Keywords: warping function, torsion constant, center of twist, deformable connection, slip, imperfect interfaces, boundary element method..

In engineering practice we often come across the analysis of rectilinear members of composite structures subjected to twisting moments. Curved bridges or beams subjected to eccentric loading with respect to their cross section's shear center are the most common examples. In composite structural members, where no distortion of the cross-section occurs, the connection is often flexible in shear permitting only the longitudinal relative slip but preserving the contact. In this paper, a boundary element method (BEM) is developed for the uniform torsion of composite bars with deformable shear connection of arbitrary constant cross section. The composite bar (thin or thick walled) consists of materials in contact, each of which can surround a finite number of inclusions. The materials have different elasticity and shear moduli and can slip in the longitudinal direction of the bar. The problem is analysed by assuming a "spring-type" imperfect bonding along the interfaces of the materials in contact. A boundary value problem is formulated with respect to corresponding torsional warping function and solved employing a pure BEM approach, that is only boundary discretization is used. Numerical examples are worked out to illustrate the efficiency, the accuracy and the range of applications of the method developed. The considerable influence of the longitudinal relative slip in both the torsional rigidity and the warping of the composite cross section of the bar are pointed out.

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