Nonclassical Behavior of Thin‐Walled Composite Beams with Closed Cross Sections

This paper focuses on two nonclassical effects in the behavior of thin‐walled composite beams: elastic bending‐shear coupling and restrained torsional warping. These nonclassical effects are clarified and analyzed in some simple examples involving cantilevered beams. First, elastic bending‐transverse shear coupling is shown to be important in the analysis of beams designed for extension‐twist coupling. It is found that the lateral deflections ran be off by more than a factor of two if this coupling is ignored. This coupling stems from plies with off‐axis fibers in the beam. The presence of these plies affects significantly the modeling approach (i.e., determination of the constitutive equations) in that transverse shear must appear in the kinematics so that its coupling with bending will he exhibited in the elastic constants. This finding is in accord with “exact” beam theories which develop the beam displacement and cross sectional orientation in terms of six kinematical variables instead of the three or four found in some previously published works on composite blade modeling. A second nonclassical effect, torsional warping rigidity, is shown to be important far certain box beams having a thin‐walled, closed cross section. The importance of including these nonclassical phenomena in a complete theory is discussed in light of the magnitude of their effects for various values of configuration parameters.