Experimental modal analysis

Solid-borne mechanical vibrations result from the propagation of mechanical energy in matter in the form of waves. They can occur in any object, whatever its nature, size or mass. Mechanical structures are generally designed to maximize rigidity and minimize mass. They can be the seat of vibratory resonances, which considerably amplify their response to a given stress.

While the phenomenon of vibratory resonance is sometimes desirable – in musical instruments, for example –, it is more often than not a disturbance that needs to be controlled right from the design stage. Excessive vibration can lead to premature failure when mechanical strength criteria are exceeded, particularly in fatigue. They can also generate vibratory and/or acoustic discomfort, degrading the quality perceived by the user.

The appearance of a vibratory resonance is due to the excitation by external stresses of what is known as a vibratory mode, itself characterized by its modal parameters: natural frequency, damping ratio and modal deformation. Every mechanical structure intrinsically possesses a set of modes, theoretically infinite in number.

Numerical simulations are now widely used to predict the mechanical behavior of structures. Their aim is to guarantee, right from the design phase, that the design chosen will enable the desired specifications to be met. Modelling using the finite element method enables us to calculate the modes of a structure, as well as its response to given exciting forces. Modal analysis is a powerful tool for understanding, quantifying and controlling vibration responses.

Nevertheless, the models adopted are imperfect and incomplete, and validation by physical testing is often an essential step, particularly for the identification of damping parameters, which are often subject to significant errors in numerical predictions.

Experimental modal analysis is the process of determining the intrinsic modal parameters of a physical structure for given boundary conditions. By comparing the results with numerical predictions, the validity of the simulations can be assessed, and the model definition revised if necessary to better represent experimental reality.

Experimental modal analysis is now a mature activity, but the more complex the structure under consideration, the more demanding it becomes. Some of its aspects call for advanced concepts, such as damping coupling and the difference between complex and real modes. This article provides an in-depth description of the various theoretical and practical aspects that need to be understood and mastered to guarantee the validity of the modal parameters identified.

The general principle of experimental...