Mechanical Impedance Methods for Vibration Simulation

Realistic laboratory simulation of a system's field vibration environment has been of major concern to design and test engineers for many years. The usual practice of basing vibration design and test specifications on an envelope of the equipment base acceleration levels experienced in the field environment has often resulted in excessive levels of overtesting. This results from the large differences between the mechanical impedance of the vehicle structure and that of a fully equalized vibration shaker. Mechanical impedance effects occur naturally in a field environment. Including such effects in a laboratory vibration environment achieves more realistic conditions of similitude. A possible solution to the problem is the generation of design and test specifications that are based on the knowledge of both the acceleration and the forces transmitted to the equipment in the field environment.

This research points out the detrimental effects which result when impedance effects are not considered in relating test requirements with field measurements. The ways in which these effects can be considered are evaluated, and comparison of three impedance methods is accomplished based on a cumulative damage criterion. A test structure is used to simulate an equipment and support foundation system. Detailed finite element analysis is performed to aid in computation of cumulative damage totals. The results indicate that mechanical impedance methods can be effectively used to reproduce the field vibration environment in a laboratory test. The establishment of a well developed finite element computer model coupled with laboratory impedance measurements can eliminate the overtesting problems inherent with constant motion, infinite impedance testing.