Environmental testing is important for devices deployed in a hostile environment. Some typical environmental tests consist of immersion, drop, shock, vibration. Drop testing can be an important test to characterize the durability of a device.

Finite Element Analysis (FEA) can be used to determine potential points of failure under drop testing and characterization of the design and material before prototypes are constructed for physical testing. Potential failure points and stress maximums can be determined to aid in the selection of an appropriate material prior to prototype construction. In Figure 3-1, stress was applied to the lower edge of the battery enclosure to identify points of highest stress. This information was then used to modify the design to reduce the amount of stress in those locations. FEA can be performed in regions of interest for drop testing to reduce the possibility of failures in the design phase before material and design decisions have been finalized. Potential problem areas are corners, edges and protrusions. Using FEA, stress can be applied to these areas for initial material and design evaluation. These high stress points are areas of interest to be inspected during drop testing for potential failures.

As with any test procedure, it is important to have well defined specifications and test procedures. Table 3-1 defines the test requirements as outlined in military standard MIL-PRF-49471B(CR). The potential defects should be categorized to determine what constitutes critical, major, and minor defects. Some defects may not constitute a failure, such as items 201,202, and 203 in the table. These items do not cause the device to function improperly or adversely affect performance.

A well defined step by step procedure is important to the success and repeatability of any conducted test. In ACI's testing of the AN/PRC-112D rechargeable battery pack, the drop test procedure outlined in MIL-PRF-49471B(CR) was used:

Each Battery shall be dropped once from a height of 30± 2 inches onto a hard surface consisting of concrete. The smallest side of the battery perpendicular to the plane of the connector face and nearest to the connector (where applicable) shall be parallel to the concrete surface and facing downward upon release, but need not to be parallel upon impact.

In case of cylindrical batteries, the axis of a cylinder shall be parallel to the concrete surface upon release. The drop test shall be performed on batteries preconditioned at 130F and -20F. Tolerances of ±5º F shall apply throughout the storage period. The batteries shall be stabilized 4 hours minimum at each temperature and dropped at the ambient condition specified in 4.3 within 10 minutes after removal from the temperature chamber.

Open circuit voltage and dimension shall be checked after each drop.

This procedure has sufficient detail, however, due to the random trajectory of the sample the test cannot be repeated exactly. If this test is repeated, numerous times, a sufficient number of critical impact zones can be tested. This process accurately represents the type of random drop impact to which the production device could be subjected.

ACI's testing of the AN/PRC-112D battery pack prototype was modified from the above to include the AN/PRC-112D radio with the battery pack mounted. The set was then dropped and inspected, according to the criteria in Table 3-1, over 5 iterations. Over the first four iterations the device passed with minor scratches and burrs, on the fifth a mounting lug on the top of the case sheared from the bonded surface. The prototype was constructed from machined ABS material and bonded together (see Figure 3-2). The lug that failed was bonded to the top surface of the battery case. In a modified prototype design the lug was bonded to the top through a machined hole, which increased the resistance to shearing stress. The tests were performed again with the modified prototype. The new prototype passed all tests. The design was subsequently modified to remove the bonding of the lug, in favor of a one piece cast.

Drop testing combined with FEA are useful for the evaluation and refinement of assemblies. Detailed specifications, tolerances and test procedures aid in the effectiveness of drop testing.