Static and fatigue transverse crack initiation in thin-ply fibre-reinforced composites

Static and fatigue transverse crack initiation in thin-ply fibre-reinforced composites This work is part of the Collaborative Research Center (SFB) 880 preparing the scientific basis for future commercial aircraft characterised by low noise emission and extremely short take-off and landing capability. The presented investigations are motivated by a contour-variable leading-edge composite skin which constitutes a key component in such aircraft's high-lift system. Hereby, a more precise prediction of transverse crack onset is required as proven classic stress criteria turn out to behave very conservative due to the usage of comparatively thin plies and the existence of large bending stress gradients. A nominally 130 µm glass/epoxy prepreg is processed which actually constitutes the thinnest standard thickness, yet. However, distinctly retarded transverse crack onset is still observed. In fact, the local crack initiation stress within laminates stacked from thin laminae may considerably exceed the unidirectional strength as crack formation is energetically unfavourable. Such scenario is denoted as 'thin-ply' (behaviour) to distinguish it from classical stress triggered cracking. A coupled stress and energy criterion is developed to account for the 'thin-ply' phenomenon at lamina-level. The key difference between this criterion and existing approaches of such type is the ability to capture the effects of stress gradients and especially local shear loading. The energy release associated with sudden crack formation is determined within the framework of the finite fracture mechanics. For this purpose, a 3D finite element unit-cell is developed which is able to deal with arbitrary shell-type load conditions. The new criterion is validated by means of uniaxial tension and special large bending experiments. So far, most research on 'thin-ply' laminates is restricted to the scale of the whole laminate. This is especially true in case of fatigue investigations. In fact, impressively superior failure and fatigue characteristics are apparent in contrast to laminates stacked from thicker plies. In order to gain understanding, the new energy criterion as well as a static and a fatigue stress criterion from literature are applied to created large bending crack onset fatigue results. As a key finding, the elevated static 'thin-ply' cracking resistance cannot straightforwardly be extrapolated to fatigue conditions. It is concluded that the source of the reported superior fatigue characteristics at laminate-level is probably found in the effective suppression of secondary failure modes, like induced delamination.