Effect Of Strain Rate Dependency Research Articles

Multi-target active subspaces generated using a neural network for computationally efficient turbulent combustion kinetic uncertainty quantification in the flamelet regime

Propagating uncertainties in kinetic models through combustion simulations can provide important metrics on the reliability and accuracy of a model, but remains a challenging and numerically expensive problem especially for large kinetic models and expensive turbulent combustion simulations. Various surrogate model and dimension reduction techniques have previously been applied in order to reduce the cost of forward uncertainty propagation in combustion simulations, but these are often limited to low-dimensional, simple combustion cases with scalar solution targets. In the current work, we developed a neural network-accelerated framework for identifying a low-dimensional active kinetic subspace that applies to the entire temperature solution space of a flamelet table and can capture the mixture fraction and strain rate dependent effects of the kinetic uncertainty. We then demonstrated the computational savings enabled by this framework through a proof-of-concept, flamelet-based application in a Reynolds-averaged Sandia Flame D simulation using a chemical model for methane combustion with 217 reactions. By leveraging the large dimensional compression and low-cost scaling of the active subspace method, offloading the initial dimension reduction gradient sampling onto the laminar flamelet simulations, and accelerating the gradient sampling process with a specifically designed neural network, we were able to estimate the temperature uncertainty profiles across the solution space of the turbulent flame with strong accuracy of 70−85% using just seven perturbed solutions. Additionally, as it occurs entirely within the flamelet table, the cost of identifying the reduced subspace does not scale with the cost of the turbulent combustion model, which is a promising feature of this framework for future application to larger-scale and more complex turbulent combustion applications.

The influence of layered soil on ice-seabed interaction: Soft over stiff clay

Ice gouging is one of the main threats for the structural integrity of the offshore pipelines in the territories that floating icebergs can reach. The safe and cost-effective burial depth of pipelines for protection against the iceberg attack depends on accurate assessment of the subgouge soil deformation and the ice keel-seabed contact forces. Layered seabed soil strata that are found in many geographical locations adds to the complexity of assessing the ice keel-seabed interaction process by signifying the effect of strain rate dependence and strainsoftening in the soil layers interaction areas and ice-seabed contact zone. The existing design codes simplify the seabed condition by assuming a uniform soil strata. This in turn affects the accuracy and cost-effectiveness of the design. In this paper, large deformation finite element (LDFE) analysis was conducted using Coupled Eulerian Lagrangian (CEL) approach to investigate the free-field ice gouging in layered (soft over stiff clay) cohesive seabed. A modified Tresca soil model was coded in a user subroutine to account for the effects of strain rate and strain-softening on the mobilised undrained shear strength. A comprehensive parametric study was conducted to examine the effect of gouging features and soil shear strength profiles on the subgouge soil deformation, developed plastic shear strains and the resultant ice keel reaction forces. The analysis results showed that the seabed response to ice gouging is heavily dependent on the soil strata, where the layers interaction effects can override the usual response expected from a uniform seabed. Practical solutions were proposed to improve the pipeline design in layered seabed soils.

Inverse material characterisation of human aortic tissue for traumatic injury in motor vehicle crashes

Traumatic aortic rupture (TAR) resulting due to motor vehicle crashes (MVC) accounts for about 10%–20% fatalities. This article presents the outcome of our findings of the non-linear stress–strain response and strain rate-dependent behaviour of the aortic tissue under crash like impacts. This study uses both finite element (FE) modelling and experimental testing to enhance the understanding of injury mechanisms associated with TAR. Accurate material properties are essential for correct FE model predictions. Therefore, the objective of the current study was to experimentally characterise and identify a suitable constitutive model and model parameters for aortic tissue that can be incorporated into FE human body models for studying aortic rupture. Inverse characterisation and genetic algorithms (GA) were used to train the FE model to simulate real-life scenarios applying loading in multiple directions. A total of 32 uniaxial tensile tests were conducted on human aortic tissues along with the longitudinal and circumferential directions by loading at different strain rates ranging up to 200/s. The engineering stress–strain relationship obtained for human aorta in the longitudinal and circumferential directions via uniaxial tensile tests demonstrated an approximately bilinear behaviour and strain rate dependency in both directions. The higher stresses and modulus in the circumferential direction as compared to longitudinal direction demonstrates the anisotropic behaviour of the tissue. A constitutive model has been developed and implemented via user subroutine (UMAT) in LS-DYNA that accounts for the strain rate-dependent effects and bilinear behaviour observed in the aortic tissue. A FE model of the experimental set-up was developed, and the parameters of the model were estimated using a GA-based inverse mapping technique. The developed model enables investigation of the mechanical response of the aortic tissue under crashes and other high rate loading conditions. The work is based on the premise that a reliable constitutive model coupled with a FE model of the aorta shall help predict TARs.

High rate damage of composite material for modelling of energy absorbing structures

This paper proposes an approach for the modeling of crushing behavior of absorbing structural components, made of fiber-reinforced composite material. The effect of strain rate dependency of material properties is in the focus of this research, thus proposed material model is based on introduction of damage parameters, and high rate behavior is expressed in terms of damage rate. Stress-strain response of typical carbon fiber-reinforced composite to a wide range of impact loading rates provides data for calibration of the proposed model. Abaqus explicit solver with implemented user material model is used for direct 3D finite element simulation of impact with tubular specimen. Impact initiation process is considered in details to explain initial peak appearance on load diagram not only caused by the tube chamfer failure with consequent transfer of crushing process to a regular tubular zone, but also as effect of strain-rate dependency. Simulation shows a good agreement with typical load-displacement curves and crashworthiness values.

A Deep Learning Approach to Non-linearity in Wearable Stretch Sensors.

There is a growing need for flexible stretch sensors to monitor real time stress and strain in wearable technology. However, developing stretch sensors with linear responses is difficult due to viscoelastic and strain rate dependent effects. Instead of trying to engineer the perfect linear sensor we take a deep learning approach which can cope with non-linearity and yet still deliver reliable results. We present a general method for calibrating highly hysteretic resistive stretch sensors. We show results for textile and elastomeric stretch sensors however we believe the method is directly applicable to any physical choice of sensor material and fabrication, and easily adaptable to other sensing methods, such as those based on capacitance. Our algorithm does not require any a priori knowledge of the physical attributes or geometry of the sensor to be calibrated, which is a key advantage as stretchable sensors are generally applicable to highly complex geometries with integrated electronics requiring bespoke manufacture. The method involves three-stages. The first stage requires a calibration step in which the strain of the sensor material is measured using a webcam while the electrical response is measured via a set of arduino-based electronics. During this data collection stage, the strain is applied manually by pulling the sensor over a range of strains and strain rates corresponding to the realistic in-use strain and strain rates. The correlated data between electrical resistance and measured strain and strain rate are stored. In the second stage the data is passed to a Long Short Term Memory Neural Network (LSTM) which is trained using part of the data set. The ability of the LSTM to predict the strain state given a stream of unseen electrical resistance data is then assessed and the maximum errors established. In the third stage the sensor is removed from the webcam calibration set-up and embedded in the wearable application where the live stream of electrical resistance is the only measure of strain-this corresponds to the proposed use case. Highly accurate stretch topology mapping is achieved for the three commercially available flexible sensor materials tested.

A bilinear structural constitutive model for strain rate-dependent behaviour of human diaphragm tissue

Diaphragmatic rupture is the tear of the diaphragm muscle result from blunt or penetrating trauma and occurs in about 5% of cases of severe blunt trauma. Both finite element (FE) modelling and experimental testing have enhanced our understanding of the injury mechanisms associated with diaphragmatic rupture. A constitutive model for human diaphragm tissue is developed and implemented via user subroutine (UMAT) in LS Dyna that accounts for the strain rate-dependent effects and bilinear behaviour observed experimentally. To better understand the material properties of the human diaphragm, 16 dynamic tensile tests were conducted at three different strain rates of 65/s, 130/s and 190/s from six whole human diaphragms. The engineering stress–strain relationship obtained from these tests showed a bilinear behaviour and strain rate dependency. A strain rate-dependent bilinear stress–strain model was developed, and its parameters were optimised using a genetic algorithm-based inverse characterization method. The results demonstrate a good correlation between experiments and the model, with an average difference of 2 ± 2.8% (mean ± SD) between the optimised FE and experimental load–time curves. The material parameters found in this study can be used in dynamic simulations using FE models of diaphragm tissues.

On the Functional Role of Valve Interstitial Cell Stress Fibers: A Continuum Modeling Approach.

The function of the heart valve interstitial cells (VICs) is intimately connected to heart valve tissue remodeling and repair, as well as the onset and progression of valvular pathological processes. There is yet only very limited knowledge and extant models for the complex three-dimensional VIC internal stress-bearing structures, the associated cell-level biomechanical behaviors, and how they change under varying activation levels. Importantly, VICs are known to exist and function within the highly dynamic valve tissue environment, including very high physiological loading rates. Yet we have no knowledge on how these factors affect VIC function. To this end, we extended our previous VIC computational continuum mechanics model (Sakamoto, et al., 2016, "On Intrinsic Stress Fiber Contractile Forces in Semilunar Heart Valve Interstitial Cells Using a Continuum Mixture Model," J. Mech. Behav. Biomed. Mater., 54(244-258)). to incorporate realistic stress-fiber geometries, force-length relations (Hill model for active contraction), explicit α-smooth muscle actin (α-SMA) and F-actin expression levels, and strain rate. Novel micro-indentation measurements were then performed using cytochalasin D (CytoD), variable KCl molar concentrations, both alone and with transforming growth factor β1 (TGF-β1) (which emulates certain valvular pathological processes) to explore how α-SMA and F-actin expression levels influenced stress fiber responses under quasi-static and physiological loading rates. Simulation results indicated that both F-actin and α-SMA contributed substantially to stress fiber force generation, with the highest activation state (90 mM KCL + TGF-β1) inducing the largest α-SMA levels and associated force generation. Validation was performed by comparisons to traction force microscopy studies, which showed very good agreement. Interestingly, only in the highest activation state was strain rate sensitivity observed, which was captured successfully in the simulations. These unique findings demonstrated that only VICs with high levels of αSMA expression exhibited significant viscoelastic effects. Implications of this study include greater insight into the functional role of α-SMA and F-actin in VIC stress fiber function, and the potential for strain rate-dependent effects in pathological states where high levels of α-SMA occur, which appear to be unique to the valvular cellular in vivo microenvironment.

A comparative study of refined and simplified thermo-viscoplastic modeling of a thrust chamber with regenerative cooling

The thermostructural response of the cooling channel of a regeneratively cooled thrust chamber employed for rocket applications can be evaluated by means of analytical and numerical methods. The heat fluxes produced by the combustion inside the thrust chamber cause elevated temperatures in the copper structure composed of ligaments separating the coolant flow from the combustion gases. Those thermal loads together with the mechanical loads due to the flowing coolant and hot gases, lead to a non linear structural response. In the present work a comprehensive study of the viscoplastic phenomena is carried out and a simplified finite element model, which does not take into account strain rate-dependent effects, is adopted.The aim of the work is twofold: understanding and detail modeling of the thermo-viscoplastic phenomena occurring in the copper inner liner of a rocket thrust chamber and evaluating the degree of importance of strain-hardening rate-dependent phenomena. The present paper demonstrates that the simplified viscoplastic model adopted in this work is suitable in the preliminary design phase since the percentage difference detected with respect to rate dependent models, such as Perzyna's and Robinson's ones, is slightly larger than 10%. Furthermore, this model is particularly useful when the results of strain hardening tests, aimed at evaluating strain rate dependent properties, are not available for the chosen material.

A consistency elasto-viscoplastic damage model for progressive failure analysis of composite laminates subjected to various strain rate loadings

This paper is concerned with the development and validation of a consistency elasto-viscoplastic damage model that accounts for the strain rate-dependent plastic response and the progressive post-failure behaviour of composite materials. The model proposed is suitable for progressive failure analysis of composite materials and structures subjected to loadings at various strain rates. A strain rate-dependent yield criterion is adopted; whereas the standard Kuhn–Tucker conditions for plastic loading and unloading remain valid. Also, the plastic consistency condition for strain rate-dependent material is satisfied. The relevant strain-driven implicit numerical integration procedure for the proposed material model is developed. A tangent stiffness operator including the strain rate-dependent effects and consistent with the integration procedure developed is derived to ensure the quadratic convergence rate pertaining to the Newton–Raphson method in a finite element analysis. The implicit integration algorithm is implemented in the finite element code Abaqus through the user-defined material subroutine UMAT. The validation of the proposed model is performed on S2/8552 glass/epoxy off-axis block composite specimens subjected to compression at various strain rates. It is shown that the strain rate-dependent response of composite materials is well captured and the predicted results agree well with the experimental data reported in the literature.

セル壁構成材のひずみ速度依存性が金属発泡体の動的圧縮挙動に及ぼす影響

セル壁構成材のひずみ速度依存性が金属発泡体の動的圧縮挙動に及ぼす影響

Strain rate dependency in paratransit bus rollover

This paper examines the effect of structural steel strain rate dependency on the outcome of the FDOT/ECE-R66 bus rollover test. Bending and impact tests were performed on structural tubing in order to evaluate strain rate dependency parameters. Several mesh densities were used to investigate the influence of mesh size on the final outcome of the analysis. Two strain rate dependency formulations, Cowper-Symonds and Johnson-Cook, were calibrated and then implemented in the finite element analysis of a paratransit bus rollover. Unlike strain rate effect on other types of impact such as the frontal crash, the results show that the strain rate has no significant effect on the FDOT/ECE-R66 rollover test. Therefore, the current approach used by researchers investigating bus rollovers, to validate finite element models based on quasi-static experiments, is well-grounded and the strain rate dependent effects may be ignored for the global deformation response in the ECE-R66 bus rollover.

Static and dynamic axial crushing analysis of car frontal impact hybrid absorbers

Occupant safety and protection systems have risen as one of the foremost research fields in automotive engineering. Composite materials and synthetic foams are highly efficient car building materials due to their low weight and high strength, as well as their remarkable crashworthiness in combination with ductile metals. This paper compares the frontal crashworthiness capabilities of carbon-fiber reinforced polymers, glass-fiber reinforced polyamide, polyethylene terephthalate foam and cork conglomerates in combination with cold-formed steel polygonal tubes using quasi-static and dynamic numerical simulation verified with experimental results. Reinforcement materials are usually wrapped around the steel box, but in this research they are placed inside the box according to certain geometrical patterns and subjected to axial crushing. Non-linear effects, contacts, strain rate dependent effects, geometric imperfections and residual stresses are taken into account to achieve realistic results, which show a remarkable improvement in terms of energy absorption in most cases, and a quality agreement with experimental data. A combination of a glass-fiber reinforced polyamide padding and a steel box has offered the best results, and a detailed study of its collapse mechanism and the sensitivity of its specific energy absorption with respect to the thickness of the components is provided as well.

Prediction of Compressive Strength for Unidirectional Fiber Reinforced Plastics in Considering Effect of Strain-Rate Dependency on Mechanical Properties of Constituent Materials

This study presents an analytical model to predict compressive strength of unidirectional FRP. Proposed model considers the effect of strain-rate dependency on mechanical properties of constituent materials. The model is based on the elastic foundation model and the microbuckling model of fiber which has initial misalignment in matrix. Compressive deformation of unidirectional FRP is considered by dividing into fiber microbuckling region and plastic kinking region. Additionally, to take into consideration the change in compressive deformation mode accompanying fiber volume fraction or fiber microbuckling, A mode function is introduced. The predictions from the proposed model are compared with experimental results of unidirectional E-glass/Epoxy and T700SC/Epoxy evaluated by using the conventional split Hopkinson pressure bar method. Incorporating strain-rate dependency on compressive modulus of reinforcement calculated from composite mixture law, the predictions are found to be in good agreement with experimental results of strain-rate dependency on compressive strength. Accuracy of the prediction is improved by changing the mode function.

EFFECTS OF STRAIN RATE DEPENDENCY OF MATERIAL PROPERTIES IN LOW VELOCITY IMPACT

Impact processes are often analyzed using the coefficient of restitution which represents the kinetic energy loss during impact. In this paper the effect of strain rate dependency of the yield stress on the coefficient of restitution is investigated experimentally and numerically for the impact of a steel sphere against a steel rod. Finite Element simulations using strain-rate dependent material behavior are carried out. In addition, Finite Element simulations with elastic-plastic material behavior, which ignore the strain rate dependency, are carried out as well as elastic material behavior. Comparisons between the experiments and the simulations using strain-rate dependent material behavior show good agreement, and also prove the strong dependency of the coefficient of restitution on the strain rate dependency of the yield stress for steel. The results from both, the experiments and the simulations show also the strong influence of the wave propagation in the rod on the coefficient of restitution.

Issues in connection to the prediction of strain localization in metal sheets

The current paper deals with the prediction of strain localization in metal sheets. Especially the effect of strain rate dependency on the necking phenomenon is treated. The ability of FEM to predict strain localization during simulations of sheet forming operations is investigated. An approach for the determination of the strain rate material parameters, based on an experimentally determined FLD0, is suggested. The current approach is demonstrated for three sheet materials, and from the results the approach can be judged as promising.

Multiscale Analysis Including Strain Rate Dependency for Transient Response of Composite Laminated Shells

A multiscale micro-macro numerical procedure has been developed to model the transient responses of polymeric composite shell structures. A micromechanics model which accounts for the transverse shear stress effect, the effect of strain rate dependency, and the effect of inelasticity is used for analyzing the mechanical responses of the fiber and matrix constituents. The accuracy of the micromechanics model under transverse shear loading is verified by comparison with an existing procedure. A higher-order laminated shell theory is extended to capture the inelastic deformations of the composite shell and is implemented using the finite element technique. A detailed parametric study is conducted to investigate the influence of geometry, ply stacking sequence, material models, and loading conditions on the transient response of laminated shell structures under impact loadings.

Nonlinear transient response of strain rate dependent composite laminated plates using multiscale simulation

The effects of strain rate dependency and inelasticity on the transient responses of composite laminated plates are investigated. A micromechanics model which accounts for the transverse shear stress effect, the effect of strain rate dependency and the effect of inelasticity is used for analyzing the mechanical responses of the fiber and matrix constituents. The accuracy of the micromechanics model under transverse shear loading is verified by comparing the results with those obtained using a general purpose finite element code. A higher order laminated plate theory is extended to capture the inelastic deformations of the composite plate and is implemented using the finite element technique. A complete micro–macro numerical procedure is developed to model the strain rate dependent behavior of inelastic composite laminates by implementing the micromechanics model into the finite element model. Parametric studies of the transient responses of composite plates are conduced. The effects of geometry, ply stacking sequence, material models, boundary conditions and loadings are investigated. The results show that the strain rate dependency and inelasticity influence the transient responses of composite plates via two significantly different mechanisms.

A structural constitutive model for the strain rate-dependent behavior of anterior cruciate ligaments

A structural continuum constitutive law is formulated to illustrate the anterior cruciate ligament (ACL) mechanical behavior. The constitutive law is obtained by modifying previously proposed structural models for soft tissues to include the description of strain rate-dependent effects. The ligamentous tissue anisotropy, non-linearity, incompressibility, and strain rate related properties are taken into account. The collagen fibers, which comprise the ACL, are assumed to be the only load-bearing component of the tissue. They are oriented in different directions, undulated in the stress-free configuration and they gradually become taut upon loading. Moreover, the taut collagen fibers are characterized by a Kelvin–Voigt-type viscoelastic behavior. The fiber spatial orientation and gradual recruitment are represented statistically by probability density functions. Published experimental stress–strain data of the ACL-bone complex are used to assess the constitutive model. The model is tested by assuming that the ACL has a perfectly parallel collagen structure and undergoes an isochoric, axisymmetric deformation. Furthermore, the fiber recruitment is defined by a one-sided probability density function. The structural parameters are able to fit the stress–strain curves at different strain rates.

Stress-induced transformation behavior of a polycrystalline NiTi shape memory alloy: micro and macromechanical investigations via in situ optical microscopy

An experimental investigation of the micro and macromechanical transformation behavior of polycrystalline NiTi shape memory alloys was undertaken. Special attention was paid to macroscopic banding, variant microstructure, effects of cyclic loading, strain rate and temperature effects. Use of an interference filter on the microscope enabled observation of grain boundaries and martensitic plate formation and growth without recourse to etching or other chemical surface preparation. Key results of the experiments on the NiTi include observation of localized plastic deformation after only a few cycles, excellent temperature and stress relaxation correlation, a refined definition of “full transformation” for polycrystalline materials, and strain rate dependent effects. Several of these findings have critical implications for understanding and modeling of shape memory alloy behavior.

Effect of formaldehyde fixation on some mechanical properties of bovine bone

The risk of infection of investigators working on the biomechanics of human bone from a variety of modern pathogens including the human immunodeficiency virus or the hepatitis B virus has increased recently. New safety procedures are needed to reduce that risk. The procedure we follow in our aboratory employs brief (< 3 h) fixation in formaldehyde, and we report here the effects it has on some mechanical properties of bovine bone. Results in quasistatic loading tests were almost unaffected by our fixation protocol, but a significant decrease in impact strength was found. These results indicate that there may be some interaction between fixation and strain rate dependent effects and, therefore, some caution is needed when using common biomechanical measurement methods on fixed bone material.