Papers by Keyword: Time Dependence

Abstract: Zero-field muon spin relaxation experiments were carried out for Al-1.6%Mg₂Si and a pure aluminum in isothermal conditions between 260 and 300 K. Observed relaxation spectra were analyzed to extract the dipole width (D) values which were found to decrease with time after solution heat treatment and quenching. Time variations of D appeared to take place two stages in both samples. The stage transition times (t_II) deduced for Al-1.6%Mg₂Si were comparable to those for the Si-rich clustering stage reported for Al-Mg-Si alloys. The estimated activation energy of Si-rich clustering was 0.62 (±0.04) eV. The stage transition times (t_M) for the pure aluminum were 255, 110 and 82 min after quenching at the measuring temperatures of 260, 280 and 300 K, respectively. An Arrhenius plot of logarithmic t_M against reciprocal temperature resulted in an activation energy of 0.19 (±0.06) eV.

Abstract: In this article the time and heat dependence of the nitrogen distribution upon steel alloying of 04Cr20Ni6Mn11Mo2NVNb steel grade by the nitrided manganese of Mn85N10 grade at the holding in the air atmosphere from 2 to 30 minutes in the temperature range 1450-1550 °С was experimentally determined. It was determined that the highest degree of nitrogen transition into steel is 99% and is observed with a short holding time of 2 min (after the introduction of nitrided manganese into the steel melt) in the low-temperature region ~ 1450 °С. Further holding and / or temperature growth results in the release of nitrogen in the gaseous form, due to the thermal dissociation of the nitrogen-containing compounds contained in the melt and removal of nitrogen from the melt into the gas phase, which leads to a decrease in the degree of nitrogen transition to the steel. The key possibility for obtaining a steel of the indicated grades group with a nitrogen content of 0.45-0.6% is shown when alloying in an air atmosphere in a low-temperature region (1450-1500°C), while optimizing the holding time.

Abstract: With the development of highway tunnel engineering, the stability of the tunnel become the chief problem in designing and constructing. Tunnel surrounding rock under high stress based on the actual engineering background, model test process of excavation in soft rock tunnels under the distribution factors was finished in triaxial test mechine. Through sensor test, the whole process of tunnel became instable was reflected, and the different conditions of the change rule of surrounding rock stress was budgeted and analysed. the numerical simulation method was used to study dependency of deep tunnel with soft rock in it and the lining deformation time. in which nonlinear Drucker-Prager plastic coupling and creep constitutive model were used to describe the nonlinear viscous-elastic-plastic properties of high stress soft rock. The characteristies of tunnel lining deformation and internal force variation were studied with the above model. Soft rock thickness, soft rock tunnel location effect on inner force and deformation of the structure were discussed. The results have reference value in evaluation of long-term stability of deep tunnel.

Abstract: The displacement and stress of adjacent existing tunnel at different times after the new tunnel built are calculated by FEA models with ANSYS. The effects of each counting parameter of tunnel lining to results are analyzed, and some conclusions are educed, which can give references to the design and construction of siminiar engineerings.

Abstract: A simple design office oriented empirical model containing only two parameters has been developed to predict creep behavior of concrete made of crushed clay bricks as coarse aggregate. For this, concrete samples having three different normal compressive strengths in the range of 18.9 up to 24.0 N/mm2 are first prepared and then tested for their creep deformation. For each of the samples, a hyperbolic equation is developed from their creep-time behavior. These equations are then combined and modified according to statistical norms to finally obtain a generalized equation. Comparison of creep strain obtained from this equation with that of experimental values show that the proposed model can closely predict creep in brick aggregate made concrete.

Abstract: The uniaxial time-dependent cyclic deformation of POM/PTFE blends was studied experimentally under the stress-controlled and strain-controlled loading. The volume fraction of PTFE in the POM/PTFE blends was 20%. It was shown that the creep behavior of POM/PTFE blends is better than that of POM. In the cyclic strain-controlled experiments, the responded stress amplitude has a close relationship with the applied strain amplitude and strain rate. As a result of stress relaxation, the responded stress amplitude decreases as the hold time at peak strain increases. Ratcheting occurs in the blends subjected to asymmetric stress-controlled cyclic loading, and the ratcheting strain depends on the applied stress amplitude and stress rate. Hold time at peak stress and low stress rate lead to an increased ratcheting strain. The POM/PTFE blends will fail as the hold time at peak stress is longer than a critical value.

Abstract: Threshold voltage (Vth) was measured on 4H-SiC power DMOSFET devices as a function of temperature, gate stress, and gate stress time. Vth varied linearly with gate stress and gate stress time and inversely with temperature. This instability is explained with the trapping rate of channel electrons at or near the SiO2-SiC interface. Since the measurement scale of Vth is large in this case (it takes approx. 20 s to measure Vth), it is assumed that fast interface traps, i.e., ones closer to the interface, are already filled and do not contribute to the shift in Vth. Comparison with theoretical calculations shows the rate of carrier detrapping becomes higher with temperature and as a result the measured value of Vth approaches the theoretical value.

Abstract: Based on the experimental results of uniaxial time-dependent ratcheting behavior of SS304 stainless steel at room temperature and 973K, three kinds of time-dependent constitutive models were employed to describe such time-dependent ratcheting by using the Ohno-Abdel-Karim kinematic hardening rule, i.e., a unified viscoplastic model, a creep-plasticity superposition model and a creep-viscoplasticity superposition model. The capabilities of such models to describe the time-dependent ratcheting were discussed by comparing with the corresponding experimental results. It is shown that the unified viscoplastic model cannot provide reasonable simulation to the time-dependent ratcheting, especially to those with certain peak/valley stress hold and at 973K; the proposed creep-plasticity superposition model is reasonable when the creep is a dominant factor of the deformation, however, it cannot provide a reasonable description when the creep is weak; the creep-viscoplastic superposition model is reasonable not only at room temperature but also at high temperature.