Intralaminar and interlaminar progressive failure analyses of composite panels with circular cutouts Vinay K. Goyal a,1 , Navin R. Jaunky b,2 , Eric R. Johnson a, * , Damodar R. Ambur c,3 a Department of Aerospace and Ocean Engineering, Virginia Polytechnic Institute and State University, 215 Randolph Hall, MS 0203, Blacksburg, VA 24061-0203, USA b NASA Langley Research Center, MS 190, 8 West Taylor St., Hampton, VA 23681-2199, USA c NASA Langley Research Center, MS 188E, 2 West Reid St., Hampton, VA 23681-2199, USA Abstract A progressive failure methodology is developed and demonstrated to simulate the initiation and material degradation of a laminated panel due to intralaminar and interlaminar failures. Initiation of intralaminar failure can be by a matrix-cracking mode, a fiber-matrix shear mode, and a fiber failure mode. Subsequent material degradation is modeled using damage parameters for each mode to selectively reduce lamina material properties. The interlaminar failure mechanism such as delamination is simulated by positioning interface elements between adjacent sublaminates. A nonlinear constitutive law is postulated for the interface element that accounts for a multi-axial stress criteria to detect the initiation of delamination, a mixed-mode fracture criteria for delamination progression, and a damage parameter to prevent restoration of a previous cohesive state. The methodology is validated using experimental data available in the literature on the response and failure of quasi-isotropic panels with centrally located circular cutouts loaded into the postbuckling regime. Very good agreement between the progressive failure analyses and the experimental results is achieved if the failure analyses includes the interaction of intralaminar and interlaminar failures. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Progressive failure analyses; Delamination; Ply failure 1. Introduction The phenomenon of delamination in laminated composite structures usually originates from disconti- nuities such as matrix cracks, or the presence of free edges. Delamination may lead to reduced stiffness and strength. In some cases delamination can provide stress relief and delay final failure of the structure. The ability to accurately predict damage evolution is essential for predicting the performance of composite structures and developing reliable and safe designs. In recent years, progressive failure analyses (PFA) in composite lami- nates has been the focus of extensive research. In most of the PFA either intralaminar or delamination damage mechanism are considered. In past studies when fracture mechanics based approaches are used, an initial delaminated area and self- similar delamination growth and is often assumed [1,2]. These limitations can be overcome by positioning interface elements between adjacent laminae of a com- posite structure. With this methodology, the prediction of initiation and progression of delamination without specifying an initial delaminated area becomes natural without imposing constraints in the PFA. Moreover, with interface elements, the prediction of nonself similar delamination growth is possible. Interface elements can be used in a variety of problems involving buckling and in the presence of a delamination. Shahwan and Waas [3] investigated the interaction of buckling and delam- ination of a panel with a circular hole subjected to compressive loads. A mixed-mode damage model was developed by de Moura et al. [4] to simulate delamina- tion growth in a compression panel after it was sub- jected to a low velocity impact event. They showed that the internal delaminations reduced the compressive * Corresponding author. Tel.: +1-540-231-6699; fax: +1-540-231- 9632. E-mail addresses: vigoyal@vt.edu (V.K. Goyal), navin@nianet.org, n.r.jaunky@larc.nasa.gov (N.R. Jaunky), erjohns4@vt.edu (E.R. Johnson), d.r.ambur@larc.nasa.gov (D.R. Ambur). 1 Tel.: +1-540-230-2655. 2 Tel.: +1-757-864-3187. 3 Tel.: +1-757-864-3449. 0263-8223/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0263-8223(03)00217-4 Composite Structures 64 (2004) 91–105 www.elsevier.com/locate/compstruct