Stress Relaxation of Grouted Entirely Large Diameter B-GFRP Soil Nail

One of the potential solutions to steel-corrosion-related problems is the usage of fiber reinforced polymer （FRP） as a replacement of steel bars. In the past few decades, researchers have conducted a large number of experimental and theoretical studies on the behavior of small size glass fiber reinforce polymer （GFRP） bars （diameter smaller than 20 ram）. However, the behavior of large size GFRP bar is still not well understood. Particularly, few studies were conducted on the stress relaxation of grouted entirely large diameter GFRP soil nail. This paper investigates the effect of stress levels on the relaxation behavior of GFRP soil nail under sustained deformation ranging from 30% to 60% of its ultimate strain. In order to study the behavior of stress relaxation, two B-GFRP soil nail element specimens were developed and instrumented with fiber Bragg grating （FBG） strain sensors which were used to measure strains along the B-GFRP bars. The test results reveal that the behavior of stress relaxation of B-GFRP soil nail element subjected to pre-stress is significantly related to the elapsed time and the initial stress of relaxation procedure. The newly proposed model for evaluating stress relaxation ratio can substantially reflect the influences of the nature of B-GFRP bar and the property of grip body. The strain on the nail body can be redistributed automatically. Modulus reduction is not the single reason for the stress degradation.

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.

Transverse relaxation characteristic and stress relaxation model considering molecular chains of HTPB coating based on pre-strained thermal aging

In order to accurately describe the transverse relaxation characteristic and stress relaxation modulus of HTPB coating during pre-strain thermal aging process,a one month thermal aging test was carried out at70C with pre-strain of 0%,3%,6%and 9%,respectively.The low-field1 H NMR and stress relaxation modulus tests were carried out for HTPB coating at different aging stages.The stress relaxation model considering the molecular chains was proposed according to the changes of crosslinking chain and dangling chain of HTPB coating during pre-strain aging.The results showed that with the increase of aging time,the decay rate of transverse relaxation curve became faster,the transverse relaxation time decreased,the value of combined parameter q Mrl increased,the proportion of crosslinking chain decreased,while the proportion of dangling chain increased.Moreover,the stress relaxation modulus increased,the crosslinking network structure of HTPB coating became denser and the degree of crosslinking increased.At the initial aging stage,the pre-strain will destroy the crosslinking network structure of HTPB coating to a certain extent.With the increase of aging time,the effect of pre-strain will gradually weaken and the influence of aging on materials will gradually increase.The correlations between the stress relaxation model considering the molecular chains and the test results were more than 0.9950,which can accurately describe the stress relaxation modulus of HTPB coating during the pre-strain thermal aging process.

Stress Relaxation and Sensitivity Weight for Bi-Directional Evolutionary Structural Optimization to Improve the Computational Efficiency and Stabilization on Stress-Based Topology Optimization

Stress-based topology optimization is one of the most concerns of structural optimization and receives much attention in a wide range of engineering designs.To solve the inherent issues of stress-based topology optimization,many schemes are added to the conventional bi-directional evolutionary structural optimization(BESO)method in the previous studies.However,these schemes degrade the generality of BESO and increase the computational cost.This study proposes an improved topology optimization method for the continuum structures considering stress minimization in the framework of the conventional BESO method.A global stress measure constructed by p-norm function is treated as the objective function.To stabilize the optimization process,both qp-relaxation and sensitivity weight scheme are introduced.Design variables are updated by the conventional BESO method.Several 2D and 3D examples are used to demonstrate the validity of the proposed method.The results show that the optimization process can be stabilized by qp-relaxation.The value of q and p are crucial to reasonable solutions.The proposed sensitivity weight scheme further stabilizes the optimization process and evenly distributes the stress field.The computational efficiency of the proposed method is higher than the previous methods because it keeps the generality of BESO and does not need additional schemes.

Stress Relaxation of Chemically Treated Wood during Processes of Temperature Elevation and Decline

In order to clarify the effect of drying on structural changes of DMSO swell treated and DEA-SO2-DMSO decrystallization treated Chinese fir （Cunninghamia lanceolate） wood, the stress relaxation of treated oven-dry specimens during the processes of temperature elevation and reduction and that of treated wet specimens at constant temperature were determined. A stress decrease process and a stress increase process were observed in all stress ratio curves of wood during the processes of decreasing temperature. Untreated wood, during the process of temperature reduction under higher initial temperature conditions and during the process of temperature elevation, has a larger stress decrease than treated woods. In a wet state this trend is reversed. It indicated that the drying set made treated woods have a smaller increase in fluidity of wood constituents with increasing temperature. Some bonding between decrystallization reagents and wood molecules may occur.

Stress relaxation analysis of single chondrocytes using porohyperelastic model based on AFM experiments

Based on atomic force microscopy technique, we found that the chon- drocytes exhibits stress relaxation behavior. We explored the mechanism of this stress relaxation behavior and concluded that the intracellular fluid exuding out from the cells during deformation plays the most important role in the stress relax- ation. We applied the inverse finite element analysis technique to determine nec- essary material parameters for porohyperelastic （PHE） model to simulate stress relaxation behavior as this model is proven capable of capturing the non-linear behavior and the fluid-solid interaction during the stress relaxation of the single chondrocytes. It is observed that PHE model can precisely capture the stress re- laxation behavior of single chondrocytes and would be a suitable model for cell biomechanics.

Stress relaxation of Si/Si_(1-x)Ge_x/Si structure prepared by ion implantation and subsequent annealing process

The stress relaxation of 74Ge＋-implanted （100） silicon wafers was investigated. The implantation energy as a function of Si/Si1-xGex/Si struc-ture fluence and two kinds of thermal annealing were reported. The stress and stress relaxation after thermal annealing were calculated on the basis of Raman analysis, and were compared with those obtained from the calculation of virgin Si.

STRESS RELAXATION BEHAVIOUR OF TITANIUM

Studies of stress relaxation were carried out on commercially pure titanium.It was found that the stress relaxation behaviour relates closely to the level of deformation.The amount of re- laxed stress (?)σ_R is independent of the amplitude of stress dip in a certain range of deformation.However,it depend on the stress level at which stress relaxation begins in a cer- tain relaxation time t_R.Both back(critical)stress σ_c and flow stress σ_o vary with the strain in a similar manner.The effective stress on dislocations σ~* increases with increasing strain under low strains,but keeps constant under higher strains.

Thickness dependence of viscoelastic stress relaxation of laminated microbeams due to mismatch strain

Time-dependent behaviors due to various mismatch strains are very important to the reliability of micro-/nano-devices.This paper aims at presenting an analytical model to study the viscoelastic stress relaxation of the laminated microbeam caused by mismatch strain.Firstly,Zhang’s two-variable method is used to establish a mechanical model for predicting the quasi-static stress relaxation of the laminated microbeam.Secondly,the related analytical solutions are obtained by combining the differential method and the eigenvalue method in the temporal domain.Finally,the influence of the substrateto-film thickness/modulus ratio on the relaxation responses of the laminated microbeam subject to a step load of the mismatch strain is studied.The results show that the present predictions are consistent with the previous theoretical studies.Furthermore,the thickness dependence of stress relaxation time of the laminated microbeam is jointly determined by the intrinsic structural evolution factors and tension-bending coupling state;the stress relaxation time can be controlled by adjusting the substrate-to-film thickness/modulus ratio.

STRESS RELAXATION AND RELIABILITY EVALUATION OF SODA-LIME GLASS

Stress relaxation of glass is a dualism effect, it often lead to strength degradation in strengthened glass, but on the other hand, it improves the reliability and stress-uniformity of glasses. In this work, stress relaxation of soda-lime glass was investigated using three-point bending tests at 400-560℃ which is near the brittle to ductile transition temperature, for enhancing the safety of glass productions and exploring the most economic anneal process. The experimental results show that the speed of stress relaxation increases but the ultimate stress decreases with increasing temperature. The stress uniformity of the glass samples before and after anneal was examined using spherical indentation at arranged testing points. It indicates that the scatter of the local strength measured by the Hertzian indentation is smaller in the anneal glass than in initial specimen, so that the estimated Weibull modulus for the anneal specimen is higher. Furthermore, the strength evaluation by Hertzian indentation and statistical analysis was presented.

Microstructural and Microhardness Variation of Amorphous Fe_(78)Si_9B_(13) Alloy during Bend Stress Relaxation

The amorphous Fe78Si9B13 ribbons were bend stress relaxed at various temperature well below the crystallization temperature （Tx） for different time. The effect of pre-annealing on the subsequent bend stress relaxation was examined. The variation of the microstructure and microhardness during bend stress relaxation process was studied using X-ray diffraction （XRD）, atomic force microscopy （AFM） and Vickers microhardness test,respectively. Curvature radius of the amorphous Fe78Si9B13 ribbons decreased with increase bend stress relaxation temperature and time. The microhardness of the stress relaxed specimens increased with time at 300℃ due to the forming of nanocrystals during bend stress relaxation. The pre-annealing reduced the decrease rate of the curvature radius of stress relaxed specimens.

Indenter Size Effect on Stress Relaxation Behaviors of Surface-modified Silicon:A Molecular Dynamics Study

Long-lasting constant loading commonly exists in silicon-based microelectronic contact and can lead to the appearance of plastic deformation.Stress relaxation behaviors of monocrystalline silicon coated with amorphous SiO_(2)film during nanoindentation are probed using molecular dynamics simulation by varying the indenter’s size.The results show that the indentation force(stress)declines sharply at the initial and decreases almost linearly toward the end of holding for tested samples.The amount of stress relaxation of SiO_(2)/Si samples indented with different indenters during holding increases with growing indenter size,and the corresponding plastic deformation characteristics are carefully analyzed.The deformation mechanism for confined amorphous SiO_(2)film is depicted based on the amorphous plasticity theories,revealing that the more activated shear transformation zones(STZs)and free volume within indented SiO_(2)film promote stress relaxation.The phase transformation takes place to monocrystalline silicon,the generated atoms of Si-II and bct-5 phases within monocrystalline silicon substrate during holding are much higher than those for smaller indenter.

EFFECT OF INITIAL GRAIN SIZE ON STATIC RECRYSTALLIZA-TION SOFTENING IN Cr STEEL USING STRESS RELAXATION TECHNIQUE

Effect of initial grain size (I.G.S.) on static recrystallization softeningin Cr steel (0.77 wt. percent Cr) has been investigated through the use of interrupted hotcompression tests and stress relaxation curves from Gleeble 1500. Initial grain sizes were variedbetween 20 and 93 microns. Stress strains curves for Cr steel for different initial grain sizes andrecrystallization times have been highlighted. Similar observation was made for metadynamicrecrystallization with shorter retardation times. Statically recrystallized grain size alsoincreased as initial grain size increases. It is found that the values of initial grain size havesignificant effects on the mean flow stress and static recrystallization kinetics as well as thepeak strain values to initiate dynamic recrystallization.

Effect of Triple Annealing Treatment on Stress Relaxation of Ti-6Al-4V Alloy

The effect of triple annealing on stress relaxation of Ti-6Al-4V alloy as well as the microstructure after stress relaxation werestudied. The results showed that triple annealing treatment enhanced the resistance of stress relaxation performance, andwhen the temperature was rising, this effect became notable. The stress relaxation deformation mechanism is of dislocationcreep at 400℃ and recovery creep at 600℃.

Stress Relaxation and Creep of Straw Bales

Loading ability of straw bales was tested by using an Electronical Testing Machine. Linear regression models were proposed to describe the loading ability as a function of failure density and compressing energy. Based on an exponent model, the testing compression coefficients of straw bales were estimated by using the Levenberg-Marquardt Method. Results showed that the relation among failure density, loading ability and compressing energy was linear in the phase of high density, Loading ability of straw bales could meet the requirement for building bills .

Stress relaxation of warm frozen soil under drained or undrained conditions

To investigate the influence of drainage conditions on stress relaxation characteristics of warm frozen soil, a series of laboratory tests were carried out under drained and undrained conditions. The results indicate that confining pressure obviously influences the relaxation process of warm frozen soil. Under undrained condition, with increase in confining pressure, the critical relaxation du- ration tends to grow as well as instantaneous relaxation. But the relaxation rate is sensitive to confining pressure in the initial stage, and with further development, the effect tends to diminish. Under drained condition, the relaxation rate is greater than that under tmdrained condition in the initial stage but with the development of relaxation, the difference decreases. The volumetric defor- mation of warm frozen clay under drained condition is much larger than that under undrained condition.

Stress Relaxation Behavior of GH4169 Superalloy

GH4169 superalloy stress relaxation test was investigated to study its characteristics of stress relaxation curves at various temperatures( 550,650,and 750 ℃). These curves presented jointly two distinct stages,the stage of inner stress relaxing quickly,and the stage of inner stress relaxing slowly and closing to the stress relaxation limit. And these curves obtained could be fitted by second order exponential decay function well. Based on the experimental stress relaxation curves,the relationship between stress relaxation rate and time were derived, which showed that the higher relaxation temperature and the greater initial rate of stress relaxation. The whole process presented two different stages,the stage of stress relaxation rate falling rapidly and the stage of stress relaxation rate slowing down and tending to be constant. The relation curve between creep strain rate and stress of GH4169 superalloy can be divided into three stages,low stress stage,transition stage,and high stress stage.Both the high stage and the low stage present linear correlation.

Stress relaxation properties of calcium silicate hydrate:a molecular dynamics study

The time-dependent viscoelastic response of cement-based materials to applied deformation is far from fully understood at the atomic level.Calcium silicate hydrate(C-S-H),the main hydration product of Portland cement,is responsible for the viscoelastic mechanism of cement-based materials.In this study,a molecular model of C-S-H was developed to explain the stress relaxation characteristics of C-S-H at different initial deformation states,Ca/Si ratios,temperatures,and water contents,which cannot be accessed experimentally.The stress relaxation of C-S-H occurs regardless of whether it is subjected to initial shear,tensile,or compressive deformation,and shows a heterogeneous characteristic.Water plays a crucial role in the stress relaxation process.A large Ca/Si ratio and high temperature reduce the cohesion between the calcium-silicate layer and the interlayer region,and the viscosity of the interlayer region,thereby accelerating the stress relaxation of C-S-H.The effect of the hydrogen bond network and the morphology of C-S-H on the evolution of the stress relaxation characteristics of C-S-H at different water contents was elucidated by nonaffine mean squared displacement.Our results shed light on the stress relaxation characteristics of C-S-H from a microscopic perspective,bridging the gap between the microscopic phenomena and the underlying atomic-level mechanisms.

Stress Relaxation Behavior of a Nb-Stabilized Austenitic Stainless Steel at 550℃

Stress relaxation resistance is one of the most significant properties that are critical to the service life of the fasteners.In this study,the stress relaxation behavior of a Nb-stabilized austenitic stainless steel was investigated.It was revealed that the homogenization treatment at 1250℃could make more primary NbC particles dissolved back into the matrix and consequently get a great number of nano-sized secondary NbC carbides.Therefore,the stress relaxation resistance will be enhanced due to the effective pinning of such higher density of nano-sized secondary NbC carbides on the dislocation movement.