Identified Material Properties Research Articles

A New Approach for Measuring Viscoelastic Properties of Soft Materials Using the Dynamic Response of a Spherical Object Placed at the Sample Interface

BackgroundThere are several techniques to characterize the mechanical properties of soft materials, such as the indentation method and the method based on the application of a spherical object placed inside the sample. The indentation systems usually yield the elastic properties of materials and their mathematical models do not consider the inertia of the sample involved in motion and radiation damping, while placing an object inside the sample is not practical and this procedure can alter the mechanical properties of the sample for the method based on the application of a bubble/sphere placed inside the sample.ObjectiveA new approach for the identification of the viscoelastic properties of soft materials using the dynamic response of a spherical object placed at the sample interface was proposed.MethodsThe spherical object placed at the sample interface was pressed using an electromagnet and the dynamic response of the spherical object was tracked using a high-speed camera, while the dynamic response of the spherical object placed at the sample interface was estimated using a comprehensive analytical model. The effects of the shear modulus, viscosity, Poisson’s ratio and density of the soft sample, the radius and density of the spherical object and the damping due to radiation were considered in this mathematical model. The shear modulus and viscosity of the soft sample were determined by matching the experimentally identified and theoretically estimated responses of the spherical object.ResultsThe shear moduli and viscosities of the three phantoms with the gelatin mass ratios of 0.20, 0.25 and 0.29 were measured to be 3450, 4300 and 4950 Pa and 12.5, 14.0 and 15.0 Pa⋅s, respectively. The shear modulus and viscosity of the phantom increases as the gelatin mass ratio increases. The frequency of oscillations of the hemisphere placed at the phantom interface increases as the gelatin mass ratio increases due to stiffness increase.ConclusionsAfter matching the experimental and theoretical steady-state displacements and amplitudes of oscillations of the hemisphere at the sample interface, the comparison of the experimentally identified and theoretically predicted frequency of oscillations further confirmed the identified material properties of the samples. The approach presented here is expected to provide valuable information on material properties in biomedical and industrial applications.

Shear bond strength of adhered thin masonry veneer with traditional and polymer modified mortars

There have been numerous cases of bond failures in adhered thin masonry veneer (ATMV) applications, probably due to lack of knowledge and the lack of any standards for design and installation of ATMV. The physical and mechanical properties of three types of ATMV units and two types of mortars, one traditional and one polymer modified, and the shear bond strengths of different combinations of these stones and mortars were measured and evaluated. The outcomes are that bond strength is significantly improved when polymer modified mortar is used, while there is no clear relationship between identified material properties and bond strength.

Cell scientist to watch – Britta Trappmann

ABSTRACT Britta Trappmann studied chemistry at the University of Dortmund, Germany. She then moved to Cambridge, UK, for her PhD (2007–2011) with Wilhelm Huck and Fiona Watt, where she discovered that stem cell fate is regulated by extracellular matrix tethering. For her postdoc, she joined the lab of Christopher Chen in 2012, first at the University of Pennsylvania, USA, and then at Boston University and The Wyss Institute for Biologically Inspired Engineering at Harvard University; there, by developing in vitro models, she investigated how cells sense stiffness and also identified material properties that impact angiogenesis. In 2016, Britta became a group leader at the Max Planck Institute for Molecular Biomedicine, Münster, Germany, where her group studies how interactions between cells and their surrounding extracellular matrix regulate cell and tissue function.

Single-ply elastic properties determination in CFRP laminates using a combined ultrasonic method

Estimation of the elastic properties in composite laminates is very important. Researchers usually consider the laminate as a whole and identify its equivalent stiffness properties. However, determining the elastic properties of each layer within a laminate can provide more comprehensive information to the users. In this paper, an ultrasonic method is proposed to reconstruct the single-ply elastic properties by combing bulk and guided waves. Herein, a novel double-side-transmitted bulk wave method is developed to identify the stacking sequences, providing the necessary information for further evaluation of the single-ply elastic properties. The double-side-transmitted bulk wave method is demonstrated under various layup scenarios, allowing the detection of more layers than conventional techniques. The laser-generated guided wave velocities are then inverted by genetic algorithms (GAs) to reconstruct the single-ply elastic properties. An effective indicator, the coefficient of variation, is designed to represent the accuracy of the identified single-ply material properties. And the experimental dispersion curves are in good agreement with the theoretical dispersion curves under the identified material properties.

An ABAQUS® plug-in for generating virtual data required for inverse analysis of unidirectional composites using artificial neural networks

This paper presents a robust ABAQUS® plug-in called Virtual Data Generator (VDGen) for generating virtual data for identifying the uncertain material properties in unidirectional lamina through artificial neural networks (ANNs). The plug-in supports the 3D finite element models of unit cells with square and hexagonal fibre arrays, uses Latin-Hypercube sampling methods and robustly imposes periodic boundary conditions. Using the data generated from the plug-in, ANN is demonstrated to explicitly and accurately parameterise the relationship between fibre mechanical properties and fibre/matrix interphase parameters at microscale and the mechanical properties of a UD lamina at macroscale. The plug-in tool is applicable to general unidirectional lamina and enables easy establishment of high-fidelity micromechanical finite element models with identified material properties.

A Scientific Analysis of Gold Threads Used in Donggungbi-Wonsam(Ceremonial Robe Worn by a Crown Princess, National Folklore Cultural Heritage No.48)

This study identified material properties through scientific analysis on Jikgeumdan(satin with gold threads) from Donggungbi-Wonsam and the gold threads used in the embroidery. The Donggungbi-Wonsam’s base of gold threads were estimated to have used mulberry fiber’s Korean paper(Hanji) because non-wood-based fibers were observed. The X-ray spectrometer showed that the Tongsuseulan of Donggungbi-Wonsam was a flat gold thread of pure gold and Jikgeumdan of flat silver thread of its Saekdong and Hansam. High sulfur levels were detected in the flat silver thread, which appeared to have formed silver sulfide by either manufacturing process using sulfur or conservation environment. he dragon insignia’s embroidery is also described as two types twisted gold threads; pure gold and alloying-gold and silver. while dragon insignia’s border line is decorated with a twisted gold thread of pure gold. In particular, it was investigated that adhesives such as an animal glue, a protein-based compound by gas chromatography mass spectrometry. Additionally, XRF and Raman spectroscopy analysis on the mixture substances between the metal surface and the base paper of gold threads identified talc and quartz in the gold threads and Seokganju(hematite) in the flat silver threads.

Material Identification on Thin Shells Using the Virtual Fields Method, Demonstrated on the Human Eardrum.

Characterization of material parameters from experimental data remains challenging, especially on biological structures. One of such techniques allowing for the inverse determination of material parameters from measurement data is the virtual fields method (VFM). However, application of the VFM on general structures of complicated shape has not yet been extensively investigated. In this paper, we extend the framework of the VFM method to thin curved solids in three-dimensional, commonly denoted shells. Our method is then used to estimate the Young's modulus and hysteretic damping of the human eardrum. By utilizing Kirchhoff plate theory, we assume that the behavior of the shell varies linearly through the thickness. The total strain of the shell can then be separated in a bending and membrane strain. This in turn allowed for an application of the VFM based only on data of the outer surface of the shell. We validated our method on simulated and experimental data of a human eardrum made to vibrate at certain frequencies. It was shown that the identified material properties were accurately determined based only on data from the outer surface and are in agreement with literature. Additionally, we observed that neither the bending nor the membrane strain in an human eardrum can be neglected and both contribute significantly to the total strain found experimentally.

Controls on the rates and products of particle attrition by bed-load collisions

Abstract. River rocks round through the process of impact attrition, whereby energetic collisions during bed-load transport induce chipping of the grain surface. This process is also important for bedrock erosion. Although previous work has shown that impact energy, lithology, and shape are controlling factors for attrition rates, the functional dependence among these quantities is not settled. Here we examine these factors using a double-pendulum apparatus that generates controlled collisions between two grains under conditions relevant for bed-load transport. We also determine the grain size distributions (GSDs) of the attrition products. Two experimental results appear to support previous treatments of impact erosion as brittle fracture: (i) mass loss is proportional to kinetic energy, and this proportionality is a function of previously identified material properties; and (ii) attrition-product GSDs are well described by a Weibull distribution. Chipping results from the development of shallow and surface-parallel cracks, a process that is distinct from bulk fragmentation that occurs at higher energies. We suggest that Hertzian fracture is the dominant mechanism of impact attrition for bed-load transport. We also identify an initial phase of rapid mass loss in which attrition is independent of energy and material properties; this is a shape effect associated with removal of very sharp corners. The apparent universality of both mass loss curves and attrition-product GSDs requires further investigation. Nonetheless, these findings are useful for interpreting the contribution of in-stream attrition to downstream fining and the production of sand resulting from bed-load transport of river pebbles.

Elastic constants identification of fibre-reinforced composites by using guided wave dispersion curves and genetic algorithm for improved simulations

There is a great potential for model-based approaches in which guided waves are utilised for damage detection, localisation and size estimation. However, to be effective, the model must accurately represent guided wave propagation behaviour, especially wave velocity dependence on the angle of propagation. Unfortunately, improperly assumed or derived through homogenisation elastic material properties can lead to larger wave velocity changes than caused by damage. In order to account for this problem, the elastic constants are estimated based on experimental measurements of Lamb waves and optimisation techniques.In the proposed approach, experimental measurements of full wavefield data of propagating Lamb waves are conducted by scanning laser Doppler vibrometer. The obtained wavefield data is processed by using the 3D Fourier transform. Slicing the data at selected frequencies yields wavenumber profiles. These profiles can be directly translated to wave propagation velocity dependence on the angle of propagation. Data can also be sliced at selected angles giving dispersion curve images. At the same time, wavenumber profiles or dispersion curves can be calculated by using a semi-analytical model for the range of material properties. A genetic algorithm is used in which the best fit between experimental data and the semi-analytical model is optimised.Experiments were conducted on a unidirectional CFRP panel and the above-mentioned procedure was applied. Identified material properties were used in the in–house code of the time domain spectral element method for simulations of the propagating waves. Finally, numerical results were compared to the experimental full wavefield data showing much better accuracy than in case of application of homogenisation techniques.Furthermore, the correctness of elastic constants determined by the proposed method was validated against the elastic constants obtained through static test standards.

Robust Optimization Scheme for Inverse Method for Crystal Plasticity Model Parametrization.

A bottom-up material modeling based on a nonlocal crystal plasticity model requires information of a large set of physical and phenomenological parameters. Because of the many material parameters, it is inherently difficult to determine the nonlocal crystal plasticity parameters. Therefore, a robust method is proposed to parameterize the nonlocal crystal plasticity model of a body-centered cubic (BCC) material by combining a nanoindentation test and inverse analysis. Nanoindentation tests returned the load–displacement curve and surface imprint of the considered sample. The inverse analysis is developed based on trust-region-reflective algorithm, which is the most robust optimization algorithm for the considered non-convex problem. The discrepancy function is defined to minimize both the load–displacement curves and the surface topologies of the considered material under applying varied indentation forces obtained from numerical models and experimental output. The numerical model results based on the identified material properties show good agreement with the experimental output. Finally, a sensitivity analysis performed changing the nonlocal crystal plasticity parameters in a predefined range emphasized that the geometrical factor has the most significant influence on the load–displacement curve and surface imprint parameters.

Lamina properties non-destructive characterisation of asymmetric carbon fiber reinforced laminates

A non-destructive technique – numerical-experimental method, based on vibration tests is applied for the characterization of lamina properties of asymmetric carbon fiber reinforced laminates. The main idea of this procedure is the determination of simple mathematical models (response surfaces), using the finite element analysis at the reference points of experimental design. Therefore, a significant reduction of computational efforts can be attained. For the purpose of characterization, two asymmetric carbon fiber laminated specimens were prepared, using the prepreg manual layout technology. To evaluate the elastic material properties obtained, the numerical resonant frequency, determinated by using identified material properties, were successfully compared with experimentally measured and high accuracy with average relative error less than 1% was reached.

Numerical modeling method for UV imprint process simulation using thermoviscoelastic constitutive equations

A novel numerical modeling method for UV imprint process simulation using thermoviscoelastic constitutive equations is proposed. The purpose of the process simulation is to predict transfer errors caused by UV shrinkage when a soft mold such as polydimethylsiloxane (PDMS) is used. The proposed method introduces an imaginary physical quantity, “virtual temperature,” as a measure of UV reaction progress and then replaces the UV curing/shrinkage phenomenon with the thermoviscoelastic cooling solidification/contraction phenomenon. A series of rheometry experiments are conducted for a general UV resin to identify the material properties of the UV resin as a thermoviscoelastic material. For the purpose of verification, the proposed method with the identified material properties is applied to the simulation of the rheometry experiments. Moreover, for the sake of validation, it is also applied to an actual UV imprinting process for a micromirror array molding with a PDMS mold. These analysis results prove the effectiveness of the present method in predicting transfer errors caused by UV shrinkage.

Experimental and numerical modeling study of solidification cracking in Alloy 52M filler metal in the cast pin tear test

Finite element analysis (FEA) model of the solidification process in the cast pin tear test (CPTT) was developed using the commercial software ProCAST™. Modeling predictions of the solidification and strain accumulation sequences and of the cracking sensitive region location were validated using parallel CPTT testing of Alloy 52M filler metal. Predicted pin length dependences of the hot tearing indicator (HTI) and the effective plastic strain (EPS) were in good correlation with the experimentally determined zero to 100% CPTT cracking curve in Alloy 52 M. Sensitivity study with the developed model identified material properties and test parameters with strong influence on the accuracy of modeling predictions. Parallel computational modeling and CPTT experimentation was performed to quantify the critical strain for solidification cracking in Alloy 52M filler metal. This study demonstrated that numerical modeling of weldability tests can be used for quantification of material-specific properties related to cracking susceptibility in engineering alloys.

Hybrid testing with model updating on steel panel damper substructures using a multi‐axial testing system

SummaryThis paper describes a series of hybrid tests performed on a steel panel damper (SPD) specimen by using a multi‐axial testing system. The building under investigation adopted a three‐dimensional six‐story moment resisting frame with four SPDs installed at each story as the main seismic resisting system. The structural model was subjected to bi‐directional ground excitations of three hazard levels. The finite element analysis program “Platform of Inelastic Structural Analysis for 3D Systems” (PISA3D) was used as the analysis kernel for hybrid tests. Relevant programming extensions in PISA3D were created to support geographically distributed hybrid testing in a general‐purpose manner. An external displacement control (EDC) method was developed such that the actual boundary deformation of the specimen fixtures could be continuously measured and immediately compensated during tests. An online model updating (OMU) technique was developed and employed such that the material properties of the specimen could be identified directly from the specimen response during the test. The identified material properties were then immediately used to update those of the other relevant numerical elements to enhance the overall simulation fidelity. Superior flexibility of the underlying software architecture was well demonstrated in this series of hybrid tests since no hardcoding was used to support all the complex test settings. Test results confirmed the effectiveness of the proposed EDC method, as well as the capacity of the proposed OMU technique to satisfactorily and efficiently capture the hysteretic properties of the specimen.

Importance of asymmetry and anisotropy in predicting cortical bone response and fracture using human body model femur in three-point bending and axial rotation.

Modeling of cortical bone response and failure is critical for the prediction of Crash Induced Injuries (CII) using advanced finite element (FE) Human Body Models (HBM). Although cortical bone is anisotropic and asymmetric in tension and compression, current HBM often utilize simple isotropic, symmetric, elastic-plastic constitutive models. In this study, a 50th percentile male femur FE model was used to quantify the effect of asymmetry and anisotropy in three-point bending and axial torsion. A complete set of cortical bone mechanical properties was identified from a literature review, and the femur model was used to investigate the importance of material asymmetry and anisotropy on the failure load/moment, failure displacement/rotation and fracture pattern. All models were able to predict failure load in bending, since this was dominated by the cortical bone material tensile response. However, only the orthotropic model was able to predict the torsional response and failure moment. Only the orthotropic model predicted the fracture initiation location and fracture pattern in bending, and the fracture initiation location in torsion; however, the anticipated spiral fracture pattern was not predicted by the models for torsional loading. The results demonstrated that asymmetry did not significantly improve the prediction capability, and that orthotropic material model with the identified material properties was able to predict the kinetics and kinematics for both three-point bending and axial torsion. This will help to provide an improved method for modeling hard tissue response and failure in full HBM.

Method for the unique identification of hyperelastic material properties using full‐field measures. Application to the passive myocardium material response

Quantitative measurement of the material properties (eg, stiffness) of biological tissues is poised to become a powerful diagnostic tool. There are currently several methods in the literature to estimating material stiffness, and we extend this work by formulating a framework that leads to uniquely identified material properties. We design an approach to work with full-field displacement data-ie, we assume the displacement field due to the applied forces is known both on the boundaries and also within the interior of the body of interest-and seek stiffness parameters that lead to balanced internal and external forces in a model. For in vivo applications, the displacement data can be acquired clinically using magnetic resonance imaging while the forces may be computed from pressure measurements, eg, through catheterization. We outline a set of conditions under which the least-square force error objective function is convex, yielding uniquely identified material properties. An important component of our framework is a new numerical strategy to formulate polyconvex material energy laws that are linear in the material properties and provide one optimal description of the available experimental data. An outcome of our approach is the analysis of the reliability of the identified material properties, even for material laws that do not admit unique property identification. Lastly, we evaluate our approach using passive myocardium experimental data at the material point and show its application to identifying myocardial stiffness with an in silico experiment modeling the passive filling of the left ventricle.

Ultrasonic transmission measurements in the characterization of viscoelasticity utilizing polymeric waveguides

For the numerical simulation of acoustic wave propagation in (measurement) systems and their design, the use of reliable material models and material parameters is a central issue. Especially in polymers, acoustic material parameters cannot be evaluated based on quasistatically measured parameters, as are specified in data sheets by the manufacturers.In this work, a measurement method is presented which quantifies, for a given polymeric material sample, a complex-valued and frequency-dependent material model. A novel three-dimensional approach for modeling viscoelasticity is introduced. The material samples are designed as hollow cylindrical waveguides to account for the high damping characteristics of the polymers under test and to provide an axisymmetric structure for good performance of waveguide modeling and reproducible coupling conditions arising from the smaller coupling area in the experiment. Ultrasonic transmission measurements are carried out between the parallel faces of the sample. To account for the frequency dependency of the material properties, five different transducer pairs with ascending central frequency from to are used. After passing through the sample, each of the five received signals contains information on the material parameters which are determined in an inverse procedure. The solution of the inverse problem is carried out by iterative comparison of an innovative forward SBFEM-based simulations of the entire measurement system with the experimentally determined measurement data. For a given solution of the inverse problem, an estimate of the measurement uncertainty of each identified material parameter is calculated.Moreover, a second measurement setup, based on laser-acoustic excitation of Lamb modes in plate-shaped specimens, is presented. Using this setup, the identified material properties can be verified on samples with a varied geometry, but made from the same material.

Impact of Experimental Uncertainties on the Identification of Mechanical Material Properties using DIC

This paper is concerned with an in-depth study of the interactions between full-field measurements errors and material identification. It is a further step in a research plan that aims to create a simulation procedure of actual experiments, with the final goal of using the simulator to optimise the test set-up in terms of specimen shape, measurement technique, applied load etc. In particular,here, Digital Image Correlation (DIC) is used as a full-field technique to obtain strain and displacement fields. These maps are used as input in an inverse methodology as, for instance, the virtual fields method (VFM) to obtain the material parameters introducing uncertainties in the characterization. The purpose of this contribution is to bridge the gap between experiments and simulations, in order to obtain predictions as close as possible to reality in terms of identification error. That will be used, as final goal of the general study, to optimize numerically a test set-up configuration, giving a priori the best parameters to use to experimentally identify a specimen. In the present contribute, the operating procedure is to perform real experiments and then to reproduce them numerically. Experimental uncertainties such as noise, lighting conditions, in-plane and out-of-plane motions are treated separately and introduced in the simulator. As such, their impact on the identified material properties can be unambiguously investigated. Here, focus is on the elastic properties of aluminium specimens, i.e. the Young’s modulus and the Poisson ratio and their specific variances due to the aforementioned errors. The simulator predicts reality to a large extent.

Practical implementation of the corrected force analysis technique to identify the structural parameter and load distributions

The paper aims to combine two objectives of the Force Analysis Technique (FAT): vibration source identification and material characterization from the same set of measurement. Initially, the FAT was developed for external load location and identification. It consists in injecting measured vibration displacements in the discretized equation of motion. Two developments exist: FAT and CFAT (Corrected Force Analysis Technique) where two finite difference schemes are used. Recently, the FAT was adapted for the identification of elastic and damping properties in a structure. The principal interests are that the identification is local and allows mapping of material characteristics, the identification can be made at all frequencies, especially in medium and high frequency domains. The paper recalls the development of FAT and CFAT on beams and plates and how it can be possible to extract material characteristics in areas where no external loads are applied. Experimental validations are shown on an aluminum plate with arbitrary boundary conditions, excited by a point force and where a piece of foam is glued on a sub-surface of the plate. Contactless measurements were made using a scanning laser vibrometer. The results of FAT and CFAT are compared and discussed for material property identifications in the regions with and without foam. The excitation force identification is finally made by using the identified material properties. CFAT gives excellent results comparable to a direct measurement obtained by a piezoelectric sensor. The relevance of the corrected scheme is then underlined for both source identification and material characterization from the same measurements.

Application of Tube Hydroforming in Square Cross-Section Die for Inverse Identification Method Validation

Tube hydroforming processes are an excellent way for manufacturing reduced weight parts with complex shapes in widespread fields. Accurate numerical simulation of tube hydroforming process is particularly based on precise material parameters deduced from experimental tests. The free bulge test is widely employed for the parameter identification of tubular material behavior models by means of analytical [1] and numerical methods [2]. In this context, an inverse identification methodology using free bulge tests was developed. These tests were carried out by means of a new home-designed and manufactured bulge forming machine. The objective of this work is the validation of the inverse identification method using tube hydroforming in square cross-section die. The analysis of this particular hydroforming process with respect to material parameters is performed. For this purpose, circular section tubes made of low carbon steel S235 and aluminum alloy AA6063-O are hydroformed against square-cross sectional die using our bulge forming machine. Afterwards, FE model is constructed to simulate square-sectional hydroformed parts. The influence of some parameters, such as strain hardening exponent, anisotropy parameter and friction coefficient, on numerical square cross-sectional hydroformed part thickness is analyzed. It permits to assess the sensitivity of the thickness relative to used material parameters in the FE model. In order to validate the inverse identification procedure for both materials, experimental thicknesses along the profile of cross-sectional hydroformed parts are measured and compared with the corresponding numerical thicknesses predicted by FE model. It is proven after analyzing the obtained results that the chosen response, i.e. thickness distribution along the profile of the tube hydroforming against the square cross-section die, used for the validation is sensitive to the identified material properties. Particularly, it is demonstrated for low carbon steel S235 that numerical thickness is in good agreement with experimental data. However, for aluminum alloy AA6063-O, a discrepancy between experimental and predicted thicknesses is noticed. Anyway, it is demonstrated that inverse identification approach leads to sufficiently accurate parameters used for numerical tube hydroforming simulations. Furthermore, it seems that Hill48’s yield criterion is more suitable for describing steels plastic behavior than aluminum alloys for tube hydroforming processes. Concerning aluminum alloy, certainly the choice of appropriate yield criterion is of paramount importance on the prediction of tubular plastic behavior in tube hydroforming. Consequently, it is shown that the use of simple tube hydroforming in square-section die is suitable for the validation of FE model which is identified by inverse method using free bulge test.