Search

Important resolution properties of the high-resolution 3-axis neutron diffraction set-up were investigated in the frame of feasibility of its use for strain/stress measurements on bulk polycrystalline samples. The strain analysis was carried out by rocking the BPC-analyzer and the neutron signal registered by a point detector. The so-called rocking curve then provided a sample diffraction profile and could reflect the lattice or structural changes. In comparison with a conventional 2-axis set-up, this instrument alternative could be successfully applied, namely, in the strain/stress measurements in bulk samples exposed to an external load, e.g. in tension/compression rig, in aging machine etc. and provide average strain values over a large irradiated volume of the sample. [ABSTRACT FROM AUTHOR]

The study summarises the selection of dedicated techniques and procedures for measuring real stresses arising in shaft steelwork components due to car-shaft steelwork interactions as well as stresses at critical points of the car frame in the elevator installation in the Regis Shaft of the Wieliczka Salt Mine. The implemented solution and operational parameters of the hoisting installation in the Regis Shaft of the Wieliczka Salt Mine are summarised, the adopted measurement method and dedicated equipment are presented, focusing on the procedure for locating the critical points on the car frame and shaft steelwork. The purpose-built measurement set-up as well as the adopted stress measurement procedure and results are explained in detail. [ABSTRACT FROM AUTHOR]

The article focuses on Optical fiber sensor based on Sagnac interferomter. Topics include that the sensor is designed by combination of photonic crystal fiber (PCF) and fiber Bragg grating (FBG) and the device is used for measuring various physical parameters including temperature and stress by the changes of physical characteristics such as interference, diffraction, and polarization caused by light propagation in the optical fiber.

Wall shear stress and wall streamline direction measurements were conducted on a generic delta wing model to demonstrate the potential of the white-light oil-film interferometry in three-dimensional flows. With the proposed algorithm, an automatic oil-film thickness calculation is possible and yields reproducible and plausible wall shear stress distributions on the leeward side of the delta wing for various angles of attack. [ABSTRACT FROM AUTHOR]

Abstract The implementation of the new type of ensemble, with constant tension force in the molecular dynamic simulations, is considered. In contrast to the isostress N t T ensemble, in which the stress components act on the surface and the volume elements, the new N f T ensemble is determined by the tension forces acting on each particle in the extended space. Correspondingly, instead of the stress–strain relation used in conventional theory, a new type of constitutive equation in terms of stretch and tension becomes an objective of the molecular dynamics simulations. Comparison with the Parrinello–Rahman method is made for the general external load and the temperature applied. Results for the aluminum lattice show that the suggested approach gives an unbiased sampling of the isostress–isothermal ensemble in the LAMMPS environment. Additionally, the problem of atomistic boundary condition was renovated, due to almost identical tension and traction forces. It was shown on several examples of strains that the tension forces properly represent the type and magnitude of the atomistic external load applied to the surface, despite the absence of any quantities related to the area in the constitutive equations. An analog of Cauchy’s stress theorem in the new setting is formulated and verified by numerical simulations. [ABSTRACT FROM AUTHOR]

Cell elongation and differentiation has been shown to be modulated by topographical cues provided by grating substratum. However, little is known about the mechanisms and forces involved in the grating-induced cell elongation, due to the difficulty in fabricating soft elastic gels that allow 3-dimensional (3D) cell traction stress measurements. In this paper, we present a method to fabricate soft elastic polyacrylamide grating substrates, using an imprinted polyethylene terephthalate mould, for 3D cell traction stress measurements. Fibroblasts were observed to form protrusions in the grating grooves, and elongate and align parallel to the grating direction on the soft polyacrylamide grating substrates. Focal adhesions were also found to be aligned parallel to the grating direction as compared to cells on flat substrates, suggesting that grating grooves restrict focal adhesion growth perpendicular to the grating direction. The 3D traction stress measurements revealed that highly elongated fibroblasts on grating substrates exert anisotropic traction stresses, in the direction parallel to the grating direction. We propose that focal adhesion alignment along the grating direction may result in increased actin stress fibre formation in the direction parallel to the grating, leading to polarized traction stresses which drive cell elongation. [ABSTRACT FROM AUTHOR]

While stress is a widely utilized concept, no direct methods facilitating its measurement are currently available. In our previous work we proposed stress entropic load (SEL) as a potential new marker of stress response in the human body. However, at that time no method for SEL measurement existed. In this pilot study we devised and then tested methodology for SEL measurement. Healthy male participants were monitored by indirect calorimetry and thermography while resting and subsequently while under prolonged mental effort. The acquired data was then used to calculate the temporal development of SEL for each participant. Our results show that SEL production increased significantly in participants subjected to prolonged mental effort. Furthermore, we observed that the calculated development of SEL over time may be used to accurately determine the time point at which participants started performing stressful tasks. [ABSTRACT FROM AUTHOR]

Block-and-hole polyhedra can be derived from a bar-joint triangulation of a polyhedron by a stepwise construction: select a set of non-overlapping disks defined by edge-cycles of the triangulation of length at least 4; then modify the interior of each disk by an addition or deletion operation on vertices and edges so that it becomes either a rigid block or a hole. The construction has a body-hinge analogue. Models of many classical objects such as the Sarrus linkage can be modelled by block-and-hole polyhedra. Symmetry extensions of counting rules for mobility (the balance of mechanisms and states of self-stress) are obtained for the bar-joint and body-hinge models. The extended rules detect mechanisms in many cases where pure counting would predict an isostatic framework. Relations between structures where blocks and holes are swapped have a simple form. Examples illustrate the finer classification of isostatic and near-isostatic block-and-hole polyhedra achievable by using symmetry. The present approach also explains a puzzle in standard models of mobility. In the bar-joint model, a fully triangulated polyhedron is isostatic, but in a body-hinge version it is heavily overconstrained. When the bodies are panels with hinge lines intersecting at vertices, the overconstraints can be explained in local mechanical terms, with a direct symmetry description. A generalisation of the symmetry formula explains the extra states of self-stress in panel-hinge models of block-and-hole polyhedra. [ABSTRACT FROM AUTHOR]

Steel and composite steel-concrete bridges are subjected to random traffic loads along their life cycle which generate significant dynamic impacts. The road-roughness of asphalt pavements is one of the most important aspects that contributes to the significant increase of the stress amplitudes and hence to serious load-induced fatigue concerns. In this context, welded joints are well known as the weakest points in bridges, since they are prone to stress concentrations leading to initiation of fatigue cracks being in current design codes evaluated using primarily the nominal stress method (NSM). However, a more accurate stress definition, which considers the complexity of the stress field at the welds, becomes necessary mainly in old roadway steel bridges that are often subjected to out-of-plane stresses. In this paper, the hot-spot stress method (HSM) is used to evaluate the fatigue life of a welded joint subjected to distortion induced-fatigue, considering the vehicle speeds and a progressive deterioration model for the road pavement. The welded joint was modelled by solid elements and was integrated with a 3D dynamic bridge model using the sub modelling technique. Besides the importance of considering the bridge-vehicle dynamic interactions with the pavement road-roughness, the dynamic amplification effects on local stresses and the relatively high scatter found in the fatigue lives considering global and local approaches show that a detailed local stress definition is fundamental to evaluate the fatigue performance of existing roadway bridges. The effects of the annual traffic increase rate on the fatigue life are also discussed. [ABSTRACT FROM AUTHOR]

A site characterization program was carried out for a proposed Deep Geological Repository (DGR) project for Ontario Power Generation’s (OPG) low- and intermediate-level nuclear waste repository near Kincardine, Ontario. The repository is proposed to be constructed at approximately 680 m below ground surface within the competent argillaceous limestone of the Cobourg Formation. The in situ stress state at the project site will have significant impact on both the short- and long-term performance of repository openings, such as emplacement caverns and access shafts. As part of the site characterization program, an evaluation of the in situ stress state of the project site was conducted which consisted, primarily, of a review and synthesis of existing stress measurements conducted at various locations throughout Ontario and the midwestern U.S. Based on geomechanics data from deep boreholes and stress measurement data, a simplified FLAC3D model of the full stratigraphic profile was developed and used to simulate the influence of regional tectonic strain in the project area. In particular, this method takes into account the rock properties, such as stiffness, for discrete units at the DGR site. The model was calibrated on the basis of in situ stresses measured at Norton Mine, in a similar geological environment as the DGR site, and with site-specific borehole televiewer observations (i.e., breakouts). The model-predicted horizontal in situ stress profile showed general agreement with the observations and also showed the significant influence of discrete rock unit stiffness. [ABSTRACT FROM AUTHOR]

As a part of the DECID2 French National Project (2008–2012), construction of a large platform entirely made of smart composites was carried out and two demonstrators were installed. During a previous study, an ABAQUS model of smart composite platform was set up to perform numerical simulations that predict the mechanical behaviour of bolt-fastened platform under static three-point bending load, and a stress concentration is observed around the bolt holes. Unidirectional composites are subjected to delamination at very low stress, and this cannot be tolerated because most of the applications of pultruded structures are in civil engineering, which involves human safety. Therefore, it is essential to study the correlation of delamination onset and find a technology to enhance delamination strength. In this study, a numerical analysis was carried out to enhance the delamination strength around the bolt holes. Bidirectional fiberglass cloths were wrapped around unidirectional pultruded beams to reinforce the structure and to prevent delamination. Due to the high cost of these cloths, a study was also carried out to select an appropriate cloth material by taking into account two aspects: density and number of layers of cloths. The selected cloth was compared with conventional continuous fibreglass mat, and it was found that bidirectional fibreglass cloth material performs well in preventing delamination, even though it is expensive. [ABSTRACT FROM AUTHOR]

Copyright of FME Transactions is the property of University of Belgrade, Faculty of Mechanical Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of FME Transactions is the property of University of Belgrade, Faculty of Mechanical Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In some alloys such as 9%Cr heat resistant steels and magnesium alloys, the creep constitutive equation of the power-law requires a term of threshold stress due to the presence of second phase particles. It is necessary to establish an estimation method of C ∗ for such alloys to predict the life of their components. In this paper, the General Electric/Electric Power Research Institute (GE/EPRI) method and the reference stress method were modified to estimate C ∗ for power-law creep materials with threshold stress. The finite element method was used to verify the accuracy of the modified methods. The accuracy of the calculation equation of C ∗ in the American Society for Testing Materials (ASTM) E 1457 was also assessed. The results indicated that the modified GE/EPRI method was sufficiently exact as an engineering method. h 1 was slightly affected by the applied load and significantly affected by the threshold stress. The accuracy of the modified reference stress method increased with increased applied load and was within ±40%. The accuracy of the calculation equation of C ∗ in ASTM E 1457 was not affected by the threshold stress and the equation could be directly used for power-law creep materials with threshold stress. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, an analytical study is carried out on the work‐hardening, elastic‐plastic stress distributions in a cracked body under antiplane shear deformation. A modified Ramberg‐Osgood law is introduced to describe the material behaviour, and stress and strain fields are derived in closed form. Compared with the conventional Ramberg‐Osgood formulation, the new law includes the effect of a new parameter, κ, which allows the transition from the ideally elastic behaviour (low stress regime) to the power law behaviour (large stress regime) to be controlled, thus providing 1 more degree of freedom to better fit the actual behaviour of engineering materials. A discussion is carried out on the features of stresses and strains close to and far away from the crack tip. [ABSTRACT FROM AUTHOR]

The discrete element method (DEM) in use today generally considers the bonds that bind discrete elements into a continuum to be elasto-perfectly-brittle. Such brittle bonds only give accurate failure loads for large size structures where LEFM applies. But for small to medium size structures of quasi-brittle materials such as rocks and concretes, the presence of the fracture process zone (FPZ) has significant impacts and needs to be considered. To properly model size effects, an exponential softening contact bonds model was implemented which is tied to input fracture energy and also addresses particle size issue. This was followed by a successful study of the classical size effect problems of similar edge notched beams subjected to direct tension and three-point bending. The overestimation of failure loads by brittle bonds are then addressed. The discrete nature of DEM also facilitates the use of different particle packing for structure construction, and provided insights on how FPZ evolves under sustained loading. [ABSTRACT FROM AUTHOR]

A new methodology for the estimation of tyre-contact forces is presented. The new procedure is an evolution of a previous method based on harmonic elimination techniques developed with the aim of producing low cost dynamometric wheels. While the original method required stress measurement in many rim radial lines and the fulfillment of some rigid conditions of symmetry, the new methodology described in this article significantly reduces the number of required measurement points and greatly relaxes symmetry constraints. This can be done without compromising the estimation error level. The reduction of the number of measuring radial lines increases the ripple of demodulated signals due to non-eliminated higher order harmonics. Therefore, it is necessary to adapt the calibration procedure to this new scenario. A new calibration procedure that takes into account angular position of the wheel is completely described. This new methodology is tested on a standard commercial five-spoke car wheel. Obtained results are qualitatively compared to those derived from the application of former methodology leading to the conclusion that the new method is both simpler and more robust due to the reduction in the number of measuring points, while contact forces’ estimation error remains at an acceptable level. [ABSTRACT FROM AUTHOR]

The prestress loss is one of the main factors affecting the safety of prestressed concrete structure. While the detecting signals like sound and light are difficult to spread in steel strands, there is no effective method for prestress detection of the bonded prestressed steel strands in existing structures yet. In this paper, taking into consideration that the electromagnetic oscillation characteristic can make the signal propagate effectively on the bonded prestressed steel strands, a nondestructive prestress detection method based on the electromagnetic effect to detect oscillation frequency is proposed. In a detection circuit, the steel strands are simulated as an inductance component, in which an induced electromagnetic signal passes through the steel strands to form resonance. And then, a frequency meter is used to detect the oscillation frequency of the resonant circuit. The oscillation frequency is supposed to have relationship with the prestress loading on the steel strands. A section of steel strands with a length of 1.2 m is adopted to test the correlation of stress and oscillation frequency. Both the theoretical and experimental results show that the resonant frequency of the circuit decreases with the increase of the stress of the strand and is linear in a certain range. [ABSTRACT FROM AUTHOR]

The objective of the present paper is to propose a methodology to numerically simulate (i.e., initialize) the pre-mining stress field in complex but rather frequent situations in which the classical overburden-weight assumption and existing stress measurements are in disagreement and where the stresses strongly vary from one zone to another, even at constant depth. This methodology is illustrated by the case of a deep mine in France that exploits a 10°-dipping coal seam, in which numerous stress measurements were carried out. Virgin principal stresses in this mine have been shown to be highly heterogeneous and anisotropic. To correctly reproduce such a challenging initial stress state in a numerical model and to be able to later calculate mining-induced stresses, five distinct methods (M#1 to M#5) are successively presented and compared, along with their advantages and drawbacks. All of them are based on “fixed” boundary conditions, with null normal displacements on all lateral boundaries except on the top one, which is considered as a free surface of the Earth coinciding with the natural flat topography. The initial conditions of the model assume that the pre-mining stress components linearly depend on the Cartesian coordinates x , y and z (depth), and the Simplex Method is used to calculate the linear coefficients by minimizing the squared difference between the measured stresses and their simulated values. We show that the use of 3D stress gradients produces more realistic results than a 1D vertical stress gradient. [ABSTRACT FROM AUTHOR]

To accurately measure in situ stresses using overcore techniques, the key is to find the relationship between the stresses and measured strains on the wellbore wall and on the core at two stages, i.e. before and after overcoring. For gas shales, where there exist large pressure and temperature changes, we developed an analytical model for the stage before overcoring to study the evolution of all field variables, i.e. temperature, pressure, stress, strain and displacement (TPSSD), when an inclined wellbore is drilled in an infinite and homogenous thermo-poro-elastic media (THM) subjected to three far-field principal stresses, whose axes may not coincide with the horizontal and vertical planes. In that paper we investigated the thermal and poro-elastic effects on the strain changes around and especially at the wellbore wall. As an extension of the previous work. In this paper, we describe an analytical model to give the mechanical responses of the wellbore and core for the stage after overcoring. Strain changes at the wellbore wall are examined in detail because these are measured by the overcore method. By using the analytical solutions presented at a specific time and the strains before overcoring as an initial state, the transient solutions for the evolution of temperature, pressure, stress, strain and displacement (TPSSD) are obtained so that for the two critical stages before and after overcoring a reasonable estimate of the in situ stresses can be obtained. As well as the strain changes, the temperature and pressure at the wellbore wall, which vary with time, are also required to find in situ stresses in the inverse model. Fortunately, their effects become progressively less important with time due to the small size of overcored sample. The critical time is calculated when the thermal and porous effects can be ignored. Two cases, i.e. constant and exponentially time-varying temperature boundary conditions, are investigated. [ABSTRACT FROM AUTHOR]

This paper investigates the ability of the USAF characteristic K approach to represent the growth of cracks from small naturally occurring material discontinuities. The examples discussed in this paper are associated with two different load spectra. The first case studied involves crack growth in AA7050-T7451 under a combat aircraft load spectrum. The second case studied the crack growth in AA7050-T7451 under a civil aircraft load spectrum. It is shown that using the root mean square value of ΔK in a load block to characterize the stress intensity factor together with the NASGRO equation, with the constants determined from constant amplitude tests on long cracks, enables the growth of small cracks under these two different complex load spectra to be reasonably accurately represented. [ABSTRACT FROM AUTHOR]

Multistage mini hydraulic fracturing tests were performed in a borehole located in central Hungary in order to determine in-situ stress. At depth of about 500 to 560 meters, observed pressure versus time curve in metamorphic rock (mica schist) show a typical results. After each pressurization cycle, the fracture breakdown pressure in the first fracturing cycle is lower than the reopening pressures in the subsequent reopening and step-rate phases. It is assumed that the composition of the drilling mud and observed foliation of the mica schist have a significant influence on the pressure values. In order to investigate this problem, numerical modeling was performed using the discrete element code (ITASCA Particle Flow Code, PFC), which has been proven as an effective tool to investigate rock engineering problems associated with hydraulic fracturing. The code presented in this study enables simulating hydro-mechanically coupled fluid flow in crystalline rock with low porosity and pre-existing fractures (represented by the smooth joint contact model in PFC) in two dimensions. In this study, the sensitivity of the effect of foliation angle and fluid viscosity on the peak pressure is tested. The anomalous characteristics of the pressure behavior are interpreted in that way that the drilling mud penetrates the sub-horizontal foliation plane, it clogs the plane of weakness and makes the opened fracture tight. Eventually, the process prevents leak-off from the opened fracture that might explain the increased fracture reopening pressure in subsequent cycles. [ABSTRACT FROM AUTHOR]

Robot-assisted therapy can provide effective rehabilitation training for stroke patients. The interfacial stress between the manipulator and the grasped patient's body is very important for a rehabilitation therapy robot. This paper presents a measurement method, which is able to decouple the measurement of the three-dimensional (3-D) interfacial stress components. To implement the method, interfacial stress sensors were fabricated based on plate capacitance and a superelastic material, while a high-resolution microcapacitance measurement circuit was developed based on a minimal system microprocessor and a programmable controller, and experiments are carried out by using a 3-D stress simulation equipment. The results show that the measurement circuit is capable of measuring a range of 1 fF–30 pF capacitance with a resolution better than 1 fF, and the sensor is capable of measuring Z-direction normal compressive stress at a range of \text0-3.0 \textkgf/\textcm^2 with a sensitivity of \text101 \textfF/\textkgf/\textcm^2 and X - and Y-direction shear stress at a range of 0-1.0 \textkgf/\textcm^2 with a sensitivity of \text118 \textfF/\textkgf/\textcm^2. This method can be applied to rehabilitation therapy robots to decouple the measurement of the 3-D interfacial stress components. [ABSTRACT FROM PUBLISHER]

The constitutive models based on grain size effect are crucial for analyzing the deformation of metal foils. Previous investigations on the constitutive models concentrate on the foils whose thickness/average grain diameter ( T/ D) ratios are more than 3. In this study, the commercial pure titanium foils with thickness of 0.1 and 0.2 mm were employed as the experimental materials. The mechanical properties of foils with dimensions of nine different T/ D ratios categorized into three ranges ( T/ D < 1, 1 ≤ T/ D < 3, T/ D ≥ 3)were tested. Meanwhile, the fracture behaviors and fracture mechanisms of the samples with different T/ D ratios were compared and analyzed. Besides, three constitutive models incorporating the surface layer effect and grain boundary strengthening effect were established for the three T/ D ratio ranges correspondingly. In these models, the thickness of the surface layers is set T for T/ D < 1 foils, D for T/ D > 3, and increases with D linearly in 1 ≤ T/ D < 3. The results calculated by the three models were compared. The experiments indicate that those models are all in good agreement. [ABSTRACT FROM AUTHOR]

Neutron diffraction was used to describe the residual stress distributions in self-piercing riveted (SPR) joints. The sheet material displayed a compressive residual stress near the joint, and the stress gradually became tensile in the sheet material far away from the joint. The stress in the rivet leg was lower in the thick joint of the softer steel sheet than in the thin joint of the harder steel sheet. This lower magnitude was attributed to the lower force gradient during the rivet flaring stage of the SPR process curve. This study shows how the residual stress results may be related to the physical occurrences that happened during joining, using the characteristics curve. The study also shows that neutron diffraction technique enabled a crack in the rivet tip to be detected which was not apparent from a cross-section. [ABSTRACT FROM AUTHOR]

The multi-dimensional ultrasonic-assisted pulsed-laser surface irradiation method was introduced to relieve the residual stress in the AISI 1045 steel specimen based on the Nd:YAG pulsed-laser machine. In this stress relief process, the acoustic energy was imposed into the AISI 1045 steel specimen accompanied by the pulsed-laser surface irradiation, which could be called as the multi-dimensional ultrasonic-assisted stress relief (MDUASR). The mechanism of the MDUASR was discussed based on the thermal activation theory. In addition, the experiments for the MDUASR and the multi-dimensional ultrasonic stress relief (MDUSR) were carried out to validate the residual stress relieving effect and the energy-saving effect of the MDUASR. The results show that the MDUASR has good effect on relieving residual stress and the MDUASR can reduce the energy consumption compared with the MDUSR. Furthermore, the mechanism of the MDUASR can be attributed to the metal softening effect and the dynamic stress effect. The findings confirm that the thermal energy induced by pulsed-laser surface irradiation can be adequately utilized to enhance the metal plasticity, which results in that the residual stress relieving effect of the MDUASR can be improved compared with the MDUSR. It can be concluded that the MDUASR belongs to an energy-saving and green environmental protection technology during the manufacturing process. [ABSTRACT FROM AUTHOR]

The presented study introduces the development of the multi-reflection grazing-incidence X-ray diffraction method (MGIXD) for residual stress determination. The proposed new methodology is aimed at obtaining more reliable experimental data and increasing the depth of non-destructive stress determination below the sample surface. To verify proposed method measurements were performed on a classical X-ray diffractometer (Cu Kα radiation) and using synchrotron radiation (three different wavelengths: λ = 1.2527 Å, λ = 1.5419 Å and λ = 1.7512 Å). The Al2017 alloy subjected to three different surface treatments was investigated in this study. The obtained results showed that the proposed development of MGIXD method, in which not only different incident angles but also different wavelengths of X-ray are used, can be successfully applied for residual stress determination, especially when stress gradients are present in the sample. [ABSTRACT FROM AUTHOR]

A theoretical generalized plane-strain thermo-poro-elastic (THM) wellbore model is developed for application to the conventional overcore stress measurement method, under the situations when the thermal effect cannot be decoupled from the measured strains on the borehole walls. The THM theory developed by Coussy is used to describe the effects of both the pore pressure and the temperature on the stress and strain distributions around an inclined wellbore, which is assumed to be drilled instantly and subjected to initial stresses with arbitrary directions. The Laplace transformation is used to obtain the temperature, pressure, displacement, strain and stresses in the three-dimensional state around the wellbore. As special cases, the solutions of the above field variables are also provided for the poro-elastic (HM), thermo-elastic (TM) and pure elastic (CM) cases. Based on the THM solutions obtained, the stresses at the borehole wall, which vary with time, provide a bridge to find the in situ stresses from the measured strains at the wellbore surface. In this THM overcore stress measurement method, the temperature and the pressure (if a fluid is present) at the wellbore wall are another two parameters that need to be measured for stress inversion. [ABSTRACT FROM AUTHOR]

Modeling and experiments are used to investigate Lamb wave propagation in the direction perpendicular to an applied stress. Sensitivity, in terms of changes in velocity, for both symmetrical and anti-symmetrical modes was determined. Codes were developed based on analytical expressions for waves in loaded plates and they were used to give wave dispersion curves. The experimental system used a pair of compression wave transducers on variable angle wedges, with set separation, and variable frequency tone burst excitation, on an aluminum plate 0.16 cm thick with uniaxial applied loads. The loads, which were up to 600 le, were measured using strain gages. Model results and experimental data are in good agreement. It was found that the change in Lamb wave velocity, due to the acoustoelastic effect, for the S1 mode exhibits about ten times more sensitive, in terms of velocity change, than the traditional bulk wave measurements, and those performed using the fundamental Lamb modes. The data presented demonstrate the potential for the use of higher order Lamb modes for online industrial stress measurement in plate, and that the higher sensitivity seen offers potential for improved measurement systems. [ABSTRACT FROM AUTHOR]

The strain energy density (SED) averaged over a material dependent control volume proved to control fracture and fatigue behaviour of different materials in many engineering problems. A method to rapidly estimate the averaged SED at the tip of cracks under in-plane mixed mode I + II loading has been recently proposed. It is based on the peak stresses evaluated from finite element (FE) analyses, according to the peak stress method (PSM). Coarse finite element meshes automatically obtained from the free mesh generation algorithm available in a commercial FE code can be adopted. In the present paper Ansys® software has been adopted. The aim of the present contribution is to extend the FE approach to cracks subjected to out-of-plane mixed mode I + III loading. The advantage of the proposed approach is two-fold: there is no need of mesh refinement in the close neighbourhood of the points of singularity, so that coarse FE meshes can be adopted; moreover, geometrical modeling the control volume in FE models is no longer necessary. Bars weakened by circumferential outer cracks subjected to out-of-plane mixed mode (I + III) loading have been analysed taking into consideration different crack lengths, mode mixities and average sizes of the adopted FE meshes. A comparison between approximate values of the averaged SED according to the peak stress approach and accurate values calculated directly from the FE strain energy by adopting very refined FE meshes has been successfully performed within a range of applicability. [ABSTRACT FROM AUTHOR]

Mechanical and fracture mechanical behaviour of composites reinforced with unidirectionally aligned flax fibres is investigated experimentally and numerically. Thereby, the main points are the determination of crack paths in compact tension specimens with three different fibre directions under static as well as fatigue loading and their fatigue crack growth rate curves. Due to the pronounced orthotropic behaviour of those materials the crack path is not only governed by the stress state, but affected by the fibre direction and fibre volume fraction as well. Therefore, the well-known stress intensity factor solutions for the standard specimens are not applicable. Consequently, extensive numerical simulations have been carried out to evaluate the stress intensity factor evolution along the growing crack in order to be able to determine fatigue crack growth rate curves. An approach for calculating stress intensity factors analytically with geometry correction factor functions is conducted exemplified for one fibre direction. [ABSTRACT FROM AUTHOR]

Shape of a substrate directly influences the residual stress in thin film coatings. In this study, a method involving Fiber Bragg Grating (FBG) was used to measure residual stress in a film deposited on a cylindrical surface. An FBG has a cylindrical surface and its Bragg wavelength shifts continuously when a film is being deposited on the sensor's surface. Herein, we calculated the residual strain in the film from the wavelength shift of the Bragg grating by studying the transfer of the residual strain of the cylindrical film to the core of the optical fiber substrate during deposition. By employing the energy method, we derived expressions that related the strain in the core of fiber to the residual strain in single layer films, bilayer films, and multilayer cylindrical films. As an example, we demonstrated a detailed process for testing the stress and the strain distribution across a nickel (Ni) film electrodeposited on the surface of a nickel-phosphorus (Ni-P) alloy-coated optical fiber. The results indicated that the measured strain repeatability was less than 500 μɛ and the strain sensitivity was more than −2 × 10 pm/μɛ, when the thickness of the film was less than 5 μm. The negative sign on the strain sensitivity indicated that the tensile strain in the film produced compressive strain in the core of the optical fiber. The FBG sensor system has high test speed, and integrates measurement and signal transmission. This method provides an effective and convenient approach to measure stress in a film deposited on a cylindrical surface. [ABSTRACT FROM AUTHOR]

Degradation in planar high-k metal gate p- and n-channel MOSFETs, respectively, under negative bias temperature instability (NBTI) and positive bias temperature instability (PBTI) stress is studied using different characterization methods. Ultrafast measure stress measure (UF-MSM) method with a measurement delay of a few microseconds is used to characterize the threshold voltage shift ( \Delta VT ). Gated-diode or direct current IV is used to directly estimate the trap generation (TG) during BTI, after correcting for the measurement inconsistencies. BTI experiments are performed under DC stress at different stress bias ( V\mathrm {G-STR} ) and temperature ( T ) values also under AC stress at different pulse duty cycle (PDC) and frequency ( f ) values. Measured \Delta VT as well as TG show remarkable similarities between NBTI and PBTI stress, under both DC and AC stress. It is shown that TG dominates NBTI and PBTI degradation under both DC and AC stress. [ABSTRACT FROM PUBLISHER]

The article considers national and foreign experience of the use of multi-face bent metal columns (tubes) for the erection of tower structures for the needs of modern cities. The authors present a review of the literature concerned and try to reveal the problem trends in the field for the purpose of the further study of stress-and-strain state (SSS) of the aforesaid structural elements as well as the special features of their work under load. This study may improve the structural shape of the aforesaid multi-face metal structures, improve their design as well as the consideration of the wind load contributing much to their stress-and-strain state. The authors also consider the prospects of production and use of multi-face metal structural elements in modern construction in Russia. The special features of the design methods concerned and a comparative analysis of the SSS parameters of multi-face bent metal columns are considered, too. [ABSTRACT FROM AUTHOR]

Dentro de las tecnologías que hoy en día tienen implicaciones importantes en ámbitos como la educación, la salud y el sector productivo, se destaca el videostreaming. Esto teniendo en cuenta las ventajas que esta tecnología ofrece para el acceso a contenidos multimedia de manera remota, en vivo o fuera de línea. El objetivo de éste artículo es proponer un entorno de pruebas para el soporte del servicio de videostreaming haciendo uso de herramientas libres. Así mismo, este trabajo propone como parte de éste entorno, una herramienta para la medición de estrés llamada Hermes, la cual permite obtener los tiempos de respuesta producto de las múltiples conexiones RTSP a servidores de streaming. La metodología usada para el desarrollo de este trabajo está dividida en 4 fases: análisis de tecnologías y herramientas, configuración del entorno de videostreaming, diseño e implementación de Hermes y pruebas. Está metodología permitió la construcción del entorno de pruebas y su evaluación, a través de la herramienta de medición de estrés Hermes. Finalmente, mediante este trabajo se demuestra como el entorno de pruebas presentado, se convierte en un punto de referencia para diversos entornos de aplicación que requieran el montaje e implementación del servicio de videostreaming. [ABSTRACT FROM AUTHOR]

This article is mainly motivated by the growing needs for highly resolved measurements for wall-bounded turbulent flows and aims to proposes a spatial correction coefficient in order to increase the wall-shear stress sensors accuracy. As it well known for the hot wire anemometry, the fluctuating streamwise velocity measurement attenuation is mainly due to the spatial resolution and the frequency response of the sensing element. The present work agrees well with this conclusion and expands it to the wall-shear stress fluctuations measurements using electrochemical sensors and suggested a correction method based on the spanwise correlation coefficient to take into account the spatial filtering effects on unresolved wall-shear stress measurements due to too large sensor spanwise size. [ABSTRACT FROM AUTHOR]

The Longtan tunnel is one key tunnel along the Hurong (Shanghai-Chengdu) national roadway in the Hubei province of China. This tunnel is a typical long and deep tunnel, with a length of approximately 8700 m and a depth of approximately 500 m. Because of the complicated geological conditions and high initial stress, to compute the stability of the surrounding rock, it is very important to back analyze the material parameters and initial stress from the measured displacements. Based on a novel evolutionary neural network proposed by the author, a new back analysis method is proposed that can be used to simultaneously determine the material parameters and the initial stress. Using this new back analysis method, some typical monitoring sections of the Longtan tunnel are computed. From numerical computations using the back analysis results, it is found that the stability of the tunnel is consistent with engineering practice and that the initial stresses determined from the back analysis coincide very well with the measured values. Therefore, using the proposed back analysis method, the relevant parameters for the entire tunnel can be conveniently obtained. [ABSTRACT FROM AUTHOR]

Using area detectors for stress determination by diffraction methods in a single exposure greatly simplifies the measurement process and permits the design of portable systems without complex sample cradles or moving parts. An additional advantage is the ability to see the entire or a large fraction of the Debye ring and thus determine texture and grain size effects before analysis. The two methods most commonly used to obtain stress from a single Debye ring are the so-called $$\cos \alpha $$ and full-ring fitting methods, which employ least-squares procedures to determine the stress from the distortion of a Debye ring by probing a set of scattering vector simultaneously. The widely applied $$\sin ^2\psi $$ method, in contrast, requires sample rotations to probe a different subset of scattering vector orientations. In this paper, we first present a description of the different methods under the same formalism and using a unified set of coordinates that are suited to area detectors normal to the incident beam, highlighting the similarities and differences between them. We further characterize these methods by means of in situ measurements in carbon steel tube samples, using a portable detector in reflection geometry. We show that, in the absence of plastic flow, the different methods yield basically the same results and are equivalent. An analysis of possible sources of errors and their impact in the final stress values is also presented. [ABSTRACT FROM AUTHOR]

Stress is central to many aspects of rock mechanics, and in the analysis of in situ stress measurement data the calculation of the mean value and an assessment of dispersion are important for statistical characterisation. Currently, stress magnitude and orientation are processed separately in such analyses. This effectively decomposes the second-order stress tensor into scalar (principal stress magnitudes) and vector (principal stress orientations) components, and calculation of mean and dispersion of stress data on the basis of these decomposed components, which violates the tensorial nature of stress, may either yield biased results or be difficult to conduct. Here, by introducing tensorial techniques, we examine two calculation approaches for the mean and dispersion for stress tensors – based on Euclidean and Riemannian geometries – and discuss their similarities, differences and potential applicability in engineering practice. We compare the two approaches using stress tensor superposition and interpolation, and the analysis of actual in situ stress data. The results indicate that Euclidean and Riemannian mean tensors are in general not equal, with the disparity increasing as stress tensor dispersion increases. Both Euclidean and Riemannian approaches are shown to be capable of characterising stress dispersion, although Euclidean dispersion is scale dependent and has units of stress whereas Riemannian dispersion is a scale independent unitless number. Finally, a paradox is revealed in that despite stress tensors being Riemannian entities, it is Euclidean mean stress that is the more meaningful for engineering applications. [ABSTRACT FROM AUTHOR]

We have revealed experimentally a movement of the topological defects of optical indicatrix orientation, which takes place in CaB4O7 glass with residual mechanical stresses under application of external compressive mechanical stresses. The effect is explained basing on the equation of optical indicatrix perturbed by the combination of both residual and external mechanical stresses. The conditions for the appearance of the topological defects with the strength module |1/2| in glasses are formulated. Finally, possibilities for the appropriate topological reactions caused by the applied stresses are discussed. [ABSTRACT FROM AUTHOR]

In situ stress is one of the most important parameters in underground engineering. Due to the difficulty and weakness of current stress measurement methods in deep soft rock, a new one, rheological stress recovery (RSR) method, to determine three-dimensional stress tensor is developed. It is supposed that rock stresses will recover gradually with time and can be measured by embedding transducers into the borehole. In order to explore the relationship between the measured recovery stress and the initial stress, analytical solutions are developed for the stress measurement process with RSR method in a viscoelastic surrounding rock. The results showed that the measured recovery stress would be more close to the initial stress if the rock mass has a better rheological property, and the property of grouting material should be close to that of rock mass. Then, the RSR method, as well as overcoring technique, was carried out to measure the in situ stresses in Pingdingshan Number 1 coal mines in Henan Province, China. The stress measurement results are basically in the same order, and the major principal stresses are approximately in the direction of NW-SE, which correlates well with the stress regime of Pingdingshan zone known from the tectonic movement history. [ABSTRACT FROM AUTHOR]

The use of the acoustic method for measuring stresses in detachable joints is considered. It is characterized by improved accuracy, can be applied in an automated mode, and allows measurements in large parts in the sonic test area. The data on the instrumental facilities of the method are given and cases of their use in real designs are described. [ABSTRACT FROM AUTHOR]

The effect of formulations and procedures to deposit thin active layers based on low surface area powders on complex geometry substrates (open-cell foams) was experimentally assessed. An acid-free liquid medium based on water, glycerol, and polyvinyl alcohol was used for powder dispersion, while a dip-coating technique was chosen for washcoat deposition on 30 PPI ceramic open-cell foams. The rheological behavior was explained on the bases of both porosity and actual powder density. It was proved that the use of multiple dippings fulfills flexibility requirements for washcoat load management. Multiple depositions with intermediate flash drying steps at 350 °C were carried out. Washcoat loads in the 2.5 to 22 wt. % range were obtained. Pore clogging was seldom observed in a limited extent in samples with high loading (>20 wt. %). Adhesion, evaluated by means of accelerated stress test in ultrasound bath, pointed out good results of all the deposited layers. [ABSTRACT FROM AUTHOR]

Accurate flatness measurement of silicon wafers is affected greatly by the gravity-induced deflection (GID) of the wafers, especially for large and thin wafers. The three-point-support method is a preferred method for the measurement, in which the GID uniquely determined by the positions of the supports could be calculated and subtracted. The accurate calculation of GID is affected by the initial stress of the wafer and the positioning errors of the supports. In this paper, a finite element model (FEM) including the effect of initial stress was developed to calculate GID. The influence of the initial stress of the wafer on GID calculation was investigated and verified by experiment. A systematic study of the effects of positioning errors of the support ball and the wafer on GID calculation was conducted. The results showed that the effect of the initial stress could not be neglected for ground wafers. The wafer positioning error and the circumferential error of the support were the most influential factors while the effect of the vertical positioning error was negligible in GID calculation. [ABSTRACT FROM AUTHOR]

The article focuses on the electromagnetic radiation technique (EMR), a technique that helps to predict earthquakes. Topics discussed include EMR is used to determine crustal stress orientations, exhumation joints bring used by geologists to understand the stress direction during earthquakes, and low-frequency electromagnetic signals are emitted when rocks are fractured.

Many engineering ductile materials, such as metals, show inelastic behavior with corresponding large deformations. For these materials, the prediction of failure, defined as local material separation, is still nowadays a scientific challenge. Failure initiation, its locus and evolution, has been discussed extensively in recent literature. This work presents a bifailure specimen, especially developed to evaluate failure criteria at high and low triaxiality. In order to explore further the failure phenomenon, uncoupled models are numerically implemented in an explicit finite element code. Simulations were performed and do indicate that, for the studied examples, uncoupled models are not capable of correctly predicting failure. [ABSTRACT FROM AUTHOR]