C/C复合材料液相浸渍制备工艺及其力学性能模拟

针对酚醛先驱体C/C复合材料液相浸渍制备工艺各组分相的化学转化特性, 基于Arrhenius方程建立了C/C复合材料液相浸渍制备工艺力学模型, 详细分析了固化-炭化和石墨化两个重要的工艺阶段各组分相的体积变化规律, 得到的气孔体积分数与Micro-CT系统扫描处理的细编穿刺C/C复合材料微结构图像中气孔体积分数相吻合, 并结合均匀化方法对制备过程材料基体有效弹性模量进行了预测。结果表明: 材料基体的有效弹性模量随着致密化次数的增加而增大, 在每一次致密化过程中材料基体的有效弹性模量先增大后减小, 石墨化工艺过程中材料基体的有效弹性模量达到某一值后保持平稳。      

Wet powder impregnation for polyethylene composites: preparation and mechanical properties

An alternative processing route for polyethylene–polyethylene composites has been developed. The processing route includes an impregnation step in a suspension of polyethylene powder in propanol. For this step an impregnation and winding machine has been constructed. Further processing steps are the preparation of prepregs and hot compacting to form PE–PE composites. The influence of the impregnation and winding parameters on the fibre volume fractions of the resulting composites are discussed. The mechanical properties of unidirectional reinforced PE–PE composites parallel and perpendicular to the fibre direction are shown in the results of tensile and compression tests. Finally, the failure mechanisms are described by SEM investigations.     

Theory and experimentation on the most-recent-reference distribution

The cumulative distribution of the age of the most-recent-reference distribution is the “dual” variant of the first-citation distribution. The latter has been modelled in previous publications of different authors but the former one has not. This paper studies a model of this cumulative most-recent-reference distribution which is different from the first-citation distribution. This model is checked on JASIS and JACS data, with success. The model involves the determination of 3 parameters and is a transformation of the lognormal distribution. However we also show that the first-citation model (involving only 2 parameters and which is easier to handle), developed in an earlier paper, gives enough freedom to give close fits to the most-recent-reference data as well. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interface and impregnation relevant tests for continuous glass fibre-polypropylene composites

An experimental investigation was performed in order to compare the effect of the adhesive strength between fibres and matrix, contra the effect from the degree of impregnation on the mechanical properties of melt impregnated continuous glass fibre—polypropylene composites. Single fibre pull-out tests were performed in order to compare the interfacial bond strength. The degree of impregnation was measured using opacity and scattering from reflected light measurements of the prepregs. The testing included transverse tensile, ±45° tensile, double-notch shear, compression shear and double cantilever beam (DCB). These tests can to a certain degree be given interface relevance (InR) as well as impregnation relevance (ImR). With regard to InR sensitivity, the tests can be ranked in descending order according to prepreg transverse, laminate transverse, intralaminar, and interlaminar double-notch shear tests. With regard to ImR sensitivity, the tests can be ranked in descending order according to prepreg transverse, compressive interlaminar double-notch shear and laminate transverse tests. The measured shear and transverse modulus values showed limited relevance regarding the interface strength and degree of impregnation. The transverse and shear elastic modulus scored low regarding InR and ImR sensitivity. This was also true for the G_IC and G_ICPROP values.     

A model for thermoplastic melt impregnation of fiber bundles during consolidation of powder-impregnated continuous fiber composites

Continuous fiber thermoplastic matrix composites were fabricated using a novel powder-impregnation process that combined vacuum assisted resin transfer molding (VARTM) with compression molding. The resulting composite has an average fiber volume fraction of 65%. A model has been developed for the consolidation phase to predict the void fraction of the resulting composite. This model takes into account the fabric unit cell dimensions and material properties and assumes that tow permeability remains constant. The model is compared to experimental values for void fraction for samples prepared using a range of consolidation pressures and dwell times.     

Modeling of continuous ultrasonic impregnation and consolidation of thermoplastic matrix composites

Ultrasonic propagation was used to provide heat and pressure in order to perform impregnation and consolidation during production of thermoplastic matrix composites. For this purpose, a new experimental set-up, integrating a laboratory filament winding machine with a horn and a compaction roller, was developed.The heat transfer phenomena occurring during continuous impregnation and consolidation were simulated solving by finite element (FE) analysis the energy balance equations in 2D accounting for the heat generated by ultrasonic waves, the melting characteristics of the matrix and the movement of the thermoplastic commingled roving.The temperature distribution in the composite, predicted by the numerical simulations, was validated by temperature measurements during the production of E-glass/polypropylene cylinders, with the optimized parameters obtained by the FE analysis. The ultrasonic consolidated composite cylinders were characterized by low void content and a shear modulus comparable with that obtained by the micromechanical analysis.     

Material characterization and residual stresses simulation during the manufacturing process of epoxy matrix composites

A thermal, rheological and mechanical material characterization of an aeronautic epoxy resin from commercial prepreg is reported in this article. The kinetic of the crosslinking reaction of the resin is characterized and modeled. The specific heat, the glass transition temperature, the thermal expansion coefficients, the chemical shrinkage coefficients and the thermo-mechanical properties have been investigated as a function of temperature and degree of cure. Dynamic mechanical measurements are used to determine the gel point. Finally, the residual stresses developed during the curing process are calculated using a finite element simulation, taking into account the material properties evolutions according to proposed models. The results highlight the importance of the characterization accuracy and the associated models.     

Residual stresses formation during the manufacturing process of epoxy matrix composites: resin yield stress and anisotropic chemical shrinkage

A study of residual stresses during the curing process of thermosetting resin composites is presented. A methodology is proposed for predicting the formation and the development of the manufacturing residual stresses, this approach is based on the determination of the resin yield stress. A self-consistent model is used to determine the cure-dependent effective mechanical properties, thermal expansion and chemical shrinkage coefficients of the composite material. This model allows considering for the composite material behaviour an anisotropic chemical shrinkage, which is not represented by a classical linear model. Finally, a one dimensional cure simulation and a modelling of residual stresses formation in composite plate are made by using a finite elements code. The effects of the cure-dependent material properties and of the resin yield stress on the residual stress formation are studied.     

Theory and practice of manufacturing strategy

This paper first proposes a methodology for manufacturing strategy development based on a critical review of the theory of manufacturing strategy. The methodology captures the essential factors that must be considered, and the steps that must be taken, by a firm attempting to design a viable manufacturing strategy. An exploratory study of the process of formulating and implementing manufacturing strategy practised by three firms is conducted. Their experiences in the manufacturing strategy process are reported and analysed. While the practice of each firm is different, the process of manufacturing strategy formulation seems to be in line with the conceptual models discussed in the literature. Our findings offer some evidence that manufacturing is slowly shedding its image as the ‘missing link’ in corporate strategy. While managers of the studied firms are fully aware of the competitive advantages that manufacturing can provide, they are not totally committed to articulating a manufacturing strategy because of concerns about its successful implementation. It is proposed that further research on manufacturing strategy focus on such implementation issues as the infrastructure, corporate culture, performance measurements, decision-making and managerial styles.     

Hybrid aluminium matrix composites containing boron carbide and quasicrystals: manufacturing and characterisation

Hybrid composites of boron carbide (B₄C) and Al_62.5Cu₂₅Fe_12.5 quasicrystals (QCs) were prepared by ball milling and pressureless sintering in aluminium matrix to investigate their individual and hybrid effects on microstructural and mechanical properties. Hybrid composite contained B₄C and QCs in 3?wt-% each, making a total of 6?wt-%. For reference, specimens of pure aluminium and two composites containing 6?wt-%B₄C and 6?wt-% QCs were prepared. Microstructural characterisation was performed using optical, scanning electron microscopy and X-ray diffraction, while evaluation of mechanical properties was carried out by hardness and compression tests. Uniform dispersion of reinforcements in composites was observed along with significant increase in the mechanical properties. The composite containing 6?wt-% QCs demonstrated the highest hardness, while the hybrid composite showed better compressive properties.     

Uncertainty in the manufacturing of fibrous thermosetting composites: A review

Composites manufacturing involves many sources of uncertainty associated with material properties variation and boundary conditions variability. In this study, experimental and numerical results concerning the statistical characterisation and the influence of inputs variability on the main steps of composites manufacturing including process-induced defects are presented and analysed. Each of the steps of composite manufacturing introduces variability to the subsequent processes, creating strong interdependencies between the process parameters and properties of the final part. The development and implementation of stochastic simulation tools is imperative to quantify process output variabilities and develop optimal process designs in composites manufacturing.     

Carbon nanotube-based polymer composites: A trade-off between manufacturing cost and mechanical performance

A methodology was devised to evaluate the newly-developed carbon nanotube reinforced polymer composites by means of mechanical performance and manufacturing cost. Glass fibre reinforced-epoxy composite plates were produced having different parameters: (a) three manufacturing processes, (b) geometrical dimensions, (c) carbon nanotubes concentration in the epoxy resin and finally (d) modified resin infusion temperature. Tensile coupons were machined out of the manufactured plates and their quasi-static mechanical properties were evaluated. Three cost models were developed to assess plates and tensile coupons manufacturing cost for each different case. Optimal values were evaluated for major manufacturing parameters, driving force being the mechanical properties of interest (quality) as well as their low manufacturing cost. It is demonstrated that the added cost to manufacture such nano-reinforced composites is attributed to increase strength on the expense of ductility; the main benefit of the carbon nanotube-based polymer composites seems to be their ability to be monitored. Almost 20% added cost is paid to attain this new function of piezo-resistivity for the RTM process, while this amount further increases for the non-automated processes such as the Hand Lay-up.     

Mechanical properties of C/C composites processed by wet impregnation and P-CVI methods

Wet impregnation with phenolic resin and P-CVI methods were used to manufacture C/C composites. The influence of impregnation process of porous 2D carbon fibre substrate with resin and pyrocarbon deposited by CVD technique on mechanical properties of formed composites was studied. The results indicate that using P-CVI method large pores remain in the matrix resulting in lower mechanical strength. This fraction does not undergo any changes during thermal treatment. The flexural modulus of C/C composites depends mainly on the type of reinforcing fibres. The values of moduli measured in composites obtained by both methods do not differ significantly. Comparison of two methods of fabrication of C/C composite show that much better strengths can be achieved by forming the carbon matrix in solid state.     

一种制作高阻抗背衬材料的新方法

文章分析了已有方法制作高阻抗背衬材料的不足,阐述了制作高性能背衬材料的新方法。新的高阻抗背衬材料,声衰减性能优异,材料的结合力高。该材料应用于制作2．8MHz窄脉冲探头,带宽达到120％以上。     

Modelling the viscoelastic stress relaxation of glass fibre reinforcements under constant compaction strain during composites manufacturing

Viscoelastic stress relaxation of glass fibre reinforcements is commonly encountered in the manufacture of glass fibre reinforced polymer composites. A better understanding of the phenomenon, coupled with an ability to predict this behaviour, will aid improved manufacturing process control and tooling design. Finished product quality may also be bettered by virtue of increased knowledge of stresses acting within the composite product. This paper presents a simple Maxwell element-based model to both simulate and help explain the viscoelastic stress relaxation of glass fibre reinforcements under compressive strain compaction of layers during composites manufacturing. The model was validated against experimental data for reinforcement materials of different architecture, and good-to-reasonable predictions of the stress relaxation response were obtained.     

Smart tooling with integrated time domain reflectometry sensing line for non-invasive flow and cure monitoring during composites manufacturing

An electrical transmission line integrated into composite tooling has been developed to facilitate non-contact multipoint flow and cure monitoring during vacuum assisted resin transfer molding processing. The sensor is made of conductive aramid (Kevlar®) fibers and is an integral part of the glass fiber composite tooling. Electrostatic simulations in COMSOL® have been performed to optimize the wire spacing maximizing the sensitivity of the electric time domain reflectometry measurements. Tooling with integrated wires placed with optimal spacing has been fabricated. TDR flow monitoring experiments with the integrated sensor have been conducted and compared visually using digital image processing. TDR data obtained from the same sensor also enables measurement of the resin cure state. Fast and slow curing resin systems have been studied using the transmission line integrated tooling and differential scanning calorimetry experiments have been performed to validate the cure measurements obtained from TDR. Major benefits of the sensor implementation include elimination of ingress/egress issues associated with standard sensor integration into composite tooling as well as reduced maintenance of the non-contact solution.     

Modelling of heat transfer in thermoplastic composites manufacturing: double-belt press lamination

The process of double-belt press lamination of thermoplastic composite laminates has been modelled in terms of heat transfer and crystallization kinetics. Modelling includes temperature dependent thermal properties and non-infinite contact conductances at material interfaces. Model predictions have been compared with experimental results from double-belt press lamination of a sandwich-like glass reinforced polyethylene terephthalate (PET) material. A model for prediction of the temperature distribution along the length of the belt surface is also presented and compared with experimental results.     

Kenaf natural fiber reinforced polypropylene composites: A discussion on manufacturing problems and solutions

As industry attempts to lessen the dependence on petroleum based fuels and products there is an increasing need to investigate more environmentally friendly, sustainable materials to replace existing materials. This study focused on the fabrication of kenaf fiber reinforced polypropylene sheets that could be thermoformed for a wide variety of applications with properties that are comparable to existing synthetic composites. The research done in this study has proven the ability to successfully fabricate kenaf–polypropylene natural fiber composites into sheet form. The optimal fabrication method for these materials was determined to be a compression molding process utilizing a layered sifting of a microfine polypropylene powder and chopped kenaf fibers. A fiber content of both 30% and 40% by weight has been proven to provide adequate reinforcement to increase the strength of the polypropylene powder. The use of a coupling agent, 3% Epolene enabled successful fiber–matrix adhesion. The kenaf–PP composites compression molded in this study proved to have superior tensile and flexural strength when compared to other compression molded natural fiber composites such as other kenaf, sisal, and coir reinforced thermoplastics. With the elastic modulus data from testing, it was also possible to compare the economic benefits of using this kenaf composite over other natural fibers and E-glass. The kenaf–maleated polypropylene composites manufactured in this study have a higher Modulus/Cost and a higher specific modulus than sisal, coir, and even E-glass thereby providing an opportunity for replacing existing materials with a higher strength, lower cost alternative that is environmentally friendly.