ASTM D4728 Standard Test Method for Random Vibration Testing of Shipping Containers
The scope of ASTM D4728 is to cover the random vibration testing of filled shipping units. It can be used to test how a container performs in two ways.
- Ruggedness- the test assesses the interior packing and means of closure
- Protection- assesses if the container provides adequate protection to its cargo when exposed to random vibrations
ASTM D4728 is important because shipping containers are often exposed to many stresses throughout the distribution process. This test will mimic some of those stresses. After undergoing random vibration testing, the container can be assessed based on damage, or lack thereof. Random vibration testing has advantages compared to sinusoidal resonance dwell or sweep tests. For example, the resonance buildups that occur during random vibration tests are not as intense. As a result, unrealistic fatigue damage is minimized during random vibration testing as opposed to other types of testing.
Random vibration tests should be performed based on field data so that they are an accurate representation of shipping stresses. Testing laboratories may use actual field shipment effects to improve their confidence levels for simulated testing. Vibrations will affect the shipping container itself, its interior packaging, means of closure, and its contents. The test will allow each component to be analyzed. If the performance of any element is lacking, design modifications can be made. Modifications will help ensure that all the components of the shipping container are performing well during random vibrations. The test can be performed on any axis or orientation. However, different test levels may be used for other axis or orientations to accurately simulate field conditions of that container position.
ASTM D4728 requires four testing apparatuses.
- Vibration Test System- Also known as the shaker, the vibration test system will have a table that can support the entire test specimen (shipping container). The table should also be capable of distributing the vibrations equally across the entire test surface.
- Electronic Controls- The controls will generate the vibrations. They must be capable of producing the desired power spectral density at the table surface that is supporting the specimen.
- Closed Loop-Automatic Equalization- A closed loop controller is required for this type of testing. This means that the controller automatically generates equalized drive signals. The operator will enter the desired PSD data and the equalized vibrations will achieve the desired PSD which will maintain the closed loop control.
- Instrumentation- Instrumentation devices such as accelerometers, signal conditioners, analyzers, data display, storage devices, and control techniques are required to measure PSD at the table surface. Instrumentation may also be beneficial in measuring the response of the test specimen throughout the test. Any instrument used should be accurate +/- 5 % across the frequency range specified.
The procedure for ASTM D4728 begins with conditioning if necessary. Conditioning will be done in accordance with requirements of the applicable specification. In the event that there aren’t conditioning requirements, it is recommended to condition if the container materials are climatically sensitive. To begin the test, the specimen shill be placed on the vibration table in its normal shipping orientation. The specimen should be secured to the table or have restraining devices that will prevent the container from moving off the table. The specimen should be able to move about 10mm (0.4in) in any direction when restrained. Since random vibrations produce large but low frequency displacements in an unpredictable sequence, it is important that the vibration levels do not overshoot the PSD profile on start up. The test should be initiated below the desired PSD and increase by a specified increment until the desired test level is reached. Once the desired level is reached, then the test duration begins. Startup/build up time is not included in the test duration. The duration should be based on either the requirement of test specifications, or a predetermined period/ amount of observable damage. Finally, after testing is completed the testing laboratory will compile a comprehensive report based on the guidelines of ASTM D4728.
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