This thesis focuses on tensile-tensile fatigue of quasi-unidirectional (quasi-UD) non-crimp fabric (NCF) reinforced glass-fibre/epoxy laminates. These laminates have good fatigue performance since fibre bundles exhibit a very small amount of crimp. They are commonly used in fatigue critical structures, e.g. in spar caps of wind turbine rotor blades. The thesis is divided into three parts. The first part is devoted to the development of an applicable test specimen conﬁguration for tensile-tensile fatigue testing of the laminates. This is necessary because standard rectangular specimens tend to fail at the tab area, resulting in unrealistic fatigue life predictions. The thesis further focuses on the internal structure characterisation, fatigue damage evolution and fatigue performance of laminates reinforced with NCFs based on different stitch patterns and stitch tensions. The third part concentrates on dry and wet compaction of quasi-UD NCF preforms, with the aim of finding out how the NCF structure and preform thickness possibly affect the final thickness and fibre volume fraction (FVF) of the laminate. The first part revealed that the developed dog-bone specimen conﬁguration decreased tab failures and yielded signiﬁcantly higher fatigue lives for UD laminates when compared to the fatigue lives measured with rectangular and slightly different dog-bone specimens. Fatigue tests and mesostructural analyses performed in the second part indicated that damage evolution and the measured fatigue life were signiﬁcantly dependent on the stitch pattern, while they were not dependent on the stitch tension. The main reasons for the differences were apparently the waviness of axial ﬁbres that were dependent on the stitch pattern but not on the stitch tension. The study also revealed that the mutual location of stitch patterns in neighbouring fabrics affects the fatigue life of the laminates. Finally, the thesis showed that the dry and wet compaction of quasi-UD preform is dependent on the stitch pattern but not on the stitch tension. In addition, the fabric shift perpendicular to the axial ﬁbre bundles had a small effect on the thickness of the preform. The results of the first part suggest that the developed specimen configuration is applicable for fatigue tests and provides more reliable fatigue lives for quasi-UD laminates when compared to the reference specimens. However, even with this configuration, part of the specimens failed outside the gauge section, meaning that there is a need for further development of the specimen. The results of the second part revealed that stitch patterns that keep fibre bundles well-aligned provide the best fatigue lives for the laminates. The results also revealed that the mutual location of neighbouring fabrics might affect the fatigue life of the laminates. Thus, the locations should be the same, e.g. when the objective is to compare the fatigue lives of laminates with different stitch patterns and tensions. The results in the third part indicated that FVFs of thin test laminates for fatigue tests correspond well to the FVFs of thicker laminates used in the compaction tests.
|Translated title of the contribution||Näennäisesti yhdensuuntaisten suorakuitulujitettujen lasikuitu-epoksilaminaattien vetoväsytystestaus ja väsymisominaisuudet|
- , Supervisor
- Mikko Kanerva, Advisor
|Publication status||Published - 2017|
|MoE publication type||G5 Doctoral dissertation (article)|
- non-crimp fabrics, UD laminate, quasi-UD laminate, fatigue, S-N curve