A comparative study on the flexural performance of plasma treated polypropylene fiber reinforced cementitious composites

The flexural performance of cementitious composites incorporating low frequency cold plasma treated polypropylene (PP) fibers has been investigated in this study. Polypropylene fibers were subjected to low frequency plasmas (40 kHz) of argon and oxygen gases at various power levels (60, 120, and 180 W) for different exposure periods (0.5, 1, 2, 5, 8 and 30 min). Possible physical changes on fiber surfaces were observed by scanning electron microscope (SEM) at micrometers scale. Meanwhile, the surface wettability of polypropylene plates was monitored by means of static contact angle measurements. Modified fibers subjected to 60 W–8 min, 120 W–2 min, 180 W–2 min and 180 W–30 min of argon and oxygen plasmas which were selected from the SEM and wettability observations. These modified fibers have been used in the production of cementitious composites. The flexural performance of 40 mm × 40 mm × 160 mm prismatic samples were characterised by means of first cracking strength, flexural strength, and toughness values. Test results showed that appropriate low frequency plasma treatment conditions lead to an improvement in the flexural strength and toughness of polypropylene fiber reinforced cementitious composites. From the SEM images and wettability measurements, it was postulated that main mechanism of performance improvement is the result of physical adhesion increase between polypropylene and matrix. Adhesion improvement is attributed to the relatively closer spacing of fiber and matrix which can be explained by modification of fibers’ surface from hydrophobic to hydrophilic by plasma exposure.     

Drilling of long fiber reinforced thermoplastics—Influence of the cutting edge on the machining results

The cutting edge has a large impact on the performance of a cutting tool when drilling fiber reinforced plastics (frp). Delamination, burrs and defects at hole entry and exit pose a challenge when drilling frp. Within the presented work the influence of the cutting edge on drilled hole quality when machining long fiber reinforced thermoplastics is investigated. Major influencing determinants on the machining results when drilling long fiber reinforced thermoplastics are feed force and fiber separation. The interdependencies between cutting edge radius, feed forces and fiber separation are described in a phenomenological model.     

An approach for development of damage-free drilling of carbon fiber reinforced thermosets

A new comprehensive approach to select cutting parameters for damage-free drilling in carbon fiber reinforced epoxy composite material is presented. The approach is based on a combination of Taguchi's experimental analysis technique and a multi-objective optimization criterion. The optimization objective includes the contributing effects of the drilling performance measures: delamination, damage width, surface roughness, and drilling thrust force. A hybrid process model based on a database of experimental results together with numerical methods for data interpolation are used to relate drilling parameters to the drilling performance measures. Case studies are presented to demonstrate the application of this method in the determination of optimum drilling conditions for damage-free drilling in BMS 8-256 composite laminate. A process map based on the results is presented as a tool for drilling process design and optimization for the investigated tool/material combination.     

A mechanistic approach to investigate drilling of UD-CFRP laminates with PCD drills

Carbon fiber reinforced plastics (CFRPs) possess desirable material properties that satisfy the aerospace industry's high strength to weight ratio objective. Therefore, CFRPs are commonly used in structural parts, either alone or together with aluminum and titanium alloys. Drilling of CFRPs has been studied extensively in the literature in recent years, with special emphasis on process parameters and delamination. This study identifies mechanical properties of uni-directional CFRPs through drilling tests. Drilling of uni-directional CFRP plates with and without pilot holes has been performed, and cutting and edge force coefficients are identified. A polycrystalline diamond (PCD) drill was used in tests since this type of drill is commonly used in practice. Finally, validation tests on multi directional CFRP laminates have been performed and good results have been obtained.     

Induced drilling strains in glass fibre reinforced epoxy composites

Residual strains induced by drilling of glass-fibre reinforced polymers (GFRPs) were determined using a hybrid experimental–numerical methodology. Experimentally, a set of GFRP specimens were drilled under well-defined tensile (calibration) stresses, using an especially designed tensile test device. To remove the effect of the initial residual stresses, this methodology considers differential stress values instead of absolute ones. Numerically, the experimental procedure was simulated using the finite element method. The induced drilling strains were determined by comparing the experimental measured strains with those calculated numerically. Clear differences between the selected drilling operations could be observed and evaluated.     

Preparation of three-dimensional braided carbon fiber reinforced mullite composites from a sol with high solid content

To prepare the three-dimensional braided carbon fiber reinforced mullite (3D C/mullite) composites, an Al₂O₃-SiO₂ sol with a solid content of 20% (mass fraction) and an Al₂O₃/SiO₂ mass ratio of 2:1 was selected as the raw material. Characteristics and mullitization of the sol were analyzed throughly. It is found that the formation of mullite is basically completed at 1300 °C and the gel powders exhibit favorable sintering shrinkage. The 3D C/mullite composites without interfacial coating were fabricated through the route of vacuum impregnation-drying-heat treatment. Satisfied mechanical properties with a flexural strength of 241.2 MPa and a fracture toughness of 10.9 MPa·m^1/2 are obtained although the total porosity reaches 26.0%. Oxidation resistances of the composites at 1200, 1400 and 1600 °C were investigated. Due to the further densification of matrix, the 3D C/mullite composites show tiny mass loss and their mechanical properties are well retained after oxidation at 1600 °C for 30 min.     

Evaluation of drilling parameters on thrust force in drilling carbon fiber reinforced plastic (CFRP) composite laminates using compound core-special drills

Drilling is the mostly used secondary machining of the fiber reinforced composite laminates, while the delamination occurs frequently at the drill exit in the workpiece. In the industrial experiences, core drill shows better drilling quality than twist drill. However, chip removal is a troublesome problem when using the core drill. Conventional compound core-special drills (core-special drills and step-core-special drills) are designed to avoid the chip removal clog in drilling. But the cutting velocity ratio (relative motion) between outer drill and inner drill is null for conventional compound core-special drills. The current study develops a new device and to solve the problems of relative motion and chip removal between the outer and inner drills in drilling CFRP composite laminates. In addition, this study investigates the influence of drilling parameters (cutting velocity ratio, feed rate, stretch, inner drill type and inner drill diameter) on thrust force of compound core-special drills. An innovative device can be consulted in application of compound core-special drill in different industries in the future.     

Evolution of the health of concrete structures by electrically conductive GFRP (glass fiber reinforced plastic) composites

The function and performance of self-diagnostic composites embedded in concrete blocks and piles were investigated by bending tests and electrical resistance measurement. Carbon powder (CP) and carbon fiber (CF) were introduced into glass fiber reinforced plastic (GFRP) composites to provide electrical conductivity. The CPGFRP composite displays generally good performance in various bending tests of concrete block and piles compared to the CFGFRP composite. The electrical resistance of the CPGFRP composite increases remarkably at small strains in response to microcrack formation at about 200 μm strain, and can be used to detect smaller deformations before crack formation. The CPGFRP composite shows continuous change in resistance up to a large strain level just before the final fracture for concrete structures reinforced by steel bars. It is concluded that self-diagnostic composites can be used to predict damage and fracture in concrete blocks and piles.     

Effects of high speed in the drilling of glass fibre reinforced plastic: Evaluation of the delamination factor

High speed machining (HSM) is an outstanding technology capable of improving productivity and lowering production costs in manufacturing companies. Drilling is probably the machining process most widely applied to composite materials; nevertheless, the damage induced by this operation may reduce drastically the component performance. This work employs HSM to realize high performance drilling of glass fibre reinforced plastics (GFRP) with reduced damage. In order to establish the damage level, digital analysis is used to assess delamination. A comparison between the conventional (F_d) and adjusted (F_da) delamination factor is presented. The experimental results indicate that the use of HSM is suitable for drilling GFRP ensuring low damage levels.     

Mechanistic force modeling for milling of carbon fiber reinforced polymers with double helix tools

Carbon fiber reinforced polymers (CFRP) have emerged as the material of choice to satisfy increasing demand for lighter aircrafts. Machinability characteristics of CFRPs are quite different than those of metals; therefore, special tool designs have been developed for CFRP machining. The double helix end mill design compresses the upper and lower sides of the laminate using opposite helix angles that eliminate delamination. A mechanistic force model for double helix tools is developed based on milling force data obtained on flat end mills. The proposed model can be used to improve double helix tool designs and to optimize milling process parameters.     

外加颗粒增强表层复合材料制备方法

按照采用热源和陶瓷增强颗粒的添加方式,对外加颗粒增强表层复合材料的制备方法分类.详细介绍了堆焊、激光熔覆、激光熔射、等离子熔化-注射等颗粒增强表层复合材料的制备方法,并分析了各种制备技术的优缺点.堆焊特点是基体与表层为冶金结合,效率高.激光熔覆可以实现输入的准确控制,冷却速度快,热畸变小.但是堆焊和激光熔覆过程都存在裂纹问题.激光熔射不受基体可焊性限制,可制备颗粒增强相连续分布的表层,避免裂纹的形成.等离子熔化-注射技术与激光熔射技术类似,可以制备出增强相体积分数从0～100%连续变化的梯度复合材料.避免由于增强颗粒分布不均引起的裂纹,实现低投入、低成本运行.     

Application of acoustic emission in drilling of composite laminates

Acoustic emission (AE) technique was used to characterise drilling of composite laminates. Uni-directional glass fibre reinforced plastic (GFRP) laminates consisting of 12-layers and 16-layers (0/90)_s were drilled using a twist drill and the generated AE was monitored. Results of the investigations reveal that the complexion of the acoustic emission root mean square (AE-RMS) signal response changes from the drill entry to the exit thus giving an overall understanding about the different events that take place during drilling. Also, AE-RMS signal level increases with an increase in the applied thrust and further reveals that it is possible to evaluate the drill induced damages in composites through AE signal characterisation.     

Anisotropy in carbon fiber reinforced polymer (CFRP) and its effect on induction thermography

Lock-in induction thermography was carried out on uniaxial, biaxial, and multi-ply woven carbon fiber reinforced polymers (CFRPs) at an induction frequency of approximately 300 kHz and at modulation frequencies of 0.4–36 Hz to detect defects in them. The measurements were performed in reflection and transmission modes. In the uniaxial material, heating occurred far away from the excitation coil. Lines of minimal heating were observed. The modulation frequency dependence of the amplitude showed an inverse proportionality, indicating volume heating. The multi-ply woven CFRP plates with impacts of 3 and 5 J showed distinct contrast because of cracked fibers. The results were compared with those obtained using optically excited thermography and X-ray tomography.     

石英纤维增强陶瓷基复合材料制孔工艺研究

本试验针对目前硬质合金刀具加工石英纤维增强陶瓷基复合材料时存在的刀具磨损严重、加工质量差、效率低下等问题,对比了硬质合金刀具钻孔、PCD刀具钻孔和电镀金刚石套料钻螺旋铣磨制孔的效果,分析了切削力对制孔质量的影响。研究结果表明:纬纱纤维对X向和Y向切削力的影响明显大于经纱纤维,垂直于纬纱纤维方向的切削力较小,平行于纬纱纤维方向的切削力较大;PCD刀具钻孔质量相对较好,刀具磨损不明显,适用于石英纤维增强陶瓷基复合材料的制孔加工。      

化学镀表面改性石英纤维复合材料与因瓦合金的真空钎焊

采用化学镀镍方法对石英纤维复合材料进行了表面改性,研究了镀镍工艺对改善石英纤维复合材料焊接性的作用.结果表明,采用Ag-Cu共晶箔片对化学镀镍改性的石英纤维复合材料与因瓦合金在830℃保温10min进行真空钎焊后,形成由(Cu,Ni),Ni(s,s),Ag(s,s)等固溶组织及Ni元素与石英纤维间钉扎作用共同构成的界面连接结构,此时接头室温抗剪强度达到29MPa;这也说明,镀镍层通过扩散反应发挥了增强复合材料表面结构、实现致密冶金连接的作用.     

电镀金刚石钻头钻削碳纤维复合材料研究

碳纤维复合材料钻孔加工时极易产生分层、毛刺、撕裂等缺陷,是典型的难加工材料。针对碳纤维复合材料特点,以电镀金刚石钻头为研究对象,从钻削轴向力、钻孔出口质量等方面分析电镀金刚石钻头钻孔特点,并与硬质合金麻花钻进行对比,得出结论：电镀金刚石钻头钻削碳纤维复合材料时钻削轴向力较小,钻削质量较好,更适合于碳纤维复合材料的加工;钻头转速提高有利于减小钻孔缺陷的产生,钻削轴向力随钻头转速的升高而降低,随钻头直径的增大而增大;最后,通过多元线形回归方法得出电镀金刚石钻头钻削力经验公式。     

连续SiC纤维增强钛基复合材料界面反应研究

针对连续SiC纤维增强钛基复合材料界面反应速率、反应产物进行了研究.采用基体-纤维涂覆法和热等静压工艺,制备了连续W芯SiC纤维增强TC17复合材料.对复合材料进行不同温度、不同时间热暴露,通过SEM、TEM、EDS,表征分析了界面反应层厚度、界面处化学成分及界面反应产物类型.结果表明:C涂层能有效保护SiC纤维;界面反应层处的主要元素为Ti和C;制备状态试样的界面反应产物为TiC1-x,靠近C涂层的TiC1-x晶粒较细小,靠近基体TiC1-x晶粒较粗大;高温热暴露使界面反应加剧,反应层厚度增加,反应层的生长符合抛物线规律,反应的动力学参数为频率因子k0=1.33×10-3m·s-1/2,反应激活能Q=243.22 kJ·mol-1.     

Optimal drilling of particulate metal matrix composites based on experimental and numerical procedures

An experimental and numerical study of the evolution of cutting forces, tool wear and surface finish, measured when drilling the particulate metal matrix composite A356/20/SiCp-T6 is presented. The experimental work was developed through the continuous measurement of the cutting forces with an appropriate piezoelectric dynamometer. The wear type was identified and its evolution with cutting time was measured. Drills with polycrystalline diamond were tested. The surface finish of the holes was evaluated with a profilometer.Using the experimental results, a numerical search of optimal drilling conditions was performed. Since there are contradictory objectives, such as maximisation of tool life and minimisation of tool wear, the concept of the Pareto optimum solution is considered in the optimisation procedure. An evolution strategy is adopted to obtain the optimal solution for cutting speed, feed rate and tool life prediction with industrial interest.     

Feasibility study of the rotary ultrasonic elliptical machining of carbon fiber reinforced plastics (CFRP)

Carbon fiber reinforced plastics (CFRP) are used for various aircraft structural components because of their superior mechanical and physical properties such as high specific strength, high specific stiffness, etc. However, when CFRP are machined, rapid tool wear and delamination are troublesome. Therefore, cost effective and excellent quality machining of CFRP remains a challenge. In this paper, the rotary ultrasonic elliptical machining (RUEM) using core drill is proposed for drilling of holes on CFRP panels. This method combines advantages of core-drill and elliptical tool vibration towards achieving better quality, delamination free holes. The cutting force model and chip-removal phenomenon in ultrasonic elliptical vibration cutting are introduced and analyzed. The feasibility to machine CFRP for RUEM is verified experimentally. The results demonstrate that compared to conventional drilling (CD), the chip-removal rate has been improved, tool wear is reduced, precision and surface quality around holes is enhanced, delamination at hole exits has been prevented and significant reduction in cutting force has been achieved.     

石英纤维复合材料与因瓦合金的胶接辅助钎焊连接分析

通过表面涂覆活性胶改性的方法,实现了石英纤维复合材料与因瓦合金的胶接辅助钎焊连接.结果表明,含有钛的液态活性胶在焊接加热过程中与石英纤维复合材料表面纤维发生反应,并通过Ag-Cu共晶钎料层、铜中间层与因瓦合金获得致密连接,接头产生TiO,TiC,CuTi,Fe2Ti等化合物,其结构可表示为QFSC/TiO+Si+TiC+Cu(s,s)/CuTi+Cu(s,s)+Ag(s,s)/Cu(s,s)+Ag(s,s)+Fe2Ti/Invar.由不同钎焊温度接头的剪切性能对比试验得出,在850℃保温15min时的接头抗剪强度达到最大值44MPa.表面涂覆活性胶对钎料润湿的促进作用、活性金属元素Ti与复合材料纤维的化学反应及接头焊缝区产生的化合物生成相是影响连接性能的主要因素.