Civil Research Talks | Carlos A. Cimini Jr. | The use of invariants and non-conventional laminates for designing CFRP composites

Carlos A. Cimini Jr. | Professor, Department of Mechanical Engineering, Federal University of Minas Gerais (UFMG), Brazil

February 21, 2024 | 17h00-18h00 | Room U0.7 | Lusófona University, Lisbon, Portugal

Abstract: Carbon/epoxy composite laminate design usually relies on a large set of parameters, which are defined as properties of the material. Also, traditional designs in aerospace industry are based on Quad laminates (0°, 90°, ±45° oriented plies) with additional rules (symmetry, balance, 10% minimum of each angle, etc.) that greatly restrict the potential of the material. Using the ply as the building block, designers need to know four independent elastic constants (Ex , Ey , Gxy , and Vxy) plus a set strength allowables, depending on the failure criterion to be chosen. In the case of the more traditional failure criteria, five strength allowables are required (X, X’, Y, Y’ and S). To overcome these problems, it is introduced the use of invariants and non-conventional Double-Double (DD) laminates for composites design. A novel approach is proposed, based on only three parameters to define all those properties: one for the elastic constants and two for the failure criterion. The elastic constants can be derived based on the invariant master ply theory using the Tsai’s modulus, which is the trace of the ply stiffness matrix, considered a material property itself. The Young’s modulus in the fiber direction Ex can then be used as a single parameter to define the Tsai’s modulus. Together with the master ply stiffness matrix derived for the carbon/epoxy class of composite materials, the particular stiffness matrix can be obtained for any material of this class. Failure analysis is performed using either two (X and X’ or their corresponding strains) or, more conservatively, one parameter based on whichever is smaller. The introduction of non-conventional layups, specifically the Double-Double (DD) concept to replace the traditional Quad laminates, adds to the simplification of the design and manufacturing process. DD laminates are defined by a simple building-block of four-angle plies [±/±] continuously stacked. Symmetry and balancing are achieved by the repetition of this building-block, due to homogenization of the laminate. Furthermore, ply-drops are greatly simplified and optimization can be easily achieved due to DD continuous domain (angle), instead of the discrete domain for the Quads.