Moisture diffusion law of cracked HDPE sheath for bridge cables under alternating action of ultraviolet and fatigue loads
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摘要: 为揭示服役过程中缆索承重体系桥梁索体开裂高密度聚乙烯(HDPE)护套的水分扩散机理，首先对预制槽护套施加紫外与疲劳荷载交替作用以获得疲劳裂纹，在此基础上开展了开裂护套水分扩散试验，定义了护套等效裂纹面积，揭示了水分扩散通量与扩散系数变化规律。研究结果表明：单一紫外作用未显著改变开裂护套的水分扩散行为，而在单一疲劳荷载作用下，当疲劳加载次数从4.50×104增加至1.35×105时，水分扩散通量与扩散系数均增加了6.16%，当试件开裂面积较小时，护套的水分扩散性主要受疲劳荷载作用的影响；考虑紫外与疲劳荷载交替作用时，相比于单一疲劳荷载作用，1次交替与2次交替作用下，水分扩散通量与扩散系数均分别增加4.80%与8.57%，两者交替作用顺序亦会影响开裂护套水分扩散规律，先施加紫外作用可增强疲劳荷载作用效应，相同疲劳加载次数下水分扩散通量与扩散系数均可提升8.42%，而后施加紫外作用的试件在1次交替作用时与单一疲劳荷载作用结果相近，在2次交替作用时仅提升4.49%。可见，考虑单一紫外或疲劳荷载作用不足以充分解释缆索开裂护套服役过程中的水分扩散规律，预测索体内部水分累计规律需综合考虑紫外与疲劳荷载的耦合作用，该研究成果可为桥梁缆索的维护和修补提供依据，以延长索承桥梁寿命。Abstract: To reveal the moisture diffusion mechanism of cracked high density polyethylene (HDPE) sheaths in cable-bearing systems of bridges in service, prefabricated-slot sheaths were subjected to alternating ultraviolet and fatigue loads to generate fatigue cracks. Based on this, moisture diffusion tests on cracked sheaths were conducted, the equivalent crack area of the sheath was defined, and the variation laws of moisture diffusion flux and diffusion coefficient were revealed. Research results show that only ultraviolet exposure does not significantly change the moisture diffusion behavior of the cracked sheath. In contrast, when the fatigue load cycles increase from 4.50×104 to 1.35×105, the moisture diffusion flux and diffusion coefficient both increase by 6.16% under only fatigue load. When the crack area of the specimen is small, the moisture diffusivity of the sheath is mainly influenced by fatigue load. Considering alternating ultraviolet and fatigue loads, compared to only fatigue loads, single and two alternating actions can increase the moisture diffusion flux and diffusion coefficient by 4.80% and 8.57%, respectively. The sequence of alternating actions also affects the moisture diffusion law in the cracked sheath. Ultraviolet conducted at first can enhance the effect of fatigue load, increasing the moisture diffusion flux and diffusion coefficient by 8.42% under the same fatigue load cycles. When ultraviolet exposure is applied later, the results for single alternating action are similar to those under only fatigue loads, while two alternating actions only increase the values by 4.49%. Therefore, it is considered that only ultraviolet or fatigue loads are insufficient to fully explain the moisture diffusion law in cracked sheaths during service. Predicting the moisture accumulation law within the cable sheath requires a comprehensive consideration of the coupled effects of ultraviolet and fatigue loads. These findings can provide a basis for the maintenance and repair of bridge cables, thereby extending the service life of cable-supported bridges.
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图 2 1B型(哑铃型)多用途标准HDPE试件(单位：mm)
Figure 2. 1B-shaped (dumbbell-shaped) multipurpose standard HDPE specimen (unit: mm)
图 4 预制槽HDPE试件尺寸及开槽端部裂纹扩展示意
Figure 4. Dimensions of prefabricated slot HDPE specimens and schematic of crack propagation at slot end
图 7 水分采集仓吸水能力测试
Figure 7. Moisture absorption capacity test on moisture collection chamber
图 12 各工况下HDPE试件表面裂纹SEM图像
Figure 12. SEM images of HDPE specimen surface cracks under various conditions
图 13 开裂试件裂纹处微观结构SEM图像
Figure 13. SEM images of microstructure at crack of cracked specimen
图 14 不同工况下试件裂纹面积变化
Figure 14. Crack area variations of specimens under different conditions
图 15 FA-2与FA-3工况下试件SEM图像对比
Figure 15. Comparison of SEM images of specimens under FA-2 and FA-3 conditions
图 18 水分扩散通量与水分扩散系数-等效裂纹面积曲线
Figure 18. Moisture diffusion flux and moisture diffusion coefficient-equivalent crack area curves
图 21 试件水分扩散通量与扩散系数拟合曲线斜率变化
Figure 21. Fitted curve slope variations of specimen moisture diffusion flux and diffusion coefficient
表 1 名词解释
Table 1. Term explanation
单一因素作用 试件仅受A或F 双因素单次交替作用 试件受到A与F的交替作用，且交替顺序为A-F或F-A 双因素2次交替作用 试件受到A与F的交替作用，且交替顺序为A-F-A-F或F-A-F-A 
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表 2 HDPE试件力学性能测试
Table 2. Mechanical property testing of HDPE specimens
名称 屈服强度/MPa 弹性模量/MPa 拉伸断裂标称应变/% 样品1 32.10 1 220 52 样品2 31.80 1 180 54 样品3 32.20 1 250 55 平均值 32.03 1 217 54
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表 3 预制槽HDPE试件尺寸
Table 3. Dimensions of prefabricated slot HDPE specimens mm
试件尺寸 开槽尺寸 L W H WC LC 250 70 10 1.5 40
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表 4 仅考虑紫外或疲劳荷载作用单一因素试验工况
Table 4. Test conditions only with ultraviolet or fatigue load
工况组A的老化时间/h A-1 120 A-2 240 A-3 360 工况组F的疲劳加载次数 F-1 4.50×104 F-2 9.00×104 F-3 1.35×105
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表 5 考虑紫外与疲劳荷载作用单次交替作用试验工况
Table 5. Test conditions with single alternating action of ultraviolet and fatigue load
工况组编号 试验编号 试验顺序 老化时间/h 疲劳加载次数 AF-Ⅰ AF-1 A-F 120 1.35 ×105 AF-2 240 1.35 ×105 AF-3 360 1.35 ×105 FA-Ⅱ AF-4 A-F 240 9.00×104 AF-5 240 4.50 ×104 AF-6 240 1.35 ×105 FA-Ⅰ FA-1 F-A 120 1.35 ×105 FA-2 240 1.35 ×105 FA-3 360 1.35 ×105 AF-Ⅱ FA-4 F-A 240 9.00×104 FA-5 240 4.50×104 FA-6 240 1.35 ×105
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表 6 考虑紫外与疲劳荷载作用2次交替作用试验工况
Table 6. Test conditions with two alternating actions of ultraviolet and fatigue load
工况组编号 试验编号 试验顺序 老化时间/h 疲劳加载次数/104 AFAF-Ⅰ AFAF-1 A-F-A-F 60×2 6.75×2 AFAF-2 120×2 6.75×2 AFAF-3 180×2 6.75×2 AFAF-Ⅱ AFAF-4 A-F-A-F 120×2 4.50×2 AFAF-5 120×2 2.25×2 AFAF-6 120×2 6.75×2 FAFA-Ⅰ FAFA-1 F-A-F-A 60×2 6.75×2 FAFA-2 120×2 6.75×2 FAFA-3 180×2 6.75×2 FAFA-Ⅱ FAFA-4 F-A-F-A 120×2 4.50×2 FAFA-5 120×2 2.25×2 FAFA-6 120×2 6.75×2
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表 7 单一紫外或疲劳荷载作用下水分采集仓质量增量变化率
Table 7. Moisture collection chamber mass increment rates under only ultraviolet or fatigue load
% 组别 工况组A 工况组F 1 0.00 14.36 2 0.00 16.79 3 0.01 24.09
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表 8 紫外与疲劳荷载单次交替作用下水分采集仓质量增量变化率
Table 8. Moisture collection chamber mass increment rates under single alternating action of ultraviolet and fatigue load
% 组别 工况组AF-Ⅰ 工况组AF-Ⅱ 工况组FA-Ⅰ 工况组FA-Ⅱ 1 25.55 14.36 24.33 14.11 2 28.47 16.30 25.79 15.33 3 32.36 28.47 26.52 25.79
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表 9 紫外与疲劳荷载2次交替作用下水分采集仓质量增量变化率
Table 9. Moisture collection chamber mass increment rates under two alternating actions of ultraviolet and fatigue loads
% 组别 工况组AFAF-Ⅰ 工况组AFAF-Ⅱ 工况组FAFA-Ⅰ 工况组FAFA-Ⅱ 1 27.98 15.82 26.27 15.57 2 34.06 18.98 31.87 18.01 3 38.93 34.06 36.25 31.87
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表 10 紫外及疲劳荷载2次交替作用与单次交替作用下试件水分扩散通量与扩散系数变化率
Table 10. Moisture diffusion fluxes and diffusion coefficient variation rates of specimens under two alternating actions of ultraviolet and fatigue loads compared to single alternating action
% 组别 AFAF-Ⅰ与AF-Ⅰ FAFA-Ⅰ与FA-Ⅰ AFAF-Ⅱ与AF-Ⅱ FAFA-Ⅱ与FA-Ⅱ 组1 1.98 0.76 3.96 4.48 组2 2.67 2.34 4.31 5.17 组3 3.60 5.28 0.94 4.49
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