Experimental-numerical Approach Research Articles

Toward a New Generation of Two-Fluid Flow Models Based on the Thermodynamically-Constrained Averaging Theory

Traditional models of two-fluid flow through porous media at the macroscale have existed for nearly a century. These phenomenological models are not firmly connected to the microscale; thermodynamic constraints are not enforced; empirical closure relations are well known to be hysteretic; fluid pressures are typically assumed to be in a local equilibrium state with fluid saturations; and important quantities such as interfacial and curvilinear geometric extents, tensions, and curvatures, known to be important from microscale studies, do not explicitly appear in traditional macroscale models. Despite these shortcomings, the traditional model for two-fluid flow in porous media has been extensively studied to develop efficient numerical approximation methods, experimental and surrogate measure parameterization approaches, and convenient pre- and post-processing environments; and they have been applied in a large number of applications from a variety of fields. The thermodynamically constrained averaging theory (TCAT) was developed to overcome the limitations associated with traditional approaches, and we consider here issues associated with the closure of this new generation of models. It has been shown that a hysteretic-free state equation exists based upon integral geometry that relates changes in volume fractions, capillary pressure, interfacial areas, and the Euler characteristic. We show an analysis of how this state equation can be parameterized with a relatively small amount of data. We also formulate a state equation for resistance coefficients that we show to be hysteretic free, unlike traditional relative permeability models. Lastly, we comment on the open issues remaining for this new generation of models.

Crack closure mechanisms in residual stress fields generated by laser shock peening: A combined experimental-numerical approach

Laser shock peening (LSP) is successfully applied to retard fatigue cracks in metallic lightweight structures by introducing specific, in particular compressive, residual stress fields. In this work, experiments and a multi-step simulation strategy are used to explain the fatigue crack retarding and accelerating mechanisms within these LSP-induced residual stress fields. Crack face contact is identified as main mechanism to retard the fatigue crack as the stress distribution changes and the stress intensity factor range decreases. Crack face contact is experimentally detected by load vs. crack opening displacement (COD) curves and scanning electron microscopy (SEM) of the crack faces, as well as during numerical simulations. The convincing agreement between experiment and simulation, especially regarding the specific crack face contact areas, allowed the proper evaluation of the stress intensity factors depending on the crack length. It is found that crack closure is indeed one of the main reasons for the efficient application of LSP for fatigue crack retardation. Furthermore, the occurrence of crack closure does not indicate a zero value stress intensity factor in complex residual stress fields, as the areas of crack face contact depend strongly on the LSP-induced compressive residual stresses.

Identifying the mechanisms of concrete drying: An experimental-numerical approach

The loss of non-chemically linked water from concrete has a significant effect on its structural behaviour through a modification of its strength, Young modulus, fracture properties, creep and volume stability. The knowledge of the hydric state of concrete is therefore of upmost importance regarding the mechanical behaviour and durability potential of concrete structures. We use a non-linear diffusion equation for predicting the drying kinetics, including processes of the diffusion of water vapour and the permeation of liquid water. The multiple parameters of this model are then identified based on an in-house optimization algorithm and on a thorough experimental setup including measurements of mass loss and located relative humidity evolutions on various sample geometries. This coupled experimental and numerical approach allows identifying robust and meaningful material parameters of concrete used for drying predictions. This approach allows also identifying the mechanisms in concrete drying phenomena and processes by inverse analysis.

Thermo-mechanically coupled modelling of a single-asperity scratch on an isotropic isotactic polypropylene surface

Abstract Isotactic polypropylene (iPP) is a low cost semi-crystalline polymer that is easy to process, has a wide variety in properties and is, therefore, used in many applications. Many of these applications require enhanced wear-resistance to prolong the lifetime of the product. Essential is to first investigate the intrinsic response of the material in order to describe its friction and wear response. In this respect, a hybrid experimental-numerical approach is used to couple the intrinsic response to the single-asperity scratch response. The numerical model used is a 3D elasto-viscoplastic model based on the Eindhoven Glassy Polymer (EGP) model. For the first time a coupled thermo-mechanical EGP model is implemented in a Finite Element Method (FEM)-framework. The model is capable of accurately describing the intrinsic response of the material, which opens the door to qualitatively and quantitatively describe its frictional response and understand the damage formation mechanism (i.e. the initiation of wear). In this study, α- and β-phase iPP are studied. We show that the difference in the intrinsic response between the two phases has a significant influence on the friction and wear response. Moreover, a stick-slip phenomenon is proven to be the main responsible for the damage mechanism observed. The observed periodic “fish-scale” damage pattern results from periodic changes in resistance during the tip movement. A relation between the polymer intrinsic response and the damage formation mechanism is established. The influence of the applied load and scratch speed on damage formation is investigated as well.

Enrichment of ovine gonadotropes via adenovirus gene targeting enhances assessment of transcriptional changes in response to estradiol-17 beta†.

Gonadotropes represent approximately 5-15% of the total endocrine cell population in the mammalian anterior pituitary. Therefore, assessing the effects of experimental manipulation on virtually any parameter of gonadotrope biology is difficult to detect and parse from background noise. In non-rodent species, applying techniques such as high-throughput ribonucleic acid (RNA) sequencing is problematic due to difficulty in isolating and analyzing individual endocrine cell populations. Herein, we exploited cell-specific properties inherent to the proximal promoter of the human glycoprotein hormone alpha subunit gene (CGA) to genetically target the expression of a fluorescent reporter (green fluorescent protein [GFP]) selectively to ovine gonadotropes. Dissociated ovine pituitary cells were cultured and infected with an adenoviral reporter vector (Ad-hαCGA-eGFP). We established efficient gene targeting by successfully enriching dispersed GFP-positive cells with flow cytometry. Confirming enrichment of gonadotropes specifically, we detected elevated levels of luteinizing hormone (LH) but not thyrotropin-stimulating hormone (TSH) in GFP-positive cell populations compared to GFP-negative populations. Subsequently, we used next-generation sequencing to obtain the transcriptional profile of GFP-positive ovine gonadotropes in the presence or absence of estradiol 17-beta (E2), a key modulator of gonadotrope function. Compared to non-sorted cells, enriched GFP-positive cells revealed a distinct transcriptional profile consistent with established patterns of gonadotrope gene expression. Importantly, we also detected nearly 200 E2-responsive genes in enriched gonadotropes, which were not apparent in parallel experiments on non-enriched cell populations. From these data, we conclude that CGA-targeted adenoviral gene transfer is an effective means for selectively labeling and enriching ovine gonadotropes suitable for investigation by numerous experimental approaches.

The evolution of fracture process zones in as-received and oxidized air plasma sprayed TBCs

The evolution of the fracture process zones as precursor to fracture was elucidated in as-received and oxidized air plasma sprayed (APS) Thermal Barrier Coating (TBC) specimens. Multiple techniques of Digital Image Correlation (DIC), Acoustic Emission (AE), Finite Element Analysis (FEA) and Scanning Electron Microscope (SEM) micrographs were combined together to obtain in-depth understanding of the failure mechanisms involved in a TBC component under uniform bending stress field. Both sides of specimens were monitored simultaneously during loading and it is shown that within a uniform applied strain, the localized zones of non-symmetric strains with the different lengths were initiated and growth within the ceramic top coat perpendicular to the principle stresses. The process zones were initiated in both as-received and oxidized specimens upon yielding the substrate. However, different strain localization areas were observed on two sides of each specimen. It was observed that separations in each localized zone were resisted by cohesive tractions controlled by the material microstructural features and not the applied external load. One of these localized strains was then rotated parallel to the interface propagating until the final delamination. In the oxidized specimen, the delamination was faster and spallation of top coat ceramic layer followed immediately after cracks reached the bond coat interface. The cohesive properties were determined using a hybrid experimental-numerical approach.

Innovative integrated numerical-experimental method for high-performance multispectral Mueller polarimeters based on ferroelectric liquid crystals.

In this work, an original and effective integrated numerical-experimental approach is proposed for building a high-performance multispectral Mueller polarimeter based on ferroelectric liquid crystals (FLCs). This method relies on accurate experimental characterization of the optical components specifically selected to construct such a system, combined with a numerical procedure used to optimize it, in the spectral range of interest, by means of a global optimization function. The proposed strategy enabled the construction of an FLC-based Mueller polarimeter in transmission configuration operating between 450 and 700 nm. The robustness of this system to various optical component misalignments, as well as the conditions to keep the measurement error less than 1% over the whole spectral range of interest, have been determined experimentally. The proposed strategy is very well suited to build optimized multispectral Mueller polarimetric systems for biomedical applications for which variations of the order of a few percent in the elements of the measured Mueller matrices need to be appreciated.

Abrasive flow machining of laser powder bed-fused parts: Numerical modeling and experimental validation

Abrasive flow machining (AFM) is considered one of the best-suited candidates for the finishing of complex components made by additive manufacturing. However, the AFM process leads to mass loss and consequent alteration of the part geometry and dimensions. A numerical tool is needed to reduce the number of time-consuming and labor-intensive experimental trials and allow the efficient prediction of machining allowances for the final geometry and surface finish of AFM-processed parts. This paper presents a combined numerical-experimental approach to predict the results of such a polishing process. The model devised simulates the indentation of abrasive grains to a workpiece and takes into account the initial surface roughness of the part as well as the velocity and pressure of a polishing medium abrading the surface, the latter obtained via computational fluid dynamics (CFD) simulations. To validate the model, an experimental study has been carried out on laser powder bed-fused Ti-6Al-4 V coupons containing multiple planar faces with different surface roughnesses resulting from their variable orientations during manufacturing. With respect to material removal, simulations and experiments provided very close results, and the relatively minor discrepancies observed may be reduced by refining the calibration protocol and adjusting the boundary conditions of the model.

Study on the effect of post curing on the mode II fracture energy of structural adhesive using a parameter identification approach

In this work, a parameter identification approach was developed based on a combined experimental-numerical approach to determine the optimal set of adhesive parameters in adhesively bonded specimen subjected to various post curing treatments. End-Notched Flexure (ENF) testing was conducted to characterise the Mode II fracture property under both recommended and post curing conditions, providing benchmark data for the numerical analysis stage. Experimental results revealed that Mode II fracture energy was effectively affected by the post curing history, with higher temperature and longer curing duration leading to enhanced fracture resistance. The numerically identified Cohesive Zone Model (CZM) parameters using multi-island genetic algorithm provided good correlation in Mode II fracture energies between Finite element (FE) modelling and experimentally measured values, thus extensive experimental characterisation work to determine the adhesive parameters can be effectively eliminated.

A Novel Damage Model to Predict Ductile Fracture Behavior for Anisotropic Sheet Metal

The purpose of the present work is to investigate the fracture behavior of anisotropic sheet metal under various stress states. Notched tension and flat-grooved tension tests at 0°, 45°, and 90° directions with respect to rolling direction were carried out by a hybrid experimental–numerical approach, and then a novel damage model was proposed by coupling Hill48’s criterion. Based on this, finite element method (FEM) analysis models were established. The force–displacement responses of experiments and simulations are in good agreement, which verify the FEM models. The predictability of the damage model established for the fracture behavior of anisotropic materials was studied by comparing the fracture displacements between experiments and simulations. It is found that the predictability of novel damage model is basically consistent with predictive results. The difference of damage locations and local strain evolutions at a 45° direction is greater than the other directions. In addition, stress triaxiality does not play a predominant role in the fracture process for notched tension specimens, while it does play a predominant role for flat-grooved tension specimens.

A combined experimental and numerical approach for radiation heat transfer in semi-crystalline thermoplastics

This paper proposes a novel experimental-numerical approach for radiation heat transfer in semi-crystalline thermoplastics. The proposed experimental and numerical approaches were analyzed here for the case of IR heating of linear polyethylene (PE). The challenges for radiation transport in semi-crystalline polymers may be categorized into the two main aspects: optical heterogeneity in polymer medium due to semi-crystalline nature and, semi-transparency in certain type of such polymers, like PE. In this study, we address the temperature dependence in the thermo-optical properties of semi-crystalline thermoplastics concerning its effect as a thermal radiation parameter for radiation heat transfer modeling of such type of polymers. The temperature dependence in the optical properties of PE, namely transmittance and reflectance characteristics, were experimentally analyzed under heating condition and, the temperature-dependent thermo-optical properties of PE was determined. The experimental analyses showed that the change in the thermo-optical properties under heating is related to change in the optical scattering behavior in PE medium since it is strongly affected by its crystalline structure. The radiation heat transfer model was built based on optically homogeneous medium assumption at which the change in the amount of radiative energy absorbed by the PE medium under heating was introduced without modeling how the light scatters inside of the medium. Thus, the effect of optical scattering on the absorption characteristics of PE under varying temperature was taken into account without modeling the spatial distribution of the scattered light intensity at microscopic level, which offers computationally cost-effective numerical solutions. The accuracy of the numerical model was analyzed performing IR heating experiments where IR thermography and thermocouples were employed for the surface temperature measurements. Due to the semi-transparent nature of PE, an experimental method was developed for IR thermography of PE and its accuracy was analyzed. The experimental-numerical comparisons showed that the temperature field on PE can be closely predicted using its temperature-dependent thermo-optical properties. Thanks to the adopted comparative case study, it was demonstrated how the temperature field predictions may deviate from the experimental thermal measurements due to the ignorance of this temperature dependence. Considering any type of monochromatic or polychromatic radiation source, the combined experimental-numerical approach proposed here may be adopted for temperature field predictions and non-invasive surface temperature measurements on the radiatively heated semi-crystalline thermoplastics.-

A numerical-experimental approach towards picomechanics and picotribology: the case study of defective carbon nanotubes bundles

We present a simulation study on the peeling of carbon nanotubes bundles interacting with a flat substrate, represented by the back surface of an atomic force microscopy cantilever. A defected sample, acquired in situ using a scanning electron microscope, was investigated under different peeling configurations by finite element method simulations. The coupled computational-experimental analysis let to identify the position and the entity of a structural defect by means of reverse image correlation problem. By exploiting this defective fiber it was also possible to quantify, as indirect measure, the friction and adhesion forces between the bundle and the substrate of few pN magnitude, otherwise difficult to measure with the resolution of currently available instruments. The proposed approach can be useful to study the tribology-induced mechanical behavior of one-dimensional nanostructures as well as for real-time identification and monitoring of nanodefects for industrial applications, such as nanoelectronics.

Evaluation of the lateral displacements of a sandy slope reinforced by a row of floating piles: A numerical-experimental approach

To evaluate the multidirectional seismic behaviour of reinforced sandy slopes and to assess the effects of seismic loading directions, a pile-stabilized slope is subjected to one, two and three-dimensional seismic loading combinations. Small-scale physical models of a biaxial shaking table and three-dimensional numerical models were implemented using the 3D explicit finite difference method. Seismic loading was imposed on the numerical models in two frameworks: (1) high frequency sinusoidal harmonic excitations to simulate the physical models and (2) earthquake loading. Both near-fault and far-field earthquake records were used in the numerical modeling. The results show that the permanent lateral displacement of the slope has the highest value among the combination of the longitudinal, transverse, and vertical components (i.e., LTV) of earthquake records. Findings have also revealed a simple logarithmic relationship between some intensity measures (IMs) and the lateral displacement of a reinforced sandy slope.

Correlation of Macroscopic Fracture Behavior with Microscopic Fracture Mechanism for AHSS Sheet.

This research aims to correlate the macroscopic fracture phenomenon with its microscopic fracture mechanism for an advanced high-strength steel (AHSS) TRIP 780 sheet by applying a combined experimental-numerical approach. Six specimens with different shapes were tensioned to fracture and the main deformation areas of specimens were subjected to stress states ranging from lower to higher stress triaxiality. The final fracture surface feature for each specimen was obtained to characterize the macroscopic fracture modes at different stress states. The scanning electron microscope (SEM) fractographies of fracture surfaces were detected to reveal the microscopic fracture mechanisms. The stress triaxiality evolution was applied to correlate of fracture mode and fracture mechanism by comparing the macroscopic fracture features as well as micro-defect changes. An increase of stress triaxiality leads to voids extension and then results in a voids-dominant fracture. The micro-shear-slip tends to appear in the stress triaxiality level lower than that of pure shear stress state. The fracture behavior of a practice deformation process was the result of interplay between shear-slip fracture and void-dominant fracture. The unified relationship between average void sizes and stress triaxiality was obtained. The void growth was predicted by the Rice–Tracey model with higher precision.

Effect of delamination on the fatigue life of GFRP: A thermographic and numerical study

Delamination is the major failure mechanism in composite laminates and eventually leads to material failure. An early-detection and a better understanding of this phenomenon, through non-destructive assessment, can provide a proper in situ repair and allow a better evaluation of its effects on residual strength of lightweight structural components. Here we adopt a joint numerical-experimental approach to study the effect of delamination on the fatigue life of glass/epoxy composites. To identify and monitor the evolution of the delamination during loading, we carried out stepwise cyclic tests coupled with IR-thermography on both undamaged and artificially-damaged samples. The outcome of the tests shows that IR-thermography is able to identify a threshold stress, named damage stress σD, which is correlated to the damage initiation and the fatigue performance of the composite. Additionally, we performed FE-simulations, implementing the delamination by cohesive elements. Such models, calibrated on the basis of the experimental fatigue results, can provide a tool to assess the effect of parameters, such as the delamination size and location and composite stacking sequence, on the residual strength and fatigue life of the composite material.

A hybrid experimental-numerical approach for load rating of reinforced concrete bridges with insufficient structural properties

This article presents a non-destructive approach for load rating of reinforced concrete bridges without structural plans. The approach is found on a hybrid method, which employs vibration and live load testing coupled with numerical simulation and model updating techniques, to converge on estimate of unknown structural parameters. The material properties of bridge and the amount of reinforcing steel for calculating the bending capacity of a bridge are determined through model updating results and nondestructive approaches. The updated model is then employed to determine load effects for calculating load rating factors, and these results are combined with live load test result to arrive at the bridge capacity, and ultimately the load rating. The method is validated by testing a skewed reinforced concrete slab bridge for load rating purposes. The bridge was instrumented with accelerometers and strain gages, and the responses of the bridge under vibration and quasi-static tests were measured. Results demonstrated that the proposed method is capable of determining the bridge capacity and load rating factor with good accuracy, and not only can be used for load rating of concrete bridges without structural information, but also can be useful in condition assessment of existing concrete bridge with available as-built information.

Modeling of 3D moisture distribution in heated concrete: From continuum towards mesoscopic approach

The exposure of concrete to fire, which often leads to spalling, is a phenomenon that is not yet fully understood. Given the complexity of the thermo-hydro-mechanical phenomena and their strong interaction, a combined numerical-experimental approach is indispensable for understanding the behaviour of concrete at high temperature with respect to spalling. This paper deals with the numerical modeling of moisture distribution in heated concrete, a parameter often linked with spalling, by making use of original experimental data from in-situ neutron tomography. A homogenized continuum model has been employed for investigating various aspects, such as: speed of the drying front, moisture accumulation behind this front, experimental boundary effects etc. In addition, a fully coupled 3D thermo-hydo-mechanical (THM) model on the mesoscale is presented and used for simulating an experiment with a single aggregate for the purpose of investigating the influence of the aggregate on the drying front.

Machine-learning based temperature- and rate-dependent plasticity model: Application to analysis of fracture experiments on DP steel

Slow, intermediate and high strain rate experiments are carried out on flat smooth and notched tensile specimens extracted from dual phase steel sheets. A split Hopkinson pressure bar testing system with load inversion device is utilized to achieve high strain rates. Temperature dependent experiments ranging from 20 °C to 300 °C are performed at quasi-static strain rates. The experimental results reveal a non-monotonic effect of the temperature on the stress-strain curve for DP800 steel. For the temperatures considered, the lowest and highest curves are obtained for 120 °C and 300 °C, respectively. A modified Johnson-Cook plasticity model is developed to capture the observed unconventional effect of the strain rate- and temperature on the hardening response. It includes a von Mises yield surface, a non-associated Hill’48 flow rule, a Swift-Voce reference stress-strain curve describing the rate-independent behavior at room temperature, and a neural network function depending on the plastic strain, strain rate and temperature. This model is implemented into a material user subroutine and identified using a combination of analytical formulas along with a resilient back propagation algorithm. It is found that the obtained machine-learning based Johnson-Cook plasticity model can describe all experimental data with high accuracy, including both force-displacement and local surface strain measurements. Using a hybrid experimental-numerical approach, the loading paths to fracture are determined. It is found that the temperature not only affects the plasticity of the DP800 steel in a non-monotonic manner, but also its strain to fracture, with ductility loss of up to 30% when increasing the temperature from 20 to 120 °C, followed by a gain in ductility when increasing the temperature further to 300 °C.

Enhanced Yeast One-hybrid Screens To Identify Transcription Factor Binding To Human DNA Sequences.

Identifying the sets of transcription factors (TFs) that regulate each human gene is a daunting task that requires integrating numerous experimental and computational approaches. One such method is the yeast one-hybrid (Y1H) assay, in which interactions between TFs and DNA regions are tested in the milieu of the yeast nucleus using reporter genes. Y1H assays involve two components: a 'DNA-bait' (e.g., promoters, enhancers, silencers, etc.) and a 'TF-prey,' which can be screened for reporter gene activation. Most published protocols for performing Y1H screens are based on transforming TF-prey libraries or arrays into DNA-bait yeast strains. Here, we describe a pipeline, called enhanced Y1H (eY1H) assays, where TF-DNA interactions are interrogated by mating DNA-bait strains with an arrayed collection of TF-prey strains using a high density array (HDA) robotic platform that allows screening in a 1,536 colony format. This allows for a dramatic increase in throughput (60 DNA-bait sequences against >1,000 TFs takes two weeks per researcher) and reproducibility. We illustrate the different types of expected results by testing human promoter sequences against an array of 1,086 human TFs, as well as examples of issues that can arise during screens and how to troubleshoot them.

A stress triaxiality-dependent viscoplastic damage model to analyze ductile fracture under axisymmetric tensile loading

The aim of this paper is to gain insight for the transition of fracture initiation sites in notched tensile specimens. For this purpose, a stress triaxiality-dependent viscoplastic damage model was proposed for various types of notched tensile specimens, where the influence of stress triaxiality on damage, damage-free stress-strain curve and viscoplasticity was considered. Also, in this constitutive model, the consistency viscoplastic regularization method was used to solve the pathological mesh-dependent problem due to strain softening. A hybrid experimental-numerical approach was used to calibrate model parameters by using smooth and one type of round-notched tensile specimens. The same set of model parameters was then applied to predict ductile fracture for three other types of round-notched tensile specimens. Good agreement between experimental data and simulation results confirmed the transferability of model parameters to all types of tensile specimens considered and the ability of the proposed constitutive model to predict ductile fracture over a large range of positive stress triaxialities. This study found that at the onset of fracture critical damage parameters for all five types of tensile specimens were relatively constant, and fracture initiation sites in these specimens were associated with critical damage parameters, shifting from the center to the notch root. Therefore, the critical damage parameter was defined as a ductile fracture criterion and the fracture strain for all the five types of tensile specimens was successfully predicted.