Nondestructive test methods are used for evaluating bond integrity of low-density polyurethane spray-on foam used as cryogenic insulation on aluminum alloy surfaces

Bankston, B. F.

Clotfelter, W. M.

Duren, P. C.

English

NASA Technical Reports Server

Audio 
Oscillator
Spectrum 
Analyzer 
December 1971	 Brief 71-10507 
NASA TECH BRIEF 
Marshall Space Flight Center
	
_icv 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
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be directed to the Technology. Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Nondestructive Testing of Bond Integrit y in Foam ItisuIatioii/ItiminuIII 
Composites 
New nondestructive test methods can be used for 
evaluating the bond integrity of a low-density poly-
urethane spray-on foam used as cryogenic insulation 
on aluminum alloy surfaces. One promising method 
employs a pulse-echo audio frequency system to 
Speaker
speaker is adhesively bonded to the foam coupler. 
This speaker,which is driven with an ordinary audio-
oscillator, is an effective means of introducing audio-
vibrations into the test specimen through the foam 
coupler. A contact microphone within the lower por-
Microphone  
Foam Coupler 
Aluminum Alloy 
Foam Insulation 
Pulse Echo Audiofrequency System 
overcome the major drawback with pulse-echo ultra-
sonic detection systems; i.e., that the low-density 
foam and the air do not have sufficiently different 
acoustic impedances at ultrasonic frequences. As a 
consequence, there is too small a difference between 
the magnitude of the acdustic energy reflected from 
the aluminum alloy/foam interface when a bond 
exists and that reflected from the aluminum alloy/air 
interface regions which exist in the absence of a bond. 
The novel feature of the new audio frequency sys-
tem is the use of a foam coupler which has the same 
acoustic impedance as the foam insulation on the 
aluminum alloy faceplate. In the first experimental 
design of the system (see figure), a commercial loud-
tion of the coupler detects changes in vibrational 
patterns caused by unbonded areas in the specimen 
panel. The signal generated by the microphone is 
amplified, processed, and displayed on the oscillo-
scope of a spectrum analyzer. The frequencies above 
and below the center frequency to which the spectrum 
analyzer has been set are used as indicators of bond 
integrity. Debonded foam has greater freedom of 
motion than well bonded material, and the operating 
frequency is selected to emphasize the associated 
increase in vibrational energy. 
Since a single method would not be adequate for 
evaluating composite bond conditions covering a wide 
range of foam-metal thickness combinations, other 
(coflhu,sUcd Os erleal 
NNW
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Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19710000504 2020-03-17T02:26:18+00:00Z
complementary methods were investigated. It was 
found that debonds can be detected with audiofre-
quency through-transmission eddy current techniques. 
Voids in the foam can be found with radiographic and 
audio frequency transmission methods. Radiography 
is very sensitive to small defects in composites with 
very thin metal faceplates. It is also useful for 
evaluating questionable debond areas that are first 
located by other means. The through-transmission 
eddy current technique appears to offer greater poten-
tial for the evaluation of composites having rela-
tively thick metal plates.
Note: 
The following documentation may be obtained 
from:
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95) 
Reference: 
NASA TM-53940 (N70-17360), The Non-
destructive Evaluation of Low Density 
Foam -Aluminum Composite Materials 
Patent status: 
No patent action is contemplated by NASA. 
Source: W.M. Clotfelter,

B.F. Bankston, and

P.C. Duren
Marshall Space Flight Center

(M FS-20786) 
Brief 71-10507	 Category 06


Nondestructive testing of bond integrity in foam insulation/aluminum composites

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