Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct Deriving unidirectional lamina properties from testing on cylindrical laminated specimens P.B. Ataabadi a, ⁎ , D. Karagiozova b , M.H. Shaterzadeh a , M. Alves a a Group of Solid Mechanics and Structural Impact, Department of Mechatronics and Mechanical Systems Engineering, University of Sao Paulo, Sao Paulo 05508900, Brazil b Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Block 4, Sofia 1113, Bulgaria ARTICLEINFO Keywords: Material characterization Laminate properties UD lamina properties Shear fixture Non-flat specimens ABSTRACT The present work aims to derive the mechanical properties of a unidirectional (UD) lamina from experi- mentally measured properties of non-flat (cylindrical) laminated specimens. The specimens were cut from carbon fiber-epoxy tubes having cross-ply and angle-ply layup configurations. Special tests were designed and applied on these specimens to measure the mechanical properties of the laminate, E x , E y , ν xy, G xy, as well as tensile and shear failure strength. By using the classical laminate theory (CLT), the mechanical properties of UD lamina, E 1 , E 2 , ν 12 and G 12 were expressed explicitly as functions of the measured properties of the cross- ply laminate and in form of effective mechanical properties of general lamination configuration like the angle- ply layup. A new shear test fixture was designed in order to obtain the in-plane shear modulus and shear strength of ring-shaped composite specimens. This shear test fixture has proved to be advantageous, especially when one bears in mind that there are very few shear test methods for cylindrical components. The outcomes of this work would be useful for material characterization and quality control when standard flat coupons are not available. 1. Introduction The mechanical properties of a lamina, such as elastic moduli, Poisson’s ratio, in-plane shear modulus and failure strengths, are basic requirements to model the behavior of composite materials and analyze laminated structures, numerically and/or theoretically. Standard flat unidirectional (UD) specimens are used to measure the elastic mechanical properties and failure strength of composites ac- cording to the well-known test standards [1,2]. In structural appli- cations, however, multidirectional lay-ups, cross-ply, angle-ply and quasi-isotropic, are more desired than unidirectional lay-up, com- prising only isolated UD plies from differently orientated plies. It has been argued, however, that the properties of individual laminae in a multidirectional lay-up configuration, called in-situ properties, could be thoroughly different from UD lamina properties measured by testing on unidirectional specimens [3–5]. Testing of multidirectional specimens, especially cross-ply specimens, has been reported to assist the improvement of the failure strength characterization of UD la- mina and reduction of data scatters by using a back-out calculation [4,5]. In the back-out calculation, the lamina failure strength is re- versed from multidirectional laminate strength by using available back out factors [5–7]. The lower failure strength of the multidirectional, angle-ply or cross-ply, composite specimens in comparison with the unidirec- tional laminated specimens [5] and more robust lay-up of cross-ply specimens [4], brings several advantages leading to a more reliable measurement of the failure strength. For instance, the gripping force on tensile specimens, which may be responsible for premature failure modes in composite materials, is somewhat proportional to failure load. Therefore, the lower failure strength (failure load) of multidirectional specimens can help to avoid premature failure and measure the failure strength more accurately. Since multi- directional laminate fails at lower strengths, the results are less sensitive to the specimen preparation (in some case un-tapped specimens are allowed [5]) thus leading eventually to a lower data scatter as it has been reported for strength measurement [3,5,8]. The main advantage of testing on multidirectional specimens and back-out calculation could be, that the specimens having multi- directional layups more closely represent those in the laminates of actual structural components as the laminae may respond differ- ently to the loads when the surrounding plies are of different or- ientations [8]. The classical laminate theory has been utilized to find the ex- tensional and bending stiffness constants of laminate structures https://doi.org/10.1016/j.compstruct.2020.112382 Received 9 April 2019; Received in revised form 27 February 2020; Accepted 14 April 2020 ⁎ Corresponding author. E-mail address: Pouriabahrami@usp.br (P.B. Ataabadi). Composite Structures 245 (2020) 112382 Available online 18 April 2020 0263-8223/ © 2020 Elsevier Ltd. All rights reserved. T