Limitations of the Power Spectral Density as an Indicator of Test Severity

Abstract

This paper presents a set of experimental data comparing repetitive shock (RS) vibration, single-axis electrodynamic (ED) vibration, and multi-axis ED vibration. It was found that multi-axis testing is more severe than single-axis testing at the same level. In addition, weaknesses were found in the RS system at low frequency. Smoothing of the data or poor line resolution was also shown to change the overall severity of a test. A poor correlation was shown between the power spectral density (PSD) and the rate of natural frequency change (RFC) over a wide frequency shift. The change in natural frequency caused the initial PSD to be an ineffective indicator of test severity. Quantification of the severity of the test profile can be accomplished through characterization of the RFC.

Semi-empirical life model of a cantilevered beam subject to random vibration

“…In order to assess the validity of the subsequent model, time to failure and natural frequency shift data was recorded for two beams subjected to stationary, Gaussian, random vibration inputs [5]. An outline of the data in Ref.…”

“…An outline of the data in Ref. [5] will be provided here. Two test specimens were built from cold rolled 1018 steel and mounted to a shaker as shown in Fig.…”

“…For exam- ple, the change in natural frequency may be plotted in the time domain as depicted in Fig. 6 [5]. Examination of WHITE-HIGH from Fig.…”

“…Assessment of the model will be done using failure data of the two cantilevered beams described earlier [5]. As noted previously, two accelerometers were mounted to the beam.…”

“…Although this is a good assumption for many CCAs, the presence of cracks in structural systems will decrease the stiffness leading to a decrease in natural frequency. The change in natural frequency is well documented in the health monitoring and prognostics as well as in multi-axial vibration testing communities [5][6][7][8]. Within these communities it has been demonstrated that many structures can withstand a large change in natural frequency before failure.…”