Материал: Стандарт MIL-STD-202G

MIL-STD-202G

2.2.2Transducer. The fundamental resonant frequency of the accelerometer shall be greater than 30,000 Hz, when the accelerator is employed as the shock sensor.

2.2.3Transducer calibration. Transducers shall be calibrated in accordance with ASA STD S2.2-1959. The accuracy of the calibration method shall be at least ±5 percent over the frequency range of 2 to 5,000 Hz. The amplitude of the transducer being calibrated shall also be ±5 percent over the frequency range of 4 to 5,000 Hz.

2.2.4Linearity. The signal level of the system shall be chosen so that the acceleration pulse operates over the linear portion of the system.

2.2.5Transducer mounting. When conformance to 2.3 is required, the monitoring transducer shall be rigidly secured and located as near as possible to an attachment point of the specimen but not on the specimen itself.

2.3 Application of shock measuring instrumentation. Shock measuring instrumentation shall be utilized to determine that the correct input shock pulse is applied to the test specimen. This is particularly important where a multi-specimen test is made. Generally, the shock pulse should be monitored whenever there is a change in the test setup, such as a different test fixture, different component (change in physical characteristics), different weight, different shock pulse (change in pulse shape, intensity, or duration) or different shock machine characteristics. It is not mandatory that each individual shock be monitored, provided that the repeatability of the shock application as specified in 2.1.1 has been established.

3. SHOCK PULSES. Two types of shock pulses, a half-sine shock pulse and a sawtooth shock pulse, are specified. The pulse shape and tolerances are shown on figures 213-1 and 213-2, respectively. For single degree of freedom systems, a sawtooth shock pulse can be assumed to have a damage potential at least as great as that of the half-sine pulse if the shock spectrum of the sawtooth pulse is everywhere at least as great as that of the half-sine pulse. This condition will exist for two such pulses of the same duration if over most of the spectrum the acceleration peak value of the sawtooth pulse is 1.4 times the acceleration peak value of the half-sine pulse.

3.1 Half-sine shock pulse. The half-sine shock pulse shall be as indicated on figure 213-1. The velocity change of the pulse shall be within ±10 percent of the velocity change of the desired shock pulse. The velocity change may be determined either by direct measurement, indirectly, or by integrating (graphically or electrically) the area (faired acceleration pulse may be used for the graphical representation) under the measured acceleration pulse. For halfsine acceleration pulses of less than 3 milliseconds duration the following tolerances should apply: The faired maximum value of the measured pulse shall be within ±20 percent of the specified ideal pulse amplitude, its duration shall be within ±15 percent of the specified ideal pulse duration, and the velocity change associated with the measured pulse shall be within ±10 percent of Vi = 2AD/π . See figure 213-1. The measured pulse will then be considered a nominal half-sine pulse with a nominal amplitude and duration equal to respective values of the corresponding ideal half-sine pulse. The duration of the measured pulse shall be taken as Dm = D(.1A)/.94; where D(.1A) is the time between points at .1A for the faired measured acceleration pulse.

3.1.1 The ideal half-sine pulse. An ideal half-sine acceleration pulse is given by the solid curve. See figure 213-1. The measured acceleration pulse must lie within the boundaries given by the broken lines. In addition, the actual velocity change of the shock must be within 10 percent of the ideal velocity change. The actual velocity change can be determined by direct measurements, or from the area under the measured acceleration curve. The ideal velocity change is equal to Vi = 2AD/π ; where A is the acceleration amplitude and D is the pulse duration of the ideal pulse.

METHOD 213B

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MIL-STD-202G

3.2 Sawtooth shock pulse. The sawtooth pulse shall be as indicated on figure 213-2. The velocity change of the faired measured pulse shall be within ±10 percent of the velocity change of the ideal pulse.

3.2.1 The ideal terminal-peak sawtooth. An ideal terminal-peak sawtooth acceleration pulse is given by the solid line. See figure 213-2. The measured acceleration pulse must be within the boundaries given by the broken lines. In addition, the actual velocity change of the shock pulse must be within 10 percent of the ideal value. The actual velocity change can be determined from direct measurements, or from the area under the measured acceleration curve. The ideal velocity change is equal to Vi = PD/2; where P is the peak value of acceleration, and D is the pulse duration.

4. PROCEDURE. The test specimen shall be mounted as specified in the component specification. Whenever possible, the test load shall be distributed uniformly on the test platform in order to minimize the effects of unbalanced loads.

4.1 Basic design test. Three shocks in each direction shall be applied along the three mutually perpendicular axes of the test specimen (18 shocks). If the test specimen is normally mounted on vibration isolators, the isolators shall be functional during the test. The specified test pulse (half-sine or sawtooth pulse) shall be in accordance with figures 213-1 and 213-2, respectively, and shall have a duration and peak value in accordance with one of the test conditions of table 213-1.

TABLE 213-1. Test condition values.

1/ For half-sine shock pulses of less than 3 milliseconds duration, it is not required that the envelope fall within the tolerances specified on figure 213-1. The faired amplitude of the measured pulse shall be within ±20 percent of the ideal amplitude. The measured duration shall be within ±15 percent of the specified amplitude duration. The velocity change of the faired measured pulse shall be within ±10 percent of the ideal pulse. The duration of the pulse shall be measured at the 0.1A point on the pulse. The duration of the pulse shall be the duration measured at the 0.1A point divided by .94. Test conditions D, E, and F are principally applicable to semiconductors.

2/ Test conditions G, H, and I (sawtooth) waveforms are preferred, except for semiconductors, for which test conditions D, E, and F (half-sine) are preferred.

3/ For test condition D. Where the weight of multi-specimen and fixtures exceeds 150 pounds, there is a question as to whether the shock pulse is properly transmitted to all specimens. Due consideration should be given to the design of the test fixture to assure the proper shock input to each specimen. This also applies to test conditions E and F except that where the weight of the multi-specimen and fixtures exceeds 25 pounds.

METHOD 213B

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MIL-STD-202G

5.MEASUREMENTS. Measurements are to be made before and after the required number of shocks unless otherwise specified, and during the test if specified.

6.SUMMARY. The following details are to be specified in the individual specification:

a.Mounting method and accessories (see 2.2.5 and 4).

b.Test condition letter (see 4.1).

c.Measurements before, during, and after the test (see 5).

METHOD 213B

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