On Selection of Liner Material for Pressure Cylinders
from Composite Materials
N.G. Moroz, S.V. Lukyanets, I.K. Lebedev. (ARMOTECH)
It is revealed by practical operation of composite cylinders that most cylinder failures are
connected with formation of cracks in the liner material (independent of the material type).
The cracks appear when the cylinder is exposed to cyclic loading by the internal pressure, which
is the crucial factor of its performance and survivability. However, the nature of cracks initiation
and development is completely different in metal and polymer liners.
Liners from Polymer Materials
When analyzing the main properties of the most commonly used thermoplastics, one can
note that in terms of the requirements to processability, gas permeability, strength,
deformability, the most suitable materials for using as the material of the composite cylinder
liner are polyethylene and polyethylene terephthalate. They fall into the category of
crystallizable thermoplastics and are used in the above-mentioned designs of composite
cylinders.
Polyethylene is a light-weight elastic crystallizable material with the heat resistance of
certain grades reaching 110 °С. The melting point of certain grades is 120-135 °С, the glass
transition point is around 20 °С. The fracture strain is 300-500 %. It is characterized by good
impact resistance. The properties strongly depend on the material density. A higher density
results in improved strength, stiffness, hardness, and chemical resistance. A high creep rate is
observed in case of long-term loading. It has very high chemical resistance for practically all
solvents. The oxygen permeability coefficient is (6-9) * 10-6 cm3/(cm2 atm).
Polyethylene terephthalate (PET) is a saturated polyester of ethylene glycol and
terephthalic acid. It is a rigid-chain thermoplastic polymer. It preserves its properties when
refrigerated to -4 °К. The melting point of certain grades is 250 °С, the glass transition point is
around 78 °С. The fracture strain is 210-280 %. It is characterized by good impact resistance.
The chemical and physical structure of PET determines the possibility of close packing of
macromolecules, and, correspondingly, its ability to crystallize. It has high chemical resistance
for practically all acids. Sensitive to solvents and alkalis. The oxygen permeability coefficient is
(2-3) * 10-9 cm3/(cm2 atm).
In spite of fairly good properties and characteristics, the common disadvantage of
polymer materials is the destruction of polymers in contact with gases and liquids, connected
with the phenomenon of medium diffusion into the polymer.
It is known [1] that amorphous polymers are structurally heterogeneous bodies with the
size of heterogeneities of the order of tens of Angstroms. Heterogeneity of the structure of
amorphous polymers is not phase but fluctuating and nonequilibrium in nature. Typical structure
of a polymer (PET, polyethylene) is shown in Fig. 1, 2.
Fig. 1.
Fig. 2
Besides, it is also well known that deformation of a polymer in the yield strength domain
is significantly heterogeneous in terms of volume [2].
Fig. 3 shows a photomicrograph of a glassy PET sample conditioned under a constant
load equal to 0.6 of the yield strength. Besides shear bands, other areas of plastic deformation
called crazes are visible in the polymer (Fig. 4).
The physical pattern of craze formation in polymers is shown in Fig. 5.
The presence of a surface-active medium (such as water, air and others) in contact with
the polymer dramatically enhances the above-mentioned deformation phenomena resulting in its
destruction.
Fig. 3
Fig. 4
Stresses
Deformation
Fig.5
Fig. 5
The phenomenon essentially consists in the medium, which penetrates the material by
means of diffusion and molar transfer on the defects of the polymer structure, contributing to
increased mobility of the structural elements, acting as a lubricant. This facilitates bond breaking
in macromolecules of the polymer, loosening of its structure, its embrittlement and appearance of
local cracks.
The development of such defects is a process in time. In different media the time period it
is manifested in ranges between several hours and several days.
The described process is typically referred to as stress corrosion cracking of polymers. It
is one of the most common types of polymer fracture which determines their areas of application.
It is demonstrated in a large number of polymer studies [4] that if the level of internal
stresses in the polymer is around (0.2-0.35) of its yield strength and if the deformation is up to
0.2 %, the polymer’s diffusion characteristics remain practically the same. Raising the level of
internal stresses and deformations results in radical acceleration of the medium transfer into the
material owing to the microfractures that begin to be formed in the polymer under these stresses.
It follows from these remarks that in structures working under load, basically, the main
characteristic of polymers is the maximum allowable value of strains and tensile deformations in
the polymer which it is able to withstand during its design service life or its resistance to the slow
propagation of cracks under the design temperatures and the design initial stress in the material
expressed in hours.
Obviously, different types of media that are in contact with the polymer material in the
process of cylinder liner manufacturing or in the process of the cylinder operation, affect the
crack formation process in the polymer under load in a different way. Polymers are most
sensitive to solvents, alkalis and other active media.
Below, there is an illustration of crack formation in the cylinder liner made from
polyethylene (see Fig. 6) and polyethylene terephthalate (see Fig. 7) when exposed to alkalis.
Fig. 6
Fig. 7
Fig.8
In addition to the above, cyclic loading of polymers results in their fracture due to the socalled “decompression” effect. In this case, the medium under the increased pressure diffuses
into the polymer structure. After the load is released, the polymer is fractured by the pressure of
the medium diffused into the polymer (see Fig. 8-11).
Fig. 9
Fig. 10
Fig. 11
The temperature of the polymer at the moment of deformation also produces a significant
impact. Thus, for example, due to a high difference between the coefficients of thermal
expansion (200 * 10-6 for polyethylene, 10 * 10-6 for fiberglass), with the increase of temperature
the liner material experiences high compression stresses, which in certain cases may result in
loss of stability of the liner (see Fig. 12). Under low temperatures, the liner inside the shell
attempts to compress, but it is loaded by the applied pressure with increased stresses and
deformations, which may result in the appearance of stress corrosion cracking of the liner
material.
Fig. 12
Liners from Metal Alloys
Gas permeability of metal alloys is by 6 to 7 orders of magnitude lower than that of
polymers. (the oxygen permeability coefficient for steels is (1-2) * 10-16 cm3/(cm2 atm)) which
allows to use them in cylinder designs for gas storage at any pressure.
In case of using liners from metal alloys, under periodic loading of the cylinder, the liner
material is exposed to cyclic elasto-plastic deformations resulting in initiation and accumulation
of microdefects in the material structure and development of failures through the low-cycle
fatigue process.
A distinguishing characteristic of low-cycle fracture consists in a relatively weak
dependence of the number of cycles before fracture on the nominal stresses exceeding the yield
strength. Under these conditions low-cycle strength is evaluated with respect to deformation
criteria [6]. Typically, here the Coffin-Manson equation is used to determine the durability in the
range of 103...104 cycles.
where k = 0.4...0.7; с = 0,5 ln [1/(1– ψ)], ψ is the relative
contraction of the material,
is the number of load cycles,
deformation of the material.
is the span of plastic
It follows from this criterion that the number of cylinder load cycles before the crack
formation in the liner is the higher, the greater the material ductility characterized by the value ψ.
Dry clean air is an inactive medium which practically does not impact the fatigue strength
of metal alloys. This has served as the basis for differentiating between the corrosive and noncorrosive (in air) fatigue behaviors of alloys.
Due to the cyclic loading of the alloy in air, typically one deep and sharp crack appears in
the most weakened point. Its development mainly occurs under the impact of stress concentration
and the oxidizing action of air on the newly formed surfaces in the crack apex. In similar tests of
the alloy in electrolytes, the development of the fatigue crack occurs under the impact of the
electrochemical factor connected with the complex electrochemical pattern inside the crack.
Simultaneous appearance and development of a large number of fatigue cracks in the load area is
a distinguishing feature of corrosion fatigue.
Thus, besides the cyclic strength loading of the cylinder during its operation, cyclic
corrosion (cyclic corrosion cracking) in the liner is manifested, which in aggregate determine the
survivability of the cylinder. These types of loading are the ones that determine the choice of
metal liner material in the process of its design.
Judging from the above-mentioned remarks, we can assume that the most suitable
materials for this purpose are aluminum alloys, titanium alloys and stainless steels, being the
most corrosion-resistant materials.
However, mechanical characteristics of these materials reveal significant distinctions in
their behavior under non-stationary low-cycle loading.
Based on the assumption that under cyclic loading of the cylinder in the considered liner
materials the same span of plastic deformation
occurs, we can claim that in terms of
cyclic strength, austenitic stainless steels are the most efficient (ψ of steel = 66%, С of steel =
0.53 , ψ of titanium = 55%, С of titanium = 0.39, ψ of aluminum = 31 %, С of aluminum = 0.18).
In this case the steel efficiency in terms of low-cycle fatigue determined by the ratio of
(Nksteel|Nk1aluminum) = Сsteel /Сaluminum is 5.5 compared to the aluminum alloy and 1.95
compared to the titanium alloy. However, the performance and survivability of a metal-
composite cylinder are severely impacted by the technological features of liner manufacturing,
specific mechanical characteristics and corrosion resistance of its material.
Liners from aluminum alloys. The tendency of using strong highly ductile alloys for
these designs is justified, but, besides improvement of alloy strength and ductility, one more
criterion has to be considered, namely, increasing the weight efficiency, which results in
application of lighter materials. This criterion is met by aluminum alloys, as they are
characterized by a lower density (in particular, aluminum alloys containing lithium and
beryllium) than that of steels and titanium alloys.
At present, in spite of inferior characteristics of low-cycle fatigue, the most common
design is a liner from aluminum alloys manufactured as a solid-drawn shell similar to the process
of full metal cylinders production. In this case higher specific mechanical characteristics of
aluminum alloys and the production process play the defining role. This process allows to
manufacture the liner with a minimum quantity of technological defects which subsequently act
as concentrators of deformations in the liner during its operation in the cylinder.
Among aluminum alloys used as liner material, the alloy type 6061 has become the most
commonly used. Its chemical composition and mechanical characteristics are given in Tables
1,2, and its low-cycle fatigue parameters are shown in Fig. 12, from which it follows that when
the total deformation occurring in the liner material is up to the level of 0.35% (plastic
deformation is 0.15%), the low-cycle fatigue of this alloy is determined by the level of 1.5-2
×104 cycles.
Table 1
Al
Base
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
other
0.400.80
0.70
max
0.150.40
0.15
0.81.2
0.040.35
0.25
max
0.15
max
0.20 max
Table 2
Characteristic
Ultimate
Yield Strength,
Elongation
Strength, MPa
MPa
δ, %
ψ%
Modulus of
Elasticity,
GPa
Value
310
287
12
22
0.68
Total deformation
Plastic deformation
Elastic deformation
N cycle
Fig. 12
Corrosion resistance of aluminum and its alloys in concentrated solutions of nitric and
sulphuric acid is fairly high. In alkales the protective film on the aluminum is dissolved, and its
corrosion with hydrogen depolarization occurs. Aluminum-based alloys are not resistant in
contact with many metals and alloys. The contact with copper and its alloys, as well as with iron
and iron-based alloys, is particularly dangerous.
Stress concentrators significantly reduce fatigue strength and durability of Al alloys
under cyclic loading in air. In this case, local defects in the liner neck are the most dangerous.
Thus, for example, presence of such a defect may result in cylinder fracture in the form shown in
Fig. 13.
Fig. 13
Intensive plastic deformation of Al alloys results in heavy reduction of the size of Al, Zn
grains and of inter-metallide phase particles. As a result of the deformation, disintegration of the
supersaturated solid solution of Al occurs.
Resistance to corrosion cracking of aluminum alloys is determined by their composition
and by the distribution of inter-metallide compounds settled on grain boundaries in the process of
heat treatment.
Liners from stainless steels. Among the great variety of corrosion resistant steels and
alloys, austenitic chromium-nickel steels type Х18Н10 (304SS , 316 SS) and their modifications
have found the widest use.
Austenitic chromium-nickel stainless steels are characterized by a higher corrosion
resistance in gas and vapor-gas media under increased temperatures, as well as in certain
solutions of electrolytes formed with moisture condensation. Increased corrosion resistance of
stainless steels is based on formation of protective films on their surface, whose composition
includes chromium. This reduces their chemical corrosion in gas media and ensures steel
passivation in many electrolyte solutions.
The difference in corrosion behavior of various steel grades is connected with different
steel structures, with the presence of contaminants in the form of carbides and nitrites, and with
different physical-mechanical properties. With anode protection of stainless steels, one should
take into account their tendency to intergranular corrosion under certain conditions.
As far as conditions of operation are concerned, liner from thin sheet steel type
12Х18Н10 can be used as a component of a pressure cylinder without protection in the domain
of stray current potentials more negative than minus 0.1 V against a copper sulfate reference
electrode. Other steels of this type (12Х18Н10Т, 12Х18Н9Т, 12Х18Н9, 08Х18Н10Т) have
similar values.
Fig. 14,15 [9] show the low-cycle strength curves of steels type 18-10, from which it
follows that at the operating temperature of this steel equal to 300 С and at the level of total
deformation occurring in the material of 0.56 % (plastic deformation is 0.36 %), low-cycle
strength is determined by the level of 1.5-6.5×104 cycles. In particular, the cyclic life is 2.5 -6.5
×104 cycles in air, and 1.2- 2.5 ×104 cycles in sea water.
σ,
kg/mm
N, cycle
Fig.14
Amplitude of deformation 2
in air
in 3 % sodium chloride at
polarization of +0.2V (MSE)
Number of cycles before fracture, N cycles
Fig.15
Liners from Titanium Alloys
It is known [10] that titanium alloys have high corrosion resistance in the majority of
natural media (water and solutions of neutral salts). A corrosive medium has also an extremely
low impact on these alloys in low-cycle fatigue tests. However, corrosion susceptibility begins to
be manifested with the content of aluminum in alloys above 2%. Dry gaseous chlorine produces
strong corrosion of titanium, which creates a risk of ignition. If the chlorine contains even
insignificant amounts of moisture (around 0.005%), titanium corrosion in chlorine stops.
Titanium is stable in alkalis diluted to the concentration of 20% NaOH. In more concentrated
solutions, as well as under heating, it slowly reacts with formation of titanium acid salt—
Na2TiO3. Titanium has high corrosion resistance in many organic media, in which steels of the
12Х18Н9 type corrode. An exclusive property of titanium is its complete corrosive resistance in
sea water and marine atmosphere.
The level of cyclic strength of titanium alloys is higher than that of aluminum alloys and
is at the same level with high-strength steels.
Fig. 16 [11] shows the low-cycle strength curves of titanium alloy type Ti-6Al-4V from
which it follows that at the level of total deformation occurring in the material of up to 0.4 %
(plastic deformation is 0.2%), low-cycle strength is determined by the level of 1.5-2.5×104
cycles.
Fig. 16
Among titanium alloys used as liner material, the alloy type 6AL-4V has become the
most widely used. Its chemical composition and certain mechanical characteristics are given in
Table 3, 4.
Table 3
С
Fe
N2
O2
Al
< 0.08
< 0.25
< 0.05
< 0.2
5.5-6.76 3.5-4.5
V
H2
Ti
< 0.015
other
Table 4
Characteristic
Nominal
Ultimate
Yield
Elongation δ
Modulus of
Elongation
Strength MPa
Strength MPa
%
Elasticity GPa
ψ%
1000
910
18
114
20
value
Liners from titanium alloys and stainless steels are typically manufactured using welding
technology which results in loss of mechanical characteristics of the materials themselves and in
appearance of deformation concentrations in local areas of welds. Appearance and propagation
of low-cycle fracture cracks in welded joints essentially depends on the initial ductility and
strength of the welded joint metal and on the presence of flaws in welds. Similar to the base
material, amplitudes of elastic-plastic deformations occurring under loading with the design
spans of plastic deformations are used as the criteria of low-cycle fracture of welded joints.
Analysis of possible weld flaws demonstrates that the effective coefficient of internal
concentration of deformations, depending on the particular welded joint design (weld shapes,
presence of various types of backing rings or their absence, non-uniform thickness of joined
elements in the weld area and others) can reach the values of  = 1.3  3>25 for the metal’s
elastic domain, and around К = 1.12.7 for the plasticity and creep condition.
Results of a large number of studies demonstrate that the fatigue limit of welded joint
metal obtained using the most suitable welding method cannot be higher than 80% of the base
metal fatigue limit, and the reduction of fatigue strength occurs mainly due to internal structural
flaws in the weld. Results of studies on fatigue characteristics of butt welds demonstrate that
high-quality welds with complete penetration can exhibit acceptable fatigue properties. For lowcycle fatigue, when the number of cycles is up to 104, destructive amplitudes of deformation for
a welded joint are around 50-65% of destructive amplitudes of the base metal. This means that
the acceptable range of plastic deformation for high-ductility steels is -0.2-0.4%, and the range
for ductile aluminum alloys is 0.05-0.15%.
The condition of surface metal layers (cracks, scratch marks, dents, etc.) is essential for
metal liners, as according to the contemporary outlook it is in the surface layers, due to their
physical disparity with the base volume of metal, that the first plastic deformations occur which
result in fatigue cracks. The practice of fatigue tests has revealed that these alloys are the most
sensitive to surface roughness. Fatigue strength can greatly vary depending on the nature of their
surface treatment. It has been revealed that after abrasive grinding, especially in high-speed
mode, aluminum and titanium alloys exhibit the lowest values of fatigue strength. On the
contrary, when machined by turning with a cutting tool at low speeds of cutting and by light
cutting as finish treatment with subsequent manual polishing using a fine emery cloth, these
alloys exhibit the highest values of fatigue strength. The difference between the fatigue limits
determined for these two types of machining can be double or even threefold for the same alloys.
Welded joints of aluminum and its alloys of type АМg6 are mainly fractured by the
process of corrosion cracking. Maximum residual tensile stresses in these metals associated with
welding typically exceed the threshold level of stresses, and corrosion cracking of welded joints
occurs even in the absence of any significant working stresses. This allows to view the welding
residual stresses as one of the main factors of loading along with working residual stresses that
determine the strength of a welded structure.
Based on the assumption that only the method of welding liners from separate parts can
be applied for manufacturing ball-shaped cylinders, and taking into account the above-mentioned
remarks, one can draw the following conclusions.
1.
In light of the phenomena noted above which occur in polymer materials, using
these materials for liner designs in high pressure cylinders is highly undesirable This is due to
their high gas permeability, low level of allowable internal stresses in the polymer being in the
range of (0.2-0.35) of its yield strength, the polymer’s melting point and its corrosion cracking in
a variety of active media.
2.
From the perspective of low-cycle fatigue and of corrosion cracking resistance,
austenitic stainless steels are the most efficient among the metal alloys used in liners. With the
total deformation occurring in the liner material up to the level of 0.56%, the low-cycle fatigue of
this alloy is practically three times as high as the low-cycle fatigue of aluminum alloys and is
determined by the level of (3.5-6) ×104 cycles.
3.
High-quality welds with complete penetration, used in manufacturing liner from
austenitic stainless steel by the method of welding, can exhibit acceptable fatigue characteristics.
In terms of low-cycle fatigue, with the number of cycles of up to 1.5 х104, fracturing amplitudes
of deformation for the welded joint are around 50-65% of the base metal fracturing amplitudes,
which is 1.5-1.8 more efficient than solid-drawn aluminum alloys and by orders of magnitude
more efficient than welded joints of aluminum alloys.