DEVELOPMENT OF A FINITE-ELEMENT ANALYSIS METHODOLOGY FOR THE PROPAGATION OF DELAMINATIONS IN COMPOSITE STRUCTURES A. C. Orifici,* R. S. Thomson,** R. Degenhardt,*** C. Bisagni,**** and J. Bayandor***** Keywords: delamination, virtual crack closure technique, double cantilever beam, propagation modelling Analysing the collapse of skin-stiffened structures requires capturing the critical phenomenon of skin-stiffener separation, which can be considered analogous to interlaminar cracking. This paper presents the develop- ment of a numerical approach for simulating the propagation of interlaminar cracks in composite structures. A degradation methodology was introduced in MSC.Marc, which involved the modelling of a structure with shell layers connected by user-defined multiple-point constraints (MPCs). User subroutines were written that employ the virtual crack closure technique (VCCT) to determine the onset of crack growth and modify the properties of the user-defined MPCs to simulate crack propagation. Methodologies for the release of failing MPCs are presented and are discussed with reference to the VCCT assumption of self-similar crack growth. The numerical results obtained by using the release methodologies are then compared with experimental data for a double-cantilever beam specimen. Based on this comparison, recommendations for the future develop- ment of the degradation model are made, especially with reference to developing an approach for the collapse analysis of fuselage-representative structures. Introduction The European Commission Project COCOMAT (Improved MATerial Exploitation at Safe Design of COmposite Air- frame Structures by Accurate Simulation of COllapse) is an ongoing four-year programme that aims to exploit the large strength reserves of composite aerospace structures through a more accurate prediction of their collapse [1-2]. Accordingly, one of the COCOMAT work-packages involves the development of degradation models capable of capturing the composite damage mechanisms that contribute to structural collapse. For stiffened structures in compression, one of the most critical dam- 9 0191-5665/07/4301-0009 © 2007 Springer Science+Business Media, Inc. *School of Aerospace, Mechanical & Manufacturing Engineering, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne, Victoria, 3001, Australia. **Cooperative Research Centre for Advanced Composite Structures Ltd, 506 Lorimer St., Fishermans Bend, Victoria, 3207, Australia. ***Institute of Composite Structures and Adaptive Systems, DLR — German Aerospace Center, Lilienthalplatz 7, 38108 Braunschweig, Germany. ****Dipartimento di Ingegneria Aerospaziale, Politecnico di Milano, Via La Masa 34, 20156 Milan, Italy. *****The Sir Lawrence Wackett Aerospace Centre, School of Aerospace, Mechanical and Manufacturing Engineering, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne, Victoria, 3001, Australia. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 1, pp. 15-42, January-February, 2007. Original article submitted June 5, 2006. Mechanics of Composite Materials, Vol. 43, No. 1, 2007