Gunfire shock tests are performed to provide a degree of confidence that materiel can structurally and functionally withstand the relatively infrequent, short duration transient high rate repetitive shock input encountered in operational environments during the firing of guns.
519.8-1 SCOPE
519.8-1 SCOPE
519.8-1 SCOPE
This Annex provides the option of using predicted gunfire vibration in the form of a Sine-on-Random (SOR) spectrum when measured data are not available. Information in this Annex is to ensure materiel mounted in an aircraft with onboard guns can withstand the predicted environmental acceleration levels caused by (1) pulse overpressures emitting from the muzzle of the gun impinging upon materiel support structure, and (2) structure-borne vibration. The first portion of this Annex constitutes a reformatting of Method 519.5, Gunfire Vibration, Aircraft, in MIL-STD-810F with a limited number of enhancements. The second portion of this Annex briefly describes two methodologies for taking a predicted spectrum and generating a form of “shock pulse” acceleration time history of arbitrary length whose ASD estimate matches the predicted spectrum and can be implemented using Method 525.2 Time Waveform Replication (TWR). The two methodologies represent time domain and frequency domain implementations of standard Fourier based techniques.
Note that the predicted spectrum has a continuous part and a discrete part consisting of a fundamental frequency/amplitude and three additional components with frequencies at harmonic ratios to the fundamental. The SOR is a more limited and specific terminology and implies vibration controller implementation.
MIL-STD-810H METHOD 519.8 ANNEX C
PART 1 - SPECTRUM PREDICTION METHODOLOGY FOR PRELIMINARY MATERIEL DESIGN
519.8-1 SCOPE
519.8-1 SCOPE
1.2 Application
Use this Method to evaluate the structural and functional performance of materiel likely to be exposed to a gunfire shock environment in its lifetime. This test Method is applicable when materiel is required to demonstrate its adequacy to resist a “gunfire schedule” environment without unacceptable degradation of its structural integrity and functional performance (“gunfire schedule” here refers to the firing rate, the number of rounds fired in a given firing, and the number of firing events). The gunfire environment may be considered to be a high rate repetitive shock having form of a substantial transient vibration produced by (1) an airborne gun muzzle blast pressure wave impinging on the materiel at the gun firing rate, (2) a structure-borne repetitive shock transmitted through structure connecting the gun mechanism and the materiel, and/or a combination of (1) and (2). The closer the materiel surface is to direct pressure pulse exposure, the more likely the measured acceleration environment appears as a repetitive shock producing “very short” rise time and rapid decay of materiel response, and the less role the structure-borne repetitive shock contributes to the overall materiel response environment. The farther the materiel surface is from direct pressure pulse exposure, the more the measured acceleration environment appears as a structure-borne high rate repetitive shock (or a substantial transient vibration) with some periodic nature that has been filtered by the structure intervening between the gun mechanism and the materiel. Repetitive shock applied to a complex multi-modal materiel system will cause the materiel to respond (1) at forced frequencies imposed on the materiel from the external excitation environment, and (2) to the materiel’s resonant natural frequencies either during or immediately after application of the external excitation. Such response may cause:
- a. Materiel failure as a result of increased or decreased friction between parts, or general interference between parts.
- b. Changes in materiel dielectric strength, loss of insulation resistance, variations in magnetic and electrostatic field strength.
- c. Materiel electronic circuit card malfunction, electronic circuit card damage, and electronic connector failure. (On occasion, circuit card contaminants having the potential to cause short circuits may be dislodged under materiel response to gunfire environment)
- d. Permanent mechanical deformation of the materiel as a result of overstress of materiel structural and non-structural members.
- e. Collapse of mechanical elements of the materiel as a result of the ultimate strength of the element being exceeded.
- f. Accelerated fatiguing of materials (low cycle fatigue).
- g. Potential piezoelectric activity of materials.
- h. Materiel failure as a result of cracks and fracture in crystals, ceramics, epoxies, or glass envelopes.
MIL-STD-810H
METHOD 519.8
519.8-1.2 Application
This procedure is essential for accurate time trace replication of single point input to materiel that may be characterized as an in-service measured gunfire shock. Because of the repetitive non-stationary nature of the gunfire shock environment, this is possibly the only known procedure that will provide precision in laboratory testing. Acceleration is considered the measurement variable in the discussion to follow, although other variables may be used, provided the dynamic range of the measured materiel response is consistent with the dynamic range of the force exciter used as the test input device. Testing is performed in order to ensure materiel physical and functional integrity during a specific measured gunfire shock event, and to provide confidence that materiel will demonstrate the same integrity under similar in-service events.
519.8-1.2 Application
This Annex addresses a method for laboratory gunfire shock replication under TWR based upon SRS specification alone. This application begins with a single deterministic shock pulse generated based upon the SRS “sine-beat” or “damped sine” vendor procedure which is then concatenated to the duration of the specified “gunfire event”. In general an SRS determined over the deterministic concatenated pulse train will not be within tolerance as specified for shock in Method 516.8 for the lower natural frequencies. There are a variety of means of bringing the concatenated pulse train within tolerance. One such procedure is go generate a bandlimited random time history, scaling to a correct amplitude based upon SRS estimate considerations and then adding the bandlimited random time history to the concatenated deterministic pulse train file. This results in a form of impulsive shock random time history. The resulting time history should provide an SRS estimate within SRS tolerance called out in Method 516.8 for standard shock. It is noted that the advantage accrued via the SRS procedure is that SRS based laboratory test tolerances are readily available and easily understood (see Method 516.8). However, in TWR execution of the test, time history test tolerances under TWR, Method 525.2, must be observed. That is both the SRS tolerances for overall test satisfaction according to the specification and TWR test tolerances for execution of the test are applicable, with SRS test tolerances of first concern for specification satisfaction. The major limitation of the procedure is that SRS information at
MIL-STD-810H METHOD
519.8-1.2 Application
This Annex is applicable only for aircraft gunfire and materiel mounted in an aircraft with onboard guns. Guidance in this Annex is to be used only if in-service measured materiel response data are not available, or will not be available in the early stages of a development program. This Annex is not intended to justify the use of SOR or narrowband random-on-random for cases in which measured data display a broadband spectra along with components at discrete frequencies. Use the information in this Annex only if it is vital to the design of the materiel. If there is a possibility of obtaining early measurements of the materiel response mounted on the in-service platform, supplant the severities developed using the information in this Annex with the severities estimated from the materiel response under in-service measurements, and one of the other procedures used for testing. In particular, if the measured materiel response in-service environment has the character of high level broadband random vibration with no characteristics conducive to application of Procedure II or Procedure III, then:
- a. Apply Procedure I in the form of transient vibration, or
- b. Submit the test item to a specified level of high level broadband random vibration (based on ASD estimates of the measured in-service materiel response) over a period of time, consistent with low cycle fatigue assumptions in accelerated testing or as specified in the test plan (see Method 514.8).
This technique is based upon obtaining the predicted SOR spectrum, using the four sine components in phase to develop the envelope of the form of a pulse, and using the predicted spectrum as stationary random vibration that can be enveloped to provide a pulse form time trace that can be used for preliminary design of materiel where no addition information is available. This technique is not intended to develop a pulse that can be concatenated and used for testing under TWR.
519.8-1.2 Time Domain Windowed Pulse (TDWP)
1.2.1 Introduction
This ad hoc methodology uses the fact that predicted spectrum has a fundamental frequency and it is assumed that this fundamental frequency dominates the time history generation. A properly normalized exponential form window having a period corresponding to the fundamental period of the predicted spectrum allows time domain windowing of a stationary time history with the predicted continuous/discrete spectrum and generates a form of “repeated shock” at the gun firing rate. The three harmonics of the fundamental represented in the predicted spectrum provide correct amplitudes for the discrete spectrum. This methodology is effective because in general the frequency content of the discrete spectrum is fundamentally “stochastically independent” of the frequency content of the continuous spectrum with the exponential window providing the correct fundamental frequency and the three harmonics are unaffected when viewed as convolution of the exponential window in the frequency domain with the harmonic frequencies.
1.2.2 Outline of Implementation
The following steps are outlined for implementation. A detailed discussion is found in Reference 6.1 k.
- a. Generate a normalized deterministic pulse of exponential form of duration of the period of the fundamental of the predicted spectrum
- b. Generate a stationary random bandlimited Gaussian time history of arbitrary duration
- c. Shape the spectrum of the band limited time Gaussian time history to the shape of the continuous spectrum and add discrete components at the three harmonic frequencies.
- d. Multiply the deterministic exponential pulse block times the stationary time history with the correct continuous/discrete spectrum block by block
- e. Compute the ASD estimate and compensate for any resulting harmonic distortion
- f. Execute the time history under TWR according to the LCEP gunfire event description
MIL-STD-810H
METHOD 519.8 ANNEX C
1.3 Limitations
This Method provides limited information with regard to the prediction of input levels to materiel based only on the gun parameters and the geometrical configuration between the gun and materiel. Procedure III is provided for purposes of preliminary materiel design when no other information is available. The shock form of time trace information generated in Procedure III may be tested under Time Waveform Replication (TWR) as a recommended practice. It may not be possible to replicate some operational service gunfire materiel response environments because of impedance mismatches. In particular, laboratory fixture limitations or other physical constraints may prevent the satisfactory application of gunfire-induced excitation to a test item in the laboratory. In addition, this Method:
- a. Does not provide guidelines for separating air-borne from structure-borne excitation input to materiel. It is important that a trained structural dynamicist examine the structural configuration and any measured data to determine the transmission path(s) from the gun excitation source to the materiel.
- b. Does not provide guidance on techniques for isolation of the materiel from the source of excitation.
- c. Does not provide guidance on materiel design to avoid unacceptable structural or functional materiel degradation during gun firing, e.g., shock isolation.
- d. Does not include the repetitive shock effects experienced by large extended materiel, e.g., airframe structural systems over which varied parts of the materiel may experience spatially correlated external excitation. For this type of repetitive shock, with degrees of input and response spatial correlation from the external excitation, specialized tests based on experimentally measured data must be employed.
- e. Does not include provisions for performing gunfire tests at high or low temperatures including the extreme temperature environment directly related to the gunfire pressure wave emission and subsequent materiel absorption of this thermal energy. Perform tests at standard ambient temperature unless otherwise specified. However, thermal energy generated from the gun blast pressure wave may be an important design consideration for materiel close to the gun muzzle.
- f. Is not intended to simulate blast pressure or acoustic effects on materiel as a result of exposure to gunfire environment. This Method assumes materiel acceleration as the measurement variable but does not limit consideration to other materiel input/response variables, e.g., force.
- g. In general, it provides limited guidance on materiel response to gun excitation from simultaneous firing of more than one gun.
- h. Does not address benign gunfire shock environments where materiel input or response may be a form of transient random vibration, with peak root-mean-square levels below the levels of materiel qualification to stationary random vibration as determined by the square root of the area under the Autospectral Density Estimate (ASD).
- i. Does not provide guidance on “single shot” gunfire response from large guns e.g., Navy ship guns. For such gunfire response representing basically a single shock to materiel, guidance in Method 516 is applicable.
519.8-1.3 Limitations
This Annex is not intended to justify the use of SOR or narrowband random-on-random for cases in which measured data displays a broadband spectra along with components at discrete frequencies.
519.8-1.3 Random Modulated Harmonic Pulse (RMHP)
1.3.1 Introduction
This ad hoc methodology uses the fact that predicted spectrum has an associated harmonic pulse composed of the four fundamental frequencies. It is assumed that these four discrete frequency components add and are of zero phase with respect to one another. This construction provides a form of “harmonic pulse”. Since the continuous and discrete spectrums are stochastically independent of one another they can be added to provide the exact predicted spectrum. However, as it is easy to demonstrate the resulting “additive” model does not have time history characteristics of a measured gunfire pulse such as high kurtosis. If the generated pulse is multiplied by the Gaussian time history with the appropriate continuous spectrum i.e., the deterministic pulse is modulated by the random time history, and any distortion from the predicted spectrum is removed in the frequency domain the resulting time history has a form or random time history with a degree of kurtosis differing from typical random vibration kurtosis. Analysis of the generated time history reveals an underlying “random pulse” where the fundamental and harmonic components have a non-zero relative phase. The stochastic generation must be such that the fundamental and three harmonic frequencies coincide with FFT Fourier frequencies (which is not a severe limitation).
1.3.2 Outline of Implementation
The following steps are taken for implementation. A detailed discussion is found in Reference 6.1.1.
- a. Generate a deterministic pulse composed of zero phase summation of the fundamental and three harmonic components of the predicted spectrum. Choose the digital sequence sample rate such that it has the harmonic frequencies coincide with FFT Fourier frequency lines i.e., the Fourier frequencies.
- b. Generate a stationary random bandlimited Gaussian time history of arbitrary duration
- c. Shape the stationary random bandlimited Gaussian time history to the continuous spectrum provided by the prediction.
- d. Modulate the deterministic pulse by multiplying it by blocks of stationary random time history
- e. Using the FFT over a long portion of time history correct the harmonic components lying on the Fourier frequencies to the correct amplitudes leaving the phase as random (this may be a iterative procedure)
- f. Execute the time history under TWR according to the LCEP gunfire event duration
Note: This methodology introduces kurtosis differing from stationary random vibration kurtosis into the testing and is random in the sense that the underlying pulse in the generated time history generally is un-symmetric and does not have zero boundary conditions.
1.1 Purpose
This Annex provides (1) pre-processing procedures for Method 525.2 (SESA) TWR laboratory test for gunfire shock environment, (2) an illustration of direct reproduction (in a laboratory test) of in-service measured materiel input/response time trace data on a force exciter under Method 525.2, (3) test tolerance limit assessment for guidelines provided in Method 525.2 and a rationale and procedure for stochastic generation of gunfire shock time traces from measured information. This annex assumes that the testing facility is fully qualified to perform the Single- Exciter/Single-Axis (SESA) Procedure in Method 525.2. For potential extensions of Procedure I to either Multi- Exciter/Single-Axis (MESA) or Multi-Exciter/Multi-Axis (MEMA), use guidelines in Method 527.2.
519.8-1.1 Purpose
The purpose of Procedure II is to provide an alternative gunfire shock testing philosophy when only an SRS computed from a gunfire event is provided. In cases in which a measurement time history is available Procedure I must be used without exception. An estimate of the available time history SRS cannot be used to provide an alternative gunfire test to Procedure I, utilizing Procedure II. To implement this procedure the following three pieces of information are required:
- An SRS (termed a “Target SRS”) and associated damping as computed over a specified natural frequency bandwidth. (This SRS may be from a measurement previously processed from time history data that is no longer available or from an SRS that is of a generic nature).
- The fundamental firing rate of the gun.
- The duration of the "gunfire event" indicating the number of pulses to be concatenated for the proposed TWR testing (as defined per the LCEP).
As is likely to be the case, the SRS display, i.e., natural frequency versus maximax SRS estimate, may extend beyond the test frequency bandwidth for the TWR implementation. This is particularly true for low natural frequency display and very high natural frequency display. For low natural frequency display, this may be the result of processing a gunfire time history or defining a generic reference extending much beyond the period of the firing rate. For high natural frequency display this may be a result of gunfire response extending well beyond electrodynamic exciter capabilities.
Procedure II requires input from Method 516.8 relative to (1) proper synthesis of deterministic time histories from SRS estimates with representative shock duration, and (2) laboratory test SRS tolerance limits. In addition, since the testing will be implemented under TWR, TWR testing options from Method 525.2 may also be applicable. From Method 516.8 the SRS “sine-beat procedure” or “damped sine procedure” provide an algorithmic methodology for generating a single deterministic pulse time history from the SRS estimate. Typically, this generated pulse matching the SRS will extend well beyond the period related to the gun firing rate.
An SRS estimate provided for a single pulse from a gunfire shock event is unacceptable for gunfire shock testing and any such proposed testing relative to single pulses must follow the philosophy and guidelines of Method 516.8.
This Annex assumes that the testing facility is fully qualified to provide a deterministic pulse based upon SRS considerations as outlined in Method 516.8, synthesizing the pulse train and providing additional manipulation in order to perform the Single-Exciter/Single-Axis (SESA) TWR Procedure in Method 525.2 with the analytically generated reference time history.
519.8-1.1 Purpose
This Annex provides the option of using predicted gunfire vibration (Sine-on-Random [SOR]) data (when measured data are not available), to ensure materiel mounted in an aircraft with onboard guns can withstand the predicted environmental acceleration levels caused by (1) pulse overpressures emitting from the muzzle of the gun impinging upon materiel support structure, and (2) structure-borne vibration. (This Annex constitutes a reformatting of Method 519.5, Gunfire Vibration, Aircraft, in MIL-STD-810F with a limited number of enhancements.) This Annex also provides the option for using high level random vibration (measured data are available) when the measured data spectrum displays no outstanding discrete harmonic components.
519.8-1.1 Purpose
The purpose of Part 2 of Annex C is to provide ad hoc methods for taking a predicted form of SOR spectrum and generating a time history having the form of a repeated shock with an ASD estimate that matches both the continuous and discrete spectrum. The repeated shock has the same period as the frequency of the fundamental harmonic of the predicted spectrum.
There are at least two ad hoc methodologies for generating repetitive shock time histories matching a form of SOR spectrum. Both methodologies are described briefly in this Annex. The first methodology is termed a Time Domain Windowed Pulse (TDWP) and the second methodology is termed a Random Modulated Harmonic Pulse (RMHP). The form of implementation in both methodologies is strikingly similar but differs in fundamental ways. TDWP relies upon a time domain windowing with an exponential window that has frequency components outside the principal gunfire bandwidth. The RMHP relies upon frequency domain implementation and development of a random pulse form with correct frequency domain properties. The methodologies are termed ad hoc since there is no attempt to “fit” a gunfire measurement time history. Both techniques provide a time history that has the outward appearance of a repetitive pulse gunfire environment.