Fracture behavior and analysis of fiber reinforced concrete beams

Fiber reinforced concrete beams with varying notch depths and different volume fractions of steel fibers were tested. The results were analyzed to examine the applicability of various fracture mechanics approaches including: critical stress intensity factor, J-integral, critical crack opening displacement, compliance technique for determining the slow crack growth and R-curve analysis. Attempts were made to identify a fracture parameter which is independent of test-specimen geometry and which can correctly predict the effects of fiber addition. R-curve method appears to be promising.     

Microstructure and properties of CNTs reinforced CVI-pyrocarbon matrix

Pyrocarbon (PyC) matrices were prepared in two kinds of quartz fiber preforms by chemical vapor infiltration (CVI), and then the fibers were leached by HF. Effects of CNTs on the microstructures and mechanical properties of the quartz fiber reinforced carbon composites and PyC matrices, as well as the interface behaviors of the fiber reinforced composites, were discussed. Randomly oriented CNTs reinforced PyC micro-composites account for the pseudo ISO structure and contribute to the mechanical properties of the PyC matrix. Relative strength between reinforcement and matrix and interface bonding significantly affect the mechanical behaviors of the quartz fiber reinforced pyrocarbon composites: Quartz fiber with low strength and strong interface bonding result in limited strengthening effect on flexural strength of the fiber reinforced composite; low strength unidirectional quartz fiber and weak interface bonding in a much stronger matrix result in limited strengthening effect on tensile strength of the composite.     

Experimental dynamic analysis of polymer-based nanocomposite beams under low-velocity impact loading

An experimental study was performed on low-velocity impact response of nanocomposite beams reinforced with different loadings of nanoclay (0, 3, 5 or 7 wt%). Using two commercially organo-modified nanoclays (Cloisite 30B and Cloisite 15A), as well as two types of manufacturing methods (hand lay-up and vacuum assisted resin transfer molding), a set of composite/nanocomposite beam samples was prepared. To apply identical boundary conditions along the width of the beam, a cylindrical impactor was selected, whose lateral surface was in contact with the beam. The comparative results of the low-velocity impact (LVI) and quasi-static tests for the beam with 3 wt% Cloisite 30B showed that energy absorption capacity of dynamic loading and quasi-static one was equal to 20.75 and 40 J, respectively. Therefore, it was deduced that the energy absorption capacity of an LVI sample was approximately twice as much as that of a quasi-static one. In addition, the smaller after-impact damage to the beams with a higher content of nanoclay was justified as a result of the impermeable nature of nanoclays. Moreover, it was observed that addition of Cloisite 15A significantly increased the energy absorption capacity of the composite than that of the Cloisite 30B. However, increasing the weight percentage of Cloisite 15A had no significant effect on energy absorption capacity of nanocomposite. The reason of the last result has been described in the paper with a precise study of the microstructure of the materials.     

Synthesis and characterization of CNTs/POM nanocomposite acetabular hip cup

Polyoxymethylene (POM) considered as the most appropriate alternative for ultra-high molecular weight polyethylene (UHMWPE) in the hip joint replacement application due to their biocompatibility, high mechanical properties, and cheapness. The wear is the main cause of the failure in the hip joint and the wear resistance of UHMWPE is still better than the wear resistance of POM. This research aims to improve the wear behavior of POM by blending it with 0.02?wt% of functionalized carbon nanotubes (CNTs) and using paraffin oil dispersion technique to obtain a uniform dispersion. The injection molding and machining process were used to produce the new (CNTs/POM) nanocomposite acetabular hip cup which has a high wear performance. The wear rate of the CNTs/POM cups was evaluated using a total leg joint’s simulator at 1,000?N for 3 million cycles under serum-based lubricated conditions. Moreover, the wear mechanism of cups was examined by scanning electron microscopy as well as the dispersion of CNTs inside the cup matrix. The results show that the wear resistance of POM cup has been improved by adding functionalized CNTs ～402% and ～221%, when compared with a virgin POM and UHMWPE, respectively, because of increasing the melting temperature and crystallinity degree.     

碳纳米管增强环氧／石墨复合材料的制备与性能研究

采用粉末冶金法制备了碳纳米管增强环氧／石墨复合材料,并研究了酸洗处理对复合材料弯曲强度、硬度和导电性能的影响。结果表明：与未处理碳纳米管相比,酸处理的碳纳米管增加了环氧／石墨复合材料的弯曲强度和硬度,降低了电阻率。酸处理的碳纳米管增强环氧／石墨复合材料的弯曲强度达到21．9MPa,比未添加碳纳米管时提高了近22％;同时复合材料的硬度达到最大值21．7HS,比未添加碳纳米管时提高了近10％;复合材料的电阻率达到了最小值45036μΩ·cm,比未添加碳纳米管时复合材料的电阻率降低了近17％。     

钢-混凝土组合梁纵向抗剪非线性分析

在钢混凝土组合梁中,栓钉传递的剪力作用在混凝土板上,当横向钢筋配筋率低于一定值时,混凝土板会发生纵向剪切破坏,从而会导致组合梁的极限承载力降低,不能充分发挥组合梁的优势。采用了ANSYS软件对组合梁进行了非线性有限元分析,并结合试验结果探讨了横向钢筋对组合梁极限受弯承载力的影响,同时对组合梁的挠度及滑移进行了分析。计算结果表明,为保证组合梁混凝土板不发生纵向抗剪破坏,横向钢筋配筋率应不小于0.6%,利用给出的模型可反映出栓钉对混凝土板的作用。     

Micromechanical properties of hydroxyapatite nanocomposites reinforced with CNTs and ZrO2

This study examined the mechanical properties, wettability, and tribology of hydroxyapatite (HA)–zirconia (ZrO₂)–carbon nanotube (CNTs) ceramic nanocomposites (with various CNT ratios (x): 1, 5, and 10 wt%). HA–ZrO₂–CNT-x powders were hydrothermally synthesized. Hot isostatic pressing (HIP) and cold isostatic pressing were used to manufacture solid and dense tablets; consolidation was performed by sintering the nanocomposites under Ar gas at 1150 °C during HIP. The microstructure and morphology of the nanocomposites were characterized via transmission electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffractometry, Fourier transform infrared (FTIR), and scanning electron microscopy. The effects of ZrO₂ and CNTs on the mechanical characteristics of the nanocomposites were examined via nanoindentation, reciprocating wear, and Vickers hardness tests. The microhardness of HA–ZrO₂–CNT-1% and HA–ZrO₂–CNT-5% increased by 36.8% and 66.67%, respectively, compared with that of pure HA. The nanohardness of the HA–ZrO₂–CNT-1%, HA–ZrO₂–CNT-5%, and HA–ZrO₂–CNT-10% samples was 8.3, 9.65, and 8.02 Gpa, and the corresponding elastic modulus was 83.72, 114.34, and 89.27 GPa, respectively. Both of these parameters were higher than those of pure HA. However, in the nanocomposite reinforced with 10% CNT, as opposed to those with lower CNT ratios, their values were lower. Additionally, HA–ZrO₂–CNT-10% was the most hydrophilic nanocomposite synthesized in this study with a contact angle of 48.8°.     

VARI成型泡沫夹层复合材料界面质量控制方法研究

采用本体树脂涂覆、胶膜和辅助织物粘贴在泡沫加工表面的方法分别改善胶接界面,解决了真空树脂浸渗工艺(VARI)制备的非屈曲碳纤维织物(NCF)/聚甲基丙烯胺(PMI)泡沫夹芯复合材料中泡沫与蒙皮的脱粘问题。结果表明,在相同VARI工艺参数条件下,采用890树脂涂覆的方法可以达到大约80%区域面积的良好粘接界面,对应的抗平拉强度提高了30%;同时胶膜和辅助织物粘贴方法则可达到接近100%改善胶接界面质量的效果,其对应的抗平拉强度则分别提高了76%和56%。     

Synthesis and characterization of transparent alumina reinforced polycarbonate nanocomposite

Transparent nanocomposite films were fabricated by blending a concentrated nanocomposite formed by in-situ polymerization of polycarbonate in the presence of alumina nanowhisker with a high molecular weight polycarbonate. Polycarbonate was grafted to the alumina nanowhisker surface to improve nanofiller dispersion and load transfer to polymer matrix. Fourier transform infrared spectroscopy confirmed the functionalization of the nanowhisker with polycarbonate. Samples produced using functionalized alumina exhibited improved dispersion compared to the raw alumina nanowhiskers in the PC. Functionalization of alumina nanowhisker enhanced tensile properties (Young’s modulus and tensile strength) relative to the pure polycarbonate and blends produced with raw alumina nanowhisker. Additionally, the nanocomposite formed using in-situ polymerization showed small decreases in transparency in the visual range compared to the base polymer with increased absorption in the UV range. The effect of reaction temperature during in-situ polymerization on the properties of the nanocomposite was investigated. Higher reaction temperature resulted in improved dispersion and sharp increases in tensile modulus and shear strength.     

Parametric finite element analysis of FRP reinforced concrete beams in fire and design guidelines

FRP bars are made of innovative materials, and use of these bars in residential and commercial buildings and infrastructure could result in their increased applications. This requires establishment of fire resistance of the FRP bar RC. This paper describes the results of a parametric study that was carried out on hybrid and carbon FRP bar RC beams. The influence of concrete strength and load ratio on the high temperature performance of beams was investigated. The study used finite element modelling and was conducted with the help of numerical models that were calibrated previously by the authors against the data of experimentally tested beams. It was found that the beam strength and stiffness reduce in the same proportion between two consecutive load ratios and are nearly uninfluenced by the concrete strength. The amount of load was found to be a critical factor for the beam thermal resistance. Preliminary guidance for FRP RC beam design in fire situation is provided on the basis of findings of the study. Copyright © 2013 John Wiley & Sons, Ltd.     

Fabrication and characterization of hollow nanofibrous PA6 yarn reinforced with CNTs

Core-sheath nanofibrous yarns were obtained through electrospinning of polyamide 6 (PA6) solution containing different concentrations of multi-wall carbon nanotubes (MWNTs) as sheath and PVA multifilament as the yarn core. By dissolving PVA, for obtaining conductive hollow nanofibrous PA6/MWNTs yarn, two types of porosity could be obtained including hollow central tube due to the structure of hollow yarn and nano-porous areas embedded in electrospun nanofibers. SEM results showed that the diameters of nanofibers were varying in the range of 103–145 nm obeying MWNTs concentrations and TEM results revealed that the MWNTs were embedded in nanofiber matrix as straight and aligned form. DSC analysis showed that electrospinning process caused the formation of less-ordered γ phase in nanofibers. The electrical conductivity of yarns increased from 10^?13 S m^?1 to 2.4?×?10^?6 S m^?1 with increasing the concentration of nanotubes from 0 wt.% to 7 wt.%.     

Experimental characterization of concrete beams elements reinforced by long fiber chips

This work deals with the experimental study of the mechanical behavior by three-point bending tests and shear (fracture shear) of concrete beam elements reinforced by long fibers. These fibers come from machined steel parts waste. Bending and shear tests are conducted on prismatic specimens [10 × 20 × 120] cm³. Compressive strength tests are carried out on cylinders of 16 cm diameter and 32 cm height. The fibers are characterized in terms of mechanical strength and tear strength. The composition of the concrete is determined by the experimental method called ‘Dreux-Gorisse.’ Two different fiber contents were used for this study (W = 0.6%, 0.8%) and a control concrete BT (W = 0%) of the same composition as the matrix is prepared as a reference. The comparison of the different results obtained shows that long fibers provide a significant ductility to the material after concrete cracking. Also, the used fibers limit diagonal shear cracks and improve the strength and rigidity in bending and shear.     

Study of modal parameters and vibration signatures of notched concrete prisms

In the present study, vibration tests have been carried out on notched concrete prisms. The tests have been conducted to study modal parameters and vibration signatures in order to enhance the knowledge of monitoring concrete structures. Notch (artificially introduced crack) of constant widths and varying depths has been introduced in the concrete prisms at different locations. The specimens have been subjected to impact excitations by dropping a specific weight from a fixed height and at a particular location. The natural frequencies of notched and intact prisms have been experimentally measured. The frequency response functions (FRF) as obtained from multichannel pulse analyzer have been synthesized to evaluate the fundamental mode shape of vibration of the prisms by a curve fitting method. These curves have been further post-processed to obtain the modal curvature values. Pattern Recognition Scheme is applied to synthesize vibration signatures for the evaluation of the curvature differential values. The curvature differential values corresponding to different sensor locations in both intact and notched specimens have been obtained. Experimental results with different depth of flaws suggest that natural frequency alone cannot be a reliable parameter for the detection of damage in concrete beams with shallow depth of flaws. The modal curvature values have been found to be a reasonably good indicator of the location of the damage. The curvature differential values show the extent of damage but found to be dependent on the sensor location from the position of notch.     

Preparation and characterization of tough and highly resilient nanocomposite hydrogels reinforced by surface-grafted cellulose nanocrystals

Despite many strong and tough hydrogels have been fabricated according to the energy dissipating mechanism, they usually lack high resilience due to the presence of large hysteresis. Herein, poly (N-vinylpyrrolidone) grafted cellulose nanocrystal (CNC-g-PVP) was used as special multifunctional physical crosslinkers to fabricate tough and highly resilient nanocomposite hydrogels. CNC-g-PVP with varying loading was incorporated into chemically crosslinked polyacrylamide (PAM) networks by in-situ radical polymerization to give PAM/CNC-g-PVP nanocomposite hydrogels. Robust cooperative hydrogen bonds existed between the surface-grafted PVP chains and the PAM matrix, which could rupture to dissipate energy upon deformation and recover instantly on the removal of stress. This unique energy dissipating mechanism led to excellent mechanical performance of the hydrogels. Their tensile elastic modulus, toughness, and compressive strength are 1.4–1.8, 2.1–3.0, and 1.44–2.73 times of pure PAM hydrogel, respectively. Moreover, the hydrogels exhibit low hysteresis, high resilience (ca. 97%) under cyclic tensile loading-unloading and good recovery of hysteresis (ca. 90%) under cyclic compressive loading-unloading.     

A new improved high‐order theory for analysis of free vibration of sandwich panels

A new improved high‐order theory is presented to investigate the dynamic behavior of sandwich panels with flexible core. Shear deformation theory is used for the face sheets, whereas the three‐dimensional elasticity theory is used for the core. Displacements in the core are assumed as polynomial with unknown coefficients. Inertia forces, moments of inertia and shear deformations in the core, and the face sheets are taken into consideration. Unlike the previous improved theory, the in‐plane normal and shear stresses in the core are considered. The governing equations and the boundary conditions are derived by Hamilton's principle. Closed form solution is achieved using the Navier method and solving the eigenvalues. The numerical results of present analysis compared with the available results in the literature. It indicates that the present new modified theory is more accurate than the other developed theories for sandwich panels. In this study, the variation of the nondimensional natural frequency with respect to the various parameters is presented. POLYM. COMPOS., 31:2042–2048, 2010. © 2010 Society of Plastics Engineers     

Development and characterization of nanosoy‐reinforced dextran nanocomposite membranes

Crosslinked nanocomposite membranes were developed by in situ reaction of dextran and soy protein isolate nanoparticles (nanosoy). The formation of a covalent bond between the reducing end of dextran and the amine groups of nanosoy (NS) leads to the in situ crosslinking. The NS particles employed for this study were 5–15 nm in size, as observed in the high‐resolution transmission electron microscopy micrograph. Glycerol addition assisted in the plasticization of the membranes, thus improving their flexibility and handling features. The effect of polymer composition on the extent of crosslinking, morphology, and flexibility of the films was investigated. Field emission scanning electron microscopy and atomic force microscopy revealed that single‐phase, homogeneous membranes are obtained within a specific composition of dextran/NS/glycerol (D/NS/G). The degree of crosslinking was evaluated by Raman spectroscopy and gel content measurements. The crystallinity of the D/NS/G membranes was found to increase monotonically as the NS content in the blend increased. An increase in tensile strength and decrease in Young's modulus was observed with an increase in NS content up to 28%, due to the reinforcing effect of NS and the plasticizing effect of glycerol playing roles simultaneously in the system. The reinforcing effect of the NS assisted in the formation of high‐strength nanocomposite membranes. Furthermore, they were characterized to analyze their thermal behavior. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44655.     

Peel strength improvement of foam core sandwich beams by epoxy resin impregnation on the foam surface

The interfacial adhesion characteristics between foam cores and faces affect much the structural integrity of foam core sandwich structures. The peel strength between the face plate and the foam core is one of the appropriate parameters for the interfacial characteristics of sandwich structures and its peel energy is also measured for the interfacial characterization. The peel strength is the first peak force per unit width of bondline required to produce progressive separation, and the peel energy is the amount of energy per unit bonding area associated with a crack opening. In this study, to improve the peel strength between the foam core and the face plate of foam core sandwich beams, the surfaces of foam core sandwich beams were resin-impregnated. Then the peel strength as well as peel energy of resin impregnated polyurethane foam core sandwich beams were measured by the cleavage peel test and compared with those of the same sandwich beams without surface resin impregnation on the foam surface.     

Numerical modelling of carbon fibre‐reinforced polymer and hybrid reinforced concrete beams in fire

Investigation on the fire resistance of fibre‐reinforced polymer (FRP) reinforced concrete (RC) is essential for increased application of FRP bars in the construction industry. Experimental tests for determining the fire resistance of RC elements tend to be expensive and time‐consuming. Although numerical models provide an effective alternative to these tests, their use in case of FRP RC structures is hindered because of the insufficient constitutive laws for FRP bars at elevated temperatures. This paper presents the details of a numerical modelling work that was carried out for simply supported carbon FRP (CFRP) and hybrid (steel‐FRP) bar RC beams at elevated temperatures. Constitutive laws for determining temperature‐dependent strength and stiffness properties of CFRP bars are proposed. Numerical models based on finite element modelling were employed for the rational analysis of beams using the proposed constitutive laws. The behaviour of concrete was simulated by means of a smeared crack model based on the tangent stiffness solution algorithm. The employed numerical models were validated against previous experimental results. The theoretical rebar stresses were calculated in both the FRP and steel bars, and the differences are discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Preparation of MWCNT reinforced epoxy nanocomposite and examination of its mechanical properties

Abstract

Multiwalled carbon nanotube (MWCNT) reinforced and unfilled epoxy resins were prepared. Viscosity tests were run on the MWCNT filled and unfilled resins, comparative tensile, three point bending and Charpy impact tests were performed. The viscosity of MWCNT filled epoxy resin has increased by from three to one order of magnitude from the lowest to higher shear rate compared to unfilled epoxy. Owing to this, the simple cast specimen preparation was not applicable in the case of MWCNT filled epoxy; instead of that, a resin injection method was used. According to the results of the mechanical tests, tensile strength has decreased by 4·6% caused probably indirectly by nanotube filling of the resin, but Young's modulus, bending modulus and Charpy impact strength, which characterise resistance against dynamic loads, have increased by 12, 10 and 20% respectively.     

A sandwich structure graphite block with excellent thermal and mechanical properties reinforced by in-situ grown carbon nanotubes

A graphite block derived from natural graphite flakes (NGF) has high thermal conductivity (TC) but poor mechanical properties. An effort to overcome this shortcoming was made by introducing carbon nanotubes (CNTs) onto the NGF surface by chemical vapor deposition (CVD). A block with a CNT–NGF–CNT sandwich structure was then prepared by hot-pressing at 2973 K. The new structure improved bend strength (increasing 52.2%) of the block, while maintaining the TC in the direction perpendicular to hot-pressing.