Virtual Crack Closure Technique and fail release approach: an effective finite element implementation for delamination growth phenomena

Within the frame of the MAAXIMUS project, an interlaminar damage of known dimension and position has been considered as a pre-existing defect within a stiffened panel (embedded bay delamination). Its evolution, induced by a static compressive load, has been simulated in ANSYS by implementing a user subroutine based on the combined use of the Virtual Crack Closure Technique and of a fail release approach. It has been observed that, even thought the procedure defined was very effective in predicting the delamination growth initiation, a strong dependency of the results on the load step and element size was introduced during the growth phase. Aim at reducing computational efforts required for delamination growth analyses and at achieving realistic predictions in terms of growth whatever the mesh density may be, the cause of the dependency of the results on load step size and mesh density has been investigated. Then, a convergence check has been introduced into the non linear iterative flow in order to allow an automatic adjustment of the load step size to be obtained on the basis of the overbalance existing at each iteration between the energy level computed at the delamination front and the threshold level fixed by the growth criterion adopted. Furthermore the presence of peaks of Energy Release Rate where corner were induced in the delamination front has been identified and mitigated through the introduction of weight factors into VCCT formulae to take into account the presence of a delaminated area of complex shape. The results obtained with the novel approach proposed have been validated against literature data and have been found to be in good agreement with them in terms of both delamination growth rate and debonded area, showing the effectiveness of the enhancements introduced when dealing with growth phenomena.