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Design and fabrication of high performance composite pressure vessels
Theoretical and Applied Fracture Mechanics 10 (1988) 157-163 North-Holland
157
DESIGN AND FABRICATION OF H I G H PERFORMANCE COMPOSITE PRESSURE VESSELS G.S. GER, D.G. HWANG, W.Y. CHEN and S.E. HSU Chung Shan Institute of Science and Technology, Lung-Tan, Taiwan 32500, Republic of China
The manufacturing of composite materials has gained momentum in Taiwan, the Republic of China, over recent years. Fiber-reinforced plastics (FRP) pressure vessels have been made successfully by filament winding. Reviewed in this work are the design consideration manufacturing technology and inspection procedure. This involves theoretical analysis, material selection, mandrel design, tooling, processing know-how and proof tests.
1. Introduction
Advancements in fiber-reinforced composites have significantly enhanced the performance and use of pressure vessels [1,2]. High performance vessels with a high strength-to-weight ratio are being widely used in both military and commercial applications [3,4]. This is being made possible by improved filament winding techniques that involve the winding of resin impregnated fibers over a rotating mandrel. Laydown path of the fibers is accurately determined and monitored by controlling the combined rotational and translational motion of the mandrel. In addition to requirements on precision winding, pressure vessel performance [5-7] will also
depend on design criteria, selection of materials, fabrication methods and qualification tests. The variables and parameters that would affect and optimize the performance of a pressure vessel will be discussed in this communication.
2. Material selection
To begin with, a fiber/resin system should be first selected to meet design requirements. Refer to Table 1 for the properties of the commercially available fibers. Considerations are given to their specific strength and stiffness, cost, manufacturability and machinability. Resin also affects the load carrying capacity of the composite depending
Table 1 Status of available fibers Manufacturer
Fiber ident,
Fila. dia. (1~)
Specific weight
Fiber modulus (Msi)
Fiber ten. str. (Ksi)
Owens Coming Toho
E-glass S2-glass Carbon fiber ST-I-6000 ST-II-6000 ST-III-6000 Carbon fiber T300-6000 T400-6000 Graphite fiber AS4 Graphite fiber T300 Kevlar 49
11 9.1
2.57-2.59 2.46
10.5 12
500 550
7 7 7
1.77 1.77 1.77
34 34 34
525 600 625
7 7 7
1.76 1.70 1.80
33 34 35
510 600 600
7
1.77
34
450
12
1.44
19
545
Torayca
Hercules Union Carbide DuPont
0167-8442/88/$3.50 © 1988, Elsevier Science Publishers B.V. (North-Holland)
158
G.S. Ger et al. / High performance composite pressure vessels
upon whether disturbances are transmitted by shear, compression, transverse tension or flexure. Performance studies have shown that missile or rocket motor casings made from graphite or carbon/epoxy are superior to those made from fiber glass/epoxy. Significant increase in the quantity P V / W that stands for the product of pressure times volume over weight were found for the carbon fiber systems. This translates into greater strength and stiffness, higher velocity and altitude and range of the missiles or rockets. In this connection, this study focuses attention on carbon fiber with an intermediate modulus of 34 Msi (TOHO ST-III-6000) as shown in Table 1 while the resin is sufficiently flexible (Epon 828/ERL 4206/Tonox 6040): The skirt material is made of S-glass fabric/epoxy being considerably less in cost. A perforated SAE 1086 high strength and ductility sheet is used as a shim to sustain the high local stresses and enhance the bolt joint efficiency for the skirt [8,9].
3. Design consideration The design procedure follows the flow chart in Fig. 1. The dome helical and cylindrical hoop thickness were determined from the internal pressure whereas the cylindrical helical thickness was chosen from the external loads so as to avoid buckling. Other desirable features involve high bending stiffness and low circumferential expansion. Specific limits, however, were not set. Other factors that can influence the performance of a vessel will be discussed subsequently.
3.1. Analytical approach As the filament-wound composites behave the same way as that of the angle-ply laminates, laminate theories may be used. The macromechanics approach assumes that each lamina of the composite is made of an anisotropic and homogeneous monolayer. Such a layer consists of fila-
Case Configuration
Tension Compression
Loading Analysis
Buckling,etc.
MatmialSelection
Design
L
r
Bending Test
I I
AnalyticalMethods WindingPattm'n Dome Contour Joint,etc. Mandrel Liner
Subscal¢ Vessels HydroburstTest Compressive Test,
R~inforccmem Matrix
~-~
Winding Proc.ess,etc.
NDT
etC.
Proof Tests
L
r
Machining Assembling
Product
] I
Fig. 1. Flow chart of research concept.
MEOP
G.S. Ger et a L / High performance compositepressure vessels
ments oriented at a plus and minus angle. It can also be unidirectional. The mechanical properties E l l , E 2 2 , 1£12, /£21 ( = g12E22/Ell) and G12 are determined experimentally while the analysis is carried out on a layer by layer basis. As estimate of the fiber stresses in the internally pressurized cyhndrical vessel is made by neglecting the influence of the resin. The fibers carry only tensile loads. Such an approach is known as netting analysis. The NASTRAN computer code is applied in conjunction with the criterion of "Maximum Stress" for determining the integrity of the vessel. 3.2. Winding pattern A series of stress analyses were performed to determine the fiber orientation, winding (lay-up) sequence, interspersement, stress ratio, and the wall thickness of the pressure vessel. Based on this information, the winding patterns are made systematically over the mandrel. As the buckling strength depends sensitively on the helical thickness, additional windings were added for safety measures. Since these layers were not required in the domes, they were extended through the skirts as shown in Fig. 2. With this design, the extra helicals serve as a foundation for the skirts and
159
yield a stiffer, stronger case-to-skirt joint than a conventional skirt which were composed of local cloth layers extending only a short distance into the cylinder. 3.3. Dome contour For high performance vessels, integral heads are essential, and the dome contours and related polar bosses are critical in vessel design. An important consideration of dome design is to increase volume without increasing dome build-up. Hoop stresses are to be kept within acceptable limits. A balanced Geodesic-Isotensoid (GI) contour design is adopted such that the fiber path is made tangent to the polar boss [10], Fig. 3. Each point on the path is defined by its meridional and circumferential radii, r I and r2, respectively. These radii are related to the X-, Y-coordinates as follows:
[1 + (r' 213J2 y.
r1 = _
r2=-
,
X[1 + (y,)2] 1/2 y, ,
(2)
where Y' and Y" are, respectively, the first and
[ 2ct/SAE 1086 Metal shim/S-glass fabric/2ho ] - - -FWD l
79.48 51.18
AFT .
16.14 - -
0.19 TYP.-
Y Joint
Liner [ 2ho/2Cq2ho/4OV2ho ]
Z
[ 2c~/Wafer/2et] Metal Fitmg
1 6.1I
J ~3 DIA
Metal Fitmg
F
19.69 DIA.
!
....2'. I /
Fig. 2. The configuration of studied vessel.
160
G.S. Ger et al. / High performance composite pressure vessels Y
FILAMENT PATH
DIRECTION POINT
..
(x,y)
) DIRECTION
M E R / D I A N T H R O U G H POINT
p
Fig. 3. Geometryfor geodesicpath. second derivatives of Y with respect to X. The ratio r1
--
r2
=
XY"
y'[l+(y')
2]
= 2 - 2 tan2a
(3)
guarantees a balanced stress state with a being the winding angle. The geodesic path is described by X sin a = constant = X0 ,
(4)
where X0 is the boss diameter. Equations (3) and (4) may yield the coordinates of Geodesic-Isotensoid contour. Additional strengthening of the dome is necessary as failure during hydrostatic tests will occur if reinforcements are not added. Local wafer reinforcements in the dome build-up area have been employed. An optimum design stress ratio may also be invoked to yield the highest performance efficiency per pound of material used in the vessel.
integrally wounded vessels. Fiber damage during part removal, dimensional tolerances and residual stresses are factors that require attention. In particular, the mandrel must resist sagging due to its weight and applied winding tension. Sufficient strength must also be retained when curing takes place at elevated temperatures. Utilizing a sandPVA mandrel for both liner and case fabrication, tooling requirements on many filament wound casings can be reduced, particularly when the case size is small enough to allow the use of a sand mandrel and when only a small number of units are to be built. A full-scale sand-PVA pressure vessel mandrel is shown in Fig. 4. 4.2. L i n e r
The liner was stacked by laying up NBR and V-44 rubber on the sand mandrel to an over-
4. Fabrication
Product quality and performance are affected by the fabrication procedures and processes. Of particular significance are the mandrel, liner and casing. ~1. Man~el
Special care should be given to the design of the mandrel and selection of material [11] for
Fig. 4. A full-scalesand-PVA mandrel.
G.S. Ger et al. / High performance compositepressure vessels
stocked thickness, and then bagged and cured in autoclave at 150 psi and 300°F for five hours. The exterior surface was then machined to the required external contour. Prior to winding the composite vessel, an epoxy compatible primer was applied over the entire surface of the liner in order to ensure a good liner-to-case bond.
161
2. The completed case was then bagged, and the cure process was executed in the autoclave. After machining to the desired diameter, the sand mandrel was washed out by using hot water. A finished case is shown in Fig. 5.
5. Testing 4.3. Case fabrication 5.1. Nondestructive testing
A wet filament wound technology with a target of 0.62 fiber volume content was utilized. Resin content was controlled by employing an impregnating drum. The McClean-Anderson numericalcontrolled winding machine with a maximum capacity of 6 meters in length and 1.5 meters in diameter was used to execute the case winding. Both the hoops and helicals were wound at a 12 tow/in./ply end density. After winding the first two hoop plies in the cylindrical section, the first two helicals were wound from dome to dome as shown in Fig. 2. Then the second two hoop plies were interspersed between the helicals in the cylindrical section. A graphite cloth wafer was installed between the first and second helical on each dome to prevent possible boss blowout failure. After winding the second helical, the skirt mandrels were installed and the third helical was wound from skirt to skirt. A Y-joint rubber was installed locally in the skirt joint region prior to winding the third helical so that the stress concentration can be alleviated and the joint strength can be improved. The final two hoop plies were then wound over the third helical with SAE 1086 metal shim and S-glass fabric locally interspersed in the skirt regions. The stacking sequence for the cylindrical dome and skirt parts are also shown in Fig.
Fig. 5. Prooftest set-up.
There are numerous nondestructive testing (NDT) techniques to detect defects in composites. These imperfections may include voids, resin crazing (matrix failure), fiber debonding, cracks, delamination, fiber breaking, and composite fracture. Although the NDT techniques are well developed for metals, the application of NDT techniques to composites is still comparatively limited. In this study, the vessels were examined by the X-ray method. The film records were compared to the well-established accept/reject criterion for each defect. 5.2. Proof test
Hydrostatic proof tests were made to each vessel. Figure 5 shows the proof test set-up. The supply water through the hydraulic lines is pressurized by an air compressor. A solenoid valve switches the water input between line pressure (MEOP) and the high pressure water supply. A waste-water valve connected to the other end of the specimen is used to purge trapped air. 5.3. Subscale hydroburst test
Design and fabrication of large filament wound pressure vessels often necessitate the testing of subscaled vessels for evaluating materials, design concepts, and manufacturing techniques [12-14]. Configuration alternation including changes of materials, processes, and design features, can be evaluated using data gathered from the subscale vessels. By varying the diameter, cylinder length and dome contour, the hydroburst tests for subscale pressure vessels were conducted. Figure 6 shows the different dimensions of various types of subscale vessels. The results of hydroburst tests are summarized in Table 2. The failure modes of
G.S. Ger et al. / High performance composite pressure vessels
162
Table 3 Comparison of test results for metal and carbon fiber vessel Metal SAE 4130 Weight (case only) (kg) Theoretical max. pressure (psi) Max. effective operation pressure (MEOP) (psi) Safety factor Burst pressure (psi) Pressure/weight
Carbon Fiber ST-III-6000
134
98.4
1880
3120
1564 1.2 1906 14.2
1564 2.0 3052 c 31.0
Fig. 6. The subscale vessels. c Average data from these tests.
cylinder, hoop, dome curve, and boss blowout which can be predicted by using" N A S T R A N computer code were presented in the hydroburst test. One of the dome curve failure is shown in Fig. 7. The hydroburst te_sts were also conducted of the full-scale pressure vessel to confirm the final design with desired requirements. Refer to a comparison of the test data for the carbon fiber vessel with those for the metal. Note that the ratio of 1.66 for the theoretically predicted maximum pressure for the carbon fiber vessel and metal vessel agreed closely with the corresponding ratio of 1.60 measured experimentally, Table 3. The bending test in Fig. 8, the compressive tests, and the static firing tests in Fig. 9 were
Fig. 7. The failure of a studied vessel.
Table 2 Subscale hydroburst test results N u m b e r of represented vessels
1
2
3
4
Reinforcement
S-glass 500 yields a a-21 b ho a-3/ a-21 ho-41
E-glass 250 yields a-41 ho-21
0.0096 0.0069 0.258 4950 4250 boss blowout failure
0.011 0.0095 0.341 2800 3000 Cylindrical failure
Carbon ST-III-6000 ho-21 a-21 ho-21 a-2 ho-2 0.0057 0.0044 0.603 2500 2750 D o m e curve failure
Carbon ST-III-6000 ho-2 l a-21 ho-21 a-41 ho-21 0.0053 0.0042 0.603 2500 2830 Cylindrical and hoop failure
Winding sequence
Helical winding thickness (in/layer) Hoop winding thickness (in/layer) Db / Dc Design pressure (psi) Burst pressure (psi) Failure mode
act = hefical winding; ho = hoop winding b 1 = layer
G.S. Ger et al. / High performance composite pressure vessels
163
Acknowledgments The authors would like to thank their colleagues who assisted in manufacturing and testing of the vessels.
References Fig. 8. Bending test of a vessel.
Fig. 9. Static firing test of a vessel.
conducted to monitor the performance of the pressure vessels.
6. Conclusions Fiber-reinforced composite pressure vessels used for rocket motors and other applications offer weight savings and enhance performance. Recent advances in fiber manufacturing techniques provided additional improvement in strength and modulus of the reinforced fibers. Optimization of pressure vessel performance involves the combined consideration of material selection, design, analysis, processing and test. The present study describes the filament winding techniques used in Taiwan, the Republic of China, for manufacturing composite pressure vessels. Additional improvements are anticipated as progress is being made in design and manufacturing techniques.
[1] N. Christensen and E. Wolcott, "Development and fabrication of a graphite/epoxy motor case for air launch missile applications", 29th National SAMPE Symposium, p. 1335 (1984). [2] S.W. Beckwith, G.D. Walker and J.B. Schutz, "Filament wound case (FWC) graphite/epoxy pressure vessel response to environmental conditioning", 31st International SAMPE Symposium, p. 1330 (1986). [3] A.K. Munjal, "Use of filament winding in manufacturing high-quality aerospace composite components", 31st International SAMPE Symposium, p. 1504 (1986). [4] M.E. Huber, "Filament winding of aircraft engine Nacelle components", 29th National SAMPE Symposium, p. 1184 (1984). [5] A.K. Munjal, "Use of fiber-reinforced composites in rocket motor industry", 17th National S A M P E Technical Conf., p. 371 (1985). [6] M.H. Young and B.A. Lloyd, "Rocket case performance optimization", 17th National S A M P E Technical Conf., p. 482 (1985). [7] A.K. Munjal and S.B. Kulkarni, "Characterization of filament wound Kevlar and glass composites for rocket motor applications", 29th National SAMPE Symposium, p. 324, (1984). [8] R.L. Grover and S.J. Zitek, "Development of lug attachment methods for air-launched composite motor cases", 29th National SAMPE Symposium, p. 1349 (1984). [9] D.G. Hwang, et al., "A study in metallic interlayer-reinforced glass cloth/epoxy mechanical joint", 5th Internat. Conf. on Composite Materials, ICCM-V, (1985). [10] G. Lubin, Handbook of Composites, Filament Winding, Van Nostrand Reinhold, New York (1982). [11] E. Calius and G.S. Springer, "Selecting the process variables for filament winding", 31st International SAMPE Symposium, p. 891 (1986). [121 N.R. Dajani, "Hydroburst tests of carbon/epoxy pressure vessels subjected to uniform and simulate aeroheat gradient temperatures", 31st International SAMPE Symposium, p. 1519 (1986). [13] N.L. Newhouse and W.D. Humphrey, "Development of the standard test evaluation bottle (STEB)", 17th National S A M P E Technical Conf., p. 554 (1985). [14] W.D. Humphrey and N.L. Newhouse, "The standard test and evaluation bottle (STEB) five years later", 31st International SAMPE Symposium, p. 1383 (1986).
157
DESIGN AND FABRICATION OF H I G H PERFORMANCE COMPOSITE PRESSURE VESSELS G.S. GER, D.G. HWANG, W.Y. CHEN and S.E. HSU Chung Shan Institute of Science and Technology, Lung-Tan, Taiwan 32500, Republic of China
The manufacturing of composite materials has gained momentum in Taiwan, the Republic of China, over recent years. Fiber-reinforced plastics (FRP) pressure vessels have been made successfully by filament winding. Reviewed in this work are the design consideration manufacturing technology and inspection procedure. This involves theoretical analysis, material selection, mandrel design, tooling, processing know-how and proof tests.
1. Introduction
Advancements in fiber-reinforced composites have significantly enhanced the performance and use of pressure vessels [1,2]. High performance vessels with a high strength-to-weight ratio are being widely used in both military and commercial applications [3,4]. This is being made possible by improved filament winding techniques that involve the winding of resin impregnated fibers over a rotating mandrel. Laydown path of the fibers is accurately determined and monitored by controlling the combined rotational and translational motion of the mandrel. In addition to requirements on precision winding, pressure vessel performance [5-7] will also
depend on design criteria, selection of materials, fabrication methods and qualification tests. The variables and parameters that would affect and optimize the performance of a pressure vessel will be discussed in this communication.
2. Material selection
To begin with, a fiber/resin system should be first selected to meet design requirements. Refer to Table 1 for the properties of the commercially available fibers. Considerations are given to their specific strength and stiffness, cost, manufacturability and machinability. Resin also affects the load carrying capacity of the composite depending
Table 1 Status of available fibers Manufacturer
Fiber ident,
Fila. dia. (1~)
Specific weight
Fiber modulus (Msi)
Fiber ten. str. (Ksi)
Owens Coming Toho
E-glass S2-glass Carbon fiber ST-I-6000 ST-II-6000 ST-III-6000 Carbon fiber T300-6000 T400-6000 Graphite fiber AS4 Graphite fiber T300 Kevlar 49
11 9.1
2.57-2.59 2.46
10.5 12
500 550
7 7 7
1.77 1.77 1.77
34 34 34
525 600 625
7 7 7
1.76 1.70 1.80
33 34 35
510 600 600
7
1.77
34
450
12
1.44
19
545
Torayca
Hercules Union Carbide DuPont
0167-8442/88/$3.50 © 1988, Elsevier Science Publishers B.V. (North-Holland)
158
G.S. Ger et al. / High performance composite pressure vessels
upon whether disturbances are transmitted by shear, compression, transverse tension or flexure. Performance studies have shown that missile or rocket motor casings made from graphite or carbon/epoxy are superior to those made from fiber glass/epoxy. Significant increase in the quantity P V / W that stands for the product of pressure times volume over weight were found for the carbon fiber systems. This translates into greater strength and stiffness, higher velocity and altitude and range of the missiles or rockets. In this connection, this study focuses attention on carbon fiber with an intermediate modulus of 34 Msi (TOHO ST-III-6000) as shown in Table 1 while the resin is sufficiently flexible (Epon 828/ERL 4206/Tonox 6040): The skirt material is made of S-glass fabric/epoxy being considerably less in cost. A perforated SAE 1086 high strength and ductility sheet is used as a shim to sustain the high local stresses and enhance the bolt joint efficiency for the skirt [8,9].
3. Design consideration The design procedure follows the flow chart in Fig. 1. The dome helical and cylindrical hoop thickness were determined from the internal pressure whereas the cylindrical helical thickness was chosen from the external loads so as to avoid buckling. Other desirable features involve high bending stiffness and low circumferential expansion. Specific limits, however, were not set. Other factors that can influence the performance of a vessel will be discussed subsequently.
3.1. Analytical approach As the filament-wound composites behave the same way as that of the angle-ply laminates, laminate theories may be used. The macromechanics approach assumes that each lamina of the composite is made of an anisotropic and homogeneous monolayer. Such a layer consists of fila-
Case Configuration
Tension Compression
Loading Analysis
Buckling,etc.
MatmialSelection
Design
L
r
Bending Test
I I
AnalyticalMethods WindingPattm'n Dome Contour Joint,etc. Mandrel Liner
Subscal¢ Vessels HydroburstTest Compressive Test,
R~inforccmem Matrix
~-~
Winding Proc.ess,etc.
NDT
etC.
Proof Tests
L
r
Machining Assembling
Product
] I
Fig. 1. Flow chart of research concept.
MEOP
G.S. Ger et a L / High performance compositepressure vessels
ments oriented at a plus and minus angle. It can also be unidirectional. The mechanical properties E l l , E 2 2 , 1£12, /£21 ( = g12E22/Ell) and G12 are determined experimentally while the analysis is carried out on a layer by layer basis. As estimate of the fiber stresses in the internally pressurized cyhndrical vessel is made by neglecting the influence of the resin. The fibers carry only tensile loads. Such an approach is known as netting analysis. The NASTRAN computer code is applied in conjunction with the criterion of "Maximum Stress" for determining the integrity of the vessel. 3.2. Winding pattern A series of stress analyses were performed to determine the fiber orientation, winding (lay-up) sequence, interspersement, stress ratio, and the wall thickness of the pressure vessel. Based on this information, the winding patterns are made systematically over the mandrel. As the buckling strength depends sensitively on the helical thickness, additional windings were added for safety measures. Since these layers were not required in the domes, they were extended through the skirts as shown in Fig. 2. With this design, the extra helicals serve as a foundation for the skirts and
159
yield a stiffer, stronger case-to-skirt joint than a conventional skirt which were composed of local cloth layers extending only a short distance into the cylinder. 3.3. Dome contour For high performance vessels, integral heads are essential, and the dome contours and related polar bosses are critical in vessel design. An important consideration of dome design is to increase volume without increasing dome build-up. Hoop stresses are to be kept within acceptable limits. A balanced Geodesic-Isotensoid (GI) contour design is adopted such that the fiber path is made tangent to the polar boss [10], Fig. 3. Each point on the path is defined by its meridional and circumferential radii, r I and r2, respectively. These radii are related to the X-, Y-coordinates as follows:
[1 + (r' 213J2 y.
r1 = _
r2=-
,
X[1 + (y,)2] 1/2 y, ,
(2)
where Y' and Y" are, respectively, the first and
[ 2ct/SAE 1086 Metal shim/S-glass fabric/2ho ] - - -FWD l
79.48 51.18
AFT .
16.14 - -
0.19 TYP.-
Y Joint
Liner [ 2ho/2Cq2ho/4OV2ho ]
Z
[ 2c~/Wafer/2et] Metal Fitmg
1 6.1I
J ~3 DIA
Metal Fitmg
F
19.69 DIA.
!
....2'. I /
Fig. 2. The configuration of studied vessel.
160
G.S. Ger et al. / High performance composite pressure vessels Y
FILAMENT PATH
DIRECTION POINT
..
(x,y)
) DIRECTION
M E R / D I A N T H R O U G H POINT
p
Fig. 3. Geometryfor geodesicpath. second derivatives of Y with respect to X. The ratio r1
--
r2
=
XY"
y'[l+(y')
2]
= 2 - 2 tan2a
(3)
guarantees a balanced stress state with a being the winding angle. The geodesic path is described by X sin a = constant = X0 ,
(4)
where X0 is the boss diameter. Equations (3) and (4) may yield the coordinates of Geodesic-Isotensoid contour. Additional strengthening of the dome is necessary as failure during hydrostatic tests will occur if reinforcements are not added. Local wafer reinforcements in the dome build-up area have been employed. An optimum design stress ratio may also be invoked to yield the highest performance efficiency per pound of material used in the vessel.
integrally wounded vessels. Fiber damage during part removal, dimensional tolerances and residual stresses are factors that require attention. In particular, the mandrel must resist sagging due to its weight and applied winding tension. Sufficient strength must also be retained when curing takes place at elevated temperatures. Utilizing a sandPVA mandrel for both liner and case fabrication, tooling requirements on many filament wound casings can be reduced, particularly when the case size is small enough to allow the use of a sand mandrel and when only a small number of units are to be built. A full-scale sand-PVA pressure vessel mandrel is shown in Fig. 4. 4.2. L i n e r
The liner was stacked by laying up NBR and V-44 rubber on the sand mandrel to an over-
4. Fabrication
Product quality and performance are affected by the fabrication procedures and processes. Of particular significance are the mandrel, liner and casing. ~1. Man~el
Special care should be given to the design of the mandrel and selection of material [11] for
Fig. 4. A full-scalesand-PVA mandrel.
G.S. Ger et al. / High performance compositepressure vessels
stocked thickness, and then bagged and cured in autoclave at 150 psi and 300°F for five hours. The exterior surface was then machined to the required external contour. Prior to winding the composite vessel, an epoxy compatible primer was applied over the entire surface of the liner in order to ensure a good liner-to-case bond.
161
2. The completed case was then bagged, and the cure process was executed in the autoclave. After machining to the desired diameter, the sand mandrel was washed out by using hot water. A finished case is shown in Fig. 5.
5. Testing 4.3. Case fabrication 5.1. Nondestructive testing
A wet filament wound technology with a target of 0.62 fiber volume content was utilized. Resin content was controlled by employing an impregnating drum. The McClean-Anderson numericalcontrolled winding machine with a maximum capacity of 6 meters in length and 1.5 meters in diameter was used to execute the case winding. Both the hoops and helicals were wound at a 12 tow/in./ply end density. After winding the first two hoop plies in the cylindrical section, the first two helicals were wound from dome to dome as shown in Fig. 2. Then the second two hoop plies were interspersed between the helicals in the cylindrical section. A graphite cloth wafer was installed between the first and second helical on each dome to prevent possible boss blowout failure. After winding the second helical, the skirt mandrels were installed and the third helical was wound from skirt to skirt. A Y-joint rubber was installed locally in the skirt joint region prior to winding the third helical so that the stress concentration can be alleviated and the joint strength can be improved. The final two hoop plies were then wound over the third helical with SAE 1086 metal shim and S-glass fabric locally interspersed in the skirt regions. The stacking sequence for the cylindrical dome and skirt parts are also shown in Fig.
Fig. 5. Prooftest set-up.
There are numerous nondestructive testing (NDT) techniques to detect defects in composites. These imperfections may include voids, resin crazing (matrix failure), fiber debonding, cracks, delamination, fiber breaking, and composite fracture. Although the NDT techniques are well developed for metals, the application of NDT techniques to composites is still comparatively limited. In this study, the vessels were examined by the X-ray method. The film records were compared to the well-established accept/reject criterion for each defect. 5.2. Proof test
Hydrostatic proof tests were made to each vessel. Figure 5 shows the proof test set-up. The supply water through the hydraulic lines is pressurized by an air compressor. A solenoid valve switches the water input between line pressure (MEOP) and the high pressure water supply. A waste-water valve connected to the other end of the specimen is used to purge trapped air. 5.3. Subscale hydroburst test
Design and fabrication of large filament wound pressure vessels often necessitate the testing of subscaled vessels for evaluating materials, design concepts, and manufacturing techniques [12-14]. Configuration alternation including changes of materials, processes, and design features, can be evaluated using data gathered from the subscale vessels. By varying the diameter, cylinder length and dome contour, the hydroburst tests for subscale pressure vessels were conducted. Figure 6 shows the different dimensions of various types of subscale vessels. The results of hydroburst tests are summarized in Table 2. The failure modes of
G.S. Ger et al. / High performance composite pressure vessels
162
Table 3 Comparison of test results for metal and carbon fiber vessel Metal SAE 4130 Weight (case only) (kg) Theoretical max. pressure (psi) Max. effective operation pressure (MEOP) (psi) Safety factor Burst pressure (psi) Pressure/weight
Carbon Fiber ST-III-6000
134
98.4
1880
3120
1564 1.2 1906 14.2
1564 2.0 3052 c 31.0
Fig. 6. The subscale vessels. c Average data from these tests.
cylinder, hoop, dome curve, and boss blowout which can be predicted by using" N A S T R A N computer code were presented in the hydroburst test. One of the dome curve failure is shown in Fig. 7. The hydroburst te_sts were also conducted of the full-scale pressure vessel to confirm the final design with desired requirements. Refer to a comparison of the test data for the carbon fiber vessel with those for the metal. Note that the ratio of 1.66 for the theoretically predicted maximum pressure for the carbon fiber vessel and metal vessel agreed closely with the corresponding ratio of 1.60 measured experimentally, Table 3. The bending test in Fig. 8, the compressive tests, and the static firing tests in Fig. 9 were
Fig. 7. The failure of a studied vessel.
Table 2 Subscale hydroburst test results N u m b e r of represented vessels
1
2
3
4
Reinforcement
S-glass 500 yields a a-21 b ho a-3/ a-21 ho-41
E-glass 250 yields a-41 ho-21
0.0096 0.0069 0.258 4950 4250 boss blowout failure
0.011 0.0095 0.341 2800 3000 Cylindrical failure
Carbon ST-III-6000 ho-21 a-21 ho-21 a-2 ho-2 0.0057 0.0044 0.603 2500 2750 D o m e curve failure
Carbon ST-III-6000 ho-2 l a-21 ho-21 a-41 ho-21 0.0053 0.0042 0.603 2500 2830 Cylindrical and hoop failure
Winding sequence
Helical winding thickness (in/layer) Hoop winding thickness (in/layer) Db / Dc Design pressure (psi) Burst pressure (psi) Failure mode
act = hefical winding; ho = hoop winding b 1 = layer
G.S. Ger et al. / High performance composite pressure vessels
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Acknowledgments The authors would like to thank their colleagues who assisted in manufacturing and testing of the vessels.
References Fig. 8. Bending test of a vessel.
Fig. 9. Static firing test of a vessel.
conducted to monitor the performance of the pressure vessels.
6. Conclusions Fiber-reinforced composite pressure vessels used for rocket motors and other applications offer weight savings and enhance performance. Recent advances in fiber manufacturing techniques provided additional improvement in strength and modulus of the reinforced fibers. Optimization of pressure vessel performance involves the combined consideration of material selection, design, analysis, processing and test. The present study describes the filament winding techniques used in Taiwan, the Republic of China, for manufacturing composite pressure vessels. Additional improvements are anticipated as progress is being made in design and manufacturing techniques.
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