At present, the various kind of used sensors perform a punctual monitoring of the structure and, therefore, need to be used in a high number in order to insure an adequate control. This aspect brings to the fact that a prohibitive number of wires, necessary for the data transmission, is needed that makes often practically impossible to employ the necessary number of sensors.

Another important point is related to the use of internal sensors that require the presence of particular housings within the structure that can easily introduce weak points. Moreover, the devices used for data collection are usually quite complex and need high skilled personnel both to carry out the necessary measurements, as well as to perform the acquired data manipulation and interpretation. As a consequence, the installation and managing costs of such systems are, at present, rather expensive, especially if compared to the sensor duration. As a result, health monitoring of structure is nowadays scarcely spread.

Among various solutions, at present, the most spread sensors used in structural monitoring are strain gauges, piezoelectric sensors and fiber optic sensors.

Strain gauges are applied on the external surface of the structure and their basic working principle is based on the transformation of a deformation into an electrical resistance variation. They consist on a grid of electrically conductive wires that change their conductivity when strained. The strain gauge sensibility is defined as the ratio between the electrical resistance variation and the wires length. The critical point in the gauges application is to guarantee excellent adhesion between them and the structure to which they are applied, so that the strain gauges can follow the structure deformation without influencing or limiting it. Semiconductor strain gauges consist on semiconductor piezoelectric materials in which a variation of the applied stress generates a variation of their electrical conductivity. They are characterized by very small dimensions, very high sensibility to temperature variation and a very small range in which the gauge factor is constant.

Piezoelectric sensors convert tensile, compressive and shear stresses into electromotive forces. The application of such stresses on piezoelectric crystals which are cut in a particular way, causes electrical charges to be generated on their surfaces producing, as a result, a measurable electrical potential difference. Nevertheless, the piezoelectric sensors generate an electrical response only in case of variation of the applied load, i. e. when a variation of the mechanical stress appear. Therefore, they are very difficult to calibrate in static conditions.

Fiber optic sensors transmit the light emitted from a source through two glass fibers: a transmitting one and a receiving one. This monitoring technique is based on the analysis of the variation of light propagation into the fibres, when these are mechanically stressed. Such interesting system is highly expensive and need the presence of hollow spaces inside the structure in which allocate the fibers. Therefore, at present, their application is limited to very few and highly expensive new buildings.

A new sensor typology has been recently proposed that makes use of composite materials sensors which contain an electrically conductive element (usually made of carbon fibres) whose electrical resistance change as a function of the stress applied to them. As previously seen in the case of strain gauges, such elements can either be applied to the external surface of the structure to be monitored, or they can be manufactured with specific configurations that make them suitable for internal application as structural sensing elements. At present, this kind of sensors need to be equipped with electrical wires for data collection and this fact strongly limits their use in the health monitoring of structures.

APPLICATION FIELDS The invention is a sensor made of polymer composite material able to detect the stress/strain or damage occurring in a structure which it is integrated in or is integral with and it is characterized by the fact that no electrical connection is needed for data collection, but it is a wireless system. It is evident the high versatility of the developed product, which is suitable for many different applications. Such versatility derives from the use of composite material, which can be design and manufactured to suit any specific application. The invention, therefore, can be easily used in all those applications where, at present, polymer composite materials are used; such as in aeronautical, nautical, automotive and leisure time industries and in civil engineering. The invention, as a non binding example, can be used as smart structural element in the form of laminate, pultruded or extruded component, or as sandwich skins, etc. In particular here, as a non binding example, the use of the invention in the form of a pultruded rod to be used in civil structures as replacement of steel reinforcing elements, is proposed. So-manufactured rods would perform both the functions of structural reinforce and sensor of the structure in which they are inserted.

OBJECT OF THE INVENTION Structure safety is an always increasing important topic and target. At present, in the field of constructions, there is a lack of powerful systems able to predict and prevent the onset of dangerous situations, that can occur either due to excessive and/or sudden stress or due to the mechanical characteristics of the structure and the materials used to built it. Moreover, nowadays, health monitoring systems that can be at the same time sufficiently reliable and cheap, do not exist. At present, the only solution to increase structure safety would be to apply different monitoring systems, but this solution is almost impossible considering the high amount of sensors that would be needed (with their costs) and the large number of complicated connections necessary to acquire their signals. Think to an house or an office from whose walls come out a quantity of wires and connections: even if it is a technical possible solution, nevertheless it is not practically acceptable.

The invention can solve these problems since offers cheap sensors, that are fully integrated within the structure and that can easily provide all the necessary information to know precisely the present and past stress state of the structure, so to insure its safety.

The invention presents the possibility to use a single element that can behave contemporarily as a structural element and a sensor and that is characterized by a wireless connection to the data acquisition system. The invention can be produced by using the most common polymer composite manufacturing techniques and, in particular, it can be produced to be used either as a smart reinforcing system or as a sensor applied to an external surface of the structure. If employed as smart reinforcing element, the invention is able to offer the same structural functions of the element that it is replacing and, at the same time, it works as a sensor too. If used as external sensor, instead, the invention insure a longer working life than the other conventional sensors since it is stronger and harder. The invention working principle is based on the possibility to convert a mechanical stress into an electrical impedance and/or electrical resistance variation of its sensing part. The invention dialogs with the acquisition system via a wireless technique, which means that there is no need of any electrical wire, making, therefore, the invention placement into and/or on the structure easier, since it does not require the predisposition of electrodes or external contacts. From the aesthetical point of view, the invention looks like a common polymer composite material element. As any composite material, it is characterized by a large flexibility in its realization and use, so that it can be used as a structural part of vehicles, civil structures or as other various industrial products.

The above description allows to state that the invention constitutes an health monitoring system fully integrated within the structure and, therefore, it does not introduce any weak points or defects. Moreover, thank to its structural properties and to the lack of electric wires, it shows a longer working life than other sensors, at lower costs.

DETAILED DESCRIPTION The innovation introduced by the invention is the possibility to unify many different tasks into a single product.

These tasks are : 1. the ability to describe the damage and/or the stress/deformation state of a structure, so that it works as a sensor; 2. the ability to bear loads, so that it works like a structural element; 3. the lack of any wires for the connection to the data acquisition system, so that any piece is independent from the others (see FIG. 1) The invention can, therefore, substitute the traditional sensors used for structural health monitoring, thus avoiding the possible need of introducing in the structure sensors which are extraneous elements, thanks to its load-bearing abilities, and thus avoiding the need of knowing the exact point in which to install the sensor, in order to allow the opportunity of knowing the effective health state of a structure ; this knowledge allows to avoid that sudden failures, which are due to a hardly detectable decay, could cause damages or could kill people.

The invention is made of polymer composite material (FIG. 1A) internally containing an electrically conducting element (FIG. 1B). Such element constitutes the sensing part of the invention and it is connected to an electronic circuit (FIG. 1C), which collects the sensor electrical variations and communicate them, via a wireless connection, to the external acquisition system (FIG. 1L). Basically, the application of a load to the structure, into which the invention is inserted or attached, causes a variation of a physical quantity of the invention, which can be its electrical impedance or its electrical resistance, to which corresponds a variation of the electrical signal transmitted by the electronic circuit.

The invention can provide always an electrical response whether the structure is statically or dynamically loaded or subjected to a load variable in time and, moreover, it can supply information of the load history of the structure in which it is inserted or attached, by correlating the electrical impedance and/or the electrical resistance variation to the applied load by a mathematical law. As previously reported, the invention can supply information of the load history of the structure which it is inserted in or attached to, since some electrically conducting elements can memorize the maximum load to which the structure has been subjected during its

working life. The"invention memory ability"is in the form of a"residual"electrical impedance and/or electrical resistance variation which is a function of the maximum stress level ever carried by the structure, and that can be detected and evaluated by the acquisition system even long time after that the event happened.

More details of the invention composition are here reported.

Part A (FIG. 2A) constitutes the structural part of the invention and it can be realized, as a non binding example, by fiber glass. In the following the materials that constitutes Part A of the invention will be called "reinforcement". The predominant task of the reinforcement is to bear the applied load and, therefore, it has to offer the mechanical properties necessary to suit the particular application. In the case that the reinforcement is an electrically conductive element, it has to provide electrical conductivity variation in the range of interest in the specific application. The reinforcement is always kept together by a matrix that can be a thermoplastic or thermosetting polymeric resin or a ceramic. Part A can be realized by using different types of reinforcement that can be made, as a non binding example, by organic fibers (i. e. aramidic, carbon or graphite fibers), or inorganic ones, as a non binding example, glass, ceramic, basalt, or metal fibers or clothes, whiskers, particles or mats made by every kind of material (polymeric, metallic or ceramic). The reinforcement, together with the matrix, can be formed to assume the geometrical shape that is necessary to bear the load in any kind of specific application.

Part B (FIG. 2B) constitutes the sensing part of the invention and can be realized, as a non binding example, by carbon fibres and/or carbon particles. In the following, the materials that constitutes Part B of the invention will be called"conductive elements". Part B can be made by different conductive materials, which means that it can be formed by conductive fibres and/or conductive clothes and/or conductive particles and/or conductive whiskers and/or conductive mat of every kind. The conductive material used to realize the conductive element is always kept together by a matrix that can be a thermoplastic or thermosetting polymeric resin or a ceramic. The matrix and the conductive materials become a conductive element that is one part of the electric/electronic circuit. The whole system, in the following will be called"variable electrical impedance and/or resistance element". The variable electrical impedance and/or resistance element is characterized by a peculiar electrical impedance and/or electrical resistance value and by peculiar mechanical properties to suit the specific application. It is combined to the reinforcement to form a single piece: in particular, it can be embedded into the reinforcement and placed, as a non binding example, parallel to the invention main axis (FIGS. 3A and 3B) or placed in particular allocations depending on the invention peculiar geometrical configuration and application (FIG. 3C), or it can be joined to the reinforcement by other techniques, as a non binding example, silkscreen process. The variable electrical impedance and/or electrical resistance element must present a variation in its electrical impedance and/or electrical resistance value which is a function of the load applied to the structure, following a well-known mathematical law that can include the case in which the conductive elements break, giving rise to the opening of the electrical circuit in which it is an electrical resistor. The possibility to measure the electrical impedance and/or electrical resistance variation is of major importance to guarantee the self- diagnosing property of the invention.

Depending on the type of conductive material used, the variable electrical impedance and/or electrical resistance element can offer two different behaviours: 4a) a guard behaviour, as an example, typical of long fibres or clothes, 4b) a describing behaviour, as an example, typical of particles and whiskers, 4a) the guard behaviour (FIG. 4A) allows to follow the health state of the material until a fixed mechanical stress, above which the variable electrical impedance and/or electrical resistance element can collapse. With this sensor, an electrical impedance or electrical resistance variation corresponds to a mechanical stress increase, such variation become steeper in correspondence to a mechanical crisis, and goes to infinity when the electric circuit is finally opened. The invention is designed so that the variable electrical impedance and/or electrical resistance element breaks when the mechanical load reaches a fixed value, providing, therefore, a certain alarm on the structure mechanical health state. It is important to remark that, when the variable electrical impedance and/or electrical resistance element breaks, the reinforcement can still bear the load applied to the invention.

Such behaviour is typical, as an example, of long carbon fibres.

4b) the describing behaviour (FIG. 4B) allows to monitor the stress/strain state of the material during its whole working life. The increase of electrical impedance and/or electrical resistance variation proportionally corresponds to the mechanical stress applied to the invention, allowing, as a consequence, to perform a real time monitoring of the structure which the invention is inserted in or applied to. When the load is removed from the invention, the electrical impedance and/or electrical resistance of the variable electrical impedance and/or electrical resistance element can be different from the original one, which was present before the load application. Such behaviour depends on the type of conductive material used to realize the variable electrical impedance and/or electrical resistance element and allows to describe the load history of the structure which the invention has been inserted in or applied to. In particular, it is possible to know whether the structure, during its working life, has been subjected to loads that can in some way jeopardize its structural integrity. Such behaviour

is typical, as an example, of a variable electrical impedance and/or electrical resistance element made of carbon particles.

Part C (FIG. 2C) consists of an electronic circuit which is electrically connected to the variable electrical impedance and/or electrical resistance element that constitutes the sensitive part of the invention, and it is fully integrated in the invention itself. It is realized by using a wireless circuital system following four principal producing and working principles that allow the data transmission in different specific application needs: 5a transponder, 5b transponder/sender, 5c remote-sender, 5d independent-sender.

5a) the transponder version is characterized by an absolute absence of any internal battery; it is constituted by a receiving antenna (as a non binding example, dipole, coil) which is the secondary circuit of an imaginary transformer which has air as air gap. The primary circuit of the transformer is a part of the remote system/circuit of data acquisition and will be successively only briefly described, since it is not a claim of the present patent.

The primary circuit transmits to the secondary circuit (the transponder on the invention, FIG. 5.0) the energy sufficient to activate the electric/electronic system of the invention, which can, in turn, evaluate the sensitive element state (FIG. 2C) and feed the other circuits successively described. The remote power feed system, which receives energy from the primary circuit, is integrated in the invention (FIG. 5.0) (remote feed) and can be activated by either low or high frequencies oscillations generated by the primary controlling circuit (FIG. 6a).

After having received sufficient energy, the feed circuit (FIG. 5.0) administrates, makes stable and supplies one or more current flows and/or voltages to let the invention work efficiently.

A control voltage is used to check and measure the electrical impedance and/or electrical resistance of the sensing part of the invention (FIG. 5.1), by using either a voltage and/or a current decay method or the balancing of one or more Wheatstone bridges. Said bridge/bridges are balanced by a dedicated circuit, which, if necessary, can act automatically (FIG. 5.2). The electrical signal deriving from the decay and/or the bridges is successively amplified by another circuit (FIG. 5. 3) that can reveal even a very small electrical impedance and/or electrical resistance variation and amplify it until it becomes an easily recognizable and measurable electrical quantity. This analogical signal can be converted either into a digital one by passing through another circuit (FIG. 5.4) or into a frequency (voltage-frequency converter, FIG. 5.4. 2). The signal which has been codified according to one of the previously cited method, is sent to the modulator (FIG. 5.5) and, successively, to the part of the circuit dedicated to data transmission (FIG. 5.6).

5b) trasponder/sender version: such version differs from the previous version because it has an autonomous internal feed system (for non binding example a rechargeable battery) which can work together with the remote power feed system. Said system can, in addition, recharge the battery cited in the non binding example. The presence of a stopgap battery system allows grater distances between the invention and the remote measuring system of sensor parameters.

5c) remote-sender version: this version integrates exclusively a feed battery in the invention and no remote power feed circuit is presented. It answers to a remote interrogation, performed via radio or via magnetic/electromagnetic field and transmits the electrical parameters of the invention.

5d) independent-sender version: this version, which uses any combinations of the above described feed solution, even singularly considered, can cyclically transmit the information either independently or only by request, following programmable timetables, which depend on the specific application and the invention duration (as a non binding example: 2 transmissions per day, 48 transmissions per day offew milliseconds each, etc.).

The remote measuring system/circuit (FIG. 6A) is here only briefly described since it is not claimed in the present patent. It can be realized in different more or less complex solutions. The most suitable and reliable solution is to employ a portable pc or a palm computer connected to a specifically designed circuit that provides the receiving, transmitting, excitation and remote power feed functions. By emitting an excitation and/or feed signal and by controlling it from this device (FIG. 6.1) it is possible to check and/or to feed the invention, that can, in turn, emit a complex signal containing all the information that it can generate, including temperature information that can derive from a temperature signal placed on the invention itself. The circuit integrated in the computer receives and elaborates the signal and passes the information to other parts of the computer. At this point, it is possible to visualize the invention parameters and to draw graphs of the invention parameters as function of time.

Part D (FIG. 2D) is constituted by a electrical connection system made of an electrically conductive material that can be integrated and/or applied to the invention in order to close the electrical circuit formed by two or more variable electrical impedance and/or electrical resistance elements and by the electronic circuit (FIG. 2C).

After its insertion, the electrical connection system, is protected from the environment by covering it with particular resins. Instead of using the electrical connection system it is possible, as a non binding example, to use a single variable electrical impedance and/or electrical resistance element connected to the electronic circuit (FIG. 7E) or the use of an electrically conductive wire to connect the variable electrical impedance and/or electrical resistance element to the electronic circuit.

The invention is realized by integrating into a single piece the reinforcement, the conductive materials, the electronic circuit and, in case, the plug/electrical connection system. Both the reinforcement and the electrically conductive materials are embedded into a thermoplastic or thermosetting matrix or a ceramic one. The variable electrical impedance and/or electrical resistance element made of conductive materials forms an electric circuit in which an electric current can easily flow. The invention can be designed and realized with peculiar geometrical shapes and mechanical properties in order to suit any specific application. As a non binding example, it is possible to make the invention in form of profiles with various geometrical sections (FIG. 8A), profiles with various geometrical sections that have a stiffening core (FIG. 8B), laminates (FIG. 8C), sandwich structures (FIG. 8D) or to make structural parts. The invention can work under tensile and/or compressive and/or bending and/or shear conditions and the monitoring task is guaranteed by the variation of the electrical impedance and/or electrical resistance of the electric circuit, which is formed by the variable electrical impedance and/or electrical resistance elements and the electronic circuit as a function of the load applied to the invention. Said variation can be due to a possible breakdown of the variable electrical impedance and/or electrical resistance elements, while the mechanical behaviour of the invention is guaranteed by an adequate constitutive law.

PRODUCT EXAMPLE The invention can be realized according to a peculiar geometrical shape and by using a suitable reinforcement so that it can be used as concrete reinforcement element in concrete structures. Under these conditions, the invention can simply replace steel rods during a building construction.

Before being loaded, a first electrical measure of the electrical resistance is performed, which provides the reference value R 0 which will successively be compared with the other values measured during the invention working life. In fact, after loading, the invention new electrical resistance value is Rl. If the load is removed, a new reference value is found, R [Oresl], which is higher than the previous R 0 and indicates that the structure has been already loaded. If the building is successively subjected to normal working loads, R_ [Oresl] will again be the value found after the complete unloading of the structure. But, if a new load, exceeding the normal loading conditions is applied, the invention will present a new electrical resistance value, called R_2, which is higher than R0 and Rl. If this exceeding load is then removed, a new reference value would be found R<BR> [Ores2], which is higher that R_ [Oresl] and indicates that the structure has been subjected to an exceptional load. Any time the invention is subjected to anomalous or exceeding loads, a new actual electrical resistance value, say R_3, is found, which is much higher that those values usually acquired during the normal working conditions. Moreover, when the anomalous loading is removed the invention keeps memory of it in its residual electrical resistance, which is shifted to a new R_ [ORes3] value, that is again higher than all the others previously recorded values and that can indicate whether a damage threshold has been reached and/or exceeded during the event. Such facility allows to be informed of the maximum load ever experienced by the structure, without any need to perform continuous long-lasting acquisitions. All measurements are carried out via a wireless system.