Nonlinearities due to Joint Friction and Clearance in a Structural Dynamic Test Fixture

Abstract

Modal survey testing of the Space Station Common Module Prototype in a test fixture designed to simulate Shuttle Orbiter constraints revealed a number of sources of nonlinearities in the measured frequency response functions. First, the test fixture original design, which utilized spherical bearing joints and an airbag suspension system for offloading the test article weight, is described in detail in order to provide a basis for understanding the test data. Several experimental and analytical lowest-frequency mode shapes are compared, and differences are pointed out. One of the basic modes predicted by the finite element model was not observed at all in the experimental data. Then the measured acceleration/force frequency response functions are examined for different shaker force amplitudes and air pressure levels in the airbag offload system. It is shown that the character of the response functions, and the mode shapes and frequencies of the Common Module, varied depending on the test parameters. Static stiffness and stick-slip measurements for the fixture-to-test-article interfaces were made to provide insight into the behavior of the bearing mechanisms. Careful examination of the experimental mode shapes, frequency responses, static stiffness curves, and bearing stick-slip data led to a determination that the spherical joint interfaces between the Common Module test article and the test fixture were not functioning as designed. Friction in some interface joints prevented relative motion between the test article and fixture that was essential for simulating Shuttle Orbiter constraints of the Common Module in its flight configuration. In other instances, clearances between the spherical bearings and the surfaces of the test article interface structures caused highly nonlinear response as a function of load at the interface. The severity of the unintended joint friction and clearances and the resulting nonlinearities made necessary a redesign of the modal test fixture in the interface regions before the flight constraints of the test article could be adequately simulated.