FIELD

This invention relates to a vessel for use in the containment of pressurised fluids, and to its method of construction. In embodiments, the fluids are gases. Such vessels may be suitable for storing and transporting fluids, in aerospace applications, for example.

BACKGROUND

Pressure vessels are a known solution to the storage and transport of fluids at high pressures. Such vessels are used to store gaseous fluids such as oxygen, hydrogen, carbon dioxide and nitrogen. Gases such as Liquified petroleum gas (LPG) are known to be used as fuels in the automotive sector, and as such require pressure vessels to store the gas in the vehicle. More recently, hydrogen has gained interest for its potential as a fuel for automotive and aerospace applications. The term ‘fluids’ is used herein to refer to both gases and also liquids, and the pressure vessels of the invention as described herein are also equally suitable for storing pressurised liquids.

The storage of fluids at high pressures is structurally demanding. Typical service pressures are in the order of 30 MPa. When considering hydrogen storage, typical storage pressures increase to 70 MPa when stored at ambient temperatures. By design, the failure pressure of a composite pressure vessel is typically 2 to 3 times the required service pressure. When taking such factors of safety into account, typical failure or “burst” pressure of a composite reinforced hydrogen storage vessel is in the region of 140 MPa to 210 MPa. The pressure vessels as described herein are suitable for storing fluids at these pressures, but it is envisaged that the vessels according to the invention may also be suitable for storing fluids at higher pressures.

In many applications, the weight of a pressure vessel is an important consideration. Heavy weights of vessels are undesirable when considered in the context of transport, due to the associated higher costs and environmental impact.

Metallic pressure vessels are common. They provide a robust, damage-tolerant solution for storing fluids at high pressures. However, metals have a low specific strength (or strength to mass ratio). As a result, the vessels that are produced must be relatively heavy if they are to have a sufficient structural strength. This is undesirable in many industries, but is a particular concern in the aerospace sector, where the weight of the vehicle has a direct impact on its fuel efficiency. Composite materials are a known solution to this problem. There are various composite production methods and materials applicable to manufactures of pressure vessels. Fibres of carbon, glass and aramid are known to be used in filament winding processes in order to produce pressure vessels. Composites, such as carbon fibre reinforced plastics, have a high specific strength. Furthermore, the orientation of the reinforcement, in terms of its strength, can be tailored to make efficient use of the material. This results in significant weight savings when compared to metallic vessels.

The filament winding process involves winding bundles of fibres around a frame, forming a wall comprising multiple layers of bundles. Resin may be applied to the fibres either before, or after, the wall is formed. The resin is subsequently cured. A known problem associated with the filament winding process is that it produces a layered or lamellar arrangement of the composite reinforcement. For instance, when using carbon fibres, the lamellar arrangement of the carbon fibres results in a material that has a low interlaminar strength. As a result, pressure vessels produced by filament winding are particularly prone to damage from impacts. An impact caused by dropping the article or from collision with a foreign object can cause damage to the composite structure.

In a lamellar structure, damage to the matrix region between the layers can result in cracks that propagate between the laminations of carbon fibres. Such interlaminar cracks are not easily identified on visual inspection and can lead to catastrophic failures when such composite structures are in service. As a result, the design of composite pressure vessels is often compromised by the high safety factors that are used to account for potential damage - for example, requiring many additional layers of material to be used as a redundant safety measure.

The present invention seeks to reduce or overcome one or more of the deficiencies associated with the prior art.

It is recognised that the containment of fluids at high pressures is structurally demanding on the vessels that are used. It is also recognised that minimising weight is an important design objective. Composite materials and manufacturing processes can lead to reductions in vessel weight. The composite manufacturing processes that are most applicable to such vessels result in structures that are inherently prone to damage. This is often accounted for in the design of composite vessels by using high factors of safety. This means that, while significant weight savings are achieved, the full potential weight savings are often not realised.

Textile composite manufacturing processes have been used to improve the damage tolerance of composite pressure vessels. Braiding is particularly suited to the manufacture of pressure vessels. The braiding process provides interlacement of the carbon fibre reinforcement; however, this interlacement is limited and overall, the structure remains lamellar by nature. Three-dimensional braiding methods can produce structures that are reinforced through the thickness of the material. However, the process is slow and complex and are therefore difficult to produce products that are commercially viable.

BRIEF DESCRIPTION OF THE INVENTION

According to a first aspect of the invention, we provide a pressure vessel defining a volume for containing a pressurised fluid, the volume being at least partially surrounded by a vessel wall, the vessel wall having: a fibrous wall having an inner surface and an outer surface, the fibrous wall including multiple layers of fibres, each layer comprising multiple bundles of wound fibres, wherein multiple stitches are formed, each stitch being made by a strand that penetrates the outer surface and forms a loop extending between the outer surface and the inner surface effectively providing a double length of strand within the fibrous wall.

The loop may be held in place at the inner surface by a locking strand, or the loop may be held in place at the outer surface by a locking strand.

The vessel wall may be curved.

The fibre bundles forming the fibrous wall may be formed of one of: carbon, glass or aramid.

The strands forming the stitches may be fibres of one of: carbon, glass or aramid.

The vessel wall may further include a polymeric matrix disposed between the inner surface and outer surface, interspersed with the fibrous wall.

The fibres bundles may be co-mingled with a thermoplastic material.

The fibres forming a first layer of the multiple layers in the fibrous wall may be of a different material than the fibres forming a second one of the multiple layers.

The bundles of fibres forming a layer of the fibrous wall may be arranged transverse to a lengthwise direction, around the vessel, at an angle offset from the lengthwise direction in the range of 10 to 70 degrees.

The bundles of fibres forming a layer of the fibrous wall may be arranged in a lengthwise direction. The pressure vessel may be suitable for storing pressurised hydrogen.

The pressure vessel may further include a first end part at a first end of the vessel wall, and a second end part at the second end of the wall, the second end being opposite the first end.

The fibrous wall may surround a liner, the liner providing a substantially impermeable barrier.

The stitches may be formed in rows.

According to a second aspect of the invention, we provide 11. A method of forming a pressure vessel, the method including: forming a fibrous wall of multiple layers of fibres by, for each layer between an inner surface and an outer surface, winding one or more bundles of fibres onto a frame; and applying multiple stitches in the fibrous wall, each stitch formed of a strand that penetrates the outer surface and forms a loop extending between the outer surface and the inner surface effectively providing a double length of strand within the fibrous wall.

The frame may include: a shaft supported on multiple bearings; and a pair of supports fixed to the shaft, each support providing multiple pins extending outwardly from the shaft; and winding the one or more bundles of fibres onto the frame may include sequentially winding a bundle of fibres around a pin on a first one of the supports, and subsequently winding the bundle of fibres around a second pin on the other one of the supports.

Each pin may provide an abutment formation, and winding the bundles of fibres onto the pins of the frame at the innermost layer may comprise winding the bundles of fibres onto the pins at a position at or adjacent the abutment formation.

Applying each stitch may include: using a needle supporting the strand, inserting the strand through the outer surface, moving the needle towards the inner surface thereby drawing the strand to the inner surface, and retracting the needle so as to leave at least a portion of the strand disposed between the outer surface and the inner surface.

Alternatively, applying each stitch may includes: using a needle supporting a strand, inserting the strand through the inner surface, moving the needle towards the outer surface thereby drawing the strand to the outer surface, and retracting the needle so as to leave at least a portion of the strand disposed between the outer surface and the inner surface.

Retracting the needle may further comprise retracting a portion of the strand with the needle, so that the strand forms the loop extending between the outer surface and the inner surface.

On inserting the strand, a first part of the strand may be located in a groove on the needle, such that when the needle is retracted, the first part of the strand is able to pass within the groove and a second part of the strand is held by frictional resistance with the fibrous wall.

The method may further including the step of: inserting a locking strand through the loop formed by the strand, at the inner surface, so as to resist removal of the strand.

Applying multiple stitches to form one or more rows may include the steps of: applying the multiple stitches simultaneously so that multiple loops are formed at the inner surface, and inserting the locking strand may involve inserting the locking strand through the multiple loops.

The method may further including the step of: inserting a locking strand through the loop formed by the strand, at the outer surface, so as to resist removal of the strand.

Applying multiple stitches to form one or more rows may include the steps of: applying the multiple stitches simultaneously so that multiple loops are formed at the outer surface, and inserting the locking strand may involve inserting the locking strand through the multiple loops.

The method may include the step of: forming a polymeric matrix between the inner surface and outer surface, such that the matrix is interspersed with the fibrous wall.

According to a third aspect of the invention, we provide a frame for forming a textile preform, for use in a composite structure, the frame comprising: multiple supports, each support providing multiple pins extending outwardly, and each pin being secured to the support at a first end of the pin, each pin providing an abutment formation at a position spaced from the first end of the pin, configured such that in use, where a fibre bundle is positioned adjacent the abutment formation and between the abutment formation and a second end of the pin, the abutment formation resists movement of the fibre bundle towards the first end.

According to a fourth aspect of the invention we provide a pin for a frame for forming a textile preform for a composite structure, the frame being of the type comprising multiple supports to which multiple pins are securable such that the pins extend outwardly from the support, the pin providing: an abutment formation at a position spaced from a first end of the pin, the abutment formation being formed between a first portion of the pin having a relatively larger diameter towards the first end of the pin, and a second portion having a relatively smaller diameter towards a second end of the pin.

The abutment formation may be provided by a step formed at a point of transition between the first portion and second portion of the pin, the step forming an end of an inset notch in the body of the pin.

The abutment formation may be provided by a step formed at a point of transition between the first portion and second portion of the pin, the second portion of the pin extending from the step to the second end of the pin.

The abutment formation is provided by a projection extending outwardly from the body of the pin, the projection forming the first portion of the pin, and the second portion of the pin extending from the projection towards the second end of the pin.

Further embodiments of the invention are set out below by way of example, with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE FIGURES

In order that the present disclosure may be more readily understood, preferable embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIGURE 1 is a top view of a vessel preform embodying the present disclosure;

FIGURE 2 is a cross-section view of the vessel preform;

FIGURE 3 is a schematic showing a stitch in a fibrous wall; FIGURE 4 is a perspective view of a frame for use in methods of forming a pressure vessel, embodying aspects of the present disclosure;

FIGURE 5 is a front view of a partially wrapped frame during the process of forming the vessel preform;

FIGURE 6 is a schematic view of the fibrous wall during the stitch insertion phase;

FIGURE 7 is a schematic of the process of forming multiple stitches;

FIGURE 8 is a top view showing lengthwise windings and hoop windings;

FIGURE 9 is a schematic of the pressure vessel having a first end and a second end;

FIGURE 10 is a cross-sectional schematic a pressure vessel illustrating the vessel wall surrounding a liner; and

FIGURE 11 is a cross-sectional view of a portion of a frame and a pin of the frame, for forming the fibrous wall.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present invention seeks to address the issues outlined above by providing a pressure vessel and a method of producing a pressure vessel using a composite material with a stitched through thickness reinforcement. The method of producing the pressure vessel includes creating a textile preform using a rotatable preforming apparatus (generally a frame), orientating the reinforcing fibres in axial (i.e., lengthwise), transverse (i.e. angularly offset from the lengthwise direction) and/or in a hoop pattern (i.e. wrapping the fibres around the periphery of the vessel similar to a band), according to a specified design, and finally providing the through thickness reinforcement through a stitching process.

An advantage of the described technology is that by reducing the level of impact damage sustained from any single impact, by limiting the spread of damage over the surface and between the layers of the composite reinforcement, the vessel is less likely to require replacement and/or mending, or the frequency of mending or replacing the vessel will be lower. In addition, since vessel damage may have serious repercussions (when storing pressurised fluids), additional measures may be taken to minimise the risk or impact of such damage, including increasing the thickness of reinforcement of the structure of the vessel. The additional reinforcement may take the form of additional layers of fibrous material used to create the vessel walls, and this adds to the cost, and weight, and resources required to produce the vessel. By reducing the likelihood and severity of any impact damage, this reduces the resources required to produce a safe pressure vessel.

With reference to figures we describe a pressure vessel 11 and a method of forming a vessel. The pressure vessel 11 defines a volume 12 for containing a pressurised fluid, the volume 12 being at least partially surrounded by a vessel wall 14. The vessel wall 14 (illustrated in figure 2) provides a fibrous wall 14a with an inner surface 14b and an outer surface 14c. The fibrous wall 14a includes multiple layers of fibres 26, each layer comprising multiple bundles of wound fibres 28.

A pressure vessel preform 10 is illustrated in figure 1. Multiple stitches are provided in the outer surface 14c, each stitch 18 being made by a strand 20 that penetrates the outer surface 14c and extends between the outer surface 14c and the inner surface.

Each stitch 18 provides through thickness reinforcement of the vessel wall 14 at the location of the stitch 18. In this way, damage caused to the vessel wall 14 by an impact, for example, is limited as delamination damage reaches the point of the stitch 18. The stitch 18 effectively holds the layers of the wall together at that position, and unless the stitch 18 snaps or stretches, the delamination will halt at that point. Therefore, the stitches 18 effectively define regions 16 of the outer surface 26b of the pressure vessel 11 , such that damage caused by an impact to a given region 16 is unlikely to spread beyond the stitches 18 surrounding that part of the vessel wall 14. Where the stitches 18 are provided in rows, for example, borders 17 may be defined between adjacent regions 16 of the outer surface 26b.

The stitches 18 extend between the outer 14c and inner 14b surfaces of the pressure vessel wall 14 providing the through thickness reinforcement. This reinforcement, when formed as rows of stitches 18, reinforces the wall along the borders 17 between the regions 16.

The volume of material that is formed directly below the region 16 on the outer surface 14c of the vessel wall 14, within the borders 17 defining that portion 16 of the wall, remains a lamellar structure and is not affected or altered by the stitching process (other than at the specific points of insertion through the wall).

So, in general terms, the stitches 18 provide a resistance to advancing cracks or delamination, limiting the ability of such damage or defects to progress between the regions 16. For a crack to progress between one region and an adjacent region, the force of impact must be sufficient to allow the delamination to progress beyond the stitch (i.e by breaking the stitch). Therefore, the stitch 18 acts to prevent the layers in the structure from separating.

For instance, if an impact occurs to the wall of the pressure vessel, damage may occur to structure. In a composite pressure vessel that is not reinforced in the through thickness of the material, the damage, particularly delamination, is likely to extend significantly beyond the impact site. In a pressure vessel that has been reinforced thorough stitching, the extent of the delamination is limited. This is because the stiches resist the progress of the damage. Therefore, the damage is contained within the volume of material below the regions that are in the area of the impact. Furthermore, the stitches resist the progress of the same damage when the impact damaged vessel is subjected to in service load cycles.

Figures 4 to 7 illustrate a method of forming a pressure vessel preform 10, and subsequently the pressure vessel 11 , in accordance with the present disclosure, as described in more detail below. Figures 11 and 12 illustrate an example pressure vessel incorporating the pressure vessel preform 10 as described.

The term pressure vessel as used here is intended to apply to any vessel for use in containing a pressurised fluid. Such fluids can be gases or liquids, for example. Example gases which may be stored under pressure in such a vessel may be one of, but not limited to, hydrogen, nitrogen, or oxygen.

In embodiments of the described technology, the multiple bundles of wound fibres 28 forming the fibrous wall 14a are formed of carbon fibres. The strands 20 forming the stitches 18 may also be formed of fibres of carbon.

In other embodiments, the multiple bundles of wound fibres 28 may be formed of glass or aramid, for example. The strands 20 may also be formed of glass or aramid, as an alternative to carbon. The strands 20 may be formed of the same material as the multiple bundles of wound fibres 28 forming the fibrous wall 14a or may alternatively be formed of a different material. In embodiments of the technology, the strands 20 used to form the stitches 18 may be of different materials - for example, a subset of the strands 20 formed of carbon, and other strands 20 formed of glass or aramid.

Where it is stated that the volume 12 is at least partially surrounded by a vessel wall 14, it is meant that the wall 14 forms a portion of the boundary around the volume 12. As shown in the figures, the vessel wall 14 may form an outer perimeter of the volume 12. Upper and lower ends of the pressure vessel 11 may be formed by first end part 32 and second end part 34 which, combined with the vessel wall 14, surround the volume 12 entirely.

The pressure vessel 11 in the examples shown in the drawings is cylindrical. To achieve this, the vessel wall 14 may be an open cylinder, for example. Generally, the vessel wall 14 is substantially curved. The vessel wall 14 may have a uniform cross-sectional shape along its length or height, or it may be non-uniform (for example, the vessel wall 14 may be barrel-shaped, or conical). In other embodiments the pressure vessel 11 may be a cuboid or cube in shape, and in which case the vessel walls 14 may be non-curved. As shown in figure 3, each stitch 18 comprises a double length of strand 20 formed as a loop 22. The strand 20 extends through the fibrous wall 14a, between its outer surface 14c and inner surface 14b. In embodiments, the strand 20 extends through the entirety of the width of the fibrous wall 14a and forms a loop 22 at the inner surface 14b, and is held in place at the inner surface 14b by a locking strand 24. A further length of the strand 20 returns from the inner surface 14b to the outer surface 14c, creating a stitch 18 held in place by the locking strand 24. The locking strand 24 prevents the two lengths of strand 20 from being pulled back through the fibrous wall 14a, so that the stitch 18 cannot readily be removed. By a “double length” of strand 20, we mean that there are effectively two portions of the strand 20 passing through the fibrous wall 14a between the outer and inner surfaces 14c, 14b, forming each stitch 18.

It should be understood that in other embodiments, the stitches may be applied between the inner 14b and outer surfaces 14c so as to form a loop 22, starting from the inner surface 14b and extending outwards to the outer surface 14c - i.e. , in the opposite direction to the layout described above.

In other embodiments, the strands 20 may be formed using a tufting process. In such embodiments the strand 20 may be only partially inserted into the fibrous wall 14 (i.e., the strand 20 does not extend entirely through the wall 14). The fibrous wall 14a retains the strand as the stitches 18 are applied. Alternatively, the locking strand 24 may be omitted, and the loop 22 may be clamped at the inner surface 14b by other means. For example, a foam pad may be used to retain the loop temporarily, by pressing the pad against the inner surface 14b of the wall. Subsequently, the pad is removed once the stitch 18 is fixed in position.

In embodiments of the described technology a polymeric matrix is disposed between the inner surface 14b and outer surface 14c, interspersed with the fibrous wall 14a. In embodiments the polymer forming the matrix may be a thermoset material such as epoxy, polyester, vinyl ester, cyanate esters, polyimides and phenolics. A wall formed of the fibrous wall 14a combined with a polymeric matrix is shown generally at 36. In other embodiments the matrix may be a thermoplastic material such as polypropylene, polyamide, polyphenylene sulfide or polyetheretherketone. Where a thermoplastic material is used as the matrix, a co-mingled fibre and polymer bundle may be used.

Figure 3 shows the arrangement of multiple layers 26 in the fibrous wall 14a. The innermost layer 26a forms the inner surface 14b and the outer most layer 26b forms the outer surface 14c. In the embodiments of the present disclosure the multiple layers 26 are formed of the same material, i.e. a carbon fibre composite. In further embodiments, a first layer of the multiple layers 26 in the fibrous wall 14a may be of a different material than the fibres forming a second one of the multiple layers 26. As is visible in figures 1 , 7 and 8, the fibre bundles 28 in one or more layers 26 of the fibrous wall 14a, may be formed in various patterns. For example, the fibre bundles 28 may be formed as lengthwise windings 29a, transverse (i.e. , angularly offset) windings 29b or braided windings 29d. As shown in figure 3, the multiple bundles of wound fibres 28 forming a layer 26a 26b of the fibrous wall 14a may be arranged to be in a transverse formation (see 29b) around the vessel, at an angle in the range of approximately 30-35 degrees relative to a lengthwise direction of the vessel wall 14. As can also be seen in figure 3, other layers of bundles of fibres 28 are arranged in a lengthwise direction (see 29a).

Figure 7 shows an example of the outermost layer 26b of the fibrous wall 14a. In embodiments of the technology, the wall is formed of multiple lengthwise bundles of fibres 28. The regions 16 of the of the outer wall may aligned with respective bundles of fibres 28, for example. So, if the bundles of fibres 28 in the outermost layer are formed lengthwise, rows of stitches 18 may be formed lengthwise of the pressure vessel 11 , for example.

With reference to figures 4 to 7, a method of forming portions of a pressure vessel 11 is described. A fibrous wall 14a of multiple layers of fibres 26 is formed. Each layer is formed, from the first layer which is the inner layer 26a, forming an inner surface 14b of the fibrous wall, to an outer layer 26b which forms an outer surface 14c. The bundles of fibres 28 forming each layer are wound onto a frame 52 (of a mandrel, for example), in turn. Once the desired number of layers 26 have been applied to the frame 52, multiple stitches 18 are then applied to the fibrous wall 14a. As mentioned above, the stitches form multiple borders 17, so as to divide the outer surface 14c into multiple regions 16. Typically, the stitches 18 are applied in rows. In embodiments, the stiches 18 may be applied in a uniform pattern across the outer surface 14c of the fibrous wall 14a. In some embodiments, the stitches 18 are formed in a grid-like pattern, i.e., as multiple spaced rows.

Figure 4 illustrates the frame 52. The frame consists of a shaft 54, supported on multiple bearings. Attached to the shaft 54 are a pair of supports 58, each 58 providing multiple pins 60 extending outwardly from the shaft 54.

When winding the multiple bundles of wound fibres 28 of fibres onto the frame 52, the fibre bundles 28 are wound sequentially. Firstly a bundle of fibres 28 is wrapped around a pin 60 on a first support 58. Subsequently the bundle of fibres 28 is wound around a second pin 60 on a second support of the supports 58. The selection of the sequence of pins 60 determines the orientation of the fibre bundles within the layer of the fibrous wall 14a. The winding process continues until the desired thickness of the fibrous wall 14a is achieved. The process of applying each stitch 18 to the fibrous wall 14a is explained with reference to figures 6 and 7. Figure 6 shows a needle 30. The needle 30 supports a strand 20 and is used to insert the strand 20 through the outer surface 14c. The needle moves towards the inner surface 14b and draws the strand 20 with it. The needle 30 is then retracted, leaving at least a portion of the strand

20 disposed between the outer surface 14c and the inner surface 14b.

In embodiments of the technology, as the needle 30 is retracted from the fibrous wall 14a, a portion of the strand 20 is retracted with the needle 30. In other words, the strand 20 is drawn from the outer surface 14c through the wall, to the inner surface 14b, and then the strand 20 continues with the needle 30 back to the outer surface 14c from which it is then withdrawn from the wall. The material of the strand 20 is left in place within the fibrous wall 14a, extending from the outer surface 14c through to the inner surface 14b. A loop 22 is formed between the outer surface 14c and the inner surface 14b.

When the needle 30 holding the strand 20 is inserted into the fibrous wall 14a, a first part of the strand 20 is located in a groove 21 or slot on the needle 30. Preferably, and as illustrated, the needle 30 is formed with two sides, so that first and second parts of the strand 20 lie on either side of the needle 30. When the needle 30 is retracted, the first part of the strand 20, disposed on the side of the needle 30 that provides the groove 21 , can pass through the groove 21 with minimal resistance. By contrast, a second part of the strand 20, which is not held within such a groove 21 , is held by frictional resistance with the fibrous wall 14a. The second part of the strand 20 is frictionally held by the fibrous wall 14a, whereas the groove 21 allows the needle 30 to slide past the first part of the strand 20. As a result, a loop 22 is formed at the inner surface 14b. The groove

21 is formed as a slot lying lengthwise of the needle 30, providing a recess adapted to surround a portion of the strand 20.

In embodiments of the present technology a locking strand 24 is inserted through the loop formed by the strand 20, at the inner surface 14b. The locking strand 24 effectively holds the loop of the strand 20 so that a portion of the strand 20 is held at the inner surface 14b. In this way, removal of the strand 20 is resisted.

In embodiments the strand 20 may be retained at the inner surface 14b temporarily using a foam support. In this case a loop 22 is formed at the inner surface 14b. The loop 22 enters the foam support and is trapped during the stitching process. This process does not require a locking strand 24 and is of particular use when the access to the inner surface 14b is limited, so that a locking strand 24 cannot be inserted.

Figure 7 shows the application of multiple stitches 18 to form each of the borders 17. The multiple stiches 18 are applied simultaneously so that multiple loops 22 are formed at the inner surface 14b. In other words, multiple needles 30 are provided, each supporting a different respective strand 20. The needles 30 each act to insert the strand 20 through the fibrous wall 14a as described, aligned in a row (typically lengthwise of the fibrous wall). In this way, the stitches 18 are formed in a row, along the length of the vessel wall 14.

The locking strand 24 is then inserted through the multiple loops 22 formed at the inner surface 14b. The locking strand 24 may be inserted by using a rapier 25. The rapier 25 supports a locking strand 24 and is inserted into the volume 12 from one of the ends of the fibrous wall 14a. The rapier 25 is operable to pass from a first end of the fibrous wall to a second end, passing through the loops 22 of the strands 20. Once at the second end, the locking strand 24 is retained (by a clamp, or simply by the loops 22 of the strand(s) 20 being pulled tight by movement of the needle 30 when retracting. In embodiments, the locking strand 24 may be cut at this point. The rapier 25 is operated to move back towards its starting position, drawing it back from within the loops 22, leaving the locking strand 24 retained within the loops 22.

In other embodiments strand could be inserted by a projectile, or a jet of water or air. In such embodiments a rapier 25 is not required.

The fibre bundles 28 in one or more layers 26 of the fibrous wall 14a, may be wound in patterns of lengthwise windings 29a, transverse (i.e. , angularly offset) windings 29b or braided windings 29d.

Lengthwise windings 29a are substantially aligned with the lengthwise axis X (illustrated in figure 2) of the vessel (i.e., positioned substantially parallel in the case of the vessel wall 14 being cylindrical). The lengthwise windings 29a can be substantially aligned with the axis X. In embodiments of the technology, the lengthwise windings 29a are within a tolerance of around 10 degrees from axial alignment; in other words, they are angularly offset from the lengthwise axis X by no more than 10 degrees.

The lengthwise windings 29a align with the axial principal stresses in a pressurised vessel. The alignment of the lengthwise windings 29a and the axial principal stresses is desirable, as there are material strength losses associated with offsetting the orientation of the reinforcement away from the principal stresses. In a typical filament winding or braiding process the axial windings are typically at around 20 degrees. Therefore, a weight saving can be achieved by aligning the fibres closer to the direction of principal stress.

Transverse windings 29b, which are angularly offset from the lengthwise direction, are typically orientated at angles of between 10 and 70 degrees from the lengthwise axis X of the vessel 11. Preferably, the angle of offset is between 15 and 45 degrees, and more preferably, around 30-35 degrees. The transverse windings 29b are wound onto a pin 60 on a first support 58. A pin 60 on a second support 58 is then selected for supporting and winding the fibre bundle. The second pin 60 is chosen such that the fibre bundle 28 is offset from the lengthwise axis X.

Braided windings 29d are formed by interlacing the fibre bundles 28. The braided windings 29d are typically produced by a braiding machine. A braided fabric consisting of multiple braided windings 29d, may be produced by the braiding machine and subsequently placed onto the frame 52. Alternatively, the braiding machine can be used to wrap the braided windings 29d directly onto the frame 52. The fibre bundles 28 forming the braided windings 29d follow an undulating path, with the individual fibre bundles 28 passing over or under adjacent fibre bundles 28 within which the bundle is braided. Braided windings 29d can be used in combination with the generally lengthwise windings 29a and axially offset windings 29b (applied in layers one on top of another, for example).

Figure 8 shows multiple hoop windings 29c. The hoop windings 29c of the fibrous wall 26 are formed by wrapping one or more bundles 28 of fibres around the vessel wall 14 in a direction generally perpendicular to the lengthwise direction of the vessel wall 14. In other words, the hoop windings 29c extend around the vessel wall 14 at a constant radial offset from the central lengthwise axis X. The hoop windings 29c are typically applied after the lengthwise windings 29a and angularly offset windings 29b are wound onto the frame 52. The hoop windings 29c are then wrapped around the periphery of the pressure vessel preform 10, in a circumferential direction. As is known in the art, once the fibrous wall 14a is formed, the vessel wall 14 is completed by provision of a polymeric matrix 36 as mentioned above, formed between the inner surface 14b and outer surface 14c, such that the matrix is interspersed with the fibrous wall 14a. In embodiments the polymeric matrix may be formed through a liquid resin infusion process. In other embodiments the matrix may be contained within the fibres prior to the winding steps. Such methods employ the use of a pre-impregnated tows or a co-mingled tow.

Figure 9 shows the pressure vessel 11. In embodiments of the technology, the pressure vessel 11 is closed by a first end part 32 and a second end part 34. One or both of the first and second end parts 32, 34 may be formed integrally with the vessel wall 14. Alternatively, one or both of the first and second end parts 32, 34 may be formed separately to the vessel wall 14, and subsequently joined and sealed to the vessel wall 14 in a fluid tight manner.

Figure 10 shows the vessel wall 14 surrounding a liner 40, the liner 40 providing a substantially impermeable barrier for containing fluid within the vessel 11. In embodiments the liner 40 is a metallic or polymeric material. A portion of a frame 52 according to embodiments of the technology is illustrated in Figure 11. The frame 52 includes multiple pins 60, of which a single pin 60 is illustrated. The pins 60 each provide a first end 62 and a second end 64. In use, the first end 62 of each pin 60 is secured to the support 58, and the second end 62 extends outwardly away from the support 58. As shown in the figures, the support 58 in this example is circular, and the pins 60 are secured around the periphery of the support 58, extending radially outwardly, aligned with the centre point of the circular support 58. Preferably, each pin 60 is generally round in cross-section.

In embodiments of the described technology, each pin 60 provides a first portion 60a having a relatively larger diameter towards the first end 62 of the pin, and a second portion 60b having a relatively smaller diameter towards the second end 64 of the pin. The relative transition between the larger diameter and smaller diameter provides an abutment formation 66 that resists movement of fibre bundles wrapped around the pin 60 in the direction towards the first end 62 of the pin. In this way, the fibre bundle 28 forming the innermost layer 26a of the fibrous wall 14a, is prevented from slipping down the pin 60 as the fibrous wall 14a is formed. This is particularly important at the stage of inserting the stitches 18 using the needles 30, at which point the fibre bundles 28 may otherwise be forced downwards along the pins 60, towards the first ends 62 of the pins.

The abutment formation 66 may take a variety of forms. In embodiments, and as shown in figure 11 , the abutment formation 66 is provided by a step formed at a point of transition between the first portion 60a and second portion 60b of the pin 60, the step forming an end of an inset notch in the body of the pin 60. The notch effectively provides a radially-inset portion (i.e. , a stepped portion), having a smaller radius than the surrounding portions along the length of the pin 60. The fibre bundle 28 forming the innermost layer 26a can be located on the pin 60 at the position of the notch 66, so that the stepped end of the notch - at which the diameter of the pin 60 transitions from a larger diameter (at 60a) to a small diameter (at 60b), forms a ridge that abuts the fibre bundle 28. In this way, movement of the fibre bundle 28 towards the first end 62 of the pin 60 is prevented or resisted.

In another embodiment, the portion of the pin with smaller diameter 60b, extends between the point of the abutment formation 66 and the second end 64 of the pin 60. In this way, the pin 60 is formed with a step at a position of the abutment formation 66, between a first portion 60a having a relatively larger diameter at the first end 62, to a second portion 60b having a relatively smaller diameter, extending to the second end 64. The step between the larger diameter portion and the smaller diameter portion acts in the same way as described above, acting to prevent the fibre bundle 28 of the innermost layer 26a from slipping down the pin 60.

In another embodiment, the pin 60 may be formed with a body of regular diameter along its length and provides a projection that extends outwardly from the body of the pin 60 to form a step formed outwardly from the pin body. In this embodiment, the projection provides an abutment formation 66, formed between a portion of relatively larger diameter 60a at the position of the projection, and a portion of relatively smaller diameter 60b formed along the length of the pin between the wall and the second end 64 of the pin 66. The projection preferably extends around the circumference of the pin 60. The projection may be formed as a flange, for example.

Preferably, in these embodiments, the abutment formation 66 is formed as a well-defined step. In other words, the stepped part is formed with a square cut, so as to provide a distinct radially- extending surface to abut the fibre bundles 28. In this way, when the fibre bundles 28 are positioned at the portion of smaller diameter 60b, the fibre bundles abut the abutment formation 66.

When wrapping the first layer 26a of the fibre bundles 28 around the pins 60, the fibre bundles 28 are wound into the portion of smaller diameter 60b, which in the embodiments shown is formed as a notch, or radially-inset portion. Subsequent layers 26 of fibre bundles 28 are wound onto the pins 60 at positions between the abutment formation 66 and the second end of the pin 64.

In other embodiments of the technology (not shown), the abutment portion is formed as a relatively smooth transition between the portions of larger and smaller diameter 60a, 60b. For example, the transition may be tapered, or curved, rather than being provided as a distinct step.

In each of these embodiments, the purpose of the abutment formation 66 is to react to the forces exerted on the fibrous wall 14a during stitch insertion. As a result the fibrous wall 14a does not move inwards on the frame 52. Instead, the fibrous wall 14a is held by the recess of the abutment formation 66.

In embodiments of the technology, the abutment portion 66 may provide a step of around 1-2mm, such that the portion of the pin 60 having a relatively larger diameter 60a is 1-2mm greater in diameter than the portion having a relatively smaller diameter 60b. In other embodiments, the abutment portion 66 may provide a step of more than 2mm.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The invention may also broadly consist in the parts, elements, steps, examples and/or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and/or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein. Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure.

Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and/or to encompass equivalents.

CLAUSES

Certain preferred features are set out in the following numbered clauses.

1. A pressure vessel defining a volume for containing a pressurised fluid, the volume being at least partially surrounded by a vessel wall, the vessel wall having: a fibrous wall having an inner surface and an outer surface, the fibrous wall including multiple layers of fibres, each layer comprising multiple bundles of wound fibres, wherein multiple stitches are formed at the outer surface, each stitch being made by a strand that penetrates the outer surface and extends between the outer surface and the inner surface.

2. A pressure vessel according to clause 1 , wherein each stitch comprises a double length of strand forming a loop.

3. A pressure vessel according to clause 2, wherein the strand extends to the inner surface and the loop is held in place at the inner surface by a locking strand.

4. A pressure vessel according to any preceding clause, wherein the vessel wall is curved.

5. A pressure vessel according to any preceding clause, wherein the fibre bundles forming the fibrous wall and/ or the strands forming the stitches are formed of one of: carbon, glass or aramid.

6. A pressure vessel according to any preceding clause, wherein the vessel wall further includes a polymeric matrix disposed between the inner surface and outer surface, interspersed with the fibrous wall.

7. A pressure vessel according to any one of clauses 1 to 5, wherein the bundles of fibres are co-mingled with a thermoplastic material. 8. A pressure vessel according to any preceding clause, suitable for storing pressurised hydrogen.

9. A pressure vessel according to any preceding clause, further including a first end part at a first end of the vessel wall, and a second end part at the second end of the wall, the second end being opposite the first end.

10. A pressure vessel according to any preceding clause, wherein the fibrous wall surrounds a liner, the liner providing a substantially impermeable barrier.

11. A pressure vessel according to any preceding clause, wherein the stitches are formed in one or more rows.

12. A method of forming a pressure vessel, the method including: forming a fibrous wall of multiple layers of fibres by, for each layer between an inner surface and an outer surface, winding one or more bundles of fibres onto a frame; and applying multiple stitches in the outer surface of the fibrous wall.

13. A method according to clause 12, wherein the frame includes: a shaft supported on multiple bearings; and a pair of supports fixed to the shaft, each support providing multiple pins extending outwardly from the shaft; wherein winding the one or more bundles of fibres onto the frame includes sequentially winding a bundle of fibres around a pin on a first one of the supports, and subsequently winding the bundle of fibres around a second pin on the other one of the supports.

14. A method according to clause 12 or clause 13, wherein applying each stitch includes: using a needle supporting a strand, inserting the strand through the outer surface, moving the needle towards the inner surface thereby drawing the strand to the inner surface, and retracting the needle so as to leave at least a portion of the strand disposed between the outer surface and the inner surface.

15. A method according to clause 14, wherein retracting the needle further comprises retracting a portion of the strand with the needle, so that the strand forms a loop extending between the outer surface and the inner surface.

16. A method according to clause 15 wherein, on inserting the strand, a first part of the strand is located in a groove on the needle, such that when the needle is retracted, the first part of the strand is able to pass within the groove and a second part of the strand is held by frictional resistance with the fibrous wall.

17. A method according to clause 15 or clause 16, further including the step of: inserting a locking strand through the loop formed by the strand, at the inner surface, so as to resist removal of the strand.

18. A method according to clause 17, wherein: applying multiple stitches to form the or each row includes: applying the multiple stitches simultaneously so that multiple loops are formed at the inner surface, and inserting the locking strand involves inserting the locking strand through the multiple loops.

19. A method according to any one of clauses 12 to 18, further including the step of: forming a polymeric matrix between the inner surface and outer surface, such that the matrix is interspersed with the fibrous wall.

20. A method according to clause 13, or any one of clauses 14 to 19 where dependent directly or indirectly on clause 13, wherein each pin provides an abutment formation, and wherein winding the bundles of fibres onto the pins of the frame at the innermost layer comprises winding the bundles of fibres onto the pins at a position at or adjacent the abutment formation.

21. A frame for forming a textile preform, for use in a composite structure, the frame comprising: multiple supports, each support providing multiple pins extending outwardly, and each pin being secured to the support at a first end of the pin, each pin providing an abutment formation at a position spaced from the first end of the pin, configured such that in use, where a fibre bundle is positioned adjacent the abutment formation and between the abutment formation and a second end of the pin, the abutment formation resists movement of the fibre bundle towards the first end.

22. A pin for a frame for forming a textile preform for a composite structure, the frame being of the type comprising multiple supports to which multiple pins are securable such that the pins extend outwardly from the support, the pin providing: an abutment formation at a position spaced from a first end of the pin, the abutment formation being formed between a first portion of the pin having a relatively larger diameter towards the first end of the pin, and a second portion having a relatively smaller diameter towards a second end of the pin. 23. A pin according to clause 22, wherein the abutment formation is provided by a step formed at a point of transition between the first portion and second portion of the pin, the step forming an end of an inset notch in the body of the pin. 24. A pin according to clause 22, wherein the abutment formation is provided by a step formed at a point of transition between the first portion and second portion of the pin, the second portion of the pin extending from the step to the second end of the pin.

25. A pin according to clause 22, wherein the abutment formation is provided by a projection extending outwardly from the body of the pin, the projection forming the first portion of the pin, and the second portion of the pin extending from the projection towards the second end of the pin.