4.1 Shipping containers and the interior packaging materials are used to protect their contents from the hazards encountered in handling, transportation, and storage. Shock is one of the more troublesome of these hazards. Free-fall drop testing, while easy to perform, often understresses the test specimen by subjecting it to drops which are not perpendicular to the dropping surface.
Note 1: For example, testing has shown that non-perpendicular drops, 2° off perpendicularity, result in 8 % lower acceleration into the test specimen resulting from the impact energy dispersing in several axes.4
4.1.1 Controlled shock input by shock machines provides a convenient method for evaluating the ability of shipping containers, interior packaging materials, and contents to withstand shocks. Simulated free-fall drop testing of package systems, which have critical elements, has produced good results where the frequency of the shock pulse is at least three times that of the package system's natural frequency.
4.2 As in most mechanical shock test procedures, fixturing of the package on the shock test machine may have significant influence on the test results. Typically, packages will be firmly held on the table by securing some type of cross member(s) across the top of the package. Care should be taken that any pressure resulting from such fixturing should be minimal, particularly when the container being tested is corrugated or some other similar material.
4.2.1 In cases where low-acceleration, long-duration responses are anticipated, any fixturing can potentially influence packaged item response and can possibly alter any correlation between this test method and free-fall drop testing. Where such correlation is desired, the package can be tested without it being fixed directly to the table. Note that in such circumstances, the shipping container can vigorously rebound from the table and can, if not otherwise controlled, present a safety problem for operators. Fixing the shipping container to the shock machine table is most often recommended for safety and convenience, but accuracy and precision of this test method should not be compromised by such fixturing.
Note 2: A rigid package system with a natural frequency above 83 Hz requires a shock pulse shorter than the 2-ms (nominal) duration currently available with many of today's shock machines:
| where: |
|
ds | = | shock pulse duration, s, |
|
fs | = | shock pulse frequency, Hz, and |
|
fp | = | package system frequency, which may be determined by Test Methods D999. |
Similarly, a shock machine using an input shock pulse duration of 3 ms would only be effective with package system frequencies below 56 Hz.
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