70811_26.pdf

CHAPTER 26, PART I

SHOCK TESTING

MACHINES

Richard H. Chalmers

INTRODUCTION

Equipment must be sufficiently rugged to operate satisfactorily in the shock and vibra-

tion environments to which it will be exposed and to survive transportation to the site

of ultimate use. To ensure that the equipment is sufficiently rugged and to determine

what its mechanical faults are, it is subjected to controlled mechanical shocks on shock

testing machines. Mechanical shock is a nonperiodic excitation (e.g., a motion of the

foundation or an applied force) of a mechanical system that is characterized by sud-

denness and severity, and it usually causes significant relative displacements in the sys-

tem. The severity and nature of the applied shocks are usually intended to simulate

environments expected in later use or to be similar to important components of those

environments. However, a principal characteristic of shocks encountered in the field is

their variety. These field shocks cannot be defined exactly. Therefore shock simulation

can never exactly duplicate shock conditions that occur in the field.

There is no general requirement that a shock testing machine reproduce field

conditions. All that is required is that the shock testing machine provide a shock test

such that equipment which survives is acceptable under service conditions. Assur-

ance that this condition exists requires a comparison of shock test results and field

experience extending over long periods of time. This comparison is not possible for

newly developed items. It is generally accepted that shocks that occur in field envi-

ronments should be measured and that shock machines should simulate the impor-

tant characteristics of shocks that occur in field environments or have a damage

potential which by analysis is shown to be similar to that of a composite field shock

environment against which protection is required.

A shock testing machine (frequently called a shock machine ) is a mechanical

device that applies a mechanical shock to an equipment under test. The nature of the

shock is determined from an analysis of the field environment. Tests by means of

shock machines usually are preferable to tests under actual field conditions for four

principal reasons:

1. The nature of the shock is under good control, and the shock can be repeated

with reasonable exactness. This permits a comparative evaluation of the equip-

ment under test and allows exact performance specifications to be written.

2. The intensity and nature of shock motions can be produced that represent an

average condition for which protection is practical, whereas a field test may

involve only a specific condition that is contained in this average.

26.1

26.2

CHAPTER TWENTY-SIX, PART I

3. The shock machine can be housed at a convenient location with suitable facilities

available for monitoring the test.

4. The shock machine is relatively inexpensive to operate, so it is practical to per-

form a great number of developmental tests on components and subassemblies in

a manner not otherwise practical.

SHOCK-MACHINE CHARACTERISTICS

DAMAGE POTENTIAL AND SHOCK RESPONSE SPECTRA

The damage potential of a shock motion is dependent upon the nature of an equip-

ment subjected to the shock, as well as upon the nature and intensity of the shock

motion. To describe the damage potential, a description of what the shock does to an

equipment must be given—a description of the shock motion is not sufficient. To

obtain a comparative measure of the damage potential of a shock motion, it is cus-

tomary to determine the effect of the motion on simple mechanical systems. This

is done by determining the maximum responses of a series of single degree-of-

freedom systems (see Chap. 2) to the shock motion and considering the magnitude

of the response of each of these systems as indicative of the damage potential of the

shock motion. The responses are plotted as a function of these natural frequencies.

A curve representing these responses is called a shock response spectrum, or

response spectrum (see Chap. 23). Its magnitude at any given frequency is a quanti-

tative measure of the damage potential of a particular shock motion to a single

degree-of-freedom system with that natural frequency. This concept of the shock

response spectrum originally was applied only to undamped single degree-of-

freedom systems, but the concept has been extended to include systems in which any

specified amount of damping exists.

The response of a simple system can be expressed in terms of the relative dis-

placement, velocity, or acceleration of the system. It is customary to define velocity

and acceleration responses as 2

f ) 2 times the maximum relative displace-

ment response, where f is frequency expressed in hertz. The corresponding response

curves are called displacement, velocity, or acceleration shock response spectra. A

more detailed discussion of shock response spectra is given in Chap. 23.

Of the three motion parameters (displacement, velocity, and acceleration)

describing a shock spectrum, velocity is the parameter of greatest interest from the

viewpoint of damage potential. This is because the maximum stresses in a structure

subjected to a dynamic load typically are due to the responses of the normal modes

of the structure, that is, the responses at natural frequencies (see Chap. 21). At any

given natural frequency, stress is proportional to the modal (relative) response

velocity. 1 Specifically,

f and (2

σ max =

ν max

(26.1)

where

σ max =

maximum modal stress in the structure

ν max =

maximum modal velocity of the structural response

Young’s modulus of the structural material

ρ=

mass density of the structural material

constant of proportionality dependent upon the geometry of the

structure (often assumed for complex equipment to be 4

8) 2

26.3

SHOCK TESTING MACHINES

Of course, if the shock response spectrum for a test machine–generated shock is com-

puted solely to validate that test results comply with a specified shock response spec-

trum, or for comparison to the shock response spectra computed from measured

shocks in a service environment, then either displacement or acceleration shock

response spectra are as meaningful as a velocity shock response spectrum. However,

if the maximum stress in the structure subjected to the shock is of primary interest,

the velocity shock response spectrum is the most applicable.

MODIFICATION OF CHARACTERISTICS

BY REACTIONS OF TEST ITEM

The shock motion produced by a shock machine may depend upon the mass and fre-

quency characteristics of the item under test. However, if the effective weight of the

item is small compared with the weight of the moving parts of the shock machine, its

influence is relatively unimportant. Generally, however, the reaction of the test item

on the shock machine is appreciable and it is not possible to specify the test in terms

of the shock motions unless large tolerances are permissible. The test item acts like

a dynamic vibration absorber (see Chap. 6). If the item is relatively heavy, this causes

the shock response spectra of the exciting shock to have minima at the frequencies

of the test item; it also causes its mounting foundation to have these minima during

shock excitation at field installations. Shock tests and design factors are sometimes

established on the basis of an envelope of the maximum values of shock response

spectra. However, maximum stresses in the test item will most probably occur at the

antiresonance frequencies where the shock response spectrum exhibits minimum

values. To require that the item withstand the upper limit of spectra at these fre-

quencies may result in overtesting and overdesign. Considerable judgment is there-

fore required both in the specification of shock tests and in the establishment of

theoretical design factors on the basis of field measurements. See Chap. 20 for a

more complete discussion of this subject.

DOMINANT FREQUENCIES OF SHOCK MACHINES

The shock motion produced by a shock machine may exhibit frequencies that are char-

acteristic of the machine. These frequencies may be affected by the equipment under

test.The probability that these particular frequencies will occur in the field is no greater

than the probability of other frequencies in the general range of interest. A shock test,

therefore, discriminates against equipment having elements whose natural frequencies

coincide with frequencies introduced by the shock machine. This may cause failures to

occur in relatively good equipment whereas other equipment, having different natural

frequencies, may pass the test even though of poorer quality. Because of these factors,

there is an increasing tendency to design shock machines to be as rigid as possible, so

that their natural frequencies are above the range of frequencies that might be strongly

excited in the equipment under test. The shock motion is then designed to be the sim-

plest shape pulse that will give a desired shock motion or response spectrum.

CALIBRATION

A shock-machine calibration is a determination of the shock motions or response

spectra generated by the machine under standard specified conditions of load,

mounting arrangements, methods of measurement, and machine operation. The

26.4

CHAPTER TWENTY-SIX, PART I

purpose of the calibration is not to present a complete study of the characteristics of

the machine but rather to present a sufficient measure of its performance to ensure

the user that the machine is in a satisfactory condition. Measurements should there-

fore be made under a limited number of significant conditions that can be accurately

specified and easily duplicated. Calibrations are usually performed with deadweight

loads rigidly attached to the shock machine.

The statement of calibration results must include information relative to all fac-

tors that may affect the nature of the motion. These include the magnitude, dimen-

sions, and type of load; the location and method of mounting of the load;

factors related to the operation of the shock machine; the locations and mounting

arrangements of pickups; and the frequency range over which the measurements

extend.

SPECIFYING A SHOCK TEST

Two methods of specification are employed in defining a shock test: (1) a specifica-

tion of the shock motions (or response spectra) to which the item under test is sub-

jected and (2) a specification of the shock machine, the method of mounting the test

item, and the procedure for operating the machine. 3

The first method of specification can be used only when the shock motion can be

defined in a reasonably simple manner and when the application of forces is not so

sudden as to excite structural vibration of significant amplitude in the shock

machine. If equipment under test is relatively heavy, and if its normal modes of

vibration are excited with significant amplitude, the shock motions are affected by

the load; then the specified shock motions should be regarded as nominal. If compa-

rable results are to be obtained for tests of different machines of the same type, the

methods of mounting and operational procedures must be the same.

The second method of specification for a shock test assumes that it is impractical

to specify a shock motion because of its complexity; instead, the specification states

that the shock test shall be performed in a given manner on a particular machine.

The second method permits a machine to be developed and specified as a standard

shock testing machine. Those who are responsible for the specification then should

ensure that the shock machine generates appropriate shock motions. This method

avoids a difficulty that arises in the first method when measurements show that the

shock motions differ from those specified. These differences are to be expected if

load reactions are appreciable and complex.

A shock testing machine must be capable of reproducing shock motions with

good precision for purposes of comparative evaluation of equipment and for the

determination as to whether a manufacturer has met contractual obligations. More-

over, different machines of the same type must be able to provide shocks of equiva-

lent damage potential to the same types of equipment under test. Precision in

machine performance, therefore, is required on the basis of contractual obligations

and for the comparative evaluation of equipments even though it is not justified on

the basis of a knowledge of field conditions.

Sometimes equipment under test may consistently fail to meet specification

requirements on one shock machine but may be acceptable when tested on a different

shock machine of the same type. The reason for this is that small changes of natural fre-

quencies and of internal damping, of either the equipment or the shock machine, may

cause large changes in the likelihood of failure of the item. Results of this kind do not

necessarily mean that a test has been performed on a faulty machine; normal variations

of natural frequencies and internal damping from machine to machine make such

26.5

SHOCK TESTING MACHINES

changes possible. However, standard calibrations of shock machines should be made

from time to time to ensure that significant changes in the machines have not occurred.

SHOCK TESTING MACHINES

CHARACTERISTIC TYPES OF SHOCKS

The shock machines described below are grouped according to the types of shocks

they produce. When a machine can be classified under several headings, it is placed

in the one for which it is primarily intended. One characteristic shared by all shock

machines is that the motions they produce are sudden and likely to create significant

inertial forces in the item under test. The types of shock shown in Fig. 26.1 are clas-

sified as ( A ) through ( D ), simple shock pulses, whose shapes can be expressed in a

practical mathematical form; ( E ), single complex shock; and ( F ), a multiple shock. In

contrast to a simple shock pulse specification, the motions illustrated in Fig. 26.1 ( E )

and ( F ) often are the result of a shock test in which the shock testing machine, the

method of mounting, and machine operations were specified.

Velocity Shocks. A velocity shock is produced by a sudden change in the net

velocity of the structure supporting the item under test. When the duration of the

shock is short compared to the periods

of the principal natural frequencies of

the item under test, a velocity shock is

said to have occurred. Figure 26.1 A

shows a nearly instantaneous change in

velocity. The shocks shown in Fig. 26.1 B,

C, and D are also considered velocity

shocks if the above shortness criterion is

met. Velocity shocks produce substantial

energy at the principal natural frequen-

cies of the item under test. This is illus-

trated in Figs. 26.2 and 26.3, which show

the shock response spectra (computed

with a zero damping ratio) for the half-

sine and sawtooth acceleration pulses in

Fig. 26.1 A and B, respectively. Note in

both cases that the values of the velocity

shock response spectra are uniform at

all frequencies below about Tf

0.2.

Hence, from Eq. (26.1), they have the

potential to cause substantial damage to

the basic structure of the item under

test, assuming the item has natural fre-

quencies below f

0.2/ T Hz.

Displacement Shocks. Some shock

test machines produce a sequence of

two or more velocity shocks with equal

and opposite velocity magnitudes such

that the test item experiences no net

velocity change. For example, the half-

FIGURE 26.1 Characteristic types of shocks.

( A ) Velocity shock, or step velocity change. ( B )

Simple half-sine acceleration shock pulse. ( C )

Rectangular force pulse. ( D ) Sawtooth accelera-

tion pulse. ( E ) Single complex shock. ( F ) Multi-

ple shock.

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