A Pure Bending Device to Test Elastic Beams in the Nonlinear Region

Background Standard bending tests usually give valid measurement results only in the linear region, rely on some beam model for evaluation, and transfer the load in some discrete points that localize the stress field. Pure bending tests can provide a solution to these issues and are therefore worth studying. Objective To provide a deeper insight into the mechanics and possible evaluation methods of the pure bending tests. Methods A pure bending test rig with a working principle similar to that of the Hill device [1] was developed. The mechanics of the device were analyzed, and possible issues were identified, including the effect of a parasitic force arising from friction. The bending moment is calculated from the loading force, while the beam curvature is measured by circle fitting to beam points obtained from digital photographs. Results Different evaluation methods of curvature measurement were tested, and the effect of measurement errors was analyzed with Monte Carlo simulations. The magnitude of the parasitic force was approximated and used for correction of the obtained raw moment-curvature relationship. The device and proposed methodology was validated on steel beam specimens of different lengths and a glass fiber laminate specimen for larger deformations. Conclusion The study provides a validated measurement and evaluation method for pure bending tests capable of handling large deflections. It is demonstrated that while parasitic forces appear in these devices, the bending characteristic can be obtained accurately with the proposed correction procedure. In the elastic region, the measurements were in good agreement with the standard three-point bending results.