Publications

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Six degree of freedom (6-DOF) subsystem/component testing is becoming a desirable method, for field test data and the stress environment can be better replicated with this technology. Unfortunately, it is a rare occasion where a field test can be sufficiently instrumented such that the subsystem/component 6-DOF inputs can be directly derived. However, a recent field test of a Sandia National Laboratory system was instrumented sufficiently such that the input could be directly derived for a particular subsystem. This input is compared to methods for deriving 6-DOF test inputs from field data with limited instrumentation. There are four methods in this study used for deriving 6-DOF input with limited instrumentation. In addition to input comparisons, response measurements during the flight are compared to the predicted response of each input derivation method. All these methods with limited instrumentation suffer from the need to inverse the transmissibility function.

Recent advances in 6DOF testing has made 6DOF subsystem/component testing a preferred method because field environments are inherently multidimensional and can be better replicated with this technology. Unfortunately, it is rare that there is sufficient instrumentation in a field test to derive 6DOF inputs. One of the most challenging aspects of the test inputs to derive is the cross spectra. Unfortunately, if cross spectra are poorly defined, it makes executing the tests on a shaker difficult. In this study, tests were carried out using the inputs derived by four different inverse methods, as described in a companion paper. The tests were run with all 6DOF as well with just the three translational degrees of freedom. To evaluate the best way to handle the cross spectra, three different sets of tests were run: with no cross terms defined, with only the coherence defined, and with the coherence and phase defined. All of the different tests were compared using a variety of metrics to assess the efficacy of the specification methods. The drive requirements for the different methods are also compared to evaluate how the specifications affect the shaker performance. A number of the inverse methods show great promise for being able to derive inputs to a 6DOF shaker to replicate the flight environments.

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The vibration excitation mechanisms for structures in service are typically multi-directional. However, during product testing conducted in a lab setting the standard practice is to replicate these environments with three orthogonal single axis vibration tests. Recent advances in technology have made it possible to perform multi-axis simulations in the laboratory. Simultaneous multi-axis excitation can result in different stress states, rates of damage accumulation, and peak accelerations and strains than those resulting from sequential single axis testing. Accordingly, a series of experiments were run on a plate structure to investigate and quantify these differences. The experiments included single and multiple axis tests with different excitation amplitudes. The single axis tests were performed on both uniaxial and multiaxial shaker systems. The control levels, response energy, modal behavior, and peak accelerations were compared for each test condition. The data illustrates the differences between the structural response for single and multi-axis tests and enables an objective comparison between testing conducted on single and multiple axis shaker systems.

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