Interlaminar/interfiber Failure of Unidirectional GFRP used for Wind Turbine Blades

A unidirectional glass fiber/epoxy composite material system used for wind turbine blades was characterized under multi-axial loading by cutting specimens in varying off-axis angles relative to the fiber direction. In addition, Iosipescu shear tests were performed on both symmetric and asymmetric specimens, where the latter were used to suppress premature axial splitting of the test specimens. The tests were performed in both the 1-2 and 1-3 material coordinate planes. Strain gauges and Digital Image Correlation were used to record the stress-strain responses. A new approach was used to define a “failure initiation strength” by analyzing the recorded stress-strain curves. The experimentally determined failure stresses were compared to the predictions of the maximum stress, Tsai-Wu and Northwestern University (NU) failure criteria. It was found that by using the approach of analyzing the stress-strain curve to define a point of material failure initiation, it was possible to obtain a good theoretical fit to the experimental data using both the Tsai-Wu and the NU failure criteria.