Post-Resonance Backward Whirl in a Jeffcott Rotor with a Breathing Crack Model

Abstract

Propagation of fatigue cracks is one of the major causes for catastrophic damage in rotor systems. Such cracks have an unusual property in that they can open and close continuously and synchronously with the shaft rotational speed which was termed as a breathing mechanism. This problem has not been adequately analyzed from nonlinear and rotor dynamic perspectives in the literature in spite of a large number of papers that have appeared over the last two decades. Separately from this, backward whirl (where the precession is opposite to the rotational direction) can be dangerous and can lead to catastrophic failures in rotor systems. Interestingly, a new backward whirl phenomenon at start-up and coast down operations in cracked rotor systems with open crack models was observed in a recent publication. This phenomenon has been numerically and experimentally verified with an open crack to directly appear after the passage through the critical speed. Building on these recent findings, there is an imperative need to further investigate this phenomenon in rotor systems associated with open and breathing crack models from nonlinear and rotor dynamic perspectives. For start-up and coast down operations at constant angular acceleration, the model of the cracked rotor system with open or breathing crack models becomes a linear time-varying (LTV) system. Here, we also numerically verify the existence of this new backward whirl (BW) phenomenon in a cracked rotor with a breathing crack model via numerical simulation. Results indicated that a wide zone of BW rotational speeds is observable after the passage through the critical speed due to appearance of the breathing crack in the considered Jeffcott rotor system.