MANUFACTURE OF TAPERED COMPOSITE PANELS

A composite manufacture assembly is disclosed including a pair of closed mould tools for pressing a composite laminate panel therebetween, the closed mould tools having opposing inner surfaces that define an inner cavity that decreases in height along a span of the inner cavity in accordance with a curved rational function, the decrease in height corresponding to a designed thickness of the composite panel. The composite laminate panel including a stack of fibre-reinforced composite plies; a plurality of sets of ply drops spaced along a span of the composite laminate panel at which a corresponding set of plies terminate such that the thickness of the composite laminate panel varies along the span in accordance with the position and thickness of the sets of ply drops, each set of ply drops comprising one or more ply drops.

CROSS RELATED APPLICATION

This application claims priority to United Kingdom Patent Application GB 2309717.3, filed Jun. 28, 2023, the entire contents of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a composite manufacture assembly and a method of manufacturing a composite laminate panel.

BACKGROUND OF THE INVENTION

Aircraft components are increasingly made from fibre-reinforced composite materials due to the high specific properties (i.e. the material property per mass density of material) compared to conventional materials, such as metals. Fibre-reinforced composite structures are typically formed of layered plies stacked to form a composite structure of the required structure.

A reduction in the thickness of composite components can be achieved by terminating plies at discrete locations, otherwise referred to as ply drops, that reduce the thickness of the component according to the thickness of the ply. This can provide significant weight and cost benefits, but also introduces stress discontinuities that need to be carefully managed.

Design guidelines for introducing ply drops are generally quite conservative to ensure these discontinuities are accounted for in the overall structure both by overdesigning the component and following design guidelines on staggering the ply drops over a minimum length of the component. These design guidelines are continually reviewed and updated to adjust the ramp rate of the ply drops in order to provide more design freedom when developing composite components.

These design principles can become more conservative as new manufacturing techniques are adopted, such as closed moulding, and this can offset some of the advantages of using these manufacturing techniques.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a composite manufacture assembly, comprising: a pair of closed mould tools for pressing a composite laminate panel therebetween, the closed mould tools having opposing inner surfaces that define an inner cavity that decreases in height along a span of the inner cavity in accordance with a curved rational function, the decrease in height corresponding to a designed thickness of the composite panel; the composite laminate panel comprising: a stack of fibre-reinforced composite plies; a plurality of sets of ply drops spaced along a span of the composite laminate panel at which a corresponding set of plies terminate such that the thickness of the composite laminate panel varies along the span in accordance with the position and thickness of the sets of ply drops, each set of ply drops comprising one or more ply drops; each set of ply drops positioned along a respective span portion of the span of the panel, each respective span portion defining a portion of the span of the panel at which the designed thickness decreases in dependence on the thickness of the respective set of plies to be terminated; and wherein the position of each set of ply drops along a respective span portion between a mid-thickness position and a zero-thickness position of the designed thickness decrease.

A second aspect of the invention provides a method of manufacturing a composite laminate panel, comprising: designing a profile of the composite laminate panel, the profile defining a designed thickness of the panel decreasing along a span of the composite laminate panel according to a curved rational function; determining a layup of a series of fibre-reinforced composite plies to form the composite laminate panel, each composite ply having a respective thickness; determining a number of sets of ply drops at which respective sets of plies terminate required to provide a thickness decrease of the composite laminate panel along the span in accordance with the rational function; determining a set of respective span portions along the span of the composite laminate panel, each respective span portion defining a portion of the span of the panel at which the designed thickness decreases in dependence on the thickness of the respective set of plies to be terminated, determining the position of each set of ply drops along a respective span portion between a mid-thickness position and a zero-thickness position of the designed thickness decrease; and laying up a set composite plies to form a composite laminate panel preform according to the determined layup and position of the plurality of sets of ply drops.

Advantageously, this systematic means of determining the position of the ply drops helps to reduce any discrepancies between the designed profile of the composite panel and the moulding tool, for instance a closed moulding tool where constraints on the composite panel preform can be particularly tight, whilst minimising the formation of areas with low fibre volume fraction and other defects.

The decrease in height between ends of the span of the inner cavity may have an overall slope of 0.5% or less, optionally 0.3% or less, and optionally 0.2% or less. Similarly, the decrease in height of the designed composite laminate panel may have an overall slope of 0.5% or less, optionally 0.3% or less, and optionally 0.2% or less.

The distance between each of the sets of ply drops may increase or decrease along the span of the composite laminate panel.

The designed thickness may decrease along the span portion by the thickness of the respective set of plies to be terminated at the respective span portion.

The plurality of sets of ply drops may comprise at least five sets of ply drops.

The position of each set of ply drops along a respective span portion may be between a quarter-thickness position and a zero-thickness position of the designed thickness decrease.

The curved rational function may be a conic function.

The curved rational function may be one of a hyperbolic function, parabolic function, elliptical function or polynomial function.

Each set of ply drops may comprise a single ply drop.

Each set of ply drops may comprise a plurality of ply drops.

The ply drops within each set of ply drops may define a slope of 2.5% or more.

The composite laminate panel may have an upper composite ply and a lower composite ply, and wherein each ply drop is a termination of a composite ply located between the upper and lower composite plies.

The plies to be terminated may have a stacking sequence in the through-thickness direction of the panel, and the spanwise order in which the plies are terminated is different to the stacking sequence.

The composite laminate panel may have curvature in two orthogonal directions such that the height of the inner cavity decreases in height along a width of the inner cavity.

The height along the width of the inner cavity may decrease in accordance with a curved rational function.

The composite laminate panel may have a ratio of span-to-thickness and width-to-thickness of at least 100:1.

The composite laminate panel may be an aerodynamic skin panel.

The composite laminate panel is an aircraft skin panel.

The composite laminate panel may be part of an aircraft wingtip or winglet.

The method may comprise: providing a pair of closed mould tools having opposing inner surfaces that, when the closed mould tools are pressed together, define an inner cavity having the same profile as the designed profile of the composite laminate panel; placing the composite laminate panel between the closed mould tools; and pressing the closed mould tools together to consolidate the composite laminate panel preform.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG.1shows an existing aircraft1with port and starboard fixed wings2,3, engines9, a fuselage4with a nose end5and a tail end6, the tail end6including horizontal and vertical stabilising surfaces7,8. The aircraft1is a typical jet passenger transport aircraft but the invention is applicable to a wide variety of fixed wing aircraft types, including commercial, military, passenger, cargo, jet, propeller, general aviation, etc. with any number of engines attached to the wings or fuselage.

Each wing2,3of the aircraft1has a cantilevered structure with a length extending in a span-wise direction from a root to a tip, the root being joined to the aircraft fuselage4. The wings include a main fixed wing portion10and a wing tip device11outboard of the main fixed wing portion10.

The wing tip device11shown is a winglet11awith an upwardly projecting lifting surface (alternatively referred to as a sharklet), although it will be appreciated the invention is applicable to a wide range of highly curved composite panels. These may include aircraft components such as portions of the wing2,3, alternately shaped wing tip devices10, fuselage panels and the like, as well as composite panels for other industries, such as vehicle body panels.

The thickness of composite panels used in such structural applications is typically varied to tailor the static and dynamic performance of the panel, which thereby requires management of the composite laminate thickness. The standard practice has been to manage laminate thickness changes by dropping plies in a ‘terracing’ manner, partly to simplify the end-to-end tasking and processing of the laminates panels, with any detrimental effects of the ply drops considered on a localised level to help prevent any significant stress concentrations.

An example of terracing a series of ply drops is seen inFIG.2, in which the thickness of the outer skin panel15of the winglet11ais decreased in batches at a series of ply terraces29.FIG.3shows a close-up of an example ply terrace29, in which successive ply drops30are spread across a relatively short distance D of the composite laminate panel20to provide a decrease in the thickness of the composite laminate panel20. In this example only two plies25are dropped, although it will be appreciated that a ply drop terrace29may comprise many more ply drops30, for example more than five. Each ply drop30is generally accompanied by a resin pocket31or other defect(s) potentially detrimental to structural performance. Design guidelines for ply drops30typically specify minimum spacing between the ply drops30along the span S of the composite laminate panel20to reduce stress concentrations, with slopes of between 2.5% and 10% typical (i.e. ramp rates of thickness-to-length of between 1:40 and 1:10).

Composite manufacture techniques and processes continually evolve and improve, such that minimising ply drop spacing may no longer be a prime driver, and may, if fact, introduce unwelcome constraints, especially within the context of closed moulding manufacturing processes, such as Resin Transfer Moulding (RTM), and Same Qualified Resin Transfer Moulding (SQRTM).

This invention relates to a systematic means of determining the position of ply drops30in a manner that can help provide structural performance benefits, weight savings, and material utilization benefits, as well as ease the creation of closed mould tools.

To achieve this, the overall profile of a composite panel20is designed, such that the curvature of the outer surfaces of the composite panel20and the thickness T of the composite panel20are predetermined.FIG.4shows an example of a designed composite panel20in which the profile of the upper surface20aincludes a curved portion (Sx), with a thickness T of the composite panel20that varies in the span direction. Incorporated into this design may be the configuration of the fibre-reinforced composite plies25for forming the composite panel20, such that the lay-up of the composite panel20is known in terms of the material composition of the plies25, the stacking sequence of the plies25, and the individual thicknesses of each ply25. The plies25are continuous fibre-reinforced composite plies25, although chopped fibre-reinforced composites plies25, composite material tapers, filaments and veils may be utilised.

As previously discussed, current practice has been to semi-arbitrarily space the ply drops30in a manner that simplifies the end-to-end tasking and processing of composite laminate panels, whilst meeting the structural performance requirements. This can result in inefficient use of materials and overdesigning of the structure.

The designed composite panel20shown inFIG.4has a designed thickness T that varies along a section Sx of its span S according to a curved rational function, f (x). In the present example, the curved rational function is an arc of an ellipse, although a non-exhaustive list of rational functions includes conic functions, hyperbolic functions, parabolic functions, and high-order polynomial functions (i.e. second order and above), with it being appreciated there may be overlap between these terms in some instances. The function may be non-circular.

The information related to the designed layup of the fibre-reinforced composite plies25and the profile of the composite laminate panel20can then be used to determine the required number of ply drops30to achieve the decrease in thickness across the span S of the designed composite laminate panel20. For instance, the combined thickness of all plies25to be terminated will be equal to the decrease in thickness of the composite laminate panel20. The plies25may each have the same thickness, or there may be plies25having different thicknesses to other plies25.

The position P of the ply drops30is then determined according to a set of design principles, illustrated byFIG.5, that help minimise manufacturing defects that can be particularly prevalent in closed mould manufacturing processes where the constraints on the geometry of the composite part are generally greatest.

Specifically, a set of span portions S1, S2, S3, S4are defined along the span of the composite laminate panel20such that each ply drop30a,30b,30c,30dis associated with a respective span portion S1-S4. In other words, a first ply drop30ais associated with a first span portion S1, and so on. The span portions S1-S4are calculated by identifying a span portion S1-S4for each ply drop30a-dat which the change in designed thickness of the designed composite panel20is the same as the thickness of the respective set of plies25a,25b,25c,25dto be terminated, such that the thickness of a first set of plies25ato be terminated is the same as the change in designed thickness along the respective span portion S1.

It will be appreciated that in some examples, the designed change in thickness will not exactly correlate to the thicknesses of the plies25, for example the number of plies25required to exactly match the thickness variation may not be an integer. In such cases, the span portions S1-S4may be increased or decreased in size to account for this, although any change will be proportional for each span portion S1-S4.

The position P1, P2, P3, P4of each ply drop30a-dwill be at a position along the respective span portion S1-S4, and between a mid-thickness position (SeeFIG.6A) and a zero-thickness position (SeeFIG.6C) along the respective span portion S1-S4of the designed thickness decrease. For reference, the mid-thickness position is the spanwise location from one end of the span portion to the opposing end at which the change in thickness is half the total change in thickness between the ends. Said differently, each of the ply drops30will be cut in half by the profile of the designed composite panel20. Meanwhile, the zero-thickness position is the spanwise location at the end of the span portion at the lowest thickness position of the span portion. It will be understood that the exact position will be subject to manufacturing tolerances, which are preferably no more than 10 mm, and may be less than 5 mm.

In this manner, the positions P1-P4of the ply drops30a-dare determined in a manner that aims to optimize the location of the ply drops30a-dand thereby improve the overall quality of the composite panel20. In particular, a position P1-P4of the ply drops30a-dat a mid-thickness location minimizes the discrepancies with the ply drops30a-dand the variation in thickness of the designed composite panel20. Whereas a zero-thickness position can help prevent the formation of voids and dry fibres. The ply drops30a-dwill generally be positioned between the mid-thickness and zero-thickness positions.

The preferred ‘thickness position’ of the ply drops30a-dwill generally be a function of composite panel20thickness. A relatively thicker panel20(or position of the same panel20) can be more tolerant to integrating the terminated ply25a-dinto the ply stack than a thin panel20, and therefore a position towards the zero-thickness position may be preferable, whereas the thin panel20may require the ply drops30a-dto be position closer to the quarter-thickness position (SeeFIG.6B).

The position P1-P4of the ply drops30a-dwill generally be determined in a manner that ensures the fibre volume fraction remains relatively constant throughout the panel20(i.e. biasing towards the zero-thickness position), so as to avoid resin dry areas and the like, whilst not over-constraining as a result of the discrepancy between the designed profile of the composite panel20(i.e. biasing towards the mid-thickness position). The preferred position will generally be between the quarter-thickness and zero-thickness positions (SeeFIGS.6B and6C).

It will be understood that each span portion S1-S4is associated with a single ply drop30, with each span portion span portion S1-S4arranged adjacent one another so as to be continuous across the span S.

It will be appreciated that the preferred positioning of the ply drops30may be determined in an iterative process given the number of variables in the layup (e.g. ply orientation, ply thickness, material, and the specific plies to be dropped, and the sequence in which they are dropped) and the competing structural demands. For instance, among the many variables, it will be appreciated that different plies25may be terminated, or in a different sequence, through the thickness of the designed composite panel20(i.e. in some examples, the plies25may be terminated non-sequentially in the through-thickness direction) and/or there may be continuous plies25extending across the span portion S1-S4located either side of one or more of the plies25that are terminated. Any of the plies25may be terminated, although typically this will not include outer plies due to the increased prevalence of surface delamination and other associated defects, such that the upper composite ply25iand a lower composite ply25jdoes not terminate to form a ply drop30but instead each ply drop30is a termination of a composite ply25located between the upper and lower composite plies25i,25j(SeeFIG.7).

The resulting arrangement of the ply drops30a-30dfrom this process is illustrated byFIG.7, in which it can be seen that, prior to consolidation between the opposing closed mould tools50, the composite preform120has an overall profile that generally approximates the profile of the designed composite panel20(the designed profile of the composite panel20indicated by the profile of the closed mould tools50) in a manner that best ensures the final quality of the part. In this manner, the potentially detrimental effects of discrepancies between the composite preform120and the closed mould tools50caused at the ply drop30locations are minimized, such as the occurrence of resin rich areas, dry fibres, and inter-ply fibre interleaving.

Upon finalizing the composite preform120, the closed mould tools50are pressed together to consolidate and cure the composite laminate panel preform120(SeeFIG.8) to form the final, cured composite laminate panel220(SeeFIG.9). The pair of closed mould tools50have opposing inner surfaces51that, when the closed mould tools50are pressed together, define an inner cavity having the same profile as the designed profile of the composite laminate panel20, and thereby the composite laminate panel preform120once consolidated.

It will be appreciated that the benefits of the present invention are greater the more complex the curvature of the designed composite panel20, firstly in terms of achieving a higher quality composite part, and secondly in terms of providing a methodology that can speed up decision relating to the positioning of the ply drops30and thereby increase the manufacturing and processing rates, as well as potentially help in narrowing the variables in any optimisation of the structure. The invention may therefore be particularly advantageous for non-circular profiles in which the curvature of the composite part20varies along the span.

In the present case, reference to the span of the panel20,120,220refers to a panel20,120,220in which the span is greater than the width and thickness of the panel20,120,220. The span S, the curved portion Sx, and the span portions S1-S4may each have a length at least a magnitude greater than the thickness of the panel20,120,220. The composite laminate panel20,120,220may have a ratio of span-to-thickness and width-to-thickness of at least 100:1.

Typical wing tip devices11may be more than 2 metres in length, with ply thicknesses generally ranging between 0.2 millimetres and 0.5 millimetres, with an decrease in height between ends of the curved portion Sx defining an overall slope of 0.5% or less (i.e. a ramp rate of thickness-to-length of 1:200 or less), or perhaps 0.3% or less (i.e. a ramp rate of thickness-to-length of 1:333 or less), or even 0.2% or less (i.e. a ramp rate of thickness-to-length of 1:500 or less).

The examples described above refer to the termination of individual ply drops30a-din each span portion S1-S4. In alternative examples, each span portion S1-S4may include a plurality of ply drops30a-dreferred to as a set of ply drops30that are tightly packed such that each set of ply drops30defines a slope of 2.5% or more (i.e. a ramp rate of thickness-to-length of 1:40 or more). Such an arrangement may find a compromise between the improved correlation between the outer profile of the unconsolidated composite laminate panel preform120and the closed mould tools50, and processing expediency of the layup process. In this case, it is intended that the total thickness of all ply drops30in a set of ply drops30is taken into account when determining the mid-thickness, quarter-thickness and zero-thickness positions.

Whilst the abovementioned examples describe the terminate of plies25across the span S of a panel20,120,220, it will be appreciated that the curvature may extend in two orthogonal directions such that particular advantage is found in using the design methodology to determine the position of ply drops30in the width direction in addition to the span direction of the panel20.

As discussed previously, the plies25may be terminated in a non-sequential order.FIG.10shows an example of such a non-sequential ply25termination arrangement. Specifically, the plies25to be terminated have a stacking sequence in the thickness direction, and the spanwise order in which the plies25are terminated is different to the stacking sequence.

Further examples of possible rational functions are illustrated inFIGS.11ato11d.FIG.11ashows ply drops30arranged along the span S of the panel20according to a convex parabolic function,FIG.11bshows ply drops30arranged along the span S of the panel20according to a convex hyperbolic function,FIG.11cshows ply drops30arranged along the span S of the panel20according to a concave parabolic function, andFIG.11dshows ply drops30arranged along the span S of the panel20according to a concave hyperbolic function. It will be appreciated that for non-circular rational functions, such as those shown above, the distance between the ply drops30(or sets of ply drops30) may increase or decrease continually along the span S of the composite laminate panel20,120,220.

Where the word ‘or’ appears this is to be construed to mean ‘and/or’ such that items referred to are not necessarily mutually exclusive and may be used in any appropriate combination.