Damage progression in unidirectional glass fibre reinforced composites manufactured of a non-crimp fabric subjected to tension-tension fatigue is investigated, and a quantitative explanation is given for the experimentally observed stiffness degradation. The underlying damage-mechanisms are examined using three distinct microscopic analyses, and the transverse crack density is measured. It is documented that the stiffness loss in fatigue is directly related to fibre fractures in the load-carrying axial fibre bundles, initialised by interface debonding and cracking in the transverse backing bundles. A simple stiffness spring model validates the stiffness loss observed. A fatigue damage scheme is presented, which suggests that damage initiates due to failure of the backing bundle causing a stress concentration in the axial loadcarrying fibres. This stress concentration, along with fretting fatigue, gives rise to axial fibre fractures and a loss of stiffness, eventually leading to final failure. The uniqueness of the present work is identification of the mechanisms associated with tension fatigue failure of unidirectional non-crimp fabrics used for wind turbine blades. The observed damage mechanisms need further attention and understanding in order to improve the fatigue life-time of unidirectional glass fibre reinforced non-crimp fabrics.

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