Anisotropic Material Properties Research Articles

Effect of anisotropic characteristics on the mechanical behavior of asphalt concrete overlay

Asphalt concrete (AC) overlays placed over old asphalt pavement have become an alternative to repairing and reinforcing pavements. The strength contributed by the AC overlay is strongly influenced by the anisotropic properties of the pavement material. This study was conducted to analyze the influence of anisotropy, modulus gradient properties, and the condition of the AC overlay and old pavement contact plane on the mechanical behaviors of AC overlays, as well as to quantify the influence of the degree of anisotropy on the mechanical behaviors of AC overlay by a sensitivity analysis (SA). The mechanical behaviors of the AC overlay were numerically obtained using the three-dimensional finite element method with the aid of ABAQUS, a commercial program. Variations in the AC overlay’s modulus as a function of temperature as well as the contact state between the AC overlay and AC layer were considered. The SA is based on standardized regression coefficients method. Comparing the mechanical behavior in terms of surface deflection, stress, and strain of the anisotropy model against those corresponding to the isotropic model under static loads show that the anisotropic properties had greater effects on the mechanical behavior of the AC overlay. In addition, the maximum shear stress in the AC overlay was the most significant output parameter affected by the degree of anisotropy. Therefore, future research concerning the reinforcement and repair of pavements should consider the anisotropic properties of the pavement materials.

Impact of interlayer on the anisotropic multi-layered medium overlaying viscoelastic layer under axisymmetric loading

• An analytical solution for analyzing the mechanical behavior of multi-layered medium is proposed. • Computational efficiency of proposed solution is higher than ABAQUS . • Interlayer condition and anisotropic property significantly affected on the mechanical behavior of multi-layered medium. Multi-layered media is the most common structure in artificial and natural surroundings, and it is considered as a good candidate for mathematical model. The anisotropic properties and viscoelastic properties of layer materials have been widely used in multi-layered medium. Furthermore, the interlayer condition between the layers also plays an important role in the mechanical behavior of multi-layered medium. Despite many numerical approaches having been applied in these analyses, only few analytical solution were developed to analyze the effects of these three issues at the same time. The objective of this study is to develop an analytical solution that can be used for analyzing the impact of interlayer conditions on the mechanical behavior of anisotropic multi-layered medium overlaying viscoelastic layer subjected to axisymmetric loading, but the anisotropy property was approximated as transversely isotropy. The interlayer condition between the adjacent layers will also be considered. Details of the mathematical derivation, implementation and verification of the proposed analytical solution were presented. Subsequent numerical results demonstrate that both viscoelastic, transverse isotropy and interlayer condition could substantially contribute to the mechanical behavior of the multi-layered medium.

Mechanical properties of the airway tree: heterogeneous and anisotropic pseudoelastic and viscoelastic tissue responses.

Airway obstruction and pulmonary mechanics remain understudied despite lung disease being the third cause of death in the United States. Lack of relevant data has led computational pulmonary models to infer mechanical properties from available material data for the trachea. Additionally, the time-dependent, viscoelastic behaviors of airways have been largely overlooked, despite their potential physiological relevance and utility as metrics of tissue remodeling and disease progression. Here, we address the clear need for airway-specific material characterization to inform biophysical studies of the bronchial tree. Specimens from three airway levels (trachea, large bronchi, and small bronchi) and two orientations (axial and circumferential) were prepared from five fresh pig lungs. Uniaxial tensile tests revealed substantial heterogeneity and anisotropy. Overall, the linear pseudoelastic modulus was significantly higher axially than circumferentially (30.5 ± 3.1 vs. 8.4 ± 1.1 kPa) and significantly higher among circumferential samples for small bronchi than for the trachea and large bronchi (12.5 ± 1.9 vs. 6.0 ± 0.6 and 6.6 ± 0.9 kPa). Circumferential samples exhibited greater percent stress relaxation over 300 s than their axial counterparts (38.0 ± 1.4 vs. 23.1 ± 1.5%). Axial and circumferential trachea samples displayed greater percent stress relaxation (26.4 ± 1.6 and 42.5 ± 1.7%) than corresponding large and small bronchi. This ex vivo pseudoelastic and viscoelastic characterization reveals novel anisotropic and heterogeneous behaviors and equips us to construct airway-specific constitutive relations. Our results establish necessary fundamentals for airway mechanics, laying the groundwork for future studies to extend to clinical questions surrounding lung injury, and further directly enables computational tools for lung disease obstruction predictions. NEW & NOTEWORTHY Understanding the mechanics of the lung is necessary for investigating disease progression. Trachea mechanics comprises the vast majority of ex vivo airway tissue characterization despite distal airways being the site of disease manifestation and occlusion. Furthermore, viscoelastic studies are scarce, whereas time-dependent behaviors could be potential physiological metrics of tissue remodeling. In this study, the critical need for airway-specific material properties is addressed, reporting bronchial tree anisotropic and heterogeneous material properties.

A COMPUTATIONAL STRATEGY TO EXAMINE THE PROFILE EFFECTS OF MICROPRISM REGIONS ON THE OVERALL ANISOTROPY OF HUMAN ENAMELS

In this study, our attention is mainly on elaborating a computational strategy to effectively predict the influence of prism profiles on the overall anisotropic property of human enamels (HEs). At first, two distinct schemes are developed separately with the aid of the polynomial fitting technique and the general power functions to mathematically describe the practical irregular and simplified regular profiles of enamel prisms. Hereafter, two parametric piecewise formulas, which facilitate the definition of anisotropic material properties of finite elements at different locations and make the numerical simulation of HE microstructures consisting of irregularly shaped prisms feasible, are presented to describe the orientation of hydroxyapatite (HAP) crystallites embedded in microprisms. The effective anisotropic moduli over a representative unit cell (RUC) under the periodic displacement constraint is concisely introduced according to the micromechanics, and a computational strategy is established to calculate these moduli numerically. Finally, the evaluations in the open literature are employed to demonstrate the validity of the elaborated computational strategy, and more investigations are conducted and yield the conclusions such that the material property of the inter-prism regions as well as the prism shapes plays a crucial role in determining the overall anisotropy of HEs.

Theoretical prediction and atomic kinetic Monte Carlo simulations of void superlattice self-organization under irradiation

Nano-structured superlattices may have novel physical properties and irradiation is a powerful mean to drive their self-organization. However, the formation mechanism of superlattice under irradiation is still open for debate. Here we use atomic kinetic Monte Carlo simulations in conjunction with a theoretical analysis to understand and predict the self-organization of nano-void superlattices under irradiation, which have been observed in various types of materials for more than 40 years but yet to be well understood. The superlattice is found to be a result of spontaneous precipitation of voids from the matrix, a process similar to phase separation in regular solid solution, with the symmetry dictated by anisotropic materials properties such as one-dimensional interstitial atom diffusion. This discovery challenges the widely accepted empirical rule of the coherency between the superlattice and host matrix crystal lattice. The atomic scale perspective has enabled a new theoretical analysis to successfully predict the superlattice parameters, which are in good agreement with independent experiments. The theory developed in this work can provide guidelines for designing target experiments to tailor desired microstructure under irradiation. It may also be generalized for situations beyond irradiation, such as spontaneous phase separation with reaction.

Light-weight shape and topology optimization with hybrid deposition path planning for FDM parts

FDM (fused deposition modeling) parts demonstrate anisotropic properties between the in-layer raster and transverse directions. However, structural optimization of FDM parts has rarely taken the deposition path and thus the anisotropic material properties into consideration. This work proposes a methodology that integrates optimal hybrid deposition paths with the shape and topology optimization and brings all aspects under a unified level set framework. A dedicated sensitivity analysis is performed on both the shape and path variables, which ensures the optimality of the derived design solutions. Effectiveness of the proposed method is demonstrated through a number of 2D and 3D numerical examples.

Finite Element Analysis of Tricuspid Valve Deformation from Multi-slice Computed Tomography Images.

Despite the growing clinical interest in the tricuspid valve (TV), there is an incomplete understanding of TV biomechanics which is important in normal TV function and successful TV repair techniques. Computational models with patient-specific human TV geometries can provide a quantitative understanding of TV biomechanic. Therefore, this study aimed to develop finite element (FE) models of human TVs from multi-slice computed tomography (MSCT) images to investigate chordal forces and leaflet stresses and strains. Three FE models were constructed for human subjects with healthy TVs from MSCT images and incorporated detailed leaflet geometries, realistic nonlinear anisotropic hyperelastic material properties of human TV, and physiological boundary conditions tracked from MSCT images. TV closure from diastole to systole was simulated. Chordal lengths were iteratively adjusted until the simulated TV geometries were in good agreement with the "true" geometries reconstructed from MSCT images at systole. Larger chordal forces were found on the strut (or basal) chords than on the rough zone chords and the total forces applied on the anterior papillary muscles by the strut chords were higher than those on the posterior or septal papillary muscles. At peak systolic pressure, the average maximum stress on the middle sections of the leaflets ranged from 30 to 90kPa, while the average maximum principal strain values ranged from 0.16 to 0.30. The results from healthy TVs can serve as baseline biomechanical metrics of TV mechanics and may be used to inform TV repair device design. The computational approach developed could be one step towards developing computational models that may support pre-operative planning in complex TV repair procedures in the future.

Polarization-Dependent Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR): Infrared Nanopolarimetric Analysis of Structure and Anisotropy of Thin Films and Surfaces.

Infrared techniques enable nondestructive and label-free studies of thin films with high chemical and structural contrast. In this work, we review recent progress and perspectives in the nanoscale analysis of anisotropic materials using an extended version of the atomic force microscopy-infrared (AFM-IR) technique. This advanced photothermal technique, includes polarization control of the incoming light and bridges the gap in IR spectroscopic analysis of local anisotropic material properties. Such local anisotropy occurs in a wide range of materials during molecular nucleation, aggregation, and crystallization processes. However, analysis of the anisotropy in morphology and structure can be experimentally and theoretically demanding as it is related to order and disorder processes in ranges from nanoscopic to macroscopic length scales, depending on preparation and environmental conditions. In this context IR techniques can significantly assist as IR spectra can be interpreted in the framework of optical models and numerical calculations with respect to both, the present chemical conditions as well as the micro- and nanostructure. With these extraordinary analytic possibilities, the advanced AFM-IR approach is an essential puzzle piece in direction to connect nanoscale and macroscale anisotropic thin film properties experimentally. In this review, we highlight the analytic possibilities of AFM-IR for studies on nanoscale anisotropy with a set of examples for polymer, plasmonic, and polaritonic films, as well as aggregates of large molecules and proteins.

Cross-Sectional Design and Analysis of Multiscale Carbon Nanotubes-Reinforced Composite Beams and Blades

The present work addresses with the cross-sectional design and analysis of fiber-reinforced multiscale composite beams of general cross-sectional shape and arbitrary anisotropic material properties and investigates the effect of carbon nanotubes (CNTs) on their stiffness properties. The three-dimensional strain field was formulated in terms of one-dimensional strains and a three-dimensional warping displacement. The bulk material properties of the multiscale composite were predicted using Halpin–Tsai equations and fiber micromechanics. The carbon nanotubes were assumed to be uniformly distributed and randomly oriented throughout the polymer matrix. The variational asymptotic beam section (VABS) was used to numerically evaluate the stiffness and mass matrices of four test cases: strip, circular pipe, box beam and airfoil. The influence of CNTs weight percentage and volume fraction of fibers was investigated through a detailed parametric study. The numerical results indicate that the inclusion of a small weight percentage of carbon nanotubes in the polymer matrix is sufficient to induce a significant improvement in stiffness properties.

Drilling Strategy for Thick Carbon Fiber Reinforced Polymer Composites (CFRP): A Preliminary Assessment

Carbon fiber reinforced polymer or CFRP composites are the epitome of high-performance materials in lightweight design. However, their machinability can be problematic due to non-homogenous and anisotropic material properties. This preliminary assessment emphasizes drilling strategy by using mechanical drilling and laser machining on 25.4 mm thick CFRP, which has not been investigated so far. In mechanical drilling, three drilling strategies were applied with the same parameters in order to assess the feasibility of drilling thick CFRP. The laser machining experiments were conducted to identify the potential of fiber laser machines to cut thick CFRP due to their superior laser beam quality. The results showed that choosing the appropriate drilling strategy in mechanical drilling is essential for reducing damage when drilling thick CFRP. Significant damage occurred in all experiments. The results are useful to define the relationships between machining parameters related to mechanical/laser drilling and hole/cut quality.

Multi-axial damage and failure of medical grade carbon fibre reinforced PEEK laminates: Experimental testing and computational modelling.

Orthopaedic devices using unidirectional carbon fibre reinforced poly-ether-ether-ketone (PEEK) laminates potentially offer several benefits over metallic implants including: anisotropic material properties; radiolucency and strength to weight ratio. However, despite FDA clearance of PEEK-OPTIMA™ Ultra-Reinforced, no investigation of the mechanical properties or failure mechanisms of a medical grade unidirectional laminate material has been published to date, thus hindering the development of first-generation laminated orthopaedic devices. This study presents the first investigation of the mechanical behaviour and failure mechanisms of PEEK-OPTIMA™ Ultra-Reinforced. The following multi-axial suite of experimental tests are presented: 0° and 90° tension and compression, in-plane shear, mode I and mode II fracture toughness, compression of ±45° laminates and flexure of 0°, 90° and ±45° laminates. Three damage mechanisms are uncovered: (1) inter-laminar delamination, (2) intra-laminar cracking and (3) anisotropic plasticity. A computational damage and failure model that incorporates all three damage mechanisms is developed. The model accurately predicts the complex multi-mode failure mechanisms observed experimentally. The ability of a model to predict diverse damage mechanisms under multiple loading directions conditions is critical for the safe design of fibre reinforced laminated orthopaedic devices subjected to complex physiological loading conditions.

Atomistic origins of the differences in anisotropic fracture behaviour of LiTaO3 and LiNbO3 single crystals

The search of new designs for communication technologies is driven by the required high data transfer rates. Piezoelectric single crystals such as LiTaO3 and LiNbO3 have qualified as materials for efficient and precise frequency filters also considered for the new 5G standards. To ensure optimal high frequency functionality, single crystal wafers are grown and cut in particular directions. However, due to the high anisotropic physical properties of these brittle materials, the structural integrity and reliability of devices are affected by the crystal orientation with respect to the occurring thermo-mechanical stresses. In this work, the anisotropic fracture response of LiTaO3 and LiNbO3 single crystals is investigated through combination of (i) toughness measurements in notched micro-cantilevers along weak cleavage planes and (ii) atomistic modelling of cleavage fracture energies using density functional theory. It is demonstrated that differences in fracture behaviour between LiTaO3 and LiNbO3 can be explained by the stronger chemical bonding in LiTaO3 as compared to LiNbO3 within the loaded crystallographic planes. The knowledge on the alignment of tough as well as weak planes (i.e. cleavage planes) can be used to tailor the design of single crystal based functional components, aiming to exhibit enhanced mechanical reliability without compromising the functionality.

Magnetic resonance elastography inversions: Technical challenges and recent developments

Magnetic resonance elastography (MRE) is a phase contrast based MRI imaging technique that can quantitatively and non-invasively measure full 3D vector displacement data from propagating acoustic waves in vivo. The data acquired allow the calculation of local values of complex shear modulus and the generation of images that depict the viscoelastic properties of tissue. Acquisitions at different frequencies can capture the dispersive behavior of these quantities, and advanced approaches attempt to map anisotropic and poroelastic properties of materials. We will discuss current approaches for the inversion of MRE data, focusing on the issues and assumptions involved, and highlight some of the challenges faced in such inversions, particularly in thin or small structures in which wave propagation is dominated by waveguide effects. We will also present results based on neural network inversions, which may have lower repeatability error and be more resistant to noise than some current approaches.

Straightforward measurement of anisotropic thermal properties of a Bi2Se3 single crystal

We demonstrate here a simple measurement protocol which allows the thermal properties of anisotropic crystalline materials to be determined. This protocol is validated by the measurement of Bi2Se3, a layered material consisting of covalently bonded sheets with weak van der Waals bonds between each layer, which has highly anisotropic thermal properties. Thermoreflectance microscopy measurements were carried out on a single-crystal Bi2Se3 sample, firstly on the bare sample and then after capping with a 100 nm thick gold layer. Whereas on the bare sample lateral heat diffusion is dominated by the in-plane thermal diffusivity, on the metal-capped substrate heat diffusion perpendicular to the sample surface dominates. Using a simple theoretical model, we show how this double measurement protocol allows the anisotropic thermal conductivity coefficients of bulk Bi2Se3 to be evaluated.

Neural network based model for estimation of the level of anisotropy of unbound aggregate systems

Abstract Directional dependency of the materials properties in granular soils is an integral component of the analysis and design of pavement foundations. Laboratory determination of such anisotropic properties is often overlooked by design engineers due to the lack of available protocols, equipment, time constraints, and budgetary issues. This research strives to bridge this gap by investigating an alternative approach to provide a practical and reliable framework to estimate the level of anisotropy of unbound granular materials. To achieve this objective, an experiment design was developed to establish a comprehensive aggregate feature database. Nonlinear and cross anisotropic material properties were determined using Variable Dynamic Confining Pressure (VDCP) stress path tests in the laboratory. Particle geometry was characterized using the Aggregate Imaging System (AIMS). Scale parameters and shape parameters of the form, angularity and textural properties of the particles were incorporated in the aggregate database to account for the shape-induced anisotropy of particulate soils. Moisture state and density parameters were also incorporated in the database for further post processing. Several neural network models with different architectures were developed, and the performances of the models were assessed based on an unseen set of data. The impact of the neural network topologies on the performance of the models were also investigated in this research effort. The results indicate that increasing the number of hidden layers has negligible impact on the performance of the models. Additionally, a series of sensitivity analyses were performed to investigate the level of contribution of aggregate features on the anisotropic behavior of unbound granular materials. The results of the sensitivity analysis underscores the influence of aggregate angularity, fines content, surface macro-texture, as well as the parameters of the stress path tests, on the orthogonal load distribution capacity of aggregate layers.

Quantitative imaging of anisotropic material properties with vectorial ptychography.

Following the recent establishment of the formalism of vectorial ptychography [Opt. Lett.40, 5144 (2015)OPLEDP0146-959210.1364/OL.40.005144], first measurements, to the best of our knowledge, are reported in the optical range, demonstrating the capability of the proposed method to map the four parameters of the Jones matrix of an anisotropic specimen, and therefore to quantify a wide range of optical material properties, including power transmittance, optical path difference, diattenuation, retardance, and fast-axis orientation.

Identification of anisotropic plasticity properties of materials using spherical indentation imprint mapping

In this paper, a new inverse method is proposed to identify the anisotropic plasticity properties of materials using solely the residual imprint mapping in spherical indentation. This method does not need the entire load-displacement (P-h) curve in spherical indentation experiment. The stress strain curves along transverse and longitudinal directions of the indented specimens can be simultaneously identified using single spherical indentation test. The new method is based on weighting the average and difference amounts, S̅ and ∆S of the residual imprint snapshots along transverse and longitudinal directions. The proper orthogonal decomposition is used to correlate the effective imprint snapshots with material constitutive parameters, and the inverse problem is solved by the “Interior-point” optimization algorithm. We finally applied the new method on the SiCw/A6061. The well-posed solution of material properties is achieved, and the anisotropic plasticity parameters obtained from indentation and uniaxial tests show good agreement. Besides, the influence of weighting factor and experiment error on the numerical results are also investigated. Results indicate the proposed inverse method is very effective and reliable.

Positive and negative birefringence of materials in microwave region

We present a set of experiments with microwaves that enable one to distinguish between isotropic and anisotropic properties of wooden materials. We also determine the magnitude and the sign of the birefringence of uniaxial materials. Due to the visible structure of wood and possible direct observation of the effects on the wave propagation, solid wood boards and particle boards can be used as persuasive representatives of positively and negatively birefringent materials, respectively. Suggested experiments can easily be used for demonstration purposes or laboratory work at the undergraduate or graduate level in optics courses.

Surface integrity of water jet guided laser machining of CFRP

Carbon fiber reinforced plastics (CFRP) is now applied widely in various industries because of its excellent properties. Although conventional processing methods, such as milling and abrasive water jet, are the dominating techniques used, laser cutting of CFRP is still attractive owing to the wear-free, contactless and flexible processing. However, the anisotropic material properties of CFRP, the issues related to the heat affected zone (HAZ) and delamination during laser processing, are major obstacles in its application. In order to reduce the heat damage caused by laser, a compound machining method, water jet guided laser technique, was adopted to cut CFRP in this paper. In addition, we make a comparative investigation for conventional laser beam machining and water jet guided laser machining with an identical material. The difference of kerf geometry, HAZ, surface morphology and efficiency was discussed within this research. The experiments showed that the HAZ of water jet guided laser is smaller than conventional laser beam machining, but much less efficient. Water jet guided laser technique is a potential processing method for CFRP if the efficiency is improved.

Crystallographic crack propagation rate in single-crystal nickelbase superalloys

Single-crystal nickel-base superalloys are often used in the hot sections of gas turbines due to their good mechanical properties at high temperatures such as enhanced creep resistance. However, the anisotropic material properties of these materials bring many difficulties in terms of modelling and crack growth prediction. Cracks tend to switch cracking mode from Mode I cracking to crystallographic cracking. Crystallographic crack growth is often associated with a decrease in crack propagation life compared to Mode I cracking and this must be taken into account for reliable component lifing. In this paper a method to evaluate the crystallographic crack propagation rate related to a crystallographic crack driving force parameter is presented. The crystallographic crack growth rate is determined by an evaluation of heat tints on the fracture surface of a specimen subjected to fatigue loading. The complicated crack geometry including two crystallographic crack fronts is modelled in a three dimensional finite element context. The crack driving force parameter is determined by calculating anisotropic stress intensity factors along the two crystallographic crack fronts by finite-element simulations and post-processing the data in a fracture mechanics tool that resolves the stress intensity factors on the crystallographic slip planes in the slip directions. The evaluated crack propagation rate shows a good correlation for both considered crystallographic cracks fronts.