Saturation Stress Research Articles

Investigation on multi-temperature short-term stress and creep relaxation of Sn-Ag-Cu-In solder alloys under the effect of rotating magnetic field

In this study, using one single specimen and viscous strain εv of ≤0.0001, the short-term stress and creep relaxation tests are developed and performed on SAC(307)-2In alloy before and after applying rotating magnetic field (RMF), to predict the creep mechanisms at different temperatures and strain rates. The global strain rate εp is shown to follow the same power-law with stress exponents of 4.8–8.1, while the activation energies Q were 55.7–51.2 kJ/mol, indicating the climb-controlled dislocation creep enhanced by pipe diffusion mechanism. Because of the grain refinement after applying RMF, significant enhancement has been detected in viscous strain εv, elastic strain εe, delayed strain εd, young modulus E, stress exponents n and activation energy Q, whereas the thermal properties are rather sensitive to it. Moreover, significant drop in stress-time relationship rather than saturation stress was observed at different strain rates after applying RMF, indicating the viscoelastic response of creep, which has good predictions for next-generation electronic devices and engineering alloy design due to the contribution of εv/εe ratio.

Stability Analysis of Near-Wellbore Reservoirs Considering the Damage of Hydrate-Bearing Sediments

The stability of hydrate-bearing near-wellbore reservoirs is one of the key issues in gas hydrate exploitation. In most previous investigations, the damage evolution process of the sediment structure and its effect on near-wellbore reservoir stability have been neglected. Therefore, the damage variable is introduced into a multi-field coupled model based on continuous damage theory and multi-field coupling theory. A thermo-hydro-mechanical-chemical (THMC) multi-field coupling mathematical model considering damage of hydrate-bearing sediments is established. The effects of damage of hydrate-bearing sediments on the thermal field, seepage field, and mechanical field are considered. Finally, the distributions of hydrate saturation, pore pressure, damage variable, and effective stress of a near-wellbore reservoir in gas hydrate exploitation by depressurization are calculated, and the stability of a hydrate-bearing near-wellbore reservoir is analyzed using the model. Through calculation and analysis, it is found that structural damage of hydrate-bearing sediments has an adverse effect on the stability of hydrate-bearing near-wellbore reservoirs. The closer to the wellbore, the worse the reservoir stability, and the near-wellbore reservoir stability is the worst in the direction of minimum horizontal ground stress.

Deformation behaviour of an as-cast nickel base superalloy during primary hot working above and below the gamma prime solvus

Primary hot working behaviour of an as-cast nickel base superalloy has been evaluated by carrying out isothermal hot compression tests both above and below the gamma prime solvus. The flow (yield σ0, peak σp, steady state σss) stress of the material was found to be highly temperature sensitive when deformed below the gamma prime solvus. The coherent gamma prime precipitates present in the material were found to be sheared by the moving dislocations in this regime. The material was relatively insensitive to temperature changes above the gamma prime solvus and exhibits discontinuous dynamic recrystallization (DDRX) when deformed between 1100 °C – 1140 °C and 6 × 10−3 to 10−2/sec with a characteristic efficiency value of 48% as defined by dynamic materials modeling. The kinetics of DDRX was evaluated by carrying out hot compression tests up to a strain of 140% under peak efficiency conditions. The friction corrected flow curve was used to estimate the critical strain (εc) for DDRX initiation and the saturation stress (σsat) for dynamic recovery. The fraction recrystallized (XDDRX) estimated using the flow curve analysis was found to correlate well with the quantitative microstructural observations. The activation energy map developed using the back stress corrected flow stress and the true stress exponent revealed that glide along with diffusion controlled climb as the rate controlling mechanism. Side-pressing test was used to validate the experimental findings and also to establish a suitable thermo-mechanical processing schedule to achieve an order of grain refinement. The results obtained are presented and discussed here.

Avrami Kinetic-Based Constitutive Relationship for Armco-Type Pure Iron in Hot Deformation

The work presents a full mathematical description of the stress-strain compression curves in a wide range of strain rates and deformation temperatures for Armco-type pure iron. The constructed models are based on a dislocation structure evolution equation (in the case of dynamic recovery (DRV)) and Avrami kinetic-based model (in the case of dynamic recrystallization (DRX)). The fractional softening model is modified as: X = ( σ 2 − σ r 2 ) / ( σ d s 2 − σ r 2 ) considering the strain hardening of un-recrystallized regions. The Avrami kinetic equation is modified and used to describe the DRX process considering the strain rate and temperature. The relations between the Avrami constant k ∗ , time exponent n ∗ , strain rate ε ˙ , temperature T and Z parameter are discussed. The yield stress σ y , saturation stress σ r s , steady stress σ d s and critical strain ε c are expressed as the functions of the Z parameter. A constitutive model is constructed based on the strain-hardening model, fractional softening model and modified Avrami kinetic equation. The DRV and DRX characters of Armco-type pure iron are clearly presented in these flow stress curves determined by the model.

An efficient method of analysis of heat transfer during plane strain upsetting of a viscoplastic strip

AbstractA solution for the transient heat transfer during plane strain upsetting of a viscoplastic strip has been developed using a Lagrangian coordinate system and the Green's function available in the literature. A general approximate solution for stress and velocity found elsewhere has been used. In contrast to conventional viscoplastic models adopted in theoretical analyses of metal forming processes, the model considered in the present paper includes a saturation stress. This model used in the conjunction with the maximum friction law predicts the localization of plastic deformation in the vicinity of the friction surface. In turn, localized plastic deformation and temperature are responsible for the generation of a very narrow layer with drastically modified microstructure near the friction surface. The appearance of such layers is reported in numerous experimental works.

Analysis of the influence factors of tight reservoir start-up pressure difference

Tight gas reservoir is not only characterized by low permeability, but also by high water saturation. This paper was studied on the basis of tight reservoir microscopic pore structure analysis, through gas depletion-drive development simulation experiments of constant rate production and constant pressure difference two ways, analyzed the influence factors of reservoir starting pressure difference, to explore the change law of tight reservoir start-up pressure. The study suggests that the pore structure throat radius of tight reservoir is small, and the throat occupies a large proportion of pore volume, and the throat is not only the seepage channel but also the important storage space. The experiment also shows that the throat radius, water saturation and effective stress are the main factors to determine the starting pressure difference of reservoir. So in the development process, some measures have been taken to reduce or control the start-up pressure near wellbore area such as well location optimization, perforating optimization, increasing fracturing fluid flowback rate and drainage gas recovery measures optimization, these measures have achieved positive effect in Sulige gas field.

Construction and Verification of the Constitutive Model of Pure Copper Deformation at Elevated Temperatures

The deformation behavior of pure copper was studied in hot compression tests in the temperature range of 773–1173 K and strain rate interval of 0.001–1.0 s–1, the corresponding flow stress curves were plotted. The new method to calculate critical and saturation stresses was devised, quantitative analysis of strain hardening and dynamic softening was presented, a three-stage constitutive model was constructed to predict the flow stress of pure copper. As predicted and measured flow stress comparison indicate, the physical constitutive model can accurately characterize hot deformation of pure copper. With dynamic recovery and/or recrystallization. Numerical simulation of an upsetting process is carried out by implementing the constitutive model into commercial software. This model can be put to practical use and be quite promising for improving efficiency of a hot forging process for pure copper components.

Telescopic Shearing of Non-Newtonian Fluids with a Saturation Stress

A closed form solution for quasi-static telescopic shearing of non-Newtonian fluids between two concentric cylinders is derived and then analysed. The paper focuses on qualitative features of the solution. In particular, it is shown that the qualitative behaviour of the solution in the vicinity of the inner friction surface is controlled by the dependence of the quadratic invariant of the stress tensor on the quadratic invariant of the strain rate tensor as the latter approaches infinity. In particular, no solution at sticking may exist if the quadratic invariant of the stress tensor approaches a finite value as the quadratic invariant of the strain rate tensor approaches infinity. In this case, it is necessary to construct the solution at sliding. This solution is singular, and the exact asymptotic representation of the quadratic invariant of the strain rate tensor in the vicinity of the friction surface depends on the exact dependence of the quadratic invariant of the stress tensor on the quadratic invariant of the strain rate tensor as the latter approaches infinity.

Water Saturation Stress in Mimosa scabrella Seedlings

ABSTRACT The stress caused by water saturation has negative consequences for plants. The search for species tolerant to flooded soils can make feasible the use of areas with these characteristics. The objective of this study was to observe the effect of water saturation levels on the initial growth of Mimosa scabrella Benth seedlings. Newly germinated seedlings were subjected to three water depths (without saturation, 1/3, and 2/3 above the pot capacity) in a greenhouse environment. After 173 days, it was verified that for most of the variables there was no difference between the control treatment and the two saturation levels. In addition, the analysis of seedling quality demonstrated the ability of the species to develop and survive in this condition. Therefore, it is inferred that bracatinga is a promising species for cultivation under water saturation conditions.

Numerical investigations on random close packings of cylindrical particles with different aspect ratios

Random close packings (RCP) of different shaped cylindrical particles under optimal vibration conditions were numerically reproduced using discrete element method. Corresponding characteristic macro and micro properties such as packing density, coordination number (CN), radial distribution function (RDF), particle orientation, orientation randomness and forces/stresses as a function of particle aspect ratio (AR) were systematically characterized and analyzed. And the container wall effects on the packing were also comprehensively considered. The results show that with the increase of AR, the packing density of each RCP structure first increases to a maximum (0.70 with AR = 1) and then decreases. The RDF analysis shows the particle arrangement of local structure varies with AR. The container wall is only in effect within three layers for equiaxed cylinders (AR = 1) near the boundary and just one layer for other shaped cylinders. The non-equiaxed cylinders especially elongated ones exhibit a strong tendency to horizontal alignment. With the growth of AR, the nematic order parameter of the packings first decreases to a minimum and then increases. Both particle position and orientation distributions demonstrate that the obtained packing structures are random. Force analyses indicate that the strong forces follow an exponential distribution, while those weak forces follow a power-law distribution, which is comparable to the force distributions of spherical particles. Compared with other cylindrical particles, the force orientation distribution in the packing of equiaxed cylinders is more homogenous. The peak position of the hydrostatic stress distribution is the same for different shaped cylinders. Moreover, with the increase of the packing density, the percentage of strong stresses increases. With the AR deviating from 1, the static vertical stresses present a gradually larger saturation stress in the same depth.

The characteristics and mechanisms of the recent catastrophic landslides in Africa under IPL and WCoE projects

This paper described the recent catastrophic landslides in Sierra Leone, DR Congo, and Nigeria and their probable mechanisms. The landslides occurred at the peak of rainy season on steep mountains predisposed to failure by geologic, climatic, hydrologic, and tectonic factors. The landslides killed over 500 people and displaced more than 2000 people. The DR Congo landslide occurred a few days after the Sierra Leone landslide of August 14, 2017. The Sierra Leone mudslide occurred in the Regent area of Freetown on the steep slopes of Mount Sugar Loaf. Sugar Loaf forms a part of extensive and forested highlands south of the city. The cities of Freetown, Lokoja in Nigeria, Conakry in Guinea, and other cities in Africa have grown in recent times to almost encircle vulnerable highlands that are prone to failure. The internal friction angles of the slope materials were low at all saturation and normal stress levels used in the study. The result correlated with the stability analysis which showed that the factor of safety along the potential sliding surfaces was at critical value at failure. The number of casualties following a slip of the weak slope materials increased because of the housing pattern and the lack of effective drainages in urban cites in Africa. These factors are important considerations for future prevention and mitigating measures aimed at saving Africa from catastrophic slope failures.

Serrated Flow and Work-Hardening Behavior of China Low Activation Martensitic Steel (CLAM)

Serrated flow and work-hardening behavior (by Voce equation) of China low activation martensitic (CLAM) steel were investigated in the temperature range of 300–923 K (27 °C to 650 °C). The steel exhibited serrated flow at intermediate temperatures of 573–623 K (300–350 °C) and elevated temperatures higher than 773 K (500 °C), respectively. Type A, A + B, A + C, A + D, and mild serrations were identified. The observed serrations, the plateau/peak in flow stress and a minimum in ductility suggested the occurrence of dynamic strain aging (DSA), a phenomenon due to interactions between solute atoms and dislocations during plastic deformation, at intermediate temperatures, while the mild serrations observed at high temperatures were ascribed to a combined effect of tensile deformation and dynamic recovery. The variations of work-hardening parameters including saturation stress σ S , initial stress σ I and rate parameter n V with respect to temperatures also implied the dominance of DSA at intermediate temperatures. CLAM steel in present investigation generally displayed two-stage work-hardening behavior consisting of transient stage and stage-III. Good correlations had been presented between work-hardening parameters obtained by Voce equation and the respective experimental tensile properties, suggesting adequate applicability of Voce relationship for CLAM steel.

History Dependence of the Microstructure on Time-Dependent Deformation During In-Situ Cooling of a Nickel-Based Single-Crystal Superalloy

Time-dependent plastic deformation through stress relaxation and creep deformation during in-situ cooling of the as-cast single-crystal superalloy CMSX-4® has been studied via neutron diffraction, transmission electron microscopy, electro-thermal miniature testing, and analytical modeling across two temperature regimes. Between 1000 °C and 900 °C, stress relaxation prevails and gives rise to softening as evidenced by a decreased dislocation density and the presence of long segment stacking faults in γ phase. Lattice strains decrease in both the γ matrix and γ′ precipitate phases. A constitutive viscoplastic law derived from in-situ isothermal relaxation test under-estimates the equivalent plastic strain in the prediction of the stress and strain evolution during cooling in this case. It is thereby shown that the history dependence of the microstructure needs to be taken into account while deriving a constitutive law and which becomes even more relevant at high temperatures approaching the solvus. Higher temperature cooling experiments have also been carried out between 1300 °C and 1150 °C to measure the evolution of stress and plastic strain close to the γ′ solvus temperature. In-situ cooling of samples using ETMT shows that creep dominates during high-temperature deformation between 1300 °C and 1220 °C, but below a threshold temperature, typically 1220 °C work hardening begins to prevail from increasing γ′ fraction and resulting in a rapid increase in stress. The history dependence of prior accumulated deformation is also confirmed in the flow stress measurements using a single sample while cooling. The saturation stresses in the flow stress experiments show very good agreement with the stresses measured in the cooling experiments when viscoplastic deformation is dominant. This study demonstrates that experimentation during high-temperature deformation as well as the history dependence of the microstructure during cooling plays a key role in deriving an accurate viscoplastic constitutive law for the thermo-mechanical process during cooling from solidification.

Improvement of pressure-saturation changes estimations from time-lapse PP-AVO data by using non-linear optimization method

First-order approximations of seismic parameters in AVO intercept and gradient changes are not sufficient to estimate changes in saturation and effective stress accurately. Analysis from an unconsolidated reservoir and a compacting reservoir show that we need to consider higher order terms in seismic parameters to reduce the inaccuracy of the estimates. Furthermore, improper approximation of rock physics parameters increases the error in the estimates. Here, we implement a non-linear optimization method to estimate changes in saturation and pressure using reflectivity equation directly. We test the applicability of this non-linear optimization method on synthetic data for both the above-mentioned reservoir scenarios over a wide range of realistic saturation and pressure changes. We observe that the inversion results using the new method are reasonably good for both reservoir scenarios.

Effects of Swaddled and Sponge Bathing Methods on Signs of Stress and Pain in Premature Newborns: Implications for Evidence-Based Practice.

To determine the effects of sponge baths and swaddled bathing on premature infants' vital signs, oxygen saturation levels, crying times, pain, and stress levels. This study was a clinical trial with a crossover design. Data were conducted in the neonatal intensive care unit of a public hospital in Denizli, Turkey. A total of 35 premature infants, who were born at 33-37 weeks gestation with a birth weight <1,500 g, were enrolled in the study. Two bathing methods were applied at 3-day intervals. Vital signs and oxygen saturation levels were measured before and at minutes 1, 5, 15, 30 after bathing. Infants' bathing was video recorded to assess pain and stress behaviors. The pain and stress behaviors of infants were evaluated by independent observers. A significance level of .05 was used for all statistical analyses. There were statistically significant differences between bathing methods on vital signs, oxygen saturation levels, and crying times. Levels of stress and pain according to bathing type were significantly higher in the sponge bath condition (p<.05). Swaddled bathing has a positive effect on the infant's vital signs, oxygen saturation levels, crying time, and level of stress and pain compared to the sponge bath condition. Swaddled bathing is a harmless and safe nursing practice.

Chlorophyll to Carbon Ratio Derived From a Global Ecosystem Model With Photodamage

AbstractPhytoplankton harvests light by integrating chlorophyll in protein‐pigment complexes (photosystems) that are variable in number and size. In ecosystem models, the capacity of light harvesting is described as the pool of chlorophyll. Since most of the variability in phytoplankton chlorophyll content is driven by acclimation to changing nutrient and light conditions, photoacclimation is generally parameterized as a regulation of chlorophyll synthesis with changing light. However, photosystems can also be degraded, and of the few process‐based models that have been proposed in the literature for the representation of their degradation and repair, none of them have been extended to more realistic conditions offered by pelagic biogeochemical models. We proposed three potential parameterizations to treat the degradation of photosystems as a function of light intensity and included them as a source of variation in the size of the chlorophyll pool in Regulated Ecosystem Model, version 2 (REcoM2). These model versions provided chlorophyll values highly correlated with satellite chlorophyll and accurate patterns of the chlorophyll to carbon ratio at global scale. The improvement in the prediction of the ratios was remarkable in scenarios where cells are exposed to periods of low light conditions. By isolating the effects of light and nutrients on the variability of the chlorophyll to carbon ratio and the growth rate, we observed a potential reduction in photosynthetic performance under simultaneous light saturation and nutrient stress. This effect, whose strength depends on the presence of photoprotective mechanisms and the degree of nutrient limitation, allowed to assess the role of photodamage and photoprotection under stress conditions.

Influence of Thermal Treatment on the Thermal Conductivity of Beishan Granite

Granitic rocks are potential rock types for hosting high-level radioactive waste (HLW) repositories at depth. A better understanding of rock thermal conductivity is essential to develop HLW repositories successfully. In this work, experimental investigations on the thermal conductivity of thermally treated Beishan granite were conducted. Disk specimens preconditioned at 105 °C were heated to different temperatures (200, 300, 400, 550, 650, and 800 °C) and then cooled to room temperature for testing. Conventional physical properties such as bulk density, porosity, and P-wave velocity were measured under the effect of thermal treatment. Scanning electron microscope was used to characterize thermally induced microcracks in the rock. Thermal conductivities of the treated specimens under dry and water-saturated conditions were determined using the transient plane source method, and the effect of water saturation on the thermal conductivity was investigated. The influences of temperature and axial compression stress on the thermal conductivity were also studied. Results indicate that the thermal conductivity of the specimens depends strongly on the thermal treatment temperature. The thermal conductivity decreases nonlinearly with applied temperature, because of growth and propagation of microcracks in the specimens. On the other hand, water saturation plays an important role in increasing the thermal conductivity. In addition, significant differences exist in the thermal conductivity behaviors of the specimens when subjected to different ambient temperatures and compression stresses. Based on the experimental data, models considering the effect of porosity were established for describing the effects of water saturation, ambient temperature, and compression stress on the thermal conductivity of thermally treated rock.

Effect of thermo-mechanical treatment on tensile properties of reduced activation ferritic-martensitic steel

Effect of thermo-mechanical processing on tensile behaviour of reduced activation ferritic martensitic (RAFM) steel has been assessed and compared with those of the normalized and tempered steel. The thermo-mechanical treatment (TMT) was carried out by performing hot rolling and ageing in the austenite phase field, air cooled followed by tempering. TMT of the steel increased its hardness compared to the normalized and tempered condition. Tensile strength of the steel increased on TMT over the investigated temperature range of 300–923 K and more importantly was accompanied with increase in ductility. The strength increase on TMT was accomplished with the refinement of microstructure of the steel having finer martensite lath size and intra-granular precipitates. The tensile flow behaviour of steel has been assessed in the frame work of Voce's constitutive equation. The strength related parameters of the constitutive equation i.e. initial stress and saturation stress increased on TMT, while the recovery related parameter (nv) decreased marginally. The enhancement of tensile strength and ductility of the steel on TMT is considered due to microstructural refinement and its enhanced stability as reflected in appreciable lower relaxation and creep rates and the delayed in onset of tertiary stage of creep deformation.

Analysis of Flow Behavior of an Nb-Ti Microalloyed Steel During Hot Deformation

The hot flow behavior of an Nb-Ti microalloyed steel is investigated through hot compression test at various strain rates and temperatures. By the combination of dynamic recovery (DRV) and dynamic recrystallization (DRX) models, a phenomenological constitutive model is developed to derive the flow stress. The predefined activation energy of Q = 270 kJ/mol and the exponent of n = 5 are successfully set to derive critical stress at the onset of DRX and saturation stress of DRV as functions of the Zener–Hollomon parameter by the classical hyperbolic sine equation. The remaining parameters of the constitutive model are determined by fitting them to the experiments. Through substitution of a normalized strain in the DRV model and considering the interconnections between dependent parameters, a new model is developed. It is shown that, despite its fewer parameters, this model is in good agreement with the experiments. Accurate analyses of flow data along with microstructural analyses indicate that the dissolution of NbC precipitates and its consequent solid solution strengthening and retardation of DRX are responsible for the distinguished behaviors in the two temperature ranges between T < 1100 °C and T ≥ 1100 °C. Nevertheless, it is shown that a single constitutive equation can still be employed for the present steel in the whole tested temperature ranges.

Suction and rate-dependent behaviour of a shear-zone soil from a landslide in a gently-inclined mudstone-sandstone sequence in the Sichuan basin, China

Bedding-controlled landslides in alternated mudstone and sandstone beds in cataclinal slopes are common in the Sichuan basin and in the Three Gorges area in China. They are characterised by very gentle dip angles of the beds (<10°), and usually exhibit slow creep movements throughout the year. However, during the rainy season, they can have sudden accelerations and undergo large displacements. In this work, the influence of saturation degree, displacement rate and stress state on the shear strength of the shear-zone soil was investigated. The results of the displacement-controlled ring-shear tests show a rate-weakening behaviour even at low rates of displacement. Dually, the shear-controlled tests show that the displacement rate can increase dramatically in response to a slight increase of the shear stress. The threshold stress that triggers large displacement is equal to the rate-dependent residual shear strength of the soil.