Method and photodetector device for locating abnormal voids in low density material

Howard, W. D.

Vukelich, E. K.

English

NASA Technical Reports Server

NATIONAL AERONAUTiCS AND SPACE ADMINISTRATION 
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U. S. Pa ten t  No. 8 
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NASA Patent C a s e  No. : 
NOTE - I f  this patent covers an invention made by a corporate 
employee of a NASA Contractor, the following is applicable: 
Pursuant to Section 305(a) of the National Aeronautics and 
Space A c t ,  the name of the Administrator of NASA appears on 
the first page of the patent; however, the name of the actual 
inven to r  (author) appears a t  the heading of Column No. 1 of 
the Specification, following the words . . with respect t o  
YesIjf No 0 
Enclosure 3 (THRU) 
(CODE) 
copy of Patent crited above 5 
19 
(CATEGORY) 
2 (NASA CR OR TMX OR AD NUMBER) 
U 
https://ntrs.nasa.gov/search.jsp?R=19710019517 2020-03-17T03:11:37+00:00Z
Inventors 
Appl. No. 
Filed 
Patented 
T. 0. Baine 
Deputy Administrator of the National 
Aeronautics and Space Administration in 
respect to an invention of; 
Edward K. Vukeaich, Flathead County, 
Mont.; William D. Howard, Huntsville, Ala, 
838,630 
July 2,1969 
Apr. 13,1971 
[561 References Cited 
UNITED STATES PATENTS 
2,939,963 6/1960 Rideout ........................ 250/2 19 
3,206,603 9/1965 Mauro .......................... 250/219X 
3,224,324 12/1965 Coppock et al ............... 356/209 
3,327,849 6/1967 Sorbie .......................... 250/223X 
3,460,143 8/1969 Vaccaro ....................... 356/209X 
Primary Examiner-Walter Stolwein 
Attorneys-L. D. Wofford, Jr., Wayland H. Riggins and 6. T. 
McCoy 
METHOD AND DEVICE FOR DETECTING VOIDS 
5 Claims, 2 Drawing Figs. 
US. CI ........................................................ 356/209, 
25012 19 
Int. CI ......................................................... GOln 21/48 
Field of Search ............................................ 2501222, 
223(B),219(DF),219(S);356/201,209-211, 
237; 73/104, 105 
IN LOW-DENSITY MATEMAE 
ABSTRACT: A method and device for detecting abnormal 
voids in a low-density material such as polyurethane foam 
insulation wherein light is beamed into the material and the 
light reflected from the material is received by a 
photodetector which converts the light into a voltage level 
proportional to the light intensity received by the detector. 
The intensity of the reflected light decreases when the light is 
beamed into low-density material having an abnormal void 
therein and the void is indicated by resultant reduction in 
voltage output of the photodetector. 
b 
PATENTED APR 1 3 1971 
FIG. I 
- It 
13 
FIG. 2 
E. K. VUKELICH 
W. D. HOWARD 
INVENTORS 
3,574.470 
1 
M E T ~ O D  AND DEVICE FOR DETECTING VOIDS IN 
LOW-DENSITY MATERIAL 
ORIGIN OF THE lNVENTION 
The invention described herein was made in the 
performance of work under a NASA contract and is subject to 
the provisions of Section 305 of the National Aeronautics and 
Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 
BACKGROUND OF THE INVENTION 
This invention relates to a method and means for detecting 
defects in low-density material and more particularly to a 
method and device for photoelectrically detecting abnormal 
voids in a low-density material such as low-density foam 
insulation. 
Among the materials currently being used for thermal 
insulation purposes is low-density foam material, an example 
of which is polyurethane "spray-on foam insulation." One 
area of extensive use of low-density foam insulation is found in 
the fabrication of space vehicles wherein very effective and 
lightweight insulation is required in the insulation of tanks 
holding cryogenic propellants such as liquid hydrogen and 
liquid oxygen. 
Since the effectiveness of the spray-on foam insulation is a 
critical matter in the manufacturing of space vehicles, 
improved techniques are needed for detecting voids in the 
insulation after it is applied. Of course, the detection method 
must be nondestructive to the insulation. Prior experience in 
nondestructive detection tests of low-density foam insulation 
has shown that known techniques for nondestructive testing, 
such as ultrasonic vibration, are not completely reliable when 
employed on low-density foam because the electrical and 
acoustical impedance of low-density foam is so close to those 
of air. Therefore, abnormal voids in the foam insulation are 
not indicated with the reliability desired when these prior 
testing methods are used. 
SUMMARY OF THE INVENTION 
The invention comprises a method and device for detecting 
voids in low-density foam material wherein a light beam is 
transmitted into the material that has been applied on a 
backing panel such as an aluminum wall of a propellant tank. 
The light reflected from the low-density foam and the backing 
panel is received by a high-sensitivity photovoltaic detector 
and is converted into a voltage level proportional to the light 
intensity received by the detector. A reduction in the intensity 
of the reflected light occurs when the light is beamed into 
foam having voids therein large enough to constitute 
significant defects in the insulation effectiveness of the foam. 
The device of the invention comprises a housing unit 
containing a light source, a light detector and a voltage 
amplifier. The light is transmitted into the material being 
tested through a port in the housing and the reflected light is 
,received by the detector in a second port in the housing. The 
housing unit may be mounted on an automatic scanning 
9 U.S.C. 2457). 
d 
2 
These and other objects of the invention will become 
apparent upon reference to the following specification, 
attendant claims and drawing. 
BRIEF DESCRIPTION OF THE DRAWING 
FIG. 1 is a cross-sectional view of the void detection device 
overlying foam insulation applied to a backing panel with the 
circuitry shown in block diagram; and 
FIG. 2 is a view indicating the type of recording produced 
lo  by the void detector and indicating voids present in a low- 
5 
density foam material. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, therein is shown a layer of low-density 
foam insulation 11 applied to a backing 13 which may be an 
aluminum wall section of a container such as a propellant 
tank. The foam 11 would typically be polyurethane spray-on 
foam insulation of low density. 
The device for testing or inspecting the foam 11 for void 
defects within the foam material comprises a housing unit 15 
containing a light source 17, a light detector 19 and an 
amplifier 21. The light source 17 may comprise an 
incandescent lamp and in one embodiment a General Electric 
25 No. 47 lamp was used. The light source 17 is mounted in a 
compartment 23 that is fitted in an opening 25 in the housing 
unit 15. The compartment has a port 27 through which the 
light is directed into the foam 11. 
The light detector 19 receives light reflected from the foam 
30 material 11 and converts the reflected light into a voltage level 
proportional to the light intensity received by the light 
detector. One embodiment of the invention comprised a 602 
photovoltaic detector marketed by the Electro-Nuclear 
35 Laboratories, Inc. The light detector 19 is movably mounted 
in an elongated slot 29 in the housing unit 15. The slot 29 
permits adjustment of the separation between the light source 
17 and the light detector 19. 
A light beam 31 is transmitted into the foam material 11 
40 from the light source 17. The beam 31 is diffused within the 
foam material and some of the light is reflected from the 
material. When the foam is substantially uniform in density, 
texture and color the light is diffused evenly within the mater- 
ial and the intensity of the reflected light received by the 
45 detector 19 remains essentially constant. It has been found, 
however, that the intensity of the reflected light received by 
the detector 19 decreases when the foam material under 
inspection has a void therein of a size that would seriously 
impair the insulation effectiveness of the material. 
The amplifier 21 receives the voltage signal from the light 
detector 19 and amplifies the voltage and applies the 
amplified voltage to a threshold detector 33, the latter being 
connected to a recorder 35. In previous operational 
5 5  embodiments the intensity of the light from the source 17 and 
the amplifier gain were adjusted so that the threshold detector 
passed a voltage of -15 volts to the recorder when the light 
was beamed into sound foam and 0 volts when the light was 
beamed into a defective area containing a void. 
I5 
20 
50 
system-including 
depicting the internal structure of the foam insulation being 
tested for voids. 
Accordingly, it is a general object of the present invention 
to provide an improved method and device for detecting 
defects in lowdensity foam material. 
Another object of the invention is to provide a relatively 
simple, economical and more reliable means for detecting 
voids in polyurethane spray-on foam insulation. 
Another object of the invention is to provide a means for 
detecting voids in low-density foam material that will provide 70 
a recording depicting the internal structure of the foam mater- 
ial and which means overcomes the prior difftculties of 
detecting voids in lowdensity foam material caused by the 
similarity of the electrical and acoustical impedance of low- 
density foam and that of air. 
recorder for providing a C-scan recording 60 The housing unit 15 may be mountedon a moveable arm 37 
of an automatic scanning system and systematically scanned 
across the foam panel to produce a C-scan recording depicting 
the internal structure of the foam insulation. A portion of such 
a recording is shown in FIG. 2. Voids in the material are 
65 indicated by the blank areas such as 39 and 41. 
We claim: 
1. The method of detecting abnormal voids in a low-density, 
spray-on foam material having an opaque backing on one 
surface thereof comprising: 
a. positioning a light source and a photodetector adjacent 
the surface of said material opposite said opaque backing 
so that said light source and said photodetector are 
protected from external light; 
b. transmitting a beam of light directly from said light 
source into said material whereby said light is diffused 75 
3,574,470 
3 4 
within said material and some of said light is reflected 
from said material; and 
c. detecting with means including said photodetector 
significant variations in the intensity of light reflected 
from said material and thereby detecting abnormal voids 5 
in said material. 
c. a plurality of spaced openings in said housing extending 
from the interior of said housing to said exterior surface; 
d. said light source being mounted in alignment with one of 
said openings and said photodetector being mounted in 
alignment with another of said openings; 
e. said light source and said photodetector being directed 
2. The method of claim 1 including scanning a outwardly of said exterior surface whereby said light 
source is adapted to transmit light into said material and 
the said photodetector is adapted to receive light 
reflected from said 
for adjusting the spacing between said light 
and said photodetector; 
predetermined area of said lowdensity material by moving 
said light source and said photodetector over said material 
while continuously transmitting a beam of light directly from 10 
said light source into said material and continuously detecting 
significant variations in the intensity of light reflected from 
said material. 
indicating abnormal voids in said material and delineating the 15 
location of such voids. 
4. The method of claim 1 wherein said low-density material 
is polyurethane foam material and said opaque backing is 
metal. 
spray-on foam material comprising: 
Y 
f. 
g. means for amplifying a signal from said photodetector; 
i, 
; 
3. The method of claim 2 including producing a recording h. a recorder associated with said amplifier; 6 
including said amplifying for applying a 
predetermined voltage to said recorder when said light 
source is transmitting light into sound material and 
applying a zero voltage to said recorder when said light 
abnormal void therein; and 
j. an arm attached to said housing for moving said housing in 
a scanning motion over the surface of the material being 
inspected. 
5. A device for detecting abnormal voids in a Iow-density, 20 Source is transmitting light a material having an 
a. a housing containing a light source and a photodetector; 
b. said housing having a flat exterior surface adapted to 
contact and move over the surface of the material being 
inspected; 25 
30 
35 
40 
45 
50 
55 
60 
65 
IO 
15 


Method and device for detecting voids in low density material Patent

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