Toughness and characterization of reactive powder concrete with ultra-high strength

A number of three-point bending and fracture tests of 200 MPa-level reactive powder concrete (RPC) with the various fiber contents have been conducted to probe the nature and characteristics of toughness of RPC200. The contribution of the embedded fibers to improving the crack-resistant capacity, energy absorption capacity and toughness with various deformation mechanisms has been analyzed. Taking account of that the first-crack deformation, peak-load deformation and their improvement varied with the fiber contents and that the deformation mechanism affected differently the performance at the first crack and the peak load, we took the peak-load deformation of plain RPC200 as the reference deformation to measure the toughness of fibered RPC200. Two toughness indices T _2(n−1)(n) and FT _2(n−1)(n) have been formulated based on P-δ responses and P-CMOD responses. The indices quantify the toughness of RPC200 with the various deformation mechanisms relative to perfectly elastoplastic materials by setting the toughness level 2(n−1) as the initial reference. It is shown that the toughness index T _2(n−1)(n) reflects the function of fibers to improve the toughness of RPC with the deformation throughout specimens, but overestimates the contribution to enhancing the toughness in post-peak periods. It underestimates, on other hands, the contribution to improving the toughness in the period from the first crack to the peak load. In contrast, the toughness index FT _2(n−1)(n) properly presents the capability that fibers absorb energy and constrain crack propagation in the matrix when the deformation is concentrated on the open crack. The proposed index unveils the contribution of fibers to toughening RPC200 both in the period from the first-crack to the peak load and in the period of post peak. This characterization method not only reveals the nature of toughness but also levels the toughness of RPC200. It could provide a way to establish an objective toughness characterization for RPC200 and facilitate its applications. Supported by the New Century Excellent Talents Program (Grant No. NCET-05-0215), the Natural Science and Engineering Research Council of Canada (Grant No. PGS D2 2006), the Laboratory Innovation Plan of Beijing Science and Education Committee (Grant No. JD102900671) and the National Basic Research Project of China (“973” Project) (Grant No. 2002CB412705)     

Effect of coagulation temperature on structures and properties of poly(<Emphasis Type="Italic">p</Emphasis>-phenylene benzobisoxazole) (DHPBO) fibers during dry-jet wet-spinning process

The effect of coagulation temperature on the morphology, microstructures and mechanical properties of dihydroxy poly(p-phenylene benzobisoxazole) (DHPBO) fibers was investigated during dry-jet wet-spinning process, in which the coagulation bath concentration and drawn ratio were kept as 10 wt% of PPA in water and 1.7, respectively. The structures and mechanical properties of the as-spun DHPBO fibers were characterized by FTIR, XRD, SEM, and single fiber tensile testing. The results indicated that in PPA/H₂O coagulation system, when the coagulation temperature was 25°C, highly crystallized DHPBO as-spun fibers possessing fine crystallites, circular and smooth morphology, and excellent mechanical properties could be achieved. Supported by the National Natural Science Foundation of China (Grant No. 50673017), Shanghai Leading Academic Discipline Project (Grant No. B603) and the Program of Introducing Taleuts of Discipline to University of People’s Republic of China (“111” Program) (Grant No. 111-2-04)     

Introduction of fluorin into PBO polymer chains: Toward higher thermal stability and lower dielectric constant

A series of novel fluorinated benzoxazole polymers (6FPBO's) with high thermal stability and low dielectric constant were synthesized by copolymerization of 1,3-diamino-4, 6-dihydroxybenzene dihy-drochloride (DAR), 1,4-benzenedicarboxylic acid (PTA) and various amount of 4'4- (hexafluoroisopro-pylidene) bis (benzoic acid) (BIS-B-AF) in the medium of polyphosphoric acid (PPA). 6FPBO fibers were then obtained via dry-jet wet-spinning technique and characterized by means of Fourier transform infrared (FTIR) spectra, thermogravimetric analysis (TGA), single fiber tensile testing machine and scanning electron microscopy (SEM). FTIR spectrum of 6FPBO fibers indicated that the fluorine groups had been incorporated into PBO molecular chains successfully. TGA curves revealed that 6FPBO fibers possessed high thermal stability just as pure PBO fibers. Moreover, dielectric constant spectrum of 6FPBO exhibited that the polymers had low dielectric constant, especially in the range of high- frequency.     

超高性能轻质混凝土的力学性能及微观结构

超高性能轻质混凝土(ultra-high performance lightweight concrete,UHPLC)是一种由高强水泥浆体、漂珠和纤维组成的密度低于1 950 kg/m~3的新型水泥基结构材料。本文研究了不同养护制度及养护龄期对UHPLC抗压强度、轴拉力学性能和弯曲性能的影响,最后利用扫描电镜观察了UHPLC中漂珠的微观形貌。结果表明:随着龄期增长,UHPLC的抗压强度、轴拉性能和弯曲性能均提高,并出现明显的应变强化现象,说明养护龄期对UHPLC基体强度和纤维-UHPLC基体的界面黏结强度均有显著提高作用;高温蒸汽养护3 d可促进UHPLC基体早期强度发展,使UHPLC的抗压强度和弯曲性能迅速达到标准养护28 d时的水平,显著缩短养护龄期,但对纤维-基体界面黏结强度的贡献不大,黏结强度仍主要受龄期影响;UHPLC实测密度为1 815.2 kg/m~3,100 mm立方体抗压强度达103.1 MPa,极限抗拉强度达7.60 MPa,极限拉应变达0.431%,出现明显的应变强化现象,弯曲峰值强度达22.43 MPa,满足了RPC160的抗折强度要求,实现了水泥基结构材料轻质高强的目标。     

硅烷涂层提升钢纤维-砂浆界面性能的试验研究

为了提升钢纤维-砂浆界面的黏结性能,采用9种基于硅烷的表面处理剂对钢纤维进行浸渍处理并高温固化成膜;埋置于水泥砂浆圆柱体试块中,开展单根纤维拉拔试验,获得拉拔荷载-位移曲线. 试验结果表明,采用不同的硅烷涂层对钢纤维进行表面改性,可以不同程度地改善钢纤维-砂浆界面的黏结性能;拉拔峰值荷载最高增加5.75倍,拉拔能耗最多增加2.48倍. 硅烷Z6011和Z6020及复合涂层能够较大幅度地提升界面黏结强度,主要增加钢纤维与砂浆界面的化学黏结力;硅烷Z6030和Z6040及复合涂层对界面黏结强度的提升幅度相对较小,主要增加界面滑移摩擦力. 采用扫描电子显微镜（SEM）研究界面黏结性能的提升机理,发现硅烷涂层使得界面过渡区的微观结构更致密,显著提升了钢纤维-砂浆之间的黏结性能.     

高温后RPC立方体抗压强度退化规律研究

为摸清活性粉末混凝土(RPC)的高温爆裂情况及高温后立方体抗压强度的退化规律,对300个70.7 mm×70.7 mm×70.7 mm的RPC立方体试件和120个40 mm×40 mm×160 mm的RPC棱柱体试件进行高温试验与高温后抗压试验,考察纤维种类、纤维掺量、温度、尺寸效应等因素对RPC立方体抗压强度及受压破坏特征的影响。结果表明：单掺钢纤维体积率为2%或单掺聚丙烯纤维体积率为0.3%时可以有效防止RPC发生爆裂;钢纤维可以有效提高RPC高温后立方体抗压强度并改善其受压破坏特征,聚丙烯纤维对抗压强度有不利影响． 高温后RPC立方体抗压强度随经历温度的升高呈先增大后减小的变化规律,通过回归分析,建立了RPC立方体抗压强度随温度变化的计算公式．     

Molecular dynamics simulation of the test of single-walled carbon nanotubes under tensile loading

Molecular dynamics (MD) simulations were performed to do the test of sin-gle-walled carbon nanotubes (SWCNT) under tensile loading with the use of Bren-ner potential to describe the interactions of atoms in SWCNTs. The Young’s modulus and tensile strength for SWCNTs were calculated and the values found are 4.2 TPa and 1.40―1.77 TPa, respectively. During the simulation, it was found that if the SWCNTs are unloaded prior to the maximum stress, the stress-strain curve for unloading process overlaps with the loading one, showing that the SWCNT’s de-formation up to its fracture point is completely elastic. The MD simulation also demonstrates the fracture process for several types of SWCNT and the breaking mechanisms for SWCNTs were analyzed based on the energy and structure be-havior.     

Anisotropic plastic deformation behavior of as-extruded ZK60 magnesium alloy at room temperature

The anisotropic plastic deformation behavior of as-extruded ZK60 magnesium alloy at room tempera-ture (RT) was investigated by compressive and tensile testing in different directions, i.e. the loading axis oriented at 0°, 45° and 90° to the extrusion direction. The relationship between texture and plastic deformation behavior were examined. The results show that the extruded ZK60 alloy exhibits a strong ring fiber texture. The mechanical properties are strongly orientation dependent. In tension testing, the 0° specimen exhibited higher yield strength and lower elongation. In compression testing, however, ZK60 alloy exhibited almost the same yield strength in three directions. The anisotropic plastic defor-mation behavior is due to strong fiber texture and the lower symmetry of the hexagonal close packed (hcp) structure of ZK60 alloy. The correlation between texture and mechanical behaviour offers the possibility to improve the mechanical properties of magnesium alloy by optimization of the material production process.     

Micro powder injection molding—large scale production technology for micro-sized components

Micro powder injection molding (μPIM), a miniaturized variant of powder injection molding, has advantages of shape complexity, applicability to many materials and good mechanical properties. Co-injection molding has been realized between metals and ceramics on micro components, which become the first breakthrough within the PIM field. Combined with the prominent characteristics of high features/cost ratio, micro powder injection molding becomes a potential technique for large scale production of intricate and three-dimensional micro components or microstructured components in microsystems technology (MST) field. Supported by National Basic Research Program of China (Grant No. 2004CB719802) and Hi-Tech Research and Development Program of China (Grant No. 2006AA03Z113)     

圆台大端纤维的拔出力分析

贝壳珍珠母中的文石片具有中间薄两端厚的特殊形貌,受此启发,把复合材料中的纤维设计成两端部为圆台状的大端异型体,可使复合材料的强韧性得以改善。本文考虑均匀分布和含量相同的圆台大端纤维和平直纤维增强复合材料,利用剪滞理论分别计算了2种纤维的与材料强韧性密切相关的拔出力。计算结果表明:圆台大端纤维的最大拔出力明显大于平直纤维的最大拔出力;基体和纤维的材料常数以及几何尺寸对最大拔出力有较大影响;在不改变基体和纤维物性时,只要合理选取形体优化设计的圆台大端纤维以及适当增加界面摩擦系数,也可使复合材料的强韧性增大。     

Design and fabrication of erbium doped photosensitive fibers

A kind of erbium doped photosensitive fiber (EDPF) was proposed and fabricated, whose core was made of double layers named photosensitive layer and erbium doped layer. The double-layer core design can overcome difficulties in fabrication of EDPF with single core design, i.e. the conflict between the high consistency rare earth doping and high consistency germanium doping. A sample was fabricated through the modified chemical vapor deposition method combined with solution doping technique. The peak absorption coefficient was 48.80 dB/m at 1.53 μm, the background loss was lower than 0.1 dB/m, and the reflectivity of the fiber Brag gratings (FBG) written directly on the sample fiber was up to 97.3% by UV-writing technology. Moreover, a C band tunable fiber laser was fabricated using the sample fiber, in which a uniform FBG was written directly on EDPFs as a reflector. A single wavelength lasing with a maximum wavelength tuning range of 1555.2–1558.0 nm was achieved experimentally. Within this tuning range, the full-width at half maximum (FWHM) of the laser output was smaller than 0.015 nm and the side mode suppression ratio (SMSR) was better than 50 dB. Supported by the National High Technology Research and Development Program of China (863 Project) (Grant No. 2007AA01Z258), the National Natural Science Foundation of China (Grant No. 60771008), Program for New Century Excellent Talents in University (Grant No. NCET-06-0076), Beijing Natural Science Foundation (Grant No. 4052023), and the Beijing Jiaotong University Foundation (Grant No. 2006XM003)     

The Young's moduli prediction of random distributed short-fiber-reinforced polypropylene foams using finite element method

The elastic moduli of short-fiber-reinforced foams depend critically on the fiber content and fiber length, as well as on the fiber orientation distribution. Based on periodic tetrakaidecahedrons, the finite element models with short-fiber reinforcement were proposed in this paper to examine the effects of the fiber content and fiber length on Young's modulus. The fiber length distribution and fiber orientation distribution were also considered. The proposed models featured in a three-dimensional diorama with random short-fiber distribution within or on the surfaces of the walls and edges of the closed-cells of polypropylene (PP) foams. The fiber length/orientation distributions were modeled by Gaussian prob-ability density functions. Different fiber volume fractions, different lengths, and different distributions were investigated. The predicted Young's moduli of the PP foams with short-glass-fiber or short-carbon-fiber reinforcement were compared with other theoretic and experimental results, and the agreement was found to be satisfactory. The proposed finite element models were proved to be acceptable to predict the Young's moduli of the grafted closed-cell PP foams with short-fiber reinforcement.     

Ultralight X-type lattice sandwich structure (II): Micromechanics modeling and finite element analysis

The methods of homogenization and finite elements are employed to predict the effective elastic constants and stress-strain responses of a new type of lattice structure, the X-structure proposed by the authors in a companion paper. It is shown that in most cases the predictions by the equivalent homogenization theory agree well with the experimental and 3-dimensional finite element calculated results. The theoretical and numerical study supports the argument that the X-structure is superior to the pyramid lattice structure in terms of mechanical strength. Supported by the National Basic Research Program of China (“973” Project) (Grant No. 2006CB601202), the National Natural Science Foundation of China (Grant Nos. 10632060, 10825210), the National “111” Project of China (Grant No. B06024) and the National High-Tech Research and Development Program of China (“863” Project) (Grant No. 2006AA03Z519)     

Study on the characteristics of erbium-doped waveguide ring laser

A theoretical model of the erbium-doped waveguide ring laser is established according to the theory of erbium-doped waveguide amplifier and the transmission matrix of waveguide directional coupler. The influence of bend radius, coupling coefficient and doped erbium ion concentration on the characteristics of waveguide ring laser is investigated. It is shown that due to the co-action of waveguide bend loss and other relevant loss there is an optimal bend radius which can provide simultaneously low threshold pumping power and high laser light output power. As one part of the resonator’s loss, the laser light coupling coefficient of directional coupler has an impact on the laser property. The analysis indicates that the laser achieves the high output power when the coupling coefficient is about 0.2. The threshold pumping power is the minimum when the doped erbium ion concentration is 0.85×10²⁶ m⁻³. Increasing the concentration of doped erbium ions will enhance the output power of laser light as long as the concentration doesn’t introduce remarkable up-conversion effect. The results give a good theoretical basis for the design and fabrication of erbium-doped waveguide ring laser devices. Supported by the National Natural Science Foundation of China (Grant No. 60807015), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 200801411037), the Young Teacher Cultivation Foundation of Dalian University of Technology (Grant No. 893210) and Doctor Start-up Foundation of Dalian University of Technology (Grant No. 893322)     

Enhanced flexural performance of epoxy polymer concrete with short natural fibers

Epoxy polymer concrete (EPC) has found various applications in civil engineering. To enhance the flexural performance of EPC, two kinds of short natural fibers with high specific strength (sisal fibers and ramie fibers) have been incorporated into EPC. The results of mechanical tests show that a small loading of natural fibers (0.36 vol%) can significantly increase the flexural strength of EPC by 25.3% (ramie fibers) or 10.4% (sisal fibers). This enhancement is achieved without any sacrifice of compressive strength of EPC. The reinforcing effects of short natural fibers on the flexural properties and compressive properties of EPC decrease with further increase in fiber content, due to the insufficient wetting of fibers by epoxy resin which results in poor interfacial bonding. The reinforcing mechanisms of short natural fibers are explored according to the observation of fracture surfaces and micromechanical modelling. It is found that the parallel model based on the rule of mixture can be a good approximation to describe the improvement in flexural strength of the short natural fiber reinforced EPC at low fiber volume fractions.     

Bending characteristic of a cantilevered magnetostrictive film-substrate system

The bending problem of a film-substrate cantilever with arbitrary film-to-substrate thickness ratio is solved exactly by employing the force equilibrium equation, and then the optimization and application of the bending characteristic of the magne-tostrictive cantilever is discussed. Furthermore, the influence of geometrical and physical parameters of the two cantilever components on the maximum free-end deflection of the cantilever is addressed. The results indicate that as the substrate thickness is kept constant, the greater film-to-substrate stiffness ratio will induce a larger deflection, while for the case of fixed total cantilever thickness, the optimal cantilever deflection is independent of the physical parameters of the materials such as Young’s modulus and Poisson’s ratio.     

最大界面剪切应力的估算与芳纶复合材料界面粘结的表征

研究以估算的最大界面剪切应力，表征芳纶Ｋｅｖｌａｒ４９复合材料的界面粘结。依据Ｇｒｅｓｚｃｚｕｋ模型和单纤维拔出实验数据估算最大界面剪切应力，研究纤维表面处理和基体改性对界面粘结的影响。选用不同活性基团表面的芳纶Ｋｅｖｌａｒ４９和两种耐高温树脂基体组成强粘结界面体系的研究结果表明，实验数据与理论曲线拟合很好。     

纤维-基体早龄期界面性能的影响因素

通过自行改造的测试系统,实现了对早龄期纤维增强水泥基复合材料体系中纤维–基体界面性能的测定,考察了龄期、基体组成、埋入深度、纤维表面特征等因素对界面性能的影响规律.结果表明:随着龄期的增加,界面的黏结强度逐渐增加,相对密实的基体组成有利于提高界面性能;随着埋入深度的增加,纤维–基体的界面黏结强度降低;而三角形截面及化学沉积改性的纤维与基体之间具有更高的界面黏结强度.     

UV处理对改善Kevlar纤维粘结性的影响

采用UV处理Kevlar纤维以改善它与树脂基体间的界面粘结性能．研究了处理时间及光敏剂对处理效果的影响。同时,通过XPS和SEM技术研究Kevlar纤维表面化学组成和表面结构的变化，通过纤维断裂拉伸实验研究UV处理对纤维抗拉强度的影响。结果表明：UV处理对纤维的损伤较小．但表面极性基团增加，比表面积增大．从而提高了Kevlar纤维／UP树脂的界面粘结强度。通过UV处理，材料的拉剪强度可提高16．5％。     

Laboratory investigation on mechanisms of stress wave propagations in porous media

A number of porous models having the similar statistical characteristics of pores and physical properties with natural sandstones have been produced using reactive powder concrete (RPC) and polystyrenes. Spit-Hopkinson-Pressure -Bar tests and CT scans have been carried out on the models with the various porosities to probe the performance of wave propagations and the responses of pores and the matrix during wave propagations. It is shown that porosities significantly influence wave propagations. For an identical impact strain rate, the greater the porosity is, the larger the amplitude of the reflected wave appears, the more the peak in the reflected wave presents, and the smaller the amplitude of the transmitted wave turns out. A single peak emerges in the reflected wave when the porosity falls down to 5%. The larger the impact strain rate, the much remarkable the phenomena. The energy-dissipated ratio of porous models, i.e., W _J /W _I , linearly increases with the increment of porosities. The ratio is sensitive to the impact strain rate. Differences in the performance of wave propagations and energy dissipation result from the varied mechanisms that pores response to impacts. For the porosity less than 10%, the mechanism appears to be a process fracturing the matrix to generate new surfaces or pores. Energy has primarily been dissipated in creating new surfaces or pores. No apparent pore deformation takes place. The impact strain rate takes little effect on pore geometry. For the porosity of 15% or more, the mechanism works depending on the impact strain rate. When a low impact strain rate applies, the mechanism still appears to crack the matrix to generate surfaces or pores, but the amount is lower as compared to the case with a low porosity. If a large impact stain rate applies, the mechanism combines both fracturing the matrix and deforming the pores, with the deforming pores predominating. The vast majority of energy has been dissipated to deform pores. Only high porosity and impact strain rate can bring significant deformation to the pores. The proposed eccentricity of pores is capable of characterizing the geometry of pores and its change during wave propagations. Supported by the Natural Sciences & Engineering Research Council of Canada (PGS-D2-2006), the National Basic Research Program of China (“973” Project) (Grant No. 2002CB412705), and the New Century Excellent Talents Program of the Ministry of Education of China (NCET-05-0215)