31 Energy Dissipation of a System with Foam to Metal Interfaces 325 31.2.2 Solid Mass Details Three solid masses of different diameters are used in the experiments to determine the effects of snugness of fit on the frequency and energy dissipation characteristics of the system. The nominal, or no gap, specimen has a diameter of 7.62 cm, so that it fits snugly in the foam specimens. The 1.5875 mm gap specimen has a diameter of 7.46 cm so that there is a gap between the solid mass and the foam. The 3.175 mm gap specimen has a diameter of 7.30 cm. All three solid masses are 10 cm in length and are made from 6061-T6 Aluminum. They have a recessed area in the top that can accommodate a triaxial accelerometer. 31.2.3 Specimen Assembly The solid mass is placed in both halves of the foam cups, with markings on the cups being lined up to help limit the variability in alignment from assembly to assembly. Then, the cups are placed in the can, with markings on the cups and the can being aligned to help with the repeatability of assembly. The steel plate is placed on top of the foam and solid mass assembly in the can, after which a load cell is placed on top of the assembly. A preload is applied using a press until the reading on the load cell is approximately the nominal preload. The retaining ring is tightened to maintain the preload, the press is released, and Ministack is ready for testing. 31.2.4 Test Specifications and Data Collection During testing, Ministack is oriented so that the plate on the bottom is flat on the shaker. In this orientation the specimen is being excited in the direction of the load path. The energy dissipation comes from the foam rubbing against the metal interface as well as the interface between the two sets of foam for the cases when there is contact. A constant acceleration sine sweep base excitation is run from 500 to 3000 Hz at a rate of 3 octaves per minute (an octave represents a doubling of frequency). The sweep rate allows for the test article to experience several cycles at each frequency, allowing for sufficient data at each frequency to estimate the natural frequency and energy dissipation. Four different amplitudes, 1 g, 2 g, 5 g and 10 g, are used to determine the effects of excitation amplitude on the response of the test article. A triaxial accelerometer measures data in the three mutually perpendicular directions at the top of the solid mass. A uniaxial accelerometer attached to the baseplate is used to control the input to the structure. The data measured at the triaxial accelerometer at the top of the solid mass and the control accelerometer are used to calculate transfer functions between the responses of the three different axes at the top of the solid mass and the input accelerations. The maximum amplitude of the transfer function gives Q, the amplification factor of the input at the natural frequency. The amplification factor, Q, can then be used to calculate the energy dissipation of Ministack using Eq. 31.1 derived in [15]. Q A2 b f 2 n (31.1) Where fn is the natural frequency of the first axial mode in Hz, Ab is the amplitude of the excitation in g’s and Q is the amplification factor of the response. A configuration is comprised of one set of the two lengths of foam cups and one of the three solid masses. A test series was run for all possible combinations. Between series of tests on each configuration, the setup was disassembled and then reassembled. This reassembly allows for determination of the assembly to assembly variation of the response of the test article. The test sequences are enumerated in Table 31.1. In all assemblies in Table 31.1, the sequence of tests was always repeated to determine if vibrating the specimen changed the dynamic characteristics. The final test assembly 3 was run to see if the order in which the amplitudes were applied affected the dynamics.
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