【作者】 徐艳华；

【导师】 胡红；

【作者基本信息】 东华大学 ， 纺织工程， 2011， 博士

【摘要】 本课题是对自主研发的一种新型的经纱与纬纱呈交织状态的双轴向纬编织物及其增强复合材料的拉伸性能进行研究。由于该织物中既有针织物的线圈结构,又有机织物的交织结构,又称之为机织针织复合结构(Co-woven-knitted Fabric,简称CWK织物)。该织物将机织和针织两种结构结合起来,同时融合两种结构的性能和特点。与常见的双轴向针织物相比,最大的不同是CWK织物中的经纱与纬纱有交织。鉴于CWK织物性能与结构参数之间的密切联系,首先对CWK织物的结构进行了深入的研究,确定其编织工艺,并设计了其相应的编织装置和给纱装置；随后在普通针织横机上进行改造,并编织出CWK织物,实现了该新型结构织物的编织,验证了其编织的可行性,同时对CWK织物的上机编织工艺加以分析和讨论,内容包括编织机件的配置及配合、各种纱线的张力调节、经纱位置的控制及前后机头的同步控制等等,并进行了实际上机编织操作,逐步进行编织工艺和编织机件的完善和优化。在编织工艺优化之后,选用了玻璃纤维粗纱作经纱和纬纱,高强涤纶丝作针织纱,进行了CWK织物的编织,并确定了适用于高性能纤维CWK织物的编织工艺。首次编织出该种织物后,对其拉伸性能进行的试验,初步了解其力学性能,结果发现其经向和纬向的力学性能差异较大,这将影响该类织物的应用范围。在此基础上,重新选择经纱、纬纱和针织纱,适当调整了纬纱的衬入密度,重新编织出CWK织物。改进后的CWK织物的经向和纬向的拉伸性能进行试验之后,发现两个方向的力学性能的差异得到极大的改善。该类织物的力学行为同时具有机织结构和针织结构的性能,可以看作是两种结构的复合。同时,该类织物的轴向力学性能可以通过调整经纱、纬纱的线密度和衬入密度来改变,以达到应用上的需要。此外,在同一台样机上用同样的纱线,编织出纬编双轴向多层织物(MBWK),将其拉伸性能与CWK织物的性能进行对比研究。结果显示两者的轴向拉伸性能有很大的相似性,在拉伸过程中,先由其中的经纱和纬纱,在较低的伸长下,承担较高的负荷直至破坏,然后再由针织结构,在较高的伸长下,承担较低的负荷,直至针织结构破坏。另外,CWK织物中由于经纱和纬纱的屈曲,对经纱和纬纱拉伸强力的发挥有一定的影响,但CWK织物中经纱和纬纱的交织使得针织纱的“预牵伸”作用更为突出,进而针织纱对经纱和纬纱的捆绑作用更好,经纱层与纬纱层之间更为紧密。以CWK织物作为增强材料制成复合材料,在对其拉伸性能试验和分析后,发现CWK织物增强复合材料的轴向拉伸性能表现出比较好的线性,其拉伸强度主要该方向的经纱或纬纱强力及其衬入密度决定；CWK织物增强复合材料的轴向拉伸力学性能的差异,可以通过其预制件的纬纱和经纱衬入密度的合理设计来减小；CWK织物增强复合材料经纬向拉伸曲线都存在拐点,这是由于经纬纱交织导致纱线弯曲的结果,经纱的弯曲程度高于纬纱；与普通纬编针织物增强复合材料相比,CWK织物增强复合材料具有较高的初始模量和断裂强度；CWK织物增强复合材料,在横向拉伸时,通常是沿着纵行方向发生断裂,在纵向拉伸时,通常是沿着横列方向发生断裂；在45°斜向拉伸时,断口沿纬纱方向。45°斜向的拉伸强度和模量最低,但拉伸能量吸收最高。同时对MBWK织物增强复合材料的拉伸性能进行了对比研究,研究表明由于经纱和纬纱的交织,降低了CWK织物的中经纱和纬纱强力的发挥,但在两种织物均制成复合材料之后,MBWK增强复合材料中的经纱和纬纱的屈曲比MBWK织物中的经纱和纬纱的屈曲有明显增大,因而MBWK织物增强复合材料的轴向性能与CWK织物增强复合材料的轴向性能基本相同。CWK织物由针织纱、经纱和纬纱三部分组成。在一定的假设条件下,选取了CWK织物结构单胞及体积代表元RVE,利用几何分析的方法,建立了CWK织物三维几何模型。通过分段计算可得到经纱、纬纱和针织纱的长度,理论预测与实际试验值吻合良好,证实了所建模型的有效性和实用性。成型后的纤维体积分数通过织物结构参数和实测纱线长度进行计算,所得实验值与模型计算理论值有很好的一致性,进一步说明所建模型的有效性和实用性。讨论了在给定的织物紧度下,随着针织纱与衬纱线密度之比的变化,总纤维体积分数的变化趋势。在给定的较大的织物紧度下,随着针织纱与衬纱线密度之比的增大,总纤维体积分数先是逐渐下降到一个最小值,然后再增大。在针织纱与衬纱线密度之比由小变大时,针织纱的纤维百分含量随之增大,而衬纱的纤维百分数随之减小；在给定的较小的织物紧度下,随着针织纱与衬纱线密度之比的增大,总纤维体积分数也是由小逐渐增大。这提供了一个给定针织纱或衬纱的纤维百分含量的CWK织物,如何去选择针织纱或衬纱的线密度的依据。成型后的双轴向交织纬编织物增强复合材料的三维模型与织物的几何模型基本一致。利用所建立的CWK织物三维结构模型,根据经、纬纱纤维束及针织纱在RVE中的几何形态、各纱束每一微元段在RVE总体坐标的位置,确定了各系统纱线的取向度；将固化后的经、纬纱纤维束及针织纱看作为纤维单向复合材料,采用体积平均的分析方法,进行细观力学分析,对经纱、纬纱、针织纱和基体的局部刚/柔度矩阵进行分析,计算出各自在局部坐标下的柔度和刚度；利用混合率法则和等应力及等应变边界条件的最佳系数组合对四个子元的刚/柔度进行组合,从理论上得到材料的总体刚/柔度矩阵和工程弹性常数。并用MATLAB语言编程,实现了CWK织物增强复合材料弹性常数理论预测模型的计算,分析表明采用这种分析方法所得到的弹性常数理论预测值与试验结果吻合良好。通过对CWK织物及其增强复合材料的力学性能的试验和理论分析与预测,对这种织物及其增强复合材料的力学性能有了较全面的理解,为以后更深入地研究和设计这种织物及其复合材料的结构和性能奠定了良好的基础,同时本文的分析方法对其他类似材料的分析具有一定的借鉴作用。更多还原

【Abstract】 The tensile properties were studied for a self-developed novel type of biaxial weft knitted fabric, in which warp and weft yarns introduced the knitting structure are in a state of interweaving, and its reinforced composites in this paper. The fabric was named also co-woven-knitted (CWK) fabric because it includes both the knitting structure and the woven structure. The CWK fabric has comprehensive performances of both woven fabric and knitted fabric, because of combining knitted structure and woven structure into one. Compared with the traditional biaxial knitted fabric, the biggest difference is that the warp and weft yarns are weaving in the CWK fabric.In view of the close relationship between the CWK fabric performance and its structural parameters, depth research on the structure of the CWK fabric to determine the weaving process and design its weaving and feeding equipments on the flat knitting machine. According to the weaving process of the CWK fabric, a co-woven-knitted machine was reformed from a manual V-bed flat knitting machine to producing the CWK fabric, on which the CWK fabric was produced with polyester yarns. This demonstrated the validity of the weaving method and the feasibility of the CWK fabric. While the weaving, the process of the CWK fabric was analyzed and discussed, including the configuration and positions and motion cooperate of weaving parts, the establishment for the yarn guide devices, layout for the yarn guide path, all kinds of yarn tension, the precise control for warp position and adjustment of synchronization control for the heads. During the course of the actually machine operation, gradually improving and optimizing of weaving and knitting process and mechanical parts were carried.Based on the weaving process above, the CWK fabric was fabricated with high tenacity polyester yarn for knitting yarn, the same glass filaments for warp and weft. After many weaving experiment, the best knitting process was determined for high performance fiber CWK fabric. Tensile tests were conducted in the CWK fabric coursewise and walewise direction. It was found that the mechanical properties were quite different between the warp and weft direction. This will affect the application of the CWK fabric. The CWK fabric was fabricated again with re-selecting the warp, weft and knitting yarn, increasing linear density of warp, reducing linear density of the knitting yarn and decreasing the number inserted into fabric. After tensile properties were tested, it was found the mechanical properties are greatly improved in both directions. Mechanical behavior of the CWK fabrics is in between woven structure and knitted structure. Meanwhile, the axial mechanical properties of these fabrics can be adjusted by changing the density of warp and weft yarn to meet the application needs.In addition, the multi-layer biaxial weft knitted fabric (MBWK fabric) was produced using the same yarn, in the same depth of stitch cams for knitting yarn on the same flat knitting machine. The tensile tests were carried on the fabric for comparative study of the tensile properties with the CWK fabric. The results showed that axial tensile properties have great similarities between two kinds of biaxial weft knitted fabrics. During the course of drawing, the two fabrics have the same performance as that first by drawing on the direction of the warp or weft, in low elongation, bear high load, until the warp or wefts damage, then to the knitted fabric, at higher elongation, undertake low load, until the knitting structural damage. Comparative studies also found that, compared with the MBWK fabric, the buckling of warp and weft yarns in the CWK fabric played certain effect on the tensile strength of the CWK fabric. But the pre-coating draft function that the interweaving of the warp and weft yarns carried on the knitting yarn in CWK fabric is more outstanding. Thus, bind effect that the knitting yarn to the warp and weft yarns is better, the contacts between the warp layer and wefts layer is closer. This shows the performance of resisting delamination of the CWK fabric is better than that of the MBWK fabric.The tensile tests were carried on the composite material, which were made using the CWK fabric as reinforcement. The tensile properties of the composites showed good linearity performance. The tensile strength is mainly depends on the yarn density and the tensile strength of weft and warp yarns in the tensile direction. Differences between in the axial tensile mechanical properties can be decreased through the selecting and designing of the linear density and inserted density of the weft and warp yarns. Due to the bending of the weft and warp yarns, there has an inflection point in the tensile curves of the CWK fabric reinforced composites. And degree of the bending of warp yarn is higher than the weft yarn. Compared to ordinary knitted fabric reinforced composites, the CWK fabric reinforced composites have higher initial modulus and fracture strength. In the transverse stretching, fracture is generally along the wale direction. In the longitudinal tension, usually along the course direction was broken. In the 45°diagonal tension, broken is in course direction, and have minimum tensile strength and modulus, the highest tensile energy. Meanwhile, tensile tests were carried on the MBWK fabric reinforced composites for comparative study. The result showed that the axial performance of CWK fabric reinforced composites is basically the same as that of it. This is because that, in the course of the manufacture for composites, buckling of the weft and warp yarns in the MBWK fabric is increased, while buckling of the weft and warp yarns in the CWK fabric is reduced.Under certain assumptions, unit cell and representative volume element (RVE) were selected for the CWK fabric reinforced composites, established three-dimensional geometric model of the CWK fabric by the geometric analysis method, obtained length of warp, weft and knitting yarns according the geometric model. Theoretical predictions of the yarn length agree well with the measured test values, confirming the effective and practical of the model. The fiber volume fraction is calculated through the fabric structure parameters and the measured yarn length after forming. The experimental data and theoretical model are in good agreement, which further shows that effective and practical of the model. The total fiber volume fraction trends was discussed, under a given fabric tightness, with changing in the ratio of the knitting yarn to the inserted yarn density. This provides a basis of how to choose yarn linear density for a given fiber percentage content of CWK fabric reinforced composites.The 3D model of the composite is basically same as the CWK fabric after molding. Thus, According to the 3D model of the CWK fabric, the geometric shape and position of the inserted yarns and knitting yarns in the RVE can be determined. So the orientation of various yarn were determined. After molding three kinds of yarns and metrin can be seen as unidirectional fiber composite. Using the average volume analytical and the micromechanics analysis methods, obtained the local rigid/flexibility matrix of the warp, weft yarn, knitting yarn and the matrix. Then the overall material stiffness/flexibility matrix and the elastic constants were analysis using the mixed rate law, the equivalent stress and strain boundary conditions and the best factor combination. In order to realize the calculating of theoretical elastic constants for the CWK fabric reinforced composites, MATLAB language computing program is compiled. The results shows that theoretical predictions of elastic constants in good agreement with the experimental results.There has a more overall understanding to the mechanical properties of the CWK fabric and its reinforced composites through experimental tests and theoretical analysis and prediction. This provided a good foundation for more in-depth study and design of the CWK fabric and its composites in structure and properties. At the same time, the analysis method can be used as a reference for the analysis of other similar materials.更多还原