Recall that the x axis (i. e., the axial coordinate) for homogeneous, isotropic beams is generally chosen as the locus of cross-sectional shear centers. This choice is frequently denoted as the “elastic axis” because it structurally uncouples torsion from both transverse shearing deformation and bending. Thus, transverse forces acting through this axis do not twist the beam. However, even for spanwise uniform composite beams, when transverse shear forces act through any axis defined as the locus of a cross-sectional property, it is still possible that these forces will twist the beam
because bending-twist coupling may be present. For the type of composite-beam analysis presented herein, we still choose the x axis to be along the locus of shear centers; but, for composite beams, this choice uncouples only torsion and transverse shear deformation. Therefore, although transverse shear forces acting through the x axis do not directly induces twist, the bending moment induced by the shear force still induces twist when bending-twist coupling is present.