Lug with pocket and/or relief

A joining part with a wing comprises a through bore defining a fastening direction perpendicular to the wing. The wing comprises at least one non-through pocket configured to reduce the stress gradient in a peripheral zone of the through bore and/or at least one relief configured to reduce the stress gradient in a peripheral zone of the through bore, the non-through pocket forming a cavity in the thickness of the wing and comprising a curved side at a distance from the peripheral edge of the through bore. The joining part may be part of a lug, such as, for an aircraft fitting.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of the French patent application No. 1463215 filed on Dec. 23, 2014, the entire disclosures of which are incorporated herein by way of reference.

BACKGROUND OF THE INVENTION

The present invention relates to a joining part with a wing comprising a through bore.

The present invention generally relates to the field of connectors or attachments for joining different structural elements together, involving a single fastening direction. It applies, in a non-limiting manner, to the field of aircraft construction.

In conventional practice, numerous joining parts are used in aeronautics and, more particularly, in the construction of an aircraft, for example, for making structural joins, attaching equipment, or engines, landing gear, and so on.

By way of example, use is made of numerous lugs in aircraft, these lugs belonging to aircraft fittings, shackles or even connecting rods commonly used to join various elements of the aircraft structure.

Although there may be varied lug models, such lugs typically have a wing comprising a through bore defining a fastening direction perpendicular to the wing of the lug. The fastening direction corresponds, for example, to the axis of a screw or any other type of fastener inserted into the through bore.

Given the loads and forces taken up by the lug, stress concentrations arise over the peripheral zone around the through bore.

The stresses are at their maximum at the edge of the through bore and may lead to the lug prematurely suffering from fatigue.

In particular, risks of cracks or splits are observed at the edge of the bore, these cracks then potentially propagating into the peripheral zone around the through bore, in the linking parts of the lug located on either side of the through bore.

A known solution is described in document FR 2 997 143, which aims to limit the stress concentrations in the immediate vicinity of a through bore of a lug wing.

Document FR 2 997 143 thus makes provision for the formation of a through hole in the wing of the lug. This through hole is arranged along the longitudinal median axis of the wing.

Furthermore, through orifices are added between the bore and the aforesaid hole, offset on either side of the longitudinal median axis of the wing.

SUMMARY OF THE INVENTION

The present invention aims to improve the structure of a joining part to enhance its ability to withstand the appearance and the propagation of cracks in the part.

To that end, the present invention relates to a joining part with a wing comprising a through bore defining a fastening direction perpendicular to the wing.

According to the invention, the wing comprises at least one non-through pocket configured to reduce the stress gradient in a peripheral zone of the through bore and/or at least one relief configured to reduce the stress gradient in a peripheral zone of the through bore, said non-through pocket forming a cavity in the thickness of the wing and comprising a curved side at a distance from the peripheral edge of the through bore.

By seeking to reduce the stress gradient around the through bore by means of the addition of a non-through pocket and/or of a relief it is possible to reduce the observed stress peak and thus to delay the start of cracks at the edge of the through bore and the propagation of these cracks in the linking part.

The reduction in the stress gradient enables homogeneous stress values to be obtained in the peripheral zone of the bore, thus limiting the propagation of the cracks in the joining part.

This results in a beneficial effect not just on the weight of the joining part but also on its service life and dimensions, in terms of damage tolerance.

According to various features and diverse embodiments of the invention, which may be taken in isolation or in combination:

the non-through pocket forms a cavity in the surface of the wing, the cavity having a base extending in a plane substantially parallel to a plane of the wing;

the through bore is arranged in an end portion of the wing and centered on the intersection of a longitudinal median axis of the wing and of a transverse axis and said at least one non-through pocket and/or said at least one relief are located solely on a portion of the wing extending on the same side of said transverse axis, opposite a free end of the wing;

said at least one non-through pocket extends on either side of the longitudinal median axis;

said at least one non-through pocket and/or said at least one relief have a side extending parallel to a longitudinal edge of said wing;

said curved side of said at least one non-through pocket extends over an angular sector of between 30° and 180°;

said curved side of said at least one non-through pocket extends over a portion of an arc of a circle concentric with said through bore;

said at least one relief has a rib part extending substantially parallel to a longitudinal edge of said wing and an end part forming an angle of between 0 and 90° with said rib part;

said at least one non-through pocket and/or said at least one relief are symmetrical relative to a longitudinal median axis of said wing;

the joining part constitutes a lug of an aircraft fitting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIG. 1, a summary will first be given of the distribution of stresses in a joining part with a wing comprising a through bore.

In the remainder of the description, the term “comprising” does not exclude other elements or steps, and the singular article does not exclude the plural.

As clearly illustrated inFIG. 1, the joining part10comprises a wing11comprising a through bore12.

This type of joining part is used for joining different structural elements together, requiring a single joining point, achieved by means of a single fastening (not shown) on the axis of the through bore12.

Owing to the presence of the through bore12in the joining part10, the fastening direction Z is defined perpendicularly to the wing11, which extends in a plane X, Y.

This type of joining part can be found in various types of joints used, in particular, in aeronautics.

The joining parts, attaching various structures of an aircraft and also called fittings, comprise a lug with one or more wings that are each provided with a through bore in order to interact with a fastening.

It is thus estimated that, in an aircraft, more than a thousand lugs, corresponding to elementary joining points, are used at various points of the structure.

Consequently, the improvement of their performance levels and service life, and also any, albeit minimal, gain in weight, are of significance.

It is possible to identify various types of lug commonly used in aircraft: first, lugs formed as an isolated part, or lugs secured by one of their distant ends to their support.

This type of lug can be found, in particular, at the end of long connecting rods or, alternatively, in a shackle.

Other types of lug are secured by insertion into ribbed panels or involve one or more edge flanges that may be affixed to structural parts using assembly screws.

The illustrative diagram ofFIG. 1applies to any type of lug where a through bore12is present in a wing11and where the joining function is provided by means of a single fastening (screw, rod, bolt).

A stress concentration is observed, during use, in the peripheral zone of the through bore12.

A concentration peak is observed at the peripheral edge12aof the through bore12.

A curve thus illustrates, as a function of the distance r from the center c of the through bore12, the value of the tangential stress σθθobserved in the wing11and, more particularly, in the peripheral zone around the through bore12.

InFIG. 1, the curve in fine lines illustrates, by way of example, a tangential stress gradient observed in the peripheral zone of the through bore12as a function of the distance r from the center c of the through bore12a.

Thus, the maximum tangential stress σmaxis located at the edge12aof the through bore12, i.e., at a distance D/2 from the center c of the through bore12, where D is the diameter of the through bore12.

The value of the tangential stress σθθvaries greatly between the peripheral edge12aof the through bore12and a peripheral edge11aof the wing11of the joining part10.

Owing to this significant level of stress, cracks may appear at the peripheral edge12aof the through bore12, and they are likely to propagate in the wing11in the direction of a radius of the through bore12.

A description will now be given with reference toFIGS. 2 and 3of a first embodiment of a joining part10that makes it possible to reduce the crack appearance and propagation times.

The wing11comprises at least one non-through pocket21or at least one relief22configured to reduce the stress gradient, and thus the value of the maximum tangential stress σmaxin the peripheral zone of the through bore12.

In the embodiment illustrated inFIG. 2, and in a non-limiting manner, the wing11comprises a non-through pocket21and two reliefs22.

The non-through pocket21and the reliefs22are configured to reduce the stress gradient, as illustrated on the curve ofFIG. 1(in dark lines).

A stress gradient is thus observed that has a tangential stress peak σ′maxthat is less than the tangential stress peak σmaxobserved in the absence of a pocket and/or of a relief. Furthermore, the value of the tangential stress σθθis more homogeneous in the peripheral zone of the through bore12.

This reduction in the stress gradient thus results in an extension of the service life before the appearance of cracks or splits at the peripheral edge12aof the through bore12and a reduction in the speed of propagation of the cracks in the peripheral zone of the through bore12.

The positioning of the non-through pocket21and of the reliefs22, and also their dimensions, are configured to obtain a reduction in the stress gradient and thus in the value of the maximum tangential stress σmax.

Thus, the dimensions of the joining part10take account not only of the width W of the wing11, the diameter D of the bore12and the thickness T of the wing11, but also the thickness T1at the non-through pocket21, which is less than the thickness T of the wing11, and the thickness T2of the reliefs22, which is greater than the thickness T of the wing11. The choice of these parameters makes it possible to modify the mechanical behavior of the joining part10and thus to reduce the value of the stresses in the peripheral zone of the through bore12, which corresponds to the most loaded zone of the joining part10during use.

More precisely inFIG. 2, in this first embodiment the through bore12is arranged in an end portion11bof the wing11.

The through bore12is centered on the intersection c of a longitudinal median axis X of the wing11and of a transverse axis Y perpendicular to the longitudinal median axis X.

The center c of the through bore12merges with the fastening direction Z perpendicular to the wing11, corresponding to the direction of a fastening (not shown).

The non-through pocket21and the reliefs22are located only in a portion of the wing11that extends on the same side of the transverse axis Y, opposite a free end11cof the wing11.

The non-through pocket21extends on either side of the longitudinal median axis X.

The non-through pocket21thus forms a cavity in the surface of the wing11, with a base21′ extending in a plane substantially parallel to the plane X, Y of the wing11of the joining part10.

It will be noted that a non-through pocket21may also be arranged on each face of the wing11, preferably symmetrically relative to the plane X, Y of the wing11of the joining part10.

As clearly illustrated inFIG. 3, the non-through pocket21thus forms a cavity in the thickness T of the wing11such that the thickness T1at the non-through pocket21is less than the thickness T of the wing11.

In other words, the non-through pocket21corresponds to a non-through thinner zone in the wing11.

The reliefs22are, in this embodiment, arranged on either side of the non-through pocket21, between the non-through pocket21and a longitudinal edge11dof the wing11.

As clearly illustrated inFIG. 3, the reliefs22are in this case constituted respectively by a rib of which the thickness T2is greater than the thickness T of the wing11.

Furthermore, the non-through pocket21and the reliefs22each have a side that extends parallel to the longitudinal edge11dof the wing11.

More precisely, in the embodiment illustrated inFIG. 2, the non-through pocket21comprises two longitudinal sides21aextending substantially parallel to the longitudinal median axis X. The two longitudinal sides21aof the non-through pocket21are parallel respectively to two longitudinal edges11dof the wing11.

In this embodiment, the distance between each longitudinal side21aof the non-through pocket21and the longitudinal edge11dof the wing11is substantially identical.

Furthermore, the two reliefs22each have a rib part22aof which the sides extend parallel to the longitudinal edges11dof the wing11and, in this case, parallel to the longitudinal median axis X of the wing11.

Here, each rib part22ais rectilinear and has a width P in the transverse direction Y of the wing11.

In this embodiment, the distance between each rib part22aand the longitudinal edge11dof the wing11is substantially identical.

Each relief22has, furthermore, in this case, and in a non-limiting manner, an end part22bforming a non-zero angle relative to the rib part22aand oriented toward the through bore12in the wing11.

The orientation of the end part22bof each relief22corresponds to an orientation in the direction of the principal maximum stress around the through bore12. The reliefs22thus play a stiffening role, improving the mechanical strength of the wing11.

More generally, the end part22bmay form an angle of between 0 and 90° with the rib part22a, such that the relief22may be rectilinear or have an inclined end part22b.

Moreover, the non-through pocket21comprises a curved side21bextending, in this case, over a portion of an arc of a circle concentric with the through bore12.

The curved side21bof the non-through pocket21extends over an angular sector of at least 30° and, more generally, a sector that may be between 30° and 180°.

In the embodiment illustrated inFIG. 2, the angular sector of the curved side21bof the non-through pocket21extends, by way of example, substantially over 120°.

It will be noted that the angular sector varies as a function of the ratio W/D, where W corresponds to the width of the wing11and D to the diameter of the through bore12.

Generally, when the dimensions of the joining part10described above are chosen, the transverse section is kept substantially constant in the wing11, irrespective of the transverse plane considered.

Thus, at the transverse axis Y, passing through the center c of the through bore12, the section S of the wing11is equal to:
S=(W−D)×T

The section of the wing11taken in other transverse planes parallel to the transverse axis Y remains substantially constant at this value of section S.

Thus, as illustrated inFIG. 3, the dimensions of the reliefs22, the non-through pocket21, including the various thicknesses T1, T2, T of the wing11, are chosen so as to meet the criterion of maintenance of the value of the section S.

In addition to these parameters, the dimensions of the joining part10also take account of the distance L separating the curved side21bof the through bore12and the distance between each longitudinal side21aof the non-through pocket21and the corresponding longitudinal edge11dof the wing11.

It will also be noted that, preferably, the positioning of the non-through pocket21and of the reliefs22is symmetrical relative to the longitudinal median axis X of the wing11.

Moreover, the non-through pocket21and the reliefs22are, in this case, symmetrical relative to the longitudinal median axis X of the wing11.

Furthermore, the peripheral zone around the through bore12that is affected by the load introduced at the fastening axis extends in a portion of the wing11located at a maximum distance from the center c of the through bore12equal to 1.57×W, where W is the width of the wing11(see the definition of the size of the peripheral zone subject to stresses by THEOCARIS, in “The stress distribution in strip loaded in tension by means of a central pin,” J. Applied Mech. 85-90-1940).

It is thus advantageous to provide one or more non-through pockets21and one or more reliefs22in a zone of the wing11that does not extend beyond this maximum distance of 1.57×W.

The design of a joining part10of this type will take account of all the parameters described above.

It will likewise take account of the value of the shortest distance L between the non-through pocket21and the through bore12, which in this case corresponds to the distance L between the curved side21bof the non-through pocket21and the peripheral edge12aof the through bore12.

Moreover, the dimensions of the joining part10will take account of the width P of the rib part22aof the reliefs22and the shortest distance M between the reliefs22and the through bore12.

In particular, the shortest distances L, M as defined above may be dimensioned as a function of the shortest distance B existing between the through bore12and the peripheral edge11aof the wing11.

It will be noted that, in the embodiment illustrated inFIG. 2, the free end11cof the wing11has a semi-circular form around the through bore12: the center of the free end11cmay advantageously be offset (upward, in the example illustrated inFIG. 2) relative to the center c of the through bore12, the effect of this slight offset being to reduce the stress peak σmax.

Here, the shortest distance B that exists between the through bore12and the peripheral edge11aof the wing corresponds to the value of the width of the linking part (W−D)/2 on the transverse axis y.

The shortest distance M between the reliefs22and the through bore12is advantageously less than the shortest distance B between the through bore12and the peripheral edge11aof the wing11and greater than one quarter of the shortest distance B between the through bore12and the peripheral edge11aof the wing11.

Similarly, the shortest distance L between the non-through pocket21and the through bore12is advantageously greater than or equal to one third of the radius D/2 of the through bore12.

Moreover, the distance between the longitudinal edges11dof the wing11and the longitudinal sides21aof the non-through pocket21and the distance between the longitudinal edges11dof the wing11and the rib part22aof the reliefs22may likewise be variable.

What is more, the provision of non-through pockets21in the joining part10makes it possible to reduce the weight of said joining part, which offers a significant advantage within the context of an aeronautical application, where several hundreds or thousands of lugs are used.

In particular, it will be noted that the addition of reliefs22as envisaged in the embodiment inFIG. 2makes it possible further to reduce the thickness T1of the wing11at the non-through pocket21because the presence of the reliefs22has a greater effect on the reduction of the value of the maximum tangential stress σmaxthan the presence of the non-through pocket21. In a case such as this, the weight of the joining part10may be further reduced.

All the parameters described above are thus taken into account in terms of the dimensions of the joining part10illustrated inFIGS. 2 and 3.

By way of example, a joining part of this type may be a lug of an aircraft fitting, which may represent up to 80% of the cases of lugs used in an aircraft.

FIG. 4, furthermore, illustrates a second embodiment of a joining part10.

This joining part10is similar to the example described above with reference toFIG. 2since it likewise involves a wing11comprising a through bore12defining a fastening direction perpendicular to the wing11.

Features inFIG. 4andFIG. 2that are similar thus bear the same numerical references and have no need to be described again in detail at this point.

The joining part10in this case further comprises a fastening flange40allowing mounting of the joining part10, for example by bolting (indicated in schematic form by axes (or screws)41), to a structure such as an aircraft door.

In this embodiment, the wing11comprises a non-through pocket21configured to reduce the stress gradient in the peripheral zone of the through bore12.

To that end, it will be noted that, as in the embodiment inFIG. 2, the non-through pocket21is located solely on a portion of the wing11that extends on the same side of the transverse axis Y, opposite a free end11cof the wing11.

The non-through pocket21furthermore extends on either side of the longitudinal median axis X of the wing11.

It comprises two sides21aeach parallel to a longitudinal edge11dof the wing11.

The non-through pocket21also comprises a curved side21bextending opposite the through bore12.

In this case, the radius of the curved side21bof the non-through pocket21is greater than the radius D/2 of the through bore12, increased by the shortest distance L between the non-through pocket21and the through bore12.

Thus, in this embodiment, the curved side21bof the non-through pocket21does not extend over a portion of an arc of a circle concentric with the through bore12.

Naturally, the examples described above with reference toFIGS. 2 to 4are given only for purely illustrative purposes.

Thus, the number of non-through pockets21and of reliefs22may be different.

Furthermore, the wing11of the joining part10may have only one or several reliefs22.

The joining parts may, moreover, be produced by machining, but also using different fabrication techniques, including molding, stamping, additive layer manufacturing or, furthermore, by linear friction welding.

Advantageously, it is possible to use different materials when fabricating the wing11of the joining part10and, for example, different materials for fabricating the reliefs22.

It will further be noted in the embodiment illustrated inFIG. 4that the positioning of non-through pockets21and/or of reliefs may take account also of the presence of the fastening flange40and of the fastening stresses of the joining part10at the fastening flange40.

Of course, numerous modifications may be made to the illustrative embodiments described above without departing from the context of the invention.