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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Coronal heating by the resonant absorption of Alfven waves: The effect of viscous stress tensor

The time-dependent linearized magnetohydrodynamics (MHD) equations for a fully compressible, low-beta, viscoresistive plasma are solved numerically using an implicit integration scheme. The full viscosity stress tensor (Braginskii 1965) is included with the five parameters eta(sub i) i = 0 to 4. In agreement with previous studies, the numerical simulations demonstrate that the dissipation on inhomogeneities in the background Alfven speed occurs in a narrow resonant layer. For an active region in the solar corona the values of eta(sub i) are eta(sub o) = 0.65 g/cm/s, eta(sub 1) = 3.7 x 10(exp -12) g/cm/s, eta(sub 2) = 4 eta(sub 1), eta(sub 3) = 1.4 x 10(exp -6) g/cm/s, eta(sub 4) = 2 eta(sub 3), with n = 10(exp 10)/cu cm, T = 2 x 10(exp 6) K, and B = 100 G. When the Lundquist number S = 10(exp 4) and R(sub 1) much greater than S (where R(sub 1) is the dimensionless shear viscous number) the width of the resistive dissipation layer d(sub r) is 0.22a (where a is the density gradient length scale) and d(sub r) approximately S(exp -1/3). When S much greater than R(sub 1) the shear viscous dissipation layer width d(sub r) scales as R(sub 1)(exp -1/3). The shear viscous and the resistive dissipation occurs in an overlapping narrow region, and the total heating rate is independent of the value of the dissipation parameters in agreement with previous studies. Consequently, the maximum values of the perpendicular velocity and perpendicular magnetic field scale as R(sub 1)(exp -1/3). It is evident from the simulations that for solar parameters the heating due to the compressive viscosity (R(sub 0) = 560) is negligible compared to the resistive and the shear viscous (R(sub 1)) dissipation and it occurs in a broad layer of order a in width. In the solar corona with S approximately equals 10(exp 4) and R(sub 1) approximately equals 10(exp 14) (as calculated from the Braginskii expressions), the shear viscous resonant heating is of comparable magnitude to the resistive resonant heating.

Ofman, L.↗
NASA NTRS1961

Experimental Investigation at Mach Number 3.0 of the Effects of Thermal Stress and Buckling on the Flutter of Four-Bay Aluminum Alloy Panels with Length-Width Ratios of 10

Skin-stiffener aluminum alloy panels consisting of four bays, each bay having a length-width ratio of 10, were tested at a Mach number of 3.0 at dynamic pressures ranging from 1,500 psf to 5,000 psf and at stagnation temperatures from 300 F to 655 F. The panels were restrained by the supporting structure in such a manner that partial thermal expansion of the skins could occur in both the longitudinal and lateral directions. A boundary faired through the experimental flutter points consisted of a flat-panel portion, a buckled-panel portion, and a transition point at the intersection of the two boundaries. In the region where a panel must be flat when flutter occurs, an increase in panel skin temperature (or midplane compressive stress) makes the panel more susceptible to flutter. In the region where a panel must be buckled when flutter occurs, the flutter trend is reversed. This reversal in trend is attributed to the panel postbuckling behavior.

Dixon, Sidney C.↗
NASA NTRS1988

Effects of mechanical stress or abscisic acid on growth, water status and leaf abscisic acid content of eggplant seedlings

Container-grown eggplant (Solanum melongena L. var esculentum Nees. 'Burpee's Black Beauty') seedlings were conditioned with brief, periodic mechanical stress or abscisic acid (ABA) in a greenhouse prior to outdoor exposure. Mechanical stress consisted of seismic (shaking) or thigmic (stem flexing) treatment. Exogenous ABA (10(-3) or 10(-4)M) was applied as a soil drench 3 days prior to outdoor transfer. During conditioning, only thigmic stress reduced stem elongation and only 10(-3) M ABA reduced relative growth rate (RGR). Both conditioning treatments increased leaf specific chlorophyll content, but mechanical stress did not affect leaf ABA content. Outdoor exposure of unconditioned eggplant seedlings decreased RGR and leaf-specific chlorophyll content, but tended to increase leaf ABA content relative to that of plants maintained in the greenhouse. Conditioning did not affect RGR of plants subsequently transferred outdoors, but did reduce stem growth. Seismic stress applied in the greenhouse reduced dry weight gain by plants subsequently transferred outdoors. Mechanical stress treatments increased leaf water potential by 18-25% relative to that of untreated plants.

NASA Discipline Plant Biology↗
NASA NTRS2016

A Piezoelectric Shear Stress Sensor

In this paper, a piezoelectric sensor with a floating element was developed for shear stress measurement. The piezoelectric sensor was designed to detect the pure shear stress suppressing effects of normal stress generated from the vortex lift-up by applying opposite poling vectors to the: piezoelectric elements. The sensor was first calibrated in the lab by applying shear forces and it showed high sensitivity to shear stress (=91.3 +/- 2.1 pC/Pa) due to the high piezoelectric coefficients of PMN-33%PT (d31=-1330 pC/N). The sensor also showed almost no sensitivity to normal stress (less than 1.2 pC/Pa) because of the electromechanical symmetry of the device. The usable frequency range of the sensor is 0-800 Hz. Keywords: Piezoelectric sensor, shear stress, floating element, electromechanical symmetry

Kim, Taeyang↗
DOE OSTI2022

Effect of Applied Stress on the Selective Oxidation of Alloy 600 Exposed to PWR Primary Water and Rhines Pack Environments

A mechanistic understanding of corrosion and oxidation processes is crucial to ensure long-term resistance to stress corrosion cracking of Ni-base structural alloys in pressurized water reactor (PWR) primary and secondary systems. Aqueous corrosion of Ni-base alloys under PWR primary water conditions was previously compared to gaseous oxidation with comparable oxygen potentials. However, those studies considered only stress-free conditions, which removes a critical parameter for understanding stress corrosion behaviors. A significant knowledge gap exists for the role of applied stresses on selective internal oxidation in gaseous environments. A micro-4-point-bend jig was designed to facilitate stress in a Rhines Pack set-up. Selective oxidation at different stress and strain conditions was investigated in a series of Rhines Pack experiments for Alloy 600. Focused ion beam milling enabled the extraction of site-specific specimens containing selected high-energy grain boundaries from tensile and compressive stress regions. Analytical transmission electron microscopy was employed to analyze the microstructure and chemical composition of the resulting oxides. Protective surface oxide films and selective intergranular oxides were observed at different conditions. The implications of these fundamental differences are discussed relative to intergranular attack and stress corrosion cracking mechanisms in PWR primary water.

stress, 4-point-bend, selective oxidation, Alloy 6↗
NASA NTRS2003

A dynamic model of stress and sustained attention

This paper examines the effects of stress on sustained attention. With recognition of the task itself as the major source of cognitive stress, a dynamic model is presented that addresses the effects of stress on vigilance and, potentially, a wide variety of attention performance tasks.

Review↗
NASA NTRS1986

Effect of texture on magnetoacoustic stress measurement in steel

The effect of texture on the general stress measurements of a heavily cold rolled steel plate is analyzed. Changes in the acoustic natural velocity for plate samples with their long axes perpendicular, parallel, and in a 40 degree direction with respect to the rolling axis were measured; graphs of the velocity variations for the three samples are provided. It is observed that the negative minimum change in the acoustic natural velocity under residual compression for the perpendicular sample is about half that of the parallel and 40 degree samples indicating that the functional form of the velocity curve is influenced by the structural properties of the sample; however, the stress measurements are unaffected by the texture.

Namkung, M.↗
NASA NTRS1979

An analysis of thermal stress and gas bending effects on vibrations of compressor rotor stages

A preliminary study conducted by Kerrebrock et al. (1976) has shown that the torsional rigidity of untwisted thin blades of a transonic compressor can be reduced significantly by transient thermal stresses. The aerodynamic loads have various effects on blade vibration. One effect is that gas bending loads may result in a bending-torsion coupling which may change the characteristics of the torsion and bending vibration of the blade. For a general study of transient-temperature distribution within a rotor stage, a finite-element heat-conduction analysis was developed. The blade and shroud are divided into annular elements. With a temperature distribution obtained from the heat-conduction analysis and a prescribed gas bending load distribution along the blade span, the static deformation and moment distributions of the blade can be solved iteratively using the finite-element method. The reduction of the torsional rigidity of pretwisted blades caused by the thermal stress effect is then computed. The dynamic behavior of the blade is studied by a modified Galerkin's method.

Chen, L.-T.↗
NASA NTRS1988

The stimulation of arachidonic acid metabolism in human platelets by hydrodynamic stresses

The effects of stimulating human platelets by thrombin and by hydrodynamic stresses on the platelets' arachidonic acid metabolism were investigated using (1-C-14)-arachidonic acid label and a specially designed viscometer that ensured laminar shear flow with a nearly uniform shear rate throughout the flow region. It was found that platelets activated by thrombin formed principally thromboxane A2, 12-hydroxy 5,8,10-heptadecatrienoic acid and 12-hydroxy 5,8,10,14-eicosatetraenoic acid (12-HETE). On the other hand, platelets activated by shear, formed only 12-HETE (although arachidonic acid metabolism was stimulated); no cyclooxygenase metabolites were detected. Results indicate that platelets may greatly increase their 12-HETE production when activated by passage through a high-stress region of the circulation, such as an atherosclerotic stenosis.

Rajagopalan, Sridhar↗
DOE OSTI2020

Investigation of Bone Growth in Additive-Manufactured Pedicle Screw Implant by Using Ti-6Al-4V and Bioactive Glass Powder Composite

In this study, we optimized the geometry and composition of additive-manufactured pedicle screws. Metal powders of titanium-aluminum-vanadium (Ti-6Al-4V) were mixed with reactive glass-ceramic biomaterials of bioactive glass (BG) powders. To optimize the geometry of pedicle screws, we applied a novel numerical approach to proposing the optimal shape of the healing chamber to promote biological healing. We examined the geometry and composition effects of pedicle screw implants on the interfacial autologous bone attachment and bone graft incorporation through in vivo studies. The addition of an optimal amount of BG to Ti-6Al-4V leads to a lower elastic modulus of the ceramic-metal composite material, effectively reducing the stress-shielding effects. Pedicle screw implants with optimal shape design and made of the composite material of Ti-6Al-4V doped with BG fabricated through additive manufacturing exhibit greater osseointegration and a more rapid bone volume fraction during the fracture healing process 120 days after implantation, per in vivo studies.

stress-shielding effect↗
NASA NTRS1982

A finite element stress analysis of spur gears including fillet radii and rim thickness effects

Spur gear stress analysis results are presented for a variety of loading conditions, support conditions, fillet radii, and rim thickness. These results are obtained using the SAP IV finite-element code. The maximum stresses, occurring at the root surface, substantially increase with decreasing rim thickness for partially supported rims (that is, with loose-fitting hubs). For fully supported rims (that is, with tight-fitting hubs), the root surface stresses slightly decrease with decreasing rim thickness. The fillet radius is found to have a significant effect upon the maximum stresses at the root surface. These stresses increase with increasing fillet radius. The fillet radius has little effect upon the internal root section stresses.

Chang, S. H.↗
NASA NTRS2013

The Effect of Pre-Stressing on the Static Indentation Load Capacity of the Superelastic 60NiTi

Superelastic nickel-titanium alloys, such as 60NiTi (60Ni-40Ti by wt.%), are under development for use in mechanical components like rolling element bearings and gears. Compared to traditional bearing steels, these intermetallic alloys, when properly heat-treated, are hard but exhibit much lower elastic modulus (approx.100 GPa) and a much broader elastic deformation range (approx.3 percent or more). These material characteristics lead to high indentation static load capacity, which is important for certain applications especially space mechanisms. To ensure the maximum degree of elastic behavior, superelastic materials must be pre-stressed, a process referred to as "training" in shape memory effect (SME) terminology, at loads and stresses beyond expected use conditions. In this paper, static indentation load capacity tests are employed to assess the effects of pre-stressing on elastic response behavior of 60NiTi. The static load capacity is measured by pressing 12.7 mm diameter ceramic Si3N4 balls into highly polished, hardened 60NiTi flat plates that have previously been exposed to varying levels of pre-stress (up to 2.7 GPa) to determine the load that results in shallow but measurable (0.6 m, 25 in. deep) permanent dents. Hertz stress calculations are used to estimate contact stress. Without exposure to pre-stress, the 60NiTi surface can withstand an approximately 3400 kN load before significant denting (>0.4 m deep) occurs. When pre-stressed to 2.7 GPa, a static load of 4900 kN is required to achieve a comparable dent, a 30 percent increase. These results suggest that stressing contact surfaces prior to use enhances the static indentation load capacity of the superelastic 60NiTi. This approach may be adaptable to the engineering and manufacture of highly resilient mechanical components such as rolling element bearings.

DellaCorte, Christopher↗
NASA NTRS1992

Effect of tensile mean stress on fatigue behavior of single-crystal and directionally solidified superalloys

Two nickel base superalloys, single crystal PWA 1480 and directionally solidified MAR-M 246 + Hf, were studied in view of the potential usage of the former and usage of the latter as blade materials for the turbomachinery of the Space Shuttle main engine. The baseline zero mean stress (ZMS) fatigue life (FL) behavior of these superalloys was established, and then the effect of tensile mean stress (TMS) on their FL behavior was characterized. A stress range based FL prediction approach was used to characterize both the ZMS and TMS fatigue data. In the past, several researchers have developed methods to account for the detrimental effect of tensile mean stress on the FL for polycrystalline engineering alloys. These methods were applied to characterize the TMS fatigue data of single crystal PWA 1480 and directionally solidified MAR-M 246 + Hf and were found to be unsatisfactory. Therefore, a method of accounting for the TMS effect on FL, that is based on a technique proposed by Heidmann and Manson was developed to characterize the TMS fatigue data of these superalloys. Details of this method and its relationship to the conventionally used mean stress methods in FL prediction are discussed.

Kalluri, Sreeramesh↗
NASA NTRS1990

Effect of tensile mean stress on fatigue behavior of single-crystal and directionally solidified superalloys

Two nickel base superalloys, single crystal PWA 1480 and directionally solidified MAR-M 246 + Hf, were studied in view of the potential usage of the former and usage of the latter as blade materials for the turbomachinery of the space shuttle main engine. The baseline zero mean stress (ZMS) fatigue life (FL) behavior of these superalloys was established, and then the effect of tensile mean stress (TMS) on their FL behavior was characterized. At room temperature these superalloys have lower ductilities and higher strengths than most polycrystalline engineering alloys. The cycle stress-strain response was thus nominally elastic in most of the fatigue tests. Therefore, a stress range based FL prediction approach was used to characterize both the ZMS and TMS fatigue data. In the past, several researchers have developed methods to account for the detrimental effect of tensile mean stress on the FL for polycrystalline engineering alloys. However, the applicability of these methods to single crystal and directionally solidified superalloys has not been established. In this study, these methods were applied to characterize the TMS fatigue data of single crystal PWA 1480 and directionally solidified MAR-M 246 + Hf and were found to be unsatisfactory. Therefore, a method of accounting for the TMS effect on FL, that is based on a technique proposed by Heidmann and Manson was developed to characterize the TMS fatigue data of these superalloys. Details of this method and its relationship to the conventionally used mean stress methods in FL prediction are discussed.

Kalluri, Sreeramesh↗
NASA NTRS1976

Effect of laminate construction on residual stresses in composites

The effects of ply stacking sequence and ply orientation on the magnitude of lamination residual stresses in graphite/polyimide angle-ply laminates was studied. Embedded strain gage techniques were used to measure thermal strains in laminates of different layup, and residual strains were determined by comparing thermal strains in the angle-ply laminates with those of a unidirectional laminate. The ply stacking sequence did not have an effect on the magnitude of residual strains.

Daniel, I. M.↗