Crack detection method

Abstract

To provide a piezoelectric FRP sensor which is flexible, can be deformed into various shapes, can be used as a structural member, and can be used without power supply, and a crack detection system and method using the same.SOLUTION: A piezoelectric FRP sensor 10 has an FRP sheet 15 in which a resin 12 is reinforced with a non-conductive continuous fiber 14a and an electrode 18 bonded to both sides of the FRP sheet. In addition, piezoelectric particles 16 are dispersed in the resin and polarized in the resin. The crack detection system 100 includes a piezoelectric FRP sensor 10 attached to a non-conductive structure 5, a voltage detection device 20, and a crack analysis device 24. The voltage detection device 20 detects the output voltage V that occurs without power in one or more pairs of electrodes located so as to be opposed to both surfaces of the FRP sheet 10. The crack analysis device 24 analyzes a crack 4 of the structure from the change in the output voltage V.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a piezoelectric FRP sensor that outputs a voltage without power supply and a crack detection means using the same.

Background Art

[0002] In Japan, bridges built during the period of high economic growth in the 1950s are aging, and their repair and reinforcement are important. Therefore, various crack detection technologies have been proposed (for example, Patent Document 1).

[0003] The "AE sensor" of Patent Document 1 has a sensor body mounted on a flexible plate. The sensor body includes a composite oscillator, an electronic circuit that processes the AE signal detected by the composite oscillator, and a communication circuit that transmits the processed data processed by the electronic circuit to the outside. Further, the electronic circuit includes an amplifier, a filter, an AD converter, a counter, and a memory.

[0004] On the other hand, fiber reinforced composite materials have the characteristics of being lighter and stronger, with excellent mechanical properties and high specific strength, while having a lower density than metal materials such as iron and aluminum. Therefore, in recent years, as a structural member to replace aluminum alloys, it has been used for the reinforcement of aging bridges, and for the reinforcement of aircraft, small ships, automobiles, wind power generation blades, etc. Hereinafter, the fiber reinforced composite material is simply referred to as "composite material" or "FRP".

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The AE sensor described in Patent Document 1 has electronic and communication circuits in addition to the composite oscillator, and therefore requires a power supply and cannot be used without power. Furthermore, due to its complex structure and lack of flexibility, it cannot be used as a structural member to reinforce bridges, aircraft, small vessels, automobiles, wind turbine blades, etc.

[0007] This invention was devised to solve the aforementioned problems. In other words, the object of this invention is to provide a piezoelectric FRP sensor that is flexible, can be deformed into various shapes, can be used as a structural member, and can be used without power supply, as well as a crack detection means using the same. [Means for solving the problem]

[0008] According to the present invention, a piezoelectric FRP sensor is provided, comprising an FRP sheet in which the resin is reinforced with non-conductive continuous fibers, and electrodes bonded to both sides of the FRP sheet, wherein piezoelectric particles are dispersed within the resin and subjected to polarization treatment within the resin.

[0009] Furthermore, according to the present invention, the piezoelectric FRP sensor attached to a non-conductive structure, A voltage detection device for detecting an output voltage generated without power supply to one or more pairs of electrodes positioned opposite each other on both sides of the FRP sheet, A crack detection system is provided, which includes a crack analysis device that analyzes cracks in the structure from changes in the output voltage.

[0010] Furthermore, according to the present invention, the above piezoelectric FRP sensor is attached to a non-conductive structure. The output voltage generated without power supply to one or more pairs of electrodes positioned opposite each other on both sides of the FRP sheet is detected. A crack detection method is provided for analyzing cracks in a structure based on changes in the output voltage. [Effects of the Invention]

[0011] According to the present invention, since the piezoelectric FRP sensor has an FRP sheet in which the resin is reinforced with non-conductive continuous fibers, it is flexible and can be deformed into various shapes, and can be used as a structural member itself.

[0012] Furthermore, the FRP sheet has electrodes that are tightly bonded to both sides, and piezoelectric particles are dispersed and polarization-treated within the resin. As a result, when the structure to which the piezoelectric FRP sensor is attached is displaced due to vibration or the like, a tensile or compressive force acts on the piezoelectric FRP sensor, generating an output voltage between the electrodes without any power supply. Therefore, crack analysis equipment can be used to analyze cracks in structures from changes in output voltage. [Brief explanation of the drawing]

[0013] [Figure 1] This is an explanatory diagram of the piezoelectric FRP sensor according to the present invention. [Figure 2] This is a schematic perspective view of a piezoelectric FRP sensor. [Figure 3] This is an overall configuration diagram of the crack detection system according to the present invention. [Figure 4] This is an explanatory diagram of a crack detection method. [Figure 5] This is an explanatory diagram of a bending vibration test using a piezoelectric FRP sensor. [Figure 6] This is an explanatory diagram of a crack depth detection test using a piezoelectric FRP sensor. [Figure 7] This is an explanatory diagram of a hypothetical test based on the results of Example 2. [Figure 8] This is an explanatory diagram of a four-point bending test using a piezoelectric FRP sensor. [Figure 9] This diagram shows the relationship between the load and unloading and the output voltage of the piezoelectric FRP sensor. [Figure 10] This is a test result showing the relationship between crack depth and output voltage. [Figure 11] The results shown are from a test conducted using the four-point bending test apparatus shown in Figure 8. A downward load was applied to a crack-free specimen, and the load was gradually increased until cracks occurred in the specimen.

Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described in detail based on the accompanying drawings. In each figure, the same reference numerals are given to the common parts, and redundant explanations are omitted.

[0015] FIG. 1 is an explanatory view of a piezoelectric FRP sensor 10 according to the present invention. In this figure, (A) is a plan view of the piezoelectric FRP sensor 10, (B) is a sectional view taken along the line B - B of (A), (C) is a partially enlarged view of (B), and (D) is a principle diagram of the piezoelectric FRP sensor 10.

[0016] In FIGS. 1(A), (B), and (C), the piezoelectric FRP sensor 10 is a prepreg sheet for a composite material in which a resin 12 is reinforced with reinforcing fibers 14. Hereinafter, when the piezoelectric FRP sensor 10 is not necessary, it is simply abbreviated as "sensor 10".

[0017] The resin 12 is preferably a flexible thermoplastic resin. The thermoplastic resin is, for example, a polyamide resin. The resin 12 before molding, that is, the thermoplastic resin, is flexible but solidified. Note that the resin 12 may be a thermosetting resin before curing.

[0018] The reinforcing fiber 14 is a non - conductive continuous fiber 14a. The non - conductive reinforcing fiber 14 is, for example, a fabric or unidirectional fibers of mullite fiber, glass fiber, or SiC fiber. The reason for using "non - conductive fiber" is that in the case of conductive fiber, the generated electric power (or electric charge) is dispersed through the reinforcing fiber 14, and the extraction efficiency from the electrode 18 is reduced. Therefore, carbon fibers (and CFRP) with low electrical resistivity are not the subject of the present invention. The reason for using "fabric or unidirectional fibers" is to enhance the function as a reinforcing material. Furthermore, when applying a DC voltage to a prepreg to perform polarization treatment, if conductive fibers are present within the prepreg, the charge will selectively move along the conductive fibers, preventing the ceramic from being polarized. Therefore, it is necessary to use non-conductive fibers to allow the charge to selectively move along the ceramic particles during the polarization treatment.

[0019] In Figures 1(A), (B), and (C), the piezoelectric particles 16 are dispersed within the resin and subjected to polarization treatment within the resin. The piezoelectric particle 16 has a median diameter (d 50 The particles should preferably be KNN or PZT ceramic particles with a size of 0.5 μm or more and 0.7 μm or less.

[0020] In Figure 1(D), the left figure shows the molecules of piezoelectric particles 16 before polarization, and the right figure shows the molecules of piezoelectric particles 16 after polarization. "Polarization" refers to the presence of an electric dipole moment due to an uneven distribution of charge between two atoms in a molecule or throughout the molecule as a whole. Before polarization, the positive and negative ions cancel each other out, resulting in an even charge distribution. In contrast, after polarization, some of the positive ions are displaced, creating an uneven charge distribution in the molecule. The piezoelectric particles 16 within the resin need to be polarized uniformly throughout. Therefore, in the piezoelectric FRP sensor 10 of the present invention, after the prepreg sheet is formed, a DC voltage is applied between both sides of the sheet to polarize the piezoelectric particles 16 within the resin.

[0021] Figure 2 is a schematic perspective view of the piezoelectric FRP sensor 10. In this figure, the piezoelectric FRP sensor 10 has an FRP sheet 15 in which resin 12 is reinforced with non-conductive continuous fibers 14a, and electrodes 18 that are tightly bonded to both sides of the FRP sheet 15. Piezoelectric particles 16 are dispersed in the resin and polarization treatment is performed within the resin. The electrodes 18 may be coated with gold electrodes by sputtering, for example. Alternatively, conductive tape (for example, copper tape) may be used as the electrodes 18. With this configuration, the piezoelectric FRP sensor 10 is flexible and moldable overall, provided that the resin 12 is a flexible thermoplastic resin. Note that a thermosetting resin before curing can also be used as the resin 12.

[0022] Figure 3 is an overall configuration diagram of the crack detection system 100 according to the present invention. In this figure, the crack detection system 100 includes a piezoelectric FRP sensor 10, a voltage detection device 20, and a crack analysis device 24. The piezoelectric FRP sensor 10 is attached to the surface of a non-conductive structure 5. The piezoelectric FRP sensor 10 has one or more pairs of electrodes 18 positioned opposite each other on both sides of the FRP sheet 15. The structure 5 is, for example, a bridge girder. The voltage detection device 20 is, for example, a data logger, and detects the output voltage V generated by one or more pairs of electrodes 18 without supplying power. The voltage detection device 20 may also be a detection device such as an LED light. The crack analysis device 24 is, for example, a computer (PC), which analyzes cracks 4 in the structure 5 from changes in the output voltage V.

[0023] The crack detection method according to the present invention comprises steps S1 to S3. In step S1, the piezoelectric FRP sensor 10 described above is attached to the non-conductive structure 5. In step S2, the output voltage V generated without power supply is detected by one or more pairs of electrodes 18 positioned facing each other on both sides of the FRP sheet 15. In step S3, the cracks 4 in structure 5 are analyzed based on the change in output voltage V.

[0024] Figure 4 is an explanatory diagram of the crack detection method. In the upper diagram of each figure, there is a crack 4 on the lower surface of structure 5, and a piezoelectric FRP sensor 10 is attached to the upper surface of the crack. Furthermore, (A) and (C) show a bending state in which tensile stress acts on the lower surface and compressive stress acts on the upper surface, while (B) shows a bending state in which compressive stress acts on both the lower and upper surfaces.

[0025] The diagrams below each figure show the output voltage V generated at the piezoelectric FRP sensor 10. As shown in these figures, in (A), the output voltage V increases when a bending force is applied, and the amount of increase varies depending on the depth of the crack 4. Conversely, in (B), the output voltage V decreases when a bending force is applied, and the amount of decrease varies depending on the depth of the crack 4. Also, in (C), the change in output voltage V is small when the load increases, but the output voltage V decreases sharply as the crack 4 progresses. Therefore, based on these characteristics, cracks 4 in the structure 5 can be analyzed from changes in the output voltage V.

[0026] The following describes some examples. [Examples]

[0027] Figure 5 is an explanatory diagram of a bending vibration test using the piezoelectric FRP sensor 10.

[0028] Figure 5(A) is an overall diagram of the test apparatus. In this diagram, 30 is the vibration exciter, 32 is the data logger, 34 is the voltage recording device, and 36 is the laser displacement meter. In this example, test specimen 1 is a glass fiber reinforced polymer (GFRP) with a thickness of 0.2 mm, a width of 40 mm, and a length of 100 mm. The piezoelectric FRP sensor 10 is approximately 0.2 mm thick, 40 mm wide, and 40 mm long, and was attached to test specimen 1 at a position 10 to 50 mm from the left edge in the diagram.

[0029] In the test, the left end of the test specimen 1 was gripped by the vibration excitation device 30, with the right end of the test specimen 1 remaining free. A vertical vibration was applied to the left end of the test specimen 1 at a frequency of 50 Hz, and its amplitude was varied. The displacement of the left end of the test specimen 1 was measured using a laser displacement meter 36. This test shows that the piezoelectric FRP sensor 10 near the left end of test specimen 1 experiences alternating tensile and compressive forces when the left end of test specimen 1 is cantilevered.

[0030] Figure 5(B) shows the test results illustrating the relationship between time and output voltage V. In this figure, the horizontal axis represents time, and the vertical axis represents the output voltage V of the piezoelectric FRP sensor 10. The three curves in the figure show the change in output voltage V when the amplitude is 0.2, 0.4, and 0.6 mm. From this figure, it can be said that the piezoelectric FRP sensor 10 of the present invention utilizes the piezoelectric effect, and therefore has high sensitivity to changes in stress and strain that occur at an accelerating rate, making it effective for detecting vibrating test specimens.

[0031] Figure 5(C) shows the test results illustrating the relationship between amplitude and output voltage V. In this figure, the horizontal axis represents amplitude, and the vertical axis represents the output voltage V of the piezoelectric FRP sensor 10. The black circles in the figure indicate the fluctuation range of the output voltage V in Figure 5(B). This figure shows that the amplitude of the output voltage V has a linear relationship that is almost proportional to the amplitude, indicating a high correlation.

[0032] From the above-described embodiment, it can be seen that when the structure 5 is subjected to an external force and vibrates with different amplitudes, the amplitude of the structure 5 can be detected from the amplitude of the output voltage V of the piezoelectric FRP sensor 10. [Examples]

[0033] Figure 6 is an explanatory diagram of a crack depth detection test using a piezoelectric FRP sensor 10.

[0034] Figure 6(A) is an overall diagram of test specimen 2, with the top view being the top view and the bottom view being the side view. The same vibration exciter 30, data logger 32, voltage recorder 34, and laser displacement meter 36 as in Example 1 were used for the test. In this example, test specimen 2 is a cement board with a thickness of 8 mm, a width of 15 mm, and a length of 70 mm. The piezoelectric FRP sensor 10 is approximately 0.2 mm thick, 10 mm wide, and 10 mm long, and was attached to the upper surface of test specimen 2 at a position 13 to 23 mm from the left edge in the diagram. In addition, a strain gauge (not shown) was attached to the lower surface of test specimen 2 corresponding to the piezoelectric FRP sensor 10.

[0035] The weight 38 was fixed to the right end of test specimen 2, which remained a free end, and a crack 4 extending across its entire width was made on the upper surface 19 mm from the right end of test specimen 2. Three types of test specimen 2 were prepared: no crack, crack depth of 1 mm, and crack depth of 2 mm. The left end of the test specimen 2 was gripped by the vibration device 30 at a distance of 12 mm, and the left end of the test specimen 2 was vibrated vertically with the same amplitude, while the frequency was varied.

[0036] Figure 6(B) shows the test results illustrating the relationship between frequency and output voltage V. In this figure, the horizontal axis represents frequency, and the vertical axis represents the amplitude of the output voltage V. The three curves in the figure represent the cases of no crack, a crack depth of 1 mm, and a crack depth of 2 mm. These results indicate that as crack 4 deepens, the dynamic strain (accelerating strain) increases around the crack during vibration, while the dynamic strain decreases at the cantilever end (near the left end in the diagram). Therefore, when the piezoelectric FRP sensor 10 is attached near the cantilever end (cantilever side), the amplitude of the output voltage V decreases as crack 4 deepens. This characteristic makes it possible to detect the crack depth by measuring the amplitude of the output voltage V.

[0037] From the above-described embodiment, it can be seen that when the structure 5 vibrates due to external forces of different frequencies, the depth of the crack 4 in the structure 5 can be detected from the amplitude of the output voltage V of the piezoelectric FRP sensor 10. Furthermore, crack detection is possible even when the object vibrates under external force of the same frequency. For example, in Figure 6(B), if the object vibrates at 42Hz, the reading for no crack is 0.2mV, for a 1mm crack it is 0.1-0.15mV, and for a 2mm crack it is 0.00-0.05mV, allowing the depth of the crack to be detected.

[0038] Figure 7 is an explanatory diagram of a hypothetical test based on the results of Example 2. In Figure 7(A), test specimen 2 is the same cement board as in Example 2, and it is assumed that there is a crack 4 extending across its entire width on the free end side of its upper surface. In this example, the cantilevered side (left end) is the vibrating end, and the piezoelectric FRP sensor 10 is attached to the cantilevered side and the location of the crack 4, respectively.

[0039] Figure 7(B) shows the relationship between crack depth and detected voltage in this case. In the vibrating object (test specimen 2), the strain is greater at the crack location and decreases as you move away from crack 4. Furthermore, it is expected that this difference will increase as the crack depth increases. Therefore, if the crack 4 is located away from the sensor 10 (A), the amplitude of the output voltage V decreases as the crack 4 deepens, and if the crack 4 is located directly below the sensor (B), the amplitude of the output voltage V increases in proportion to the length of the crack.

[0040] Therefore, it is possible to determine whether the crack 4 is propagating around the sensor 10 or at a location away from the sensor 10. In other words, when the structure 5 vibrates due to external forces of the same or different frequencies, the location of the crack propagation in the structure 5 can be detected from the change in the amplitude of the output voltage V of the sensor 10. [Examples]

[0041] Figure 8 is an explanatory diagram of a four-point bending test using the piezoelectric FRP sensor 10. In this example, specimen 3 is a cement board with a thickness of 15 mm, a width of 15 mm, and a length of 100 mm. The lower surface A and B of specimen 3 were supported, and a downward load L was repeatedly applied and removed to the upper surface C and D. The distance between support points A and B was 60 mm, and the distance between load points C and D was 40 mm. A crack 4 extending across the entire width was created at the center of support points A and B on the underside of test specimen 3. Three types of test specimen 3 were prepared: no crack, crack depth of 0.5 mm, and crack depth of 2 mm. The piezoelectric FRP sensor 10 was approximately 0.2 mm thick, 10 mm wide, and 10 mm long, and was attached to the center of load points C and D on the upper surface of the test specimen 2.

[0042] Figure 9 shows the relationship between the load and unloading and the output voltage V of the piezoelectric FRP sensor 10. In this figure, the horizontal axis represents time, and the vertical axes represent the load (right vertical axis) and the output voltage V (left vertical axis). The thin lines in the figure show the change in load, and the thick lines show the change in output voltage V. Figure 9(A) shows the relationship between the load and unloading cycle and the output voltage V, while Figure 9(B) shows the relationship between the repeated loading and unloading cycle and the output voltage V. This figure shows that a positive output voltage V is generated when a load L is applied, and a negative output voltage V is generated when the load is removed. Furthermore, it can be seen that the repeated loading and unloading correspond precisely to the output voltage V, and that the loading (ON) and unloading (OFF) of the load can be detected by the output voltage V of the piezoelectric FRP sensor 10.

[0043] From the above-described embodiment, it can be seen that when the structure 5 is repeatedly subjected to loading and unloading, the loading and unloading of the load can be detected from the change in the output voltage V of the piezoelectric FRP sensor 10.

[0044] Figure 10 shows the test results illustrating the relationship between crack depth 4 and output voltage V. This diagram shows that as crack 4 deepens, the voltage increases, and the crack depth can be detected from the magnitude of the output voltage V. Therefore, when the structure 5 is repeatedly subjected to loading and unloading, the depth of the crack 4 in the structure 5 can be detected from the change in the output voltage V of the piezoelectric FRP sensor 10. [Examples]

[0045] Figure 11 shows the test results obtained by using the four-point bending test apparatus shown in Figure 8, applying a downward load L to a test specimen 3 without cracks 4, and gradually increasing the load until cracks 4 occur in the test specimen 3. In this figure, the horizontal axis represents time, and the vertical axes represent load (right vertical axis) and output voltage V (left vertical axis). The thick lines in the figure show changes in load, and the thin lines show changes in output voltage V.

[0046] In this figure, the output voltage V remains almost constant (0mV) as the load increases from 0 to approximately 180N, and then rapidly generates a negative output voltage V when the load reaches approximately 180N. This result shows that the moment of crack propagation can be detected by detecting a significant negative voltage value. This can be inferred to be because compression was applied to the length of the sensor 10 when the crack occurred.

[0047] Therefore, when the load on structure 5 increases, the moment of crack propagation in structure 5 can be detected by detecting a significant negative voltage value in the output voltage V of the piezoelectric FRP sensor 10.

[0048] The piezoelectric FRP sensor 10 described above has the following advantages. (1) High formability (flexibility) Because the FRP sheet 15 is a flexible sheet, it is easy to mold afterwards and can be used for parts and components of various shapes.

[0049] (2) Reinforcement function and voltage generation function The piezoelectric FRP sensor 10 has both a reinforcing function and a voltage generation function. Because the FRP sheet 15 uses continuous fibers, it can also function as a reinforcing material for the structure 5. In other words, since it provides reinforcement and generates an output voltage V between electrodes without power supply, it can be used as a material that combines the functions of a reinforcing material with health monitoring and energy harvesting. Examples of use include bridge reinforcement, repair materials for aircraft structural members, and sensor materials. By combining these materials with IoT sensors, they can function as both a reinforcing material and a sensor power source. Furthermore, a piezoelectric FRP sensor 10 can be used as both a reinforcing material and a sensor for health monitoring.

[0050] According to the embodiments of the present invention described above, since the piezoelectric FRP sensor 10 has an FRP sheet 15 in which the resin is reinforced with non-conductive continuous fibers 14a, it is flexible and can be deformed into various shapes and can be used as a structural member itself.

[0051] Furthermore, the FRP sheet 15 has electrodes 18 that are tightly bonded to both sides, and piezoelectric particles 16 are dispersed in the resin and polarized within the resin. As a result, when the structure 5 to which the piezoelectric FRP sensor 10 is attached is displaced by vibration or the like, a tensile or compressive force acts on the piezoelectric FRP sensor 10, generating an output voltage V between one or more pairs of electrodes without power supply. Therefore, the crack analysis device 24 can analyze the cracks 4 in the structure 5 from the change in output voltage V.

[0052] It should be noted that the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. [Explanation of Symbols]

[0053] V Output voltage, 1, 2, 3 Test specimen, 4 Crack, 5 Structure, 10 Piezoelectric FRP sensor (sensor), 12 Resin, 14 Reinforced fiber, 14a Continuous fibers, 15 FRP sheets, 16 Piezoelectric particles, 16a KNN ceramic particles, 18 electrodes, 20 voltage detection device, 24 Crack analysis device, 30 Vibration device, 32 Data logger, 34 Voltage recording device, 36 Laser displacement meter, 38 Weight, 100 Crack Detection System

Claims

1. A piezoelectric FRP sensor is attached to a non-conductive structure, comprising an FRP sheet reinforced with non-conductive continuous fibers and electrodes bonded to both sides of the FRP sheet, wherein piezoelectric particles are dispersed within the resin and polarization treatment is performed within the resin. The output voltage generated without power supply is detected at one or more pairs of electrodes positioned opposite each other on both sides of the FRP sheet. The cracks in the structure are analyzed from the changes in the output voltage. A crack detection method for detecting the amplitude of a structure from the amplitude of the output voltage when the structure vibrates with different amplitudes due to an external force.

2. A piezoelectric FRP sensor is attached to a non-conductive structure, comprising an FRP sheet reinforced with non-conductive continuous fibers and electrodes bonded to both sides of the FRP sheet, wherein piezoelectric particles are dispersed within the resin and polarization treatment is performed within the resin. The output voltage generated without power supply is detected at one or more pairs of electrodes positioned opposite each other on both sides of the FRP sheet. The cracks in the structure are analyzed from the changes in the output voltage. A crack detection method for detecting the depth of a crack in a structure from the amplitude of the output voltage when the structure vibrates due to an external force of the same or different frequencies.

3. A piezoelectric FRP sensor is attached to a non-conductive structure, comprising an FRP sheet reinforced with non-conductive continuous fibers and electrodes bonded to both sides of the FRP sheet, wherein piezoelectric particles are dispersed within the resin and polarization treatment is performed within the resin. The output voltage generated without power supply is detected at one or more pairs of electrodes positioned opposite each other on both sides of the FRP sheet. The cracks in the structure are analyzed from the changes in the output voltage. A crack detection method for detecting the location of a crack in a structure when the structure vibrates due to an external force of the same or different frequencies, by detecting the change in the amplitude of the output voltage.

4. A piezoelectric FRP sensor is attached to a non-conductive structure, comprising an FRP sheet reinforced with non-conductive continuous fibers and electrodes bonded to both sides of the FRP sheet, wherein piezoelectric particles are dispersed within the resin and polarization treatment is performed within the resin. The output voltage generated without power supply is detected at one or more pairs of electrodes positioned opposite each other on both sides of the FRP sheet. The cracks in the structure are analyzed from the changes in the output voltage. A crack detection method that detects the loading and unloading of a load from changes in the output voltage when the aforementioned structure is repeatedly subjected to loading and unloading.

5. A piezoelectric FRP sensor is attached to a non-conductive structure, comprising an FRP sheet reinforced with non-conductive continuous fibers and electrodes bonded to both sides of the FRP sheet, wherein piezoelectric particles are dispersed within the resin and polarization treatment is performed within the resin. The output voltage generated without power supply is detected at one or more pairs of electrodes positioned opposite each other on both sides of the FRP sheet. The cracks in the structure are analyzed from the changes in the output voltage. A crack detection method for detecting the crack depth of a structure from the change in output voltage when the structure is repeatedly subjected to loading and unloading of a load.

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