The combined effect of the crack and the unbalance force vector angle on the values and locations of the whirl amplitudes at the critical whirl speeds for a cracked rotor-bearing-disk system is numerically and experimentally investigated here. The strongly nonlinear time-periodic equations of motion, which are analogous to Mathieus equation, of the cracked system with an open crack model are formulated according to the finite element time-periodic stiffness matrix. The whirl response during the passage through the critical speeds is obtained via numerical simulation for different unbalance vector angles with respect to the crack opening direction. It is found that the variation in the unbalance force vector angle with respect to the crack opening direction significantly alters the peaks of the critical whirl amplitudes and their corresponding critical whirl speeds. Consequently, the critical speeds of the cracked rotor are either shifted to higher or lower values according to the unbalance force vector angle value. These significant numerical simulation observations are also verified via robust experimental results.

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