Application of commercial coating to exterior of sandwich panel structures for determining presence of voids in brazed plates is discussed. Procedure for applying coating material and method of conducting nondestructive tests are explained. Illustrations are included to show appearance of voids

Gibson, J. C.

English

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Panel No. 2
	
Panel No. 3 
Figure 1. 
July 1972	 B72-10374 
NASA TECH BRIEF 
Langley Research Center 
NASA Tech Briefs announce now 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 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Nondestructive Testing for Braze Voids in Thin Panels by 
Use of Special Coatings 
Panel No. 1
—I 
fIA	
'F 1.l5in. 
(Typ.)
2 in. )
B:
 D
Figure 2. 
The problem: 
The laminae of single-sandwich panel structures used 
for pressure containment are bonded together by brazing. 
In the brazing process, voids may be left between the 
laminae, and when the panel is subsequently subjected 
to differential pressure, these voids become points of 
maximum stress. 
The solution: 
A commercially available coating (Stresscoat or its 
equivalent) has demonstrated substantial advantages in 
the nondestructive detection of braze voids in single-
panel structures. This coating applied directly to the 
panel will indicate the areas of high stress over braze 
voids by simple inspection. The method is more direct 
than the ultrasonic, radiographic, or infrared void 
detection techniques which require an additional step 
of interpretation of the recorded data.
How it's done: 
The panels are first cleaned with toluene, spray 
painted with silver paint (Krylon or its equivalent) to 
gage stress coating thickness, and then painted with the 
stress coating. The curing period for the coating ranges 
from 12 to 24 hours for the coating thickness of 
0.13 to 0.25 mm (5 to 10 mil). Maximum sensitivity is 
obtained after about 24 hours of curing. 
The stress coating is calibrated by placing a weight 
on a coated cantilever beam. The minimum uniaxial 
cracking stress is proportional to the distance from the 
load application to the first crack observed in the coat-
ing. The biaxial stress characteristic of the voided areas 
is only in error a maximum of 15 percent from the 
minimum uniaxial calibration stress. 
The panels are hydrostatically pressurized in any 
convenient increments until visual cracks appear in the 
coating above the braze fin voids. (The panels are not 
(continued overleaf) 
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pressurized while the surface is being examined for 
cracks.) A flashlight, with the beam directed parallel to 
the surface, aids the visual observation of the surface 
for stress cracks. 
To evaluate this technique, three 6.35-by-10.16 cm 
(2.5-by-4 in) plate-fm panels were fabricated with known 
braze voids (Figure 1). The panels were fabricated from 
0.38 mm (0.015 in) Hastelloy X plates with 1.91-by-
2.54-by-0.15 mm (20R-0.075-0.100-0.006 in) Hastelloy 
X fins. After being cleaned and spray painted, the 
panels were painted with Stresscoat ST-75. The 75 
designates the ambient test temperature; which in these 
tests was 21°C (70 0F). To obtain greater sensitivity, 
hydrostatic pressure was applied in 446 x iø N/rn2 
(50 psig) increments until cracks appeared above all of 
the fin braze voids. These cracks appeared at the pressure 
and location given in the table of test results. Figure 2 
shows the three panels with the stress coating cracked 
above all the braze voids.
TEST RESULTS
Stresscoat 
Sensitivity (Mm. 
Panel	 Pressure Void Cracking stress) 
No.	 N/rn x 10  (psig) Location N/rn 
1	 5.6 (800) B and C 8 x 10	 N/rn2 
(11,600 psi) 
8.72 (1250) E and F 
10.4 (1500) A and E 
11.8 (1700) D 
2	 3.2 (450) C 
3.6 (500) E 
3.9 (550) B and 0 
4.8 (675) C 
5.3 (750) A 
3	 2.0 (275) C 9.45 x 10	 N/rn2 
(13,700 psi) 
2.5 (350) A 
2.9 (400) E 
3.2 (450) D 
4.2 (600) B 
The braze void locations shown are as noted in Figure 1. 
Void F is unique to panel #1.
Note: 
No further documentation is available. Specific ques-
tions, however, may be directed to: 
Technology Utilization Officer 
Langley Research Center 
Langley Station/Mail Stop 139A 
Hampton, Virginia 23365 
Reference: B72-10374 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
Patent Counsel 
Langley Research Center 
Mail Code: 456 
Langley Station 
Hampton, Virginia 23365 
Source: J. C. Gibson of 
AiResearch Manufacturing Co. 
under contract to
Langley Research Center 
(LAR-1 0486)
to 
B72-10374	 Category 08 


Nondestructive testing for braze voids in thin panels by use of special coatings

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