基于数字图像技术的纤维织物面内渗透率表征

材料渗透率的表征受其结构空间离散性和求解方式准确性的严重影响。基于数字图像技术,评估了纤维织物渗透率的空间分布,并探讨了阶梯铺层对灌注工艺的影响。首先,从恒压单向注射实验的视频流中动态提取了流动前沿的流速分布和流动前沿角,通过织物渗透率与结构的关系仅一次实验便可求得纤维织物的面内局部渗透率分布;其次,利用正态分布函数拟合,建立了基于数字图像技术的纤维织物面内主方向渗透率张量的求解方法,并利用该方法研究了编织形式对渗透率的影响;最后,研究了阶梯铺层和恒定铺层对灌注过程的影响。结果表明:建立的基于数字图像技术的渗透率表征方法可以通过一次实验同时获取面内主方向上的渗透率及其空间离散型;在恒定铺层下缎纹织物渗透率随着纤维层数增大而增大,从厚铺层向薄铺层的灌注方式可以达到最优的灌注时间。      

In-plane permeability characterization of a unidirectional flax/paper reinforcement for liquid composite molding processes

Principal in-plane permeabilities of a unidirectional flax/paper reinforcement are characterized in terms of reinforcement material and manufacturing parameters at a constant fiber volume fraction (V_f). ANOVA result shows that surface density of the unidirectional flax layer is the most important parameter on the mean and variance of the K₁ permeability. On the other hand all four studied parameters are concluded to affect the K₂ permeability. The K₁ permeability is found close to that of a twill weave flax fiber fabric reported in the literature and only one order of magnitude lower than a plain weave glass fiber fabric. Impregnation of the reinforcement with epoxy resin shows that a large area of the molded plaques was dominated by capillary forces during resin injection. This means capillary number and subsequently the resin injection velocity should be optimized for reducing void content in the final composite.     

Evaluation of through-thickness permeability and the capillary effect in vacuum assisted liquid molding process

Through-thickness penetration under vacuum assistance is crucial for resin film infusion (RFI) and vacuum assistant resin transfer molding (VARTM) process. In this paper, values of the through-thickness unsaturated permeability (TTUP) and capillary pressure (P_c) are estimated based on the infiltration velocity in preforms of carbon fiber fabric and glass fiber fabric, respectively, measured by a specially designed apparatus. It reveals that, for the through-thickness permeation, the P_c values generally decrease with increasing fiber content. Relatively accurate TTUP can be obtained by counting P_c into the permeation dynamics. If P_c is neglected, liquids with good-wettability, such as silicone oil, tend to result in larger TTUPs. The corrected TTUPs show good agreement according to Carman–Kozeny, Gutowski modified Carman–Kozeny equation, and Gebart model, respectively. The resultant permeability resistance parameters of the preforms indicate that the penetration in carbon fabric bed is slower than in glass fabric bed. However, for fiber volume fraction more than 60%, the corrected TTUPs show no significant difference for all the preforms.     

Study of nesting induced scatter of permeability values in layered reinforcement fabrics

Time consumption is the major drawback of many existing set-ups for permeability measurement. This drawback has prevented many researchers from studying the statistical distribution of experimentally obtained permeability values. To overcome this problem—while maintaining the necessary accuracy—an automated central injection rig for permeability identification called ‘PIERS set-up’ (permeability identification using electrical resistance sensors) was developed. The PIERS set-up was used to characterize two typical glass fiber reinforcements. The test results demonstrated the presence of large scatter in the identified permeability values and hence the necessity to consider a statistical distribution of permeability values. The existence of a permeability distribution implies that a small number of measurements will not be sufficient to fully characterize the material. This paper discusses the necessary number of measurements to adequately characterize the statistical distribution and investigates the origins of the observed scatter. The paper also shows that the nesting of layers during stacking and mold closing is a major source of the observed variations in permeability values.     

Structurally stitched NCF CFRP laminates. Part 2: Finite element unit cell based prediction of in-plane strength

Based on experimental investigations on structurally stitched non-crimp fabric (NCF) carbon fiber/epoxy laminates under in-plane tension, compression and shear loading [1], a finite element based unit cell model was developed to estimate the in-plane strength of NCF laminates taking into consideration the yarn diameter, the stitching pattern and direction as well as the load type. Depending on these parameters, regions with undisturbed and disturbed fiber orientations leading to resin pockets as well as local changes of the fiber volume fraction are taken into account in the model.The comparison of experimental and numerical results showed that the strength of structurally stitched NCF laminates under in-plane tension, compression or shear loading can be predicted with an acceptable accuracy. The overall mean deviation between simulation and experiment observed was between 8% and 13%.     

国产CCF300碳纤维单向织物液体成型工艺性及其复合材料力学性能

选取国产碳纤维CCF300所制备的2种单向织物,单向无纬织物U3160及单向无屈曲织物KUC160,分别对其预成型体进行压缩特性和渗透特性测试,以研究2种单向织物的液体成型工艺性,并采用树脂传递模塑(RTM)工艺制备2种单向织物/双马来酰亚胺树脂基复合材料,测试并对比其面内力学性能。结果表明:预成型体压缩试验中,嵌套效应受压力及织物层数影响较大,压力越高、层数越多,嵌套效应越显著。U3160织物的嵌套效应较KUC160织物更为明显,在较高压力下,KUC160织物预成型体的纤维体积分数较U3160织物的下降了约20%。渗透率测试结果表明:相比U3160织物,KUC160织物0°方向的渗透率较高,而90°方向的渗透率有所降低;这是由于经编线的绑缚作用能促进0°方向的宏观流动,而阻碍90°方向的微观渗透。此外,KUC160织物的经编线与U3160织物的纬向纱线的导流作用也对渗透率有影响。力学性能试验结果表明:相比U3160织物增强复合材料,KUC160织物增强复合材料0°方向的拉伸、弯曲和压缩性能均有所下降,拉伸强度和弯曲模量降幅最大,分别约为11%和21%;而层间剪切强度有小幅提高,增幅约为8%。      

In-plane permeability of sheared fabrics

Shear is the main mode of deformation in the draping of fabrics over complex mould geometries in composites manufacturing. Hence, the measurement and prediction of the in-plane permeability of sheared fabrics is a crucial task for the design of resin transfer moulding and other composites processing techniques of complex shaped articles. A mathematical model has been developed and applied to predict the in-plane permeability in the two principal directions and the angle of the flow ellipse for sheared assemblies of bi-directional woven fabrics that are in-plane isotropic to flow when unsheared. Modelling was accompanied by in-plane permeability measurements for unsheared and sheared woven fabric assemblies, and a comparison of this experimental permeability data with the proposed model proved encouraging. A study into the change of the areal density of different woven fabrics with shear angle has also been included.     

Structurally stitched NCF CFRP laminates. Part 1: Experimental characterization of in-plane and out-of-plane properties

The insertion of local through-thickness reinforcements into dry fiber preforms by stitching provides a possibility to improve the mechanical performance of polymer-matrix composites perpendicular to the laminate plane (out-of-plane). Three-dimensional stress states can be sustained by stitching yarns, leading to increased out-of-plane properties, such as impact resistance and damage tolerance. On the other hand, 3D reinforcements induce dislocations of the in-plane fibers causing fiber waviness and the formation of resin pockets in the stitch vicinity after resin infusion which may reduce the in-plane stiffness and strength properties of the laminate.In the present paper an experimental study on the influence of varying stitching parameters on in-plane and out-of-plane properties of non-crimp fabric (NCF) carbon fiber/epoxy laminates is presented, namely, shear modulus and strength as well as compression after impact (CAI) strength and mode I energy release rate. The direction of stitching, thread diameter, spacing and pitch length as well as the direction of loading (which is to be interpreted as the direction of the three rail shear loading or the direction of crack propagation in case of mode 1 energy release rate testing) were varied, and their effect on the mechanical properties was evaluated statistically.The stitching parameters were found to have ambivalent effect on the mechanical properties. Larger thread diameters and increased stitch densities result in enhanced CAI strengths and energy release rates but deteriorate the in-plane properties of the laminate. On the other hand, a good compromise between both effects can be found with a proper selection of the stitching configurations.     

Effect of fabric compaction and yarn waviness on 3D woven composite compressive properties

3D-woven fabrics incorporate through-thickness reinforcement and can exhibit remarkable inter-laminar properties that aid damage suppression and delay crack propagation. However, distortions in the internal architecture such as yarn waviness can reduce in-plane properties, especially in compression. The degree of yarn waviness present in a 3D woven fabric can be affected by a range of factors including weave parameters and manufacturing-induced distortions such as fabric compaction. This paper presents a thorough analysis of the effect of fabric compaction and yarn waviness on the mechanical properties and failure mechanisms of an angel-interlock fabric in compression. Tests were conducted on coupons moulded to different volume fractions and data compared to previous measurements of local yarn angle. Major findings show the importance of yarn straightness on compressive strength and how this can be affected by optimising moulding thickness. Failure initiation was also found to be heavily influenced by weave style and yarn interlacing.     

Temperature Dependence of Resin Flow in a Resin Film Infusion (RFI) Process by Ultrasound Imaging

Ultrasonic imaging in the C-scan mode was used in conjunction with the amplitude of the reflected signal to measure the temperature dependence of resin flow rate in single layers of woven carbon fabric. The RFI samples were vacuum-bagged and scanned in a water tank at 50°C, 60°C, 70°C, and 80°C. The measured flow rates were plotted versus inverse viscosity to determine the permeability in the thin film, non-saturated system. The permeability values determined in this work were consistent with permeability values reported in the literature. Capillary flow was not observed at the temperatures and times required for pressurized flow to occur. The flow rate at 65°C was predicted from the measured flow rates, and then measured in a 10-layer laminate. The investigation demonstrates that ultrasonic imaging in the C-scan mode in conjunction with the amplitude of the reflected signal is an effective method for measuring resin flow through fabric.     

Influence of fabrics’ design parameters on the morphology and 3D permeability tensor of quasi-unidirectional non-crimp fabrics

This research addresses the effects of quasi-UD non-crimp fabric (NCF) design parameters on the fabric architecture and on the permeability tensor. These fabrics are designed for the Liquid Resin Infusion (LRI) of large and thick composite parts. Three fabrics’ parameters intended to bring a flow enhancement to the NCF are investigated: the stitch spacing, the stitch pattern and the weft tow lineal weight. Image analysis is undertaken to characterize the morphology of non-crimp fabric composite. A new continuous permeability measurement method based on compressive tests is proposed to relate the permeability of the quasi-UD NCF to the design parameters during the infusion process. The latter are proven to influence significantly both the fabric architecture and the permeability tensor coefficients.     

Influence of the fabric architecture on the variations in experimentally determined in-plane permeability values

Fabric non-uniformities can be described by the geometric parameters R_p/a₀, which refers to the average ratio of in-plane dimension of fibre tows R_p and fibre tow spacing a₀ and characterises the fibre-void distribution in the fabric, and the standard deviation of the inter-fibre-angle σ_α. For three different woven fabrics and a stitch-bonded non-crimp fabric, it was found that in general the higher R_p/a₀, the lower is σ_α, with an approximately linear relation between R_p/a₀ and σ_α. Woven fabrics show mainly macro-scale non-uniformities determined by the fibre tow mobility, which is limited by the void space between the tows. The mainly mesoscopic non-uniformities in non-crimp fabrics are imposed by the stitching pattern, which again determines the values of R_p/a₀ and σ_α. The principal values and the orientation of the principal axes for the global in-plane permeabilities were determined based on series of experiments that were evaluated statistically. All relative variations (standard deviations/mean values) of the principal permeability values were found to be in the range between 9.1% and 29.3%. The variation in the angle describing the orientation of the principal permeability axes increases with increasing relative permeability variations. Analysis of the relation between permeability variations and geometrical fabric parameters suggests that the more homogeneous the fibre-void distribution in the fabric, i.e., the higher R_p/a₀ and the lower σ_α, respectively, the lower the local permeability variations and the more similar the flow front shape to a perfect ellipse. The global permeability variations, on the other hand, increase with the local variations until they reach a maximum and then decrease again. Quantitative comparison of measured permeability variations and the results of stochastic simulations indicates that, while the results are in the same order of magnitude, the stochastic model is not detailed enough for quantitatively accurate prediction of permeability variations.     

Characterization of textile permeability as a function of fiber volume content with a single unidirectional injection experiment

Textile permeability is a fundamental property to describe preform impregnation in Liquid Composite Molding (LCM) processes. It depends on textile architecture and fiber volume content (FVC). Conventional methods to measure in-plane permeability are based on radial or unidirectional injection experiments performed at fixed FVC. A complete characterization involves a series of tests and requires several material samples. This study presents a novel approach to characterize permeability as a function of FVC through a unique unidirectional injection experiment with a preform containing different FVC sections. The same experimental set-up as in conventional unidirectional unsaturated permeability measurements is used with a second pressure transducer embedded in the mold in addition to the one located at the inlet gate. A fast algorithm is developed to exploit the data from the two sensors and automatically derive the permeability distribution without any need of visual flow front observations. The methodology is validated with a random fiber mat and a woven fabric. Results show that accurate permeability characterization can be achieved for both kinds of textiles.     

碳纤维织物力学性能和冲压成形实验研究

在实际成形过程中,碳纤维复合材料往往处于复杂的应力状态,开展近于真实载荷环境下的力学试验分析,能够更准确地认识实际应用中材料的成形性能和变形机理.为获得碳纤维织物的基本力学特性,设计了平纹碳纤维织物拉伸试样及成形试样,进行了单轴拉伸、双轴拉伸、镜框剪切试验和方盒冲压成形实验研究,对比了不同双拉比及纱线取向对力学性能及成形性能的影响.研究结果表明:碳纤维织物具有高度的非线性、各向异性和双拉耦合特性,即经纬向纤维的力学性能会相互影响;剪切变形是成形过程中的主要变形模式,当剪切角达到临界锁死角时,织物发生起皱现象;同种织物不同纱线取向试样表现出不同的成形性能,因此可以根据零件几何形状选择合适纤维取向的织物,从而减少缺陷,优化成形零件的力学性能.研究结果为后续建立碳纤维织物本构模型和成形仿真奠定了基础.     

Nondestructive Evaluation of Anisotropy in Composite Materials Via Acoustic Birefringence

A linearly polarized ultrasonic shear wave propagating in fiber-reinforced composites produces an elliptical vibration pattern due to birefringence. The acoustic birefringence of shear waves is analyzed and applied for evaluation of in-plane stiffness anisotropy of uni-axial nonfabric, biaxial, and quadra-axial fabric composite materials. The parameters of the elliptical motion are determined by measurements of the amplitude and phase of the transmitted/reflected shear wave as functions of the receiver polarization angle. The strength of birefringence is derived from the measured elliptical pattern and used to quantify the in-plane stiffness asymmetry in glass and carbon fiber-reinforced nonfabric and fabric composites. The technique is shown to be sensitive enough to detect damage induced variation in stiffness anisotropy. The correlation between the asymmetry of shear stiffness and orientation of impact cracking in composites is also discussed.     

Importance of in-plane shear rigidity in finite element analyses of woven fabric composite preforming

A finite element made of woven unit cells under biaxial tension and in-plane shear is proposed for the simulation of fabric forming. The simulation is made within an explicit dynamic approach and is based on a simplified dynamic equation accounting for tension and in-plane shear strain energy. The biaxial tensile properties (given by two surfaces) and the in-plane shear properties (given by a curve) can be determined both by biaxial tensile tests and picture frame experiments or obtained by mesoscopic 3D finite element analyses of the woven unit cell. The interior load components of the proposed finite element are calculated explicitly and simply from the tensions and shear torque on four woven cells. The results obtained by the simulations of a hemispherical forming process on a very unbalanced fabric are compared to experiments. It is shown that the tension strain energy permits to describe the asymmetry of the response but that the computation of wrinkles and of the deformed states when the locking angle is exceeded needs to take the in-plane shear stiffness and its evolution with shear angle into account.     

Numerical and experimental determination of in-plane elastic properties of 2/2 twill weave fabric composites

This paper presents the in-plane elastic properties of 2/2 twill weave, T300 carbon/epoxy, woven fabric composite plates, obtained by both finite element analysis and experiments. A micromechanical, three-dimensional (3D) finite element model of the single layer unit cell of a 2/2 twill weave fabric composite is built, and a homogenization process is implemented. A unit cell is chosen such that it encloses the characteristic periodic repeat pattern in the fabric weave. Detailed geometry together with construction procedures for this new model are developed by using ANSYS Parametric Design Language (APDL). In this respect, the scope for altering the weave and yarn parameters is facilitated. Standard tensile and rail shear tests with modifications are performed for this kind of woven fabric composite. Elastic mechanical properties determined by experiments are presented, and the finite element model is verified. Satisfactory correlation between the predicted and experimental results are obtained.     

无弯曲纤维织物面内渗透率的结构相关性

建立无弯曲纤维织物(Non-crimped fabrics, NCF)的几何结构单胞, 应用树脂在纤维束内与束间耦合流动的模型, 数值模拟树脂的细观流动行为, 结合Darcy定律, 计算单胞的面内等效渗透率, 并对计算方案进行验证。在此基础上, 探讨织物的纤维束间距离、纤维束高度以及束内渗透率等细观结构参数与单胞面内等效渗透率之间的关系。结果表明: 单胞面内等效渗透率随纤维束间距离的增大而增大, 其倒数的对数之间呈正的线性关系; 纤维束高度对单胞面内等效渗透率的影响类似于纤维束间距离对其的影响; 单胞面内等效渗透率随纤维束内渗透率的增加而线性增加。      

尼龙无纺布结构化增韧层增韧碳纤维/环氧树脂复合材料的湿热力学性能

采用尼龙无纺布(PNF)作为结构化增韧层,制备了PNF层间增韧改性的U3160碳纤维增强3266环氧树脂(U3160-PNF/3266)复合材料,研究了U3160-PNF/3266复合材料的面内力学性能及湿热老化后的力学性能变化,并分析了复合材料湿热老化前后的层间形貌。结果表明:PNF增韧层的引入并未导致复合材料面内力学性能的下降,与未增韧的U3160碳纤维增强3266环氧树脂(U3160/3266)复合材料相比,增韧复合材料U3160-PNF/3266的90°拉伸性能有所提高。而湿热老化处理对U3160-PNF/3266复合材料的基体和界面性能影响相对明显,尤其是尼龙纤维与树脂基体之间的界面结合性能,湿热老化处理后增韧复合材料的90°压缩和层间剪切性能保持率均明显低于未增韧复合材料的。     

Material property variations and defects of carbon/carbon brake disks monitored by ultrasonic methods

Several ultrasonic techniques were applied to carbon/carbon brake disks for the evaluation of spatial variations in material properties that are attributable to the manufacturing process. In a carbon/carbon brake disk manufactured by a combination of pitch impregnation and vapor infiltration methods, the spatial variation of ultrasonic velocity was measured and found to be consistent with the nonuniform densification behavior in the manufacturing process. Low frequency (e.g. 1–5 MHz) through-transmission scans were used for mapping out the material property inhomogeneity. These results were compared with that obtained by dry-coupling ultrasonics. A good correlation was found between ultrasonic velocity and material density on a set of small blocks cut out of the disk. Pulse-echo C-Scans (10–25 MHz) were used to image near-surface material property anomalies associated with certain steps in the manufacturing process. Ultrasonic velocities in the in-plane directions were affected more by the relative contents of fabric and chopped fiber, and less by the void content.