Simulation of high-density pulse current-induced stress relaxation

The phenomenological equation relating pulse current-induced relaxation of elastic tensile stresses to current density, initial stresses, and temperature is proposed. The equation allows approximation of experimental data on positive and negative tensile stress relaxation with the above parameters. Translated from Problemy Prochnosti, No. 2, pp. 113–120, March–April, 2007.     

Effect of a high-density current pulse on the relaxation of shear stresses

The procedure and several results of experimental studies on the high-density current pulse-induced relaxation of shear stresses are presented. A significant nonthermal current effect on shear strain kinetics has been established. The strain rate of the metal in shear under the action of a current pulse is shown to be determined by the viscosity factor, like under high-rate inelastic deformation. Translated from Problemy Prochnosti, No. 6, pp. 44–53, November–December, 2008.     

Experimental evaluation of pulse electric current effect on residual stresses in composite-to-copper joints

Experimental method for evaluation of residual stress distribution in carbon fiber reinforced carbon composite jointed to copper is presented. Pulse electric current passed through composite-to-copper joint essentially reduces the level of residual stresses. __________ Translated from Problemy Prochnosti, No. 4, pp. 79–86, July–August, 2008.     

拉-拉疲劳载荷下钛合金湿喷丸的残余应力松弛及再次喷丸工艺EI北大核心CSCD

采用湿法喷丸强化工艺(wet shot-peening)对TC4钛合金表面进行处理,研究高、低周的拉-拉疲劳过程中合金残余应力松弛规律,探讨再次喷丸工艺(re-shot-peening,RSP)对疲劳寿命的影响。结果表明:在拉应力载荷状态下,残余压应力依然发生松弛现象。疲劳载荷水平对喷丸TC4钛合金残余压应力场(CRSF)的松弛速率、松弛程度和松弛范围具有重要影响。高周疲劳(HCF)过程中残余应力松弛主要发生在近表层0~30μm,松弛速率较慢。低周疲劳(LCF)过程中残余应力松弛发生在0~80μm,范围更大,速率更快。RSP周期对于TC4钛合金的疲劳寿命也具有较大影响。在25%和50%初始喷丸疲劳寿命进行RSP处理会显著提高疲劳寿命,而在75%初始喷丸疲劳寿命处进行RSP处理对于疲劳寿命基本没有影响。此外,RSP的强化效果与疲劳载荷水平相关,对于高周疲劳寿命提高明显。     

Comparing various fitting models to construct the tensile relaxation modulus master curve of asphalt mixes

This study was to compare the relative ability of seven common fitting models, i.e. Pure Power Law (PPL), Generalized Power Law (GPL), Modified Power Law (MPL), Modified Power Law Series (MPLS), Standard Sigmoid (SS), Generalized Logistic Sigmoid (GLS) and Prony Series (PS), to construct the tensile relaxation modulus master curve of dense graded asphalt mixes. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with three replicates. Direct tension relaxation modulus tests were conducted on the specimens at four different temperatures using the trapezoidal loading pattern at a low level of input strain. Tensile relaxation modulus master curves were constructed using all the fitting models together with the numerical shifting technique. Finally, both the graphical and statistical comparisons were made among the fitting models, and the best one was found to be PS, followed by MPLS, GLS, MPL, SS, GPL and PPL.     

A Boussinesq wedge analysis of the tensile stress region in a rail head beneath compressive and traction point loads

Investigations into the causes of damage to rail heads and on rolling contact fatigue, date back a considerable time and are described in a large number of expertise literature on the many different aspects of this complex problem. The present paper is prompted by the naïve observation that fatigue crack propagation is more likely to occur in a tensile stress environment than in a highly compressive one and it therefore seeks to locate a region of radial or longitudinal tensile stresses on a rail head during the rectilinear motion of the train, providing favourable conditions for mode I or mixed‐mode crack propagation.     

Predicting the tensile relaxation modulus of asphalt mixes based on the mix design and environmental factors

The main objective of this study was to predict the tensile relaxation modulus of asphalt mixes, without having to perform the common relaxation modulus tests, by developing a predictive model based on the mix characteristics, ageing condition, temperature and loading time. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with four replicates. Uniaxial tensile relaxation modulus tests were conducted on the specimens at four temperatures using the trapezoidal loading pattern at a low level of strain. Tensile relaxation modulus master curves of all the experimental combinations were constructed by the sigmoidal model. Statistical analysis of variance and regression analysis was performed on the test data and a predictive model was developed. Finally, the predictive model was verified using a group of measured values other than those used for the development of the model, and it was found that the predicted values correlated well with the measured ones.     

Evaluation of the high-density pulse current effect on the plastic strain rate in metals

A direct (nonthermal) high-density pulse current effect on the plastic strain rate in the metal under loading and on the relaxation of elastic stresses in it is evaluated. The evaluation is based on existing notions of electronic conductivity and thermally activated dislocation motion-controlled plastic strain mechanisms in metals. The effect of pulse current on the plastic strain rate is shown to become more and more pronounced with current density, elastic stress levels, and temperature; at low current densities and applied loads its effect is inessential. __________ Translated from Problemy Prochnosti, No. 2, pp. 105–113, March–April, 2006.     

A novel method for pulse shaping of Split Hopkinson tensile bar signals

Split Hopkinson bar (SHB) techniques are commonly used to experimentally characterise materials at high strain-rates. One important aspect of high-strain rate characterisation using SHBs is the necessity to tailor the input pulse to the needs of the material to be tested. Here, a new method to shape the input pulse, specifically developed for tensile SHBs (SHTB), is presented. The new method overcomes several challenges of existing designs, allows for a controlled adjustment of the pulse rising time, and significantly reduces wave dispersion effects.     

Concurrent ratcheting and stress relaxation at the notch root of steel samples undergoing asymmetric tensile loading cycles

The current study intends to develop a framework model to assess ratcheting and stress relaxation at the notch root of 1045 steel samples over asymmetric loading cycles. The framework involves the Ahmadzadeh‐Varvani (A‐V) kinematic hardening rule to control ratcheting progress and Neuber rule to accommodate for local stress and strain components at the vicinity of notch root. Plastic strain at notch root was first coupled with its counterpart in the A‐V model to establish a relation between local stress and backstress components. Calculated local stress and strain values at turning points enabled the A‐V model to assess ratcheting strain over each loading cycle. The stepwise drop in stresses at peak‐valley tips of hysteresis loops at the notch root was associated to coupled framework of the A‐V model and Neuber rule through constancy in local strain while ratcheting progressed over each cycle. This relaxed out the local stresses at tips of hysteresis loops to position on Neuber hyperbolic curve. Predicted ratcheting values at notch root of various diameters closely agreed with those of measured in steel samples over stress cycles.     

Thermal expansion and stress relaxation of metal-matrix composites

The coefficient of thermal expansion (CTE) of a series of Al-6%Si matrix samples, with reinforcements of carbon, SiC, Al₂O₃, or boron fibres, cloths, or ceramic particles was measured in the range 60°–220°C with a dilatometer. The anisotropy of the CTE was measured and found to be very large for specimens unidirectionally reinforced with carbon fibre. Relaxation of stresses due to the different thermal expansion of matrix and reinforcement was studied by using the bending of asymmetrically reinforced samples and the magnitude of the stresses evaluated using bending beam theory.     

Size effect in tensile strength caused by stress fluctuations

The randomness of microstructure heterogeneous materials leads to creation of microscopic random stress fields within the bulk of the material under loading. Although in average the microscopic stresses coincide with the macroscopic (e.g., externally applied) stress, the local differences (stress fluctuations) can be high, the magnitude increasing with the volume of the heterogeneous material. In the case of uniform macroscopic loading, Gaussian stress fluctuations lead to a size effect in which the tensile strength reduces as square root of logarithm of the sample size. In practice, however, the macroscopic tensile stress fields are usually nonuniform. In this case, failure is determined by the maximum value of the macroscopic stress with the scale effect controlled by the minimum degree of the macroscopic stress decrease from its maximum. Therefore, a second model is proposed which accounts for a linear stress variation. Comparison of both models with the experimental data on macroscopic strength and stress variations in dog-bone shaped samples (scale range of 1:32), shows that the model based on the assumption of uniform macroscopic stresses can only explain part of the experimental data with unrealistic values of the fitting parameters. The model which takes into account the linear part of the macroscopic stress distribution offers reasonably good accuracy. This serves as another indication that macroscopic stress nonuniformity plays a crucial role in the mechanism of size effect.     

Experimental analysis of piezomagnetic behaviour of ferromagnetic steels subjected to tensile stress

The purpose of this paper is to explore the correlation between the mechanical actions and the piezomagnetic field variations of ferromagnetic steels. The continuous examination of changes in the piezomagnetic behaviour was performed during tensile tests of Q235 and Q345 steels. The results obtained allowed the division of the magnetic behaviour of the investigated steels into four stages corresponding to four distinct loading stages. The piezomangtic fields demonstrate apparently different variation characteristics in different loading stages. Furthermore, the field intensity firstly reaches its extremum before the yield point. The variation of the ratio between the stress of the extreme point and the yield strength shows regular changes. The characteristics of the piezomagnetic responses are discussed in terms of the mechanical and microstructural condition of the steel. Additionally, a comparison of the piezomagnetic field variations in Q235 and Q345 steels demonstrates that the piezomagnetic properties are unique attributes of the material, which may result from the different chemical composition and microstructure of the two steels.     

硅泡沫材料的长期应力松弛实验研究

对硅泡沫材料的长期应力松弛进行了实验研究,得出了硅泡沫材料应力保持率与松弛时间的关系,并用现代分析测试技术对硅泡沫材料由应力松弛引起的变化进行了分析.     

Effect of electric current pulse on preparation of nano Y2O3

ABSTRACT

The effect of electric current pulse (ECP) on preparing nano Y₂O₃ powder was initially investigated in this paper. From the determination of thermogravimetric analysis, differential scanning calorimetry and X-ray diffraction, it was revealed that the composition of non-calcined precursor had changed due to the application of ECP. In addition, the scanning electron microscope results indicated that Y₂O₃ powders fabricated by ECP treatment had less aggregate and were in nano size. Therefore, the ECP treatment is a promising way to prepare nano Y₂O₃ powder.

This paper is part of a themed issue on Materials in External Fields.     

Void evolution in nanocrystalline metal film under uniform tensile stress

A theoretical model to describe the nucleation and growth of voids at triple junctions of nanocrystalline metal film under uniform tensile loading is suggested. The void growth rate controlled by grain boundary diffusion under the combined influence of void surface energy, grain boundary interface energy and elastic energy stored in the solid is evaluated. Stress relaxation during uniform tension deformation is finally discussed; the effective stress relaxation distance is also calculated. The stress relaxation not only suppresses the nucleation of voids and cracks, but also influences the void growth rate.     

On the analysis of stress state in elliptical tensile neck

The tensile necking in cylindrical and plate specimens is described based on a modification of the Bridgman theory for an isotropic case. A method of determination of principal stress trajectories in an elliptical tensile neck is put forward, which involves transformation of the initial coordinate grid by means of conformal mappings. Expressions are derived for the principal stresses and the radius of curvature of stress trajectories. Based on assumptions that depart from the Bridgman theory, an approximate analytical solution is obtained for the stress distribution in specimens with an elliptical cross-section. The new solution belongs to the one-parameter family of solutions which includes the Bridgman’s and Davidenkov–Spiridonova’s solutions. Translated from Problemy Prochnosti, No. 4, pp. 19–28, July–August, 2009.     

Observation of self-magnetic field relaxations in Bi2223 and Y123 HTS tapes after over-current pulse and DC current operation

The development of power transmission lines based on long-length HTS tapes requires the production of high quality tapes. Due to fault conditions, technical mistakes and human errors during the operation of a DC power transmission line, an over-current pulse, several times larger than the rated current, could occur. To study the effect of such over-current pulses on the transport current density distribution in the HTS tapes, we simulated two start-up scenarios for one BSCCO and two YBCO tapes. The first start-up scenario is an initial over-current pulse during which the transport current was turned on rapidly, rising to 900 A during the first milliseconds, then reduced to a 100 A DC current. The second start-up scenario is normal operation, and involved increasing the transport current slowly from 0 A to 100 A at a rate of 1 A/s. For both scenarios, we then measured the vertical component of the self-magnetic field by means of a Hall probe above the tape, and afterward, by solving a linear equation of the inverse problem we obtain the current density profiles. We observe a change of the self-magnetic field above the edge of the BSCCO and YBCO tapes during 30 min after the 5 ms of over-current pulse and during the normal operation. The current density profiles are peaked in the centre for over-current pulse, and more peaked around the edge of the HTS tape for normal operation, which means that the limited time over-current pulse changes the current density profiles of the HTS tapes. We observe also a loop of current for YBCO tapes and we show the role of the HTS tape stabilizer.     

拉应力对玻璃纤维增强聚合物基复合材料介电强度的影响

在拉应力条件下, 测试了聚合物基体和单向玻璃纤维增强聚合物基复合材料的介电强度, 探索了聚合物基体和玻璃纤维/聚合物复合材料的介电强度与拉应力的关系, 提出并证明了聚合物基体的介电强度与拉应力呈负指数关系, 复合材料中纤维与基体的界面是影响材料介电强度的主要因素。     

冷轧带肋钢筋应力松弛试验机的研制

针对冷轧带肋钢筋材料在高应力(高于松弛下限)作用下会产生流变(晶格变形、滑移),应力将随时间衰减的特性,研制该电子应力松弛试验机。该机采用MaxTest标准化模块软件系统进行应力、变形和位移闭环的自动控制,同时在结构设计上通过使用三爪夹具保证防滑,通过结构力学的核算来保证其刚度,从而具有防止试样打滑、精确地负荷和形变控制、连续并适时地绘制松驰力-试验时间曲线的特点,并设计在意外因素(停电、断电)下继续进行试验的措施,充分满足标准要求和实际工作需要。