JP3482468B2 - Strain measurement sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a strain measuring sensor for measuring strain of a structure or a material.

[0002]

2. Description of the Related Art Conventional metal foil strain gauges and strain measurement sensors using optical fibers are used to attach or embed the sensor to a structure or material and use a special measuring instrument to measure strain in real time. It is something to measure. Such a strain measuring sensor is suitable for measuring the history of transient strain, and is widely used in industry. However, these sensors are basically based on real-time measurement, and there is a problem that it is difficult to measure the history of the maximum strain received by the structure or material under the actual use environment. In addition, conventional metal foil strain gauges and optical fiber strain measurement sensors have large dimensions and may impair mechanical properties and manufacturing processability in order to be embedded in materials such as FRP. There was a problem.

On the other hand, as a method for measuring the maximum strain history of the composite material under the actual use environment, for example, fiber breakage and electric resistance change accompanying microscopic damage development in the fiber reinforced composite material are utilized. Studies on the measured methods have been reported. (For example, JP-A-11-21789
No. 3, gazette, N.P. Muto et al. J. Am. Cre
am. Soc. , 76 [4], 875-79 (199
3), Park Sung-Han et al., Proc. Of the 8th Mechanical Material and Material Processing Technology Conference of the Japan Society of Mechanical Engineers p. 21-9-220 etc.)

This is a method of measuring strain or microscopic damage from an increase in electric resistance of carbon fiber due to increase in stress and a change in electric resistance due to successive breakage of fiber. However, in this method, the conductive path is formed by the contact between the carbon fibers, the measurement accuracy of the sensor, the measurement variation, the reproducibility and the resolution are low, and the theoretical value for determining the relationship between the electrical resistance and the fiber breakage rate is used. There was a problem that various analyzes were complicated. Further, since the breakage of the carbon fiber occurs in the high strain region, there is a problem that the measurable range of the strain is limited to the high strain region.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of such a situation and is capable of performing a high-performance strain measurement which is small, lightweight and excellent in handleability, and has a maximum strain history measured. The purpose is to provide a possible strain measuring sensor.

[0006]

A strain measuring sensor of the present invention which solves the above-mentioned problems is a fiber bundle obtained by bundling a plurality of continuous fibers integrated by a base material such as fiber-reinforced composite material, resin, metal or ceramics. Strain measurement of composite structures
A sensor, wherein each of the continuous fibers is isolated on its surface.
An edge film layer is formed to insulate each fiber in the fiber bundle from each other.
Becomes Te, sequential breaking of the continuous fiber with increasing strain, by measuring the change in electrical resistance, the strain measurement, it is characterized in that and stored.

As a method for integrating and compounding a plurality of fiber bundles obtained by bundling the continuous fibers, the fiber bundle is embedded in a material such as a fiber-reinforced composite material, resin, metal or ceramics which is a base material to be integrated. Method, or a fiber-reinforced composite material, which is a base material of the fiber bundle, resin, metal,
It is possible to adopt a method in which the material is sandwiched between materials such as ceramics and is pressed and molded by hot pressing to integrate and compound. Further, on the surface of the continuous fiber, heat treatment,
By forming a surface layer by applying chemical treatment, coating treatment, etc., and by using this as an insulating film between each fiber present in the fiber bundle, there is no fiber contact and a conductive path after fiber breakage is formed. And a strain measurement sensor with high measurement accuracy can be obtained.

Further, the continuous fiber is subjected to a heat treatment or a chemical treatment to control the strength distribution of the single fiber, and a single fiber bundle or a plurality of fiber bundles obtained by bundling the continuous fiber are combined to obtain the entire continuous fiber. It is desirable because the strength distribution of the fiber can be controlled. At that time, by using two or more kinds of fiber bundles having different strength distributions,
A strain measurement sensor applicable in a wide strain range can be used. Further, the continuous fiber used in the present invention,
Although it is not particularly limited, it is desirable that it has a certain degree of conductivity, and silicon carbide fibers and the like can be suitably applied.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The strain measuring sensor of the present invention comprises:
A structure in which a fiber bundle obtained by bundling a plurality of continuous fibers is integrated and compounded by a base material such as resin, metal, or ceramics,
The continuous breakage of continuous fibers due to an increase in strain is measured as a change in the electrical resistance of the structure. Since the conductivity of the continuous fibers is used, the fibers may have a certain degree of conductivity. It is desirable, but within the measurable range,
The value of conductivity is not particularly limited.

Further, since the strain history is measured by utilizing the strength distribution of the fibers, the number of fibers needs to be plural, but the specific number of fibers is not limited. Then, in the present invention, the surface of the continuous fiber is subjected to heat treatment, chemical treatment, coating treatment or the like to form a surface layer, which is an insulating film between the respective fibers present in the fiber bundle, It is possible to form a sensor with higher resolution. When the insulating film is not formed on the fiber surface, conductive paths are formed by fiber contact, and these conductive paths reduce the measurement accuracy, measurement variability, reproducibility, and resolution of the sensor, and the theoretical analysis is complicated. Becomes

By forming an insulating film on the fiber surface, the performance of the sensor can be remarkably improved. As the method for forming the insulating film, for example, in the case of silicon carbide ceramic fibers, the surface is thermally oxidized, the sol-gel method or C
If the insulating film layer is formed by the VD method or the like, and if it is carbon fiber, the combined treatment of surface silicon carbide and thermal oxidation, the combined treatment of surface boron carbide and thermal oxidation, sol-gel method and CV
Various methods are conceivable, such as forming an insulating film skin by the D method, but the specific method is not specified.

In the present invention, as described above, the phenomenon that continuous fibers are successively broken as the strain increases is measured by the change in electrical conductivity. Therefore, the measurable strain range is determined by the volume fraction of the fibers contained in the sensor and the single fiber strength distribution of the fibers. The single fiber strength distribution of continuous fibers can be controlled by subjecting the fibers to, for example, heat treatment or chemical treatment. Further, it is possible to highly control the strength distribution of the fibers by combining a plurality of fiber bundles controlled in this way. This makes it possible to design sensors having various specifications such as a sensor compatible with a wide strain range and a sensor having high sensitivity in a narrow strain range.

[0013]

EXAMPLES Hereinafter, the strain measuring sensor of the present invention will be described in more detail based on examples, but the present invention is not limited to these examples. Table 1 shows various characteristics of the strain measurement sensors according to Examples 1 to 7 and Comparative Example of the present invention.

(Examples 1 to 6 and Comparative Example ) The strain measuring sensors according to Examples 1 to 6 and Comparative Example are
The film-type sensor, which is intended to be used by being attached to a structure or material, has a configuration as shown in the conceptual diagram of FIG. In the figure, 1 is a strain measurement sensor, 2 is a fiber bundle in which a plurality of continuous fibers are arranged in a flat plate shape, 3 is an insulating base, and 4 is electrodes provided on both ends of the flat plate fiber bundle 2 by sputtering or the like. . As continuous fibers in these examples, Si containing a trace amount of Ti is used.
-C-O ceramic fiber (Ube Industries, Ltd.)
Made, product name Tyranno Lox-M grade, diameter about 11μ
m) was used. A polyimide resin is used as the base material that serves as an insulating base.

In the comparative example , the silicon carbide type fiber is used as it is. In the example 1 , the fiber is used in air.
This is an example in which an oxide film is formed on the surface by performing an oxidation treatment at 1000 ° C. for 1 hour. In addition, in Examples 2 to 5 , after performing an oxidation treatment at 1000 ° C. for 1 hour in the air, a heat treatment was further performed in argon gas at different heat treatment temperatures and heat treatment times to prevent the surface from being oxidized. This is an example in which the strength of the fiber is controlled. In Table 1, the fiber strength distribution in each of the examples is shown by the Weibull coefficient (scale parameter, shape parameter) of the fiber when the gauge length is 5 mm. It can be seen that detailed control is possible. In Example 6 , the fibers prepared in Examples 2 and 5 in Table 1 were used in the same ratio and integrated to form a sensor.

[0016]

[Table 1]

The fiber bundle thus obtained (80 fibers)
(0 pieces) were arranged in one direction, sandwiched by a thermoplastic polyimide film as an insulating material, and then pressure-molded using a hot press heated to 370 ° C. The silicon carbide fiber / polyimide composite material thus produced has a fiber volume ratio of 10%. Then, the obtained unidirectional composite material was cut into a piece having a width of 2 mm and a length of 5 mm, and the electrode 4 was formed by sputtering to fabricate a strain measurement sensor 1 as shown in FIG. The elastic modulus of the obtained strain measurement sensor was about 22 GPa.

The characteristics of the strain measuring sensor obtained as described above were evaluated by the following methods. A trial strain measurement sensor and a commercially available metal foil strain gauge for comparison were adhered to a stainless steel (SUS304) tensile test piece with a width of 10 mm, a thickness of 4 mm and a length of 200 mm using a cyanoacrylate adhesive. The electrical resistance between the electrodes of the strain measurement sensors of the examples and comparative examples was measured using a digital multimeter while applying a tensile load to the tensile test piece using a hydraulic servo type material testing machine.

As a result, the relationship between the strain (%) obtained in the strain measurement sensors according to Examples 1 to 3 and the comparative example and the rate of change in electrical resistance showed the characteristics shown in FIG. Here, the electrical resistance change rate is the ratio (R / R0) of the electrical resistance value (R) in the strain state at the time of measurement to the electrical resistance value (R0) of the initial value. Even before breaking, the silicon carbide fiber has a large electric resistance with an increase in strain, and in the low strain region, a gradual increase in the electric resistance change rate is recognized, but when the fiber breakage starts, the electric resistance change rate is Increase significantly.

In the comparative example , the rate of change in electric resistance is about 3 to 4 even if the fiber is completely broken, and the measurement sensitivity as a sensor is low. In addition, the resistance change due to fiber breakage is small and the resolution is low. This is because even if the fibers are completely broken, a conductive path is formed due to the contact between the fibers. As described above, the formation of the conductive path between the fibers causes a decrease in the measurement accuracy, measurement variation, reproducibility, and resolution of the sensor.

On the other hand, as shown in Example 1 , by forming an insulating film on the fiber surface, the change in electrical resistivity accompanying an increase in strain becomes significantly large, and as a result, the sensitivity and resolution of the sensor are increased. It is possible to improve remarkably. In Example 1 , the thermal oxidation treatment is performed as the method for forming the insulating film, but it goes without saying that the method for forming the surface insulating film is not limited to this.
Various methods such as sol-gel method and oxide coating such as CVD method can be applied.

Considering that the increase of the electric resistance is caused only by the fiber breakage, the reciprocal of the electric resistance change rate (R
0 / R) and the cumulative breaking probability F (σ) of the fiber with respect to the target gauge length, the following relationship is established. F (σ) = 1−R0 / R For many carbon fibers and ceramic fibers, the cumulative fiber breakage probability F with respect to the target gauge length (L)
(Σ) can be represented by a Weibull distribution with two parameters. At this time, the cumulative breakage probability F (σ) of the fiber can be expressed by Equation 1 using the Weibull coefficient of the fiber (scale parameter: σ0, shape parameter: m).

[0023]

[Equation 1]

From the above viewpoint, FIG. 3 shows Examples 1 to 5 and
It is the figure which plotted the relationship of the strain (%) and (1-R0 / R) obtained by the strain measurement sensor concerning the comparative example . Note that, in FIG. 3, the change in the electrical resistance of the fiber due to the increase in strain that occurs before breakage is shown by subtraction. As is clear from this figure, (1-R0 /
When the value of R) is 0.05 or less and 0.95 or more,
The change in the value of (1-R0 / R) due to the change in strain is small, and the accuracy of the sensor decreases. Therefore, for the sensor prototyped here, the value of (1-R0 / R) is
The range of 0.05 to 0.95 was set as the guaranteed range of the sensor.

In the fiber breakage detection type sensor, the strength distribution of the fiber determines the measuring range of the sensor. for that reason,
If the commercially available silicon carbide fiber is used as it is, there is a drawback that the strain measurement range is limited to a high strain range and a narrow range. In order to eliminate this drawback, in Examples 2 to 5 , the silicon carbide fibers having an oxide film formed on the surface were further subjected to a high temperature heat treatment in an argon gas to reduce the strength of the fibers while controlling the strength. This made it possible to obtain a strain measurement sensor capable of controlling the strain measurement range of the sensor, and capable of measuring strain over a wide range.

In Example 6 , two types of fiber bundles having different strength distributions are used to provide a strain measuring sensor applicable in a wide strain range. FIG. 4 shows the characteristics of strain (%) and electric resistance change rate (R / R0) obtained by the strain measurement sensor according to the seventh embodiment. The cumulative breakage probability of fibers in a sensor using k kinds of fiber bundles having different fiber strength distributions as in this example can be expressed by the following mathematical formula 2.

[0027]

[Equation 2] By using this relational expression, it is possible to design sensors with different strain measurement ranges.

(Embodiment 7 ) FIG. 5 schematically shows Embodiment 8 of the present invention, in which a strain measuring sensor is applied as an embedded sensor. Strain measuring sensor 5 according to the present embodiment
Relates to an embedded sensor in which the fiber bundle 9 constituting the measurement sensor is embedded in a structure or a material and used.
In the figure, 6 is a material in which a fiber bundle 9 in which a plurality of continuous fibers are bundled is embedded, and in the present example, it is composed of a laminated plate of carbon fiber reinforced composite material (Toray T800H / 3631). The laminated structure of the composite material is (0 ° / 90 ° /
0 °) symmetrical stacking, as shown in FIG.
In this example, a fiber bundle 9 obtained by bundling a plurality of continuous fibers made of silicon carbide based fibers was embedded in the 0 ° layer 8 as in Examples 1 to 7, and electrodes 10 were formed at both ends to obtain a strain measurement sensor 5. The laminate 7 on both sides of the composite material is a 0 ° layer. The specifications of the strain measurement sensor of Example 7 created are as follows.
As described in Table 1.

The strain measuring sensor of Example 7 was different from the other strain measuring sensors shown in Table 1, and the gauge length of this strain measuring sensor was 100 mm in the tensile test described later, and correspondingly the same. The average strength (Weibull scale parameter) is smaller due to the volume effect as compared with the fiber under the heat treatment condition. The fiber volume fraction of the strain measuring sensor was set to 2% so that the elastic modulus of the strain measuring sensor was close to the elastic modulus of the 90 ° composite layer.

A tensile test piece having a thickness of 2 mm, a width of 12.5 mm and a length of 200 mm was obtained from the composite material in which the silicon carbide fiber was embedded. A glass fiber composite material tab was adhered to both ends of the test piece at a position of 50 mm, and this portion was fixed by a hydraulic chuck to perform a tensile test. Therefore, the parallel length of the test piece, that is, the gauge length of the strain measuring sensor is 100 mm.

Nominal strain (%) of the composite material when a tensile load is applied to the strain measuring sensor according to this embodiment,
FIG. 6 shows the relationship between the strain measurement values (%) measured by the strain measurement sensor. As shown in this figure, reflecting the characteristics of the strain measurement sensor, the fracture of the silicon carbide fiber forming the strain measurement sensor started at a strain of about 0.1%, and an increase in electrical resistance was observed. And, as shown in the figure, the strain measured by the strain measurement sensor is
Although it coincides with the nominal strain of the composite material up to the middle, when the strain exceeds 0.55%, the strain value measured by the strain measuring sensor becomes significantly larger than the nominal strain. This is because the number of fractures of the silicon carbide fiber increased due to the partial strain increase due to the opening of the matrix cracks generated in the 90 ° layer of the composite material. As described above, the strain measurement sensor according to the present embodiment has a function as a damage sensing sensor in addition to a function as a simple strain measurement sensor.

In the seventh embodiment, the sensor material is embedded in the composite material, but the fiber of the strain measuring sensor may be directly embedded in the composite material and integrally molded. In this case, since the influence of the strain measuring sensor on the mechanical properties can be almost ignored, it becomes possible to realize a smart composite material having a damage sensing or strain measuring function.

[0033]

As described above, according to the present invention,
A structure in which a fiber bundle obtained by bundling a plurality of continuous fibers is integrated and compounded by a base material such as resin, metal, or ceramics,
It is possible to obtain a small and lightweight strain measurement sensor which is capable of measuring and storing the strain by measuring the change in the electrical resistance of the sequential breakage of the fiber due to the increase of the strain and which is excellent in handleability. In addition, the strain measurement sensor according to the present invention is a fiber reinforced composite material, a resin, a metal, a ceramics, or the like, which is used to integrate and combine a continuous fiber bundle into a fiber bundle. It is also possible to integrate these by embedding them and have these fibers function as a strain measuring sensor.

Further, in the strain measuring sensor according to the present invention, a surface layer is formed on the surface of the continuous fiber by using heat treatment, chemical treatment, coating treatment or the like, and the surface layer is formed between the fibers present in the fiber bundle. The use of this insulating film prevents the formation of conductive paths between fibers and enables the formation of a strain measuring sensor with higher resolution. Further, the strain measuring sensor according to the present invention adjusts the single fiber strength distribution by subjecting continuous fibers to heat treatment or chemical treatment, and by combining these fiber bundles singly or in combination, the fiber strength distribution can be determined. By highly controlling, it becomes possible to arbitrarily design a strain measurement sensor having various specifications such as a strain measurement sensor applicable to a wide strain region.

[Brief description of drawings]

FIG. 1 is a conceptual diagram for explaining a configuration of strain measurement sensors according to Examples 1 to 6 and a comparative example of the present invention.

FIG. 2 is a diagram showing the characteristics of strain (%) and electric resistance change rate obtained by the strain measurement sensors according to Examples 1 to 3 and Comparative Example of the present invention.

FIG. 3 shows strains (%) and (1-R0) obtained by strain measurement sensors according to Examples 1 to 5 and Comparative Example of the present invention.
It is a diagram which shows the relationship of / R).

FIG. 4 is a diagram showing the strain (%) and the electrical resistance change rate (R / R) obtained by the strain measurement sensor according to Example 6 of the present invention.
It is a diagram which shows the characteristic of 0).

FIG. 5 is a conceptual diagram for explaining the configuration of a strain measurement sensor according to a seventh embodiment of the present invention.

FIG. 6 is a nominal strain (%) of a composite material when a tensile load of a strain measurement sensor according to Example 7 of the present invention is applied.
And a strain measurement value (%) measured by the strain measurement sensor.

[Explanation of symbols]

1, 5 strain measurement sensor 2,9 Continuous fiber bundles (silicon carbide fibers, etc.) 3 insulation base 4, 10 electrodes 6 Carbon fiber reinforced composite material 7 0 ° layer of carbon fiber reinforced composite material 8 90 ° layer of carbon fiber reinforced composite material

Continuation of the front page (56) Reference JP-A-7-72023 (JP, A) JP-A-11-223565 (JP, A) JP-A-9-5175 (JP, A) JP-A-10-122985 (JP , A) JP 7-201374 (JP, A) JP 10-9905 (JP, A) JP 8-178765 (JP, A) JP 2000-55748 (JP, A) (58) Survey Fields (Int.Cl. ⁷ , DB name) G01B 7/16 G01L 1/20

Claims (5)

(57) [Claims] 1. A strain measuring sensor having a structure in which a fiber bundle obtained by bundling a plurality of continuous fibers is integrated and compounded by a base material such as a fiber-reinforced composite material, resin, metal or ceramics.
Each of the continuous fibers has an insulating film layer on its surface.
It is formed by insulating between each fiber in the fiber bundle, the sequential breakage of the continuous fiber with the increase of strain, by measuring the strain, by measuring the strain, Characteristic strain measurement sensor. 2. The strain measuring sensor according to claim 1, wherein the fiber bundle is integrated by being embedded in a material such as a fiber-reinforced composite material, resin, metal, or ceramics which is a base material. 3. The fiber bundle is sandwiched by a material such as a fiber-reinforced composite material as a base material, resin, metal, or ceramics,
The strain measuring sensor according to claim 1, wherein the strain measuring sensor is integrated and compounded by molding by hot pressing. 4. A heat treatment or a chemical treatment is applied to the continuous fibers to adjust the insulation coating of the short fibers and the strength distribution of the fiber bundles, and the single or plural fiber bundles obtained by bundling the continuous fibers. The combination is controlled to control the strength distribution of the entire continuous fiber.
The strain measuring sensor according to any one of 3 to 4 . 5. A strain measurement sensor of the continuous fibers according to claim 1-4, wherein either a silicon carbide fiber.