In this paper, a lengthwise fracture analysis of a viscoplastic inhomogeneous cantilever beam of circular cross-section acted upon by a torsion moment is presented. The beam is with an arbitrary number of concentric lengthwise cracks. The time-dependent mechanical behaviour of the beam is treated by a viscoplastic model representing a series of units which are combinations of springs, dashpots and frictional sliders. Due to the sliders, the model units start to deform one after another with increasing of the external load. Constitutive laws of the viscoplastic model are obtained and applied to treat the beam mechanical behaviour when deriving time-dependent strain energy release rates for the lengthwise cracks.