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Патент USA US2403999

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T. iA. READ m.
Filed Feb. 22‘, 1944
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Patented July 16,_ 1946
Thomas A. Read, Herbert I. Fusfeld, and Sumner
W. Kitchen, Philadelphia, Pa.
Application February 22, 1944, Serial N0. 523,430
4 Claims.
(Cl. ’i3-69)
(Granted under the act of March 3, 1883, as
amended April 30, 1928; 370 O. G. 757)
The invention described herein may be manu
factured and used by or for the Government for
ing same are shown by the accompanying draw
governmental purposes without the payment to us
of any royalty thereon.
Our invention relates to the testing of metals
and it has particular reference to methods for
detecting ilaws, cracks and other defects in metal
ings wherein:
Fig. l is a diagrammatic showing of brass car
tridge case testing equipment which incorporates
the principles of this invention;
Fig. 2 is an enlarged section view showing
preferred mechanical constructions for Fig. l’s
case support and driving magnet;
Broadly stated, the object of our invention is
Fig. 3 is a horizontal section on line 3-3
to provide improved procedure by which the pres 10 through the top plate and circular air gap of the
ence of internal flaws in metal specimens of vari
ous compositions and shapes may be detected
positively, quickly and reliably.
driving magnet of Fig. 2;
Fig. 4 is a showing of further details of the
strain gauge which is attached to the test speci
A more speciñc object is to make special ad
men side ;
aptation of our improved testing method to the 15
Figs. 5, 6 and 7 are curves illustrative of cer
examination of metal specimens which are cylin
tain principles upon which our improved test
drical in shape and non-magnetic in character.
method is based;
Another object is to provide a method of test
Fig. 8 illustrates alternative forms which Fig. l’s
ing the brass cases of fired and other artillery
case driving source and amplitude measuring
cartridges for the presence of dangerous season 20 means may take;
cracks. '
Fig. 9 shows further variations of the basic
system of Fig. 1; .and
A further object is to provide a reliable test
for the susceptibility of fired and resized cartridge
Figs. 10, 11 and12 show alternative arrange
cases to splitting or other fracture on subsequent
ments by which the amplitude of case vibration
25 may be registered and measured.
A still further object is to provide a test methodV
by which it is possible prior to resizing to reject
In Fig. l we have shown our improved testing
all cartridge cases which after being resized and
facilities as organized for detecting the presence
reconditioned will crack on subsequent firing.
The improved metal testing method of our in 30 of flaws in a brass cartridge case I5. For clarify
ing the description to follow it will be assumed
vention is predicated on our discovery that the
that this case is designed for use in a 75 mm.
“damping” characteristics of vibrated metal
artillery piece. As the description proceeds it
specimens will sharply reiiect the presence of in
will become evident that other sizes and shapes of
ternal iiaws if the specimen vibrations have such
cartridge cases may be tested with equal facility,
intensified amplitude as to set up in the metal
and that metal specimens of compositions other
peak stresses which are far higher than any here
than brass and of shapes and contours other than
tofore employed by tests of the “sonic” character
represented at I5 likewise lend themselves to
here considered.
test by our improved flaw detecting method here
In practicing our invention we attain the fore
in disclosed.
going and other objects by exciting the metal
Essential elements of the equipment shown in
specimen to longitudinal vibration at its natural
Fig. 1 include: (ai) a case vibrating magnet M;
or resonant frequency; by so building up the
(b) a source B-C of alternating current power
amplitude of this resonant vibration that the
for energizing a driving winding I6 oi the mag
peak stress thereby induced in the metal has an
net; (c) apparatus including a strain gauge S for
exceedingly high intensity (of the order of ñve to
measuring the amplitude of vibration that is inten thousand pounds per square inch for brass
duced in the test case I5; and (d) current and
cartridge case metal); by measuring the damp
voltage measuring instruments A and V for giving
ing capacity (i. e. the internal friction) at this
the ratio of the driving power input to the strain
high amplitude of vibration; and by comparing
gauge output.
the observed measurement with known stand
ards for purposes of classifying the metal in the
specimen either as sound or as defective.
The case driving magnet M has the construc
Illustrative embodiments of our improved test
tion best shown in Figs. 2-3. A central core I8
ing method and of apparatus suitable for practic
of magnetically soft iron is fastened to a lower
plate IQ of the same material and substantially
square in shape. Extending upwardly from each
of the four edges of this lower plate are side
plates 2S, also of magnetic iron. Fitted into the
square opening formed by the upper edges of
these side plates is an iron top plate 2l having in
its center a round Opening somewhat larger than
described are found to have a natural vibration
frequency of the order of about four thousand
cycles per second, and in conducting tests on such
cases source B-C thus should be capable of sup
the top >of the central core I8.
plying case driving energy at four thousand cycles
per second and further be adjustable through one
or two hundred cycles above and below the stated
The ’ utility of this adjustment will Ibe
come further evident as the description proceeds.
The step down transformer T is for the purpose
The annular space between magnet core I8 and
the surrounding plate metal 2| constitutes an airl
gap through which a direct current winding 24
causes te flow unidirectional magnetic flux from
of converting alternating current energy of the
the central core I8 in all radial directions to the
source B-C into energy of the lowered voltage
surrounding top plate 2l. This core Winding 2li
and relatively high amperage which is required
for satisfactory operation of the case driving
moderate-voltage potential readily availalble at
is continuously excited by any suitable source of
direct current, designated in Fig. 1 by the termi-`
nal “-i-” and “-.” Energizing leads 25;?5 are
brought from winding 24 to the outside of mag
net M in any suitaßlole manner suchv as that indi
cated in Fig. 2,
magnet M.
ers on an insulating tube about 8 inches in diam
20 eter, and having a secondary made up of a single
The outside diameter of the magnet’s central
core I8 is slightly less than the inside mouth
diameter of the cartridge case l5 to be tested,
while the inside diameter of the central opening
in the magnet’s top plate 2l is somewhat larger 25
than the mouth diameter of the same case.
permits the case mouth to ht down into the mag
net’s annular gap in a manner clearly- indicated
In one design which has proven sat- ,
isfactory this transformer makes use of a primary
having 8S turns of No. 18 wire wound in two lay
copper strip 2 inches lv'v'ide by .020 inch thick
wound as a single turn around the same tube.
The particular transformer used is of the air core
This copper strip of 2 inches by .020 inch cross
’ section is, in the test equipment now described,
continued from the transformer T into the air
gap of the driving magnet M where it constitutes
by Figs. 1-2-3 and to a depth of slightly over 11/2
the single turn driving winding shownV at I6 in
inches. In this position there is passed through 30 Figsv 2_3. For producing `cartridge case vibra
the ease mouth metal the radial magnetic flux
tions of the elevated order required by our test
earlier referred to as being set up by direct cur
method currents of the order of several hundred
rent winding 2li. Preferably this unidirectional
amperes are supplied .by the secondary of this
magnetic force has an intensity of the order of
transformer T.
10,000 oersteds.
The load thus presented to the power source
Positioned between the case mouth metal'and
B--C is found to be predominantly inductive, and
the-magnet plate metal 2| is the alternating cur
without corrective means results in an objection
rent driving winding I5 earlier mentioned. Pref
ably low power factor. To compensate for this
erably this winding takes the form of the repre
use is made of a capacitor 35 series connected in
sented single turn of copper strip 'secured inside 4 the transformer primary `circuit as shown in Fig.
the opening in plate 2l in any suitable manner
l.’ By choosing the capacitive rea'ctance of ele
such as shown in Figs.- 2-3. There thin layers of
ment 3S to equal the inductive reactance of trans
insulation 28 separate the copper strip I5 from
former T and its connected load, the power fac
the. plate metal and at the same time mechani
tor of energy drawn from the alternating .current
cally support the Astrip through a «bonding thereof
source B-C may be made substantially unity.
to the metal 2 l .
Under these conditions the driving magnet M is
For holding the cartridge case I5 in_the repre
observed tol impose a load on the source B-C
sented test position use may be made of any suit
which approximates a pure resistance of about 12
able means which impart mechanical support
without interfering with case vibration. Such
The described magnet M and energizing sources
means may take the form of a support plate 33
therefor constitute an “eddy current” metal
held at suitable distance above the top of magnet
drive by which there is exerted on the case mouth
M Iby corner uprights 3d and having a central
mechanical forces which alternately act upward
opening somewhat larger than the outside diam
ly and downwardly This action results from the
eter of the cartridgey case VHi. Two or more 55 fact that the alternating 4current in the driving
strands of piano wire 3l are stretched at moder
winding l5 induces corresponding`- eddy currents
ate tension across this opening «between securing
in the case mouth metal. The interaction of
screws S2. By restraining downward movement
these induced eddy currents with the radially
of the case head rim, these wires 3l mechanically
flowing unidirectional magnetic flux produces on
suspend the case in the test position represented. 60 the case mouth metal an alternating mechanical
force »which is 4directed along the axis of this case.
This alternating mechanical force reverses its
For causing the just described magnet M to
direction in step with the frequency of the cur
drive the cartridge case i5 at its natural rate of
rent from driving source B-C, and when the fre
vibration use is made of the facilities which the 65 quency thereof is chosen to match the test case’s
left portion of Fig. i represents. These facilities
natural frequency of vibration there may be pro
include the earlier named source B-C of alter
duced in the case longitudinal vibrations of the
hating current power plus a step down trans
extremely elevated magnitude which the test
former T, a capacitor 36, the vearlier mentioned
method of our invention requires.
amine-ter A and a frequency meter F.
Since the case driving forces are directed alter
Source B-C may take any suitable form capa
nately in opposite directions at a frequency hav
ble of supplying up to about one kilowatt of power
ing the stated value of around four thousand
at a frequency which exactly matches the natural
cycles per second, the average force of each of.
or resonant rate of longitudinal vibration for the
these complete cycles is zero and this makes it
cartridge case l5. Cases ofthe 75 mmf size here' 75 possible to support the case in a position in the
magnet gap bythe light Wires'indicated at 3I
ment possible use is made ofythe amplifier' 38.
in engagement with the rim at the head of the
Thisampliñer may be any one of a number >of
commercially available forms, and for this rea
son no attempt to show details has been made.
Exciting its input terminals is the potential
appearing across the resistor 43. .Each change
therein is magniñed many times by the ampliñer
For measuring the amplitude of the mechanical
vibrations thus induced in the test case I5 use
i's made of the earlier named strain gauge S plusv
and impressed upon a suitable measuring device
an amplifier 38 plus the earlier named volt
meter V.
indicated as voltmeter V.
As is more clearly shown in Fig. 4, the strain
gauge S consists of a series of back and forth
When-appropriately calibrated, this voltmeter
can thus be made to yield a direct indication of
the amplitude at which the cartridge case I5 is
loops of electrical resistance wire 4!) mechanically
being vibrated yduring practice of the improved
bonded together by suitable insulating material
test method of our invention. Conveniently this>
and further bonded to the wall side of the car 15 indication may be in terms of the millivolts of
tridge case I5 under test. As a result of this
potential which appear across resistor 43, and
bonding any change in the length of the test
in connection with certain test data later pre
metal I5 produces a corresponding changeV in the
sented use is made 0f a voltmeter indication ex
mechanical length of the‘gauge loops 40.
pressed in such terms.
These loops are connected in series and the 20
The combination just described is thus so or~Ã
ganized that complete absence yofvibration on
resistance variations of’ each which accompany
changes in its effective length are additively com-V
the part of the test case I 5 will cause meter V to
bined to give a total resistance change which
give what may be termed a “zero” indication;
lends itself to ready measurement or detection.
that vVibration of moderate amplitude on the part
of case I5 will produce an intermediate reading
Strain gauges of the type shown at S are commer
by voltmeter V; and that vibration of high am-~
cially available, and one design found especially
plitude by ca'se I5 will produce acorrespondingly
suitable has a static resistance of 500 ohms and
high reading by thevoltmeter.
a strain sensitivity of 3.5. By strain sensitivity
is meant the ratio of the fractional change in re
sistance of the gauge to the fractional change in
gauge length.
‘ For most effective response to longitudinal vi
. bration of the test case I5 the strain gauge S "
In applying the just described test equipment
of Fig. 1 to ñred ’l5 mm. brass cartridge cases we
are able to predetermine with high accuracy all
of those cases in the tested group which will rup
should be attached at some intermediate point
between the two case ends, such as is sho-wn in
ture upon resizing, reloading and subsequent fir
Figs. 1_2. This particular location was selected
after experimentation which showed that in lon»
ing, and which of the cases in the group will with
stand the reñring without rupture or other fail
ure of the case metal.
gitudinal Vibrations of the character here con
The need for such determination has been felt
sidered the effect is analogous to simple compres
sion and stretching from the two case ends. This 40 in a practical way for a long time. Splits which
have been obtained in the firing of resized brass
results in a point known as the displacement
cases are now ascribed to the presence of season
“node,” at lwhich mechanical movement of the
cracks. These cracks are caused by the at
case divides and further at which there is no
tacks of the products of the burning of the pro
change of mechanical position.
For cartridge cases of the 75 mm. design shown, 45 pellant powder during the period between the
ñring of the round and the cleaning of the case.
this point is approximately one-third of the case
Such cleaning is subject to considerable delay
length distance from the head end. Mounting
by reason of the fact that in many instances the
of the strain gauge S at this point results in the
cases lay out in the ñeld exposed to weather for
maximum movement between the two gauge
ends; and while other positions are found also 50 long periods of time after their original iiring,
and when subsequently subjected to resizing op
to give indications of the case vibration amplitude
erations preparatory to a second use there fre
their effect is relatively less than the nodal point
quently have developed minute cracks, :particu
mounting here shown.
larly on the side Wall interiors.
For converting variations in strain gauge re
These are so'inconspicuous and diflicult to de
sistance into corresponding variations in poten 5,5
tect that they go completely unnoticed through
tial, use is made of a battery or other direct cur
the entire sequence of resizing operations and are
rent source 42 of constant potential connected
only discovered upon reiiring of the case, when
through the strain gauge loops 40 in series with
their presence then results in longitudinal splits
a resistor 43.
As long as the strain gauge remains static the 60 or other failure of the case. Such failures not
only endanger -personnel ñring the weapon but
current flow through resistor 43 is constant and
by reason of loss in chamber pressure they so alter
the voltage ‘appearing thereacross remains un
the 'ballistic performance of the fired projectile
changed. Each decrease in strain gauge resist
that aiming becomes inaccurate and unreliable.
ance which accompanies an effective compression
of the cartridge case I5 raises the resistor cur 65 In addition the escaping gases erode the weapon’s
chamber and firing pin causing early malfunc
rent and produces a corresponding rise in volt
tion of the weapon.
age between the resistor terminals. Similarly,
Until the advent of our invention, no method
each increase in strain gauge resistance which
was available for reliably separating the good
accompanies an effective stretching of the test
case I5 produces a corresponding drop in resistor 70 from the bad prior to case resizing or at any time
prior to case reuse. With our method, however,
current and a resultant lowering of the resistor
the desired selection can be made quickly, re
terminal voltage.
liably and effectively.
The magnitude of these changes in resistor
In Vusing the earlier described equipment of
voltage is relatively small and in order to increase
Fig. 1_ each of the cartridge cases to be testedis-A
it to‘such an >extent as to make ready measure
subjected to onlytwo preparatory operations.
The primer is punched out of the case head, .and
the mouth of the case is restored substantially to
spending elongation in the side wall metal dabove
the nodal point.
During resonant vibration, therefore, a simple
of the case mouth ñts into the `annular gap of .the
“bellows” action takes place wherein the case
metal at the nodal point and on both sides there
of (with the exception of the case extremities)
alternately shortens under compression and
lengthensr under tension. Although the nodal
driving magnet M and is there supported by a
point completely lacks longitudinal motion, some>
its original circular shape.
. So prepared the case then is lowered through
the ‘opening Vin the support `plate 33 (see Fig. 2)
to the `represented position where the’wall Vmetal
resting of the case head rim on the supporting 10 transverse expansion and contraction of the case
circumference appears there to be present.
wires 53|. The strain gauge S is now attached to
For the relatively low intensities of induced case
the case >side wall >at the location between the
vibration which have heretofore been used in at
two ends 0f thev case determined as earlier
- Direct current is now applied to the magnetM’s
tempting to test material specimens, it is ob
served that a defective specimen exhibits prac
tically the same “damping” capacity as does a
central core winding 2e, and alternating current
sound specimen. In the curve of Fig. 6 such rela
from source B-C is applied to transformer T and
tively low vibration amplitudes are indicated by
thence tothe case driving winding IS. ri'his di
the vertical line 45, and typically they have re
rect current supply is adjusted to a value prede
termined as suitable for the test and which yields 20 sulted in peak stresses in the test metal of the
order of five hundred pounds per square inch or
the required intensity of uni-directional flux in
the circular gap surrounding the magnet’s central
We have discovered that when the amplitude
core head I8.
of the longitudinal test case vibrations is in
The frequency of the alternating current from
creased to a substantially higher value, such as
source B-C is, by the aid of meter F, roughly ad
is designated at 46 in Fig. 6, a defective case will
justed to what is expected will match the reso
show a substantially higher damping than does a
nant frequency-of vibration for the case I5. An
sound case. By damping is meant the internal
observation at meter V of the resulting amplitude
friction of the case metal, and one convenient
of case vibration is taken, and holding the case
driving current as shown by meter A at a con 30 measure thereof is given by the ratio of the case
stant value, the frequency of the alternating driv
ing current is varied'in small steps in both direc
tions `until .the amplitude meter V shows a maxi
driving force to the resulting vibration amplitude
as indicated through the strain gauge S by Fig.
i’s voltmeter V.
During the remainder of this .
description a quantity proportional to that ratio
be designated as the “sonic test coefficient.”
This maximum indicates that the frequency of 35
Observations made by us show that a sound
the alternating driving current now exactly
test specimen has a damping-amplitude curve of
matches the resonant frequency of vibration for
the relatively flat character shown at 41 in Fig. 6;
the test case i5. This indication follows from
thatspecimen metal having defects of moderate
the well known resonance curve illustrated in Fig.
quantity has a relatively steeper curve such >as is
5 which shows how the test case I5 responds in 40
shown at 48; that the presence of fewer defects
vibration amplitude to driving current frequen
causes the curve to be less steep, as shown at 49;
cies above and below the resonant value which
and that defects present in larger number result
the dotted vertical line designates.
in a curve of the greater steepness shown at 50.
Explanation has already been given of how the
Accordingly, in operating the Fig. 1 equipment
eddy 'currents induced in the case mouth metal
the driving current supplied from source B-C
interact with the uni-directional flux iiowing
to the magnet winding I6 is next increased to
through lthat metal to set up in the metal mechan
such an extent that the resulting resonant vi
ical- forces which alternately act upwardly and
bration of the test case l5 rises to an amplitude
downwardly instep with the reversals of current
of the high order shown at 46 in Fig. 6.
mum reading.
induced in the case mouth metal.
At frequencies ~
of the order of four thousand cycles per second
the duration of each of these forces is extremely
That high order amplitude is known to be at
short. Each upwardly acting force pulse tends
tained when meter V gives a reading correspond
ing to about 12 millivolts input to the amplifier.
It is accompanied by peak stresses in the case
to compress the side wall metal above the case
_ metal of the order of from live to ten thousand
mouth, while each downwardly acting force tends
to stretch or elongate it.
pounds per square inch. Correlation of the read
ings of meter V' with such stresses can be made
The inertia of the total case mass prevents the
in any one of a number of manners so well known
complete case from following these pulsations,
to the metals testing art that description at'this
point isdeemed unnecessary.
andv even though applied at one end only their
At these high stress amplitudes of longitudinal
effect is to set up longitudinal vibrations toward (Si)
vibration there is a relatively wide variation in
and away from an intermediate “nodal” point
the damping capacities of sound and defective
along the case length. Were both ends of a, sim
cartridge cases. As Fig. 6 indicates, the greater
ple vcylindrical test specimen to be open this point
the defects the higher the damping and hence the
would `be substantially midway. Closure of the
more driving force that is required to induce the
top end by the case head shifts this neutral point
high vibration amplitude.
upwardly to about two-thirds of the total distance
Since for a given Vibration amplitude there is
from the case mouth.
a more or less direct relation between the speci
Our observations show that at resonant fre
men damping and the driving power require
quency for Vthe case each compressive action in
ments, those cartridge cases which are defective
the case side wall below the nodal point is accom- `
require considerably more power from driving
panied by a similar compressive action in the
source B-C than do cases which are sound. One
case side wall above the nodal point; likewise,
measureof this power is the reading of ammeter
that each side wall stretch or elongation below_
_ A. It is found that the impedance of ¿the case;
the nodal point ls also accompanied by corre
' driving circuit remains substantially constant re
gardless of whether the case is sound or defective,
and for this reason the reading of ammeter A
may be assumed to be directly proportional to
This data shows an excellent correlationÍ be
tween the sonic test coefiicient and the incidence
the driving power input.
the group was a result of its selected components
for it was expected that most of the cases with
of ñring splits. The high percentage of splits in
a coemcient above 0.50 would split.l f
In order that some variation in the values of
test vibration amplitude may be permitted, we
prefer to analyze comparative test results
through a ratio earlier termed as the “sonic test
coefficient.” This coeñîcient is the quotient of the
ampere current reading by meter A to the milli
volt potential appearing across resistor 43 as read
In Fig. 8 we have represented alternative forms
of case driving power supply and of vibration
amplitude measuring means. These diiîer fromthe corresponding elements earlier described in
connection with Fig. 1 in the manners now to be
by meter V. For test system constants having
pointed out.
one particular set of values, new o-r sound cases
exhibit a coeiiicient value of approximately 0.30;
fired cases having defect contents within toler
ances acceptable for retiring show coeñicients of
0.50 and below; and fired cases having defects
suiiicient to cause rupture after resizing and sub
must be capable of generating a frequency which
matches the resonant vibration frequency of the
test case I5 and should in addition be adjustable
through a small range on either side of theres
onant value. It feeds into the power ampli
fier 53.
This power amplifier 53 corresponds to the
source B-C of Fig. 1 and should have an output
capacity of approximately one kilowatt; in ener
gizing transformer T it performs exactly the same
sequent firing have test coefficients above 0.50.
This relation is best indicated by the curve of
Fig. 7 wherein the horizontal dotted line indi
cates the point at which acceptable cases should
be separated from unacceptable ones on the basis
of proven test results.
One set of data which establishes the foregoing
sonic test correlation is presented by the accom
panying Table X.
Some test correlation
Sonic test
Sonic test
0. 368
0. 760
. 400
. 400
. 417
. 433
. 778
. 779
. 784
. 796
. 437
. 442
. 450
. 450
. 451
. 460
. 467
. 468
. 500
. 800
. 804
. 825
. 842
. 847
. 850
. 886
. 888
. 919
. 933
. 940
. 500
. 516
. 544
. 550
. 568
. 577
. 591
. 592
. 616
. 636
1. 02
1. 14
1. 32
1. 33
. 952
. 955
. 989
. 990
In this Fig. 8 arrangement adjustment of the
power driving frequency, as measured at meter‘
F, is eifected at the driving oscillator, while ad
justment of the driving power current, as meas
ured by meter A, is effected at the power ampli
fier. Both of these adjustments are manual and
. 650
1. 35
. 651
1. 37
. 659
1. 38
. 660
1. 44
. 661
. 666
1. 50
l. 60
. 668
. 676
1. 64
. 679
2. 00
l. 60
. 685
. 705
. 708
2. 35
2. 66
4. 00
. 728
5. 34
. 7 54
O-No defect.
x~Longitudinal crack.
-I--Tran sverse rupture.
The seventy-five ñred brass cartridge cases of
75 mm. size which are identified under the “case”
column were examined for the presence of season
In order to obtain an amplitude measurement
for the vibrating case I5, switch 60 is first thrown
to the upward position wherein its output circuit
feeds into resistor 43 and the tuned-plate ampli
ñer 55.» That amplifier is manually adjusted for
the resonant case frequency, and under these
conditions it functions to magnify the voltage
are made to meet the requirements earlier de
scribed in connection with Fig. 1.
Looking next at the frequency amplitude meas
uring means of Fig. 8, these employ a tuned-plate
amplifier 56, a cathode ray oscillograph 51, a beat
frequency oscillator 58, a potential comparing de
vice 59 and a transfer switch 60.
' function as does the Fig. l source.
. 106
Look first at the case driving power supply
means of Fig. 8, it makes use of a power ampli
ñer 53 and a driving oscillator 511. This oscillator
fluctuations across resistor 43 which the strain
gauge S produces.
So magnified, these fluctuations aretransmit
ted to the cathode ray oscillograph 51 where they
cause to be traced on the oscillograph screen (not
shown) a visual showing of the case vibration
55 wave form and amplitude. In the system of Fig.
1 only the latter quantity can be indicated. The
former occasionally is of value and can be pro
videdby the Fig. 8 arrangement,
The amplitude of the case vibrationwave hav
60 ing been noted on oscillograph 51, transfer switch
60 is shifted to the downward position where the
amplifier 56 has transmitted thereto “compari
son” oscillations from the beat frequency oscil
lator 58. This oscillator is manually adjusted to
the resonant frequency of the test case I5 and it
causes oscillograph 57 to trace a wave of the same
frequency as is present in case I5.
cracks by our improved sonic test method. This
examination gave the “sonic test coefñcients”
oscillograph may be varied by device 59. Adjust
which Table X lists. These eases were then fired
at 12% excess pressure in a 75 mm. weapon hav
ing a worn chamber. 53% of the cases split.
None of the cases with a sonic test coeiü‘cient less
ment from that device is now so made that the
wave traced by oscillograph 57 has the same am
plitude as did the wave there traced when ampli
ñer 55 was connected with the strain gauge S.
than 0.55 split.
All but one of those with. the
coefficient above 1.00 failed during firing.
The amplitude of this wave as show by the
Under these “matched wave” conditions the
75 voltage reading at indicator V of the comparing
device 59 is now noted.
of the strain gauge S earlier described as. being
mechanically attached to the case side wall and
This reading corre
sponds to that directly obtained by voltmeter V
varying its resistance in step with the elonga
in the organization of Fig. 1.
tions and contractions oí the side wall metal.
For purposes of computing “sonic test coeiì
cients” the modiñed organization of Fig. 8k thus Cl While exceedingly satisfactory such a strain
gauge is not, however, the only device which
is the full equivalent of the basic organization
lends itself to registration of amplitude measure
shown in Fig. 1.
During its vibration, a 75 min. case emits a
It has been seen that Fig. 8’s driving. power sup
ply facilities require that the frequency of the
supplied driving power be adjusted at oscillator
54 to match the resonant frequency of the tested
case I5. The need for such manual adjustment
may be dispensed with through use of driving
power equipment organized as shown in Fig. 9.~
There Fig'. S’s driving oscillator ‘54 is replaced
by a phase shifting network 62. The input termi
nal's of this network are directly connected with
the output terminals of an amplifier 'I2 having 20
highly audible sound, and the intensity of this
sound has been observed to vary in direct pro
portion to the amplitude of the vibration. In
the arrangement of Fig. l0 advantage of this
fact is taken by the use oi a microphone 3G placed
to receive the sound waves induced by the longi
tudinal vibrating movements of the case.
This microphone 65 may be of the conven- .
tional carbon-granule type used in commercial
telephones, or the condenser type used in radio
broadcasting, in which case itr sets up resistance
automatic volume control, while the output ter
variations analogous to those produced by the
minals of phase shifter 62 lead directly to the
strain gauge S. This similarity makes possible
input terminals of power ampliñer 53.
a direct substitution in the electrical circuits of
The power ampliiier 53 of. Fig. 9 corresponds
Fig. l, for example, of the microphone 66 for
to the similarly identiiied amplifier of Fig. 8 and 25 the strain gauge S and results in the apparatus
constitutes the source of alternating current drive
organization which Fig. l0 shows.
energy impressed upon transformer T and there
The organization of Fig. 10 employs battery
by transmitted to the driving winding I6 of mag
¿52 in the same manner as does the strain gauge
net MÍ The strain gauge ampliner `33 of Fig. 9,
S of the earlier Views. It is possible to eliminate
in turn, corresponds in all respect to the simi 30 this battery by employing a dynamic type of
larly marked ampliñer of Fig. 1 and indicates the
microphone in the manner shown at 57 in Fig. 1l.
strain gauge output at meter V.
Such a device generates its own potential and
In operation ofthe Fig. 9 system, each vibra
for this reason is suitable for directly exciting
tion of the tested case I5 varies the resistance
`the ampliiier 38 without recourse to the earlier
of strain gauge S, produces a corresponding
shown battery 42 and resistor 43.
change in the voltage across resistor 43, causes
Fig. l2 shows a further arrangement for con
an ampliiied measure of this change to appear at
verting the case vibrations into amplitude pro
the output terminals ofV ampliñer ‘I2 and hence
portional changes in potential and supplying that
at the input terminals of phase shifter 62. There
potential to amplifier 33. In Fig. l2 use is made
such displacement and timingV ís introduced as 40 of an electrical pick up device 63 analogous to
proves most effective for exciting the power am
pliñer 53. Since this excitation is in the form
of pulsations which recur at the case’s resonant
frequency, the cycles of case vibration once set
up are self-propagating through a feed back or
that used in phonographs. Such. a device 69
also generates its own voltage and hence is suit
able for direct connecting to the input terminals
regenerative action.
Once, started, this action continues indeñnitely
and it automatically adjusts the frequency of
which vibrate. at above the audible sound range
may also have their vibration amplitudes detected
of amplifier 3.8.
It should be pointed out that test specimens
by the just described apparatus of Figs.A 10-11-12.
the case driving voltage from amplifier 53 to an
exact matching relation with the resonant fre
quency of the vibrated case I5.
From the foregoing it will be seen that we have
In order to start this regenerative vibration
provided improved procedure by which the pres
control, it is merely necessary to set up some
ence of internal flaws in metal specimens may
electrical or mechanical disturbance, such as clo
be detected reliably, quickly andeasily; that we
sure of a switch 64 in the supply circuit for the
have made special provision for examining metal
magnet’s direct current winding 24. Once the
specimens which are cylindrical in shape; that
sequence of regenerative actions above discussed,
we have provided a method for testing the brass
has been started, these actions `continue until
cases of ñred artillery cartridges for the presence
some break is made in the power supply or feed
back circuits.
The automatic volume control desired in am
pliñer 'I2 is such that oscillation of the system in
the manner described above is possible when the
sonic test coeinicent of the cartridge case is sub
stantially above the acceptance value; it further
is such that the amplitude ofV oscillation is suc
cessively greater for cases of lower sonic test co
efficients until the maximum permissible power
from amplifier 53 is attained for a case with a
of dangerous season cracks; that we have devel
oped a reliable test for the susceptibility of fired
and resized cartridge cases to splitting on subse
quent rlring; and that we have perfected a test
method by which it is possible prior to resizing
to reject all cases which after being resized and
reconditioned will‘fracture on firing.
The testing yapparatus for '75 mm. cartridge
cases which we have shown by way of illustra
tion may with very slight modiñcation be adapted
to the testing of artillery cartridge cases of other70 sizes and forms. Such adaptation consists in
as 0.30.
selecting mechanical dimensions of the case sup
porting frame and the driving magnet M to ac
F1os. 10-11-12
commodate the particular size of cartridge case
In all of the thus far shown arrangements for
desired to be tested, and in choosing electrical
indicating vibration amplitude', use has been made
characteristics of the driving power supply and
test coeñicient below the acceptance value, such
vibration measuring facilities which are appro
priate for the selected case size.
Our improved sonic testing method further
lends itself to use with metal specimens other
than cartridge cases and may with comparable
success be applied to the detection of internal
ilaws in cylindrical specimens which are open
at both ends.
Nor are cylindrical specimens the only type
men exhibits a damping capacity substantially
different from that of a sound-brass specimen,
and measuring the damping capacity of said test
ed specimen at said high stress producing ampli
tude for purposes of classifying the brass thereof
either as sound or as defective to an observed
3. In a method of testing metal for the pres
ence of defects, the steps which comprise induc
which can be tested, for upon the making of 10 ing in a specimen of said metal mechanical forces
modiñcations immediately apparent to those
skilled in the art, specimens of other forms and
shapes may also be subjected to flaw determina
which repeatedly reverse themselves and by which
said specimen is excited to longitudinal vibra
tion, adjusting the frequency oi' this vibration to
the specimen’s natural frequency of resonance,
tion tests by the here disclosed method of excit
ing the specimen to resonant frequency vibra 15 measuring the amplitude of this resonant fre
quency vibration to indicate the resulting peak
tion at amplitudes sufliciently intense to reñect
stresses which are set up in the specimen metal,
specimen iiaws, measuring the “sonic test co
intensifying this resonant frequency vibration’s
efficient” under these conditions and comparing
amplitude until said indicated peak stresses at
the observed value with standards established
for specimens of the same form and material. 20 tain a given high value at which a defective
metal specimen exhibits a damping capacity
Our inventive improvements are therefore eX
sharply differing from that of a sound-metal
tensive in their adaption and are not to be re
specimen,v and measuring the energy that is re
stricted to the speciñc form here disclosed by
way of illustration.
We claim:
1. In a method of testing metal for the pres
ence of defects, the steps which' comp-rise excit
ing a specimen of said metal to longitudinal vi
quired to produce said given-stress-valve ampli
25 tude of resonant vibration whereby to indicate
the relative damping capacity of the specimen and
therefrom to determine whether the metal of said
bration, adjusting the frequency of this longitu
specimen is sound or is defective.
4. In a method of testing metal for the pres
specimen metal, intensifying this resonant fre
longitudinal vibration, adjusting the frequency of
men of cartridge case brass for the presence of
required to produce the intensified amplitude
vibration by which said given high value of meas
ured peak stress is yielded, and dividing said
given-value stress measurement into said electri
dinal vibration to the specimen’s natural fre 30 ence of defects, the steps which comprise elec
trically inducing in a specimen of said metal me
quency of resonance, measuring the amplitude of
chanical forces which repeatedly reverse them
this resonant frequency vibration to indicate the
selves and by which said specimen is excited to
resulting peak stresses which are set up in the
quency vibration’s amplitude until said indicated 35 this vibration to the specimen’s natural frequency
of resonance, measuring the amplitude of this
peak stresses attain a predeterminedly high value
resonant frequency vibration and the resulting
at which a defective-metal specimen exhibits a
peak stresses which are set up in the specimen
damping capacity sharply differing from that of a
metal, intensifying this resonant frequency vibra
sound-metal specimen, and measuring said damp
ing capacity at the so intensiñed amplitude of 40 tion’s amplitude until said measured peak stresses
attain a given high value at which a defective
vibration whereby to determine whether the
metal specimen exhibits a damping capacity sub
metal of said tested _specimen is sound or is defec
stantially differing from that of a sound-metal
specimen, measuring the electrical energy that is
2. In a method of testing a cylindrical speci
defects, the steps which comprise exciting said
specimen to longitudinal vibration, adjusting the
frequency of this longitudinal vibration to the
specimen’s natural frequency of resonance, meas
uring the amplitude of this resonant frequency
vibration to indicate the resulting peak stresses
which are set up in the specimen brass, intensify
ing this resonant frequency vibration’s amplitude
until said indicated peak stresses attain high val
ues typiñed by several thousand pounds per
square inch and at which a defective-brass speci
-cal energy measurement whereby to indicate the
relative damping capacity of the specimen for
purposes of classifying the metal thereof either
as sound or as a defective to an observed degree.
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