Fatigue longevity and evolving material properties of Flax-epoxy laminates is examined under constant strain amplitude cycling. Several recent fatigue studies on Flax-composites, all of which were conducted under stress-control, found that certain laminates demonstrate a modulus increase over fatigue life, even while accumulating internal damage and permanent deformation. This study investigates whether such fatigue-stiffening phenomena are also observed under strain-control. Specimens of four layups ([0]₁₆, [0/90]_4S, [$\pm$45]_4S, and [0/45/90/$-$45]_2S) are tested under 5 Hz and strain ratio $R_{ϵ}=\frac{{ϵ}_{min}}{{ϵ}_{max}}$=0.1. Strain-life ($ϵ$–$N$) plots are generated, which are found to follow a consistent trend that can be modelled by a linearised relationship with identifiable parameters. No evidence of stiffening is observed. All specimens demonstrate stiffness degradation, thereby contradicting previous studies. The extent of degradation is proportional to loading applied. A directly proportional relationship is observed between strain-rate and measured modulus. It is proposed that the reported modulus increase in existing literature does not reflect any physical improvement of material stiffness, but is a consequence of stress-amplitude controlled loading conditions. As such, strain-controlled cycling may be more appropriate for fatigue studies on natural fibre composites.

在恒定的应变幅度循环下，研究了亚麻-环氧树脂层压板的疲劳寿命和材料性能的变化。最近在亚麻复合材料上进行的几项疲劳研究均在应力控制下进行，发现某些层压材料即使在累积内部损伤和永久变形的同时，其疲劳寿命中的模量也会增加。这项研究调查是否在应变控制下也观察到这种疲劳强化现象。四个上铺的样本（[0] ₁₆，[0/90] _4S，[$\pm$45] _4S和[0/45/90 /$-$45] _2S）在5 Hz和应变比下进行测试${\mathrm{[R}}_{ϵ}=\frac{{ϵ}_{分}}{{ϵ}_{最高}}$= 0.1。应变寿命$ϵ$–$ñ$）生成图，发现图遵循一致的趋势，该趋势可以通过具有可识别参数的线性关系来建模。没有观察到变硬的迹象。所有样本均显示出刚度下降，从而与先前的研究相矛盾。退化程度与施加的载荷成比例。在应变率和测得的模量之间观察到直接比例关系。建议在现有文献中报告的模量增加不会反映出材料刚度的任何物理改善，而是应力-振幅控制的载荷条件的结果。因此，应变控制循环可能更适合于天然纤维复合材料的疲劳研究。