Interlaminar structure and properties of carbon fiber/epoxy composites toughened with copolymerized nylon fiber veil after cryogenic treatment
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摘要: 本文通过自主搭建超低温/室温循环研究平台，探究了超低温浸泡时间和超低温/室温循环次数对碳纤维/环氧树脂(CF/EP)复合材料各项力学性能的影响。之后将不同面密度共聚尼龙纤维网纱(PAV)插入到复合材料层间，研究在液氮中超低温浸泡16 h后复合材料层间韧性的变化。结果表明，未增韧情况下超低温浸泡16 h后CF/EP复合材料的Ⅰ型层间断裂韧性(GⅠC)、Ⅱ型层间断裂韧性(GⅡC)、拉伸强度和弯曲强度分别下降了46.2%、22.9%、17.7%和3.2%。而当插入面密度为8 gsm的PAV时，与未增韧样相比CF/EP复合材料的GⅠC在室温下和超低温浸泡16 h后分别提升了49.1%和114.0%；当插入面密度为24 gsm的PAV时，增韧样的GⅡC相比于未增韧样在室温下和超低温浸泡16 h后分别提升了140.2%和178.0%。此外PAV的插入并未对CF/EP复合材料的弯曲性能和层间剪切强度造成明显影响。研究表明，PAV的增韧机制主要是尼龙树脂的拔出、基体塑性形变以及引发裂纹的偏转。本研究改善了超低温处理后的CF/EP复合材料层间韧性，对推动超低温贮箱复合材料化进程具有积极意义。Abstract: In this paper, a research platform for cryogenic/room temperature cycles was built to explore the effects of cryogenic immersion time and cryogenic/room temperature cycle times on the mechanical properties of carbon fiber/epoxy(CF/EP)composites. Then, copolymerized nylon fiber veil (PAV) with different areal densities was inserted into the interlaminar of the composites, and the interlaminar toughness of the composites was studied after 16 h of cryogenic immersion in liquid nitrogen. The results demonstrate that the interlaminar fracture toughness of mode I(GⅠC), mode Ⅱ(GⅡC), tensile strength and flexural strength of CF/EP composites decline by 46.2%, 22.9%, 17.7% and 3.2%, after 16 h of cryogenic immersion without toughening. The GⅠC of CF/EP composites is increased by 49.1% and 114.0% at room temperature and after 16 h of cryogenic immersion, respectively, when PAV with 8 gsm areal densities were inserted. The GⅡC is increased by 140.2% and 178.0% at room temperature and after 16 h of cryogenic immersion, when PAV with 24 gsm areal densities were inserted. In addition, the insertion of PAV didn't significantly affect the flexural properties and interlaminar shear strength of CF/EP composites. Our findings suggest that the toughening mechanism of PAV is mainly the pull-out of nylon resin, plastic deformation of matrix, and the deflection of initiating cracks. This study improves the interlayer toughness of CF/EP composites after cryogenic treatment, which is of positive significance for promoting the compositing process of cryogenic fuel tanks.
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图 1 碳纤维/环氧树脂(CF/EP)复合材料制备工艺流程图
Figure 1. Schematic diagram of the preparation process of carbon fiber/epoxy(CF/EP)composites
图 2 CF/EP复合材料超低温/室温循环过程温度随时间的变化
Figure 2. Temperature variation with time during cryogenic/room temperature cycling of CF/EP composites
图 3 不同面密度共聚尼龙纤维网纱形貌图：(a)共聚尼龙纤维网纱宏观图；(b) 8 gsm；(c) 16 gsm；(d) 24 gsm
Figure 3. The morphology of copolymerized nylon fiber veil with different areal densities: (a)d iagram of copolymerized nylon fiber veil; (b) 8 gsm; (c) 16 gsm; (d) 24 gsm
图 4 (a)尼龙纤维网纱的DSC曲线 (b) YY23环氧树脂、CF/EP复合材料线性热膨胀系数随温度的变化
Figure 4. (a) DSC of nylon copolymer fiber veil (b) Variation of linear coefficient of thermal expansion with temperature for YY23 epoxy and CF/EP composites
图 5 不同超低温处理后CF/EP复合材料力学性能测试结果
Figure 5. Mechanical properties of CF/EP composites after different cryogenic treatment
图 6 不同面密度PAV层间增韧CF/EP复合材料的GⅠC测试结果
Figure 6. GIC test results of PAV interlayer toughened CF/EP composites with different areal densities
图 7 不同面密度PAV层间增韧CF/EP复合材料的Ⅰ型断裂面SEM图
Figure 7. SEM images of mode I fracture surfaces of PAV interlayer toughened CF/EP composites with different areal densities
图 8 不同面密度PAV层间增韧CF/EP复合材料的GⅡC测试结果
Figure 8. GⅡC test results of PAV interlayer toughened CF/EP composites with different areal densities
图 9 不同面密度PAV层间增韧CF/EP复合材料的Ⅱ型断裂面SEM图
Figure 9. SEM images of mode Ⅱ fracture surfaces of PAV interlayer toughened CF/EP composites with different areal densities
图 10 不同面密度PAV层间增韧CF/EP复合材料的弯曲性能
Figure 10. Flexural properties of PAV interlayer toughened CF/EP composites with different areal densities
图 11 不同面密度PAV层间增韧CF/EP复合材料的层间厚度：(a) Ref；(b) PA8；(c) PA16；(d) PA24
Figure 11. Interlayer thickness of PAV interlayer toughened CF/EP composites with different areal densities: (a) Ref; (b) PA8;(c) PA16; (d) PA24
图 12 不同面密度PAV层间增韧CF/EP复合材料的层间剪切强度
Figure 12. ILSS of PAV interlayer toughened CF/EP composites with different areal densities
表 1 线性热膨胀系数对比表
Table 1. Comparison table of linear coefficients of thermal expansion
Sample YY23 CF/EP PA8-CF/EP CTE.m/(×10−6·℃−1) 31 17 45 Notes：CTE.m is mean coefficient of thermal expansion；YY23 is YY23 epoxy；CF/EP is carbon fiber/epoxy composite；PA8-CF/EP is carbon fiber/epoxy composite toughened by copolymerized nylon fiber veil with 8 gsm areal densities 
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