Laser-based repair of carbon fiber reinforced plastics

The recent progress in laser system technology enables innovative techniques for machining of carbon fiber reinforced plastics (CFRPs). A representative application is the layer-by-layer removal of damaged composite material to provide a cavity for refilling with repair plies. Results show that it is possible to achieve a reliable and automatable removal rate to perform arbitrary repair cavity geometries, obtaining a relevant time-reduction with respect to the conventional manual grinding process. The combination of modern UV-laser sources with a scanning technology enabling deflection speeds up to 2 m/s, suppresses heat affected zones (HAZs) and detachment of fibers from the polymer matrix. A method for the selective removal of surface matrix without damaging the fibers beneath is also presented.     

预浸料工艺制备氧化铝纤维增强氧化铝陶瓷基复合材料

氧化物/氧化物陶瓷基复合材料的增强体和基体均由氧化物构成,不存在氧化问题,是长寿命高可靠性构件的理想选材,可在1000~1300℃的高温环镜中可长期使用。本文借鉴树脂基复合材料单向纤维湿法预浸料制备工艺,通过配置氧化铝粉体料浆在缠绕式湿法预浸机上制备了单向氧化铝纤维预浸料,然后预浸料经铺层模压和高温热处理获得了氧化铝纤维增强氧化铝陶瓷基复合材料,同时对复合材料性能进行了表征。结果表明,氧化铝粉体料浆的固含量在50vol%,料浆溶剂中水和丙三醇的比例为3:1,纤维的走丝速度为6m/min,滚筒平行进度为0.5mm时可获得无缝隙,无纤维重叠、表面平整光滑的预浸料。通过预浸料铺层热压成型制备的复合材料拉伸强度高达208.2MPa,弯曲强度为386.7MPa。和料浆涂刷二维纤维布工艺相比,力学性能大幅度提高,且预浸料工艺具有易存储、操作简单、适于工业化生产等优势。     

Maximum cutting speed in laser cutting of fiber reinforced plastics

A simple one-parameter thermal model, predicting the maximum feed rate still resulting in a through cut as a function of beam power, focal spot diameter and thickness of worked material is presented and discussed for CO₂ laser cutting of composites. An extensive experimental program, including glass, carbon and aramide fabric reinforced polyester resin was carried out by varying all the parameters involved in the model. An excellent agreement is shown between experimental data and theoretical predictions. In addition, a criterion for cut quality classification, based on kerf geometry and heat affected zone size is formulated to help in selecting the optimum cutting conditions in order to obtain the best cut quality.     

Fracture detection in concrete by glass fiber cloth reinforced plastics

Two types of carbon (carbon fiber and carbon powder) and a glass cloth were used as conductive phases and a reinforcing fiber, respectively, in polymer rods. The carbon powder was used for fabricating electrically conductive carbon powder-glass fiber reinforced plastic (CP-GFRP) rods. The carbon fiber tows and the CP-GFRP rods were adhered to mortar specimens using epoxy resin and glass fiber cloth. On bending, the electrical resistance of the carbon fiber tow attached to the mortar specimen increased greatly after crack generation, and that of the CP-GFRP rod increased after the early stages of deflection in the mortar. Therefore, the CP-GFRP rod is superior to the carbon fiber tow in detecting fractures. Also, by reinforcing with a glass fiber cloth reinforced plastic, the strength of the mortar specimens became more than twice as strong as that of the unreinforced mortar.     

Sheet forming process of carbon fiber reinforced plastics for lightweight parts

The use of carbon fiber reinforced plastics is increasing markedly, particularly in aircraft bodies, but the time required to manufacture CFRP parts should be shortened to realize mass production. The stamping of solidified CFRP sheets can reduce the production time and may increase the flexibility of the manufacturing process. A new sheet forming process for solidified CFRP, in which a CFRP sheet is sandwiched by dummy metallic sheets during stamping, is proposed. The dummy metallic sheets act as protective materials as well as media for heating the CFRP sheet. The results of applying the proposed process are presented.     

Rapid 3D microwave printing of continuous carbon fiber reinforced plastics

Using microwave heating in 3D printing of continuous carbon fiber reinforced plastic (CCFRP) instead of the traditional resistive heating constitutes a new approach for the additive production of high performance composite components. Without the intrinsic slow speed and contact-needed heat transfer disadvantages, the instantaneous and volumetric heating benefits of microwave allows the fabrication of composites at higher speed. This paper presents the 3D microwave printing technology for CCFRP and investigates the mechanical properties of the tensile specimens that have been printed with different speeds. The printing process and mechanical properties of printed specimens are investigated and discussed.     

Fracture strength of alumina fiber reinforced aluminum wire with and without a torsional pre-strain

The influence of torsion applied prior to tensile testing on the fracture strength of Nextel™ 610 alumina continuous-fiber reinforced aluminum matrix composite wire is investigated. It is shown that internal stresses that develop in the composite upon twisting exert a dominant influence on the derived failure criteria, which exceeds that of fiber misalignment resulting from the gradual inclination during twisting of the fibers with respect to the wire axis. At low and intermediate levels of twisting up to a surface shear strain of 10%, fracture occurs when the sum of externally applied and internal stresses reaches locally the fracture stress of the untwisted material. For higher surface shear deformations, two possible fracture-stress vs surface shear strain dependencies are predicted, depending on the general fracture behavior of the composite qualified in terms of damage tolerance. Experiment shows that the fracture strength of the reinforced wire used in this study follows the behavior ascribed to a nondamage-sensitive material.     

Swift and inverse Swift effect in alumina fiber reinforced aluminum wires

The change in length during torsion is examined (i.e. the Swift effect) of an aluminum wire reinforced with 45% continuous Nextel^®610 alumina fibers at ambient temperature, and also its rotation under applied tensile stress after pre-twisting (i.e. the inverse Swift effect). The wire is shown to shorten when twisted away from its initial configuration in which all fibers are parallel. The shortening is proportional to the square of the twisting angle over a large range of angles. Reversal of the shear from a given pre-twist elongates the wire again, up to its initial length when zero cumulated twist is reached. In the inverse Swift experiment the wire increasingly untwists as the external load is increased. Analytical models for both Swift and inverse Swift behavior of the wire are presented, and are shown to be in good agreement with the experiments.     

Quantification of metallic coating failure on carbon fiber reinforced plastics using acoustic emission

Carbon fiber reinforced plastic substrates were subsequently coated with an electrochemically applied nickel and an electroplated copper layer. The coating-substrate system was loaded in four-point bending and the acoustic emission from coating failure was recorded during loading. The acoustic emission signals were analyzed using pattern recognition and frequency analysis techniques. This approach yields three distinguishable types of acoustic emission signals, which are correlated to three different failure mechanisms: i) nickel cracking ii) copper cracking and iii) delamination between the two coating layers. To confirm the correlation between the types of acoustic emission signals and the respective failure mechanisms and to assess the validity of the acoustic emission method to describe mechanical failure, the micro-mechanical fracture energies released during mechanical loading were calculated based on microscopic measurements of the crack progress utilizing scanning electron microscopy and scanning acoustic microscopy. These energies were compared to the associated acoustic emission signals energy for each failure mechanism. We found the calculated micro-mechanical energy values to be proportional to the measured accumulated acoustic emission energy of the associated acoustic emission signal type. We conclude that the reported failure classification method offers the possibility to compare fracture toughness values in multilayered coatings with multiple failure types, derived solely from acoustic emission energies.     

Comparative study on the wear behavior of long and short glass fiber reinforced plastics

In recent years, the fiber reinforced composites have been used more in tribological applications where the sliding surfaces requires a high wear resistance and a low co-efficient of friction. The growth of GFRP is significantly higher than that of steel. No engineer or designer can ignore the growth of GFRP, but the decision to use a new material is difficult, yet important. The comparative tribological performance of short and long glass fiber-epoxy composites, under varying load and sliding velocities, is reported in this investigation. Besides conventional weighing, the coefficient of friction, contact temperature, and wear rate were determined. The worn surfaces of the specimens were examined by a Scanning Electron Microscope (SEM). The wear mechanisms and the transitions that govern the tribological behavior of the composites between them are discussed in detail. It was found that the epoxy reinforced with a long glass fiber exhibited a reduced wear rate than the short glass fiber.     

Laser dressing of alumina grinding wheels

High-power lasers are being explored as a non-contact-type dressing tool for alumina grinding wheels. The alumina grinding wheel surface underwent melting and/or vaporization on the surface when laser-dressed, forming a modified layer on the surface. Refinement of the grain size took place. The individual particles that formed on the surface had well-defined faceted structures. Microcutting edges were generated on the individual grains and particles, which can act as cutting edges for efficient grinding. The results of x-ray diffraction and pole figure analysis suggested that the formation of these faceted structures was due to the preferential orientation of the grains after dressing. This paper was presented at the fourth International Surface Engineering Congress and Exposition held August 1–3, 2005 in St. Paul, MN.     

增强纤维对超薄超硬树脂砂轮切割偏摆问题的影响

针对树脂超薄超硬砂轮切割偏摆幅度较大的问题,研究碳纤维、玻璃纤维及两者混杂对树脂超薄超硬切割砂轮切割偏摆的影响,并对其影响机理进行分析。试验结果表明:加入碳纤维有利于提高砂轮的抗折强度,加入玻璃纤维有利于降低砂轮的切割阻力,碳纤维和玻璃纤维两者混杂可有效降低砂轮的切割偏摆幅度。当加入体积分数为15%的直径为7.6 μm、长度为100 μm的碳纤维和体积分数为5%的直径为15.0 μm、长度为120 μm的玻璃纤维时,可有效降低砂轮的切割偏摆幅度,其偏摆幅度最小可达0.006 7 mm,满足生产要求,切割偏摆性能达到最优。      

Design of eddy current-based dielectric constant meter for defect detection in glass fiber reinforced plastics

This paper presents the design of an eddy current testing probe for inspection of non-conductive glass fiber reinforced plastics. Because the magnetic field contains information pertaining to the permittivity of materials under test, eddy current testing offers the possibility of flaw detection in non-conductive materials through detection of the difference in permittivity between the intact part and the defective part of each material. We analytically investigated the design of a probe suitable for dielectric constant measurements. Experimental studies proved that the proposed probe can detect slit defects and flat-bottomed holes located 2 mm away from the surface of the glass fiber reinforced plastic samples.     

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.     

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。     

Turning of glass–fiber reinforced plastics materials with chamfered main cutting edge carbide tools

In this paper, the machinability of high-strength glass–fiber reinforced plastics (GFRP) materials in turning with chamfered main cutting edge of P and K type carbide tools have been investigated experimentally. Chip formation mechanisms have been obtained with respect to tip's geometries and nose radii. Experimental results for cutting forces were also taken with GFRP as the workpiece material. Force data from these tests were used to estimate the empirical constants of the mechanical model and verify its prediction capabilities. The results show good agreement between the predicted and measured forces. In this study, the nose radius R = 0.3 mm induces a decrease of the cutting force and the smallest cutting force values was achieved in the case of C_s = 20°, _S1(_S2) = −10°(10°) and R = 0.3 mm. Comparing the different P and K type of tools, K type tool is better than P type of chamfered main cutting edge tools. The theoretical values of cutting forces were calculated and compared with the experimental results; the forces predicted by this model were consistent with the experimental values.     

长玻璃纤维增强聚丙烯复合材料拉伸试样纤维取向模拟研究

介绍采用Moldflow软件对长玻璃纤维增强聚丙烯复合材料拉伸件的纤维取向及拉伸性能进行模拟研究,对模拟后的纤维取向、拉伸模量、泊松比进行了分析,结果表明:沿流动方向,靠近浇口部位的平均纤维取向不佳,远离浇口部位的标距尾部和过渡部位的平均纤维取向较好,垂直流动方向上平均纤维取向存在分层;纤维取向整体呈现明显的皮芯结构;试样的拉伸模量和泊松比随纤维取向的变化呈现出与其相一致的趋势。     

An investigation of workpiece temperature variation of helical milling for carbon fiber reinforced plastics (CFRP)

Better prediction about the temperature distribution of workpiece has a great significance for improving performance of cutting process, especially relating to the workpiece of carbon fiber reinforced plastics (CFRP). In this paper, a heat transfer model is developed to investigate the temperature distribution of CFRP workpiece in helical milling process. Depending on characteristics of helical milling, two kinds of heat sources have been presented, the geometrical shapes of which are modeled as semicircle arc and line. The complex trajectory of each heat source relative to the stable workpiece has been studied. Based on the analysis, unsteady state three-dimensional governing equation of heat transfer in CFRP workpiece with adiabatic boundary condition is proposed. The solution procedure of this nonhomogeneous heat transfer equation consists of two steps: it is transformed into homogeneous equation according to the heat transfer theory firstly; and then the homogeneous equation is solved using the separation of variables. Basing on the solution of the homogeneous equation, the temperature distribution resulting from the moving semicircle arc heat source and the line heat source has been studied detailedly. In order to calculate the heat generation in the helical milling process, a cutting force model is presented and the heat partition transferring into the CFRP workpiece is solved using the Conjugate Gradient Method. A series of tests of helical milling for CFRP are conducted, and the experiment results agree well with the results calculated by the predicted model. This model can be extended to optimize the cutting condition and restrain the thermal damage of the CFRP workpiece.