Filamentary fabric made with slip-net principle principle for pressurized suit

Kyser, A. C.

Schuerch, H.

English

NASA Technical Reports Server

4 
3 ARC-R-221,# 
 PRESSURIZED STRUCTURES OF HIGH MOBILITY L/ 
J/ 
SEPTEMBER 2 2 ,  19661  ' 
! 
BY 
! A .  C.  KYSER 
H .  SCHUERCH 
3 " 1 Prepared Under C o n t r a c t  N A S 7 - 4 2 7 A  
ASTRO- RESEARCH CORPORATION 
2 
SANTA BARBARA, C A L I F O R N I G  
P . O .  BOX 4 1 2 8  
TELEPHONE: 9 6 3 - 3 4 2 3  
https://ntrs.nasa.gov/search.jsp?R=19670030608 2020-03-16T16:48:15+00:00Z
TABLE OF CONTENTS 
ABSTRACT 
I. INTRODUCTION 
11. GENERAL DISCUSSION OF SLIP-NET PRINCIPLE 
111. STABILITY CONSIDERATIONS 
Page 
1 
IV. EXPERIMENTAL 5 
V. CONCLUSIONS 6 
REFERENCES 8 
FIGURES 9 
ii 
, "' 
PRESSURIZED STRUCTURES OF HIGH MOBILITY 
By A. C. Kyser and H. Schuerch 
Astro Research Corporation 
ABSTRACT 
A method is presented for constructing a cylinder having low 
bending stiffness and zero restoring moment from the bent condition 
throughout a wide range of bending deformations. The behavior of 
the structural mechanism which carries the pressure load and ac- 
commodates the bending is described and possible applications to 
space suit components are shown. 
I. INTRODUCTION 
A requirement exists for pressure-resistant structures that 
can be made to undergo large geometrical changes, while inflated, 
with a minimum of associated resistance to deformation. Tubular 
members'having this property could be used to advantage, for example, 
in rocket-motor fuel lines (to reduce the flange moments, as dis- 
cussed in Reference 1). Other potential applications may be found 
in pneumatic conveyors, temporary pipelines, etc. More complex 
shapes and mobility requirements exist for the pressure constraint 
layers of high altitude protection garments and space suits (Ref. 3 ) .  
A theoretical basis for pressurized structures of zero stiff- 
ness can be approached from two points of view: 
Since the stiffness due to pressurization is of primary 
concern, it can be postulated that this stiffness will 
vanish if the membrane forming the structure can be deformed 
in such a fashion that the enclosed volume remains constant 
during the deformation. This postulate arises from the fact 
that the work of deformation which is associated with pressure 
stiffness must reappear in the form of potential energy in 
the contained gas and therefore must be represented by a 
volumetric change; alternatively, 
the equilibrium conditions of a filamentary membrane with 
internal pressure and externally applied loads can be formu- 
lated and'the force-deformation characteristics can be 
derived from consideration of statics. From these con- 
siderations, the kinetic and elastic properties of the 
membrane can be inferred which will tend towards mini- 
mizing the resistance against deformation. 
This second approach has been pursued in the work subject of 
the present report. 
11. GENERAL DISCUSSION OF SLIP-NET PRINCIPLE 
In the conventional types of flexible, pressure-resistant 
tubing, such as convoluted tubing or braid-reinforced rubber tubing, 
the structural material is arranged in a way such that the longitudi- 
nal loads are carried along curved load paths. The local normal 
curvature of these load paths is established (ideally) by the in- 
ternal pressure and the tension stresses in the structural material. 
When a bending moment is applied to bend the tubing, a gradient in 
the tension loading develops across the cross section of the tube,' 
and the local load-path curvature changes, producing the bending 
deformation. The elastic properties of this general type of struc- 
ture characteristically include a bending stiffness which is pro- 
portional to pressure and which increases with deformation, a 
distinct equilibrium shape (except for minor hysterisis) for each 
value of applied bending moment, and a tendency to fold (becoming 
creased on the inside and flattened on the outside) at a bend radius 
of several tube diameters. 
The structural concept presented here, which has been given 
the name of "slip-net" tubing, differs from the conventional approach 
in that the structural material is permitted to slip locally with 
respect to the surface so as to redistribute itself to reduce (or, 
ideally, eliminate) the longitudinal 'tension gradients which give 
rise to the restoring moment in the deformed tube. This sliding 
action, with its associated friction, completely changes the charac- 
teristics of the structure, with the result that the behavior of 
the tube under bending approximates that of a distributed plastic 
hinge. 
The construction of the slip-net tubing is shown in Figure 1. 
The structural wall is made of three separate, independent layers 
of (essentially) unidirectional filamentary material, of which two 
are wrapped as opposing, mirror-image sets of helical load-paths, 
while the third is circumferential. The helical layers are free to 
slide, independently, over the surface and to stretch in the "cross- 
grain: direction in order to accommodate the bending deformation of 
2 
the tube. This sliding freedom gives the desired bending com- 
pliance. The circumferential layer is required to stabilize the 
cross section of the tube, as described later. Both the circum- 
ferential layer and the impermeable inner liner (which is re- 
quired to confine the pressurized fluid) must have enough compliance 
in the longitudinal direction to remain unstressed as the tube is 
bent. 
During the bending deformation, the surface deforms in a way 
such that the original helical paths become steeper at the outside 
of the bend and less steep at the inside. 
helix-angle distribution can be observed by bending a helically- 
wound cylinder on which the intersections of the opposing helices 
have been pinned to prevent sliding. 
element is a rhomboidal cell with a fixed perimeter. As indicated 
in Figure 2, the cells at the outside of the bend become longer and 
narrower, while those on the inside become shorter and wider. The 
resulting fiber pattern has geodesic curvature which is concave 
toward the inside of the bend and therefore produces a tension gradi- 
ent that increases the load in the outer fibers and unloads the 
inner fibers. The net result is that the deformed tube develops a 
restoring moment: the longitudinal stress resultant, T cosy/a , 
increases across the cross section from inside to outside, since 
To > Ti , cosy > cosyi , and a. > ai . 
This general type of 
Here the unit structural- 
0 
The effect of the sliding of the two helical wraps relative to 
each other is to reduce the geodesic curvature and, therefore, to 
redistribute the longitudinal structure in a way such that the 
gradient of the longitudinal stress resultant is reduced through 
changes in all three components: 
the spacing. 
tube with a section of a helically-wound isotensoid toroid. 
3 is a photograph of a toroid which was designed according to Refer- 
ence 2 to have a geodesic winding pattern and (therefore) uniform 
fiber tension. 
spacing (dimension a of Figure 2) is greatest at the outside of the 
bend, while the helix angle y is greatest at the inside. 
relations are basically different from those discussed for the bent 
helically-wound tube with pinned intersections. 
the tension, the helix angle, and 
Figure 
It is instructive to compare the bent portion of the 
It can be seen in this photograph that the fiber 
These 
Because both the fiber-packing density and the longitudinal 
(i.e., equatorial) component of fiber tension on the toroid are 
greater at the inside of the bend, the center of effort of the 
longitudinal components of fiber tension at any meridian lies 
toward the inside from the center of area of the cross section. 
For the toroid in the photograph the center of effort is displaced 
from the center of area by 41 percent of the mean radius of the 
3 
cross section. It must be concluded that if the bent tube had 
the same curvature and fiber distribution as the toroid shown, 
the internal structural forces would act to increase the bending 
deformation, rather than decrease it, which is to say it would 
exhibit a negative restoring moment. Such a tendency has been 
observed in varying degrees in the models which have been built. 
It has been found that the negative restoring moment can be counter- 
acted by incorporating sufficient cross-grain stiffness in the 
sliding layers. 
111. STABILITY CONSIDERATIONS 
In applying the slip-net principle, it is necessary to take 
precautions to avoid certain instabilities which arise as a result 
of the sliding freedom. One potential instability involves the 
tendency of the helical fibers to bunch together to form a helical 
crease in the surface of the tube. Such a crease can grow with 
progressively more fibers sliding into it, until the tube is grossly 
distorted. A similar tendency which can appear when the tube is 
sharply bent involves the spreading of fibers on both layers to form 
a rhomboidal area over which the surface is unsupported. Eventually 
the gap becomes great enough to allow the circumferentially-wrapped 
bladder to push its way completely through the helical structure. 
Both of these instabilities can be eliminated by incorporating in 
each of the layers sufficient cross-grain stiffness to prevent large 
local variations in fiber spacing. 
Another instability which has been observed in this type of 
device occurs when there is insufficient stiffness associated with 
the circumferential dimension of the tube, as is the case with a 
cylinder which has only helical fibers. Such a tube will grow in 
diameter in one section and shrink in an adjacent section, as the 
filamentary layers slide from the region of high curvature and low 
tension to the region of low curvature and high tension (See Fig. 12). 
This tendency is readily controlled with a layer of circumferential 
fibers to constrain the cross section to a given perimeter. 
Since a circumferential layer is required for stability reasons, 
the helix angle y is a free parameter which can, in principle, 
have any value less than the ''hose angle", 54.7O (above which the 
circumferential wrap would be loaded in compression). No attempt 
has been made to find an "optimum" value for this angle or to explore 
the likely range. It can be seen, however, that there are counter- 
acting effects which indicate that a best value might be found. 
Thus the steep helix (small y )  requires more sliding and more lateral 
stretch to accommodate a given bend. It can, however, slide more 
4 
e a s i l y  because of t h e  smal ler  t e n s i o n  and normal cu rva tu re ,  which 
produce t h e  normal p ressu re  t h a t  g ives  rise t o  t h e  s l i d i n g  re- 
s i s t a n c e .  The shallow h e l i x ,  on t h e  o t h e r  hand, r e q u i r e s  less 
s l i d i n g  b u t  has  h igher  r e s i s t a n c e  t o  s l i d i n g .  
I V .  EXPERIMENTAL 
I n  o rde r  t o  ga in  experience wi th  t he  s l i p - n e t  concept,  s e v e r a l  
pro to type  tubes  w e r e  f a b r i c a t e d .  
Dacron f i b e r ;  f i g u r e s  8 - 1 0  show tubes  made from Teflon f i b e r  i n  
var ious  p o s i t i o n s  of bending, 
three i n d i v i d u a l  l a y e r s  of k n i t t e d  fabr ic ,  us ing  a h e l i x  angle  of 
4 5  degrees.  
Figures  5 - 7 show tubes  made from 
These tubes  w e r e  cons t ruc ted  w i t h  
Kni t ted  cons t ruc t ion  was chosen f o r  these prototypes because of 
two p r o p e r t i e s  which are of s p e c i a l  importance i n  t h i s  a p p l i c a t i o n .  
Kni t ted  shee t ing  i s  r e a d i l y  f a b r i c a t e d ,  and, p a r t i c u l a r l y  w i t h  a 
knit- and-purl  p a t t e r n ,  makes an e s p e c i a l l y  smooth and s l i p p e r y  "uni-  
d i r e c t i o n a l "  l a y e r .  More important ,  however, i t s  b i d i r e c t i o n a l  
e l a s t i c  p r o p e r t i e s  a r e  such t h a t  t h e  " f ibers"  ( i . e . ,  t h e  "whales" 
of t h e  f a b r i c )  show a s t rong  tendency t o  remain uniformly spaced. 
T h i s  tendency a r i s e s  from t h e  f a c t  t h a t  t he  u n i d i r e c t i o n a l  sheet i s  
shortened where t h e  spacing i s  l a r g e  and r e l a x e s  where t h e  fibers 
a r e  bunched. Such a "Po i s son ' s- ra t io"  e f f e c t  r e s u l t s  i n  a s t rong  
s t a b i l i z i n g  r e a c t i o n  t o  t h e  tendency t o  form c reases  and wa i s t s .  
Each l a y e r  of t he  tubes  was k n i t  i n  a long s t r i p  which was 
wound i n  p l a c e  on a c y l i n d r i c a l  form. 
t o  behave e s s e n t i a l l y  l i k e  a u n i d i r e c t i o n a l  f a b r i c ,  and then sewn 
toge the r  a long t h e  s e a m .  The l i n e r  i s  a " c i g a r e t t e  r o l l "  tube  of 
s u r g i c a l  rubber shee t ing ,  a s  shown inc t h e  photographs. 
can be bent e a s i l y  by hand, w i t h  an i n t e r n a l  pressure  of 30 p s i ,  
around i t s  own diameter ( 4 . 5  inches)  and then  s t r a igh tened .  
T h e  s t r i p  was stretched so as  
These models 
Bending- st i f fness  measurements w e r e  made on the  prototype 
s l i p- n e t  tube  models by suppor t ing  one end on a f ixed  clamp, and 
applying t o  t h e  o t h e r  end a pure bending moment through a dead-weight- 
pu l l ey  arrangement a s  shown i n  Figure 11. The most important f a c t o r  
i n  t h e  behavior of t h i s  tube  appears t o  be the  f r i c t i o n a l  r e s i s t a n c e  
t o  s l i d i n g  between the  l a y e r s .  
t he  r e s t o r i n g  moment w a s  zero  for  e s s e n t i a l l y  t he  e n t i r e  range of 
bending deformation (see Figs .  5 ,  6 ,  8 and 9 > ,  whether t h e  deformed 
condi t ion  was reached by inc reas ing  o r  decreas ing  t h e  bending defor-  
mation. When a pure bending moment was app l i ed ,  t h e  tube  would n o t  
respond u n t i l  t he  bending moment reached a th resho ld  va lue ,  a t  which 
' p o i n t  t h e  i n t e r l a m i n a r  shea r  loads  would overcome t h e  s ta t ic  f r i c t i o n  
I n  a l l  t h e  models which w e r e  made, 
5 
and the tube would start to bend. The bending deformation would 
then proceed through approximately a radian on a tube that was 
about eight diameters in length until the limits of the apparatus 
were reached. The threshold bending moment was such that M/pnr3 0.2 
throughout the entire range of deformation, for the tube made of 
Dacron roving. This tube showed a slight tendency to bend more easily 
than to straighten; the threshold moments were lower and the subse- 
quent travel was greater when the bend radius was being decreased. 
A tube made without the third "circumferential layer" is shown 
in Figure 12. It exhibits the "pinch" instability discussed in 
Section 111. 
In addition to the tube models, several prototypes of more 
complex structures were constructed. 
Figure 1B shows a structure involving a bifurcation, such ,as 
would be required for a fingered glove or hip-leg portion of a space 
suit. Figure 14 shows the same structure in an unconstrained, de- 
formed mode under a 7 psi internal pressure. 
Finally, a full-sized shoulder-arm joint was fabricated and 
patched into the constraint layer of a regulation space suit, obtained 
for this purpose from Houston Manned Spacecraft Center. Figure 15 
shows the limits of upward arm mobility at 5 psi internal pressure 
obtained with conventional (link-net) and the slip-net design. The 
conventional portions of the suit have been retouched to obscure 
details for reasons of classification. 
, 
It was observed, that the side made from slip-net fabric 
exhibited markedly better mobility. Overhead "reach" was improved 
by approximately 3 "  without compromise on other position limits. 
Further, a much better conformance to the human shape was achieved 
in all positions examined, effectively reducing the bulk of the suit. 
V. CONCLUSIONS 
The slip-net principle appears to offer a practical approach to 
. -  the problem of reducing the bending stiffness of pressurized tubular 
members. Experiments with using this principle in the restraint 
layer of a space suit have shown the concept to be very promising in 
improving the mobility of critical joints, since it offers the 
attractive combination of low bulk (in contrast to that associated 
with convolutions and bearings), low bending stiffness, and zero re-. 
storing moment in the deformed condition. 
Further work is required, both theoretical and experimental, 
to establish quantitatively the performance characteristics obtain- 
able with this concept. Particularly, more work is required to 
realize the full potential mobility characteristics of complex 
shapes typical in space suit applications. 
REFERENCES 
1. 
2, 
3 .  
Weathers, T. M. and Weingart, 0 . :  Flexible Filament-Wound 
Fluid Transfer Lines for High-pressure Liquid Rocket 
Engines. AIAA 6th Structures and Materials Conference, 
April 5-7, 1965. 
Burggraf, 0. R. and Schuerch, H.: Analysis of Axisymmetric, 
Rotating Pressurized Filamentary Structures. NASA TN D-1920, 
May 1963. 
Johnston, R, S., Correale, J, V., and Radnofsky, M. I.: 
Space’ Suit Development Status. 
Report 5-102, December 1965. 
NASA Manned Spacecraft Center 
r 
8 
FIGURE CAPTIONS 
' 1. Construction of S1ip.Net Tubing 
2. Geometry of Unit Structural Cells on Helically-Wound 
Tube with Pinned Inte'rsections 
3 .  Geodesically Wound Toroid 
4. Displacement of Center o f .  Effort of Fiber Forces From 
Center of Area of Cross Section 
5. Pressurized. Three-Layer Slip-Net Tube Made From "Dacron" - /- 
Straight 
6. . Pressurized Three-Layer Slip-Net Tube Made From "Dacron" - 
Bent 
Pressurized Three-Layer Slip-Net Tube Made From "Dacron" - 
Curled 
Pressurized Three-Layer Slip-Net Tube Made From "Teflon" - 
Straight 
7, 
. '  
8. 
9. Pressurized Three-Layer Slip-Net Tube Made From "Teflon" - 
Bent 
10. Pressurized Three-Layer Slip-Net Tube Made From "Teflon" - 
Curled 
11. Bending Test Setup with "Dacron'@ Tube 
12. Pressurized Two-Layer Tube with "Pinch" Instability 
13. Bifurcated Slip-Net Structure - 7 PSI Internal Pressure 
. 14, Deformation of Bifurcated Slip-Net Structure - . 7  PSI 
Internal Pressure 
15. Operational Space Suit with Slip-Net Shoulder Joint .. 
Relative Mobility Demonstration at 5 PSI Internal Pressure 
9 
UNIDIRECTIONAL HELICAL LAYERS :r 
UN I D 1  RECTI  ONAL C I  RCUMF'ERENTIAL 
LAYER 
END CAP 
RUBBER L I N E R  
L HELICAL SEAMS 
FIGURE 1. CONSTRUCTION OF S L I P- N E T  TUBING 
10 
C E L L  GEOMETRY AT / O U T S I D E  O F  BEND 
A X I S  
M M 
CELL GEOMETRY AT 
I N S I D E  O F  BEND 
F I G U R E  2. GEOMETRY O F  U N I T  STRUCTURAL C E L L S  ON HELICALLY-WOUND 
TUBE WITH P I N N E D  I N T E R S E C T I O N S  
11 
F I G U R E  3 .  GEODESICALLY WOUND TOROID 
q-. --AI- 
. E q u a t o r i a l  P lane  \ 2 T 
O u t s i d e  R a d i u s  
F I G U R E  4 .  DISPLACEMENT O F  CENTER O F  EFFORT O F  F I B E R  FORCES 
FROM CENTER O F  AREA O F  CROSS S E C T I O N  
12 
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F I G U R E  11. B E N D I N G  TEST S E T U P  WITH "DACRON" TUBE 
19 
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20 
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FIGURE 13. BIFURCATED SLIP-NET STRUCTURE - 7 PSI 
INTERNAL PRESSURE 
21 
FIGURE 14. DEFORMATION OF BIFURCATED SLIP-NET STRUCTURE 
- 7 PSI INTERNAL PRESSURE 
22 
2 
3 
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23 


Pressurized structures of high mobility

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