Cushion Vibration Testing

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Cushion Vibration Testing

Transcript of Cushion Vibration Testing


2. Cushion Vibration Testing: Comparing Sine vs. Random Vibration Excitation of Different Spring-Mass Models ABSTRACT Cushion materials are used as mechanical springs in many protective package systems for shock and vibration mitigation. For larger and more fragile products, vibration characteristics of cushions are more important than shock characteristics. To design an effective vibration attenuation system, the engineer must know the vibration characteristics of the product and the cushion materials in question. This paper discusses the effect of utilizing sinusoidal and random vibration input to collect the cushion response data. Two different spring-mass models are utilized with two different cushion materials. Recommendations are offered for efficient use of the data for vibration attenuating package design. Sample packages are tested using the data and best results are analyzed. INTRODUCTION Protective package systems are often designed with shock mitigation in mind and very little else, including vibration. This is unfortunate, especially for large, heavy, and expensive products. Large products are rarely subjected to high intensity shock levels due to drops or impacts. They are, however, subjected to high intensity shock levels due to drops or impacts. They are, however, subjected to lots of vibration input because they must travel from where they are produced to where they are used. This travel always involves some level of excitation to the product in the form of mechanical vibration because the excitation comes from the vehicles in which the product travels. An improperly designed package system can amplify this vehicle-induced excitation to levels that cause disastrous damage to products in the form of fatigue, loosened components, mis-aligned assemblies, and similar results. Most cushioned crate designers concede that vibration protection of the products that go into the crates is very important. However, they rarely consider vibration because most cushion manufacturers don't furnish the data necessary for the design, and even if they did, most crate designers admit that they wouldn't be able to use the information successfully even if it were furnished. Surprisingly enough, the same song was being sung by crate designers a generation ago when cushion (impact) curves were first being developed and disseminated throughout the protective packaging community. It didn't take long for crate designers to figure out how to use the curves and to design very efficient shock mitigating systems - 2 - 3. using the information. The same thing is likely to occur with cushion vibration data once it becomes more widely available. Historically, the development of cushion vibration data using the amplification- attenuation plot (Figure 1) has been a common practice in the protective packaging business, although no formal standard exists. The ASTM D101 sub-committee, assigned to come up with a standard, floundered during the early to mid-1980's before declaring it an impossible task. Inconsistencies in the data depending on variations in top-loading were given as the reason. The only widely publicized cushion vibration data was found in MIL Handbooks 304 A and B2 . A complete procedure for data collection was also included. In addition, MIL Handbook 304 B also contained a long and well thought out explanation of exactly how to collect and use the data. Westpak has experienced substantial success over the past several years using a very simple fixture to develop data in the compression-compression model as well as the compression-tensile model (Figure 2). In addition, the use of random vibration rather than sinusoidal input has made the data easier to collect and more accurate to use. The effect of using various models as well as different types of vibration input (sine vs. random) will be discussed. A design is given for a simplified fixture for the C-C and T-C models. Finally, the effect of sinusoidal vs. random vibration input on the data and its accuracy will be explored. Figure 1 Static Loading - 3 - 4. BACKGROUND The first documented use of a fixture to obtain vibration information for cushion materials comes from the Lansmont Corporation in the mid-1970's3 . The data was generated on a variety of different cushion materials for the Air Force Packaging Evaluation Agency and later published as MIL Handbook 304A and the revised version 304 B2 . The fixture utilized an enclosed test block with rollers designed to guide it in the x and y axes while leaving it free to move in the z axis. Cushion samples were restrained above and below the enclosed test block (Figure 3). Figure 2 Typical Cushion Vibration Test Setups Figure 3 Roller Block Cushion Vibration Fixture (Source 2) - 4 - 5. The cushion materials were stressed in a compression-compression manner. When the responsible ASTM D10 sub-committee attempted to standardize this procedure, they found widely differing results based on slight differences in the top-load applied by the fixture. This appeared to be true especially for closed cell cushion materials, but would also apply to a lesser degree to open cell materials. As the top-load increased slightly, the results would be a marked increase in the natural frequency of the system. Sinusoidal input was used typically at 0.5 G 0-to-peak amplitude swept over the frequency band of 3-200 Hz. The resulting transmissibility plots (Figure 4) were assimilated into an amplification-attenuation plot (Figure 5) in order to have the data in a format that could be utilized for package design purposes. This format showed natural frequency of the cushion material on the vertical axis and static stress loading of the cushion on the horizontal axis. These two variables, one dependent and one independent, are the ones that can be utilized most effectively in package cushion design for vibration attenuation. Figure 4 Multiple Transmissibility Plots (Different Cushion Loadings) - 5 - 6. Figure 5 Amplification-Attenuation Plot The data is utilized by identifying the lowest natural or resonant frequency of the product and then drawing a horizontal line across the amplification-attenuation plot. The intersection of this horizontal line with the attenuation zone for the cushion material identified the loading that would provide vibration attenuation at that frequency (Figure 6). Figure 6 Cushion Vibration Design from Amplification-Attenuation Plot 2 (5 cm) Thick Cushion - 6 - 7. PROTECTIVE PACKAGE CUSHION VIBRATION DESIGN The lowest product natural frequency of the product was of greatest concern because higher natural frequencies would be placed further into the attenuation zone based on the slope of the plot. Stiffer cushion materials normally had amplification-attenuation plots that would be of a relatively high frequency nature. EPS (expanded polystyrene) foam and some densities of polypropylene foam material were typical of this situation. Using these materials, the highest natural frequencies were identified and an attempt was made to place the loading in the one-to-one zone such that the cushion material would not amplify at product natural frequencies. This was not as good a scenario as loading the cushion material in the attenuation zone but at least it would avoid amplification of the vibration input through the cushions at the product natural frequencies. Compression-Compression Model Using Sine vs. Random - To conduct this investigation, a new fixture was designed which utilizes the 8-inch (20 cm) cube enclosed test block described in ASTM D41683 . This block is free floating in a plywood tube that is fastened to the table of the vibration test machine. One cushion sample is placed below the test block and another above it; the response accelerometer is located inside the test block. On top of the top cushion a fixture is placed which has the ability to be rigidly fixed to the vibration table. The setup is schematically shown in Figure 7 and a photo of the actual test device is shown in Figure 8. Figure 7 Cushion Vibration Test Fixture, C-C Model Figure 8 Test Fixture Photo C-C Model - 7 - 8. In this configuration, various cushion materials were subjected to sinusoidal sweep tests conducted over the frequency band of 5-200 Hz at a rate of 1 octave per minute while the input acceleration level was held constant at 0.5 G. Transmissibility plots were drawn for each static stress loading which ranged from 0.2 to 2.0 lb/in2 (14 to 141 g/cm2 ). The materials utilized included 2-inch (5 cm) thick polyethylene foam laminate material, 1.7 lb/ft3 (.037 g/cc) density, as well as 2-inch (5 cm) thick open cell polyurethane foam material, 2.6 lb.ft3 (.042 g/cc) density. The vibration testing was repeated on each of the samples using a random vibration input. The spectrum of the input covered the frequency band of approximately 5-200 Hz at an amplitude level of approximately .5 G rms. The same cushion materials were utilized in the same fixture over the same static stress loading range. Once collected, the data was plotted and overlaid on the same amplification- attenuation plot for sine vs. random treatment of the same cushion materials and thicknesses using the C-C model. The data is presented graphically for peak resonance values in Figures 9 and 10. Figure 9 Compression-Compression Model 2 (5 cm) Thick Polyurethane Foam Sine and Random Resonant Frequencies - 8 - 9. Figure 10 Compression-Compression Model 2 (5 cm) Thick Polyethylene Foam Sine and Random Resonant Frequencies - 9 - 10. Compression-Tensile Model Using Sine vs. Rando