Application of internal hydrostatic pressure to seam-welded circular cylindrical tanks having corner-welded, flat, circular ends forms large thin-walled high quality tanks. Form limits expansion of cylindrical portion of final tank while hemispherical ends develop freely; no external form or restraint is required to fabricate spherical tanks

Erickson, A. L.

Guist, L. R.

English

NASA Technical Reports Server

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197 	 B72-10025 
NASA TECH BRIEF 
Ames Research Center 
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 
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. 
Simple Method for Forming Thin-Wall Pressure Vessels 
A novel method that has been developed for form-
ing thin-wall pressure vessels for spacecraft also ap-
pears to be applicable to the commercial fabrication 
of a wide variety of tanks.
seam-welded tubing (0.04-cm wall) from an initial 
diameter of 15.2 cm to a final diameter of 21.6 cm. 
The resulting cylinders were highly uniform in wall 
thickness, remarkably free of geometric imperfections, 
Final Tank, 
w
\ 
Pressure Line 
The need for reducing tank construction costs for 
the Space Shuttle was clearly evident from the begin-
ning of the program, for it was realized that nearly 
one third of the direct cost per flight could be repre-
sented by the expendable hydrogen tanks unless their 
manufacture could be accomplished more cheaply. 
The results of a series of feasibility studies revealed 
that current manufacturing techniques or contem-
plated developments would not reduce costs signifi-
cantly. 
However, a novel concept was prompted by con-
clusions derived from results of an experimental study 
on buckling of hydrostatically formed cylinders. In 
the course of this study, cylinders were formed by 
hydrostatic expansion of type 304 stainless steel,
and uniformly work hardened to a yield strength of 
about 62 meganewtons/m 2 (90,000 psi). Accordingly, 
it appeared that this combination of effects of plastic 
expansion would be uniquely favorable to the concept 
of hydrostatic formation of large thin-walled high 
quality tanks. 
The new method for forming thin-wall pressure 
vessels merely requires application of internal hydro-
static pressure to a seam-welded circular cylindrical 
tank that has corner-welded, flat, circular ends. As 
shown in the diagram, a form is used to limit expan-
sion of the cylindrical portion of the final tank while 
the hemispherical ends develop freely. No external 
form or restraint of any kind is required to fabricate 
spherical tanks. Some research has been conducted 
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19720000025 2020-03-17T03:23:55+00:00Z
on the possibility of free-forming tanks with cylin-
drical sections; however, these techniques are not 
perfected at this time. 
Two types of weld joints for attachment of the 
cylindrical section to the flat circular ends of the orig-
inal tanks have been used with metal thickness of the 
order of 0.03cm or less (0.012 inch). The ends were 
formed with a shallow flange to facilitate welding 
the thin material; a simple corner butt weld was used 
with thicker material. In experiments performed thus 
far, the 15.2-cm tubes were the standard, seam-
welded, bead-rolled type 304 S.S. tubing; larger dia-
meters have been formed from sheet stock, and have 
butt-welded longitudinal seams. 
When pressure is applied to the original cylindrical 
tanks, expansion occurs relatively uniformly over the 
cylindrical surfaces, but wrinkles develop at the 
corner butt welds; however, these wrinkles are com-
pletely smoothed out in the final pressurization stages. 
In tests conducted with confining end plates attached 
to the cylindrical outer form to prevent bulging of 
the ends during initial expansion, the wrinkling was 
nearly completely eliminated, but it is not known 
whether this technique provides advantages in the 
weld or material integrity of a finished tank. 
In general, cylindrical and spherical tanks formed 
in this way are expanded to a final diameter corre-
sponding to about a 40% plastic elongation in the hoop 
direction at maximum diameter. This results in a 
work-hardened yield stress of 85% to 95% of the ulti-
mate stress for the materials used. 
Spherical tanks are formed by the free expansion of 
cylinders; for example, a cylinder 15.2 cm in diameter, 
14.7-cm long, with walls 0.05-cm thick can be "blown" 
into a 21.6-cm sphere.
Notes: 
1. Rupture pressures fQr cylinders and spheres are 
close to theoretical values: cylindrical vessels can 
withstand nearly one-half the pressure of spherical 
vessels owing to the fact that cylindrical hoop stress 
should be exactly twice the maximum tensile stress 
in the hemispherical end. 
2. Spherical tanks as large as 129 cm in diameter 
have been fabricated by this method. 
3. The outside surfaces of the tanks are remarkably 
smooth and can be finished-to a high polish. 
4. 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 TMX 62 043, 
A Simplified Method of Forming Thin-Walled 
Pressure Vessels. 
5. No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94085 
Reference: B72-10025 
Patent status: 
No patent action is contemplated by NASA. 
Source: A. L. Erickson and L. R. Guist 
Ames Research Center 
(ARC-1051 1) 
Category 08


Simple method for forming thin-wall pressure vessels

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