Abstract:
The invention relates to a process for the manufacture of reinforced-reinforced pressure vessels and the product produced therefrom. A ductile metal vessel liner, being formed with at least one nozzle, is wrapped with a first composite layer of resin-impregnated fibers. Pressure is applied to the vessel sufficient to yield the liner but is insufficient to cause deformation of the nozzle. The liner retains a residual compressive stress critical for withstanding cyclical service pressures. A second composite layer is applied over the first layer to increase the strength of the vessel sufficient to maintain the vessel&#39;s integrity at very high cyclical service pressures (such as about 0-10,000 psig). The vessel is then pressure tested at 1.5 times the intended service pressure to ensure vessel integrity.

Description:
FIELD OF THE INVENTION  
         [0001]    A process for the manufacture of resin-impregnated fiber-reinforced structural composite vessels and the product resulting therefrom. More particularly, pressure vessels are wrapped in stages with a resin-fibers matrix and are subjected to autofrettage pre-stressing technique to increase the vessel&#39;s cycle life and burst strength.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is known to wrap a variety of underlying shapes with fibers embedded in a matrix of resin so as to form reinforced-reinforced plastic composite products, or FRP. The fiber acts as the structural portion wrapped over a normally weak shape such as a liner for a pressure vessel.  
           [0003]    One such example is the manufacture of reinforced-reinforced pressure vessels by wrapping hollow, substantially non-structural pressure vessel liners with fibers resulting in a vessel having burst pressure and cyclical loading capabilities beyond that of the liner alone. Fibers wrapped about the vessel liner act in tension when the vessel is pressurized.  
           [0004]    Conventionally, a multiplicity of fibers spooled into tows are passed through heated resin baths containing catalyzed resins prior to being mechanically wrapped onto the vessel liner. The configuration of the winding is dependant upon the speed of rotation of the vessel liner and the rate of travel of the tow-dispensing apparatus. The most common configurations are helical, in which the tows are at a significant angle from the axis of the object being wrapped; circumferential, in which the tows are wound hoop-wise around the object; and polar, in which the tows are wrapped in the direction of the longitudinal axis of the object.  
           [0005]    The resin is permitted to dry and is then cured. Curing relates to the process by which the resin is allowed to achieve its final chemical state and effect its purpose to provide reinforcement to the liner. Curing or chemical poly-condensation, is the formation of polymers from monomers with the release of water or another simple substance. Curing is usually performed at elevated temperatures however, room temperature may be sufficient for some types of resins.  
           [0006]    In some cases the resulting vessel is pressed into service after curing.  
           [0007]    In other known processes, the liner is pre-stressed using a technique called autofrettage wherein the liner is plastically deformed (yielded) within the fiber-wrap for imparting a compressive residual stress into the liner and thereby increasing the vessel&#39;s structural characteristics under pressure.  
           [0008]    In more detail, a liner forms a fluid nozzle structure for providing access to the liner&#39;s bore. In the autofrettage process, the cured fiber-wrapped liner is placed inside a protective housing and liquid is injected through the fluid nozzle structure, over-pressurizing the liner within the wrap and causing it to yield. When the fluid pressure is released, the tension in the liner diminishes and then becomes compressive at ambient pressure (pre-stressed). Accordingly, as the pressure rises once again, in service, the stress on the liner progressively reduces from compressive, through zero, then finally becomes tensile at a peak stress less than that previously experienced at the same pressure. Cyclical tensile stress is a major factor in precipitating fatigue stress failures and thus, with an initial compressive stress, each pressure cycle results in a lower maximum tensile stress in the liner and thus improves its fatigue strength.  
           [0009]    It is known to apply autofrettage pressures of 6000 psig to pre-stress aluminum-lined, fiber-wrapped tanks which are safely operable at service pressures of about 3000 psig.  
           [0010]    However, increased use of alternative fuels to fuel vehicles, such as compressed natural gas and hydrogen, and the requirement for ever greater fuel range, has created a need for lightweight, safe tanks with even greater capacity. One method for avoiding increasing tank size and weight, new tanks must be capable of containing fuel at higher service pressures, upwards of 10,000 psig. It is possible to provide reinforced-reinforced tanks capable of such pressures but without pre-stressing, the cyclical life is too short to be of commercial value. Accordingly a pre-stressing technique is suggested.  
           [0011]    Using the prior art process, autofrettage pressures in the range of about 20,000 to 25,000 psig would be required in order to adequately yield an aluminum liner to produce a vessel capable of safely maintaining integrity at a cyclic service pressure of 10,000 psig. Unfortunately, at these high pressures the boss forming the nozzle and the internal threads used to attach various fittings to the tank liner, also yield. As the nozzle structure yields, its dimensions no longer accept conventionally sized valve bodies and fittings. The dimensional changes in this portion of the tank are not predictable, require close fitting tolerances and therefore one is unable to compensate for such deformation when originally sizing the nozzle structure. The deformation of the nozzle structure is related to the surface area of the nozzle, as provided by the nozzle&#39;s port size.  
           [0012]    One approach to minimize deformation and enable high autofrettage pressures would be to make the nozzle opening much smaller, thereby reducing the service area and lowering the stress. From a practical standpoint however, this does not solve the problem as the smaller openings would no longer be able to accept current valve bodies. Note that more and more components are integrated into the valve bodies, and therefore must be of sufficient size to incorporate components such as solenoid-controlled valves, manual override valves, pressure regulators and temperature control devices.  
           [0013]    Further, attempts to thicken the walls or fiber-wrap the boss have only provided marginal increases in strength of the nozzle structure, insufficient to eliminate yield.  
           [0014]    U.S. Pat. No. 4,571,969 to Tomita discloses an alternative to single high pressure autofrettage for steel liners operating at service pressures of about 100-200 ksig. Tomita suggests that application of a single pressure is generally disadvantageous in that it does not always produce the required residual tangential compressive stresses in alloy steel cylinder bores. Further, Tomita states that high pressure autofrettage deforms the bore of the cylinder so that it is no longer employable in its intended use, where bore size must be maintained within narrowly prescribed ranges. Tomita teaches a cyclical autofrettage technique applicable to alloy steel cylinders in which the vessel is subjected to multiple lower pressure cycles in order to avoid large diameter dilations and the associated tolerance problems. The rate of production would diminish and costs would escalate if each cylinder were to be cyclically pressurized until a suitable strength was achieved. Further, aluminum liners have a lower yield stress than alloy steel and therefore do not require the extreme pressures suggested by Tomita.  
           [0015]    Thus, there remains a need for a process by which a reinforced-reinforced pressure vessel can be produced, capable of maintaining its integrity at service pressures upwards of 10,000 psig, with a nozzle structure port and threads which can be reliably sized and retain their dimensions necessary to accept a variety of valve bodies and fittings. Ideally, the process should be accomplished with a minimum number of steps for efficient rates of production.  
         SUMMARY OF THE INVENTION  
         [0016]    The current invention addresses the unsuitability of the existing single cycle autofrettage processes for manufacturing very high pressure reinforced-reinforced vessels for storing fuel gas. High pressure autofrettage is associated with disadvantages including imposing unpredictable and significant deformation of the bore of the vessels threaded nozzles. The novel process also uses a single pressure cycle, but it achieves the objectives of forming a residual compressive stress in the vessels liner using lower pressures, and standard autofrettage equipment by implementing a unique two stage resin-impregnated fiber wrapping process to produce pressure vessels capable of reliably withstanding high service pressures in the range of 10,000 psig.  
           [0017]    In a broad aspect of the invention, then a process for manufacturing a reinforced-reinforced high pressure vessel comprises the steps of:  
           [0018]    wrapping the liner with a first composite layer of predetermined strength;  
           [0019]    applying a predetermined first pressure to the bore, the first pressure being greater than the design pressure and sufficient to yield the liner within the first composite layer and produce residual compressive stress therein; and  
           [0020]    wrapping the liner with a second composite layer so that when the design pressure is applied, the strain in the liner is constrained so that stress in the liner is below yield.  
           [0021]    Preferably the first pressure and the strength of the first composite layer are complementary. A maximum pressure is set which would not damage the nozzle and further that the strength of the first composite layer is set such that the liner can be plastically deformed at a pressure less than that maximum pressure.  
           [0022]    More preferably, the ductile metal liner is aluminum and the pressure used to autofrettage the liner is in the order of 15,000 psig, resulting in a product pressure vessel which, in combination with the strength provided by the second composite layer, is capable of cyclical use at design pressures of 0-10,000 psig. More preferably, the pressure vessel is tested using cyclical pressure tests at 1.25-1.5 times design pressure in order to meet safety standards.  
           [0023]    Using this novel autofrettage process, the user of the vessel produced thereby can be confident that the vessel has been subjected to at least 1.5 times the service pressure, even with the higher service pressures now sought in the industry. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a cross-sectional view of a cutaway end of a reinforced-reinforced pressure vessel illustrating a threaded nozzle structure fitted with a generic valve assembly;  
         [0025]    [0025]FIG. 2 is a flow chart describing the wrapping, drying and curing process of the prior art which may or may not include autofrettage to produce a conventional reinforced-reinforced vessel;  
         [0026]    [0026]FIG. 3 is a two-section graph illustrating, on the bottom a cross-section of the vessel liner and a nozzle, and on the top the stress profile resulting from autofrettage according to FIG. 2, for conventional pressures (solid line) and at very high pressures (dashed lines) for an appropriately stronger-walled vessel;  
         [0027]    [0027]FIG. 4 is a flow chart describing an embodiment of the present invention wherein the liner is wrapped a first time, cured, pressurized to yield, cured, and wrapped a second time;  
         [0028]    [0028]FIG. 5 a  is a graph illustrating the stress curve for the wall of the liner for autofrettage and demonstrating permanent plastic deformation;  
         [0029]    [0029]FIG. 5 b  is a graph illustrating the elastic, reversible stress curve for the pre-stressed wall of the liner during normal pressure cycling and indicating an acceptably low peak stress;  
         [0030]    [0030]FIG. 6 is a graphical representation of the manufacturing steps over time including staged wrapping of the vessel liner, the pressure cycles and the resulting stress curves for the liner; and  
         [0031]    [0031]FIG. 7 is an exaggerated and fanciful cross-sectional view of a vessel. The circle represents the unstrained liner. Two partial arcs represent the strain or deflection in the liner after the first and second wrapping, demonstrating the change in the strain and thus the reduced stress in the liner after the second wrapping. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]    Having reference to FIG. 1, a portion of a fiber-wrapped vessel liner  10 , is illustrated having a cylindrical body portion  11  (see FIG. 3), hemispherical ends  12  formed thereon and at least one boss  13  formed into a threaded nozzle structure or nozzle  14  protruding axially from a hemispherical end  12 . The nozzle  14  is shown fitted with a generic valve body  15 . A composite layer  16  comprising a matrix of fibers and resin is wrapped about the cylindrical body protion and partially around the hemispherical end  12 .  
         [0033]    It is understood that reference to a “wrap” or “layer” suggests as many layers of fibers and resin as necessary to obtain the desired strength between distinct process steps. Accordingly, a first wrap or composite layer could include thousands of actual wraps of fibers.  
         [0034]    Having reference to FIG. 2, the prior art process is shown in which metal lined fiber-wrapped pressure vessels are produced. The ductile metal liner  1  is first wrapped at  21  with heated resin impregnated fibers. The wrapped liner is allowed to air dry and then is heated in a hot air oven at about 125 degrees Celsius to cause polycondensation or curing of the resin at  21 .  
         [0035]    Optionally, the cured, fiber-wrapped liner may then be subjected to autofrettage at  22 . Autofrettage is the process of pre-stressing the ductile metal within the composite layer so as to leave a residual compressive stress therein, increasing the ultimate burst pressure and the peak tensile stress for improving fatigue strength. Referring to FIG. 5 a , the autofrettage process  22  comprises placing the cured fiber-wrapped liner inside a protective housing and over-pressurizing the liner to impose stresses at  23  and causing it to yield at  24 . When the pressure is released at  25 , the tension in the liner diminishes and then becomes compressive  26  at ambient pressure (pre-stressed) due to the constraint imposed by the overwrap. Accordingly, as shown in FIG. 5 b , on re-pressurization at  27 , the stress on the liner is first progressively reduced from compressive, then passes through zero, then finally becomes tensile. Pressure cycles causing tensile stress are a major factor in precipitating fatigue stress failures. With the initial pre-compressive stress, each pressure cycle (pressurization  27  and depressurization  28 ) results in a lower maximum tensile stress at  29  in the liner and thus improves its fatigue strength.  
         [0036]    This prior art process is performed once and the product vessel is presumed ready for service.  
         [0037]    Having reference to FIG. 3, pressure P, imposed during autofrettage, strain the liner  10  and results in liner stress. The fiber-reinforcement constrains and prevents the liner from reaching the ultimate tensile strength at which the liner would lose its integrity. Due to the shape and design of the walls of the liner  10 , the imposed stresses  30  in the main cylindrical body  11  are greater (above yield) than in the hemispherical ends  12  or the nozzle  14 . Typically, an aluminum lined pressure vessel designed for use at 3000 psig is pressurized at 6000 psig to produce sufficient yielding of the cylindrical portion  11  of the aluminum liner  10 .  
         [0038]    The foregoing constitutes the state of the art for fiber-wrapped liners.  
         [0039]    Continuing to refer to FIG. 3, in order to produce lightweight aluminum lined pressure vessels designed for use at high pressures such as about 10,000 psig, the yield pressure Pa used for autofrettage would have to be in the order of 20,000 or 25,000 psig to achieve sufficient yield in the liner. While the composite layer  16  is appropriately strengthened to prevent the cylindrical body  11  from reaching ultimate tensile strength, design constrains on the area of the nozzle  14  limit similar strengthening. Accordingly, under the prior art techniques, pressures in this range however, imposed stresses  31  at the nozzle  14  result in excessive dilations and permanent deformation in the diameter of the threaded nozzle  14 . Such deformation subsequently interferes with the insertion of valve body fittings  15 , machined by their respective manufacturers to fixed tolerances. Further, the nozzle  14  and valve bodies  15  are typically quite large for accommodating the growing need for more and more components like solenoid operated valves to control the flow of gases out of the pressure vessel, control circuitry to operate the valves, manual override valves and temperature regulation devices.  
         [0040]    The process of the present invention reduces the pressure required for autofrettage by using a unique two stage wrapping process and thus maintains the inside diameter of the threaded nozzle  14  within tolerance limits.  
         [0041]    Having reference to FIGS. 4 and 6, an aluminum liner  10  is first partially wrapped at  40  in a first composite layer  16   a . Simply, a partial wrapping  40  results in a first composite layer  16   a  which provides only enough reinforcement so as to enable the cylindrical liner  10 ,  11  to be yielded at pressures which are less than those which are damaging to the nozzle  14 . The amount of wrapping of the first composite layer  16   a  and an autofrettage or first pressure Pa are predetermined to enable the liner  10 ,  11  to yield at the predetermined pressure but sufficiently reinforce the liner  10 ,  11  to prevent liner failure. For a vessel having service pressures of about 10,000 psig, an autofrettage pressure used Pa (about 15,000 psig) would be lower than that which would be required using the processes of the prior art (about 25,000 psig) to produce a similarly pre-stressed pressure vessel liner. The first composite layer  16   a  is permitted to dry sufficiently to be subjected to autofrettage.  
         [0042]    The partially wrapped liner is pressurized at  20  at the first, autofrettage pressure Pa, causing the liner  10  to yield. The lower pressure ensures that the diameter of the threaded boss is maintained within allowable tolerance limits.  
         [0043]    The liner  10  is then wrapped at  42  with a second composite layer  16   b  of resin and fibers. The subsequent second composite layer  16   b,  or additional layers, provides additional reinforcement to the liner  10  and additional constraint to limit the magnitude of the strain imposed on the liner  10  at testing and design pressures. The amount of wrapping in the second composite layer  16   b  is calculated to increase the strength of the vessel sufficiently to meet the designed service pressure Ps.  
         [0044]    The resin is then cured at  44  according to the known techniques of the prior art.  
         [0045]    The residual stress present in the liner  10  and the overall strength provided by the second or additional and subsequent composite layers  16   b  increase the burst strength of the vessel and minimize the cyclical peak stress. As shown in FIG. 7, the amount of strain ε 2  following the second wrapping is less than the strain ε 1  following the first partial wrapping and autofrettage, assuming a constant pressure P.  
         [0046]    Having reference to FIG. 6, after the autofrettage  42 , the addition  43  of the second composite layer  16   b  and the curing  44 , the resulting product vessel  45  can be subjected to repetitive pressure cycle testing PT 1 , PT 2 , PT 3 , and so on, to ensure that the fiber-wrapped liner meets accepted performance standards such as those set forth by the Canadian Standards Association. Thereafter, the vessel is ready for commercial cyclical service at the intended service pressure.  
         [0047]    In one example of the first embodiment of the invention, an extruded aluminum liner of about 325 mm outside diameter would be partially wrapped in a 4.5 mm thick composite layer of carbon fibers pre-impregnated with epoxy resin and allowed to air dry. Such fibers include carbon or graphite fibers available from Mitsubishi Rayon JP as Grafil 34-700. A suitable epoxy resin is that obtained from Shell, as Epon 826/Epi CURE 9551. Autofrettage could be performed at approximately 15,000 psig, suitable to yield the aluminum liner. The liner would then be wrapped with a second composite layer of a further 4.5 mm in thickness using the same resin-impregnated fibers, followed by curing at 125 degrees Celsius for two hours. The fiber-wrapped liner would thereafter be subjected to a certification testing program. The Canadian Standards Association requires such vessels to be subjected to 11250 cycles at 1.5 times service pressure. For a service pressure of 10,000 psig, the fiber-wrapped liners were tested at 15,000 psig.