Experimental modal analysis

Modal analysis methods are relatively recent investigative methods, which have been implemented to establish and/or improve knowledge of the dynamic model of real structures. Indeed, the significant parameters for representing the dynamic behavior of a linear structure, whatever its complexity, are "concentrated" in a small number of modal parameters: natural frequencies, damping and associated eigenforms. The dynamic behavior of the structure under specific excitation conditions in the absence of any modeling requires only knowledge of these parameters. This is why experimental modal analysis has become, thanks to advances in computer science and instrumentation, a preferred method of investigation in the field of structural dynamics .

The first instigators of this technique were aircraft manufacturers, who were faced with the crucial problem of aircraft "flutter". This phenomenon, due to aeroelastic coupling between the air and the aircraft structure, causes self-excited vibrations at certain speeds, which can lead to the destruction of the aircraft. It can be predicted if the dynamic characteristics of the structure are known, i.e. eigenvectors, eigenfrequencies and generalized damping, generalized masses (modal masses).

The first methods developed in the 1950s-60s were modal appropriation methods, which involved applying a set of harmonic exciting forces to the structure, suitably distributed in amplitude and phase, giving a response from the structure proportional to an eigenmode of the associated conservative system.

These methods, which are still in use, have the advantage of being highly reliable, as the experimenter "sees" the identified mode, and the quality of appropriation can be qualified using the phase criterion. However, they are very cumbersome to implement and require substantial investment.

The constant quest for quality improvement in all fields where mechanics are involved has led designers to use experimental modal analysis as a privileged tool to gain a better understanding of the dynamic behavior of structures. For this reason, modal analysis techniques have gone far beyond the aeronautical field, to cover structures in the transport sector (motor vehicles, railways, ships, etc.), civil engineering structures (bridges, cooling towers, group blocks, etc.) and, more generally, all equipment likely to be subjected to severe vibration. A whole methodology has thus been developed downstream of modal analysis, concerning, for example, dyna-mic substructuring or the recalibration of finite element calculation...