56 D. E. Soine et al. Simple aluminum plates exhibit low damping, leading to long shock test ringdown times on the order of 150 msec or greater. To better simulate field environments, a shorter ringdown time is often desired. Damping devices such as “damping bars” and constrained layer approaches have been developed in response to this need [2]. In addition, the force pulse applied to the resonant plate by the projectile is modified by selection of the projectile mass, velocity, and “programmer material.” This activity to optimize the force input to the resonant plate is critical to adequately excite the bending mode of the plate while limiting the response at higher test assembly natural frequencies. Aerospace component qualification test specifications are tailored to these test methods and are usually specified by a simple straight-line SRS profile and a ±6 dB tolerance band. The typical SRS specification has a +12 dB/octave slope up to the “knee frequency,” where it flattens to zero slope at a specified SRS acceleration level. In the test lab, a plate bending frequency that corresponds to the knee frequency is chosen to perform the shock test [3]. Though these methods have generally proven successful for single-axis testing, there is a desire to expand resonant fixture shock techniques to provide a multi-axis response with higher fidelity to the field environment. For a resonant fixture to provide a controlled shock response in more than one axis requires a test assembly designed to have fundamental responding modes with more sophisticated, multi-axis motion [4,5]. These test assemblies must be designed and optimized via computer modeling and simulation. Sometimes, one of several responding modes will drive an undesirable response at the test article. A dynamic vibration absorber could be a design feature useful in disrupting that undesirable modal response [6]. Testing and Analysis In this investigation, the effect of a cantilevered-mass dynamic absorber on the bending mode of the resonant plate was observed. The cantilevered mass was made up of a cylindrical pedestal and square plate mass, which when assembled was known to have a fixed base natural frequency near 500 Hz and a weight of 9.4 pounds; see Figure 1. The resonant plate dimensions were 23.75 x 23.75 x 1.25 inches, with an approximate natural frequency of 500 Hz. The plate was suspended by ropes in front of an air gun, with two rods bolted to the plate to enable the plate to be leveled with cords when loaded with the dynamic absorber; see Figure 2 below. No damping bars were installed. To measure the shock event, an accelerometer was placed in the center of the plate, where a test article would ordinarily be installed for a uniaxial shock test. The cantilevered mass was bolted to the plate with precision spacers between the pedestal and the plate, since earlier investigations demonstrated that the precision spacers linearize the interface at the base of the pedestal and improve correlation with a finite element model. Setup testing was performed to select the projectile weight (35 pounds,) projectile velocity (17 feet/second,) and programmer material (two1/4 inch pieces of felt) for the rest of the testing. Fig. 1 Cylindrical pedestal and square plate.
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