Compressive Stress Changes Research Articles

Three-dimensional constitutive relations for granular materials based on the dilatant double shearing mechanism and the concept of fabric

An extension of a three-dimensional model proposed by Anand and Gu (2000) for amorphous granular materials to include the effects of initial and induced anisotropy is presented in this paper. The proposed model can also be considered as a three-dimensional generalization of a model recently developed by Zhu et al. (2005) for the planar deformation of granular materials. The main ingredients of the model include the dilatant double shearing mechanism ( Spencer, 1964; Mehrabadi and Cowin, 1978), the concept of fabric ( Oda, 1972), and an extension of the Mohr–Coulomb yield criterion ( Shield, 1955; Spencer, 1982) to three dimensions. The constitutive equations are implemented in the finite element program ABAQUS/Explicit ( ABAQUS, 2001) by developing a user-material subroutine to conduct numerical triaxial compression tests for samples of granular materials with different initial anisotropy. The numerical results agree with the observed behavior and show that the extended constitutive model is capable of capturing the strength anisotropy of granular materials. Employing the anisotropic model developed here, we have also repeated the numerical simulation of the stress state in a static conical sand pile conducted earlier by Anand and Gu (2000). We find that fabric has little or no influence on the vertical stress distribution except at the base of the sand pile where the peak value of this stress is slightly higher than that predicted by the model of Anand and Gu (2000) which does not include the effects of fabric. We also find that the direction of the principal compressive stress changes from vertical at points away from the center of the pile to almost horizontal at points close to the center of the pile. This result provides a possible explanation for the observed dip in the vertical stress distribution in sand piles.

Calculation of the compression index and precompression stress from soil compression test data

Abstract When compressing soil, there is a characteristic relationship between compressive stress and volume change that can be used to define important soil mechanical properties. Two defining features can be determined – the compression index (Cc – the modulus of the slope of the linear virgin compression curve) and the precompression stress ( σ ′ p – the transition point between the elastic rebound curve and the virgin compression curve). These are indicators of compressibility and stress history respectively. The purpose of this paper is to evaluate different ways of estimating these indicators based on laboratory test data. Repacked soils with a range of textures were subjected to sequential compressions of 50, 100 and 200 kPa, which provided two compression characteristics with “known” σ ′ p of 50 and 100 kPa. Three functions were fitted to the measured test data (fourth-order polynomial, symmetrical logistic sigmoidal and asymmetrical Gompertz sigmoidal). Values of Cc were estimated by linear regression (for the data later fitted with a polynomial function) or by the tangent at the inflection point derived from model parameters (logistic and Gompertz functions). Three estimates of σ ′ p were calculated for each of the three functions: the standard Casagrande method (C), the intercept of the virgin compression curve and the initial (no stress) horizontal line (V–I), and the point of maximum curvature (MC) derived from the curvature function (κ). The accuracy of estimating σ ′ p and the magnitude of Cc generally increased with clay content. Estimates of Cc based on sigmoidal curves did not differ greatly from the linear regression estimate. Sigmoidal curves yielded σ ′ p estimates with lower absolute deviations from known values than polynomial-based estimates. The MC calculation based on the Gompertz function gave the most accurate estimate of σ ′ p . The lower asymptote of sigmoidal curves may also correspond to the water-filled pore space. Thus, despite the fact that all three functions fitted the measured data equally well, characteristics based on the sigmoidal curves were deemed to be most appropriate. The greater accuracy of the prediction of σ ′ p favoured the Gompertz function. Wider applicability of this was further checked with data selected from an independent database on subsoil compaction. We recommend fitting the Gompertz function to measured soil compression characteristic test data, and to define Cc objectively as the modulus of the slope of the tangent at the inflection point, providing this lies within the measured data range, and σ ′ p as the point of maximum curvature as defined by κ.

Influence of Proteoglycan on Time-Dependent Mechanical Behaviors of Articular Cartilage under Constant Total Compressive Deformation

Articular cartilage has biphasic property based on high water content. It is generally believed that the proteoglycan supports the compressive load, but the detailed loading mechanism has not yet been clarified. In this study, first we observed the changes in compressive stress and strain of articular cartilage under constant total compressive deflection. We evaluated the changes in modulus of elasticity, which was estimated from the stress-strain relation in equilibrium state. To examine the role of proteoglycan in compressed articular cartilage, we compared the time-dependent viscoelastic behaviors in both the intact cartilage and the cartilage treated with chondoroitinase ABC under constant total compressive deformation. We could confirm that the peak stress after compression and the modulus of elasticity at equilibrium were reduced after the digestion of proteoglycan. Next, we observed the changes in local strain in both articular cartilage specimens with and without chondroitinase treatment by monitoring the position of stained chondrocyte in the confocal laser scanning microscope. These visualized images indicated that the local strain changed time-dependently and depth-dependently. The digested cartilage showed the quicker change in movement and larger thinning in surface layer than the intact cartilage. These results indicate that the proteoglycan contributes to the compressive load-carrying capacity and controls the permeability.

Atmospheric Stability of E-Beam Deposited Optical Thin Film Stacks

ABSTRACTOptical thin films with SiO2-TiO2 stack were prepared by the technology of electron beam (e-beam) evaporation with ion beam assistant deposition (IBAD). The mechanical (stress) and optical properties of as-deposited thin films were studied as a function of exposure time in the atmospheric environment. Exposing to the air at the ambient temperature causes incremental compressive stress and spectrum profile changes, which is related to the absorption of water moisture into the films. Making a dense film is, therefore, a practical approach to improve structural stability of thin films and then the performance of optical devices.

The effect of bond coat grit blasting on the durability and thermally grown oxide stress in an electron beam physical vapor deposited thermal barrier coating

Photo-stimulated luminescence Piezo-spectroscopy (PLPS) is being developed as a non-destructive technique for thermal barrier coatings (TBC). In this study, the evolution of photo-stimulated luminescence with thermal cycling was systematically investigated from the thermally grown oxide (TGO) in a production TBC, which consists of an electron beam physical vapor deposited (EB-PVD) 7 wt.% Y 2O 3–ZrO 2 top coat, a grit blasted (Ni,Pt)Al bond coat and a CMSX-4 superalloy substrate. The change of compressive stress in the TGO layer on the bond coat with thermal cycling was calculated from the wavelength shift of the luminescence spectra. The compressive stress increased from 1.0–2.2 GPa in the as received state to 2.8–3.3 GPa at 10 cycles, then gradually decreased to 1.2–1.9 GPa until 500 cycles and remained at this level until TBC spallation. Other fluorescence spectra characteristics, such as the width of R1 and R2 peaks and their relative intensity, were also evaluated. These PLPS measurements on TBCs with grit blasted bond coats are compared with previous measurements on similar TBC system but with non-grit blasted bond coats. It is concluded that the initial increase in stress is associated with the formation of a continuous oxide layer. The lower stress of the specimens with the grit blasted bond coats compared to that of the as-coated bond coats is associated partly with the greater surface roughness. And the fast decline in compressive stress is the result of bond coat surface rumpling facilitated by the initially rougher surface. The lifetime of the TBCs with grit blasted bond coats varies over a narrow range, 600–750 cycles with an average of 675 cycles, which is related to their consistent bond coat surface roughness.

Mechanical initiation of intervertebral disc degeneration.

Mechanical testing of cadaveric lumbar motion segments. To test the hypothesis that minor damage to a vertebral body can lead to progressive disruption of the adjacent intervertebral disc. Disc degeneration involves gross structural disruption as well as cell-mediated changes in matrix composition, but there is little evidence concerning which comes first. Comparatively minor damage to a vertebral body is known to decompress the adjacent discs, and this may adversely affect both structure and cell function in the disc. In this study, 38 cadaveric lumbar motion segments (mean age, 51 years) were subjected to complex mechanical loading to simulate typical activities in vivo while the distribution of compressive stress in the disc matrix was measured using a pressure transducer mounted in a needle 1.3 mm in diameter. "Stress profiles" were repeated after a controlled compressive overload injury had reduced motion segment height by approximately 1%. Moderate repetitive loading, appropriate for the simulation of light manual labor, then was applied to the damaged specimens for approximately 4 hours, and stress profilometry was repeated a third time. Discs then were sectioned and photographed. Endplate damage reduced pressure in the adjacent nucleus pulposus by 25% +/- 27% and generated peaks of compressive stress in the anulus, usually posteriorly to the nucleus. Discs 50 to 70 years of age were affected the most. Repetitive loading further decompressed the nucleus and intensified stress concentrations in the anulus, especially in simulated lordotic postures. Sagittal plane sections of 15 of the discs showed an inwardly collapsing anulus in 9 discs, extreme outward bulging of the anulus in 11 discs, and complete radial fissures in 2 discs, 1 of which allowed posterior migration of nucleus pulposus. Comparisons with the results from tissue culture experiments indicated that the observed changes in matrix compressive stress would inhibit disc cell metabolism throughout the disc, and could lead to progressive deterioration of the matrix. Minor damage to a vertebral body endplate leads to progressive structural changes in the adjacent intervertebral discs.

Evolution of residual stress in plasma-enhanced chemical-vapor-deposited silicon dioxide film exposed to room air

Considerable increase in compressive stress was observed in silicon dioxide films upon prolonged exposure to atmospheric moisture. The compressive stress change upon exposure to atmosphere consisted of a reversible change and an irreversible change. The irreversible component was found to be due to structural changes in the Si–O–Si network. The reversible change in stress is due to water molecules absorbed on walls of the pores.

Tribological and optical properties of hydrogen-free amorphous carbon films with varying sp3/sp2 composition

Unhydrogenated amorphous carbon films were deposited at different temperatures on a single silicon wafer by a process known as Filtered Cathodic Vacuum Arc. A transition from diamond-like properties to graphitic properties was observed at the substrate temperature of 200 °C. This change of properties upon transition includes a change in compressive stress, frictional coefficient and density. The variation of results is consistent with the subplantation model, which emphasizes the role of carbon atoms trapped in the subsurface layers in the evolution of a dense sp3-rich phase at room temperature. At higher temperature (>200 °C) diffusion of carbon atoms to the surface of the evolving film releases internal stress and leads to the formation of a graphitic sp2 phase. Based on x-ray reflectivity measurements, it was found that a thin interlayer formed between the tetrahedral amorphous carbon film and the silicon substrate contributes greatly to the good fitting of the spectra. The thickness of this interlayer is independent of the temperature. The optical properties were found to undergo a more gradual transition with the deposition temperature. Copyright © 1999 John Wiley & Sons, Ltd.

N2+ Implantation Approaches for Improving Thermal Stability of Cu/Mo/Si Contact Structure

We have suggested N2+ ion modification method to improve the thermal stability of Mo thin film by implanting 3 ×1017 N2+ ions/cm2 with very low acceleration energy of 20 keV. The Mo film modified by N2+ ions (Mo–N2+) keeps microcrystalline after annealing at 600°C and performs excellent diffusion barrier against Cu atoms at 700°C for 30 min. The stress evolution of the Mo–N2+ thin film during the annealing process indicates that highly compressive stress changes to low tensile stress at 600 °C for 30 min.

Influence of yttrium implantation on growth stresses developed in FeO scales formed on pure iron at 800°C.

Abstract In-situ internal stress determinations by X-ray diffraction have been performed during pure iron oxidation ( p (O 2 )=2×10 −3 Pa, T =800°C). The compressive stress, initially present in a blank substrate, due to surface preparation, is completely released at 400°C. On yttrium implanted iron no stress is initially present in the substrate. Under oxidising conditions, the in-situ compressive stress level determined in the FeO scale during oxidation does not strongly depend on the presence of implanted yttrium. On blank and implanted specimens, the compressive stress changes from −400±80 to −150±100 MPa after 30 h oxidation. However, after cooling to room temperature, blank specimens show compressive residual stresses, while implanted samples show tensile residual stresses. Our results also indicate that epitaxial relationships between the oxide scale and the substrate can be a source of stress in an oxide scale. A way to lower compressive stresses can be the use of yttrium ion implantation.

Streaming potentials during the confined compression creep test of normal and proteoglycan-depleted cartilage.

The streaming potential response of cartilage in the confined compression creep configuration was assessed theoretically and measured experimentally in normal and proteoglycan-depleted tissue. The analytical solution, using the linear biphasic continuum model including electrokinetics and assuming homogeneous material properties, predicted that: (i) the peak streaming potential is delta V = ke x delta sigma, where ke is the electrokinetic coefficient and delta sigma is the change in compressive stress; (ii) the potential is maintained at 95 to 100% of the peak value for 0 < t < 0.10 tau, where tau is the gel diffusion time constant; and (iii) during short times, 0 < t < 0.01 tau, 90% of the peak streaming potential occurs over a region extending 23% into the tissue sample. Experimentally, adult bovine cartilage disks, 0.5 mm thick, were subjected to step changes of compressive stress. The measured changes in potential indicated a linear response for changes in stress up to 0.10 MPa. The ke of normal cartilage, estimated from the short time (0 < t < 2 sec) change in potential, was -1.65 +/- 1.25 mV/MPa. Digestion of cartilage by chondroitinase ABC resulted in an increased (less negative) ke of -0.75 +/- 0.70 mV/MPa and a 33 +/- 29% depletion of anionic glycosaminoglycan, whereas digestion with trypsin resulted in a further increase in ke to +1.74 +/- 0.95 mV/MPa and a 98 +/- 1 % depletion of glycosaminoglycan. The streaming potential measurement may be a useful addition to the widely used confined compression creep test to assess cartilage material properties.

On the influence of soil strength brittleness and nonlinearity on slope stability

Abstract The influence of soil shear strength brittleness and the nonlinearity of the shear strength envelope on factors of the safety of slope stability is investigated for two homogeneous slopes using polygonal and circular trial slip surfaces and three different procedures based on the limit equilibrium method. The results of the investigation show that a slower rate of post peak strength decrease in brittle soil could cause a larger difference in the factors calculated for brittle and ideally plastic soil. Also the nonlinearity of soil strength with changes in compressive stresses is less influential on the factors calculated for soil with a smaller basic angle of friction in drained conditions. For coarse grained soil this difference could be large and cause a discrepancy between the inferred shear strength at slope collapse and the measured strength in the laboratory.

Correlation of silica glass properties with the infrared spectra

Infrared (IR) spectroscopy has been used to interpret structural changes in silica glasses. Specifically, a shift of the Si–O stretching band in IR spectra is used to monitor changes in average Si–O–Si bond angle in the glass structure. A similar structural change is induced by the change of fictive temperature, hydrostatic pressure or compressive stress, with the average Si–O–Si bond angle decreasing with the increase of these parameters. It is anticipated that these similar structural changes would produce a similar change in glass properties. In order to confirm this expectation, HF etch rates of silica glasses were measured as a function of fictive temperature and stress. The experimental results on HF etch rates, together with changes in other glass properties in the literature, were compared with the change in glass structure revealed by IR spectroscopy. It was found that the similar structural change is accompanied by the consistent changes in a variety of glass properties. Monitoring the IR spectra of a silica glass sample, therefore, can be used to deduce changes in glass properties.

Reexamination of Service Load Lmit Compressive Stress in Prestressed Concrete Members

One of the main reasons for setting permissible stresses in prestressed concrete flexural members is to insure that serviceability criteria are met. However, the allowable compressive stress at service seems to be unjustified. The issue of allowable compressive stress has not been emphasized in the past, since it seldom controlled in design. But now the compressive stress limit has become a critical factor and more restrictive in modern designs, such as spliced girder bridges, long-span beams in office buildings, and segmental box girders. This paper discusses the proposed ACI 318-95 Building Code and AASHTO Bridge Specifications allowable compressive stress changes. It is suggested that the ACI allowable compressive stress be 0.60 f' c due to load combinations of transient nature, plus the corresponding prestress forces, and 0.45 f' c due to effective prestress plus permanent (dead) loads. The reasons for the proposed changes and several examples are presented. It is shown that these changes do not adversely affect the acceptable safety margin against compression failure or deflection and fatigue limits

Electrical Resistance and Stress of Bilayer Co(P)/Cu and Ni(P)/Cu Thin Films

The electrical resistance of the metallic bilayer was measured in situ using a resistance furnace. In the isothermal resistance measurements the sheet resistance was measured as a function of time, up to 20 h, at the constant temperature of 400°C. Results show that the sheet resistance of Co(P)/Cu system stays constant for 19 h, while that for the Ni(P)/Cu system continuously changes with time. In the dynamic resistance measurements the sheet resistance was measured as a function of temperature, from 25 to 500°C at the heating rate of 10°C/min. Results show that the dependence of the sheet resistance on temperature for the bilayer Ni(P)/Cu and Co(P)/Cu thin films is at first linear and then deviates from linearity as the temperature increases from room temperature to 500°C. This dependence exhibited irreversibility. The stress of the bimetallic bilayers was measured in situ as a function of temperature in the range from 50 to 500°C using a modified Flexus Z2400S apparatus. Results show that the initial stress at room temperature is compressive. During the first heating cycle the stress state remains compressive and the stress level at the maximum temperature of 450°C is slightly higher than the stress in the as‐deposited film. During the cooling cycle the compressive stress changes into the tensile and continues to accumulate with the decreasing temperature, reaching a maximum value at room temperature.

地震に伴う地下水・地球化学現象

Hydro-geochemical anomalies associated with large earthquakes are reviewed and discussed from the viewpoints of (1) the response of confined groundwater to minute deformations of crustal rocks, (2) ground-water anomalies related to earthquake source mechanisms, (3) origins of groundwaters and dissolved gases in them, and (4) factors to cause groundwater gas anomalies.Theoretical models for deformations of water-saturated porous or cracked rocks under the undrained condition indicate that compressive and extensive stress changes induce increasing and decreasing changes in the pore water pressure, respectively. The pore pressure change is to be the same order of the external stress change, but the maximum change of pore pressure cannot exceed the stress change. These are consistent with observed responses of confined groundwaters to the earth tides.Coseismic groundwater anomalies accompanied by the 1946 Nankaido earthquake and the 1923 Great Kwanto earthquake are directly related to their residual fields of megathrusting faults. Preseismic groundwater anomalies of these great earthquakes commonly suggest that megathrusting fault motions began to start preceding main seismic events.Chemical compositions of groundwater gases are controlled by admixing of the dissolved atmospheric air with the subsurface gases enriched in CH4, He and others. Groundwater gas anomalies can be described by the change in mixing ratio of the two different gases and/or by the compositional change of the admixing subsurface gas itself. Such an admixing process could be affected by anomalous pore pressure distributions induced by the deformation of crustal rocks related to earthquakes. H2 gas anomalies are distinctive among subsurface gas anomalies reported so far, but the origins are not fully understood. The characteristics of H2 anomalies are discussed.

Compression testing of carbon fibre-reinforced plastics exposed to humid environments

A compression test jig has been developed which may be used in the measurement of the compressive strength of both unidirectional and multidirectional carbon fibre-reinforced plastics. The jig has the merit that small specimens may be tested without the necessity of end-tabbing. This feature makes its use in test programmes where the material undergoes environmental exposure prior to tests particularly advantageous. The results of tests on the compressive strength of waisted unidirectional specimens support the utility of the test technique in comparison with alternative methods. The results of compression tests on unwaisted specimens of multidirectional material demonstrates that the test technique is able to detect changes in compressive stress due to environmental exposure and test conditions.

The kinetics of chemically induced nonequilibrium swelling of articular cartilage and corneal stroma.

An electromechanical model for charged, hydrated tissues is developed to predict the kinetics of changes in swelling and isometric compressive stress induced by changes in bath salt concentration. The model focuses on ionic transport as the rate limiting step in chemically modulating electrical interactions between the charged macromolecules of the extracellular matrix. The swelling response to such changes in local interaction forces is determined by the relative rates of chemical diffusion and fluid redistribution in the tissue sample. We have tested the model by comparing the experimentally observed salt-induced stress relaxation response in bovine articular cartilage and corneal stroma to the response predicted by the model using constitutive relations for the concentration dependent material properties of the tissues reported in a related study. The qualitatively good agreement between our experimental measurements and the predictions of the model supports the physical basis of the model and demonstrates the model's ability to discriminate between the two soft connective tissues that were examined.

爆発硬化した高マンガンオーステナイト鋳鋼繰返し衝撃材の残留応力

Explosion hardened high manganese austenitic cast steel is being tried for rail crossing recently. From the previous studies, it became clear that high tensile residual stress was generated in the hardened surface layer by explosion and the surface tensile residual stress changed into the compressive stress of about 500MPa after the repetition of impact.In this study, the distribution of biaxial residual stress in the explosion hardened high manganese austenitic cast steel subjected to repeated impact loads was examined and compared with those of original and shot peened steels.The results obtained are summarized as follows.(1) The residual stress distributions in the impact surface layer of the original, explosion hardened and shot peened steels were similar qualitatively as given in the following. The surface compressive stress changes into tension at the depth of 0.01mm and the tensile stress in the surface layer shows a remarkable variation as the depth increases and changes into compressive stress in the inner layer.(2) It seems that the residual stress distribution mentioned above is produced by the combined effects of surface hardened layer, the transverse deformation in impact surface and uneven deformation at hardened layer.(3) The high tensile residual stress in the explosion hardened surface layer did not increase by repeated impact loading, and no macrocrack was osberved in the impact surface layer.

A Study of Metal Flow Ahead of Tool Face With Large Negative Rake Angle

Although the chip formation mechanism by a tool having a large negative rake angle is not well known, it is very important to make the process clear in order to get high quality in finished surfaces. In this paper, the behavior of material ahead of a tool face with a large negative rake angle is examined by means of low speed machining on lead. The deformation process of the material is investigated by the deformation study combining a finite element method with a grid line method. During cutting, the deformation process of grid lines which were drawn on the sides of testpieces was observed through a side glass which restricted the side flow of material. Cutting force was measured by a dynamometer consisting of an elongated octagonal ring with strain wire gages. As a result it was found that the shear stress on the slip line of maximum increment of shear strain is nearly constant, but the compressive stress changes along the line. It was concave near the top of cutting edge and convex near the surface of the test piece. The position of the change of polarity in the slope shifted depending on the rake angle of the tool. This phenomenon is considered to have close relation with the stagnant tip, which decides not only the size of chip, but also whether or not a chip will be formed. Flow lines of material and the deformed region ahead of tool faces with different negative rake angles were also obtained and they were compared with each other.