注射条件对LCM工艺非饱和流动特性影响

双尺度多孔纤维预制体填充过程中延迟浸润的非饱和流动现象,对基于树脂流过区域为完全饱和区域的充模理论及模拟方法提出了挑战。通过控制体/有限单元（CV/FE）法结合沉浸函数实现了液体模塑成型工艺（LCM）中非饱和填充浸润的数值模拟,并对比了恒压下的实验结果,验证了其可靠性。分析讨论了注射口压力、流量和液体黏度对双尺度多孔纤维织物非饱和填充浸润特性的影响。结果表明:在允许误差内,该数值模拟结果可靠,可用于分析讨论各因素对双尺度多孔织物非饱和流动特性的影响;填充浸润过程中,纤维织物内部非饱和区域长度并非保持不变,而是随着填充浸润的进行经历了4个变化过程;不同注射条件下,压力、流量及黏度对非饱和流动特性影响不同。研究结果对合理控制注射条件及流体特性实现双尺度多孔纤维预制件的完全浸润具有指导意义。      

双尺度纤维织物二维非饱和流动的数值模拟与实验

液体模塑成型工艺(LCM)中非饱和流动的填充模拟对于在虚拟空间中快速、高效地优化工艺参数具有重要意义。采用了一种模拟双尺度纤维织物在等温条件下非饱和流动的双尺度计算模型,通过引入沉浸函数求解宏观-微观流动控制方程组,同时考虑了在微观浸渍中毛细压力的影响,在有限元/控制体积算法中实现了对非饱和流动的数值模拟。随后对三向缝合纤维织物进行了二维径向填充实验,将实验结果与数值模拟的预测值对比。结果表明,该计算模型可以较精确地模拟双尺度纤维织物中的非饱和流动。在此计算模型的基础上,讨论了流体黏度、注射流量及纤维束孔隙率对非饱和填充浸润的影响。结果表明,不同流体黏度、注射流量及纤维束孔隙率对纤维织物填充过程中非饱和区域长度、入口压力曲线及填充时间影响不同。研究结果可以对合理预测纤维织物的浸润及树脂填充过程中入口压力提供指导。     

恒压下树脂在双尺度多孔介质中的流动特性

基于复合材料液态模塑(LCM)工艺过程中存在半饱和区域的实验现象以及对预制体双尺度效应的逐步认识, 一些学者提出用沉浸模型来研究双尺度多孔介质的不饱和流动。通过体积均匀化方法描述了双尺度多孔介质复合材料液态模塑工艺模型的特征, 得到含有沉浸项的双尺度多孔介质的质量守恒方程, 并采用有限元法对方程进行数值求解, 通过具体算例计算了考虑双尺度效应时恒压树脂注射下不同时段的压力分布状态, 得到树脂在填充过程中流动前沿半饱和区域从出现到消失的过程, 采用不同注射压力进行模拟并比较。结果表明, 与单尺度多孔介质模型不同, 双尺度多孔介质模型更能反映实际树脂填充过程中出现的半饱和区域现象。     

层间纳米纤维膜对玻纤预制体渗流特性的影响

为了研究层间纳米纤维膜对玻纤织物渗流特性的影响,使用超景深三维显微镜表征了纳米纤维含量对玻纤织物微观结构的影响,采用径向法测量了纳米纤维膜夹层玻纤织物预制体的渗透率,重点分析了纳米纤维含量对玻纤织物预制体渗流模式的影响。结果表明:玻璃纤维束间的毫米尺度区域被纳米纤维膜填充而离散成微米尺度区域;预制体孔隙率及渗透率值均随着纳米纤维含量的增加而减小;随着纳米纤维含量的增加,复合预制体表现出的各向异性程度逐渐减小;树脂宏观流动前沿内部分饱和区域面积比例随纳米纤维含量的增加而增大;相同纳米纤维含量预制体的部分饱和区域面积比例随注入时间的增加呈先增大后减小趋势。     

风力发电机叶片用玻璃纤维织物的渗透率

通过改变纤维层数来改变纤维织物的孔隙率,采用一维饱和流动方法测量了风力发电叶片用玻璃纤维织物(WindstrandTM)三个方向(x、y和z)的饱和渗透率大小.考察了孔隙率、模具尺寸以及纤维方向(平行于和垂直于2％的纤维束两个方向)对其饱和渗透率的影响.结果表明:渗透率随孔隙率的降低而迅速降低；当孔隙率为34.6％～54.7％时,模具尺寸对y向饱和渗透率影响不大；改变2％的纤维束方向(由平行到垂直),当孔隙率为34.6％～54.7％时,对饱和渗透率有一定影响.当孔隙率为45％～55％时,玻璃纤维织物x和y方向的饱和渗透率约为z方向饱和渗透率的3～7倍.     

风力发电机叶片用玻璃纤维织物的渗透率

通过改变纤维层数来改变纤维织物的孔隙率, 采用一维饱和流动方法测量了风力发电叶片用玻璃纤维织物(Windstrand^TM)三个方向(x、y和z)的饱和渗透率大小。考察了孔隙率、模具尺寸以及纤维方向(平行于和垂直于2%的纤维束两个方向)对其饱和渗透率的影响。结果表明: 渗透率随孔隙率的降低而迅速降低; 当孔隙率为34.6%~54.7%时, 模具尺寸对y向饱和渗透率影响不大; 改变2%的纤维束方向(由平行到垂直), 当孔隙率为34.6%~54.7%时, 对饱和渗透率有一定影响。当孔隙率为45%~55%时, 玻璃纤维织物x和y方向的饱和渗透率约为z方向饱和渗透率的3~7倍。     

不同纤维堆积状态下饱和渗透率实验研究

通过调节压力控制纤维体积分数, 用单向饱和流动法对纤维层厚度方向的饱和渗透率进行了测定。考察了纤维体积分数随压力的变化规律, 并分别研究了纤维体积分数、纤维层数、铺层方式和纤维种类对渗透率的影响。结果表明, 当纤维体积分数超过某临界值后其值随压力增大的幅度减小。单向和正交铺层时层数变化对渗透率没有影响, 而平纹布渗透率随着纤维层数的增加而减小。此外, 纤维种类的改变对渗透率有明显影响。      

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。      

测试条件对集束纤维单向渗透率的影响

为掌握渗透率测试的影响因素,自行研制了单向纤维集束的浸渗特性测试系统,对比了液体沿单向纤维集束的饱和与非饱和渗透特性,分析了不同压力作用形式、纤维体积分数、液体种类对非饱和渗透率测试结果的影响。结果表明：相同条件下纤维集束的饱和渗透率大于非饱和渗透率测试结果;当外压较小时毛细作用对非饱和渗透率测试结果的影响显著;当纤维体积分数增加时集束纤维的渗透率呈减小趋势;相同压力差下真空驱动测得的渗透率比正压驱动测试结果大。     

树脂传递模塑工艺中的非饱和流动过程模拟与实验研究

针对编织类纤维增强体的纤维束之间与纤维束内孔隙的双尺度特点,建立了平纹织物的细观结构模型,并推导了汇函数的数学表达式。建立了局部细观流动特征的非饱和流动控制方程,利用有限元/控制体积方法求解,得到了局部饱和度分布。与实验进行比较,吻合较好。      

Modelling microscopic flow in woven fabric reinforcements and its application in dual-scale resin infusion modelling

A physical unit cell impregnation model is proposed for the micro-scale flow in plain woven reinforcements. The modelling results show a characteristic relationship between tow impregnation speed, the surrounding local macro-scale resin pressure and the tow saturation within the unit cell. This relationship has been formulated into a mathematical algorithm which can be directly incorporated into a continuum dual-scale model to predict the ‘sink’ term. The results using the dual-scale model show a sharp resin front in inter-tow-pore spaces and a partially saturated front region in intra-tow-pore spaces. This demonstrates that the impregnation of fibre tows lags behind the resin front in the macro pore spaces. The modelling results are in agreement with two reported experimental observations. It has been shown that the unsaturated region at the flow front could increase or have a fixed length under different circumstances. These differences are due to the variation in tow impregnation speed (or the time required for the tow to become fully impregnated), the weave architecture and the nesting and packing of plies. The modelling results have also demonstrated the drooping of the inlet pressure when flow is carried out under constant injection rates. The implementation of the algorithm into a dual-scale model shows coherence with a single-scale unsaturated model, but demonstrates an advantage in flexibility, precision and convenience in application.     

Flow modeling of the VARTM process including progressive saturation effects

Modeling of vacuum based liquid composite molding methods (e.g., VARTM) relies on good understanding of closely coupled phenomena. The resin flow depends on the preform permeability, which in turn depends on the local fluid pressure; the preform compaction behavior, and the membrane stresses in the vacuum bag. It has also been shown that for many preforms there is a significant unsaturated region behind the flow front, and that the flow in this region affects the overall flow behavior of the process. Studies of preform compaction have shown that the preform stiffness, as well as being non-linear and exhibiting significant hysteresis, is dependant on the fluid saturation. For this reason most researchers model the preform compaction based on the pressure-compaction behavior of saturated preforms during unloading. This assumption leads to an effective discontinuity in preform thickness at the flow front, which is not observed in actual experiments. In this paper an improved compaction model incorporating the saturation dependence of the compaction pressure in the partially saturated region, is used in a one-dimensional model of the VARTM process. The model gives physically more realistic results for the thickness in the flow front region, and an improved model for the consolidation of the preform at the end of infusion.     

RTM工艺注模过程边缘效应模拟分析

RTM工艺需将纤维预制体预置到模具中,由于纤维预制体结构不均匀性和模具形状、尺寸等影响,极易产生边缘效应。边缘效应会严重影响树脂流场发展和压力场分布。本文作者采用等效渗透系数方法模拟边缘效应,得到了其影响下的树脂流动前峰曲线和压力场。研究表明:一方面边缘效应可能导致不期望的树脂流场发展而形成工艺缺陷——干斑;另一方面可以利用边缘效应提高工艺效率:常流率注射时减小合模压力和注射压力,常压力注射时可以减少注模时间。      

Variations in unsaturated flow with flow direction in resin transfer molding: An experimental investigation

The dual-scale nature of fiber preforms due to the presence of large continuous gaps between fiber tows gives rise to the unsaturated flow in resin transfer molding (RTM) process which is characterized by a droop in the injection pressure history due to the delayed absorption of fiber tows (the ‘sink’ effect). In this study, we experimentally investigate the effect of change in flow direction on the unsaturated flow in three anisotropic dual-scale fiber mats. A series of 1-D mold-filling experiments involving a constant flow rate were conducted for a unidirectional woven fiber-mat, a biaxial stitched mat, and a triaxial stitched fiber-mat along with a reference single-scale random mat. In the case of the unidirectional mats, the droop in the inlet-pressure history, signifying the strength of the sink effect, is found to be strongest for flow along the micro-channels aligned with fiber tows. The droop, and hence the sink effect, is observed to weaken progressively for flow-directions at 45° and 90° to this principal direction. In the case of the biaxial and triaxial mats, the situation is more complex due to the multi-layer construction of such mats: maximum droop is found when mats are oriented at a 45° angle with respect to the fiber-mat coordinate, and it weakens in the 0° and 90° directions. The unsaturated flow effect is also quantified by measuring percentage deviation in the area under the experimental curve from that of the predicted curve. A clear correlation between the droop (through the percentage deviation) and the permeability along a flow direction in the unidirectional mats is observable, though such a relationship eludes the triaxial mat. The effect of unsaturated flow on liquid-front progress during the 1-D experiment was also studied. In contrast to the reference single-scale random mat where the observed front progress closely follow the prediction based on the single-scale physics, a small difference was observed between the observed and predicted front progress for the three dual-scale mats considered. However the difference was too small to yield any significant correlation with the flow direction.     

A two-phase flow model to simulate mold filling and saturation in Resin Transfer Molding

This paper addresses the numerical simulation of void formation and transport during mold filling in Resin Transfer Molding (RTM). The saturation equation, based on a two-phase flow model resin/air, is coupled with Darcy’s law and mass conservation to simulate the unsaturated filling flow that takes place in a RTM mold when resin is injected through the fiber bed. These equations lead to a system composed of an advection–diffusion equation for saturation including capillary effects and an elliptic equation for pressure taking into account the effect of air residual saturation. The model introduces the relative permeability as a function of resin saturation. When capillary effects are omitted, the hyperbolic nature of the saturation equation and its strong coupling with Darcy equation through relative permeability represent a challenging numerical issue. The combination of the constitutive physical laws relating permeability to saturation with the coupled system of the pressure and saturation equations allows predicting the saturation profiles. The model was validated by comparison with experimental data obtained for a fiberglass reinforcement injected in a RTM mold at constant flow rate. The saturation measured as a function of time during the resin impregnation of the fiber bed compared very well with numerical predictions.     

四步法三维编织预制件的渗透率

采用径向法测量了三维编织预制件的渗透率, 利用修正注入半径的循环方法, 基于Visual C + + 计算了渗透率的大小, 主渗透率方向分两步确定: 一是通过选择阈值的图像处理运算提取流动前沿; 二是采用Matlab中的非线性最小二乘法将所得轮廓进行椭圆拟合, 并计算该椭圆长轴方向与编织方向的夹角。结果表明: 编织参数影响渗透率, 当编织角一定时, 随着纤维体积含量的增加, 主渗透率降低; 当纤维体积含量一定时, 随着编织角的增加, 主渗透率降低, 但主渗透率的方向不随编织角的变化而改变, 始终保持与编织轴向一致。