English
Italiano
MIL STD 810 G – Test Method 516.6 – Shock
SCOPE
Purpose
Shock tests are performed to:
provide a degree of confidence that materiel can physically and functionally withstand the relatively infrequent, non-repetitive shocks encountered in handling, transportation, and service environments. This may include an assessment of the overall materiel system integrity for safety purposes in any one or all of the handling, transportation, and service environments;
determine the materiel’s fragility level, in order that packaging may be designed to protect the materiel’s physical and functional integrity; and
test the strength of devices that attach materiel to platforms that can crash.
Application
Use this method to evaluate the physical and functional performance of materiel likely to be exposed to mechanically induced shocks in its lifetime. Such mechanical shock environments are generally limited to a frequency range not to exceed 10,000 Hz, and a time duration of not more than 1.0 second. (In most cases of mechanical shock the significant materiel response frequencies will not exceed 4,000 Hz and the duration of materiel response will not exceed 0.1 second.) The materiel response to the mechanical shock environment will, in general, be highly oscillatory, of short duration, and have a substantial initial rise time with large positive and negative peak amplitudes of about the same order of magnitude. The peak responses of materiel to mechanical shock will, in general, be enveloped by a decreasing form of exponential function in time. In general, mechanical shock applied to a complex multi-modal materiel system will cause the materiel to respond to (1) forced frequencies imposed on the materiel from the external excitation environment, and (2) the materiel’s resonant natural frequencies either during or after application of the excitation. Such response may cause:
materiel failure as a result of increased or decreased friction between parts, or general interference between parts;
changes in materiel dielectric strength, loss of insulation resistance, variations in magnetic and electrostatic field strength;
materiel electronic circuit card malfunction, electronic circuit card damage, and electronic connector failure. (On occasion, circuit card contaminants having the potential to cause short circuit may be dislodged under materiel response to shock.);
permanent mechanical deformation of the materiel as a result of overstress of materiel structural and non-structural members;
collapse of mechanical elements of the materiel as a result of the ultimate strength of the component being exceeded;
accelerated fatiguing of materials (low cycle fatigue);
potential piezoelectric activity of materials, and
materiel failure as a result of cracks in fracturing crystals, ceramics, epoxies, or glass envelopes.
Limitations
This method does not include:
The effects of shock experienced by materiel as a result of pyrotechnic device initiation. For this type of shock see Method 517.1, Pyroshock.
The effects experienced by materiel to very high level localized impact shocks, e.g., ballistic impacts. For this type of shock, devise specialized tests based on experimental data, and consult Method 522.1, Ballistic Shock.
The high impact shock effects experienced by materiel aboard a ship due to wartime service (from nuclear or conventional weapons). Consider performing shock tests for shipboard materiel in accordance with MIL-S-901 (paragraph 6.1, reference c).
The effects experienced by fuse systems. Perform shock tests for safety and operation of fuses and fuse components in accordance with MIL-STD-331 (paragraph 6.1, reference d).
The effects experienced by materiel that is subject to high pressure wave impact, e.g., pressure impact on a materiel surface as a result of firing of a gun. For this type of shock and subsequent materiel response, devise specialized tests based on experimental data and consult Method 519.6, Gunfire Shock.
The shock effects experienced by very large extended materiel, e.g., building pipe distribution systems, over which varied parts of the materiel may experience different and unrelated shock events. For this type of shock, devise specialized tests based on experimental data.
Special provisions for performing shock tests at high or low temperatures. Perform tests at room ambient temperature unless otherwise specified. Guidelines found in this section of the standard, however, may be helpful in setting up and performing shock tests at high or low temperatures.
Testing of materiel worn on or attached to humans.
Time Waveform Replication (TWR) methodology. The specifics of TWR are defined in Methods 525 and 527.
TEST PROCESS
Procedure I – Functional Shock
Step 1. Select the test conditions and calibrate the shock test apparatus as follows:
Select accelerometers and analysis techniques that meet or exceed the criteria outlined in paragraph 4.3 of paragraph 6.1, reference a.
Mount the calibration load to the shock test apparatus in a configuration similar to that of the test item. If the materiel is normally mounted on vibration/shock isolators, ensure the corresponding test item isolators are functional during the test. If the shock test apparatus input waveform is to be compensated via input/output impulse response function for waveform control, exercise care to details in the calibration configuration and the subsequent processing of the data.
Perform calibration shocks until two consecutive shock applications to the calibration load produce waveforms that meet or exceed the derived test conditions consistent with the test tolerances in paragraph 4.6.2.2 for at least the test direction of one axis.
Remove the calibration load and install the test item on the shock apparatus.
Step 2. Perform a pre-shock operational check of the test item. If the test item operates satisfactorily, proceed to Step 3. If not, resolve the problems and repeat this step.
Step 3. Subject the test item (in its operational mode) to the test shock input.
Step 4. Record necessary data to show that the shock met or exceeded desired test levels within the specified tolerances in paragraph 4.6.2.2. This includes test setup photos, test logs, and photos of actual shocks from the transient recorder or storage oscilloscope. For shock and vibration isolated assemblies inherent within the test item, make measurements and/or inspections to assure these assemblies did not impact with adjacent assemblies. If required, record the data to show that the materiel functions satisfactorily during shock.
Step 5. Perform a post test operational check on the test item. Record performance data. If the test item does not operate satisfactorily, follow the guidance in paragraph 4.3.2 for test item failure.
Step 6. Repeat Steps 2, 3, 4, and 5 two additional times for each orthogonal test axis if the SRS form of specification is used (a total of three shocks in each orthogonal axis). If the classical shock form of specification is used, subject the test item to both a positive and a negative input pulse (a total of six shocks in each orthogonal axis). If one or both of the test pulse’s time history or SRS falls outside the pulse time history tolerance or the SRS test tolerance, continue to tailor the pulses until both test tolerances are met. If both test tolerances cannot be met simultaneously, choose to satisfy the SRS test tolerance.
Step 7. Perform a post test operational check on the test item. Record performance data, document the test sequence, and see paragraph 5 for analysis of results.
Procedure II – Materiel to be Packaged
Step 1. Calibrate the shock machine as follows: Mount the calibration load to the test apparatus in a configuration similar to that of the actual test item. Use a fixture similar in shape and configuration to the shock attenuation system that will support the materiel in its shipping container. The fixture should be as rigid as possible to prevent distortion of the shock pulse input to the test item. If the test apparatus input waveform is to be compensated via input/output impulse response function, exercise care to details in the calibration configuration and the subsequent processing of the data. Perform calibration shocks until two consecutive shock applications to the calibration load reproduce waveforms that are within the test tolerance specification.
Step 2. Remove the calibration load and install the actual test item on the shock apparatus.
Step 3. Perform a pre-shock operational test of the test item. If the test item operates satisfactorily, proceed to Step 4. If not, resolve the problems and repeat this step.
Step 4. Subject the test item to the test pulse.
Step 5. Record necessary test data to include test setup photos, test logs, and photos of the actual test pulse from a transient recorder or storage oscilloscope.
Step 6. For classical trapezoidal shock waveform, repeat Steps 3, 4, and 5once in each direction for three orthogonal axes with positive and negative polarity (six shocks total). For a complex shock waveform, repeat Steps 3, 4, and 5 once in each of the three orthogonal axes (three shocks total).
Step 7. Perform a post shock operational test of the test item. See paragraph 5 for analysis of results. Document the results, including plots of the measured test response waveforms and any pre- or post-shock operational anomalies.
Procedure III – Fragility
This test is designed to build up in severity until a test item failure occurs, or a predetermined goal is reached. It may be necessary to switch axes between each shock event unless critical axes are determined prior to test. In general, all axes of importance will be tested at the same level before moving to another level. The order of test activity and the calibration requirements for each test setup should be clearly established in the test plan. It is also desirable to pre-select the steps in severity based on knowledge of the materiel item or the test environment, and document this in the test plan. Unless critical stress thresholds are analytically predicted and instrumentation used to track stress threshold build-up, there is no rational way to estimate the potential for stress threshold exceedance at the next shock input level. The following procedures, one for a classical pulse and the other for a complex transient, are written as if the test will be conducted in one axis alone. In cases where more test axes are required, modify the procedure accordingly.
Classical Pulse. This part of the procedures assumes that the classical pulse approach is being used to establish the fragility level by increasing the drop height of the test item, thereby increasing the ΔV directly. The fragility level is given in terms of the measurement variable-peak acceleration of the classical pulse.
Step 1. Mount the calibration load to the test apparatus in a configuration similar to that of the actual test item. Use a fixture similar in configuration to the interface of the shock attenuation system (if any) that will support the materiel. The fixture should be as rigid as possible to prevent distortion of the shock pulse input to the test item.
Step 2. Perform calibration shocks until two consecutive shock applications to the calibration load reproduce the waveforms that are within the specified test tolerances. If response to the calibration shock is nonlinear with respect to shock input level, other test procedures may need to be applied to establish materiel fragility levels depending upon the extent of the nonlinearity prior to reaching the “stress threshold.”
Step 3. Select a drop height low enough to assure that no damage will occur. For drop heights other than those in Table 516.6-IV, the maximum velocity change can be taken to be
ΔV = 2√2gh
Where:
ΔV = maximum test item velocity change, cm/s (in/s)
(assumes full resilient rebound of test item)
h = drop height, cm (in)
g = acceleration of gravity 981 cm/s2 (386 in/s2)
Step 4. Mount the test item in the fixture. Perform an operational check and document the pre-test condition. If the test item operates satisfactorily, proceed to Step 5. If not, resolve the problems and repeat this step.
Step 5. Perform the shock test at the selected level, and examine the recorded data to assure the test is within tolerance.
Step 6. Visually examine and operationally check the test item to determine if damage has occurred. If the test item does not operate satisfactorily, follow the guidance in paragraph 4.3.2 for test item failure.
Step 7. If it is required to determine the fragility of the test item in more than one axis, proceed to test the item (Steps 4-6) in the other axes (before changing the drop height).
Step 8. If the test item integrity is preserved, select the next drop height.
Step 9. Repeat Steps 4 through 8 until the test objectives have been met.
Step 10. Perform a post shock operational test of the test item. See paragraph 5 for analysis of results. Document the results, including plots of the measured test response waveforms, and any pre- or post-shock operational anomalies.
Synthesized Pulse. This part of the procedure assumes that the fragility level is some function of the peak acceleration level determined from a maximax acceleration SRS of a complex transient. For a complex transient specified in the time domain, this procedure could use the peak acceleration of the time history to define the fragility level.
Step 1. Mount the calibration load to the test apparatus in a configuration similar to that of the actual test item. Use a fixture similar in configuration to the interface of the shock attenuation system (if any) that will support the materiel. The fixture should be as rigid as possible to prevent distortion of the shock pulse input to the test item.
Step 2. Perform calibration shocks until two consecutive shock applications to the calibration load reproduce maximax acceleration SRS that are within the specified test tolerances. If response to the calibration shock is nonlinear with respect to shock input level, other test procedures may need to be applied to establish materiel fragility levels, depending upon the extent of the nonlinearity prior to reaching the “stress threshold.”
Step 3. Select a peak maximax acceleration SRS level low enough to assure no damage will occur.
Step 4. Mount the test item in the fixture. Inspect and operationally test the item to document the pre-test condition. If the test item operates satisfactorily, proceed to Step 5. If not, resolve the problems and repeat this step.
Step 5. Perform the shock test at the selected level, and examine the recorded data to assure the test maximax acceleration SRS is within tolerance.
Step 6. Visually examine and operationally check the test item to determine if damage has occurred. If so, follow the guidance in paragraph 4.3.2 for test item failure.
Step 7. If it is required to determine the fragility of the test item in more than one axis, proceed to test the item in the other axes (before changing the peak maximax acceleration SRS level).
Step 8. If the test item integrity is preserved, select the next predetermined peak maximax acceleration SRS level.
Step 9. Repeat Steps 5 through 8 until the test objectives have been met.
Step 10. Perform a post shock operational test of the test item. See paragraph 5 for analysis of results. Document the results, including plots of the measured test response waveforms and any pre- or post-shock operational anomalies.
Procedure IV – Transit Drop
Step 1. After performing a visual inspection and operational check for baseline data, install the test item in its transit or combination case as prepared for field use (if measurement information is to be obtained, install and calibrate such instrumentation in this Step). If the test item operates satisfactorily, proceed to Step 2. If not, resolve the problems and repeat this step.
Step 2. From paragraph 4.6.5.1 and Table 516.6-VI, determine the height of the drops to be performed, the number of drops per test item, and the drop surface.
Step 3. Perform the required drops using the apparatus and requirements of paragraphs 4.6.5 and 4.6.5.1 and Table 516.6-VI notes. Recommend visually and/or operationally checking the test item periodically during the drop test to simplify any follow-on evaluation that may be required. If any degradation is noted, see paragraph 4.3.2.
Step 4. Document the impact point or surface for each drop and any obvious damage.
Step 5. Following completion of the required drops, visually examine the test item(s), and document the results.
Step 6. Conduct an operational checkout in accordance with the approved test plan. See paragraph 5 for analysis of results.
Step 7. Document the results for comparison with data obtained in Step 1, above.
Procedure V – Crash Hazard Shock Test
Step 1. Secure the test item mount to the shock apparatus by its in-service mounting configuration. Use a test item that is dynamically similar to the materiel, or a mechanically equivalent mockup. If a mockup is used, it will represent the same hazard potential, mass, center of mass, and mass moments about the attachment points as the materiel being simulated. (If measurement information is to be collected, mount and calibrate the instrumentation.)
Step 2. Perform two shocks in each direction (as determined in paragraph 2.3.3) along three orthogonal axes of the test item for a maximum of 12 shocks.
Step 3. Perform a physical inspection of the test setup. Operation of the test item is not required.
Step 4. Document the results of the physical inspection, including an assessment of potential hazards created by either materiel breakage or structural deformation, or both. Process any measurement data according to the maximax acceleration SRS or the pseudovelocity SRS.
Procedure VI – Bench Handling
Step 1. Following an operational and physical checkout, configure the item as it would be for servicing, e.g., with the chassis and front panel assembly removed from its enclosure. If the test item operates satisfactorily, proceed to Step 2. If not, resolve the problems and repeat this Step. Position the test item as it would be for servicing. Generally, the test item will be non-operational during the test.
Step 2. Using one edge as a pivot, lift the opposite edge of the chassis until one of the following conditions occurs (whichever occurs first).
The lifted edge of the chassis has been raised 100 mm (4 in) above the horizontal bench top.
The chassis forms an angle of 45° with the horizontal bench top.
The lifted edge of the chassis is just below the point of perfect balance. Let the chassis drop back freely to the horizontal bench top. Repeat using other practical edges of the same horizontal face as pivot points, for a total of four drops.
Step 3. Repeat Step 2 with the test item resting on other faces until it has been dropped for a total of four times on each face on which the test item could be placed practically during servicing.
Step 4. Visually inspect the test item.
Step 5. Document the results.
Step 6. Operate the test item in accordance with the approved test plan. See paragraph 5 for analysis of results.
Step 7. Document the results for comparison with data obtained in Step 1, above.
Procedure VII – Pendulum Impact
Step 1. If required, perform a pretest operational checkout in accordance with the test plan. Install accelerometers and other sensors on the test item, as required.
Step 2. Place the test item on the platform with the surface that is to be impacted projecting beyond the front end of the platform so that the specimen just touches the vertical surface of the bumper.
Step 3. Pull back the platform so that the center of gravity of the pack is raised to the prescribed height, and then release it to swing freely so that the surface of the container impacts against the bumper. Unless otherwise specified, the vertical height is a drop of 23cm (9 in) that results in a velocity of 214cm/sec (7 ft/sec) at impact.
Step 4. Examine the test item and record obvious damage. If the container is undamaged, rotate it 180 degrees and repeat Step 3. When the test is conducted to determine satisfactory performance of a container or pack, and unless otherwise specified, subject each test item to one impact to each side and each end that has a horizontal dimension of less than 3m (9.8 ft).
Step 5. Record any changes or breaks in the container, such as apparent racking, nail pull, or broken parts, and their locations. Carefully examine the packing (blocks, braces, cushions, or other devices) and the contents, and record their condition. If required, perform a post test operational checkout in accordance with the test plan. See paragraph 5 for analysis of results.
Procedure VIII – Catapult Launch/Arrested Landing
Step 1. Mount the test item to its shock/vibration fixture on the shock device for the first test axis.
Step 2. Attach instrumentation as required in the approved test plan.
Step 3. Conduct an operational checkout and visual examination in accordance with the approved test plan. If the test item operates satisfactorily, proceed to Step 4. If not, resolve the problems and repeat this step.
Step 4.
If no measured field data are available, apply short transient sine waves of several cycles to the test item in the first test axis. (Each short transient sine wave of several cycles represents a single catapult or arrested landing event.) Follow each burst by a rest period to prevent unrepresentative effects. Operate the test item in its appropriate operational mode while bursts are applied. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
If measured field data are available, either apply the measured response data under exciter system wave form control (see Method 519.6, Annex A), or process the catapult as two shocks separated by a transient vibration, and the arrested landing as a shock followed by a transient vibration.
Step 5. If the test item has not malfunctioned during testing, conduct an operational checkout and visual examination in accordance with the approved test plan. If a failure has occurred, it may be desirable to perform a thorough visual examination before proceeding with the operational checkout to avoid initiating additional hardware damage. When a failure occurs, consider the nature of the failure and corrective action along with the purpose of the test (engineering information or contractual compliance) in determining whether to restart the test or to continue from the point of interruption. If the test item does not operate satisfactorily, follow the guidance in paragraph 4.3.2 for test item failure.
Step 6. Repeat Steps 1 through 5 for the second test axis.
Step 7. Document the test results including amplitude time history plots, and notes of any test item operational or structural degradation. See paragraph 5 for analysis of results.
NOTE: Tailoring is essential. Please, ask to your confidence laboratory for further details about tailoring of test methods.