Mechanical Shock Testing

The Qualtest EMI Test Facility has expanded from 4,100 to 8,400 square feet! Two shielded EMI Test Enclosures were also added along with more equipment and test personnel to support the increased demand for this important area of testing. In addition, test capability was expanded to include DOD-STD-1399 Section 070, which uses a Helmholtz coil to generate a DC Magnetic Field of 1600 A/m. Our Helmholtz coil has a test area of 50 cm3 but larger test items can be accommodated by using multiple position exposures. 

Shocks along with impulse or constant loads are some of the common environments experienced by all equipment installed in aircraft. The tests described by Section 7 of the DO-160 standard apply shocks or constant loads to the equipment under test in order to simulate events encountered as part of normal aircraft operations. When applied in the prescribed manner, the operational shock tests of Section 7 verify that equipment will continue to function within specified standards during events such as wind gusts, landing or taxiing. Crash safety tests verify that equipment will not present a hazard to personnel by detaching from its mounting or separating into projectiles during an emergency landing. These tests can be divided into three general types:

• Operational Shock
• Crash Safety (Impulse)
• Crash Safety (Sustained)

The specific parameters and type of test to be applied is determined according to the equipment category. Four categories are defined in Section 7 (A, B, D & E). The category selection for equipment is usually chosen by the prime contractor or specifying authority who has knowledge of the ultimate installation location and aircraft type(s) and should be stated in the relevant equipment specification.  The category chosen will basically determine whether the standard or low-frequency variant of either operational shocks or crash safety will apply. As with any shock or acceleration test, a rigid fixture simulating the in-service installation and allowing easy attachment to the test equipment mounting pattern will help to insure a compliant and smooth flowing test sequence.

For the Operational Shock test, a terminal peak sawtooth shock pulse is applied to the equipment under test (EUT) with am amplitude of 6 g’s peak and a nominal duration of 11, 20 or 100 mS according to the defined equipment category. Equipment is normally tested in an operating or power-applied state. If planning to perform this test on a vibration exciter, there are two considerations. First, the required velocity and displacement will increase substantially with the longer duration (low-frequency) shocks. Secondly, the effects of any applied pre and post compensation may alter the temporal and spectral characteristics of the reference pulse. These effects are best illustrated by viewing the pseudo velocity shock response spectrum (SRS) associated with the compensated pulse. DO-160 also allows for applying an equivalent SRS to replace the terminal peak sawtooth pulse. Since these shocks are to be applied to each direction of each orthogonal axis, a good illustration of the axis definition for the EUT provided in the test procedure will help insure that the testing flows smoothly as each axis is completed.

Crash safety (if applicable) is performed using both the impulse and sustained procedures. For the impulse procedure, the amplitude for the terminal peak sawtooth pulse is specified as 20g’s peak. As with the operational shock procedure, the nominal duration varies according to the equipment category between 11 and 100 mS.  Whether performed on a drop table or shaker, the availability of a dummy load or mass simulator with a similar center of gravity (CG) will aid in the setup and performance of these shocks by utilizing this unit to apply shocks during the setup phase. A dummy load may also be substituted for any electro-mechanical components mounted on or within the equipment case as long as it represents the same weight and CG to the assembly.

The sustained crash safety test is normally performed using a centrifuge or sled, but in special cases it may be acceptable to simulate the inertial effects by apply equivalent forces statically through the CG of the EUT. When performing the test using a centrifuge, remember that the direction of loading will be opposite to the direction of acceleration with respect to the rotational axis. Once again, a clear axis definition for the EUT will aid in the setup and performance of the test since all six directions of the aircraft mount orientation must be tested (Forward, Aft, Up, Down, Side [1], Side [2]). If the orientation of the equipment within the aircraft is not known, then the random for each of the equipment’s three orthogonal axes is defined in a table provided in Section 7. Acceleration (G’s) are calculated according to the relationship of the centrifuge swing arm radius, angular rotation (radians/sec), and the revolutions per minute (RPM) of the centrifuge. Crash safety is typically performed on non-operating equipment unless the equipment specification states otherwise. See more…!