RTM主要缺陷的研究进展

针对树脂传递模塑(RTM)成型过程中气泡、干斑两种主要缺陷,综述了RTM成型过程中气泡和干斑的形成过程、产生原因和解决方法。并介绍了国内外针对这两种缺陷的研究现状。     

复合材料多隔板框梁结构的RTM工艺成型

复合材料多隔板框梁结构复杂,内外形精度要求高,本文研究了采用树脂传递模塑成型(RTM)工艺实现该类结构的精确制造。介绍了RTM成型模具的设计、预制体的制备及其注胶工艺的改进,并讨论了它们对于RTM制件质量的影响。结果表明,整体线型注胶口利于大型复杂结构的树脂注胶;适当增大注胶压力,并进行保压,有利改善制件表面质量;控制预制体和树脂中的气泡有利改善制件内部质量;预制体转角区增加额外填充材料可以避免该区域的富树脂及其脱落引起的表面缺陷。最后对其力学性能进行了测试分析。     

真空辅助RTM工艺中孔隙的形成机理及实时控制研究

根据国内外的研究成果对真空辅助树脂传递成型(RTM)工艺过程中气泡的形成机理进行了分析总结,同时介绍了几种真空辅助RTM实时过程控制方法.研究结果表明,实时过程控制能够减少孔隙形成的几率,改善流动前沿均匀性,缩短注胶时间,是提高复合材料产品质量的重要方法.     

RTM复合材料中分层缺陷的超声表征及量化分析

以ZT7H/QY8911碳纤维增强树脂基RTM工艺成型复合材料为研究对象,通过预埋单层聚四氟乙烯薄膜(PTFE)模拟不同尺寸(?=3 mm,6 mm,10 mm)和不同深度(近上表面、中间深度、近下表明)的分层缺陷,制备了RTM复合材料超声检测试样。基于水浸式超声反射法对缺陷进行超声A-Scan、B-Scan和C-Scan检测表征,并对检测结果进行量化分析。结果表明:超声反射法对RTM复合材料层压板中分层缺陷具有定性、定量检测能力,超声A-Scan检测信号中幅值、相位、时域渡越等特征量与缺陷的尺寸、深度、性质等具有相关性,超声B-Scan检测可以对缺陷深度进行表征,超声C-Scan可以对缺陷尺寸进行表征,深度方向检测分辨率达1个铺层厚度,缺陷尺寸的检测偏差≤1.0 mm。     

编织结构复合材料RTM成型流场仿真模拟

针对航空发动机用编织结构复合材料树脂传递模塑料(RTM)成型工艺,开展了增韧树脂基体的工艺特性及编织结构预制体的渗透特性研究,并结合PAM–RTM软件对RTM成型工艺方案进行了研究,研究结果表明,1304增韧环氧树脂RTM成型的工艺窗口期仅有25 min,8步法编织结构经向渗透率远大于纬向和Z向渗透率。通过PAM–RTM软件对RTM成型树脂注射流道和工艺参数进行设计,获得了航空发动机用高韧性编织结构复合材料RTM成型工艺方法。     

RTM工艺过程缺陷产生机理分析

本文全面分析了RTM工艺过程中缺陷产生的原因,并讨论了如何根据缺陷的特征找出问题的根源。     

预成型体质量对桨叶RTM成型工艺缺陷的影响

为考察预成型体质量对桨叶RTM成型工艺缺陷的影响,本文从预成型模具以及预成型工艺两方面对XX型复合材料螺旋桨桨叶的预成型体技术进行了研究。结果表明,在旧预成型模具的基础上改进的新型模具能精确控制预成型压力,从而可以保证预成型体的尺寸精度和质量。相比于湿法定型,达到相同的定型效果,干法定型所需的定型剂用量要少很多,后续利用RTM工艺制备得到的样件缺陷较少。     

RTM工艺中硅橡胶气囊模具的应用研究

由于用传统RTM工艺整体成型复杂结构复合材料构件有一定困难，所以用硅橡胶气囊模具代替RTM工艺中的金属阳模，并对这一工艺过程进行研究。通过硅橡胶力学性能试验，得出用硅橡胶的拉伸、撕裂、剥离强度等性能指标，可以满足气囊模具的要求。利用硅橡胶气囊辅助RTM工艺，可以制作纤维体积含量较高的复合材料构件。与其他模具相比，硅橡胶模具具有很多优点。研究表明，纤维体积含量随气囊内压力的增加而增加。     

RTM工艺制造玻璃钢检查井盖

本文系统阐述了玻璃钢检查井盖的材料选择。结构设计和利用RTM成型方法生产检查井盖的工艺流程，分析了制品主要缺陷的产生原因。研究树脂体系特性及充模流动与工艺缺陷形成的关系。合理地选择注胶工艺参数。     

复合材料RTM成型工艺参数的研究

对树脂传递模塑(RTM)成型工艺进行了具体的研究,对所得制品进行了力学性能测试,并研究分析了不同注胶温度和注胶压力对制品力学性能的影响.证明了RTM工艺过程中注胶压力越小,树脂对纤维的浸润也越充分,制品的力学性能也相应的较好.提高注胶温度在使树脂粘度降低的同时也不利于RTM工艺的排气,从而影响制品的力学性能.     

Processing/performance relationships considering voids in structural reaction injection molding

The causes of void content and its resulting effect on the material properties of structural reaction injection molded (SRIM) composites were investigated. Plaques were molded under different combinations of five two-level factors, and the resulting effect of each factor on void content was determined. The geometry of the glass reinforcement had the largest effect, as woven mats caused double the void content of random mats. Evacuating the mold cavity proved to greatly reduce void content as well. Varying the injection rate and shot size had little effect. Material property testing was conducted on groups of specimens with similar void content. Tension testing revealed no decrease in strength with increasing void content. One million cycles of tension-tension fatigue at 20% of the ultimate strength caused no decrease in strength at any void content, while fatigue at 30% caused premature failure. Impact testing showed that only very high void contents had a substantial effect on performance. Environmental conditioning showed moisture absorption proportional to void content and furthermore significantly reduced the residual tensile strength after a single thermal cycle to 0°C. These results suggest that void content in conjunction with a weak interface may have been responsible for limiting material properties.     

Influence from process parameters on void formation in resin transfer molding

The influence of different process variables on the void content in resin transfer modling (RTM) has been investigated experimentally. The moldings were made in a flat mold filled by a parallel flow from one edge of the laminate to the other. The viods were found concentrated in a narrow region close to the ventilation side of the laminate. The void volume fraction in this region was almost constant and dropped over a short distance to basically no voids in the rest of the laminate. Micrographs from cross sections in different directions revealed that the voids were of two different types, long cylinderical bubbles inside the fiber bundles. An efficient way of reducing the void content was to use vacuum assistance during mold filling. This technique was benefical both for the magnitude of the void content and for the extent of the void region. The void content with the highest level of vacuum assistance (≈︁ 1 kPa absolute pressure), was practically negligible. Strong indications for void generation by mechanical entrapment at the flow front was found. The lowering of the void content with vacuum assistance can be interpreted as aresult of compression of voids when the vacuum is released and a higher mobility of voids created at a lower pressure.     

Effect of packing on void morphology in resin transfer molded E‐glass/epoxy composites

Effects of applying a packing pressure on void content, void morphology, and void spatial distribution were investigated for resin transfer molding (RTM) E‐glass/epoxy composites. Packing pressures of zero and 570 kPa were respectively applied to center‐gated composites containing 17.5% randomly oriented, E‐glass fiber preform. Radial samples of these disk‐shaped composites were utilized to evaluate voidage via microscopic image analysis. Two adjacent surfaces were cut from each molded disk in order to evaluate void presence from both through‐the‐thickness and planar views. The packed composite was found to contain almost 92% less void content than the unpacked composite. While void fractions of 2.2 and 2.6% were measured, respectively, from the through‐the‐thickness and planar surfaces of the unpacked composite, only 0.2% void content was observed in the packed composite from both surfaces. Digital images obtained from through‐the‐thickness surface showed that average void size dropped from 59.3 μm in the unpacked composite to 31.7 μm in the packed composite. A similar reduction in average void size from 66.7 to 41.1 μm was observed from the planar surfaces. Circular voids were found to experience higher removal rates at 99%, followed by cylindrical and elliptical voids at 83 and 81%, respectively; while irregular voids show slightly lower void removal rates at 67%. Void proximity to fiber bundles was also observed to affect void reduction as voids located inside fiber tows experience lower void reduction rates. Along the radial direction of the molded disks, removal of voids with different proximities to fibers seems to depend on their arrangement at the end of the filling stage. These findings are believed to ascertain packing as an effective void removal method for RTM and similar liquid composite molding processes. POLYM. COMPOS., 26:614–627, 2005. © 2005 Society of Plastics Engineers     

Void fraction measurement and calculation model of vertical upward co-current air–water slug flow

The focus of this paper is on the measurement and calculation model of void fraction for the vertical upward co-current air–water slug flow in a circular tube of 15 mm inner diameter. High-speed photography and optical probes were utilized, with water superficial velocity ranging from 0.089 to 0.65 m·s^-1 and gas superficial velocity ranging from 0.049 to 0.65 m·s^-1. A new void fraction model based on the local parameters was proposed, disposing the slug flow as a combination of Taylor bubbles and liquid slugs. In the Taylor bubble region, correction factors of liquid film thickness C_δ and nose shape C_Z^* were proposed to calculate α_TB. In the liquid slug region, the radial void fraction distribution profiles were obtained to calculate α_LS, by employing the image processing technique based on supervised machine learning. Results showed that the void fraction proportion in Taylor bubbles occupied crucial contribution to the overall void fraction. Multiple types of void fraction predictive correlations were assessed using the present data. The performance of the Schmidt model was optimal, while some models for slug flow performed not outstanding. Additionally, a predictive correlation was correlated between the central local void fraction and the cross-sectional averaged void fraction, as a straightforward form of the void fraction calculation model. The predictive correlation showed a good agreement with the present experimental data, as well as the data of Olerni et al., indicating that the new model was effective and applicable under the slug flow conditions.     

Monte-Carlo simulations of surface and gas phase diffusion in complex porous structures

A new procedure for estimating surface diffusivities and tortuosities within realistic models of complex porous structures is reported. Our approach uses Monte-Carlo tracer methods to monitor mean-square displacements for molecules restricted to wander on pore walls within model random mesoporous solids typical of those used as adsorbents, heterogeneous catalysts, and porous membranes. We consider model porous solids formed from initial packings of spheres with unimodal, Gaussian, or bimodal distributions of size; changes in pellet porosity are achieved by increasing microsphere radii and by randomly removing spheres from highly densified packings in order to simulate densification and coarsening, respectively. Geometric tortuosities for the surface phase reached large values at void fractions near 0.04 and 0.42 for densified solids; the surface tortuosity gave a minimum value of 1.9 at a void fraction of ∼0.26. These high tortuosities correspond to percolation thresholds for the void and solid phases, which in turn reflect packing densities at which each phase becomes discontinuous. Surface tortuosities for coarsened solids at low void fractions were similar to those in densified solids; however, at void fractions above ∼0.3, surface tortuosities of coarsened solids increased only gradually with void fraction, because coarsening retains significant overlap among spheres at void fractions above those giving disconnected solids in densified structures. Simulations of bulk diffusion within voids were used to compare the transport properties and connectivity of the void space with those of surfaces that define this void space. Surface and void tortuosities were similar, except for void fractions near the solid percolation threshold, because unconnected solid particles interrupt surface connectivity but not gas phase diffusion paths. Surface and void tortuosities were also similar for channels within linear chains of overlapping hollow spheres as both tortuosities increased with decreasing extent of sphere overlap. These simulations provide a basis for estimates of surface and void tortuosities, which are essential in the interpretation and extrapolation of diffusion rates in complex porous media. Surface and void diffusivity estimates differed significantly from those obtained from lattice and capillary models of complex porous structures.     

An experimental study of gas void fraction in dilute alcohol solutions in annular gap bubble columns using a four-point conductivity probe

The influence of alcohol concentration on the gas void fraction in open tube and annular gap bubble columns has been investigated using a vertical column with an internal diameter of 0.102 m, containing a range of concentric inner tubes, which formed an annular gap; the inner tubes had diameter ratios from 0.25 to 0.69. Gas (air) superficial velocities in the range 0.014–0.200 m/s were investigated. Tap water and aqueous solutions of ethanol and isopropanol, with concentrations in the range 8–300 ppm by mass, were used as the working liquids. Radial profiles of the local void fraction were obtained using a four-point conductivity probe and were cross-sectionally averaged to give mean values that were within 12% of the volume-averaged gas void fractions obtained from changes in the aerated level. The presence of alcohol inhibited the coalescence between the bubbles and consequently increased the mean gas void fraction at a given gas superficial velocity in both the open tube and the annular gap bubble columns. This effect also extended the range of homogeneous bubbly flow and delayed the transition to heterogeneous flow. Moreover, isopropanol results gave slightly higher mean void fractions compared to those for ethanol at the same mass fraction, due to their increased carbon chain length. It was shown that the void fraction profiles in the annular gap bubble column were far from uniform, leading to lower mean void fractions than were obtained in an open tube for the same gas superficial velocity and liquid composition.     

Void formation model and measuring method of void formation condition during hot pressing process

For resin matrix composites, voids are common defects that can seriously deteriorate the properties of the composite parts. Thus, the elimination of voids is a crucial element in controlling the manufacturing process of composite parts. This article focuses on void formation originating from hygroscopic water for resin matrix composite laminates prepared with hot pressing process. The Kardos void formation model was developed to analyze the critical resin pressure for the initiation of voids, and the influencing factors were investigated experimentally to validate the modified model. It is found that resin pressure and gel temperature are the two key parameters to control void defects and that entrapped air in prepreg stacks must be considered in the void formation model. Furthermore, a simple method was established to measure the relationship between porosity and the processing parameters, and the void formation conditions of the resin and the prepreg stack were also studied. The theoretically predicted void formation conditions and the experimental results were compatible for the studied cases. These results are valuable for eliminating void defects, optimizing processing parameters, and enhancing the performance of composite parts. POLYM. COMPOS., 31:1562–1571, 2010. © 2009 Society of Plastics Engineers     

Effects of maximum void size and aggregate characteristics on the strength of mortar

The effects of the maximum void size and aggregate shape and roughness on the flexural strength of high strength mortar were investigated. Substantial reductions in the maximum void size and air content of quartz aggregate mortars resulted in flexural strength increases. These increases in flexural strength were somewhat lower than predicted by Griffith's theory, thus suggesting that the maximum void size did not act as the critical flaw controlling the flexural strength. Factors relating to the cement-aggregate bond, including aggregate shape and roughness, appeared to affect the flexural strength more than the maximum void size.     

微通道内伴有化学吸收气液两相流的空隙率与压力降

微通道内气液两相流空隙率与压力降对微反应器的热质传递性能有显著影响,是微反应器的重要设计参数。采用高速摄像仪和压力测量系统分别对矩形微通道内单乙醇胺水溶液化学吸收CO₂过程的空隙率和压力降进行了研究,考察了弹状流下气液两相流量与化学反应速率对空隙率及压力降的影响。结果表明:当液相流量一定时,微通道内空隙率和压力降均随着气相流量的增大而增大,空隙率随化学反应速率的增大而减小,压力降随化学反应速率的增大而增大;当气相流量一定时,随着液相流量和化学反应速率的上升,微通道内空隙率下降,而压力降上升。提出了微通道内伴有化学吸收的空隙率和压力降的半理论预测模型,模型平均误差分别为15.79%和11.12%,显示了良好的预测性能。     

Experimental characterization of autoclave-cured glass-epoxy composite laminates: Cure cycle effects upon thickness,void content,and related phenomena

This article presents results from over 100 experimental autoclave curing fiber-glass-epoxy composite laminate curing runs. The primary objective was to verify shrinking horizon model predictive control—SHMPC—for thickness and void content control, using readily available secondary measurements. The secondary objective was to present and analyze the extensive experimental results obtained through this verification. Seven series of curing runs (16 per series) were performed, with cure settings governed by partial- or full-factorial orthogonal array based design of experiments. Through t-tests and two-way analysis of variance, it was found that pressure magnitude had the largest influence on laminate thickness and void content, while first hold duration/temperature, pressure application duration, and run delay influenced void content more than thickness. Thinner laminates with lesser void contents resulted from pressure application before the second temperature ramp. Prepreg age also affected thickness and void content. Photomicrographs revealed not one large void, but void clusters. Interrupted autoclave cure cycles revealed that significant laminate thickness reduction occurred during all curing cycle stages. The percentage of resin weight loss through laminate sides increased with pressure magnitude and application duration. Ten test curing runs indicated that SHMPC met difficult thickness targets while minimizing void content.