Composite laminate and load-introduction component for a load-introduction joint

Abstract

A composite laminate for connection to at least one attachment component in a load-introduction joint, said composite laminate comprising an opening that is adapted to receive a load-introduction component of said load-introduction joint, wherein a prefailure indication element is provided in the region of said opening, said prefailure indication element being modifiable by said load-introduction component in a prefailure mode of said load-introduction joint for indication of an impending function-affecting failure of said composite laminate. The invention is further related to a corresponding load-introduction component.

Claims

1. A composite laminate for connection to an attachment component in a load-introduction joint, the composite laminate comprising: an opening that comprises an inner peripheral sidewall and extends between opposing sides of the composite laminate, the opening being adapted to receive a load-introduction component of the load-introduction joint therethrough such that the load-introduction component comes at least partly in direct contact with the inner peripheral sidewall of the opening in a normal operation mode of the load-introduction joint; and a prefailure indication element that is arranged inside the opening on a portion of the inner peripheral sidewall of the opening, the prefailure indication element being modifiable by the load-introduction component in a prefailure mode of the load-introduction joint from a first condition in the normal operation mode of the load-introduction joint to a second condition in the prefailure mode for indication of an impending function-affecting failure of the composite laminate, wherein the prefailure indication element is compressed or partially destructed by the load-introduction component of the load-introduction joint in the prefailure mode from the first condition in the normal operation mode to the second condition in the prefailure mode, wherein the prefailure indication element comprises at least one prefailure indication projection that is arranged on the inner peripheral sidewall of the opening, wherein the at least one prefailure indication projection is arc-shaped, and wherein the at least one arc-shaped prefailure indication projection is arranged between a first and a second arc-shaped axial recess provided inside the opening on the inner peripheral sidewall of the opening.

2. The composite laminate of claim 1, wherein the opening comprises a first opening diameter when the prefailure indication element is in the first condition in the normal operation mode and a second opening diameter when the prefailure indication element is in the second condition in the prefailure mode, and wherein the first opening diameter is no more than 50% of the second opening diameter.

3. The composite laminate of claim 1, wherein the prefailure indication element is provided inside the opening at least on a peripheral side of the opening that is arranged diametrically opposed to a main load direction of a main load that is applicable to the composite laminate in operation.

4. A load-introduction joint, comprising: an attachment component; a load-introduction component; and a composite laminate that is mounted to the attachment component via the load-introduction component, the composite laminate comprising: an opening that comprises an inner peripheral sidewall and extends between opposing sides of the composite laminate, wherein the opening accommodates at least partly the load-introduction component such that the load-introduction component is at least partly in direct contact with the inner peripheral sidewall of the opening in a normal operation mode of the load-introduction joint; and a prefailure indication element that is arranged inside the opening on a portion of the inner peripheral sidewall of the opening, the prefailure indication element being modifiable by the load-introduction component in a prefailure mode of the load-introduction joint from a first condition in the normal operation mode of the load-introduction joint to a second condition in the prefailure mode for indication of an impending function-affecting failure of the composite laminate, wherein the prefailure indication element of the composite laminate is compressible or partially destructible by the load-introduction component in the prefailure mode from the first condition in the normal operation mode to the second condition in the prefailure mode and comprises at least one prefailure indication projection that is arranged on the inner peripheral sidewall of the opening of the composite laminate, wherein the at least one prefailure indication projection is arc-shaped, and wherein the at least one arc-shaped prefailure indication projection is arranged between a first and a second arc-shaped axial recess provided inside the opening of the composite laminate on the inner peripheral sidewall of the opening.

5. The load-introduction joint of claim 4, wherein the opening of the composite laminate comprises a first opening diameter when the prefailure indication element is in the first condition in the normal operation mode and a second opening diameter when the prefailure indication element is in the second condition in the prefailure mode, and wherein the first opening diameter is no more than 50% of the second opening diameter.

6. The load-introduction joint of claim 4, wherein the prefailure indication element of the composite laminate is provided inside the opening of the composite laminate at least on a peripheral side of the opening that is arranged diametrically opposed to a main load direction of a main load that is applicable to the composite laminate in operation.

7. A composite laminate for connection to at least one attachment component in a load-introduction joint, the composite laminate comprising: an opening extending between opposing sides of the composite laminate and having a central axis thereof, the opening being adapted to receive a load-introduction component of the load-introduction joint; and a prefailure indication projection defined in and extending along a portion of a peripheral sidewall of the opening, the prefailure indication projection having a first position and a second position, the projection being compressed or partially destructed by the load-introduction component when in the second position relative to when in the first position for indication of an impending function-affecting failure of the composite laminate, wherein the prefailure indication projection comprises a projection defined between axially opposing first and second recesses that extend radially away from the central axis into the peripheral sidewall of the opening, and wherein the prefailure indication projection extends radially toward the central axis between the first and second recesses.

8. The composite laminate according to claim 7, wherein the prefailure indication projection has a thickness of 50% to 90% of a thickness of the composite laminate adjacent to the load-introduction component when the prefailure indication projection is in the first position.

9. The composite laminate according to claim 7, wherein the prefailure indication projection is arranged diametrically opposed to a main load direction of a main load that is applicable to the composite laminate in operation.

10. The composite laminate according to claim 7, wherein the prefailure indication projection is formed to reduce a given contact surface between the load-introduction component and the composite laminate when the prefailure indication projection is in the first position.

11. The composite laminate according to claim 7, wherein the at least one prefailure indication projection is an arc-shaped prefailure indication projection that extends adjacent to the load-introduction component about less than an entire circumference of the load-introduction component when the arc-shaped prefailure indication projection is in the first position.

12. The composite laminate according to claim 11, wherein the at least one arc-shaped prefailure indication projection extends in a thickness direction parallel to the central axis of the opening between the first and second recesses.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Preferred embodiments of the invention are outlined by way of example in the following description with reference to the attached drawings. In these attached drawings, identical or identically functioning components and elements are labeled with identical reference numbers and characters and are, consequently, only described once in the following description.

(2) FIG. 1 shows a sectional view of a load-introduction joint with a composite laminate and a load-introduction component according to a first embodiment of the invention,

(3) FIG. 2 shows a top view of the composite laminate of FIG. 1, seen in direction of an arrow II of FIG. 1,

(4) FIG. 3 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2,

(5) FIG. 4 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows IV-IV of FIG. 2,

(6) FIG. 5 shows a sectional view of the composite laminate of FIG. 2 and FIG. 3 in a normal operation mode and in the prefailure mode, seen in direction of arrows V-V of FIG. 3,

(7) FIG. 6 shows the composite laminate and the load-introduction component of FIG. 1 in a first possible function-affecting failure mode,

(8) FIG. 7 shows the composite laminate and the load-introduction component of FIG. 1 in a second possible function-affecting failure mode,

(9) FIG. 8 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a second embodiment,

(10) FIG. 9 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a third embodiment,

(11) FIG. 10 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a fourth embodiment,

(12) FIG. 11 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a fifth embodiment,

(13) FIG. 12 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a sixth embodiment,

(14) FIG. 13 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to a seventh embodiment,

(15) FIG. 14 shows a sectional view of the composite laminate of FIG. 1 and FIG. 2, seen in direction of arrows III-III of FIG. 2, according to an eighth embodiment,

(16) FIG. 15 shows a simplified sectional view of the load-introduction joint of FIG. 1, with a load-introduction component according to a second embodiment,

(17) FIG. 16 shows a sectional view of the load-introduction component of FIG. 1 according to a third embodiment,

(18) FIG. 17 shows a sectional view of the load-introduction component of FIG. 1 according to a fourth embodiment,

(19) FIG. 18 shows a sectional view of the load-introduction component of FIG. 1 according to a fifth embodiment,

(20) FIG. 19 shows a sectional view of the load-introduction component of FIG. 1 according to a sixth embodiment, and

(21) FIG. 20 shows a sectional view of the load-introduction component of FIG. 1 according to a seventh embodiment.

DETAILED DESCRIPTION OF THE INVENTION

(22) FIG. 1 shows a load-introduction joint 1 according to the present invention, which comprises a composite laminate 6 that is connected to at least one, and illustratively two attachment components 4, 5 by means of at least one associated load-introduction component. The latter is adapted to introduce loads that are applied to the composite laminate 6 into the attachment components 4, 5.

(23) By way of example, the load-introduction joint 1 can be used to connect a rotor blade of a multi-blade rotor of a rotary-wing aircraft to a rotor head of said multi-blade rotor. In this case, the composite laminate 6 is associated with, or integrated into, the rotor blade and the attachment components 4, 5 are associated with the rotor head and, e.g., integrated into a flexbeam thereof.

(24) According to one aspect, the load-introduction component comprises a bolt 2 with a bolt shaft 2a and three bushings 8, 9, 23. A first bushing 8 is arranged in an associated opening 4a provided in the attachment component 4, a second bushing 9 is arranged in an associated opening 5a provided in the attachment component 5, and a third bushing 23 is arranged in an associated opening 6a provided in the composite laminate 6. All three bushings 8, 9, 23 are equipped with corresponding openings 8a, 9a, 23a, respectively, wherein the bolt shaft 2a with an outer circumference 2b is arranged. The bolt 2 is secured in the bushings 8, 9, 23 by means of a nut 3 and, thus, secures the bushings 8, 9, 23 to the attachment components 4, 5 and the composite laminate 6 on the one hand and, on the other hand, attaches the composite laminate 6 to the attachment components 4, 5.

(25) In operation of the load-introduction joint 1, a main load F that is, e.g., applicable to the composite laminate 6 in a main load direction 10, which illustratively corresponds to a longitudinal extension of the composite laminate 6, is introduced from the composite laminate 6 via the bushing 23 into the bolt 2, i.e. the bolt shaft 2a, and from the bolt shaft 2a via the bushings 8, 9 into the attachment components 4, 5, respectively. Thus, in a strict sense the third bushing 23 defines the load-introduction component in the load-introduction joint 1 with respect to the composite laminate 6.

(26) However, it should be noted that use of the three bushings 8, 9, 23 is only optional and not mandatory for realization of the present invention. In fact, the three bushings 8, 9, 23 can alternatively be implemented as only one or two bushings or, still alternatively, be omitted, if the underlying dimensions of the openings 4a, 5a, 6a provided in the attachment components 4, 5 and the composite laminate 6 match the underlying dimensions of the bolt shaft 2a, i.e. its outer circumference 2b, at least within usual predetermined manufacturing tolerances that allow to press-fit or screw the bolt 2 into the attachment components 4, 5 and the composite laminate 6. In this case, the bolt 2 defines the load-introduction component of the load-introduction joint 1. Therefore, the load-introduction component of the load-introduction joint 1 is hereinafter referred to with the reference sign “2”, for simplicity.

(27) Illustratively, the composite laminate 6 with the opening 6a comprises an upper side 6b and an opposed lower side 6c and is preferably embodied similar to a bearing laminate. On the upper side 6b, the attachment component 4 with the opening 4a is arranged. Therefore, the attachment component 4 is hereinafter referred to as the “upper” component, for simplicity and brevity. On the lower side 6c, the attachment component 5 with the opening 5a is arranged. Therefore, the attachment component 5 is hereinafter referred to as the “lower” component, for simplicity and brevity.

(28) The opening 4a of the upper component 4 has dimensions that preferably match corresponding dimensions of an outer circumference 8b of a sleeve-like area 8c of the first bushing 8, at least within usual predetermined manufacturing tolerances in analogy to what is said above. It should be noted that the sleeve-like area 8c illustratively merges into a flange-like extension arranged essentially between the upper component 4 and the nut 3. However, this flange-like extension is only optional and, therefore, not specifically referenced, for simplicity and clarity of the drawings.

(29) The opening 5a of the lower component 5 has dimensions that preferably match corresponding dimensions of an outer circumference 9b of a sleeve-like area 9c of the second bushing 9, at least within usual predetermined manufacturing tolerances in analogy to what is said above. It should be noted that the sleeve-like area 9c illustratively merges into a flange-like extension arranged essentially between the lower component 5 and a bolt head 2d of the bolt 2. However, this flange-like extension is only optional and, therefore, not specifically referenced, for simplicity and clarity of the drawings.

(30) According to one aspect of the present invention, the opening 6a of the composite laminate 6 is provided with a prefailure indication element 7. More specifically, this prefailure indication element 7 is preferably provided in the region of the opening 6a, preferentially at least on a mainly loaded side thereof, and modifiable by the load-introduction component 2 in a prefailure mode of the load-introduction joint 1 for indication of an impending function-affecting failure of the composite laminate 6. Such a modification of the prefailure indication element 7 by the load-introduction component 2 in the prefailure mode comprises, for instance, an at least partial compression and/or destruction of the prefailure indication component 7.

(31) In other words, the prefailure indication element 7 is adapted to fail without affecting the overall function of the load-introduction joint 1. However, after such a failure of the prefailure indication element 7, a subsequent function-affecting failure of the load-introduction joint 1 as such must reasonably be expected. Therefore, the load-introduction joint 1 is considered being in a “prefailure mode” after failure of the prefailure indication element 7.

(32) Preferably, the prefailure indication element 7 is provided at least on a peripheral side 6g of the opening 6a that is arranged diametrically opposed to the main load direction 10 of the main load F and, thus, defines the mainly loaded side of the opening 6a. The prefailure indication element 7 can be realized by one or more prefailure indication projections and is, illustratively, realized by a single prefailure indication projection 7a that is arranged in the opening 6a. In the region of the single prefailure indication projection 7a, the opening 6a has dimensions that preferably match corresponding dimensions of an outer circumference 23b of a sleeve-like area 23c of the third bushing 23, at least within usual predetermined manufacturing tolerances in analogy to what is said above.

(33) According to a first embodiment, the single prefailure indication projection 7a is arc-shaped and provided only on the mainly loaded side of the opening 6a, as illustrated in greater detail in FIG. 2. This arc-shaped prefailure indication projection 7a is preferably arranged between a first and a second arc-shaped axial recess 7b, 7c provided in the region of the opening 6a.

(34) FIG. 2 shows the upper side 6b of the composite laminate 6 of FIG. 1, which is provided with the opening 6a and the prefailure indication element 7 with the prefailure indication projection 7a. The latter illustratively comprises a maximal length 11e in the main load direction 10 of FIG. 1.

(35) The opening 6a has a center 6d and an opening diameter 11c. The center 6d is illustratively arranged on a longitudinal axis of the composite laminate 6, which is coaxial to the main load direction 10 of FIG. 1, by way of example.

(36) The composite laminate 6 comprises longitudinal edges between its upper side 6b and its lower side 6c of FIG. 1, which are illustratively oriented in parallel to the main load direction 10 and from which only the—in FIG. 2—upper longitudinal edge is labelled with the reference sign 6f. The composite laminate 6 further comprises end edges between its upper side 6b and its lower side 6c of FIG. 1, which are illustratively oriented perpendicular to the main load direction 10 and from which only the—in FIG. 2—left end edge is shown and labelled with the reference sign 6e.

(37) Between the end edge 6e and the opening center 6d, an end edge distance 11a is embodied. Between the longitudinal edge 6f and the opening center 6d, a longitudinal edge distance 11b is embodied. The relation between the end edge distance 11a or the longitudinal edge distance 11b and the opening diameter 11c is preferably less than 2 and, illustratively, equal to 1.

(38) Furthermore, according to one aspect of the present invention, the length 11e of the prefailure indication projection 7a is determined dependent on the opening diameter 11c. More specifically, the length 11e preferably amounts to at least 0.5% of the opening diameter 11c, if the latter is greater than 40 mm. If, however, the opening diameter 11c is less than 4 mm, the length 11e preferably amounts maximally to 50% of said opening diameter 11c. More generally, the length 11e is preferentially comprised in a range from 0.2 mm to 2 mm.

(39) FIG. 3 shows the composite laminate 6 with the opening 6a and the prefailure indication element 7 with the prefailure indication projection 7a of FIG. 1 and FIG. 2 in greater detail. FIG. 3 further illustrates the axial recesses 7b, 7c of FIG. 1 that delimit the prefailure indication projection 7a.

(40) Illustratively, the composite laminate 6 exhibits between its upper and lower sides 6b, 6c a thickness 11d and the prefailure indication projection 7a exhibits a projection thickness 7d. Preferably, this projection thickness 7d comprises 50% to 90% of the thickness 11d of the composite laminate 6, and illustratively amounts to 50% thereof.

(41) FIG. 4 shows the composite laminate 6 with the opening 6a of FIG. 1 to FIG. 3 to further illustrate the opening diameter 11c and the projection thickness 7d of the prefailure indication projection 7a according to FIG. 3.

(42) FIG. 5 shows the composite laminate 6 of FIG. 1 to FIG. 4 in a normal operation mode and in the prefailure mode according to the present invention. In the normal operation mode, the bolt shaft 2a of the bolt 2 of FIG. 1 and/or the sleeve-like area 23c of the bushing 23 of FIG. 1 are arranged in an initial position 12a in the opening 6a of the composite laminate 6 and the prefailure indication element 7, i.e. the prefailure indication projection 7a of FIG. 1 to FIG. 4, is intact, i.e. unmodified.

(43) In contrast thereto, the prefailure indication projection 7a is modified i.e. at least partly compressed and/or destroyed, in the prefailure mode, wherein the bolt shaft 2a of the bolt 2 of FIG. 1 and/or the sleeve-like area 23c of the bushing 23 of FIG. 1 are arranged in a prefailure indication position 12b. In this prefailure indication position 12b, a gap 13 is formed between the composite laminate 6 and the bolt 2 and/or the bushing 23 in the opening 6a. This gap 13 is preferably adapted to allow for an easy and reliable detection of the prefailure mode.

(44) It should be noted that the gap 13 as such is not directly detectable, as it is covered by the upper and lower components 4, 5 in FIG. 1, However, if the gap 13 is formed a relative positioning of the composite laminate 6 with respect to these upper and lower components 4, 5 is changed and easily detectable.

(45) FIG. 6 and FIG. 7 show the composite laminate 6 of FIG. 5 in the prefailure mode. For simplicity, only the bolt shaft 2a of the bolt 2 of FIG. 1 is respectively illustrated in the opening 6a of the composite laminate 6 and arranged in the prefailure indication position 12b according to FIG. 5.

(46) As indicated above, the prefailure mode alerts and indicates an impending function-affecting failure of the composite laminate 6. Such a function-affecting failure can e.g. be caused by transversal ruptures 14a occurring in the composite laminate 6 according to FIG. 6, and/or by longitudinal ruptures 14b occurring in the composite laminate 6 according to FIG. 7. However, by detecting the prefailure mode by means of the gap 13, an impending occurrence of such transversal and/or longitudinal ruptures can be predicted and counter-measures can be taken in advance, e.g. by exchanging the composite laminate 6.

(47) It should be noted that in the composite laminate 6 according to the first embodiment, which is described as such in detail above with reference to FIGS. 1 to 4, the prefailure indication element 7 and, more particularly, the prefailure indication projection 7a is implemented by forming the axial recesses 7b, 7c inside the opening 6a. These axial recesses 7b, 7c are illustratively C-shaped in cross-section and preferably produced during manufacture of the composite laminate 6, e.g. by adequately forming layers that are lying upon each other. Alternatively, the axial recesses 7b, 7c can be formed by producing elongated blind holes in the opening 6a. Furthermore, only the single main load direction 10 was assumed. However, other shapes and production techniques and also more than one main load directions are also contemplated, as described below with reference to FIGS. 8 to 14.

(48) FIG. 8 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7a that is provided in the opening 6a and now embodied according to a second embodiment. In this second embodiment, the prefailure indication projection 7a is arc-shaped and arranged between a first and a second axial chamfer 15a, 15b that are provided on each axial end of the opening 6a. However, these axial chamfers 15a, 15b, and consequently also the arc-shaped prefailure indication projection 7a, are only provided on a mainly loaded side of the opening 6a, i.e. illustratively adjacent to the end edge 6e.

(49) FIG. 9 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7a with the axial recesses 7b, 7c that are provided in the opening 6a and now embodied according to a third embodiment. In this third embodiment, the axial recesses 7b, 7c, and consequently also the prefailure indication projection 7a, are again arc-shaped and only provided on a mainly loaded side of the opening 6a, i.e. illustratively adjacent to the end edge 6e, like in the first embodiment. Alternatively, they can be annular in the case of multiple possible load directions.

(50) However, the axial recesses 7b, 7c are smaller than in the first embodiment. Furthermore, the prefailure indication projection 7a comprises at least one undercut 16 that is adapted to divide said at least one prefailure indication projection 7a into first and second projection sections. Thus, a more even pressure distribution on the prefailure indication projection 7a can be achieved in operation, or more pressure near the end edge 6e of the composite laminate 6 is achievable so that a higher strength against bending moments can be realized.

(51) In other words, by dividing the at least one prefailure indication projection 7a into first and second projection sections, two prefailure indication projections are created, which are illustratively arranged in parallel to the longitudinal extension of the composite laminate 6. More generally, two or more prefailure indication projections can be defined by peripheral recesses and/or undercuts that are provided in the region of the opening 6a, independent on whether the peripheral recesses and/or undercuts are oriented in parallel, perpendicular or transversely to the longitudinal extension of the composite laminate 6. Such a configuration can likewise be obtained by thinning out the composite laminate 6 in the region of the prefailure indication element 7.

(52) FIG. 10 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7a that is provided in the opening 6a and now embodied according to a fourth embodiment. In this fourth embodiment, the prefailure indication projection 7a is arranged between a first and a second axial chamfer 15a, 15b that are provided on each axial end of the opening 6a, like in the second embodiment. However, in contrast to this second embodiment, the axial chamfers 15a, 15b, and the prefailure indication projection 7a are now annular. This is particularly useful if multiple load directions are possible instead of the single main load direction 10 of FIG. 1.

(53) FIG. 11 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7a that is provided in the opening 6a and now embodied according to a fifth embodiment. In this fifth embodiment, the prefailure indication projection 7a is illustratively arc-shaped like in the first embodiment. However, in contrast to the first embodiment, the prefailure indication projection 7a now comprises at least one prefailure layer 17 that is mounted into the opening 6a. Preferably, the prefailure layer 17 comprises a laminate section with one or more dummy layers that define the prefailure indication projection 7a.

(54) More specifically, in the composite laminate 6 an underlying bearing force is mainly carried by layers with high stiffness and high strength in the normal direction of the bearing stress. The bearing stress in these layers can be increased by locally replacing some of the layers with dummy layers that have a low stiffness and low strength in the normal direction of the bearing stress, thereby defining the prefailure layer 17. This can, e.g., be reached by replacing some of the 0° layers, which are oriented parallel to the main load direction 10 of FIG. 1, with other layers, e.g., 90° layers, or non-FRP layers.

(55) FIG. 12 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7a that is provided in the opening 6a and now embodied according to a sixth embodiment. In this sixth embodiment, the prefailure indication projection 7a is again arc-shaped and only provided on a mainly loaded side of the opening 6a, i.e. illustratively adjacent to the end edge 6e, like in the first embodiment. However, the prefailure indication projection 7a is now arranged between a first and a second tangential recess 18 provided in the region of the opening 6a on said mainly loaded side. Illustratively, the tangential recesses 18 are ear-shaped.

(56) FIG. 13 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7a that is provided in the opening 6a and now embodied according to a seventh embodiment. In this seventh embodiment, the prefailure indication projection 7a is again annular and, thus, particularly suitable for applications with more than one main load direction.

(57) However, in contrast to the sixth embodiment, multiple prefailure indication projections 7a are now arranged between multiple tangential recesses 18 that are preferably arranged equidistantly around the opening 6a. The number of tangential recesses 18 and corresponding prefailure indication projections 7a is application-specific and amongst others dependent on an underlying number of possible load directions.

(58) FIG. 14 shows the composite laminate 6 of FIG. 1 with the prefailure indication element 7 having the prefailure indication projection 7a that is provided in the opening 6a and now embodied according to an eighth embodiment. In this eighth embodiment, the prefailure indication projection 7a is illustratively arc-shaped like in the first embodiment. However, in contrast to the first embodiment, the prefailure indication projection 7a now comprises a constituting material that is weaker than a constituting material of the composite laminate 6.

(59) It should be noted that the prefailure indication element 7 is always described above with reference to FIGS. 1 to 14 as being realized by the composite laminate 6. However, other realizations are also possible and likewise contemplated, as described hereinafter with reference to FIGS. 15 to 20.

(60) FIG. 15 shows the load-introduction joint 1 of FIG. 1 with the upper and lower components 4, 5, the composite laminate 6, the associated load-introduction component and the prefailure indication element 7. As described above, the load-introduction component is either embodied by the bolt 2 with the bolt shaft 2a, or by the bushing 23, which are here exemplarily illustrated as a single component, for simplicity and clarity of the drawings.

(61) According to one aspect of the present invention, the prefailure indication element 7 is provided on a cylindrical section of the load-introduction component, i.e. on an outer circumference 2b of the bolt shaft 2a of the bolt 2 or on an outer circumference 23b of the bushing 23. Preferably, the prefailure indication element 7 comprises at least one and, preferentially, multiple annular recesses 19 provided on said cylindrical section. Thus, a contact surface between the bolt 2, or the bushing 23, and the composite laminate 6 in the opening 6a can be reduced in analogy to what is said above.

(62) It should be noted that such a reduction of the contact surface can be achieved by performing multiple different modifications to the bolt 2 or the bushing 23. Therefore, various modified bolts 2 and bushings 23 are shown in FIGS. 16 to 20 and described below, by way of example. However, it should be noted that corresponding modifications can likewise be applied to either one of the bolt 2 or the bushing 23, so that in each one of FIGS. 16 to 20 respectively only a single component is shown that simultaneously and schematically represents the bolt 2 and the bushing 23.

(63) FIG. 16 represents on the one hand the bolt 2 of FIG. 15 with the bolt shaft 2a, which is at least partly embodied with a varying or reduced outer circumference 2c, e.g. by means of an annular recess. The reduced outer circumference 2c and the normal outer circumference 2b of the bolt shaft 2a as such are preferably arranged concentrically.

(64) On the other hand, FIG. 16 represents the bushing 23 of FIG. 15, which comprises the opening 23a. The latter and the sleeve-like area 23c of the bushing 23 are preferably arranged concentrically.

(65) FIG. 17 represents on the one hand the bolt 2 of FIG. 15 with the bolt shaft 2a, which is at least partly embodied with at least one peripheral cut-out 20 provided on its cylindrical section, i.e. on the outer circumference 2b of the bolt shaft 2a. On the other hand, FIG. 17 represents the bushing 23 of FIG. 15, which comprises said at least one peripheral cut-out 20 in its sleeve-like area 23c.

(66) In both cases, the at least one peripheral cut-out 20 is arranged in parallel to a middle axis 20a of the bolt 2 or the bushing 23. Furthermore, by way of example two peripheral cut-outs 20 are provided and arranged in mirror-symmetry relative to each other.

(67) FIG. 18 represents on the one hand the bolt 2 of FIG. 17 with the bolt shaft 2a, which is at least partly embodied with multiple peripheral cut-outs 20 provided on its cylindrical section, i.e. on the outer circumference 2b of the bolt shaft 2a. On the other hand, FIG. 18 represents the bushing 23 of FIG. 17, which comprises said multiple peripheral cut-outs 20 in its sleeve-like area 23c.

(68) In both cases, the multiple peripheral cut-outs 20 are preferably arranged equidistantly around the middle axis 20a of the bolt 2 or the bushing 23. The bolt 2 or bushing 23 according to FIG. 18 is particularly suitable for applications with more than one main load direction.

(69) FIG. 19 represents on the one hand the bolt 2 of FIG. 15 with the bolt shaft 2a, which is at least partly embodied with a varying or reduced outer circumference 2c, e.g. by means of an annular recess. The reduced outer circumference 2c and the normal outer circumference 2b of the bolt shaft 2a as such are preferably arranged eccentrically.

(70) On the other hand, FIG. 19 represents the bushing 23 of FIG. 15, which comprises the opening 23a. The latter and the sleeve-like area 23c of the bushing 23 are preferably arranged eccentrically. Accordingly, the sleeve-like area 23c comprises a thicker side that defines an indication projection 21.

(71) FIG. 20 represents on the one hand the bolt 2 of FIG. 15 with the bolt shaft 2a, which is at least partly embodied with a varying or reduced outer circumference 2c, e.g. by means of an annular recess. The reduced outer circumference 2c and the normal outer circumference 2b of the bolt shaft 2a as such are preferably arranged concentrically with the normal outer circumference 2b being elliptical.

(72) On the other hand, FIG. 20 represents the bushing 23 of FIG. 15, which comprises the opening 23a. The latter is annular and arranged concentrically to the sleeve-like area 23c of the bushing 23, which is preferably elliptical.

(73) It should be noted that the above described, preferred embodiments are merely described to illustrate possible embodiments of the present invention, but not in order to restrict the present invention thereto. Instead, multiple modifications and variations of the invention are possible and should, therefore, also be considered as being part of the invention. In particular, combinations of the above described variations are possible and should, thus, be considered as being covered by the present invention. In particular, such variations and modifications are readily available to the person skilled in the art and may become necessary dependent e.g. on an underlying number of load directions and other application-specific parameters, such as a predetermined edge distance, number of bolts and/or bushings to be used, and so on.

REFERENCE LIST

(74) 1 load-introduction joint 2 bolt 2a bolt shaft 2b bolt shaft outer circumference 2c reduced bolt shaft outer circumference 3 nut 4 first attachment component 4a first attachment component opening 5 second attachment component 5a second attachment component opening 6 composite laminate 6a composite laminate opening 6b composite laminate upper side 6c composite laminate lower side 6d composite laminate opening center 6e composite laminate end edge 6f composite laminate longitudinal edge 6g peripheral opening side 7 composite laminate prefailure indication element 7a prefailure indication projection 7b upper axial recess 7c lower axial recess 7d prefailure indication projection thickness 8 first bushing 8a first bushing opening 8b first bushing outer circumference 8c first bushing sleeve-like area 9 second bushing 9a second bushing opening 9b second bushing outer circumference 9c second bushing sleeve-like area 10 main load direction 11a end edge distance 11b longitudinal edge distance 11c composite laminate opening diameter 11d composite laminate thickness 11e prefailure indication projection length 12a initial bolt/bushing position 12b prefailure indication bolt/bushing position 13 prefailure gap 14a transversal composite laminate rupture 14b longitudinal composite laminate rupture 15a upper axial chamfer 15b lower axial chamfer 16 undercut 17 prefailure layer 18 tangential recesses 19 peripheral recesses 20 peripheral cut-outs 20a middle axis 21 bushing indication projection 23 third bushing 23a third bushing opening 23b third bushing outer circumference 23c third bushing sleeve-like area F main load