ISO/TR 13086-4:2019
(Main)Gas cylinders — Guidance for design of composite cylinders — Part 4: Cyclic fatigue of fibres and liners
Gas cylinders — Guidance for design of composite cylinders — Part 4: Cyclic fatigue of fibres and liners
This document addresses the topic of cyclic fatigue of structural reinforcing fibres as used in composite cylinders, and cyclic fatigue of structural and non-structural liners in these cylinders. This document provides a basic level of understanding of these topics.
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TECHNICAL ISO/TR
REPORT 13086-4
First edition
2019-09
Gas cylinders — Guidance for design
of composite cylinders —
Part 4:
Cyclic fatigue of fibres and liners
Reference number
©
ISO 2019
© ISO 2019
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Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Background . 1
5 Cyclic fatigue evaluation . 2
6 Elements of cyclic fatigue . 2
6.1 Service conditions and requirements . 2
6.1.1 Temperature and moisture . 2
6.1.2 Pressure . 3
6.1.3 Pressure cycles . 3
6.2 Test conditions and specimens . 4
6.3 Fibre materials and their fatigue properties . 6
6.3.1 Materials . 6
6.3.2 Material properties and data . 6
6.3.3 Hybrid construction . 7
6.4 Liner materials and their fatigue properties . 8
6.4.1 Materials used . 8
6.4.2 Material properties and data . 8
6.4.3 Issues with localized strain differences . 8
6.5 Resin materials and their fatigue properties . 9
6.6 Composite/liner load sharing . 9
6.7 Autofrettage . 9
6.8 Analysis methods .10
6.9 Leak before burst (LBB) .11
6.10 Damage tolerance .11
6.11 Aging and environment .11
6.12 Counting cycles .11
6.13 Combining cycles.13
6.14 Qualification testing .14
7 Summary and conclusions .14
Annex A (informative) Equivalent pressure cycling .15
Bibliography .20
Foreword
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This document was prepared by Technical Committee ISO/TC 58, Gas cylinders, Subcommittee SC 3,
Cylinder design.
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iv © ISO 2019 – All rights reserved
TECHNICAL REPORT ISO/TR 13086-4:2019(E)
Gas cylinders — Guidance for design of composite
cylinders —
Part 4:
Cyclic fatigue of fibres and liners
1 Scope
This document addresses the topic of cyclic fatigue of structural reinforcing fibres as used in composite
cylinders, and cyclic fatigue of structural and non-structural liners in these cylinders. This document
provides a basic level of understanding of these topics.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
NOTE Terms and definitions related to gas cylinders can be found in ISO 10286.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http: //www .electropedia .org/
— ISO Online browsing platform: available at https: //www .iso .org/obp
4 Background
Composite cylinders began service in the 1950s, initially as rocket motor cases with glass fibre
reinforcement. This soon led to glass fibre pressure vessels with rubber liners, and then to glass
fibre pressure vessels with metal liners. Metal liners were typically either aluminium alloy or steel.
Eventually, new structural fibres, such as aramid and carbon, came into use for reinforcing pressure
vessels. Today, typical reinforcements for composite gas cylinders are glass and carbon, either
individually or together as a hybrid. Typical liner materials are steel, aluminium alloy or polymers, for
example, high-density polyethylene (HDPE) or polyamide (PA); other materials may be acceptable.
Each of these materials is subject to cyclic fatigue based on the type of service and the construction of
the cylinder. Cylinders used in transport service generally see full range cycles, with a limited number
of cycles per year. Cylinders used as fuel containers would typically see up to three pressure cycles
per day for fleet vehicles, and less for private vehicles. Cylinders used in stationary applications such
as refuelling cascades could see a very large number of partial cycles in a year. Some cylinders could
see a combination of these conditions. Stationary cylinders used for fuel cells or emergency breathing
applications could see a very limited number of cycles. Design working pressures for high pressure
cylinders are typically in the range of 20 bar to 1 100 bar. Cylinders for liquified gases such as propane
may operate at pressures up to 20 bar, and normally see fewer pressure cycles.
The different reinforcing fibres have different fatigue lives for a given stress or strain range. Liner
materials will also have different fatigue lives for a given stress or strain range. The load-sharing
characteristics of a liner material with a given reinforcement will affect their fatigue lives. An
autofrettage cycle is used with metal lined cylinders to improve fatigue life. The low modulus of
elasticity of polymer liner materials often results in the liner being in compression when the cylinder is
pressurized, so their fatigue life could be very high. Welds in a liner, whether it is metal or polymer, can
affect the fatigue life due to the different mechanical properties in a weld and in heat affected zones.
Surface quality and conditions such as roughness will affect cyclic fatigue, particularly crack initiation.
Autofrettage generally blunts cracks, and adds surface compression, which will improve fatigue life.
Evaluation and understanding of cyclic fatigue will lead to improved designs and reduce the risk of
cyclic fatigue failures without the need to overdesign the cylinders or conduct extensive qualification
testing on each new design.
5 Cyclic fatigue evaluation
Cyclic fatigue of composite cylinders can be addressed with an understanding of:
— service conditions and requirements;
— test conditions and specimens;
— fibre materials and their fatigue properties;
— liner materials and their fatigue properties;
— resin materials and their fatigue properties;
— composite/liner load sharing;
— autofrettage;
— analysis methods;
— leak before burst (LBB);
— damage tolerance;
— aging and environment;
— counting and combining different cycles;
— qualification testing.
6 Elements of cyclic fatigue
6.1 Service conditions and requirements
6.1.1 Temperature and moisture
Service conditions depend largely on location and usage of the cylinder. If the cylinders are located
and used outdoors, they must be able to withstand ambient conditions. Common conditions include
temperature ranges from −40 °C to +85 °C (−40 °F to +185 °F), which include higher temperature
exposure due to solar input and storage in confined spaces. This may include use in a vehicle or shipment
in a rail car where direct sunlight will raise temperatures within the storage compartment. Surface
absorptivity and emissivity of the cylinder can affect solar input to the cylinder and its equilibrium
temperature. It is less common to require operation in temperatures to -55 °C (−67 °F), and in some
cases to even lower temperatures.
Moisture levels in outdoor locations would range from very high to very low depending on ambient
conditions. Some cylinders are actually located in a water bath. Moisture itself generally does not affect
fatigue of structural materials used in cylinders, but can cause corrosion, which could affect fatigue
life. Moisture can also be absorbed into polymer liners, and resulting property changes should be
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understood by the cylinder designer. Moisture can also bring in chemicals that could affect material
strength and fatigue properties, particularly those glass fibres that are not corrosion resistant.
Some cylinders are maintained in a controlled environment, such that temperature and moisture are
monitored and controlled. However, conditions must be guaranteed if a controlled environment is
required to meet fatigue requirements. Otherwise, it is best to assume the cylinders will be exposed to
worst-case conditions.
Temperature and moisture changes from a reference point can cause dimensional changes in the
cylinder components, which would likely result in stresses within the cylinder. These stresses can
result from either transient or steady-state conditions of temperature and moisture, as shown by
[2]
Newhouse .
6.1.2 Pressure
Working pressures typically range from 5 bar to 20 bar for liquified gas applications, and up to
1 100 bar for compressed gas applications, with allowance for pressure increases due to temperature
increases. The maximum allowable working pressure in stationary applications, more commonly
known as the design pressure or maximum service pressure for this application, is the maximum
pressure the cylinder can be exposed to. The pressure may be at the design limit regardless of the
service temperature.
In transportable and vehicle fuel container applications, the working pressure is the settled pressure at
15 °C, and can increase up to about 130 % of the rated working pressure during extreme temperature
conditions. Operating pressures will be below the rated working pressure when ambient temperature
drops below 15 °C. Note that in North America, the reference temperature is usually 21,1 °C (70 °F).
Cylinders in various applications can also be subject to test pressures that are generally 150 % of
the rated working pressure, but can range from 125 % to 167 % of the rated working pressure, with
generally not more than 50 such cycles over a lifetime. Although some cylinder standards or regulations
allow pressurizing to test pressure during fill, cylinders should not be filled with more gas than would
settle to working pressure at 15 °C.
6.1.3 Pressure cycles
Some applications require only a limited number of cycles in a lifetime, so fatigue evaluation is not a
significant concern. Such applications include emergency breathing cylinders, and fuel containers for
fuel cells providing power when primary power is out of service. It can also include applications where
the cylinder is in limited use, and could only experience one or two pressure cycles in a month.
Transportable cylinders are generally designed for a specified lifetime, either limited or non-limited,
and qualified by conducting a specified number of cycles. A typical qualification test requirement is
12,000 cycles to test pressure, or in dedicated gas service 24,000 cycles to maximum developed
pressure, for a non-limited life. For a limited life, cycling 250 times to test pressure, or in dedicated gas
service 500 times to maximum developed pressure, per year of design life is a common requirement.
Specific standards could require more or less cycles. Transportable cylinders are generally not expected
to be filled more than once a day, and cycling to the test pressure provides a margin of safety.
Vehicle fuel tanks, containing either natural gas or hydrogen, could be filled two to three times a day in
fleet use, such as in buses or medium- and heavy-duty trucks. This is the basis for qualification testing
of 750 cycles to 1 000 cycles per year used in fuel container standards.
Stationary cylinders, generally referred to as pressure vessels, could be subject to a high number
of pressure cycles. One such application is use as a refuelling cascade for natural gas and hydrogen
powered vehicles. These cylinders could be in use continually as vehicles are brought in for refuelling,
resulting in a high number of cycles per day. In some cases, the cylinders could be refuelled from another
fuel reservoir, such as from a pipeline, as soon as the pressure begins to drop. These cylinders can see
a very high number of partial cycles. Some cylinders can see a high number of partial cycles, combined
with a given number of full cycles, in the course of a day.
6.2 Test conditions and specimens
Testing is generally conducted at ambient temperature. Care should be taken to avoid testing at
temperatures that would affect test results. Consideration should be given to actual conditions, and
how that can affect fatigue results.
Low temperatures can increase strength of the material being tested, but can also cause embrittlement
that would decrease the fatigue life. High temperatures can decrease the strength of material being
tested. Extreme temperatures will also affect load share between liner and overwrap materials due
to differences in thermal coefficient of expansion, and will also affect stress distribution if hybrid
construction is used for the composite overwrap. For example, as temperature decreases, an aluminium
alloy liner would tend to decrease interface pressure with the composite overwrap, causing the liner to
carry a larger percentage of the pressure load. Analysis would need to be conducted to evaluate the
effect of temperature on stresses and strains within an actual cylinder.
Testing with liquid vs. gas to pressurize a cylinder results in the same pressure on the inside of the
cylinder, and therefore the same stress in the cylinder. However, there can be temperature differences
resulting from the use of different fluids, depending on energy to compress the fluid. This could also
be a consideration for the service conditions, although filling and discharge are generally over a longer
time period in service compared with testing.
Fibre strength in the helical and hoop directions is the basic design criteria for design of the composite
overwrap for the cylinder. As cylinder design pressure increases, laminate thickness is increased in
order to maintain the stress and strain at the same level in the helical and hoop directions. Although
the peak fibre stresses generally remain the same, the radial compressive stress increases in the inner
part of the laminate. This change in stress conditions can have a significant effect on the fatigue life
of the composite and of a metal liner. Therefore, consideration should be given to test pressure versus
service pressure when evaluating fatigue life.
Options for test specimens to evaluate laminate strength and fatigue resistance include flat coupons,
tube sections, and cylinders. Each option has advantages and disadvantages. As the test specimen gets
closer to the actual product configuration, the results will be more valid, but more difficult to obtain.
Flat coupons can include unidirectional specimens and cross-plied laminates. These specimens could be
suitable for comparisons between fibres as to strength and fatigue properties, but would generally not
be suitable from which to predict cylinder performance directly. Loading would only be in the principle
direction, unlike the three-dimensional loading of a pressure cylinder. If loaded in tension, consider the
stress concentrations caused by the grips, and the geometry of the specimen, including edge effects. If
loaded in bending, consider that the specimen loading is further removed from the type of loading seen
in a cylinder. Nevertheless, the ability to quickly test comparative specimens can have some value.
A flat coupon would not be suitable for evaluating the interaction between a metal liner and a composite
overwrap.
[3] [4]
Tube specimens can include unidirectional specimens and cross-plied laminates. NOL or ASTM
rings are one option for unidirectional tubular specimens. Tube specimens can also be wound with
helical and/or hoop layers over a longer cylindrical mandrel. Cross-plied tube specimens could be
tested in axial tension using end grips that interface with tube ends that have additional reinforcement
to avoid grip failures. That is, the tube would have similarity to a flat tensile specimen with wider or
thicker ends (i.e. a “dog-bone” specimen).
Tubular specimens can also be tested using internal pressure. The resultant would be hoop stress if the
pressure source was contained within a double ended piston, so that axial load was contained within
the piston. Alternatively, the tube could experience both hoop load and axial tension if the tube ends
were closed such that the end closures would apply tension to the tube, such as when doing an axial
tension test, but using the internal pressure for loading.
A tubular specimen loaded in either axial tension or in hoop loading has advantages over a flat specimen
given that it is testing of a curved specimen, but the single direction loading has limitations. As with
a flat specimen, it is suitable for comparisons between fibres as to strength and fatigue properties,
4 © ISO 2019 – All rights reserved
and would give more representative results, but is still not as accurate as an actual specimen. A
tubular specimen loaded in both axial tension and hoop loading will have an even greater fidelity, with
consideration to the level the laminate reflects the construction of an actual cylinder.
Cylinder specimens give the best fidelity when assessing strength and fatigue life. However, the relative
cost can make them less attractive for a study involving many specimens. Subscale cylinder specimens
offer an option for good fidelity at a lower cost than full scale cylinders.
Figure 1 shows how fatigue results can vary with the choice of test specimen. The upper line, with data
[5]
from Mandell , reflects use of a unidirectional carbon fibre reinforced specimen loaded in tension.
[6]
The middle line, with data from Liber and Daniel , reflects use of a flattened tube with a symmetric
laminate having longitudinal fibre layers and ±45-degree layers loaded in tension. This construction
results in a more complex laminate, with more complex loading within the laminate. The data from this
specimen shows a reduction in fatigue life compared with the test specimen using unidirectional fibre.
The lower line reflects cyclic pressure testing of high pressure gas cylinders. These cylinders have a
more complex laminate and loading within the laminate than the test specimens of Mandell and of Liber
and Daniel. The lower line reflects a reduction in life compared with the other two specimens, but it does
contain some conservativism. The points plotted reflect test cycles conducted, but not necessarily with a
resulting test failure. It therefore represents a lower limit on fatigue life, rather than an average life.
Key
x
Mandell X Log cycles (10 )
Liber and Daniel Y Fraction of ultimate strength
pressure vessel
Figure 1 — Fatigue results using different configuration test specimens
The data presented in Figure 1 reflects what was stated above, that cyclic fatigue performance
depends on laminate construction, method of loading and other factors. Resin selection and laminate
construction can also affect results, as load transfer through the wall is dependent on radial laminate
properties. It is therefore necessary to demonstrate cyclic fatigue performance on a representative
gas cylinder to properly address fatigue performance in service due to pressure cycling. Note that the
lines from Mandell and Liber and Daniel reflect mean values, while the pressure vessel line reflects the
methodology of this report.
The data presented in Figure 1 also indicates that cyclic fatigue testing can be accelerated by increasing
the upper pressure limit. Sufficient cyclic fatigue data, over a range of stress levels, is needed to get
representative results.
6.3 Fibre materials and their fatigue properties
6.3.1 Materials
Common composite reinforcing materials include glass, aramid and carbon fibres, generally filament
wound with an epoxy or vinyl ester resin matrix. Other resin matrix materials could be suitable. Other
reinforcing fibres could be available, but none have developed as being viable alternatives to glass,
aramid and carbon fibre at this time.
Glass fibre was the first to be developed and was in use in the 1950s and 1960s. The most commonly
used grade for gas cylinders is ECR-glass. This is fundamentally an E-glass, but has enhanced corrosion
resistance resulting from removal of boron from the glass formulation. Other grades of glass fibre
are suitable, but are less widely used. Glass fibre is essentially a super-cooled liquid, and is subject to
creep flow and surface cracking. It has the least resistance to fatigue failure of the three commonly
used fibre types.
Aramid fibre (aromatic polyamide) was developed in the 1960s and came into use in gas cylinders in
the 1970s. It has greater strength, lower density and improved fatigue resistance compared with glass
fibre. It has a long-chain molecular structure, with very high strength in the longitudinal direction, but
relatively weak transverse properties.
Carbon fibre suitable for use in gas cylinders was developed in the 1960s and 1970s. It came into
widespread use in commercial gas cylinders in the 1990s. Carbon fibre is more of a crystalline
structure, and is generally processed from a PAN precursor. It has higher tensile strength and modulus
than glass and aramid fibre. Carbon fibre has the best fatigue resistance of the commonly used fibre
reinforcements, but is more sensitive to mechanical impacts.
6.3.2 Material properties and data
Table 1 provides typical properties for glass, aramid and carbon fibres. Actual fibres used could have
higher or lower values, particularly for strength and modulus, depending on the characteristics of
the fibre.
Table 1 — Typical fibre properties
Property ECR-Glass Aramid Carbon
a
Tensile strength, MPa (ksi) 1 500 (220) 2 500 (360) 4 500 (650)
b
Working strength, MPa (ksi) 430 (63) 830 (120) 2 000 (290)
Tensile modulus, GPa (msi) 72 (10,5) 131 (19) 220 (32)
Density, g/cc (pounds per cubic inch) 2,55 (0,092) 1,44 (0,052) 1,80 (0,065)
a
Nominal design fibre strength in the hoop direction of a pressure vessel at minimum burst
pressure.
b
Nominal design fibre strength in the hoop direction of a pressure vessel at service pressure.
NOTE ECR refers to corrosion resistant E-glass, from which boron has been removed as a constituent.
[5]
Figure 2 compares nominal cyclic fatigue for glass, aramid and carbon fibre.
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Key
carbon X Log (cycles to fail, N)
aramid Y Maximum stress/static strength
glass
Figure 2 — S/N data for carbon, aramid and glass reinforcement
6.3.3 Hybrid construction
Some gas cylinders are manufactured using hybrid construction. That is, using two or more different
reinforcing fibres in the gas cylinder. This could be a combination of a glass and carbon fibre, or it could
be a c
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