Interlaminar and intralaminar damage evolution in composite stiffened panels under compressive loads: a global/local approach

The damage phenomenology of composite laminate is extremely complex and may involve different geometric scales in the initiation and progression of damage, and different typologies of failure. In recent years, delaminations and intralaminar damage onset and propagation have been extensively investigated separately [1-3] both numerically and experimentally and when possible analytical models have been developed. However, results of a few works [4-5] have shown that the correct simulation of the damage phenomenology for composite laminate structure can be achieved only by considering interlaminar and intralaminar damage interaction. In fact, to neglect this interaction would mean not only an incorrect prediction of the structural stiffness in presence of damage but also a significant overestimation of the residual structural strength. A numerical procedure has been developed and implemented in the parametric language of the ANSYS © finite element code. Within this procedure both delamination growth and progressive damage onset and evolution have been taken into account. The interlaminar damage evolution is based on the evaluation of the Strain energy Release Rate at the delamination front [7] computed by means of the modified virtual crack closure technique (MVCCT) and on releasable connections which allow the debonding between adjacent sub-laminates to be simulated. Furthermore, an iterative numerical procedure has been introduced to simulate the progressive matrix and fibre breakage by adopting respectively the Hashin’s failure criteria [8] to check the stress state and instantaneous degradation rules for the reduction of the damaged material properties [3]. The effectiveness of this procedure has just been proved on flat composite laminate plates [4]. In the present work the interlaminar-intralaminar interaction phenomenon is analysed for a more complex structure: a stiffened composite panel with a circular embedded delamination. In order to reduce the computational efforts requested for the numerical analysis a global/local approach has been used to connect differently modeled substructures. Results obtained by numerical approaches characterised by a different degree of accuracy (only interlaminar damage, damage interaction) are compared with experimental results [9] thus highlighting the intralaminar/interlaminar damage interaction effects on the residual stiffness and strength of the structure under consideration.