Torsional behavior of laminated bamboo tubes: Experiments, numerical calibration, and surrogate modeling

This study investigates the monotonic torsional response of laminated bamboo tubes and develops a surrogate model of their behavior. Fifteen specimens representing five geometric configurations were tested in pure torsion using epoxy end blocks designed to limit slip and premature end damage. Three-dimensional digital image correlation (3D-DIC) was used to determine specimen rotation and nominal outer-surface shear strain with reduced sensitivity to machine compliance. The specimens exhibited an initially approximately linear response followed by nonlinear hardening up to peak torque and brittle longitudinal splitting. Across the 15 tests, the mean ultimate shear stress, ultimate shear strain, and elastic shear modulus G 12 were 15.41 MPa, 2.33%, and 1116 MPa, respectively. A shell finite-element model implemented in OpenSees, combining an equivalent isotropic J 2 model with a predefined longitudinal interface, was calibrated against the measured torque–rotation curves. The calibrated model was then used to simulate 1000 geometries sampled within a constrained design space. These responses were represented by a surrogate model defined up to the failure rotation, with closed-form expressions derived for the torsional stiffness, yield torque, hardening parameters, and failure rotation. The resulting surrogate model provides a compact representation of the finite-element-generated monotonic response and supports rapid parametric studies, beam-type finite-element analyses, and comparative design-space exploration.