1

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

ii

© Copyright 2011

by

Felicia Fiore

All Rights Reserved

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CONTENTS LIST OF SYMBOLS ........................................................................................................ iv 

LIST OF TABLES ............................................................................................................. v 

LIST OF FIGURES .......................................................................................................... vi 

1.  Introduction/Background ............................................................................................. 1 

1.1  Composite Structure & Material ........................................................................ 2 

1.1.1  Importance of Fibers & Matrix .............................................................. 3 

1.1.2  Composite Benefits ................................................................................ 3 

1.1.3  Composite Disadvantages ...................................................................... 5 

1.2  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 

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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

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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 

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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 

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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.

viii

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.

 

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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 thin-

walled 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.

2

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 co-

polymers, 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]

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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].

4

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.

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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].

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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

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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 pre-

impregnated 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.

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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:

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(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.

2.72 1.50.96 4.25

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).

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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.

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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

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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 2.72 Equasi [0-45-45-0] (psi)× 106 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° 0.74 CTE11 1.46E-06 CTE22 1.46E-06 CTE333 1.67E-05 Thickness (in) 0.0075 density (lb/in3) 0.056

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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 stress-

strain 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.

14

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.

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 was the known value of 63,900 psi and 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.

(2)

Max stress (

Figure 5: Nastran Model-Composite Pressure Vessel

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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.

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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.

CAPS

Tamb

Pin

Protective

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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.

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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

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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.

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Figure 8: Nastran Model Pin=5.1 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.

Max stress 54,467 psi

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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

22

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

23

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.

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6. References

[1] Lekhnitskii, S. (1963). Theory of elasticity of an anisotropic body. Journal of the

Franklin Institute .

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Appendix A

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Appendix B

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