Test Simulation of Fighter Aircraft Maneuver Load Spectra

Abstract

This paper studies the use of power spectral density (PSD) and related random data analysis techniques in the test laboratory to characterize low frequency loading spectra output response, measured from a specimen, for the purpose of comparing test machine responses. In recent years digital techniques have been developed that simulate aircraft maneuver spectrum loading in the time domain. In this study one of these techniques is used to generate the input load spectrum signal to an electrohydraulic closed-loop test machine for fatigue testing of graphite/epoxy to titanium step lap bonded joints at various frequencies and PSD shapes. The output signal was measured directly from the specimen by recording on tape the dynamic strain gage signal. This signal then was processed to determine the PSD response and root mean square (RMS) time history. These data were compared with the recorded strain amplitude time history and the input load amplitude, frequency, and PSD information. A qualitative correlation was found between variations in specimen fatigue life, output signal waveform and amplitude, and variations in PSD. PSD is a measure of loading energy in the time domain and can be influenced both by load amplitude and waveform variations. Thus, the way a fatigue test machine processes an input load and frequency signal can affect the measured fatigue life of certain materials.