A Modified Lattice Model of the Reversible Effect of Axial Strain on the Critical Current of Polycrystalline REBa2Cu3O7-δ Films

The strain effect on the critical current is one of the most important properties for polycrystalline YBa2 Cu3O7-δ （REBCO, RE： rare earth） films, in which the reversible effect is intrinsic in the range of strain 0 and the irreversible strain εirr. By introducing the applied strain, a modified grain boundaries （GBs） in the REBCO film is developed. lattice model combining the strain and misorientation of A good agreement of the calculation on the lattice model with the experimental data shows that the lattice model is able to well describe the reversible effect of axial strain on the critical current of the REBCO film, and provides a good understanding of the mechanism of the reversible effect of the strain. Moreover, the effects of the crystallographic texture of the REBCO film and the residual strain εr on the variation of the critical current with the applied strain are extensively investigated. Furthermore by using the developed lattice model, the irreversible strain εirr of the REBCO film can be theoretically determined by comparing the calculation of the critical current-strain curve with the experimental data.

Multi-axial strain-stiffening elastic potentials with energy bounds:explicit approach based on uniaxial data

According to the well-known models for rubberlike elasticity with strain- stii^ening effects, the unbounded strain energy is generated with the unlimitedly growing stress when the stretch approaches certain limits. Toward a solution to this issue, an explicit approach is proposed to derive the multi-axial elastic potentials directly from the uniaxial potentials. Then, a new multi-axial potential is presented to characterize the strain-stiffening effect by prescribing suitable forms of uniaxia] potentials so that the strain energy is always bounded as the stress grows to infinity. Numerical examples show good agreement with a number of test data.

Crack initiation stress and strain of jointed rock containing multi-cracks under uniaxial compressive loading: A particle flow code approach

The ratio of crack initiation stress to the uniaxial compressive strength(SCI,B/SUC,B) and the ratio of axial strain at the crack initiation stress to the axial strain at the uniaxial compressive strength(B,UCB,CI,A,A/SSSS) were studied by performing numerical stress analysis on blocks having multi flaws at close spacing's under uniaxial loading using PFC3 D. The following findings are obtained: SCI,B/SUC,B has an average value of about 0.5 with a variability of ± 0.1. This range agrees quite well with the values obtained by former research. For joint inclination angle, β=90°,B,UCB,CI,A,A/SSSS is found to be around 0.48 irrespective of the value of joint continuity factor, k. No particular relation is found betweenB,UCB,CI,A,A/SSSS and β; however, the average B,UCB,CI,A,A/SSSS seems to slightly decrease with increasing k. The variability ofB,UCB,CI,A,A/SSSS is found to increase with k.Based on the cases studied in this work,B,UCB,CI,A,A/SSSS ranges between 0.3 and 0.5. This range is quite close to the range of 0.4to 0.6 obtained for SCI,B/SUC,B. The highest variability of ± 0.12 forB,UCB,CI,A,A/SSSS is obtained for k=0.8. For the remaining k values the variability ofB,UCB,CI,A,A/SSSS can be expressed within ± 0.05. This finding is very similar to the finding obtained for the variability of SCI,B/SUC,B.

Simultaneous measurement of axial strain and temperature based on twisted fiber structure

In this Letter, an alternative solution is proposed and demonstrated for simultaneous measurement of axial strain and temperature. This sensor consists of two twisted points on a commercial single mode fiber introduced by flame-heated and rotation treatment. The fabrication process modifies the geometrical configuration and refractive index of the fiber. Different cladding modes are excited at the first twisted point, and part of them are coupled back to the fiber core at the second twisted point. Experimental results show distinct sensitivities of 34.9 pm/με with 49.23 pm/℃ and -36.19 pm/με with 62.99 pm/℃ for the two selected destructive interference wavelengths.

A CONTINUUM MODEL FOR AXIAL-STRAIN-INDUCED TORSION IN SINGLE-WALL CARBON NANOTUBES

Using the atomistic-based finite-deformation shell theory, we analytically investigate the coupling between the axial deformation and the torsion in single-wall carbon nanotubes. We find that the axial-strain-induced torsion（ASIT） response is limited only to chiral nanotubes. This response is affected by chiralities and radii of carbon nanotubes. Our results are similar to that of molecular dynamic simulations reported in the literatures.

In-plane Tensile Behaviors of Bi-axial Warp-Knitted Composites under Quasi-static and High Strain Rate Loading

The in-plane tensile behaviors of bi-axial warp-knitted(BWK) composites under quasi-static and high strain rates loading were experimentally analyzed in this article. The tensile tests were conducted along warp direction( 0°) and weft direction( 90°) at quasi-static rate of 0. 001 s^(-1) and high strain rates ranging from 1 450 to 2 540 s^(-1),respectively. It is found that the significant strain rate sensitivity can be observed in the stress-strain curves of BWK composites. The fracture morphologies of BWK composites demonstrate that the tensile failure modes are shear failure and fiber breakage under the quasi-static testing condition while interface failure and fibers pullout are at high strain rates.

Mechanical Behavior of Polyurethane Polymer Materials under Triaxial Cyclic Loading:A Particle Flow Code Approach

Polyurethane polymer grouting materials were studied with conventional triaxial tests via the particle flow code in two dimensions（PFC^（2D）） method, and the simulation results agreed with the experimental data. The particle flow code method can simulate the mechanical properties of the polymer. The triaxial cyclic loading tests of the polymer material under different confining pressures were carried out via PFC^（2D） to analyze its mechanical performance. The PFC^（2D） simulation results show that the value of the elastic modulus of the polymer decreases slowly at first and fluctuated within a narrow range near the value of the peak strength; the cumulative plastic strain increases slowly at first and then increases rapidly; the peak strength and elastic modulus of polymer increase with the confining pressure; the PFC^（2D） method can be used to quantitatively evaluate the damage behavior of the polymer material and estimate the fatigue life of the materials under fatigue load based on the number and the location of micro-cracks. Thus, the PFC^（2D） method is an effective tool to study polymers.

Entire deformational characteristics and strain localization of jointed rock specimen in plane strain compression

Shear band (SB), axial, lateral and volumetric strains as well as Poisson’s ratio of anisotropic jointed rock specimen (JRS) were modeled by Fast Lagrangian Analysis of Continua (FLAC). Failure criterion of rock was a composited Mohr-Coulomb criterion with tension cut-off. An inclined joint was treated as square elements of ideal plastic material beyond the peak strength. Several FISH functions were written to automatically find the addresses of elements in the joint and to calculate the entire deformational characteristics of plane strain JRS. The results show that for moderate joint inclination (JI), strain is only concentrated into the joint governing the behavior of JRS, leading to ideal plastic responses in axial and lateral directions. For higher JI, the post-peak stress-axial and lateral strain curves become steeper as JI increases owing to the increase of new SB’s length. Lateral expansion and precursor to the unstable failure are the most apparent, resulting in the highest Poisson’s ratio and even negative volumetric strain. For lower JI, the entire post-peak deformational characteristics are independent of JI. The lowest lateral expansion occurs, leading to the lowest Poisson’s ratio and positive volumetric strain all along. The present prediction on anisotropic strength in plane strain compression qualitatively agrees with the results in triaxial tests of rocks. The JI calculated by Jaeger’s formula overestimates that related to the minimum strength. Advantages of the present numerical model over the Jaeger’s model are pointed out.

Effect of height of rock specimen on strain localization, precursor to failure and entire deformational characteristics

Patterns of shear band, precursors to shear failure occurring in strain-softening stage, axial, lateral and volumetric strains as well as Poisson＇s ratio of plane strain rock specimens in compression for different heights were investigated by use of Fast Lagrangian Analysis of Continua（FLAC）. A material imperfection closer to the lower-left corner of the specimen was prescribed. For finer mesh, the imperfection was modeled by four null elements, while it was modeled by a null element for coarser mesh. FISH functions were written to calculate the entire deformational characteristics of the specimen. In elastic stage, the adopted constitutive relation was linear elastic; in strain-softening stage, a composite Mohr-Coulomb criterion with tension cut-off and a post-peak linear constitutive relation were adopted. Height of rock specimen does not influence shear band＇s pattern （including the thickness and inclination angle of shear band）. The slopes of the post-peak stress-axial strain curve, stress-lateral strain curve, lateral strain-axial strain curve, Poisson＇s ratio-axial strain curve and volumetric strain-axial strain curve depend on the height. Hence, the slopes of these curves cannot be considered as material properties. Nonlinear deformation prior to the peak stress is a kind of precursors to shear failure, which is less apparent for shorter specimen. For the same axial strain, lower lateral expansion is reached for shorter specimen, leading to lower Poisson＇s ratio and higher volumetric strain. The maximum volumetric strain of longer specimen is less than that of shorter specimen. The conclusions drawn from numerical results using finer mesh qualitatively agree with those using coarser mesh.

Analyses of axial,lateral and circumferential deformations of rock specimen in triaxial compression

The axial,lateral and circumferential strains were analyzed for a rock specimen subjected to shear failure in the form of a shear band bisecting the specimen in triaxial compression.Plastic deformation of the specimen stemmed from shear strain localization initiated at the peak shear stress.Beyond the onset of strain localization,the axial,lateral and circumferential strains were decomposed into two parts,respectively.One is the elas- tic strain described by general Hooke's law.The other is attributable to the plastic shear slips along shear band with a certain thickness dependent on the internal length of rock. The post-peak circumferential strain-axial strain curve of longer specimen is steeper than that of shorter specimen,as is consistent with the previous experiments.In elastic stage, the circumferential strain-axial strain curve exhibits nonlinear characteristic,as is in agreement with the previous experiment since confining pressure is loaded progressively until a certain value is reached.When the confining pressure is loaded completely,the circumferential strain-axial strain curve is linear in elastic and strain-softening stages.The predicted circumferential strain-axial strain curve in elastic and strain-softening stages agrees with the previous experiment.

Volume Change of Heterogeneous Quasi-brittle Materials in Uniaxial Compression

The volumetric strain was categorized into elastic and plastic parts. The farmer camposed of axial and lateral strains is uniform and determined by Hooke＇s law ; however, the latter consisting of axial and lateral strains is a fuaction af thickness af shear band determined by grndieat-dependeat plasticity by cansidering the heterngeneity of quasi- brittle materials. The non- uniform lateral strain due to the fact that shear band was farmed in the middle of specimen was averaged within specimen to precisely assess the volumetric strain. Then, the analytical expression for volumetric strain was verified by comparison with two earlier experimental results for concrete and rack. Finally, a detailed parametric study was carried out to investigate effects of constitutive parameters （ shear band thickness, elastic and softening rnoduli ） and geometrical size of specimen（ height and width of specimen ） on the volume dilatancy.

Axial Tensile Testing of Single Fibres

The mechanical and damage properties of single fibres used in fibrous composite have gained tremendous importance in recent years. These properties are used in determination of effective properties of composites by micromechanics. These are also used in the micromechanical damage modeling. Further, these properties are used as an indicator of the excellence of product by manufacturers. In the present study the axial tensile modulus, ultimate strength and failure strain of single fibres are determined for carbon and glass fibres. ASTM D3379-75 standard is followed and a number of fibers are tested for statistical analysis. The axial tensile moduli measured are 246.7 GPa and 93.3 GPa, respectively and strength are 3031.6 MPa and 2035.9 MPa, respectively for carbon and glass fibres. Further, the respective axial tensile failure strains are 0.0137 and 0.0224. The error in the measurement of axial modulus is below 8% while for axial tensile strength is below 1%.

Stress field of orthotropic cylinder subjected to axial compression

Based on the constancy hypothesis of material volume, the circumferential and radial stresses of a cylinder specimen are analyzed when the cylinder is subject to a loading along the axial direction. The circumferential and radial stress distribution is a power function of radius parameter when the constitutive relation of specimen material is orthotropic. The stress distribution is a quadratic function of radius parameter for transversely isotropic material. Along the cylinder axial line, the circumferential and radial stresses are maximum and equal to each other. In the circumference boundary surface, the radial stress is zero and the circumferential stress value is minimal. The failure theory of maximum tensile circumferential strain is applied to calculate the critical axial loading. The circumference-boundary-layer failure criterion of orthotropic cylinders is described with the Hill-Tsai strength theory. The obtained strength theory is related to axial stress and mechanical properties of specimen material and to the specimen axialdeformation strain rate and the change rate of strain rate.

剪切大变形条件下心墙沥青混凝土渗透特性研究

为研究心墙沥青混凝土在大剪切变形条件下的渗透特性,基于自主研发的三轴剪切轴向渗透试验系统,对70号、110号沥青混凝土开展了不同围压、渗透压组合的三轴剪切渗透试验,并提出了基于全局优化算法的沥青混凝土渗透系数反演最优模型。试验结果表明:剪胀效应引起裂隙的张开与扩展,使沥青混凝土的渗透系数大幅增加。低围压下,轴向应变从8%增加至30%,渗透系数变化幅度可达2个数量级;应变状态和应变增量影响沥青混凝土的渗透性,渗透系数随应变场的变化而改变。压密阶段,随轴向应变的增加,体积增量比为负值,渗透系数减小。裂隙发育阶段,体积增量比为正值,渗透系数增大;围压对沥青混凝土的渗透性有显著影响。两种标号沥青混凝土试样的防渗安全轴向应变阈值在低围压时较小,随着围压的增加呈递增趋势。当围压加载至1.0 MPa,轴向应变增加至30%后,沥青混凝土的渗透系数仍满足规范要求。

列车荷载作用下饱和路基翻浆冒泥特性研究

研究目的:重载列车轴重的提高增大了路基不同深度处的动应力,由此引发的翻浆冒泥等病害日益严重。本文开展精细化模型试验,针对重载列车荷载作用下饱和粉质黏土路基的翻浆冒泥特性进行研究,分析循环荷载幅值及加载频率对试样轴向应变、超孔隙水压力及细颗粒迁移特性等性质的影响。研究结论:(1)循环荷载幅值对粉质黏土路基翻浆冒泥特性具有显著的影响,随着循环荷载幅值的增加,试样产生的轴向应变及超孔隙水压力增大,路基翻浆冒泥程度也不断增加;(2)加载频率对路基翻浆冒泥特性有一定影响,随着加载频率的增加,试样的最终轴向应变及超孔隙水压力均有所减小,试验结束后翻冒的泥浆质量减少;结合试验现象来看,加载频率的增加并不能加剧翻浆冒泥的程度,但能显著缩短翻浆冒泥发生的时间;(3)循环荷载作用下路基内部的超孔隙水压力梯度驱动路基土体细颗粒迁移,从而产生翻浆冒泥病害;(4)本研究成果可为重载铁路路基翻浆冒泥病害的整治提供参考。

THE CONSTITUTIVE RELATION OF CRACK-WEAKENED ROCK MASSES UNDER AXIAL-DIMENSIONAL UNLOADING

An accurate and efficient numerical method for solving the crack-crack interaction problem is presented. The method is mainly by means of the dislocation model, stress superposition principle and Chebyshev polynomial expansion of the pseudo-traction. This method can be applied to compute the stress intensity factors of multiple kinked cracks and multiple rows of periodic cracks as well as the overall strains of rock masses containing multiple kinked cracks under complex loads. Many complex computational examples are given. The dependence of the crack-crack interaction on the crack configuration, the geometrical and physical parameters, and loads pattern, is investigated. By comparison with numerical results under confining pressure unloading, it is shown that the crack-crack interaction under axial-dimensional unloading is weaker than those under confining pressure unloading. Numerical results for single faults and crossed faults show that the single faults are more unstable than the crossed faults. It is found from numerical results for different crack lengths and different crack spacing that the interaction among kinked cracks decreases with an increase in length of the kinked cracks and the crack spacing under axial-dimensional unloading.

土工格室加筋正常固结粉质黏土应力应变响应

土工格室加筋正常固结粉质黏土对工程建设意义重大。基于土工格室与填土的相互作用机制和增量法,采用弹塑性理论、邓肯-张双曲线模型、修正剑桥屈服函数以及剪胀方程,推导了土工格室加筋正常固结黏土的应力应变响应计算模型,进行了土工格室加筋正常固结黏土的三轴固结排水剪切试验,实测不同围压下的应力应变关系,采用三轴试验结果验证了所提理论模型的正确性和可靠性。此外,采用所提理论模型进行了参数敏感分析,研究了填土的力学性质参数、格室的网格尺寸和力学性质参数对于加筋土应力应变响应的影响。研究结论表明,与砂砾料相似,土工格室约束作用对于正常固结黏土的内摩擦角影响较小,而黏聚力有所增加;随着轴向应变的增加或围压的减小,加筋效果逐渐明显;加筋正常固结黏土的强度和刚度随着非线性弹性常数k的增加、强度参数φ的增加和Rf的减小而增大,常数n对于加筋土应力应变响应影响较小。本研究成果可为工程建设提供理论借鉴。

循环荷载下加筋砂土累积应变特性试验研究

为研究加筋在砂土循环加载过程中产生的影响,以高密度聚乙烯(HDPE)网格作为加筋材料,对实际工程场地所取砂土试样开展动三轴试验,研究不同加筋层数和不同加筋间距下砂土轴向累积应变的演化趋势,并对筋材在试验过程中发挥影响作用的机制进行分析。研究结果表明:加筋能减少砂土轴向累积应变,加筋层数越多,对轴向累积应变的削减作用越强,但通过增加筋材层数带来的加筋效果增幅逐渐减小;不同加筋间距下轴向累积应变曲线均符合改进Monismith模型,在间距为80 mm时,轴向累积应变显著降低。加筋在砂土中产生影响作用的方式主要有两种:一是通过自身的弹性性能发挥作用,筋材的弹性变形恢复,导致其附近砂土颗粒的位移减少;二是在筋土界面处产生更好的咬合作用以形成似黏聚力,从而产生影响。研究结果可为加筋作用机理的深入研究提供参考。

FRP约束UHPC圆形短柱轴心受压性能研究

为研究纤维增强复合材料(FRP)种类、FRP层数和钢纤维体积掺量对超高性能混凝土(UHPC)圆形短柱轴压性能的影响,对21个FRP约束UHPC圆形短柱进行轴心受压试验,分析试件的失效破坏特征及受力机理,研究各参数对试件极限强度和极限应变的影响规律.研究结果表明:FRP层数的增加可以提高UHPC圆形短柱的极限强度,试件C12、C22、C32的极限强度比C11、C21、C31分别提高了17.8%、25.4%、23.4%;钢纤维体积掺量的增加可以使UHPC圆形短柱的极限强度和极限应变得到提高,并可以在一定程度上改善试件的脆性,试件C31的极限强度和极限应变比试件C21分别提高了2.9%和15.1%,比试件C11分别提高了4.7%和50%;在FRP层数和钢纤维体积掺量相同的情况下,碳纤维增强复合材料(CFRP)对圆形短柱极限强度的改善程度明显优于玻璃纤维增强复合材料(GFRP),试件C11、C12和C13的极限强度比试件G11、G12和G13的分别提高了9.7%、7.8%和7.2%;考虑钢纤维的约束影响,提出FRP约束UHPC圆形短柱的抗压强度和极限应变的计算模型,并进一步给出FRP约束UHPC的本构模型.

循环球应力作用下黄土变形特性研究

针对西安某地铁车站基坑Q3原状黄土,开展偏应力恒定的循环球应力加/卸载试验,对不同固结应力与不同动应力幅值条件下原状黄土的轴向变形与体应变发展规律进行分析。结果表明:循环球应力作用下,黄土会产生一定的不可逆塑性变形与可逆弹性变形,其变形增长规律与偏压程度和动应力幅值成正比,与固结应力成反比;轴向变形规律符合Monismith C L幂函数模型,且其模型参数与固结应力条件具有显著的相关性;原状黄土在加载初期体应变累积较明显,之后逐渐趋于稳定;固结应力比对体应变影响较小,固结球应力对体应变发展具有抑制作用,表明浅层土体更容易产生压缩变形。