Void Inclusions Research Articles

Numerical evaluation of strain transfer model for steel-reinforced optical fiber cable embedded in a cylindrical concrete beam with two void inclusions

In distributed optical fiber sensors, strain transfer is usually described through a simplified one-dimensional equation, derived from continuum mechanics. This equation, in which a parameter known as the strain-lag parameter takes into account the cable’s geometric and mechanical characteristics, establishes a relationship between the measured longitudinal strain profile within the optical fiber and the real strain profile occurring in the host material. In the case of steel-reinforced optical fiber cables, appreciated for their resistance to breakage during on-site instrumentation, a notable discrepancy between the measured and actual strain profiles is revealed especially in the presence of a strain gradient, indicating that the ability to transfer strain from the host material to the optical fiber is restrained using this type of cable. This paper assesses numerically the strain transfer model for steel-reinforced optical fiber sensors in the presence of a strain gradient generated by two void inclusions in a concrete beam. The good accuracy of the strain transfer model is observed by the comparison with a 3D finite element simulation. However, the result points out the critical necessity of precisely determining the strain-lag parameter.

Competing failure modes in void and inclusion NiCoCrFeCu alloy under tensile simulation using molecular dynamic

This study exhibits dual effects from the parallel existence of inclusion and void on mechanical characteristics and deformation response for NiCoCrFeCu alloy based on an MD simulation of a tensile test. The pure Cu-inclusion and the defect rate 1.0 reinforce the strength of NiCoCrFeCu HEA, while pure void causes damage quickly of HEA. The nuclear deformation forms at the void root and boundary between the inclusion and workpiece for defect samples, which occurs in random zones for monolithic pieces. The deformation density for the sample combined void and inclusion is always the lowest because the boundary prevents the fusion between the hole-induced and workpiece-induced deformation. The defect rates are a critical factor in deciding the phase transition and deformation development during the strain stages, and the defect rate of 1.0 is the ladder. The tensile strength and strain energy increase as the defect rates increase from 0.43 to 1, then enormously decrease as the defect rate to 1.5, while Young’s modulus declines with an increase in defect rate. Moreover, the results also show that the effect of the void position and the divided number of voids with the same void volume in each material sample is severe. The tensile strength and Young’s modulus increase with the increase of the void number; the model with two voids in the X direction is subjected to higher stress than those with two holes in the Y direction. Finally, the mechanical parameters, such as the tensile strength and Young’s modulus, all decrease as the void radius and inclusion radius increase, and the sample can enhance elongation when adding an inclusion with a radius of 30 Å. The occupation of the twin boundary interferes with the development of dislocation, reducing the total dislocation length.

Modeling of frontal polymerization of carbon fiber and dicyclopentadiene woven composites with stochastic material uncertainty

A novel finite element modeling framework is proposed for simulating dicyclopentadiene (DCPD) frontal polymerization in carbon fiber (CF) woven composites. A mesoscale CF/DCPD representative volume element (RVE) model was developed with two triggering directions, resulting in out-of-plane and in-plane DCPD polymerization. The Arrhenius equation coupled with a modified Prout-Tompkins autocatalytic model was used to evaluate the dynamic DCPD polymerization process. The rate and degree of DCPD cure were calculated by the material property uncertainties of DCPD with and without random void inclusions (3 % used as a reference in this work). The DCPD frontal polymerization in CF/DCPD composite was strongly influenced by the triggering direction. In the RVE model considered in this work, DCPD frontal polymerization was slightly faster in the in-plane (warp/weft yarn) directions than the out-of-plane (thickness) direction; polymerization occurs first in DCPD resin and is followed by CF/DCPD interface. Material property uncertainty and void inclusion had significant effects on both out-of-plane and in-plane DCPD polymerization. A large variation in DCPD material properties and the presence of void inclusions significantly delayed the rate and degree of DCPD frontal polymerization. This work provides the preliminary estimation of the frontal polymerization of DCPD-based composites and guides the structural applications of these materials.

Influence of void contour on the elastic behavior of parts produced by material extrusion

The significant void content is the most noticeable flaw of 3D printed thermoplastic structures. The voids run parallel to the printed filaments, resulting in non-ellipsoidal geometries that are directly related to the printing variables. Until recently, only numerical solutions for accurate simulation of void geometry have been published, while analytical models have been limited to solutions of ideal geometries such as long cylinders. The impact of the geometry of void inclusions on the elastic behavior of acrylonitrile butadiene styrene (ABS) produced by material extrusion is examined in this study, and a unique semi-analytical method for real void geometries is proposed using, as basis, finite element (FE) solutions of Eshelby’s tensor. The FE solutions of Eshelby’s tensor are obtained by employing the FE approach to solve the elasticity problem of a large matrix containing one void inclusion. The numerical solutions are then used to evaluate the effective elastic properties using the effective field methods. The results of the semi-analytical method are in good agreement with numerical and experimental results.

On the Effect of Hot Rolling on Inclusion Size and Distribution in a Cast AISI 1070 Steel Railroad Wheel

The goal of this work is to examine the effect of hot deformation on shrinkage porosity and nonmetallic inclusions in an AISI 1070 grade steel industrially produced wheel casting. Steel cleanliness is an important consideration as it influences the mechanical properties of the final product. A high density of porosity and inclusions have been shown to be detrimental for mechanical properties, especially during hot rolling. Using a laboratory-scale rolling mill, cast preforms were subjected to a 66% cumulative reduction to determine the effect of thermomechanical processing on void closure and inclusions that may produce anisotropy in mechanical properties. Quantitative automated feature analysis, AFA, of inclusion type, size, morphology, and distribution was conducted utilizing an Aspex PICA 1020 scanning electron microscope to determine differences in inclusions and shrinkage porosity in the as-cast and as-rolled conditions. The results were compared with previously reported impact toughness values which indicated a trend with MnS projected length and average impact toughness in the T-L orientation. Reduction in shrinkage porosity was also verified utilizing 3D micro-X-ray CT scans. The AFA results showed a decrease in shrinkage porosity from 177 ppm in the as-cast condition to less than 35 ppm after rolling. Pores were in general much smaller and widely distributed after hot rolling and this would suggest improved impact properties. Analysis of nonmetallic inclusions revealed three primary categories of inclusions that included MnS, Al2O3, and complex inclusions that mainly consisted of MnS with an Al2O3 core, with small quantities of mixed silicates of Mn and Al and calcium aluminates (CaAl2O4).

Enhanced reactivity by energy trapping in shocked materials: reactive metamaterials for controllable output

Through the use of carefully designed numerical experiments on an explosive system, that use predictive models for subcomponents and multi-material simulation, we demonstrate enhanced reactivity by energy trapping in regions of the reactive flow that were previously shocked. Particles and inclusions are placed in designed patterns in an explosive matrix. New capabilities in additive manufacture make it possible to consider novel designs, that we refer to as ‘reactive metamaterials’. For a fixed amount of energy delivered by a shock impactor, an explosive that normally would not detonate, will detonate when particles are included. Enhanced reactivity correlates precisely with a change in the partition of energy from kinetic to internal, via reflective processes and flow stagnation in high pressure systems. We analyse cases associated with high shock impedance tantalum particles, and void inclusions, individually and placed in a test array. High impedance reflectors trap energy in regions of pre-shocked material. Whereas void shock collapse causes depressurisation of the material along with rapid material flow, and high pressure spot formation related to the jet impact blast. We analyse how these limiting cases of high impedance particle arrays and void arrays partition specific kinetic and internal energy, during the shock impact transient on the system of matrix explosive and embedded particle/voids. Both generate specific flow fields, pressure and temperature cycles in the matrix material over interval times, determined by the particle/void size and placement. Design variations of the configurations presented here can be tested by both experiment and simulation, and can be searched for optimal designs, aided by modern machine learning search methods.

Measurements and Analysis of Partial Discharges at HVDC Voltage with AC Components

This paper presents the methodology for phase-resolved partial discharge measurements in HVDC systems with a DC voltage containing trace AC harmonics or a DC voltage ripple. The measurement result of partial discharges is an indicator of the current condition of the high-voltage power devices’ insulation system. The voltage waveforms in HVDC systems are not ideal DC, because different disturbances occurring naturally in these systems can affect the DC voltage. The AC harmonics related to the AC source voltage, and the voltage ripples provided by the power converter topology, can be found in the HVDC voltage. This paper proposes a novel approach to partial discharge measurement in DC networks. The synchronization to the particular AC harmonics appearing in DC voltage was applied to the PD measurements. The analyses were performed on the model sample containing a void inclusion, which was placed between electrodes fed by the DC voltages with the imposed chosen AC harmonics. Two scenarios were analyzed at a constant DC level: one with a variable AC magnitude and the second with a variable frequency of an AC source adjusted to the harmonics: 50, 150, 300, and 350 Hz. It was observed that the superimposed AC voltage component resulted in an intensification of PDs.

Effects of void and inclusion sizes on mechanical response and failure mechanism of AlCrCuFeNi2 high-entropy alloy

The effects of various void sizes, inclusion sizes, and strain rates on the mechanical response, deformation behavior, and failure mechanism of AlCrCuFeNi2 high-entropy alloy samples with a pre-void/inclusion under the tension are investigated via the molecular dynamics. The results reveal that there exists a critical value of mechanical parameters such as the tensile strength and Young’s modulus in the tension of sample with an inclusion size of 15 Å, where the mechanistic parameters are changed. Meanwhile, the mechanical parameters decrease under increasing the void size and reducing the strain rate. The deformation behavior discloses that the void and inclusion are the principal cause of initial strain, and the shear bands are propagated inside the workpiece along the direction of an angle of 45° related to the tensile axis. The transformation from the FCC phase to other structures such as HCP and amorphous has occurred during deformation. Also, the HCP phase of inclusion shows relatively unstable, which is demonstrated by a rapid phase transformation of inclusion under a small strain. Finally, the dislocation evolution mechanism exhibits that it begins to nucleate around the void and inclusion, and the dislocations are then moved to free surfaces with increased strain.

On the respect to the Hashin-Shtrikman bounds of some analytical methods applying to porous media for estimating elastic moduli

In this work, some popular analytic formulas such as Maxwell (MA), Mori-Tanaka approximation (MTA), and a recent method, named the Polarization approximation (PA) will be applied to estimate the elastic moduli for some porous media. These approximations are simple and robust but can be lack reliability in many cases. The Hashin-Shtrikman (H-S) bounds do not supply an exact value but a range that has been admitted by researchers in material science. Meanwhile, the effective properties by unit cell method using the finite element method (FEM) are considered accurate. Different shapes of void inclusions in two or three dimensions are employed to investigate. Results generated by H-S bounds and FEM will be utilized as references. The comparison suggests that the method constructed from the minimum energy principle PA can give a better estimation in some cases. The discussion gives out some remarks which are helpful for the evaluation of effective elastic moduli.
 Keywords:
 Maxwell approximation; polarization approximation; Mori-Tanaka approximation; effective elastic moduli; porous medium.

Multiscale seismic imaging with inverse homogenization

SummarySeismic imaging techniques such as elastic full waveform inversion (FWI) have their spatial resolution limited by the maximum frequency present in the observed waveforms. Scales smaller than a fraction of the minimum wavelength cannot be resolved, and only a smoothed, effective version of the true underlying medium can be recovered. These finite-frequency effects are revealed by the upscaling or homogenization theory of wave propagation. Homogenization aims at computing larger scale effective properties of a medium containing small-scale heterogeneities. We study how this theory can be used in the context of FWI. The seismic imaging problem is broken down in a two-stage multiscale approach. In the first step, called homogenized FWI (HFWI), observed waveforms are inverted for a smooth, fully anisotropic effective medium, that does not contain scales smaller than the shortest wavelength present in the wavefield. The solution being an effective medium, it is difficult to directly interpret it. It requires a second step, called downscaling or inverse homogenization, where the smooth image is used as data, and the goal is to recover small-scale parameters. All the information contained in the observed waveforms is extracted in the HFWI step. The solution of the downscaling step is highly non-unique as many small-scale models may share the same long wavelength effective properties. We therefore rely on the introduction of external a priori information, and cast the problem in a Bayesian formulation. The ensemble of potential fine-scale models sharing the same long wavelength effective properties is explored with a Markov chain Monte Carlo algorithm. We illustrate the method with a synthetic cavity detection problem: we search for the position, size and shape of void inclusions in a homogeneous elastic medium, where the size of cavities is smaller than the resolving length of the seismic data. We illustrate the advantages of introducing the homogenization theory at both stages. In HFWI, homogenization acts as a natural regularization helping convergence towards meaningful solution models. Working with fully anisotropic effective media prevents the leakage of anisotropy induced by the fine scales into isotropic macroparameters estimates. In the downscaling step, the forward theory is the homogenization itself. It is computationally cheap, allowing us to consider geological models with more complexity (e.g. including discontinuities) and use stochastic inversion techniques.

Development of locally homogeneous finite element model for simulating the mesoscale structure of asphalt mixture

A locally homogeneous finite element (FE) model is proposed to account for the heterogeneous internal structures of asphalt mixtures in this study. For this model, the internal structure of the asphalt mixture was divided into several homogeneous parts based on digital image processing (DIP) technology. Within each part, one aggregate or air void inclusion was embedded in asphalt mortar, and the integral material properties were homogenized. By comparison with the currently used mesoscale heterogeneous FE models, the proposed locally homogeneous model was proven to be able to effectively consider the local enhancement of the aggregates especially at low frequencies and also consume much less computational time. In addition, the locally homogeneous asphalt mixtures were coupled into the macroscale asphalt pavement model to analyze the load-bearing capacity of the pavements. Comparing with a fully homogeneous pavement model, the computational results showed that the proposed locally homogeneous model can effectively represent the loading-bearing responses of asphalt pavements. This study indicates that this proposed FE simulation approach is able to balance the computational time and simulation effectivity and thus form a base for providing an efficient and robust platform for future development in the multiscale simulation of asphalt pavements.

Boron nitride inclusions within adhesive joints: Optimization of mechanical strength and bonded defects detection

Non Destructive Testing (NDT) by active InfraRed Thermography (IRT) of bonded Carbon Fibers Reinforced Plastic (CFRP) laminates is a very challenging issue. Adding Boron Nitride (BN) particles within the joint material provides an interesting solution to better capture bonding defects. Though increasing additives leads to the improvement of NDT, the validation of this solution requires the mechanical characterization of the new joint material. In this way, the influence of particles on the mechanical properties of the adhesive joint is first investigated via double strap lap shear tests for different BN volume fraction. At the same time, controlled size void inclusions are generated within composite assemblies in order to evaluate the IRT ability to detect bonded defects inside BN-based adhesives. It is demonstrated that thermal data post-processing methods such as Singular Values Decomposition (SVD) and Principal Component Thermography (PCT) help to get the best trade-off between mechanical performance and defect detection.

Control of elastic shear waves by periodic geometric transformation: cloaking, high reflectivity and anomalous resonances

A doubly periodic geometric transformation is applied to the problem of out of plane shear wave propagation in a doubly periodic perforated elastic medium. The technique leads to the design of a system of radially anisotropic and inhomogeneous shells surrounding the void inclusions, that can be tuned to give the desired filtering properties.For a regular transformation, the transformed elastic system displays the same dispersion properties than the original homogeneous one, but for overlapping and unfolding transformations new filtering properties can be obtained, which include anomalous resonances at zero and finite frequencies. Low-frequency homogenisation reveals how it is possible to tune the phase and group velocity in the long-wave limit at any value, or to obtain a zero frequency band gap for Neumann boundary conditions. The dispersion properties of the medium are studied both semi-analytically by the multipole expansion and numerically by the finite element methods.Several applications are shown, including the transmission problem throughout a grating of void inclusions and an interface in a waveguide, where the capability of the proposed model is quantitatively demonstrated by computing the transmitted power flow. Finally, we gave a demonstration of defect modes in a waveguide and of tuning the transformation for the research of Dirac points.

Surface acoustic wave modes in two-dimensional shallow void inclusion phononic crystals on GaAs

The possibility to control supersonic acoustic wave propagation is intriguing, but when modeling phononic crystal devices, supersonic surface acoustic waves are mired by radiative attenuation and, hence, eschewed in many device designs. In this paper, we study supersonic surface acoustic wave modes in shallow hole phononic crystals computationally with respect to the three bulk wave sound barriers of cubic (001) GaAs. From a first principles modeling approach of linear elasticity, the finite element method, and with the aid of characterization parameters for systematic modal categorization, detailed nuances are observed for supersonic surface waves propagating along the [110]-direction of GaAs with a periodically patterned surface. Modes of interest are distinguished by possessing both strain energy and squared polarization ratios above defined thresholds. The square array of shallow inclusions imparts a metamaterial surface layer effect that results in marked changes in the dispersion, the bulk wave hybridization, and the modal interactions of the surface modes in the Γ-X direction of the phononic crystal, which are characterized by their modal profiles and attenuation via bulk wave radiation. From these findings, we propose an extended sound cone concept to accommodate supersonic surface acoustic waves with low attenuation. Furthermore, at frequencies above the shear vertical bulk dispersion line, well-bounded surface acoustic wave modes are revealed, and the phenomenon of these supersonic modes with limited bulk wave coupling is explored. From these detailed band structures, the systematic method of mode characterization reveals deeper insights into modes that exist in shallow phononic crystals on cubic GaAs.

Additive manufacturing in repair: Influence of processing parameters on properties of Inconel 718

Laser engineered net shaping (LENS™), a metal additive manufacturing process, was employed to repair internal defects in Inconel 718. The technique involves milling slot/s around defect zone and subsequently refill the cavities with a material. This study investigates the effectiveness of two slots with different geometric shapes, rectangular and trapezoidal, to repair internal cracks and understand the influence of deposition orientation on mechanical properties of the repaired components. Trapezoidal shaped cross-section showed good fusion between deposit material and the part along the sidewalls without any void inclusion. Build-direction, especially diagonal-deposition, influenced coefficient of friction and compound-wear for non-heat-treated samples.

Detection of void and metallic inclusion in 2D piezoelectric cantilever beam using impedance measurements

The aim of current work is to improve the existing inverse methodology of void-detection based on a target impedance curve, leading to quick-prediction of the parameters of single circular void. In this work, mode-shape dependent shifting phenomenon of peaks of impedance curve with change in void location has been analyzed. A number of initial guesses followed by an iterative optimization algorithm based on univariate method has been used to solve the problem. In each iteration starting from each initial guess, the difference between the computationally obtained impedance curve and the target impedance curve has been reduced. This methodology has been extended to detect single circular metallic inclusion in 2D piezoelectric cantilever beam. A good accuracy level was observed for detection of flaw radius and flaw-location along beam-length, but not the precise location along beam-width.

On the application of the Eshelby-Cheng effective model in a porous cracked medium with background anisotropy: An experimental approach

Understanding the effect of cracks in elastic media is important for hydrocarbon recovery, especially in nonconventional reservoirs. Consequently, due to the presence of oriented cracks on these types of reservoirs, an anisotropic behavior can be induced. In terms of seismic or ultrasonic velocities, this means that elastic waves propagating on regions with oriented cracks have their velocities varying with the propagation and polarization directions. Thus, the analysis of the seismic wave velocities in a cracked medium can be used as a tool for reservoir characterization. For this reason, there is a variety of mathematical models to describe transversely isotropic cracked medium as well as the design of several experiments to test these models. We have experimentally analyzed the theoretical predictions of Eshelby-Cheng’s first-order model. For this proposal, we measured P- and S-wave ultrasonic velocities in 17 anisotropic samples. All samples indicate weak background anisotropy due to layering deposition; i.e., they are vertical transversely isotropic (VTI) media. Sixteen synthetic anisotropic samples with different crack densities and aspect ratios were simulated by penny-shaped void inclusions in a homogeneous porous matrix made with cement and sand. An uncracked sample, with weak VTI anisotropy, was constructed for reference. The crack densities and aspect ratios ranged from 0 to 0.102 and from 0 to 0.52, respectively. All measurements were performed in a dry condition. From the experimental and theoretical velocities, we calculated the Thomsen’s parameters and correlated them with the crack density. An efficient flowchart was developed to make feasible and clear the inversion of the output Eshelby-Cheng’s effective elastic coefficients in effective velocities. Our results suggest that the anisotropy increases with crack density. In general, we noted that the best fit between the Eshelby-Cheng’s model and the experimental results occurs when the crack density and aspect ratio were lower than 0.1 and 0.32, respectively, and it is largely dependent on the type of crack porosity’s equation used in the inversion of effective stiffness coefficients in the elastic effective velocities.

Analytical Solutions and Stress Concentration Factors for Annuli With Inhomogeneous Boundary Conditions

Analytical displacement and stress fields with stress concentration factors (SCFs) are derived for linearly elastic annular regions subject to inhomogeneous boundary conditions: an infinite class of the mth order polynomial antiplane tractions or displacements. The solution of the Laplace equation governing the out-of-plane problem covers both rigid and void circular inclusions forming the core of the annulus. The results show first that the SCF and the loading order are inversely proportional. In particular, the SCF approaches value 2 when either the outer boundary of the annulus tends to infinity or the order of the polynomial loading increases. Second, the number of peculiar points on the inner contour having null stress increases with the increasing loading order. The analytical solution is confirmed and extended to noncircular enclosures via finite element analysis by exploiting the heat-stress analogy. The results show that the closed-form solution for a circular annulus can be used as an accurate approximation for noncircular enclosures. Altogether, the results shown can be exploited for analyzing complex loading conditions and/or multiple rigid or void inclusions for enhancing the design of hollow and reinforced composites materials.

A New Equivalent Statistical Damage Constitutive Model on Rock Block Mixed Up with Fluid Inclusions

So far, there are few studies concerning the effect of closed “fluid inclusions” on the macroscopic constitutive relation of deep rock. Fluid-matrix element (FME) is defined based on rock element in statistical damage model. The properties of FME are related to the size of inclusions, fluid properties, and pore pressure. Using FME, the equivalent elastic modulus of rock block containing fluid inclusions is obtained with Eshelby inclusion theory and the double M-T homogenization method. The new statistical damage model of rock is established on the equivalent elastic modulus. Besides, the porosity and confining pressure are important influencing factors of the model. The model reflects the initial damage (void and fluid inclusion) and the macroscopic deformation law of rock, which is an improvement of the traditional statistical damage model. Additionally, the model can not only be consistent with the rock damage experiment date and three-axis compression experiment date of rock containing pore water but also describe the locked-in stress experiment in rock-like material. It is a new fundamental study of the constitutive relation of locked-in stress in deep rock mass.

Piezoelectric T-matrix approach and multiple scattering of electroacoustic waves in thin plates

Metamaterial-enhanced harvesting (MEH) of wave energy in thin plates and other structures has appeared recently for powering small sensors and devices. To support continued MEH concept development, this paper proposes a fully coupled T-matrix formulation for analyzing scattering of incident wave energy from a piezoelectric patch attached to a thin plate. More generally, the T-matrix represents an input–output relationship between incident and reflected waves from inclusions in a host layer, and is introduced herein for a piezoelectric patch connected to an external circuit. The utility of a T-matrix formalism is most apparent in scenarios employing multiple piezoelectric harvesters, where it can be re-used with other T-matrices (such as those previously formulated for rigid, void, and elastic inclusions) in a multiple scattering context to compute the total wavefield and other response quantities, such as harvested power. Following development of the requisite T-matrix, harvesting in an example funnel-shaped metamaterial waveguide structure is predicted using the multiple scattering approach. Enhanced wave energy harvesting predictions are verified through comparisons to experimental results of a funnel-shaped waveguide formed by placing rigid aluminum inclusions in, and multiple piezoelectric harvesters on, a Lexan plate. Good agreement with predicted response quantities is noted.