MAC and ROTMAC in structures with closely spaced modes

Abstract

[EN] In models with closely spaced or repeated modes, small perturbations in mass or stiffness can cause significant rotations of the mode shapes within their local subspace. The rotation angle is dependent on both the closeness of the modes and the magnitude and location of the perturbation. Consequently, low MAC (modal assurance criteria) values may be obtained in the case of closely spaced modes, even in the case of low mass and stiffness discrepancies. The concept of ROTMAC and the relative angle between the mode shapes provides information of the shear and serves as an indicator of mass discrepancies between models. In this paper, simulations of two spring-mass systems are performed modifying the closeness of the modes, in order to study its effects in the MAC and the ROTMAC.