Some Non-conventional Boundary Conditions (From Marshmallows to Plungers: Who Would Have Guessed)

Abstract

When performing a modal test, a number of mechanisms are available to approximate the free-free boundary condition. Some test setups may be more difficult to implement than others, but ultimately the effect of the boundary condition on the actual modes of the system are of concern. An expensive setup may not be any better than an equivalent, inexpensive “Rube Goldberg” approach. In lieu of an expensive setup, a low cost, easily deployed configuration may be improvised from common objects not native to the test lab.

In this paper, several different boundary conditions are employed with two being very, very unconventional. The effects of the supports on the flexible modes are explored through testing with multiple support configurations. Natural frequencies and mode shapes of the modes (flexible and rigid body modes) are presented. Drive point frequency response functions for the different support configurations are compared. Some other considerations are discussed relative to the general items of concern when performing these types of free-free modal tests.