12.6 Standards

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12.6.1 Types of Standards

There are two types of standards: (1) those which specify arbitrary shock pulses (IEC, ISO, MIL STD

810 C, …), and (2) those which require test tailoring (GAM EG 13, 1986; DEF STAN, 1999; MIL STD

810 F, 2000; NATO, 2000).

For the first case, the most frequently specified shock shapes are the half-sine, the TPS, and the

rectangular (or trapezoidal) waveforms. In these standards, a table proposes several values of levels and

durations, with preferred combinations (for example, 30g, 18 msec or 50g, 11 msec).

To take account of the limitations of test facilities and unavoidable signal distortions, the shock carried

out is regarded as acceptable if the time acceleration signal lies between two tolerance limits. Two shocks

included within these limits can, however, have very different effects (which can be evaluated with the

SRS; Lalanne, 2002b).

Over-test

Under-test for

2

2

1

1

f fp f

Over-test for

FIGURE 12.27 Examples of SRS that are difficult to envelop with the SRS of a simple shock. (Source: Lalanne, Chocs

Mecaniques, Hermes Science Publications. With permission.)

TABLE 12.7 Deriving a Shock Test Specification from the SRS of the Real

Environment

Specification Determined by

Shock waveform Comparing the shape of the SRS of the real environment

(reference) to that of the SRS of the simple shape shocks

(half-sine, TPS or rectangular waveform)

Shock amplitude The SRS amplitude at high frequency

Shock duration Writing that the abscissa of the first point which reaches

the value of the asymptote at high frequencies

(amplitude of shock) is the same for the reduced SRS

of the chosen simple shock and the reference SRS

12-24 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

Some examples of standards are given in

Figure 12.28 and Figure 12.29.

In the second case, the test specification is

preferably written from real measurements corresponding

to the life profile of the material. The

data to be used can be any of the following, in the

preferential order:

* Functional real environment time history

measurements of the material.

* Data measured under similar conditions

and estimated to be representative.

* Data issued from prediction or calculation.

* Default values (fallback levels), obviously

more arbitrary in character, to be used if

measured data not available (classical pulse

shock or SRS).

Derivation of the test specification and

subsequent test should be carried out, in the

preferential order as follows:

* For measured data of the same event, if the

measured pulse shapes are very similar, use

direct reproduction of the measured data

under shaker waveform control (if possible).

If the measured shock shapes are very

different, use the following method.

* For measured data of different shock events

use a synthesis of measurements using

SRS (see Section 12.5.2). Test on shaker

with SRS control if possible. If not possible,

test on shock machine with a classical

pulse having the same SRS.

* If there is no measured data of the real shock, but measured data under similar conditions, use the

method as above.

* If there are no measured data, fallback levels and provisional values are to be replaced by results of

measurement as soon as possible.

The transformation shock spectrum-signal has an infinite number of solutions, and very different

signals can have identical response spectra. Standards often require specifying in addition to the

spectrum other complementary data such as the duration of the signal time, the velocity change during

the shock or the number of cycles (less easy), in order to deal with the spectrum and the couple

amplitude/duration of the signal at the same time (see Section 12.10.4).

It is not correct to decompose a SRS into two separate domains in order to be able to meet a shock

requirement (a low frequency component and a high frequency component). If the specimen has no

significant low natural frequency, it is permissible to allow the low frequency domain of the SRS to fall

out of tolerance in order to satisfy the high frequency part of the requirement.

The tolerance on the SRS amplitude should be, for example (MIL STD 810 F), 2 1.5 dB, þ3 dB over the

specified frequency range; a tolerance of þ3 dB, þ6 dB being permissible over a limited frequency range.

It is generally required to determine the positive and negative spectra (absolute acceleration or relative

displacement) at Q ¼ 10; at at least 1/12-octave frequency intervals.

Integration

time

Nominal pulse

Limits of

tolerances

1.5 D 1.2 A

0.8 A

A

0.4 D

2.4 D = T1

0.1 D

D D

6 D = T2

FIGURE 12.28 Half-sine pulse (NATO Stanag 4370,

AECTP 403). T1: minimum time during which the pulse

shall be monitored for shocks produced using a

conventional shock-testing machine; T2: minimum

time during which the pulse shall be monitored for

shocks produced using a vibration generator.

Ideal Sawtooth Pulse

Tolerance Limits

0.15 A

0.3 D D

0.15 A

0.3 A

0.07 D

0.02 A

1.15 A

A

0.05 A

0.05 A

FIGURE 12.29 TPS pulse (MIL STD 810 F). D:

duration of nominal pulse; A: peak acceleration of

nominal pulse.

Mechanical Shock 12-25

© 2005 by Taylor & Francis Group, LLC

In the absence of accurate information on the number of shocks which the material will undergo in its

service life, a minimum is often required of three shocks in both directions along each of the three

orthogonal axes, a total of 18 shocks.

12.6.2 Installation Conditions of Test Item

* The test item should be mechanically fastened to the shock machine, directly by its normal means

of attachment or by means of a fixture.

* The mounting configuration should enable the test item to be subjected to shocks along the

various axes and directions as specified.

* External connections necessary for measuring purposes should add minimum restraint and mass.

* The fixture should not modify the dynamic behavior of the test item.

* Material intended for use with isolators should be tested with its isolators.

* The direction of gravity or any loading factors (mechanisms, shock isolators, etc.) must be taken

into account by compensation or by suitable simulation.

12.6.3 Uncertainty Factor

An uncertainty factor may be added to the resulting envelope if confidence in the data is low or in order

to take account of the dispersion of levels in the real environment when the data set is small. This factor

can be arbitrary, of the order of 3 to 6 dB, for example, or determined from a reliability computation,

taking account of the statistical distributions of the real environment and of the material strength

(Lalanne, 2002d).

It is important that all uncertainties be clearly defined and that uncertainties are not superimposed

upon estimates that already account for uncertainties.

Note: The purpose of the test is to demonstrate that the equipment has at least the specified strength at

the time of its design. However, for obvious reasons of cost, this demonstration is generally conducted

only on one specimen. To take into account the variability of the strength of the material, it is possible to

increase the test severity by applying a “test factor.” This second factor depends on the number of tests to

be conducted and on the coefficient of variation of the material strength (Lalanne, 2002d).

12.6.4 Bump Test

A bump test is a test in which a simple shock is repeated many times (DEF STAN, 1999; IEC, 1987b; AFNOR,

1993). Standardized severities are proposed. For example, half-sine, 10 g, 16 ms, 3000 bumps (shock) per

axis, 3 bumps a second.

The purpose of this test is not to simulate any specific service condition. It is simply considered that it

could be useful as a general ruggedness test to provide some confidence in the suitability of equipment for

transportation in wheeled vehicles. It is intended to produce in the specimen effects similar of those

resulting from repetitive shocks likely those encountered during transportation.

In this test, the equipment is always fastened (with its isolators if it is normally used with isolators) to

the bump machine during conditioning.

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