Direct Stress Measurements Research Articles

Measurement of internal stress in Radiata pine sapwood during drying using an improved online sensor

Abstract Internal stress in wood as a result of kiln drying is an important factor that affects the quality of the final product. Internal stress is inherently difficult to measure and it is usually either assessed through destructive sample evaluation after drying, or is predictively modelled using extensive material property relations which require much effort to obtain. This study describes improvements of the design, accuracy, and precision of a previously constructed stress sensor for the direct measurement of internal stress in wood during drying. The sensor is designed to be placed directly into a wood block and it records online internal uniaxial stress. Lurethane® was found to be the best material for transmitting internal stress to the sensor because reduced creep was observed, compared with Teflon® which was originally used. Stress modelling showed that a linear relation could be theoretically obtained between the stress from the sensor and the actual primary axis stress that is likely to occur in an intact wood block. Accordingly, the response of a sensor placed in a certain area could be successfully calibrated. However, rotation of the sensor away from the primary stress axis caused a decrease in recorded stress but did not affect stress trends during drying, provided the angle remained constant. Finally, stress results from drying Pinus radiata D. Don at 70°C are also described and discussed.

XRD total pattern fitting applied to study of microstructure of TiO2films

New XRD total pattern fitting softwareMSTRUCTwas used to study the microstructure of magnetron-deposited TiO2thin films.MSTRUCTis an extension of theFOXprogram for structure determination from powder diffraction data.MSTRUCTmakes corrections for refraction and absorption, residual stress, and preferred orientation that are necessary for thin-film analysis using the parallel-beam geometry and an asymmetric detector scan with small angles of incidence. The program also corrects for crystallite size broadening in terms of log-normal distribution, two models of strain (phenomenological and dislocation models), as well as the influence of stacking faults in the most common cubic and hexagonal structures. The microstructure results obtained by this study show that during crystallization of the amorphous TiO2films, tensile stresses were generated resulting in anisotropic shifts of diffraction peaks. This was confirmed byin situcrystallization and direct stress measurements. The consideration of the stress effect in terms of the weighted Reuss-Voigt model improved the fits significantly. The stresses were found to depend systematically on the TiO2film thickness, and their values determined by total pattern fitting agree well with the values measured directly by XRD stress analysis.

Texture and Strain Experiments at OPAL

In response to the development of new materials and the application of materials and components in new technologies the direct measurement, calculation and evaluation of textures and residual stresses has gained worldwide significance in recent years. Non-destructive analysis for phase specific residual stresses and textures is only possible by means of diffraction methods. The determination of global texture and the local variation of texture for example by inhomogeneous deformation are very important due to the coherence between the texture and the physical and mechanical properties of materials.

Response of buried steel pipelines subjected to relative axial soil movement

The performance of buried steel pipelines subjected to relative soil movements in the axial direction was investigated using full-scale pullout testing in a soil chamber. Measured axial soil loads from pullout testing of pipes buried in loose dry sand were comparable to those predicted using guidelines commonly used in practice. The peak values of axial pullout resistance observed on pipes buried in dense dry sand were several-fold (in excess of 2 times) higher than the predictions from guidelines; the observed high axial pullout resistance is primarily due to a significant increase in normal soil stresses on the pipelines, resulting from constrained dilation of dense sand during interface shear deformations. This reasoning was confirmed by direct measurement of soil stresses on pipes during full-scale testing and numerical modeling. The research findings herein suggest that the use of the coefficient of lateral earth pressure at-rest (K0) to compute axial soil loads, employing equations recommended in common guidelines, should be undertaken with caution for pipes buried in soils that are likely to experience significant shear-induced dilation.

Intraplate stress state from finite element modelling: The southern border of the Spanish Central System

An elastic finite element approach has been used with the dual aim of determining the most appropriate reference state of stress, namely a uniaxial strain state or a lithostatic state, and refining the understanding of the Iberian intraplate stresses. A cross-section model with an average crustal rheology and a flat topography has been analysed first in order to evaluate the influence of boundary conditions and rheological properties in the reference and tectonic stress states. The uniaxial and lithostatic states are obtained by including the overburden weight and a compressive horizontal load, which equals the uniaxial and lithostatic stress respectively, and provided that Poisson's ratio equals ~ 0.5 in the lithostatic state. On the other hand, a tectonic state with a σ H > σ V regime is reproduced by adding a horizontal constant load. Subsequently, constraints on the magnitude of the predicted Cenozoic stresses along a NW–SE cross-section in the southern border of the Spanish Central System (in the Variscan granitic basement of El Berrocal) have been estimated incorporating the topographic loading, lithological variations and the most recent far tectonic stresses. The deep geological structure has been established from gravity modelling and geological data. To simulate the active strike-slip to uniaxial extension regimes in the interior of Iberian Peninsula, a lithostatic initial state has to be considered and a tectonic load in the range of 15–20 MPa has to be applied. The gradient of maximum horizontal stress originated under these conditions is in the range of 30–35 MPa km − 1 . These results are in accordance with the estimated intraplate tectonic stress, the force along the convergent plate boundary of Eurasia–Africa, the lithospheric strength of Iberia, and the direct measurements of stresses.

Long-range internal stresses in monotonically and cyclically deformed metallic single crystals

Abstract Selected experimental measurements and theoretical predictions for the magnitude of long-range internal stress in monotonically and cyclically deformed metals are assessed and recently developed, spatially-resolved X-ray micro-beam techniques for direct measurements of long-range internal stress are discussed. The results of previously reported differential-aperture X-ray microscopy spatially-resolved measurements of long-range internal stress in dislocation-cell interiors in monotonically deformed copper are compared with predictions and analyses associated with the composite model of deformation. In addition, the results of volume-integrating X-ray line-profile measurements and spatially-resolved differential-aperture X-ray microscopy measurements of strains in <100> oriented copper single crystals that were cyclically deformed to pre-saturation (without persistent slip bands) are presented.

Some basic equations for the next generation of surface testers solving the problem of pileup, sink-in, and making area-function calibration obsolete

In the present paper the formulas for the stiffnesses of a new and more general surface tester concept are given and discussed. The concept is based on the idea that the next generation of surface testers will provide the means to use all degrees of freedom of movement a probe on a sample surface could perform. Thus, in addition to the ordinary normal stiffness, lateral and tilting stiffness are measured, as well as twisting stiffness, and then used in the subsequent parameter determination of the investigated materials. It is shown in the paper that such a concept would not only solve classical problems such as “pileup” and “sink-in” completely, but it would also supersede the need of area-function calibration for the indenter tips and allow direct measurement of local intrinsic and residual stresses, anisotropy, and many other things, too.

孔内計測について

Technologies, results and problems to be solved on the borehole measurements are briefly reviewed with particular attention to the measurement of crustal deformation. Detection of tilt and strain change associated with very low frequency tremors at depth in the southwestern Japan should be noted as one of the best topics. Reduction of noises due to meteorological and hydrological origins has been discussed for the past thirty years and remains still incomplete. In order to reduce such noises, deep borehole can be one of the solutions. Two major questions to the conventional hydraulic fracturing method have been raised for the past twenty years. Several kinds of stiff pressurizing system can be solutions for the measurement of crustal stress at depth shallower than 2 or 3 km. For the case deeper than 3 km, usual stress measurement system cannot be applied because of borehole breakouts or drilling induced fractures. In such cases, further development of measurement method is necessary for direct measurement of stress.

STUDY OF ACCURATE HIGH-SPEED TENSION TESTING METHOD

The high measurement accuracy in dynamic tension testing is required for designs and numerical simulations based on the accurate modeling of stress–strain relations at various strain-rates. The non-coaxial Hopkinson bar method (NCHBM) is one of the recently proposed methods for dynamic tension tests. In this study, the direct measurement of both stress and strain was conducted by using the strain gauges affixed on the special specimen with load cell part. The stress–strain relations obtained by this revised NCHBM were compared with those obtained by the normal NCHBM. In order to verify the experimental results, some numerical models were examined by using the FEM code LS-DYNA, in which the whole finite-element model of the apparatus and the plate-type specimen were made in detail. Furthermore, new appliances for fixing specimen were examined by experiment. The target material employed in this study is SUS316.

Image-based finite element modeling of alveolar epithelial cell injury during airway reopening

The acute respiratory distress syndrome (ARDS) is characterized by fluid accumulation in small pulmonary airways. The reopening of these fluid-filled airways involves the propagation of an air-liquid interface that exerts injurious hydrodynamic stresses on the epithelial cells (EpC) lining the airway walls. Previous experimental studies have demonstrated that these hydrodynamic stresses may cause rupture of the plasma membrane (i.e., cell necrosis) and have postulated that cell morphology plays a role in cell death. However, direct experimental measurement of stress and strain within the cell is intractable, and limited data are available on the mechanical response (i.e., deformation) of the epithelium during airway reopening. The goal of this study is to use image-based finite element models of cell deformation during airway reopening to investigate how cell morphology and mechanics influence the risk of cell injury/necrosis. Confocal microscopy images of EpC in subconfluent and confluent monolayers were used to generate morphologically accurate three-dimensional finite element models. Hydrodynamic stresses on the cells were calculated from boundary element solutions of bubble propagation in a fluid-filled parallel-plate flow channel. Results indicate that for equivalent cell mechanical properties and hydrodynamic load conditions, subconfluent cells develop higher membrane strains than confluent cells. Strain magnitudes were also found to decrease with increasing stiffness of the cell and membrane/cortex region but were most sensitive to changes in the cell's interior stiffness. These models may be useful in identifying pharmacological treatments that mitigate cell injury during airway reopening by altering specific biomechanical properties of the EpC.

A Sensor for Direct Measurement of Internal Stress in Wood During Drying: Experimental Tests Toward Industrial Application

In this study, an internal drying stress sensor was developed based on a silicon micromachined pressure gauge inserted in a cylindrical Teflon shell. Teflon provides protection and works as a medium between the gauge and wood. In this configuration, the pressure gauge has unidirectional, axis-selective sensitivity. When inserted in a drill hole at a given depth in the wood, the sensor can directly measure the compressive stress produced by shrinkage in the diameter of the hole. Based on research in the last 5 years, a preindustrial version has been developed that has been tested in laboratory drying experiments. Results show that the trend of the stress during the drying process can be visualized, and critical points can be determined such as stress reversal time and the effect of the conditioning phase on the internal stress. The sensors thus have potential to be used in the drying control on industrial scale once the remaining problems are solved.

EXPERIMENTAL DETERMINATION OF THE STRESS EVOLUTION IN A PREBENT STEEL VERY HIGH-PERFORMANCE CONCRETE BEAM BY MEANS OF AN ACTIVE STRESSMETER

This experimental research allowed to emphasize two main factors for the quality of the measurements by active stress-meters: on one hand, the accuracy of the location and the orientation of the jacks considering the severe conditions during the casting and in contrast, and the reliability with time of the embedded strain gages considering the possible presence of important electric disturbances in the environment. However, during 4 months, the quality of the strain signals used for the control turned out satisfactory and the obtained values are close to the theoretical computation. The evolution of the pressure during the same period shows a good regularity and a good agreement between the measurements and the values computed by the two-function method using the CEB-MC90 model code in its 1999 version for the prediction of concrete creep and shrinkage11 and the model of Yue and Taerwe10 for the creep recovery law (maximal difference of only 0.6 MPa after 4 months of recording). The aim of the direct measurement of stresses can be considered as reached, the prediction of the stress state in the beam according to the two-function method being validated with a precision of ±1.5% (uncertainty of about 1 MPa for a devise designed for a stress range up to 35 MPa).

Direct Measurements of Critical Stresses and Cracking in Thin Films of Colloid Dispersions

Useful films can be formed by drying colloidal dispersions, but the negative capillary pressure generated often promotes cracks. Complex lateral flows during drying compromised previous measurements of the pressure required for cracking. Here we report data for the onset of cracking, and the additional cracks that appear at higher pressures, from high-pressure ultrafiltration experiments on homogeneously compressed films. A comparison of the data with expectations from theory confirms that cracking is controlled by elastic recovery, though an energy criterion only provides a lower bound. Our experiments also identify the role of flaws as nucleation sites that initiate cracks.

High-energy synchrotron X-ray analysis of residual plastic strains induced in shot-peened steel plates

Shot peening has long been recognized as a method for treating metal surfaces in order to improve their resistance to crack initiation and propagation. In view of shot-peening control, it would be useful if methods for direct measurement or estimation of residual stresses and depths of deformed layers were available. The focus of the present paper is on the presentation of high-energy synchrotron X-ray diffraction that allows better understanding of shot-peening residual strain profiles. Furthermore, it attempts to show that considerable elucidation of shot-peening effects can be attained if plastic strain profiles ( eigenstrains) are considered, rather than residual stresses. Indeed, bending analysis shows that residual stresses depend on the sample geometry, and on the thickness in particular. In contrast, the experimental analysis presented in this paper suggests that the plastic strain profile induced in a particular material, on the other hand, is a function of the peening intensity alone and does not depend on the specimen thickness.

Gradient Heat Flux Sensors for Thermophysical Measurements

Opportunities and advantages of gradient heat flux sensors for direct heat flux measurement, temperature, and shear stress measurement are shown.

High pressures and asymmetrical stresses in the scoliotic disc in the absence of muscle loading.

BackgroundLoads acting on scoliotic spines are thought to be asymmetric and involved in progression of the scoliotic deformity; abnormal loading patterns lead to changes in bone and disc cell activity and hence to vertebral body and disc wedging. At present however there are no direct measurements of intradiscal stresses or pressures in scoliotic spines. The aim of this study was to obtain quantitative measurements of the intradiscal stress environment in scoliotic intervertebral discs and to determine if loads acting across the scoliotic spine are asymmetric. We performed in vivo measurements of stresses across the intervertebral disc in patients with scoliosis, both parallel (termed horizontal) and perpendicular (termed vertical) to the end plate, using a side mounted pressure transducer (stress profilometry)MethodsStress profilometry was used to measure horizontal and vertical stresses at 5 mm intervals across 25 intervertebral discs of 7 scoliotic patients during anterior reconstructive surgery. A state of hydrostatic pressure was defined by identical horizontal and vertical stresses for at least two consecutive readings. Results were compared with similar stress profiles measured during surgery across 10 discs of 4 spines with no lateral curvature and with data from the literature.ResultsProfiles across scoliotic discs were very different from those of normal, young, healthy discs of equivalent age previously presented in the literature. Hydrostatic pressure regions were only seen in 14/25 discs, extended only over a short distance. Non-scoliotic discs of equivalent age would be expected to show large centrally placed hydrostatic nuclear regions in all discs. Mean pressures were significantly greater (0.25 MPa) than those measured in other anaesthetised patients (<0.07 MPa). A stress peak was seen in the concave annulus in 13/25 discs. Stresses in the concave annulus were greater than in the convex annulus indicating asymmetric loading in these anaesthetised, recumbent patients.ConclusionIntradiscal pressures and stresses in scoliotic discs are abnormal, asymmetrical and high in magnitude even in the absence of significant applied muscle loading. The origin of these abnormal stresses is unclear.

A method for estimating wall friction in turbulent wall-bounded flows

We describe a simple method for estimating turbulent boundary layer wall friction using the fit of measured velocity data to a boundary layer model profile that extends the logarithmic profile all the way to the wall. Two models for the boundary layer profile are examined, the power-series interpolation scheme of Spalding and the Musker profile which is based on the eddy viscosity concept. The performance of the method is quantified using recent experimental data in zero pressure gradient flat-plate turbulent boundary layers, and favorable pressure gradient turbulent boundary layers in a pipe, for which independent measurements of wall shear are also available. Between the two model profiles tested, the Musker profile performs much better than the Spalding profile. Results show that the new procedure can provide highly accurate estimates of wall shear with a mean error of about 0.5% in friction velocity, or 1% in shear stress, an accuracy that is comparable to that from independent direct measurements of wall shear stress. An important advantage of the method is its ability to provide accurate estimates of wall shear not only based on many data points in a velocity profile but also very sparse data points in the velocity profile, including only a single data point such as that originating from a near-wall probe.

Measurement of dynamic full-field internal stresses through surface laser Doppler vibrometry

We present a method for evaluating internal dynamic stresses in a solid vibrating body from measurements of surface motion. The method relies on the same mathematics as boundary element method: A boundary reciprocity integral represents interior motion as a surface integral of boundary motion times the Green’s function. The surface motions are measured with a laser vibrometer rather than simulated, giving a direct measurement of internal motions and internal dynamic stresses. Experimental results on a flexing beam demonstrate that stresses measured in this fashion match those calculated from elementary theory.

Stress dependence of the Raman spectrum of polycrystalline barium titanate in presence of localized domain texture

The stress dependence of the Raman spectrum of polycrystalline barium titanate (BaTiO3, BT) ceramics has been examined with microprobe polarized Raman spectroscopy. The angular dependence of the Raman spectrum of the tetragonal BT crystal has been theoretically established, enabling us to assess the stress dependence of selected spectral modes without the influence of crystallographic domain orientation. Upon considering the frequency shift of selected Raman modes as a function of orientation between the crystallographic axis and the polarization vector of incident and scattered light, a suitable instrumental configuration has been selected, which allowed a direct residual stress measurement according to a modified piezospectroscopic procedure. The analysis is based on the selection of mixed photostimulated spectral modes in two perpendicular angular orientations.

Direct Measurement of Electromigration-Induced Stress in Interconnect Structures

This paper reports the first direct experimental measurement of electromigration-induced stress in aluminum interconnects, using a series of microrotating stress sensors. The build-up of stress gradients in interconnect metallization, which is concomitant with backstress, has been previously investigated theoretically, but experimental verification using optical or X-ray techniques has proven more difficult. These initial results show a compressive-stress gradient along the line of stress sensors, consistent with that predicted by conventional mass-transport theory. Additionally, these measurements are in qualitative agreement with the mathematical model proposed by Korhonen et al. (1993), with a reduced-stress and increased-diffusion coefficient. These differences are discussed, and the causes postulated. The limited resolution of previous techniques restricts their ability to obtain a detailed characterization, whereas, in principle, this new technique can be scaled to the end of the International Technology Roadmap for Semiconductors. These preliminary findings suggest that the presented technique will provide a valuable tool for the investigation of back-end-of-line interconnect stress in the future.