Available online at www.sciencedirect.com ScienceDirect Comput. Methods Appl. Mech. Engrg. 371 (2020) 113311 www.elsevier.com/locate/cma Geometrically nonlinear analysis of sandwich FGM and laminated composite degenerated shells using the isogeometric finite strip method Mohammad Amin Shahmohammadi, Mojtaba Azhari ∗ , Mohammad Mehdi Saadatpour, Saeid Sarrami-Foroushani Department of Civil Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran Received 1 January 2020; received in revised form 20 July 2020; accepted 20 July 2020 Available online xxxx Abstract Isogeometric analysis (IGA) has become a strong tool for analysis of shell structures, which can be considered as a powerful alternative to the standard finite element method (FEM). Besides several advantages of IGA over FEM in geometric nonlinear analysis of the shells, it has some drawbacks corresponding to tensor-product of B-spline and NURBS surfaces. One of the solutions to eliminate or at least reduce the effects of the mentioned shortcomings is to combine IGA with the other methods, such as FEM. In the present paper, a novel version of the finite strip method (FSM) is developed to carry out the geometric nonlinear analysis, which employs a combination of FEM and IGA in the transversal and longitudinal directions, respectively and is named isogeometric B-spline finite strip method (IG-SFSM). The strip elements have been formulated based on the Degenerated-Shell method for Updated-Lagrangian (UL) approach. Then, it is examined by some examples incorporating various geometries, boundary conditions (BCs), and material properties, such as laminated composites and sandwich FGMs. The results exhibited that, to achieve the complicated nonlinear equilibrium paths of the shells, IG-SFSM requires a lower number of degrees of freedom (DOFs) and, consequently, computational efforts comparing to the other common methods, such as finite element and mesh-free methods. c ⃝ 2020 Elsevier B.V. All rights reserved. Keywords: Isogeometric B-spline finite strip; Geometric nonlinear analysis; Sandwich FGM; Laminated composite; Updated-Lagrangian (UL); Equilibrium path 1. Introduction Shell structures are among the widely used structural components in various engineering branches such as civil, mechanical, and aerospace engineering, because of their high strength against the in-plane compression loadings. Depending on their application, the shells may be made of various types of materials, such as laminated composites and functionally graded materials (FGMs). The important point is that the compressive in-plane loads in shells lead to various types of instabilities, such as snap-through and snap-back, which can be considered as a weak-point. The intensity of the instabilities is very dependent on the material and geometrical properties and boundary conditions ∗ Corresponding author. E-mail address: mojtaba@iut.ac.ir (M. Azhari). https://doi.org/10.1016/j.cma.2020.113311 0045-7825/ c ⃝ 2020 Elsevier B.V. All rights reserved.