12 Mechanical Shock Christian Lalanne

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Engineering Consultant

12.1 Definitions ............................................................................ 12-2

Shock † Simple (or Perfect) Shock † Half-Sine Shock †

Versed-Sine (or Haversine) Shock † Terminal Peak

Sawtooth Shock or Final Peak Sawtooth Shock †

Rectangular Shock

12.2 Description in the Time Domain ...................................... 12-3

12.3 Shock Response Spectrum .................................................. 12-4

Need † Shock Response Spectrum Definition †

Response of a Linear One-Degree-of-Freedom System †

Definitions † Standardized Shock Response Spectrum †

Choice of Damping † Shock Response Spectra

Domains † Algorithms for Calculation of the Shock

Response Spectra † Choice of the Digitization Frequency

of the Signal † Use of Shock Response Spectra for the

Study of Systems with Several Degrees of Freedom

12.4 Pyroshocks ........................................................................... 12-17

12.5 Use of Shock Response Spectra .......................................... 12-18

Severity Comparison of Several Shocks † Test

Specification Development from Real Environment Data

12.6 Standards .............................................................................. 12-24

Types of Standards † Installation Conditions of Test

Item † Uncertainty Factor † Bump Test

12.7 Damage Boundary Curve ................................................... 12-26

Definition † Analysis of Test Result

12.8 Shock Machines ................................................................... 12-28

Main Types † Impact Shock Machines † Shock

Simulators (Programmers) † Limitations † Pneumatic

Machines † High Impact Shock Machines † Specific

Test Facilities

12.9 Generation of Shock Using Shakers .................................. 12-44

Principle Behind the Generation of a Simple Shape Signal

versus Time † Main Advantages † Pre- and

Postshocks † Limitations of Electrodynamic

Shakers † The Use of Electrohydraulic Shakers

12.10 Control by a Shock Response Spectrum ........................... 12-52

Principle † Principal Shapes of Elementary Signals †

Comparison of WAVSIN, SHOC Waveforms, and

Decaying Sinusoid † Criticism of Control by a Shock

Response Spectrum

12.11 Pyrotechnic Shock Simulation ........................................... 12-58

Simulation Using Pyrotechnic Facilities † Simulation

Using Metal-to-Metal Impact † Simulation Using

Electrodynamic Shakers † Simulation Using

Conventional Shock Machines

12-1

© 2005 by Taylor & Francis Group, LLC

Summary

Transported or on-board equipment is very frequently subjected to mechanical shocks in the course of its useful

lifetime (in material handling, transportation, etc.). This kind of environment, although of extremely short

duration (from a fraction of a millisecond to a few dozen milliseconds), is often severe and cannot be neglected.

What is presented in this chapter is summarized here. After a brief recapitulation of the shock shapes most

widely used in tests, the shock response spectrum (SRS) is presented with its numerous definitions and calculation

methods. The main properties of the spectrum are described, showing that important characteristics of the

original signal can be drawn from it, such as its amplitude or the velocity change associated with the movement

during a shock.

The SRS is the ideal tool for comparing the severity of several shocks and for drafting specifications. Recent

standards require writing test specifications from real environment measurements associated with the life profile of

the material (test tailoring). The process that makes it possible to transform a set of recorded shocks into a

specification of the same severity is detailed.

Packages must protect the equipment contained within them from various forms of disturbance related to

handling and possible free fall drop and impact onto a floor. A method to characterize the shock fragility of the

packaged product, using the “damage boundary curve” (DBC), and to choose the characteristics of the cushioning

material constituting the package is described.

The principle of shock machines that are currently most widely used in laboratories is described. To reduce costs

by restricting the number of changes in test facilities, specifications expressed in the form of a simple shock (halfsine,

rectangle, sawtooth with a final peak) can occasionally be tested using an electrodynamic exciter. The problems

encountered, which stress the limitations of such means, are set out together with the consequences of modifications,

that have to be made to the shock profile on the quality of the simulation.

Determining a simple shape shock of the same severity as a set of shocks on the basis of their response

spectrum is often a delicate operation. Thanks to progress in computerization and control facilities, this

difficulty can sometimes be overcome by expressing the specification in the form of a response spectrum and by

controlling the exciter directly from that spectrum. In practical terms, as the exciter can only be driven with a

signal that is a function of time, the software of the control rack determines a time signal with the same

spectrum as the specification displayed. The principles of composition of the equivalent shock are described with

the shapes of the basic signals commonly used, while their properties and the problems that can be

encountered, both in the generation of the signal and with respect to the quality of the simulation obtained, are

emphasized.

Pyrotechnic devices or equipment (cords, valves, etc.) are frequently used in satellite launchers due to the

very high degree of accuracy that they provide in operating sequences. Shocks induced in structures by explosive

charges are extremely severe, with very specific characteristics. It is shown that they cannot be correctly

simulated in the laboratory by conventional means and that their simulation requires specific tools.

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