Influence of moisture absorption on the mechanical properties of unidirectional flax fibre composites
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摘要: 为探明亚麻纤维增强树脂复合材料(FFRPs)在长期潮湿环境中的力学性能变化规律，基于真空辅助树脂模塑传递成型(VARTM)手段制备了的干燥状态的单向FFRPs(纤维体积分数为40vol%)。试验研究了 FFRPs在30°C、80%相对湿度(RH)环境中放置5天、35天、86天后拉伸力学性能变化。结果表明，FFRPs在湿热环境中吸水量近似符合一维情况下的Fickian第二定律，饱和吸水量为5.3%左右。FFRPs在垂直于纤维方向上的拉伸强度和模量随吸湿度增加递减，断口微观分析表明吸湿降低了纤维基体界面结合性能。然而，FFRPs在纤维方向上的拉伸强度并未因吸湿降低，反而在吸湿过程呈现出先减小后增加的趋势，此变化规律文献中尚未报道：相比干燥状态，放置5天后拉伸强度下降5.7%；放置35天后拉伸强度增加18.7%；放置86天时样品已处在饱和吸湿状态，拉伸强度略微减小，但仍比干燥状态增加13.7%。FFRPs在纤维取向上拉伸强度变化可解释为多因素共同作用的结果。Abstract: This paper investigated the evolution of mechanical properties of flax fibre reinforced polymer compo-sites (FFRPs) conditioned in humid condition for a long term. Dry unidirectional FFRPs having fibre volume fraction of 40vol% were manufactured via vacuum assisted resin transfer moulding (VARTM). FFRPs were conditioned in 30°C, 80% relative humidity (RH) for different time (5 days, 35 days and 86 days) and their tensile properties were tested and analyzed. The results demonstrate that the moisture absorption of FFRPs fairly follows one dimensional Fickian’s second law. The equilibrated water content is around 5.3%. Tensile strength and modulus perpendicular to fibre direction decrease with moisture absorption. Fracture morphology shows that fibre-matrix bonding strength decreases after moisture absorption. Tensile strength in fibre direction is not degraded by moisture absorption, and exhibits a trend featured by a first drop followed by an increase, which has not been reported in literatures. Tensile strength in fiber direction decreases by 5.7% after being conditioned in humid for 5 days, and increases by 18.7% after 35 days. Further absorption of moisture up to 86 days (already saturated) causes a slight decrease in tensile strength but is still 13.7% higher than that at dry state. The change trend of tensile strength in fibre direction during moisture absorption can be explained as a consequence of averaging effects of several factors.
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Key words:
- plant fibre composites /
- flax fibres /
- moisture absorption /
- tensile properties /
- durability
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图 1 单向亚麻纤维织物(左)和亚麻纱线结构(右)
Figure 1. Unidirectional (UD) flax fiber fabric (left) and structure of flax yarn (right)
图 2 获取不同含水量亚麻纤维纱线的方法
Figure 2. Method to obtain flax yarns with different water contents
F—Tensile force
图 3 纤维预干燥及真空辅助树脂传递模塑成型(VARTM)过程中的温度和压力周期
Figure 3. Temperature and pressure cycle in fiber pre-drying and vacuum assisted resin transfer moulding (VARTM) process
图 4 亚麻纤维增强树脂复合材料(FFRPs)吸湿实验数据及Fickian拟合吸湿曲线
Figure 4. Experimental and Fickian’law fitted weight uptake of flax fibre reinforced polymer composites (FFRPs) in humid
RH—Relative humidity
图 5 不同含水量亚麻纤维纱线的典型拉伸应力-应变曲线
Figure 5. Representative tensile stress-strain curves of flax yarns having different water contents
图 6 [90°]FFRPs样品在不同吸湿时间下的典型拉伸应力-应变曲线
Figure 6. Representative tensile stress-strain curves of [90°] FFRPs specimens at different conditioning time
σ—Tensile stress
图 7 [90°] FFRPs样品在不同吸湿时间下的拉伸力学性能
Figure 7. Evolution of tensile properties of [90°] FFRPs specimens at different conditioning time
图 8 [90°] FFRPs样品不同吸湿时间下(30℃、80%RH)的断裂面微观形貌
Figure 8. Fracture morphologies of [90°] FFRPs specimens conditioned in humid (30℃, 80%RH)
图 9 [0°] FFRPs样品在不同吸湿时间下的拉伸力学性能
Figure 9. Evolution of tensile properties of [0°] FFRPs specimens at different conditioning time
图 10 [0°] FFRPs样品在不同吸湿时间下的典型拉伸应力-应变曲线
Figure 10. Representative tensile stress-strain curves of [0°] FFRPs specimens at different conditioning time
图 11 [0°] FFRPs样品在不同吸湿时间下的刚度变化
Figure 11. Stiffness evolution of [0°] FFRPs specimens at different conditioning time
图 12 影响吸湿后[0°] FFRPs样品拉伸强度的因素趋势线
Figure 12. Trend lines that would influence on tensile strength of [0°] wet-conditioned FFRPs specimens
图 13 干湿状态下[0°] FFRPs样品的断裂面形貌比较
Figure 13. Comparison of fracture morphologies of [0°] dry and wet-conditioned FFRPs specimens
表 1 [0°]和[90°]FFRPs样品的计算Fickian扩散系数
Table 1. Calculated Fickian diffusion coefficients of [0°] and [90°] FFRPs
$ {D}_{\mathrm{e}} $1/(10−7 mm2·s−1) $ {D}_{\mathrm{c}} $2/(10−7 mm2·s−1) $ {M}_{\mathrm{m}} $3/% [0°] 2.40 1.84 5.41 [90°] 3.27 2.74 5.25 Notes: $ {D}_{\mathrm{e}} $—Diffusion coefficient calculated from experimental data; $ {D}_{\mathrm{c}} $—Corrected diffusion coefficient; $ {M}_{\mathrm{m}} $—Absorption of water at saturation. 
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表 2 亚麻纤维纱线的拉伸性能 (n≥20)
Table 2. Tensile properties of tested flax yarns (n≥20)
Water content/% Strength/MPa Modulus1/GPa Elongation at break/% ~0.5 486.0±74.9 22.0±2.9 2.5±0.2 ~3.0 517.2±92.2 16.4±2.9 3.4±0.5 ~5.5 544.1±81.2 14.4±1.9 4.1±0.4 ~50.0 598.8±136.4 10.7±0.9 5.6±0.9 Notes: Modulus1 was calculated as the gradient of the regression line between 100 MPa and 200 MPa; n—Sample size of each condition.
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