Characterization Of Anisotropic Materials Research Articles

Anisotropic Material Characterization of Human Cervix Tissue Based on Indentation and Inverse Finite Element Analysis.

The cervix is essential to a healthy pregnancy as it must bear the increasing load caused by the growing fetus. Preterm birth is suspected to be caused by the premature softening and mechanical failure of the cervix. The objective of this paper is to measure the anisotropic mechanical properties of human cervical tissue using indentation and video extensometry. The human cervix is a layered structure, where its thick stromal core contains preferentially aligned collagen fibers embedded in a soft ground substance. The fiber composite nature of the tissue provides resistance to the complex three-dimensional loading environment of pregnancy. In this work, we detail an indentation mechanical test to obtain the force and deformation response during loading which closely matches in vivo conditions. We postulate a constitutive material model to describe the equilibrium material behavior to ramp-hold indentation, and we use an inverse finite element method based on genetic algorithm (GA) optimization to determine best-fit material parameters. We report the material properties of human cervical slices taken at different anatomical locations from women of different obstetric backgrounds. In this cohort of patients, the anterior internal os (the area where the cervix meets the uterus) of the cervix is stiffer than the anterior external os (the area closest to the vagina). The anatomic anterior and posterior quadrants of cervical tissue are more anisotropic than the left and right quadrants. There is no significant difference in material properties between samples of different parities (number of pregnancies reaching viable gestation age).

Quantitative analysis of QSI and LVI damage in GFRP unidirectional composite laminates by a new ultrasonic approach

Abstract Our work is focused on a new experimental approach for the comparison between Quasi Static Indentation (QSI) damage and Low-Velocity Impact (LVI) damage in polymer composites starting from the results of ultrasonic goniometric immersion tests. In particular, the comparison is performed through the analysis of the additional anisotropy induced by the damage in unidirectional Glass Fiber-Reinforced Polymer (GFRP) composites due to QSI and LVI damage tests performed with a low level of the employed energy. To this aim, we ultrasonically reconstruct the acoustic curves (velocity curves and slowness curves) before and after the damage. Ultrasonic experiments are performed by using a goniometric ultrasonic immersion device designed and built at our laboratory, aimed at the mechanical characterization of anisotropic materials. We highlight differences and similarities between QSI and LVI damage starting from the analysis of the variations of the acoustic behavior and by using a suitable anisotropic damage model developed in the framework of the Continuum Damage Mechanics theory. The proposed experimental approach can be suitably developed for in situ investigations on low-velocity impact damage in polymer composite components.

Evaluation of Multiple Reflections in the Characterization of Anisotropic Materials by Through Transmission Ultrasonic Technique

Ultrasonic through-transmission techniques have been widely used along decades for determining the elastic constants of anisotropic composite materials. This work presents a new methodology that takes into account not only the transmitted waves but also the multiple reflections inside the sample. The superposition of different reflected waves are theoretically and experimentally identified and excluded, which avoids the distortion caused by pulse superposition. Additionally, the results of the Christoffel tensor optimization are evaluated in the reflected waves, improving the results validation. Experimental results using unidirectional carbon-matrix composite samples used in the aeronautical industry are presented.

Characterization of Material Damage by Ultrasonic Immersion Test

We present an innovative non-destructive experimental technique for a quantitative determination of the damage level in structural components. In particular, we study the propagation of ultrasonic waves in an isotropic material specimen before and after a mechanical fatigue incremental test. To this aim, we use an innovative goniometric device for ultrasonic immersion tests, designed and built for the mechanical characterization of anisotropic materials. In the first test, performed in absence of damage, we characterize the mechanical behavior of the isotropic material by the measure of time of flight (TOF) of ultrasonic waves, and then by determining the so-called natural velocities. In the next ultrasonic tests, performed after each fatigue test, we determine the response of the damaged material by measuring again the velocity of ultrasonic waves: the comparison between these velocities and the natural velocities allows for the evaluation of the level of damage in the material. Moreover, through the goniometric ultrasonic immersion device it is possible to relate the damage to the stress-induced anisotropyacquired by the specimen, which we characterize by identifying the acoustic axes and the dependence of the velocity of the ultrasonic waves on the direction of propagation.

Thermal characterization of anisotropic materials by integral transforms taking into account the thermal coupling with the sample-holder

In this paper, a method for thermal characterization of anisotropic materials taking into account the coupling between the sample and its holder is presented. The originality of this work is to use temperature measurements made by infrared camera on the edges of the sample and introduce them as boundary conditions to get rid of the presence of the support. Using the principle of superposition and adapted integral transforms, it is then possible by an iterative method to obtain simultaneously the material properties in its different anisotropy directions by a least squares method applied to the spatial harmonics of the temperature field. This method is validated and compared with a classical Fourier-Cosine transform through a simulation before being applied to experimental measurements.

Finite element modeling of impulsive excitation and shear wave propagation in an incompressible, transversely isotropic medium

Elastic properties of materials can be measured by observing shear wave propagation following localized, impulsive excitations and relating the propagation velocity to a model of the material. However, characterization of anisotropic materials is difficult because of the number of elasticity constants in the material model and the complex dependence of propagation velocity relative to the excitation axis, material symmetries, and propagation directions. In this study, we develop a model of wave propagation following impulsive excitation in an incompressible, transversely isotropic (TI) material such as muscle. Wave motion is described in terms of three propagation modes identified by their polarization relative to the material symmetry axis and propagation direction. Phase velocities for these propagation modes are expressed in terms of five elasticity constants needed to describe a general TI material, and also in terms of three constants after the application of two constraints that hold in the limit of an incompressible material. Group propagation velocities are derived from the phase velocities to describe the propagation of wave packets away from the excitation region following localized excitation. The theoretical model is compared to the results of finite element (FE) simulations performed using a nearly incompressible material model with the five elasticity constants chosen to preserve the essential properties of the material in the incompressible limit. Propagation velocities calculated from the FE displacement data show complex structure that agrees quantitatively with the theoretical model and demonstrates the possibility of measuring all three elasticity constants needed to characterize an incompressible, TI material.

Thermal Characterization of Anisotropic Materials at High Temperature Through Integral Methods and Localized Pulsed Technique

New applications in aerospace or energy industries require the development of new materials at high temperature exhibiting high anisotropic properties. Their thermal characterization requires the development of specific experimental benches. In this article, a new experiment is presented which allows one to estimate through only one experiment the three diffusivities of an orthotropic material at high temperatures without the need of vacuum. The estimation procedure is very fast and accurate due to using, on the one hand, integral transforms that allows one to get rid of the spatial distribution of the flash energy, and on the other hand, an infrared camera that provides a large amount of experimental data. And thanks to the use of a nonlinear parameter estimation and estimations made directly on Fourier transforms of the temperature field, the heat flux stimulation is no longer necessary to be Dirac in time. To validate the method and the experimental facility, measurements were performed on a Ti–6Al–4V alloy from room temperature up to $$1000\,^{\circ }\text{ C }$$ . In addition, particular attention has been paid to the thermal coupling that can appear between the low conducting materials and the air, and a criterion has been established to determine if the in-plane thermal diffusivity measurements can be affected or not.

Neural network constitutive modelling for non‐linear characterization of anisotropic materials

AbstractThis paper presents a new technique of neural network constitutive modelling for non‐linear characterization of anisotropic materials. The proposed technique, based on a recently developed energy‐based characterization framework, derives the variations of the external work applied to and the strain energy induced in a specimen. The error between the variations of the energies is subsequently applied to correct the neural network properties by using a modified backpropagation algorithm. Unlike the conventional techniques for neural network constitutive modelling, the proposed technique develops models by quantifying the deformation of the specimen on a continuum basis. This allows the neural network constitutive models to be constructed from a single load test of one specimen. Numerical examples first examine the effect of specimen geometries and loading conditions. The effect of noise in the experimental measurements is subsequently investigated while having the applicability for non‐linear constitutive behaviour shown thereafter. The application for anisotropic materials is finally demonstrated by modelling a unidirectional lamina based on the measurements of a biaxial load test on a balanced laminate. Copyright © 2010 John Wiley & Sons, Ltd.

The dual-mode heat flow meter technique: A versatile method for characterizing thermal conductivity

The dual-mode heat flow meter technique was developed for steady-state thermal characterization by optimizing the curve fit between the experimental temperature profile and the prediction from a simple analytical solution taking into account conductive as well as radiative heat transfer. The validation results were seen to be in good agreement with published literature values and demonstrated the versatility of the method. Moreover, the technique is ideally suited for characterization of anisotropic materials without necessitating any additional information about the nature of anisotropy and lends itself as an attractive alternative for characterization of novel materials with engineered transport properties.

Systematic errors for a Mueller matrix dual rotating compensator ellipsometer

The characterization of anisotropic materials and complex systems by ellipsometry has pushed the design of instruments to require the measurement of the full reflection Mueller matrix of the sample with a great precision. Therefore Mueller matrix ellipsometers have emerged over the past twenty years. The values of some coefficients of the matrix can be very small and errors due to noise or systematic errors can induce distored analysis. We present a detailed characterization of the systematic errors for a Mueller Matrix Ellipsometer in the dual-rotating compensator configuration. Starting from a general formalism, we derive explicit first-order expressions for the errors on all the coefficients of the Mueller matrix of the sample. The errors caused by inaccuracy of the azimuthal arrangement of the optical components and residual ellipticity introduced by imperfect optical elements are shown. A new method based on a four-zone averaging measurement is proposed to vanish the systematic errors.

Fourier Transform Infrared Spectroscopic Imaging of Anisotropic Poly(Vinylidene Fluoride) Films with Polarized Radiation

The technique of Fourier transform infrared (FT-IR) spectroscopic imaging with focal plane array detectors has proved to be a powerful technique for rapid chemical visualization of samples with a lateral resolution up to about 10 mum. However, the potential of FT-IR imaging for the characterization of anisotropic materials can be significantly enhanced by using polarized radiation. This issue will be addressed in the present communication, which reports for the first time imaging investigations based on the FT-IR polarization spectra of poly(vinylidene fluoride) films that have been uniaxially elongated below and above the threshold temperature of the II(alpha) --> I(beta) phase transition.

Computational design of multiaxial tests for anisotropic material characterization

AbstractAlthough anisotropic materials provide more capabilities for mission‐ and application‐tailored design and functional flexibility to final structures than regular isotropic materials, the directional behavior of the anisotropic materials further complicates their inelastic and damage behavior. Such a non‐linear behavior can be effectively observed and characterized by multiaxial testing, but how to design a multiaxial test for material characterization given a specimen remains an untouched issue. This paper presents a methodology that numerically designs the loading path of a multiaxial testing machine to characterize anisotropic materials. The multiaxial test must be able to exhibit quantities used to characterize materials as distinctly as possible. The proposed methodology formulates distinguishability and uniqueness as such quantities by first analyzing the specimen on a continuum basis with finite element method and then applying singular value decomposition. Associating the distinguishability and uniqueness with the informativeness of the loading path, the design problem is formulated such that an effective loading path can be found efficiently by a standard optimization method. Numerical examples first investigate the validity of the distinguishability and the uniqueness as performance measures to evaluate loading paths. The efficacy of the proposed methodology has been then confirmed by analyzing it with and applying it to design problems. Copyright © 2007 John Wiley & Sons, Ltd.

Elastic characterization of anisotropic materials by speckle interferometry

In this paper we present an experimental procedure for the elastic characterization of thin anisotropic plates. The procedure allows the determination of the elastic constants of isotropic materials (metals and ceramics) and orthotropic materials (composite laminae). Moreover, the flexural compliances of completely anisotropic uncoupled materials (most of composite laminates) can also be measured. The tests are carried out by applying a concentrated force to the specimen supported by three spheres. The stress components are evaluated by simply measuring the applied load, while the strain fields are measured by digital phase-shifting speckle interferometry. The experimental procedure is entirely controlled by a virtual instrument developed for this purpose in the National Instruments LabVIEW® environment, which runs on a personal computer interfaced with a standard black and white CCD camera. By means of the speckle interferometer, the whole field of the out-of-plane displacements is acquired; the curvatures, and hence the strain components, are obtained by two subsequent numerical differentiations. The numerical processing of the experimental data was carried out in the Mathematica™ environment. The results obtained on a steel specimen and on a composite laminate are reported.

Elastic Characterization of Anisotropic Materials by Speckle Interferometry

Elastic Characterization of Anisotropic Materials by Speckle Interferometry

Time and Temperature Dependence of the Viscoelastic Properties of PEEK/IM7

Understanding viscoelastic properties of composite materials is essential for the design and analysis of many advanced structures. However, experimental viscoelastic characterization of anisotropic materials can be complicated because of the number of independent parameters to be evaluated. Recently, an approach leading to the 3-D viscoelastic characterization of transversely isotropic materials using a reduced number of measured parameters has been developed. The model reduces the number of independent parameters to describe the viscoelastic behavior of transversely isotropic materials, which greatly simplifies the experimental procedures. Based on this recently developed model, the present article evaluates time and temperature effects on the viscoelastic properties of a fiber reinforced lamina. The experimental investigation is conducted on sub-scale specimens loaded in flexure, using Dynamic Mechanical Analysis (DMA) equipment. Ultimately, the approach presented in this work will allow the construction of master curves for the independent viscoelastic parameters that characterize a transversely isotropic material. Therefore, the experimental technique presented in this work provides a means for the study of viscoelastic properties of fiber reinforced composites, and constitutes a valuable contribution to the understanding of time and temperature dependence of these mechanical properties.

Time and temperature dependence of the viscoelastic properties of CFRP by dynamic mechanical analysis

Understanding viscoelastic properties of composite materials is essential for the design and analysis of many advanced structures. Viscoelastic material characterization is critical in designs that incorporate specified levels of damping, and to the understanding of processing problems. However, experimental viscoelastic characterization of anisotropic materials can be complicated because of the number of independent parameters to be evaluated. Recently, an approach leading to the 3-D viscoelastic characterization of transversely isotropic materials using a reduced number of measured parameters has been developed. Based on this recently developed model, the present work evaluates time and temperature effects on the viscoelastic properties of unidirectional carbon fiber reinforced laminae and cross-ply laminates. The experimental investigation is conducted on subscale specimens loaded in flexure, using dynamic mechanical analysis (DMA) equipment. The results presented demonstrate that DMA equipment offers good potential to study changes in viscoelastic properties of transversely isotropic laminae and laminates, related to temperature and frequency. This work provides a means for the study of viscoelastic properties of fiber-reinforced composites, and, therefore, constitutes a valuable contribution to the understanding of time and temperature dependence of these mechanical properties.

Generalized ellipsometry for the characterization of anisotropic materials: influence of the sample adjustment on the extracted optical indices

A new method to extract accurately the optical properties of anisotropic materials and multilayer structures is presented. The main difference compared to the literature is that we measure the Fourier coefficients of the ellipsometric signal α and β vs. the sample azimuth at one analyzer position and adjust the optical properties of the sample directly on α and β by a regression algorithm. Measurements are made in rotating polarizer configuration at various wavelengths. In these conditions, the measurement behavior for the different optical and instrumental parameters can be clearly identified almost for uniaxial crystals. The tilt of the optical axis from the normal of the sample surface breaks the symmetry of the oscillations of α and β vs. the azimuth. The oscillation amplitude is directly related to the ordinary and extraordinary indices. Measurement is also extremely sensitive to the sample surface horizontality, which can induce large errors on the optical axis orientation and on all the extracted optical parameters. Our method includes directly the alignment imperfection that is calibrated on isotropic sample. In these conditions the accuracy of the method is around ±0.002 on the optical indices for transparent samples.

A full-field approach for the elastic characterization of anisotropic materials

In the paper a procedure is described whereby the characterization of isotropic and anisotropic laminae is carried out by full-field measurement of the surface rotations under proper flexural loads. The analytic formulation does not necessarily imply a numerical calibration and the elastic constants are determined by integrating the whole experimental data on the surface of the specimen. Measurements were carried out by phase-stepped speckle interferometry using a shearometer based on Michelson design. The experimental instrumentation, including the loading device, and the procedures for the manipulation of experimental data are detailedly described; experimental results obtained on a steel specimen and a composite laminate are also reported.

Photothermal characterization of anisotropic materials with buried principal axes

We study theoretically and experimentally the ability of photothermal techniques to thermally characterize the anisotropic materials whose principal axes are not contained at the sample surfaces. We first determine the thermal diffusivity tensor when the directions of the principal axes are known. Inversely, we calculate the directions of the principal axes in a sample whose thermal diffusivities are given. Measurements validating our theory are presented for a sample of pyrolytic graphite (PG), which has a high anisotropy, by means of two photothermal techniques: IR radiometry and the mirage effect.

Computationally efficient representation for elastostatic and elastodynamic Green's functions for anisotropic solids.

A computationally efficient representation of the three-dimensional elastostatic and elastodynamic Green's functions for anisotropic solids is derived by solving the Christoffel equation in terms of delta functions. The representation is also applicable to other wave equations. The method is applied to calculate the transient and the static displacement field due to a point source in infinite and semi-infinite anisotropic cubic solids. For elastodynamic calculations in anisotropic solids, our representation saves the computational time by a factor of about 1000 over the conventional Fourier-Laplace representation. In the elastostatic case, the computational efficiency of our method is much more than the conventional Fourier representation but comparable to the methods of and and Lothe in specific cases. I. INTRQDUCTIGN We derive an integral representation for the elastodynarnic Green's function in terms of a 5 function. In this representation, even in the general anisotropic 3D (three-dimensional} case, only a 1D integration needs to be done numerically. The integration does not involve oscillatory functions or singularities. This reduces the problem of numerical convergence. The CPU time required for calculating the elastodynamic Green's function is only about, ' of that required for the conventional Fourier-Laplace representation. ' Qur technique is also applicable to other wave equations such as the electromagnetic or acoustic wave equation. We also calculate the elastostatic Green's function by taking the static limit of the elastodynamic response of a solid. Qur method is much more efticient than the conventional Fourier representation. However, the computational eKciency of our method is comparable to that of Barnett for an infinite solid and to that of and Lothe for calculations of surface displacements in a semi-infinite solid. The elastodynamic Green's function is useful for calculating physical properties of solids involving longwavelength phonons. Presently, there is a strong interest in wave-form-based ultrasonics for nondestructive characterization of anisotropic materials. These techniques measure the response of a material to an elastic pulse. which is very well modeled in terms of the elastodynamic Green's function. The elastostatic Green's function is useful for calculating stress distribution in solids containing defects and discontinuities. ' In 1attice statics calculations and the computer simulation of lattice defects, the elastostatic Green's function is needed to fix the asymptotic limit of the lattice distortions. ' The Green's function for an isotropic solid is usually' calculated by using the Fourier-I. aplace representation in the wave-vector — frequency space. This is adequate for traditional materials which could be approximated as isotropic. Modern composite materials are highly anisotropic. In this case the conventional Fourier representation requires a 4D integration over oscillatory functions for the elastodynamic Green's function and 3D integration for the elastostatic Green's function. In many applications, we need to calculate the Green's function for bounded solids (containing free surfaces of interfaces). The Fourier-Laplace representation of the Green's function in such cases is poorly convergent and its evaluation is CPU intensive. A powerful technique for calculating the 3D elastostatic Green's function was suggested by Barnett. This technique is very well suited for calculation of the Green's function for infinite solids. For this case, the computational e%ciency of our method is about the same as that of Barnett. An extension of Stroh's formalism has been developed by and Lothe for bounded solids, but it is applicable only to 2D problems. Qur representation retains its simplicity even for 3D bounded solids. For example, our expression for the Green's function for a semi-infinite solid satisfies the free-surface boundary con