Soft Inclusions Research Articles

Effects of particle inclusions on cracking in ultrathin barrier films

The effects of particle inclusions with different mechanical properties on the development of cracks in ultra-thin barrier films have been studied. Two types of particles, alumina and poly(styrene-b-methyl methacrylate) with large differences in their elastic modulus were used, representing both hard and soft inclusions. Particles were seeded onto polyethylene naphthalate substrates and subsequently coated with atomic layer deposited Al2O3 as a barrier film. Ca corrosion tests used to determine the performance of the barrier films revealed that soft polymer particle inclusions severely affect the development of cracking in barrier films as compared to stiff ceramic particles. Tensile tests also confirmed that the barrier films with polymer particle inclusions have the lowest value of critical onset strain. Parametric simulations, conducted by varying the elastic modulus of the particle inclusions over a wide range, resulted in a larger strain energy release rate near particle inclusions which promotes crack formation and negatively impacts the performance of barrier films with softer particle inclusions. Thus, the combination of high residual stresses in the barrier films with the presence of soft particle inclusions is found the most detrimental for the development of high-performance ultrathin barrier films.

Thermoelastic problem of two arbitrarily-shaped inclusions

In this paper, we study the plane thermoelastic problem of an infinite matrix containing two interacting inclusions of arbitrary shapes for a uniform heat flux applied on the matrix remotely. We propose a numerical scheme for calculating the temperature and stress fields in the inclusion-matrix system by using the principle of superposition and the Faber series method. A group of numerical examples is given to demonstrate the stress field around two soft inclusions (softer than the matrix) of triangular and square shapes. The results show that the interfacial thermal stress concentration may be relieved by improving the thermal conductivities of the soft inclusions. In particular, we find that for soft inclusions with higher thermal conductivities (higher than the matrix), the interfacial thermal stresses could even reduce slightly when the inclusions approach each other. Additionally, it is shown that the thermal load-induced interaction between the inclusions decays much slower than the mechanical load-induced interaction between the inclusions as the inclusions move away from each other.

Homogenization of an elastodynamic system with a strong magnetic field and soft inclusions inducing a viscoelastic effective behavior

In this paper we study homogenization of a linear system of elastodynamics in an elastic body with soft inclusions, which is embedded in a highly oscillating magnetic field. We show two limits behaviors according to the magnetic field. On the one hand, if the magnetic field has at least two different directions at the interface between the hard phase and the soft phase, then the limit of the displacement in the hard phase is independent of time, so that the magnetic field induces an infinite effective mass. On the other hand, if the magnetic field has a constant direction ξ at the interface, then the limit of the displacement in the hard phase involves an additional displacement in the direction ξ which is solution to an elastodynamics equation with a memory mass, a memory stress tensor and memory external forces depending on the initial conditions, which read as time convolutions with some kernel. When the magnetic field has the same direction ξ in the soft phase with smooth inclusions, we prove that the space-average of the kernel is regular and that the limit of the overall displacement in the direction ξ is solution to a viscoelasticity equation.

Denoising RF Data via Robust Principal Component Analysis: Results in Ultrasound Elastography.

Ultrasound data often suffers from an excessive amount of noise especially from deep tissue or in synthetic aperture imaging where the acoustic wave is weak. Such noisy data renders Time Delay Estimation (TDE) inaccurate in the context of ultrasound elastography. Herein, a novel two-step elastography technique is presented to ensure accurate TDE while dealing with noisy ultrasound data. In the first step, instead of one, we acquire several Radio-Frequency (RF) frames from both pre- and post-deformed positions of the tissue. We stack the frames collected from pre- and post-deformed planes in separate data matrices. Since each set is collected from the same level of tissue compression, we assume that the Casorati data matrices exhibit underlying low-rank structures, which are sought by taking the low-rank and sparse decomposition framework into account. This Robust Principal Component Analysis (RPCA) approach removes the random noise from the datasets as sparse error components. In the second step, we select one frame from each denoised ensemble and employ GLobal Ultrasound Elastography (GLUE) to perform the strain elastography. We call the proposed technique RPCA-GLUE. Our preliminary validation of RPCA-GLUE against simulation phantoms containing hard and soft inclusions proves its robustness to large noise. Substantial improvement in Signal-to-Noise Ratio (SNR) and Contrast-to-Noise Ratio (CNR) has also been observed. Simulation results show that in the presence of large noise, the proposed method substantially improves CNR from 5.0 to 22.6 in a soft inclusion and from 2.2 to 21.7 in a hard inclusion phantom.

Experimental quantification of the effect of oil based drilling fluid contamination on properties of wellbore cement

Portland cement hydration is a chemically sensitive process impacted by temperature, amount of water and the presence of any potential contaminants. Drilling fluids contaminated cement hydration products will therefore have altered microstructures, impacting porosity, permeability, and the overall mechanical properties of the cement sheath. The objective of this study is to quantify the impact of drilling fluid contamination on the ability of cement to provide zonal isolation. Class-H Portland cement-based slurries were prepared according to the API 10B RP. The cement slurry was contaminated with oil based drilling fluid, at 5%, 10%, and 30% by volume of cement casted into core and cured at 95oC and 95% RH for a minimum of 30 days. The microstructural characterization, porosity, permeability, and unconfined compressive strength (UCS) were obtained at ambient conditions, unless specified otherwise. The results provide a striking demonstration of the degradation of wellbore cement performance imposed by oil based drilling fluid contamination. The overall degradation of cement cores is shown to be minimal for contamination up to 5%; to be considerable for 10% contamination, and to be detrimental for 30% contamination and above. Microstructural observations show that the drilling fluid forms relatively non-reactive, soft inclusions within hydrated cement matrix. The interfaces between cement and the fluid inclusions potentially contribute to porosity increase due to the presence of microfractures that emerge at the boundaries. The permeability increase with 10% contamination is roughly a factor of 6 and with 30% contamination is a factor over 130. The permeability is shown to decrease substantially with increasing confining stress for neat cement, but this stress-dependent permeability reduction is much less pronounced for contaminated cement. The samples contaminated by 30% had UCS reduction by a factor 3–4 at ambient temperature.

C-Elastography: In Vitro Feasibility Phantom Study

C-Elastography (CE) is a new ultrasound technique that locally maps the non-linear elasticity of soft tissue using low-frequency (150–250 Hz) shear waves generated by the acoustic radiation force (ARF). CE is based on a recent finding that the magnitude of the ARF in an isotropic tissue-like solid is related linearly to a third-order modulus of elasticity, C, which is responsible for the coupling between deviatoric and volumetric constitutive behaviors. The main objective of the work described here was to examine the feasibility of using and performance of C-elastography in differentiating and characterizing soft tissue via a pilot study on ex vivo tissue and tissue-mimicking inclusions cast in a gelatin block. In this vein, the CE technique deploys a combination of ultrasound motion sensing and 3-D visco-elastodynamic simulation to estimate the non-linear modulus C. As ultrasound focusing inherently confines the ARF to a small region, CE provides the means for measuring C within O(mm3) volumes. Equipped with such data analysis, we performed in vitro CE experiments on agar-based, xenograft and normal breast tissue samples embedded in a gelatin matrix. The compound C-elastograms indicate marked (and sharp) C-contrast, with average values of 1.9 and 5.6 at push points inside the featured soft and hard inclusions, respectively.

Second gradient substitution model for high contrast bi-phase structure

Lightweight structures with soft inclusion material, such as hollow core slabs, foam sandwich wall, pervious pavement ... are widely used in construction engineering for sustainable goals. Voids and soft inclusion can be modeled as a very soft material, while the main material is modeled with its original rigidity, which is so much higher than inclusion's one. In consequence, highly contrast bi-phase structure attracts the interests of scientists and engineers. One important demand is how to build a homogeneous equivalent model to replace the multi-phase structure which requires much resources and time to perform structure analysis. Various homogenization schemes have succeeded in establishing a homogeneous substitution model for composite materials which fulfill the scale separation condition (characteristic length of heterogeneity is very small in comparison to structure dimensions). Herein, elastic stiffness matrix of a homogeneous model which replaces a bi-phase material is computed by a higher-order homogenization scheme. A non-homogeneous boundary condition (a polynomial inspired from Taylor series expansion) is used in computation. Homogeneous substitution model constructed from this computation process, can give engineers a fast and effective tool to predict the behavior of bi-phase structure. Instead of a classical Cauchy continuum, second gradient model is selected as a potential candidate for substituting the composite material behavior because of the separation scale (volume ratio of inclusion to matrix phase reaches unit).
 Keywords:
 generalized continuum; second-gradient medium; higher-order homogenization; non-homogeneous boundary conditions; representative volume element.

Predicting post-impact compression strength of composite structures using the inverse method

The application of carbon fiber-reinforced laminated composites is restricted due to the dramatic decrease of compression strength after impact, which can reach 70% at most. So far, there is no method of predicting the post-impact material performance that has been widely accepted by researchers, aircraft manufactures and aviation safety administrations. Recently, an inverse method was proposed for post-impact strength prediction by modeling impact damage as a soft inclusion with apparent material properties from experiment. This study aims to explore the inverse method in compression-after-impact (CAI) strength prediction of aircraft structures. First, a model and computer program to implement the validation of the inverse method was developed to determine the damage area properties. Then, experiments were conducted with the help of digital image correlation technology and the optimization program to measure the displacement field around damage zones and calculate the apparent material properties. Finally, a strategy of predicting post-impact compression strength of laminated composites was proposed based on the investigation of the compression properties of T700/M21.

Finite Element Study of Hard and Soft Inclusions in Hard/soft Matrix

Hard inclusions in the soft matrix are common in metal matrix composites. On the other hand, soft inclusions in the hard matrix can be found for instance in plastics like acrylonitrile-butadiene-styrene (ABS). This work focuses on the finite element study of hard inclusions in the soft matrix and soft inclusions in the hard matrix. Equivalent von-Mises Stress and Equivalent plastic strain are evaluated. Hard inclusions in the soft matrix produces higher plastic deformation than soft inclusions in the hard matrix but stresses are higher for soft inclusions in the hard matrix than for soft inclusions in the hard matrix along with plastic deformatio

Are Large Particles of Iron Detrimental to Properties of Powder Metallurgy Steels?

It is experimentally shown that a removal of particles exceeding 100 microns in size from iron powders typically used in the fabrication of medium density powder metallurgy steels has a weak effect on apparent density, flowability and compressibility of blends as well as on density and strength of green bodies. An elimination of such particles, i.e., cutting off a heavy tail of a size distribution histogram at the 100 μm threshold, improves a compositional uniformity of sintered materials, but has no noticeable beneficial effect upon the strength of a final product, which is likely be determined by a fraction of pores and their shapes. A presence of soft pearlitic inclusions hardly matters unless their number density becomes so large that a 3D continuity (integrity) of a hard martensitic matrix is lost. This finding suggests that such an expensive preparatory step as sieving away large particles from as-received mixtures would bear no technological advantages. It was experimentally found that an attempt to lower the threshold below 100 μm noticeably worsened apparent density, flowability and compressibility.

Grain- to Reservoir-Scale Modeling of Depletion-Induced Compaction and Implications on Production Rate

Summary The reduction of pore pressure caused by depletion can induce significant reservoir compaction and loss of permeability, especially in unconsolidated reservoirs. In this paper, we develop a numerical approach on the basis of computer-based simulations of unconsolidated rock samples subjected to mechanical tests that replicate the one-dimensional (1D) strain depletion path and allow for a prediction of permeability loss. The 1D strain stress path is a good approximation for long and thin conventional reservoirs with a compliant caprock. The numerical sample consists of crushable stiff and soft grains (proxies for sand and shale) simulated with the discrete element method (DEM) coupled with the bonded-particle model (BPM). Model parameters are calibrated through numerical single-grain-crushing tests which reproduce the experimentally measured sand strength. Grain crushing induced by the uniaxial strain stress path results in a pronounced reduction of porosity and permeability, which manifests more readily for samples with large grain size. The change of particle-size distribution indicates that high effective stresses cause grain crushing and production of a significant amount of fines that lower permeability. Simulation results indicate that the presence of soft grains and inclusions (e.g., shale fragments) facilitates grain crushing. Reservoir simulations—incorporating the change of porosity and permeability as a compaction table—show that the compaction can enhance cumulative production due to compaction drive but also reduces production rate by impairing the reservoir permeability. This multiscale numerical workflow bridges grain-scale compaction behavior and field-scale reservoir production.

Effect of superimposed compressive stresses on rolling contact fatigue initiation at hard and soft inclusions

A semi-analytical framework is introduced for the evaluation of favorable residual stresses for delaying rolling contact fatigue initiation. Subsurface stress histories are applied to a micro-scale model accounting for isolated inclusions of different types and geometries. Micro-scale von Mises stresses are calculated based on Eshelby’s method and considered as an estimator of crack initiation due to plastic deformation. The most critical cases in terms of micro-scale plasticity are identified, and the effect of macro-scale compressive residual stresses is considered. Finally, optimized residual stresses are determined that minimize the maximum attained micro-scale von Mises stress at different depths.

Computational second-order homogenization of materials with effective anisotropic strain-gradient behavior

A computational homogenization method to determine the effective parameters of Mindlin’s Strain Gradient Elasticity (SGE) model from a local heterogeneous Cauchy linear material is developed. The devised method, which is an extension of the classical one based on the use of Quadratic Boundary Conditions, intents to correct the well-known non-physical problem of persistent gradient effects when the Representative Volume Element (RVE) is homogeneous. Those spurious effects are eliminated by introducing a microstructure-dependent body force field in the homogenization scheme together with alternative definitions of the localization tensors. With these modifications, and by a simple application of the superposition principle, the higher-order stiffness tensors of SGE are computed from elementary numerical calculations on RVE. Within this new framework, the convergence of SGE effective properties is investigated with respect to the size of the RVE. Finally, a C1-FEM procedure for simulating the behavior of the effective material at the macro scale is developed. We show that the proposed model is consistent with the solutions arising from asymptotic analysis and that the computed effective tensors verify the expected invariance properties for several classes of anisotropy. We also point out an issue that the present model shares with asymptotic-based solutions in the case of soft inclusions. Applications to anisotropic effective strain-gradient materials are provided, as well as comparisons between fully meshed structures and equivalent homogeneous models.

A fast and efficient numerical prediction of compression after impact (CAI) strength of composite laminates and structures

Abstract In this paper, an equivalent damage model is proposed to quickly predict the compression after impact (CAI) strength of composite laminates and stiffened panels. Low-velocity impact (LVI) and CAI tests at various energy levels are carried out on laminates. Based on the measured impact damage sizes, the model simplifies the damage area into concentric circles with different stiffness and strength properties by adopting the soft inclusion method. Progressive failure analysis and virtual crack closure technique (VCCT) are used to simulate the intra-laminar and inter-laminar failure of impacted laminates under axial compression in commercial finite element software ABAQUS. The predicted failure modes and CAI strength of laminates are in good agreement with the experiment results from this paper and the literature at various impact energies, which proves the validity of the model. Whilst the numerical model achieves high computational efficiency, the effect of mass scaling on computational efficiency and accuracy is evaluated, which provides a reference for the parameter choice in engineering application. The model is further applied to predict the compressive failure load of the stiffened composite panel with skin bay impact damage as a practical application, showing this modelling approach is also suitable to composite structures.

Water Stability of Light Weight Soil Amended with Expanded Polystyrene Beads

To reveal the durability of light weight soil ground under water environment conditions, the influence laws of soaking time and dry–wet cycle times on mechanical properties of light weight soil are researched by density tests, unconfined compressive strength tests, water absorption tests and mass loss rate tests. Due to chemical compositions in the light weight soil interacting with each other, a kind of network cemented structure containing soft inclusions (expanded polystyrene beads) is formed in the specimens. Enough water has been provided for the mixed soil in the physical–chemical reaction process in the experiments, hydrolysis and hydration of cement, ion exchange, hardening reaction and carbonation action occur continuously. Under long term soaking conditions, the physical properties of light weight soil are extremely stable, the network cemented structure is the physical basis for the hydrophobicity. When soaking time is more than 90 days, the unconfined compressive strength of the specimens will reach a stable value gradually. Similarly, light weight soil does not appear cracking phenomenon, and keep the mechanical properties and material density stable under long term dry–wet cycle. Thus light weight soil has good water stability and durability, it is recommended that light weight soil can be widely used in civil engineering, such as ground improvement, soft subgrade engineering, embankment heightening, slope repair, excavation backfill, pipeline backfilling and so on.

Numerical Analysis of an Edge Crack Isotropic Plate with Void/Inclusions under Different Loading by Implementing XFEM

In the present work, the effect of various discontinuities like voids, soft inclusions and hard inclusions of the mixed-mode stress intensity factor (MMSIF), crack growth and energy release rate (ERR) of an edge crack isotropic plate under different loading like tensile, shear, combine and exponential by various numerical examples is investigated. The basic formulation is based on the extended finite element method (XFEM) through the M interaction approach using the level set method. The effect of single and multi voids and inclusions with position variation on MMSIF and crack growth are also investigated. The presented results would be applicable to enhancing the better fracture resistance of cracked structures and various loading conditions.

A Clinical, Radiographic and Histological Study of Unerupted Teeth in Dogs and Cats: 73 Cases (2001-2018).

Lack of dental eruption may be accompanied by development of dentigerous cysts and has also been rarely associated with neoplasia. However, little information is available on prevalence of unerupted teeth and associated lesions in dogs and cats. The main objective of this study was to describe the epidemiologic data of canine and feline dental patients with unerupted teeth, and assess the prevalence of associated dentigerous cysts and tumors. Secondary aims included the evaluation of possible factors implicated in cystic development, and description of the histological features of dentigerous cysts. Medical and dental records, intraoral photographs, intraoral radiographs of client-owned dogs and cats with clinically missing teeth examined between 2001 and March 2018 were reviewed. Collected data included signalment, reason for presentation, number, type, depth of inclusion and angulation of unerupted teeth, presence of cystic lesions or tumors, abnormalities affecting involved teeth, histopathological findings, performed treatment and outcome. Seventy-three animals (69 dogs and 4 cats) with 113 unerupted teeth were included. The most frequent unerupted tooth in dogs was the first premolar teeth (78%), followed by the canine and third molar teeth. Dentigerous cysts were diagnosed associated with 48 (44.4%) teeth in dogs and one out of five unerupted teeth in cats. The affected teeth in dogs were predominantly in horizontal inclination (40%) and in soft tissue inclusion (77%). Brachycephalic canine breeds were overrepresented. The only unerupted tooth in boxer dogs was the first premolar tooth (32 teeth). Ninety percentage of boxers with unerupted teeth developed associated lesions (25 dentigerous cysts and one tumor). Two ameloblastomas (one in a dog and one in a cat) and one osteosarcoma (in a dog) were diagnosed in association with three unerupted teeth. Histology was essential in diagnosing two odontogenic cysts not evident on radiographs. In all cases that were followed-up, treatment (i.e., extraction, extraction and surgical curettage, or operculectomy) appeared successful. Untreated dentigerous cysts showed progression at re-examination. None of the unerupted teeth without evidence of cyst at the time of diagnosis showed incipient cystic development. None of the evaluated factors were associated with lack of eruption and/or development of associated lesions.

Theoretical analyses of nanocrack nucleation near the main crack tip in nano and micro crystalline materials

Nano or micro crack nucleation in the process of metallic fracture is a common phenomenon and it affects fracture behaviors of the materials. The mechanisms of nanocrack nucleation are different for metals with nanoscale and microscale grain sizes. In this paper, according to the deformation mechanisms of materials with different grain sizes, theoretical models are established to investigate nanocrack nucleation near a main crack tip under mode I load. The theoretical solution is presented based on the distributed dislocation technique. For nanocrystalline metals, the process of nanocrack nucleation due to grain boundary sliding is analyzed and the effect of grain boundary structure on nanocrack nucleation is studied. The results show that nanocracks may be preferentially nucleated at the triple junctions which are located on the upper or lower sides of the main crack plane, instead of the front of the crack tip. A larger nanocrack may be nucleated at triple junctions with a larger characteristic abutting angle of the triple junctions. For microcrystalline metals, nanocrack nucleation due to lattice dislocation pileup is considered. The problem of inclusion cracking due to lattice dislocation pileup is analyzed. The results show that lattice dislocation pileup causes the entire soft inclusion to crack easily, but a relatively small nanocrack can be only nucleated in the hard inclusion.

Homogenization Approach and Bloch-Floquet Theory for Band-Gap Prediction in 2D Locally Resonant Metamaterials

This paper provides a detailed comparison of the two-scale homogenization method and of the Bloch-Floquet theory for the determination of band-gaps in locally resonant metamaterials. A medium composed by a stiff matrix with soft inclusions with 2D periodicity is considered and the equivalent mass density of the homogenized medium is explicitly obtained both for in-plane and out-of-plane wave propagation through two-scale asymptotic expansion. The intervals of frequency where the effective mass is negative identify the band-gaps of the material. The Bloch-Floquet problem is then considered and, through an asymptotic analysis, its is shown that it leads to the same prediction of the band-gaps. The results are confirmed by some examples and the limits of the asymptotic approach are explicitly given and numerically verified.

Effective behavior of composites with combined kinematic and isotropic hardening based on additive tangent Mori–Tanaka scheme

The goal of the present work is to propose a multi-scale approach for composite materials which accounts for kinematic hardening in the phases. For that purpose, the additive/sequential interaction rule and tangent linearization of viscoplastic response proposed for elastic–viscoplastic material can be extended in a straightforward manner. A two phase composite where each phase is elastic–viscoplastic is considered. The viscoplastic flow is governed by a J2 flow theory with an overstress. To find the overall behavior of the composite, a Mori–Tanaka model is applied. Numerical validation of the proposition is carried out by considering a representative volume element with 30 inclusions. Various configurations have been tested: hard or soft inclusion cases with or without isotropic hardening. It is shown that the quality of the model predictions is not affected by the introduction of the kinematic hardening component in the local constitutive behavior. Namely, in most cases considered in the paper the overall stress–strain response as well as the average stress–strain response per phase is accurately estimated. It has been also verified that the obtained backstress components are consistent with the ones predicted by Finite element calculations with ABAQUS Software.