Characterization of transverse tensile, interlaminar shear and interlaminate fracture in CF/EP laminates with 10 wt% and 20 wt% silica nanoparticles in matrix resins

The transverse tensile properties, interlaminar shear strength (ILSS) and mode I and mode II interlaminar fracture toughness of carbon fibre/epoxy (CF/EP) laminates with 10 wt% and 20 wt% silica nanoparticles in matrix were investigated, and the influences of silica nanoparticle on those properties of CF/EP laminates were characterized. The transverse tensile properties and mode I interlaminar fracture toughness (G_IC) increased with an increase in nanosilica concentration in the matrix resins. However, ILSS and the mode II interlaminar fracture toughness (G_IIC) decreased with increasing nanosilica concentration, especially for the higher nanosilica concentration (20 wt%). The reduced G_IIC value is attributed to two main competing mechanisms; one is the formation of zipper-like pattern associated with matrix microcracks aligned 45° ahead of the crack tip, while the other is the shear failure of matrix. The ratio of G_IIC/G_IC decreased with the concentration of silica nanoparticles, comparable with similar CF/EP laminates with dispersed CNTs in matrix. Fractographic studies showed that interfacial failure between carbon fibre and epoxy resin occurred in the neat epoxy laminate, whereas a combination of interfacial failure and matrix failure occurred in the nanosilica-modified epoxy laminates, especially those with a higher nanosilica concentration (20 wt%).     

Effect of particles on interlaminar crack growth in cross-plied carbon-fibre/epoxy laminates

This paper discusses the effect of particulate additions on the mode-I and mode-II interlaminar fracture toughness of a cross-plied, carbon-fibre-reinforced, epoxy-resin laminate. Particles of graphite, silicon carbide and polyethylene were mixed with the epoxy resin prior to laminating with woven carbon-fibre cloth. Tests have been performed on double cantilever beam (DCB) and end-notched-flexure (ENF) specimens to obtain the critical-strain energy-release rates,G _IC andG _IIC, for the laminates with and without particulate additions. The dependences of the values ofG _IC andG _IIC on the crack length are also considered. The results indicate that the interlaminar-fracture-toughness (mode-I and mode-II) values of the CFRP laminate increase with increases in the particle content up to about 3%, and thereafter they decrease with further increases in the particle content. This was thought to be due to an increase in multiple-crack formation in the middle region of the cracked-plate samples. Furthermore, mode-I tests indicate that the propagation values ofG _IC are dependent on the crack length.     

Influence of electrospun Nylon 6,6 nanofibrous mats on the interlaminar properties of Gr-epoxy composite laminates

The present work aims at investigating the influence of electrospun Nylon 6,6 nanofibrous mat to interlaminar strength. Mode I and the Mode II fracture mechanics of virgin and nanomodified laminates are investigated. Nanomodified laminates are fabricated by interleaving an electrospun nanofibrous mats in laminate mid-plane. Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests are performed on both virgin and nanomodified configuration. Results show that electrospun nanofibrous mat is able to increase by 23.2% the mechanical energy absorbing capability and by about 5% the G_IC. ENF tests reveal that the nanofibrous mats contribute to improve the maximum stress before the material crisis (6.5% of increment) and a measurable increment of (8.1%) the maximum mechanical energy that can be absorbed by the material during the crack propagation is registered. The acoustic emission (AE) technique is used to monitor both DCB and ENF tests. The AE information highlight that the nanofibrous mats mitigate the interlaminar matrix failure on both the fracture modes.     

Improvement of interlaminar mechanical properties of CFRP laminates using VGCF

In order to improve the interlaminar mechanical properties of CFRP laminates, hybrid CFRP/VGCF laminates have been fabricated by using a newly-developed method, i.e., powder method, where the powder of vapor grown carbon fiber (VGCF) is added at the mid-plane of [0°/0°]₁₄ CFRP laminates. Experimental results of double cantilever beam (DCB) tests indicate the improvement on the interlaminar mechanical properties of Mode-I fracture behavior with much higher critical load P_C and fracture toughness G_IC with VGCF interlayer. Crack propagation and fracture surface have also been observed to interpret this improvement mechanism. Moreover, based on experimental G_IC, numerical simulations using finite element method (FEM) with cohesive elements have been carried out to analyze the delamination propagation. The interlaminar tensile strength of hybrid CFRP/VGCF laminates, which is obtained by matching the numerical load–COD (crack opening displacement) curves to experimental ones, is higher than that of base CFRP laminates.     

Monitoring of delamination onset and growth during Mode I and Mode II interlaminar fracture tests using guided waves

A delamination monitoring method was proposed to characterize Mode I and Mode II delamination onset in carbon fiber/epoxy (CF/EP) composite laminates through interrogation of guided waves activated and captured using piezoelectric actuators and sensors in a pitch–catch configuration. Mode I and Mode II interlaminar fracture tests were conducted using double cantilever beam (DCB) and end notch flexure (ENF) specimens to evaluate the proposed method. The changes in wave propagation velocity and wave magnitude (or attenuation), and the degree of waveform similarity between excitation and response signals, were calculated as delamination-sensitive wave parameters and plotted versus displacement recorded using a materials testing system. The kink points determined from wave parameter–displacement curves agreed well with the deviation from linearity (NL), visual observation (VIS) and maximum load (Max) points, which are often used in conventional methods for determining interlaminar fracture toughness. The propagation characteristics of the A₀ wave mode in a low frequency range were demonstrated to have high sensitivity to Mode I and in particular Mode II delamination onset in CF/EP composite laminates. It was concluded that the guided waves propagating in the DCB and ENF specimens were capable of determining Mode I and Mode II interlaminar fracture toughness, complementing current practices based on visual inspection or trivial interrogation using load–displacement curve alone.     

Effect of delamination failure in crashworthiness analysis of hybrid composite box structures

In the present paper the effects of delamination failure of hybrid composite box structures on their crashworthy behaviour will be studied and also their performance will be compared with non-hybrid ones. The combination of twill-weave and unidirectional CFRP composite materials are used to laminate the composite boxes. Delamination study in Mode-I and Mode-II with the same lay-ups was carried out to investigate the effect of delamination crack growth on energy absorption of hybrid composite box structures. The end-loaded split (ELS) and double-cantilever beam (DCB) standard test methods were chosen for delamination studies. In all hybrid composite boxes the lamina bending crushing mode was observed. Regarding the delamination study of hybrid DCB and ELS the variation of the specific energy absorption (SEA) versus summation of G_IC and G_IIC were plotted to combine the effect of Mode-I and Mode-II interlaminar fracture toughness on the SEA. From this relationship it was found the hybrid laminate designs which showed higher fracture toughness in Mode-I and Mode-II delamination tests, will absorb more energy as a hybrid composite box in crushing process. The crushing process of hybrid composite boxes was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental result.     

Fabrication and characterization of thermo-responsive GO nanosheets with controllable grafting of poly(hexadecyl acrylate) chains

A series of novel poly(hexadecyl acrylate)-grafted-graphene oxides (PHDA-g-GOs) were fabricated as thermo-responsive GO nanosheets via diazonium addition and surface-initiated atom transfer radical polymerization (SI-ATRP). Various spectroscopic and microscopic evidences confirm the successful fabrication of the thermo-responsive GO nanosheets. The thermo-responsive property and thermal stability of the PHDA-g-GOs were determined by DSC and TGA. Furthermore, we have demonstrated the ability to systematically tune the chain length of polymer molecules covalently bonded to GO nanosheets by SI-ATRP. Comparing the thermo-responsive GO samples, the ?H _m and ?H _c increased as the molar ratio of monomer to initiator increased. Moreover, the T _mo, T _co, ?H _m and ?H _c values of the PHDA-g-GO3 (with a molar ratio of monomer to initiator as 1000:1) are 31.2, 31.4 °C, 79 and 76 J/g, respectively. Meanwhile, the thermo-responsive GO nanosheets have good thermal stability and shape stability. Therefore, the thermo-responsive GO nanosheet is a very promising functional material and can be used in more areas, such as the fabrication of temperature-sensitive drugs, reagents, fibers and textiles, solar energy storage and temperature sensor.     

Non-BCS Temperature Dependence of Energy Gap in Thin Film Electron-Doped Cuprates

We investigate the dependence of the energy gap (G) on the temperature (T) for the electron-doped high-temperature superconductors. The following compounds, in the form of the thin films, have been taken into consideration: La_2?x Ce _x CuO₄ (LCCO), Pr_2?x Ce _x CuO₄ (PCCO), and Nd_2?x Ce _x CuO₄ (NCCO). It was found that G(T) deviates from the BCS prediction more, if a concentration of cerium assumes the lower values. For the lowest concentration (in the case of LCCO and NCCO), the function G(T) is not quite like the BCS curve, which is connected with the existence of the residual Nernst region. Next, it has been pointed out that the NCCO superconductor becomes structurally unstable for the maximum concentration of cerium, which is leading to the anomalous dependence of the energy gap on the temperature and the induction of the wide Nernst region.     

Interlaminar Fracture Toughness of CF/PEI and GF/PEI Composites at Elevated Temperatures

An experimental study has been conducted to assess temperature effects on mode-I and mode-II interlaminar fracture toughness of carbon fibre/polyetherimide (CF/PEI) and glass fibre/polyetherimide (GF/PEI) thermoplastic composites. Mode-I double cantilever beam (DCB) and mode-II end notched flexure (ENF) tests were carried out in a temperature range from 25 to 130°C. For both composite systems, the initiation toughness, G _IC,ini and G _IIC,ini, of mode-I and mode-II interlaminar fracture decreased with an increase in temperature, while the propagation toughness, G _IC,prop and G _IIC,prop, displayed a reverse trend. Three main mechanisms were identified to contribute to the interlaminar fracture toughness, namely matrix deformation, fibre/matrix interfacial failure and fibre bridging during the delamination process. At delamination initiation, the weakened fibre/matrix interface at elevated temperatures plays an overriding role with the delamination growth initiating at the fibre/matrix interface, rather than from a blunt crack tip introduced by the insert film, leading to low values of G _IC,ini and G _IIC,ini. On the other hand, during delamination propagation, enhanced matrix deformation at elevated temperatures and fibre bridging promoted by weakened fibre/matrix interface result in greater G _IC,prop values. Meanwhile enhanced matrix toughness and ductility at elevated temperatures also increase the stability of mode-II crack growth.     

Thermodynamic properties of (TeO<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>(MoO<Subscript>3</Subscript>)<Subscript>1–<Emphasis Type="Italic">n</Emphasis></Subscript> glasses

The thermal behavior of (TeO₂) _n (MoO₃)_1–n (n = 0.75, 0.85, 0.90) tellurite glasses has been studied by differential scanning calorimetry in the range from T = 300 to T = 850 K and heat capacity has been measured in the temperature range. The thermodynamic characteristics of the devitrification process and glassy state have been determined. The experimental data obtained have been used to evaluate the standard thermodynamic functions of the system in glassy and supercooled liquid states: heat capacity C _p °(T), enthalpy H°(T)–H°(320), entropy S°(T)–S°(320), and Gibbs function G°(T)–G°(320) in the temperature range 320–630 K. The composition dependences of the glass transition temperature and thermodynamic functions for the glasses have been obtained. The thermal and thermodynamic properties of the tellurite glasses have been compared to those of previously studied (TeO₂) _n (WO₃)_1–n and (TeO₂) _n (ZnO)_1–n glasses.     

A dimensionless criterion of dispersion for crystalline solids

A dimensionless criterion of dispersion for crystalline solids is proposed based on a comparison of the excess free energy in the compact and disperse state: D_L=12σ/Gb²pL, where σ is the surface energy, G is the shear modulus, b is the Burgers vector, p is the dislocation density, and L is the particle size. According to this, the critical size of particles corresponding to D_L=1 is approximately the same (L₁=500 nm) for all metals. Classification of disperse crystalline solids with respect to the parameter D_L is more justified thermodynamically than characterization by the particle size.     

Improved fracture toughness in carbon fibre epoxy composite through novel pre-preg coating method using Epoxy Terminated Butadiene Nitrile rubber

A novel pre-preg coating method was used to improve the interlaminar fracture toughness in carbon fibre epoxy composite laminates, using reactive liquid rubber. The Epoxy Terminated Butadiene Nitrile (ETBN) liquid rubber incorporated between pre-pregs using automatic draw bar coating technique. Experimental test results reveal that by adding ETBN in small quantities in the range of 15.55–22.66 g/m², inter laminar critical energy release rates (G_IC and G_IIC) can be improved up to 140% in mode-I loadings and 32% in mode-II loadings respectively. It was confirmed that the effect of ETBN rubber concentration in carbon epoxy pre-preg system on interlaminar fracture toughness under mode-I and mode-II loadings, was discussed by on the bases of fractographic observations and mechanism considerations using SEM.     

Error tolerance for the recognition of faulty strings in a regulated grammar using fuzzy sets

To overcome the limitations of context-free and context-sensitive grammars, regulated grammars have been proposed. In this paper, an algorithm is proposed for the recognition of faulty strings in regulated grammar. Furthermore, depending on the errors and certainty, it is decided whether the string belongs to the language or not based on string membership value. The time complexity of the proposed algorithm is O(|G _R ² |·|w|), where |G_R| represents the number of production rules and |w| is the length of the input string, w. The reader is provided with numerical examples by applying the algorithm to regularly controlled and matrix grammar. Finally, the proposed algorithm is applied in the Hindi language for the recognition of faulty strings in regulated grammar as a real-life application.     

玻璃纤维-碳纤维混杂增强PCBT复合材料层合板的制备及低速冲击性能

采用环状对苯二甲酸丁二醇酯(CBT)预浸料,利用真空袋辅助热压工艺制备了玻璃纤维机织布-碳纤维机织布/聚环状对苯二甲酸丁二醇酯(GF-CF/PCBT)混杂复合材料层合板。利用双悬臂梁(DCB)和三点端部开口弯曲(3ENF)试验对连续纤维增强PCBT复合材料层合板的层间强度做出评估。同时,利用低速冲击试验结合Abaqus/Explicit有限元仿真重点考察了混杂纤维增强PCBT复合材料层合板的低速冲击性能。试验结果表明:尽管CF/PCBT复合材料层合板具有优异的层间性能,当冲击能量为114.3J时,由于CF自身的脆性,CF/PCBT复合材料层合板被完全穿透,而GF-CF/PCBT混杂复合材料层合板只在表面形成凹痕。与纯CF增强PCBT复合材料层合板相比,铺层形式为[CF/GF/CF]25的GF-CF/PCBT混杂复合材料层合板的抗冲击损伤能力提高2倍。仿真得到的云图显示,冲击引起的应力在CF中的分布区域要明显大于在GF中的分布区域。     

Wave spectrum of a conducting cylinder in an isotropic plasma

The boundary value problem of the propagation of an electromagnetic field along a cylindrical conductor in an isotropic plasma medium has been solved by the impedance method. The boundedness of the wave spectrum of such a guiding structure has been shown. The spectrum includes fast intrinsic wave E ₀₁ and extrinsic hybrid waves HE _nm and EH _nm , both fast and slow ones, their countable set being determined by the azimuthal index.     

How much is a quantum controller controlled by the controlled system?

We consider unitary transformations on a bipartite system A × B. To what extent entails the ability to transmit information from A to B the ability to transfer information in the converse direction? We prove a dimension-dependent lower bound on the classical channel capacity C(A ← B) in terms of the capacity C(A → B) for the case that the bipartite unitary operation consists of controlled local unitaries on B conditioned on basis states on A. If the local operations are given by the regular representation of a finite group G we have C(A → B) = log |G| and C(A ← B) = log N where N is the sum over the degrees of all inequivalent representations. Hence the information deficit C(A → B) ? C(A ← B) between the forward and the backward capacity depends on the “non-abelianness” of the control group. For regular representations, the ratio between backward and forward capacities cannot be smaller than 1/2. The symmetric group S _n reaches this bound asymptotically. However, for the general case (without group structure) all bounds must depend on the dimensions since it is known that the ratio can tend to zero. Our results can be interpreted as statements on the strength of the inevitable backaction of a quantum system on its controller.     

Magnetic-Memory Effects in High-Purity Beryllium

We study the magnesium-thermal beryllium condensate in the course of natural aging after several procedures of treatment by weak constant magnetic fields. The amplitude-time dependences of the effective shear modulus (G _ef ~ f ²), low-frequency internal friction, the parameter r, and the ratio of dislocation velocities V _afef /V ₀ are analyzed. It was revealed that the hysteresis in the f ²(γ) curves disappears after ~120 h of natural aging and the sign of r(γ) and V _afef /V ₀(γ) is inverted. The nature of the detected effects is connected with the appearance of microcracks in the stage of elastic deformation and their healing as a result of the viscous motion of unpinned dislocations in the field of point defects in the basal slip plane.     

Correlation functions of a nonisotropic binary mixture in the Percus-Yevick theory

The correlation properties are studied of binary mixtures in the critical state of vaporization in external field. The investigations are based on the system of Percus-Yevick integral equations for pair correlation functions G ^αβ(r ₁, r ₂) and the theory of scaling in critical phenomena. Expressions are obtained for pair correlation functions and correlation radius R _c , which are analyzed in various extreme cases.     

Mixed Bending-Tension (MBT) test for mode I interlaminar and intralaminar fracture

A new Mixed Bending-Tension (MBT) test is proposed for mode I fracture of laminated composites. The MBT specimen consists of a relatively small pre-cracked laminate adhesively bonded to pin-loaded steel beams. This design reduces significantly the bending stresses that prevent successful application of DCB tests to certain laminates. The MBT was here applied to carbon/epoxy unidirectional [0°]₂₆ and [90°]₂₆ laminates with starter delaminations. Interlaminar initiation G_IC values of [0°]₂₆ laminates agreed well with previous DCB test results, while [90°]₂₆ laminates exhibited 50% higher values. Significant lengths of fairly planar intralaminar crack propagation were seen in the latter laminates. The results showed a fibre bridging related R-curve, which was more pronounced in [0°]₂₆ laminates. The consistency of the present results indicates that the MBT opens new possibilities for the interlaminar and intralaminar mode I fracture.