Keywords: skin-stringer delamination, composite laminate, mixed mode fracture, energy release rates.

Stiffened composite panels and shells are often used in different aircraft structural design. The deformations associated with post-buckling response can produce severe bending and shear loads at the skin-stringer interface. It can cause stiffener separation from the skin. Therefore, accurate computational methods are required to analyze and predict skin-stiffener debonding failures [1,2,3].

To analyze the skin-stringer debonding a fracture mechanics approach has been used based on 3D solid or shell finite element analysis results. A combined shell/3D modelling technique was developed in [4]. The 3D solid finite element model is used only in vicinity of the delamination front and the remainder of the structure is modelled using plate or shell elements. Such shell/3D technique can combine the computational efficiency of a shell finite element model with the accuracy of the full three-dimensional solution in the area of delamination.

The total number degrees of freedom can be reduced by the use of 2D plate or shell finite elements instead of 3D solid elements. Such approach was used in several papers [1,2,3]. It was shown that by using the shell or plate elements sufficient accuracy can be achieved in calculation of energy release rates. In the present paper for analysis of the skin-stringer debonding the 2D shell finite elements are employed.

For analysis of skin-stringer delamination the fracture criterion and fracture parameters for the mixed mode loading conditions are obtained. There are different experimental methods and fracture criterions for the mixed mode loading conditions of composite laminates. Review on this subject was given in [5]. There a compound version of the compact tension shear (CTS) specimen was proposed, which covers all in-plane mixed mode loading conditions starting from pure mode I through any mixed mode I/II ratio up to pure mode II loading. The CTS specimen is used in the present paper to obtain fracture toughness of carbon/epoxy laminate at mixed mode I/II loading conditions. These fracture properties are used to model the skin-stringer separation problem.

In the papers [2,3] the skin-stringer separation analysis in composite stiffened panels was performed employing shell elements. In the present paper the skin-stringer delamination is analysed employing a simplified model - the flat plate with one stringer. The initial delamination is modelled by Teflon insertion between the flange and skin of the panel. The stringer is constructed from the flange and web (blade). This is the so-called blade type stringer. Such model is chosen for experimental determination of fracture parameters at the skin-stringer interface. The model loaded in tension mode is analysed employing 2D shell elements. The energy release rates are calculated at the crack font on skin-stringer interface. For this a Modified Virtual Crack Closure Integral (MVCCI) method and a simplified the so- called B-technique is employed [1]. It was shown that at the crack front there is mixed mode loading conditions where the dominant is crack opening mode. Therefore, the simplified model (flat plate with one stringer) can be used in experimental determination of delamination fracture parameters at the skin-stringer interface.

1

J.T. Wang, I.S. Raju, "Strain energy release rate formulae skin-stiffener debond modeled with plate elements", Engineering Fracture Mechanics, 54(2), 211-228, 1996. doi:10.1016/0013-7944(95)00088-7

2

R.S. Thomson, M.L. Scott, "Modelling delaminations in postbuckling stiffened composite shear panels", Computational Mechanics, 26, 75-89, 2000. doi:10.1007/s004660000154

3

J.W.H. Yap, M.L. Scott, R.S. Thomson, D. Hachenberg, "The analysis of skin-to-stiffener debonding in composite aerospace structures", Composite Structures, 57, 425-435, 2002. doi:10.1016/S0263-8223(02)00110-1

4

R. Krueger, T.K. O'Brien, "A shell/3D modeling technique for the analysis of delaminated composite laminates", Composites. Part A, 32, 25-44, 2001. doi:10.1016/S1359-835X(00)00133-0

5

R. Rikards, F.-G. Buchholz, H. Wang, A.K. Bledzki, A. Korjakin, H.-A. Richard, "Investigation of mixed mode I/II interlaminar fracture toughness of laminated composites by using CTS type specimen", Engineering Fracture Mechanics, 61, 325-342, 1998. doi:10.1016/S0013-7944(98)00068-X

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