Mechanical Stress Field Research Articles

The Influence of Mechanical Stress Micro Fields around Pores on the Strength of Elongated Etched Membrane.

The investigation of the mechanical properties of polymer track-etched membranes (TMs) has attracted significant attention in connection with the extended region of their possible applications. In the present work, the mechanical stress fields around the pores of an elongated polyethylene terephthalate TM and around the 0.3 mm holes in model polymer specimens were studied in polarized light and with the finite element method. A break-up experiment showed the controlling role of stress field interaction in the forming of a microcrack system and the generation of a main crack with rupture of the TM (or model pattern). This interaction depended on the relative distance between the pores (holes) and their orientation. The results of the calculations of the pore distribution function over the surface of the TM via the net method and wavelet analysis are presented. The fractal character of pore distribution was established. The geometric characteristics of the TM pore system as initial defects are inherited by obtaining TM-based composites.

Thermomechanical Analysis of Steel-to-Timber Connections under Fire and the Material Density Effect

This work presents a thermomechanical numerical analysis of a steel-to-timber connection with dowels in tension when exposed to fire using ANSYS ® software. Three different wood density materials were considered. The connection is built by a three-dimensional model with a thermomechanical boundary condition. A nominal temperature–time curve, ISO 834, was used to simulate the fire effect. Numerical simulation to determine the field of thermal and mechanical stresses was performed using a combined problem. A temperature field was imposed for a given time instant of fire exposure, calculated through a thermal analysis in a transient regime. This temperature profile was coupled to an incremental tensile load, allowing the determination of the maximum mechanical resistance of the connection. According to this methodology, the load-bearing capacity of the connections in each fire rating will be determined. In addition, the numerical results allow verification of the wood density influence on the mechanical resistance of the connection exposed to fire. In conclusion, the load-bearing capacity decreases with fire exposure and with lower material density. With the proposed methodology, the effect of the wood density on the heat transferred through the connection under fire can be verified, and a thermomechanical complex model is proposed to solve and analyze this type of problem, which is the great motivation in this work. The numerical methodology represents well the thermomechanical behavior of the connection under fire. This procedure can be used, considering other different parameters, to improve the design and allow the study of the connection behavior as an alternative to the experimental tests.

(Invited) Micromechanical Aspects of GaN Hemt Performance and Reliability

Performance and reliability of microelectronic devices are governed by the mechanical strain. Typically, strain engineering implies uniformly distributed strain. However, we offer a different perspective by hypothesizing that very small but localized strain (or stress) may have significant impact on the overall behavior of AlGaN/GaN high electron mobility transistors (HEMTs). Micro to nanoscale confined mechanical stress fields may develop unavoidably and are ignored because their spatial average is insignificant. We exploit high resolution techniques such as transmission electron microscopy (TEM) and micro-Raman spectroscopy to spatially resolve the stress field in GaN HEMTs to demonstrate the stress localization and then radiation effects.To study stress localization effects on the global electrical characteristics, we introduced a highly localized strain relief by milling a 20×30 μm2 micro trench about 70 μm deep on the backside of an 800×840 μm2 size HEMT die. The resulting local relaxation of in-plane residual strain was mapped using micro-Raman technique. Our results show that a decrease of only 0.02% strain can decrease the overall output saturation current up to ~20 %. The drop of output current is attributed to reduced two-dimensional electron gas (2DEG) sheet carrier density and electron mobility due to the strain relief in the device layers. However, the mechanistic process of strain relief also causes defect generation at the interfaces, which increases leakage current. Our technique for localized strain re-distribution could be an effective tool to surrogate the influence of inherent localized strain build-up across the channel of electronic devices.To study radiation effects, we exposed the HEMTs to gamma rays (up to 10 megaRads dose at 180 kiloRads per hour). We report the thermal and mechanical responses in terms of the changes in lattice strain and temperature, which were simultaneously characterized by changes in phonon frequency of E2 (high) and A1 (LO) from on- state and unpowered/pinched off reference states. Lower doses of radiation improved the electrical properties, however degradation initiated at about 1 megaRads. We observed about 16% decrease in saturation current and 6% decrease in transconductance at the highest dose. However, leakage current increase by 3 orders of magnitude was the most notable radiation effect. We observed temperature increase by 40% and mechanical stress increase by a factor of three at dose of 10 megaRads compared to the pristine devices. Spatial mapping of mechanical stress along the channel identifies the gate region as mechanically affected area, whereas the thermal degradation was mostly uniform. Transmission electron microscopy showed contrast changes reflecting high vacancy concentration in the gate region. These findings suggest that localized stress (mechanical hotspots) may increases vulnerability to radiation damage by accommodating higher concentration of defects that promote leakage current.Finally, we present experimental results on a novel concept of electron wind force driven room temperature annealing of GaN and SiC devices. This technique was applied on Gamma irradiated HEMT and SiC diodes. We demonstrate full recovery of the HEMTs irradiated to 10 mega-Rads gamma radiation. About 200% performance improvement was recorded for the SiC diodes. Our hypothesis is that mechanical stress waves are instrumental in eliminating interfacial defects.

Application of smart responsive materials in phosphopeptide and glycopeptide enrichment

蛋白质的磷酸化和糖基化作为研究最广泛的两种翻译后修饰(PTMs),在疾病的早期无创诊断、预后和治疗评估中表现出越来越大的潜力。蛋白质的异常磷酸化和糖基化经常被用于临床蛋白质组学研究和疾病相关生物标志物的发现。目前已有多种材料被开发用于磷酸化肽和糖肽的富集研究,其中,智能响应材料由于具有独特的响应特性,已被陆续报道用于磷酸化肽和糖肽的富集。智能响应材料可对外界刺激做出响应,发生结构和性质上的变化,将光、电、热、机械等信号转化为生物化学信号。响应分子是决定智能响应材料响应特性的先决条件,它们在不同刺激条件下(如温度、pH、光、机械应力、电磁场等)的可逆异构化将导致材料的宏观物理和化学性质的动态变化。与传统材料相比,智能响应材料可以可逆地“打开”和“关闭”,具有更好的可调控性。由于引起智能材料响应的刺激信号对其性能具有重要的影响,综述根据施加的刺激种类对智能响应材料进行分类,具体分为外源性响应材料和内源性响应材料,且分别总结了外源性响应材料、内源性响应材料以及内外源共同响应材料在磷酸化肽和糖肽富集方面的工作。此外,综述对智能响应材料在磷酸化肽和糖肽富集方面的发展前景进行了展望,并且提出了智能响应材料在其他蛋白质翻译后修饰方面的应用中存在的挑战。

Stress and Strain State Analysis of Crack Front in Dissimilar Metal Welded Joints with Dual Field of Mechanical Heterogeneity and Residual Stress

The micro-mechanical state at the crack front is one of the key factors affecting the stress corrosion cracking (SCC) growth behavior. The mechanical heterogeneity and residual stress in the dissimilar metal welded joint (DMWJ) induce the micro-mechanical state at the crack front to become more complex. The sandwich model and dual-field model of the DMWJ with inner surface axial crack were established in this study. The stress and strain states at the crack front with different crack locations and lengths under the interaction of the mechanical property and the residual stress were investigated. The results show that a more accurate evaluation of stress and strain states can be obtained when using the dual-field model to describe the material mechanical property and residual stress of the DMWJ. The sandwich model overestimates the crack driving force including the stress and strain at the crack front. The tensile stress in the middle of shallow cracks is smaller than that at both ends, while the tensile stress in the middle of deep crack is larger than that at both ends. The variation trend of the tensile stress and normal strain at the crack apex is basically the same as that of the residual stress with the crack depth. However, there is almost no normal plastic strain in the initial stage of crack propagation due to the small residual stress in the initial stage.

Study on the failure mechanism in shale-sand formation based on hybrid finite-discrete element method

During the hydraulic fracturing in shale formation, the mechanical properties and stress field caused by sand and shale interbedding were the major factors affecting the hydraulic fracture propagation. In this paper, a hydraulic fracturing simulation model was proposed based on the hybrid finite-discrete element method, and the factors affecting the fracture propagation were analyzed. The simulation results indicated that the fracture width experienced a sudden decrease when the hydraulic fracture interacted with a bedding plane. The reservoir with high stress difference and Young’s modulus was favorable for the opening of bedding plane, and high injection rate was favorable for the longitudinal expansion of hydraulic fracture. This was mainly because the lower stress difference and Young’s modulus was, the opening degree of the bedding plane was much smaller, and the less the energy consumption of opening the bedding surface was.

Nanoscale Stress Localization Effects on the Radiation Susceptibility of GaN High‐Mobility Transistors

Nanoscale localized mechanical stress fields develop unavoidably in microelectronic devices due to structural and processing aspects. Their global average is too small to influence bandgap or mobility, but it is proposed that stress localization can influence defect nucleation sites under radiation. This is investigated on gallium nitride high‐electron‐mobility transistors (GaN HEMTs). Using transmission electron microscopy, we spatially resolved the stress field in the AlGaN layer for both pristine and 10 Mrad gamma‐irradiated HEMTs. The quantitative nanobeam electron diffraction and geometric phase analysis indicate that tensile stressed localizations experience higher radiation‐induced strain. This finding is explained by the tensile stress dependence of the carrier concentration and mobility in the AlGaN layer. Since gamma radiation damage is inflicted by high‐energy electrons only, localized regions of higher tensile stress in the AlGaN layer are expected to be more susceptible to gamma rays.

Simulation of tectonic stress field and prediction of tectonic fractures in shale reservoirs: A case study of the Ansai area in the Ordos Basin, China

The Chang 7 oil-bearing layers from the Ansai area of the Ordos Basin are abundant in oil and gas resources. These layers are chiefly comprised of shales interbedded with tight sandstone. In the regions where the exploration level is relatively low and the structures are gentle in the Ansai area, the distribution of fractures in shale reservoirs and tight sandstone reservoirs is significant for predicting sweet spots and determining the success rate of hydraulic fracturing. In this research, the results acquired from the scanning electron microscope (SEM), cores, thin sections, and image logging determined the developmental characteristics of fractures. Moreover, the Himalayan stress field was modeled using ANSYS, whereas the two-factor method is used to predict the distribution of tectonic fracture by combining stress field and fracture criterion. The results indicate a gradually decreasing trend of the maximum principal stress from east to west. The minimum principal stress in the western part is less than that in the eastern part. The differential stress that can indicate the degree of tectonic deformation ranges from 17 Mpa to 25 MPa. The fracture density ranges from 0 strip/m to 1.5 strip/m. In addition, the areas with more fractures are mainly located in the western and southeastern parts, where shale oil is relatively rich. Similarly, the comparison of different maps illustrates that the distribution of tectonic fracture is related to mechanical parameters and stress fields.

Comprehensive numerical characterization of a piezoelectric composite with hollow metallic inclusions using an adaptable random representative volume

This paper investigates a piezoelectric composite made up of a piezoceramic matrix with irregularly scattered random-sized hollow metal inclusions. This composite can be produced using a combination of porous ceramic fabrication methods and microcapsule transfer techniques that were previously utilized to deliver nanoparticle medicines in biomedical applications. The effective material characteristics were computed using a numerical homogenization methodology that included finite element analysis, random representative volume element, perfect contact between distinct phases, special linear boundary conditions, and the Hill–Mandel principle. To imitate the random distribution of inclusions inside a particle-filled composite, the relationship describing the inclusion’s volume fraction was adjusted by adding a regularity coefficient. Thus, a new representative volume was designed to study the effect of the non-uniform distribution of randomly sized inclusions on the effective coefficients of the composite. An analysis of the effective properties of this new composite system with metalized pores surfaces (SMPS) revealed the following features. Due to the presence of metallic impurities, the resulting electric induction field or mechanical stress field inside the piezoelectric phase is scattered, which raises the transverse piezoelectric moduli with the increase of the inclusion’s volume fraction. Also, the electrical permittivity coefficients increase steadily with the increase in the portion of the metallic impurities. According to the findings, the volume fraction of the inclusion can be considered the most critical parameter to modify the electromechanical characteristics of the SMPS. The effects of the regularity coefficient on the equivalent properties increase as the inclusion’s volume fraction increases. The effective dielectric constants decrease as the degree of regularity of the composite structure grows, increasing the effectiveness of the piezoelectric transducer in transverse energy harvesting and motor applications. The findings of the newly constructed representative volume were verified using a basic periodic unit-cell and a random representative volume based on the random sequential adsorption (RSA) method. The developed structure of the representative volume can be applied to simulate a wide range of composites in the form of solid particles. Piezoelectric transducers constructed from the proposed piezocomposite are predicted to function well in piezoelectric motors and transverse piezoelectric sensors.

Exact Analytical Solution of the Nonstationary Problem of Calculation of Temperature and Mechanical-Stress Fields in a Two-Layer Cylinder and Its Application for the Verification of Numerical Models

Exact Analytical Solution of the Nonstationary Problem of Calculation of Temperature and Mechanical-Stress Fields in a Two-Layer Cylinder and Its Application for the Verification of Numerical Models

Material Removal of Hip Stem Prosthesis Using Bio-Inspiration from Trabecular Bone

In the hip stem prostheses domain, stress shielding represents a major issue due to its potential implant loosening effect. This phenomenon occurs when the implant is much stiffer than the bone itself. A way to reduce this stiffness difference is to remove material from the implant. In addition, this solution leads to mass reduction of the implant, improving the patient quality of life. To insure the implant will withstand the loading, material must be distributed in a specific way. Bones use the same objectives of lightness and mechanical efficiency while using a minimal amount of material. In addition, these biological structures are well spread and reliable in living beings, especially in mammals and avian species. In this paper, a method bio-inspired by trabecular bone structure is proposed to remove material from parts considering their mechanical stress field. In Nature, trabecular bone is defined as a graded porous material with bone material locally oriented along the local stresses’ direction. To mimic this natural behavior, the proposed method generates local porosities bio-inspired in position, shape, size and orientation. To evaluate the proposed method performance, experimental tests were carried out on a hip prosthesis stem. Test results demonstrated that the method can be used to reduce the stiffness of the prosthesis while withstanding the applied constrains.

FEA of Drug-Eluting Stents and Sensitivity Analysis of a Continuum Damage Model for the Degradation of PLGA Coating.

Drug-Eluting Stents (DES) are commonly used in coronary angioplasty operations as a solution against artery stenosis and restenosis. Computational Bioengineering allows for the in-silico analysis of their performance. The scope of this work is to develop a DES Digital Twin, focusing on the mechanical integrity of its biodegradable coating throughout the operational lifecycle. The implementation leverages the Finite Element Method (FEM) to compute the developed mechanical stress field on the DES during the inflation/deflation stage, followed by the degradation of the polymer-based coating. The simulation of the degradation process is based on a Continuum Damage Mechanics (CDM) model that considers bulk degradation. The CDM algorithm is implemented on the NX Nastran solver through a user-defined material (UMAT) subroutine. For benchmarking purposes and to compare with the baseline design of the BioCoStent project, this conceptual study implements an alternative stent design, to study the effect of the geometry on the developed stresses. Additionally, the effect of the degradation rate on the polymer-based coating's lifecycle is studied via sensitivity analysis.

Modelling the Laser Cladding of Geometrically More Complex Tracks and Its Experimental Verification

In this paper, a methodology for depositing wear- and corrosion-resistant layers of geometrically more complex tracks on metal substrates is presented. The corresponding mathematical model includes the temperature field produced by the laser beam, the field of residual mechanical stresses, and the efficiency of utilization of the delivered powder material. The computations are realized using the finite element method, with a substantial improvement in processing the time-variable geometry of the investigated system being found, based on the introduction of two specific matrices that characterize both the surface on which the tracks are cladded as well as the track itself. The proposed technique is illustrated by cladding an angled helix on a metal surface. Selected results are successfully verified by experiments.

Simulation of nonstationary hydrogen diffusion processes near the crack tip in a field of inhomogeneous mechanical stresses

An elastic-plastic isotropic body is investigated, weakened by a rectilinear crack directed along the abscissa axis, under the action of stresses symmetric with respect to its plane. The hydrogen concentration near the crack tip is calculated. An approximate solution of this problem is constructed under the condition that the distribution of hydrostatic stresses along the crack extension is approximated by a parabola. For a numerical solution, a method of the third order of accuracy with a two-sided estimate of the main term of the local error is proposed.

Dynamic mechanical interaction between injection liquid and human tissue simulant induced by needle-free injection of a highly focused microjet

This study investigated the fluid–tissue interaction of needle-free injection by evaluating the dynamics of the cavity induced in body-tissue simulant and the resulting unsteady mechanical stress field. Temporal evolution of cavity shape, stress intensity field, and stress vector field during the injection of a conventional injection needle, a proposed highly focused microjet (tip diameter much smaller than capillary nozzle), and a typical non-focused microjet in gelatin were measured using a state-of-the-art high-speed polarization camera, at a frame rate up to 25,000 f.p.s. During the needle injection performed by an experienced nurse, high stress intensity lasted for an order of seconds (from beginning of needle penetration until end of withdrawal), which is much longer than the order of milliseconds during needle-free injections, causing more damage to the body tissue. The cavity induced by focused microjet resembled a funnel which had a narrow tip that penetrated deep into tissue simulant, exerting shear stress in low intensity which diffused through shear stress wave. Whereas the cavity induced by non-focused microjet rebounded elastically (quickly expanded into a sphere and shrank into a small cavity which remained), exerting compressive stress on tissue simulant in high stress intensity. By comparing the distribution of stress intensity, tip shape of the focused microjet contributed to a better performance than non-focused microjet with its ability to penetrate deep while only inducing stress at lower intensity. Dynamic mechanical interaction revealed in this research uncovered the importance of the jet shape for the development of minimally invasive medical devices.

Clinical assessment of a biophysical model for distinguishing tumor progression from radiation necrosis

The efficacy of an imaging-driven mechanistic biophysical model of tumor growth for distinguishing radiation necrosis from tumor progression in patients with enhancing lesions following stereotactic radiosurgery (SRS) for brain metastasis is validated. We retrospectively assessed the model using 73 patients with 78 lesions and histologically confirmed radiation necrosis or tumor progression. Postcontrast T1-weighted MRI images were used to extract parameters for a mechanistic reaction-diffusion logistic growth model mechanically coupled to the surrounding tissue. The resulting model was then used to estimate mechanical stress fields, which were then compared with edema visualized on FLAIR imaging using DICE similarity coefficients. DICE, model, and standard radiographic morphometric analysis parameters were evaluated using a receiver operating characteristic (ROC) curve for prediction of radiation necrosis or tumor progression. Multivariate logistic regression models were then constructed using mechanistic model parameters or advanced radiomic features. An independent validation was performed to evaluate predictive performance. Tumor cell proliferation rate resulted in ROC AUC=0.86, 95% CI: 0.76-0.95, P<0.0001, 74% sensitivity and 95% specificity) and DICE similarity coefficient associated with high stresses demonstrated an ROC AUC=0.93, 95% CI: 0.86-0.99, P<0.0001, 81% sensitivity and 95% specificity. In a multivariate logistic regression model using an independent validation dataset, mechanistic modeling parameters had an ROC AUC of 0.95, with 94% sensitivity and 96% specificity. Imaging-driven biophysical modeling of tumor growth represents a novel method for accurately predicting clinically significant tumor behavior.

Design and Analysis of an Advanced Three-Point Bend Test Approach for Miniature Irradiated Disk Specimens

Candidate tungsten armor materials in a magnetic confinement fusion device must be able to withstand thermal variation that leads to internal stresses caused by the impinging heat load. In addition, the thermomechanical properties of these materials are degraded by irradiation-induced defect accumulation. Fission reactor–based irradiation data are used to predict the fusion neutron damage and property change. This study examines the motivation and design of a custom-designed three-point bend test for neutron-irradiated disk specimens that are 3 mm in diameter to be able to define the flexural strength of advanced tungsten materials, alloys, and composites—and to the extent that embrittlement occurs after neutron irradiation. The theory provided shows a calculation for the flexural deflection and shear deflection due to the small-geometry constraints. A finite element deformation analysis is performed to evaluate the mechanical stress field of disk bend specimens. The stress values above 80% of the maximum stress are concentrated in 2.4 mm of the 3.0-mm length of the centerline across the tungsten disk diameter. A bend test fixture has been designed and fabricated to enable testing of these specimens with precisely engineered tolerance and minimal machine compliance. This fixture will be able to be placed inside a universal testing frame at elevated temperatures for the mechanical property evaluation of future neutron-irradiated disk specimens.

Development of a Magneto-Mechanical Bench and Experimental Characterization of Magneto-Rheological Elastomers

Magneto-rheological elastomers (MREs) belong to the class of smart materials whose mechanical properties can be controlled by an external magnetic field. These materials can be integrated into mechatronic systems and submitted to multiple loadings such as temperature, mechanical stress, and magnetic field. Thus, this work is dedicated to the development of a magneto-mechanical bench and on first experimental characterizations of hard MREs taking multiphysics coupling into account. Regarding the mechanical loading, the experimental setup is able to create a uniaxial tensile stress in case of low strain (< 1%) without friction effect. In regards to the magnetic loading, a magnetic circuit made of a strong permanent magnet has been designed to impose a variable and a homogeneous magnetic field strength up to 41 kA/m. Experimental analysis has been performed on silicone rubber filled with 36%vol. of NdFeB particles. The purpose was first to investigate the evolution of the Young modulus with or without magnetic field. Results obtained from measurements show that the developed test bench is able to depict the mechanical behavior and phenomena linked to rubber-like material.

Thermal and electrical characterization of poly(vinyl)alcohol)/poly(vinylidene fluoride) blends reinforced with nano-graphene platelets

Nano-graphene/polymer composites can function as pressure induced electro-switches, at concentrations around their conductivity percolation threshold. Close to the critical point, the pressure dependence of the electron tunneling through the polymer barrier separating nano-graphene's results from the competition among shortening of the tunneling length and the increase of the polymer's polarizability. Such switching behavior was recently observed in polyvinyl alcohol (PVA) loaded with nano-graphene platelets (NGPs). In this work, PVA is blended with α-poly(vinylidene fluoride) (PVdF) and NGPs. Coaxial mechanical stress and electric field render the nano-composite piezoelectric. We investigate the influence of heterogeneity, thermal properties, phase transitions and kinetic processes occurring in the polymer matrix on the macroscopic electrical conductivity and interfacial polarization in casted specimens. Furthermore, the effect of electro-activity of PVdF grains on the electric and thermal properties are comparatively studied. Broadband Dielectric spectroscopy is employed to resolve and inspect electron transport and trapping with respect to thermal transitions and kinetic processes traced via Differential Scanning Calorimetry. The harmonic electric field applied during a BDS sweep induces volume modifications of the electro-active PVdF grains, while, electro-activity of PVdF grains can disturb the internal electric field that free (or bound) electric. The dc conductivity and dielectric relaxation was found to exhibit weak dependencies.

Investigation of mechanical behavior of buried DN110 polyethylene pipe with a scratch defect under land subsidence

In this work, a rigorous 3D finite element (FE) model was established to investigate the mechanical behavior of buried DN110 High-density polyethylene (HDPE) pipe with a scratch defect under land subsidence. Parametric effects, including soil gravity, defect geometric, operating pressure, transition section length, and land subsidence displacement, were investigated to analyze the significance of each factor on mechanical stress field. Then the land subsidence displacement was selected as the evaluation index of the ultimate bearing capacity (UBC) of the pipeline. A model was developed to discuss the function relationship between the land subsidence displacement and other parameters.