Pre-stress Values Research Articles

Cortisol responses to social evaluation in 10- to 15-year-old boys and girls

This study investigated Cortisol responses and perceived stress of 10–15-year-olds to a computerized paradigm including elements of social evaluation, unpredictability, and uncontrollability. Both age and sex differences were examined. Participants were 52 children and adolescents (23 boys, mean age = 12.5 years). Over the course of an approximately 2-h testing session participants were exposed to a computerized testing paradigm, the social evaluative stress test (SEST), that lasted for 50 min and includes elements of social evaluation, unpredictability and uncontrollability. Seven saliva samples were obtained to measure cortisol concentrations before, during and after the SEST, to provide pre-stress values, cortisol reactivity to the stressor and recovery after stress. In addition, subjective emotional stress experiences were recorded. The results showed no effect of age on cortisol responses. Furthermore, although both sexes reported experiencing the paradigm as (equally) stressful, only boys reacted with significant cortisol increases (M = 163%). To our knowledge, this is the first computerized stressor that induces cortisol responses in 10- to 15-year-old boys. Whether the girls' perceived stress results in the activation of other biological systems, such as the sympathetic nervous system as well as in differential activation of brain regions, remains to be determined. Future studies investigating sex differences in stress reactivity during adolescence should include neuroimaging, as well as psychophysiological measures, to unveil some of the mechanisms behind the current findings.

Effective Prestressing Force Testing Method of External Prestressed Bridge Based on Frequency Method

A practical project is taken for the example to analyze the application of frequency method in the effective prestress test of external prestressed bridge. In the process of solving the dynamic differential equation of tension cable, the effects of boundary conditions on the natural frequencies of external prestressed tendons are analyzed. The analysis results show that the fundamental frequency of the prestressed tendons under fixed boundary condition is higher than that under hinged-support support boundary condition, and the relative variance decreases with the increase of tensioning force. With the same tensioning force, the relative variance decreases with the increase of cable length. In the practical test, only the upper and lower bound limits can be obtained according to the measured prestressed tendon frequencies, while the real effective prestress values are between them. For external prestressed tendons, a kind of short tension cable, the frequency method can produce certain error in calculating the effective prestress.

FBG-Based Creep Analysis of GFRP Materials Embedded in Concrete

This paper presents a typical study regarding the creep interaction behavior between prestressed glass fiber reinforced polymer (GFRP) bar and concrete when this GFRP bar is subjected to a constant external pullout force. A number of optical fiber Bragg grating (FBG) sensors were mounted on GFRP bar surface by using an innovative installation method to measure strain distributions. Test results indicate that the complicated interaction at GFRP bar-concrete interface can be evaluated using a transitional factor. Variation trends of this transitional factor indicate three typical zones characterized by different strain/stress variation trends of the GFRP bar when prestress values are sustained at specific levels. These three typical zones include stress release zone, stress transition zone, and continuous tension zone. Test results also suggest that the instant stress loss at the interaction interface between concrete and GFRP bar was quite limited (less than 5%) in present test. Contributed proportion of each GFRP bar section was obtained to represent the creep behavior of the GFRP bar embedded in concrete. This investigation improved the understanding of the short-term interaction behavior between prestressed GFRP bar and concrete.

Strength Calculation Method for Cross-Section of Reinforced Concrete Flexural Member Using Curvilinear Concrete Stress Diagram of EN-2

This article is continuation of earlier work of the author. Apractical method for cross-section strength calculation of flexural reinforced concrete members with rectangular compression zone using curvilinear concrete stress-strain diagrams of Eurocode 2 (EN-2) is presented in this article. The method can be used for strength calculation of members with flanges and reinforcement in compression zone as well. Curvilinear EN-2 stress diagrams are readily superseded by polynomials that can be easily integrated are proposed by the author. It gives opportunity for mathematically accurate, without changing of curvilinear diagram by arbitrary diagrams, determination of value and location for resultant of concrete compression zone stresses and location of neutral axes as well. Calculation is performed for any chosen strain of the layer subjected to the maximum compression. The said strain may be of greater or less value in comparison to that corresponding to the concrete strength. In this way the maximum strength of the member can be estimated when the compression zone strain value varies within chosen interval. The method is suitable in both cases for the members which are commonly (not abundantly) and abundantly reinforced if the neutral axes is located within the cross-section, i. e. within the interval up to the centre of the tensile reinforcement cross-sectional area. Strength of members reinforced with high strength reinforcement which is not sufficiently prestressed or not prestressed at all can be determined by calculation. Formulae can be used for calculation of stress-strain state in persistent situations (not ultimate ones) and for calculation of steel prestress value as well. Rectangular compression zone parameters required for member strength calculation using curvilinear stress diagram are presented in the article. Results obtained by the proposed method are compared with the results obtained using EN-2 formulae.

Nucleation and arrest of dynamic slip on a pressurized fault

Elevated pore pressure can lead to reactivation of slip on pre‐existing fractures and faults when the static Coulomb failure is reached locally. As the pressurized region spreads diffusively, slip can accumulate quasi‐statically (paced by the pore fluid diffusion) or dynamically. In this work, we consider a prestressed fault with a locally peaked, diffusively spreading pore pressure field to study (1) conditions leading to the escalation of slip and nucleation of dynamic rupture and (2) rupture run‐out distance before it is arrested. Nucleation appears in this model when the fault friction decreases from its peak value with slip, while arrest of dynamic propagation is imminent on aseismic faults (i.e., such that prestressτb is less than the residual fault strength τrat ambient conditions). When fluid overpressure is a small‐to‐moderate fraction of the ambient value of normal effective stress (and prestress is large enough for fault slip to be activated by overpressure), dynamic rupture always nucleates, and the nucleation length increases with decreasing prestress practically independently of the overpressure value. Transition from the ultimately unstable (τb > τr) to the ultimately stable (τb < τr) fault loading is marked by a strong increase of the nucleation length (∝1/(τb − τr)2) as τb approaches τr from above. For aseismic faults (τb < τr), no dynamic rupture is nucleated at large fluid overpressures for all but the smallest values of prestress. The largest run‐out distances of dynamic slip on aseismic faults correspond to overpressure/prestress just sufficient for slip activation. In such cases, the dynamically accumulated slip can lead to enhanced, dynamic fault weakening, resulting in a sustained dynamic rupture and generating a large earthquake. This is consistent with field observations when the largest injection‐induced seismicity occurred after fluid injection ended.

Anomalous statistics of aftershock sequences generated by supershear ruptures

Most earthquake ruptures propagate with speeds smaller than the Rayleigh wave velocity of the medium. These are called sub- Rayleigh ruptures. However, under suitable conditions, segments of otherwise sub- Rayleigh seismogenic ruptures can occasionally accelerate to speeds higher than the local shear wave velocity, giving rise to so-called supershear ruptures. The occurrence of supershear ruptures is usually associated with a locally higher value of pre-stress on the fault segment compared to the sub-Rayleigh segments of the same fault. Additionally, shear stress changes generated by the supershear rupture are radiated out unattenuated to distances comparable to the depth of rupture instead of rapidly decaying at much smaller distances from the rupture. This leads to aftershocks being distributed away from the fault on the supershear segment. This study attempts to verify whether these pre- and postseismic stress conditions and the resultant spatial aftershock distributions lead to discernible features in the statistical properties of the aftershock sequences of the earthquakes known to be associated with supershear ruptures. We analyze the Gutenberg-Richter scaling, the modified Omori law and Båth’s law for the aftershock sequences of two supershear mainshocks: the 1979 <em>M</em><sub>W</sub> 6.5 Imperial Valley (California) and 2002 <em>M</em><sub>W</sub> 7.9 Denali (Alaska) earthquakes. We observe that the <em>b</em>-value is always higher in the supershear zone than the rest of the sequence. We also observe that there is no systematic trend in the exponent of the modified Omori law when comparing the aftershocks in the supershear zone with the rest of the aftershocks. We argue that the <em>b</em>-value anomaly can be explained in terms of the off-fault distribution of aftershocks around the supershear segment of the rupture.

Modeling and experimental characterization of the stress, strain, and resistance of shape memory alloy actuator wires with controlled power input

Recently, a novel power controller has been presented that simultaneously controls electric power input and measures resistance of a commercially available Flexinol shape memory alloy wire. This work exploits the new power controller by plotting shape memory alloy stress, strain, and resistance versus Joule heating power instead of input voltage or current. Heating power is directly related to shape memory alloy temperature, whereas standard constant voltage or constant current inputs cause more or less heating as the resistance of the wire changes due to phase transformation. A simple experimental setup consisting of a 50-µm-diameter Flexinol wire mounted in series with the tip of a compliant cantilever beam is used to systematically study the shape memory alloy behavior. Actuator performance is reported for a range of prestress values and actuation frequencies. All the experimental data are compared with simulated behavior that is derived from a free energy–based numerical model for shape memory alloy material. Additionally, a new resistance model based on the simulated wire temperature and phase fractions is introduced and compared to experimental data. Exploiting a multifunctional power controller and an improved understanding of the resistance change during phase transformation will help enable the use of shape memory alloy wires in simultaneous sensing and actuating applications.

Influence of Needle Insertion Speed on Backflow for Convection-Enhanced Delivery

Fluid flow back along the outer surface of a needle (backflow) can be a significant problem during the direct infusion of drugs into brain tissues for procedures such as convection-enhanced delivery (CED). This study evaluates the effects of needle insertion speed (0.2 and 1.8 mm/s) as well as needle diameter and flow rate on the extent of backflow and local damage to surrounding tissues. Infusion experiments were conducted on a transparent tissue phantom, 0.6% (w/v) agarose hydrogel, to visualize backflow. Needle insertion experiments were also performed to evaluate local damage at the needle tip and to back out the prestress in the surrounding media for speed conditions where localized damage was not excessive. Prestress values were then used in an analytical model of backflow. At the higher insertion speed (1.8 mm/s), local insertion damage was found to be reduced and backflow was decreased. The compressive prestress at the needle-tissue interface was estimated to be approximately constant (0.812 kPa), and backflow distances were similar regardless of needle gauge (22, 26, and 32 gauge). The analytical model underestimated backflow distances at low infusion flow rates and overestimated backflow at higher flow rates. At the lower insertion speed (0.2 mm/s), significant backflow was measured. This corresponded to an observed accumulation of material at the needle tip which produced a gap between the needle and the surrounding media. Local tissue damage was also evaluated in excised rat brain tissues, and insertion tests show similar rate-dependent accumulation of tissue at the needle tip at the lower insertion speed. These results indicate that local tissue damage and backflow may be avoided by using an appropriate insertion speed.

Prenatal cortisol exposure predicts infant cortisol response to acute stress

Experimental animal findings suggest that early stress and glucocorticoid exposure may program the function of the hypothalamic-pituitary-adrenal (HPA) axis in the offspring. The extension of these findings to human development is not yet clear. A prospective longitudinal study was conducted on 125 mothers and their normally developing children. Amniotic fluid was obtained at, on average, 17.2 weeks gestation; infant behavior and cortisol response to a separation-reunion stress was assessed at 17 months. Amniotic fluid cortisol predicted infant cortisol response to separation-reunion stress: infants who were exposed to higher levels of cortisol in utero showed higher pre-stress cortisol values and blunted response to stress exposure. The association was independent of prenatal, obstetric, and socioeconomic factors and child-parent attachment. The findings provide some of the strongest data in humans that HPA axis functioning in the child may be predicted from prenatal cortisol exposure.

地山の吸水膨張による山岳トンネルの路盤隆起現象とその対策工に関する研究

Some mountain tunnels that have suffered from ground heaving require some countermeasures in service, but these designs often depend on empirical methods. Therefore, reasonable countermeasures against heaving become a significant problem. In this study, swelling is focused on as the mechanism of ground heaving, and the model experiment and numerical analysis that express swelling are carried out. As the result, the model test can simulate the heaving phenomenon which progress as time goes on, and it is confirmed that downward rock bolts curb the heaving by the effect of stiffness improvement of tunnel structure. Additionally, the effect of downward rock bolts specifications such as the number of bolts, their diameter, length, and the value of prestress are estimated quantitatively.

Research of Self Pre-Stress Test of Concrete-Filled Steel Tube Arch Rib

Indirect test method of installing vibrating wire strain gauges as sensors in outer surface of steel tube rib is adopted to carry out the research of concrete-filled steel tube (CFST) self pre-stress test for a half-through CFST arch bridge with a main span of 240m. The results show that indirect test method is a practical approach for self pre-stress test, and the measured value of self pre-stress from this method is much smaller than that of being theoretically calculated by concrete free expansion ratio.

Research of Self Pre-Stress Test of Concrete-Filled Steel Tube Arch Rib

Indirect test method of installing vibrating wire strain gauges as sensors in outer surface of steel tube rib is adopted to carry out the research of concrete-filled steel tube (CFST) self pre-stress test for a half-through CFST arch bridge with a main span of 240m. The results show that indirect test method is a practical approach for self pre-stress test, and the measured value of self pre-stress from this method is much smaller than that of being theoretically calculated by concrete free expansion ratio.

Prestress Scratching on SiC Ceramic

Prestress scratching tests have been performed on polished surface of SiC ceramics under a Rockwell diamond indenter moving with uniform speed and constant normal load. Combining with acoustic emission testing, the scratching depth and the surface/subsurface damage were investigated under the action of different prestress values. The results showed that the existence of prestress should shorten the depth of median cracks and reduce the subsurface damage, moreover, with increasing prestress in a proper range, the depth of median crack was decreased and the propagation length of lateral crack was limited.

The Distribution of Shear Stress along the Anchoring Section of Prestressed Bolt

In this paper the distribution of shear stress along the anchoring section of prestressed bolt was presented. The distribution of shear stress along the anchoring section of prestressed bolt was analyzed with the plane elastic theory. The analytical solution of shear stress along the anchoring section was deduced. Several cases of shear stresses were analyzed with the analytical solution. The results show that shear stress of prestressed bolt are not uniform distribution along the anchoring section; it is has no relations with the prestress value where the shear stress peak value is; the shear stress is at a certain range along the anchoring section; the shear stress peak value increases with the increase of prestress value;the larger the diameter of the anchoring section is, the less of shear stress peak value is.

Experimental Research on the Permanent Prestress of Transverse Prestressing Bundle for the Xiangjiang Yinpenling Bridge

In order to assess the service states of the permanent prestress of transverse prestressing bundle for the Xiangjiang Yinpenling Bridge (XYB), the permanent prestress values are measured. Appearance census of the diseases of transverse prestressing bundles for the cable-stayed part, the east and west incremental launching girder parts of the XYB are carried out. Three transverse prestressing bundles of the most serious corrosion are investigated by windowing damage inspection. The measured results and the analysis of the permanent prestress for the XYB show that the average permanent prestress of the three transverse prestressing bundles is about 300MPa to 550MPa, which do not meet the requirements of the design tension stress. It suggests that the appropriate treatment measures should be taken.

Wind-Induced vibration responses of prestressed double-layered spherical latticed shells

This paper focuses on the wind-induced vibration response of prestressed double-layered spherical latticed shell (PDSLS) structures by adopting time-domain analysis method. Welch spectrum analysis method is used to make precision evaluation of power spectrum of fluctuating wind speed time history simulated by weighted amplitude wavelet superposition (WAWS) method and linear filtering method of auto-regression (AR) model. Results show that the two methods produce little precision difference, but AR method is far more efficient than WAWS and is more suitable for wind speed simulation of PDSLSs. The effect of various parameters on the wind-induced vibration response of PDSLS structures are comprehensively investigated, including rise-span ratio, span, shell thickness, elastic constraint stiffness, prestress value, with or without cables and cable layout scheme. Results show that rise-span ratio and span are the major factors that affect wind-induced vibration response of PDSLSs. When cables are set, the wind vibration coefficient of nodal vertical displacement becomes smaller and more equally distributed, which demonstrates that PDSLSs are less sensitive to fluctuating wind effect than common latticed shell structures without cables. Finally, based on the envelopment concept and with the maximum dynamic and average wind-induced displacement responses as control indicators, the calculating method for global wind vibration coefficient (GWVC) of PDSLSs is proposed and the value with usual design parameters is given. Meanwhile, when the structure is made static analysis by means of the equivalent static wind load obtained from GWVC, the obtained internal member force response is relatively accordant with the actual response got from time-history analysis, and is a little safer.

Theoretical Analysis and Application Research on Grid String Branch Structure

According to the thought of beam string structure, pre-stressing structure was presented that it was joined by rigid grid and flexible cable by pole support. It was grid string branch structure. A calculation model of string branch grid was founded. Static and dynamic characteristics of the structure that it was acted by different pre-stress value were analyzed. Through the analysis, the structure displacement and internal forces of rod are improved obviously depending on pre-stress value. The pre-stress value is larger, structure displacement and internal forces of rod are lower. Natural vibration characteristics of the structure are almost not changing, while pre-stress value is changing. When the structure is time history analysis, its response curve accords with general law of structural vibration. Node displacement response of up and bottom chord nodes are not occurred retardation effect.

Scratching of Al 2O 3 under pre-stressing

Pre-stressing scratching tests have been preformed on polished surfaces of Al 2O 3 ceramic under a Rockwell diamond indenter which moved with uniform speed and constant normal load to investigate how the pre-stress contributes to the material removal mechanism. With the measurement of acoustic emission signals as well as indenter tangential forces, surface damages and cross-section of grooves of Al 2O 3 ceramic were evaluated under the action of different values of pre-stress. It was found that the scratched groove width was increased with the increasing of pre-stress when same normal loads were applied. The existence of pre-stress tends to restrain the crack propagation along the direction of pre-stress, and obvious plastic deformation at the bottom of scratched groove has been observed. Moreover, the fluctuation of tangential force was obviously enhanced, and the magnitude of tangential force in the test of pre-stress was higher than that of without pre-stress. The acoustic emission signals showed that fewer damages were produced in the process of scratching with an appropriate pre-stress. However, the continuing increase of pre-stress would aggravate the machining process.

Fatigue tests on SMA bars in span control

The estimation of the fatigue lifetime of copper-based shape memory alloy (SMA) specimens in the form of bars is investigated in view of their use in passive devices for structural control applications. In the envisaged application, the pre-stress value assigned to the SMA bars is selected so that it is only slightly modified when strain variations occur within an operative range. This condition requires that the fatigue cycles are performed in span control, conversely to the common practice of driving the tests in load control to study the fatigue behaviour of traditional metals such as steel. Several experiments are carried out at different temperatures for different strain ranges and the results are arranged with the aim of building suitable fatigue models.

Computational Insights Into Cytoskeletal Rheology

Cytoskeletal rheology is critical in the interaction of cells with their environment. It influences cell migration, mechanotransduction, and the ability of the cell to transmit externally applied forces. Models for cell mechanics have progressed considerably in recent years, and are now capable of simulating the effects of thermal fluctuations, cross-linking, and stresses generated by motor activity, all thought to be important determinants of cytoskeletal viscoelastic behavior. At the same time, model systems such as reconstituted actin gels have been used to gain insight into the factors that give rise to the unique cytoskeletal behavior. Of interest in this regard are: (i) the unique power-law rheology observed in the linear stress regime, (ii) nonlinear strain stiffening, (iii) fluidization that occurs above some critical level of stress, beyond which the modulus falls precipitously, and (iv) the role of prestress on cell stiffness. Here we present results from recent models that address each of these phenomena. We show through modeling that (i) thermal fluctuations of actin filaments are important only at low levels of prestress and small values of the ratio of persistence length to distance between cross-links, (ii) power-law rheology appears to emerge as a consequence of motor-induced prestress and cross-links, (iii) in the strain-stiffening regime, prestress is the primary determinant of network modulus, (iv) cross-link rupture rather than unfolding accounts for network remodeling and stress relaxation behavior, (v) motor activity generates prestress, but also has a strong influence on network morphology and the formation of stress fibers. [Support from the Sumsung Scholarship Program (to TYK) and the NIH (GM076689) is gratefully acknowledged.]