Space Station Freedom is being developed for a service life of up to thirty years. As a consequence, the design requirements for the seals to be used are unprecedented. Full scale testing to assure the selected seals can satisfy the design requirements are not feasible. As an alternative, a sub-scale test program has been developed by MSFC to calibrate the analysis tools to be used to certify the proposed design. This research has been conducted in support of the MSFC Integrated Seal Test Program. The ultimate objective of this research is to correlate analysis and test results to qualify the analytical tools, which in turn, are to be used to qualify the flight hardware. This research is totally focused on O-rings that are compressed by perpendicular clamping forces. In this type of seal the O-ring is clamped between the sealing surfaces by loads perpendicular to the circular cross section

Bower, Mark V.

English

NASA Technical Reports Server

N9 083
.. .._ _ ,_.
1992
NASA / ASEE SUMMER FACULTY FELLOWSHIP PROGRAM
MARSHALL SPACE FLIGHT CENTER
THE UNIVERSITY OF ALABAMA
DESIGN AND ANALYSIS OF SEALS FOR EXTENDED SERVICE LIFE
Prepared By: Mark V. Bower, Ph.D., P.E.
Academic Rank: Assistant Professor
Institution and Department: The University of Alabama in Huntsville
Department of Mechanical and Aerospace Engineering
NASA / MSFC:
Laboratory:
Division:
Branch:
Propulsion Laboratory
Component Development Division
Mechanical Systems Development Branch
MSFC Colleagues: Thomas D. Bechtel
Brian K. Mitchell
IV
https://ntrs.nasa.gov/search.jsp?R=19930008094 2020-03-17T09:12:19+00:00Z

Introduction
Space Station Freedom is being developed for a service life of up to thirty years. As a consequence,
the design requirements for the seals to be used are unprecedented. Full scale testing to assure the selected
seals can satisfy the design requirements are not feasible. As an alternative, a sub-scale test program[4] has
been developed by MSFC to calibrate the analysis tools to be used to certify the proposed design. This
research has been conducted in support of the MSFC Integrated Seal Test Program. The ultimate objective of
this research is to correlate analysis and test results to qualify the analytical tools, which in turn, are to be
used to qualify the flight hardware.
Sosh are simple devices that are in widespread use. The most common type of seal is the O-ring.
O-rings seals are typically rings of rubber with a circular cross section. The rings are placed between the
surfaces to be sealed usually in a grove of some design. The particular designs may differ based on a number
of different factors. This research is totally focused on O-rings that are compressed by perpendicular
clamping forces. In this type of seal the O-ring is clamped between the sealing surfaces by loads
perpendicular to the circular cross section.
In spite of the relative simplicity of the O-ring it does not lend itself to analysis. Which is why O-
rings have been so frequently designed based on handbook values, without extensive analysis. O-ring
analysis is complicated by the inherent nonlinearities in the problem. The O-ring problem involves nonlinear
geometric effects; due to the contact, or moving boundary, problem and the large deformations. It also
involves nonlinear material effects; due to the material nonlinearities at large deformations (hyperelasticity)
and the viscous behavior of the material (viscoelasticity). Cune_t advancements in computational methods
have led to the development of the tools that are capable of handling the O-ring problem.
Specific Problem Addressed
There are four basic design parameters that are to be considered in the development of an O-ring
seal: O-ring cross sectional diameter, grove design, O-ring squeeze, and material. In this research a single
O-ring diameter was considered. Grove design is typically selected from one of four fundamental grove
shapes: rectangular - no side wall contact, rectangular - side wall contact, dove tail, and iudf-dove tail.
Tkis research addresses both types of rectangular groves, and the half-dove tail design. The O-ring squeeze
levels range from 10% to as much as 50%. In this research three squeeze levels were considered: 15, 25, and
40%. The materials studied were selected from candidate materials for Space Station Freedom: Viton (Parker
V747) and Sificone (Parker $383).
Method of Approach
The designs considered in this research were analyzed using a commercial finite element analysis
code: ABAQUS by Hibbitt, Karlsson, and Soreusen, Incorporated. Preliminary model development was
accomplished using EMS and IFEM by Intergraph and translation to ABAQUS and post processing using
PATRAN by PDA. ABAQUS is a multipurpose finite element program developed without the classical
assumptions of small displacements and rotations. It was used in this _h because of its ability to analyze
contact problems, nonlinear material behavior, and viscoelastic response.
Three finite element models were developed. Although O-rings present natural symmetries that may
be used advantageously to reduce model size, symmetry was not used in any of the models developed. This
was motivated by the desire to include axisymme_c behavior and pressure loads on internal surfaces. In the
model development care was exercised to limit the number of triangular elements in the models. Further care
was exercised to limit the skewness of the elements, maintain uniform element size, and maintain element
aspect ratios near one. The models developed are shown in Figure I and the significant model characteristics
listed in Table 1.
Table L ]b'_it_ Eksmmst ld[odd (_am'mctm'i_
Model I N=_ofNodm To_Nmn_,'ofmemaB Numba' of T_dsr Eiamsm _-
Coenm | 202 86 4
I 633 244 s
........... _l_ino_, --I ..... 1295 ........... S40 ........ 16 ........
Coerse Model ln_ Model rme Model
]_L O-Rinll gadt_ Eiemed: Modds.
T_ mm_ ]robtemv,'nmdy_d mmu_ u_ of q_d purl_ _ _ _AQ_.
_ nKlui_ deSmil_m of the rigid m=1_c_ _mth_ nmdeL The side wall contact - r_-mngulm*Ip'ow md
the half dove-tail grove are shown in Figure 2. Note in the figure that the grove dismmsi_ chanp baud ms
the squeeze med. The rigid mn'fa_ are defmed thmegh use ofline and arc segme_. The lett_ in the
figure deeignate mdl_ of the segngnts md the numbers iadicate the tearers of the ===L Ti_ comdinmm of
theae pointa me listed ia Table8 2 md 3.
__ ,
3":'TO_ | _E _
Side Wall Contact- Half Dove'Tail
Rectangular- Grove
Figure 2. Candidate Grove Designs as Investigated.
Results
PATRAN and a specialpurpoeeprogramwere usedtoanalyzethen_mfl_geaeratedby ABAQUS.
A separatetrmudatorwas usedtotrmudatetheresultsintoPATRAN. Thistranslatoratthistimedoe8not
supporthecemactelementor appanmtlytheaxisymmetricelementsinABAQUS. PATRAN was usedto
produceplotsofthedeformedgeometri_ee,asshown inFigure3,and contourplotsofthestressesintheO-
ring. Figure 3 shows three typical deformed geometries, 15% squeeze with no side wall _ 15%
squeeze with side wall contact, and25% squeeze with side wall coutact. Note the smalle_ umta_ are_ on
the _ surfaces for the higher squeeze. This is due to the different grove dimensions between the
mode_
IV-2 %
OF PO0_ Qb_J._!,
The translator flies were used to calculate a von Mises stress and the strain eaergy at each node.
These values were also plotted using PATILAN.
Table 2.
Position
A
Center- 1
B
C
Center - 2
D
E
Center - 3
F
G
Center - 4
H
Side Wall Contact - Rectangular Grove Coordinates Relative to O-Ring Center.
15% Squeeze
X
-0.1395 0.0965
-0.1395 0.0915
-0.1345 0.0915
-0.1345 -0.1125
-0.1095 -0.1125
-0.1095 -0.1375
0.1095 -0.1375
0.1095 -0.1125
0.1345 -0.1125
0.1345 0.0915
0.1395 0.0915
0.1395 0.0965
All units are in inches.
25_ Squeeze
x y
-0.1600 0.0685
-0.1600 0.0635
-0.1550 0.0635
-0.1550 -0.1125
-0.1300 -0.1125
-0.1300 -0.1375
0.1300 -0.1375
0.1300 -0.1125
0.1550 -0.1125
0.1550 0.0635
0.1600 0.0635
0.1600 0.0695
-0.1960 0.0275
-0.1960 0.0225
-0.1910 0.0225
-0.1910 -0.1125
-0.1660 -0.1125
-0.1660 -0.1375
0.1660 -0.1375
0.1660 -0.1125
0.1910 -0.1125
0.1910 0.0225
0.1960 0.0225
O. 1960 0.0275
Table 3. Half Dove-Tail Grove Coordinates Relative to O-Ring Center.
Position
A
Center- 1
B
C
Center - 2
D
E
Center - 3
F
G
Center - 4
H
All nnits are in inches.
15% Squeeze 25% Squeeze 40_ Squeeze
x y x y x y
-0.1162 0.0965
-0.1162 0.0915
-0.1116 0.0895
-0.1970 -0.1023
-0.1742 -0.1125
-0.1742 -0.1375
0.0835 -0.1375
0.0835 -0.1125
0.1085 -0.1125
O. 1085 0.0915
0.1135 0.0915
O.1135 0.0965
-0.1402 0.0685
-0.1402 0.0635
-0.1356 0.0615
-0.2086 -0.1023
-0.1857 -0.1125
-0.1857 -0.1375
0.1075 -0.1375
0.1075 -0.1125
0.1325 -0.1125
0.1325 0.0635
0.1375 0.0635
0.1375 0.0685
-0.1810 0.0275
-0.1810 0.0225
-0.1764 0.0205
-0.2311 -0.1023
-0.2082 -0.1125
-0.2082 -0.1375
0.1475 -0.1375
0.1475 -0.1125
0.1725 -0.1125
0.1725 0.0225
0.1775 0.0225
0.1775 0.0275
IV-3
155 No sick wall Contgt 155 Side Wan Contact 23_ Side Wan Contgt
3. Typical Deformed Gecmetrim.
Conclusions
ABAQUS is an excellent tool for addressing seal performance. However, at tiffs time the program is
not capable of _ hypereh_c-viscoela_c amdyses. For viscoelastic malyses thz pmgnun is limited
to elastic_vismettstic analyses. This _ the nonlinear aspect of the ¢catact problem and the amociated
defonmtions, but does not includethe nonlinearmaterialbehaviormmociatedwith the kqle
de_
A second, potentially less tronbkmome, limitation of the code is the limitation to a _
depmdmt mmm'ial _. In tlw code the sheaz arem relaxatima modulus and the bulk _ nduatkm
modulus differ only by a scale factor (which can be zero). This may not be an acctLrate description of the
material behavior.
Acknowledgment
The author wishes to express his thanks to Mr. Brian Mitchell and Mr. Thomas Bechtel for a
productive and enjoyable summer. It has truly been a pleasure to work with people who are motivated and
dedicated to do their jobe, willing to go the extra mile to masu_ethat the job ia done right. Thanks m aim
givan to Mr. Pat Rodgers (ED24) for his support and advice. Further flanks are offered to Dr. Roy Sullivan
(ED24) for Im iavalmble mdmace. IXOYZ.
References
4
5
6
ABAQUS Theory Manual, Version 4.8, Hibbitt, Kadsson, & Soremm, Inc., Pawtucket, RL 1989.
ABAQUS Users' Manual, Version 4.8, Hibbitt, Karlmon, & Soremea, Inc., Pawtudr_ RI, 1989.
g,revelea, D.W. 'van, Properties of Polymers: Their Correlation with Chemical Stmcm_ Theh-
Numeti_tl Estimation and Prediction from Additive Group Contributions, 3rd Edition, Elsevier
Scieafific PubLishing Company, Amsterdam, 1990.
Mitchell, B.K. and Flatt, L.W., "Design Parameter Test Plan for MSFC Integrated Seal Test
Program," NASA/MSFC, 1992.
Stavennan,A. J. and SchwarzJ,F., "linearDeformationBehaviorof High Polymers,"in Die
ray k Der Hochpotymm : und Techae gi  
yon llochpolymm'en Werkstoffen, pp.l-121. Edited by: Stuart, H.A., Springer-Vedag, Berlin,
1956.
Staverman, A. J. and Schwar/l, F., "Non Linear Deformation Behavior of High Polymm," in Die
Physik Der Hochpolymm'en: Tbeorie und Molekulare Deutung Technologischer F.,igeasdmftm
yon Hochpolymerea Wefkstoft'e_, pp.126-158. Edited by: Stuart, H.A., Springer-Vedag, Berlin,
1956.
IV-4 _


Design and analysis of seals for extended service life

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