The effect of fabric and fiber tow shear on dual scale flow and fiber bundle saturation during liquid molding of textile composites

In Liquid Composite Molding (LCM) processes, a fibrous reinforcement preform is placed or draped over a mold surface, the mold is closed and a resin is either injected under pressure or infused under vacuum to cover all the spaces in between the fibers of the preform to create a composite part. LCM is used in a variety of manufacturing applications, from the aerospace to the medical industries. In this manufacturing process, the properties of the fibrous reinforcement inside the closed mold is of great concern. Preform structure, volume fraction, and permeability all influence the processing characteristics and final part integrity. When preform fabrics are draped over a mold surface, the geometry and characteristics of both the bulk fabric and fiber tow bundles change as the fabric shears to conform to the mold curvature. Numerical simulations can predict resin flow in dual scale fabrics in which one can separately track the filling of the fiber tows in addition to flow of resin within the bulk fabric. The effect of the deformation of the bulk fabric due to draping over the tool surface has been previously addressed by accounting for the change in fiber volume fraction and permeability during the filling of a mold. In this work, we investigate the effect of shearing of the fiber tows in addition to bulk deformation during the dual scale filling. We model the influence of change in fiber tow characteristics due to draping and deformation on mold filling and compare it with the results when the fiber tow deformation effect is ignored. Model experiments are designed and conducted with a dual scale fabric to characterize the change in permeability of fiber tow with deformation angle. Simulations which account for dual scale shear demonstrate that the tow saturation rate is affected, requiring longer fill times, or higher pressures to completely saturate fiber tows in areas of a mold with high local shear. This should prove useful in design of components for applications in which it is imperative to ensure that there are no unfilled fiber tows in the final fabricated component.     

Reinforcing hydroxyapatite/thermosetting epoxy composite with 3-D carbon fiber fabric through RTM processing

Bioactive ceramic/polymer composites have been developed in the orthopaedic field in recent years. In this work, three-dimensional (3-D) carbon fiber fabric is used to reinforce hydroxyapatite (HA)/thermosetting epoxy composite and epoxy resin through resin transfer molding (RTM) processing. It is found that the 3-D carbon fiber fabric can be impregnated with epoxy and HA-containing epoxy resin, and HA is distributed gradually along the depth direction in fiber-reinforced HA/epoxy composite, although HA is dispersed evenly in epoxy resin by surface modification of silane coupling agent. The impact toughness and flexural strength of fiber-reinforced epoxy and fiber-reinforced HA/epoxy composites are much higher than those of epoxy and HA/epoxy composite. The impact toughness of both fiber-reinforced composites decreases while the flexural strength and the flexural modulus increase with fiber volume ratio. The impact toughness of the fiber-reinforced HA/epoxy composite is higher, while the flexural strength and modulus are lower than those of the fiber-reinforced epoxy composite at the same fiber volume ratio. The flexural strength of the both composites is higher than, and their flexural modulus is close to, those of the human cortical bone. The in vitro cytotoxicity test with L929 fibroblasts shows that the addition of HA diminished the toxicity of epoxy resin.     

Experimental and numerical investigation of fabric impact behavior

This paper presents experimental and numerical research regarding blunt trauma resistance of ten fabrics made of high strength fibers. Fabrics of various architecture were examined, including plain woven fabrics, unidirectional laminates and multiaxial fabrics. The fabrics were compared with respect to the depth of the depression formed and the amount of energy transferred to the backing during projectile impact. Absolute values of mentioned parameters were compared, as well as their values after normalization with respect to thickness and areal density of the fabrics. A numerical method for estimating the amount of energy transferred to the backing was proposed.Normalized results, obtained experimentally and numerically, proved that most of the analyzed fabrics provide a similar level of protection, but the best blunt trauma resistance is given by multiaxial fabrics and the least by plain woven fabrics. This study has also shown that the depth of the depression in the backing material is an insufficient parameter in describing protective properties of fabric against blunt trauma. It is possible that impacts into ballistic packages composed of different fabrics with the same depth of depression may cause completely dissimilar injuries because of the amount of energy transferred to the backing material.     

碳纤维复合材料超声振动辅助RTM工艺的浸润特性

研究了织物类型、纤维体积分数和超声振动对树脂在碳纤维织物中流动特性的影响规律,设计了超声振动辅助RTM工艺过程中单向渗透率测量装置,开展了16组渗透率测试实验,并结合COMSOL软件仿真分析了织物中的树脂流动特性。研究表明,在相同纤维体积分数水平下,斜纹编织物的纤维束间隙通道比平纹织物的更宽,2/2斜纹编织织物渗透率比平纹织物提高了约21.5%。纤维体积分数与织物渗透率呈负相关,其函数关系与半经验公式Kozeny-Carman(KC)方程吻合较好。树脂流动过程中加入超声振动,其超声空化效应、加速度效应和微射流效应作用于纤维丝束表面,提高了织物渗透率约58.2%。有限元仿真模拟了椭圆形和近矩形纤维束截面设计的织物模型的流动过程,结果发现近矩形纤维束截面高流速区域范围更广,流体向纤维布夹层浸渍的速度分量更大。超声作用于织物纤维可能带动纤维丝束蠕动,使纤维束截面趋于近矩形状,从而提高了树脂对纤维织物的浸润性。上述研究结果对优化碳纤维复合材料成型工艺和成型性能具有一定的指导意义。      

Experimental characterization of voids in high fibre volume fraction composites processed by high injection pressure RTM

Replacing autoclave processes is a well-known industry drive in the composites community. One of the most recognized candidates for this replacement is high injection pressure resin transfer moulding (HIPRTM), because it is both an out of autoclave process and because the high processing pressures can, hypothetically, reduce the size of voids, thereby reducing void content. In order to clarify this issue, this paper presents our results on the size distribution and total void fraction of composites containing high fibre volume fractions (>60%) composites produced by HIPRTM. To substantiate this work we present a comparative study considering both autoclave and RTM at lower pressure/fibre volume fractions. Results show that HIPRTM is able to produce high fibre volume fraction parts at very low void content (<0.05%) and is comparable to autoclave results. Future work should study the mechanical properties of these laminates in order to clarify further the limits of HIPRTM.     

尼龙无纺布增韧RTM复合材料的湿热老化性能

采用尼龙无纺布(PNF)作为结构化增韧层,利用树脂传递模塑(RTM)工艺制备了PNF层间增韧改性的碳纤维增强环氧树脂基复合材料(U3160-PNF/3266),研究了U3160-PNF/3266复合材料的吸湿特性及湿热老化对其耐热性能的影响。结果表明:增韧前后复合材料具有相似的吸湿动力学特性,但在吸湿初期,U3160-PNF/3266复合材料具有更大的吸湿速率,达到饱和吸湿后,U3160-PNF/3266复合材料的饱和吸湿率约为0.96%,略大于非增韧复合材料U3160/3266的0.87%。随着湿热老化时间的增加,两种复合材料的玻璃化转变温度均逐渐降低,并随着吸湿率的饱和而趋于平稳,达到饱和吸湿后,U3160-PNF/3266和U3160/3266复合材料的玻璃化转变温度分别下降了约15%和14%。     

国产F-12芳纶织物面内剪切性能

对国产F-12芳纶织物的剪切性能进行了研究,通过像框剪切试验和偏轴拉伸试验对其进行了测试,设计了相应的像框剪切夹具,利用数字图像相关方法（DIC）采集织物加载过程的变形场,获得了剪切载荷-剪切角度的变化曲线和剪切锁紧角,通过正则化处理对两种测试结果进行了对比分析;利用数值方法对织物剪切性能进行了计算,建立了一种非正交各向异性的杆-壳模型用于模拟织物剪切变形,编制了VUMAT子程序用于计算纱线方向的改变和应力更新,通过与试验结果的对比验证了计算模型的准确性。结果表明：织物的剪切行为具有明显的非线性特征,试样夹持部分的结构形式会对试验结果产生影响,未抽丝试样的剪切载荷明显大于抽丝试样的剪切载荷,二者的剪切锁紧角近似相等;正则化处理后,相框剪切的剪切载荷大于偏轴拉伸的剪切载荷,而锁紧角小于偏轴拉伸的锁紧角;试验结果与仿真结果对比,仿真结果与试验结果吻合较好。     

Experimental and analytical investigation of reinforced high strength concrete continuous beams strengthened with fiber reinforced polymer

Carbon and glass fiber reinforced polymer (CFRP and GFRP) are two materials suitable for strengthening the reinforced concrete (RC) beams. Although many in situ RC beams are of continuous constructions, there has been very limited research on the behavior of such beams with externally applied FRP laminate. In addition, most design guidelines were developed for simply supported beams with external FRP laminates. This paper presents an experimental program conducted to study the flexural behavior and redistribution in moment of reinforced high strength concrete (RHSC) continuous beams strengthened with CFRP and GFRP sheets. Test results showed that with increasing the number of CFRP sheet layers, the ultimate strength increases, while the ductility, moment redistribution, and ultimate strain of CFRP sheet decrease. Also, by using the GFRP sheet in strengthening the continuous beam reduced loss in ductility and moment redistribution but it did not significantly increase ultimate strength of beam. The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate load enhancement ratio in the same beam. An analytical model for moment–curvature and load capacity are developed and used for the tested continuous beams in current and other similar studies. The stress–strain curves of concrete, steel and FRP were considered as integrity model. Stress–strain model of concrete is extended from Oztekin et al.’s model by modifying the ultimate strain. Also, new parameters of equivalent stress block are obtained for flexural calculation of RHSC beams. Good agreement between experiment and prediction values is achieved.     

Experimental measurement of wrinkle formation during draping of non-crimp fabric

A rig and image analysis methodology is described to characterise wrinkle formation during draping of non-crimp fabrics. The circular fabric blank is draped over a male hemispherical mould, partly constrained by a circular clamping ring around the periphery of the blank. The three-dimensional shape of the fabric is derived from a shape-from-focus analysis of a stack of photographs of the deformed blank. Wrinkles are identified from the deviation of the shape from a smoothed shape. Wrinkle formation is strongly dependent on the fabric architecture and increases progressively with increased punch displacement. Triaxial fabrics have the highest wrinkle amplitude, unidirectional and 0/90° biaxial fabrics the lowest amplitude. The clamping force reduces the wrinkling for some fabrics but, for the maximum force applied, is not effective at eliminating wrinkling.     

碳纤维丝束结构对碳纤维/酚醛复合材料烧蚀性能的影响EI北大核心CSCD

以酚醛树脂为基体,以平纹碳布和短切碳纤维两种结构形式的碳纤维为增强剂,制备碳纤维增强的碳/酚醛复合材料。采用氧/乙炔烧蚀实验对复合材料的耐烧蚀性能进行了对比性研究,采用电子拉力试验机对复合材料的弯曲性能进行表征,采用扫描电镜对复合材料烧蚀形面进行观察,并通过固体火箭发动机对复合材料的烧蚀性能进行考核验证。研究结果表明:以这两种结构形式的碳纤维为增强剂制备的碳/酚醛复合材料,其氧乙炔质量烧蚀率的大小与碳纤维丝束的大小具有正相关的特性,碳纤维丝束越小碳纤维质量烧蚀率越低,当碳纤维增强剂处于单丝状态时,复合材料的氧乙炔质量烧蚀率达到最低为0.046 g/s,并且碳纤维的型号规格对复合材料氧乙炔质量烧蚀率的影响变小。固体火箭发动机实验表明,单丝状态下的碳纤维/酚醛复合材料的抗烧蚀冲刷性能明显优于束状碳纤维/酚醛复合材料。     

Monte Carlo simulation of the effect of fiber characteristics on creep life of a fiber tow

Fatigue of reinforced ceramics at elevated temperatures was numerically evaluated with a fiber dominated, power-law creep model. A Monte Carlo simulation of fiber creep in a uniaxially loaded tow was used to examine the influence of fiber radius, elastic modulus, and strength on creep respose. The simulation permitted variation of both the average magnitude and dispersion of fiber characteristics while maintaining constant power-law creep parameters. A linear increase in creep life was predicted for an increase in mean fiber radius, and a linear decrease in creep life was predicted for an increase in the standard deviation of fiber radii. A linear increase in creep life was predicted for both an increase in mean fiber elastic modulus and standard deviation of elastic moduli. Characteristic fiber strength and Weibull modulus were predicted to have a significant effect on creep life of a SiC fiber low. An increase in either the characteristic strength or Weibull modulus was predicted to result in an increase in creep life.     

Modeling the impact of capillary pressure and air entrapment on fiber tow saturation during resin infusion in LCM

Traditionally, capillary effects have been neglected when modeling the filling stage of Liquid Composite Molding processes. This simplification is justified because the inlet resin pressures are much higher than the capillary pressure. This simplification is also acceptable when impregnating fabrics in which their fiber tows saturate at the same rate as the bulk preform. However, this assumption is questionable for fabrics that exhibit dual scale in which the fiber tows saturate at a much slower rate than the bulk preform. In such cases, the capillary pressure can influence the time to saturate a fiber tow significantly and impact the overall impregnation dynamics. Since the flow front velocity inside the fiber tows is significantly smaller than the flow around them, it is important to include the capillary pressure that may aid the saturation of the tow. In this paper, we modify our existing simulation that can predict the filling of the bulk preform and the saturation of the fiber tows to include the capillary forces at the fiber tow level. Important parameters are identified and grouped in non-dimensional form. A parametric study is conducted to examine the role of these dimensionless parameters on the overall tow saturation levels. The modeling is extended to include the effect of entrapped air inside the tows on the overall saturation of the preform. An experimental technique using the optical properties of vinyl ester and glass fiber was used to qualitatively validate the proposed model.     

Dry friction characterisation of carbon fibre tow and satin weave fabric for composite applications

Composites forming processes such as resin transfer moulding (RTM) typically involve a preforming step in which dry fabric material is deformed. Frictional forces in tool–fabric and fabric–fabric contacts determine the fabric deformation behaviour to a large extent. Previous investigations of the frictional behaviour of fibrous materials were mostly performed on a particular scale, i.e. microscopic (filament), mesoscopic (tow), or macroscopic (fabric). This study aims to provide a coupling between these scales by means of friction experiments on both carbon tows and carbon fabric in contact with metal counterfaces. The frictional behaviour of both materials on metal was measured on a capstan and a flat plate-friction setup. The frictional behaviour of fabric was comparable to that of single tows for matching pressures based on the mesoscopic contact area with the metal counterface. Furthermore, the agreement of the results forms a validation of both friction characterisation methods.     

Experimental investigation on the ball burnishing of carbon fiber reinforced polymer

The polymer-based materials are generally used in all industrial applications. Even if polymer bars can be machined easily, they need surface finishing treatment after the machining process. The ductile properties make coarse roughness on the surface that causes the elastic structure of polymer materials. The carbon fiber reinforced composite materials differ from polymer-based materials by high strength and stiffness. Its structure exhibits similar performances such as metallic materials. The effect of ball burnishing on the surface quality of rod workpiece was investigated in this study. To enhance the surface quality of high strength carbon fiber reinforced polymer rod workpiece, burnishing process was performed in three different parameters (force, feed rate, number of passes) and under four different mediums (dry, wet, boron oil, and mineral oil) conditions. The results of the experiments were analyzed, and optimum burnishing parameters were determined and discussed in detail. The best surface roughness value of the CFRP material used in the experiments was obtained as burnishing force: 250 N, feed rate: 0.05 mm/rev, four passes and wet medium. According to the results of variance analysis, it was found that the CFRP workpiece is the important leading factor for surface roughness with a contribution ratio of 62.47%.     

复合材料RTM十字型接头疲劳性能对比实验研究

测试了复合材料RTM十字型接头疲劳性能,分析对比RTM、缝纫RTM和共胶接3种不同成型工艺十字接头的疲劳强度,结果表明:RTM成型十字接头具有良好的疲劳性能,缝纫RTM接头仅为前者的1/4,而先固化后胶接接头最低.研究还发现,复合材料疲劳寿命对成型工艺非常敏感,导致试验结果存在严重分散性.RTM和缝纫RTM的接头破坏都是从根部富树脂区的尖端率先产生裂纹,然后裂纹沿着富树脂区和玻璃布的界面向下扩展,直至接头的整体破坏;先固化后胶接接头则表现为中间层板和T接头的胶层拉伸破坏.     

Experimental investigation of steel fiber reinforced concrete box beams under bending

Reduction of dead weight of a reinforced-concrete (RC) structure without too much concession in its load carrying capacity has always been an attractive study subject because it engenders (1) a decrease in dimensions of the members, (2) a decrease in the reinforcement steel, and (3) a decrease in lateral inertia forces during severe earthquakes. In this study, nine RC beams of outer dimensions of 300 × 300 × 2000 mm, six of which are box beams, designed and produced using a C20 class steel fiber concrete, (SFRC) with the commonly used steel fiber type of Dramix-RC-80/0.60-BN at a dosage of 30 kg/m³, are tested under bending. The mechanical behaviours of all these nine beams under bending are recorded from the beginning of the test till the ultimate failure of the tensile reinforcement in a two-point beam-loading setup. The proportions of (1) loss in ultimate load versus reduction in dead weight and (2) (ultimate experimental load)/(ultimate theoretical load) of the SFRC box beams are determined for two different box thicknesses. Dimensionless behaviour relationships of all the SFRC beams are determined, and the experimentally obtained relationship between the ratio of (actual ultimate load)/(theoretical ultimate load) and the ratio of (wall thickness)/(beam height) for the SFRC box beams is expressed diagrammatically.     

Experimental investigation of link between growth and decay of fiber Bragg gratings

We report here an experimental investigation for establishing and quantifying a link between the growth and decay characteristics of fiber Bragg gratings. One of the key aspects of our work is the determination of the defect energy distribution from the grating characteristics measured during their fabrication. We observe a strong correlation between the growth-based defect energy distribution and that obtained through accelerated aging experiments, paving the way for predicting the decay characteristics of fiber Bragg gratings from their growth data. Such a prediction is significant in simplifying the postfabrication steps required to enhance the thermal stability of fiber Bragg gratings.