Response of curved composite panels subjected to combined thermal and mechanical loads

Experimental and numerical results are presented for the buckling and postbuckling responses of unstiffened curved composite panels subjected to mechanically-applied axial end shortening and a uniform temperature increase. Thermal stresses are induced in the panel by increasing the temperature while constraining axial displacement. Three different lamination sequences and four different loading conditions were considered. Panels with a quasi-isotropic laminate, a circumferentially stiff laminate, and an axially stiff laminate were tested with end shortening alone, and with combinations of end shortening and a uniform temperature increase. Numerical results were generated using a geometrically nonlinear finite element analysis code. The circumferential thermal expansion of the stainless steel end fixture altered the curved-end boundary conditions during thermal loading. Therefore, correlation of the experimental results and numerical predictions required different curved end boundary conditions during thermal loading than during end-shortening. Introduction Future high speed aircraft are expected to operate at elevated temperatures and be constructed of laminated composite materials. Therefore, the thermally induced buckling and postbuckling responses of curved composite panels are of interest. Experimental and numerical studies have provided an understanding of the Post-Doctoral Research Associate, National Research Council ^Professor, Dept. of Engineering Science and Mechanics, Associate Fellow AIAA *Head, Structural Mechanics Branch, Fellow AIAA Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. response of curved composite panels subjected to mechanically-applied axial compression loads, including the study by Knight and Starnes. Thermal buckling of curved composite panels has been studied numerically by Huang and Tauchert and Librescu et al. However, experimental studies of thermal buckling are rare even for isotropic materials. Anderson and Card tested stainless steel ring-stiffened cylinders subjected to pure mechanical bending and an applied nonuniform temperature distribution until buckling occurred. Ross, Hoff, and Horton uniformly heated steel cylinders with the ends restricted from axial and circumferential expansion, resulting in compressive thermal stresses that eventually led to buckling. Ari-Bur, Baruch, and Singer tested aluminum-alloy cylindrical shells preloaded in axial compression and torsion, then heated along two opposite generators. Thompson and Richards tested a square titanium matrix composite hat-stiffened panel subjected to both a uniform temperature and inplane temperature gradients. Tests were conducted by first allowing the panel to reach an equilibrium temperature, then by applying a compressive load. Data were presented only after an equilibrium temperature had been achieved. No numerical correlation was presented for that study. The current study presents experimental results and numerical correlations for unstiffened curved composite panels subjected to combinations of axial end-shortening and a uniform temperature increase. Limited experimental results appeared in ref. 1 along with details of the refinements to the numerical model that were required to obtain a reasonable correlation with experiments. Additional experimental results are presented in the present paper, which demonstrate the importance of knowing the boundary conditions that arise due to the interaction of mechanical and thermal effects. 624 American Institute of Aeronautics and Astronautics Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.