BRUEL & KJAER 1022 - Page 43

BRUEL & KJAER 1022

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20

10

0

50

100

200

Cjs

300

400

Fig. 3.18b. Recording

of

the

mechanical

strain

of

a bar. Measured

with

an

arrangement as

shown

in

Fig. 3.18a.

By

using

the

built-in

meter

on

the

Amplifier

Type

2603,

the

output

voltage

of

the

Accelerometer

can

be

observed

and

the

acceleration

calculated

from

the

Accelerometer's

sensitivity

curve.

From

this

the

driving

force

can

be

calculated,

using

Newton's

equation

F = m X

a,

where

F =

the

driving

force

,

m

=

the

mass,

and

a =

the

acceleration

.

To

measure

the

mechanical

strain

in

the

object

under

test

a

Strain

Gage

is

used.

This

is a

pick-up

device

which

is

comprised

of

a

looped

resistance

(or

resis

·

tances)

sandwiched

between

insulating

m-aterial

which

is

cut

in

the

form

of

a

strip

,

and

which

can

be

glued

on

to

the

test

object.

The

object

when

subjected

to

mechanical

strain

is

distorted

and

this

will

alter

the

Gage

resistance,

the

variation

being

registered

by

a

sensitive

measuring

bridg{'

arrangement,

e.g.

the

Strain

Gage

Apparatus

Type

1516. (

For

further

informa

-

tion

refer

to

manual

).

The

output

voltage

from

the

Strain

Gage

Apparatus

is

then

fed

to

the

input

of

the

Level

Recorder

of

the

Type

3308, to

give

an

automatic

recording.

An

example

of

such

a

recording,

taken

on

a

thin

metal

bar,

showing

the

mechanical

strain

and

indicating

its

resonant

frequency

, is

shown

in

Fig.

3.18b.

41