Zero-stress State Research Articles

High-cycle fatigue enhancement of dissimilar 2017A-T451/7075-T651 Al alloy joint fabricated by a single pass FSW without any post-processing

This study investigates the microstructural, mechanical, high-cycle fatigue, and fracture behaviour of a dissimilar 2017A-T451/7075-T651 Al alloy joint produced by single-pass friction stir welding (FSW) without post-processing. Residual stress analysis revealed low tensile residual stresses in the longitudinal direction on the crown side and compressive residual stress in the transverse direction and on the root side of the weld, indicating an almost zero-stress state. High-cycle fatigue tests showed the superior fatigue endurance of the FSWed sample due to the synergistic effect of expressive grain refinement and compressive residual stress in the transverse direction. Fractographic observations confirmed a brittle type II failure with low fracture toughness in the base material 7075-T651 alloy. In contrast, both the FSWed samples and the base 2017A-T451 alloy displayed a ductile transition with type I failure, resulting in higher average fracture toughness values. The direction and propagation of the fatigue crack in the FSWed sample were influenced by the microstructure, with the crack completely bypassing the stir region consisting of larger crystal grains that acted as the crack arrester. This study highlights the high potential of single-pass FSW technology for producing thick dissimilar Al alloy joints with enhanced high-cycle fatigue life without requiring post-treatment. The findings of this study can benefit industries that require high-performance lightweight structures with improved fatigue life, such as aerospace and transportation.

Increased arterial pressure volume index and cardiovascular risk score in China

Background and objectiveThe new non-invasive arterial stiffness indices, arterial pressure volume index (API) is explored as a novel marker of residual stress in the wall of the peripheral muscular arteries at zero-stress state in clinical settings. The present study aimed to study the association of API with cardiovascular disease (CVD) risk in China (China-PAR).MethodsAccording to China-PAR score, participants were divided into three groups: low risk (< 5%), medium risk (5–9.9%), and high risk (≥ 10.0%). API ≥ 31 was defined as high API, and the incidences of high API were compared. Logistic regression models were used to analyze the risk factors of high API and high risk China-PAR categories. The association between China-PAR and API was analyzed by restrictive cubic spline.ResultsThe study included 4311 participants. After adjustments for confounding factors, high API was independent factor associated with high risk China-PAR categories, and the probability of high API was 1.366 times higher than that in normal API subjects. While, the independent factors associated with high API were BMI, blood pressure and heart rate. Furthermore, API had a significant U-shaped association with China-PAR. CVD risk was lowest with API of 19 units, the fastest increase at 26 units and the flattest starting point at 59 units.ConclusionAPI, an indicator of arterial stiffness and residual stress, had a U-shaped association with China-PAR score and might play an important role in predicting CVD risk in Chinese natural populations.

Residual strains in ascending thoracic aortic aneurysms: The effect of valve type, layer, and circumferential quadrant

The stress distribution in ascending thoracic aortic aneurysms is determined by the mechanical properties, geometry, loading conditions, and zero-stress state of the aneurysmal aorta. Our objective was to fully characterize the zero-stress state of the aneurysmal aorta in twelve tricuspid aortic valve patients and eight (age/aortic diameter-matched) bicuspid aortic valve patients, for which little data are available. Opening angles and residual stretches were measured for the intact wall and individual layers according to quadrant and were similar in the two patient groups. The intact-wall and medial opening angles were comparable; their circumferential but not their axial ones peaked in the left lateral quadrant, with non-significant regional differences in the other layers. The intima’s circumferential opening angles were the highest of all layers (∼300 deg) and the adventitia’s the lowest (∼165 deg), with lesser layer differences in the axial opening angles. Upon radially cutting aortic rings, the released circumferential residual stretches were tensile (of large magnitude) externally and compressive (of small magnitude) internally, unlike the axial residual stretches released when cutting intact-wall strips, whose magnitude was small externally and large internally. Nevertheless, large circumferential compressive residual stretches were released in the adventitia upon layer dissection, counteracting the large circumferential tensile stretches of the intact wall externally. Moreover, the large axial tensile residual stretches of the intima counteracted the large axial compressive stretches of the intact wall internally. These layer-specific residual stretches may moderate the in-vivo stress gradients across wall thickness, serving as a protective mechanism against aortic dissection or rupture.

An improved parameter fitting approach of a planar biaxial test including the experimental prestretch

Planar biaxial testing is a popular experimental technique for characterizing and comparing biological soft tissues. A correct identification of the different stress states of the tissue sample is therefore essential. However, the difference between the zero-stress reference state and the sample state prior to the loading cycle caused by the mounting, preconditioning and preloading is often not considered. The importance of this difference, caused by prestretch, is investigated by simulating virtual planar biaxial experiments, either assuming an ideal test with a single deformation gradient or using finite element modeling to simulate a rake-based experiment. Multiple parameter fitting methods are used to estimate the material properties based on the available experimental data. These methods vary based on how they approximate the zero-stress state: either the prestretch is ignored, or the loads are zeroed after the preload has been reached, or the unknown prestretch values are included into the optimization function. The results reveal the high necessity of assessing the stress-free state when analyzing a planar biaxial test. The material fitting including the prestretch outperforms the other methods in terms of correctly describing the mechanical behavior of the tested material. It can be extended to correct for the boundary effects induced by the gripping mechanisms, providing a more accurate, yet more computationally expensive estimate of the material properties.

Mechanical behavior of crimped collagen fibers under uniaxial tension

Knowledge of the highly nonlinear behavior of crimped collagen fibers has a great importance for medical diagnosis. This behavior can be associated with the gradual straightening of fibers and the amount of fibers that bears load. To model this phenomenon, we propose an explicit scalar potential, based on the Weibull distribution, in terms of the fiber bundle strength. The total strain is given in terms of the stress and is defined by sum of the strain of crimped fibers, arising from the proposed potential, and the strain of completely straightened fibers by using the Hookean model. Besides three physically meaningful parameters, further two new parameters that have intuitive interpretations were considered. The proposed model was compared with the Freed-Rajagopal model and the Holzapfel-Gasser-Ogden model for verifying its ability to fit the measured data from bovine tendon samples under uniaxial tension. Our model better described the highly nonlinear behavior of the tendon. Experimental data from literature were also used for validation of the parameters of the proposed model. The estimated parameters were consistent with those found in the literature. The effectiveness of the proposed model and Freed-Rajagopal model in describing the stress-strain curve with different initial range of strain with the zero-stress state was also investigated. Unlike the Freed-Rajagopal model, the proposed model fits well in all cases, including those with wide range of strain.

Prestress Self-Equilibrium Force-Finding Method for Cable-Supported Grid Structures Considering Zero-Stress State Form-Finding and the Construction Process

The cable-supported grid structure (CSGS) is a hybrid structure combined with rigidity and flexibility. Its formed state is closely related to the zero-stress state, construction process, and prestress distribution. The prestress self-equilibrium force-finding method is proposed in this paper to continuously conduct zero-stress state form-finding, the construction process, and prestress state force-finding analysis. As the first engineering project adapting CSGS with an internal compression ring, Shanghai Pudong Football Stadium was taken as the analysis object. The structural finite element model was established in ANSYS, structural components were divided into prestress and ordinary components to form a prestress self-equilibrium system, and the equivalent temperature difference was applied to the prestress components as prestress. The Newton–Raphson method was used for geometric nonlinear analysis. The analysis results show that the internal forces and displacements of the formed structure are consistent with the design state, which proves the validity of the method and provides significant guidance for the actual construction. The method proposed in this paper can effectively and accurately obtain the zero-stress state configuration, structural response during construction, and the prestress distribution after forming in a single analysis, without analyzing the above three separately, which improves the analysis efficiency.

Mechanical Properties and Structural Optimization of Continuous Welded Rail on Super-Long-Span Suspension Bridges for High-Speed Railway

In order to ensure driving safety and comfort, it is necessary to figure out the complex interaction between continuous welded rail (CWR) and suspension bridges for high-speed railway. A spatial finite element model for a 1092 m main span suspension bridge was established based on the bridge-track interaction theory. A specific correction method was put forward to keep the rail in a zero-stress state when just laid. Three rail expansion joint (REJ) layout schemes were proposed according to practical engineering experience. Both static and dynamic analysis methods were used to evaluate the feasibility of these schemes. The results show that the REJ should be laid at the position with a distance away from the primary beam end, and the beam with more substantial integral stiffness should be preferentially selected. For the recommended scheme, the REJ expansion reaches more than 380 mm under expansion load. The factors affecting the REJ expansion from major to minor are temperature, earthquake, rail fracture, braking, and bending load. The superposition effect of the above factors is suggested to be considered in the selection of REJ range.

3D biomechanical properties of the layered esophagus: Fung-type SEF and new constitutive model.

Most current studies on the passive biomechanical properties of esophageal tissues directly use the exponential strain energy function (SEF) to fit and calculate the constants of the constitutive equation. In the context of the extensive application of exponential SEF, in-depth research on the exponential SEF is still lacking. The purpose of this study is to combine the exponential function with the polynomial SEF to obtain the most suitable constitutive equation to describe the three-dimensional passive behavior of the esophagus. fresh pig esophagus with a length of 13cm in the middle position was selected as esophageal samples. The esophageal sample was separated into muscular layer and mucosal layer with surgical scissors. Stretch-inflation mechanical tests of the intact esophagus, esophageal muscular, and esophageal mucosa were carried out on a triaxial test machine. The external radius, axial force, and internal pressure were recorded simultaneously. The seven-parameter Fung-type SEF and several new SEFs combining polynomials and exponents were used to fit the experimental data curves. The stretch-inflation test data and the morphometric parameters at the zero-stress state of the layered esophagus were obtained. The new SEF with polynomial and exponential combination is more suitable to describe describing the three-dimensional passive biomechanical properties of esophageal tissue. Among them, New-Fung13 SEF is more suitable for describing the passive biomechanical properties of intact esophageal tissue, Sokolis-Fung13 SEF is more suitable for the esophageal muscle layer, and New-Fung10 SEF is more suitable for the esophageal mucosa. The constitutive parameters of the optimal constitutive model for each layer of the esophagus were obtained.

Layer-Specific Residual Deformations and Their Variation Along the Human Aorta.

This study described the regional distribution of layer-specific residual deformations in fifteen human aortas collected during autopsy. Circumferentially and axially cut strips of standardized dimensions from the anterior quadrant of nine consecutive aortic levels were photographed to obtain the zero-stress state for the intact wall. The strips were then dissected into layers that were also photographed to obtain their zero-stress state. Changes in layer-specific opening angle, residual stretches, and thickness at each aortic level and direction were determined via image analysis. The circumferential and axial opening angles of the intima were ∼240 deg and ∼30 deg, respectively, throughout the aorta; those of the adventitia were ∼150 deg and -20 deg to 70 deg. The opening angles of the intact wall and media were similar in either direction. The circumferential residual stretches of the intima and the axial residual stretches of the media showed high values in the aortic arch, decreasing in the descending thoracic aorta and increasing toward the iliac artery bifurcation, while the axial residual stretches of the adventitia increased distally. The remaining residual stretches did not vary significantly with aortic level, suggesting an intimal role in determining circumferential, as well as medial and adventitial roles in determining axial residual stretches. We conclude that the tensile residual stretches released in the intima and media upon separation, and the compressive residual stretches released in the adventitia may moderate the inverse transmural stress gradients under physiologic loads, resulting from the >180 deg circumferential opening angle of the intact wall.

Space\u2013Time Variational Multiscale Isogeometric Analysis of a tsunami-shelter vertical-axis wind turbine

We present computational flow analysis of a vertical-axis wind turbine (VAWT) that has been proposed to also serve as a tsunami shelter. In addition to the three-blade rotor, the turbine has four support columns at the periphery. The columns support the turbine rotor and the shelter. Computational challenges encountered in flow analysis of wind turbines in general include accurate representation of the turbine geometry, multiscale unsteady flow, and moving-boundary flow associated with the rotor motion. The tsunami-shelter VAWT, because of its rather high geometric complexity, poses the additional challenge of reaching high accuracy in turbine-geometry representation and flow solution when the geometry is so complex. We address the challenges with a space–time (ST) computational method that integrates three special ST methods around the core, ST Variational Multiscale (ST-VMS) method, and mesh generation and improvement methods. The three special methods are the ST Slip Interface (ST-SI) method, ST Isogeometric Analysis (ST-IGA), and the ST/NURBS Mesh Update Method (STNMUM). The ST-discretization feature of the integrated method provides higher-order accuracy compared to standard discretization methods. The VMS feature addresses the computational challenges associated with the multiscale nature of the unsteady flow. The moving-mesh feature of the ST framework enables high-resolution computation near the blades. The ST-SI enables moving-mesh computation of the spinning rotor. The mesh covering the rotor spins with it, and the SI between the spinning mesh and the rest of the mesh accurately connects the two sides of the solution. The ST-IGA enables more accurate representation of the blade and other turbine geometries and increased accuracy in the flow solution. The STNMUM enables exact representation of the mesh rotation. A general-purpose NURBS mesh generation method makes it easier to deal with the complex turbine geometry. The quality of the mesh generated with this method is improved with a mesh relaxation method based on fiber-reinforced hyperelasticity and optimized zero-stress state. We present computations for the 2D and 3D cases. The computations show the effectiveness of our ST and mesh generation and relaxation methods in flow analysis of the tsunami-shelter VAWT.

Determination of active members and zero-stress states for symmetric prestressed cable–strut structures

A prestressed cable–strut structure is generally flexible and exhibits strong coupling between its stress state and configuration. The zero-stress state offers the basis for design and analysis of cable–strut structures and has significant influence on the prestress state and the load state. Here, a computational method is proposed for seeking zero-stress states of symmetric cable–strut structures. By evaluating distributed static indeterminacy and symmetry representations using group theory, the active member with proper importance index and high-order symmetry is chosen from different types of members. Moreover, natural lengths and the involved elongations of the members are established from the initial prestresses and geometric properties. Then, based on the Newton method and the Moore–Penrose inverse theory, internal forces of the members are actively reduced. The structural configuration and tangent stiffness matrix are iteratively updated during the whole process from the prestress state to the zero-stress state. The feasibility and accuracy of the proposed approach are verified by some numerical examples, whereas the results are compared with analytical solutions and FEM simulation. The results show that one zero-stress configuration is associated with a specific prestress state, and the process between zero-stress state and prestress state is reversible. This work has theoretical significance for the design of novel cable–strut structures and provides a reference for the construction process of prestressed cable–strut structures in practical applications.

A low-distortion mesh moving method based on fiber-reinforced hyperelasticity and optimized zero-stress state

In computation of flow problems with moving boundaries and interfaces, including fluid–structure interaction, moving-mesh methods enable mesh-resolution control near the interface and consequently high-resolution representation of the boundary layers. Good moving-mesh methods require good mesh moving methods. We introduce a low-distortion mesh moving method based on fiber-reinforced hyperelasticity and optimized zero-stress state (ZSS). The method has been developed targeting isogeometric discretization but is also applicable to finite element discretization. With the large-deformation mechanics equations, we can expect to have a unique mesh associated with each step of the boundary or interface motion. With the fibers placed in multiple directions, we stiffen the element in those directions for the purpose of reducing the distortion during the mesh deformation. We optimize the ZSS by seeking orthogonality of the parametric directions, by mesh relaxation, and by making the ZSS time-dependent as needed. We present 2D and 3D test computations with isogeometric discretization. The computations show that the mesh moving method introduced performs well.

Finite element simulation of human trachea: Normal vs. surgically treated and scaffold implanted cases

Trachea experiences mechanical loads under physiological conditions. Surgery and tissue engineering change its mechanical conditions and may cause remodeling or damage. Here, we mechanically simulated one healthy trachea (from a 53-year-old brain-dead man) and as two examples, evaluated the impacts of surgery and scaffold implantation. Before patient's death, CT-scanning was performed at two respiratory pressures (0 and + 40 cm H2O for constructing 3D tracheal geometry at zero-stress state and model validation, respectively). After harvesting trachea, MRI, mechanical testing and hyperelastic modeling of tracheal tissues (cartilage, smooth muscle, and connective tissue) were performed. We simulated tracheal deformation and stress distribution under three conditions, including normal, 30%-resection of tracheal length and anastomosis (surgery) or its substitution with a cylindrical scaffold. In all models, the greatest stresses were produced in cartilaginous rings. Surgery-related extension reduced the physiologic cross-sectional deformability and increased stress levels in response to respiratory pressures, compared to normal trachea. For instance, maximum von-Mises stress in normal and operated trachea reached 355 kPa and 3.228 MPa, respectively. Scaffold was less deformable than normal trachea, though stress levels in both models were relatively close. This study provides an insight into the mechanics of human trachea and the impacts of surgery or scaffold implantation.

Variation of Axial Residual Strains Along the Course and Circumference of Human Aorta Considering Age and Gender.

Our understanding of aortic biomechanics is customarily limited by lack of information on the axial residual stretches of the vessel in both humans and experimental animals that would facilitate the identification of its actual zero-stress state. The aim of this study was thus to acquire hitherto unreported quantitative knowledge of axial opening angle and residual stretches in different segments and quadrants of the human aorta according to age and gender. Twenty-three aortas were harvested during autopsy from the aortic root to the iliac bifurcation and were divided into ≥12 segments and 4 quadrants. Morphometric measurements were taken in the excised/curled configuration of rectangular strips considered to be under zero-stress using image-analysis software to study the axial/circumferential variation of axial opening angle, internal/external residual stretch, and thickness of the aortic wall. The measured data demonstrated: (1) an axial opening angle peak at the arch branches, decreasing toward the ascending and to a near-constant value in the descending thoracic aorta, and increasing in the abdominal aorta; (2) the variation of residual stretches resembled that of opening angle, but axial differences in external residual stretch were more prominent; (3) wall thickness showed a progressive diminution along the vessel; (4) the highest opening angle/residual stretches were found in the inner quadrant and the lowest in the outer quadrant; (5) the anterior was the thinnest quadrant throughout the aorta; (6) age caused thickening but greatly reduced axial opening angle/residual stretches, without differences between males and females.

Regional distribution of layer-specific circumferential residual deformations and opening angles in the porcine aorta

Information on the layer-specific residual deformations of aortic tissue and how these vary throughout the vessel is important for understanding the regionally-varying aortic functions and pathophysiology, but not so much can be found in the literature. Toward this end, porcine aortas were sectioned into eighteen rings, with one ring from each anatomical position radially cut to obtain the zero-stress state for the intact wall and the other ring dissected into intimal-medial and adventitial layers; these rings were then radially cut to reach the zero-stress state for the intima-media and adventitia. Peripheral variations in internal/external circumferences, thickness, and opening angle of the intact wall and its layers were measured through image analysis at the no-load and zero-stress states. Intact wall and layer circumferences at both states significantly declined along the aorta, as did intact wall and intimal-medial but not adventitial thickness. Adventitia exhibited the greatest opening angles, approaching 180 deg all over the aorta. The opening angles of the intima-media and intact wall were quite similar, with the highest values in the ascending aorta, the lowest at the diaphragm, and increasing subsequently. Bending-related residual stretches were released by radial cutting that were compressive internally and tensile externally, displaying distinct axial variation for the intima-media and intact wall, and non-significant variation for the adventitia. Evidence is provided for the release upon layer separation of compressive stretches in the intima-media and of tensile stretches in the adventitia, whose values were smallest in the descending thoracic aorta and highest near the iliac artery bifurcation.

Computational method for in-situ finite element modeling of inflatable membrane structures based on geometrical shape measurement using photogrammetry

Inflatable membrane structures with the advantages of light weight, large span, long duration and aesthetical appearance have gained considerable attentions from buildings, super-pressure balloons, airships and deployable antennas. This paper presents a novel computational method for in-situ finite element modeling of inflatable membrane structures based on geometrical shape measurement using photogrammetry for further structural analysis. Firstly, kinematic equations of membrane link structure simplified from the measured inflatable membrane structure are established on force equilibrium between membrane internal and external forces to calculate the tensile forces. Then, computational method for solving zero-stress state is proposed through unloading internal pressure based on equilibrium matrix theory. Finally, true finite element model is established through numerical simulation of reloading internal pressure with considering the zero-stress state as initial model. For verifying purpose, inflated forming and normal working tests of an ethylene-tetrafluoroethylene (ETFE) air-inflated cushion structure were carried out. By comparing geometrical shapes and stress distributions of ETFE cushion between numerical and experimental results, good agreements with 0.72% and 4.76% maximum errors are acquired. These findings strongly confirm the need of in-situ finite element modeling of inflatable membrane structures undergoing long-term service and provide an effective computational method for further structural analysis.

Mechanical analysis of intestinal contractility in a neonatal maternal deprivation irritable bowel syndrome rat model

The aims of the present study are to investigate biomechanical properties and provide mechanical analysis of contractility in ileum and colon in a neonatal maternal deprivation (NMD) irritable bowel syndrome (IBS) rat model. Mechanical testing was done on segments from ileum and colon in 25 IBS rats and 13 Control rats. Morphometric data were obtained from digitized images of the segments at no-load and zero-stress states. Pressure and diameter changes were measured during flow and ramp distensions under active and passive experimental conditions. Circumferential stresses (force per area) and strains (deformation) were computed with referenced to the zero-stress state. The contraction frequency was analyzed. Contraction thresholds and maximum contraction amplitude were calculated in terms of mechanical stress and strain. Compared with controls, the IBS rats had lower body weight (P < 0.01), smaller colonic opening angle (P < 0.05), higher colonic contraction frequency (P < 0.05 and P < 0.01) and lower contraction thresholds of pressure, stress and strain in both ileum and colon (P < 0.05 and P < 0.01). The maximum contraction pressure, stress and strain did not differ between IBS and Control groups (P > 0.05). In conclusion, the pressure, stress, and strain to evoke contractility in ileum and colon were lower whereas the frequency of induced colon contractions was higher in NMD IBS rats compared to normal rats. Furthermore, zero-stress state remodeling occur in colon in NMD IBS rats. Further studies on the association between intestinal biomechanical properties, hypersensitivity and afferent signaling in the IBS animal models are warranted.

Reference-Free Traction Force Microscopy Platform Fabricated via Two-Photon Laser Scanning Lithography Enables Facile Measurement of Cell-Generated Forces.

Cells sense and respond to the physical nature of their microenvironment by mechanically probing their surroundings via cytoskeletal contractions. The material response to these stresses can be measured via traction force microscopy (TFM). Traditional TFM platforms present several limitations including variable spatial resolution, difficulty in attaining the full three-dimensional (3D) deformation/stress profile, and the requirement to remove or relax the cells being measured to determine the zero-stress state. To overcome these limitations, we developed a two-photon, photochemical coupling approach to fabricate a new TFM platform that provides high-resolution control over the 3D placement of fluorescent fiducial markers for facile measurement of cell-generated shear and normal components of traction forces. The highly controlled placement of the 3D marker array provides a built-in, zero stress state eliminating the need to perturb the cells being measured while also providing increased throughput. Using this platform, we discovered that the magnitude of cell-generated shear and normal force components are linked both spatially and temporally. The facile nature and increased throughput of measuring cell-generated forces afforded by this new platform will be useful to the mechanotransduction community and others.

Medical-image-based aorta modeling with zero-stress-state estimation

Because the medical-image-based geometries used in patient-specific arterial fluid–structure interaction computations do not come from the zero-stress state (ZSS) of the artery, we need to estimate the ZSS required in the computations. The task becomes even more challenging for arteries with complex geometries, such as the aorta. In a method we introduced earlier the estimate is based on T-spline discretization of the arterial wall and is in the form of integration-point-based ZSS (IPBZSS). The T-spline discretization enables dealing with complex arterial geometries, such as an aorta model with branches, while retaining the desirable features of isogeometric discretization. With higher-order basis functions of the isogeometric discretization, we may be able to achieve a similar level of accuracy as with the linear basis functions, but using larger-size and fewer elements. In addition, the higher-order basis functions allow representation of more complex shapes within an element. The IPBZSS is a convenient representation of the ZSS because with isogeometric discretization, especially with T-spline discretization, specifying conditions at integration points is more straightforward than imposing conditions on control points. The method has two main components. 1. An iteration technique, which starts with a calculated ZSS initial guess, is used for computing the IPBZSS such that when a given pressure load is applied, the medical-image-based target shape is matched. 2. A design procedure, which is based on the Kirchhoff–Love shell model of the artery, is used for calculating the ZSS initial guess. Here we increase the scope and robustness of the method by introducing a new design procedure for the ZSS initial guess. The new design procedure has two features. (a) An IPB shell-like coordinate system, which increases the scope of the design to general parametrization in the computational space. (b) Analytical solution of the force equilibrium in the normal direction, based on the Kirchhoff–Love shell model, which places proper constraints on the design parameters. This increases the estimation accuracy, which in turn increases the robustness of the iterations and the convergence speed. To show how the new design procedure for the ZSS initial guess performs, we first present 3D test computations with a straight tube and a Y-shaped tube. Then we present a 3D computation where the target geometry is coming from medical image of a human aorta, and we include the branches in the model.

Rail neutral temperature monitoring using non-contact photoluminescence Piezospectroscopy: A field study at high-speed rail track

A novel non-contact stress sensing method using the photoluminescence piezospectroscopy (PLPS) method was applied in full-scale field test at high-speed rail tracks. The fluorescence spectra of the aluminum oxide (α-Al2O3) were collected from different locations on bare thermite weld surface for about 1.5 days. Prominent α-Al2O3 signals (i.e., R1/R2 peaks) were observed with the very short measurement time of 0.01 s, which is preferable in mobile sensing applications. The kernel spectrum estimation (KSE) method was developed as a postprocessing technique to determine R1/R2 peak locations accurately. The R1/R2 time histories showed strong correlation with those of strain gauges mounted on the rail near the thermite welds. The piezospectroscopy coefficients were determined at −30.01 cm−1/GPa for R1 and –32.11 cm−1/GPa for R2, calibrated with the strain gauge measurements. Prior to the field test, a laboratory test was conducted to determine the R1/R2 zero-stress states using a pulverized thermite weld sample. The zero-stress state was determined at 14,404.99 cm−1 for R1 and 14,434.09 cm−1 for R2. Using these parameters, rail neutral temperature was determined at full-scale high-speed tracks based on the absolute rail stress measured with PLPS without additional field calibrations.