Modal test data adjustment for interface compliance

Abstract

Modal tests are ideally performed in configurations that represent boundary conditions that are both appropriate (facilitate application of analytical methods) and easily modeled (fixed, free, or interfaced to hardware of known stiffness). Due to the constraints of the Ares 1-X program, test fixtures that would guarantee an idealized boundary were unavailable for the desired subsystem test configurations (cantilevered). The decision was made to test subsystems in a configuration as near cantilevered as could be achieved within program constraints. It was decided to test Super Stack 1 (SS 1) and Super Stack 5 (SS5) - the two tested subsystems - resting on the test facility floor. In addition, the configuration for SS5 would include non-flight transportation / access hardware between it and the floor. Since the facility floor and non-flight hardware were of unknown stiffness, analytical methods were developed to address the effects of this compliance on the measured data. A method was developed to identify interface motion from test measurements, judge when interface motion would affect test results significantly, and adjust the test results to represent those of a fixed-base configuration. ©2010 Society for Experimental Mechanics Inc.