Aerospace systems and components are designed and qualified against several operational environments. Some of these environments are climatic, mechanical, and electrical in nature. Traditionally, mechanical test specifications are derived with the goal of qualifying a system or component to a suite of independent mechanical environments in series. True operational environments, however, are composed of complex, combined events. This work examines the effect of combined mechanical shock and vibration environments on response of a dynamic system. Responses under combined environments are compared to those under single environments, and the adequacy/limitations of conventional, single environment test approaches (shock only or vibration only) will be assessed. Test integration strategies for combined shock and vibration environments are also discussed.
Qualification of products to their vibration and shock requirements in a laboratory setting consists of two basic steps. The first is the quantification of the product's mechanical environment in the field. The second is the process of testing the product in the laboratory to ensure it is robust enough to survive the field environment. The latter part is the subject of the “Boundary Condition for Component Qualification” challenge problem. This paper describes the challenges in determining the appropriate boundary conditions and input stimulus required to qualify the product. This paper also describes the step sand analyses that were taken to design a set of hardware that demonstrates the issue and can be used by round robin challenge participants to investigate the problem.
Vibration and shock qualification testing of components can be an expensive and time-consuming process. If the component is small, often two or more units can be mounted on a fixture and tested simultaneously to reduce test time. There is an inherent danger in simultaneously testing two or more identical components as the fundamental natural frequencies and mode shapes of the individual components will be nearly identical with some slight variation due to manufacturing variability. Testing in this manner can create a situation where closely spaced vibration modes produce unwanted interference between the two units under test. This phenomenon could result in a case where one unit is over-tested while the other is under-tested. This paper presents some experimental results from simultaneously testing pairs of components which show distinct interference between the units. Some analysis will also be presented showing how variations in the components can alter the intended test response, potentially impacting component qualification.
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.
Qualification of complex systems typically involves testing the components individually in shock and vibration environments before assembling them into the system. When the components are secured to a fixture on the shaker table, the mechanical impedance of the boundary condition is quite different from that of the next level of assembly. Thus the modes of the component under test are not excited in the same way that they are excited in the system using the typical methods for defining input specifications. Here, the boundary condition impedance is investigated and quantified using substructuring techniques. Also, fixture inputs are derived to overcome the impedance differences and excite a component in the same way it is excited in the next level of assembly.