Observations of microscale tensile fatigue damage mechanisms of composite materials for wind turbine blades

In the presented study, it has been studied how tensile/tensile fatigue damage will develop in quasi-unidirectional non-crimp fabric based glass fibre epoxy matrix composites. A material system conventionally used as the load carrying laminates in wind turbine blades. It will be demonstrated by stopping a tensile/tensile fatigue test before the final material failure, how 3D X-ray computer tomography (x-ray CT) can be used for determining fibre failure inside the composite material. Due to the rather coarse bundle-structure of the non-crimp fabrics investigated, a relative large cross section area of the test-sample is necessary in order for testing a representative material volume. Therefore, the x-ray CT technique is used in a scout and zoom test-setup where a scout scan of the 15 mm large cross-section is used for identifying the damage region, while the subsequently zoom scan of a 2-3 mm field of view region is used for determine the fibre failures. This scan-setup is non-destructive, and it is therefore possible to obtain a number of scans during the fatigue damage development. Thereby, it is found that the fibre failure are mainly occurring close to points where the load carrying uni-directional fibre bundles are in close contact with the crossing points of secondary oriented fibre bundles, the so-called backing bundles.