Patent Description:
Metal matrix composites (MMCs) are composite materials comprising elements of a first material (in the form of fibres or particles) distributed within a matrix of a second, metallic material. The physical properties of the MMC are a function of the interaction between the first material and the second (matrix) material. MMCs in which the first material is in the form of continuous fibres are known as "continuous fibre MMCs".

Aircraft components, particularly components for commercial airliners, are preferably as lightweight as possible, whilst also being low cost and easy to manufacture. However; many aircraft components must withstand extreme environments during operation, and must be very reliable. Moreover, the failure modes of aircraft components must be predictable and well-understood. Continuous fibre MMCs offer the promise of lighter, easier to manufacture load-bearing components, but the reliability and failure modes of MMC materials are not sufficiently well-understood for use in commercial aircraft structures. The present invention seeks to address this. In particular, it seeks to provide design concepts which enable the failure mode and time of failure of a continuous fibre MMC structure to be planned. <CIT> describes using controlled lamination of tape-like strips or sheet materials in order to fabricate structural components having inherent fracture arrestment capabilities.

A first aspect of the present invention provides a load-bearing structure configured to, during operation of the structure, transfer load from a first part of the structure to a second part of the structure via a load path. The load-bearing structure comprises a matrix material, a plurality of longitudinal first reinforcing elements embedded in the matrix material, and a plurality of longitudinal second reinforcing elements embedded in the matrix material. The long axis of each first reinforcing element is substantially aligned with a first direction and the long axis of each second reinforcing element is substantially aligned with a second direction, the second direction being substantially perpendicular to the first direction. The structure has a predefined crack-propagation region configured to control the propagation of a crack in the structure. The crack-propagation region either comprises multiple first reinforcing elements and does not comprise any second reinforcing elements; or comprises multiple second reinforcing elements and does not comprise any first reinforcing elements.

Optionally, the load-bearing structure further comprises a plurality of longitudinal third reinforcing elements embedded in the matrix material, wherein the long axis of each third reinforcing element is substantially aligned with a third direction, the third direction being substantially perpendicular to the first direction and substantially perpendicular to the second direction.

Optionally, the load path is substantially within a plane defined by the first direction and the second direction.

Optionally, the load path is substantially parallel to the first direction, and the crack-propagation region comprises multiple first reinforcing elements and does not comprise any second reinforcing elements.

Optionally, a length of each first reinforcing element is at least <NUM>% of a dimension of the structure along the first direction and a length of each second reinforcing element is at least <NUM>% of a dimension of the structure along the second direction.

Optionally, the crack-propagation region is longitudinal and has a long axis parallel to the first direction.

Optionally, although not claimed, the location of the crack-propagation region is selected such that the presence of cracks in the crack-propagation region does not prevent operation of the structure to transfer load from the first part to the second part.

Optionally, the load-bearing structure further comprises a predefined crack-termination region of the structure which does not contain any reinforcing elements, wherein the crack-termination region is adjacent to the crack-propagation region and is configured to limit the propagation of a crack in the structure.

Optionally, a region of the load-bearing structure disposed between the crack-termination region and a surface of the structure is transparent.

Optionally, the load-bearing structure further comprises an opening which extends through the structure, the structure is configured to transfer load from the opening to the second part of the structure via the load path, and the crack-propagation region is adjacent the opening.

Optionally, although not claimed, the opening is configured to engage with a further structure during operation of the structure such that load is transferred from the further structure to the structure via the opening.

Optionally, the crack-propagation region is an internal region of the load-bearing structure, such that the crack-propagation region does not comprise any part of any surface of the load-bearing structure.

Optionally, the crack-propagation region comprises a part of at least one surface of the load-bearing structure.

Optionally, a transparent layer is provided on a part of a surface of the load-bearing structure which is comprised in the crack-propagation region.

Optionally, each first reinforcing element and each second reinforcing element comprises a continuous element.

Optionally, the matrix material is aluminium.

Optionally, each first reinforcing element and each second reinforcing element comprises one of: an aluminium oxide fibre, a carbon fibre; a silicon carbide fibre.

Optionally, the load-bearing structure is an aircraft structure.

An aircraft comprising a load-bearing structure according to the first aspect.

The examples described herein relate to load-bearing structures. Each example load-bearing structure is configured to, during operation of the structure, transfer load from a first part of the structure to a second part of the structure via a load path. Each example structure comprises a matrix material, a plurality of longitudinal first reinforcing elements embedded in the matrix material, and a plurality of longitudinal second reinforcing elements embedded in the matrix material. The long axis of each first reinforcing element is substantially aligned with a first direction and the long axis of each second reinforcing element is substantially aligned with a second direction which is substantially perpendicular to the first direction. Each example structure has a predefined crack-propagation region configured to control the propagation of a crack in the structure. The predefined crack-propagation region comprises multiple first reinforcing elements and does not comprise any second reinforcing elements.

By virtue of the load-bearing structure having first longitudinal reinforcing elements and substantially perpendicular second longitudinal reinforcing elements (that is, the load-bearing structure is multi-directionally reinforced), the load-bearing structure has a high resistance to the propagation of material damage to the structure in regions of the structure where both first and second longitudinal reinforcing elements are present. However; all load-bearing structures will eventually experience fatigue failures after having been subjected to enough load cycles. It is expected that the manner of failure of a multi-directionally reinforced load-bearing structure will be difficult to predict. In certain applications, such as aircraft structures, being able to predict with a high degree of certainty the manner in which a load-bearing structure will fail is very important.

The load-bearing structures according to the invention are configured to fail in a controlled and predictable manner, as well as being bi-directionally reinforced. This is achieved by providing a pre-defined crack propagation region within each structure. The crack-propagation region is uni-directionally reinforced, and thereby guides the propagation of a crack within the region along a direction parallel to the reinforcing elements present in the crack-propagation region. The location and orientation of the crack-propagation region may be selected such that cracks in this region do not prevent the structure from performing its load transmitting function. The manner in which the load-bearing structures according to the invention achieve these advantages will now be explained in more detail with reference to the accompanying figures.

<FIG> is a plan view of an example load-bearing structure <NUM> according to the invention. <FIG> are each cross-sections through the structure <NUM> along the line A-A. The load-bearing structure <NUM> may be configured to be comprised in a vehicle, or any other mechanical system or larger structure. In some examples the structure <NUM> is an aircraft structure, meaning that it is configured to be comprised in an aircraft.

The structure <NUM> is configured to, during operation of the structure <NUM>, transfer load L (indicated by a block arrow in the figures) from a first part <NUM> of the structure <NUM> to a second part <NUM> of the structure <NUM> via a load path. "Operation" of the structure <NUM> means that the structure <NUM> is receiving a load (e.g. from a further component or structure engaged with the first part <NUM>) of a type that the structure <NUM> is designed to transfer. Where the structure <NUM> is designed to be connected to one or more further components or structures in order to perform its function, operation of the structure <NUM> should be understood to mean that such connections are present. Operation of the structure <NUM> may (but need not) mean operation of a vehicle or other mechanical system or structure in which the structure <NUM> is comprised.

The first part <NUM> of the structure, at which the load L is received, may be located at the location of a connection between the structure <NUM> and a further component or structure. Such a location may be, for example, at an edge of the structure <NUM> and/or may be the location of a connection feature such as a lug, a fastener hole, bracket, a spigot or the like. Such a location may be on a surface of the structure <NUM>. The second part <NUM> of the structure, to which the load L is transferred, may similarly be the location of a connection between the structure <NUM> and a further component or structure (which may or may not be the same further component or structure to which the first part <NUM> is connected). In the illustrated example, the structure <NUM> is configured to transfer the load L from the left-hand end (with reference to the orientation shown in <FIG>) of the structure <NUM> to the right-hand end of the structure <NUM>.

The structure <NUM> is formed from a metal matrix composite (MMC). The composite comprises a plurality of longitudinal first reinforcing elements <NUM> embedded within a metal matrix <NUM>, and a plurality of longitudinal second reinforcing elements <NUM> also embedded within the metal matrix <NUM>. The matrix material <NUM> may be aluminium, titanium, or any other metallic material. Preferably the matrix material is relatively lightweight, low cost, and easy to machine.

A "longitudinal" element is considered to be any structure having one dimension that is significantly longer than any other dimension. For example wires, fibres, tapes, threads and the like are all considered to be longitudinal elements. The long axis of each first reinforcing element is substantially aligned with a first direction x, and the long axis of each second reinforcing element is substantially aligned with a second direction y. The first direction x and the second direction y are substantially perpendicular to each other.

The structure <NUM> has a predefined crack-propagation region <NUM>, which is configured to control the propagation of a crack in the structure. The crack-propagation region <NUM> comprises multiple first reinforcing elements <NUM> and does not comprise any second reinforcing elements <NUM>. Thus, the structure <NUM> is bi-directionally reinforced in regions other than the crack-propagation region <NUM>, and is uni-directionally reinforced in the crack-propagation region <NUM>. The crack-propagation region <NUM> is longitudinal and has a long axis parallel to the first direction x.

The location of the crack-propagation region <NUM> relative to the rest of the structure <NUM> is selected such that the presence of one or more cracks in the crack-propagation region <NUM> does not prevent operation of the structure <NUM> to transfer load from the first part <NUM> to the second part <NUM>. For example, the location of the crack-propagation region <NUM> may be selected such that the main load path through the structure does not pass through the crack-propagation region. Furthermore, the crack-propagation region comprises first reinforcing elements <NUM> which are aligned with the load path, meaning that any cracks in the matrix material <NUM> in the crack-propagation region <NUM> should not significantly reduce the ability of the crack-propagation region to transfer the load L.

As can be seen from <FIG>, in the particular illustrated example the crack-propagation region does not extend for the full thickness (in the direction z) of the structure <NUM>, but instead is present only in a central part. Accordingly, the crack-propagation region <NUM> is an internal region of the structure <NUM> such that the crack-propagation region <NUM> does not comprise any part of any surface of the structure <NUM>. In other examples the crack-propagation region <NUM> may comprise a part of at least one surface of the load-bearing structure <NUM>, and/or may extend for the full thickness of the structure <NUM>. The extension in the z direction of the crack-propagation region <NUM> and/or its location along the z direction (i.e. top, middle, bottom relative to the structure <NUM>) may be selected according to the requirements of the particular intended application of the structure <NUM>. For example, the arrangement shown in <FIG> may be particularly suitable when it is desired to minimise the effect of the crack-propagation region <NUM> on the strength of the structure <NUM>. Alternatively, an arrangement in which the crack-propagation region <NUM> comprises a part of at least one surface of the structure <NUM> may be suitable if it is desired for any cracks in the crack-propagation region to be visible from an external surface of the structure <NUM>.

The manner in which the crack-propagation region <NUM> operates to control the propagation of a crack in the structure will be described in more detail below with reference to <FIG>.

Each of the first and second reinforcing elements <NUM>, <NUM> may be a continuous element, meaning that it comprises a single element which extends across a significant portion of the structure <NUM>. Each first reinforcing element <NUM> may have a length which is at least <NUM>% of a dimension of the structure <NUM> along the first direction x. Each second reinforcing elements <NUM> may have a length which is at least <NUM>% of a dimension of the structure <NUM> along the second direction y. In the illustrated example, the length of each first reinforcing element <NUM> is substantially equal to a dimension of the structure <NUM> in the first direction x and the length of each second reinforcing element <NUM> which is not interrupted by the crack-propagation region <NUM> is substantially equal to a dimension of the structure in the second direction y, although this need not be the case in other examples. The first and second reinforcing elements <NUM>, <NUM> are substantially straight when embedded in the matrix material <NUM>. Other examples are possible in which the first reinforcing elements <NUM> are curved when embedded in the matrix material, and/or the second reinforcing elements <NUM> are curved when embedded in the matrix material. The first and second reinforcing elements <NUM>, <NUM> may comprise a flexible material (although it will be appreciated that flexing of the reinforcing elements <NUM>, <NUM> is substantially or entirely prevented when the reinforcing elements <NUM>, <NUM> are embedded in the matrix material <NUM>).

In the illustrated example, each first reinforcing element <NUM> and each second reinforcing element <NUM> comprises a fibre. The first and second reinforcing elements <NUM>, <NUM> may, for example, comprise aluminium oxide fibres, silicon carbide fibres, galvanic-coated carbon fibres, or any other high strength fibres. The material composition and properties of the second reinforcing elements <NUM> may be (but need not be) substantially identical to the material composition and properties of the first reinforcing elements <NUM>.

In the illustrated example, the first reinforcing elements <NUM> are evenly distributed throughout the volume of the structure <NUM>. The second reinforcing elements <NUM> are similarly evenly distributed, except for in the crack-propagation region <NUM> where the second reinforcing elements <NUM> are not present at all. In other examples the distribution of the first reinforcing elements <NUM> may not be even, and/or the distribution of the second reinforcing elements <NUM> may not be even. For example, the density of first reinforcing elements <NUM> (that is, the number of first reinforcing elements <NUM> per unit area in the x-y plane) may be higher in regions of the structure <NUM> which are expected to experience higher loading during operation of the structure <NUM>.

Each of the first reinforcing elements <NUM> and each of the second reinforcing elements <NUM> may be under tension. The first and second reinforcing elements <NUM>, <NUM> being under tension may be advantageous for manufacturing the structure, as it can facilitate maintaining a desired arrangement of the fibres of the first and second reinforcing elements during the manufacturing process. Additionally, tension in the first and/or second reinforcing elements <NUM>, <NUM> provides an internal compressive pre-load in the structure <NUM>, which enhances the ability of the structure <NUM> to resist crack initiation.

In some examples the structure <NUM> may additionally comprise a plurality of longitudinal third reinforcing elements <NUM>. <FIG> shows such an example of the structure <NUM>. The long axis of each third reinforcing element <NUM> is substantially aligned with a third direction z, which is substantially perpendicular to both the first direction x and the second direction y. The third reinforcing elements <NUM> are not present in the crack-propagation region <NUM>. Apart from their direction, the third reinforcing elements <NUM> have substantially the same features as the first and second reinforcing elements <NUM>, <NUM>.

Other examples (not illustrated) are possible in which the structure <NUM> comprises a plurality of reinforcing elements which are substantially aligned with the first direction x and a plurality of reinforcing elements which are substantially aligned with the third direction z, but no reinforcing elements which are substantially aligned with the second direction y; or in which the which the structure <NUM> comprises a plurality of reinforcing elements which are substantially aligned with the second direction y and a plurality of reinforcing elements which are substantially aligned with the third direction z, but no reinforcing elements which are substantially aligned with the first direction x. It should be appreciated that the first and second reinforcing elements referred to in the claims need not be the first reinforcing elements <NUM> and the second reinforcing elements <NUM>, but instead may be the first reinforcing elements <NUM> and the third reinforcing elements <NUM>, or the second reinforcing elements <NUM> and the third reinforcing elements <NUM>.

<FIG> show a second example load-bearing structure <NUM> according to the invention. <FIG> is a plan view of the structure <NUM>, <FIG> is a cross-section taken along the line A-A which extends through a crack-propagation region of the structure <NUM>; and <FIG> is a cross-section taken along the line B-B which extends through a crack-termination region of the structure <NUM>. Many features of the example structure <NUM> are substantially identical to the corresponding features of the example structure <NUM> of <FIG>. These features have been given the same reference numbers and will not be further discussed.

The key difference between the example load-bearing structure <NUM> of <FIG> and the example load-bearing structure <NUM> of <FIG> is that the structure <NUM> additionally comprises a predefined crack-termination region <NUM>. The crack-termination region <NUM> does not contain any second reinforcing elements <NUM> and also does not contain any first reinforcing elements <NUM>. In some examples no reinforcing elements of any type or orientation are present in the crack termination region <NUM>. In such examples the crack-termination region <NUM> may comprise only the matrix material <NUM>.

The example structure <NUM> comprises a crack-propagation region <NUM>, which is substantially the same as the crack-propagation region <NUM> of the example structure <NUM> except that it has a greater thickness (along the z-direction), and it comprises part of the upper surface (with reference to the orientation shown in <FIG>) of the structure <NUM>, as can be seen from <FIG>. The crack-propagation region <NUM> is therefore located adjacent the upper surface of the structure <NUM>. The crack-propagation region <NUM> extends in the z-direction for only part of the thickness in the z-direction of the structure <NUM>, so unlike the crack-propagation region <NUM>, the crack-propagation region <NUM> is not centrally-located in the structure along the z-direction. In other examples the crack-propagation region <NUM> may be centrally-located in the structure <NUM> along the z-direction, or may be disposed toward the lower surface of the structure <NUM>, or may be adjacent the lower surface of the structure <NUM>.

The crack-termination region <NUM> is adjacent to the crack-propagation region <NUM>. In the illustrated example, the crack-termination region <NUM> is adjacent an end of the crack-propagation region <NUM>. That is, the crack-termination region <NUM> and the crack-propagation region <NUM> are immediately next to each other along the first direction x. The first reinforcing elements <NUM> which are present in the crack-propagation region <NUM> terminate at the boundary between the crack-propagation region <NUM> and the crack termination region <NUM>. The location of the crack-termination region <NUM> relative to the rest of the structure <NUM> is selected such that the presence of one or more cracks in the crack-termination region <NUM> does not prevent operation of the structure <NUM> to transfer load from the first part <NUM> to the second part <NUM>. For example, the location of the crack-termination region <NUM> may be selected such that the main load path through the structure <NUM> does not pass through the crack-termination region <NUM>.

Similarly to the crack-propagation region <NUM>, the crack-termination region <NUM> comprises part of the upper surface (with reference to the orientation shown in <FIG>) of the structure <NUM> and is therefore located adjacent the upper surface of the structure <NUM>. The crack-termination region <NUM> extends in the z-direction for only part of the thickness in the z-direction of the structure <NUM>, so is not centrally-located in the structure along the z-direction. In other examples the crack-termination region <NUM> may be centrally-located in the structure <NUM> along the z-direction, or may be disposed toward the lower surface of the structure <NUM>, or may be adjacent the lower surface of the structure <NUM>. It may generally be expected that the crack-termination region <NUM> has substantially the same location relative to the z dimension of the structure <NUM> as the crack-propagation region <NUM>, although this need not be the case in all examples.

In the illustrated example the crack-termination region <NUM> is a cylindrical region arranged such that the long-axis of the cylinder is parallel to the thickness of the structure <NUM> (the z-direction). In other examples the crack-termination region <NUM> may have a different shape, such as a sphere. The crack-termination region <NUM> is configured to facilitate preventing the further propagation of cracks which propagate into the crack-termination region <NUM> from the crack-propagation region <NUM>. The boundary between the crack-termination region <NUM> and adjacent regions of the structure <NUM> other than the crack-propagation region <NUM> may be reinforced in order to prevent the further propagation of cracks which propagate into the crack-termination region <NUM>. For example, this boundary may be reinforced by providing longitudinal boundary reinforcing elements <NUM> which extend around the circumference of the crack-termination region <NUM> except where the crack-termination region is adjacent the crack-propagation region <NUM>. The properties of the boundary reinforcing elements <NUM> may be substantially similar to the properties of the first, second and third reinforcing elements <NUM>, <NUM>, <NUM> described above.

The location of the crack-propagation region <NUM> and the crack-termination region <NUM> adjacent a surface of the structure <NUM> confers the advantage that cracks in either of these regions may be visible (e.g. to a human observer, or a camera-based monitoring device arranged to view the surface) from outside of the structure <NUM>. A load-bearing structure according to the invention which has this arrangement may therefore facilitate monitoring the structural health of the structure. In some examples a transparent layer or coating may be provided on the upper surface of the structure <NUM>, either across the entire surface or only on the parts comprised in the crack-propagation region <NUM> or the crack-termination region <NUM>. Such a coating is configured to protect the underlying structure <NUM> whilst maintaining visibility of the damage state of the crack-propagation region <NUM> and the crack-termination region <NUM>.

<FIG> show a further example load-bearing structure <NUM> according to the invention. The structure <NUM> is shown in perspective in <FIG>. A plan view of the structure <NUM> is provided in <FIG>, and <FIG> is a cross-section taken along the line A-A. Many features of the example structure <NUM> are substantially identical to the corresponding features of the example structure <NUM> of <FIG> and/or the example structure <NUM> of <FIG>. Like the version of the example structure <NUM> shown in <FIG>, the example structure <NUM> comprises third reinforcing elements <NUM> in addition to first and second reinforcing elements <NUM>, <NUM>. Features of the example structure <NUM> which are common with the example structure <NUM> and/or the example structure <NUM> have been given the same reference numbers and will not be further discussed.

The example structure <NUM> comprises an opening <NUM>, and is configured to transfer load from an edge of the opening <NUM> (this edge forms the first part <NUM> of the structure <NUM>) to an edge of the structure <NUM> (this edge forms the second part <NUM> of the structure <NUM>). The opening <NUM> extends completely through the structure <NUM> (that is, the opening has the form of a through-hole). Other examples are possible in which the opening <NUM> does not extend completely through the component <NUM>, but instead has the form of a recess. In the illustrated example the opening <NUM> is circular, although that need not be the case in other examples. The opening <NUM> is configured to engage with a further structure during operation of the load-bearing structure <NUM> such that load is transferred from the further structure to the load-bearing structure <NUM> via the opening <NUM>. For example, the opening <NUM> may be configured to receive a pin, spigot, bearing, or the like, such that loads are transferred into the structure <NUM> from the pin, spigot, bearing or the like, via the opening <NUM>. The structure <NUM> is configured to receive loads acting radially with respect to the axis of the opening <NUM>.

The structure <NUM> comprises two crack-propagation regions 16a, 16b, each of which has a substantially similar configuration to the other, and to the example crack-propagation region <NUM> discussed above. Each of the crack-propagation regions 16a, 16b is adjacent the opening <NUM>. The crack-propagation regions 16a, 16b are located such that the opening <NUM> is between the crack-propagation regions 16a, 16b.

The structure <NUM> further comprises four crack-termination regions 27a, 27a', 27b, 27b'. Each of the crack-termination regions 27a, 27a', 27b, 27b' is located adjacent a different end of a crack-propagation region 16a, 16b. The crack-termination regions 27a, 27a' are located adjacent opposite ends of the upper (with respect to the orientation shown in <FIG>) crack-propagation region 16a, and the crack-termination regions 27b, 27b' are located adjacent opposite ends of the lower crack-propagation region 16b. Each of the crack-termination regions 27a, 27a', 27b, 27b' has a substantially similar configuration to each other crack-termination region 27a, 27a', 27b, 27b' of the structure <NUM>, and to the example crack-termination region <NUM> discussed above.

The crack-propagation regions 16a, 16b and the crack-termination regions 27a, 27a', 27b, 27b' are located centrally in the structure <NUM> with respect to the z dimension, and do not extend for the full thickness of the structure <NUM>. The crack-propagation regions 16a, 16b include regions of the structure <NUM> where cracks might be expected to form, in particular the upper and lower edge regions of the opening <NUM>. However; neither the crack-propagation regions 16a, 16b nor the crack-termination regions 27a, 27a', 27b, 27b'overlap with the main load path between the first part <NUM> and the second part <NUM> of the structure <NUM>. This means that the structure <NUM> should still be able to operate to transfer load from the first part <NUM> to the second part <NUM> even when cracks are present in any or all of the crack-propagation and crack-termination regions.

The manner in which the crack-propagation (and, if present, crack-termination regions) of load-bearing structures according to the invention function to control the propagation of cracks will now be explained with reference to <FIG> is a plan view of the part of the structure <NUM> which includes the opening <NUM> and the crack-propagation regions 16a, 16b and crack-termination regions 27a, 27a', 27b, 27b.

A crack <NUM> is present in the structure <NUM>. It extends within the lower crack-propagation region 16b and the lower right-hand crack-termination region 27b'. The crack initiated at the point P on the edge of the opening <NUM>, in the lower crack-propagation region 16b. The crack <NUM> initially propagated substantially radially away from the opening <NUM>, until it encountered one of the first reinforcing elements <NUM><NUM>. Further propagation of the crack <NUM> in the radial direction was prevented by the first reinforcing element <NUM><NUM>. The crack <NUM> therefore continued to propagate substantially along the first direction x, guided between the first reinforcing element <NUM><NUM> and the adjacent first reinforcing element <NUM><NUM>. It will be appreciated that the presence of the two first reinforcing elements <NUM><NUM>, <NUM><NUM> constrains the propagation path of the crack <NUM>.

The first reinforcing elements <NUM><NUM>, <NUM><NUM> end at the boundary between the crack-propagation region 16b and the crack-termination region 27b'. When the crack <NUM> reached this point, its path was therefore no longer constrained. In the illustrated example, upon reaching the crack-termination region 27b' the crack <NUM> divided into several smaller sub-cracks which propagated throughout the crack-termination region 27b'. Each sub-crack ceased propagating upon encountering a boundary reinforcing element <NUM> at the boundary of the crack-termination region 27b'.

The shape and size of the crack-propagation region(s) and, if present, the crack-termination region(s) of an example load-bearing structure according to the invention are selected according to the particular intended application of that structure, to ensure that the energy which is causing the cracking will have been dissipated by the time the crack reaches the distal edge of a crack-propagation region, or if present, a crack-termination region. In examples in which the structure does not include any crack-termination regions, a given crack-propagation region may need to be longer in the first direction x than if an adjacent crack-termination region were present.

The crack-propagation regions (and, if present, crack-termination regions) of load-bearing structures according to the invention thereby function to guide cracks which form in the structure along selected pathways, which may be away from a main load path through the structure. This makes crack propagation in structures according to the invention highly predictable, and ensures that the structure will still be able to operate to transfer loads even after a crack has formed. These beneficial effects of structures according to the invention, combined with the inherent benefits of metal matrix composite materials, make such structures especially suitable for aerospace applications.

<FIG> shows an example aircraft <NUM> comprising one or more load-bearing structures according to the invention. The aircraft <NUM> has a fuselage <NUM> to which a pair of wings 502a, 502b is mounted. An engine 503a, 503b and a main landing gear 504a, 504b is mounted to each of the wings. In particular, each wing 502a, 502b comprises a gear rib, to which the respective main landing gear is attached. The gear ribs are load-transferring components, and may each comprise a load-bearing structure according to the invention. One or more components of the engine mounting mechanism by which each engine 503a, 503b is mounted to the corresponding wing 502a, 502b may also comprise load-bearing structures according to the invention. In general, any load-bearing structure of the aircraft <NUM> which is configured to transfer a load from a first part of the structure to a second part of the structure may advantageously be a load-bearing structure according to the invention.

Although the invention has been described above with reference to one or more preferred examples or embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Although the invention has been described above mainly in the context of a fixed-wing aircraft application, it may also be advantageously applied to various other applications, including but not limited to applications on vehicles such as helicopters, drones, trains, automobiles and spacecraft.

Claim 1:
A load-bearing structure (<NUM>, <NUM>, <NUM>) configured to, during operation of the structure, transfer load (L) from a first part (<NUM>) of the structure (<NUM>, <NUM>, <NUM>) to a second part (<NUM>) of the structure via a load path, the load-bearing structure comprising:
a matrix material (<NUM>);
a plurality of longitudinal first reinforcing elements (<NUM>) embedded in the matrix material (<NUM>), wherein the long axis of each first reinforcing element (<NUM>) is substantially aligned with a first direction; and
a plurality of longitudinal second reinforcing elements (<NUM>) embedded in the matrix material (<NUM>), wherein the long axis of each second reinforcing element (<NUM>) is substantially aligned with a second direction, the second direction being substantially perpendicular to the first direction;
wherein the structure (<NUM>, <NUM>, <NUM>) has a predefined crack-propagation region configured to control the propagation of a crack in the structure, and wherein the crack-propagation region (<NUM>, 16a, 16b, <NUM>) comprises multiple first reinforcing elements (<NUM>) and does not comprise any second reinforcing elements (<NUM>) or comprises multiple second reinforcing elements (<NUM>) and does not comprise any first reinforcing elements (<NUM>).