DE102019111042A1 - Structure monitoring system and structure monitoring method - Google Patents

Abstract

In order to improve the detection of defects (14), such as cracks (40, 42), breaks or delamination, in composite layer components (12), a structure monitoring system (10) or method is proposed. The system (10) monitors the fiber composite layer structure (12) by means of a sensor layer (18) made of electroactive polymer (24), in short: EAP. The different defects (14) have different characteristic voltage curves (U) and can therefore be distinguished by means of an evaluation unit (32). In addition, a control unit (36) can operate the EAP sensor layer (18) as a bending actuator (38) in order to counteract any deformation of the fiber composite layer structure (12).

Description

The invention relates to a structure monitoring system for monitoring a fiber composite layer structure. The invention also relates to a structure monitoring method.

Various hardware technologies for structural health monitoring (SHM) are known for composite airframes of commercial aircraft.

Some known challenges for SHM technology are the complex wear and tear mechanisms that occur in a composite laminate, the large areas to be covered, and ensuring that all critical areas are covered. In addition, the continuous acquisition, reliable signal collection and the previously required interpretation of the signals using software represent major challenges.

Damage mechanisms within a composite laminate can include cracks in the skin, breaks (in other words, cracks that extend through the entire fiber composite layer structure) and delamination of the layers in the skin.

A well-known technology that has been extensively researched in recent years is what is known as acousto ultrasonic technology. Although this technology is promising with regard to the direct integration of the measurement hardware in the fiber composite layer structure, the effort required to introduce the required large number of cables, sensors, etc. is too high from a practical point of view.

An ultrasonic transmitter sends ultrasonic waves in the fiber composite layer structure. The waves propagate radially from the transmitter and are detected by a number of sensors.

If the composite component is undamaged, the sensors respond in a known manner. Should a defect occur on the way from the transmitter to one of the sensors, such as, for example, a crack, a break or a delamination, it is easy to see that the signal received by the sensor has changed. If the defect is between two sensors, both sensors will detect a change. Interpretation software has been developed that tries to calculate the location of the defect. Multiple transmitters are often used to simplify this process.

One advantage of this system is that hardly any parts have to be integrated into the fiber composite layer structure. However, the large number of sensors and transmitters, which are supposed to ensure that all relevant areas are covered, can hardly be arranged in an uncomplicated manner.

Another challenge is the correct attachment of these devices, usually by bonding. For this purpose, processes with special surface treatments have been developed that must be carried out very precisely. Long-term stability also plays a role. Another challenge is to manage the correct supply of all sensors and transmitters and the reliable collection of signals. A self-testing system is required for this.

Other technologies have been proposed based on various miniaturized sensors built into the fiber composite layer structure or optical fibers. All these known technologies, however, insert “foreign objects” into the fiber composite layer structure, which on the one hand makes production more difficult and on the other hand increases the complexity of the component.

• • US 2010/0 201 384 A1
• • EP 3 222 514 A1
• • WO 2001 / 0 039 253 A2
• • US 2006/0 123 918A1
• • US 8 384 398 B2
• • US 2011 / 0 089 958 A1
• • US 2012/0 197 482 A1
• • US 2009 / 0 246 892 A1
• • DE 11 2015 003 110 T5
• • US 5 869 189 A
• • WO 2010/0 004 324 A1
• • WO 2017 / 0 141 207 A2
• • US 5 305 507 A
• • US 2017/0 168 021 A1
• • EP 2 559 656 A1
• • US 9 329 021 B1
• • US 9 358 757 B2
• • DE 10 2011 122 481 B4
• • US 6 809 462 B2

For the state of the art, reference is also made to the following documents:

• • US 2010/0 201 384 A1
• • EP 3 222 514 A1
• • WO 2001/0 039 253 A2
• • US 2006/0 123 918A1
• • US 8 384 398 B2
• • US 2011/0 089 958 A1
• • US 2012/0 197 482 A1
• • US 2009/0 246 892 A1
• • DE 11 2015 003 110 T5
• • U.S. 5,869,189 A
• • WO 2010/0 004 324 A1
• • WO 2017/0 141 207 A2
• • U.S. 5,305,507 A
• • US 2017/0 168 021 A1
• • EP 2 559 656 A1
• • US 9 329 021 B1
• • US 9 358 757 B2
• • DE 10 2011 122 481 B4
• • US 6,809,462 B2

The invention is based on the object of improving composite layer components, especially with regard to possible defects such as cracks, breaks or delamination.

The object is achieved by the subject matter of the independent claims. Preferred developments are the subject of the dependent claims.

The invention creates a structure monitoring system for monitoring a fiber composite layer structure, the structure monitoring system having a fiber composite layer structure to be monitored, the at least one fiber-reinforced polymer layer and a sensor layer laminated to it, which contains an electroactive polymer, the sensor layer being formed with a voltage change when its sensor layer thickness changes and comprises an evaluation unit which is designed to detect a voltage change caused by the sensor layer and to determine based thereon whether the fiber composite layer structure has a defect.

It is preferred that the sensor layer is embedded between two fiber-reinforced polymer layers.

It is preferred that the sensor layer is designed as a cover layer of the fiber composite layer structure.

It is preferred that the sensor layer is designed to generate a change in tension as a function of the bending direction when the fiber composite layer structure is bent.

It is preferred that the sensor layer is designed to generate a positive change in tension when the fiber composite layer structure is bent with a bending load that only causes elastic deformation in a direction away from the sensor layer.

It is preferred that the sensor layer is designed to generate a negative voltage change when the fiber composite layer structure is bent with a bending load that only causes elastic deformation in one direction towards the sensor layer.

It is preferred that the sensor layer is designed to produce a greater change in tension when the fiber composite layer structure is bent with a bending load that causes the defect than when it is bent elastically.

It is preferred that the sensor layer is designed to generate a greater change in tension with the same sign than with elastic bending in the same direction when the fiber composite layer structure is bent with a bending load causing a microcrack, crack or break as the defect.

It is preferred that the sensor layer is designed to generate a stress change with the opposite sign than with elastic bending in the same direction when the fiber composite layer structure is bent with a bending load causing delamination as the defect.

It is preferred that the sensor layer has at least two electrode layers and an electroactive polymer layer arranged between them, the electrodes being electrically connected to the evaluation unit.

It is preferred that the evaluation unit is designed to determine the type of defect based on the voltage changes in the sensor layer.

The structure monitoring system preferably comprises a memory device for storing the voltage measured by the sensor layer and / or the data determined by the evaluation unit.

The structure monitoring system preferably comprises a control unit which is designed to apply electrical voltage to the sensor layer in such a way that the sensor layer functions as a bending actuator in order to counteract the deformation of the fiber composite layer structure.

The invention creates an aircraft with a fiber composite component and a preferred structure monitoring system for monitoring the fiber composite component, in particular the primary structure and / or the secondary structure of the aircraft.

The invention creates a structure monitoring method for monitoring a fiber composite layer structure, in particular an aircraft, by means of a preferred structure monitoring system, in which the evaluation unit determines a deformation of the fiber composite layer structure caused by a bending load on the basis of the tension changes in the sensor layer, whether and, if so, what type of defect in the fiber composite layer structure caused by the bending load.

It is preferred that the control unit applies an electrical voltage to the sensor layer in such a way that the sensor layer functions as a bending actuator in order to counteract the deformation of the fiber composite layer structure.

• 1 eine Ansicht eines Verbundschichtbauteils bei Verformung;
• 2 eine Ansicht des Verbundschichtbauteils aus 1 bei Defektbildung;
• 3 eine Ansicht des Verbundschichtbauteils aus 1 bei Verformung; und
• 4 eine Ansicht des Verbundschichtbauteils aus 1 bei Delamination.

Embodiments are explained in more detail with reference to the accompanying schematic drawings. It shows:

• 1 a view of a composite layer component during deformation;
• 2 a view of the composite layer component 1 with defect formation;
• 3 a view of the composite layer component 1 at deformation; and
• 4th a view of the composite layer component 1 with delamination.

It will be on 1 to 4th Referring to one embodiment of a structural monitoring system 10 demonstrate. The structure monitoring system 10 comprises a fiber composite layer structure 12 by the structural monitoring system 10 on the occurrence of a defect 14th should be monitored.

The fiber composite layer structure 12 comprises a plurality of fiber reinforced polymer layers 16 and a sensor layer 18th . The sensor layer 18th is for example as a lower cover layer 20th educated. The sensor layer 18th can alternatively also be used as an upper cover layer 22nd be formed or between two polymer layers 16 be embedded.

The polymer layer 16 is formed in a conventional manner and can, for example, be a glass fiber reinforced polymer layer (GFRP), carbon fiber reinforced polymer layer (CFRP), aramid fiber reinforced polymer layer (AFRP) or the like.

The sensor layer 18th consists of electroactive polymer 24 . The sensor layer 18th a voltage change is therefore formed due to a deformation ΔU depending on the deformation of the polymer layer 16 to evoke.

The sensor layer 18th contains a laminated electrode layer 26th attached to the bottom polymer layer 16 is laminated, an electroactive polymer layer 28 , which is variable in thickness, and a free electrode layer 30th that is exposed. In the case that the sensor layer 18th is embedded, is also the free electrode layer 30th on the adjacent polymer layer 16 laminated on.

The structure monitoring system 10 further comprises an evaluation unit 32 that is formed, the voltage change ΔU to record and evaluate. The evaluation unit 32 for example a computer 34 be.

The structure monitoring system 10 further comprises a control unit 36 that also through the computer 34 can be formed. The control unit 36 is designed to apply an electrical voltage to the sensor layer 18th in such a way that the sensor layer 18th as a bending actuator 38 acts that a deformation of the fiber composite layer structure 12 counteracts.

The following is how the structure monitoring system works 10 explained in more detail.

As in 1 The fiber composite layer structure is shown deforming 12 when a bending load F is applied to it. The bending load F acts in the direction of the sensor layer 18th . The bending load F is chosen so that only elastic deformation occurs.

The top polymer layers are thereby 16 compressed while the lower polymer layers 16 and the sensor layer 18th be stretched. The extension of the sensor layer 18th calls for a reduction in the thickness of the sensor layer 18th out that an electrical measurement voltage U caused. The course of the measurement voltage U is an example on the evaluation unit 32 shown.

The voltage change ΔU the measuring voltage U is positive in the present case. In other words, the measurement voltage increases U on. The evaluation unit 32 can affect the voltage change ΔU and / or the measurement voltage U can be taught in so that the evaluation unit outputs the voltage curve 1 recognizes as free of defects.

As in 2 can be represented in the fiber composite layer structure 12 , if the bending load F is too high or if the stress is repeated frequently, micro-cracks initially 40 in the inner polymer layers 16 within the fiber composite layer structure 12 form. A micro crack 40 is an example of a defect 14th . With a micro-crack 40 the affected polymer layer works 16 outwardly intact, although an internal weakening has already occurred.

Due to the weakening of the microcracks 40 affected polymer layers 16 the fiber composite layer structure deforms 12 near the micro-cracks 40 more, so that the change in length of the sensor layer 18th increases. Thus the measuring voltage increases U due to the greater rate of change, it increases faster than with an intact fiber composite layer structure 12 . In other words is the voltage change ΔU the measuring voltage U greater than with elastic deformation. The course of the measurement voltage U is an example on the evaluation unit 32 shown.

If the bending load F increases further, as in 2 shown below, the microcracks can develop 40 to Cracks 42 expand. A crack 42 is also an example of the defect 14th . The crack 42 is characterized by the fact that the affected polymer layer 16 has lost its structural integrity in at least one direction (thickness or along a line). The course of the measurement voltage U is an example on the evaluation unit 32 shown.

It should be noted that the crack 42 can expand to a fracture. When the break is the fiber composite layer structure 12 completely destroyed and no longer has any structural integrity.

Due to the further weakening of the fiber composite layer structure 12 is the deformation of the sensor layer 18th even bigger; in other words, the size of the voltage change increases ΔU even further to.

Based on the behavior of the measurement voltage described U can the evaluation unit 32 be taught in so that the evaluation unit 32 Micro cracks 40 and cracks 42 can distinguish.

As in 3 The fiber composite layer structure is shown deforming 12 when a bending load F is applied to it. The bending load F acts in the direction away from the sensor layer 18th . The bending load F is chosen so that only elastic deformation occurs.

The top polymer layers are thereby 16 stretched while the lower polymer layers 16 and the sensor layer 18th be compressed. The shortening of the sensor layer 18th calls for an increase in the thickness of the sensor layer 18th out that an electrical measurement voltage U caused. The course of the measurement voltage over time U is an example on the evaluation unit 32 shown.

The voltage change ΔU the measuring voltage U is negative. In other words, the measurement voltage drops U from. The evaluation unit 32 can affect the voltage change ΔU and / or the measurement voltage U can be taught in so that the evaluation unit outputs the voltage curve 3 recognizes as a defect-free bend in the opposite direction.

4th shows delamination 44 the sensor layer 18th than the defect 14th . But it could also be another layer or polymer layer 16 be delaminated. Due to the compression of the sensor layer 18th the sensor layer comes off 18th from the polymer layer 16 and bulges.

The sudden change in the geometry of the sensor layer 18th causes an abrupt change in the measurement voltage U . In particular, the sign of the voltage change is reversed ΔU abruptly around. This causes the delamination 44 by a kink 46 in the course of the measuring voltage U on the evaluation unit 32 visible. Thus, the delamination also has 44 one from the evaluation unit 32 recognizable signature.

In addition, the control unit 36 a voltage on the sensor layer 18th create it as a bending actuator 38 acts and the bending load F counteracts. The control unit 36 initiates the application of the voltage in particular when a specified limit bending load has been exceeded. Thus, the formation of microcracks can occur 40 better counteracted.

An electroactive polymer (EAP) can be, for example, a layer composite that has two electrodes and an elastic polymer layer located between them. For the electrodes (conductive layer), an electrically conductive polymer such as poly (3, 4 ethylene-dioxithiophene) / polystyrene sulfonate, also known as PEDOT / PSS, can be used.

A thermoplastic polyurethane elastomer (TPU) such as Epurex LPT 4207 CU-T can be used for the elastic polymer. This polymer shows a compressive strength of 50 MPa and can therefore basically be used as a layer in a structural composite component.

A special type of EAP with a flat polymer and thin elastic electrodes is called a dielectric polymer. When the two electrodes are disconnected from a voltage source, the thickness of the EAP is in an expanded state, also called an "inactive state". When the electrodes are connected to a voltage source, they attract each other through electrostatic pressure (Coulomb forces) and press the polymer layer between them, so that the EAP contracts in the direction of its thickness. The compression can be between 10% and 35% and can be even greater for other EAP. At the same time there is an expansion in the plane due to the Poisson's number.

The basic idea is a device and method for detecting defects in composite layers, such as cracks, breaks and delaminations. The previously unused property of EAP is used here, namely that a change in length of an EAP layer results in a change in thickness of the EAP layer. This in turn can cause a voltage change between the EAP electrodes.

In this way, attaching foreign objects to the structure can be avoided. Furthermore, no sound waves need to be sent, received and interpreted. The EAP layer is very thin and material compatible with typical composite layers.

If the fiber composite layer structure is not subjected to any mechanical stress, the fiber composite layer structure is undeformed. This corresponds to a certain electrical potential between the EAP electrodes. The electrical potential is therefore constant over time.

When a bending load is applied to the fiber composite layer structure, bending deformation is caused. This results in a compression of the upper fiber reinforced layers and a stretching of the lower fiber reinforced layers as well as the EAP layer. This results in a change in length of the EAP layer compared to the undeformed state. This change in length also causes a change in thickness of the EAP layer, which is mechanically very small, but is fully measurable as a change in stress. The electrical voltage increases.

In the event of major deformation of the composite laminate, the first microcracks can form in the fiber composite layer structure. These are not open cracks and indicate the beginning of the crack. A corresponding change in length of the EAP layer generates a further increase in the electrical voltage.

With repeated or increased stress, open cracks can eventually form in the fiber-reinforced layer next to the EAP layer. This marks the beginning of a break. A correspondingly steeper rise in the voltage generated by the EAP layer can be measured. The EAP layer is not yet affected.

When a bending load is applied to the fiber composite layer structure in the other direction, bending deformation is caused in the other direction. This results in stretching of the upper fiber reinforced layers and compression of the lower fiber reinforced layers as well as the EAP layer. This results in a change in length of the EAP layer compared to the undeformed state. This change in length also causes a change in thickness of the EAP layer, which is mechanically very small, but is fully measurable as a change in stress. The electrical voltage drops in the process.

If the composite laminate is deformed too much, layers such as the EAP layer can delaminate. This also changes the length of the EAP layer. A similar process occurs if one of the fiber-reinforced layers next to the EAP layer peeled off. The previous drop in electrical voltage is followed by a sharp rise, which is an indication of delamination.

The EAP layer is fully compatible with carbon fiber reinforced and glass fiber reinforced composites and can be integrated into the composite laminate by placing strips or fiber laying heads. Furthermore, the EAP layer can be divided into strips in order to enable spatial resolution.

The invention advantageously allows crack, breakage and delamination detection with the same device and without bonding devices to the skin or the component. The EAP layer allows weight reduction and is always active. Furthermore, the EAP layer is compatible with composite components.

List of reference symbols

10

Structural monitoring system

12

Fiber composite layer structure

14th

malfunction

16

Polymer layer

18th

Sensor layer

20th

lower top layer

22nd

upper cover layer

24

electroactive polymer

26th

laminated electrode layer

28

electroactive polymer layer

30th

free electrode layer

32

Evaluation unit

34

computer

36

Control unit

38

Bending actuator

40

Micro crack

42

Crack

44

Delamination

46

Kink

F.

Bending load

U

Measuring voltage

ΔU

Voltage change

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• US 2010/0201384 A1 [0011]
• EP 3222514 A1 [0011]
• US 2006/0123918 A1 [0011]
• US 8384398 B2 [0011]
• US 2012/0197482 A1 [0011]
• DE 112015003110 T5 [0011]
• US 5869189 A [0011]
• WO 2010/0004324 A1 [0011]
• US 5305507 A [0011]
• US 2017/0168021 A1 [0011]
• EP 2559656 A1 [0011]
• US 9329021 B1 [0011]
• US 9358757 B2 [0011]
• DE 102011122481 B4 [0011]
• US 6809462 B2 [0011]

Claims (15)

Structure monitoring system (10) for monitoring a fiber composite layer structure (12), the structure monitoring system (10) having a fiber composite layer structure (12) to be monitored, the at least one fiber-reinforced polymer layer (16) and a sensor layer (18) laminated to it, which has an electroactive polymer (24 ), the sensor layer (18) being designed to cause a change in voltage (ΔU) when its sensor layer thickness changes, and comprising an evaluation unit (32) designed to detect a change in voltage (ΔU) caused by the sensor layer (18) and to determine based thereon whether the fiber composite layer structure (12) has a defect (14). Structure monitoring system (10) according to one of the preceding claims, characterized in that the sensor layer (18) is embedded between two fiber-reinforced polymer layers (16) or that the sensor layer (18) is designed as a cover layer (22) of the fiber composite layer structure (12). Structure monitoring system (10) according to one of the preceding claims, characterized in that the sensor layer (18) is designed to generate a tension change (ΔU) as a function of the bending direction when the fiber composite layer structure (12) is bent. Structure monitoring system (10) according to one of the preceding claims, characterized in that the sensor layer (18) is designed, when the fiber composite layer structure (12) is bent, with a bending load (F) causing only an elastic deformation in a direction away from the sensor layer (18) to generate a positive voltage change (ΔU). Structure monitoring system (10) according to one of the preceding claims, characterized in that the sensor layer (18) is formed when bending the fiber composite layer structure (12) with a bending load (F) causing only elastic deformation in one direction towards the sensor layer (18) to generate negative voltage change (ΔU). Structure monitoring system (10) according to one of the preceding claims, characterized in that the sensor layer (18) is designed, when bending the fiber composite layer structure (12) with a bending load (F) causing the defect (14), a greater stress change (ΔU) than with elastic To produce bending. Structure monitoring system (10) according to one of the preceding claims, characterized in that the sensor layer (18) is formed when bending the fiber composite layer structure (12) with a micro-crack (40), a crack (42) or a break (43) as the Defect (14) causing the bending load (F) to generate a greater change in tension (ΔU) with the same sign than with elastic bending in the same direction. Structure monitoring system (10) according to one of the preceding claims, characterized in that the sensor layer (18) is designed to cause a change in tension (F) when the fiber composite layer structure (12) is bent with a delamination (44) causing the defect (14). ΔU) with the opposite sign than with elastic bending in the same direction. Structure monitoring system (10) according to one of the preceding claims, characterized in that the sensor layer (18) has at least two electrode layers (26, 30) and an electroactive polymer layer (28) arranged between them, the electrode layers (26, 30) being connected to the evaluation unit ( 32) are electrically connected. Structure monitoring system (10) according to one of the preceding claims, characterized in that the evaluation unit (32) is designed to determine the type of defect (14) based on the characteristic voltage changes (ΔU) of the sensor layer (18). Structure monitoring system (10) according to one of the preceding claims, characterized by a memory device for storing the voltage measured by the sensor layer (18) and / or the data determined by the evaluation unit (32). Structure monitoring system (10) according to one of the preceding claims, characterized by a control unit (36) which is designed to apply electrical voltage to the sensor layer (18) in such a way that the sensor layer (18) functions as a bending actuator (38) to prevent the deformation of the To counteract fiber composite layer structure (12). Aircraft with a fiber composite component which contains a fiber composite layer structure (12) and a structure monitoring system (10) according to one of the preceding claims for monitoring the fiber composite component, in particular the primary structure and / or the secondary structure of the aircraft. Structure monitoring method for monitoring a fiber composite layer structure (12), in particular of an aircraft, by means of a structure monitoring system (10) according to one of the preceding claims, in which the evaluation unit (32) determines a deformation of the fiber composite layer structure (12) caused by a bending load (F) on the basis of the sequence of stress changes (ΔU) of the sensor layer (18) whether and, if so, what type of defect (14) has arisen in the fiber composite layer structure (12) due to the bending load (F). Structural monitoring procedure according to Claim 14 , characterized in that the control unit (36) applies an electrical voltage to the sensor layer (18) in such a way that the sensor layer (18) functions as a bending actuator (38) to counteract the deformation of the fiber composite layer structure (12).

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