Determination of stresses, strains and failure types in multidirectional laminates under pure bending

Curved structures made of fibre-reinforced plastics tend to show an out-of-plane failure type, when loaded under bending. One method to determine the related tensile strength in thickness direction, is the standardised unfolding test related to ASTM D6415 with L-profile specimens. However, the standard shall be used for unidirectional materials only, which is not the case for multidirectional laminates. Therefore, this publication presents methods to determine the layer-wise stress and strain state as well as occurring failure types of multidirectional laminates. An extension of the Lekhnitski formulae is presented and its results are validated by comparison with high resolution strain measurements by Digital Image Correlation. Based on the analytical solution, a failure analysis using Cuntze's Failure Mode Concept is conducted. It was revealed, that the failure load and position can be predicted accurately, if a failure type related strength -in-plane or out-of-plane failure- is used. In the presence of 45°-layers higher deviations occur between test and prediction and therefore limits the validity of the presented analytical failure analysis.