Development Of Material Model Research Articles

Biomechanical properties and microstructure of human ventricular myocardium

In the multidisciplinary field of heart research it is of utmost importance to identify accurate myocardium material properties for the description of phenomena such as mechano-electric feedback or heart wall thickening. A rationally-based material model is required to understand the highly nonlinear mechanics of complex structures such as the passive myocardium under different loading conditions. Unfortunately, to date there are no experimental data of human heart tissues available to estimate material parameters and to develop adequate material models. This study aimed to determine biaxial extension and triaxial shear properties and the underlying microstructure of the passive human ventricular myocardium. Using new state-of-the-art equipment, planar biaxial extension tests were performed to determine the biaxial extension properties of the passive ventricular human myocardium. Shear properties of the myocardium were examined by triaxial simple shear tests performed on small cubic specimens excised from an adjacent region of the biaxial extension specimens. The three-dimensional microstructure was investigated through second-harmonic generation (SHG) microscopy on optically cleared tissues, which emphasized the 3D orientation and dispersion of the myofibers and adjacent collagen fabrics. The results suggest that the passive human LV myocardium under quasi-static and dynamic multiaxial loadings is a nonlinear, anisotropic (orthotropic), viscoelastic and history-dependent soft biological material undergoing large deformations. Material properties of the tissue components along local microstructural axes drive the nonlinear and orthotropic features of the myocardium. SHG microscopy investigation revealed detailed information about the myocardial microstructure due to its high resolution. It enabled the identification of structural parameters such as the fiber and the sheet orientations and corresponding dispersions. With this complete set of material data, a sophisticated material model and associated material parameters can be defined for a better description of the biomechanical response of the ventricular myocardium in humans. Such a model will lead to more accurate computational simulations to better understand the fundamental underlying ventricular mechanics, a step needed in the improvement of medical treatment of heart diseases. Statement of SignificanceUnfortunately, to date there are no experimental data of human heart tissues available for material parameter estimation and the development of adequate material models. In this manuscript novel biaxial tensile and shear test data at different specimen orientations are presented, which allowed to adequately capture the direction-dependent material response. With these complete sets of mechanical data, combined with their underlying microstructural data (also presented herein), sophisticated material models and associated material parameters can be defined for the description of the mechanical behavior of the ventricular myocardium in humans. Such models will lead to accurate computational simulations to better understand the fundamental underlying ventricular mechanics, a step needed in the improvement of medical treatment of heart diseases.

Shock Compression Modeling of Distended Mixtures

Development of material models to describe the thermodynamic shock states of distended mixtures is motivated by the need to understand how these materials respond over large compression ranges starting from low-pressure mechanical crush, extending into extreme thermodynamic states, and subsequent release to low-pressure. A material modeling approach is presented, which is comprised of a thermodynamically consistent equation-of-state (EOS) within a mixture-modeling framework. This modeling approach enables the investigator to describe the dynamic response of distended mixtures over a large compression range. The EOS model is applied to shock Hugoniot data on tungsten carbide, tantalum pentoxide, and calcite-water mixtures.

Elastic behaviour characterisation of TRIP 700 steel by means of loading–unloading tests

The elastic behaviour of TRIP 700 steel under plastic deformation is analysed. The analysis is carried out by means of classical tensile test and loading–unloading cyclic tests. These tests have been performed using high deformation strain gages, which enable an accurate and continuous measurement of strain.An elastic modulus reduction of 20% is observed for 12% plastic deformation. Furthermore, non-linear unloading and loading paths have been found in this work. This is an important difference with respect to other authors and opens new possibilities for the development of new material models to improve the prediction of the post-forming springback of industrial parts, which is an important issue for the automotive industry.

Comparison of approaches to model viscoelasticity based on fractional time derivatives

Two approaches to describe a constitutive fractional Zener model at large strain are presented. Both viscoelastic Zener models consist of a nonlinear elastic spring and a fractional Maxwell element in parallel. The fractional Maxwell element represents the viscoelastic behaviour of the formulation. Here, development of a new fractional viscoelastic material model under consideration of finite strain theory is presented. Additionally, the constitutive equations based on two different algorithmic approaches to capture the fractional time integration within this material model are derived. The consideration of fractional elements enables the characterization of highly inelastic, time dependent materials with relatively few material parameters. For the fractional element, a material parameter α determines the transition of the rheological element’s behaviour between spring (α=0) and dashpot (α=1). Accuracy and efficiency of a classical (non-recursive) and a new recursive algorithm to handle the fractional elements have been verified and validated by the comparison of several finite element (FE) simulations to material test results. The simulations found on finite strains and an implicit time integration scheme. Finally, a FE moulding simulation, found the extended constitutive model for the explicit time integration scheme has been carried out to illustrate the performance for large scale simulations in comparison to a real forming process and found to be quite efficient.

A non-linear orthotropic hydrocode model for ultra-high molecular weight polyethylene in impact simulations

This paper presents detailed experimental characterization of quasi-static anisotropic directional strength properties as well as the shock behavior of ultra-high molecular weight polyethylene (UHMWPE) for the development of an advanced material model for this class of materials. Specifically, we consider Dyneema® HB26 – pressed from uni-directional (UD) tapes in a 0/90° stacking sequence. A material model based on a constitutive law with orthotropic, non-linear strength, shock response, composite failure and softening criteria is presented. A set of material parameters is derived for applications in hydrocodes (here: ANSYS AUTODYN). High- and hypervelocity impact tests with different impact velocities are used for preliminary validation and discussion of the predictive capabilities in view of future application.

Analysis of the behavior of multiple dowel timber connections in fire

Being a combustible material, timber has not benefited from recent advances in fire research and associated computational work. However, as the design of timber structures is moving from prescriptive to performance based, there is a need for the development of material models that can be used in numerical simulations to predict fire resistance. A model based on continuum mechanics is therefore presented for the determination of the load bearing capacity of multi-dowel timber connections under fire. The model applies the classical flow theory of plasticity. It takes into account the orthotropic elastic behavior, the plastic anisotropic hardening, the large plastic deformations, and the effect of heat. Qualitatively, the predicted results agree with the experimental ones. The model can be easily implemented as a subroutine in existing finite element software.

Stress and Deformation Analysis of Concrete Face Rockfill Dam Based on ABAQUS

A new analysis method of stress and deformation of concrete face rockfill dam is established based on ABAQUS. This new approach includes secondary development of Duncan E-B material model and secondary development of Goodman frictional model of contact between concrete face and cushion. Meanwhile, contact characteristics is considered among concrete face and between concrete face and plinth. According to dam prototype observation, the analytic results are in good agreement with calculation results from the practical engineering, which indicates that the analysis method can be used widely.

Special Section on Advances in Ballistic Impact and Crashworthiness Response of Aerospace Structures

analytical methods needed to be used to determine the correct material properties for the sand. In addition, studies were conducted to determine the best methods of simulating the boundary conditions present in the test. In “Ballistic Impact Testing of Aluminum 2024 and Titanium 6Al-4V for Material Model Development,” Pereira et al. present asetofballistictestsonaluminumandtitanium flatpanelspecimens. The goal of the experimental program was to develop a set of highquality impact data suitable for use in impact simulations conducted using these materials. The ballistic limit and residual projectile velocity after impact were determined for each of the specimens. Cylindrical projectiles were used for the smaller specimens, whereas projectiles meant to simulate the features of an actual engine fan blade were used for the larger specimens. The effects of parameters suchasfrictionandprojectilehardnessontheimpactresponseofthe panels were also determined. In “Macromechanical Approach to Modeling Barely Visible Damage in Braided Composites,” Blinzler and Binienda describe a computationally efficient methodology developed to analyze the architecturally dependent impact damage in braided polymer composites. The braided composite is simulated as a series of parallel laminated composites. In the presented paper, the application of the method to model barely visible damage under impact conditions is presented. Specifically, the application of cohesive elements between layers of the braid to simulate the damage before penetration in the braided composites is presented. In “Analytical Model and Numerical Analysis of the Elastic Behavior of Triaxial Braided Composites,” Zhang et al. describe the development of a detailed mesomechanical finite-element model of a triaxially braided polymer matrix composite. The procedures used to determine the detailed geometry of the finite-element model based on detailed examination and measurements of micrographs of the actual composites are presented. Methods to determine theeffectiveelasticpropertiesoftheimpregnated fibertows,including the effects of fiber undulation, are described. The effective elastic properties of the braided composite are presented. The effects of factors such as edge effects and fiber tow undulation on the overall elastic response of the braided composites are discussed. In “Enhancements to Modeling Dry Fabrics for Impact Analysis,” Deivanayagam et al. present methods to model aircraft engine fan containment systems composed of dry Kevlar fabric. A constitutive model is described, and the coupon level tests required to characterize the constitutive model are discussed. Furthermore, the simulation of large-scale ballistic impact tests of structures composedoftheKevlarfabricarepresented,andcomparisonsbetweenthe experimental results and the analytical simulations are presented.

Transients and Evolution in NiTi

Many of the applications that seek to utilize shape memory alloys for their unique set of properties inevitably must deal, on some level, with the dimensional instability that is inherent to these materials under cyclic thermomechanical loading conditions. As a result, a better understanding of the transient and evolutionary behavior of a shape memory alloy is critical to both the successful design of useful actuation systems and development of accurate material models that can adequately capture the types of dimensional instability that can arise during component design. To this end, a set of experiments were conducted wherein the temperature cycling excursion was held fixed while the applied stress was varied. The results indicated that the extent of strain evolution produced under the initially applied stress has a significant impact on both the amount of transient that is observed as well as the rate of evolution observed under subsequent stress levels. In particular, lowering the applied stress to 50 MPa after cycling under an initial stress of 75 MPa did not stabilize the strain. However, lowering the applied stress to 50 MPa after cycling under an initial stress of 150 MPa produced a nearly saturated strain/temperature response. The thermomechanical observations are discussed in terms of the nature of strain evolution and its connection to the concept of a local/global minimization of the energy of the system, however, the exact mechanisms associated with these strain evolutions were not determined.

Development of a strain rate dependent material model of human cortical bone for computer-aided reconstruction of injury mechanisms

Computer-aided methods such as finite-element simulation offer a great potential in the forensic reconstruction of injury mechanisms. Numerous studies have been performed on understanding and analysing the mechanical properties of bone and the mechanism of its fracture. Determination of the mechanical properties of bones is made on the same basis used for other structural materials. The mechanical behaviour of bones is affected by the mechanical properties of the bone material, the geometry, the loading direction and mode and of course the loading rate. Strain rate dependency of mechanical properties of cortical bone has been well demonstrated in literature studies, but as many of these were performed on animal bones and at non-physiological strain rates it is questionable how these will apply in the human situations. High strain-rates dominate in a lot of forensic applications in automotive crashes and assault scenarios. There is an overwhelming need to a model which can describe the complex behaviour of bone at lower strain rates as well as higher ones. Some attempts have been made to model the viscoelastic and viscoplastic properties of the bone at high strain rates using constitutive mathematical models with little demonstrated success. The main objective of the present study is to model the rate dependent behaviour of the bones based on experimental data. An isotropic material model of human cortical bone with strain rate dependency effects is implemented using the LS-DYNA material library.We employed a human finite element model called THUMS (Total Human Model for Safety), developed by Toyota R&D Labs and the Wayne State University, USA. The finite element model of the human femur is extracted from the THUMS model. Different methods have been employed to develop a strain rate dependent material model for the femur bone. Results of one the recent experimental studies on human femur have been employed to obtain the numerical model for cortical femur. A forensic application of the model is explained in which impacts to the arm have been reconstructed using the finite element model of THUMS. The advantage of the numerical method is that a wide range of impact conditions can be easily reconstructed. Impact velocity has been changed as a parameter to find the tolerance levels of injuries to the lower arm. The method can be further developed to study the assaults and the injury mechanism which can lead to severe traumatic injuries in forensic cases.

Kraft ECF pulp bleaching: A review of the development and use of techno-economic models to optimize cost, performance, and justify capital expenditures

This work evaluates the findings of various research efforts and describes the development, use, and benefits of applying techno-economic models of different types to various production and bleach plant scenarios. During the late 1980s and early 1990s, researchers determined that bleach plants were releasing potentially significant quantities of chlorinated organics into receiving waters. This led to extensive research to develop bleaching chemicals to eliminate chlorine and hypochlorite from the bleaching sequence. Over time, 100% chlorine dioxide substitution for chlorine became the industry standard (today’s elemental chlorine free [ECF] bleaching), and research efforts shifted toward chemical optimization of the ECF bleaching sequence. Several delignification stage and brightening stage models were developed and refined for use in bleach plant simulations to optimize chemical performance. Advances in computing power and the availability of packaged spreadsheet and simulation programs likewise led to the development of detailed material and energy balance models of the bleach plants. These simulations are used to minimize bleaching and pulp production costs, and to assist engineers in the development of justifications for capital expenditures. These models have been used to predict operating conditions that could lead to or reduce scale formation. They have also been extensively used to predict environmental performance of both new and modified bleach plants.

Effect of Strain Rate on the Material Properties of Human Liver Parenchyma in Unconfined Compression

The liver is one of the most frequently injured organs in abdominal trauma. Although motor vehicle collisions are the most common cause of liver injuries, current anthropomorphic test devices are not equipped to predict the risk of sustaining abdominal organ injuries. Consequently, researchers rely on finite element models to assess the potential risk of injury to abdominal organs such as the liver. These models must be validated based on appropriate biomechanical data in order to accurately assess injury risk. This study presents a total of 36 uniaxial unconfined compression tests performed on fresh human liver parenchyma within 48 h of death. Each specimen was tested once to failure at one of four loading rates (0.012, 0.106, 1.036, and 10.708 s-1) in order to investigate the effects of loading rate on the compressive failure properties of human liver parenchyma. The results of this study showed that the response of human liver parenchyma is both nonlinear and rate dependent. Specifically, failure stress significantly increased with increased loading rate, while failure strain significantly decreased with increased loading rate. The failure stress and failure strain for all liver parenchyma specimens ranged from -38.9 kPa to -145.9 kPa and from -0.48 strain to -1.15 strain, respectively. Overall, this study provides novel biomechanical data that can be used in the development of rate dependent material models and the identification of tissue-level tolerance values, which are critical to the validation of finite element models used to assess injury risk.

Numerical modelling of ductile damage evolution in tensile and bending tests of timber structures

The characteristic values for strength and stiffness of all sorts of timber products are based on the assumption of a linear relation between stress and strain prior to failure and consequently verification of the load-bearing capacity of individual members in a construction is also based on a similar linear relation. Such an approach is very conservative and ill suited for performance-based design, which requires a full analysis of the structure with the possibility of moment and/or stress redistribution within parts of the structure. The development of material models that encompass the complex behaviour of wood is therefore necessary. The present work presents a model formulated within the frameworks of plasticity and continuum damage mechanics (CDM). It applies the classical flow theory of plasticity to formulate ductile failure of wood in compression and damage mechanics for the brittle failure modes. It takes into account the orthotropic elastic behaviour, the plastic anisotropic isotropic hardening, the isotropic ductile damage, and the large plastic deformations. The model was used to predict the initiation and growth of ductile damage in tensile and bending tests on different timbers types. Good agreement was found between the predictions of the model and the experimental results.

Development of a new nonlinear numerical material model for woven composite materials accounting for permanent deformation and damage

Due to their draping, stiffness, improved ductility and damage tolerance properties woven composites are being increasingly used for the construction of crash-relevant structural parts. Textile composites may depict a nonlinear behavior along several directions. Moreover, considerably-thick composite structures are likely to be used in order to increase energy absorption and to comply with the crash validation criteria. Therefore, a nonlinear numerical material model for textile composite materials has been developed for shells and thick shells. The model has been implemented as a user-defined subroutine (UMAT) in the LS-DYNA finite element code featuring with explicit time integration. The nonlinear behavior until failure is modeled in each in-plane material direction by a user-defined load curve or the Ramberg–Osgood equation. A plasticity formulation coupled with the nonlinearity accounts for permanent deformations. The failure is predicted using either a maximal stress criterion or the quadratic Tsai–Wu criterion. In order to model damage propagation, different post-failure damage definitions have been developed and implemented for each main in-plane material direction. A smeared formulation ensures the mesh independence in the presence of strain localization. The model has been assessed using characterization tensile and compressive tests on plain-weave and twill-weave carbon fiber composites.

Model-scale ice — Part A: Experiments

This paper is presenting novel model-scale ice property measurements for grain size, elastic strain-modulus, compressive and tensile specimen tests. The testing and analyzing procedure is targeted to define the basic material behavior accurately to understand the material behavior for the future development of a numerical material model. Additionally, the model-scale ice thickness and the bending strength (following ITTC) are determined to classify the ice properties. The experiments consist of systematic in-situ tests to identify the model-scale ice properties in a format suitable for numerical simulations. The elastic strain-modulus is determined on the intact level ice sheet based on the load displacement relationship of the infinite plate deflection. All specimens are cut with a template to minimize dimensional variations. The specimens are loaded with a linear drive at constant speed while displacement and force are recorded. The resulting load–displacement curves indicate good repeatability. The experiments are conducted over a time of 4h–5h in the keeping phase, where the cooling system is adjusted to maintain the mechanical ice properties, and the obtained results do not show a dependency on the time of testing. A linear-elastic finite element model is used to reproduce the plate bending measurements for the elastic strain-modulus determination. Therewith, it is found that the actual elastic strain-modulus is 27% larger than in plain stress theory due to stresses in thickness direction. Additionally, the approximate yield strength of the model-scale ice is investigated and is found to be significantly lower than the determined maximum stresses in compression, tension and bending. Consequently, this paper contributes to a deeper understanding of the mechanics of model-scale ice, and a procedure is shown how the mechanical parameters can be determined by systematic experiments and analyses.

Introduction of Materials Modelling into Processing Simulation

The introduction of materials modelling into computer-aided engineering (CAE) processing simulation has become popular in recent years, whereas the fundamental challenge lies in the development of material models that can calculate the properties essential for processing design and simulation. This paper reviews the recent development of such models and the material data that can be calculated include physical, thermophysical, and mechanical properties, as well as phase transformation kinetics. The calculated material data has been used as input to numerous CAE packages for the simulation of casting, welding, forming and heat treatments. Two case studies are presented here, one on the simulation of residual stress in linear friction welding of titanium alloys, and the other on the prediction of distortion and residual stress in heat-treated large steel rings.

Experimental Investigation of Strain Behaviour of Heated Cement Paste and Concrete

ABSTRACTThe material degradation of concrete subjected to fire events has a severe influence on the load‐carrying capacity of support structures. Spalling of concrete layers, exposing the reinforcement bars and degradation of the material properties (Young's modulus, compressive strength) may lead to significant damage of the reduced cross‐section and, therefore, cause failure of the structure. In order to understand the stress build‐up at the heated surface caused by thermal expansion due to fire loading, finally leading to damage and spalling of concrete, the strain behaviour of cement paste and concrete exposed to combined thermo‐mechanical loading is the focus of this work. Hereby, the evolution of thermal strains, Young's modulus and Poisson's ratio with increasing temperature are investigated experimentally. For this purpose, the specimens are loaded uniaxially while the temperature is increased up to 800 °C. The obtained results provide the proper basis for the development of realistic material models, allowing more sophisticated simulations of structures exposed to fire.

An experimental system for high temperature X-ray diffraction studies with in situ mechanical loading

An experimental system with in situ thermomechanical loading has been developed to enable high energy synchrotron x-ray diffraction studies of crystalline materials. The system applies and maintains loads of up to 2250 N in uniaxial tension or compression at a frequency of up to 100 Hz. The furnace heats the specimen uniformly up to a maximum temperature of 1200 °C in a variety of atmospheres (oxidizing, inert, reducing) that, combined with in situ mechanical loading, can be used to mimic processing and operating conditions of engineering components. The loaded specimen is reoriented with respect to the incident beam of x-rays using two rotational axes to increase the number of crystal orientations interrogated. The system was used at the Cornell High Energy Synchrotron Source to conduct experiments on single crystal silicon and polycrystalline Low Solvus High Refractory nickel-based superalloy. The data from these experiments provide new insights into how stresses evolve at the crystal scale during thermomechanical loading and complement the development of high-fidelity material models.

Parameter-refreshed Chaboche model for mild steel cyclic plasticity behaviour

The development of a steel material model in order to follow the cyclic plastic behaviour is essential in seismic design. The authors present an improved Chaboche model for the cyclic mechanical behaviour of structural steel. The model is based on the combination of nonlinear kinematic and multi-linear isotropic hardening with a parameter refreshing method. This model can consider the different physical phenomena of steel bahviour under cyclic loading. The developed model is verified by experimental results; the calculation of model parameters, the effect of the applied constants and the calibration are discussed.

902 応力軟化を考慮した異方性超弾性モデルの開発(OS9 オーガナイズドセッション《発電機器の材料・構造解析と評価》)

902 応力軟化を考慮した異方性超弾性モデルの開発(OS9 オーガナイズドセッション《発電機器の材料・構造解析と評価》)