JP2002303504A - Stress measurement sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small, lightweight, easily-handleable, and high- performance fiber fracture detection type stress measurement sensor. SOLUTION: When continuous fibers of silicon carbide based fiber are subjected to oxidation treatment in the air, an oxide coating is formed on the surface. Heat difference treatment or chemical treatment is applied for regulating the strength distribution of a single filament. A continuous fiber bundle 2 obtained in this way is arranged in one direction, and the continuous fiber bundle is integrated/combined to be molded with a base material such as a fiber reinforcing composite material serving as an insulating material, resin, metal or ceramic. When electrodes are attached to the obtained one directional composite member, a stress measurement sensor 1 is formed. In the silicon carbide based fiber, electric resistance is increased even before its fracture according to an increase in the stress, and in a low stress area, a gentle increase of the electric resistance fluctuation ratio is observed. However, the electric resistance fluctuation ratio is increased greatly once the fracture is started in the fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventional metal foil type strain gauges and strain measurement sensors using optical fibers have a sensor attached or embedded in a structure or material, and the strain is measured in real time using a special measuring instrument. It is to measure. Such a strain measurement sensor is suitable for measuring the history of transient strain, and is widely used in industry. However, these sensors are based on real-time measurement, and there is a problem that it is difficult to measure the history of the maximum strain received by a structure or a material under an 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 workability in order to be embedded in a material such as FRP. There was a problem.

[0003] On the other hand, as a method for measuring the maximum strain history of a composite material under an actual use environment, for example, a fiber breakage and a change in electric resistance of a fiber-reinforced composite material accompanying microscopic damage progression are used. Studies on the measurement methods have been reported. (For example, Japanese Patent Application Laid-Open No. H11-21789)
No. 3, N. Muto et al., J. Am. Cream. Soc., 76 [4], 875-
79 (1993), Park Norikazu et al., The 8th Mechanical Material of the Japan Society of Mechanical Engineers
Proceedings of the Material Processing Technology Lecture, p.2l9-220 etc.)

This is a method for measuring strain or microscopic damage from an increase in the electrical resistance of a carbon fiber with an increase in stress and a change in the electrical resistance with a successive breakage of the fiber. However, in this method, due to the formation of a conductive bus due to the contact between carbon fibers, the measurement accuracy, measurement variation, reproducibility, and resolution of the sensor are low, and a theoretical method for determining the relationship between the electrical resistance and the fiber breakage rate. There was a problem that complicated analysis was complicated. In addition, 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 circumstances, and is capable of measuring a compact, lightweight, high-performance strain with excellent handleability, and measuring the maximum strain history. It is an object to provide a possible strain measuring sensor.

[0006]

A strain measuring sensor according to the present invention, which solves the above-mentioned problems, has a structure in which a fiber bundle obtained by bundling a plurality of continuous fibers is integrated with a base material such as resin, metal, and ceramics to form a composite structure. The magnitude of the strain is measured and stored by measuring the sequential breakage of the fiber accompanying the increase in the strain as a change in the electrical resistance of the structure.

[0007] As a method of integrating and compounding a fiber bundle obtained by bundling a plurality of continuous fibers, a fiber bundle is embedded in a material such as a fiber-reinforced composite material, a resin, a metal, or a ceramic material serving as a base material. A fiber-reinforced composite material, a resin, a metal,
A method of sandwiching and sandwiching between materials such as ceramics and press-molding with a hot press to integrate and composite can be adopted. In addition, a heat treatment,
A surface layer is formed by performing chemical treatment, coating treatment, etc., and by using this as an insulating film between fibers present in the fiber bundle, fiber contact is eliminated, 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, a chemical treatment, or the like to adjust the strength distribution of the single fiber, and a single fiber bundle or a plurality of fiber bundles obtained by bundling the continuous fiber is combined to form a whole. This is desirable because the strength distribution of continuous fibers can be controlled. At that time, by using two or more types of fiber bundles with different strength distribution,
A strain measurement sensor applicable in a wide range of strain can be used. Further, the continuous fiber used in the present invention,
Although not particularly limited, it is desirable to have a certain degree of conductivity, and silicon carbide fibers or the like can be suitably applied.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The strain measuring sensor of the present invention
A fiber bundle obtained by bundling a plurality of continuous fibers is integrated and composited with a base material such as resin, metal, and ceramics.
In order to measure the sequential breakage of the continuous fiber with the increase in strain as a change in the electrical resistance of the structure, and to utilize the conductivity of the continuous fiber, the fiber must have a certain degree of conductivity. Desirable, but within measurable range,
The value of the conductivity is not particularly limited.

Further, in order to measure the strain history using the fiber strength distribution, the number of fibers needs to be plural, but the specific number of fibers is not limited. In the present invention, the surface of the continuous fiber is subjected to heat treatment, chemical treatment, coating treatment and the like to form a surface layer, and this is used as an insulating film between the fibers present in the fiber bundle, It is possible to form a sensor with higher resolution. If an insulating film is not formed on the fiber surface, conductive buses due to fiber contact will be formed, and these conductive buses will reduce the measurement accuracy, measurement dispersion, reproducibility and resolution of the sensor, and the theoretical analysis will be complicated. Becomes

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

In the present invention, as described above, the phenomenon that the continuous fiber breaks sequentially 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 fiber contained in the sensor and the single fiber strength distribution of the fiber. The single fiber strength distribution of the continuous fiber can be controlled by subjecting the fiber to, for example, heat treatment or chemical treatment. Further, by combining a plurality of fiber bundles controlled in this manner, it is possible to control the fiber strength distribution to a high degree. This makes it possible to design sensors having various specifications, such as a sensor corresponding to a wide strain region and a sensor having a high sensitivity in a narrow strain region.

[0013]

EXAMPLES Hereinafter, the strain measuring sensor of the present invention will be described in more detail with reference to 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 8 of the present invention.

(Examples 1 to 7) The strain measuring sensors according to Examples 1 to 7 relate to a film type sensor which is assumed to be used by being attached to a structure or a material. It is as shown in the figure. In the figure,
Reference numeral 1 denotes a strain measuring sensor, 2 denotes a fiber bundle in which a plurality of continuous fibers are arranged in a flat plate shape, 3 denotes an insulating base, and 4 denotes electrodes provided on both ends of the flat fiber bundle 2 by sputtering or the like. In these examples, as the continuous fiber, a ceramic fiber having a Si—CO composition containing a trace amount of Ti (trade name: Tyranno-Lox-M, manufactured by Ube Industries, Ltd., having a diameter of about 11 μm) was used. In addition, a polyimide resin is used as a base material serving as an insulating base.

In Example 1, a silicon carbide fiber was used as it was. In Example 2, the fiber was subjected to an oxidation treatment in air at 1000 ° C. for 1 hour to form an oxide film on the surface. This is an example in which is formed. In addition, Examples 3 to 6
After performing the oxidation treatment in air at 1000 ° C. for 1 hour,
Further, in this example, the heat treatment is performed in an argon gas while changing the heat treatment temperature and the heat treatment time to control the fiber strength without oxidizing the surface. In Table 1, the fiber strength distribution in each example 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. Example 7 is a sensor in which the fibers prepared in Examples 3 and 6 in Table 1 were used at the same ratio, and were integrated into a sensor.

[0016]

[Table 1]

The fiber bundle (80 pieces) thus obtained
No. 0) were arranged in one direction, sandwiched by a thermoplastic polyimide film as an insulating material, and then pressure-formed using a hot press heated to 370 ° C. The fiber volume ratio of the silicon carbide fiber / polyimide composite material thus produced is 10%. Then, the obtained unidirectional composite material was cut into a width of 2 mm and a length of 5 mm, and an electrode 4 was applied by sputtering, thereby producing 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 prototype strain measurement sensor and a commercially available metal foil type strain gauge for comparison were adhered to a stainless steel (SUS304) tensile test piece having a width of 10 mm, a thickness of 4 mm, and a length of 200 mm using a cyanoacrylate adhesive. Using a hydraulic servo-type material testing machine, the electric 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 pieces.

As a result, the relationship between the strain (%) obtained in the strain measuring sensors according to Examples 1 to 4 and the rate of change in electric resistance showed characteristics as shown in FIG. here,
The electric resistance change rate is the ratio (R / R ₀ ) of the electric resistance value (R) in the strain state at the time of measurement to the initial electric resistance value (R ₀ ). Even before the silicon carbide fiber breaks, the electrical resistance increases with an increase in strain, and a gradual increase in the rate of change in electrical resistance is observed in the low strain region. Increase significantly.

In Example 1, the rate of change in electrical resistance is about 3 to 4 even when the fiber is completely broken, and the measurement sensitivity as a sensor is low. Further, 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 bus is formed due to the contact between the fibers. As described above, the formation of the conductive bus between the fibers causes a decrease in measurement accuracy, measurement variation, reproducibility, and resolution of the sensor.

On the other hand, as shown in Example 2, by forming an insulating film on the surface of the fiber, the change in the electrical resistivity accompanying the increase in strain becomes extremely large, and as a result, the sensitivity and resolution of the sensor are reduced. It is possible to significantly improve. In Example 2, the thermal oxidation treatment is performed as a method of forming the insulating film. However, as a matter of course, the method of forming the surface insulating film is not limited thereto.
Various methods such as oxide coating by a sol-gel method or a CVD method can be applied.

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

[0023]

(Equation 1)

FIG. 3 is a diagram plotting the relationship between the strain (%) obtained by the strain measuring sensors according to Examples 1 to 6 and (1-R ₀ / R) from the above viewpoints. In FIG. 3, the change in the electrical resistance of the fiber accompanying the increase in the strain occurring before the fracture is not shown. As is clear from this figure, the value of (1-R ₀ / R) is
Below 0.05 and above 0.95, the change in the value of (1-R ₀ / R) due to the change in strain is small, and the accuracy of the sensor decreases. Therefore, the value of (1−R ₀ / R) is 0.05 to 0.95 for the prototype sensor here.
Is the guaranteed range of the sensor.

In a fiber breakage detection type sensor, the fiber intensity distribution determines the measurement range of the sensor. for that reason,
When commercially available silicon carbide fibers are used as they are, there is a disadvantage that the strain measurement range is limited to a high strain region and a narrow range. In order to solve this drawback, Examples 3 to 6 show that the silicon carbide fiber having an oxide film formed on its surface is further subjected to a high-temperature heat treatment in an argon gas to reduce the fiber strength while controlling the fiber strength. This made the sensor a sensor, whereby the measured strain range of the sensor could be controlled, and a strain measuring sensor capable of measuring strain over a wide range could be obtained.

In the seventh embodiment, a strain measurement sensor applicable to a wide range of strain is obtained by using two types of fiber bundles having different strength distributions. FIG. 4 shows the characteristics of the strain (%) and the rate of change in electric resistance (R / R ₀ ) obtained by the strain measuring sensor according to the seventh embodiment. The cumulative breaking probability of the fibers in the sensor using k kinds of fiber bundles having different fiber strength distributions as in this embodiment can be expressed by the following equation (2).

[0027]

(Equation 2) By using this relational expression, sensors having different strain measurement ranges can be designed.

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

The strain measuring sensor of Example 8 differs from the other strain measuring sensors shown in Table 1 in that the gauge length of this strain measuring sensor is 100 mm in a tensile test described later, and correspondingly the same. The average strength (Weibull scale parameter) is smaller than that of the fiber under the heat treatment condition due to the volume effect. The fiber volume ratio 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 ° layer of the composite material.

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 tab of a glass fiber composite material was adhered to both ends of the test piece at a position of 50 mm, and this portion was fixed with a hydraulic chuck to perform a tensile test. Therefore, the length of the parallel portion of the test piece, that is, the length of the gauge of the strain measurement sensor is 100 mm.

The nominal strain (%) of the composite material when a tensile load is applied to the strain measuring sensor according to the present embodiment,
FIG. 6 shows the relationship between the strain measured value (%) by the strain measuring sensor. As shown in this figure, reflecting the characteristics of the strain measurement sensor, the breakage of the silicon carbide fiber forming the strain measurement sensor started at about 0.1% strain, and an increase in electrical resistance was observed. And, as shown in the figure, the strain measured by the strain measurement sensor is
Up to a certain point, the strain coincides with the nominal strain of the composite material. However, 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 breaks of the silicon carbide fiber was increased due to a partial increase in strain due to the opening of a matrix crack generated in the 90 ° layer of the composite material. Thus, the strain measurement sensor according to the present embodiment has a function as a damage sensing sensor in addition to a function as a mere strain measurement sensor.

In the eighth embodiment, the sensor is embedded in the composite material. However, it is also possible to embed the fiber of the strain measurement sensor directly in the composite material and integrally mold the fiber. In this case, since the influence of the strain measurement sensor on the mechanical properties can be almost ignored, it is possible to realize a smart composite material having a damage sensing or strain measurement function.

[0033]

As described above, according to the present invention,
A fiber bundle obtained by bundling a plurality of continuous fibers is integrated and composited with a base material such as resin, metal, and ceramics.
By measuring the change in the electrical resistance of the successive breakage of the fiber with the increase in strain, a small, lightweight, and easy-to-handle strain measurement sensor capable of measuring and storing the strain can be obtained. In addition, the strain measuring sensor according to the present invention, when integrating and compounding a fiber bundle obtained by bundling a plurality of continuous fibers, a fiber reinforced composite material, a resin, a metal, a material such as ceramics, these continuous fibers The fibers can be integrated by embedding them, and these fibers can also function as strain measurement sensors.

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 a heat treatment, a chemical treatment, a coating treatment or the like, and the surface layer is formed between each fiber existing in the fiber bundle. The formation of the conductive bus between the fibers can be prevented by forming the insulating film as described above, and a strain measuring sensor with higher resolution can be formed. Further, the strain measurement sensor according to the present invention adjusts the single fiber strength distribution by performing heat treatment or chemical treatment on the continuous fiber, and singly or combines a plurality of these fiber bundles, thereby controlling the fiber strength distribution. By performing advanced control, 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 the drawings]

FIG. 1 is a conceptual diagram illustrating a configuration of a strain measurement sensor according to Examples 1 to 7 of the present invention.

FIG. 2 is a diagram showing characteristics of strain (%) and electric resistance change rate obtained by strain measuring sensors according to Examples 1 to 4 of the present invention.

FIG. 3 is a diagram showing a relationship between strain (%) and (1−R ₀ / R) obtained by strain measuring sensors according to Examples 1 to 6 of the present invention.

FIG. 4 shows strain (%) and rate of change in electric resistance (R / R) obtained by the strain measurement sensor according to Example 7 of the present invention.
FIG. 4 is a diagram showing characteristics of R ₀ ).

FIG. 5 is a conceptual diagram illustrating a configuration of a strain measurement sensor according to an eighth embodiment of the present invention.

FIG. 6 shows a nominal strain (%) of the composite material when a tensile load is applied to the strain measuring sensor according to the eighth embodiment of the present invention.
FIG. 4 is a diagram showing a relationship between a strain measurement value (%) and a strain measurement sensor.

[Explanation of symbols]

 1, 5 strain measurement sensor 2, 9 continuous fiber bundle (silicon carbide fiber etc.) 3 insulating base 4, 10 electrode 6 carbon fiber reinforced composite material 7 0 ° layer of carbon fiber reinforced composite material 8 90 of carbon fiber reinforced composite material ° layer

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2F063 AA25 DC08 EC03 EC07 EC22 FA14 FA18 2F070 AA20 BB10

Claims (7)

[Claims] 1. 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, ceramics, etc., and the continuous fibers are successively broken with an increase in strain. Is measured by measuring a change in electrical resistance, thereby measuring and storing the strain. 2. The strain measuring sensor according to claim 1, wherein the fiber bundle is integrated by embedding the fiber bundle in a material such as a fiber reinforced composite material, a resin, a metal, or a ceramic serving as a base material. 3. The fiber bundle is sandwiched between a material such as a fiber-reinforced composite material, a resin, a metal, and a ceramic serving as a base material,
2. The strain measuring sensor according to claim 1, wherein the strain measuring sensor is integrated and compounded by press molding with a hot press. 4. A surface layer is formed by subjecting the surface of the continuous fiber to heat treatment, chemical treatment, coating treatment, or the like, and this is used as an insulating film between fibers present in the fiber bundle. The strain measuring sensor according to claim 1. 5. The continuous fiber is subjected to a heat treatment, a chemical treatment, or the like to adjust a single fiber strength distribution, and a single fiber bundle or a plurality of fiber bundles obtained by bundling the continuous fibers is combined to form a whole. The strain measurement sensor according to any one of claims 1 to 4, wherein a strength distribution of the continuous fiber is controlled. 6. The strain measuring sensor according to claim 1, wherein the continuous fiber is a silicon carbide fiber. 7. The strain measuring sensor according to claim 1, wherein two or more types of fiber bundles having different intensity distributions are used.