The purpose of mechanical environment testing is to prove that designs can withstand the loads imparted on them under operating conditions. This is dependent not only on the test article construction but also on the loads imparted through its boundary conditions. Current practices develop environment test specifications from field responses using a single degree of freedom input control with no consideration for the mild to severe deviations from the field motion caused by the laboratory boundary condition. Test specifications are considered conservative with the assumption that most of the steps taken to generate them (e.g., straight-line envelopes and adding 3 dB) result in appropriately conservative specifications. However, without an accurate quantifiable measure of conservatism, designs can be easily mis-tested yielding unnecessarily high costs. Previous work showed a modal model for components excited through base-mounted fixtures to generate specifications with much lower uncertainty and with guaranteed quantifiable conservatism. The method focused on reproducing in-service modal energy in the test configuration by controlling the 6 degree-of-freedom input motion. That work generated test specifications with enough conservatism to account for unit-to-unit variability in the damping of the test article. This paper focuses on generating conservative specifications while considering resonant frequency shifts as a parameter for unit-to-unit variability.
The outline for this presentation includes: Motivation, Test hardware and loads, Modal test of RC on 6 DOF test fixture, and Analysis--develop one specification accounting for unit-to-unit variability and develop independently tailored test specifications for unit-to-unit variability.
The main point of mechanical environment testing is to prove that designs can withstand the loads imparted on them while being exposed to in-service conditions. This is dependent not only on the test article construction, but also the loads imparted through its boundary conditions. Current practices for developing environment test specification are typically based on inadequate information reduced to single input point control with large uncertainty as compared to the field environment. Yet the test specifications are considered conservative, with the assumption that most of the adjustment for uncertainty is conservatism. For base mounted components, a modal model is presented that can be used to generate specifications with much lower uncertainty and with guaranteed quantifiable conservatism. In this method, the modal energies in the fixed base modes of the article due to the in-service loads are determined. Using the fixed base modes of the test article as a basis, the test specification is derived by determining what fixture motion is required to emulate the in-service environment. The specification method accounts for frequency shifts between the in-service and test configurations. Variability in nominal test articles can be included in the derivation of the test specifications. Real hardware under in-service environment loads and in a ground test fixture and loading configuration are considered.
The Box Assembly with Removable Component (BARC) structure was developed as a challenge problem for those investigating boundary conditions and their effect on structural dynamic tests. To investigate the effects of boundary conditions on the dynamic response of the Removable Component, it was tested in three configurations, each with a different fixture and thus a different boundary condition. A “truth” configuration test with the component attached to its next-level assembly (the Box) was first performed to provide data that multi-axis tests of the component would aim to replicate. The following two tests aimed to reproduce the component responses of the first test through multi-axis testing. The first of these tests is a more “traditional” vibration test with the removable component attached to a “rigid” plate fixture. A second set of these tests replaces the fixture plate with flexible fixtures designed using topology optimization and created using additive manufacturing. These two test approaches are compared back to the truth test to determine how much improvement can be obtained in a laboratory test by using a fixture that is more representative of the compliance of the component’s assembly.
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.