Keywords: delamination, damage tolerance, composites, linear approach, optimization, finite element method.

The mechanical behaviour of composite structures with delaminations has been widely investigated in the literature by performing experimental tests and by developing non-linear predictive numerical tools [1,2,3,4] based on the virtual crack closure technique (VCCT), the modified virtual crack closure technique (MVCCT) and the cohesive zone models (CZM) for the calculation of the energy release rate. However, as a result of the high computational cost, the use of non-linear techniques for delamination growth simulations is still not applicable to the preliminary design and in optimisation phases.

In this context, an alternative, linear delamination growth initiation approach is proposed. The approach presented is strongly connected to the numerical methodology already introduced and validated in [5]. This methodology, based on the linearized buckling evaluation and on the application of the energy balance principle, has been implemented using the ANSYS code and proved to be able to reasonably predictable, by means of a limited number of linear analyses, the location along the delamination front and the load at which a delamination may propagate.

Comparisons with the ABAQUS non-linear results, for a stiffened composite panel with a bay embedded delamination, are introduced to demonstrate the affordability of the proposed approach, when the delamination depth falls within a range of relatively small values (<=15% of the total skin thickness). Such a situation is quite representative of manufacturing defects and the damage induced by low velocity impact events. Furthermore, the excellent compromise between the very limited computational effort and the accuracy of the results, have suggested as a possible application to perform an optimization in combination with the linear approach for the determination of the damage tolerance of stiffened composite panels with an arbitrary impact induced delamination.

These kinds of optimization activities, have been demonstrated feasible only by the very relevant reduction in computational time provided by the linear approach presented.

1

J.D. Whitcomb, "Parametric Analytical Study of Instability-Related Delamination Growth", Composite Science and Technology, 25(1), 18-48, 1986. doi:10.1016/0266-3538(86)90019-9

2

K.F. Nilsson, J.C. Thesken, P. Sindelar, A.E. Giannakopoulos, B.A. Storakers, "Theoretical and Experimental Investigation of Buckling Induced Delamination Growth", Journal of Mech. Phys. Solids, 41(4), 749-782, 1993. doi:10.1016/0022-5096(93)90025-B

3

A. Riccio, F. Scaramuzzino, P. Perugini, "Influence of Contact Phenomena on Embedded Delamination Growth in Composites", AIAA Journal, 41(5), 933-940, 2003. doi:10.2514/2.2029

4

E. Greenhalgh, C. Meeks, C.A. Clarke, J. Thatcher, "The effect of defects on the performance of post-buckled CFRP stringer-stiffened panels", Composites Part A: Applied Science and Manufacturing, 34(7), 623-633, 2003. doi:10.1016/S1359-835X(03)00098-8

5

A. Riccio, M. Zarrelli, M. Giordano, "A Linear Numerical Approach to Simulate the Delamination Growth Initiation in Stiffened Composite Panels", Journal of Composite Materials, 44(15), 1841-1865, 2010. doi:10.1177/0021998310364108

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