Quantification of Dynamic Differences Between Boundary Conditions for Environment Specification Improvement

Abstract

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.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Appendix 1: Additional Model Details

The free modes of the models up to 2000 Hz are shown in the following figures. The free modes of the truth system (including the component) are shown in Fig. 11.12, and the free modes of the subassembly without the component are shown in Fig. 11.13. The free modes of the component in the fixture are shown in Fig. 11.14, and the free modes of the fixture alone are shown in Fig. 11.15. The free modes of the component itself are shown in Fig. 11.16.

Fig. 11.12

Free modes of system (with component)

Fig. 11.13

Free modes of subassembly (without component)

Fig. 11.14

Free modes of component in fixture

Fig. 11.15

Free modes of fixture

Fig. 11.16

Free modes of component

As noted in Sects. 11.3.2 and 11.3.3, the contribution of component response from each subassembly/fixture mode was calculated. The contributions from Sect. 11.3.2, where the input frequency was close to a subassembly mode, are shown in Fig. 11.17 for the subassembly, Fig. 11.18 for the fixture with the original input, and Fig. 11.19 for the fixture with the modified input. The contributions from Sect. 11.3.3, where the input frequency was close to a subassembly mode, are shown in Fig. 11.20 for the subassembly, Fig. 11.21 for the fixture with the original input, and Fig. 11.22 for the fixture with the modified input. Note that for both cases, the figure showing the subassembly contributions has the same y-scale as the figure showing the fixture contributions with the modified input, however the figure showing the fixture contributions with the original input has a significantly different y-scale.

Fig. 11.17

Excitation of component from each subassembly mode with excitation frequency near a subassembly mode

Fig. 11.18

Excitation of component from each fixture mode (original input) with excitation frequency near a subassembly mode

Fig. 11.19

Excitation of component from each fixture mode (mod. input) with excitation frequency near a subassembly mode

Fig. 11.20

Excitation of component from each subassembly mode with excitation frequency near a fixture mode

Fig. 11.21

Excitation of component from each fixture mode (original input) with excitation frequency near a fixture mode

Fig. 11.22

Excitation of component from each fixture mode (mod. input) with excitation frequency near a fixture mode