rates of 10, 100, and 1000 mm/sec, respectively. We observed the tissue is showing a viscoelastic behavior and become stiffer under increasing strain rates. We developed a nonlinear viscoelastic rate-dependent constitutive model to be calibrated with the test results. The material parameters for this constitutive model have been validated for the above tested results. The model was examined against other rates and agrees well. The study will provide new insight into a better understanding of the rate-dependency behavior of the brain tissue under dynamic conditions. The work is a step forward in understanding the material characteristics of brain tissue for TBI analysis and prediction under loading or high kinematical motions.

scholarly journals Determination of Forming Limits Based on Finite Element Simulations

2021 ◽  
Author(s):  
Fuhui Shen ◽  
Kai Chen ◽  
Junhe Lian ◽  
Sebastian Münstermann
Keyword(s):  
Finite Element ◽  
Damage Mechanics ◽  
Tensile Tests ◽  
Finite Element Simulations ◽  
Experimental Results ◽  
Uniaxial Tensile ◽  
Anisotropic Plasticity ◽  
Forming Limits ◽  
Dog Bone ◽  
Spatio Temporal

Two categories of experiments have been performed to obtain the experimental forming limits of a ferritic stainless steel from uniaxial to equibiaxial tension, including Nakajima tests and tensile tests of flat specimens with different geometries of the central hole as well as the notched dog bone. The plasticity behavior of the investigated material is described using an evolving non-associated anisotropic plasticity model, which is calibrated based on experimental results of uniaxial tensile tests along different loading directions. A damage mechanics model is calibrated and validated based on the global force and displacement response of tensile tests. Finite element simulations of the Nakajima tests and the tensile tests of various geometries have been performed using the anisotropic material model. A novel spatio-temporal method is developed to evaluate the forming limits under different stress states by quantitatively characterizing the plastic strain distribution on the specimen surface. The forming limits have been independently determined from finite element simulation results of tensile specimens and Nakajima specimens using the spatio-temporal evaluation method. The forming limits obtained from numerical simulations of these two types of experiments are in good agreement with experimental results.

Elasto-viscoplastic constitutive equations for the description of glassy polymers behavior at constant strain rate

2006 ◽  
Vol 129 (1) ◽  
pp. 29-35 ◽  
Author(s):  
Fahmi Zaïri ◽  
Moussa Naït-Abdelaziz ◽  
Krzysztof Woznica ◽  
Jean-Michel Gloaguen
Keyword(s):  
Constitutive Equations ◽  
Constant Strain Rate ◽  
Tensile Tests ◽  
Glassy Polymers ◽  
Uniaxial Tensile ◽  
Model Parameters ◽  
Strain Rates ◽  
State Variable ◽  
Rubber Toughened ◽  
Compressive Tests

In this study, a modelization of the viscoplastic behavior of amorphous polymers is proposed, from an approach originally developed for metal behavior at high temperature, in which state variable constitutive equations have been modified. A procedure for the identification of model parameters is developed through the use of experimental data from both uniaxial compressive tests extracted from the literature and uniaxial tensile tests performed in this study across a variety of strain rates. The numerical algorithm shows that the predictions of this model well describe qualitatively and quantitatively the intrinsic softening immediately after yielding and the subsequent progressive orientational hardening corresponding to the response of two polymers, amorphous polyethylene terephthalate and rubber toughened polymethyl methacrylate.

Inverse Mechanical Characterization of Tissue Engineered Heart Valves

2008 ◽  
Author(s):  
Martijn A. J. Cox ◽  
Jeroen Kortsmit ◽  
Niels J. B. Driessen ◽  
Carlijn V. C. Bouten ◽  
Frank P. T. Baaijens
Keyword(s):  
Heart Valves ◽  
Soft Tissues ◽  
Mechanical Characterization ◽  
Tensile Tests ◽  
Material Behavior ◽  
Uniaxial Tensile ◽  
Mechanical Conditioning ◽  
Indentation Tests ◽  
Tissue Engineered Heart Valves

Over the last few years, research interest in tissue engineering as an alternative for current treatment and replacement strategies for cardiovascular and heart valve diseases has significantly increased. In vitro mechanical conditioning is an essential tool for engineering strong implantable tissues [1]. Detailed knowledge of the mechanical properties of the native tissue as well as the properties of the developing engineered constructs is vital for a better understanding and control of the mechanical conditioning process. The nonlinear and anisotropic behavior of soft tissues puts high demands on their mechanical characterization. Current standards in mechanical testing of soft tissues include (multiaxial) tensile testing and indentation tests. Uniaxial tensile tests do not provide sufficient information for characterizing the full anisotropic material behavior, while biaxial tensile tests are difficult to perform, and boundary effects limit the test region to a small central portion of the tissue. In addition, characterization of the local tissue properties from a tensile test is non-trivial. Indentation tests may be used to overcome some of these limitations. Indentation tests are easy to perform and when indenter size is small relative to the tissue dimensions, local characterization is possible. We have demonstrated that by recording deformation gradients and indentation force during a spherical indentation test the anisotropic mechanical behavior of engineered cardiovascular constructs can be characterized [2]. In the current study this combined numerical-experimental approach is used on Tissue Engineered Heart Valves (TEHV).

scholarly journals Embedded axonal fiber tracts improve finite element model predictions of traumatic brain injury

2019 ◽  
Vol 19 (3) ◽  
pp. 1109-1130 ◽  
Author(s):  
Marzieh Hajiaghamemar ◽  
Taotao Wu ◽  
Matthew B. Panzer ◽  
Susan S. Margulies
Keyword(s):  
Traumatic Brain Injury ◽  
Finite Element ◽  
Brain Injury ◽  
Strain Rate ◽  
Brain Tissue ◽  
Brain Injuries ◽  
Characteristic Curve ◽  
Strain And Strain Rate

AbstractWith the growing rate of traumatic brain injury (TBI), there is an increasing interest in validated tools to predict and prevent brain injuries. Finite element models (FEM) are valuable tools to estimate tissue responses, predict probability of TBI, and guide the development of safety equipment. In this study, we developed and validated an anisotropic pig brain multi-scale FEM by explicitly embedding the axonal tract structures and utilized the model to simulate experimental TBI in piglets undergoing dynamic head rotations. Binary logistic regression, survival analysis with Weibull distribution, and receiver operating characteristic curve analysis, coupled with repeated k-fold cross-validation technique, were used to examine 12 FEM-derived metrics related to axonal/brain tissue strain and strain rate for predicting the presence or absence of traumatic axonal injury (TAI). All 12 metrics performed well in predicting of TAI with prediction accuracy rate of 73–90%. The axonal-based metrics outperformed their rival brain tissue-based metrics in predicting TAI. The best predictors of TAI were maximum axonal strain times strain rate (MASxSR) and its corresponding optimal fraction-based metric (AF-MASxSR7.5) that represents the fraction of axonal fibers exceeding MASxSR of 7.5 s−1. The thresholds compare favorably with tissue tolerances found in in–vitro/in–vivo measurements in the literature. In addition, the damaged volume fractions (DVF) predicted using the axonal-based metrics, especially MASxSR (DVF = 0.05–4.5%), were closer to the actual DVF obtained from histopathology (AIV = 0.02–1.65%) in comparison with the DVF predicted using the brain-related metrics (DVF = 0.11–41.2%). The methods and the results from this study can be used to improve model prediction of TBI in humans.

scholarly journals Strain Rate-Dependent Constitutive and Low Stress Triaxiality Fracture Behavior Investigation of 6005 Al Alloy

2018 ◽  
Vol 2018 ◽  
pp. 1-14 ◽  
Author(s):  
Yong Peng ◽  
Xuanzhen Chen ◽  
Shan Peng ◽  
Chao Chen ◽  
Jiahao Li ◽  
...  
Keyword(s):  
Flow Stress ◽  
Plastic Strain ◽  
Strain Rate ◽  
Fracture Behavior ◽  
Equivalent Plastic Strain ◽  
Stress Triaxiality ◽  
Tensile Tests ◽  
Uniaxial Tensile ◽  
Strain Rates ◽  
Stress States

In order to study the dynamic and fracture behavior of 6005 aluminum alloy at different strain rates and stress states, various tests (tensile tests at different strain rates and tensile shearing tests at five stress states) are conducted by Mechanical Testing and Simulation (MTS) and split-Hopkinson tension bar (SHTB). Numerical simulations based on the finite element method (FEM) are performed with ABAQUS/Standard to obtain the actual stress triaxialities and equivalent plastic strain to fracture. The results of tensile tests for 6005 Al show obvious rate dependence on strain rates. The results obtained from simulations indicate the feature of nonmonotonicity between the strain to fracture and stress triaxiality. The equivalent plastic strain reduces to a minimum value and then increases in the stress triaxiality range from 0.04 to 0.30. A simplified Johnson-Cook (JC) constitutive model is proposed to depict the relationship between the flow stress and strain rate. What is more, the strain-rate factor is modified using a quadratic polynomial regression model, in which it is considered to vary with the strain and strain rates. A fracture criterion is also proposed in a low stress triaxiality range from 0.04 to 0.369. Error analysis for the modified JC model indicates that the model exhibits higher accuracy than the original one in predicting the flow stress at different strain rates. The fractography analysis indicates that the material has a typical ductile fracture mechanism including the shear fracture under pure shear and the dimple fracture under uniaxial tensile.

Methods for the Experimental Evaluation of True Stress-Strain Curves After Necking of Conventional Tensile Specimens: Exploratory Investigation and Proposals

2012 ◽  
Author(s):  
Grace Kelly Q. Ganharul ◽  
Nick de Brangança Azevedo ◽  
Gustavo Henrique B. Donato
Keyword(s):  
Real Time ◽  
Weighted Average ◽  
Tensile Tests ◽  
Plastic Instability ◽  
True Stress ◽  
Experimental Results ◽  
Uniaxial Tensile ◽  
High Definition ◽  
Stress Strain

Numerical elastic-plastic simulations have undergone significant expansion during the last decades (e.g. refined fracture mechanics finite element models including ductile tearing). However, one limitation to increase the accuracy of such models is the reliable experimental characterization of true stress-strain curves from conventional uniaxial tensile tests after necking (plastic instability), which complicates the direct assessment of the true stress-strain curves until failure. As a step in this direction, this work presents four key activities: i) first, existing correction methods are presented, including Bridgman, power law, weighted average and others; ii) second, selected metals are tested to experimentally characterize loads and the geometric evolution of necking. High-definition images are used to obtain real-time measurements following a proposed methodology; iii) third, refined non-linear FEM models are developed to reproduce necking and assess stresses as a function of normalized neck geometry; iv) finally, existing correction methods are critically compared to experimental results and FEM predictions in terms of potential and accuracy. The experimental results evaluated using high-definition images presented an excellent geometrical characterization of instability. FEM models were able to describe stress-strain-displacement fields after necking, supporting the exploratory validations and proposals of this work. Classical methodologies could be adapted based on experiments to provide accurate stress-strain curves up to failure with less need for real-time measurements, thus giving further support to the determination of true material properties considering severe plasticity.

Validation of the Hardening Behaviors for Metallic Materials at High Strain Rate and Temperature by Using the Taylor Impact Test

2016 ◽  
Vol 715 ◽  
pp. 153-158
Author(s):  
Ming Jun Piao ◽  
Hoon Huh ◽  
Ik Jin Lee ◽  
Hyung Won Kim ◽  
Lee Ju Park
Keyword(s):  
Tensile Tests ◽  
Thermal Softening ◽  
Experimental Results ◽  
Uniaxial Tensile ◽  
Strain Rates ◽  
Metallic Materials ◽  
Softening Effect ◽  
Impact Tests ◽  
Wide Range ◽  
Taylor Impact

This paper is concerned with the validation of the dynamic hardening behaviors of metallic materials by comparing numerical and experimental results of the Taylor impact tests. Several uniaxial tensile tests are performed at different strain rates and temperatures by using three kinds of materials: 4130 steel (BCC); OFHC copper (FCC); and Ti6Al4V alloy (HCP). Uniaxial material tests are performed at a wide range of strain rates from 10−3 s−1 to 103 s−1. Moreover, tensile tests are performed at temperature of 25 °C and 200 °C at strain rates of 10−3 s−1, 10−1 s−1, and 102 s−1, respectively. A modified Johnson–Cook type thermal softening model is utilized for the accurate application of the thermal softening effect at different strain rates. The hardening behaviors of the three materials are characterized by comparing the seven sequentially deformed shapes of the projectile from numerical and experimental results of Taylor impact tests.

Determining Representative Stress and Representative Strain in Deriving Indentation Flow Curves Based on Finite Element Analysis

2005 ◽  
Vol 297-300 ◽  
pp. 2152-2157 ◽  
Author(s):  
Eun Chae Jeon ◽  
Min Kyung Baik ◽  
Sung Hoon Kim ◽  
Baik Woo Lee ◽  
Dong Il Kwon
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Indentation Test ◽  
Plastic Property ◽  
Tensile Tests ◽  
Uniaxial Tensile ◽  
Flow Curves ◽  
Element Analysis ◽  
Representative Strain ◽  
Instrumented Indentation Test

A new method [1] to evaluate indentation flow curves using an instrumented indentation test has been applied to many materials for several years. Though the method produces relatively good results compared to uniaxial tensile tests, a few parameters had not been verified by theoretical or numerical methods. In this study, proportional constants of representative strain and representative stress were verified using finite element analysis and proven to be unaffected by the elastic property and strain level. The constants were generally dependent on the plastic property; however, one combination of the constants is independent of all properties. The values of this combination are consistent with early research and produced overlapping indentation flow curves with uniaxial curves.

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