CUSHION VIBRATION TESTING

COMPARING SINE VS. RANDOM VIBRATION EXCITATION OF DIFFERENT

SPRING-MASS MODELS

PREPARED BY

CHRISTINA L. WONG AND

HERBERT H. SCHUENEMAN, CP-P/MH

FEBRUARY, 1997

PRESENTED BY

WESTPAK, INC. 83 Great Oaks Blvd., San Jose, CA 95119

(408) 224-1300 FAX (408) 224-5113 www.westpak.com

________________________________________________

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

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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

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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)

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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)

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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

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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

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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

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Figure 10

Compression-Compression Model 2” (5 cm) Thick Polyethylene Foam

Sine and Random Resonant Frequencies

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Compression-Tensile Model Using Sine vs. Random - This model consists of a plate securely fastened to the table of a vibration test machine. On the plate is mounted an 8” x 8” x 2” (20 x 20 x 5 cm) thick cushion material while another plate with two rods attached is placed on top. This latter platen is used to secure additional weights to achieve the necessary static stress loading, which range from 0.2 to 2.0 lb/in2 (14 to 141 g/cm2). The response accelerometer is located at the center of this platen. This model is shown graphically in Figure 11 and a photo of the setup is shown in Figure 12.

Figure 11

Tension-Compression Cushion Vibration Test Setup

Figure 12 Test Fixture Photo T-C Model

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In this configuration, the two cushion materials were subjected to the same test profile as in the C-C model. Sinusoidal vibration tests were conducted using a sweep rate of 1 octave per minute over the frequency band of 5-200 Hz at an amplitude of 0.5 G. For each static stress loading, transmissibility graphs were plotted displaying the resonant frequencies. The same polyethylene and polyurethane cushion materials were used during this testing. This process was repeated using random vibration input as described earlier. The data is presented graphically in Figures 13 and 14.

Figure 13

Tension-Compression Model 2” (5 cm) Thick Polyethylene Foam

Sine and Random Resonant Frequencies

Figure 14

Tension-Compression Model 2” (5 cm) Thick Polyurethane Foam

Sine and Random Resonant Frequencies

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Protective Package System - After collecting the data generated from both C-C and C-T models, a resonant search test was conducted using random vibration input on a bare product, which in this case was computer processing unit. (Both sine and random excitations of products are allowed by ASTM D35805.) A number of components located within the unit were monitored with a response accelerometer in order to determine the frequency at which excitation would occur. This process was repeated in each axis of the product. Plots representing the lowest natural frequencies in each axis are shown in Figures 15, 16, and 17.

Figure 16

Lowest Product Natural Frequency in Y-Axis

Figure 15

Lowest Product Natural Frequency in Z-Axis

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Once establshowswere t

Figure 17 Lowest Product Natural Frequency in X-Axis

product resonances were mapped, the lowest product resonance was used to ish the appropriate static loading to cushion the product during transit. Table 1 a summary of this data. A design was fabricated and the two cushion materials ested in the package system.

Table 1 Summary of Product Resonant Frequency Data

and Cushion Target Values

Load Bridging Treatment

Load Bridging Ratios Pallet Stiffness Levels

Low Medium High 1 0.421 0.837 0.832 2 0.651 0.937 0.888 3 0.739 0.962 0.925 4 0.899 0.970 0.925 5 1.000 1.000 1.000

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Test Results - The most striking result to occur during the testing was the similarity in the response of the both cushion materials to sine and random vibration input using the C-C model. As Table 2 shows, the peak acceleration response at the natural frequency of the cushion was nearly identical to the two different cushion materials. Predicted versus actual data is shown in Figure 18 for the polyethylene foam material and in Figure 19 for the polyurethane foam (C-C model).

Table 2 Summary of Cushioned Product Vibration Response Data C-C Model

Product Orientation

Loading lb/in2

Predicted Value from

Cushion Test Hz

Target Resonance Hz

Package Resonance Hz

POLYETHYLENE FOAM Base down 2.0 19 14 17 Side down 1.3 27 19 18 Back down 1.3 27 19 18

POLYURETHANE FOAM Base down 2.1 24 14 18 Side down 2.1 24 19 19 Back down 2.1 24 19 19

Figure 18

Compression-Compression Model 2” (5 cm) Thick Polyethylene Foam

Random Vibration Excitation

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Random vrandom vibloadings. Ttest block wspring-mas(Figures 20

Figure 19 Compression-Compression Model 2” (5 cm) Thick Polyurethane Foam

Random Vibration Excitation

s. Sine - Another significant result was the lack of instability noted during ration testing of the cushion materials, especially at the higher static stress esting conducted with sinusoidal vibration input produced instability in the hile sweeping from high frequencies to low frequencies at resonance of the

s system. This instability was not present using the random vibration input and 21).

Figure 20

Package Vibration Response Using Sinusoidal Input

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Figure 21

Package Vibration Response Using Random Input

Another difference noted between the sine and random vibration procedures was the significant reduction in time necessary to generate the data using random vibration. A typical sine sweep from 5-200-5 Hz at a rate of 1 octave per minute would take approximately 20 minutes to complete once the fixture was properly set up. In contrast to this, it takes less than one minute to collect the data using the random vibration procedure even allowing for the settling of the response spectrum that normally occurs during the first few loops of data analysis. CONCLUSIONS The first conclusion that can be drawn from the data was the remarkable similarity in the results obtained from sine vs. random vibration excitation of various cushion materials using different models. This result should be expected but was gratifying to see nonetheless. It tends to validate the ability of the models to properly evaluate the vibration response of cushion materials. Equally gratifying was the lack of instability noted during vibration testing that characterized previous tests using the “roller fixture” (Figure 3). No variability based on fixture pre-loading was noted. The fixture itself had the ability to pre-load each sample in a consistent fashion for each test. The remarkable similarity of the test rests from the two different fixtures (C-C and T-C) suggests that the materials have similar tension and compression properties. The data suggests that when using materials that have similar tension and compression

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properties that data collected using the C-C model can be used in an application where tension is important such as in a floating deck system on a pallet or crate. The authors do not suggest eliminating the tension-compression model from future cushion vibration consideration. The data is much too preliminary for that conclusion. More study in this area will definitely be necessary in order to determine the proper fixture method for cushion vibration determination. However, these results are very promising and offer hope for the successful development of a consensus test standard. The results of using the data on an actual package design show the amount of variability that can be anticipated when other factors are involved in the response of a spring-mass system to vibration input. These variables include, but are not limited to, the following: 1. The friction involved in the displacement of a product within an actual package

system during vibration response. 2. The single-axis response of the mass during cushion material vibration response

testing. That is to say, the product is unguided and able to respond in all axes during the vibration testing whereas the cushion materials are restrained to one axis response only.

3. The shape of cushions used in an actual package design are not normally right

rectangular prisms as are used in the cushion material evaluation. Package cushion designs are used in the cushion material evaluation. Package cushion designs are often tapered or trapezoidal shaped. This shape factor lends another unknown to the package vibration response testing.

4. Column buckling will have an effect when cushions become relatively tall and

narrow relative to their foot print. This will tend to lower the natural frequency of a given spring-mass system.

In summary, we believe that our research has shown the following: • Random vibration can be an effective tool in determining the vibration performance

of cushion materials. • The design of the fixture is important, but not critical, in determining vibration

properties. Top loading variances can be eliminated with a properly designed fixture and careful test setup.

• The data generated by random vibration excitation of cushion materials in the C-C

model fixture can be used to effectively design a good vibration damping package system with reasonable accuracy.

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References: 1. American Society for Testing and Materials, Committee D-10 on Packaging, West

Conshohocken, PA 2. MIL-HDBK 304B, "Package Cushioning Design", Packaging Evaluation Agency

(AFALD/PTPT), Wright Patterson AFB, OH. 31 Oct. 1978 3. Personal conversation with D. E. Young, then the laboratory manager for Lansmont

Corporation, Charlotte, Michigan 4. ASTM D1596-91, Standard Test Method for Dynamic Shock Cushioning

Characteristics of Packaging Material, American Society for Testing and Materials, West Conshohocken, PA Vol 15.09, 1996

5. ASTM D3580-95, Standard Test Methods for Vibration (Vertical Linear Motion) Test

of Products, American Society for Testing and Materials, West Conshohocken, PA Vol 15.09, 1996

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