Abstract Summary
Modal parameters estimated through in field dynamic testing define the inherent characteristics of real-world structures, being therefore employed as reference information for various purposes, including the assessment of structural damage, the evaluation of operational and environmental effects, and the calibration of realistic numerical models. Among natural frequencies, damping ratios and mode shapes, the latter have been proved far more effective in localizing structural damage given their spatial dependency on the nodal coordinates of vibrating systems. Most of modal analysis applications resort to the real part of these quantities for vibration-based damage identification of structural systems, assuming them as classically damped. However, one must be aware that the classical viscous damping assumption is idealistic in case of real-world structures because they are often made up of multiple and heterogeneous interconnected parts and the damping matrix cannot be considered as proportional to mass and stiffness matrices. Indeed, the mode shapes of real physical systems are complex in most cases, and this complexity cannot be ignored as it can adversely affect the correct identification of ongoing damage mechanisms. Based on the above considerations, the present work intends to shed light on the relationship existing between structural damage and modal complexity, taking into account that the imaginary content of experimentally identified modal vectors can derive from the simultaneous occurrence of factors other than damage, such as mass loading effects, measurement noise and high modal density, among others. To this end, numerical investigations are carried out in order to track the variation of complex mode shapes in structures subjected to damage scenarios of various extent and to infer about the generalization of a damage index, recently proposed by the authors, that relies on the weighted difference of the imaginary content of complex eigenmodes to detect, locate and assess the structural damage and its evolution path.
