High performance toughened cyanate ester resin with low injection temperature for RTM process

A high‐performance modified cyanate resin system with low injection temperature for fabricating advanced composites via resin transfer molding (RTM) was developed, which was made of bisphenol A dicyanate ester (BADCy) and diallyl phthalate (DAP). The processing characteristics, mechanical, and thermal properties of the resin were studied, and the effect of the content of DAP on the processing and performance parameters was discussed. The results show that the processing properties of the modified cyanate system are dependent on the content of DAP. All the formulations studied in this paper have good processing characteristics; their injection temperatures are between 30 and 40°C and the pot life is about 20 hr at 50°C. The cured resins exhibited good thermal stability, excellent toughness, and good hot–wet resistance, suggesting that the toughened cyanate resin is a potential high‐performance RTM matrix for advanced composites. Copyright © 2008 John Wiley & Sons, Ltd.     

基于行波分离和角度域衰减的地震波叠前逆时成像条件

为提高陡倾角复杂构造区地震波逆时成像质量,提出一种逆时偏移成像条件.以二维模型为例,采用波印廷矢量实现方向行波的波场分离和地层反射角度的计算,将炮点和检波点的上行波场和下行波场、左行波场和右行波场两两作互相关计算,去除形成逆时偏移噪声的分量,同时将反射角度的余弦函数作为权函数引入互相关逆时成像条件中,进一步实现角度域逆时噪声的衰减.研究表明,炮点和检波点相反传播方向波场的互相关计算是形成偏移噪声的主要原因,相同传播方向的行波波场两两作互相关成像并组合叠加,可以在有效压制偏移噪声的同时,保持对陡倾角和水平界面的成像能力,再在角度域实现逆时噪声衰减,进一步提高逆时成像质量,实际地震资料验证了方法的有效性.所提的逆时成像条件可为复杂构造区高精度逆时成像提供重要的方法指导.     

PREDICTION OF FILLING TIMES OF POROUS CAVITIES

A recently proposed implicit scheme for tracking the filling front during liquid impregnation into porous moulds is extended to provide ‘one-shot’ predictions for the time to completely fill the mould and the location of the last point to fill. With general boundary conditions applied at the filling gates, it is shown that the time to fill and the location of the last point to fill can be predicted on solving, at most, two linear systems of equations (of size determined by the spatial discretization). This result is confirmed by numerical filling experiments that show, for a variety of mould cavities, that ‘one-shot’ solutions agree exactly with filling time and location predictions obtained with multi-time- step simulations.     

Novel DOPO-based flame retardants in high-performance carbon fibre epoxy composites for aviation

Two novel, halogen-free, phosphorus-based oligomeric flame retardants are investigated in the commercial epoxy resin RTM6 and ∼70 wt.% carbon fibre RTM6 composites (RTM6-CF) with respect to pyrolysis and fire behaviour. The flame retardants are based on 9,10-dihydro-9-oxy-10-phosphaphenanthrene-10-oxide (DOPO) units linked to the star-shaped aliphatic ground body tetra-[(acryloyloxy)ethyl] pentarythrit (DOPP), or heterocyclic tris-[(acryloyloxy)ethyl] isocyanurate (DOPI), respectively. The glass transition temperature is reduced by adding DOPP and DOPI, but the mechanical properties of the composites (e.g. interlaminar shear strength (ILSS) and G_c in mode I and II) remain unchanged. Decomposition models are proposed based on mass loss, evolved gas analysis (TG–FTIR) and condensed product analysis (hot stage cell within FTIR). The fire behaviour is investigated comprehensively (UL 94, limiting oxygen index (LOI) and cone calorimeter). Both flame retardants act in the gas phase through flame inhibition and in the condensed phase through charring. The UL 94 of RTM6 is improved from HB to V-1 and V-0; the LOI from 25% to 34–38%. Peak heat release rate (PHRR) and total heat evolved (THE) are lowered by 31-49% and 40–44%, respectively. Adding CF increases the residue, reduces the THE, but suppresses the charring due to RTM6 and flame retardants. Thus the THE of RTM6-CF is reduced by about 25% when DOPI and DOPP are added. However, UL 94: V-0 and LOI of 45% and 48% are achieved with ∼0.6 wt.% phosphorus.     

Plasma treatment of carbon fibers: Non-equilibrium dynamic adsorption and its effect on the mechanical properties of RTM fabricated composites

The effect of oxygen plasma treatment on the non-equilibrium dynamic adsorption of the carbon fabric reinforcements in RTM process was studied. 5-Dimethylamino-1-naphthalene-sulfonylchloride (DNS-Cl) was attached to the curing agent to study the change of curing agent content in the epoxy resin matrix. Steady state fluorescence spectroscopy (FS) analysis was used to study this changes in the epoxy resin at the inlet and outlet of the RTM mould, and XPS was used to study the chemical changes on the carbon fiber surfaces introduced by plasma treatment. The interlaminar shear strength (ILSS) and flexural strength were also measured to study the effects of this non-equilibrium dynamic adsorption progress on the mechanical properties of the end products. FS analysis shows that the curing agent adsorbed onto the fiber surface preferentially for untreated carbon fiber, the curing agent content in the resin matrix maintain unchanged after plasma treatment for 3 min and 5 min, but after oxygen plasma treatment for 7 min, the epoxy resin adsorbed onto the fiber surface preferentially. XPS analysis indicated that the oxygen plasma treatment successfully increased some polar functional groups concentration on the carbon fiber surfaces, this changes on the carbon fiber surfaces can change the adsorption ability of carbon fiber to the resin and curing agent. The mechanical properties of the composites were correlated to this results.     

Interphase formation of a resin transfer molded silica-phenolics composites subjected to dynamic impregnation process

In this study interphase formation of a resin transfer molded (RTMed) silica-phenolics composites subjected to dynamic impregnation process was investigated. The solvent effect on the interphase formation of silica fiber-phenolics composites was evaluated. UV-vis spectra, XPS and electron paramagnetic resonance (EPR) techniques were used to characterize the competitive adsorption of the components of phenolics solution onto silica reinforcement surfaces. Interlaminar shear strength (ILSS) and void content of silica-phenolics composites were also measured. It has been found that phenolics forms distribution gradient over RTM mold with respect to isomeric composition under the effect of solvent, which result, to varying extent, in the inhomogeneity of void content and thus ILSS of a final product. For the first time, it has been shown that the RTM process of silica-phenolics composites is highly solvent-dependent. Our work gives an insight into the role of organic solvent in a RTM solution impregnation processing and provides useful information and trends relating microscopic to macroscopic behavior.     

A randomized method for one‐step extrapolation in reverse time migration

Reverse time migration has drawn great attention in exploration geophysics because it can be used successfully in areas with large structural and velocity complexity. But its computational cost is considerably high. This paper concerns the fast implementation of the optimized separable approximation of the two‐way propagator, which is the most computational expensive step in reverse time migration. On the basis of the low‐rank property of the propagator and the idea of randomized algorithm, a randomized method is introduced for optimized separable approximation‐based one‐step extrapolation. Numerical results of approximating the propagator show that the randomized method is more efficient than the conventional interpolation method. At the same time, numerical experiments of wavefield extrapolation show that the proposed method is much more accurate than the conventional finite‐difference plus pseudo‐spectrum scheme. Copyright © 2014 John Wiley & Sons, Ltd.     

Design and evaluation of a downconverter based on MicroTCA.4

Modern low-level RF(LLRF) control systems of particle accelerators are designed to achieve extremely precise field amplitude and phase regulation inside the accelerating cavities. The RF field signal is usually converted to an intermediate frequency(IF) before being sampled by ADC. As the down-conversion is an important procedure of digital signal processing in LLRF system, designing a high performance and broad band downconverter compatible with various accelerators is important. In this paper, the design of a downconverter based on Micro TCA and its performance evaluation on different frequency points are presented. The major design objective of this module is a wider operating frequency range and more flexibility in application.     

Performance of glass woven fabric composites with admicellar-coated thin elastomeric interphase

Adequate stress transfer between the inorganic reinforcement and surrounding polymeric matrix is essential for achieving enhanced structural integrity and extended lifetime performance of fiber-reinforced composites. The insertion of an elastomeric interlayer helps increase the stress-transfer capabilities across the fiber/matrix interface and considerably reduces crack initiation phenomena at the fiber ends. In this study, admicellar polymerization is used to modify the fiber/matrix interface in glass woven fabric composites by forming thickness-controlled poly(styrene-co-isoprene) coatings. These admicellar interphases have distinct characteristics (e.g. topology and surface coverage) depending on the surfactant/monomer ratios used during the polymerization reaction. Overall, the admicellar coatings have a positive effect on the mechanical response of resin transfer molded, E-glass/epoxy parts. For instance, ultimate tensile strength of composites with admicellar sizings improved 50–55% over the control-desized samples. Interlaminar shear strength also showed increases ranging from 18 to 38% over the same control group. Interestingly, the flexural properties of these composites proved sensitive to the type of interphase formed for various admicellar polymerization conditions. Higher surface coverage and film connectedness in admicellar polymeric sizings are observed to enhance stress transfer at the interfacial region.