Modeling of Dynamic Delamination in L-Shaped Composite Brackets

One of the widely used geometrically complex parts in recent civil passenger aircrafts is the L-shaped composite brackets connecting ribs to skins. Due to the sharp curved geometry, interlaminar opening stresses are induced and delamination occurs under considerable mode-mixities at the corner. Dynamic phenomena during delamination initiation and propagation of L-shaped beams are investigated using dynamic (explicit) finite element analysis in conjunction with cohesive zone methods (CZM). In ABAQUS a sequential explicit analysis followed by static (implicit) solution is used where the solution duration is considerably reduced. The thickness of the specimens is varied from 1.0 mm to 4.0 mm while the inner radius is kept same. Loading is applied parallel to one of the arms quasi-statically. Even though the crack is at the very middle of the specimen, this specific loading type yields variable traction fields and mode-mixities in the two sides of the crack in which delamination occurs under shear stress dominated loading on one crack tip and opening stress dominated loading on the other. It is observed that the delamination propagation is highly dynamic even though the loading is quasi-static. The speed of the delamination under shear dominated loading at one side can reach 800 m/s and under normal stress dominated loading is 50 m/s in dedicated thickness levels. In addition, moving elasto-dynamic radial compressive waves along the interface are observed. An important observation for design applications, a typical solution of adding more plies to the laminate might yield failure transition to a secondary crack nucleating at the arm and propagating towards the center crack.