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.     

Modeling of capillary flow in shaped polymer fiber bundles

Moisture transporting in fiber assembly is one of the critical factors affecting physiological comfort. In this study, we investigated at the capillary flow in complex geometries representative of the void spaces formed between fibers in shaped polymer fiber bundles. Dynamic process of liquid creeping in capillary is analyzed based on Reed and Wilson vertical wicking model. Critical equivalent radius values of capillary tubes in polymer fiber assembly are discussed here. In the cases of round, criss-cross and triangle shape fiber, Reed and Wilson model is integrated with shaped fiber bundle mathematical simulation model (MFB) to calculate the dynamic curve of liquid arising. Instantaneous wicking velocity, wicking height, wicking flux, and characters are compared to figure out that the wicking effect of shaped polymer fiber is quite better than normal round one.     

Simulation of the reinforcement compaction and resin flow during the complete resin infusion process

Resin infusion (a.k.a. VARTM) is one of the LCM processes, for which liquid resin is drawn into dry reinforcements. Significant cavity thickness changes occur during processing, due to the flexibility of the vacuum bag used as one side of the tool, and the complex stress balance within the laminate. While the magnitude of thickness change is often small, the influence is significant on reinforcement properties. Changes in permeability during filling and post-filling have the potential to significantly affect the process. To simulate this behaviour, it is important to accurately model compaction and unloading of reinforcement in dry and wet states. A series of tests were completed to determine compaction behaviour of an isotropic glass fibre mat. From these tests several non-linear elastic compaction models have been determined, and applied within a resin infusion simulation which addresses pre-filling, filling and post-filling. This simulation was then used to assess different post-filling strategies.     

碳纳米管-玻璃纤维/环氧层板双真空灌注工艺及性能

针对碳纳米管(CNT)-玻璃纤维/环氧树脂体系, 采用传统的真空灌注工艺(VARIM)和双真空灌注工艺(DVARIM)制备复合材料层板, 分析了不同工艺方法下层板缺陷状况, 测试了层板的弯曲性能和层间剪切性能, 并结合树脂性能和纤维/树脂界面粘结状况观察, 探讨了DVARIM对CNT分布的影响及碳管的增强机制。结果表明: 与传统的VARIM相比, DVARIM能增加纤维的间距, 提高树脂对纤维的浸润能力, 减小纤维束内的孔隙缺陷; 添加质量分数为0.05％的酸化CNT后层板性能提高, 而且采用DVARIM性能提高更明显; 不同灌注工艺对CNT的分布产生影响, 从而改变了CNT对纤维/树脂界面粘接的影响, 同时这种影响与织物结构的紧密程度有关。      

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 pressure dependent solubility in a thermoset resin for simulating pressure accelerated thermo-oxidation tests

This paper presents an approach for modeling high pressure thermo-oxidative conditioning, for an epoxy resin material.A coupled reaction–diffusion chemical “mechanistic” model of thermo-oxidation with classical boundary conditions, successfully applied for samples aged under air at atmospheric pressure but failing at high pressures, has been enhanced by formulating a new nonlinear sorption law, correlated with the results of Elastic Indentation modulus (EIT) measures at 5 bars of pure oxygen, and validated for pressures up to 8 bars of pure oxygen.The approach allows justifying the employment of the oxygen pressure as a pertinent accelerating parameter for thermo-oxidation degradation: moreover the proposed model can be employed for the design and the optimization of accelerated thermo-oxidation tests.     

Dynamic capillary impact on longitudinal micro-flow in vacuum assisted impregnation and the unsaturated permeability of inner fiber tows

This paper addresses issues of the synergetic dynamic effect of capillary force on the longitudinal impregnation driven by external pressures, especially under vacuum assistance. An apparatus was designed to detect the axial infiltration along unidirectional fiber bundles which were all aligned closely to give a representation of micro-flow channel of inner fiber tows. The external driving pressures were controlled sufficiently low, 20–60 kPa, on the order of capillary pressures. Based on the analysis of infiltration velocities under different external pressures, dynamic capillary pressures can be determined experimentally. The results showed that capillary pressures, the most important force of microscopic flow through inner fiber yarns, acted as a drag force on the infiltration flow for vacuum assisted penetration into unidirectional fiber bundles. This unique drag effect is very different from traditional unsaturated infiltration, different from the compressed air driving permeation and the theoretical calculated data in this paper. Moreover, values and even signs of the dynamic capillary pressures varied with the fiber fraction of the assemblies as well as the fluid types. Further analysis demonstrated that the function of capillary pressure was closely related to the capillary number (Ca), acting as drag force when Ca larger than a critical value, and as a promotive force with smaller Ca. Consequently, unsaturated permeabilities of the unidirectional fiber bundles were estimated by taking consideration of both dynamic and quasi-static capillary pressures.     

真空压力下树脂沿织物铺层厚度渗透速率

为了考察树脂膜熔渗(RFI)工艺过程中树脂在高温条件下沿织物铺层厚度方向的不饱和渗透特性, 应用自行设计的测试系统, 考察了液体沿织物铺层厚度方向流动前锋的影响因素, 测试并分析了液体沿玻璃纤维铺层厚度方向渗透速率的主要影响因素及其变化规律。结果表明, 液体沿纤维织物厚度方向流动为宏观上的一维流动。 真空压力增大、 树脂温度升高、 纤维体积分数减小, 均可使液体的渗透速率加快。另外, 对比发现, 70℃ E-51 环氧树脂沿玻璃纤维铺层厚度方向的渗透特性与室温下硅油的渗透特性基本相当。      

Effects of fiber tow misalignment on the engineering properties of plain weave textile composites

Effects of fiber tow phase angle of the adjacent layers on the engineering properties of plain weave textile composites were studied. Unit cell analyses were performed for two-layer unit cell models with different fiber tow phase angles, and multi-field macroelements were used to reduce computer resource requirements. Results indicated that the engineering properties of plain weave textile composites can vary significantly according to the manner in which the layers are stacked.     

深海纤维增强树脂复合材料圆柱耐压壳力学性能的研究进展

纤维增强树脂复合材料因其轻质和优异力学性能,使其成为了关键深海战略材料,已应用在深海潜水器重大装备。其中,耐压壳是保证深海潜水器安全和稳定的重要部件,与潜水器的质量要求和总体性能密切相关。本文重点关注纤维增强树脂深海圆柱耐压壳,综述其结构设计特性、力学性能试验表征及数值模拟研究进展,并指出目前存在的问题,以期为今后深海复合材料圆柱耐压壳的设计及力学分析提供选材依据。      

热压罐工艺树脂压力测试系统适用性实验研究

采用玻璃纤维织物/环氧树脂E51预浸料研究热压罐工艺树脂压力在线测试系统适用性,分析了树脂黏度对测试系统动态反应特性的影响,进而测试了热压罐工艺零吸胶条件下等厚、非等厚及L形预浸料铺层内部的树脂压力,并与理论模型计算树脂压力对比,以分析测试系统的准确性。结果表明: 树脂黏度明显影响系统的动态反应特性,当树脂黏度低于25 Pa·s时树脂压力测试系统具有较高的动态反应特性,满足复合材料内部树脂压力测试需求; 对于所研究的各种形式的预浸料铺层,树脂压力测试值与理论值有很好的一致性,当热压罐压力达到0.5 MPa,相对误差约为2%,验证了系统的准确性。     

Effect of capillary properties on the sensitivity enhancement in capillary/fiber optical sensors

The sensitivity enhancement observed in fluorescence signals when a conventional fiber optical sensor is coupled with a quartz or glass capillary results from the partial reflection of the radiation at the sample/internal wall interface and from the internal reflection of the refracted portion within the capillary wall. Thus, the length and absorbing properties of the capillary as well as the nature of the surrounding medium affect the enhancement significantly. To interpret the dramatic changes in enhancement observed experimentally when the absorbing properties of the capillary were changed, a partial reflective waveguide model is reported.     

基于图像处理的碳纤维增强树脂基复合材料固化压力-缺陷-力学性能建模与评估

利用热压罐成型工艺制备了不同固化压力条件下的碳纤维增强树脂基复合材料层合板,分析了超声相控阵C扫描图像与微观缺陷的对应关系,研究了固化压力、孔隙缺陷及力学性能之间的关联规律。结果表明：利用超声C扫描图像差异能够表征孔隙等缺陷含量,在本实验条件下,固化压力由0 MPa提高到0.6 MPa,复合材料孔隙率降低96.7%,拉伸强度（TS）和层间剪切强度（ILSS）分别提高56.1%和68.8%。在此基础上,对不同固化压力条件下制备的复合材料层合板的超声相控阵C扫描图像进行图像处理并定义成型质量指数,实现了基于C扫描图像对孔隙缺陷的定量表征。最后,通过对孔隙缺陷检测、力学性能测试及图像定量化评价结果进行数学拟合,建立了基于图像处理的固化压力-缺陷-力学性能之间的数学关联模型（CPDMP模型）,并给出了成型质量指数阈值为81%,及可接受的孔隙率应不高于1.1%,相应的固化压力应不低于0.35 MPa。     

Modeling the hysteretic behavior of the capillary pressure in partially saturated porous media: a review

In this review, the phenomenology of hysteresis is discussed, including both empirical and mathematical models, and some examples are presented. The focus lies on soil-moisture hysteresis, where the capillary pressure exhibits different values depending on the initial state of saturation. An historical overview is given of the investigation of this phenomenon, of various empirical models, and also of some mathematical approaches to soil-moisture hysteresis. All these studies are aimed at accurately fitting experimental results—not only the main hysteresis curves but also the inner hysteresis curves that occur upon re-wetting and re-drying. Finally, a comparison is made to another field in which hysteresis appears, the deformation of pseudoelastic bodies such as shape memory alloys.     

Modeling of measured target pressure profiles in three hypervelocity impact experiments

A 24 g aluminum sphere was shot at a sparse array of cylinders with nominal initial projectile velocity of 4 and 5 km/s. Pressure profiles were measured with cased carbon resistor gages at two locations in a projectile impacted water filled cylinder and two of its neighbors on three shots. The pressure maxima were in the 1–13 kbars range. The experiments are modeled with the ALE3D code and several techniques are used to concentrate zoning at places of interest. There is excellent agreement between the measured and calculated pressure profiles for two shots and good agreement for the third. Comparison of the calculated pressure profiles with those from more refined calculations for two shots suggest that we are near convergence with respect to zone size.     

Modeling pressure relationships of inspired air into the human lung bifurcations through simulations

Applied pressure on human lung wall has great importance on setting up protective ventilatory strategies, therefore, estimating pressure relationships in terms of specific parameters would provide invaluable information specifically during mechanical ventilation (MV). A three-dimensional model from a healthy human lung MRI is analyzed by computational fluid dynamic (CFD), and results for pressure are curve fitted to estimate relationships that associate pressure to breathing time, cross section and generation numbers of intended locations. Among all possible functions, it is observed that exponential and polynomial pressure functions present most accurate results for normal breathing (NB) and MV, respectively. For validation, pressure-location curves from CFD and results from this study are compared and good correlations are found. Also, estimated pressure values are used to calculate pressure drop and airway resistance to the induced air into the lung bifurcations. It is concluded that maximum pressure drop appeared in generation number 2 and medium sized airways show higher resistance to air flow and that resistance decreased as cross sectional area increased through the model. Results from this study are in good agreement with previous studies and provide potentials for further studies on influence of air pressure on human lung tissue and reducing lung injuries during MV.     

Effect of basalt fiber hybridization on the impact behavior under low impact velocity of glass/basalt woven fabric/epoxy resin composites

The low velocity impact behavior of E-glass/basalt reinforced hybrid laminates, manufactured by resin transfer moulding technique, was investigated. Specimens prepared with different stacking sequences were tested at three different impact energies, namely 5 J, 12.5 J and 25 J. Residual post-impact mechanical properties of the different configurations were characterized by quasi static four point bending tests. Post-impact flexural tests have been also monitored using acoustic emission in order to get further information on failure mechanisms. Results showed that basalt and hybrid laminates with an intercalated configuration exhibited higher impact energy absorption capacity than glass laminates, and enhanced damage tolerance capability. Conversely, the most favorable flexural behavior was shown by laminates with symmetrical sandwich-like configuration (E-glass fiber fabrics as core and basalt fiber fabrics as skins).