Increase In Stress Rate Research Articles

Effect of thermal radiation on convective heat transfer in MHD boundary layer Carreau fluid with chemical reaction

The temperature dependent thermophysical fluid properties have numerous aspects in different industries and engineering processes in which heat transmission is based on fluid flow. For such heat transmission processes, heat transmission system is highly fluctuated with variation of viscosity. Thus, the aim of this study is to investigate the transfer of heat in magnetized Carreau fluid with chemical reaction and under influence of thermal radiation over nonlinear stretching/shrinking surface. Additionally, we have incorporated variable heat dependent thermophysical properties to analyze the heat transfer in magnetized Carreau fluid. Set of flow governing non linear PDE’s are obtained using Carreau fluid tensor and boundary layer approximation (BLA) theory. Dimensionless set of ODE’s are obtained using suitable similarity transforms. Shooting method in conjunction with Newton's method have been utilized to solve the problem. It is noted that when stretching Bge 0 is significant with strictly increasing mass suction S shear stress rate increase with minor levels and sharp increase has been observed in Nusselt number, whereas in shrinking case B<0 shear stress and heat transfer coefficient values are improved raising the value of S mass suction. Further, raising the values of power law index n produce reduced skin friction over stretching surface Bge 0 while skin friction dramatically enhance in shrinking case B<0. It is observed that raising the non-linearity m values for stretching or shrinking, skin friction and Nusselt number considerably improved. Moreover, computational outcomes of the study are validated with already published previous results and the results obtained in this study are found in good agreement.

Experimental Study on Dynamic Splitting Characteristics of Carbon Fiber Reinforced Concrete.

Due to the non-uniform tension and compression strength of concrete, carbon fiber can be added to concrete to improve its static tensile behavior and increase the tension–compression ratio. In view of the destructive consequences of impacts and explosions, it is necessary to study the dynamic responses of carbon fiber reinforced concrete (CFRC) structures. Therefore, the effects of the stress rates and carbon fiber contents on the dynamic tension behavior of CFRC were investigated in this paper. The dynamic splitting tests of concrete with the fiber contents of 0, 0.1, 0.2, and 0.3% were carried out by using a split Hopkinson pressure bar (SHPB) device with a diameter of 74 mm. We found that with the increase of fiber content, the static tensile strength of CFRC increases obviously, but the increased amplitude tends to decrease. The dynamic tensile strength and dynamic increase factor (DIF) both increase with the increase of stress rate, but the growth rate slows down, showing an obvious rate effect. The rate sensitivity of ordinary concrete is higher than CFRC. There are significant differences in the influence of carbon fiber on the dynamic and static strength of concrete. In the design of concrete mixing proportion, the content of carbon fiber should be appropriately selected to meet the requirements of dynamic and static mechanical properties.

Statistical evidence on the effect of production changes on induced seismicity

AbstractDepletion of the Groningen gas field has induced earthquakes, although the north of the Netherlands is a tectonically inactive region. Increased seismic activity raised public concern which led the government to initiate a number of studies with the aim of understanding the cause(s) of the earthquakes. If the relationship between production and seismicity were understood then production could be optimized in such a way that the risk of induced seismicity would be minimal. The main question remains how production is correlated with induced seismicity. The Minister of Economic Affairs of the Netherlands decided to reduce production starting from 17 January 2014, specifically in the centre of the gas field as it has the highest rates of seismicity, the largest-magnitude events and the highest compaction values of the field.A reduction in production could possibly lead to a reduced rate of compaction. Additionally a reduction of production rate could lead to a reduced stress rate increase on the existing faults and consequently fewer seismic events per year. One might envisage a ‘bonus effect’ in the events reduction in the sense that the total number of events will be less, with the same total production smeared out over a longer period. This is as yet unclear.In this paper we apply different statistical methods to look for evidence supporting or disproving a decrease in the number of seismic events due to production reduction.

Quasi-static and ratcheting properties of trabecular bone under uniaxial and cyclic compression

The quasi-static and ratcheting properties of trabecular bone were investigated by experiments and theoretical predictions. The creep tests with different stress levels were completed and it is found that both the creep strain and creep compliance increase rapidly at first and then increase slowly as the creep time goes by. With increase of compressive stress the creep strain increases and the creep compliance decreases. The uniaxial compressive tests show that the applied stress rate makes remarkable influence on the compressive behaviors of trabecular bone. The Young's modulus of trabecular bone increases with increase of stress rate. The stress-strain hysteresis loops of trabecular bone under cyclic load change from sparse to dense with increase of number of cycles, which agrees with the change trend of ratcheting strain. The ratcheting strain rate rapidly decreases at first, and then exhibits a relatively stable and small value after 50cycles. Both the ratcheting strain and ratcheting strain rate increase with increase of stress amplitude or with decrease of stress rate. The creep model and the nonlinear viscoelastic constitutive model of trabecular bone were proposed and used to predict its creep property and rate-dependent compressive property. The results show that there are good agreements between the experimental data and predictions.

Low cycle fatigue and ratcheting behavior of 35CrMo structural steel at elevated temperature

Low cycle fatigue and ratcheting deformation of 35CrMo steel under uniaxial stress-controlled loading were investigated at 500°C. The effects of stress rates for 0.125MPa/s, 0.5MPa/s, 2.5MPa/s, 10MPa/s, 25MPa/s and 40MPa/s, and peak stresses σmax for 200MPa, 300MPa, 400MPa and 500MPa, were discussed in detail. Results indicated that the ratcheting strain curve versus cyclic number is similar to traditional static creep curve with three stages and the ratcheting stress rate is lower than the static creep rate with the same peak stress. Shakedown behavior occurs when σ̇ is greater than or equal to 2.5MPa/s for σmax=200MPa, but ratcheting happens for the other loads tested according to the method proposed by the Committee on Three Dimensional Finite Element Stress Evaluation (C-TDF). Moreover, the viscous stress component σv increases significantly with the decrease of stress rate, which represents the ratcheting deformation of 35CrMo steel is obviously time-dependent under high temperature. Furthermore, stress rate has little effect on fatigue life when the stress rate is less than 2.5MPa/s, while the fatigue life increases exponentially with the increase of stress rate when the stress rate is greater than 2.5MPa/s at σmax=500MPa. It is of interest that the maximum fracture strains for various stress rates remain approximately constant, and the tested average value is 11.6%.

Ratcheting fatigue behavior of Zircaloy-2 at room temperature

Nuclear core components of zirconium alloys experience asymmetric stress or strain cycling during service which leads to plastic strain accumulation and drastic reduction in fatigue life as well as dimensional instability of the component. Variables like loading rate, mean stress, and stress amplitude affect the influence of asymmetric loading. In the present investigation asymmetric stress controlled fatigue tests were conducted with mean stress from 80 to 150 MPa, stress amplitude from 270 to 340 MPa and stress rate from 30 to 750 MPa/s to study the process of plastic strain accumulation and its effect on fatigue life of Zircaloy-2 at room temperature. It was observed that with increase in mean stress and stress amplitude accumulation of ratcheting strain was increased and fatigue life was reduced. However, increase in stress rate led to improvement in fatigue life due to less accumulation of ratcheting strain.

Ratcheting Strain Accumulation Due to Asymmetric Cyclic Loading of Zircaloy-2 at Room Temperature

Asymmetric stress cycling leads to accumulation of plastic strain which is called as ratcheting strain. The problem is generally associated with nuclear fuel cladding materials used in nuclear power plants and in pressurized pipe lines. In the present investigation asymmetric stress controlled fatigue tests were conducted with three different parameters namely, mean stress, stress amplitude and stress rate (keeping two parameter constant and varying third parameter) to see the plastic strain accumulation and its effect on fatigue life of Zircaloy-2 at room temperature. The tests were conducted with mean stress varying from 80 to 150 MPa, stress amplitude varying from 270 to 340 MPa and stress rate varying from 30 to 750 MPa/s respectively. The experimental outcomes show that with increase in mean stress and stress amplitude, the ratcheting strain accumulation increases with reduction in fatigue life. However, increase in stress rate leads to improvement in fatigue life of the material due to small ratcheting strain accumulation. Cyclic softening is observed during initial cycles for lower values of mean stress and stress amplitude and cyclic hardening is predominant for higher values of mean stress and stress amplitude. However, cyclic hardening is observed with increase in stress rate.

Load rate of facet joints at the adjacent segment increased after fusion.

Background:The cause of the adjacent segment degeneration (ASD) after fusion remains unknown. It is reported that adjacent facet joint stresses increase after anterior cervical discectomy and fusion. This increase of stress rate may lead to tissue injury. Thus far, the load rate of the adjacent segment facet joint after fusion remains unclear.Methods:Six C2–C7 cadaveric spine specimens were loaded under four motion modes: Flexion, extension, rotation, and lateral bending, with a pure moment using a 6° robot arm combined with an optical motion analysis system. The Tecscan pressure test system was used for testing facet joint pressure.Results:The contact mode of the facet joints and distributions of the force center during different motions were recorded. The adjacent segment facet joint forces increased faster after fusion, compared with intact conditions. While the magnitude of pressures increased, there was no difference in distribution modes before and after fusion. No pressures were detected during flexion. The average growth velocity during extension was the fastest and was significantly faster than lateral bending.Conclusions:One of the reasons for cartilage injury was the increasing stress rate of loading. This implies that ASD after fusion may be related to habitual movement before and after fusion. More and faster extension is disadvantageous for the facet joints and should be reduced as much as possible.

Depth and rate dependent mechanical behaviors for articular cartilage: Experiments and theoretical predictions

An optimized digital image correlation (DIC) technique was applied to investigate the depth-dependent mechanical properties of articular cartilage and simultaneously the depth-dependent nonlinear viscoelastic constitutive model of cartilage was proposed and validated. The creep tests were performed with different stress levels and it is found that the initial strain and instantaneous strain increase; however the creep compliance decreases with the increase of compressive stress. The depth-dependent creep strain of cartilage was obtained by analyzing the images acquired using the optimized DIC technique. Moreover the inhomogeneous creep compliance distributions within the tissues were determined at different creep time points. It is noted that both creep strain and creep compliance with different creep times decrease from cartilage surface to deep. The depth-dependent creep compliance increases with creep time and the increasing amplitude of creep compliance decreases along cartilage depth. The depth-dependent and stress rate dependent nonlinear stress and strain curves were obtained for articular cartilage through uniaxial compression tests. It is found that the Young's modulus of cartilage increases obviously along cartilage depth from superficial layer to deep layer and the Young's modulus of different layers for cartilage increases with the increase of stress rate. The Poisson's ratio of cartilage increases along cartilage depth with given compressive strain and the Poisson's ratio of different layers decreases with the increase of compressive strain. The depth-dependent nonlinear viscoelastic constitutive model was proposed and some creep data were applied to determine the parameters of the model. The depth-dependent compressive behaviors of cartilage were predicted by the model and the results show that there are good agreements between the experimental data and predictions.

Uniaxial ratcheting behavior of sintered nanosilver joint for electronic packaging

Uniaxial ratcheting behavior and the fatigue life of sintered nanosilver joint were investigated at room temperature. All tests were carried out under stress-controlled mode. Force–displacement data were recorded during the entire fatigue lifespan by a non-contact displacement detecting system. Effects of stress amplitude, mean stress, stress rate, and stress ratio on the uniaxial ratcheting behavior of the sintered nanosilver joint were discussed. Stress-life (S–N) curves of the sintered joints were also obtained. The Smith–Watson–Topper (SWT) model, the Gerber model and the modified Goodman model, all of which took effect of mean stress into consideration, were compared for predicting the fatigue life of the sintered joint. Both the ratcheting strain and its rate increased with increasing stress amplitude or mean stress. The increase in stress amplitude and mean stress both reduced the fatigue life of the sintered joint, while the fatigue life prolonged with the increase in stress rate and stress ratio. The modified Goodman model predicted the fatigue life of the sintered joints well.

Dynamic fatigue of two-dimensional carbon/carbon composites

The dynamic fatigue of a chemical vapor infiltration 2D-C/C composite was investigated in three-point flexure using different stress rates from 0.07 to 70MPas−1. Flexure strength of the specimens increased in proportion to the increase of stress rate. From the ln σ–ln σ̇ diagram, stress corrosion factor n was obtained. The n value of 58 in the present work indicated that slow crack growth controlled the fracture process. The fracture under different stress rates shows characteristics of the slow crack growth, which agree with the result of dynamic fatigue.

Experimental Studies on Elastic Modulus and Flexural Strength of Sea Ice in the Bohai Sea

This paper deals with measurements of the elastic modulus and the flexural strength of sea ice collected at nine sites in the Bohai Sea in the two consecutive winters of 2008–2010. Experimental results show that both elastic modulus and flexural strength increase with a decrease of brine volume and have an exponential relationship with brine volume. The elastic modulus is independent of stress rate, but the flexural strength increases linearly with the increase of stress rate. This paper presents a single-parameter (i.e., brine volume) exponential equation for the elastic modulus and a two-parameter (i.e., brine volume and stress rate) exponential equation for the flexural strength, respectively. Finally, the relationship between the elastic modulus and the flexural strength is discussed.

Uniaxial ratcheting and low-cycle fatigue failure behaviors of AZ91D magnesium alloy under cyclic tension deformation

Uniaxial ratcheting and low-cycle fatigue failure behaviors of hot-rolled AZ91D magnesium alloy were studied by uniaxial cyclic stress-controlling tension deformation experiments. The effects of stress amplitude, mean stress and stress rate on the uniaxial ratcheting response and fatigue life of the hot-rolled AZ91D magnesium alloy were analyzed. Results show that (1) the ratcheting strain and ratcheting strain rate of the hot-rolled AZ91D magnesium alloy both increase with the increase of stress amplitude or mean stress; (2) increasing stress rate will decrease the ratcheting strain and ratcheting strain rate of the hot-rolled AZ91D magnesium alloy; (3) the increase of stress amplitude and mean stress can both reduce the fatigue life of the hot-rolled AZ91D magnesium alloy, while the fatigue life will be prolonged with the increase of stress rate.

Viscoelastic properties of Kevlar-29 fabric tape strength member

Kevlar ®-29 strength members find wide application in tow lines, mooring lines, oil rig pendants and underwater sensor systems. In this paper, the dynamic mechanical properties of Kevlar-29 fabric tape strength member are characterized by semi-empirical means. A series of transient and harmonic tests were conducted for a typical configuration taking into account of time scales and operating conditions of practical interest. Constitutive relations based on three-parameter and five-parameter spring-dashpot discrete models were used for extracting the properties from transient tests. The parameter values obtained through transient tests were transformed to complex modulus in the frequency domain. Comparison in the frequency domain was made between the results obtained through transient tests and harmonic tests. Effects of preload and stress rates were studied. It is observed that storage modulus increases and loss factor decreases with increase in preload. At very low frequencies, analysis of the transient tests using discrete models shows lower storage modulus values compared to harmonic tests and hence does not provide reliable estimation. At higher frequencies, i.e., above 8 Hz, five-parameter discrete model based on transient tests can adequately estimate both the storage modulus and loss modulus. The transient tests indicate a rise in the loss factor at mid frequencies whereas the harmonic tests give almost flat response. Also the modulus of Kevlar fabric tape increases with increase in stress rate. Nonlinear behaviour is noticed throughout the tensile deformation response.

Aftershock decay, productivity, and stress rates in Hawaii: Indicators of temperature and stress from magma sources

We examined dozens of aftershock sequences in Hawaii in terms of Gutenberg‐Richter and modified Omori law parameters. We studied p, the rate of aftershock decay; Ap, the aftershock productivity, defined as the observed divided by the expected number of aftershocks; and c, the time delay when aftershock rates begin to fall. We found that for earthquakes shallower than 20 km, p values &gt;1.2 are near active magma centers. We associate this high decay rate with higher temperatures and faster stress relaxation near magma reservoirs. Deep earthquakes near Kilauea's inferred magma transport path show a range of p values, suggesting the absence of a large, deep magma reservoir. Aftershock productivity is &gt;4.0 for flank earthquakes known to be triggered by intrusions but is normal (0.25 to 4.0) for isolated main shocks. We infer that continuing, post‐main shock stress from the intrusion adds to the main shock's stress step and causes higher Ap. High Ap in other zones suggests less obvious intrusions and pulsing magma pressure near Kilauea's feeding conduit. We calculate stress rates and stress rate changes from pre‐main shock and aftershock rates. Stress rate increased after many intrusions but decreased after large M7–8 earthquakes. Stress rates are highest in the seismically active volcano flanks and lowest in areas far from volcanic centers. We found sequences triggered by intrusions tend to have high Ap, high (&gt;0.10 day) c values, a stress rate increase, and sometimes a peak in aftershock rate hours after the main shock. We interpret these values as indicating continuing intrusive stress after the main shock.

A model emitting dislocation group from crack tip with stress singularity and its application to brittle—ductile transition

Taking into account the stress singularity near the crack tip, computer simulation of dislocation emission and motion has been carried out. A model is proposed in which the source emitting the dislocation group is located near by the crack tip. The numerical method has been used by programming to adjust time increment automatically. By this model and the analytical method, the converged solution has been obtained. The main results are as follows: The region where any dislocation does not exist along the slip plane near the stressed source, namely, dislocation free zone (DFZ) is found to appear. Also it has been found that inverse pile-up of dislocations against the tip of DFZ will appear. The formula is obtained correlating the maximum dislocation density with DFZ length. With increase of stress rate and decrease of the value of μ/τ 0 ∗ , the inverse pile-up at the tip of DFZ becomes more significant. Based on these results, a new fracture criterion for brittle fracture is proposed assuming critical local stress requisites within DFZ, where high stress concentration is induced by dynamic inverse pile-up of dislocations.

Experimental investigation of the effects of stress rate and stress level on fracture in polystyrene

The effects of stress rate and stress level on fatigue crack propagation in compression-moulded single-edge notched specimens (0.25 mm in thickness) of polystyrene are reported. Values of the stress rate\(\dot \sigma\) are obtained from the formula\(\dot \sigma\) = 2v(σmax − σmax),, wherev is the frequency and σmax, σmin are the maximum and minimum stresses of the fatigue cycle. Different levels of\(\dot \sigma\) are achieved by changing the frequency while keeping σmax, σmin at fixed values. The effect of the stress level is investigated by keeping\(\dot \sigma\) and σmin constant and varying σmax andv. The results show that when the kinetic data are plotted as Δl/Δt against the energy release rateG1, a relatively small effect of the stress rate is observed. If the same data are treated as Δl/ΔN againstG1, a decrease in Δl/ΔN with test frequency is seen. The increase in the level of σmax results in a higher crack speed. The critical crack length is found to be practically the same for all stress-rate experiments. A decrease in the critical crack length is observed with the increase in stress level. Analysis of craze distribution around the crack path shows that the extent of crazing decreases with the increase in stress rate and increases with the increase in stress level. For all experimental conditions, the ratio of the second moment to the square root of the fourth moment of the histograms of craze density along directions normal to the crack path is found to be constant throughout the slow phase of crack propagation. This result supports a self-similarity hypothesis of damage evolution proposed in the crack layer model.

Experiments on Anisotropic and Rate-Sensitive Strain Ratio and Modulus of Columnar-Grained Ice

Abstract Anisotropic and rate-sensitive characteristics of the ratio of lateral strain to axial strain in addition to the rate-sensitive effective modulus for columnar-grained freshwater ice and sea ice from the Arctic, have been investigated. Tests were carried out at −20°C, for conditions of no microcracking under uniaxial loads (normal to the length of the grains) in the stress rate range of 1 × 10−3 MN · m−2 s−1 to 1 × 102MN · m−2 s−1 or an equivalent strain rate range of 1 × 10−7 s−1 to 1 × 10−2 s−1. With increase in stress rate, the ratio increased from about 0.2 to 0.3 in the plane parallel to the columns, whereas it decreased from about 0.65 to 0.3 in the plane normal to the columns. A closed-loop controlled test system in association with foil gages, mounted directly on the specimen, was used. A clip-on displacement gage was used to evaluate the accuracy of the foil gages.