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A thin-wall, low-z pressure vessel
Nuclear Instruments and Methods in Physics Research A284 (1989) 405-408 North-Holland, Amsterdam
405
A THIN-WALL, LOW-Z PRESSURE VESSEL David, A. JENKINS and Charles L. CHANDLER
Virginia Polytechnic Istetute and State Unmersety, Blacksburg, Virginia 24061, USA
Received 10 May 1989 A thin-wall vessel has been constructed for use as a container for a pressurized gas target in a nuclear experiment . The objective of the design of the container was to combine high strength with low energy loss for the passage of highly ionizing particles. Target containers, constructed from a graphite-fiber reinforced epoxy composite, had a wall thickness of 20 mg/cm2 and yielded at an average pressure of 3400 kPa. 1. Introduction Experiments with gas targets which detect low energy particles require a thin-wall, low-Z target container m order to minimize the energy loss of particles in the target wall . The vessel described here was developed as part of a program to examine possible configurations for a target in an experiment which studies photodisintegration of light nuclei . The photdisintegration target is a gas surrounded by a system of wire chambers and scintillator counters which measure the direction and energy of the reaction products . The configuration of an experiment at the University of Illinois is shown in fig. 1 . Photons, passing along the axis of the target container, can react with a nucleus to produce charged particles that are detected by three layers of wire chambers and a scintillator samdwich . In order to register as an event, a particle produced in photodisintegration must pass through the target gas, the wall of the container and the wire chamber volume into the scintillators . A count in the scintil-
Proton
Fig. 1 . Target-detector layout for photodisintegration expert ment . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
Table 1 Properties of materials Material Polyetherimide residen a~ Kapton b) Aluminum `) HAVAR d) Graphite epoxy composite et
Yield strength [N/mm2 ] 105 (7% strain) 69 (3% yield) 462 2067 (0 .2% offset) 1380-2760
Density [gm/cm' ] 1.2 1.42 2.7 8.3 -1 .5
Zav
-6 -6 13 - 27 -6
Marketed as Danar 1000 from Dixon Industries, Division of Bunnell Plastics . b) Dupont. `) 7075 T6 Alloy . d) Hamilton Precision Metals, age hardened . e) Hexcel Corporation, a)
lators then triggers a readout of the wire chambers from which the particle trajectory can be derived. The event rate can be increased by raising the target pressure . But then, since a higher pressure will require thicker walls to strengthen the target container, escaping particles will lose more energy. Materials must be used for the walls combine high strength with low energy loss for passing particles. The properties of materials which have been used for this purpose are listed in table 1 . Of special interest is the use of a graphite composite material which combines high strength with low atomic number . We report here the construction of a pressure vessel made from a laminate of a graphite-fiber reinforced composite material . There are many examples of thin-walled target-containers for nuclear experiments . The construction of a thin-walled, high-pressure gas target has been described by Brotschi, Menetrey and Watson [1]. They report that
406
AA Jenkins, C L. Chandler / A thin-wall, low-Z pressure oessel
their cylinder, 2 cm to diameter and constructed from 8 lim thick Kapton, held a pressure of 600 kPa. Vacuum vessels have been constructed from graphite composites at Fermilab, [2] a thin-wall pressure vessel made with a graphite composite is being developed for the SALAD detector at the University of Alberta [3]. 2. Graphite epoxy composite The graphite composite material used to form the cylinder was composed or high strength carbon fibers of about 7-8 li m in diameter imbedded in an epoxy resin. The material is available commercially to a sheet form called prepreg which is flexible and can be formed to the desired shape before being heat-cured to obtain the final product. To form a cylinder, the material is wrapped on an aluminum mandre . The prepreg, on the mandrel, is then cured to form a cylinder with an inner diameter equal to the diameter of the mandrel. The strength of the graphite composite is greatest along the direction of the carbon fiber . The strength perpendicular to the fiber depends on the tensile strength of the epoxy matrix which is much less than the strength of the carbon fiber. A more uniform strength in two direction can be fabricated by making a laminate of two sheets of prepreg with the graphite fibers oriented at a relative angle so as to maximize the strength of the material in perpendicular directions . To minimize the wall thickness, we have worked with one sheet of prepreg using the minimum thickness that is commercially available. The orientation of the composite material is such that the graphite fibers wrap circumferentially around the cylinder, as shown in fig. 2, to strengthen the tube in the hoop direction. The stress along the axis of the cylinder is minimized by constraining the ends of the cylinder. The cylinder is 14 .6 cm long and 6.3 cm in diameter . The wall consists of a 76 lt m thick layer of prepreg and an inner layer of 13 lim polyetherimide film . The inner liner of polyetherimide provides a continuous unbroken surface that prevents the gas or liquid in the cylinder
75 p m
~--- 14 6 cm
Fig. 2. Graphite fibers
ni
graphite fiber
~j
cylindrical configuration
from escaping through manufacturing defects in the graphite composite. 3. Fabrication The polyetherimide film was measured, cut and wrapped around the aluminum mandrel overlapping itself circumferentially by 0.5 cm . A small amount of epoxy resin (Epon 828 with curing agent MPDA) was applied along the overlapping seam . The aluminum mandrel was coated with release agent (Miller Stephenson) to prevent resin overrun which would cause the cylinder to adhere to the aluminum mandrel. The layer of graphite-epoxy prepreg was then placed over the polyetherimide film and wrapped around the mandres with the reinforcing graphite fibers running in the hoop direction. A slightly larger overlapping seam of approimately 1.3 em was necessary to ensure strength of the tube, as the shear strenth of the epoxy resin is only 1/30 that of the tensile strength of the graphite-epoxy composite. The seam of the graphite-epoxy does not overlap the seam of the polyetherimide film . Since the graphite epoxy prepreg is tacky, once contact is made with the polyetherimide film the structure will remain in place. Care was taken at each step to make certain that there were no wrinkles in any of the layers of the cylinder . The ends of the cylinder are then reinforced by adding a 1 .9 cm wide layer of prepreg with the fibers running in the axial direction and extending inward from the end of the cylinder . A second layer of 1 cm wide prepreg, with the fibers parallel to the original layer (in the hoop direction), was added at the ends of the cylinder, and overlapping 2.5 em with the overlap at a different position from the overlap of the first layer. The cylinder was then ready to be cured. Three to four layers of highly porous teflon coated fiberglass fabric, each about 76 lim thick, was placed around the cylinder. A nylon vacuum bag was then placed over the glass fabric. The layers of fiberglass transmit vacuum and air, bleed off a small amount of resin, and prevent the resin from soaking through to the nylon vacuum bag causing vacuum bag failure. The teflon coating on the fabric ensures that the glass fabric will release from the cylinder after the cure . The vacuum bag was made air tight, a vacuum hose was connected, vacuum was applied and the assembly was placed in an autoclave The temperature was slowly increased to 115 ° C. At this point the epoxy resin has a very low viscosity and trapped air is squeezed out of the structure. After 45 min the pressure around the bag was increased from atmospheric pressure to 585 kPa to ensure that the individual graphite fibers and the seams are pressed tightly together. Also, some of the remaining trapped air is squeezed out. After 15 min the tem-
D.A .
Jenkins, C.L. Chandler / A thin-wall, low-Z pressure vessel
407
4. Test results Pressure tests were conducted by filling the cylinder with water and then applying pressure using a hydrostatic test pump (Richard Dudgeon Model 4A). Nine samples were teste; failure pressures of the samples varied from 2400 to 4000 kPa with an average of 3400 kPa. The yield strength for the cylinder can be deduced from the relation for the circumferential tensile stress of a longitudinal section in a cylindrical shell : a = pR/t
Fig. 3.
Apparatus for pressure test of cylinder.
perature was increased slowly to 177 ° C. After setting at 177 ° C for 2 h , the epoxy resin has gelled, hardened and formed cross links. The polyetherimide film has also become bonded to the graphite epoxy composite. The structure was then complete, the temperature and pressure were lowered, and the assembly was removed from the autoclave. The structure became hard at 177'C when the aluminum mandrel was several thousandths of an inch larger than at room temperature because fo thermal expansion. Thus a room temperature the cylinder, having shrunk only negligibly in the circumferential direction, easily slides off the aluminum mandrel. The cylinder was tested by placing it between two end plates to form a leak-proof container in which water could be injected under pressure . The test cylinder is shown in fig. 3. The aluminum end plates were held in place by four 8 mm diameter bolts; sleeves on the bolts prevented the end plates from crushing the cylinder when the nuts were tightened on the bolts. Grooves 3 mm deep and 0.5 mm wide in the end plates held the cylinder in position while epoxy was applied to form a leak-proof point. Thus the end plates served two functions; they formed a leak-proof container for pressure testing and, with the bolts, constrained the cylinder to minimize the stress along the axis of the cylinder . After curing in the autoclave, the cylinder was trimmed to fit in the test fixture. The recessed groove of one end cap was filled with a low viscosity high grade epoxy resin (Epon 828, Curing Agent U) and the test fixture was assembled. After resin hardens, the fixture was taken apart and the other groove was filled with resin, the fixture was reassembled and the resin was allowed to harden . The cylinder was then ready for testing.
where p is the pressure on the inner surface of the cylinder, t is the thickness of the shell and R is the inner radius of the cylinder . Letting p = 3400 kPa, R = 3.2 cm and t = 89 p for the test cylinder, a yield strength of 1240 N/mm z is deduced . This yield strength is somewhat lower than the data for graphite epoxy composite in table 1 which lists the yield strenth in the fiber direction. The graphite epoxy used in constructing the cylinder had a yield strength of 1380 N/mmz. A graphite epoxy with yield strength of 2760 N/mm z , which is available commercially, would allow operation of the cylinder at a higher pressure . In order to compare the graphite composite with other materials, the thickness of material required for a cylinder of the same geometry which would hold 3400 kPa was calculated for each of the materials listed in table 1 . Then, for the sake of comparison, both the energy of a proton which could pass perpendicularly through the container wall and the multiple scattering of a 15 MeV proton in the wall were calculated . The results are presented in table 2. The graphite laminate is superior to the other organic materials, polyethermimide and Kapton in this comparison. While the energy loss in the graphite laminate is about the same as in aluminum and HAVAR, multi-
Table 2 Proton range and multiple scattering . The thickness t of different materials which can form a cylinder to hold 3400 kPa are compared . The minimum energy of a proton which will pass through thickness t and the multiple scattering of a 15MeV proton in thickness t are tabulated. Material
Minimum energy Multiple scattering [mg/cm z ] [MeV] [mr ad] Polyetherimide 125 10 26 Kapton 220 14 35 Aluminum 64 6 24 HAVAR 44 5 26 Graphite laminate 20 4 10 t = pR /a
408
D A . Jenkins, C. L. Chandler / A thin-wall, low-Z pressure vessel
ple scattering is much less because of the lower Z of the laminate .
and the Virginia Tech Center for Composite Materials and Structures .
Acknowledgements
References
We thank Scott Cain, John Fernando and Jay Oustrich for their assistance to this project and the Hexcel Corporation for supplying the graphite laminate . Work was supported in part by NSF grant PHY-8520738
[1] U. Brotschi, A. Menetrey and R.H . Watson, Nucl. Instr and Meth. 129 (1975) 39 . [2] R. Kephart, private communication . [3] 1 Soukup, private communication .
405
A THIN-WALL, LOW-Z PRESSURE VESSEL David, A. JENKINS and Charles L. CHANDLER
Virginia Polytechnic Istetute and State Unmersety, Blacksburg, Virginia 24061, USA
Received 10 May 1989 A thin-wall vessel has been constructed for use as a container for a pressurized gas target in a nuclear experiment . The objective of the design of the container was to combine high strength with low energy loss for the passage of highly ionizing particles. Target containers, constructed from a graphite-fiber reinforced epoxy composite, had a wall thickness of 20 mg/cm2 and yielded at an average pressure of 3400 kPa. 1. Introduction Experiments with gas targets which detect low energy particles require a thin-wall, low-Z target container m order to minimize the energy loss of particles in the target wall . The vessel described here was developed as part of a program to examine possible configurations for a target in an experiment which studies photodisintegration of light nuclei . The photdisintegration target is a gas surrounded by a system of wire chambers and scintillator counters which measure the direction and energy of the reaction products . The configuration of an experiment at the University of Illinois is shown in fig. 1 . Photons, passing along the axis of the target container, can react with a nucleus to produce charged particles that are detected by three layers of wire chambers and a scintillator samdwich . In order to register as an event, a particle produced in photodisintegration must pass through the target gas, the wall of the container and the wire chamber volume into the scintillators . A count in the scintil-
Proton
Fig. 1 . Target-detector layout for photodisintegration expert ment . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
Table 1 Properties of materials Material Polyetherimide residen a~ Kapton b) Aluminum `) HAVAR d) Graphite epoxy composite et
Yield strength [N/mm2 ] 105 (7% strain) 69 (3% yield) 462 2067 (0 .2% offset) 1380-2760
Density [gm/cm' ] 1.2 1.42 2.7 8.3 -1 .5
Zav
-6 -6 13 - 27 -6
Marketed as Danar 1000 from Dixon Industries, Division of Bunnell Plastics . b) Dupont. `) 7075 T6 Alloy . d) Hamilton Precision Metals, age hardened . e) Hexcel Corporation, a)
lators then triggers a readout of the wire chambers from which the particle trajectory can be derived. The event rate can be increased by raising the target pressure . But then, since a higher pressure will require thicker walls to strengthen the target container, escaping particles will lose more energy. Materials must be used for the walls combine high strength with low energy loss for passing particles. The properties of materials which have been used for this purpose are listed in table 1 . Of special interest is the use of a graphite composite material which combines high strength with low atomic number . We report here the construction of a pressure vessel made from a laminate of a graphite-fiber reinforced composite material . There are many examples of thin-walled target-containers for nuclear experiments . The construction of a thin-walled, high-pressure gas target has been described by Brotschi, Menetrey and Watson [1]. They report that
406
AA Jenkins, C L. Chandler / A thin-wall, low-Z pressure oessel
their cylinder, 2 cm to diameter and constructed from 8 lim thick Kapton, held a pressure of 600 kPa. Vacuum vessels have been constructed from graphite composites at Fermilab, [2] a thin-wall pressure vessel made with a graphite composite is being developed for the SALAD detector at the University of Alberta [3]. 2. Graphite epoxy composite The graphite composite material used to form the cylinder was composed or high strength carbon fibers of about 7-8 li m in diameter imbedded in an epoxy resin. The material is available commercially to a sheet form called prepreg which is flexible and can be formed to the desired shape before being heat-cured to obtain the final product. To form a cylinder, the material is wrapped on an aluminum mandre . The prepreg, on the mandrel, is then cured to form a cylinder with an inner diameter equal to the diameter of the mandrel. The strength of the graphite composite is greatest along the direction of the carbon fiber . The strength perpendicular to the fiber depends on the tensile strength of the epoxy matrix which is much less than the strength of the carbon fiber. A more uniform strength in two direction can be fabricated by making a laminate of two sheets of prepreg with the graphite fibers oriented at a relative angle so as to maximize the strength of the material in perpendicular directions . To minimize the wall thickness, we have worked with one sheet of prepreg using the minimum thickness that is commercially available. The orientation of the composite material is such that the graphite fibers wrap circumferentially around the cylinder, as shown in fig. 2, to strengthen the tube in the hoop direction. The stress along the axis of the cylinder is minimized by constraining the ends of the cylinder. The cylinder is 14 .6 cm long and 6.3 cm in diameter . The wall consists of a 76 lt m thick layer of prepreg and an inner layer of 13 lim polyetherimide film . The inner liner of polyetherimide provides a continuous unbroken surface that prevents the gas or liquid in the cylinder
75 p m
~--- 14 6 cm
Fig. 2. Graphite fibers
ni
graphite fiber
~j
cylindrical configuration
from escaping through manufacturing defects in the graphite composite. 3. Fabrication The polyetherimide film was measured, cut and wrapped around the aluminum mandrel overlapping itself circumferentially by 0.5 cm . A small amount of epoxy resin (Epon 828 with curing agent MPDA) was applied along the overlapping seam . The aluminum mandrel was coated with release agent (Miller Stephenson) to prevent resin overrun which would cause the cylinder to adhere to the aluminum mandrel. The layer of graphite-epoxy prepreg was then placed over the polyetherimide film and wrapped around the mandres with the reinforcing graphite fibers running in the hoop direction. A slightly larger overlapping seam of approimately 1.3 em was necessary to ensure strength of the tube, as the shear strenth of the epoxy resin is only 1/30 that of the tensile strength of the graphite-epoxy composite. The seam of the graphite-epoxy does not overlap the seam of the polyetherimide film . Since the graphite epoxy prepreg is tacky, once contact is made with the polyetherimide film the structure will remain in place. Care was taken at each step to make certain that there were no wrinkles in any of the layers of the cylinder . The ends of the cylinder are then reinforced by adding a 1 .9 cm wide layer of prepreg with the fibers running in the axial direction and extending inward from the end of the cylinder . A second layer of 1 cm wide prepreg, with the fibers parallel to the original layer (in the hoop direction), was added at the ends of the cylinder, and overlapping 2.5 em with the overlap at a different position from the overlap of the first layer. The cylinder was then ready to be cured. Three to four layers of highly porous teflon coated fiberglass fabric, each about 76 lim thick, was placed around the cylinder. A nylon vacuum bag was then placed over the glass fabric. The layers of fiberglass transmit vacuum and air, bleed off a small amount of resin, and prevent the resin from soaking through to the nylon vacuum bag causing vacuum bag failure. The teflon coating on the fabric ensures that the glass fabric will release from the cylinder after the cure . The vacuum bag was made air tight, a vacuum hose was connected, vacuum was applied and the assembly was placed in an autoclave The temperature was slowly increased to 115 ° C. At this point the epoxy resin has a very low viscosity and trapped air is squeezed out of the structure. After 45 min the pressure around the bag was increased from atmospheric pressure to 585 kPa to ensure that the individual graphite fibers and the seams are pressed tightly together. Also, some of the remaining trapped air is squeezed out. After 15 min the tem-
D.A .
Jenkins, C.L. Chandler / A thin-wall, low-Z pressure vessel
407
4. Test results Pressure tests were conducted by filling the cylinder with water and then applying pressure using a hydrostatic test pump (Richard Dudgeon Model 4A). Nine samples were teste; failure pressures of the samples varied from 2400 to 4000 kPa with an average of 3400 kPa. The yield strength for the cylinder can be deduced from the relation for the circumferential tensile stress of a longitudinal section in a cylindrical shell : a = pR/t
Fig. 3.
Apparatus for pressure test of cylinder.
perature was increased slowly to 177 ° C. After setting at 177 ° C for 2 h , the epoxy resin has gelled, hardened and formed cross links. The polyetherimide film has also become bonded to the graphite epoxy composite. The structure was then complete, the temperature and pressure were lowered, and the assembly was removed from the autoclave. The structure became hard at 177'C when the aluminum mandrel was several thousandths of an inch larger than at room temperature because fo thermal expansion. Thus a room temperature the cylinder, having shrunk only negligibly in the circumferential direction, easily slides off the aluminum mandrel. The cylinder was tested by placing it between two end plates to form a leak-proof container in which water could be injected under pressure . The test cylinder is shown in fig. 3. The aluminum end plates were held in place by four 8 mm diameter bolts; sleeves on the bolts prevented the end plates from crushing the cylinder when the nuts were tightened on the bolts. Grooves 3 mm deep and 0.5 mm wide in the end plates held the cylinder in position while epoxy was applied to form a leak-proof point. Thus the end plates served two functions; they formed a leak-proof container for pressure testing and, with the bolts, constrained the cylinder to minimize the stress along the axis of the cylinder . After curing in the autoclave, the cylinder was trimmed to fit in the test fixture. The recessed groove of one end cap was filled with a low viscosity high grade epoxy resin (Epon 828, Curing Agent U) and the test fixture was assembled. After resin hardens, the fixture was taken apart and the other groove was filled with resin, the fixture was reassembled and the resin was allowed to harden . The cylinder was then ready for testing.
where p is the pressure on the inner surface of the cylinder, t is the thickness of the shell and R is the inner radius of the cylinder . Letting p = 3400 kPa, R = 3.2 cm and t = 89 p for the test cylinder, a yield strength of 1240 N/mm z is deduced . This yield strength is somewhat lower than the data for graphite epoxy composite in table 1 which lists the yield strenth in the fiber direction. The graphite epoxy used in constructing the cylinder had a yield strength of 1380 N/mmz. A graphite epoxy with yield strength of 2760 N/mm z , which is available commercially, would allow operation of the cylinder at a higher pressure . In order to compare the graphite composite with other materials, the thickness of material required for a cylinder of the same geometry which would hold 3400 kPa was calculated for each of the materials listed in table 1 . Then, for the sake of comparison, both the energy of a proton which could pass perpendicularly through the container wall and the multiple scattering of a 15 MeV proton in the wall were calculated . The results are presented in table 2. The graphite laminate is superior to the other organic materials, polyethermimide and Kapton in this comparison. While the energy loss in the graphite laminate is about the same as in aluminum and HAVAR, multi-
Table 2 Proton range and multiple scattering . The thickness t of different materials which can form a cylinder to hold 3400 kPa are compared . The minimum energy of a proton which will pass through thickness t and the multiple scattering of a 15MeV proton in thickness t are tabulated. Material
Minimum energy Multiple scattering [mg/cm z ] [MeV] [mr ad] Polyetherimide 125 10 26 Kapton 220 14 35 Aluminum 64 6 24 HAVAR 44 5 26 Graphite laminate 20 4 10 t = pR /a
408
D A . Jenkins, C. L. Chandler / A thin-wall, low-Z pressure vessel
ple scattering is much less because of the lower Z of the laminate .
and the Virginia Tech Center for Composite Materials and Structures .
Acknowledgements
References
We thank Scott Cain, John Fernando and Jay Oustrich for their assistance to this project and the Hexcel Corporation for supplying the graphite laminate . Work was supported in part by NSF grant PHY-8520738
[1] U. Brotschi, A. Menetrey and R.H . Watson, Nucl. Instr and Meth. 129 (1975) 39 . [2] R. Kephart, private communication . [3] 1 Soukup, private communication .