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MIL STD 810 G – Test Method 514.6 – Vibration
SCOPE
Purpose
Vibration tests are performed to:
Develop materiel to function in and withstand the vibration exposures of a life cycle including synergistic effects of other environmental factors, materiel duty cycle, and maintenance. This method is limited to consideration of one mechanical degree-of-freedom at a time. Refer to Method 527 for further guidance on multiple exciter testing. Combine the guidance of this method with the guidance of Part One and other methods herein to account for environmental synergism.
Verify that materiel will function in and withstand the vibration exposures of a life cycle.
Application
General. Use this method for all types of materiel except as noted in MIL-STD-810G, Part One, paragraph 1.3, and as stated in paragraph 1.3 below. For combined environment tests, conduct the test in accordance with the applicable test documentation. However, use this method for determination of vibration test levels, durations, data reduction, and test procedure details.
Purpose of test. The test procedures and guidance herein are adaptable to various test purposes including development, reliability, qualification, etc. See Annex A for definitions and guidance.
Vibration life cycle. Table 514.6-I provides an overview of various life cycle situations during which some form of vibration may be encountered, along with the anticipated platform involved. Annex A provides definitions and engineering guidance useful in interpreting and applying this method. Annexes B – E provide guidance for estimating vibration levels and durations and for selection of test procedures. International Test Operations Procedure (ITOP) 1-2-601 (paragraph 6.1, reference d), , includes an assortment of specific ground vehicle and helicopter vibration data.
Manufacturing. The manufacture and acceptance testing of materiel involves vibration exposures. These exposures are not directly addressed herein. It is assumed that the manufacturing and acceptance process completed on the materiel that undergo environmental testing are the same as the process used to produce deliverable materiel. Thus, the environmental test materiel will have accumulated the same damage prior to test as delivered materiel accumulates prior to delivery. The environmental test then verifies the field life of delivered materiel. When a change is made to the manufacturing process that involves increased vibration exposure, evaluate this increased vibration exposure to ensure the field life of subsequent materiel is not shortened. An example might be pre-production materiel completely assembled in one building, whereas production units are partially assembled at one site and then transported to another site for final assembly. Changes in the manufacturing vibration environment should be evaluated with regard to the need for design and (re)qualification. (See Annex B)
Environmental Stress Screening (ESS). Many materiel items are subjected to ESS, burn-in, or other production acceptance test procedures prior to delivery to the government, and sometimes during maintenance. As in basic production processes, it is assumed that both the test units and the field units receive the same vibration exposures, so that environmental test results are valid for the field units. Where units do not necessarily receive the same exposures, such as multiple passes through ESS, apply the maximum allowable exposures to the items used for environmental test as pre-conditioning for the environmental tests. (See Annex A, paragraph 2.1.6, and Annex B, paragraph 2.3.)
Limitations
Safety testing. This method may be used to apply specific safety test requirements as coordinated with the responsible safety organization. However, vibration levels or durations for specific safety related issues are not provided or discussed.
Platform/materiel interaction. In this method, vibration requirements are generally expressed as inputs to materiel that is considered to be a rigid body with respect to the vibration exciter (platform, shaker, etc.). While this is often not true, it is an acceptable simplification for smaller materiel items. For large materiel items, it is necessary to recognize that the materiel and the exciter vibrate as a single flexible system. There is no simple rule to determine the validity of this assumption (see Annex A, paragraph 2.4). Further, proper treatment of a given materiel item may vary with platform. An example might be a galley designed for an aircraft. For the operational environment (installation on an operating aircraft), consider the galley structure as aircraft secondary structure, and design and test accordingly. Design subassemblies within the galley (e.g., coffee maker) for vibration levels based on guidance of Annex D, and tested in accordance with Procedure I. When packaged for shipment, the packaging, galley, and subassemblies are considered a single materiel item, and tested accordingly. Another example is a shelter transported to the field as a pre-assembled office, laboratory, etc. Consider the shelter as large materiel and develop accordingly. A suitable test would be the large assembly transport test of paragraph 4.4.3. Where impedance mismatch between platform/materiel and laboratory vibration exciter/test item are significantly different, force control or acceleration limiting control strategies may be required to avoid unrealistically severe vibration response (see paragraph 4.2). Control limits should be based upon field and laboratory measurements. For sensitive materiel for which over-conservative testing philosophy must not be applied, force or acceleration limiting control is an option. In certain cases in which the field measured response is well defined on a small component, the duration of the vibration is short, and then execution of the laboratory test under open loop waveform control based upon the field measured data is an option.
Environmental Stress Screening (ESS). This method does not contain guidance for selection of ESS exposures. Some discussion is in Annex A, paragraph 2.1.6, and Annex B, paragraph 2.3.
TEST PROCESS
Procedure I – General vibration
Step 1. Conduct a fixture modal survey and verify that fixture design is compliant with recommended practices, and meets any test defined requirements that may have been provided in the item-specific test plan (see paragraph 6.1, references aa, dd, and ee).
Step 2. Mount the test item to the test fixture in a manner dynamically representative of the life cycle event simulated.
Step 3. Install sufficient transducers on or near the test item/fixture/vibration exciter combination to measure vibration at the test item/fixture interface, to control the vibration exciter as required by the control strategy, and measure any other required parameters. Mount control transducer(s) as close as possible to the test item/fixture interface. Ensure that the total accuracy of the instrumentation system is sufficient to verify that vibration levels are within the tolerances of paragraph 4.2.2, and to meet additionally specified accuracy requirements.
Step 4. Conduct a test item modal survey, if required.
Step 5. Perform a visual inspection of the test setup.
Step 6. Apply low level vibration to the test item/fixture interface. If required, include other environmental stresses.
Step 7. Verify that the vibration exciter, fixture, and instrumentation system function as required.
Step 8. Apply the required vibration levels to the test item/fixture interface. Apply additional environmental stresses as required.
Step 9. Monitor vibration levels and, if applicable, test item performance continuously through the exposure. If levels shift or a failure occurs, shut down the test in accordance with the test interruption procedure (paragraph 4.3.2). Determine the reason for the shift and proceed in accordance with the test interruption recovery procedure (paragraph 4.3.2).
Step 10. When the required duration has been achieved, stop the vibration.
Step 11. If the test plan calls for additional exposures, repeat Steps 5 through 10 as required by the test plan before proceeding.
Step 12. Inspect the test item, fixture, vibration exciter, and instrumentation. If failure, wear, looseness, or other anomalies are found, proceed in accordance with the test interruption recovery procedure (paragraph 4.3.2).
Step 13. Verify that the instrumentation functions as required, and perform an operational check of the test item as required per the test plan. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
Step 14. Repeat Steps 1 through 13 for each required excitation axis.
Step 15. Remove the test item from the fixture and inspect the test item, mounting hardware, packaging, etc., for any signs of visual mechanical degradation that may have occurred during testing. See paragraph 5 for analysis of results.
Procedure II – Loose cargo transportation
Step 1. Place the test item(s) on the package tester within the restraining fences in accordance with paragraph 2.2 of Annex C.
Step 2. Install instrumentation to measure the rotational speed of the package tester. Ensure the total accuracy of the instrumentation system is sufficient to meet specified accuracy requirements.
Step 3. After determining the number of possible test item orientations and corresponding test time (paragraph 3.1d), operate the package tester for the prescribed orientation duration (Annex C, paragraph 2.2).
Step 4. Perform a visual inspection of the test item and an operational check. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure. Otherwise, proceed to Step 5.
Step 5. Reorient the test item(s) and/or the fencing/impact walls in accordance with paragraph 3.1d(1) and Annex C, paragraph 2.2b.
Step 6. Operate the package tester for the next prescribed duration.
Step 7. Perform a visual inspection of the test item and an operational check. If the test item fails to operate as intended, see paragraph 5 for analysis of results, and follow the guidance in paragraph 4.3.2 for test item failure.
Step 8. Repeat Steps 5-7 for the total number of orientations.
Step 9. Perform a final visual inspection of the test item and an operational check. See paragraph 5 for analysis of results.
Procedure III – Large assembly transport
Step 1. Mount the test item(s) on/in the test vehicle as required in the test plan.
Step 2. If required, install transducers on or near the test item sufficient to measure vibration at the test item/vehicle interface, and to measure any other required parameters. Protect transducers to prevent contact with surfaces other than the mounting surface.
Step 3. Subject the vehicle containing the test item to the specified test conditions in Annex C, paragraph 2.3, or as otherwise specified in the test plan.
Step 4. Perform a visual inspection of the test item and an operational check. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
Step 5. Repeat Steps 1 through 4 for additional test runs, test loads, or test vehicles as required by the test plan.
Step 6. Perform a final visual inspection of the test item and an operational check. See paragraph 5 for analysis of results.
Procedure IV – Assembled aircraft store captive carriage and free flight
Step 1. With the store suspended within the test chamber and the instrumentation functional, verify that the store suspension system functions as required by measuring the suspension frequencies.
Step 2. If required, conduct a test item modal survey.
Step 3. If required, place the test item in an operational mode and verify that it functions properly. Perform a visual inspection of the test setup.
Step 4. Apply low level vibration to the vibration exciter/store interface(s) to ensure the vibration exciter and instrumentation system function properly. For acceleration feedback control, use an initial input level 12 dB down from the required forward test monitor transducer spectrum. For force feedback control, use a flat force spectrum where the response at the test monitor accelerometer is at least 12 dB below the required test monitor value at all frequencies. For bending moment feedback control, use an initial input level that is 12 dB down from the required test monitor transducer spectrum.
Step 5. Adjust the vibration exciter(s) such that the test monitor transducers in the excitation axis meet the test requirements. For acceleration control, identify the test monitor transducer spectrum peaks that exceed the input spectrum by 6 dB or more (frequencies may differ fore and aft). For force feedback control, identify major peaks from the force measurements to check monitor accelerometer transfer functions. For both cases, equalize the input spectra until the identified peaks equal or exceed the required test levels. The resulting input spectra should be as smooth and continuous as possible while achieving the required peak responses. (It is not necessary to fill in valleys in the test monitor transducer spectra; however, it is not acceptable to notch out the input in these valleys.) For bending moment control raise and shape the input spectrum until it matches the required spectrum (peaks and valleys).
Step 6. When the input vibration is adjusted such that the required input response (R1) is achieved, measure the off-axis response(s) (R2, R3). Verify that off-axis response levels are within requirements using the following equations. If the result obtained from the equation is greater than the value established for the equation, reduce the input vibration level until the achieved input and off-axis response levels are less than or equal to the appropriate constant. Apply these equations at each peak separately. Use the first equation for testing that requires vibration application in two separate mutually perpendicular axes, and use the second equation for testing that requires vibration application in three separate mutually perpendicular axes. Refer to paragraph 4.2.2.4 for additional guidance.
(R1/A1 + R2/A2) ≤ 2
or
(R1/A1 + R2/A2 + R3/A3) ≤ 3
Where:
Ri = Response level in g2/Hz or (N-m)2/Hz or (in-lb)2/Hz for i = 1 – 3, and
Ai = Test requirement level in g2/Hz or (N-m)2/Hz or (in-lb)2/Hz for i = 1 – 3
For example:
For testing that requires vibration application in three, separate, mutually-perpendicular axes, and the vibration is being applied in the vertical axis, use the equation below as follows:
(R1/A1 + R2/A2 + R3/A3) ≤ 3
Where:
R1 = Vertical axis response level
A1 = Vertical axis requirement level
R2 = Horizontal axis response level
A2 = Horizontal axis requirement level
R3 = Longitudinal axis response level
A3 = Longitudinal axis requirement level
For vibration being applied in either the horizontal and longitudinal axis, repeat the above process.
3 = (R1/A1 + R2/A2 + R3/A3)
Where:
R1 = Horizontal axis test requirement level
A1 = Horizontal axis response level
R2 = Vertical axis test requirement level
A2 = Vertical axis response level
R3 = Longitudinal axis test requirement level
A3 = Longitudinal axis response level
For vibration being applied in the longitudinal axis, use the equation below as follows:
3 = (R1/A1 + R2/A2 + R3/A3)
Where:
R1 = Longitudinal axis test requirement level
A1 = Longitudinal axis response level
R2 = Vertical axis test requirement level
A2 = Vertical axis response level
R3 = Horizontal axis test requirement level
A3 = Horizontal axis response level
Step 7. Verify that vibration levels are as specified. If the exposure duration is 1/2 hour or less, accomplish this step immediately after full levels are first applied, and immediately before scheduled shut down. Otherwise, accomplish this step immediately after full levels are first applied, every half-hour thereafter, and immediately before scheduled shut down.
Step 8. Monitor the vibration levels and test item performance continuously through the exposure. If levels shift, performance deviates beyond allowable limits, or failure occurs, shut down the test in accordance with the test shut down procedure (paragraph 3.1b(10)). Determine the reason for the anomaly and proceed in accordance with the test interruption recovery procedure (paragraph 4.3).
Step 9. When the required duration has been achieved, stop the vibration.
Step 10. If the test plan calls for additional exposures, repeat Steps 3 through 9 as required by the test plan before proceeding.
Step 11. Inspect the test item, fixture, vibration exciter, and instrumentation. If failure, wear, looseness or other anomalies are found, proceed in accordance with the test interruption recovery procedure (paragraph 4.3).
Step 12. Verify that the instrumentation functions as required and perform an operational check of the test item for comparison with data collected in paragraph 4.5.1.2. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
Step 13. Repeat Steps 1 through 12 for each required excitation axis.
Step 14. Remove the test item from the fixture and inspect the test item, mounting hardware, packaging, etc., for any signs of visual mechanical degradation that may have occurred during testing. 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.