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Axonal injury occurs during trauma when tissue-scale loads are transferred to individual axons. Computational models are used to understand this transfer and predict the circumstances that cause injury. However, these findings are limited by a lack of validating experimental work examining the mechanics of axons in their in situ state. As a first step towards validation for dynamic stretch, we use contactin-associated protein (Caspr), expressed at the nodes of Ranvier, as a fiduciary marker of quasistatic axonal stretch. We measured changes in the distance between immunolabled Caspr pairs along axons as a function of tissue-level stretch in chick embryo spinal cords harvested from different developmental periods. We then identified and characterized broken axons and adapted a kinematic model published previously by our group ( Singh et al., 2015 ) to estimate average strain thresholds for axon mechanical failure. The distance between Caspr pairs increased with stretch, though not as much as predicted by simple continuum mechanics. For equivalent tissue stretch, greater numbers of broken axons were found at later stages of development. In adapting our kinematic model to predict a breaking threshold strain, we found that breaking thresholds decrease with development stage. When thresholds were split and classified based on kinematic behavior, non-affine, uncoupled axons had higher strain thresholds than affine, coupled axons, corroborating thresholds predicted from in vitro and in vivo preparations. These results provide a valuable launching point for generating more accurate multi-scale models in primary central nervous system injury. [ABSTRACT FROM AUTHOR]

Biomechanical imaging techniques based on acoustic radiation force (ARF) have been developed to characterize the viscoelasticity of soft tissue by measuring the motion excited by ARF non-invasively. The unknown stress distribution in the region of excitation limits an accurate inverse characterization of soft tissue viscoelasticity, and single degree-of-freedom simplified models have been applied to solve the inverse problem approximately. In this study, the ARF-induced creep imaging is employed to estimate the time constant of a Voigt viscoelastic tissue model, and an inverse finite element (FE) characterization procedure based on a Bayesian formulation is presented. The Bayesian approach aims to estimate a reasonable quantification of the probability distributions of soft tissue mechanical properties in the presence of measurement noise and model parameter uncertainty. Gaussian process metamodeling is applied to provide a fast statistical approximation based on a small number of computationally expensive FE model runs. Numerical simulation results demonstrate that the Bayesian approach provides an efficient and practical estimation of the probability distributions of time constant in the ARF-induced creep imaging. In a comparison study with the single degree of freedom models, the Bayesian approach with FE models improves the estimation results even in the presence of large uncertainty levels of the model parameters. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Traumatic injury to axons in white matter of the brain and spinal cord occurs primarily via tensile stretch. During injury, the stress and strain experienced at the tissue level is transferred to the microscopic axons. How this transfer occurs, and the primary constituents dictating this transfer must be better understood to develop more accurate multi-scale models of injury. Previous studies have characterized axon tortuosity and kinematic behavior in 2-dimensions (2-D), where axons have been modeled to exhibit non-affine (discrete), affine (composite-like), or switching behavior. In this study, we characterize axon tortuosity and model axon kinematic behavior in 3-dimensions (3-D). Embryonic chick spinal cords at different development stages were excised and stretched. Cords were then fixed, transversely sectioned, stained, and imaged. 3-D axon tortuosity was measured from confocal images using a custom-built MATLAB script. 2-D kinematic models previously described in Bain et al. (J Biomech Eng 125(6):798, ) were extended, re-derived, and validated for the 3-D case. Results showed that 3-D tortuosity decreased with stretch, exhibiting similar trends with changes in development as observed in the 2-D studies. Kinematic parameters also displayed similar general trends. Axons demonstrated more affine behavior with increasing stretch and development. In comparison with 2-D results, a smaller percentage of the populations of 3-D axons were predicted to follow pure non-affine behavior. The data and kinematic models presented herein can be incorporated into multi-scale CNS injury models, which can advance the accuracy of the models and improve the potential to identify axonal injury thresholds. [ABSTRACT FROM AUTHOR]

Acoustic radiation force (ARF) creep imaging applies step ARF excitation to induce creep displacement of soft tissue, and the corresponding time-dependent responses are used to estimate soft tissue viscoelasticity or its contrast. Single degree of freedom (SDF) and homogeneous analytical models have been used to characterize soft tissue viscoelasticity in ARF creep imaging. The purpose of this study is to investigate the fundamental limitations of the commonly used SDF and homogeneous assumptions in ARF creep imaging. In this paper, finite element (FE) models are developed to simulate the dynamic behavior of viscoelastic soft tissue subjected to step ARF. Both homogeneous and heterogeneous models are studied with different soft tissue viscoelasticity and ARF configurations. The results indicate that the SDF model can provide good estimations for homogeneous soft tissue with high viscosity, but exhibits poor performance for low viscosity soft tissue. In addition, a smaller focal region of the ARF is desirable to reduce the estimation error with the SDF models. For heterogeneous media, the responses of the focal region are highly affected by the local heterogeneity, which results in deterioration of the effectiveness of the SDF and homogeneous simplifications. [ABSTRACT FROM AUTHOR]

Biomechanical imaging techniques have been developed for soft tissue characterisation and detection of breast tumours. Harmonic motion imaging (HMI) uses a focused ultrasound technology to generate a harmonic radiation force in a localised region inside a soft tissue. The resulting dynamic response is used to map the local distribution of the mechanical properties of the tissue. In this study, a finite element (FE) model is developed to investigate the effect of global boundary conditions on the dynamic response of a soft tissue during HMI. The direct-solution steady-state dynamic analysis procedure is used to compute the harmonic displacement amplitude in FE simulations. The model is parameterised in terms of boundary conditions and viscoelastic properties, and the corresponding raster-scan displacement amplitudes are captured to examine its response. The effect of the model's global dimensions on the harmonic response is also investigated. It is observed that the dynamic response of soft tissue with high viscosity is independent of the global boundary conditions for regions remote to the boundary; thus, it can be subjected to local analysis to estimate the underlying mechanical properties. However, the dynamic response is sensitive to global boundary conditions for tissue with low viscosity or regions located near to the boundary. [ABSTRACT FROM AUTHOR]

Models that can predict battery cells' thermal and electrical behaviors are necessary for real-time battery management systems to regulate the imbalance within battery cells. This work introduces a Gaussian Process Regression (GPR)-based data-driven framework that succeeds the Multi-Scale Multi-Dimensional (MSMD) modeling structure. The framework can make highly accurate predictions at the same level as full-order full-distribution simulations based on MSMD. A pseudo-2D model is used to generate training data and is combined with a process that shifts computation burdens from real-time battery management systems to lab data preparation. The testing results highlight the reliability of the GPR-based data-driven framework in terms of accuracy and stability under various operational conditions. [ABSTRACT FROM AUTHOR]

A finite element method is employed to numerically evaluate the stiffness and energy absorption properties of an architecturally hybrid composite material consisting of unidirectional and random glass fiber layers. An LS-DYNA finite element model of a composite hollow square tube is developed in which the position of the random fiber layers varies through the thickness. The assessment of the stiffness and energy absorption is performed via three-point impact and longitudinal crash tests at two speeds, 15.6 m/s (35 mph) and 29.0 m/s (65 mph), and five strain rates, ε = 0.1 s-1, 1s-1, 10 s-1, 20 s-1, and 40 s-1. It is suggested that strategic positioning of the random fiber microstructural architecture into the hybrid composite increases its specific absorption energy and, therefore, enhances its crashworthiness. The simulation data indicate that the composite structure with outer layers of unidirectional lamina followed by random fiber layers is the stiffest due to the considerable superior specific energy absorption of the random fiber micro-architecture. Moreover, it is illustrated that the specific energy absorption increases with the increased ratio of impact contact area over cross-section area. Of all the parameters tested the thickness of the unidirectional laminate on the specific energy absorption does not appear to have a significant effect at the studied thickness ratios. [ABSTRACT FROM AUTHOR]

Organic and inorganic fiber reinforced composites with various fiber orientation distributions and fiber geometries are abundantly available in several natural and synthetic structures. Inorganic glass fiber composites have been introduced to numerous applications due to their economical fabrication and tailored structural properties. Numerical characterization of such composite materials is necessitated due to their intrinsic statistical nature, since elaborate experiments are prohibitively costly and time consuming. In this work, representative volume elements of unidirectional random filaments and fibers are numerically developed in PYTHON to enhance accuracy and efficiency of complex geometric representations encountered in random fiber networks. A modified random sequential adsorption algorithm is applied to increase the volume fraction of the representative volume elements, and a spatial segment shortest distance algorithm is introduced to construct a 3D random fiber composite with high fiber aspect ratio (100:1) and high volume fraction (31.8%). For the unidirectional fiber networks, volume fractions as high as 70% are achieved when an assortment of circular fiber diameters are used in the representative volume element. [ABSTRACT FROM AUTHOR]

The mechanical properties of random chopped fiber composites are analyzed using micromechanical principles. A progressive damage model is adopted to investigate the damage and failure of the material. A representative volume element is generated numerically based on microscopic observations that capture the complex mesostructure of the random chopped fiber composite specimens. Sequentially, the mechanical properties are obtained using a micromechanics approach, particularly, the homogenization method. The underlying hypothesis insinuates that damage mechanisms such as matrix cracking, fiber damage, and interfacial debonding are responsible for the damaged behavior of the composite. Matrix cracking and fiber damage are modeled by progressive degradation of their respective stiffnesses. The interfacial debonding is modeled with a cohesive zone model. The prediction of uniaxial tensile response is compared with experimental data. [ABSTRACT FROM AUTHOR]

Abstract: Numerous experimental and computational methods have been developed to estimate tissue elasticity. The existing testing techniques are generally classified into in vitro, invasive in vivo and non-invasive in vivo. For each experimental method, a computational scheme is accordingly proposed to calculate mechanical properties of soft biological tissues. Harmonic motion imaging (HMI) is a new technique that performs radio frequency (RF) signal tracking to estimate the localized oscillatory motion resulting from a radiation force produced by focused ultrasound. A mechanical model and computational scheme based on the superposition principle are developed in this paper to estimate the Young''s modulus of a tissue mimicking phantom and bovine liver in vitro tissue from the harmonic displacement measured by HMI. The simulation results are verified by two groups of measurement data, and good agreement is shown in each comparison. Furthermore, an inverse function is observed to correlate the elastic modulus of uniform phantoms with amplitude of displacement measured in HMI. The computational scheme is also implemented to estimate 3D elastic modulus of bovine liver in vitro. [Copyright &y& Elsevier]

Abstract: In this paper, the cone nanoindentation on soft film/hard substrate and hard film/soft substrate material systems was systematically investigated with the FE method. Based on the elastic–perfectly plastic material model, two typical sets of mechanical parameters (i.e., Young’s modulus and Poisson’s ratio) and initial yield stress were configured in such a way that the piling-up and sinking-in phenomena during nanoindentation can be observed. The mechanical responses of soft film/hard substrate and hard film/soft substrate material systems to the indenter’s maximum displacement were obtained by changing the ratio between the indenter’s maximum displacement and thin-film thickness, h max/t, from 0.05 to 0.45 (at 0.05 increments) and by comparing FE analysis results with the corresponding bulk materials’ mechanical responses. Finally, the effect of substrate properties on the calculated mechanical properties of thin films and the piling-up and sinking-in phenomena were studied. It was found that, when Young’s modulus of a thin film was smaller than that of its substrate, the calculated Young’s modulus of the thin film increased with the indenter’s maximum displacement; and that, when Young’s modulus of a thin film was larger than that of its substrate, the calculated value decreased with the indenter’s maximum displacement. For the configured hard film/soft substrate material system, the calculated hardness of the thin film started to decrease only when h max/t was equal to 0.40. It is concluded that the calculated Young’s modulus of thin films responds more sensitively to the mechanical properties of substrates than does the calculated hardness. Also, the relationship of the indenter’s contact depth and its maximum displacement deviates from the linear relationship noted for thin-film materials. The piling-up of the soft film/hard substrate material and the sinking-in of the hard film/soft substrate material are more severe than those of the corresponding soft and hard bulk materials. [Copyright &y& Elsevier]

Abstract: Finite elements (FE) provide a numerical method to calculate complex nanoindentation problems. Correlation of FE analysis with experimental data on nanoindentation may lead to improved characterization of the mechanical properties of thin films and coating systems. In this study, a model of the friction contact at the interface of thin films and substrates is established using FE analysis about a cone indenter, which imitates a Berkovich nanoindenter. Finite element nanoindentation simulations were performed at three different interface friction contact conditions. The following conclusions were depicted through the study of the simulation data. First, for increasing values of the friction coefficient, the indenter’s force versus displacement response of the film/substrate-friction-contact (F/SFC) model coincides with the response of the film/substrate-perfectly-bonded (F/SPB) model. Second, when the indenter’s maximum displacement is less than 10% of the film thickness the deformed nanoindentation area is concentrated under the indenter tip for both F/SFC and F/SPB models. Third, a mechanical response is generated along the F/SFC interface while the mechanical response along the F/SPB interface is negligible. Finally, the nanoindentation simulation data indicate that the calculated mechanical properties intrinsically depend on the interfacial contact conditions of the film/substrate even when the maximum displacement of the indenter is controlled within the 10% of the thin-film thickness. [Copyright &y& Elsevier]

The quantitative study of mechanical properties at nanoscale is motivated by the increasing demand for miniaturization of engineering and electronic components, development of nanostructured materials, thin film technology, and surface science. There are only a few existing, or projected, technologies for conducting such measurements at nanoscale structures, and nanoindentation is the leading candidate. Finite elements (FE) provide a numerical method to calculate the indentation problem. In this article, the mechanical properties of a thin film material system, namely thermally oxidized silicon grown on silicon substrate, SiO2/Si, is characterized with nanoindentation measurements and calculated via FE analysis. Infinite elements in FE nanoindentation analysis are employed to allow for actual specimen size simulation and produce accurate results. It is concluded that the nanoindentation measurements coupled with the proposed FE analysis provide precise mechanical characterization of the tested thin films. For the featured thermally oxidized silicon grown on silicon substrate, the hardness and the reduced Young's modulus are directly determined from Berkovich nanoindentation measurements as H = 9.23 ± 0.30 GPa and E = 63.11 ± 0.26 GPa, respectively. From the FE analysis, the following mechanical parameters are obtained: hardness, 10.63 GPa, Young's modulus, 65.20 GPa, initial yield stress, 6.50 GPa, and Poisson's ratio, 0.35. A FE analysis of a cube corner nanoindenter is also conducted and the results are compared to the Berkovich simulations. [ABSTRACT FROM AUTHOR]

MEMS (MicroElectroMechanical Systems) are composed of thin films and composite nanomaterials. Although the mechanical properties of their constituent materials play an important role in controlling their quality, reliability, and lifetime, they are often found to be different from their bulk counterparts. In this paper, low-k porous silica thin films spin coated on silicon substrates are studied. The roughness of spin-on coated porous silica films is analyzed with in-situ imaging and their mechanical properties are determined using nanoindentation. A Berkovich type nanoindenter, of a 142.3 deg total included angle, is used and continuous measurements of force and displacements are acquired. It is shown, that the measured results of hardness and Young's modulus of these films depend on penetration depth. Furthermore, the film's mechanical properties are influenced by the properties of the substrate, and the reproduction of the force versus displacement curves depends on the quality of the thin film. The hardness of the studied low-k spin coated silica thin film is measured as 0.35∼0.41 GPa and the Young's modulus is determined as 2.74∼2.94 GPa. [ABSTRACT FROM AUTHOR]

The complex fracture behavior of a cross-ply composite cantilever beam with artificially embedded delamination is investigated analytically, numerically, and experimentally. The analysis of the cantilever beam is divided into two geometric configurations: the global bending of the undelaminated cantilever, and the local buckling of the delaminated part. A finite element model developed in ANSYS is used to concurrently analyze the effects of contact zone and delamination in the aforementioned asymmetrically loaded structure. The obtained experimental data are correlated and compared with the findings of the FEM simulations. All numerical, analytical, and experimental results illustrate that the fracture behavior of the laminate cantilever beam is dominated by mode II, mainly due to the effect of a large contact zone. The latter is determined by geometric and loading parameters. The dominance of mode H over mode 1, leads to the initiation and propagation of an interfacial crack rather than an intralayer one. Furthermore, experimental evidence indicates that crack kinking during propagation depends on the architecture of the specimens. [ABSTRACT FROM AUTHOR]

Studies the maximization of mode I critical delamination fracture toughness (G[subIC]) in cross ply graphite-epoxy laminates using Design of Experiments and Taguchi arrays. Main and interactive factor effects on G[subIC]; Effect of interfaces on delamination propagation; Response surface for balanced laminates.

An analytical model, supported by numerical simulations, which describes the behavior of the energy release rate of composite laminates subjected to fatigue overloads is presented. The model establishes that accumulation of damage due to intermittent surge in applied load reduces the energy release rate, G, of the material. The effect of constant amplitude, regularly applied overloads on the energy release rate of [0/90]n graphite/epoxy composites appears to diminish as the structural integrity reduces over time, suggesting the effects of overloads is more severe during the initiation of delamination growth. The results obtained in this study have no restrictive assumptions regarding the specimen or the delamination thickness, T and h, respectively, and are presented over varying mode-mixity and h/T ratios. Simulations are correlated and compared with data from performed experiments confirming the model’s functionality and predictive capability. Results of this investigation shed light on the nature and extent of damage accumulation during spectral fatigue. [ABSTRACT FROM AUTHOR]

Studies the behavior of delaminated graphite/epoxy composites under variable amplitude compressive loading. Measurements of the change in delamination length; Vulnerability of laminated composites to damage by delamination; Use of specimens with unidirectional and cross ply lay-up configurations.

Herein, state‐of‐the‐art multiscale modeling methods have been described. This research includes notable molecular, micro‐, meso‐, and macroscale models for hard (polymer, metal, yarn, fiber, fiber‐reinforced polymer, and polymer matrix composites) and soft (biological tissues such as brain white matter [BWM]) composite materials. These numerical models vary from molecular dynamics simulations to finite‐element (FE) analyses and machine learning/deep learning surrogate models. Constitutive material models are summarized, such as viscoelastic hyperelastic, and emerging models like fractional viscoelastic. Key challenges such as meshing, data variability and material nonlinearity‐driven uncertainty, limitations in terms of computational resources availability, model fidelity, and repeatability are outlined with state‐of‐the‐art models. Latest advancements in FE modeling involving meshless methods, hybrid ML and FE models, and nonlinear constitutive material (linear and nonlinear) models aim to provide readers with a clear outlook on futuristic trends in composite multiscale modeling research and development. The data‐driven models presented here are of varying length and time scales, developed using advanced mathematical, numerical, and huge volumes of experimental results as data for digital models. An in‐depth discussion on data‐driven models would provide researchers with the necessary tools to build real‐time composite structure monitoring and lifecycle prediction models. [ABSTRACT FROM AUTHOR]

Investigates the growth behavior of interlayer and intralayer cracks in delaminated cross-ply laminates subjected to fatigue compression. Effects of mode mixity and position of the delamination in the formation of the cracks; Application of the mode-dependent fatigue growth law to the prediction of the behavior of the intralayer and interlayer cracks.

Nanoindentation of single microscale fibers is a challenging task due to the dimensional similarity of the probing instruments to the single-fiber cross sections. Algorithms customarily used in nanoindentation equipment assume that the indentation occurs in flat surfaces, thus simplifying the local geometry and approximating material properties in curved specimens. A modified Curved Area Function (mCAF) is developed using finite element analysis and tested using nanoindentation measurements of high-performance single microfibers. The mCAF accounts for the dimensions and curvature in the transverse direction of the cylindrical microfiber in conjunction with the contact depth and the impression of the indented area. The transverse direction indentation modulus of a 12 μ m of Kevlar ® KM2 fiber was estimated as 7.76 ± 0.22 GPa. The computational results were corroborated with experimental measurements performed on KM2 single fibers and agreed with literature findings that assumed minor differences in testing equipment, area analysis, and projected surface area. Two geometry-related coefficients C 0 and C 1 were determined that facilitate simulation of fiber nanoindentations with diameters of d = 7 , 15 , 30 , 40 , and 50 μ m with indentation depths up to, and including, 2 μ m . The mCAF provided a narrow measurement error of ± 0.22 GPa (2.8%) when compared to published studies using the semi-infinite plane approximation, reinforcing the suitability of the developed model. [ABSTRACT FROM AUTHOR]

White matter (WM) consists of bundles of long axons embedded in a glial matrix, which lead to anisotropic mechanical properties of brain tissue, and this complicates direct numerical simulations of WM viscoelastic response. The detailed axonal geometry contains scales that range from axonal diameter (microscale) to many diameters (mesoscale) imposing an additional challenge to numerical simulations. Here we describe the development of a 3D homogenization model for the central nervous system (CNS) that accounts for the anisotropy introduced by the axon/neuroglia composite, the axonal trace curvature, and the tissue dynamic response in the frequency domain. Homogenized models that allow the incorporation of all the above factors are important for accurately simulating the tissue's mechanical behavior, and this in turn is essential in interpreting non-invasive elastography measurements. Geometric and material parameters affect the material properties and thus the response of the brain tissue. More complex, orthotropic, or anisotropic material properties are to be considered as necessitated by the 3D tissue structure. An assembly of micro-scale 3D representative elemental volumes (REVs) is constructed, leading to an integrated mesoscale WM finite element model. Assemblies of microscopic REVs, with orientations based on geometrical reconstructions driven by confocal microscopy data are employed to form the elements of the WM model. Each REV carries local material properties based on a finite element model of biphasic (axon-glial matrix) unidirectional composite. The viscoelastic response of the microscopic REVs is extracted based on geometric information and fiber volume fractions calculated from the relative distance between the local elements and global axonal trace. The response of the WM tissue model is homogenized by averaging the shear moduli over the total volume (thus deriving effective properties) under realistic external loading conditions. Under harmonic shear loading, it is proven that that the effective transverse shear moduli are higher than the axial moduli when the axon moduli are higher than the glial. Methodologically, the process of using micro-scale 3D REVs to describe more complex axon geometries avoids the partition process in traditional composite finite element methods (based on partition of finite element grids) and constitutes a robust algorithm to automatically build a WM model based on available axonal trace information. Analytically, the model provides unmatched simulation flexibility and computational power as the position, orientation, and the magnitude of each tissue building block is calculated using real tissue data, as are the training and testing processes at each level of the multiscale WM tissue. • Brain and spinal cord injuries relate to cell-level mechanical stresses and strains but MR elastography (MRE) data are on mm scale. • A histology-informed numerical model of the viscoelastic response of myelinated axons under harmonic shear is developed. • Novelty lies in microscopic element assemblies with orientations based on reconstructions driven by confocal microscopy data. • An algorithm is created to automatically build a tissue model of multiple myelinated axons based on axonal trace data. • Homogenized tissue response is achieved by volume averaging under external loading conditions typical of MRE. [ABSTRACT FROM AUTHOR]

Attaining accurate representative geometry for a finite element analysis at various scales can be a challenging research task. Added complications arise when the geometry is representing a manufactured or biological composite. In this study, a representative geometry of linear fibrous composites was created at the microscale and is employed to reproduce crystalline microfibril stacking of Kevlar to form a single fibril joined by a non-ordered crystal structure. The structure can have ellipsoidal or rectangular microfibrils stacked in ellipsoidal or rectangular fibrils and may have any desired packing ratio within the 1-100% range. In order to build a fibrous structure along a path, a Random Walk methodology was used. Since the directionality of the fibers is random, but always stepping from one side of the path toward the other, the fibers can wind around each other and tangle or terminate if needed. Another key concept of this method is the addition of a rotation matrix operation for the path of the fibers. This allows the path around the three local coordinates to be in a linear or sinusoidal direction. The resultant geometry produced can represent the tortuous path nanofibrils undergo. Moreover, rotation about the path axis allows for the twisted geometry of ring spun yarn, and metal cable to be reproduced. Inclusion of spherical objects to the path of the fibers has been accommodated to reproduce fiber projection around, or end at, an obstruction in their path. This approach allows representing impurities at the fibril, fiber, or yarn level of composite fabric manufacturing. [ABSTRACT FROM AUTHOR]

A multiscale numerical approach is established to model damage in random glass fiber composites. A representative volume element of a random glass fiber composite is employed to analyze microscale damage mechanisms, such as matrix cracking and fiber-matrix interfacial debonding, while the associated damage variables are defined and applied in a mesoscale stiffness reduction law. The macroscopic response of the homogenized mesoscale damage model is investigated using finite element analysis and validated through experiments. A case study of a random glass fiber composite plate containing a central hole subjected to tensile loading is performed to illustrate the applicability of the multiscale damage model. [ABSTRACT FROM AUTHOR]

Mechanical damage to axons is a proximal cause of deficits following traumatic brain injury and spinal cord injury. Axons are injured predominantly by tensile strain, and identifying the strain experienced by axons is a critical step toward injury prevention. White matter demonstrates complex nonlinear mechanical behavior at the continuum level that evolves from even more complex, dynamic, and composite behavior between axons and the “glial matrix” at the microlevel. In situ, axons maintain an undulated state that depends on the location of the white matter and the stage of neurodevelopment. When exposed to tissue strain, axons do not demonstrate pure affine or non-affine behavior, but instead transition from non-affine-dominated kinematics at low stretch levels to affine kinematics at high stretch levels. This transitional and predominant kinematic behavior has been linked to the natural coupling of axons to each other via the glial matrix. In this paper, a transitional kinematic model is applied to a micromechanics finite element model to simulate the axonal behavior within a white matter tissue subjected to uniaxial tensile stretch. The effects of the transition parameters and the volume fraction of axons on axonal behavior is evaluated and compared to previous experimental data and numerical simulations. [ABSTRACT FROM AUTHOR]

A finite element (FE) model is employed to investigate the dynamic response of soft tissues under external excitations, particularly corresponding to the case of harmonic motion imaging. A solid 3D mixed 'u-p' element S8P0 is implemented to capture the near-incompressibility inherent in soft tissues. Two important aspects in structural modelling of these tissues are studied; these are the influence of viscous damping on the dynamic response and, following FE-modelling, a developed state-space formulation that valuates the efficiency of several order reduction methods. It is illustrated that the order of the mathematical model can be significantly reduced, while preserving the accuracy of the observed system dynamics. Thus, the reduced-order state-space representation of soft tissues for general dynamic analysis significantly reduces the computational cost and provides a unitary framework for the 'forward' simulation and 'inverse' estimation of soft tissues. Moreover, the results suggest that damping in soft-tissue is significant, effectively cancelling the contribution of all but the first few vibration modes. [ABSTRACT FROM AUTHOR]

Abstract: A finite element model was built to simulate the dynamic behavior of soft tissues subjected to sinusoidal excitation during harmonic motion imaging. In this study, soft tissues and tissue-like phantoms were modeled as isotropic, viscoelastic, and nearly incompressible media. A 3D incompressible mixed u–p element of eight nodes, S1P0, was developed to accurately calculate the stiffness matrix for soft tissues. The finite element equations of motion were solved using the Newmark method. The Voigt description for tissue viscosity was applied to estimate the relative viscous coefficient from the phase shift between the response and excitation in a harmonic case. After validating our model via ANSYS simulation and experiments, a MATLAB finite element program was then employed to explore the effect of excitation location, viscosity, and multiple frequencies on the dynamic displacement at the frequency of interest. [Copyright &y& Elsevier]

Abstract: The elastic properties of random chopped fiber-reinforced composites (RAFCs) are of paramount importance for their sound application in lightweight structures. Mass production of random chopped fiber-reinforced composite (RaFC) at a fraction of the cost of composite laminates establishes RaFCs as alternate candidate materials for manufacturing lightweight components in the automotive industry. Nevertheless, understanding and modeling of their mechanical and fracture properties are still fields of active research, yet to be exhausted. In this paper, methods to generate an RVE for random fiber or particle reinforced composites numerically are reviewed. A modified random sequential absorption algorithm is proposed to generate a representative volume element (RVE) of a random chopped fiber-reinforced composite (RaFC) material. It is assumed that the RVE represents the composite material within the framework of elasticity. The RVE thus created is analyzed to obtain the mechanical properties of the composite material by using finite element analysis (FEA). RVE generation uses both straight and curved fibers so as to achieve high fiber volume fractions (VFs) that are extremely difficult to obtain by using straight fibers alone. This work extends the capability of RVE generation of RaFCs to higher volume fractions, here 35.1% is illustrated, which are in the range of values employed in industrial applications. [Copyright &y& Elsevier]

Abstract: The overall elastic properties of fiber reinforced composite are of primary importance for practical applications. In order to obtain the overall elastic properties, a homogenization procedure based on continuum micro-mechanics is usually applied to a representative volume element (RVE) representative of the whole composite. In this study, we first employ a modified random sequential adsorption algorithm to generate the complex geometry of a random fiber composite. Second, we investigate the effect of the interaction between two over-crossing fibers on the overall elastic properties of the composite. Third, we evaluate the overall elastic material properties of the composite using the finite element method for continuum micro-mechanical analysis. [Copyright &y& Elsevier]

Effective methods are needed to fabricate the next generation of high-performance graphene-reinforced polymer matrix composites (G-PMCs). In this work, a versatile and fundamental process is demonstrated to produce high-quality graphene-polymethylmethacrylate (G-PMMA) composites via in situ shear exfoliation of well-crystallized graphite particles loaded in highly-viscous liquid PMMA/acetone solutions into graphene nanoflakes using a concentric-cylinder shearing device. Unlike other methods where graphene is added externally to the polymer and mixed, our technique is a single step process where as-exfoliated graphene can bond directly with the polymer with no contamination/handling. The setup also allows for the investigation of the rheology of exfoliation and dispersion, providing process understanding in the attainment of the subsequently heat injection-molded and solidified G-PMC, essential for future manufacturing scalability, optimization, and repeatability. High PMMA/acetone concentration correlates to high mixture viscosity, which at large strain rates results in very-high shear stresses, producing a large number of mechanically-exfoliated flakes, as confirmed by liquid-phase UV–visible spectral analysis. Raman spectroscopy and other imaging evince that single- and bi-layer graphene are readily achieved. Nevertheless, a limit is reached at high mixtures viscosities where the process becomes unstable as non-Newtonian fluid behavior (e.g. viscoelastic) dominates the system. Characterization of microstructure, morphology, and properties of this new class of nanostructured composites reveals interesting trends. Observations by transmission electron microscopy, scanning electron microscopy, and helium ion microscopy of the manufactured G-PMCs show uniform distributions of unadulterated, well-bonded, discontinuous, graphene nanoflakes in a PMMA matrix, which enhances stiffness and strength via a load-transfer mechanism. Elastic modulus of 5.193 GPa and hardness of 0.265 GPa are achieved through processing at 0.7 g ml−1 of acetone/PMMA for 1% wt. starting graphite loading when injected into a sample mold at 200 °C. Mechanical properties exhibit 31% and 28.6% enhancement in elastic modulus and hardness, respectively, as measured by nano-indentation. [ABSTRACT FROM AUTHOR]

Motivated by the need to interpret the results from a combined use of in vivo brain Magnetic Resonance Elastography (MRE) and Diffusion Tensor Imaging (DTI), we developed a computational framework to study the sensitivity of single-frequency MRE and DTI metrics to white matter microstructure and cell-level mechanical and diffusional properties. White matter was modeled as a triphasic unidirectional composite, consisting of parallel cylindrical inclusions (axons) surrounded by sheaths (myelin), and embedded in a matrix (glial cells plus extracellular matrix). Only 2D mechanics and diffusion in the transverse plane (perpendicular to the axon direction) was considered, and homogenized (effective) properties were derived for a periodic domain containing a single axon. The numerical solutions of the MRE problem were performed with ABAQUS and by employing a sophisticated boundary-conforming grid generation scheme. Based on the linear viscoelastic response to harmonic shear excitation and steady-state diffusion in the transverse plane, a systematic sensitivity analysis of MRE metrics (effective transverse shear storage and loss moduli) and DTI metric (effective radial diffusivity) was performed for a wide range of microstructural and intrinsic (phase-based) physical properties. The microstructural properties considered were fiber volume fraction, and the myelin sheath/axon diameter ratio. The MRE and DTI metrics are very sensitive to the fiber volume fraction, and the intrinsic viscoelastic moduli of the glial phase. The MRE metrics are nonlinear functions of the fiber volume fraction, but the effective diffusion coefficient varies linearly with it. Finally, the transverse metrics of both MRE and DTI are insensitive to the axon diameter in steady state. Our results are consistent with the limited anisotropic MRE and co-registered DTI measurements, mainly in the corpus callosum, available in the literature. We conclude that isotropic MRE and DTI constitutive models are good approximations for myelinated white matter in the transverse plane. The unidirectional composite model presented here is used for the first time to model harmonic shear stress under MRE-relevant frequency on the cell level. This model can be extended to 3D in order to inform the solution of the inverse problem in MRE, establish the biological basis of MRE metrics, and integrate MRE/DTI with other modalities towards increasing the specificity of neuroimaging. [ABSTRACT FROM AUTHOR]

The evolution of crystalline domain orientation vs. mechanical properties of aramid fibers under mechanical loading was investigated for initial crystalline domain orientations between 16.7° and 9.7°. The latter resulted in a broad range of longitudinal moduli between 66 GPa and 119 GPa but tensile strengths in the narrow range of 3.5–4.0 GPa. Mechanical conditioning up to 90% of the fiber tensile strength increased the initial modulus converging to 100 GPa which corresponds to a stable crystalline domain orientation of 11.6°, while the unloading modulus at stresses near the tensile strength converged to ∼165 GPa which approaches the theoretical modulus of 220 GPa. On the contrary, the tensile strength remained unchanged with increasing crystalline domain orientation, and was shown to be independent of the fiber gauge length, thus implying that failure is not due to flaws obeying weakest link statistics. Instead, short gauge length tests (200 μm) showed failure initiation at the fiber skin followed by crack propagation at the skin-core interface, resulting in extrusion of the fiber core, which points to the skin-core interface as an important factor limiting the tensile strength of this class of fibers. [ABSTRACT FROM AUTHOR]

Kevlar KM2 600 denier yarn is often twisted to improve yarn load at failure. It is noted in ASTM-D-7269 that twisting KM2 past five twists per inch can cause a lower load at failure. Understanding the physics that govern the load decrease at higher twist ratios can lead to improved yarn designs, possibly stronger than the peak values currently seen at three turns per inch. Increasing yarn strength would enable enhanced usage of Kevlar yarn, improving chord and fabric structures. This paper describes the testing and finite element analysis methods used to probe yarn tenacity as a function of twist per inch. The performed tests indicate that strength decreases as the yarn is twisted past three twists per inch and are in agreement with previously conducted trials. The finite element model results were compared to test data performed in this study and ones reported in literature. Employing the validated simulation data, yarn damage and failure pictorials were produced. The frames showing the yarn at varying displacements illustrate the yarn failure propagation at varying twist-per-turn values. It is suggested that yarn softening as a function of twist per turn is attributed to higher strains of the outer filaments, at large amounts of twist, than of the core filaments. Previous work has shown a dependency of local filament strength to its yarn radius. The focus of this paper is to derive a comprehensive filament model, using finite element analysis, that incorporates the yarn strain gradient and is experimentally verified. [ABSTRACT FROM AUTHOR]

Modeling Kevlar yarn response as a function of twist requires creating a model at the filament level that incorporates capturing the mechanical interaction of numerous fibers. The inherent complexity of building a multiscale interwoven fibrous structure manually is prohibitive for such an endeavor; therefore, computer-aided simulations are preferred. In this study, a random walk methodology was employed to generate a fibrous structure along the axis of a yarn. Since the directionality of the fibers is randomly oriented along the axis, the fibers can wind around each other and tangle or terminate on demand. The resultant geometry can represent the tortuous path that yarn filaments experience. Yarn twist can be introduced through imposing a rotation matrix to the geometry or conducting an initial analysis that applies the preload. The analysis method employed in this paper captures the correct prestress of the twisted yarns at zero, three, and ten twists per inch. The analysis then loaded the yarn until filament fracture occurred. The predicted ultimate load was within 5% for all three twists per inch analyzed. The zero twists per inch linear response matched test results to within 5%. [ABSTRACT FROM AUTHOR]