Microscale investigation into mechanical behaviors of heat-bonded nonwoven geotextile using DEM

Heat-bonded nonwoven geotextiles (HBNGs) made from synthetic fibers are widely used in engineering practices. One of the challenges on the way is to link the properties of fibers and the fabric's microstructure to the deformation and failure mechanisms of HBNGs. In this study, a random distribution geometry method was developed to reproduce the complex fibrous structure of HBNG. A piecewise linear model was adopted to reproduce the nonlinear stress-strain relationships of single fibers. The present method has been successfully applied in the simulation of uniaxial and biaxial tensile tests and puncture test. The orientation distribution of fibers and the mechanical behaviors (e.g., deformation, strain localization, force-strain relationship) of HBNG specimen were reasonably simulated. Specifically, the hourglass shape during uniaxial tensile test, the axisymmetric deformation pattern during biaxial tensile test and the trumpet shape during puncture test were all well reproduced. The present method provides an applicable tool to study the complicated mechanical behaviors of HBNG and is also helpful to obtain a better understanding of its deformation and failure mechanisms.     

弯曲试验确定岩石单轴抗拉强度的新方法

岩石的单轴抗拉强度是岩石的一个重要力学参数。四点弯曲试验法是获取岩石单轴抗拉强度的一种间接方法,但四点弯曲试验的传统公式计算出来的单轴抗拉强度往往比实际的单轴抗拉强度大得多,有时甚至是直接拉伸试验抗拉强度的2~3倍。因此,需要对传统公式进行改进,得出新的抗拉强度计算公式,并在四点弯曲试验中获得与实际比较符合的单轴抗拉强度。在岩石拉压模量不同的基础上,考虑岩石破坏前的塑性变形行为,通过弹塑性力学的方法精确地推导出四点式弯曲试验的单轴抗拉强度计算公式,并分析了新的抗拉强度计算公式的结果,获得与实际情形比较符合的岩石单轴抗拉强度。     

Biaxial tensile behavior of spunbonded nonwoven geotextiles

Geotextiles can be successfully employed for any geotechnical application when they are able to sustain pre-defined levels of tensile stresses. The biaxial tensile test has an advantage over other tensile test methods in that it does not allow “necking” during deformation which simulates the operational conditions of geotextiles under confined stresses. In this study, the model for uniaxial tensile behavior of nonwovens has been modified to investigate the biaxial tensile behavior of spunbonded geotextiles. The model has included the effect of fiber re-orientation, stress-strain behavior of constituent fibers, and physical characteristics of nonwovens when the geotextile specimen is laterally constrained. A comparison is made between predicted and experimental stress-strain curves obtained from previous work (Bais-Singh and Goswami, 1998). Theoretical findings of biaxial tensile behavior obtained using the layer theory are also critically discussed. In addition, it has been revealed that fiber re-orientation is a key factor in translating the random spunbonded nonwoven geotextiles to anisotropic structures under defined biaxial tensile stresses.     

Unified calculation method and its application in determining the uniaxial mechanical properties of concrete

This paper presents a unified calculation method and its application in determining the uniaxial mechanical properties of concrete with concrete strengths ranging from 10 to 140 MPa. By analyzing a large collection of test results of the uniaxial mechanical properties of normal-strength, high-strength and super high-strength concrete in China and performing a regression analysis, unified calculation formulas for the mechanical indexes of concrete are proposed that can be applied to various grades of concrete for determining the size coefficient, uniaxial compressive strength, uniaxial tensile strength, elastic modulus, and strain at peak uniaxial compression and tension. Optimized mathematical equations for the nonlinear stress-strain relationship of concrete, including the ascending and descending branches under uniaxial stress, are also established. The elastic modulus is almost constant throughout the elastic stage for the ascending branches of the stress-strain relationship for concrete. The proposed stress-strain relationship of concrete was applied to the nonlinear finite element analysis of both a steel-concrete composite beam and a concrete-filled steel tubular stub column. The analytical results are in good agreement with the experiment results, indicating that the proposed stress-strain relationship of concrete is applicable. The achievements presented in this paper can be used as references for the design and nonlinear finite element analysis of concrete structures.     

耐碱玻纤网格与聚丙烯混凝土复合加固方柱的轴压试验研究及承载力计算

通过16个基于耐碱玻纤网格与聚丙烯混凝土复合加固方柱的轴压性能试验,得到了各试件的极限承载力与破坏形态,测得了各试件混凝土中部纵向压应力-应变曲线、试件压应力-耐碱玻纤网格中部应变曲线,分析了加固层混凝土中聚丙烯纤维含量、耐碱玻纤网格包裹方式等参数对试件轴压性能及破坏机理的影响。通过引入合理的基本假定和强度模型,提出了耐碱玻纤网格与聚丙烯混凝土复合加固方柱的轴压承载力计算方法,并将所提计算方法及相关文献计算方法得到的理论计算值与试验结果进行了比较。结果表明:试件主要破坏形态为纵向压劈破坏,裂缝最先出现在试件角部区域并向两端发展,部分试件出现加固层剥离现象; 相较于耐碱玻纤网格间隔包裹试件,全包试件表现出更好的抗压性能及延性; 随着加固层混凝土中聚丙烯纤维含量的增加,试件极限承载力呈先提高后降低的趋势; 采用所提计算方法得到的理论计算值与试验结果吻合较好,且理论计算值均小于试验值,说明所提公式具有一定安全储备,可供工程设计参考使用。     

钢纤维混凝土抗拉性能试验研究

研究了钢纤维体积率、钢纤维长径比、钢纤维类型对钢纤维混凝土劈裂抗拉强度、轴心抗拉强度及轴心受拉应力-应变全曲线的影响规律。试验采用两端埋设钢筋的变截面轴心受拉试件并在普通万能试验机上加辅助刚性架,进行钢纤维混凝土轴心受拉应力-应变全曲线测试。研究表明,随着钢纤维体积率、长径比的增大,钢纤维混凝土劈裂抗拉强度、轴心抗拉强度呈线性增大规律。采用设计的试验装置可测得钢纤维混凝土轴心受拉应力-应变全曲线,分析了纤维掺量和基体混凝土强度对轴心受拉应力-应变全曲线的影响规律。根据试验数据分析,提出了钢纤维混凝土轴心抗拉强度与劈裂抗拉强度间的关系式,提出了钢纤维混凝土轴心受拉应力-应变全曲线的解析表达式。     

Nonwoven geotextiles from nettle and poly(lactic acid) fibers for slope stabilization using bioengineering approach

This article deals with needle-punched nonwoven geotextiles prepared from nettle and poly(lactic acid) fibers in different weight proportions for potential slope stabilization application using bioengineering approach. The geotextiles were tested for tensile strength, biodegradability, and enhancement of soil fertility. The tensile strength of the geotextiles was found to decrease with addition of stronger nettle fibers. This apparently surprising behavior was explained in the light of theoretical tensile mechanics of nonwovens. Further, the nettle fibers displayed higher biodegradability than the poly(lactic acid) fibers, and when buried under soil, all the geotextiles exhibited a loss in tensile strength. Interestingly, the fertility of the soil was remarkably improved after biodegradation of poly(lactic acid) fibers. Overall, the nonwoven geotextiles prepared in this work were found to be promising for slope stabilization application.     

土工直接拉伸试验装置的研制及应用

针对现有土工直接拉伸试验装置的不足,研制了一套卧式直接拉伸试验装置;装置由拉伸加载系统、数字液压伺服控制系统、数据采集分析系统3部分组成。试验装置可开展多种易成型材料的直接拉伸试验,能精确测试材料的抗拉强度并能给出拉应力-应变曲线;巧妙设计了加载夹具解决了试验材料与拉伸装置的连接难题;能精确连续控制试验拉力,并能获得材料抗拉强度峰后段的拉应力-应变曲线。使用新研制的试验装置开展了黏土、铁晶砂相似材料和水泥砂浆的直接拉伸试验,试验结果表明：在黏土直接拉伸试验所涉及的含水率范围内,当干密度保持不变时,黏土的抗拉强度随含水率的增加逐渐降低,而峰值拉应变则随含水率的增加逐渐增加;黏土达到峰值强度后,并不会突然断裂而是存在一个软化段,此时土体仍然具有一定的承载能力;铁晶砂相似材料的抗拉强度随着松香酒精溶液浓度的增加而提高;随灰砂比的提高,水泥砂浆的抗拉强度和拉伸弹性模量均相应提高。     

土工织物散体桩桩体大三轴试验研究

对密实状态的土工织物散体桩桩体进行大三轴试验,研究其在不同围压和不同聚丙烯土工编织布筋材强度下的应力–应变特性,并据此提出桩体强度及模量理论计算公式。研究结果表明:土工织物散体桩桩体在三轴压缩下呈剪胀破坏,其剪切带上主要是筋材的横向筋丝产生断裂,且桩体剪切角与碎石的莫尔–库仑理论破裂角比较接近;在加载初期,土工织物散体桩应力–应变曲线下凹,而后近似线性增长至峰值应力,随后应力随应变减小并趋于稳定,表现为应变软化特性;同一筋材强度、不同围压的土工织物散体桩桩体强度所对应的轴向应变值比较接近;土工织物散体桩桩体的似黏聚力随筋材强度呈较好的线性增长关系,其较碎石的似黏聚力大很多,而筋材对桩体碎石的内摩擦角影响不大;建立了土工织物散体桩桩体强度及模量的理论计算公式,并采用试验值对理论公式进行修正,经修正后的结果与试验值吻合很好。     

钛-钢复合板拉伸力学性能研究

为了研究钛钢复合板材的力学性能,对TA2+Q235B复合板材进行拉伸剪切试验.基于复合比的影响,通过试验分析得到钛钢复合板的基本力学性能指标.结果表明,钛钢复合板的应力应变曲线与复合比的大小有关.随着拉伸强度的加大,复合比增大,屈服阶段逐渐消失,弹性模量减小,但钢材的抗拉强度变化并不明显.本文基于钛钢复合板的抗拉力学试...     

Puncture resistance of polyester (PET) and polypropylene (PP) needle-punched nonwoven geotextiles

It is common practice to use needle-punched nonwoven geotextiles as puncture protection for geomembranes against sharp objects like gravel or stones in either the soil above or the underlying soil/rock below. There are several design and experimental methods available for geotextile selection in this regard. None, however, directly address the type of resin or fiber from which the geotextile is made. This paper does exactly that insofar as a direct comparison of similar mass per unit area polyester (PET) versus polypropylene (PP) geotextiles are concerned. Furthermore, two types of PP geotextiles are evaluated; one made from continuous filaments and the other from staple fibers. Three different size and shaped puncture probes are used in the testing program. All three are ASTM Standards, i.e., D4833, D5495 and D6241.The test results clearly indicate that geotextiles made from PP fibers outperform those made from PET fibers at all masses evaluated. Clearly, the present trend of using PP resin for heavy nonwoven protection geotextiles seems justified on the basis of these test results. In addition, the continuous filament PP and staple fiber PP geotextiles performed equivalently over all mass ranges for the three different types of puncture tests.     

Changes in filtration opening size of woven geotextiles subjected to tensile loads

A modified hydrodynamic sieving technique in which the geotextile is subjected to a tensile load is described. This load may be either uni-axial or bi-axial. To date tests have been conducted on two different woven slit-film polypropylene geotextiles and the results illustrate a marked change in the filtration opening size of the geotextiles as the tensile load is increased. The opening size of the thicker geotextile decreased with increasing biaxial load, whereas the opposite occurred for the thinner of the two geotextiles. The geotextiles were loaded up to only about 10% of their minimum ultimate tensile strength and the filtration opening size changed by up to 28%. It is suggested that this effect cannot be ignored in applications where there are in-plane tensile stresses.     

Effect of bituminous impregnation on nonwoven geotextiles tensile and permeability properties

Geosynthetics interlayer systems are effective techniques to control reflective cracking in damaged pavements. It comprises the inclusion of nonwoven geotextiles between the damaged layer and the new overlay of the pavement to reduce the propagation of cracks and to extend pavement life. However, the success of this technique depends directly on the understanding of the geotextile's behavior when impregnated with asphalt. This paper evaluates different nonwoven geotextiles frequently used in anti-reflective cracking systems, focusing on initial stiffness gain and permeability reduction after asphalt impregnation. Fresh and impregnated samples of polyester and polypropylene nonwoven geotextiles were tested. Cationic rapid setting emulsified asphalt was used as asphalt binder. Wide-width tensile tests were carried out based on the specification of ABNT - NBR 12824 (1993). Water vapor transmission tests were conducted according to ASTM E 96M (2005). Results of tensile tests on impregnated geotextiles showed a significant increase on tensile strength values, probably due to the inter contact of the fibers. Results also showed high increase in strength values at strain levels less than 0.05% and decrease on stiffness gains with increase of strains. Water vapor transmission tests demonstrated that cationic asphalt emulsion applied on nonwoven geotextiles allows a drastic reduction in permeability values to turn nonwoven geotextiles into a low permeability barrier.     

Investigation of tensile strength on alkaline treated and untreated kenaf geotextile under dry and wet conditions

Geosynthetics or geotextile is used for aggregate separation, soil reinforcement, filtration, drainage and moisture or liquid barriers in geotechnical applications. Because of the environmental issues, a bio-based material is introduced as a sustainable construction material. The kenaf fibre is a bio-based material available in the tropical countries. It can be potentially used as a geotextile because of its high tensile strength. This paper presents the tensile strength characteristics of kenaf geotextile, manufactured with and without sodium hydroxide (NaOH) treatment. The tensile strength of kenaf geotextile was determined by using the wide-width strip test based on the ASTM D4595-17 standard. Because the kenaf fibre has a high water absorption capability, the effect of wet and dry conditions on tensile behaviour of kenaf textile was studied. Two patterns of woven kenaf with two different opening sizes between their yarns (0 × 0 and 2 × 2 mm)—plain and incline patterns were studied. In addition, the tensile strength of the kenaf geotextiles, buried in natural ground, was examined after a one-year period. The tensile strength of kenaf geotextiles was higher for the smaller spaces between the yarns. Furthermore, the tensile strength and elongation were lower under wet condition. The alkaline treatment (6% concentration of NaOH) significantly improved the tensile strength of the woven kenaf geotextile. The tensile strength of the treated kenaf geotextile was higher than that of the untreated one, for both short and long-term conditions, showing the advantage of NaOH treatment.     

Brazilian test: stress field and tensile strength of anisotropic rocks using an analytical solution

Tensile strength of rock is among the most important parameters influencing rock deformability, rock crushing and blasting results. To calculate the tensile strength from the indirect tensile (Brazilian) test, one must know the principal tensile stress, in particular at the rock disc center, where a crack initiates. This stress can be assessed by an analytical solution. A study of this solution for anisotropic (transversely isotropic) rock is presented.

The solution is given explicitly. The key expansion coefficients are obtained from a complex-valued 2×2 matrix equation. The convergence of the solution is greatly improved by a new procedure. It is shown that the dimensionless stress field depends only on two intrinsic parameters, E′/E and b. The stress at the center of the disc is given in charts as a function of these parameters (and the angle θ_b between the direction of applied force and the plane of transverse isotropy). Furthermore, a new, reasonably accurate, approximate formula for the principal tension at the disc center, (0,0), is derived from the analytical solution:

where

The elastic parameters of rock in two perpendicular directions were measured in the laboratory. The result of the stress analysis was applied in calculating the indirect tensile strength of gneiss, which has a well-defined foliation plane (transversely isotropic). When the results were compared with the tensile strength of rock obtained by using a conventional formula that assumes isotropic material, there was a significant difference. Moreover, good agreement was observed for the tensile strength calculated from the stress charts and the proposed formula, when compared with other published stress charts.     

盐岩拉伸破坏力学特性的试验研究

利用MTS815 Flex Text GT岩石力学试验系统,并利用PCI-Ⅱ声发射测试系统及SMZ1000体视显微镜和CCD实时摄像系统,采用间接拉伸与直接拉伸两种试验方法,对层状盐岩拉伸破坏力学特性进行了综合试验研究。研究获得了两种测试方式的破坏全过程曲线,揭示了间接拉伸、直接拉伸强度相互关系,及其与单轴抗压强度的相互关系。研究表明,直接拉伸试验测得的层状盐岩抗拉强度低于间接拉伸试验,直接拉伸试验得到的结果更加真实反映盐岩的抗拉强度特性,建议尽可能采用直接拉伸试验方法测试盐岩抗拉强度;研究得到了两种拉伸破坏方式的声发射空间分布特征,揭示了与之对应的受力状态和导致破坏的损伤演化规律;不同拉伸破坏方式的岩石断面形貌研究表明,间接拉伸以穿晶断裂为主,直接拉伸以沿晶断裂为主,这种断裂破坏方式的差异是导致不同拉伸测试方式强度差异的原因。     

再生混凝土单轴力学性能指标统一计算方法

对大量再生混凝土单轴力学性能试验资料进行整理分析,基于统计分析方法,在借鉴普通混凝土力学模型的基础上,对不同取代率下再生混凝土的各种力学性能指标进行统一分析,提出了适用于不同取代率下再生混凝土的立方体抗压强度、轴心抗压强度、轴心抗拉强度、劈裂抗拉强度、弹性模量、轴心受压峰值应变和轴心受拉峰值应变的统一计算公式,建立了再生混凝土单轴受压、受拉的应力 应变全曲线表达式,并将公式计算结果与试验结果进行对比。结果表明:计算结果与试验结果吻合较好;提出的再生混凝土单轴力学性能指标统一计算公式精度较高。     

玄武岩筋抗拉性能试验研究

通过对φ8 mm的玄武岩筋(BFRP筋)进行拉伸试验,观察了BFRP筋的破坏模式和试验过程中的现象,测定了其极限抗拉强度、弹性模量和伸长率等力学指标,根据试验结果绘制了BFRP筋的应力—应变曲线,结果表明:BFRP筋应力—应变曲线呈直线,破坏时没有预兆,没有屈服平台,是一种典型的脆性材料。     

The influence of uniaxial tensile strain on the pore size and filtration characteristics of geotextiles

The influence of uniaxial tensile strain on the pore size distribution and filtration characteristics of geotextiles is studied. An experimental apparatus was designed and used to conduct tests for pore size distribution, flow rate through the geotextiles and the gradient ratio. Four geotextiles made of polypropylene (two heat-bonded nonwoven and two slit film woven) were studied. Throughout the test series, the geotextiles were stretched to maintain 5%, 10% and 20% in-plane uniaxial strains. The strained specimen test results were compared with those from unstrained specimen. The experimental results illustrate the pore size and the mean flow rate through the plain geotextiles increase with the increase in tensile strain. The differences in changed percentages for apparent opening size and flow rate between the two nonwoven geotextiles are much higher than those between the two woven geotextiles. The increase in tensile strain results in reduction in the gradient ratio for the soil–geotextile system. This effect is more pronounced for nonwoven geotextiles. More testing is recommended to gain a deeper understanding into tensile strain effect on various geotextiles.     

高强箍筋约束高强混凝土轴心受压应力-应变全曲线研究

为研究高强箍筋约束高强混凝土的受压力学性能,进行了高强箍筋约束高强混凝土棱柱体试件的轴心受压试验,分析了配箍率、箍筋屈服强度和箍筋形式对约束高强混凝土应力-应变曲线的影响。结果表明：高强箍筋在约束混凝达到其峰值应力时并未屈服,根据其受力机理提出了有效侧向应力和相应的高强箍筋应力的计算方法;高强箍筋可在约束混凝土应力-应变曲线下降段提供更有效的侧向约束,曲线下降较为平缓。在试验研究和理论分析的基础上,提出了约束高强混凝土的强度和变形的计算式,建议了高强箍筋约束高强混凝土的应力-应变全曲线方程,理论曲线与实测曲线吻合较好。