The response of the beam remains linear and elastic almost to the stage when the stress in the extreme fibre has reached the value of the yield stress. From there on further increases of the load induce progressive yielding of the cross-section until the whole section has yielded and the plastic hinge has fully developed. The slightest increase of load beyond this point will trigger the plastic hinge into rotation, that will continue over a short space of time and lead to very large deformations, ending in failure. No fracture is expected to occur before the hinge location becomes quite large, because structures designed by simple plastic theory are required to be made of a ductile steel and designed to remain ductile.