Fatigue durability test of quasi-isotropic carbon/epoxy laminates in marine environment

Fiber-reinforced polymer (FRP) composites are utilized as a substitute material for metal structures in maritime industries. The durability of composites depends on varying harsh marine environments. The continuous exposure of FRPs to the harsh marine environment affects their static and dynamic properties. In this study, the impact of moisture absorption on the fatigue characteristics of autoclave-cured quasi-isotropic carbon/epoxy laminates is examined. Specimens were aged in artificial seawater under ambient, sub-zero, and humid conditions for 365 d. The fatigue test was conducted at different stress levels on aged laminates. The S-N curve, stiffness degradation, damage accumulation, and rate of damage growth using energy dissipation method were evaluated based on the experimental results. The results showed that fatigue property of quasi-isotropic carbon/epoxy specimens largely depends on aging conditions. The S-N curve of aged specimens showed a greater fatigue life at higher stress levels than the pristine specimen. Under different aging conditions and varying stress levels, specimens exhibited a standard damage curve. A total of 60–70% damage was developed during the initial stages of fatigue loading. The presence of moisture, multiple cracks, and interphase debonding between fibers and matrix were responsible for the reduction in fatigue life of quasi-isotropic carbon/epoxy specimens.

纤维增强聚合物复合材料（Fiber-reinforced polymer, FRP）被应用于海事工业中，作为金属结构的替代材料。复合材料的耐久性能取决于所处的严苛海洋环境条件。FRP持续暴露于严苛海洋环境中，会对其静态与动态力学性能造成影响。本研究针对热压罐固化准各向同性碳/环氧层合板，探究了吸湿性对其疲劳特性的影响。试样被置于人工海水中，分别在室温、亚低温及高湿环境下进行了为期365天的老化处理。对老化后的层合板试样开展了不同应力水平下的疲劳试验。本研究基于试验结果，对S-N曲线、刚度退化规律、损伤累积过程以及采用能量耗散法得到的损伤增长速率进行了分析评估。结果表明，准各向同性碳/环氧试样的疲劳性能在很大程度上取决于老化环境条件。老化试样的S-N曲线显示，在较高应力水平下，其疲劳寿命优于未老化的原始试样。在不同老化条件与应力水平下，试样均呈现出标准的损伤曲线变化规律。在疲劳加载的初始阶段，试样即产生了总损伤的60%~70%。水分的存在、多裂纹萌生以及纤维与基体间的界面脱粘，是导致准各向同性碳/环氧试样疲劳寿命降低的主要原因。