Description

The invention relates to a sensor sheet for detecting damages of a composite material compris ing a substrate having at least one sensing area comprising at least one conductive path having at least one conducting sensor trace arranged on the substrate for indicating damages to the substrate within the sensing area by a change in electrical resistance. Further, the invention relates to a composite material comprising a base layer and a fiber reinforced composite layer, wherein at least one conductive path having at least one conducting sensor trace is embedded within the fiber reinforced composite layer. Further aspects of the invention relate to a detection system comprising the composite material, a method for producing such a sensor sheet and the use of the composite material.

In order to assess the status and/or integrity of a wall or container it is known in the art to form a sensor comprising a conductive trace on the wall. The conductive trace has a known electrical resistance. If the wall is damaged, the conductive trace is also damaged and thus the electrical properties are changed. A control device may then assess the status of the wall by measuring electrical resistance of the conductive trace. If the measured resistance is equal to a previously determined reference value, the wall is intact. If the resistance has changed, in particular, if the resistance is increased or the trace is no longer conducting, the wall has been damaged. Flow- ever, in such an arrangement it is not possible to detect the location of the damage. In order to quickly locate the damage, a sensor system having spatial resolution would be helpful. In order to provide spatial resolution, it is known to provide several sensor traces which are inde pendently connected to a control device. Damage to such a wall at a specific location will only affect one of the sensor traces so that the location is then limited to the area covered by the respective sensor trace. Flowever, wiring and control devices become more complex.

An example application is a wall of a cooled container or a refrigerated truck trailer where dam age often occurs due to fork-lift assisted loading and unloading of merchandise. The damage is usually detected too late due to an increase in the inner room temperature. The consequences of such damage are important because refrigerated consumer goods may not be suitable for retail any more after a temperature increase and for the trailer owner who must replace the full trailer if there is no proof of who is responsible for the damage.

A refrigerated trailer is typically made of an insulating material protected by a layer of composite material. A composite material is here defined as a polymer matrix reinforced with fibers such as fibers made of glass, polymer, wood, basalt or carbon. There is a need for a detection system which may detect damages to a wall such as a wall of a container or a truck trailer and which is able to locate the position of the damage within a few cm and/or able to detect the time of the damage incident within a few seconds.

US 7,921,727 B2 discloses a sensing system for use in monitoring the structural health of a structure such as a polymeric matrix composite structure. The system includes a sensor formed form a conductive ink containing carbon nanofibers and a polymeric resin. The conductive ink is applied to the structure to be monitored in form of a grid pattern. The grid lines are preferably electrically isolated from each other by means of a nonconductive coating. A data acquisition system for acquiring and evaluating data from the sensor is connected to the sensor by two wires per line and two wires per column of the grid. Strain and/or damage is detected by meas uring a change in conduction.

US 7,271 ,723 B2 discloses intrusion detection sensor panels, wherein the entire panel functions to detect intrusions across its entire surface without gaps. The detection sensor panel has an embedded optical fiber or fine electric wire at a specific density, so that if a part of the panel is broken or there is an intrusion from the outside, the severance of the optical fiber or fine wire is detected. The panels may be used to enclose a space. Each of the panels enclosing the space may be connected to an intrusion recording unit by means of a signal cable.

US 2010/0201519 A1 discloses a breach detection system for containers which includes at least two panels. The panels are configured for attachment to a container and have an electrical circuit mounted thereto. The electrical circuit is used to detect an intrusion through any portion of the panel. A connector electrically connects the electrical circuits of two panels of the con tainer. In one embodiment, the electrical circuit comprises an electrical path having a serpentine configuration. The electrical conductor forming the electrical path may be porous and may be formed from a wire mesh or screen. Further, the electrical circuit is provided with bypass resis tors. An interruption of the electrical path causes current to flow through bypass resistors.

WO 2017/179716 A1 discloses an elastic sheet material which comprises a force sensor. The sheet material elasticity may be adjusted to a desired value by forming alternating incisions in a kirigami structure. For force sensing, a strain gauge may be attached to the sheet material or may be formed directly on the sheet material.

US2010/171518 A1 discloses a probe apparatus for non-destructive testing to detect and locate structural flaws in a composite laminate. The apparatus comprises an array of elongate conduc tive resistance elements affixed to a surface of an insulating substrate which may be heated by applying power. Further, the apparatus comprises means for measuring a time dependent re sistance change of each element when power is applied to said element. Defects may then be located by observing propagation of a thermal wave through the composite material. The appa ratus may be embedded into a composite material panel if permanent monitoring is required. Detection systems relying on the propagation of heat waves are mostly suited for fully insulating materials and are thus not optimal for composite materials comprising glass fibers. Further, the detection method is limited to small dimensions as thermal diffusion hinders detection of defects for larger dimensions.

The publication “Built-in Sensor Network for Structural Health Monitoring of Composite Struc ture” by Xinlin P. Quing et al., Journal of intelligent material systems and structures, vol. 18, no 1 , 10 January 2007, pages 39-49, relates to the implementation of a built-in sensor network based on piezoceramic pads on a composite structure. The publication proposes arranging the sensor network on a thin dielectric layer (SMART layer) which may be integrated into a compo site structure. The SMART layer may comprise uniform miniature holes for resin to flow easily. The proposed sensor structure is not suitable for larger structures and cannot be produced easi ly in a roll-to-roll process.

The known detection systems are not suitable for seamless integration in composite material layers which comprise a polymer matrix reinforced with fibers. Further, the known systems are difficult to scale in size. It is thus an object of the invention to provide a sensor sheet for detect ing damages of a composite material which is suitable for integration within a composite materi al layer. Further, it is an object of the present invention to provide a sensor sheet which may help to determine the position where the damage has occurred while at the same time providing a simple wire connection to a control device. Still further it is an object of the invention to pro vide a composite material which is configured for detecting damages.

A sensor sheet for detecting damages of a composite material comprising a substrate having at least one sensing area is proposed. The sensing area comprises at least one conductive path having at least one conducting sensor trace arranged on the substrate for indicating of damages to the substrate within the sensing area by a change in electrical resistance. The substrate fur ther comprises a plurality of openings which are at least arranged within the at least one sens ing area, wherein the conductive path is arranged around the openings and wherein the area of each of the openings is in the range of 0.1 cm ² and 31000 cm ² , the distance between two adjacent openings is in the range of 0.5 cm and 100 cm and the total area of the openings with respect to the area of the substrate is in the range of from 30% to 99%.

Further, the at least one sensing area comprises a first power supply trace and a second power supply trace and multiple conductive sensor traces, wherein each conductive sensor trace is connected to the first power supply trace at a first end and connected to the second power sup ply trace at a second end so that a resistor network is formed, wherein a cut of one conductive sensor trace causes a specific change to the overall resistance of this resistor network which may be detected by a single measurement of the electrical resistance between the first power supply trace and the second power supply trace. The at least one sensing area is preferably configured and arranged such that the sensing ar- ea(s) cover essentially the entire surface of the sensor sheet/ the substrate of the sensor sheet. Preferably, at least 80 % of the total surface of the sensor sheet is covered by the sensing ar- ea(s), more preferably at least 90% and most preferably at least 95%. Further, it is preferred that the openings are distributed over the entire are of the sensor sheet or at least over the en tire area covered by the sensing area(s).

The sensor sheet is in particular suited for integration within composite materials consisting of or comprising a polymer matrix reinforced with fibers.

In order to allow the integration of the sensor sheet inside a composite material, openings are provided in the substrate which are, for example, arranged in a honeycomb pattern fitting exact ly the geometry of the at least one conductive sensor trace. The sensor sheet can be seamless ly integrated within a composite material comprising a polymer matrix reinforced with fibers to detect damages to the composite material. In particular, the plurality of openings in the sub strate allows the resin which is used to impregnate the fibers to penetrate the sensor sheet so that the sensor sheet becomes an integral part of the composite material and undesired chang es in the material properties of the composite material are avoided or at least reduced.

The area of each of the openings is in the range of 0.1 cm ² and 31000 cm ² , preferably from 1 to 1300 cm ² . In case of circular shaped openings, the diameter of the openings is preferably in the range of from 0.5 cm to 100 cm, especially preferred in the range of from 1 cm to 20 cm.

The area of an opening has an influence on the smallest size of damage which can be detected by means of the sensor sheet. All traces have to be routed around the openings. This means that any damage to a composite material which is only occurring at a location within one of the openings cannot be detected. Thus, the area and/or the size (e.g. diameter in case of circular openings) is preferably chosen to be smaller than a predetermined minimum damage size (de tection limit) for a cut of the composite material so that damages having at least this predeter mined minimum size may be reliably registered by the sensor sheet. Also, the minimum dis tance between any point within a sensing area and the respective conductive sensor trace must be smaller or equal to the predetermined minimum damage size (detection limit).

Preferably, the predetermined minimum damage size for a cut of a composite material is in the range of from 1 cm to 20 cm, especially preferred in the range of from 5 cm to 12 cm. This al lows in particular the detection of cuts to a composite material which may be caused by a colli sion with a fork of a forklift.

Also, the total area, the area of each of the openings as well as the spatial distribution of open ings determines the influence of the sensor sheet on the material properties of a composite ma terial into which the sensor sheet is embedded. A larger total area reduces the influence of the sensor sheet on the material properties of the resulting composite material. Also, a smaller dis tance between two adjacent openings reduces the influence of the sensor sheet on the material properties of the resulting composite material. The distance between two adjacent openings is defined as the shortest distance between the borders of two adjacent openings.

The total area of the openings with respect to the total area of the substrate is in the range of from 30% to 99%, preferably in the range of from 40 to 80% and particularly preferred from 50% to 70%.

The distance between two adjacent openings is preferably in the range of from 0.1 cm to 100 cm, especially preferred in the range of from 0.5 cm to 20 cm. The distance between two open ings may be chosen identical or different from the size (diameter in case of circular shape) of the openings.

A density of openings in the substrate is, for example, in the range of from 1 to 5000 per m ² , preferably in the range of 10 to 500 per m ² .

Preferably, the at least one conductive sensor trace is arranged in a serpentine pattern and is arranged around the openings of the substrate. By routing the conductive sensor trace in such a pattern, a large area may be covered by a single sensor trace which reduces the number of electrical circuits required to monitor a given area of the sensor sheet or a composite material comprising the sensor sheet.

The at least one sensing area comprises two power supply traces and multiple conductive sen sor traces forming multiple conductive paths, wherein each conductive sensor trace is connect ed to a first power supply trace at a first end and connected to a second power supply trace at a second end. Preferably, the electrical resistance of the power supply traces is different from the electrical resistance of the conductive sensor traces.

The electrical resistance of a trace may be adjusted by changing the shape, in particular the thickness and/or width of the trace and/or by changing the material. Different conductive materi als have different specific electrical resistances. Preferably, the electrical resistance of the pow er supply traces is chosen to be less than the electrical resistance of the sensor traces.

The power supply traces and the multiple conductive sensor traces connected to the power supply traces form a resistor network comprising multiple conductive paths. Damage to a specif ic one of the conductive sensor traces (a specific one of the conductive paths), wherein, for ex ample, the respective conductive sensor trace is cut, will cause a specific change to the overall resistance of this resistor network which may be detected by a single measurement of the elec trical resistance between the two power supply traces. Thus, a registered damage must be with in the area in which this specific conductive sensor trace is located. This allows providing spatial resolution for damage detection while requiring only a single resistance measurement. Such a measurement is preferably performed at an end of the power supply traces by providing suitable electrical contacts at an end of the respective power supply traces. The power supply traces and the multiple conductive sensor traces form a series-parallel re sistance circuit of the electrical resistance R(sensor) of a conductive sensor trace and the elec trical resistance R(supply) of a section of the power supply trace between two conductive sen sor traces. The last three resistors of this network (R(supply), R(sensor) and R(supply)) are mounted in series, their equivalent resistance being mounted in parallel with the second-last resistance R(sensor) of a conductive sensor trace, and so forth.

Preferably, the electrical resistance of a conductive sensor trace R(sensor) is greater than the electrical resistance R(supply) of a section of a power supply trace located between two con nections to conductive sensor traces. Preferably, a ratio between R(supply) and R(sensor) is chosen in the range of from 1 :50 to 1 :200. It is especially preferred to choose the resistance R(sensor) to be greater than 100 times the value of R(supply). For example, the electrical re sistance R(sensor) of the conductive sensor traces is chosen to equal 150 times the value of the electrical resistance R(supply) of a section of a power supply trace located between two connections to conductive sensor traces.

Preferably, the two power supply traces are arranged in parallel to each other forming a sensing area within the two power supply traces which is divided into multiple modules and each of the sensor traces is arranged within one of the modules and allows for detection of damages within the respective module.

Preferably, the sensing area is divided into 1 to 20 modules, especially preferred 3 to 10 mod ules. For example, a sensing area may be divided into 8 detection modules.

Preferably, each of the modules has a rectangular shape. Further suitable shapes include, for example, parallelograms or trapezoids. A damage occurring within such a module can be de tected and assigned to a specific one of the modules due to the resulting change in the electri cal resistance which is specific to this module.

The sensor sheet may comprise exactly one sensing area. For scaling of the sensor sheet to different sizes, more than one sensing area may be used. Preferably, the sensor sheet com prises from 2 to 100 sensing areas. Each sensing area may have several modules.

For covering large areas, more than one sensor sheet may be used. For example, a sensor sheet comprising 5 sensing areas arranged as 5 columns each having 8 detection modules may form a detection panel. For expanding the covered area, several of such detection panels may be used.

The achieved spatial resolution of such an arrangement depends on the width of the detection columns and the height of the detection modules. For example, the width of a detection column may be chosen in the range of from 10 cm to 60 cm, for example 30 cm and the height of a de- tection module may be chosen in the range of from 10 cm to 60 cm, for example 40 cm. Thus, in this example, a horizontal resolution of 30 cm and a vertical resolution of 40 cm is provided.

In order to form a detection system, the sensor sheet and/or a composite material comprising the sensor sheet is connected to a control device. The control device measures the electrical resistance by means of an electrical connection to the at least one conductive sensor trace of the sensor sheet. If power supply traces are present, the control device is preferably connected to the power supply traces.

Preferably, electrical connectors are arranged at or near an edge of the sensor sheet for electri cally contacting of the at least one conducting sensor trace. These electrical connectors may be used for connecting the sensor sheet to a control device. If present, the power supply traces may be connected between the connectors and the at least one conducting sensor trace. Pref erably, exactly two electrical connectors per sensing area are provided.

Preferably, all electrical connectors are arranged on the same edge of the sensor sheet. This allows for efficient and short electrical connections, in particular to a control device.

Preferably, the shape of the openings is a round shape, such as a circle, or is selected from a regular or irregular polygon. Especially preferred, the shape of the openings is selected from hexagons, circles, squares, triangles, and other polygons.

Preferably, all the openings have essentially the same shape and size and are uniformly distrib uted within the at least one sensing area. The term “essentially of the same shape and size” is to be understood to include openings located at the border of the sensor sheet and/or a sensing area which have a cut shape due to the finite nature of the area.

Alternatively, the openings are distributed in a non-uniform manner within the at least one sens ing area and the openings either have essentially the same size and shape or have varying shapes and sizes. The size distribution of the openings may, for example be a bimodal distribu tion or include exactly two shapes and/or sizes of openings. In case of a non-uniform distribu tion, space covering fractals and Voronoi networks are preferred.

The material of the substrate may be chosen from a non-conductive material. In particular, pol ymer sheets are suitable as substrates.

For example, the substrate may be selected from a polymer such as, for example, poly(ethyleneterephthalate) (PET), poly(ethylenenaphthalate) (PEN), polyimide (PI), thermo plastic polyurethane (TPU), poly(dimethylsiloxane) (PDMS).

Further suitable examples for substrates include paper and cardboard. Also, glass, silicone based substrates, woven fabrics, nonwoven fabrics, nonwoven mats, fiber mats and plies, in particular glass fiber mats and plies may be used. Suitable fiber mats and plies include in particular fiber mats and plies which are used for pro ducing fiber reinforced composite materials.

Suitable fabrics include, for example, cotton or polymer based woven and nonwoven fabrics.

Preferably, the thickness of the substrate is in the range of from 10 pm to 500 pm, especially preferred in the range of from 50 pm to 200 pm.

Preferably, the substrate is flexible.

The at least one conducting sensor trace and, if present, the power supply traces may be formed by printing the traces with a conductive ink onto the surface of the substrate.

Suitable conductive inks include, for example, silver based inks, carbon based inks, gold based inks, copper based inks, or semiconducting polymeric inks such as inks based on poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS), or Polyaniline.

In a further aspect of the invention a composite material is provided. The composite material comprises at least a base layer and a fiber reinforced composite layer, wherein at least one conductive path having at least one conductive sensor trace is embedded within the fiber rein forced composite layer and wherein the at least one sensor trace is configured and arranged to indicate damages to the at least one sensing area of the composite material by a change in electrical resistance.

In a first embodiment, the fiber reinforced composite layer comprises at least one sensor sheet as described herein which is embedded within the fiber reinforced composite layer, wherein the conductive path is arranged on a surface of the at least one sensor sheet.

Preferably, the fiber reinforced composite layer comprises a layer structure comprising at least a first fiber layer and the at least one sensor sheet. Preferably, the layer structure comprises in this order a first fiber layer, at least one sensor sheet as described herein, and a second fiber layer Preferably, the first fiber layer, the sensor sheet and, if present, the second fiber layer are all embedded in a polymer matrix. Further, the layer structure may comprise additional fiber layers.

The openings in the sensor sheet are present to allow the resin to penetrate the sensor sheet and glue the fibers together just as if the sensor sheet was not there. By providing openings in the sensor sheet, material that would otherwise block the resin from impregnating the fibers before the resin is cured and solidified is removed.

For electrically contacting the at least one sensor sheet, the contacts of the at least one sensor sheet are preferably located at an edge of the composite material and are exposed. Alternative- ly, it is possible to include a connector, which may be at least partially embedded within the composite layer of the composite material, wherein the connector extends the electrical con tacts to an edge of the composite material.

Preferably, the composite material comprises a single sensor sheet. Alternatively, the compo site material may comprise more than one sensor sheet, for example from 2 to 20 sensor sheets.

For covering large areas, more than one sensor sheet may be used. For example, a sensor sheet comprising 5 sensing areas arranged as 5 columns each having 8 detection modules may form a detection panel. For expanding the covered area, several of such detection panels may be used.

In a second embodiment, the fiber reinforced composite layer comprises at least a first fiber layer wherein the conductive path is arranged on a surface of the first fiber layer. Additionally, the fiber reinforced composite layer may comprise a second fiber layer. Preferably, the first and second fiber layers are arranged such that the surface of the first fiber layer carrying the con ductive path faces towards the second fiber layer so that the conductive path is embedded be tween the first and second fiber layers. Either the first or the second fiber layer may be arranged next to the base layer of the composite material.

The first and, if present, second fiber layers are all embedded in a polymer matrix.

The conductive path is, for example, arranged on the surface of the first fiber layer by means of a printing method using a conductive ink.

Preferably, printing of the at least one conductive path is performed by means of screen print ing, gravure printing, flexography, offset printing, electrophotography or inkjet printing. Depend ing on the fiber layer, a roll-to-roll process may be used.

Preferred fiber layers for directly printing the at least one conductive path include fiber mats, and plies, in particular glass fiber mats and plies.

Preferably, the at least one conductive sensor trace is arranged in a serpentine pattern by rout ing the conductive sensor trace in such a pattern, a large area may be covered by a single sen sor trace which reduces the number of electrical circuits required to monitor a given area of the composite material the sensor sheet.

Preferably, the at least one sensing area comprises two power supply traces and multiple con ductive sensor traces, wherein each conductive sensor trace is connected to a first power sup ply trace at a first end and connected to a second power supply trace at a second end. Prefera- bly, the electrical resistance of the power supply traces is different from the electrical resistance of the conductive sensor traces.

The electrical resistance of a trace may be adjusted by changing the shape, in particular the thickness and/or width of the trace and/or by changing the material. Different conductive materi als have different specific electrical resistances. Preferably, the electrical resistance of the pow er supply traces is chosen to be less than the electrical resistance of the sensor traces.

The power supply traces and the multiple conductive sensor traces connected to the power supply traces form a resistor network. Damage to a specific one of the conductive sensor trac es, wherein, for example, the respective conductive sensor trace is cut, will cause a specific change to the overall resistance of this resistor network which may be detected by a single measurement of the electrical resistance between the two power supply traces. Thus, a regis tered damage must be within the area in which this specific conductive sensor trace is located. This allows providing spatial resolution for damage detection while requiring only a single re sistance measurement. Such a measurement is preferably performed at an end of the power supply traces by providing suitable electrical contacts at an end of the respective power supply traces.

The power supply traces and the multiple conductive sensor traces form a series-parallel re sistance circuit of the electrical resistance R(sensor) of a conductive sensor trace and the elec trical resistance R(supply) of a section of the power supply trace between two conductive sen sor traces. The last three resistors of this network (R(supply), R(sensor) and R(supply)) are mounted in series, their equivalent resistance being mounted in parallel with the second-last resistance R(sensor) of a conductive sensor trace, and so forth.

Preferably, the electrical resistance of a conductive sensor trace R(sensor) is greater than the electrical resistance R(supply) of a section of a power supply trace located between two con nections to conductive sensor traces. Preferably, a ratio between R(supply) and R(sensor) is chosen in the range of from 1 :50 to 1 :200. It is especially preferred to choose the resistance R(sensor) to be greater than 100 times the value of R(supply). For example, the electrical re sistance R(sensor) of the conductive sensor traces is chosen to equal 150 times the value of the electrical resistance R(supply) of a section of a power supply trace located between two connections to conductive sensor traces.

Preferably, the two power supply traces are arranged in parallel to each other forming a sensing area within the two power supply traces which is divided into multiple modules and each of the sensor traces is arranged within one of the modules and allows for detection of damages within the respective module.

Preferably, the sensing area is divided into 1 to 20 modules, especially preferred 3 to 10 mod ules. For example, a sensing area may be divided into 8 detection modules. Preferably, each of the modules has a rectangular shape. Further suitable shapes include, for example, parallelograms or trapezoids. A damage occurring within such a module can be de tected and assigned to a specific one of the modules due to the resulting change in the electri cal resistance which is specific to this module.

The composite material may comprise exactly one sensing area. For scaling of the composite material to different sizes, more than one sensing area may be used. Preferably, the composite material comprises from 2 to 100 sensing areas. Each sensing area may have several modules.

For example, a composite material comprising 5 sensing areas arranged as 5 columns each having 8 detection modules may form a detection panel. For expanding the covered area, sev eral of such detection panels may be used.

The achieved spatial resolution of the composite material for damage detection depends on the width of the detection columns and the height of the detection modules. For example, the width of a detection column may be chosen in the range of from 10 cm to 60 cm, for example 30 cm and the height of a detection module may be chosen in the range of from 10 cm to 60 cm, for example 40 cm. Thus, in this example, a horizontal resolution of 30 cm and a vertical resolution of 40 cm is provided.

In order to form a detection system, the composite material is connected to a control device.

The control device measures the electrical resistance by means of an electrical connection to the at least one conductive sensor trace of the sensor sheet. If power supply traces are present, the control device is preferably connected to the power supply traces.

Preferably, electrical connectors are arranged at or near an edge of the composite material for electrically contacting of the at least one conducting sensor trace. These electrical connectors may be used for connecting the composite material to a control device. If present, the power supply traces may be connected between the connectors and the at least one conducting sen sor trace. Preferably, exactly two electrical connectors per sensing area are provided.

Preferably, all electrical connectors are arranged on the same edge of the composite material. This allows for efficient and short electrical connections, in particular to a control device.

The composite material layer comprises a polymer matrix reinforced with fibers. The conductive path arranged on a surface of the at least one sensor sheet or a surface of the first fiber layer is embedded within the composite material layer.

The fibers are preferably selected from glass fibers, carbon fibers, mineral fibers, basalt fibers, natural fibers, polymer fibers, aramid fibers, potassium titanate fibers, wood fibers, and mixtures thereof. The fibers are preferably provided in form of a fiber mat or ply.

The amount of fibers within the composite layer is preferably in the range of from 20% to 80% by weight of the composite layer.

Suitable polymers for impregnating the fibers and thus for forming the polymer matrix include, for example, epoxy and other thermoset polymers, polyurethanes, polyesters, polyure thane/polyester hybrid resins, thermoplastics like polyester, polyolefins, vinyl ester, and polyam ides.

The base layer of the composite material may, for example, be selected from a polymer. How ever, any other material, such as, for example, glass, metal, wood or cardboard may be used.

Suitable polymer materials include, for example, polyurethanes, and polystyrenes.

If a polymer is selected as base material, the polymer may be provided in form of a foamed ma terial.

For providing an insulated wall, for example for a refrigerated truck trailer or a cooled container, an insulating polyurethane foam material is preferably used.

The composite material may further comprise a cover layer, wherein a layer structure compris ing in this order the base layer, the fiber reinforced composite layer and the cover layer is formed.

The cover layer may be used to protect the composite material against environmental influ ences such as humidity. Further, the cover layer may be used to print information directly onto the composite material. Suitable materials for the cover layer include, for example, the materials described with respect to the base layer.

In a further aspect of the invention, a detection system comprising a control device and at least one sheet made out of or comprising one of the composite materials described herein which is connected to the control device is provided.

In order to form a detection system, the sensor sheet and/or a composite material comprising the sensor sheet is connected to a control device. The control device measures the electrical resistance by means of an electrical connection to the at least one conductive sensor trace of the sensor sheet. If power supply traces are present, the control device is preferably connected to the power supply traces. The control device is configured to perform measurements of the electrical resistance of at least one sensor trace located on a sensor sheet embedded within the composite material. Damages to the composite material will cause changes to the electrical resistance and may thus be de tected by the control device. In embodiments wherein the sensor sheet comprises a resistor network, the control device is preferably configured to locate the damage on the sensor sheet and thus on the at least one sheet based on the measured change of electrical resistance.

Preferably, the control device is configured to repeatedly perform measurement of the electrical resistance. A time interval between two measurements is preferably in the range of from 10 ms to 10 min, more preferably in the range of from 1 s to 10 s. This allows the control device to de tect the time of a damage incident within a few seconds.

Measuring the electrical resistance may comprise measurement of an ohmic resistance or may include measurement of the complex resistance (impedance). By measuring the impedance, any changes to the resistor network may be detected with more accuracy and reliability. A measurement of the electrical resistance with improved accuracy allows the use of more con ductive sensor traces per power supply trace which in turn allows for higher spatial resolution. If only the ohmic resistance is measured, a less complex measurement device may be used which reduces cost and complexity of the control device.

The control device may be configured to merely record any changes in the status of the at least one sheet. Recording such a change in status may include the position of the damage and/or the time when the damage was detected. Further, the control device may be configured to trig ger actions when a change of the status, in particular damage, is detected. For example, the control device may trigger an alarm, store data on the damage location and/or time, or may send a message to a contact person.

The at least one sheet may, for example, form an enclosure in which goods may be stored. Thus, the at least one sheet may form walls and/or a ceiling and/or a base of an enclosure. The enclosure may, for example, be configured as a storage container or as walls of a truck trailer. Preferably, the composite material is configured to provide thermal insulation.

By means of the at least one sensor sheet which is embedded within the composite material, damages such as cuts of the composite material may be detected so that the status and integri ty of an enclosure formed by the at least one sheet may be assessed.

The composite material or the detection system is preferably used as wall of a container or wall of a truck trailer.

In a further aspect of the invention, a method for producing a sensor sheet as described herein is provided. The method comprises a roll-to-roll process, wherein in a first step, a roll of sub strate is provided. In a subsequent second step, the at least one conductive sensor trace is printed onto the substrate using a conductive ink and subsequently, in a third step, the plurality of openings are cut.

In a roll-to-roll process, the material to be processed is provided on a roll and after processing, the material is re-reeled and provided on an output roll. Each step or at least one step of the method may be performed individually, wherein an intermediate output roll is provided after the respective step. Alternatively, each of the steps or at least two of the steps may be performed subsequently without re-reeling in between the process steps.

Preferably, printing of the at least one conductive trace is performed roll-to-roll by means of screen printing, gravure printing, flexography, offset printing, electrophotography, inkjet printing. Printing of the at least one conductive sensor trace may be performed, for example, in a flexog raphy based printing process in which conductive ink is selectively applied to a surface of the flexible substrate.

Preferably, the method includes printing at least two power supply traces onto the substrate. Printing of the power supply traces may be performed simultaneously with the step of printing of the at least one conductive sensor trace or may be performed as a separate step prior or after printing of the at least one sensor trace.

Printing the at least two power supply traces may be performed using the same printing method as used for printing the at least one sensor trace. Alternatively, a different printing method is used. For example, a flexographic printing process may be used for printing the at least two power supply traces and screen printing may be used for printing of the at least one sensor trace.

Printing the power supply traces separate from the at least one conductive trace allows the use of different conductive inks having different conductivity. However, it is of course possible to use the same conductive ink for printing of the power supply traces and the at least one conductive trace.

Preferably, the step of cutting the plurality of openings is performed after printing the at least one conductive trace and, if present, the at least two power supply traces, by means of a cutting roller, or by laser cutting.

In a further aspect of the invention, a method for producing a composite material according to the first embodiment as described herein is provided, wherein a layer structure comprising in this order a first fiber layer, a sensor sheet as described herein and a second fiber layer is formed by arranging the layers on top of each other and then impregnating the layer structure to form a polymer matrix. Preferably, the layer structure consists of in this order the first fiber layer, the sensor sheet as described herein and the second fiber layer. The layer structure may of course comprise further layers such as, for example, a base layer and/or a cover layer.

In another aspect of the invention, a method for producing a composite material according to the second embodiment as described herein is provided, wherein at least a first fiber layer is provided, at least one conductive sensor trace is printed onto the first fiber layer and subse quently impregnating the first fiber layer to form a polymer matrix.

The composite material may of course comprise further layers such as a base layer, a second fiber layer and/or a cover layer, wherein the first fiber layer is preferably embedded between the base layer and the second fiber layer. Preferably a composite layer of the composite material consists of the first and second fiber layer.

Preferably, the fiber layer is provided in form of a mat or a ply.

The step of printing may be carried out using any of the printing methods described with respect to the sensor sheet. Further, roll-to-roll processes are preferred, especially in cases where the fiber layer is provided in wound form.

Preferably, the fiber layer is pre-treated before printing of the at least one conductive sensor trace in order to promote adhesion of conductive ink. Suitable pre-treatments include chemical treatments, plasma treatments, flame treatments, corona treatments and combinations of two or more of said pre-treatments.

In a chemical treatment, a fiber mat is, for example, put in successive bath for instance sulfuric acid, sodium hydroxide and then rinsed with water.

Suitable plasma treatments include the use of Ar, Fh, O ₂ , CO ₂ , NH ₃ or air/atmospheric plasmas touching the fiber surface. Plasmas can, for example, be delivered through a plasma oven or plasma nozzles.

Suitable flame treatments involve the application of a gas flame on a fiber layer for a short time in the millisecond range. Gas jets may be used for dispensing the flame.

In a corona treatment, a corona discharge is used for creating radicals at the surface of the fi bers, creating chemically active surfaces. The corona discharge may be generated by a corona oven or corona treater.

Preferably, plasma treatment or corona treatment will be used.

The fiber pre-treatment can be done in a separate step before printing or also in-line without any re-reel step between pre-treatment and printing. Example

A composite material is prepared having a layer structure consisting of a base layer, and a fiber reinforced composite layer comprising fibers and resin, wherein a sensor sheet is embedded within the fiber reinforced composite layer.

An insulating panel made from rigid polyurethane foam is used as base layer. Thus, the compo site material is particular suitable as insulating wall for cooled containers or refrigerated truck trailers.

For fiber reinforcement, glass fiber plies of E-LM1810 available from Vectorply® Corporation using NEG Flybon 2026 fibers available from Nippon Electric Glass are used. NEG Hybon 2026 is a continuous filament E-glass fiber. The total fiber content in the composite layer is approxi mately 60% by weight of the composite layer.

The sensor sheet uses a 75 pm thick PET foil as substrate having a plurality of openings in the shape of hexagons which are all of the same size and are arranged in a uniform manner. A conductive sensor trace is printed onto the substrate using a silver based ink. The sensor trace is routed around the openings which are to be cut in a subsequent step. Further, the sensor trace is arranged in a serpentine pattern. After printing, openings are cut into the substrate. The total area of the plurality of openings is 63% of the total area of the sheet so that 37 % of the PET foil remains.

Elastocast® available from BASF SE which is a moisture insensitive polyurethane/polyester hybrid resin system is used as resin within the fiber reinforced composite layer.

For forming the composite layer on the base layer, two of the plies are placed onto the base layer. Subsequently, the sensor sheet is placed onto the fiber plies and finally two further fiber plies are placed onto the sensor sheet. The composite layer is then formed using the resin for example by means of vacuum infusion or compression molding.

The thickness of the fiber reinforced composite layer is approximately 3 to 4 mm.

The invention is described in more detail below on the basis of the drawings, in which:

Figure 1 shows the layer structure of a composite material capable of detecting damages,

Figure 2a shows a top view of a sensor sheet,

Figure 2b shows only the openings in the sensor sheet,

Figure 2c shows only power supply traces on the sensor sheet, Figure 2d shows only a conducting sensor trace on the sensor sheet, Figure 3a shows a sensor sheet having a single detection module, Figure 3b shows an equivalent circuit for a single detection module, Figure 4a shows a sensor sheet having a column of several detection modules, Figure 4b shows an equivalent circuit for the column of detection modules, Figure 5 shows a diagram depicting the changes in resistance upon damage of a detection module of a column,

Figure 6 shows a wall formed by a composite material, and Figure 7 shows a truck trailer comprising a damage detection system having a wall formed by the composite material.

Figure 1 shows a composite material 100 comprising a base layer 1 and a fiber reinforced com posite layer 10.

In known two-component fiber composites, the base layer 1 is typically covered by two layers of fibers 2, 4 which are impregnated with resin. In order to detect damages to the composite mate rial 100, the composite material 100 comprises a sensor sheet 3 which is embedded between a first fiber layer 2 and a second fiber layer 4. The first fiber layer 2, the sensor sheet 3, the sec ond fiber layer 4 are embedded in a polymer matrix and form the fiber reinforced composite lay er 10.

The sensor sheet 3 comprises a substrate 30 where an array of openings 32 is cut out. For de tecting damages to the sensor sheet 3 and thus to the composite material 100 a conductive path 33 comprising a conductive sensor trace 36 is located on the surface of the substrate 30. The sensor trace 36 is routed in a serpentine pattern so that essentially the entire surface of the flexible substrate 30 or at least of a sensing area 6 of the substrate 30 is covered by the con ductive sensor trace 36. The sensor trace 36 is also arranged such that it is routed around the openings 32. Thus, any damage within the sensing area 6 which is larger than the maximum distance between a point within the sensing area 6and the conductive sensor trace 36 will also affect the conductive sensor trace 36. Any damage to the conductive sensor trace 36 or the power supply trace 34 may then be detected by performing a measurement of the electrical re sistance of resistor network formed by the conductive sensor trace 36 and the power supply traces 34. In the embodiment shown in figure 1 , two power supply traces 34 are provided. A first end of the conductive sensor trace 36 is electrically connected to a first power supply trace 34a and a sec ond end of the conductive sensor trace 36 is electrically connected to a second power supply trace 34b, see also figure 3a. The electrical resistance of the conductive sensor trace 36 may be measured by measuring the electrical resistance between the two power supply traces 34.

Figures 2a to 2d show the sensor sheet 3 as explained with reference to figure 1 in top view. In Figure 2b, only the openings 32 are shown. As can be seen from figure 2b, the openings 32 have a hexagon shape and are all of the same shape and size except for cut openings 32 at the borders of the sensing area 6. This deviation is due to the finite size of the sensing area 6. All openings 32 are uniformly arranged in a hexagon pattern.

In the depicted example, most of the area of the sensor sheet 3 has been cut away to form the openings 32. This allows the resin used for forming the polymer matrix of the reinforced compo site layer 10, see figure 1 , to penetrate the sensor sheet 3. The resin can thus impregnate all of the fibers in the first and second fiber layers 2 and 4. In particular, the openings 32 allow for a uniform impregnation of the fibers of the first fiber layer 2 which is arranged between the base layer 1 and the sensor sheet 3.

Figure 2c shows only the power supply traces 34 and electrical contacts 5. The electrical con tacts 5 are arranged on an edge of the sensor sheet 3 and allow establishing of an electrical connection to the power supply traces 34 and thus to the at least one conductive sensor trace 36.

Figure 2d shows only the conductive sensor trace 36. As can be seen from figure 2d, the con ductive sensor trace 36 has a serpentine path and is routed around the openings 32. A first end 37a of the conductive sensor trace 36 is connected to the first power supply trace 34a, and a second end 37b of the conductive sensor trace 36 is connected to the second power supply trace 34b, see figure 2c and figure 1.

Figure 3a shows a sensor sheet 3 having a single detection module 7. The openings 32 in the substrate 30, see figure 1 , are not shown for better overview. Figure 3b shows an equivalent circuit for the sensor sheet 3 of figure 3a. As can be seen from figure 3a, a resistance meas urement may be made by contacting the first and second electrical contacts 5a and 5b. If the conductive sensor trace 36 or the first or second power supply traces 34a, 34b are damaged, for example when a composite material having the sensor sheet 3 embedded within is cut, the resistor network is broken and a very large or infinite resistance is measured.

Figure 4a shows a sensor sheet 3 having multiple detection modules 7 which are arranged in a column 8. The openings 32 in the substrate 30 are not shown for better overview. Figure 4b shows an equivalent circuit for the sensor sheet 3 of figure 4a. As can be seen from figure 4a, a resistance measurement may be made by contacting the first and second electrical contacts 5a and 5b. This arrangement allows providing of spatially resolved damage detection by using only two electrical contacts 5a and 5b. The equivalent electrical circuit of Figure 4b shows an alternating series-parallel resistance circuit of the electrical resistance R(sensor) of the conductive sensor traces and the electrical resistance of R(supply) of sections of the power supply traces located between two connections to conductive sensor traces. The last three resistors of this network (R(supply), R(sensor) and R(supply)) are mounted in series, their equivalent resistance being mounted in parallel with the second-last resistance R(sensor) of a conductive sensor trace, and so forth.

By following this alternating series-parallel resistance circuit it is possible to compute the value of the equivalent resistor measured between electrical contacts 5a and 5b.

Preferably, the electrical resistance of a conductive sensor trace R(sensor) is greater than the electrical resistance R(supply) of a section of a power supply trace located between two con nections to conductive sensor traces. For example, in the case of eight modules 7 as shown in figure 4a, the electrical resistance R(sensor) of the conductive sensor traces is preferably cho sen to equal 150 times the value of the electrical resistance R(supply) of a section of a power supply trace located between two connections to conductive sensor traces. The optimum ratio between the values of resistances R(sensor) and R(supply) can vary depending on the number of modules 3 per column 8.

If any of the conductive sensor traces 36 or any of the power supply traces 34a, 34b is dam aged, for example when a composite material having the sensor sheet 3 embedded within is cut, the resistor R(sensor) assigned to this module 7 is broken which can be detected by a change in electrical resistance of the resistor network. This allows drastically reducing the num ber of electrical contacts 5 necessary to provide spatial resolution, thus lowering the costs for wiring and the complexity of a control device for measuring the electrical resistance and as sessing the status of the sensor sheet 3. Further, the proposed circuit allows arrangement of the electrical contacts 5a, 5b on a single edge of the sensor sheet 3 which avoids instabilities in duced by electrical connectors potentially protruding from the middle of the composite material 100.

The diagram of figure 5 shows the electrical resistance R measured between the electrical con tacts 5a and 5b of the example as shown in figures 4a and 4b. The x-axis marks the number# of the module 7 which has been damaged and the y-axis depicts the respective measured elec trical resistance R. The modules 7 are, for example, numbered from left to right. The first de picted value (number 0) is the reference value for the undamaged resistor network.

As can be seen from the diagram of figure 5, the measured electrical resistance R is different for each case. Thus, the respective damaged module 7 can be detected by means of a single measurement using exactly two electrical contacts 5a, 5b for the entire column 8 of modules 7. Figure 6 shows a wall 310 formed by a composite material 100. The wall 310 may, for example, be used as a wall 310 of a cooled container or a refrigerated truck trailer.

The wall 310 comprises several detection panels 9. In the depicted example, the wall 310 com prises ten detection panels 9. Each detection panel 9 comprises five detection columns 8 as described with respect to figures 4a and 4b.

Each detection column 8 comprises two electrical contacts 5. By performing a single resistance measurement per column 8, a damaged detection module 7 of the detection column 8 may be identified. The shown arrangement of figure 6 allows for spatially resolved detection of damages to the wall 310 by identifying the respective damaged detection module 7.

In a typical example, the detection columns 8 have a width of 30 cm and a detection module 7 has a height of 40 cm. Thus any damage above a predetermined minimum detection limit may be registered and located with a horizontal resolution of 30 cm and a vertical resolution of 40 cm. The predetermined minimum detection limit is given by the arrangement of the at least one sensor trace 36 and of the opening 32 in the at least one sensor sheet 3, see for example figure 1.

Figure 6 further shows a part of the first two detection columns 8 of a panel 9 in an enlarged perspective view. In this perspective view, the electrical contacts 5 arranged on the edges are visible. Each detection column 8 has exactly two electrical contacts 5.

Figure 7 shows a truck trailer 300 comprising a damage detection system 200 having a wall 310 formed by the composite material 100, see figure 6. The detection system 200 further compris es a control device 210 which is electrically connected by means of an electrical connection 220 to the wall 310.

The control device 210 is configured for measuring the electrical resistance of each of the de tection columns 8 of the wall 310, see figure 6. Thus, the detection system 200 is capable of registering damage to the wall 310 and also to identify the respective module 7 which has been damaged. Thus, the location of the damage to the wall 310 may be determined. The control device 210 is, for example, configured to transmit measured data to the user or the owner of the truck trailer when damage is detected. Also, the control device 210 may be configured to record the time when the damage was detected. List of reference numerals

1 base layer

2 first fiber layer

3 sensor sheet

4 second fiber layer

5 electrical contact

5a first electrical contact

5b second electrical contact

6 sensing area

7 detection module

8 detection column

9 detection panel

10 fiber reinforced composite layer

30 substrate

32 openings

33 conductive path

34 power supply traces 34a first power supply trace 34b second power supply trace 36 conductive sensor trace 37a first end

37b second end

100 composite material

200 detection system 210 control device 220 electrical connection

300 truck trailer 310 wall