1. background

The detection of an excessive level of vibration on a given piece of equipment often leads to the stopping of use of the equipment in question, to avoid its ruin. Thus, the implementation of specific solutions to limit this phenomenon must be implemented quickly to allow the installation to function properly.

Two strategies can be considered. If the problem is generic, the development of a predictive model based on the information measured is the most reliable solution, but readjustment is a tedious step, and does not allow the calculation to be adapted to the specificity of each structure. If the problem is specific to a particular hardware, we want to be able, without the model adjustment step, to predict the effects of a modification on the modal characteristics of the structure. The technique presented here is part of this second strategy.

In the majority of cases, an attempt is made to move the natural frequencies of the structure so that they are not in the excitation frequency band of the equipment concerned. The solution currently used consists in adding mass or stiffness to a previously selected location in the structure. The method for estimating the results of a structural modification described in this document was proposed by Mr. Corus as part of his thesis work [1]. It uses jointly the results of experimental measurements carried out on the initial structure and a numerical model of the modification.

The sequence of the various operators is presented. The optimization of the location and the mechanical characterization of the modification are left to the user. Two ways of doing this are presented in this document: in section 3, the method initially developed exactly reproduces the method proposed by Mr. Corus in his thesis: the structure is described on a modal basis, in generalized coordinates, and the modification on a physical basis. This technique is used in the sdll137a and sdll137b test cases. In section 4, the structure and the modification are described by their modal basis. This technique, which is simpler to implement, was developed later for carrying out a modification study on a pipe. This technique is used in the sdll137e test case, on the same example; another major advantage of the proposed implementation is that the numerical models of the structure and the modification do not need to have coincident nodes for assembly. For information, the sdll137c and sdll137d test cases are identical to versions a and b, but use the macro command CALC_ESSAI to perform the calculation.