Finite element simulation of crack propagation and delamination in layered shells due to blade cutting

The focus of this work is on the simulation of crack propagation and delamination in layered shells due to cutting by sharp blades. The main features of these type of problems are: material non-linearities, contact, large deformations, fracture and delamination make the problem highly non-linear, suggesting to carry out the simulation in an explicit dynamics framework; the shell layered structure with possible delamination suggest the use of solid-shell elements; the very small thickness of individual layers requires a selective mass scaling to allow for an efficient implementation of solid-shell elements in explicit dynamics; the dominant crack path is determined by the known blade trajectory, which justifies the adoption of an inter-element description of fracture; the blade sharpness introduces in the problem an extremely small geometrical scale (orders of magnitude smaller than a typical element in-plane finite element size) which cannot be resolved using standard cohesive element formulations. In order to obtain an accurate prediction of crack propagation, the cutting process is here described by means of directional cohesive elements [1,2], i.e. massless string elements attached to the opening faces, dissipating the interface cohesive energy upon elongation and able to detect contact with the cutting blade. Upon contact, the string element interacts with the cutting blade and deforms, transmitting cohesive forces to the two crack flanks in the correct directions. The presence of different layers is accounted for by stacking one or more solid-shell elements per layer through the thickness. An ad-hoc procedure for topology updating has been developed to handle the through-the-thickness crack propagation. The effect of the number of introduced directional cohesive elements per opening face is critically assessed. Possible layer delamination is accounted for by introducing cohesive interfaces between layers.