JP2555525B2 - Sensor for acoustoelastic stress measurement by surface SH wave - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for stress measurement, which is used for stress measurement by surface SH waves, and which has an ultrasonic transmitter and receiver.

[0002]

2. Description of the Related Art Vibrations propagating in a solid include a longitudinal wave in which particles constituting the solid move in the sound traveling direction and a transverse wave vibrating in the direction perpendicular to the sound traveling direction. Includes SV waves that oscillate perpendicularly to the solid surface and SH waves that oscillate parallel to the solid surface. SH waves traveling along the surface are called surface SH waves.

As a method for measuring the stress of steel structures such as bridges, steel pipes, steel towers, and tanks, the phenomenon in which the propagation velocity of ultrasonic waves inside the steel material changes due to stress is used to transmit ultrasonic waves in close contact with the surface of the steel material. There is known a method of measuring stress by receiving pulsed ultrasonic waves emitted from a child by a receiver and measuring the propagation time of the ultrasonic waves.

As the stress measuring method, there are known three methods including a birefringent acoustoelastic method, a surface SH wave method, and an oblique angle SH wave method. Among them, the birefringent acoustoelastic method and the oblique angle SH wave method are plate members. The surface SH wave method can measure the stress in the vicinity of the surface of the member, while the average stress of the thickness is measured.

Further, it has been proposed to use surface SH waves in order to inspect for the presence or absence of fine cracks in the periodic inspection required for a steel material subjected to a large load such as an axle for railway vehicles. Yuki Toda et al., "Quantitative Evaluation of Fretting Fatigue Cracks on Railroad Vehicle Axles by Surface SH Waves", Non-Destructive Inspection, Volume 40, No. 3, 158-164 (March 1991, Society) Published by Japan Association for Nondestructive Inspection)).

A conventional sensor used for stress measurement by the surface SH wave method is, as shown in FIGS. 1 and 2, a transmitter that emits an ultrasonic wave and radiates it into a steel material and an ultrasonic wave that propagates in the steel material. It is used that a pair of receivers are provided, and the transducers are arranged and fixed so as to face each other obliquely inward with a certain distance therebetween. As the oscillators of the transmitter and the receiver, a transverse wave piezoelectric element obtained by Y-cutting a crystal, and other piezoelectric elements made of various ceramics are used.

As shown in FIGS. 1 and 2, the piezoelectric elements 1 and 8 of the transmitter 4 and the receiver 5 for the surface SH wave are wedges 2 made of synthetic resin such as polymethylmethacrylate resin, respectively.
9 and a backing material 12 made of synthetic resin or the like, and by embedding substantially the entire backs of the transmitters 4 and 5 in the backing member 3 except the bottoms of the wedges 2 and 9, The child 4 or the receiver 5 is configured. The angle of the transmitter 4 with respect to the bottom surface 7 of the piezoelectric element 1, that is, the incident angle θ of the ultrasonic wave on the test body 6, is an angle called a critical angle so that the refraction angle of the ultrasonic wave incident on the steel material is exactly 90 degrees. It is necessary to increase the angle of the piezoelectric element 1 of the receiver 5 to the same angle in order to improve the reception efficiency.

As shown in FIG. 3, the piezoelectric element 1 of the transmitter 4 is
A transverse wave that is parallel to the surface of the test body 6 and oscillates perpendicularly to the traveling direction is incident on the test body 6 via the acrylic resin wedge 2. The transverse wave incident on the test body 6 becomes a surface SH wave, and propagates as a transverse wave traveling with a spread of about 15 ° from the surface of the test body 6.

A part of the surface SH wave propagating in the vicinity of the surface of the test body 6 is refracted by the bottom surface of the wedge 9 of the receiver 5 and propagates in the direction of the piezoelectric element 8 to vibrate the piezoelectric element 8. Output the detection voltage.

[0010]

In order to improve the measurement accuracy in the stress measurement by the conventional surface SH wave method, the distance d between the centers of the transmitter 4 and the receiver 5 is always kept constant and the transmission is performed. It is necessary to make the bottom surfaces 7 of the child 4 and the receiver 5 closely adhere to the surface of the test body 6. Therefore, as shown in FIGS. 1 and 2, the transmitter 4 and the receiver 5 are the same back member 3 as shown in FIG.
Embedded therein, the distance between the transmitter 4 and the receiver 5 is always kept constant, and the bottom surfaces 7 of the transmitter 4 and the receiver 5 are polished to be completely flat.

An acoustic binder made of a viscous liquid is thinly applied to the bottom surfaces of the transmitter 4 and the receiver 5 which are integrally formed in this manner, and they are brought into close contact with the surface of the test body 6 to form a thin layer of the acoustic binder. The ultrasonic waves are transmitted through the as efficiently as possible. At that time, the surface of the test piece 6 is completely polished to be flat so that the parallelism between the bottom surface 7 of the transmitter 4 and the receiver 5 and the surface of the test piece 6 is within several μ, and transmission is performed with great force. It is necessary to press the sensor including the child 4 and the receiver 5 against the surface of the test body 6.

The design allowable stress of ordinary steel is about 120 MPa, but the measurement accuracy of about 10 MPa is necessary for measuring the stress of the steel structure. However, the surface of the test piece 6 and the transmitter 4
If the flatness of the bottom surface 7 of the receiver 5 is not good, the transmitter 4
Also, even if the receiver 5 is pressed against the surface of the test body 6 with a force of 3 kgf or more, the measurement accuracy of stress becomes worse than 30 to 40 MPa. It is extremely difficult to polish the surface of the test body 6 to a completely flat surface, and it is not easy to bring the entire sensor into close contact with the surface of the test body 6 with a strong force.

Therefore, according to the present invention, it is not necessary to raise the polishing accuracy of the surface of the test body 6 so much, and the sensor is pressed against the surface of the test body 6 with a relatively weak pressing force.
It is an object of the present invention to provide a sensor capable of performing stress measurement with high accuracy by closely adhering the bottom surfaces 7 of the transmitter 4 and the receiver 5 of the sensor to the surface of the test body 6 without changing the distance between the two. To do.

[0014]

In order to achieve the above object,
As a result of intensive studies, the inventors of the present invention have fixed the transmitter 4 and the receiver 5 to both sides of a connecting plate made of, for example, an elastic plate having a concave cross section and having a relatively large rigidity. By applying a stress to the bottom surface 7 of the receiver 5 so that the plane formed by both bottom surfaces can be slightly bent, the sensor transmission even on the surface of the test body 6 not polished to a perfect plane. The inventors have found that the bottom surfaces 7 of the child 4 and the receiver 5 can be brought into close contact with each other almost completely, and as a result, the accuracy of stress measurement can be easily improved, thereby completing the present invention.

That is, according to the present invention, a pair of piezoelectric elements are joined to each other on a wedge made of synthetic resin or the like, and a back member is joined to the back surface of each wedge to form a transmitter and a receiver at a fixed distance from each other. In a sensor for measuring acoustic elastic stress by surface SH waves in which the piezoelectric elements are arranged so as to face each other obliquely inward, the transmitter and the receiver are connected by a connecting plate having an appropriate elasticity, When the bottoms of the wedges of the transmitter and the receiver are pressed against the surface of the test body, the bottom of the wedge does not change on the surface of the test body without changing the distance between the transmitter and the receiver due to a slight deformation of the connecting plate. A gist is a sensor for measuring acousto-elastic stress by surface SH waves, which is characterized in that they are closely contacted with each other.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The sensor for measuring acoustic elastic stress by surface SH wave of the present invention will be described in detail with reference to the drawings. FIG. 4 is a plan view of an example of a sensor for measuring acoustic elastic stress by surface SH waves of the present invention, and FIG. 5 is a front view of the same. Reference numeral 4 is a transmitter, and 5 is a receiver. Piezoelectric elements 1 and 8 are closely attached to the upper surfaces of wedges 2 and 9 made of synthetic resin such as acrylic resin, and a backing material 12 made of thin synthetic resin or the like is attached to the back surface thereof. The wedge 9 sandwiching the piezoelectric elements 1 and 8 and the entire backing material 12 are embedded in the backing material 3 made of synthetic resin or the like to form a substantially rectangular parallelepiped transmitter 4 and receiver 5. As shown in FIG. 1, omitting the backing material 12, the wedge 2 and the piezoelectric element 1 are directly connected to the backing member 3.
It may be buried in. The two piezoelectric elements 1 and 8 are inclined inward with respect to each other, and their center lines I are V-shaped.

The inclination θ of the piezoelectric element 1 of the transmitter 4 and the piezoelectric element 8 of the receiver 5 with respect to the bottom surface 7, that is, the angle of the wedges 2 and 9 is the critical angle of the transverse wave in the case of the stress measuring sensor for steel. It is preferable that the angles are approximately equal to 25.3 °. This angle θ
Varies depending on the material of the test body 6, and the sensor for measuring acoustic elastic stress by surface SH waves of the present invention can be used not only for stress measurement of steel but also for stress measurement of metal structures other than steel. Therefore, the angle θ naturally changes depending on the material of the object to be measured.

The side faces of the transmitter 4 and the receiver 5 are respectively joined to both side faces of a connecting plate 10 made of a stainless steel plate or the like having a concave cross section. Further, as shown in FIG. 6, it is preferable that the upper surfaces of the transmitter 4 and the receiver 5 be joined to the rectangular parallelepiped pressing member 11 via cushioning materials 13 such as urethane foam resin. As a result, the bottom surfaces 7 of the transmitter 4 and the receiver 5 can be evenly brought into close contact with the surface of the test body 6 simply by pressing the pressing member 11.

The rigidity of the connecting plate 10 is such that when the sensor is pressed against the surface of the test body 6 which is slightly deviated from the perfect plane and a force of about 1 kgf is applied to the upper surface of the pressing member 11, the transmitter 4 is moved.
Alternatively, 1 so that the bottom surface 7 of the receiver 5 is along the surface of the test body 6.
It is preferable that the rigidity is such that it can be biased by a few μ to be in close contact therewith. If the rigidity is low, the bottom surface 7 of the transmitter 4 and the receiver 5 can be easily brought into close contact with the surface of the test body 6 due to a small pressing force, but the deformation at that time is large and the measurement accuracy is rather deteriorated. On the other hand, if the rigidity is too high, the bottom surface 7 cannot be brought into close contact with the surface of the test body 6 due to a small pressing force, and the accuracy of stress measurement deteriorates.

The bottom surfaces 7 of the transmitter 4 and the receiver 5 are on the same plane, and the lower surface 14 of the connecting plate 10 having a concave cross section is parallel to the surface including the bottom surfaces 7 of the transmitter 4 and the receiver 5. It is desirable to be located within 0.2 mm above the bottom surface 7. When the sensor is pressed against the surface of the test body 6 which is not completely flat, the bottom surfaces 7 of the transmitter 4 and the receiver 5 of the sensor are slightly deviated from the same plane and closely contact with the surface of the test body 6.
At this time, the transmitter 4 and the receiver 5 are connected to each other by a connecting plate 1 having a concave cross section.
Since it slightly rotates with the bent portions at both ends of the lower surface of 0 as fulcrums, when the lower surface 14 of the connecting plate 10 is separated above the bottom surfaces 7 of the transmitter 4 and the receiver 5, the rotation always causes The distance between the transmitter 4 and the receiver 5, which must be constant, fluctuates, and the error in the measurement value of the ultrasonic wave propagation time increases by the amount corresponding to the fluctuation in the distance, and accurate stress measurement is performed. Can not be. In order to measure the stress of the steel material with a measurement accuracy of about 10 MPa, it is necessary to measure the propagation time of ultrasonic waves with an accuracy of 10 ^{−4 or} less. For that purpose, the lower surface 14 of the connecting plate 10 has the transmitter 4 and the receiver 4. The bottom surface 7 of the child 5
It must be located within 0.2mm above. (Fig. 5
Also, the distance between the lower surface 14 of the connecting plate 10 and the bottom surface 7 of the transmitter 4 and the receiver 5 in FIG. 6 is more than the distance in the above preferable range in order to clearly show the vertical positional relationship between the lower surface 14 and the bottom surface 7. It is drawn much larger. )

The shape and size of the connecting plate 10 of the present invention is not limited to those shown in FIGS. 5 and 6, and the rigidity against the stress of bending and twisting which occurs when the sensor is pressed against the surface of the test body 6. Is within the range suitable for achieving the above-mentioned object, and is not limited to the above-mentioned shape, size and the like.

The distance 1 at the intersection between the transmitter 4 and the bottom surface 7 of the center line I of the piezoelectric elements 1 and 8 of the receiver 5 is not particularly limited, but is selected to be 30 mm, for example. The width d of the piezoelectric elements 1 and 8 is not particularly limited, but is preferably about 10 mm for improving the measurement accuracy.

[0023]

In order to measure the stress of a structure with the acousto-elastic stress measurement sensor using surface SH waves of the present invention, the measurement site of the test body 6 is ground into a substantially smooth flat surface, and a viscous liquid is then applied to it. Is applied thinly, the bottom faces 7 of the transmitter 4 and the receiver 5 of the sensor are brought into close contact with each other, and the transmitter 4 and the receiver 5 are approximately evenly placed on the surface of the test body 6 from above the pressing member 11.
Press with moderate pressing force. Connection plate 1 having appropriate elasticity
With a slight bending deformation of 0, the bottom surfaces 7 of the transmitter 4 and the receiver 5 are deformed along the surface of the test body 6, and the bottom surface 7 comes into close contact with the surface of the test body 6.

When an ultrasonic pulse composed of a transverse wave (SH wave) is emitted from the transmitter 4, the ultrasonic wave radiated in the test body 6 becomes a surface SH wave and propagates along the surface of the test body 6,
It is caught by the receiver 5 and detected and output as an electric signal from the piezoelectric element 8. The surface S of this ultrasonic pulse
By measuring the H-wave propagation time, the stress in the vicinity of the surface of the test body 6 can be measured.

[0025]

According to the sensor for measuring acoustic elastic stress by surface SH wave of the present invention, even if the surface of the test body 6 is relatively rough,
The sensor can be brought into close contact with the surface of the test body 6 with a relatively weak pressing force without changing the distance between the transmitter 4 and the receiver 5, and the accuracy of stress measurement by surface SH waves can be improved.

[Brief description of drawings]

FIG. 1 is a plan view of a conventional sensor for measuring acoustic elastic stress by surface SH waves.

FIG. 2 is a front view of a conventional sensor for measuring acoustic elastic stress by surface SH wave.

FIG. 3 is a front sectional view showing the propagation of surface SH waves of ultrasonic waves emitted from a transmitter.

FIG. 4 is a plan view of a sensor for measuring acoustic elastic stress by surface SH wave of the present invention.

FIG. 5 is a front view of a sensor for measuring acoustic elastic stress by surface SH waves of the present invention.

FIG. 6 is a front view of another embodiment of the sensor for measuring acoustic elastic stress by surface SH wave of the present invention.

[Explanation of symbols]

 1, 8 Piezoelectric element 2, 9 Wedge 3 Back member 4 Transmitter 5 Receiver 6 Test body 7 Bottom surface 10 Connecting plate 11 Pressing member 12 Back material 13 Elastic cushion material 14 Bottom surface

Claims (4)

(57) [Claims] Claim: What is claimed is: 1. A pair of piezoelectric elements, each of which is bonded to a wedge of synthetic resin or the like, and a rear surface member of which is bonded to a rear surface of the piezoelectric element. In a sensor for measuring acoustic elastic stress by surface SH waves in which elements are arranged so as to face each other obliquely inward, the transmitter and the receiver are connected by a connecting plate having an appropriate elasticity, and the transmitter and When the bottom surface of the wedge of the receiver is pressed against the surface of the specimen, the bottom surface adheres along the surface of the specimen without changing the distance between the transmitter and the receiver due to a slight deformation of the connecting plate. A sensor for measuring acousto-elastic stress by surface SH waves, characterized in that 2. A sensor for acousto-elastic stress measurement by surface SH waves according to claim 1, wherein the connecting plate has a concave cross section and the transmitter and the receiver are joined to both side surfaces thereof. 3. The lower surface of the connecting plate having a concave cross section is parallel to a plane including the bottom surfaces of the transmitter and the receiver and is less than 0.
The sensor for measuring acousto-elastic stress by surface SH waves according to claim 2, which is located above 2 mm. 4. The sensor for measuring acoustic elastic stress by surface SH waves according to claim 1, wherein the upper surfaces of the transmitter and the receiver are joined to the same pressing member via an elastic cushion material.