Technique applies to liquid or gas transfer through flexible ducting and proves valuable in high velocity fluid flow cases. Fluid mechanism responsible for free bellows vibrational excitation also causes flexible hose oscillation. Static pressure stress influences flexible ducting fatigue life and is considered separately

Henry, R. H.

English

NASA Technical Reports Server

June 1971
	 Brief 71-10150 
NASA TECH BRIEF 
_4V Marshall Space Flight 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. f. 20546. 
Predicting Vibrational Failure of Flexible Ducting 
A simple technique successfully predicts the oc-
curence of failure caused by flow-induced vibrations 
in bellows and other flexible hoses. The technique 
should find direct application wherever liquid or gas 
is transferred through flexible ducting, and should 
prove especially valuable in cases involving fluid flow 
at high velocity. 
Application of the technique entails computing the 
function
S i = ahpf V 2Q/2N t2, 
where a is the convolution width of the bellows, 
h is the convolution height of the bellows, 
Pf is the fluid static pressure, 
V is the fluid flow velocity, 
.Q is the dynamic amplification factor of the 
bellows, 
•N is the , number of plies in the bellows, 
is the convolution thickness per ply, and 
S i is the failure indicator, which is directly 
proportional to the peak stress on the bellows 
convolutions, as caused by the dynamic fluid 
pressure and the vortex-shedding force. 
The key to this analysis is the determination of 
the natural resonance (frequency and quality) of 
the flexible sections, together with the vortex-shedding 
frequency of the bellows convolutions. When the two 
frequencies are in close proximity, vibrational failure 
becomes probable. This condition is shown by a 
marked increase in the value of the indicator 
Although insufficient information has been com-
piled to allow complete definition of the exact value 
of S i above which vibrational failure will occur, the 
following ranges have been determined by testing
existing bellows installations and by examining previous 
cases of bellows failure:
S 1 < 8c10 4 No problem 
840 <S1 < 20x10 4 Uncertain failure 
probability 
20x10 < S 1	 Probable failure 
Notes: 
1. The fluid mechanism responsible for vibrational 
excitation of free bellows also causes oscillation 
in flexible hoses. Although the'braided covering 
of the hose restricts its number of vibrational 
mqdes, the stress indicator S i
 remains a valid 
indicator of failure probability. 
2. S i indicates dynamic stresses only. Stress caused 
by static pressure also influences fatigue life of 
flexible ducting, and must be considered separately. 
3. This information may be of iiterest to the 
petroleum and chemical industries. 
4. Requests for further information may be directed 
to:
Technology Utilization Officer 
Code A&TS-TU 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B71-10150 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. H. Henry of

North American Rockwell, Inc.

under contract to

Marshall Space Flight Center

(MFS- 16750) 
Category 06 
This document was prepared under the sponsorship of the National 
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 ownedrights.
https://ntrs.nasa.gov/search.jsp?R=19710000149 2020-03-17T02:24:01+00:00Z


Predicting vibrational failure of flexible ducting

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