Influence of backing layers on the interlaminar fracture toughness energy – Mode I – of quasi-unidirectional GFRP

Quasi-unidirectional glass fiber non-crimp fabrics consist of a unidirectional main layer in the 0° direction stabilized by a backing layer in the 90° direction with a significantly lower amount of fibers. It is known that the backing layers impair the fatigue performance of respective reinforced plastics due to their orientation perpendicular to the main load direction. They act as damage initiators for the failure of the load-carrying unidirectional fibers. In the present work, the positive influence of the backing layers on the interlaminar fracture toughness energy – Mode I – is demonstrated by tests in accordance with DIN EN 6033. The presence of backing fibers in the delamination plane can improve the values by up to 43 %. Furthermore, results from evaluation according to DIN EN 6033 and ASTM D5528 are shown to have a good agreement, if the correction factor for large displacements recommended in the ASTM standard is not applied. If it is applied, however, there is a clear gap of up to 19 % between the two standards because the DIN simply does not provide a correction for large deflections.