4.1 Test Facility
Prior to initiating any testing, review the pretest information in the test plan to determine test details (e.g., procedure, calibration load (dynamically similar materiel testing using a dynamic simulant for test waveform compensation), test item configuration, measurement configuration, gunfire level, gunfire duration, number of repetitions of gunfire event to be applied). Examine all details of the test validation procedures. Use fixturing that simulates actual in-service mounting attachments (including vibration isolators and fastener torque, if appropriate). Install all the connections (cables, pipes, etc.) in a way that they impose stresses and strains on the test item similar to those encountered in service. In certain cases, consider the suspension of the test item for low frequency apparatus to avoid complex test fixture resonances that may coincide with measured materiel gunfire response resonant frequencies.
For significant gunfire shock environments, use a test facility, including all auxiliary equipment, capable of providing the specified gunfire materiel response environments within the tolerances stated in paragraph 4.2. This will require a test facility with vendor supplied Time Waveform Replication capability able to perform testing in accordance with guidelines provided in either Method 525.2 or Method 527.2. In addition, use measurement transducers, data recording, and data reduction equipment capable of measuring, recording, analyzing, and displaying data sufficient to document the test and to acquire any additional data required. Unless otherwise specified, perform the specified gunfire tests and take measurements at standard ambient conditions as specified in Part One, paragraph 5.1.For benign non-transient vibration gunfire environments, any test facility capable of meeting the test guidelines in Method 514.8 (SESA) or Method 527.2 (MEMA) spectrum control will be suitable.
4.2 Controls
In general, acceleration will be the quantity measured to meet a specification, with care taken to ensure acceleration measurements can be made that provide meaningful data. Always give special consideration to the measurement instrument amplitude and frequency range specifications in order to satisfy the calibration, measurement and analysis requirements. With regard to measurement technology, accelerometers, strain gages, and laser Doppler vibrometers are commonly used devices for measurement. In processing shock data, it is important to be able to detect anomalies. For example, it is well documented that piezoelectric accelerometers may offset or zero-shift during mechanical shock, pyroshock, and ballistic shock (paragraph 6.1, references h and i). A part of this detection is the integration of the acceleration amplitude time history to determine if it has the characteristics of a physically realizable velocity trace. For mechanical shock various accelerometers are readily available which may or may not contain mechanical isolation. All measurement instrumentation must be calibrated to traceable national calibration standards (see Part One, paragraph 5.3.2). In addition, instrumentation to measure test item function may be required. In this case, obtain suitable calibration standards and adhere to them.
- a. Accelerometers. Ensure the following:
- (1) Amplitude Linearity: It is desired to have amplitude linearity within 10 percent from 5 percent to 100 percent of the peak acceleration amplitude required for testing. Since mechanically isolated piezoelectric accelerometers (mechanically isolated or not) may show zero-shift (paragraph 6.1, reference j), there is risk to not characterizing these devices at 5 percent of the peak amplitude. To address these possible zero-shifts, high pass filtering (or other data correction technique) may be required. Such additional post-test correction techniques increases the risk of distorting the measured shock environment. Consider the following in transducer selection:
- (a) It is recognized that mechanically isolated accelerometers may have both non-linear amplification and non-linear frequency content below 10,000 Hz (paragraph 6.1, reference j). In order to understand the non-linear amplification and frequency characteristics, it is recommended that shock linearity evaluations be conducted at intervals of 20 to 30 percent of the rated amplitude range of the accelerometer to identify the actual amplitude and frequency linearity characteristics and useable amplitude and frequency range. If a shock based calibration technique is employed, the shock pulse duration for the evaluation is calculated as:
Where is the duration (baseline) of the acceleration pulse and is the maximum specified frequency range for the accelerometer. For mechanical shock, the default value for is 10,000 Hz.
- (b) For cases in which response below 2 Hz is desired, a piezoresistive accelerometer measurement is required.
- (2) Frequency Response: A flat response within ± 5 percent across the frequency range of interest is required. Since it is generally not practical or cost effective to conduct a series of varying pulse width shock tests to characterize frequency response, a vibration calibration is typically employed. For the case of a high range accelerometer with low output, there may be SNR issues associated with a low level vibration calibration. In such cases a degree of engineering judgment will be required in the evaluation of frequency response.
- (3) Accelerometer Sensitivity: The sensitivity of a shock accelerometer is expected to have some variance over its large amplitude dynamic range.
- (a) If the sensitivity is based upon the low amplitude vibration calibration, it is critical that the linearity characteristics of the shock based “Amplitude Linearity” be understood such that an amplitude measurement uncertainty is clearly defined.
- (b) Ideally, vibration calibration and shock amplitude linearity results should agree within 10 percent over the amplitude range of interest for a given test.
- (4) Transverse sensitivity should be less than or equal to 5 percent.
- (5) The measurement device and its mounting will be compatible with the requirements and guidelines provided in paragraph 6.1, reference b.
- (6) Unless it is clearly demonstrated that a piezoelectric accelerometer (mechanically isolated or not) can meet the shock requirements and is designed for oscillatory shock (not one-sided shock pulses), recommend piezoresistive accelerometers be used for high intensity shock events in which oscillatory response is anticipated. Piezoelectric accelerometers may be used in scenarios in which levels are known to be within the established (verified through calibration) operating range of the transducer, thereby avoiding non-linear amplification and frequency content.
- b. Other Measurement Devices.
- (1) Any other measurement devices used to collect data must be demonstrated to be consistent with the requirements of the test, in particular, the calibration and tolerance information provided in paragraph 4.2.
- (2) Signal Conditioning. Use only signal conditioning that is compatible with the instrumentation requirements of the test, and is compatible with the requirements and guidelines provided in paragraph 6.1, reference b. In particular, filtering of the analog voltage signals will be consistent with the time history response requirements (in general, demonstrable linearity of phase throughout the frequency domain of response), and the filtering will be so configured that anomalous acceleration data caused by clipping will not be misinterpreted as response data. In particular, use extreme care in filtering the acceleration signals at the amplifier output. Never filter the signal into the amplifier for fear of filtering erroneous measurement data, and the inability to detect the erroneous measurement data. The signal from the signal conditioning must be anti-alias filtered before digitizing (see Method 516.8 Shock Annex A).
The complete test parameter control chains (checking, compensation, servings, recording, etc.) should not produce uncertainties exceeding one third of the tolerances specified in paragraphs 4.2.1 through 4.2.4. Because of the nature of the gunfire environment, tolerances may be given in the time, amplitude, and frequency domain according to the processing requirements of the procedure. Knowledge of the bandwidth of the reference and control time traces will be important, and an assessment of the out-of-band energy provided by limitations of impedance matching and fixture resonances will be important. In Procedures I and II, it is assumed that the test item response measurement data collected are representative of the true environment, and not a function of the local materiel configuration, e.g., local resonances that may not be controllable to the tolerances in paragraphs 4.2.1 through 4.2.4. Use test fixturing that will ensure test item response in other axes does not exceed twenty-five percent of the test item response in the test axis
when measured in the time, amplitude, or frequency domain. Methods 525.2 and 527.2 provide guidelines on test tolerance specification under TWR and, in most cases, these test tolerances will be adequate for gunfire testing. The test tolerance guidelines provided below assume stochastic ensemble processing formulation, whereby there is variation in time, but the frequency domain content remains the same over the ensemble of pulses. These test tolerance guidelines may be superseded by more time trace form appropriate guidelines in Methods 525.2 or 527.2. In conjunction with satisfaction of test tolerances, a dynamic simulant for the test materiel is initially recommended to compensate the input waveform. In addition, an appropriate time trace compensation strategy may be applied to optimize the TWR input to the stimulant, and applied in subsequent testing of the materiel.
4.2.1 MEASURED MATERIEL INPUT/RESPONSE TIME HISTORY UNDER TWR (PROCEDURE I)
- a. Time domain. Generally, reference and control time traces are perfectly correlated so that there is no requirement under Method 525.2.
- b. Amplitude domain. Ensure materiel time history major positive and negative response peaks are within ±10 percent of the measured gunfire time history peaks. Ensure the root-mean-square level of the point- by-point difference between the control and reference time traces is less than ±5 percent of the combined control/reference peak time traces for a short-time average time not to exceed 10 percent of the gun fire rate.
- c. Frequency domain. Compute a low frequency resolution average Energy Spectral Density (ESD) estimate over the ensemble created from the materiel time history response that is within ± 3dB of the average ESD estimate computed over the ensemble created from the measured gunfire time history over at least 90 percent of the frequency range. In cases in which an ensemble from the data cannot be created, compute an ASD estimate of the time history records for comparison, provided the data are appropriately windowed (usually with a 10 percent tapered cosine window, a Kaiser window or frequency averaging) to reduce spectral leakage. The tolerances for the ASD analysis are ± 3dB over at least 90 percent of the frequency range. In addition require that overall root-mean-square levels are within 10 percent.
4.2.2 SRS GENERATED SHOCK TIME HISTORY PULSE SEQUENCE UNDER TWR (PROCEDURE II)
- a. Time domain. Ensure the duration of every generated pulse is within 2.5 percent of the duration obtained from the predicted gunfire rate if stochastic ensemble generation methodology is implemented. Ensure the duration of the gun firing event is within 0.5 percent of the overall duration if the stochastic time trace generation methodology is implemented.
- b. Amplitude domain. Ensure materiel time history major positive and negative response peaks are within ±10 percent of the predicted gunfire time history peaks. Ensure that the root-mean-square level of the point-by-point difference between the control and reference time traces is less than ±5 percent of the combined control/reference peak time traces for a short-time average time not to exceed 0.1 of the gunfire pulse period.
- c. Frequency domain. For a reference synthesized pulse time history trace based on a SRS, compute the SRS of a representative pulse or series of pulses and compare to classical SRS tolerances addressed in Method 516.8.
4.2.3 STOCHASTICALLY GENERATED MATERIEL INPUT FROM PRELIMINARY DESIGN SPECTRUM UNDER TWR (PROCEDURE III)
If this procedure requires follow-on testing only time and frequency domain requirements are used.
- a. Time domain. Ensure the duration of every generated pulse is within 2.5 percent of the duration obtained from the specified gunfire rate.
- b. Frequency domain. Ensure the SOR spectrum developed for the pulses is within ±3dB of the predicted SOR spectrum over the entire frequency band of interest. In general, this will be based upon an estimate of the ASD from which the Time Domain Windowed Pulse (TDWP) or Random-Modulated-Harmonic-Pulse (RMHP) are created.
4.3 Test Interruption
If interruption occurs during gunfire shock test input, repeat that gunfire shock test input. Ensure stresses induced by the interrupted gunfire shock test do not invalidate subsequent test results. It is incumbent on all test facilities that data
from such interruptions be recorded and analyzed before continuing with the test sequence. In the case of any interruption, the test item must be re-inspected prior to restarting the test to ensure test item integrity.
Test interruptions can result from two or more situations, one being from failure or malfunction of associated laboratory test equipment. The second type of test interruption results from failure or malfunction of the test item itself during required or optional performance checks.
4.3.1 Interruption from Failure or Malfunction of Associated Laboratory Test Equipment
- a. General. See Part One, paragraph 5.11 of this Standard.
- b. Specific to this Method. If there is an unscheduled interruption, restore/replace laboratory test equipment and reinitiate the test being conducted at the time of failure or malfunction using the same test item.
4.3.2 Interruption Due To Test Item Operation Failure
Failure of the test item(s) to function as required during mandatory or optional performance checks during testing presents a situation with several possible options.
- a. The preferable option is to replace the test item with a “new” one and restart from Step 1.
- b. A second option is to replace / repair the failed or non-functioning component or assembly with one that functions as intended, and restart the entire test from Step 1.
NOTE: When evaluating failure interruptions, consider prior testing on the same test item and consequences of such.
4.4 Test Execution
The following actions along with steps, alone or in combination, provide the basis for collecting necessary information concerning the durability and function of a test item in a gunfire shock environment.
4.4.1 Preparation for Test
4.4.1.1 Pretest Checkout
After appropriate compensation of the excitation input device (with possibly a dynamic simulant), and prior to conducting the test, perform a pretest checkout of the test item at standard ambient conditions to provide baseline data. Conduct the checkout as follows:
- Step 1 Conduct a complete visual examination of the test item with special attention to stress areas or areas identified as being particularly susceptible to damage and document the results.
Step 2 Install the test item in its test fixture.
- Step 3 Conduct a test item operational check in accordance with the approved test plan, along with simple tests for ensuring the response measurement system is responding properly. If the test item operates satisfactorily, proceed to the appropriate procedure. If not, resolve the problems and repeat this Step. Document the results for compliance with information contained in Part One, paragraph 5.9.
- Step 4 If the test item integrity has been verified, proceed to the first test. If not, resolve the problem and restart at Step 1.
4.4.1.2 Procedure Overview
Paragraphs 4.4.2 through 4.4.4 provide the basis for collecting the necessary information concerning the test item in a gunfire shock environment. For failure analysis purposes, in addition to the guidance provided in Part One, paragraph 5.14, each procedure contains information to assist in the evaluation of the test results. Analyze any failure of a test item to meet the requirements of the system specifications based on the guidelines in Part One, paragraph 5.14. For test interruption, follow the guidelines in paragraph 4.3.
MIL-STD-810H
METHOD 519.8
4.4.1.3 Test Item Considerations
Test items can vary from individual materiel items to structural assemblies containing several items of materiel of different types.
- a. General. Unless otherwise specified in the individual test plan, attach the test item to the vibration exciter by means of a rigid fixture capable of transmitting the repetitive shock conditions specified. Ensure the fixture inputs repetitive shock to racks, panels, and/or vibration isolators to simulate as accurately as possible the repetitive shock transmitted to the materiel in service and to the measured gunfire shock environment. When required, ensure materiel protected from repetitive shock by racks, panels, and/or vibration isolators also passes the appropriate test requirements with the test item hard-mounted to the fixture.
- b. Subsystem testing. When identified in the test plan, subsystems of the materiel may be tested separately. The subsystems can be subjected to different gunfire shock environment levels according to the measured time trace data. In this case, ensure the test plan stipulates the gunfire shock levels from measured time trace data specific to each subsystem.
- c. Test item operation. Refer to the test plan to determine whether the test item should be in operation. Because continuous gunfire shock testing can cause unrealistic damage to the test item (e.g., unrealistic heating of vibration isolators), interrupt the excitations by periods of rest defined by the test plan and in accordance with the LCEP.
4.4.2 Procedure I - MEASURED MATERIEL INPUT/RESPONSE TIME HISTORY UNDER TWR
4.4.2.1 Controls
This procedure assumes that measured materiel input/response data are available in digital form, and this input/response data will be replicated in the laboratory on the test item. This procedure may include the concatenation of several files of measured reference time traces.
4.4.2.2 Test Tolerances
Ensure test tolerances are in accordance with those specified in paragraph 4.2.
4.4.2.3 Procedure Steps
Step 1 Precondition the test item in accordance with paragraphs 4.2 and 4.4.1.
Step 2 Choose control strategy and control and monitoring points in accordance with paragraph 2.5.
Step 3 Perform operational checks in accordance with paragraph 4.4.1.
- Step 4 Mount the test item on the vibration exciter or use some other means of suspension in accordance with paragraph 4.4.4.1.
- Step 5 Determine the time trace representation of the vibration exciter drive signal required to provide the desired gunfire shock materiel acceleration input/response on the test item. (Refer to Annex A.)
- Step 6 Apply the drive signal as an input voltage, and measure the test item acceleration response at the selected control/monitoring point.
Step 7 Verify that the test item response is within the allowable tolerances specified in paragraph 4.2.1.
- Step 8 Apply gunfire shock simulation for on and off periods and total test duration in accordance with the test plan. Perform operational checks in accordance with the test plan. If there is failure in test item operational performance, stop the test, assess the failure, and decide upon the appropriate course of action to proceed with testing to complete the test plan. Follow the guidance in paragraph 4.3.2.
Step 9 Repeat the previous steps along each of the other specified axes, and record the required information.
4.4.2.4 Analysis of Results
Refer to the guidance in Part One, paragraph 5.14, to assist in the evaluation of the test results. In addition, a display of the measured test item response time trace and analysis called for in paragraph 4.2.1 to satisfy the test tolerances.
MIL-STD-810H METHOD 519.8
4.4.3 Procedure II - SRS GENERATED SHOCK TIME HISTORY PULSE SEQUENCE UNDER TWR
4.4.3.1 Controls
This procedure assumes that generated input/response data are available in digital form, has been deterministically modeled after the SRS procedure described in Method 516.8 and the generated sample function input/response data will be replicated in the laboratory on the test item.
4.4.3.2 Test Tolerances
Ensure test tolerances are in accordance with those specified in paragraph 4.2.
4.4.3.3 Procedure Steps
- Step 1 Generate a deterministic representation of the field measured materiel input/response data. In general, this will involve an off-line procedure designed to generate an ensemble of deterministic pulses based on measured data for input to the vibration exciter as a single time trace of concatenated pulses or a single deterministic time trace (refer to Annex B).
Step 2 Precondition the test item in accordance with the test plan.
- Step 3 Choose control strategy and control and monitoring points in accordance with paragraph 2.5.
Step 4 Perform operational checks in accordance with paragraph 4.4.4.1.
- Step 5 Mount the test item on the vibration exciter (or use some other means of suspension) in accordance with paragraph 4.4.4.1.
- Step 6 Determine the time trace representation of the vibration exciter drive signal required to provide the desired gunfire shock materiel acceleration input/response on the test item.(Refer to Annex B).
- Step 7 Apply the drive signal as an input voltage, and measure the test item acceleration input/response at the selected control/monitoring point.
- Step 8 Verify that the test item response is within the allowable tolerances specified in paragraph 4.2.2.
- Step 9 Apply gunfire shock simulation for on and off periods, and total test duration in accordance with the test plan. Perform operational checks in accordance with the test plan. If there is failure in test item operational performance stop the test, assess the failure, and decide upon the appropriate course of action to proceed with testing to complete the test plan. Follow the guidance in paragraph 4.3.2.
Step 10 Repeat the previous steps along each of the other specified axes, and record the required information.
4.4.3.4 Analysis of Results
Refer to the guidance in Part One, paragraph 5.14, to assist in the evaluation of the test results. In addition, a display of the measured test item response time trace and analysis called for in paragraph 4.2.2 to satisfy the test tolerances.
4.4.4 Procedure III - STOCHASTICALLY GENERATED MATERIEL INPUT FROM PRELIMINARY DESIGN SPECTRUM UNDER TWR
4.4.4.1 Controls
This procedure assumes that the gun/materiel parameters are available for derivation of a predicted SOR test spectrum. This procedure also assumes given the predicted spectrum, a Time Domain Windowed Pulse or Random-Modulated-Harmonic-Pulse time trace can be developed having the same estimated spectrum with minimized harmonic distortion. Developing either the Time Domain Windowed Pulse (TDWP) or the Random Modulated Harmonic Pulse (RMHP) time trace requires a trained analyst and specialized software. It makes no provision for actual testing. For actual testing to the Time Domain Windowed Pulse or Random-Modulated-Harmonic-Pulse time trace use Procedure I as if stochastic simulation of a field measured environment has been performed.
Step 1 Specify the gun/materiel parameters and generate the predicted SOR spectrum (See Annex C.)
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Step 2 Generate a Time Domain Windowed Pulse or Random-Modulated-Harmonic-Pulse time trace with the specified predicted spectrum.
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Step 3 For materiel design considerations analyze the Time Domain Windowed Pulse or Random- Modulated-Pulse time trace according to procedures appropriate for a repetitive shock and use this analysis for consideration in preliminary materiel design. Typically:
(a) Transient vibration root-mean-square peak levels along with a normalized ASD estimate will be used in specifying the acceleration environment for the materiel design
(b) Single pulse ASD or equivalently ESD estimates will be made on the response to the Time Domain Windowed Pulse or Random-Modulated-Pulse time trace (either under ensemble representation or as an overall time trace) and be used in specifying a gunfire shock environment for materiel design.
- Step 4 If testing is required, generate the equivalent Time Domain Windowed Pulse or Random Modulated Harmonic Pulse time trace environment (refer to Annex C.), and go to Procedure I for testing while recording the required information.