Increase In Critical Shear Stress Research Articles

Effect of precipitates on hardening of 17-4PH martensitic stainless steel serviced at 300 °C in nuclear power plant

When 17-4PH martensitic stainless steel serviced at 300 °C for long term in nuclear power plant, the thermal aging embrittlement and hardening will be significant. It will seriously affect the safety and economic operation of nuclear power plant. In this work, Three-Dimensional Atom Probe Technology (3DAPT) was used to characterize the element distribution, and the evolution of Cu-rich precipitate and Cr-rich precipitate with service time were analyzed. The results show that with the service time increasing, the number density and size of Cu-rich precipitate and Cr-rich precipitate in the material increase obviously, which leads to the increase of critical shear stress with dislocation. The main reason to hardening is the nucleation and growth of Cu-rich precipitates and Cr-rich precipitates, which the contribution of Cu-rich precipitates fits Russell-Brown model. The contribution of Cr-rich precipitates to hardening increases with the service time, and is greater than that of Cu-rich precipitates.

Experimental investigation on hole erosion behaviors of chemical stabilizer treated soil

Soil erosion plays a vital role in the performance of hydraulic projections such as earth dams and embankments. To improve the erosion resistance of the soil is crucial for preventing the dam break and assuring the safety of the hydraulic projections. In this study, three chemical stabilizers, i.e., sodium silicate (SS) solution, lignosulfonate (LS) powder, and sodium polyacrylate (PAANa), are used to treat the sandy soil. A set of hole erosion tests to investigate the erosion behaviors in treated and untreated specimens. The experiments collect flux and the mass of eroded particles during the erosion process. These data then yield the relations between hydraulic shear stress and erosion rate. Erosion coefficients, i.e., the critical shear stress (CSS), and the erosion rate index (ERI), are acquired from linearly fitted relation curves between the hydraulic shear stress and the erosion rate. During the erosion experiments, we observe that the erosion continues in untreated specimens and leads to the collapse of the specimens if the experiment is not stopped. On the other hand, erosion in treated specimens stops automatically after a long erosion time, even if the hydraulic gradient is maintained. These results indicate that the stabilizers improve the anti-erosion capability of the soil specimen. The improvement becomes significantly noticeable if the stabilizer content and the curing time increase. Among the three stabilizers tested in the experiments, PAANa performs best in treating the sandy soil since the rate of increase in CSS and ERI reaches 214.4% and 85.4%, respectively, when 1.5% PAANa is added into the specimen. The rates of increase in CSS and ERI are 23.2% and 43.8%, respectively, for the specimens treated with lignosulfonate, given the same other conditions. Based on the results, the choice of the stabilizer is discussed and suggested.

Surface-erosion behaviour of biopolymer-treated soils assessed by EFA

Exocultured biopolymers are ecofriendly soil-stabilisation agents with superior particle bonding, hydrogel-formation characteristics and zero endoculture duration. However, the use of exocultured biopolymers for enhancing soil resistance against surface erosion by water flow is yet to be investigated. Using erosion function apparatus (EFA) in combination with an ultrasonic P-wave reflection monitoring device, the effect of exocultured biopolymers on the erosion parameters of critical shear stress and the erodibility coefficient was examined in this study in soils with different particle distributions. In this way, biopolymer soil treatment showed a ten-fold increase in critical shear stress along with a 90% reduction in erodibility coefficient; results which could be attributed to enhanced particle-to-particle contact and increased pore-fluid viscosity and pore clogging. The results of this study demonstrate the feasibility of using exocultured biopolymers in mitigating surface erosion of erosion-prone soils.

Erosion characteristics and horizontal variability for small erosion depths in the Sacramento-San Joaquin River Delta, California, USA

Erodibility of cohesive sediment in the Sacramento-San Joaquin River Delta (Delta) was investigated with an erosion microcosm. Erosion depths in the Delta and in the microcosm were estimated to be about one floc diameter over a range of shear stresses and times comparable to half of a typical tidal cycle. Using the conventional assumption of horizontally homogeneous bed sediment, data from 27 of 34 microcosm experiments indicate that the erosion rate coefficient increased as eroded mass increased, contrary to theory. We believe that small erosion depths, erosion rate coefficient deviation from theory, and visual observation of horizontally varying biota and texture at the sediment surface indicate that erosion cannot solely be a function of depth but must also vary horizontally. We test this hypothesis by developing a simple numerical model that includes horizontal heterogeneity, use it to develop an artificial time series of suspended-sediment concentration (SSC) in an erosion microcosm, then analyze that time series assuming horizontal homogeneity. A shear vane was used to estimate that the horizontal standard deviation of critical shear stress was about 30% of the mean value at a site in the Delta. The numerical model of the erosion microcosm included a normal distribution of initial critical shear stress, a linear increase in critical shear stress with eroded mass, an exponential decrease of erosion rate coefficient with eroded mass, and a stepped increase in applied shear stress. The maximum SSC for each step increased gradually, thus confounding identification of a single well-defined critical shear stress as encountered with the empirical data. Analysis of the artificial SSC time series with the assumption of a homogeneous bed reproduced the original profile of critical shear stress, but the erosion rate coefficient increased with eroded mass, similar to the empirical data. Thus, the numerical experiment confirms the small-depth erosion hypothesis. A linear model of critical shear stress and eroded mass is proposed to simulate small-depth erosion, assuming that the applied and critical shear stresses quickly reach equilibrium.

Non-additive increases in sediment stability are generated by macroinvertebrate species interactions in laboratory streams.

Previous studies have shown that biological structures such as plant roots can have large impacts on landscape morphodynamics, and that physical models that do not incorporate biology can generate qualitatively incorrect predictions of sediment transport. However, work to date has focused almost entirely on the impacts of single, usually dominant, species. Here we ask whether multiple, coexisting species of hydropsychid caddisfly larvae have different impacts on sediment mobility compared to single-species systems due to competitive interactions and niche differences. We manipulated the presence of two common species of net-spinning caddisfly (Ceratopsyche oslari, Arctopsyche californica) in laboratory mesocosms and measured how their silk filtration nets influence the critical shear stress required to initiate sediment grain motion when they were in monoculture versus polyculture. We found that critical shear stress increases non-additively in polycultures where species were allowed to interact. Critical shear stress was 26% higher in multi-species assemblages compared to the average single-species monoculture, and 21% greater than levels of stability achieved by the species having the largest impact on sediment motion in monoculture. Supplementary behavioral experiments suggest the non-additive increase in critical shear stress may have occurred as competition among species led to shifts in the spatial distribution of the two populations and complementary habitat use. To explore the implications of these results for field conditions, we used results from the laboratory study to parameterize a common model of sediment transport. We then used this model to estimate potential bed movement in a natural stream for which we had measurements of channel geometry, grain size, and daily discharge. Although this extrapolation is speculative, it illustrates that multi-species impacts could be sufficiently large to reduce bedload sediment flux over annual time scales in streams where multiple species of caddisfly are present.

Field based analysis of sediment entrainment in two high gradient streams located in Alpine and Andine environments

This paper presents an analysis of critical thresholds for bedload transport based on field measurements conducted in two small, high gradient streams: the Rio Cordon (Italian Alps) and the Tres Arroyos (Chilean Andes). The threshold of incipient motion was identified by using marked particles displacement and both flood and flow competence approaches. The findings are expressed in terms of Shields parameter, dimensionless discharge, and specific stream power, and are used to identify the effects of relative grain size, relative depth, and bedform resistance. Overall, particle entrainment tends to be size selective, rather than exhibiting equal mobility, and the high values of dimensionless critical shear stress observed at both study sites confirm the additional roughness effects of step–pool morphologies that are very effective in reducing the bed shear stress and causing an apparent increase in critical shear stress.

The impact of shrimp trawling and associated sediment resuspension in mud dominated, shallow estuaries

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm 2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.

EFFECTS OF VEGETATION ON GULLY FORMATION

Surface erosion is increasingly caused by forest clearance for cultivation and urbanization. It is commonly observed that gullies formed at the edge of plateau surfaces tremendously accelerate surface erosion. It is accordingly important to study the mechanism of gully formation and the effects of vegetation in order to reduce soil erosion. This paper presents a mathematical model as to how vegetation effects on gully formation in terms of a linear stability analysis. It is suggested that vegetation decelerates gully development by reducing gully spacing and flow intensification. It is found that a slight increase of critical shear stress due to vegetation prevents gully formation effectively.

Incipient Motion of Sand‐Gravel Sediment Mixtures

Experiments on the incipient motion of gravel and sand mixtures of 0:100, 10:90, 25:75, 45:55, and 100:0 percent ratios of gravel and sand, respectively, were made in a laboratory flume. A series of transport experiments was made for each of the five sediment beds and the critical shear stress for motion was calculated for 12 size ranges by interpolating or extrapolating the bed shear stress for a very small transport rate. The sand in each of the five bed mixtures began to move at nearly the same bed shear stress. All gravel sizes began to move at nearly the same bed shear stress in the 100% gravel experiments, but in the sand‐gravel experiments the gravel showed an increase in critical shear stress with increasing size. Reasons for this change in the initiation of motion of the gravel from the 100% gravel to the sand‐gravel mixtures may result from the abundant sand‐sized sediment inhibiting the formation of a coarse bed surface layer in the sand‐gravel experiments.

Calculations of the critical shear stress for motion of uniform and heterogeneous sediments

An expression for the critical shear stress noncohesive sediment is derived from the balance of forces on individual particles at the surface of a bed. The resulting equation, for a given grain size and density, depends on the near‐bed drag force, lift force to drag force ratio, and particle angle of repose. Calculated values of the critical shear stress for uniformly sized sediment correspond closely to those determined from Shields' diagram. The initial motion problem for mixed grain sizes additionally depends on the relative protrusion of the grains into the flow and the particle angle of repose. The latter decreases when the diameter of a moving grain, D, is larger than the length scale of the bed roughness, ks (D/ks &gt; 1), and increases when D/ks &lt; 1, producing a corresponding decrease or increase in critical shear stress. Using the Miller and Byrne experimental relationship between D/ks and particle angle of repose, which is consistent with Shields' definition of initial motion, we obtain results that are in good agreement with the available experimental critical shear stress data for heterogeneous beds.

Dislocation propagation in silicon from the edge of a precipitated Si3N4 film

Local mechanical stresses in the substrate created by a break in coating continuity relax at high temperatures. As a result of the relaxation dislocations are generated and move away from the boundary. Near the boundary there are formed accumulations of dislocations in the form of a set of concentric semiloops. The distribution of the dislocation semiloops over the accumulation can be used to determine critical shear stresses. It has been established that the presence of a silicon nitride film leads to an increase in critical shear stress and braking of dislocations. As a result, dislocations in the substrate beneath the film propagate a shorter distance from the film edge than dislocations propagating in the region with no coating. It has been proposed that critical shear stress decreases with increase in the size of the dislocation loop.

Effect of irradiation on the deformation of copper and copper-aluminium alloys

The effect of low-temperature irradiation (<70°C) on the deformation of pure copper, Cu-0.09 wt% Al alloy, and Cu-0.19 wt% Al alloy was investigated. Tensile specimens were prepared from single crystals of various orientations, and were exposed to a neutron fluence of 6.85 ×1019 n/cm2(E>1 MeV) at a temperature less than 70°C. All irradiated and unirradiated specimens were pulled. A large increase in critical shear stress due to irradiation was observed; the increase was smaller in Cu-0.19 wt% Al alloy than in pure copper and Cu-0.09 wt% Al alloy. Ultimate shear stress and shear strain were less influenced by irradiation. Yield points were observed in all irradiated specimens. The yield drop was large in irradiated pure copper, and decreased with the increase of aluminium content in copper-aluminium alloys. All unirradiated specimens showed a high work-hardening coefficient (n) in the beginning of the deformation, followed by a lower value. By irradiation, the first value drastically decreased, while the second remained nearly constant. Shear stress and shear strain were influenced by crystal orientation.

Teaching unit for blood pressure screening and control for secondary schools: Jim Glick, Health Education Division, State of Washington Department of Social and Health Services, Olympia, Washington

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.

The missing link in the compliance chain: Jan Gilmore, Sharon Horn Holguin, High Blood Pressure Council of Los Angeles, California

To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.

Hardening induced by additive coloration in KCl

The critical shear stress of KCl crystals has been measured as a function of the number of F-centers ( n F ) in the range of 0–10 18 cm −1 introduced by additive coloration. The introduction of the F-centers gives a marked increase of critical shear stress σ c as Δσc = (3.1−0.22 T 1 2 10 −16 n F g mm 2 , where T is the measuring temperature in °K. If the colored crystals are held in the dark below room temperature, even at nitrogen temperature, the critical shear stress increases with time until a saturation level is reached which depends upon nF and the aging temperature. The irradiation of the F-light enhances the rate of the process greatly. The hardening due to aging is a reversible function of the aging temperature. The effect of the coagulation of F-centers on the mechanical properties has also been studied. As the aging proceeds at nitrogen temperature, the absorption curve on the shorter-wavelength side of the F-band produces a broadening and a peak at 2.78 eV. Although cold working before aging does not produce a large effect, deformation at the coloration temperature after coloration gives rise to extensive effects on the broadening of the F-band in the above region, while no indication of the growth of complex centers is observed. A qualitative discussion of the present phenomena is given. This is based on the electronic process associated with the clustering of point defects, including the F-centers, in the vicinity of the dislocations.