ArticlePDF Available

Abstract and Figures

This paper describes permeability measurements for porous fabrics as influenced by strain, humidity, air flow rate, and fabric elasticity. The focus is on standard parachute fabrics, where the fabric's porosity and air permeability influence the rate of steady-state descent, and also affect the complicated fluid-structure interactions taking place during parachute opening and deployment. High strength nylon parachute fabrics showed relatively small permeability changes due to strain, humidity, and flow rate. Comparative measurements on elastomeric fabrics showed much larger changes in air flow due to fabric dimensional changes at high pressures and flow rates. Elastomeric fabrics that stretch and change permeability in response to higher pressures and flow rates may be able to reduce the "opening shock" during the parachute deployment phase.
Content may be subject to copyright.
Journal of Engineered Fibers and Fabrics 29 http://www.jeffjournal.org
SPECIAL ISSUE - July 2012 – FIBERS
Dynamic Permeability of Porous Elastic Fabrics
Phillip W. Gibson, Kenneth Desabrais, Ph.D., Thomas Godfrey, Ph.D.
U.S. Army Natick Soldier Research, Development, and Engineering Center, Natick, MA UNITED STATES
Correspondence to:
Phillip W. Gibson email: phillip.gibson@us.army.mil
ABSTRACT
This paper describes permeability measurements for
porous fabrics as influenced by strain, humidity, air
flow rate, and fabric elasticity. The focus is on
standard parachute fabrics, where the fabric’s
porosity and air permeability influence the rate of
steady-state descent, and also affect the complicated
fluid-structure interactions taking place during
parachute opening and deployment. High strength
nylon parachute fabrics showed relatively small
permeability changes due to strain, humidity, and
flow rate. Comparative measurements on elastomeric
fabrics showed much larger changes in air flow due
to fabric dimensional changes at high pressures and
flow rates. Elastomeric fabrics that stretch and
change permeability in response to higher pressures
and flow rates may be able to reduce the “opening
shock” during the parachute deployment phase.
INTRODUCTION
Air flow across porous and permeable materials can
interact with the fabric structure. For parachutes,
fabric permeability plays an important role in both
steady descent (affecting rate of descent and stability)
and in the inflation phase, affecting inflation time and
opening shock. It is during the inflation phase that
fabric permeability is least understood. Static
measurements of permeability are made under steady
state flow conditions, whereas during inflation,
dynamic permeability, as it occurs in situ, is of
interest. An additional complicating factor is the
high fabric stresses and associated deformations
occurring during inflation; applied loads have been
shown to significantly affect permeability (100 – 500
%) under limited static test conditions [1].
The porosity of parachute fabrics is an important
parameter which affects the performance of the
parachute canopy by influencing the inflation process
of the canopy as well as the overall descent rate and
stability of the parachute system. The concept of
porosity is a term in parachute technology that is
slightly different than conventional engineering use.
Porosity is used to characterize the air flow
characteristics of a parachute cloth. For the cloth
itself, “porosity,” is defined as the airflow rate
through the cloth at 0.5 inches of water pressure, as
measured in a standard test method [2]. Parachutes
commonly have slotted openings in the structure.
This is also called “porosity,” and is characterized as
the ratio of all open areas to the total canopy area [3].
High air flow and dynamic pressure can deform and
stretch the fabric layer, or even cause failure [4].
Figure 1 shows an example of a standard nylon
parachute fabric during air flow testing at two
pressure levels, and the associated fabric deformation
due to high air flow through spaces between woven
yarns. Testing under higher flow rates and pressure
loads provides information on steady state properties
of parachute fabrics that may particularly influence
canopy behavior during the inflation phase. For
example, it may be desirable to increase fabric
permeability during the canopy opening shock phase
to decrease structural loads. Instrumented flow rate
testing at high pressures, coupled with photographic
examination of fiber/yarn behavior provides new
insights into fabric behavior under these conditions.
FIGURE 1. Air permeability and fabric deformation under applied
air pressure load. Standard parachute fabric under pressure loads of
20 and 120 psi showing minimal fabric distortion, but some yarn
separation due to high air flow.
Fabrics that absorb water vapor from the atmosphere
undergo fiber swelling, which tends to close off the
Journal of Engineered Fibers and Fabrics 30 http://www.jeffjournal.org
SPECIAL ISSUE - July 2012 – FIBERS
pores in the fabric and increase the resistance to air
flow through the material. Many studies of structural
factors influencing the air permeability of fabrics
assume that gas or liquid flow takes place in the
interstices between yarns. This approach works well
for fabrics that have an open construction, where
there are large open areas between yarns. For these
open fabrics, the diameter and spacing of the yarns
are the important structural factors, since almost all
flow takes place through the interyarn pores. In more
tightly woven fabrics, there is no interyarn space, and
fluid flow takes place within the structure of the yarn
itself. In this case, the diameter and physical
arrangement of the fibers within the yarn are the most
important factors influencing the fluid flow resistance
across the textile layer (Figure 2).
FIGURE 2. Fabric construction affects magnitude of permeability
changes due to variable fiber dimensions associated with water
vapor absorption or desorption.
Of special interest is the peculiar tendency of nylon
fabrics to show decreased flow resistance (increased
permeability) at high humidity, which is counter to
the behavior of other fabrics (Figure 3). This
behavior is attributed to the tendency of nylon fibers
to swell axially rather than radially as most other
textile fibers do, thereby causing fabric pores to open
up rather than close down.
FIGURE 3. Fiber swelling significantly affects fabric permeability
in different ways depending on composition and construction.
Previous measurements on fifteen nylon fabrics [5]
revealed that this is a consistent property of nylon
fabrics and is common to many types of fabric
weaves and thicknesses. Parachute fabrics (usually
constructed of woven nylon fabric) were not
specifically addressed in this previous study. New
measurements address the magnitude of the
permeability changes associated with water
absorption for standard nylon parachute fabrics, in
the context of flow rates and pressures associated
with parachute opening and descent.
Since parachute fabric in use is under an applied
load, the strained configuration of the fabric is most
appropriate to use when measuring flow through the
fabric. The coupling between the pressure
distribution over the surface of the parachute canopy
and the change in pore dimensions due to the applied
load is anticipated to be of equal (or greater
magnitude) than the dimensional changes associated
with humidity-dependent fiber and yarn swelling. A
particular focus of this study is highly elastomeric
fabrics (as opposed to standard nylon parachute
fabrics) and the measurement of fabric permeability
under conditions of defined stress and/or strain, and
how these measurements compare to those made
under standard non-strained conditions.
Journal of Engineered Fibers and Fabrics 31 http://www.jeffjournal.org
SPECIAL ISSUE - July 2012 – FIBERS
0
1x10
6
2x10
6
3x10
6
020 40 60
Elastic
Knit Fabric
Strain (%)
Air Flow Resistance ( m
-1
)
(a) (b)
FIGURE 4. a) Pressurization sequence proceeding to failure for
highly elastic porous nonwovens; b) Fabric strain due to applied
load opens pores and increases air flow.
MATERIALS
Standard parachute fabrics were selected for testing,
along with two elastomeric porous materials, and a
reference nonwoven glass fiber filter material. The
parachute fabrics in Table I are specified according to
Parachute Industry Association (PIA) standards. The
elastomeric fabrics given in Table I are 88%
polyester / 12% spandex knit elastomeric fabric, and
an electrospun elastomeric polyurethane nonwoven
fabric.
TABLE I. Materials.
Parachute Fabrics Relative Permeability
PIA-C-7020, Type III (high air permeability)
PIA-C-7020, Type I (high air permeability)
PIA-C-7350, Type I (high air permeability)
PIA-C-44378, Type IV (low air permeability)
Other Materials
Elastomeric Nonwoven (low air permeability)
Elastic Knit Fabric (high air permeability)
Glass Fiber Filter (nonhygroscopic standard)
METHOD
A schematic of the air permeability test method is
shown in Figure 5. It is designed to be compatible
with the ASTM Standard Methods for Gas Flow
Resistance of Filtration Media (F778-07). An
alternate method for textiles, ASTM Standard Test
method for Air Permeability of Textile Fabrics
(D737-04), is commonly used for parachute fabrics,
but is more useful for quality-control testing due to
the prescribed standard pressure differential of 124.5
Pa (0.5 inches of water). ASTM D737 is also more
difficult to adapt to automated testing under a variety
of different flow rates and relative humidity
conditions.
FIGURE 5. Air flow test schematic.
There are many definitions of permeability and air
flow resistance. Usually, the permeability is defined
from Darcy's law [6]:
 
xPA
Q
xPv
KD1
(1)
KD = permeability constant (m2)
= gas viscosity (1.785 x 10-5 Pa*s for N2)
v = apparent gas flow velocity (m/s)
P = pressure drop across sample (Pa)
x = thickness (m)
Q = volumetric flow rate through the fabric (m3/s)
A = sample flow area (m2)
For textiles, although thickness measurements seem
simple, they are often problematic, and can be a large
source of error if they are incorporated into reported
measurements of Darcy permeability.
It is preferable to present the pressure-drop/flow rate
results in terms of an apparent flow resistance
defined as:
Q
PA
Kx
RD
D
(2)
RD = apparent Darcy flow resistance (m-1).
In this paper, results are given in terms of the air flow
resistance RD (m-1), rather than permeability KD (m2).
Journal of Engineered Fibers and Fabrics 32 http://www.jeffjournal.org
SPECIAL ISSUE - July 2012 – FIBERS
RESULTS
Three effects were examined: humidity, strain, and
flow rate.
Humidity
Humidity-dependent air permeability is most evident
in fabrics constructed of hygroscopic fibers such as
cotton or wool [5]. Fabric flow resistance can double
as a consequence of the water uptake by a cotton
fabric, as illustrated in Figure 6. Figure 6a shows
that the flow resistance is directly related to the
amount of water taken up by the textile (Figure 6b).
Both plots show that there is significant hysteresis in
the curves over a complete sorption and desorption
cycle, which begins at low humidity, goes to high
humidity, and returns to low humidity.
(a) Air Flow Resistance
(b) Water Vapor Sorption
FIGURE 6. (a) Hysteresis in air flow resistance as function of
relative humidity; (b) hysteresis in fabric regain (fractional water
uptake) as function of relative humidity.
Comparable humidity-dependent air flow
measurements for the four standard parachute fabrics
are shown in Figure 7. In this case, the changes in
permeability are normalized by the flow resistance
measured at 0% relative humidity (R0) to allow easier
comparison between materials.
0.5
0.7
0.9
1.1
1.3
1.5
00.2 0.4 0.6 0.8 1.0
Glass Fiber Filter
PIA-C-7020, Type III
PIA-C-7350, Type I
PIA-C-7020, Type I
PIA-C-44378, Type III
Relative Humidity (1.0 = 100% r.h.)
Normalized Air Flow Resistance
(R/R
0
)
FIGURE 7. Change in normalized air flow resistance as function
of relative humidity for several parachute fabrics compared to a
nonhygroscopic glass fiber nonwoven filter material.
The glass fiber filter material showed no humidity-
dependent permeability, as expected, due to its
nonhygroscopic nature. The four parachute fabrics
show varying degrees of sensitivity to humidity. The
least permeable fabric, PIA-C-44378, Type III,
showed the largest humidity effects. This is due to
the more tightly-woven construction and smaller pore
area of the fabric. Any dimensional changes caused
by water vapor uptake of the fabric result in a larger
percent difference in pore flow area, and have a
corresponding influence upon the air flow
measurements.
The four parachute fabrics showed similar behavior
to other nylon fabrics that have been tested
previously. Many nylon fabrics decrease air flow
resistance at higher humidities [5]. Two of the
parachute fabrics showed a decrease in flow
resistance at higher humidities, while the two other
fabrics were essentially unchanged. In all cases, the
humidity did not have a very large effect on air flow
through the fabric, with the percentage change in
flow resistance being less than 10%. As will be seen
later, other factors besides humidity had a more
significant effect on the measured air flow resistance
of these four nylon parachute fabrics.
Nonuniform Strain
Parachute fabrics are under highly nonuniform strain
during the deployment and opening phase, and also in
certain regions of the canopy during the steady-state
descent phase. Nonuniform strain is developed in the
fabric due to differential stress caused by shear
developing along the seams, at the suspension and
control line attachment points, and near sewn
openings such as flaps and slots. Stress and strain
cause the fabric to deform, twist, and stretch. This
alters the fabric pore geometry and cross-sectional
Journal of Engineered Fibers and Fabrics 33 http://www.jeffjournal.org
SPECIAL ISSUE - July 2012 – FIBERS
area, which can affect the air flow through the pores
of the fabric.
Each of the four standard parachute fabrics was
tested in the strained and unstrained condition. Strain
refers to bias deformation, where the stress is not
aligned with the woven yarn axes.
Testing in the strained and unstrained condition was
conducted simultaneously with varying humidity
levels between 0 and 1 (100% r.h.).
Figure 8 shows results for the single fabric PIA-C-
7020, one of the more permeable fabrics in the set of
four standard parachute materials.
1.15x10
7
1.25x10
7
1.35x10
7
1.45x10
7
00.4 0.8 1.2
Unstrained
Strained (Deformed on Bias)
PIA -C-7020, Type I
Relative Humidity (1.0 = 100% r.h.)
Air Flow Resistance (1/m)
FIGURE 8. Change in air flow resistance as for PIA-C-7020,
Type I parachute fabric as function of relative humidity in the
strained and unstrained condition. Hysteresis loop indicated by
arrows.
The strained test configuration shows a consistently
higher air flow resistance over all humidity levels as
compared to the unstrained fabric. This is
presumably due to the deformation of the pore area
between yarns. Deformation changed the pore from
its optimal rectangular configuration for flow (a
rectangle) to a trapezoid, which has a lower
equivalent diameter [7], and therefore a higher flow
resistance. The other parachute fabrics are not shown
here; the effects of strain deformation were even
lower for the other fabrics tested, and in all cases the
variation in air flow resistance due to relative
humidity was larger than any effects due to bias
fabric deformation.
Air Flow Rate
Standard parachute fabric air permeability testing is
carried out at a pressure difference of 0.5 inches of
water (about 150 Pa). Parachutes are used in
situations that can produce higher air flow rates and
air pressure differentials across the fabric,
particularly during the opening phase. A stagnation
pressure of 0.5 inches of water corresponds to a
steady-state descent speed of about 2800 feet/minute
(14 m/s or 30 mph).
The peak pressure developed during parachute
deployment depends on initial air speed. Personnel
parachutes that descend at rates in the range of 5 m/s
develop a dynamic differential pressure of 50 Pa
across the fabric. Transient peak pressures for
parachutes deployed under these conditions are in the
neighborhood of 5 times steady-state dynamic
pressure [8], or 300 Pa. Special-purpose cargo or
payload decelerator parachutes may be deployed at
much higher air speeds that can develop dynamic
pressures well above 1000 Pa [3].
Testing at higher flow rates and pressures than
standard conditions was carried out to determine how
much fabric permeability changed at nonstandard
conditions. Differential pressures ranged from about
4x104 Pa for the less permeable fabrics, to about
2.5x103 Pa for the most permeable fabric. Thus the
differential pressures were all at least 10 times higher
than the standard conditions of 150 Pa.
We were particular interested in the response of
elastomeric fabrics that deform, stretch, and expand
under high pressures (two examples shown in Figure
9). Elastomeric fabrics could act as an adaptive
structure to relieve excess pressure and stress during
the parachute deployment and inflation phase, when
opening shock produces high stress in parachute
seams, attachments points, and the fabric itself.
Elastomeric fabrics will recover their original shape
and air flow properties once excess pressure/stress
decreases during the parachute descent phase.
FIGURE 9. Typical expansion of two elastomeric fabrics under
high air flow conditions. Air jet flow (jet diameter smaller than
sample area) was used to illustrate fabric elasticity for the
photograph. Actual measurements were not performed with jet
flow conditions.
Journal of Engineered Fibers and Fabrics 34 http://www.jeffjournal.org
SPECIAL ISSUE - July 2012 – FIBERS
The response of the four standard parachute fabrics
and the two elastomeric porous fabrics to
nonstandard air flow rates and pressures are shown in
Figures 10-13. The volumetric flow rate through the
fabric as a function of differential pressure drop is
plotted for each fabric. The three parachute fabrics
with comparable air permeability values are shown in
Figure 10, while the least permeable parachute fabric
is shown by itself in Figure 11. The two elastomeric
fabrics are shown in Figures 12 and 13.
The shape of each curve is the most important feature
of these plots. The elastomeric materials in Figures
12 and 13 shows a “concave-up” shape, where the
fabric appears to become more permeable at higher
differential pressures, which is presumably related to
the fabric stretching in response to the higher stress.
The standard nylon parachute fabrics (Figures 10 and
11) do not show any evidence of expansion at higher
pressures.
0
0.001
0.002
0.003
01x10
3
2x10
3
3x10
3
4x10
3
PIA-C-7350, Type I
PIA-C-7020, Type I
PIA-C-7020, Type III
P (Pa)
Volumetric Flow Rate (m
3
/s)
FIGURE 10. Flow rate as a function of pressure drop for three
parachute fabrics with high air permeability.
0
0.0005
0.0010
0.0015
0.0020
01x10
4
2x10
4
3x10
4
4x10
4
5x10
4
PIA-C-44378, Type IV
P (Pa)
Volumetric Flow Rate (m
3
/s)
FIGURE 11. Flow rate as a function of pressure drop for a
parachute fabric with low air permeability.
0
0.001
0.002
0.003
01x10
3
2x10
3
3x10
3
4x10
3
Elastomeric Electrospun Polyurethane
P (Pa)
Volumetric Flow Rate (m
3
/s)
FIGURE 12. Flow rate as a function of pressure drop for an
elastomeric electrospun nonwoven fabric with low air
permeability.
0
0.001
0.002
0.003
0.8x10
3
1.2x10
3
1.6x10
3
2.0x10
3
2.4x10
3
2.8x10
3
Lycra / Spandex
P (Pa)
Volumetric Flow Rate (m
3
/s)
FIGURE 13. Flow rate as a function of pressure drop for an
elastomeric knit fabric with high air permeability.
The elastomeric fabrics deform to increase the flow
rate in response to the applied load. The fabric
structure deforms elastically as the pressure
increases. This causes both an increase in total flow
area, and an increase in the flow area of each
individual pore in the fabric.
The electrospun nanofiber nonwoven fabric shown in
Figure 12 showed the largest change in increased
flow as a function of pressure. Figure 14 shows how
the relative fiber size and inter-fiber spaces change as
a function of statically-applied biaxial strain. In this
case, the fabric was strained in two directions to the
indicated strain levels (50% and 100% strain), and
the fabric was imaged with and electron microscope.
The high strains associated with fabric deformation
reduced fiber diameter, and also greatly increased the
total pore area available for air flow through the
fabric.
Journal of Engineered Fibers and Fabrics 35 http://www.jeffjournal.org
SPECIAL ISSUE - July 2012 – FIBERS
FIGURE 14. Scanning electron microscope images of elastomeric
electrospun polyurethane nonwoven fabric under biaxial strain.
Strain directions are vertical and horizontal relative to each
microscope image.
The magnitude of change in air flow resistance
property as a function of pressure differential is
shown in Figures 15-18. The calculated air flow
resistances for the four standard parachute fabrics do
show some changes in resistance at the higher
pressures. This change in apparent resistance is
usually ascribed to the increased inertial losses
present in high air flow through a fabric layer, which
causes deviation from the linear laminar flow regime
[9].
However, the corresponding results for the two
elastomeric fabrics show very different results. The
elastomeric fabrics show a large and significant
decrease in flow resistance at higher pressures
(Figure 17 and Figure 18).
0
1x10
7
2x10
7
3x10
7
4x10
7
5x10
7
01x10
3
2x10
3
3x10
3
4x10
3
PIA-C-7350
Type I
PIA-C-7020, Type I
PIA-C-7020, Type III
P (Pa)
Air Flow Resistance (1/m)
FIGURE 15. Air flow resistance as a function of pressure drop for
three parachute fabrics with high air permeability.
0
0.3x10
9
0.6x10
9
0.9x10
9
1.2x10
9
01x10
4
2x10
4
3x10
4
4x10
4
5x10
4
PIA-C-44378, Type IV
P (Pa)
Air Flow Resistance (1/m)
FIGURE 16. Air flow resistance as a function of pressure drop for
a parachute fabric with low air permeability.
0
0.5x10
8
1.0x10
8
1.5x10
8
2.0x10
8
01x10
3
2x10
3
3x10
3
4x10
3
Elastomeric Electrospun Polyurethane
P (Pa)
Air Flow Resistance (1/m)
FIGURE 17. Air flow resistance as a function of pressure drop for
an elastomeric electrospun nonwoven fabric with low air
permeability.
Strain = 0%
Strain = 100%
Strain = 50%
Journal of Engineered Fibers and Fabrics 36 http://www.jeffjournal.org
SPECIAL ISSUE - July 2012 – FIBERS
0
0.2x10
8
0.4x10
8
0.6x10
8
0.8x10
8
1.0x10
8
0.8x10
3
1.2x10
3
1.6x10
3
2.0x10
3
2.4x10
3
2.8x10
3
Lycra / Spandex
P (Pa)
Air Flow Resistance (1/m)
FIGURE 18. Air flow resistance as a function of pressure drop for
an elastomeric knit fabric with high air permeability.
The decrease in air flow resistance shown for the two
elastomeric fabrics indicates that both materials
change their air flow properties as a function of
applied air pressure load. These materials can be
thought of as adaptive structures that change
configuration when required, and return to their
original properties when the applied stress is no
longer present.
CONCLUSIONS
The air flow resistance of four standard parachute
fabrics was minimally affected by humidity, and
strain. Measured air flow resistance was most
affected by nonstandard air flow rates and pressures.
Elastomeric fabrics can act as an adaptive structure to
relieve excess pressure during parachute opening
shock (parachute deployment/inflation phase).
Elastomeric fabrics deform in response to applied
loads, and allow more air flow to take place through
increased pore area.
Elastomeric fabrics recover original shape and air
flow properties once excess pressure/stress decreases
(parachute descent phase)
Future work in this area will address impulse and
shock loading of fabric structures. The effect of high
loading rates and shock loading will be different than
the quasi-steady-state conditions used for the
humidity, strain, and flow rate conditions covered in
this paper.
REFERENCES
[1] Payne, P.R., “The Theory of Fabric Porosity as
Applied to Parachutes in Incompressible
Flow,” Aeronautical Quarterly 29 (Aug),
1978, pp. 175-206.
[2] “Standard Test Method for Air Permeability of
Textile Fabrics,” ASTM International, D 737-
04.
[3] Knacke, T.W., “Parachute Recovery Systems
Design Manual,” Naval Weapons Center
Technical Publication 6575, Para Publishing,
Santa Barbara, 1992, pp. 5-71.
[4] Gibson, P., Schreuder-Gibson, H., “Patterned
Electrospun Fiber Structures,” International
Nonwovens Journal 13 (2), 2004, pp. 34-41.
[5] Gibson, P., Rivin, D., Kendrick, C., Schreuder-
Gibson, H., “Humidity-Dependent Air
Permeability of Textile Materials,” Textile
Research Journal 69 (5), 1999, pp. 311-317.
[6] Dullien, F., Porous Media – Fluid Transport
and Pore Structure, Academic Press, New
York, 1979, p. 157.
[7] Muzychka, Y., Yovanovich, M., “Pressure
Drop in Laminar Developing Flow in
Noncircular Ducts: A Scaling and Modeling
Approach,” Journal of Fluids Engineering 131
(11), 2009, pp. 1-11.
[8] Melzig, H., Schmidt, P., Pressure Distribution
During Parachute Inflation Phase I, Infinite
Mass Opening Case, USAF Report AFFDL-
TR-66-110, Defense Technical Information
Center, AD0482534, 1966.
http://handle.dtic.mil/100.2/AD048254
[9] Goodings, A., “Air Flow through Textile
Fabrics,” Textile Research Journal 34 (8),
1964, pp. 713-724.
AUTHORS’ ADDRESSES
Phillip W. Gibson
Kenneth Desabrais, Ph.D.
Thomas Godfrey, Ph.D.
U.S. Army Natick Soldier Research, Development,
and Engineering Center
AMSRD-NSC-SS-MS
Bldg. 3 (Research), Room 321
Natick, MA 01760-5020
UNITED STATES
... Air permeability is usually defined from Darcy's law as reported by Gibson [17]. However, for multi-ply face masks, thickness measurement can be problematic and an alternative way to present pressure drop results is expressed through the apparent airflow resistance, b, expressed as following: ...
Article
Disposable face masks are among the personal protective equipment (PPE) that highly contribute to protecting people in the context of the current COVID-19 pandemic. Health authorities recommend wearing a mask as a barrier measure to limit the spread of viral respiratory diseases. During the first waves of the pandemic, besides professional high-quality PPE, decontaminated disposable mask reuse and homemade cloth masks were allowed due to scarcities. This work introduces a simple method based on-time history of the differential pressure, and an easy to use the setup for the testing of different kinds of respiratory protective masks for the purposes of quality control and evaluation of air permeability performance. The standard mask testing method and the new proposed approach were then used to evaluate the effect of machine washing on the widely used type of disposable masks; namely the surgical (medical) face masks. The objective is to determine the number of acceptable washing cycles that this kind of mask can withstand before losing its performance in terms of breathability and airflow resistance. Other quality characteristics such as material (fibres) degradation and hydrophobicity are investigated. Degradation mechanisms due to washing cycles for the different mask constituent layers were studied by scanning electron microscopy (SEM) imaging. This work is an attempt to contribute to the determination of the reusability threshold of general-purpose disposable surgical type face masks thereby contributing to the reduction of environmental concerns. Results in terms of the studied above parameters suggest limiting the reuse of standard type surgical masks to only one machine washing cycle.
... To verify the accuracy of the new analytical model for predicting air permeability of parachute fabrics, in this study, the air permeabilities of four typical fabrics were obtained. The structural parameters and the standard fabric permeability for 125 Pa are shown in Table 1. Figure 2 shows the comparison of the calculation results of the new analytical model presented in this article and the twopoint reverse estimation method with the experimental data (Rondeau et al., 2015;Gibson et al., 2012). It can be seen from the figure that, when predicting the air permeability of parachute fabrics, the two-point reverse estimation method has a small error near two reference points. ...
Article
Full-text available
To explore the effect of structure parameters on the air permeability of parachute fabric, the parachute fabric was treated as a porous medium. A nonlinear model of the viscosity and the inertia coefficient of the parachute fabric was established based on the Ergun formula, and an an alytical model of the fabric permeability with the fabric structure parameters was established considering the air permeability of fabric with the standard differential pressure. The results, which were compared with the two-point reverse estimation method showed that the new model is more accurate and has a wider applicable dynamic pressure range. Based on this, a sensitivity analysis of the effect of the fabric structure parameters on the air permeability of parachute fabric was conducted. The results showed that (1) the air permeability of fabric decreases with the increasing fabric planar weight and increases with the increasing fiber density and fabric thickness, and (2) the fabric planar weight has a greater influence on the air permeability of the fabric when the fabric structure parameter is less than the standard structure parameter. The thickness of the fabric has a significant effect on the air permeability of fabrics when the fabric structure parameter is greater than the standard structure parameter.
... The parachute material PIA-C-7020 Type I used in the experiment [21] was investigated for calculation to validate the correctness of the air permeability model of canopy fabric namely formula (23). The main parameters are: d ¼ 7:62 Â 10 À5 m, e ¼ 0:165. ...
Article
The porosity is one of the important properties of canopy fabric, which affects the stability, aerodynamic performance and inflation performance of parachutes. The similarity criteria for canopy porosity were presented on the principle of similarity analysis. The Ergun theory of porous medium was introduced to establish the model of air permeability and differential pressure between canopies fully considering the air flow property during decelerating process of parachutes. And the differential pressure equation of micro-pore jets of fabric was established based on Bernoulli's theorem. Then, a novel model of air permeability for parachute canopy under different flying environment was proposed. The air permeability calculated by the model is in good agreement with the experiment. According to different environmental conditions, the impact analysis of air permeability and effective permeability of parachute canopies was studied. The results show that geometric and dynamic similarity are sufficient conditions for porous similarity of canopy. The air permeability and effective permeability of canopy are positively correlated with flight velocity and air density, negatively correlated with aerodynamic viscosity and less affected. There exists blockage effect in micro-pore jets of fabric.
... Darcy's air permeability tests conducted with nCel/PVP fibrous membranes (120-250 μm thickness) has shown that the permeability coefficient of "ultra" nCel/PVP membranes reduces slightly when pressure rises, but it increases for other two types of nCel/PVP materials (Figure 8). These changes are associated with the sample deformation that includes simultaneous compression and stretching of the fibrous mesh [50,51]. At low pressure drop, the stretching dominates in pure PVP, "natural" and "thick" nCel/PVP fibrous membranes, which leads to increasing pore size and air transport. ...
Article
Full-text available
Nanocellulose/polyvinylpyrrolidone (nCel/PVP) fibrous composite materials containing rod-like nanocrystalline cellulose particles with the lengths varying in the range from 100 to 2000 nm were prepared by using DC electrospinning. The particle size had a strong effect on the precursor viscosity, process efficiency, and resulting fiber diameter. The thermal crosslinking of nCel/PVP composite nanofibers with up to 1.0 : 8.0 nCel/PVP weight ratio resulted in fibrous membranes with textural, air transport, and mass swelling properties varying significantly with the size of cellulose particles. The presence of nCel particles increased the oxidation resistance of PVP during the crosslinking and affected the morphological changes of nCel/PVP fibrous membranes in aqueous solutions. Particles with the smallest size improved the strength of the membrane but decreased its mass swelling capacity, whereas the larger particles led to a more porous and flexible, but mechanically weaker, membrane structure with a higher swelling ability. Thus, by using the nCel particles of different size and shape, the properties of nCel/PVP composite fibrous membranes can be tailored to a specific application.
... Fabric porosity depends generally on the fabric and yarns constructions. Permeability measurements for porous fabrics are influenced by strain, humidity, airflow rate, and fabric elasticity [11] . Several researchers studied the relation between permeability and porosity of fabric and predicted fabric air permeability practically and theoretically [12][13][14][15][16] . ...
Article
Full-text available
Due to the inherent hydrophobic nature of Polyester fibers, fabrics formed entirely from Polyester fibers exhibit relatively poor moisture absorption and release properties introduce an uncomfortable condition to the wearer. The present study focuses on adding enhanced and durable hydrophobicity properties to Polyester fabrics, through coated using a cellulose solvent of NaOH/urea/thiourea aqueous solution at 8/8/6.5 compositions. Aqueous NaOH/urea/thiourea solution as non-derivatizing solvent broke the intra- and inter-molecular hydrogen bonding of cellulose and prevented the approach toward each other of the cellulose molecules, leading to the salvation of cellulose which the hydrophilic properties imparted to the fabric. The effect of percentage of cellulose solution coating has been studied on some physical properties such as fabric wettability, water wicking, stiffness, and air permeability, as well as the method of application of the cellulose coating. The surface characterization of the coated Polyester fabric is performed using FTIR and SEM. The results relate to coated Polyester fabric demonstrated enhanced and durable hydrophobicity and rewetting properties due to hydrophilic properties imparted to these substrates
... An increase in permeability above ΔP¼10 kPa for the multilayer fibrous membrane calcined at 500°C has been assigned to the membrane's partial stretching and increase of the distance between the fibers due to assumingly higher elasticity of this material. The shape of this membrane after the failure (photo in Fig. 6b) is consistent with the deformation of rather elastic fabric membranes [52]. The 500°C calcined fibrous alumina membranes possessed also the lowest permeability and largest air flow resistance among the other tested structures. ...
Article
Nanofibrousalumina (Al2O3)structures were fabricated from the precursor aluminum nitrate/polyvinylpyrrolidone (PVP) nanofibers prepared using a free-surface alternating current (AC) electrospinning method. Precursor nanofibers were generated at rates up to 6.4 g/hand collected as 100–300 μm thick sheets suitable for direct conversion into the nanofibrous alumina structures. The effects of process conditions and annealing temperature on the nanofiber diameter, morphology, shrinking behavior and crystalline phase formation were investigated by Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray diffraction (XRD). Textural properties of Al2O3 fibrous sheets composed of micro-/meso-porous nanocrystalline γ-alumina nanofibers with 260±90 nm diameters after the calcination at temperatures in the range from 700 °C to 1000 °C were determined from N2 adsorption/desorption isotherms. Preliminary air permeability and apparent air flow resistance studies of single sheet and multilayer nanofibrous alumina membranes were performed and compared with other porous alumina membrane structures for the evaluation of their possible usage in gas filtration, separation, and other applications.
Article
We propose a methodology for the multi-objective characterization of a light, single layer orthotropic fabric inflatable space structure. The presented approach focuses on a custom in-situ experimental quasi-static deflation, semi-empirical development of flow coefficients and structural integrity loss parameters as well as process validation with finite element analysis. The experiments are designed to investigate a collapse of a thin, deployable, one segmented inflatable structure caused by a simulated single leak and passive porous leakage through the fabric material. The acquisition of real time data with a custom designed standalone ambient conditions sensing platform and the wireless internal sensing unit furnishes pressures, temperatures, mass flow rates, dynamic strains and accelerations. Synchronously, geometric changes in the shape of the structure's representative area are recorded utilizing photogrammetric approach. The non-wireless and wireless outputs are treated with a sequence of smoothing, best-fitting and refining operations and co1mbined with the corresponding sequence of active surface area photogrammetric results. The formulation of inflatable characterization parameters and a single leak/passive porous leakage structural integrity loss ratios allows for an in depth description of the inflatable structure behavior. A closer examination of the experimental data shows that the majority of the structural integrity loss ∼97% is attributed to the sudden and relatively short leak due to puncture and the remaining ∼3% to the slow and prolonged pressure loss caused by the passive porous areal leakage through the membrane of the structure. A semi-empirical framework based on a thermodynamic equilibrium produces a series of single leak, passive porous leakage and active area coefficients to be validated with the Ls-DYNA® finite element simulation of a polyurethane coated nylon fabric inflatable structure model. We use implicit and explicit finite element analysis schemes with a control volume deflation to validate the single leak, passive porous leakage and active area coefficient curves and constants. The subsequent comparison with the experimental internal temperature, internal pressure and mass flow rates results in a 1.83%, 0.61% and 1.19% difference respectively for the single leak events. The successive contrasting with the experimental internal temperature, internal pressure and active area produces 2.04%, 1.26% and 1.68% difference for the passive porous leakage respectively. We demonstrated that the multi-objective, structural integrity loss characterization techniques for the space structures have promising capabilities for the application in inflatable structures of diverse shapes and sizes.
Article
To date, in all the Mars landing missions, a supersonic parachute has been used to decelerate the capsule's entry into Mars' thin atmosphere from supersonic to subsonic speeds, because of its low mass and high aerodynamic drag. However, when the flexible porous parachute is placed in a supersonic flow, complex interdependent phenomena are observed around the two-body configuration system. These include aerodynamic interactions between the turbulent wake with shocks and coupled interactions between the compressible flows with the flexible porous structures undergoing large deformations. The flow instability around the parachute originates from these aerodynamic interactions between the canopy shock and the capsule wake, which depend on the Mach number, Reynolds number, proximity and size ratio of the canopy to the capsule, angles of attack of the canopy and capsule, and the material properties of the canopy. The effects of these parameters on the performances of the supersonic parachutes have been studied experimentally since the late 1950s. Computational fluid dynamics (CFD) and fluid-structure interaction (FSI) approaches have been used to understand the flow physics and the driving mechanisms of the aerodynamic interactions around the parachute system. However, the underlying mechanisms of these fully coupled interactions are not clearly understood. This review presents the recent developments in the study of supersonic parachutes based on the use of CFD and FSI simulations to predict their aerodynamic performances, summarizes the progress in experimental and numerical research aiming to investigate the unsteady aerodynamics in such a FSI problem, and concludes with discussions on the future challenges in the design and application of supersonic parachutes for Mars exploration missions.
Article
Highly porous, 0.1–1.5mm thick fibrous amorphous silica (FAS) meshes with 500 ± 200 nm fiber diameters were prepared from the precursor fibers made by a high-yield free-surface alternating field electrospinning (AFES) process. A combination of mild thermal and pressure treatment of the fibrous precursor material before the calcination step was used to tailor the mechanical and transport properties of the resulting FAS structures. Compression of the as-spun material determined the resulting porosity, effective fiber diameter, and microarchitecture of the FAS structures when calcined between 600 and 1000 °C. Flexible FAS meshes and membranes revealed the tensile strength and modulus up to 1.4 MPa and 580 MPa, respectively, and Darcy's permeability coefficient in a range of 1.2×10^−12–1.6×10^−11 m^2. Taking into consideration the compression-dependent effective fiber diameter, the permeability of compressed FAS membranes fitted the models developed for two-dimensional fibrous layer architectures with partial fiber alignment within the stacked layers.
Thesis
Le travail présenté dans ce mémoire concerne le développement d’un système universel de manipulation de matériaux textiles souples. Il s’agit d’une pince de manipulation universelle qui se compose de trois techniques de manipulation, technique de vide, technique d’intrusion, technique de pincement. Cette pince universelle a été développée pour manipuler une surface textile de 100 x 100 mm². Les buts de cette pince sont les suivants: Acquérir une seule couche à partir d'un empilement de tissus.Tenir une seule couche, la transférer et la manipuler jusqu’au poste suivant.La technique de vide est la première technique développée dans notre recherche, elle se compose des organes de préhension qui sont « trois ventouses pneumatiques » dont les matériaux varient en fonction des matériaux textiles à manipuler, trois compensateurs de hauteur pour fixer les ventouses pneumatique et d'un générateur de vide pour créer le vide nécessaire grâce à un régulateur de pression. Les trois ventouses pneumatiques sont placées précisément sur les têtes d’un triangle équilatéral, au-dessus de la pièce textile. La technique d’intrusion est la deuxième technique développée dans notre recherche, cette technique est constituée de deux parties principales: Une partie qui donne le mouvement et l'actionnement des organes de préhension.Une partie de préhension qui contient des éléments de préhension qui sont des aiguilles.L’ensemble est commandé, au travers de vérins, par de l’air comprimée. La technique de pincement est la troisième technique développée dans notre recherche, elle comprend des organes de serrage opposés qui sont à mis en mouvement de façon alternative par deux vérins pneumatiques Deux types de validation des éléments constituant de la pince de préhension développée ont été réalisés avec succès, une validation statique en utilisant un support de fixation, une validation dynamique en utilisant un bras de robot. Pendant la validation statique, nous avons trouvé que la technique de vide fonctionnait très bien avec les matériaux imperméables à l’air et avec des matériaux ayant une porosité inférieure à 80% et/ou une perméabilité inférieure à 1500 L/m²/s sous 200 Pa.Pour les matériaux textiles ayant une porosité supérieure à 80% et/ou d’une perméabilité supérieure à 1500 L/m²/s sous 200 Pa, la consommation importante d’air comprimé interdit l’utilisation de cette technique et la force réelle d’attraction dépendant des propriétés du matériau manipulé suivant :La porosité, La perméabilité à l’air, La masse surfacique de matériau. Concernant la technique d’intrusion, nous trouvé que cette technique permet une manipulation efficace des matériaux textiles qui sont difficiles à manipuler par la technique de vide. Elle fonctionne très bien pour des matériaux perméables à l’air (tissus d’armure toile, tricots) alors qu’elle endommage les matériaux imperméables. Les risques liés à cette technique est le prélèvement de plusieurs couches à la fois si la profondeur de perçages des aiguilles n’est pas contrôlé précisément. Pendant la validation statique de la technique de pincement, nous avons trouvé que cette technique ne fonctionne pas bien seule.Pour résoudre ce problème, nous avons utilisé, la combinaison de deux technologies La technique d’intrusion technique La technique de pincement Et La technique de vide. La technique de pincement Les résultats trouvés pendant la validation de cette technique sont les suivants : la technologie de vide associée à la technique de serrage est la combinaison la plus efficace et la plus fiable, par contre un des inconvénients de cette technique est le contrôle de la force de serrage afin d’éviter l’endommagement de la surface de matériau manipulé. [...]
Article
Full-text available
Electrospun fibers have useful filtration properties for chemical protective clothing and filter masks. Techniques for the patterned deposition of these fibers have been developed based on varying the conductivity of the target substrate. We are investigating multilayer arrangements of patterned fibers deposited in single layers, and onto air-permeable substrates. Patterning through the depth and across the area of the deposited layers has an effect on membrane strength. These materials are possible add-on solutions to provide complete biological and chemical aerosol particle protection for air permeable garments. Enhanced filtration efficiency of woven and nonwoven fabrics will improve individual soldier protection without compromising air flow characteristics or comfort of air-permeable garments.
Article
Full-text available
Changes in fabric structure as hygroscopic fibers swell at high humidities can have a large influence on the measured air permeability of fabrics such as cotton, wool, silk, and nylon. The variation of air permeability as a function of relative humidity is of practical importance in ranking and evaluating candidate textiles for protective clothing applications. This paper describes a test method used to determine the relative humidity dependence of the air permeability of hygroscopic woven textile fabrics. The instru mentation also permits dynamic measurements during a step change in relative humid ity. Typical results are shown for woven fabrics, nonwoven battings, and novel elec trospun fiber mats.
Article
Many aspects of parachute behaviour are dominated by the permeability of the canopy cloth. Techniques employing kinematic principles to predict parachute opening behaviour, for example, must first be able to predict the air flow through the canopy material. This paper is concerned with the development of such a predictive capability. The flow of air through cloth of gauze has been studied by many workers; since the 1850’s, in connection with filters; and since the early twenties, in connection with parachutes. Workers in each of these disciplines seemed unaware of the work being done in the other. In the case of parachute cloth permeability we find the foundations being laid down by such eminent workers as Glauert in 1932 and Taylor in 1944. Yet most subsequent writers seem not to be aware of this, and since World War II, there have been many papers “rediscovering” basic principles; often with errors which could have been corrected by reference to the earlier authorities. In this paper we attempt to bring together all the work which has been done in this field, and to rationalize the data by simple mathematical modelling. Individual elements of the model have been proposed before, of course. We then examine the effect of tension on the geometric porosity of fabric, and hence the pressure loss Ap. A theoretical analysis shows that tension is likely to be a major factor. For cloths which have low porosity when unloaded our simple mathematical model shows that, for a given Ap, a stress equal to 50% of ultimate can increase the air volume flow Q by an order of magnitude. In general, the increase in permeability is greatest at the lower values of Ap, indicating that the viscous term is more influenced by tension than the dynamic term. This theoretical result is confirmed by some rather limited experiments carried out by Lashbrook and Marby. But their data also shows that a cloth made from relatively stiff glass fibers can experience a reduction in permeability when loaded, due to a “closing up” of the weave. The reasons for this are discussed.
Article
A detailed review and analysis of the hydrodynamic characteristics of laminar developing and fully developed flows in noncircular ducts is presented. New models are proposed, which simplify the prediction of the friction factor-Reynolds product f Re for developing and fully developed flows in most noncircular duct geometries found in heat exchanger applications. By means of scaling analysis it is shown that complete problem may be easily analyzed by combining the asymptotic results for the short and long ducts. Through the introduction of a new characteristic length scale, the square root of cross-sectional area, the effect of duct shape has been minimized. The new model has an accuracy of +/- 10% or better for most common duct shapes when nominal aspect ratios are used, and +/- 3% or better when effective aspect ratios are used. Both singly and doubly connected ducts are considered.
Article
Application of hydrodynamic theory to the passage of air through textile fabrics is discussed. Flow-pressure results for a group of fabrics covering a wide range of air permeabilities are recorded and the data analyzed. Laminar flow fiscosity effects as well as increased kinetic energy effects are shown to be operative, and in some instances the flow pattern is shown to exhibit a behavior attributable to the presence of pores of widely differing size in the same fabric. Loss of energy from barrier effects (shock forces) is considered, and the case of flow through multiple layers of fabric is examined. Some estimates of pore size are computed from particular experimental data. (Author)
Article
The purpose of this manual is to provide recovery system engineers in government and industry with tools to evaluate, analyze, select, and design parachute recovery systems. These systems range from simple, one-parachute assemblies to multiple-parachute systems, and may include equipment for impact attenuation, flotation, location, retrieval, and disposition. All system aspects are discussed, including the need for parachute recovery, the selection of the most suitable recovery system concept, concept analysis, parachute performance, force and stress analysis, material selection, parachute assembly and component design, and manufacturing. Experienced recovery system engineers will find this publication useful as a technical reference book; recent college graduates will find it useful as a textbook for learning about parachutes and parachute recovery systems; and technicians with extensive practical experience will find it useful as an engineering textbook that includes a chapter on parachute- related aerodynamics. In this manual, emphasis is placed on aiding government employees in evaluating and supervising the design and application of parachute systems. The parachute recovery system uses aerodynamic drag to decelerate people and equipment moving in air from a higher velocity to a lower velocity and to a safe landing. This lower velocity is known as rate of descent, landing velocity, or impact velocity, and is determined by the following requirements: (1) landing personnel uninjured and ready for action, (2) landing equipment and air vehicles undamaged and ready for use or refurbishment, and (3) impacting ordnance at a preselected angle and velocity.
Article
An experimental investigation and correlative analysis were conducted to determine the pressure distribution over the surface of parachute canopies during the period of inflation for the infinite mass case and to correlate pressure coefficients with inflating canopy shapes. Parachute canopy models of Circular Flat, 10% Extended Skirt, Ringslot, and Ribbon designs were tested under infinite mass conditions in a 9 x 10 ft low speed wind tunnel. External and internal pressure values were measured at various locations over the surface of the model canopies throughout the period of inflation, and generalized canopy profile shapes were obtained by means of photographic analysis. Pressure coefficients derived for the steady state (fully open canopy) are quite comparable to the results of previous measurements. Peak pressure values during the unsteady period of inflation were found to be up to 5 times as great as steady state values. The relationships between the pressure distribution and time for each of the canopy models deployed at free-stream velocities between 70 and 160 ft/sec. are presented.
Parachute Recovery Systems Design Manual
  • Knacket