WO2024075077A1

WO2024075077A1 - Method for monitoring structural elements in composite

Info

Publication number: WO2024075077A1
Application number: PCT/IB2023/060057
Authority: WO
Inventors: Massimiliano GABARDI; Lorenzo TOZZETTI; Stefano Faralli; Massimiliano SOLAZZI; Fabrizio Di Pasquale; David Benedetti
Original assignee: Scuola Superiore Sant'anna; Carbon Dream S.P.A.
Priority date: 2022-10-07
Filing date: 2023-10-06
Publication date: 2024-04-11

Abstract

Method for monitoring the strain and/or stress of at least one portion of a structure in composite material (10) having a body adapted to define a neutral axis (N) when loaded, comprising the steps of providing at least two fibre-optic strain sensors (11, 12), so as to provide a pair of fibre-optic strain sensors; embedding both fibre-optic strain sensors (11, 12) of the pair within the body of the structure in composite material, said fibre-optic strain sensors of the pair being mutually flanked and arranged opposite with respect to the neutral axis (N) of the structure in composite material; obtaining strain measurements from each of the fibre-optic strain sensors (11, 12) of the pair; computing, based on the obtained strain measurements, a strained configuration of the at least one portion of the structure in composite material (10) and/or the extent of applied load (F).

Description

METHOD FOR MONITORING STRUCTURAL ELEMENTS IN COMPOSITE

TECHNICAL FIELD

[001]. The present invention relates to the technical domain of monitoring structures in composite material.

[002]. In particular, the object of the present invention is a method for monitoring a structure in composite material.

[003]. The present invention also relates to a monitoring device, as well as to an assembly comprising the monitoring device and the structure in composite material to be monitored.

STATE OF THE ART

[004]. Generally, composite materials are used on a large scale and some known particular applications in the automotive, aerospace, naval industry, as well as in the industry of the production of renewable energies, such as the production of wind turbines, and/or turbo-machines, as well as in the sports sector, for example in the sector of high-performance vehicles (racing), also including racing craft and the like.

[005]. Such composite materials can be provided in panels, shells, domes, plates, slabs, beams, sticks or other elements or structures or bodies which are necessarily subjected, when in operating conditions, to high static and dynamic loads and stresses and strains.

[006]. In addition, the operating temperatures to which known structural elements in composite material are subjected can reach levels and excursions which contribute to the premature deterioration of the material.

[007], Therefore, the components made of such composite materials are subject to premature deterioration, such as in the case of competition vehicle parts, as well as aerospace vehicle shells.

[008]. In addition, structures in composite materials are typically light and thin, albeit sturdy and resistant.

[009]. Where the composite material is formed by a plurality of material layers, such as a multilayer material which drowns sheets of fabric such as pre-impregnated carbon fibre sheets ("pre-preg") in a matrix, the element is likely to incur delamination of the inner layers, a defect which may be invisible to external visual inspection.

[010]. In addition, complex phenomena of local non-linearity, such as defects due to unforeseen events, can cause a deterioration in the performance of the structure over time.

[011]. Therefore, there is a need for a solution to foresee signs of structural deterioration which could cause breakage of the structure or a portion thereof, or other dramatic consequences, at an early stage.

[012], In addition, it must be considered that typically in structures in composite materials, a deviation of the neutral axis (plane) from the axis (plane) of symmetry of the body of the structure is observed when loaded with flexion, according to different rigidity characteristics of the composite material when subjected to traction or compression.

[013], This inevitably causes measurement uncertainty and requires a fine interpretation of the information acquired by sensors located on or near parts of the structure to be monitored, irradiating it, such as piezoelectric sensors (strain gauges), ultrasonic sensors and/or X-ray sensors, or other digital imaging techniques.

[014]. Fibre Bragg Grating fibre-optic (or "FBG") sensors have also been proposed for monitoring structures because they are adapted to detect local strains without interfering with the static and dynamic behaviour of the structure itself, given the light weight and small footprint thereof.

[015], Since the measurement of the sensors is precise, in order to reconstruct strain and stress information of the entire structure, the sensor apparatus for detecting the local strain data of the structure is usually flanked with a model implemented in the structure's computer, in an attempt to compensate, by means of the computing power used for numerical modelling, the limits due to the punctuality of the measurement itself. For example, computer programs based on finite element theory ("FEM", "iFEM") can be employed.

[016], However, this type of joint solution risks obtaining an estimate of the stress/strain state of the element in composite material which undesirably incorporates the sum of the uncertainties of the two approaches employed (sensoristic and numerical).

[017], This is because the rigidity of a composite material can be, in the most general case, anisotropic and have a very high local variability depending on the portion or micro-portion of the body of the structure considered, which inevitably make the theoretical models too ideal and unable to fully grasp the local and global behaviour of the composite material when loaded.

[018]. In addition, the shapes of the bodies in composite material are usually very complex, for example because they are designed on the computer (CAD) to provide certain desired fluid dynamic performance, as can occur in the case of boat hulls.

[019]. For example, the prior art document US-2019-390985 shows a flexion sensor itself composed of a pair of FBG sensors integrated in a single body of elastomeric material and a descriptive numerical model of the dynamic behaviour of the sensor thus constructed.

[020]. Furthermore, the prior art document US-7520176 shows the application of two Fiber-Bragg-Grating type sensors to the opposite surfaces of a plurality of crosssections of a beam to be monitored which is subjected to flexion and torsion loads.

[021], Such a solution, although partially advantageous from some points of view - as in the provision of a pair of fibre-optic sensors of the Fiber-Bragg-Grating ("FBG") type to monitor, from the outside, a single cross-section of the structure - does not fully solve the problem and is in any case prone to detection uncertainties and premature deterioration of the sensors themselves.

[022]. EP 1 635 034 also discloses the application of FBG-type sensors to structures to be monitored. In this, an apparatus for detecting the radius of curvature of a structural element is more specifically disclosed. The device shows various types of structures in which optical sensors can be inserted or glued and which are arranged to be more or less permanently associated with the structure to be monitored. As shown in Fig. 2 of the aforesaid document, a measurement below the surface of the structural element can only be carried out by providing cavities in the structural element itself into which the external structural elements carrying the optical sensors can be inserted, thus causing alterations in the mechanical characteristics of the structure to be monitored, all the greater the more the optical sensors are to be inserted deep into the thickness. The optical sensors remain in any case integrated in structural elements outside the structure to be monitored. Furthermore, in the cited document the position of the optical sensors with respect to the structure to be monitored is clearly indicated as superficial and it is generally indicated that several optical sensors can be arranged in different, preferably opposite, positions. Since the optical sensors are located on structures outside the one to be monitored, and which are thus inevitably positioned exclusively on the outer surface of the structure to be monitored, the device does not allow to detect the conditions in the thickness of the structure to be monitored and only makes it possible to perform an estimate of overall changes in shape, such as the measurement of the radius of curvature.

[023]. WO 2021/041605 discloses a flexible filament to which one or more fibre-optic sensors for detecting the strain of the flexible filament are stably associated, by means of a buffer material. Also in this case, the fibre-optic sensors are positioned on the surface of the element to be monitored and no specific positions in which they must be arranged with respect to the section of the structural element to be monitored are indicated.

[024], The application on the outer surfaces of pairs of strain sensors for a given measurement point, as proposed in the aforementioned documents, provides very focused information on theoretical behaviour models which can also significantly deviate from the actual behaviour of the structural element to be monitored.

[025], The need therefore remains strongly felt to provide a solution for monitoring a body or structure made of composite material which is reliable, robust and repeatable.

[026]. At the same time, the need is felt to avoid, or at least limit to a minimum, the need to provide a dedicated numerical model for each body to be monitored, without thereby providing a decreased accuracy or a worse interpretation of the monitoring data acquired.

SUMMARY OF THE INVENTION

[027], It is an object of the present invention to overcome the drawbacks lamented with reference to the state of the art and to provide a solution to the above-mentioned needs.

[028]. This and other objects are achieved by a method according to claim 1 , as well as by a device according to claim 7, as well as by an assembly according to claim 12. [029]. Some advantageous embodiments are the object of the dependent claims.

[030]. According to an aspect of the invention, a method for monitoring the strain and/or stress of at least one portion of a structure in composite material having loads defining a neutral axis on the body, comprises the steps of:

- embedding at least two fibre-optic strain sensors, said fibre-optic strain sensors being mutually flanked and arranged, in the body section, opposite with respect to the neutral axis (not necessarily symmetrical); and

- obtaining strain measurements from each of the fibre-optic strain sensors.

[031], The method further comprises the computing step. The computing step can comprise computing a strained configuration of the at least one portion of the structure in composite material based on the obtained strain measurements. The computing step can alternatively or additionally comprise computing the extent of applied load. In fact, the load applied to the structure can be unknown, for example, dictated by environmental conditions; therefore, the method can allow to compute, i.e. , to trace, the extent of the applied load.

[032]. By virtue of the proposed solutions, it is possible to estimate the strains and stresses of at least one portion of the structure in composite material based on the obtained measurements with greater precision with respect to the use of a single sensor placed in traction or compression and used in combination with a model.

[033]. By virtue of the proposed solutions, it is possible to carry out, knowing the strains and stresses in the material in an almost continuous manner, the real-time monitoring of the structure, to detect faults due to overloading or damage to the material or to use such information to search for better performance and optimisations.

[034], The fibre-optic strain sensors can each comprise one or more Bragg gratings as detection points.

[035]. The fibre-optic strain sensors can be of the distributed type, and thus comprising a plurality of detection points, based on optical frequency domain reflectometry technique.

[036]. By means of the proposed solution with two sensors integrated within the thickness of the structure, it is possible to obtain a precise and continuous measurement of the position of the neutral axis (or plane) of the structure in composite material (or portion of the structure in composite material) over time, thus accurately tracing the precise curvature of this structure without having to pass through a constitutive model of the material, which for composite materials can be affected by great uncertainty.

[037]. By virtue of the proposed solutions, it is possible to monitor a body or a structure made of composite material of any composition in real time, in both static and dynamic conditions, without invasively interfering with the stress state of the body to be monitored.

[038]. By virtue of the proposed solutions, the sensors are prevented from being exposed to environmental conditions outside the structure to be monitored, which can also be extreme, during the monitoring operations, with the result of providing an improved measurement accuracy. At the same time, improved protection is provided to the FBG sensors, resulting in improved detection and extended service life.

[039]. At the same time, the detection quality is also improved by virtue of the integration, i.e., the incorporation of the sensors within the body of the composite material to be monitored, which consequently achieves a perfect local adhesion of the sensor to the portion to be monitored.

[040]. By virtue of the provision of at least two sensors drowned or embedded or integrated in a certain same portion of the element to be monitored, it is possible to obtain a precise measurement of both the local tensile strain and the compressive strain.

[041]. The embedding or integration of the sensors in the element or body or structure to be monitored allows to read those strains which could be invisible or inaccurate for sensors placed outside the element. DESCRIPTION OF THE FIGURES

[042], Further characteristics and advantages of the invention will be apparent in the following disclosure of preferred embodiments, illustrated merely by way of non-limiting example, with reference to the attached drawings in which:

- figure 1 schematically shows a cross-section of a structure in composite material in which a monitoring device is mounted, according to an embodiment, as well as a possible step of a monitoring method, according to a possible operating mode;

- figure 2 schematically shows a monitoring device, according to an embodiment;

- figure 3 schematically shows an assembly comprising the monitoring device of figure 2 mounted to a structure in composite material, according to a possible operating mode;

- figure 4 is a flowchart schematically illustrating a monitoring method, according to a possible operating mode.

DETAILED DESCRIPTION OF EMBODIMENTS

[043], In accordance with a general embodiment, a monitoring device 1 comprises at least one pair of fibre-optic strain sensors 11, 12.

[044]. According to a preferred embodiment, the fibre-optic strain sensors 11 , 12 of the pair are of the type comprising at least one detection site 18 including at least one Bragg grating.

[045]. In accordance with an embodiment, the fibre-optic strain sensors 11 , 12 of the pair are of the distributed type based on "Rayleigh Backscattering" and using for example optical frequency domain reflectometry technique (hereinafter also only: "OFDR", acronym for: "Optical Frequency Domain Reflectometry").

[046]. The monitoring device 1 is designed for monitoring the state of strain and/or stress of a structure in composite material 10 to be monitored.

[047], It is to be well understood that the term "structure" in composite material 10 to be monitored is intended to indicate even only a portion thereof, as well as it is intended to indicate a "body" as well as an "element" of which it is desirable to monitor the state of strain/stress. Therefore, the structure in composite material 10 does not necessarily carry out an exclusively structural function when under operating conditions.

[048]. For example, the structure to be monitored can comprise a cantilevered free end. For example, the structure to be monitored can be constrained, when in operating conditions, to another structure or another element. For example, the structure in composite material to be monitored can have a flexible body.

[049]. Therefore, the structure in composite material 10 can be or can belong to a stick, a beam, a plate, a sheet, a shell, a dome, a bottleneck, a cavity, and/or the like. For example, the structure to be monitored can comprise a plurality of changes in concavity. For example, the structure to be monitored can have an irregular shape. The shape and/or composition of the body or structure in composite material 10 can be chosen to meet various functional operating requirements.

[050]. The structure to be monitored in composite material 10, when stressed, makes it possible to define a neutral axis N, as schematically shown in figure 1.

[051], The fibre-optic strain sensors 11 , 12 of the pair are arranged mutually flanked and opposite with respect to the neutral axis N of the structure to be monitored, embedded within the thickness of the body of the structure in composite material 10.

[052], As shown for example in figure 1 , the body of the structure in composite material 10 to be monitored can have a thickness 17, and both sensors 11 , 12 are contained in the thickness 17 of the structure in composite material 10.

[053]. The sensors 11, 12 of the pair are not necessarily arranged symmetrically with respect to the neutral axis N, although in accordance with an embodiment the sensors of the pair are each at a respective distance x1 , x2 from the neutral axis N, and said distances x1 and x2 are substantially equal. Alternatively, said distances z1 and x2 are different from each other, and for example one is a fraction of the other.

[054]. The integration process of the sensors 11 , 12 in the structure in composite material 10 to be monitored envisages the definition of the characteristics of the composite material in terms of the number and thickness of the layers composing it as well as the related orientation of the fibres, the determination of the neutral axis N based on the type of stresses to which the structure in composite material 10 to be monitored will be subjected under working conditions, and finally the definition of the layers between which the sensors 11 , 12 must be interposed, chosen based on the distance from the neutral axis N and the surface of the structure in composite material 10 to be monitored to which the sensors 11 , 12 must be arranged.

[055]. As shown for example in figure 3, one fibre-optic sensor 11 of the pair is embedded in a portion which is tensile stressed, while the other fibre-optic sensor 12 of the pair is embedded in a compressive stressed portion.

[056]. It is thereby possible to obtain a precise and reliable assessment of the local state of strain of the composite material of the structure.

[057]. Preferably, the sensors 11 , and 12 are arranged such that the detection points 18 of both fibre-optic strain sensors 11 , 12 of the pair are embedded in the same crosssections of the element in composite material 10.

[058]. In accordance with a preferred embodiment, each of the fibre-optic strain sensors 11, 12 of the pair have an elongated body extending within the body of the structure in composite material to be monitored, and their elongated bodies are mutually separated and disjointed within the body of the structure to be monitored. For example, the fibre-optic sensors 11 , 12 extend substantially mutually parallel within the body of the element to be monitored along a longitudinal direction of the element 10.

[059]. In accordance with an embodiment, each fibre-optic strain sensor 11 , 12 comprises a plurality of detection points 18 distributed along the longitudinal extension thereof, i.e. , along the elongated body thereof and/or simply comprises fibre optics and uses OFDR technique based on Rayleigh Backscattering. It is thereby possible to acquire local strain/stress information on a plurality of detection sites 18 distributed in the body of the structure to be monitored along the extension of the fibre optics of the sensors 11 , 12, or continuous strain/stress information with high spatial resolution provided by the OFDR technique, obtaining an accurate estimate of the global strain/stress state of the structure to be monitored.

[060]. The detection points 18 of one sensor 11 or 12 of the pair can be arranged side flanked with the detection points 18 of the other sensor 12 or 11 of the pair, in a same cross section of the structure in composite material 10. In other words, when in operating conditions, the detection points 18 of one sensor 11 are aligned along a transverse direction X which defines a cross-section of the structure to be monitored with the detection points 18 of the other sensor 12.

[061], In the distributed detection by means of OFDR technique it is possible to define distributed pairs of measurement points 18 aligned along the transverse direction X which defines a cross-section to be monitored of the structure 10.

[062]. For example, each fibre-optic strain sensor 11, 12 comprises a fibre-optic sensor distributed along the longitudinal extension thereof which is interrogated with optical frequency domain reflectometry (OFDR) technique.

[063]. In accordance with a variant, the detection points 18 of one sensor 11 or 12 of the pair are arranged offset with respect to the detection points 18 of the other sensor 12 or 11 of the pair, along the longitudinal extension of the structure to be monitored.

[064]. The monitoring device 1 can comprise other fibre-optic sensors, in addition to said pair of fibre-optic strain sensors 11 , 12. For example, at least three fibre-optic strain sensors including said pair of fibre-optic strain sensors 11 , 12 may be provided. For example, at least two pairs of fibre-optic strain sensors including said pair of fibreoptic strain sensors 11 , 12 may be provided.

[065]. The monitoring device 1 further comprises a data processing unit 15 operatively connected with both fibre-optic strain sensors 11 , 12 of the pair. The data processing unit 15 is configured to process the information detected at least by the sensors 11 , 12. [066]. The data processing unit 15 is configured, in the monitoring device 1, to perform a real-time monitoring of the element in composite material 10. For example, when in operating conditions, the data processing unit 15 transmits control signals aimed at emitting luminous pulses of light or frequency modulated light (ODFR) in the fibre optics of the sensors 11 , 12 and subsequently acquire and process the return signal, to obtain information on the strain/stress state of the structure, in order to compute the strained configuration of the structure itself, in real time. In order for the monitoring to occur in real time, the signal acquisition occurs dynamically with an acquisition frequency sufficient to detect the dynamic behaviour of the structure in composite material 10 to be monitored which is subject to dynamic loads. For example, to obtain a real-time monitoring of a structure in composite material which undergoes variable physical effects with a frequency not exceeding 2.5 KHz, it will be sufficient to provide a data processing unit 15 having a scanning frequency of at least 5 KHz.

[067], A display can be provided which is operatively connected with the data processing unit 15, which can be used to display information on the processed strained configuration of the element to be monitored and/or to display instructions and/or signals, for example alarm signals. Alarm signals can be made by providing an acoustic signal, as an alternative or in addition to providing the display.

[068]. A memory 16 can be provided which is operatively connected with the data processing unit 15, which can be used to store the acquired and/or detected information and/or information on the time sequence of the acquisitions.

[069]. The operative connection between the fibre-optic strain sensors 11 , 12 of the pair and the data processing unit 15 can comprise at least one cabled connection extending outside the body of the element in composite material 10 by means of at least one access plug 13 provided through the body of the element in composite material to be monitored. The access plug 13 can be provided on the outer surface 19 of the structure to be monitored.

[070], In accordance with an embodiment, a memory 16 is provided which is operatively connected with the data processing unit 15 and which comprises information on: the load applied to the element in composite material to induce the stress state and/or on the mechanical constraints of the element in composite material and/or on the mechanical properties of the composite material of which the element is formed and/or on a set of allowable values of the computed strained configuration. Thereby, when in operating conditions, the data processing unit 15 can compare the detected and/or processed information with the information stored in the memory 16, as explained in more detail in this disclosure below.

[071], With reference to the above, in accordance with a general embodiment, an assembly 2 is provided comprising a structure in composite material having a neutral axis N and at least one monitoring device 1 according to any one of the previously described embodiments. [072], In particular, in said assembly 2 the fibre-optic strain sensors 11 , 12 of the pair are both embedded within the body of the structure in composite material 10, and are arranged, within the body of the structure in composite material, mutually flanked and opposite each other with respect to the neutral axis N.

[073], The structure in composite material 10 can be formed by a plurality of layers. For example, adjacent layers can be formed by two or more different materials, which for example alternate in the thickness of the structure 10. In this case, the fibre-optic strain sensors 11 , 12 of the pair can be located, i.e., embedded in the composite material between two adjacent layers of different materials.

[074], The assembly 2 can be made during the fabrication of the structure in composite material 10, by means of embedding the detection device 1 within the body of the structure in composite material, as it is being made.

[075], A monitoring method will be described below.

[076], The monitoring method is particularly adapted, although not uniquely intended, to be performed by providing at least one monitoring device 1 , according to any one of the previously described embodiments.

[077], A method for monitoring the strain and/or stress of at least one portion of a structure in composite material 10, having loads on the body defining a neutral axis N, comprises the step of providing at least one pair of fibre-optic strain sensors 11 , 12, and the step of embedding both fibre-optic strain sensors 11 , 12 of the pair within the body of the structure in composite material 10.

[078], The step of embedding can comprise drowning the fibre-optic sensors 11 , 12 of the pair in a matrix of liquid and/or viscous material.

[079], The embedding step allows the fibre-optic sensors 11 , 12 of the pair to be arranged at a certain distance or depth hi , h2 with respect to the outer surface 14 of the structure to be monitored 10.

[080]. The embedding step can comprise perforating the composite material of the structure to be monitored 10, making one or more seats inside the body of the composite material intended to receive said fibre-optic sensors 11, 12 of the pair.

[081]. The embedding step can occur by including the fibre-optic sensors 11 , 12 of the pair within the body of the structure in composite material to be monitored.

[082]. The embedding step can occur during the fabrication of the element in composite material to be monitored 10. For example, where the composite material is of the laminate type, i.e., layered, the fibre-optic sensors 11, 12 of the pair can be integrated between the layers of composite material during the forming process (e.g., during a lay-up type process).

[083]. As mentioned above, the fibre-optic strain sensors 11 , 12 of the pair are mutually flanked and arranged opposite with respect to the neutral axis N of the structure in composite material to be monitored.

[084]. The method further comprises the step of obtaining strain measurements from each of the fibre-optic strain sensors 11 , 12 of the pair, and the step of computing a strained configuration of the at least one portion of the structure in composite material 10 based on the obtained strain measurements.

[085]. Preferably, the method allows a real-time monitoring of the structure to be monitored by means of real-time computing of the strained configuration of the structure in composite material 10.

[086]. In accordance with a possible operating mode, the method further comprises the step of: evaluating whether the computed strained configuration belongs to a set of admissible configurations, for example stored within the memory 16. Thereby, if the computed strained configuration does not belong to said set of admissible configurations, then the method comprises the further step of identifying and/or discriminating a fault condition. In other words, the monitoring method can be configured to identify and discriminate a fault condition of the structure to be monitored based on the comparison between the computed strained configuration and a set of admissible values for the strained configuration or based on the comparison between the computed and allowed loads. Said admissible values can comprise one or more acceptability ranges of the strain value detected (compression and/or traction) by the fibre-optic sensors 11, 12 or of the loads estimated on the basis of the strains detected by the sensors.

[087], In accordance with a possible operating mode, the method comprises the further step of transmitting at least one alarm signal and/or intervening to secure the structure in composite material.

[088]. To determine the location and dimensioning of the monitoring device 1, the method can envisage a processing step of the data on: the load applied F to the structure in composite material 10 to induce a state of stress, and/or on the mechanical constraints of the body in composite material, and/or on the mechanical properties of the composite material of which the body itself is formed. It is thereby possible to determine, based on processed and/or stored data, the number and/or location of the detection points 18 to be provided and/or the foreseen strained configuration of the structure in composite material to be monitored.

[089]. The method can comprise the further step of inducing a stress state on the structure in composite material 10. In such a case, the fibre-optic strain sensors 11, 12 are preferably arranged in the body of the structure to be monitored 10 such that one sensor 11 or 12 of the pair measures tensile strains while simultaneously the other sensor 12 or 11 of the pair measures compressive strains.

[090]. The method can comprise the step of subjecting the structure in composite material 10 to combined loads. In such a case, more than two fibre-optic strain sensors are preferably provided, for example at least three fibre-optic strain sensors including said at least two. In other words, the fibre-optic strain sensors, flanked and embedded within the body of the structure, are provided in greater numbers than two in order to monitor the strain and/or stress when the structure in composite material is subjected to combined loads.

[091]. The step of obtaining strain measurements and the step of computing a strained configuration can be repeated several times, i.e., reiterated, in a given time interval during which said stress state is induced.

[092]. The induced stress state may be due to environmental and/or operating circumstances.

[093]. As mentioned above, the step of computing a strained configuration can be performed substantially in real time, and for example it is performed substantially simultaneously with the step of obtaining strain measurements.

[094]. As can be seen, by virtue of the characteristics described above provided disjointly or jointly with each other in particular embodiments, it is possible to meet the needs lamented above, obtaining the aforementioned advantages, and in particular:

- the integration of all the components of the fibre-optic sensors of the pair in the material is allowed, both for the detection points located above the neutral axis and for the detection points located below the neutral axis, thus avoiding exposing them to the external environment, protecting the integrity, durability of the sensors and promoting measurement accuracy;

- with the appropriate number of sensors in relation to the loads applied, it is possible to obtain a measurement of the curvature and/or displacement of the neutral plane with respect to the plane of symmetry of beams, plates and shells, regardless of the constitutive characteristics of the material;

- with the appropriate number of sensors in relation to the applied loads, it is possible to obtain a precise and continuous measurement of the position of the neutral axis (or plane) of the structure or portion of structure to be monitored over time, thus accurately tracing the precise curvature of the structure, avoiding having to provide a constitutive model of the material, which in the case of composite materials can be affected by uncertainty;

- it is possible to monitor in real time, in both static and dynamic conditions, each point of interest of the structure;

- for example, knowing the boundary conditions (information related to loads and constraints and to the material itself) identifying the working conditions of the structure, it is possible to estimate, with an adequate number of measurement points, the strains and stresses of all the material of interest with unusual accuracy and repeatability;

- for example, continuously knowing the strains and stresses in the material makes it possible to carry out real-time monitoring of the structure, detect faults due to overloading or damage to the material or use such information to search for better performance and optimisations;

- it is possible to evaluate flexion stresses and/or strains as well as those of torsion on the element or structure in composite material;

- the invention can find application in all contexts in which the search for high performance, a higher level of safety, or the constant monitoring of the stresses and strains of structures made of composite material having tensile rigidity which is different from compression rigidity or, in structures having some asymmetry in the rigidity characteristic;

- therefore, such a solution can be integrated, for example, in the carbon and/or fibreglass structures of boats, in the shafts of sail boats as well as in high- performance cars, both to improve the safety thereof and to improve the performance thereof;

- in addition, the present invention can be applied in the field of infrastructure and construction.

[095]. Of course, the combinations of features of the appended claims form an integral and integrated part of the present disclosure.

[096]. To the embodiments described above, a person skilled in the art, in order to satisfy contingent and specific needs, may make numerous modifications, adaptations and replacement of elements with others which are functionally equivalent, without however departing from the scope of the appended claims.

Claims

1. Method for monitoring the strain and/or stress of at least one portion of a structure in composite material (10) having a body adapted to define a neutral axis (N) when loaded, comprising the steps of:

- providing at least two fibre-optic strain sensors (11 , 12), so as to provide a pair of fibre-optic strain sensors;

- making said structure in composite material (10) to be monitored, embedding both fibre-optic strain sensors (11 , 12) of the pair within the thickness of the body of the structure in composite material, said fibre-optic strain sensors of the pair being arranged parallel to the neutral axis (N) and opposite each other with respect to the neutral axis (N) of the structure in composite material (10);

- obtaining strain measurements from each of the fibre-optic strain sensors (11 , 12) of the pair;

- computing, based on the obtained strain measurements, a strained configuration of the at least one portion of the structure in composite material (10) and/or the extent of applied load (F).

2. Method according to claim 1 , wherein the fibre-optic strain sensors (11 , 12) of the pair are arranged flanked in the same section of the structure in composite material (10); and wherein one sensor (11 or 12) of the pair measures tensile strains while simultaneously the other sensor (12 or 11) of the pair measures compressive strains, when the structure in composite material is stressed.

3. Method according to claim 1 or 2, wherein the step of obtaining strain measurements and the computing step are repeated several times in a given time interval.

4. Method according to any one of the preceding claims, wherein the step of making the structure in composite material (10) comprises the definition of the characteristics of the composite material in terms of the number and thickness of the layers which compose it as well as of the related orientation of the fibres, the determination of the neutral axis (N) based on the type of stresses to which the structure in composite material (10) will be subjected under working conditions, and finally the definition of the layers between which the sensors (11 , 12) chosen based on the distance at which the sensors (11 , 12) must be arranged from the neutral axis (N) and from the surface of the structure in composite material (10) to be monitored must be interposed.

5. Method according to any one of the preceding claims, wherein the computing step is performed substantially simultaneously with the step of obtaining strain measurements, with an acquisition frequency sufficient to detect the dynamic behaviour of the structure in composite material 10 to be monitored which is subject to dynamic loads, so as to perform real-time monitoring of the structure in composite material 10.

6. Method according to any one of the preceding claims, further comprising the step of:

- evaluating whether the computed strained configuration belongs to a set of admissible configurations and/or evaluating whether the computed applied load (F) is below a predeterminable threshold, so that if the computed strained configuration does not belong to said set of admissible configurations and/or if the computed applied load (F) is not less than said predeterminable threshold, then the method comprises the further step of:

- identifying and/or discriminating a fault condition.

7. Method according to any one of the preceding claims, comprising the further step of: - acquiring and/or processing information on the load applied (F) to the structure in composite material, and/or on the mechanical constraints of the structure in composite material, and/or on the mechanical properties of the composite material forming the structure to determine the number and/or the location of the detection points (18) to be foreseen and/or the spatial resolution of the distributed sensor to be obtained.

8. Method according to any one of the preceding claims, comprising the step of:

- subjecting the structure in composite material (10) to combined loads;

- in this case, providing more than two fibre-optic strain sensors (11 , 12).

9. Monitoring device (1) for monitoring the strain and/or stress of at least one portion of a structure in composite material (10) having a body adapted to define a neutral axis (N) when loaded, comprising:

-at least one pair of fibre-optic strain sensors (11, 12) adapted to be arranged flanked and embedded within the thickness of the body of the structure in composite material (10),

-a data processing unit (15) operatively connected with both fibre-optic strain sensors (11 , 12) of the pair, configured to process information detected by the sensors (11 , 12).

10. Device according to claim 8, wherein the fibre-optic strain sensors (11 , 12) of the pair are sensors each comprising a Bragg grating, i.e., they are sensors of the Fiber Bragg Grating type, each acting as a detection point (18).

11. Device according to claim 8 or 9, wherein each fibre-optic strain sensor (11 , 12) of the pair comprises a plurality of detection points (18) which are distributed along the longitudinal extension thereof.

12. Device according to any one of claims 8 to 10, wherein each fibre-optic strain sensor (11 , 12) comprises a sensor distributed along the longitudinal extension thereof, interrogated by optical frequency domain reflectometry or OFDR technique.

13. Assembly (2), comprising:

- a structure in composite material having a body adapted to define a neutral axis (N) when loaded, and

- a monitoring device (1) according to any one of claims 8 to 11 ; wherein:

- the fibre-optic strain sensors (11 , 12) of the pair are both embedded within the thickness of the body of the structure in composite material (10);

- the fibre-optic strain sensors (11, 12) of the pair are arranged parallel to the neutral axis (N) and opposite each other with respect to the neutral axis (N).

14. Assembly according to claim 12, wherein the fibre-optic strain sensors (11, 12) of the pair each have an elongated body extending within the body of the structure in composite material (10); and wherein the fibre-optic strain sensors (11 , 12) of the pair are mutually separated and disjointed within the body of the structure in composite material (10).

15. Assembly according to any one of claims 12 to 14, wherein each fibre-optic strain sensor (11 , 12) comprises a plurality of detection points (18) distributed along the longitudinal extension thereof, and wherein the detection points (18) of one sensor (11 or 12) of the pair are arranged in the same cross-sections of the element in composite material 10with respect to the detection points (18) of the other sensor (12 or 11) of the pair.

16. Assembly according to any one of claims 12 to 15, wherein the operative connection between the fibre-optic strain sensors (11 , 12) and the data processing unit (15) comprises at least one cabled connection extending outside the body of the structure in composite material (10) by means of at least one access plug (13) provided through the body of the structure in composite material.

17. Assembly according to any one of claims 12 to 16, wherein the structure in composite material (10) is formed by a plurality of layers (L1 , L2), and wherein the fibre-optic strain sensors (11 , 12) are located between two adjacent layers of different materials; and/or wherein the structure in composite material (10) comprises at least one of: a stick, a beam, a plate, a sheet, a shell, a dome, a bottleneck.

18. Assembly according to any one of claims 12 to 17, wherein the fibre-optic strain sensors (11 , 12) of the pair are arranged at different distances with respect to the neutral axis (N), and preferably one sensor (11) of the pair is arranged at a first distance (x1) from the neutral axis (N) and the other sensor (12) of the pair is arranged at a second distance (x2) from the neutral axis (N), the second distance being different from the first distance.