US3475646A

US3475646A - Spark gap light source for impact photoelasticity

Info

Publication number: US3475646A
Application number: US629749A
Authority: US
Inventors: Everett Chapman
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-04-10
Filing date: 1967-04-10
Publication date: 1969-10-28
Anticipated expiration: 1986-10-28

Description

Oct, 28, 1969 E. CHAPMAN 3,475,646

l SPARK GAP LIGHT SOURCE FOR IMPACT PHOTOELASTICITY Filed April 10, 1967 2 Sheets-Sheet l Get. 28, 1969 CHAPMAN 3,475,646

SPARK GAPLIGHT SOURCE FOR IMPACT PHOTOELASTICITY Filed April 10, 1967 .2 Sheets-Sheet 2 United States Patent 3,475,646 SPARK GAP LIGHT SOURCE FOR IMPACT PHOTOELASTICITY Everett Chapman, P.0. Box 207, West Chester, Pa. 19380 Filed Apr. 10, 1967, Ser. No. 629,749 Int. Cl. H01t 7/00 US. Cl. 315-59 7 Claims ABSTRACT OF THE DISCLOSURE A spark gap device for use in impact photoelasticity which provides for high intensity, short duration illumination and thereby permits the use of impact photoelastic specimens made of stiff plastic materials having traveling wave velocities comparable to velocities occurring in materials subject to impact. The device comprises a bank of capacitors coupled to a spark gap by circuit means whose time constant provides for the gap discharge to reach maximum value practically instantaneously.

This invention relates to photoelasticity and in particular relates to equipment for obtaining photoelastic images under impact conditions.

Conventional photoelastic work contemplates imposing a static load on a specimen, recording the resulting image, and analyzing the stress distribution. While the analysis of images attained by static loading is vitally important in stress analysis work for obtaining optimum designs, it is well known that stress distributions vary widely as between static and impact or transient conditions. For example, it has been found that many parts whose stress patterns obtained under static conditions indicate ideal design will fail under impact conditions.

Previous attempts to produce photoelastic images with a specimen loaded by impact have not accurately reproduced the conditions which prevail in materials subject to impact. One of the reasons for this has been the lack of a light source having an intensity and time duration compatible with the velocity of the traveling wave in the material. The lack of a fast light source has caused the researchers to use plastic materials as specimens having traveling wave velocities compatible with the available light sources. Such materials are relatively soft or rubbery and do not respond to impact in the same manner as stiif materials. Thus materials having wave velocities far below the velocities which actually occur in practice and which do not respond properly have made it impossible to fairly duplicate the actual conditions and fail to obtain the true impact or transient stress distribution patterns.

The present invention contemplates a light source of high intensity and of extremely short time duration which enables that use of specimens made of stitf plastics having impact response and wave velocities compatible with velocities in materials subject to impact loads. According to the invention, the time of impact and the time of appearance of the light are correlated so that the traveling wave can be stopped at the moment of impact and at finite points as it travels along the specimen and thereby obtain a series of images showing the wave as it is first generated and then as it spreads into the specimen.

charge current of the capacitors is extremely steep; for example, with the device described herein I have obtained arcs or flashes existing for less than nanoseconds.

With discharge times of the order indicated, the invention contemplates specimens made from photoelastically sensitive plastic materials such as epoxy resins having elastic moduli from approximately 50,000 to 500,- 000 p.s.i. and corresponding velocities from approximately 20,000 to 62,000 inches per second. The wave velocities of plastics of this nature correspond to wave velocities in materials and alloys commonly used for applications where the same are subject to impact loads.

The use of a spark gap having a very fast discharge time is especially advantageous for plastics of the kind mentioned and for photographing the transient image. The spectral distribution of a spark gap in air has many strong lines from about 3,500 Angstrom to 4,800 Angstrom, there being very strong blue lines, some yellow lines, and some red lines. The yellow and red lines are convenientlly filtered out, and water-clear plastics having moduli of the kind mentioned easily transmit the blue with little absorption. High speed photographic negatives are particularly sensitive to blue so that extremely sharp images can be attained.

A preferred form of the invention will be described below in connection with the following drawings, wherein:

FIGURE 1 is a perspective view of certain parts of a conventional photoelastic machine incorporating the invention;

FIGURE 2 is an end view of a spark gap device;

FIGURE 3 is a view taken along the lines 3-3 of FIGURE 2; and

FIGURE 4 is a fragmentary view of a preferred form of spark gap.

In FIGURE 1, I have shown a photoelastic machine which for the purposes of clarity has many of the components removed. The machine includes a barrel 1 rotatably mounted on the frame 2. The barrel includes a polarizer 3 and an analyzer 4. The frame 2 has an arm 5 slidably mounting the post 6, which carries the spark gap device 7.

The light from the spark gap is emitted through the aperture 10. The beam is directed to a mirror 11 on the post 12 supported by the bracket 5. The mirror reflects the beam to a concave mirror 13 which then projects the beam through the polarizer and analyzer through the specimen S, wherein the image is formed and projected to the recording camera by conventional lenses and/or mirrors.

The means for imposing an impact load on the specimen includes an upright standard 17 pivotally carrying the arm 14 on which is an impact member 15. The specimen is supported by having its lower edge rest on the platform 16 and its right hand edge firm against the inner surface of the standard 7. I have found it unnecessary to provide positive means to lock the specimen in position inasmuch as the transient conditions take place long prior to the time when forces might cause the specimen to move.

The impact is provided by swinging the arm 14 upwardly to the desired position and then releasing the same so that it swings down and the member 15 engages the left hand end of the specimen. As will be understood by those skilled in the art, the amount of force can be calculated simply by the weight and the distance through which the weight falls to make engagement.

Before going on it might be noted that the barrel 1 is of the type shown in my Patent 2,730,007, and the mirrors 11 and 13 and those mirrors and/ or lenses which convey the photoelactic image to the camera are of the type as shown in my Patent 3,293,908.

The spark gap device 7 will be described in connection with FIGUR-

ES

2, 3, and 4.

As best shown in FIGURE 2, a plurality of co-axial return, can-

type capacitors

20, 21, 22, 23, 24, and are arranged in a circular pattern about an axis; for example the axis A. Each capacitor has a pair of electrical terminals, one of which is the housing and the other a center post; for example, for the capacitor 25 the center post 25a and the housing 25b. The other capacitors have similar electrical terminals.

Within the nest of capacitors and disposed along the axis A are the spark gap electrodes and 31. As best seen in FIGURE 4, the electrode 30 is conical in form, the apex 32 of which is coincident with the axis A. The electrode 31 is in the form of a truncated cone, the flat end 33 of which faces the apex 32. The electrode 31 has a passageway 34 made up of a plurality of bores 35, 36, and 37. By varying the diameter of the hows, the divergent pattern of the passageway can be varied to suit the type of spark gap. The proportions of the passageway 34 is to allow the mirror 13 to be completely filled with light from the gap.

The circuit means interconnecting the capacitor terminals with the spark gap electrodes is of special significance in that inductance is minimized so that the time constant of the circuit is a practical minimum. The circuit means also provides for mechanically supporting both the capacitors and the spark gap electrodes.

Preferably the foregoing takes the form of a pair of

spiders

38 and 39, each having six arms which respectively extend radially between the capacitor terminals and the spark gap electrodes. The arms for the spider 39 are indicated at 40, 41, 42, 43, 44, and 45. The spider 38 has identical arms, two of which are indicated in FIGURES 3 at 46 and 47.

Corresponding arms of the two spiders face one another and lie in the same radial planes. Each arm has an aperture at its outer end; for example, the apertures 48 and 49 for the arms and 46. The apertures are coaxial. The head sections of the capacitors extend through these apertures; for example, the head 25c and post 25a of the capacitor 25.

The two

spiders

38 and 39 are spaced from one another along the axis A by insulating material which preferably comprises a pair of circular sheets of Mylar 50 and 51. The Mylar permits the arms to be spaced very closely adjacent one another, which reduces the flux linkage path and thereby minimizes the effect of inductance. The sheets 50 and 51 have appropriate apertures to accommodate the capacitor heads and the discharge electrodes. The thickness of the Mylar is determined by what is needed to insulate the capacitor charging voltage.

The terminals formed by the housings of the capacitors are electrically connected to the spider 38, and this is done simply by placing the head section of each capacitor in abutting relationship with the spider. For example, with reference to capacitor 24 in FIGURE 3, it will be seen that the nut 24d of the head section firmly abuts the arm 46.

The post terminals of the capacitors are electrically connected to the other spider 39. As shown for capacitor 22, the disk 53 is spaced across the aperture 37 and engages the spider 39 and is electrically connected with the center post 22a by the nut 54. A similar arrangement is made for the other capacitors except that the disks for the

capacitors

21, 23, and 25 are relatively wider as shown on the top of FIGURE 3 for the disk 55. Also, the latter disks have cavities accommodating connecting nuts such as the cavity 58 accommodating nut 59 on post 25a.

The

capacitors

21, 23, and 25 are provided with elongated center posts which are used for mounting the array of capacitors on the post 6. As indicated in FIGURES 2 and 3, a triangular shaped bracket 56 made of insulating material is supported on the post 6. The center posts 21a, 23a, and 25a project through apertures in the bracket and carry nuts which bear against the bracket and lock the same against the widened disks 55, 57 (FIGURE 3), and 58 (FIGURE 1).

The electrode 30 is electrically connected to the spider 38 by a threaded mounting disk 60 secured to the spider by the screws 61. The electrode 31 is electrically connected to the spider 39 by a threaded mounting disk 62 fixed to the spider 39 by the screws 63.

The triangular-shaped bracket 56 supports an adaptor 64 mounting a light collecting lens 65 and filter 66. The lens 65 collects the light from the spark gap coming through the passageway 34 and the filter 66 absorbs the yellow and red lens so that the beam going to the specimen is essentially monochromatic, i.e., blue.

I apply a capacitor potential between the

electrodes

30 and 31 which is of a magnitude that breakdown will not occur in the absence of an initiating discharge. For initiating the discharge across the gap, I use an annular electrode 70 which surrounds the electrode 30. With the application of the voltage between the electrode 70 and the electrode 30 as breakdown occurs. This triggers or causes the capacitors to discharge, and illumination energy appears in the gap.

Preferably the electrode 70 is connected to a conductor which is disposed between the Mylar sheets 50 and 51 and extends radially outwardly so that it can be connected to a high voltage source. Such a terminal is indicated at 71.

From the foregoing description it will be apparent that the

spiders

38 and 39 not only serve as low level inductance conductors but in addition serve as structural members for supporting the capacitors and spark gap electrodes. By properly contouring the arms of the spiders, the resistance of each arm can be desirably adjusted to make the circuit L/R ratio of a value to produce a high rate of rise of capacitor discharge current and to make the discharge essentially non-oscillatory.

The circuitry for the

electrodes

30 and 31 and the igniter or trigger 71 are conventional in form; for example, I have used capacitors of two microfarads each charged up to 2,000 volts from a conventional power supply, and for the igniter I have used a conventional automobile 6-volt ignition circuit. Such conventional circuitry is well understood by those skilled in the art and need not be explained in detail.

One point I want to mention, however, is that the switch to open the ignition circuit and cause the voltage to appear across the trigger 71 takes the form of a microswitch 72 mounted on the standard 7. This switch is equivalent to a breaker point in the ignition circuit.

The arm (not shown) of the switch 72 is adapted to be contacted by the adjustable screw, 73 secured to the impact device 15. The screw can be adjusted so that the switch is opened at the instant the impact member 15 contacts the specimen or opened just slightly after contact. The switch 72 and the arm 73 are adjustable for the purpose of correlating the time of impact and the discharge across the

electrodes

30 and 31 so that the spark gap discharge or illumination takes place at the exact time of impact or slightly thereafter. By correlating the illumination time with respect to the impact time, the wave can be stopped at finite points as it travels along the specimen. Thus a series of images can be obtained which show the actual stress distribution from the time the traveling wave is generated by impact and as it travels along the specimen away from the impact point.

I claim:

1. A spark gap device comprising:

a plurality of capacitors arranged in a circular pattern about an axis, and each capacitor having a pair of electrical terminals;

a pair of spark gap electrodes disposed centrally of the capacitors and spaced from one another along said axis, one of the electrodes being formed with a passageway to conduct light from the spark;

for each pair of said electrical terminals, a pair of closely spaced arms respectively electrically connected with said terminal and extending radially inwardly toward said electrode;

means commonly electrically connecting all of the arms connected with one of the corresponding capacitor terminals to one of the electrodes;

means commonly electrically connecting all of the arms connected with the other of the corresponding capacitor terminals to the other of the electrodes;

wherein all of the arms connected to said one electrode are formed from a first unitary piece of conducting, non-magnetic material and all of the arms connected to said other electrode are formed from a second unitary piece of conducting non-magnetic material; and

further including means mounting said capacitors and said one electrode on said first piece and means mounting said other electrode on said second piece.

2. A construction in accordance with claim 1 further including a trigger electrode disposed closely adjacent said other electrode.

3. A spark gap device comprising:

a plurality of capacitors arranged in a circular pattern about an axis, and each capacitor having pair of electrical terminals;

a first spider having a central bore coaxial with said axis and having a plurality of arms extending radially outwardly of the bore and terminating respectively adjacent the capacitors;

means for each arm for electrically connecting the arm with one of the terminals of its adjacent capacitor;

means for electrically connecting one of said electrodes with said first spider;

a second spider having a central bore coaxial with said axis and having a plurality of arms extending radially outwardly of the bore and terminating respectively adjacent a capacitor, the respective arms of said first and second spiders terminating adjacent a capacitor being closely adjacent one another;

for each arm of the second spider means for electrically connecting the arm with the other of the terminals of its adjacent capacitor; and

means for electrically connecting the other of said electrodes with said second spider.

4. A spark gap device comprising:

a pair of fiat, mirror image spiders spaced from one another along an axis, each spider having a central bore coaxial with said axis and each spider having a corresponding number of radially extending arms,

and pairs of corresponding arms facing one another, each arm having an aperture adjacent the outer end thereof and the apertures of the pair of facing arms being coaxial;

insulating means in the space between the spiders and each spider having a central bore coaxial with said axis;

a plurality of capacitors, each capacitor having a housing forming one of its electrical terminals and a center post forming the other electrical terminal, the capacitors being respectively disposed adjacent the outer ends of a pair of facing arms with the center post of the capacitor extending through the apertures;

means electrically connecting the housing of a capacitor to its corresponding arm and means electrically connecting the center post of the capacitor to the other of the pair and the arms forming conductive paths respectively between each housing and said central bore and between said center posts and said central bore;

a pair of spark gap electrodes adjacent said bore and spaced from one another along said axis, one of the electrodes formed with a passageway to conduct light from the spark; and

means electrically connecting said one electrode to one of said spiders and means electrically connecting the other of said electrodes to the other of said spiders.

5. A construction in accordance with claim 4, further including means connected with said one electrode adjacent the end of said passageway and mounting a pair of lenses to columnate the light from the spark.

6. A construction in accordance with claim 4, wherein said one electrode is generally in the form of a truncated cone and the other conical in form with the apex lying along said axis and facing the flat end of said one electrode.

7. A construction in accordance with claim 5, wherein said insulating means comprises a pair of sheets of Mylar and further including a conductor extending between the sheets and terminating in an annular section surrounding the other of said electrodes.

References Cited UNITED STATES PATENTS 2,298,114 10/1942 Estorff 315-59 2,730,007 l/ 1956 Chapman 8814 2,779,890 1/ 1957 Frenkcl 3l3325 X 2,911,567 11/1959 Fischer 31559 X 3,300,682 1/1967 Frungel et al. 315-56 X 3,361,930 1/1968 Blank 315-59 JOHN W. HUCKERT, Primary Examiner J. R. SHEWMAKER, 1a., Assistant Examiner US. Cl. X.R.