GB2581019A

GB2581019A - Method and device for detecting an impact event and vehicle therefor

Info

Publication number: GB2581019A
Application number: GB1918198.1A
Authority: GB
Inventors: Kreikemeier Janko; Bogenfeld Raffael; Wille Tobias
Original assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Current assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV
Priority date: 2018-12-12
Filing date: 2019-12-11
Publication date: 2020-08-05
Anticipated expiration: 2039-12-11
Also published as: FR3090106B1; ES2804911A2; ES2804911B9; ES2804911B2; DE102018131948A1; DE102018131948B4; GB201918198D0; GB2581019B; ES2804911R1; FR3090106A1

Abstract

A method for detecting an impact at a structure (100), which is made of a fibre composite material. The method involves capturing a structure vibration of the fibre composite structure by means of at least one sensor (11a, 11b, 11c), wherein the fibre composite structure has not been excited by an actuator of a structure monitoring system, detecting an impact induced vibration characteristic or signature in the structure vibration by analysing the structure vibrations with respect to the frequencies and/or amplitudes by means of an evaluation unit (12), and detecting an impact event at the structure (100) as a function of the detection of the impact induced vibration characteristic. The evaluation unit compares sensor measurements to threshold frequencies / ranges. Preferably, the vibration sensors are strain gauge, acceleration, optical or piezoelectric sensors. Vibration or amplitude / frequency spectrums with signatures or characteristics which are indicative of an impact may be predetermined from experiment or test data analysis.

Description

Method and Device for Detecting an Impact Event and Vehicle Therefor The invention relates to a method for detecting an impact event at a fibre composite structure, which is made of a fibre composite material having a fibre material and a matrix material. The invention likewise relates to a device for detecting an impact event at a fibre composite structure. The invention likewise relates to a vehicle comprising such a device for detecting an impact event.

Due to their weight-specific stability and stiffness, fibre composite materials appear indispensable for lightweight construction. Fibre composite materials generally have two essential main components, namely a fibre material on the one hand and a matrix material embedding the fibre material on the other hand. The fibre material essentially provides a fibre composite component made of such a fibre composite material with its load-bearing properties in the direction of the reinforcing fibres. By curing the matrix material embedded in the fibre material, the reinforcing fibres are forced into the predetermined load direction and, together, thus form an integral unit with the matrix material.

Due to the high potential of the fibre composite materials for lightweight construction, such materials are also used as exterior cladding elements of vehicles and thus often form parts of the exterior cladding of road vehicles or even aircrafts. It is thus known to produce wings of aircrafts, such as, for example, commercial aircrafts, of fibre composite materials, in order to be able to save weight, in addition to an improved stability.

During operation, fibre composite structures (also referred to as fibre composite component parts), which are arranged at exposed areas of the vehicle as cladding elements, can be damaged by the impact of blunt objects, which can significantly impair the load-bearing component part properties. If reinforcing fibres of the fibre composite structure are damaged or even severed by such impact events, this leads to a reduction of the stability and stiffness and can lead to the most severe accidents in particular in the case of commercial aircraft. Damages caused by an impact event -2 -can also be invisible inside the laminate and can remain undetected for a longer period of time.

At exposed positions, such as, for example, wing leading edges, it is thus known to cover these areas with an additional steel foil, in order to reduce damages to the fibre composite structure caused by such impact events. As a result, an essential advantage in the use of fibre composite materials, namely the weight reduction, is unfortunately undone. The resulting additional expenditure in the production of fibre composite structures as compared to conventional isotropic materials is thus difficult to justify.

In addition, test and monitoring systems exist for being able to detect damage to the fibre composite structures. Two different concepts for determining such damage exist, namely external test systems (non-destructive testing (NDT)) on the one hand, whereby the component is usually analysed with the help of ultrasound analyses or computer tomography, and damage can then be determined accordingly. For the most part, this is performed as part of planned inspections in the laboratory or the factory. So-called Structure Health Monitoring Systems (SHM) are furthermore known, which monitor the state of a structure from a network of sensors and actuators. A plurality of sensor systems is usually used for this purpose, which determine a wide variety of properties of the fibre composite structures and then draw a conclusion on corresponding damage to the structure on the basis of deviations.

A method and a system for detecting and localising damages in large fibre composite structures is known from WO 2013/086626 Al. For this purpose, carbon nanotubes are added, which are electrically conductive, whereby damage detection is then performed by energising the fibre composite structure and measuring electrical properties.

A method and a device for detecting damages is likewise known from WO 2011/049801 A1, whereby the fibre composite structure also has carbon nanotubes, -3 -which are electrically conductive. However, damage detection is realised via an electrical resistance measurement in this case.

A method and a device for monitoring the structural integrity is known from WO 2010/102208 A1, whereby the fibre composite structure is excited with the help of an actuator, and the vibrations resulting from the actuator are captured and analysed, whereby a conclusion is then drawn on damage to the structure in response to a change of the vibration behaviour.

A non-destructive testing method (NDT) is known from WO 2008/051953 A2, in the case of which anomalies in the fibre composite structure (for example breakages) are detected by introducing two electromagnetic wave signals and measuring the reflection.

A method for examining fibre composite laminates is known from WO 2006/009669 Al, whereby ultrasonic waves are irradiated into the component and the reflected or ultrasonically projected ultrasonic waves, respectively, are then captured by a sensor. A conclusion on damage to the fibre composite structure can then be drawn on the basis of certain characteristic deviations in the ultrasonic signals.

A method for monitoring the structural integrity of fibre composite components is known from WO 2006/004733 A1, whereby the surface of the fibre composite components is imprinted with an electrically conductive printing material and a resistance measurement of this electrically conductive print layer is then performed. If the component was damaged, at least the front surface is thus also damaged, which is reflected in a change of the resistance measurement of the additionally laid-out print layer.

The examination of structures with conventional NDT methods has the disadvantage that it can only be performed using relatively inflexible, stationary measuring systems.

For a real structure, this has the result that damage can only be determined as part of an inspection in a factory. Depending on the design, it is even possible that the complete structure cannot be accessed by the measuring instruments. The ability to -4 -monitor the state is thus only possible with limitations. The examination process is furthermore time-consuming, because the structure has to be analysed successively.

SHM systems are not subject to these limitations. Firmly integrated into the structure, they are able to monitor the state at any time. This, however, results in a different disadvantage. Such a system has to already be introduced into the structure during the manufacture or has to be applied in a complex manner subsequently. This can lead to limitations of the thermal load capacity and to geometric limitations as well as additional weight.

It is thus an aim of the present invention to specify an improved method and an improved device, by means of which damage to fibre composite structures during the use or during operation, respectively, can be determined, without having to adapt the fibre composite structures or fibre composite components for detecting. It is also an aim of the present invention to specify a method and a device for detecting damages to fibre composite structures, in the case of which the detection system used can also be subsequently introduced into the structure.

The invention provides a method according to claim 1, a device according to claim 10, and a vehicle according to claim 11.

According to claim 1, a method for detecting an impact event at a fibre composite structure is proposed, wherein the fibre composite structure (fibre composite component) is made of a fibre composite material. The fibre composite material has at least one fibre material and a matrix material, wherein the fibre material is embedded into the matrix material and the matrix material is cured. It goes without saying that the fibre composite structure can also include further elements.

According to the invention, a structure vibration of the fibre composite structure is now captured continuously by means of at least one sensor, wherein the fibre composite structure had thereby not first been excited by an actuator of a structure monitoring system. On the contrary, only the natural structure vibration of the fibre composite structure is captured by the sensor, wherein the captured structure -5 -vibration is precisely not based on an excitation by an actuator of a structure monitoring system (SHM). The structure vibration of the fibre composite structure is thereby captured continuously over time and is evaluated continuously with the help of an evaluation unit.

The structure vibration of the fibre composite structure captured within a time segment is then examined by means of an evaluation unit, in order to detect an impact event-induced vibration characteristic in the captured structure vibration by analysing the captured structure vibration with respect to the frequencies and/or amplitudes. If such an impact event-induced vibration characteristic has been detected, a conclusion is then drawn on an impact event at the fibre composite structure.

The inventors have recognized thereby that each impact event generates a characteristic structure vibration at a fibre composite structure during operation, which can be captured with the help of sensors and which can accordingly be detected by analysing the captured structure vibration. If the captured structure vibration, which was captured with the help of the sensors, has the corresponding impact event-induced vibration characteristic for impact events, and if such a characteristic can be detected in the structure vibration, a conclusion can be drawn on the presence of an impact event. The evaluation unit can thereby be embodied in such a way that it detects a corresponding impact event on the basis of the detected impact event-induced vibration characteristic and accordingly stores it in a database or outputs a corresponding warning.

With the present method according to the invention, it thus becomes possible to detect impact events at fibre composite structures during operation of the fibre composite structure, i.e. while the produced fibre composite structure is utilized or used as intended, respectively, without having to excite the structure itself by means of an actuator or having to otherwise manipulate it by means of additional measures (for example by means of energising). On the contrary, it is simply sufficient in the case of the present invention to capture a structure vibration with the help of corresponding sensors (for example strain gauges or piezo sensors), and to then -6 -examine the captured structure vibration with respect to a certain vibration characteristic, in order to draw a conclusion on such an impact event, when a vibration characteristic is present, which points towards an impact event.

In terms of the present invention, an impact event is thereby understood to be the action of one or a plurality of objects on the fibre composite structure with a certain force, in particular a destructive force. Such an impact event can thereby be destructive or non-destructive. An impact event is thereby in particular understood to be any event acting on the fibre composite structure, in the case of which objects acting on the fibre composite structure generate damage to the fibre composite structure.

The detection of an impact event thereby comprises in particular the point in time of the impact event, the type of the impact event, and/or the impact damage to the fibre composite structure. With the help of the invention, it becomes in particular possible to detect damage to the fibre composite structure as an impact event as a function of the detection of an impact event-induced vibration characteristic, whereby corresponding damage to the fibre composite structure during the use of the fibre composite structure as intended becomes detectable, based on the action of a force on the fibre composite structure.

According to an embodiment, the captured structure vibration is transformed into the frequency range (for example by means of a Fast Fourier transformation), in order to maintain the amplitude spectrum over the frequencies, wherein a certain vibration characteristic in the captured structure vibration is detected by means of the evaluation unit by analysis of the amplitude spectrum. In response to an impact event, a special characteristic of the amplitudes, which can be detected in an automated manner by means of an evaluation unit, thus appears in the amplitude spectrum, so that impact events at the fibre composite structure can be reliably detected in terms of the process.

It can be provided thereby that, for detecting the certain vibration characteristic by means of the evaluation unit, the amplitude spectrum is examined as to whether the -7 -respective amplitudes lie above a certain threshold value at certain frequencies and/or frequency ranges. It has been shown that impact events, in particular those impact events, which cause damage to the fibre composite structure, show a characteristic amplitude spectrum at certain frequencies and/or frequency ranges, wherein the amplitudes partially lie above a certain threshold value.

According to an advantageous embodiment, it cannot only be detected by means of the captured structure vibration, whether an impact event is present, but also on which type of impact event the structure vibration is based. It is thus advantageous, when a damage characteristic is detected by means of the evaluation unit as a function of the detected certain vibration characteristic. A conclusion can thus not only be drawn on the fact that an impact event with damage to the fibre composite structure is present, but also whether reinforcement fibres in the upper cover layer were damaged or whether (further) structural damages to the fibre composite structure occurred on the inside of the laminate (for example a delamination of individual fibre layers).

The damage characteristic can thereby be derived, for example, from the respective frequency range. It is thus known that damages to the reinforcement fibres lead to a characteristic amplitude curve in a first frequency range, while a delamination as a second damage type has a corresponding characteristic amplitude curve in a different, second frequency range.

According to a further embodiment, the captured structure vibration is compared to at least one predetermined structure vibration, which characterises an impact event of the fibre composite structure, by means of the evaluation unit in order to detect the impact event-induced vibration characteristic. Such predetermined structure vibrations can thereby be determined beforehand, for example by means of a laboratory test or by means of numerical methods.

It is thus conceivable that the predetermined structure vibration, which is captured in the laboratory, is displayed in the form of an amplitude spectrum in the frequency range, and is compared in this form to the captured structure vibration, which is also -8 -present as amplitude spectrum in the frequency range, in order to thus be able to identify matches in the amplitude spectrum, which characterise an impact event, in the captured structure vibrations of the fibre composite structure.

According to a further embodiment, the structure vibration is detected by means of at least one strain gauge and/or by means of at least one piezoelectric sensor element as a sensor. A strain-time curve can thereby be captured with the help of a strain gauge, which is scanned by high frequency, while a force-time curve can be captured as structure vibration with the help of a piezo sensor.

The invention also provides the device according to claim 10, wherein the device is embodied for carrying out the above-described method. For this purpose, the device has at least one sensor, which communicates with an evaluation unit in terms of signalling. The sensor thereby captures structure vibrations of the fibre compound structure and transfers them to the evaluation unit, which then detects an impact event-induced vibration characteristic by analysis of the captured structure vibration, and then identifies an impact event based on this.

It is conceivable thereby that a plurality of sensors in the form of a sensor network is used, which communicates with the evaluation unit. Such sensors can be, for example, strain gauges.

Moreover, the invention provides the vehicle according to claim 10, wherein the vehicle has a device for detecting an impact event, as described above.

It is in particular advantageous thereby when external cladding elements, such as, for example, fuselage shells or wing shells of aircrafts, are provided with corresponding sensors, which capture a structure vibration, wherein corresponding impact events based on the captured structure vibrations can then be identified with the help of the evaluation unit. -9 -

The invention will be described in an exemplary manner on the basis of the enclosed figures, in which: Figure 1 shows a schematic illustration of the device according to the invention; Figure 2 shows an exemplary illustration of two characteristic structure vibrations; Figure 3 shows a schematic illustration of an application example at a commercial aircraft.

According to the invention, Figure 1 shows the device 10, which, in the exemplary embodiment of Figure 1, has three strain gauges 11 a to 11c, which communicate with an evaluation unit 12 in terms of signalling. Each of the sensors 11 a to 11c thereby provides a strain-time curve 13, the temporal resolution of which is a function of the scanning frequency by the evaluation unit 12. The three strain gauges 11 a to are arranged at a fibre composite structure 100, which is to be monitored.

This strain-time curve is transformed into the frequency range only with the help of the evaluation unit, so that the amplitude spectrum becomes detectable. An attempt is then made to detect from the amplitude spectrum, whether characteristic features are present, which suggest an impact event.

Two such frequency spectra are shown in an exemplary manner in Figure 2. In the upper example, the strain-time curve 13 was transformed into the frequency range by means of a Fast Fourier transformation, so that the amplitudes now become visible via the involved frequencies. What can be seen in the upper spectrum a) is that in the frequency range at 22 kHz and 25 kHz, the amplitudes have a characteristic deflection 20, which allows drawing a conclusion on an impact event. Compared to a (not yet illustrated) frequency spectrum or amplitude spectrum, respectively, of a structure vibration without impact event, it can be seen that these two amplitude maxima 20a and 20b suggest damage as an impact event. It can thus not only be determined, whether an impact event has taken place, but also whether damage to the fibre composite structure has occurred due to the impact event.

Tests on fibre composite structures have in fact shown that impact damages to fibre composite structures, which result in a small fibre brakeage, created such an amplitude spectrum.

It can be seen in the lower frequency spectrum b) that the amplitude maximum is significantly more pronounced by 25 kHz than in the upper spectrum, which is related to the extent of the damage. In the lower part, a larger area of the fibre composite structure was damaged, which is shown by the respective characteristic amplitude curves at approx. 20 kHz and 25 kHz.

Damage events at fibre composite structures result in a vibration excitation of the structure. The corresponding vibrations affect tensions and strain or, in the event of impact damage, the contact force curve. Strain or contact force signals can be captured with a high measuring frequency thereby. By means of transformation into the frequency range, the associated spectrum of involved frequencies can be determined. The involved vibrations can consist of two parts. In addition to the damage-induced vibrations, the external action, such as, for example, impact damage, which is the cause for the damage, excites modes of the structure with a broad frequency band. Based on the identification of the (mostly high frequency) modes, which are induced by damage events, an identification of damage and a clear allocation of damage phenomena to a correspondingly excited frequency in the amplitude spectrum can take place.

An application example for such a device is shown schematically in Figure 3, wherein a commercial aircraft 30 was provided with a network of sensors 31 for the outer hull. The sensor network 31 thereby communicates with a corresponding evaluation unit 32 in terms of signalling, in order to thus capture the sensor signals by means of the evaluation unit 32.

A local damage accumulation can thus be performed over the service life of the structure, so that location and number of the impact events, which occurred, can be identified after a certain time. For this purpose, the location of the impact event with respect to the hull of the aircraft is recorded by means of the localisation of the respective sensor of the sensor network 31 as well as of the detected impact event. It can thus be defined, in which regions a detailed examination or a repair is required.

List of Reference Numerals: device 11 sensors 12 - evaluation unit 13 - strain-time curve - amplitude characteristic - aircraft 31 - sensor network 32 evaluation unit - fibre composite structure

Claims

Claims 1. A method for detecting an impact event at a fibre composite structure (100), which is made of a fibre composite material having a fibre material and a matrix material, wherein the method comprises the following steps: capturing a structure vibration of the fibre composite structure by means of at least one sensor (11a, 11 b, 11c), wherein the fibre composite structure has not first been excited by an actuator of a structure monitoring system, detecting an impact event-induced vibration characteristic in the captured structure vibration by analysing the captured structure vibrations with respect to the frequencies and/or amplitudes by means of an evaluation unit (12, 32), and detecting an impact event at the fibre composite structure (100) as a function of the detection of the impact event-induced vibration characteristic.
2. The method according to claim 1, wherein the detection of an impact event comprises the point in time of the impact event, a type of the impact event, and/or impact damage to the fibre composite structure.
3. The method according to claim 1 or 2, wherein the captured structure vibration is transformed into the frequency range, in order to maintain the amplitude spectrum over the frequencies, wherein an impact event-induced vibration characteristic in the captured structure vibration is detected by means of the evaluation unit (12, 32) by analysis of the amplitude spectrum.
4. The method according to claim 3, wherein for detecting the impact event-induced vibration characteristic by means of the evaluation unit, the amplitude spectrum is examined as to whether the respective amplitudes lie above a certain threshold value at certain frequencies and/or frequency ranges. 5. 6. 7. 8. 9. 10.
The method according to any one of the preceding claims, wherein a damage characteristic is detected by means of the evaluation unit (12, 32) as a function of the detected impact event-induced vibration characteristic.
The method according to any one of the preceding claims, wherein the captured structure vibration is compared to at least one predetermined structure vibration, which characterises an impact event of the fibre composite structure, by means of the evaluation unit (12, 32) in order to detect the impact event-induced vibration characteristic.
The method according to claim 6, wherein the at least one predetermined structure vibration, which characterises a certain impact event, had been determined beforehand by means of a laboratory test.
The method according to any one of the preceding claims, wherein the structure vibration is captured by means of at least one strain gauge, at least one acceleration sensor, at least one optical sensor, and/or by means of at least one piezoelectric element as a sensor (11).
The method according to any one of the preceding claims, characterised in that a force-time curve or a strain-time curve (13) is captured as structure vibration by means of the at least one sensor (11).
A device (10) for detecting an impact event at a fibre composite structure, which is made of a fibre composite material having a fibre material and a matrix material, wherein the device (10) is embodied for carrying out the method according to any one of the preceding claims, and has at least one sensor (11) for capturing a structure vibration of the fibre composite structure and an evaluation unit for detecting an impact event-induced vibration characteristic and for detecting the impact event.
11. A vehicle comprising at least one fibre composite structure, wherein the vehicle has a device (10) for detecting an impact event at one of the fibre composite structures according to claim 10.
12. The vehicle according to claim 11, characterised in that the fibre composite structure is at least partially the outer shell of the vehicle.
13. The vehicle according to claim 11 or 12, characterised in that the vehicle is a road vehicle, a watercraft or an aircraft.