Experimental-numerical Approach Research Articles

On the relation between shape imperfections of a specimen and necking growth rate under dynamic conditions

In this work, the growth rate of necks formed in dynamically loaded tensile steel samples is investigated. For that purpose, a combined experimental-numerical approach, in which the experimental results are systematically compared with finite element calculations, has been developed. The specimens have a machined sinusoidal geometrical imperfection that covers the whole gauge, introducing a characteristic wavelength in the samples. For a given cross-section diameter, specimens with 6 different gauge lengths (i.e. 6 different specimen wavelengths) were tested. Using a high-speed camera, we measured the time evolution of the radial contraction of the central section of the samples (central section of the neck), thus obtaining the growth rate of the necks. The experiments show that the speed of growth of the necks increases non-linearly with the specimen wavelength (concave-downward shape) until saturation is reached for the longest tested specimens. Numerical simulations performed for the strain rates attained in the experiments (from 900 s−1 to 2100 s−1) confirm this trend and demonstrate that the damping of short specimen wavelengths is caused by stress multiaxiality effects. Numerical simulations performed for strain rates greater than those attained in the experiments (above 7500 s−1) show that long specimen wavelengths become damped by inertia effects at sufficiently high strain rates. For strain rates greater than 7500 s−1, the maximum growth rate of the neck corresponds to an intermediate specimen wavelength defined by the joint action of stress multiaxiality and inertia on damping short and long wavelengths, respectively. Altogether, our experimental and numerical results suggest the existence of a specimen wavelength that, when inertia effects become important, determines the maximum growth rate of dynamic necks, in agreement with the predictions of the dynamic stability analyses developed by Molinari and co-workers (Fressengeas and Molinari, 1985, 1994; Mercier and Molinari, 2003, 2004).

Characterization of temperature- and rate-dependent fracture properties of fine aggregate bituminous mixtures using an integrated numerical-experimental approach

This paper employs an integrated numerical-experimental approach to evaluate the temperature- and rate-dependent fracture characteristics of four fine aggregate bituminous matrices. Two bending tests (semicircular bending, and single-edge notched beam) and one tension test (disk-shaped compact tension) were performed in the laboratory at three temperatures (−10°C, 10°C, and 25°C) and three loading rates (0.5mm/min, 1.0mm/min, and 2.0mm/min). The fracture tests were further simulated using a computational model based on the finite element method that is incorporated with material viscoelasticity and cohesive zone fracture. Two cohesive zone fracture parameters, i.e., cohesive strength and fracture energy, were determined via a calibration process between experimental and numerical results. The results obtained indicated that temperature- and rate-dependent fracture characteristics are obvious in viscoelastic bituminous mixtures, and an accurate identification of such characteristics is a key step towards the implementation of successful computational microstructure predictive models. This would lead to core insights into the effects of constituents on the overall mixture performance, with significant savings in experimental costs and time.

Wear-caused deflection evolution of a slide rail, considering linear and non-linear wear models

The research presented in this paper details an experimental-numerical approach for the quantitative correlation between wear and end-point deflection in a slide rail. Focusing the attention on slide rail utilized in white-goods applications, the aim is to evaluate the number of cycles the slide rail can operate, under different load conditions, before it should be replaced due to unacceptable end-point deflection. In this paper, two formulations are utilized to describe the wear: Archard model for the linear wear and Lemaitre damage model for the nonlinear wear. The linear wear gradually reduces the surface of the slide rail whereas the nonlinear one accounts for the surface element deletion (i.e. due to pitting). To determine the constants to use in the wear models, simple tension test and sliding wear test, by utilizing a designed and developed experiment machine, have been carried out. A full slide rail model simulation has been implemented in ABAQUS including both linear and non-linear wear models and the results have been compared with those of the real rails under different load condition, provided by the rail manufacturer. The comparison between numerically estimated and real rail results proved the reliability of the developed numerical model, limiting the error in a ±10% range. The proposed approach allows predicting the displacement vs cycle curves, parametrized for different loads and, based on a chosen failure criterion, to predict the lifetime of the rail.

A Combined Numerical–Experimental Approach to Quantify the Thermal Contraction of A356 During Solidification

A process for generating thermal contraction coefficients for use in the solidification modeling of aluminum castings is presented. Sequentially coupled thermal-stress modeling is used in conjunction with experimentation to empirically generate the thermal contraction coefficients for a strontium-modified A356 alloy. The impact of cooling curve analysis on the modeling procedure is studied. Model results are in good agreement with experimental findings, indicating a sound methodology for quantifying the thermal contraction. The technique can be applied to other commercially relevant aluminum alloys, increasing the utility of solidification modeling in the casting industry.

Coupled thermo-physical behaviour of an inorganic intumescent system in cone calorimeter testing

This article examines the thermo-physical behaviour of an inorganic-based intumescent coating, tested with bench-scale cone calorimetry, in order to promote the understanding of its intumescence and to contribute to the optimisation of its thermal insulation performance. In the test, the specimen underwent the following phenomena simultaneously: (1) thermo-kinetic endothermic water vaporisation; (2) formation of micro-scale pores in its internal volume; (3) expansion of its volume; (4) variations in thermal boundaries. These simultaneous phenomena cause several changes in internal–external conditions given to the test sample: (1) loss of mass (water molecules); (2) reduction of effective thermal conductivity owing to its porous structure; (3) increase in length of the conductive heat transfer path across its expanding volume; (4) irradiance intensification and additional heat transfer generation on its moving boundaries, exposed to the heat source and surroundings. This interacting thermo-physical behaviour impedes the heat transfer to the underlying substrate. It is therefore comprehensively explained by finite element analysis, associated with the experimental data obtained from a thermogravimetric analyser, differential scanning calorimetry, electric furnace and cone calorimeter tests. The numerical predictions agreed with the physical measurements with consistent accuracy, in terms of both histories of substrate temperature and coating-thickness expansion. This combined numerical–experimental approach enables clear interpretation on the process of intumescence, the impediment mechanism of heat transfer and the critical factors of the material’s behaviour.

A combined numerical/experimental prediction method for urban railway vibration

Railway-induced ground vibrations can cause negative effects to people/structures located in urban areas. One of the main sources of these vibrations is from the large vehicle forces generated when train wheels impact local defects (e.g. switches/crossings). The sole use of traditional in-field transfer-mobility approaches is well suited for plain-line assessments, however is more challenging when discontinuities are present, due to the generation of large magnitude impact forces. This paper presents a hybrid experimental-numerical approach that can predict ground-borne vibration levels in the presence of a variety of railroad artefacts such as transition zones, switches, crossings and rail joints on existing networks. Firstly, the experimental procedure is described, which consists of multiple single source transfer mobilities to determine the transmission characteristics between rail and nearby structures. This is then coupled with a combined multibody vehicle and track numerical model, which is capable of simulating vibration generation in the presence of railway discontinuities. The resulting model is advantageous over alternative approaches because it can account for complex railway discontinuities, while at the same time incorporating the large uncertainties associated with different soil configurations. It is used to analyse a case study, where it is shown that vibration levels are strongly dependent on vehicle speed, defect type and defect size.

Thermal evolution of residual stress in IN718 alloy subjected to laser peening

The thermal relaxation behaviors of residual stresses induced by laser peening (LP) in IN718 alloy were investigated using an integrated numerical simulation and experimental approach. LP and heat treatments (HT) were carried out after which the X-ray diffraction (XRD) technique was employed in measuring the residual stresses. Micro-structures were observed using an optic microscope (OM) and transmission electron microscope (TEM). Dislocations induced by LP were also observed by TEM and characterized using the XRD technique. The effects of the applied temperature and the exposure time on residual stress and micro-structures were investigated. The results show that the extent of the residual stresses relaxation increased accordingly with the increase in the applied temperature. The relaxation rate was initially high and tended to stabilize for a longer exposure time. Grain size evolution during the process was subsequently discussed. Furthermore, a conceivable mechanism of residual stresses thermal relaxation behavior was obtained.

New methodology for mechanical characterization of human superficial facial tissue anisotropic behaviour in vivo

Mechanical characterization of human superficial facial tissue has important applications in biomedical science, computer assisted forensics, graphics, and consumer goods development. Specifically, the latter may include facial hair removal devices. Predictive accuracy of numerical models and their ability to elucidate biomechanically relevant questions depends on the acquisition of experimental data and mechanical tissue behavior representation. Anisotropic viscoelastic behavioral characterization of human facial tissue, deformed in vivo with finite strain, however, is sparse.Employing an experimental-numerical approach, a procedure is presented to evaluate multidirectional tensile properties of superficial tissue layers of the face in vivo. Specifically, in addition to stress relaxation, displacement-controlled multi-step ramp-and-hold protocols were performed to separate elastic from inelastic properties. For numerical representation, an anisotropic hyperelastic material model in conjunction with a time domain linear viscoelasticity formulation with Prony series was employed. Model parameters were inversely derived, employing finite element models, using multi-criteria optimization.The methodology provides insight into mechanical superficial facial tissue properties. Experimental data shows pronounced anisotropy, especially with large strain. The stress relaxation rate does not depend on the loading direction, but is strain-dependent. Preconditioning eliminates equilibrium hysteresis effects and leads to stress-strain repeatability. In the preconditioned state tissue stiffness and hysteresis insensitivity to strain rate in the applied range is evident.The employed material model fits the nonlinear anisotropic elastic results and the viscoelasticity model reasonably reproduces time-dependent results. Inversely deduced maximum anisotropic long-term shear modulus of linear elasticity is G∞,maxaniso=2.43kPa and instantaneous initial shear modulus at an applied rate of ramp loading is G0,maxaniso=15.38kPa.Derived mechanical model parameters constitute a basis for complex skin interaction simulation.

Modeling of ductile fracture from shear to balanced biaxial tension for sheet metals

A ductile fracture model is proposed to describe shear fracture of sheet metals from shear to balanced biaxial tension via uniaxial and plane strain tension. The fracture criterion models plastic damage as strain-induced void nucleation, triaxiality-governed void enlargement, Lode-controlled void torsion, and shear-restrained coalescence of voids. Its flexibility is investigated by a parameter study of the ductile fracture model proposed. The fracture model is employed to describe ductile fracture behavior of an aluminum alloy AA6082 T6 (thickness: 1.0mm). Dogbone specimens are strained to characterize the strain hardening properties, while another four different specimens are tested to characterize fracture behavior in shear, uniaxial tension, plane strain tension and balanced biaxial tension. The loading processes are analyzed numerically with the stress invariant-based Drucker yield function which is for the first time specified for body-centered cubic and face-centered cubic metals. Fracture strains in various loading conditions are measured with a hybrid experimental-numerical approach. The measured fracture strains are then used to calibrate the ductile fracture model proposed. The ductile fracture model calibrated above is employed to predict the onset of ductile fracture for these four specimens. For the purpose of comparison, the predicted fracture strokes of these four loading conditions are compared with those predicted by the modified Mohr–Coulomb model (Bai and Wierzbicki, 2008), and two micromechanism-inspired criteria proposed recently (Lou et al., 2012, 2014). The comparison reveals that the proposed model predicts the fracture behavior in much better agreement compared with experimental results from shear to the balanced biaxial tension. Accordingly, the proposed ductile fracture criterion is recommended for the prediction of ductile fracture in sheet metal forming processes, optimization of forming parameters and design of tools for both solid elements and shell elements. Besides, the ductile fracture model proposed can also be applied in various bulk metal forming processes in case that the model is calibrated by proper sets of experiments.

GABAA Receptors and the Diversity in their Structure and Pharmacology.

GABAA receptors (GABAARs) are a class of ligand-gated ion channels with high physiological and therapeutic significance. In the brain, these pentameric receptors occur with diverse subunit composition, which confers highly complex pharmacology to this receptor class. An impressive range of clinically used therapeutics are known to bind to distinct sites found on GABAARs to modulate receptor function. Numerous experimental approaches have been used over the years to elucidate the binding sites of these drugs, but unequivocal identification is challenging due to subtype- and ligand-dependent pharmacology. Here, we review the current structural and pharmacological understanding of GABAARs, besides highlighting recent evidence which has revealed greater complexity than previously anticipated.

Study on Design of Pressure Chamber in a Linear-Jet Type Air Curtain System for Prevention of Smoke Spread

Death toll by smoke in fire is estimated at 70% which emphasizes the importance of smoke control system to deal with the fire smoke. In advanced countries, the studies on method to prevent smoke spread by forming the air curtain using high velocity jet flow are underway now. In this study, a linear-jet type air curtain system is proposed to prevent the smoke spread and analysis of flow characteristics of pressure chamber, which is the core component, is conducted through numerical analysis and experimental approach. Consequently, the pressure was increased in 2D functional way to input air flowrate and about 595 Pa pressure was formed at pressure chamber inlet in response to 30 m/s nozzle jet velocity.

Non-equilibrium Studies in Switching Arc Plasmas in Japan

This paper briefly introduce research work examples of non-equilibrium studies in switching arcs. In understanding arc behavior, one often assumes local thermodynamic equilibrium (LTE) condition in the arc plasma. However, actual arc plasmas are not completely and not always in LTE state because of strong temperature change temporally and spatially, and high electric field application etc. Recently, we have a collaboration work in numerical simulations and experimental approaches for decaying arcs without LTE assumption. First, our numerical model is presented for decaying arcs without chemical equilibrium assumption. Secondly, two experimental methods are introduced for measuring electron density in decaying arcs without LTE assumption: Laser Thomson Scattering method and the Schack-Hartmann method. Finally, comparison results is shown between the LTE simulation, the chemically non-equilibrium simulation, and the above experimental measurements.

Highly efficient, transparent and stable semitransparent colloidal quantum dot solar cells: a combined numerical modeling and experimental approach

A semitransparent colloidal quantum dot solar cell with high efficiency, transparency and stability is reported by coupling numerical modeling and experimental approaches.

Wind induced vibrations of a high tapered obelisk: wind tunnel tests, numerical analysis and design of countermeasures

The structure studied in this paper is a 85 meters high tapered obelisk, made of reinforced concrete for the first 9 meters and of steel welded plates for the upper 76 meters. Its bluff cross section consists of two closely spaced triangular sections that are scaled linearly with the height from ground. Having a complex bluff section and being a light structure, aerodynamic instabilities and vortex induced vibrations were investigated using a mixed experimental-numerical approach, in order to define wind loads for the structural design, and possible interventions aimed at mitigating the dynamic effects due to wind action. Exploiting the regular tapering of the obelisk section, a rigid sectional model with constant section was initially tested in wind tunnel: static aerodynamic coefficients and vortex-induced response as a function of the Scruton number were measured for several angles of attack. The experimental sectional results were successively used to assess numerically the response of the structure considering tapering, the gradient of wind speed, the modal shapes, and the correlation of wind actions. Even though the numerical analysis showed that the tapering drastically reduces aerodynamic issues, the achieved results underlined the pressing need to increase the damping of the structure in order to avoid galloping and vortex induced vibrations for wind speeds within the design range; to this aim tuned mass dampers were designed.

A numerical–experimental approach to assess emission performance of new generation engines during the cold transient

A numerical–experimental approach to assess emission performance of new generation engines during the cold transient

A new experimental-numerical approach for studying the effects of gas pressure profile on superplastic forming characteristics of Al-Mg5.6 alloy

Controlling the strain rate during superplastic forming is the main reason to implement time-variable gas pressure profiles. In the present research, a new approach is introduced for controlling the strain rate during free bulge forming. The approach was examined by performing the superplastic forming operation with the Al-Mg5.6 aluminum alloy as a valuable material in transportation industries. Free bulge forming operations were performed at 500 °C and a strain rate range of 8 × 10−5–5.7 × 10−4 s−1. The changes of the strain rate at the apex of the workpiece were numerically monitored during the deformation, and a new strategy to smooth these changes was implemented. New time-dependent pressure profiles were defined, and their influences on the forming time and uniformity of the deformed part were examined. Finally, the stepwise rising pressure function followed by a constant pressure at which the time of increasing (Tr) was between 20 and 40% of the total time needed for merely applying that constant pressure was shown to provide the best results. This type of pressure profile can also improve the uniformity of thickness and save the process time, compared with forming at a constant low pressure. Moreover, this stepwise pressure function is easy to be practically employed in various industrial applications.

Investigation of Cell-Substrate Adhesion Properties of Living Chondrocyte by Measuring Adhesive Shear Force and Detachment Using AFM and Inverse FEA.

It is well-known that cell adhesion is important in many biological processes such as cell migration and proliferation. A better understanding of the cell adhesion process will shed insight into these cellular biological responses as well as cell adhesion-related diseases treatment. However, there is little research which has attempted to investigate the process of cell adhesion and its mechanism. Thus, this paper aims to study the time-dependent adhesion properties of single living chondrocytes using an advanced coupled experimental-numerical approach. Atomic Force Microscopy (AFM) tips will be used to apply lateral forces to detach chondrocytes that are seeded for three different periods. An advanced Finite Element Analysis (FEA) model combining porohyperelastic (PHE) constitutive model and cohesive zone formulation is developed to explore the mechanism of adhesion. The results revealed that the cells can resist normal traction better than tangential traction in the beginning of adhesion. This is when the cell adhesion molecules establish early attachment to the substrates. After that when the cells are spreading, stress fiber bundles generate tangential traction on the substrate to form strong adhesion. Both simulation and experimental results agree well with each other, providing a powerful tool to study the cellular adhesion process.

Marketing with Equality: Optimization of Promotion Strategies with Social Welfare Functions

This paper studies an optimal distribution of health information for a government or Non-profit Organization under various objectives and indexes, such as equality and efficiency, using social marketing approach. To examine this, employing various social welfare functions in the field of welfare economics, we theoretically derive how much information to give to each segment. We also investigate and discuss the optimal distribution, using numerical analysis and experimental approaches. As a result, we find that the optimal solutions are determined depending on the sensitivity, importance of segments, cost, and budget parameters. Through the intervention experiment in Okinawa, our method improves some equality index. The contribution of this study is to clarify the optimal way of distribution with equality and show the possibility of applying discussions in economics to the studies of social marketing and epidemiology.

Strain localization and ductile fracture in advanced high-strength steel sheets

Abstract An experimental-numerical approach is applied to determine the strain localization and ductile fracture of high-strength dual-phase and martensitic steel sheet materials. To this end, four different quasi-static material tests were performed for each material, introducing stress states ranging from simple shear to equi-biaxial tension. The tests were analysed numerically with the nonlinear finite element method to estimate the failure strain as a function of stress state. The effect of spatial discretization on the estimated failure strain was investigated. While the global response is hardly affected by the spatial discretization, the effect on the failure strain is large for tests experiencing necking instability. The result is that the estimated failure strain in the different tests scales differently with spatial discretization. Localization analysis was performed using the imperfection band approach, and applied to estimate onset of failure of the two steel sheet materials under tensile loading. The results indicate that a conservative failure criterion for ductile materials may be established from localization analysis, provided strain localization occurs prior to ductile fracture.

A simple practical method for determination of moisture transfer coefficient of mature concrete using a combined experimental-numerical approach

In this paper, a simple practical method is introduced in which a simple weight measurement of concrete and finite element numerical analysis are used to determine the moisture transfer coefficient of concrete with a satisfactory accuracy. Six concrete mixtures with different water-to-cementitious material (w/cm) ratios and two pozzolanic materials including silica fume and zeolite were examined to validate the proposed method. The comparison between the distribution of the moisture content obtained from the model and the one from the experimental data during both the wetting and drying process properly validated the performance of the method.With the proposed method, it was also shown that the concrete moisture transfer coefficient considerably depends on the pore water saturation degree. The use of pozzolanic materials and also lowering w/cm ratio increased the moisture transfer coefficient during the initial sorption, and then, it significantly decreased with an increase in the water saturation degree.