Composite Pressure Vessel Burst Pressure Failure Verification
by Analysis and Test
by
Felicia Fiore
A Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez, Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
December, 2011
1
© Copyright 2011
by
Felicia Fiore
All Rights Reserved
ii
CONTENTS
LIST OF SYMBOLS ........................................................................................................ iv
LIST OF TABLES ............................................................................................................. v
LIST OF FIGURES .......................................................................................................... vi
1. Introduction/Background ............................................................................................. 1
1.1
1.2
Composite Structure & Material ........................................................................ 2
1.1.1
Importance of Fibers & Matrix .............................................................. 3
1.1.2
Composite Benefits ................................................................................ 3
1.1.3
Composite Disadvantages ...................................................................... 5
Composite Manufacturing Processes ................................................................. 6
2. Problem Description .................................................................................................... 8
2.1
Importance of Burst Margin Testing .................................................................. 8
2.2
Description of Composite Test Article .............................................................. 9
3. Methodology/Approach ............................................................................................. 11
3.1
Analysis ............................................................................................................ 11
3.2
Test ................................................................................................................... 15
3.2.1
Objective .............................................................................................. 15
3.2.2
Procedure.............................................................................................. 15
4. Results and Discussion .............................................................................................. 19
4.1
Analysis Results ............................................................................................... 19
4.2
Test Results ...................................................................................................... 21
5. Conclusion ................................................................................................................. 23
6. References.................................................................................................................. 24
iii
LIST OF SYMBOLS
Symbol
Engineering Unit
Description
Pact
psi
Actual Pressure, Aircraft
BF
n/a
Burst Factor
ρ
lbs/in3
Density
σ
psi
Tensile Strength
Ε
psi
Tensile Modulus
NU
n/a
Poisson ratio
UTS
psi
Ultimate Tensile Strength
MS
n/a
Margin of Safety
σy
psi
Yield Strength
σm
psi
Max Resultant Stress
Pin
psi
Temperature Corrected Internal Pressure Load
Tamb
°F
Ambient Temperature
TCF
n/a
Material Temperature Correction Factor
CACTCS
n/a
Cabin Air Conditioning Temperature Control System
iv
LIST OF TABLES
Table 1: Properties of General Engineering Materials ..................................................... 4
Table 2: Strength and Moduli Properties ........................................................................ 12
Table 3: Burst Pressure Margin Set Points ..................................................................... 17
Table 4: Resulting Margin of Safety from Pin ................................................................ 21
Table 5: Burst Pressure Test Log Sheet .......................................................................... 22
v
LIST OF FIGURES
Figure 1: Typologies of Fibre-Reinforced Composite Materials ..................................... 2
Figure 2: Hand Layup Process [7] .................................................................................. 10
Figure 3: Vacuum Bagging Process [7] .......................................................................... 10
Figure 4: Finite Element Model of Composite Pressure Vessel ..................................... 13
Figure 5: Nastran Model-Composite Pressure Vessel ..................................................... 14
Figure 6: Burst Pressure Margin Test Setup ................................................................... 16
Figure 7: Burst Pressure Test Stand................................................................................ 18
Figure 8: Nastran Model Pin=5.1 psi ............................................................................... 20
vi
ACKNOWLEDGMENT
I would like to thank my family for always supporting me in every aspect of my life.
Even when I doubt myself, they give me the motivation and strength to continue.
I would also like to thank all my professors at RPI for their assistance throughout my
degree program.
vii
ABSTRACT
This report describes results of a study designed to determine the maximum load
that can be applied to a cylindrical composite pressure vessel used in a commercial cabin
air compressor temperature control system before it exceeds a required margin of safety.
Composite pressure vessels are increasingly used in aerospace applications because of
their superiority in strength, weight advantage, and lower production costs.
A
description of the benefits and disadvantages of composites and the some of the most
common manufacturing methods is first presented. Then, a finite element model of the
vessel is used to estimate whether the current design has adequate resistance to the
internal pressures that could be encountered in actual flight conditions. Pressure loading
tests were also performed on the actual hardware to certify the part for flight use and the
results were found in good agreement with the finite element analysis.
viii
1. Introduction/Background
Composite pressure vessels have applications in many industries.
Everyday
applications include, but not limited to breathing apparatuses for medical use, hydrogen
gas storage for the automotive industry, and housings to contain rotating machinery
within the aerospace industry. Composite pressure vessels are ideal for use within the
aerospace industry because of its high strength and light weight, which helps reduce
overall cost. Most design and analysis of composite pressure vessels are based on thinwalled vessels with the ratio of the outside diameter to inside diameter being less than
1.1, anything larger than that would be considered a thick-walled pressure vessel.
The solution techniques of composite cylinders used in the mechanics community
for linear anisotropic elasticity are based on the Lekhnitskii’s theory [1].
He
investigated the plane strain case or the generalized plane strain case or the generalized
plane strain cases. Roy and Tsai [2] proposed a simple and efficient design method for
thick composite cylinders; the stress analysis is based on 3-dimensional elasticity by
considering the cylinder in the state of generalized plane strain for both open-ended
pipes and close-ended pressure vessel cases.
Sayman [3] studied the analysis of multi-layered composite cylinders under
hygrothermal loading. Xia [4] studied multi-layered filament wound composite pipes
under internal pressure and presented an exact solution for multilayered filament wound
composite pipes with resin core under pure bending.
Parnas and Katirci [5] discussed the design of fiber-reinforced composite pressure
vessels under various loading conditions based on a linear elasticity solution of the thick
walled multi-layered filament wound cylindrical shell.
1
1.1 Composite Structure & Material
A Composite material is made by combining two or more materials to give a unique
combination of properties. Composites can be comprised of a metal alloys, plastic copolymers, minerals, and wood.
Typically, composite materials are formed by
reinforcing fibers in a matrix, where the reinforcements can be fibers, particulates, or
whiskers, and the matrix materials can be metals, plastics or ceramics.
The
reinforcements can be made from polymers, ceramics, and metals. The fibers can be
continuous, long, or short. The reinforcing fibers can be long continuous fibers to short
fibers. Long continuous fibers in the direction of the load will create a composite with
properties far exceeding the matrix resin itself, whereas the short fibers yield lower
properties than continuous fibers.
For structural applications within the aerospace
industry, specifically to rotating machinery, the continuous fibers are recommended.
Short fibers are used in injection and compression molding, while continuous/long fibers
are used in filament winding, pultrusion, and roll wrapping. Figure 1 below shows the
difference in fibers used [6].
Figure 1: Typologies of Fibre-Reinforced Composite Materials [10]
2
1.1.1
Importance of Fibers & Matrix
The reinforcing fiber provides strength and stiffness to the composite while the
matrix gives rigidity and environmental resistance. The composite properties will vary
depending on the way the fibers are laid. The fiber carries the load and is strongest
along the axis of the fiber. For structural composites, the fibers carry 70 to 90% of the
load and provide the necessary stiffness, strength, and thermal stability in the composite.
The fibers also provide electrical conductivity or insulation, depending on the type of
fiber used. The matrix material is critical to the composite as it is the binder and
transfers the load to the fibers while providing rigidity and shape to the structure. The
matrix also isolates the fibers so that individual fibers can act separately which will help
prevent or slow crack propagation and protects reinforcing fibers against wear.
Choosing the right matrix material is also essential for the type of application to be used
with the composite, the matrix material can change essential performance characteristics
such as ductility, impact strength, and toughness. Thermoplastic-based composites are
used for increased toughness requirements in many aerospace applications [6].
1.1.2
Composite Benefits
The specific strength values (strength to density ratio) of composite materials are
very high and are in the ranges of 3 to 5 times that of steel and aluminum alloys which
make them very appealing in various industries, Table 1 below compares composite
properties to other standard engineering materials. The fatigue strength or endurance
limit is also much higher for composites than that of steel or alloys which generally
exhibit up to about 50%, while unidirectional carbon/epoxy composites have fatigue
strength up to almost 90% of their static strength [6].
3
Table 1: Properties of General Engineering Materials [6]
Composites can be highly resistant to corrosion and chemical exposure unlike
iron or aluminum which corrode when exposed to water and air and require special
coatings. From a design aspect, composite materials are highly preferred because of
their design flexibility. The coefficient of thermal expansion of composite structures can
be made zero by selecting suitable materials and lay-up sequence. This flexibility also
holds true to manufacturing as well since complex parts and special contours are more
feasible with composites than with metals, greatly reducing production time. From an
acoustical standpoint, composites also rein superior, as composites materials dampen
vibrations an order of magnitude better than metals. For commercial aircraft rotating
machinery applications, this proves critical as performance along with noise reduction
are both equally important.
4
1.1.3
Composite Disadvantages
Just as there are many advantages to composites there are also disadvantages.
First and more importantly composite materials tend to be very costly compared to their
metal counterparts such as steel and aluminum. There is also a lack of high volume
production methods that limits the widespread use of composites, for commercial
aircraft application this does not prove a problem since only 4 to 10 aircraft packs ship
per month due to the complexity of aircraft integration. The temperature resistance of
composite parts depends on the temperature resistance of the matrix materials, the upper
temperature limit can range from +150 to 200 deg C for high temperature plastics. Also
depending on the properties of polymers some composites might be prone to
environmental stress cracking or lower solvent/chemical resistance. Lastly Composites
absorb moisture, which can adversely affect the properties, structural integrity, and
stability of the composite [6].
5
1.2 Composite Manufacturing Processes
Composites are made available through a variety of different manufacturing
processes. The variety in resins, fibers, and core materials will lead to a unique set of
properties depending on which combination is used. Ultimately the way the materials
are designed in the product and processed also affect the final properties of the
composite structure. Different manufacturing processes will require different types of
tooling and will vary in total manufacture cost. The main composite manufacturing
processes are [6]:

Hand Lay-up which is also known as wet lay-up. This uses fibers in the form of
woven, knitted, stitched or bonded fabrics. Once the fibers have been placed in
a mould, they are impregnated by hand using rollers or brushes, the laminates
then cure under ambient conditions. Any kind of fiber resin can be used in this
process as long as it in low in viscosity in order to be workable by hand, and
tends to be low on tooling costs.

Spray Lay-up which uses a hand held spray gun which shops the fibers and then
feeds it into a spray of resin aimed at the mould. The materials then cure under
ambient conditions. This process can only make use of glass fibers and since
the laminates tend to resin rich it can add extra weight, the resin also needs to be
low in viscosity in order to be sprayable.

Vacuum Bagging which is another form of wet lay-up where the consolidation
of the laminate is improved by an application of pressure up to 1 atm. A plastic
film is sealed over the laminate and onto the mould, the air underneath is
extracted by a vacuum pump. Epoxy is primarily used in combination with any
kind of fibers. This process tends to be more expensive due to the tooling and
labor costs.

Filament winding which is the process where a band of continuous resin
impregnated rovings or monofilaments is wrapped around a rotating mandrel
6
and then cured at room temperature or in an oven.
The mandrel can be
cylindrical, round, or any shape that does not have re-entrant curvature. This is
generally used for circular or oval sectioned products, where the winding pattern
is controlled by the rotational speed of the mandrel and the movement of the
fiber feeding mechanism.

Prepregs which stands for pre-impregnated, the fabrics and fiber are preimpregnated by the materials manufacturer under heat and pressure or with
solvent, with a pre-catalysed resin.
The prepregs are laid up by hand or
machined onto a mould surface, vacuum bagged and then heated to 120-180 deg
C. Additional pressure for the molding is usually provided by an pressurized
oven which can apply up to 5 atm to the laminate. High temperature resins are
used with this process.

Resin Transfer Moulding which is a high performance production method where
everything is contained in a closed mould and there are no vapor emissions.

Rubber Pressing which is a process for forming sheet materials into products by
using one product shaped mold (usually made of aluminum or wood) and one
universal rubber cushion. A thermoplastic resin is required, the thermoplastic
prepreg sheets are heated by infrared heaters where then the sheet material is
pressed into its final form. Only one mold is required, so costs and labor time
tend to be low.

Pultrusion in which fibers pulled from a creel through a resin bath then on
through a heated die which completes the impregnation of the fiber and controls
the resin content to cure the material into its final shape as it passes through the
die.

Sandwich Constructions which are a special form of laminated composite in
which thin, strong, stiff, hard, but relatively heavy facings are combined with
thick, soft, and light weaker cores to provide a lightweight composite stronger
and stiffer than the sum of the individual stiffness and strengths. The sandwich
is comprised of two laminates (an outer and inner), a core material as a spacer,
and an adhesive for bonding of laminates.
The core can be made of paper,
honeycombs made of impregnated aramid paper, or thermoplastic foam.
7
2. Problem Description
2.1 Importance of Burst Margin Testing
In the commercial aerospace industry there are many qualification tests that are
performed on individual components to certify the aircraft. Many tests are mandated by
the FAA and involve strict requirements that the industry must test to in order to ensure
the plane is safe for flight. One critical test is burst and burst margin testing, where
multiple housings are subjected to a minimum pressure which is determined by
extensive pressure and temperature modeling and establishing a baseline normal or
single failure operating pressure and multiplying that by some factor required by the
FAA per sec 25.1438.
Burst Pressure is usually considered a worst case scenario
pressure, representative of either a single failure condition or a maximum normal
operating condition. Burst Margin testing is not a pass/fail test; rather it is more of a
safe check to find out how much margin you have in case future design changes must be
implemented. A carbon fiber pressure vessel used in the aerospace industry must ensure
that structural integrity will be maintained in the event of a failure condition on the
aircraft. The base line pressure to which the composite pressure vessel will be tested, is
determined from a single failure or maximum normal operating condition resulting in
maximum pressures which are then temperature corrected for an equivalent pressure at
room temperature which for the subsequent analysis section is referred to as internal load
pressure Pin. The burst factor is 2 for maximum pressures encountered during maximum
normal operating cases and 1.5 for maximum pressures encountered during single failure
cases. The internal pressure load at room temperature is calculated through a simple
equation which is described below:
8
𝑷𝒊𝒏 =
𝑷𝒂𝒄𝒕 ×𝑩𝑭
𝑻𝑪𝑭
(1)
Where Pact is the actual expected pressure on the aircraft, BF is the burst factor, and
TCF is the temperature correction factor to convert the actual pressure on the aircraft to
an equivalent pressure at room temperature.
The composite pressure vessel must ensure that it does not rupture after application
of the baseline internal pressure application. Once that has been proven successful, the
composite may then be subjected to burst margin testing, where the degree of structural
margin is determined. The governing case by which the baseline Burst Pressure is being
determined for the particular composite pressure vessel considered in the study is the one
resulting from a maximum operating velocity single failure condition at an altitude of
20,000 ft which requires a value of 1.5 to be used for the burst factor. The resulting
maximum pressure from that single failure condition is Pact=2.72 psi at a temperature of
198.4°F. The temperature correction factor the carbon fiber composite is 0.96, which
can then be used to determine the equivalent pressure load at room temperature Pin per
equation 1 above.
The resulting pressure Pin is considered the baseline pressure for
which burst margin testing is to be based on. The composite should be analyzed and
tested to 20% over that baseline pressure to ensure adequate design margin.
𝑃𝑖𝑛 =
2.72 × 1.5
= 4.25 𝑝𝑠𝑖
0.96
2.2 Description of Composite Test Article
The composite pressure vessel being tested and analyzed in this paper is a
composite carbon fiber composite pressure vessel which is used on the CACTCS (Cabin
air compressor temperature control system) pack on a commercial airplane. It consists
of a prepreg carbon fiber composite which is a pre-impregnated cloth, laid up by hand
inside a female mold (see figure 2).
9
Figure 2: Hand Layup Process [7]
A vacuum bag is applied to the inside to compress the material into the mold
while it is cured (see figure 3).
Figure 3: Vacuum Bagging Process [7]
The outside therefore is nice and smooth as this is in contact with the tool surface,
while the inside is rougher since it is the bagged surface. The carbon fiber composite is
specifically formulated for use in primary aircraft structures and has a maximum
continuous service temperature up to 400 deg F, and short term service temperature up to
450 deg F. It has low thermal conductivity and has excellent compression strength after
impact. It is also fluids/solvent resistant, which is critical to aircraft components in a
flammable leakage zone which may have exposure to jet fuels and other possible
corrosive liquids. The Material data sheet for industry standard carbon fiber prepreg can
be found in Appendix B. The strength properties of the prepreg can vary depending on
the type of fiber being used such as carbon or glass and the resin content percentage.
10
3. Methodology/Approach
3.1 Analysis
A finite element pre and post processor with a NASTRAN solver was utilized for
the analysis of the composite pressure vessel. Boundary conditions and operational
loads were applied so as to predict maximum stresses, or otherwise maximum failure
loads. The model was constructed of PCOMP elements [8] with a nominal thickness to
best represent the stresses. Shell elements are optimal for thin walled structures, such as
composite pressure vessels, and are similar to the solution of composite shells. Solids, a
single element in depth, tend to provide erroneous stresses, and therefore were not used
for this analysis. The model was restrained using the inertial relief option, which allows
the simulation of unconstrained structures. In an intertial relief analysis, the applied
loads are balanced by a set of translational and rotational accelerations, these
accelerations provide body forces distributed over the structure in such a way that the
sum total of the applied forces on the structure is zero. With the inertial relief analysis
boundary conditions are applied only to restrain rigid body motion, and since the
external loads are balanced by the accelerations, the reaction forces corresponding to
these boundary conditions are zero. The composite pressure vessel was modeled using
PCOMP elements, which essentially defined the structure and properties of a 4-ply
composite laminate material with an orientation of 0-45-45-0. Fiber reinforced carbon
materials typically exhibit anisotropy, where properties vary depending upon which the
geometric axis or plane they are measured along. The material properties attached to the
PCOMP elements reflect the anisotropic nature of the carbon fiber material, therefore the
modulus of elasticity and Poisson’s ratio are different in each direction.
The Max
principal stress criterion was used to observe the stress since the material properties for
carbon fiber composites are radically different in tension and compression.
This
criterion states that yield occurs when the largest principal stress exceeds the uniaxial
tensile yield strength and is governed by the equation σ1≤σy. Once the structure and the
properties of the composite were defined, the pressure load was applied to all the interior
11
faces of the composite. Table 2 below shows the properties of the materials to be used
in the analysis.
Table 2: Strength and Moduli Properties
E11 (psi)× 106
10.20
E22 (psi)×
106
10.20
E33 (psi)×
106
1.53
E@±45° (psi)× 106
Equasi [0-45-45-0] (psi)× 106
2.72
6.46
G12 (psi)× 106
0.78
G13 (psi)×
106
0.78
G23 (psi)×
106
0.78
G@±45° (psi)×
106
4.90
NU12
0.037
NU23
0.27
NU13
0.27
NU@±45°
CTE11
CTE22
CTE333
Thickness (in)
density (lb/in3)
0.74
1.46E-06
1.46E-06
1.67E-05
0.0075
0.056
12
Figure 4 below shows the finite element model of the composite pressure vessel.
The arrows show the direction of the load on the surface of FEM
Figure 4: Finite Element Model of Composite Pressure Vessel
The Ultimate Tensile Strength (UTS) and yield strength (σy) are equal to 63,900 psi
at room temperature conditions for the carbon fiber composite used in the model. This
demonstrates that the carbon fiber composite does not yield, upon reaching a stress of
63,900 psi the composite will fail. UTS referring to the point on the engineering stressstrain curve corresponding to the stress that produces fracture and material yield strength
referring to the point on the engineering stress-strain curve beyond which the material
experiences deformations that will not be completely reversed upon removal of the load.
Once all boundary conditions, material properties, and burst pressure load were input
into the model, the solver automatically revealed the maximum stress location on the
housing. Because of the geometry of the composite pressure vessel being used there was
an unrealistic large stress that was output from the model in the value of 1.15×105 psi.
13
This unrealistic stress value came from simulating bolts on mating surfaces by using
RBE (rigid body elements), essentially making them infinitely stiff. As a result this
unrealistic value was always the default maximum stress output in the Nastran model but
was of no relevance to the burst pressure analysis and was disregarded. Therefore the
real maximum stress point was on the cusp area of the housing as shown in Figure 5
below.
Max stress (𝜎𝑚 )
Figure 5: Nastran Model-Composite Pressure Vessel
The real maximum resulting stress output (located on the cusp area highlighted in
figure 5) from the FEA solver was then used to calculate Margin of Safety (MS) per the
equation below, where 𝑈𝑇𝑆 was the known value of 63,900 psi and 𝜎𝑚 was the real
maximum stress output by the model. The margin of safety value always has to be
greater than a value of 1 in order for the material theoretically not to fail.
𝑴𝑺 =
𝑼𝑻𝑺
𝝈𝒎
−𝟏
14
(2)
3.2 Test
3.2.1
Objective
The intent of this test was to determine the degree of structural margin within the
component design in the event of future system changes resulting in increased
component pressure requirements. The test item was completely enclosed within a
protective chamber during the burst pressure test to provide containment in the
event of a rupture.
3.2.2
Procedure
The Burst Pressure Margin test took place in a lab with calibrated pressure gages
and with room temperature pressurized air entering the pressure vessel.
The
equipment calibration log can be found in Appendix A. Prior to initiating test, the
composite pressure vessel was thoroughly inspected for use in the Burst Pressure
Test. The article did not have any non-conformances that might have given it any
added advantage or likewise any non conformances that might have negatively
affected the results of the test. The composite pressure vessel was capped at all
open ports to ensure there was no leakage during burst testing. The composite
structure was installed in a test setup as defined in Figure 6 below.
15
Tamb
Pin
et
CAPS
Protective
ChaCChamber
Figure 6: Burst Pressure Margin Test Setup
The test setup in Figure 6 shows the composite pressure vessel with caps attached
on all open ports. The caps were attached in a way so that no plug load is exerted
through the test unit. The pressure was applied at the location indicating where Pin
is. Tamb is the ambient temperature of the test cell which was being logged during
testing.
Photos of the test article and test setup were taken prior to testing to document
the condition of the unit prior to application of burst pressure. The application
pressure, ambient pressure, ambient temperature, and time were logged
continuously throughout testing. A pressure was applied starting at the minimum
burst pressure determined by previous failure condition analysis done on the
CACTCS system as a whole.
The internal pressure Pin was increased
incrementally according to Table 3 at ambient temperature until 20% over burst
pressure baseline was met.
16
Table 3: Burst Pressure Margin Set Points
Margin
Pressure, Pin (psig)
Baseline
4.25 +0.15/-0
105%
4.46 +0.15/-0
110%
4.68 +0.15/-0
115%
4.89 +0.15/-0
120%
5.10 +0.15/-0
Each internal pressure set was maintained for a minimum of two minutes before
moving to the next pressure interval. After the test was completed, the test article
was thoroughly inspected for any deformation or non conformance as a result of
the burst pressure application. If at all during testing, a sudden loss of pressure
was encountered or rupture was evident, the testing would have been stopped.
17
Figure 7 below shows the Burst Pressure Test stand where the composite pressure
vessel was subjected to burst margin testing.
The pressure gages were all
calibrated to the appropriate ranges to achieve the necessary accuracy for the
pressure levels we were applying.
Figure 7: Burst Pressure Test Stand
18
4. Results and Discussion
4.1 Analysis Results
Several cases were run using the Nastran solver at different internal pressure load
applications; a list of all the cases is shown in Table 3. The first model that was ran was
the internal pressure application Pin of 5.1 psi, since this was the maximum pressure to
which the vessel is to be tested as shown in Table 3. If the analysis had determined that
adequate design margin was left then it would have been safe to assume testing at lower
pressures would pass without any detriment to the composite. The analysis of the
internal pressure load of 5.1 psi was critical because if there was no margin of safety
determined at that pressure load then most likely the composite design did not have
adequate design margin able to withstand higher pressures during single failure
conditions in the aircraft. Since rigid body elements were used to simulate bolts on
mating surfaces, a maximum stress of 1.15×105 psi was output as discussed earlier in the
analysis section. Knowing that this stress was an unrealistic value, it was ignored and
the real resulting maximum stress output from the analysis was 54,467 psi which was
shown to be on the cusp area of the composite vessel, the Margin of Safety was then
calculated based on the maximum stress output and ultimate tensile strength of 63,900
psi (at room temperature) to be 0.17. This confirms that the carbon fiber composite
pressure vessel did not fail or rupture since it had a margin of safety greater than zero.
Figure 8 below shows the results of the first load analysis.
19
Max stress 54,467 psi
Figure 8: Nastran Model Pin=5.1 psi
63900 𝑝𝑠𝑖
𝑀𝑆5.1 𝑝𝑠𝑖 = 54467 𝑝𝑠𝑖 − 1 = 0.17
Table 4 below shows the values of the real maximum resulting stress on the
carbon fiber composite pressure vessel relevant to burst analysis.
As the internal
pressure application Pin is increased the stress located on the cusp on the composite
vessel starts to increase resulting in a lower margin of safety. The analysis was started at
the highest value listed in Table 3. Since the resulting margin of higher than zero, it was
safe to assume that any pressure under 5.1 psi would not have resulted in failure. So the
analysis was run with higher pressures until the margin of safety was close to zero.
20
Table 4: Resulting Margin of Safety from Pin
Pressure Application, Pin (psi)
σm (psi)
MS
5.1
54,467
0.17
5.2
55,950
0.14
5.3
56,990
0.12
5.5
59,513
0.07
The final internal pressure load application of 5.5 psi resulted in a maximum
stress of 59,513 psi and a margin of safety of 0.07. Hypothetically per the analysis the
composite pressure vessel did not fail but it may have been close to failure. This
information is critical to the test engineer prior to burst pressure testing; it indicates that
the test engineer should proceed to higher levels with caution.
4.2 Test Results
Based on the Analysis done previously, it was safe to assume that the composite
pressure vessel could safely withstand 120% of burst pressure margin level.
The
composite pressure vessel was subjected to a maximum internal pressure load of 5.2 psi
which is greater than 120% of burst pressure baseline. The composite pressure vessel
was inspected for any cracks or visible abnormalities that would indicate weakness or
stress on the composite housing and none was found. Per the analysis earlier shown, an
application pressure of 5.5 would have resulted in a margin of safety of 0.07 which
would have been close to failure or burst. The test has demonstrated that the analysis
was correct and the burst margin levels are well within a safe operating range before
burst failure. Per the test requirements section each pressure was maintained for a time
of 2 minutes minimum where the internal pressure was measured both at the beginning
and end of the 2 minute test. Table 5 below shows all the values recorded during burst
pressure testing. The pressure transducer used in the test measured in inches of water.
Therefore the maximum pressure applied was 145.32 inches of water which equals 5.25
21
psia. This was calculated using the standard conversion of 27.7 inches of water per 1
psia.
Table 5: Burst Pressure Test Log Sheet
Internal
Pressure
(Pin)
Ambient
Temp
(Tamb)
Ambient
Pressure
(Pamb)
Minimum
Time
Actual
Time
Start
"H20
Finish
"H20
Start
°F
Finish
°F
Start
psia
Finish
psia
Minutes
Minutes
115.10
115.70
78
78
14.97
14.97
2:00
2:00
125.00
125.30
78
78
14.97
14.97
2:00
2:00
132.20
131.00
78
78
14.97
14.97
2:00
2:11
137.10
138.10
78
78
14.97
14.97
2:00
2:00
143.40
145.32
78
78
14.97
14.97
2:00
2:02
22
5. Conclusion
Composites are very versatile and the ways in which they can be manufactured
are numerous and can be tailored to specific requirements of the customer application.
Composite material has proved itself to be very strong and durable yet lightweight
making it very cost effective, they can also withstand higher pressure loads without
adverse affects, which is why it is used very commonly in the aerospace field. In
particular, the carbon fiber composite used in the analysis was made specifically to
withstand environmental conditions in a commercial aircraft where it could potentially
be exposed to higher than expected pressures depending on the conditions encountered
during flight. All pressure and temperature models performed during the design phase
of an aircraft can never be as accurate as real actual flight test data therefore design
engineers must put adequate margin into their designs to account for higher pressures.
This project had the objective of determining if a maximum load 20% greater than
baseline burst pressure could be applied to a cylindrical composite pressure vessel used
in a commercial cabin air compressor temperature control system before it exceeded a
required margin of safety.
Equation 1 defined the baseline pressure to be used in the analysis and actual test
to be 4.25 psi, with the test requiring 20 percent over that baseline to be met in order to
have adequate design margin. The first loading condition examined in the finite element
element model was maximum pressure. The results revealed that indeed the composite
pressure vessel had plenty of design margin since the margin of safety was calculated to
be 0.17. The analysis proved useful for the subsequent test by providing a hypothetical
guide to how much load the composite pressure vessel could withstand. The finite
element analysis used via Nastran solver proved to be accurate, since testing
demonstrated that even when 20% over baseline burst pressure was met, the unit did not
rupture or fail.
23
6. References
[1] Lekhnitskii, S. (1963). Theory of elasticity of an anisotropic body. Journal of the
Franklin Institute .
[2] Roy, A., & Massard, T. (1988). Design of thick composite cylinders. J Press
Vessel Technologies 110, 255-262
[3] Sayman, O. (2005). Analysis of multi-layered composite cylinders under
hygrothermal loading. Composites Part A: Applied Science and Manufacturing
36, 923-933.
[4] Xia, M., & Takayanagi, H. (2001). Analysis of a multi-layered filament wound
composite pipes under internal pressure. Composite Structures 53, 483-491 .
[5] Parnas, L., & Katirci, N. (2002). Design of fiber reinforced composite pressure
vessels under various loading conditions. Composite Structures 58, 83-95 .
[6] Onder, A. (2007). First Failure Pressure of Composite Pressure Vessels.
Graduate School of Natural & Applied Sciences of Dokuz Eylul University.
[7] Home Made Composites. (2009). Retrieved from
http://www.composites.ugent.be/home_made_composites/organizing_your_com
posite_workshop.html
[8] Keith Meyer, S. R. (1987). Interactive Laminate Postprocessing of Nastran
Structural Analyses. MSC 1987 World Users Conference, (p. 15). Universal City,
California.
[9] Cytec. (2011). Cycom product datasheets. (Cytec) Retrieved from
http://www.cytec.com/engineeredmaterials/products/Datasheets/CYCOM%205250-4.pdf
[10] Wikipedia. (2011). Composite Material. Retrieved from Wikipedia:
http://en.wikipedia.org/wiki/Composite_material
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Appendix A
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Appendix B
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27
28
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