ARRANGEMENT INCLUDING A FIBER-REINFORCED COMPOSITE COMPONENT OR ASSEMBLY, AIRCRAFT OR SPACECRAFT, METHOD OF PRODUCING AN ARRANGEMENT, AS WELL AS METHOD OF MONITORING STRUCTURAL INTEGRITY

Abstract

An arrangement includes a fiber-reinforced composite component or assembly and a monitoring device. The composite component or assembly includes at least first and second reinforcing fiber formation sections stitched to each other using a yarn to connect the fiber formation sections along a seam, the yarn being electrically conductive along a length thereof. The monitoring device is configured and coupled to the yarn so as to be capable of sending an electrical input signal along at least a section of the yarn that forms the seam or part thereof and receiving a response signal on the yarn. Furthermore, an aircraft or spacecraft is provided including at least one arrangement of this type, as well as a method of producing an arrangement including a fiber-reinforced composite component or composite assembly, and a method of monitoring the structural integrity of a fiber-reinforced composite component or assembly.

Claims

1. An arrangement comprising: a fiber-reinforced composite component or composite assembly which comprises at least first and second reinforcing fiber formation sections stitched to each other using a yarn so as to connect the first and second reinforcing fiber formation sections along a seam, the yarn being electrically conductive along a length thereof, and a monitoring device configured and coupled to the yarn so as to be capable of sending an electrical input signal along at least a section of the yarn that forms the seam or part thereof and receiving a response signal on the yarn.

2. The arrangement according to claim 1, wherein the monitoring device is configured to detect or measure at least one of an ohmic resistance or an impedance of a portion of the yarn which includes the section of the yarn forming the seam or part thereof.

3. The arrangement according to claim 1, wherein at least one of: the monitoring device is configured to send a constant electrical input signal or a time-varying electrical input signal, or the monitoring device is configured to provide an input signal at intervals or within continuous time periods or continuously during an operational life of the composite component or composite assembly.

4. The arrangement according to claim 3, wherein the constant electrical input signal is a constant voltage input signal, and the time-varying electrical input signal is a time-varying voltage signal.

5. The arrangement according to claim 1, wherein within the seam, the yarn functions to support mechanical loads as well as functions as an integrated structural health sensor device.

6. The arrangement according to claim 1, wherein the seam forms part of a mechanically load-bearing structural joint.

7. The arrangement according to claim 1, wherein the yarn comprises a load-bearing yarn core as well as an electrically conductive layer provided on the yarn core.

8. The arrangement according to claim 7, wherein the electrically conductive layer provided on the yarn core comprises carbon nano-tubes.

9. The arrangement according to claim 1, wherein the yarn comprises an electrically isolating outer coating.

10. The arrangement according to claim 1, wherein the monitoring device comprises at least one electronic circuit.

11. The arrangement according to claim 10, wherein the electronic circuit is implemented using at least one semiconductor device.

12. The arrangement according to claim 1, wherein the seam is crossed by at least one further seam, wherein the at least one further seam is formed at least in part by stitching using a further yarn, the further yarn being electrically conductive along a length thereof.

13. The arrangement according to claim 1, wherein the composite assembly is formed as a shell assembly comprising a stringer coupled to a skin, wherein one of the first and second reinforcing fiber formation sections forms part of the stringer, and another one of the first and second reinforcing fiber formation sections forms part of the skin.

14. The arrangement according to claim 13, wherein the first and second reinforcing fiber formation sections form a foot of the stringer.

15. The arrangement according to claim 13, wherein the seam is crossed by at least one further seam, wherein the at least one further seam is formed at least in part by stitching using a further yarn, the further yarn being electrically conductive along a length thereof wherein the shell assembly further comprises a frame or a segment of a frame, wherein the further seam is formed by stitching through a further reinforcing fiber formation section forming part of the frame or the segment using the further yarn.

16. The arrangement according to claim 15, wherein the further reinforcing fiber formation section forms a foot of the frame.

17. An aircraft or spacecraft comprising at least one arrangement according to claim 1, wherein the composite component or composite assembly forms part of an aircraft or spacecraft structure.

18. A method of producing an arrangement including a fiber-reinforced composite component or composite assembly, the method comprising: providing at least first and second reinforcing fiber formation sections; providing a yarn that is electrically conductive along a length thereof; arranging the first and second reinforcing fiber formation sections relative to each other; stitching through the first and second reinforcing fiber formations sections using the yarn to connect the first and second fiber formation sections along a seam; providing a monitoring device and coupling the monitoring device to the yarn so as to enable the monitoring device to send an electrical input signal along at least a section of the yarn that forms at least part of the seam and to receive a response signal on the yarn.

19. A method of monitoring a structural integrity of a fiber-reinforced composite component or composite assembly including a seam formed by stitching using a yarn, wherein the method comprises monitoring a structural health status of the seam, including: sending an electrical input signal along at least a section of the yarn that forms at least part of the seam; and receiving a response signal on the yarn and evaluating the response signal.

20. The method according to claim 19, further comprising detecting at least one of over- or rupture of the yarn within the section that forms at least part of the seam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The invention will be explained in the following with reference to the schematic figures of the drawings which illustrate embodiments of the invention. Herein:

[0075] FIG. 1 shows an exemplary aircraft in which arrangements as well as methods in accordance with embodiments of the invention may be used;

[0076] FIG. 2 shows an arrangement according to a first embodiment of the invention, including an intact seam along a structural joint, in a schematic perspective view;

[0077] FIG. 3 shows the arrangement of FIG. 2, wherein a portion of the seam has been damaged and a stitching yarn has been stretched, along with a detail view 33a of the damaged seam portion;

[0078] FIG. 4 shows the arrangement of FIG. 2, wherein a portion of the seam has been damaged and a stitching yarn has been ruptured, along with a detail view 33b of the damaged seam portion;

[0079] FIG. 5 shows part of the arrangement of FIG. 2, wherein a portion of the seam has been damaged and a stitching yarn has been cut due to external action, force or impact;

[0080] FIG. 6 displays an arrangement according to a second embodiment of the invention, comprising seams running transverse to each other, in a schematic perspective view;

[0081] FIG. 7 shows a schematic cross-sectional view of a yarn of a first type which may be used in embodiments of the invention;

[0082] FIG. 8 shows a schematic cross-sectional view of a yarn of a second type which may be used in embodiments of the invention; and

[0083] FIG. 9 shows a schematic cross-sectional view of a yarn of a third type which may be used in embodiments of the invention.

[0084] The enclosed drawings are intended to illustrate embodiments of the invention so that the invention may be further understood. The drawings, in conjunction with the description, are intended to explain principles and concepts of the invention. Other embodiments and many of the advantages described may be inferred from the drawings. Elements of the drawings are not necessarily drawn to scale.

[0085] Elements, features and components which are identical or which have the same function or effect have been labeled in the drawings using the same reference signs, except where explicitly stated otherwise.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0086] FIG. 1 shows an aircraft 100 comprising a fuselage 101, wings 102, a nose 103 as well as an empennage 104 including a vertical stabilizer 105 and a horizontal stabilizer 106. Also, the aircraft 100 comprises engines 107 attached to the wings 102.

[0087] The aircraft 100 comprises a structure made at least in part from fiber-reinforced composite materials, in particular from one or more carbon or glass fiber-reinforced synthetic material(s). An exemplary structural fiber-reinforced composite assembly, which forms part of the structure of the aircraft 100, is a shell assembly 111, illustrated in FIG. 1 in exemplary and schematic manner. The shell assembly 111 is part of the fuselage 101, which comprises a plurality of further shell assemblies of the same or similar type. The shell assembly 111 comprises a skin portion stiffened, on the inner side thereof, by a plurality of stringers 20, running substantially along a longitudinal direction of the fuselage 101, and by a plurality of segments of frames 50, running substantially along a circumferential direction of the fuselage 101. In exemplary manner, a frame 50 and two stringers 20 are schematically shown in FIG. 1.

[0088] In FIG. 2, an arrangement 1 including a portion of the shell assembly 111 as well as a monitoring device 45, indicated in schematic manner by a double-dot-and-dash line, is shown. The portion of the shell assembly 111 shown is formed as a fiber-reinforced composite assembly 2 including a fiber-reinforced composite stringer 20 arranged on a fiber-reinforced composite skin 10. In the example of FIG. 2, the stringer 20 has an Omega-shaped cross-section.

[0089] Production of the composite assembly 2 of FIG. 2 is carried out as follows. Dry reinforcing fiber formation sections such as plies of dry textile reinforcing fiber material are arranged and stacked and are then stitched to each other along seams. Dry fiber material for the stringer 20 might be provided in the form of a preform. By the stitching, preforms can be assembled. In FIG. 2, a first reinforcing fiber formation section 3 for the skin 10 and a second reinforcing fiber formation section 4 for the stringer 20 are schematically indicated for illustration. It is understood, however, that further reinforcing fiber formation sections may be present in each of the skin 10 and stringer 20.

[0090] The first and second fiber formation sections 3, 4 are stitched to each other using a yarn 30 along a seam 25, so as to connect the sections 3, 4, in particular in combination with one or more further seam(s). Further, additional fiber formation sections, not illustrated in the Figure, may be connected to the first and second fiber formation sections 3, 4 in this process if desired. The stitching through the fiber formation sections 3, 4 along the seam 25 may be carried out in automated manner, e.g., using a stitching head, and using two needles for example. The stitching head may be carried and manipulated e.g., by a robot or robotic arm.

[0091] In FIG. 2, two seams are shown along the longitudinal extent of the stringer 20, joining the stringer 20 to the skin 10, wherein each of the seams runs along one of the stringer feet of the stringer 20. Additional seams may or may not be present. The seam 25 is denoted in FIG. 2 by a reference sign and will be discussed below. The other seam, or further seams, provided to join dry fiber formations of the stringer 20 and the skin 10 to form the joint between skin 10 and stringer 20, may be implemented in the same manner as described for the seam 25, or may alternatively be of other type. The seam 25 is part of a mechanically load-bearing structural joint within the assembly 2 and thus of a load-bearing structural joint in the aircraft fuselage 101.

[0092] After completion of the stitching process, the dry fiber formation sections 3, 4 can be infiltrated with curable resin, or a preform assembly comprising the sections 3, 4 can be infiltrated with the resin, for example using VAR™ or vacuum assisted resin transfer molding. Subsequently, curing is performed, e.g., using heat and/or pressure.

[0093] By the seam 25, the load-bearing structural joint of the skin 10 and stringer 20 is strengthened. In case the joint is damaged, the stitched seam 25, in the manner of so-called selective stitching, effectively helps to prevent excessive spreading of such damage and thus enhances the damage tolerance of the joint. Conventional “crack-stoppers,” such as rivet bolts, can be avoided.

[0094] In the arrangement 1, the seam 25 connecting at least the fiber formation sections 3 and 4 is formed using a yarn 30, which is electrically conductive along its length.

[0095] The yarn 30 used in the embodiment of FIG. 2 is illustrated in more detail in FIG. 7. The yarn 30 comprises a load-bearing core 301, for example formed from glass fibers or carbon fibers. In further variants, the load-bearing core 301 may be formed from a polymeric material such as Vectran™ or Dyneema®. On the core 301, an electrically conductive layer 302 formed with carbon nanotubes is deposited in the form of a coating to achieve a suitable conductivity along the length of the yarn 30. Carbon nanotubes are tube-shaped nanostructures with one wall or several walls of carbon atoms. Carbon nanotubes have high mechanical strength and are also highly electrically conductive. The layer 302 may, e.g., be formed with a carbon nanotube based material such as Tuball™.

[0096] A further, modified yarn 30′ is displayed in FIG. 8 in schematic manner. The yarn 30′ differs from the yarn 30 in that on an outer side of the conductive layer 302, an additional, electrically isolating outer coating 303 has been formed. The yarn 30′ may be used in the embodiment of FIG. 2 instead of yarn 30. The isolating outer coating 303 may be formed with an electrically insulating epoxy resin and prevents undesired current flow towards other conductive elements, and in particular towards electrically conductive filaments of the fiber formation sections 3, 4 or other conductive stitching yarns as will be described in more detail below.

[0097] FIGS. 7 and 8 also illustrate a further yarn 60 of the same type as the yarn 30, as well as a further yarn 60′ of the same type as the yarn 30′.

[0098] Another yarn 30″ is illustrated in FIG. 9 in schematic manner. The yarn 30″ comprises a load-bearing electrically conductive core 301, which may be formed with carbon fiber filaments or with a carbon nanotube-based material such as Tuball™, and an electrically isolating outer coating 303 provided on an outer side of the core 301. Also, in case of the yarn 30″, the outer coating 303 may be made with an electrically insulating epoxy resin. FIG. 9 also illustrates a further yarn 60″ of the same type as yarn 30″.

[0099] Preferably, if the fiber formation sections 3, 4 comprise electrically non-conductive fibers such as glass fibers only, yarn 30 without outer insulating coating may be used, or alternatively, a yarn corresponding to the electrically conductive core 301 of FIG. 9 without additional conductive and insulating layers may be used, for stitching. Moreover, in case the fiber formation sections 3, 4 comprise carbon fibers, which exhibit electrical conductivity, it is preferable to use a yarn 30′ or 30″ for stitching the seam 25 which is provided with an outer coating 303 for electrical insulation, wherein the coating 303 forms an enclosure around the core 301, and if present the conductive layer 302, in circumferential direction.

[0100] In case the isolating outer coating 303 is provided, the material of the coating 303 is preferably formed so as to be compatible with the matrix material used to infuse the fiber formation sections 3, 4. If the core 301 or the layer 302 forms the contact surface of the yarn to the matrix, preferably the core 301 or layer 302 is chosen such as to be sufficiently compatible with the matrix material, regarding the material of the core 301 or layer 302.

[0101] The yarn 30, 30′ or 30″ acts as a multi-functional yarn which, in the composite assembly 2, supports mechanical loads introduced into the seam 25, and, beyond this, is used as an integrated, distributed structural health sensor device for monitoring the health status of the seam 25.

[0102] The monitoring device 45 comprises, in the embodiment of FIG. 2, an electrical voltage source 40. Each of the terminals of the voltage source 40, which for example is a DC voltage source, is connected to one of the opposite ends of the conductive yarn 30, 30′ or 30″, that has been used to form the seam 25.

[0103] Furthermore, the monitoring device 45 comprises an indicator 31 for the stringer foot yarn status of the yarn 30 at the seam 25, as well as an impedance meter 32 for measuring impedance over the stitched yarn length section 37 along the seam 25 at the stringer foot. The indicator 31 in FIG. 2 forms part of an electric circuit formed with the yarn 30 running from one of the terminals of the voltage source 40 to the other and including the yarn section 37.

[0104] Accordingly, the monitoring device 45 is coupled to the yarn 30 in a manner which enables the monitoring device 45 to send an electrical input signal through the yarn 30 extending, via the seam 25, between the positive and negative terminals of the voltage source 40, and hence also along the section 37 of the yarn 30 that forms part of the seam 25. For example, the electrical input signal may a constant DC voltage. A response signal is received on the yarn 30 via the indicator 31, indicating current through the yarn 30. Further, the response of the section 37 to the input signal may be detected using the impedance meter 32 connected via a first connecting line 41 to the yarn 30 at a first location 35 close to one end of the seam 25 and connected via a second connecting line 42 to the yarn 30 at a second location 36 close to the other end of the seam 25.

[0105] In the situation shown in FIG. 2, the seam 25 and yarn 30 are intact and undamaged. Therefore, as no detrimental effect on the yarn 30 has led to an increase in the ohmic resistance or impedance thereof, the yarn status indicator 31 in FIG. 2 indicates current as expected for an undamaged yarn 30. This is illustrated in FIG. 2 by a light bulb 31 emitting light at “full power,” i.e., emitting light at a defined, strong level indicating intact yarn state. Further, in the situation of FIG. 2, the impedance meter 32 indicates low impedance over the stitched length 37 of the yarn 30 along the seam 25.

[0106] While in FIG. 2, the yarn status indicator 31 has been illustrated in schematically simplified manner as a light bulb and the impedance meter 32 as an analog instrument having a scale and a needle, it should be understood that preferably, the functionalities of the status indicator as well as of resistance or impedance measurement will preferably be implemented using one or more electronic circuit(s). The electronic circuit(s) may be implemented in miniaturized manner as integrated circuit(s), in particular using a semiconductor device or a plurality of semiconductor devices. Although this is not shown in detail in the figures, such electronic circuit(s) and the connecting lines coupling the electronic circuit(s) to the yarn or yarns 30, 30′ can be provided or arranged throughout the aircraft 100, forming a plurality of monitoring devices 45.

[0107] As an alternative to sending a constant electrical input signal in the form of a constant DC voltage, in a variant, the monitoring device 45 may be adapted to provide an electrical input signal varying over time. For instance, an intermittent DC voltage signal may be sent along the yarn 30, in order to monitor the status of the yarn 30 at regular or irregular intervals. In other examples, an AC voltage signal may be sent along the yarn 30, e.g., a sinusoidal signal. The AC voltage signal may be sent continuously or intermittently.

[0108] The monitoring device 45 may be used to provide the electrical input signal continuously during the operational life of the assembly 2. Yet, in a preferred variant, the monitoring device 45 is used at intervals during the operational life of the assembly 2, or during periods of time, to monitor the structural health of the seam 25, for example during routine inspection of the aircraft 100, e.g., during pre-takeoff inspection or “walk-around.” If the status indicator 31 or the impedance measurement by the impedance meter 32 indicates a resistance or impedance that is higher than expected for an undamaged yarn, the seam 25 can be inspected by staff, e.g., visually, in order to determine the cause thereof and the location and extent of damage. Intermittent or continuous sending of input signals and receipt as well as evaluation of response signals could in a variant be performed during flight. The monitoring device 45 may be used to implement predictive maintenance.

[0109] In FIG. 2, a skin-stringer assembly 2 of carbon-fiber reinforced plastics (CFRP) is shown, with stringer feet stitched to the skin 10. One of the stitch yarns corresponds to yarn 30, which is part of an electrical circuit, connected to the voltage source 40. The yarn status indicator 31 and the impedance meter 32 over the seam 25 enable status monitoring of the structural health status of the stitched joint. FIG. 2 shows the setup in a simplified manner in order to demonstrate the principle, with intact yarn 30.

[0110] Using the arrangement 1 schematically displayed in FIG. 2, it is possible to monitor at least the following damage mechanisms: [0111] over-straining or stretching of the yarn 30, leading to an increased electrical ohmic resistance or impedance, and rupture of the yarn 30, which prevents a signal to be passed from one end of the yarn 30 to the other, wherein both stretching and rupture can be due to an overload in the joint by over-straining the joint in the out-of-plane direction or can be due to local impact with resulting in-plane damage and peeling; and [0112] rupture of the yarn 30 due to external mechanical load or cut, e.g., when the joint is damaged by a foreign object.

[0113] In the following, some types of damage will be described with reference to FIGS. 3-5, in which some elements which are those of FIG. 2 are denoted using the reference signs of FIG. 2 with an added “a,” “b” or “c,” respectively. The reference signs of the yarn 30, the skin 10, the voltage source 40 and the sections 3, 4 that are part of the cured assembly 2, 2a-c remain the same in FIGS. 3-5.

[0114] FIG. 3 shows the arrangement 1 of FIG. 2, now denoted by reference sign 1a, with damage at one end of one of the stringer feet of stringer 20a. The damage is due to overload. In FIG. 3, the stringer foot has been torn apart from the skin 10, and a gap is visible between the stringer foot and the skin 10 at the left edge of the front stringer foot in FIG. 3. Still, the stringer 20 and the skin 10 are held together by the stitch yarns, one of which is the yarn 30. The stitch yarns, including yarn 30, have not yet ruptured in FIG. 3.

[0115] In detail 33a in FIG. 3, the stretched yarn 30 at the location of the damage is shown. For the purpose of illustration, the stretching has been exaggerated in detail 33a. It can be seen that apart from being elongated by a difference in length AL, the yarn 30 also has contracted within the distance of the open gap, by Poisson's ratio. Hence, within the gap, the cross-sectional area of the yarn 30 locally has decreased.

[0116] Both the stretching and the contraction, shown at 33a, with the length increased by ΔL and decreased yarn diameter, lead to an increase in electrical impedance ΔZ in this yarn 30, which can be measured. In this manner, the yarn 30 itself functions as an integrated sensor, in addition to the structural function of the yarn 30. The yarn 30 is thus multi-functional.

[0117] Due to the changes in diameter and the elongation within the gap, the yarn status indicator 31a, schematically shown again as a light bulb, detects reduced current by emitting reduced light. Also, the impedance meter 32a indicates increased impedance.

[0118] In FIG. 4, a situation analogous to the situation in FIG. 3 is shown, but the damage has increased, so that the stitching yarn 30 has ruptured at one location, in FIG. 4 in exemplary manner at the left edge of the stringer 20b. The yarn status indicator 31b has ceased to emit light, i.e., the bulb is dark, indicating rupture and hence interruption of the electric circuit. The impedance meter 32b indicates indefinite impedance.

[0119] FIG. 5 shows a part of the arrangement 1 of FIG. 2, now denoted by reference sign 1c, with an external damage. A tear or cut C extends through the skin 10 and through the stitched stringer foot of the stringer 20c. The integrated sensing function of the stitching yarn 30 makes it possible to capture a material rupture of this type as well, which cuts off the yarn 30. As in FIG. 4, the yarn status indicator 31c has ceased to emit light, i.e., the bulb is dark, indicating yarn rupture or cut, and the impedance meter 32c indicates indefinite impedance.

[0120] FIG. 6 shows an arrangement 1′ according to a second embodiment, comprising a composite assembly 2′ and a monitoring device 75, which in schematic manner is denoted by a double-dot-and-dash line. The assembly 2′ is formed as a skin-stringer-frame assembly, comprising a skin 10, a stringer 20 and a segment of a frame 50 extending transverse to the stringer 20. The assembly 2′ may preferably form part of a shell assembly 111, see FIG. 1. In FIG. 6, the frame 50 is formed, in exemplary manner, with an approximately Omega-shaped cross-section and comprises two frame feet joining the frame 50 to the skin 10. Portions of the frame feet and stringer feet may overlap, see FIG. 6.

[0121] In FIG. 6, the feet both of the stringer 20 and of the frame 50 are stitched to the skin 10 using electrically conductive yarns 30 and 60, respectively. The yarns 30 and 60 may be of the type described above and schematically shown in FIG. 7. In a variant, yarns 30′ or 30″ and 60′ or 60″ comprising the isolating coating 303 as described with reference to FIG. 8 may be used in the embodiment of FIG. 6. The yarns 30 and 60, 30′ and 60′, or 30″ and 60″ may be of the same type and are intact in FIG. 6. A seam 55 along one of the frame feet, formed using the yarn 60, is partially shown in FIG. 6. A further seam 56 on the opposite frame foot is shown as well.

[0122] In addition to the elements 31, 32, 40, 41, 42 of the monitoring device 45 of FIG. 2, the monitoring device 75 of the arrangement 1′ further comprises an additional frame foot yarn status indicator 61 and an impedance meter 62 to measure the yarn impedance over the stitched yarn length along the foot of the segment of the frame 50. Specifically, the impedance meter 62 is coupled to the yarn 60 at a first location 65 between one terminal of the voltage source 40 and one end of the seam 55 by a connecting line 71, and at a second location 66 between the other terminal of the voltage source 40 and the other end of the seam 55 by a connecting line 72.

[0123] In FIG. 6, the indicators 31, 61 show “full light” indicating intact yarns 30, 60, and the meters 32, 62 indicate a corresponding impedance.

[0124] In the same manner as explained above, the functions of the status indicator 61 and impedance meter 62 preferably are implemented using at least one electronic circuit, in particular using at least one semiconductor device, not shown in the figures. For example, the functions of the complete monitoring device 75 may be implemented using at least one electronic circuit. The required voltage could be provided by an appropriate supply line, replacing the schematic voltage source 40.

[0125] Thus, in FIG. 6, the seam 25 is crossed by the further seam 55, formed using the conductive yarn 60. Using the yarn 60, a reinforcing fiber formation section 5, which is part of the frame 50 or at least of one of the frame feet thereof, has been stitched to the reinforcing fiber formation section 3 of the skin 10. Afterwards, the fiber formation sections 3, 4, 5 have been infused with resin and cured, as described above for assembly 2, to obtain the composite assembly 2′.

[0126] The arrangement 1′ enables monitoring of the structural integrity of seams 25, 55 extending in two different directions, substantially transverse with respect to each other. If a damage or rupture is indicated for a seam 25 along the stringer foot, or along a stringer foot of one of several stringers 20, determining the location along the stringer 20 in question is facilitated in the arrangement 1′ of FIG. 6. Since multiple frames 50 cross the path of the stringer 20, and the transfer of mechanical loads to the stringers 20 in particular originates from the frames 50, it may be expected that a damage may in many cases affect both the foot of a stringer 20 as well as the foot of the frame 50, and hence the damage can be detected also using the yarn 60. Therefore, monitoring the status of the seams 25 and 55 facilitates determining the location of the damage.

[0127] Moreover, for instance, several additional seams 55 crossing seam 25 may be used to detect the extent of a damage, in particular of a crack, and/or to detect a growth rate or spreading rate of such damage, by performing the monitoring continuously or at appropriate intervals.

[0128] Each seam 25 or 55 may be implemented using one or more conductive yarns 30, 30′, 30″ or 60, 60′, 60″ respectively, as a stitching yarn or stitching yarns only. Alternatively, it is conceivable to use the yarn 30, 30′, 30″ or 60, 60′, 60″ in combination with other, static, non-conductive stitching yarns in the seam 25 or 55.

[0129] Input signals may be sent along the yarn 30, 30′ or 30″ and 60, 60′ or 60″ in a temporally offset manner, in order to precisely separate the responses obtained. Additionally or alternatively, the yarns 30′, 60′, 30″, 60″ provided with the isolating coating 303 may be used to implement the seams 25, 55 or at least one of them, in order to electrically separate the seams 25, 55. Further, alternatively or additionally, the yarns 30, 30′ or 30″ and 60, 60′ or 60″ may run transversely to each other, as shown in FIG. 6, but on different vertical levels in the out-of-plane direction, for separation of the yarns. One or more fiber layers may then be present between the yarns 30, 30′ or 30″ and 60, 60′ or 60″. If desired, it is even conceivable to provide an isolating fiber layer, e.g., formed using a glass-fiber formation, that separates the seams 25, 55.

[0130] Even though in FIG. 6, the seam 55 is displayed as interrupted at the location where the frame 50 crosses the stringer 20, it should be noted that, although not displayed, the yarn 60 is guided, e.g., above the stringer 20 so that the seam 55 can be continued without severing the yarn 60. In an alternative, not displayed in the figures, it is conceivable to monitor each portion of the seam 55 between two adjacent stringers 20 separately.

[0131] The assemblies 2, 2′ may each be considered components of the aircraft 100. Yet, the present invention is applicable to other components, smaller or larger than the shell assembly 111, in an aircraft or spacecraft structure, and to components or component assemblies in other fields of technology. More specifically, the assembly 2 or 2′ may form part of the fuselage 101, such as in the form of the shell assembly 111, or alternatively may form part of another shell assembly, that for instance forms part of one of the stabilizers 105, 106 or of a wing 102.

[0132] The embodiments described above with reference to FIGS. 1-8 further illustrate methods of producing composite assemblies 2, 2′.

[0133] In addition, the embodiments described above with reference to FIGS. 1-8 illustrate methods of monitoring the structural integrity of the assemblies 2, 2′ by monitoring the structural health status of the seam 25 or the seams 25 and 55, respectively, wherein an electrical input signal or electrical input signals is/are sent along the yarn(s) 30, 30′, 30″ and/or 60, 60′, 60″ and wherein a response signal or response signals is/are received on each of the yarns 30, 30′, 30″ and/or 60, 60′, 60″. The generation of the input signal as well as the receipt and evaluation of the response signal is preferably performed using one or more electronic circuits, which may be implemented as integrated circuits using semiconductor device(s).

[0134] The monitoring devices 45, 75 of various assemblies 2, 2′ of the aircraft structure may be coupled in suitable manner, and health status data obtained using the sensing yarns 30, 30′, 30″, 60, 60′, 60″ throughout the aircraft 100 may be collected and processed, e.g., within the aircraft 100, by a data collection and processing device, not shown in the figures. The data collection and processing device may be configured to provide a summary on the health status of the aircraft structure and/or a warning in case of suspected damage to flight staff or maintenance staff, e.g., via a network and a handheld device such as a laptop or smartphone. The summary or warning may be displayed in textual and/or graphical manner using a graphical user interface, including, for example, an indication of the approximate location or area of possible damage.

[0135] In all embodiments described above, the composite assembly 2, 2′ can comprise fiber formation sections 3, 4 or 3, 4, 5 including carbon fibers, which are infiltrated by a curable resin such as, e.g., epoxy resin, followed by curing. Therefore, the skin 10, the stringer 20 and the frame 50 are formed in these embodiments as elements from carbon fiber-reinforced synthetic material or CFRP (carbon fiber-reinforced plastic) elements. Alternatively, the formation sections 3, 4 or 3, 4, 5 may include glass fibers infiltrated by the curable resin, followed by curing, and in this case, the skin 10, stringer 20 and frame 50 may be formed as GFRP (glass fiber-reinforced plastic) elements.

[0136] With the invention, in particular the arrangements 1, 1′ as well as the methods of the embodiments described hereinabove, at least one or more the following advantages may be obtained: [0137] a multi-functional yarn, or multi-functional yarns, enable the implementation of stitched joints, for example between skin 10 and stringer 20 or skin 10 and frame 50, with integrated structural health monitoring (SHM) without separate SHM sensors; [0138] using stitching, the material thickness of the joined elements 10, 20, 50 may be reduced compared to riveted joints, thus the weight of the structure can be kept low, and stress concentrations under load are comparatively small due to the many stitches; [0139] drilling holes for rivets can be avoided; [0140] good compatibility of a matrix material and the yarns 30, 30′, 60, 60′ can be obtained; [0141] the same yarn(s) fulfill(s) both a static mechanical function as well as a monitoring function, and therefore is/are multi-functional; [0142] the sensor function of the yarn(s) is capable of capturing both tearing mode damage as well as rupture mode damage in the stitched joint; [0143] before rupture, damage or incipient damage can be detected, for example by detecting or measuring changes in impedance; accordingly, initial damage beyond limit load capability and well within the damage tolerance domain could be detected, too; [0144] an amount of stitching which at the same time is sufficient to effectively prevent excessive spread of damage and enables reliable detection of damage can be implemented.

[0145] Although the invention has been completely described above with reference to preferred embodiments, the invention is not limited to these embodiments but may be modified in many ways.

[0146] For instance, the invention may be useful not only in the field of aircraft or spacecraft, but also in other fields involving composite structures, such as, for example, automobile bodies, wind power plants, or pressurized tanks or vessels.

[0147] While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.

LIST OF REFERENCE SIGNS

[0148] 1 arrangement with intact seam and yarn [0149] 1a arrangement with damaged seam portion and stretched yarn [0150] 1b arrangement with damaged seam portion and ruptured yarn [0151] 1c arrangement with seam and yarn damaged by cut or tear [0152] 1′ arrangement with two intact crossing seams and yarns [0153] 2 composite component or composite assembly [0154] 2a composite component or assembly with damaged seam portion and stretched yarn [0155] 2b composite component or assembly with damaged seam portion and ruptured yarn [0156] 2c composite component or assembly with seam and yarn damaged by cut or tear [0157] 2′ composite component or assembly with two intact crossing seams and yarns [0158] 3 reinforcing fiber formation section [0159] 4 reinforcing fiber formation section [0160] 5 reinforcing fiber formation section [0161] 10 skin [0162] 20 stringer [0163] 20a stringer with damage at stringer foot joint [0164] 20b stringer with damage at stringer foot joint [0165] 20c stringer with damage at stringer foot joint due to material rupture [0166] 25 seam joining stringer foot and skin [0167] 25a seam joining stringer foot and skin, with damage [0168] 25b seam joining stringer foot and skin, with damage [0169] 25c seam joining stringer foot and skin, with damage [0170] 30 conductive yarn forming stitched seam 25 that joins stringer foot and skin [0171] 30′ conductive yarn with isolating outer coating [0172] 30″ conductive yarn with isolating outer coating [0173] 31 indicator for stringer foot yarn status [0174] 31a yarn status indicator 31 in a state indicating over-strained yarn [0175] 31b yarn status indicator 31 in a state indicating ruptured yarn [0176] 31c yarn status indicator 31 in a state indicating severed yarn due to external rupture or cut [0177] 32 impedance meter for measuring impedance over stitched yarn length at stringer foot [0178] 32a impedance meter 32 indicating increased impedance [0179] 32b impedance meter 32 indicating indefinite impedance due to ruptured yarn [0180] 32c impedance meter 32 indicating indefinite impedance due to cut yarn [0181] 33a detail of seam at damaged seam portion with stretched yarn [0182] 33b detail of seam at damaged seam portion with ruptured yarn [0183] 33c damaged seam portion with yarn that has been cut, torn or stretched and ruptured by external action [0184] 35 first location [0185] 36 second location [0186] 37 section of yarn [0187] 40 electrical voltage source [0188] 41, 42 connecting line [0189] 45 monitoring device [0190] 50 frame or frame segment [0191] 55 seam joining frame foot and skin [0192] 56 seam joining frame foot and skin [0193] 60 conductive yarn forming stitched seam 55 that joins frame foot and skin [0194] 60′ conductive yarn with isolating outer coating [0195] 60″ conductive yarn with isolating outer coating 61 indicator for frame foot yarn status [0196] 62 impedance meter for measuring impedance over stitched yarn length at frame foot [0197] 65 first location [0198] 66 second location [0199] 67 section of yarn [0200] 71, 72 connecting line [0201] 75 monitoring device [0202] 100 aircraft [0203] 101 fuselage [0204] 102 wing [0205] 103 nose [0206] 104 empennage [0207] 105 vertical stabilizer [0208] 106 horizontal stabilizer [0209] 107 engine [0210] 111 shell assembly [0211] 301 load-bearing core [0212] 302 conductive layer [0213] 303 isolating outer coating [0214] C tear or cut [0215] AL change in length [0216] AZ change in impedance