JP6793086B2 - Residual stress measurement method - Google Patents

Description

The present invention relates to a method for measuring the residual stress of an object to be inspected.

Conventionally, as a method for measuring the residual stress of an inspected object in a non-destructive manner, an X-ray diffraction method and a sound elasticity method are known as seen in Patent Document 1 and the like. The X-ray diffraction method is a method of evaluating the state of residual stress of the subject to be inspected based on the result of diffraction of X-rays applied to the subject to be inspected. The sound elasticity method is a method of evaluating the state of residual stress of the subject to be inspected based on the time until the ultrasonic waves (surface SH waves or the like) incident on the subject to be inspected are received.

Japanese Unexamined Patent Publication No. 2007-178157

The X-ray diffraction method can measure the residual stress value (absolute value) of the object to be inspected, but can only evaluate the polar surface layer (range of several μm from the surface) of the object to be inspected, and X-ray diffraction. It takes a relatively large amount of time to set the equipment used in the method and measure the residual stress. On the other hand, the sound elasticity method has a shorter measurement time than the X-ray diffraction method, and although it is possible to evaluate a deep range from the surface of the object to be inspected, in order to obtain the absolute value of the residual stress from the measurement result, , It is necessary to measure each parameter (sound velocity and sound elasticity constant) due to the material of the object to be inspected in advance. For each parameter (sound velocity and sound elastic constant), the initial sound velocity is generally measured by a sample before stress is applied under the same conditions as the object to be inspected, and the change in sound velocity according to the application of stress is obtained. Obtained by measuring.

An object of the present invention is to be relatively short without prior measurement of each parameter (sound velocity and sound elastic constant) of the inspected object required to calculate the absolute value of the residual stress of the inspected object by the sound elastic method. It is an object of the present invention to provide a residual stress measuring method capable of measuring the absolute value of the residual stress of an inspected object in time.

As a means for solving the above-mentioned problems, the present invention is a method for measuring the residual stress of an inspected object, wherein the inspected object or an inspected object made of the same material as the inspected object is subjected to an X-ray diffraction method. A conversion stress value that is a value of the stress of the specific part and has a correlation with the residual stress value by the residual stress value measuring step of measuring the residual stress value which is the value of the residual stress of a specific part and the sound elasticity method. The converted stress value is determined based on the converted stress value measuring step for measuring the above, the residual stress value measured in the residual stress value measuring step, and the converted stress value measured in the converted stress value measuring step. A calibration coefficient calculation step for calculating a calibration coefficient to be converted into a stress value, and each converted stress value obtained by measuring a plurality of parts of the inspected object by the sound elasticity method are retained based on the calibration coefficient. Provided is a method for measuring residual stress, which includes a calibration step of calibrating to a stress value.

In this residual stress measuring method, a specific part of the test object or the test object is measured by two methods, an X-ray diffraction method and a sound elastic method, and the residual stress value of the part and the converted stress correlating with the residual stress value are measured. Since the value is obtained, the calibration coefficient for converting the converted stress value into the residual stress value is calculated based on the residual stress value and the converted stress value. Therefore, by measuring a plurality of parts of the object to be inspected by the sound elasticity method that can be measured in a shorter time than the X-ray diffraction method, each converted stress value obtained by the measurement remains by using the calibration coefficient. It is possible to approximate the stress value. Therefore, in this residual stress measurement method, each parameter (sound velocity and sound elasticity constant) of the test object required to calculate the absolute value of the residual stress of the test object by the sound elasticity method is not measured in advance. Obtain the residual stress value of a plurality of parts of an inspected object in a shorter time than when measuring a plurality of parts of an inspected object only by the X-ray diffractometry and with the same accuracy as the measurement by the X-ray diffractometry. be able to.

In this case, in the residual stress value measuring step and the converted stress value measuring step, it is preferable that the specific portion of the inspected object is measured.

In this way, the calibration coefficient is obtained based on the measurement result of the same object to be inspected as the object to be measured in the calibration step, so that the accuracy of the calibration coefficient is improved. Therefore, the accuracy of calibration in the calibration process is improved.

Further, in the step of measuring the residual stress value, the X-ray is used as the residual stress value for a plurality of parts in the range of the depth range in which the ultrasonic waves used in the sound elasticity method propagate from the surface of the specific part. It is preferable to measure the average value of each residual stress value measured by the diffraction method.

In this way, the residual stress value used in the calibration coefficient calculation step becomes a value corresponding to the propagation depth of the ultrasonic waves used in the conversion stress value measurement step, so that the surface layer of the specific portion in the calibration coefficient calculation step The accuracy of the calibration coefficient is higher than that in the case where the calibration coefficient is calculated based on the residual stress value (the part shallower than the depth at which the ultrasonic waves propagate in the sound elasticity method). Therefore, the accuracy of calibration in the calibration process is improved. This method is particularly effective when there is a gradient in the residual stress value in the range of the depth at which the ultrasonic waves used in the sound elasticity method propagate from the surface of the object to be inspected.

Further, in the converted stress value measuring step, a plurality of values are measured by the sound elasticity method for different depths of the specific portion by using a plurality of ultrasonic waves having different frequencies as the converted stress values. And, based on each measured value, a stress value function indicating the relationship between the depth of the specific portion and the converted stress value is obtained, and in the calibration coefficient calculation step, the residual stress value and the stress value are performed. It is preferable to calculate the calibration coefficient based on the converted stress value when the depth is zero in the function.

In this embodiment, in the calibration coefficient calculation step, the calibration coefficient is calculated based on the converted stress value and the residual stress value when the depth is zero in the stress value function, so that the specific portion is subjected to the sound elasticity method. Compared to the case where the calibration coefficient is calculated based on the converted stress value measured at a single frequency and the residual stress value, that is, the case where the calibration coefficient is calculated based on each stress value at different depths. , The accuracy of the calibration coefficient is improved. Therefore, the accuracy of calibration in the calibration process is improved.

As described above, according to the present invention, each parameter (sound velocity and sound elasticity constant) of the test object required to calculate the absolute value of the residual stress of the test object by the sound elasticity method is measured in advance. It is possible to provide a residual stress measuring method capable of measuring the absolute value of the residual stress of the object to be inspected in a relatively short time.

It is a schematic diagram which shows the residual stress value measurement process and the conversion stress value measurement process of the residual stress value measurement method of 1st Embodiment of this invention. It is a schematic diagram which shows the relationship between the body to be inspected which is the object of measurement in the calibration process of the said residual stress measurement method, and the measurement point. It is a figure which shows the outline of the residual stress value measuring process of the residual stress measuring method of 2nd Embodiment of this invention. It is a figure which shows the outline of the conversion stress value measurement process of the residual stress measurement method of 3rd Embodiment of this invention. It is a graph which shows the relationship between a measurement depth and a stress value.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

(First Embodiment)
The method for measuring residual stress according to the first embodiment of the present invention will be described with reference to FIGS. 1 and 2. This residual stress measuring method includes a residual stress value measuring step, a converted stress value measuring step, a calibration coefficient calculation step, and a calibration step.

In the residual stress value measuring step, as shown in FIG. 1, the residual stress value σ, which is the residual stress value (absolute value) of a specific portion of the inspected object 1, is measured by the X-ray diffraction method.

In the converted stress value measuring step, as shown in FIG. 1, the converted stress value σ'of the specific part of the object 1 to be inspected (the same part as the part measured in the residual stress value measuring step) is obtained by the sound elasticity method. Be measured. The converted stress value σ'is a stress value having a correlation with the residual stress value σ. Specifically, the residual stress value σ is represented by the product of the converted stress value σ ′ and the calibration coefficient k described later. When the measurement method of the present embodiment is not applied and only the normal sound elasticity method is used for measurement, the converted stress value σ'is the state before stress is applied to the inspected object 1 (non-stress state). ), And the sound elasticity constant (rate of change in sound velocity according to the applied stress) caused by the material of the object 1 to be inspected, can be converted into the residual stress value σ. In the converted stress value measuring step of the present embodiment, the converted stress value is converted by a surface wave (surface SH wave or Rayleigh wave) propagating on the surface of the object 1 to be inspected or a region in the vicinity thereof (hereinafter, referred to as “surface region”). σ'is measured. Specifically, this converted stress value σ'is calculated based on the time until the ultrasonic wave transmitted from the transmitting unit 10 propagates as a surface wave to the receiving unit 20. The surface region means a region corresponding to a depth of about 1 to 2 wavelengths of a surface wave from the surface of the object 1 to be inspected.

In the calibration coefficient calculation step, the calibration coefficient k is calculated based on the residual stress value σ measured in the residual stress value measuring step and the converted stress value σ ′ measured in the converted stress value measuring step. The calibration coefficient k is a coefficient for converting the converted stress value σ ′ into the residual stress value σ. For example, σ = k × σ ′ based on the residual stress value σ measured by the X-ray diffraction method at the measurement point A in FIG. 2 and the converted stress value σ ′ measured by the sound elasticity method at the same measurement point A. The relational expression of is obtained.

In the calibration step, as shown in FIG. 2, a plurality of (five in FIG. 2) parts A to E of the object 1 to be inspected are measured by the sound elasticity method, and each converted stress value obtained by the measurement is measured. σ ′ is calibrated to the residual stress value σ based on the calibration coefficient k. As a result, the residual stress values σ at the plurality of parts A to E of the inspected body 1 can be obtained.

As described above, in the residual stress measuring method of the present embodiment, the residual stress value of the specific portion of the object 1 to be inspected is measured by two methods, the X-ray diffraction method and the sound elasticity method. Since σ and the converted stress value σ ′ that correlates with it are obtained, the calibration coefficient k that converts the converted stress value σ ′ into the residual stress value σ is calculated based on these residual stress value σ and the converted stress value σ ′. .. Therefore, by measuring a plurality of parts A to E of the inspected object 1 by the sound elasticity method that can be measured in a shorter time than the X-ray diffraction method, each converted stress value σ'obtained by the measurement is a calibration coefficient. By using k, it becomes possible to approximate the residual stress value σ. Therefore, in this residual stress measurement method, the inspected object 1 is inspected only by the X-ray diffraction method without performing the preliminary measurement of the inspected object 1 necessary for calculating the absolute value of the residual stress of the inspected object 1 by the sound elasticity method. The residual stress value σ of the plurality of parts A to E of the inspected body 1 can be measured in a shorter time than when measuring the plurality of parts A to E of the body 1 and with the same accuracy as the measurement by the X-ray diffraction method. Can be sought.

In the residual stress value measuring step, the average value of the residual stress value σ of a plurality of parts (for example, measurement point A and measurement point B in FIG. 2) of the object 1 to be inspected is calculated, and in the converted stress value measuring step, The average value of the converted stress values σ'of the plurality of parts of the inspected object 1 is calculated, and in the calibration coefficient calculation step, based on the average value of the residual stress value σ and the average value of the converted stress value σ'. The calibration coefficient k may be calculated.

Further, the calibration coefficient k is calculated by measuring the test object made of the same material as the test object 1 and in the same stress state by the X-ray diffraction method and the sound elasticity method, and the calibration coefficient k is used in the calibration process. May be done. That is, in the residual stress value measuring step, the specific residual stress value σ of the test piece is calculated, and in the converted stress value measuring step, the specific part of the test piece (the part measured in the residual stress value measuring step). The converted stress value σ'of the same part) is calculated, and the calibration coefficient k is calculated based on the residual stress value σ and the converted stress value σ'in the calibration coefficient calculation step. A plurality of parts A to E may be measured by the sound elasticity method, and each converted stress value σ ′ obtained by the measurement may be calibrated to a residual stress value σ based on the calibration coefficient k. This also applies to the following second and third embodiments.

(Second Embodiment)
Next, the method for measuring the residual stress according to the second embodiment of the present invention will be described. In the second embodiment, only the parts different from the first embodiment will be described, and the description of the same steps, actions and effects as in the first embodiment will be omitted.

As shown in FIG. 3, in the present embodiment, in the residual stress value measuring step, as the residual stress value σ, ultrasonic waves (surface) used by the sound elastic method from the surface of the specific portion of the inspected object 1 from the surface of the portion. The average value σave of each residual stress value σ measured by the X-ray diffractometry is measured for a plurality of parts in the range of the propagation depth (in this embodiment, the depth of one wavelength of the surface wave).

Specifically, as shown in FIG. 3, first, the residual stress value σ on the surface of the object 1 to be inspected is measured by the X-ray diffraction method. After that, the object 1 to be inspected is polished by electrolytic polishing for a first depth of d1 from the surface of the object 1 to be inspected. Then, the residual stress value σ of the new surface is measured again by the X-ray diffraction method. By repeating this operation multiple times, the average value σave of each residual stress value σ measured by the X-ray diffraction method for a plurality of parts in the range of the depth at which ultrasonic waves (surface waves) used in the sound elasticity method propagate. Is calculated.

Then, in the calibration coefficient calculation step, the calibration coefficient k is calculated based on the average value σave and the converted stress value σ ′ measured by the sound elasticity method in the converted stress value measuring step.

As described above, in the present embodiment, the average value σave is used in the calibration coefficient calculation step. That is, the residual stress value used in the calibration coefficient calculation step is a value corresponding to the propagation depth of the ultrasonic wave used in the converted stress value measurement step. Therefore, compared to the case where the calibration coefficient k is calculated based on the residual stress value σ of the surface layer of the specific part (the part shallower than the depth at which the ultrasonic waves propagate in the sound elasticity method) in the calibration coefficient calculation step. , The accuracy of the calibration coefficient k is improved. Therefore, the accuracy of calibration in the calibration process is improved. The measuring method of the present embodiment is particularly effective when there is a gradient in the residual stress value in the range of the depth at which the ultrasonic waves used in the sound elasticity method propagate from the surface of the object 1 to be inspected.

(Third Embodiment)
Next, the method for measuring the residual stress according to the third embodiment of the present invention will be described. Also in the third embodiment, only the parts different from those in the first embodiment will be described, and the description of the same steps, actions and effects as in the first embodiment will be omitted.

As shown in FIG. 4, in the present embodiment, in the converted stress value measuring step, the specific parts are different by using a plurality of ultrasonic waves (surface waves) having different frequencies as the converted stress value σ'. A stress value function F (FIG. 5) showing the relationship between the depth of the specific part and the converted stress value σ'by measuring a plurality of converted stress values σ'for the depth by the sound elasticity method and based on each measured value. See) and to do. The lower the frequency, the deeper the surface wave propagates from the surface of the inspected object 1. Therefore, by measuring the specific part by the surface waves of a plurality of different frequencies, the vicinity of the surface layer of the inspected object 1 It is possible to grasp the stress gradient in (the stress value function F is obtained). Specifically, the transmission unit 10 transmits a surface wave of the first frequency to the specific portion, and the reception unit 20 receives the surface wave. The converted stress value σ'1 is measured based on this propagation time. Next, a surface wave having a frequency lower or higher than the first frequency is transmitted from the transmitting unit 10, and this is received by the receiving unit 20. The converted stress value σ'2 is measured based on this propagation time. Then, the stress value function F is calculated based on each converted stress value σ'.

Subsequently, in the calibration coefficient calculation step, the converted stress value σ ′ when the depth is zero in the stress value function F and the residual stress value σ measured by the X-ray diffraction method in the residual stress value measurement step are used. Calculate the calibration coefficient k.

Here, in the X-ray diffraction method, the residual stress value of the portion of the polar surface layer of the object 1 to be inspected (the portion whose depth can be evaluated as substantially zero) is measured. Therefore, in the stress value function F, the calibration coefficient k is calculated based on the converted stress value σ ′ when the depth is zero and the residual stress value σ, so that the specific portion is single by the sound elasticity method. When the calibration coefficient k is calculated based on the converted stress value σ ′ measured at the frequency of, and the residual stress value σ, that is, the calibration coefficient k is calculated based on the stress values at different depths. Compared with the case, the accuracy of the calibration coefficient k is improved. Therefore, the accuracy of calibration in the calibration process is improved.

In the converted stress value measuring step, the stress value function F may be obtained based on a plurality of converted stress values σ'measured by three or more surface waves having different frequencies. By doing so, the accuracy of the calibration coefficient k is further improved.

Next, the residual stress value measuring step of the second embodiment will be described with reference to FIG. In this embodiment, a steel material was used as the object 1 to be inspected, and an ultrasonic wave having a frequency of 5 MHz was used in the conversion stress value measuring step. The propagation depth of the ultrasonic wave (surface wave) used in the sound elasticity method of the inspected object 1 is in the range of about one wavelength from the surface of the inspected object 1. In this embodiment, since the steel material is used as the inspected object 1, the propagation range of the surface wave is in the range of about 0.6 mm from the surface of the inspected object 1.

In the residual stress value measuring step of this example, first, the residual stress value σ of the surface of the specific portion of the inspected object 1 was measured by the X-ray diffraction method. The result of this measurement was −300 MPa. Since compressive stress was applied to this part, the residual stress value became a negative value.

Next, the object 1 to be inspected was polished by electrolytic polishing to a depth d1 (150 μm in this example) from the surface of the object 1 to be inspected. Then, the residual stress value σ of the new surface was measured. The result of this measurement was -250 MPa. Similarly, the residual stress value at the site of the second depth d2 (300 μm in this example) from the surface of the inspected object 1 is −200 MPa, and the third depth d3 from the surface of the inspected object 1 (this implementation). In the example, the residual stress value at the site of 450 μm) was −150 MPa, and the residual stress value at the site at the fourth depth d4 (600 μm in this example) from the surface of the inspected object 1 was −100 MPa. From this, the average value σave of the residual stress value at the specific portion of the inspected object 1 was −200 MPa. The position and number of measurement points are not limited to the above example.

From the above, the average value σave of the range of the depth at which the surface wave propagates is obtained as the residual stress value σ of the specific part of the object 1 to be inspected, and based on the residual stress value σ of the surface of the specific part. It was confirmed that the accuracy of the calibration coefficient k is higher than that when the calibration coefficient k is calculated.

Subsequently, the converted stress value measuring step of the third embodiment will be described with reference to FIGS. 4 and 5. Also in this example, a steel material was used as the inspected body 1.

In the converted stress value measuring step of this embodiment, the converted stress value σ'of the specific part was measured by ultrasonic waves (surface waves) having a frequency of 5 MHz and ultrasonic waves (surface waves) having a frequency of 2 MHz. .. The measured value of the converted stress value σ'by an ultrasonic wave (surface wave) having a frequency of 5 MHz was a value at a position where the depth from the surface of the test object 1 was about 0.6 mm, and was −500 MPa. The measured value of the converted stress value σ'by an ultrasonic wave (surface wave) having a frequency of 2 MHz was a value at a position where the depth from the surface of the test object 1 was about 1.5 mm, and was −400 MPa. From this, the stress value function F was obtained.

Next, in the calibration coefficient calculation step, the converted stress value σ'(about −567 MPa in this example) when the depth is zero in the stress value function F and the residual stress value measured in the residual stress value measurement step. The calibration coefficient k was calculated based on σ (-700 MPa in this example).

From the above, the calibration coefficient k is calculated based on the converted stress value σ ′ and the residual stress value σ when the depth is zero in the stress value function F, so that the specific portion is single by the sound elasticity method. The accuracy of the calibration coefficient k is higher than that when the calibration coefficient k is calculated based on the converted stress value σ'(-400 MPa, -500 MPa, etc.) and the residual stress value σ (-700 MPa) measured at the frequency of. It was confirmed that it would increase.

It should be noted that the embodiments disclosed this time are exemplary in all respects and are not considered to be restrictive. The scope of the present invention is shown by the scope of claims rather than the description of the above-described embodiment, and further includes all modifications within the meaning and scope equivalent to the scope of claims.

For example, in the converted stress value measuring step, the propagation time ratio having a correlation with the residual stress value σ measured by the X-ray diffraction method may be measured as the converted stress value σ'. The propagation time ratio means the ratio of the time for the surface SH wave to propagate from the transmitting unit 10 to the receiving unit 20 to the time for the Rayleigh wave to propagate from the transmitting unit 10 to the receiving unit 20.

1 Inspected object 10 Transmitter 20 Receiver

Claims (3)

It is a method of measuring the residual stress of the object to be inspected.
A residual stress value measuring step of measuring a residual stress value which is a residual stress value of a specific part of the test object or a test body made of the same material as the test object by an X-ray diffraction method.
A conversion stress value measurement step of measuring a conversion stress value which is a stress value of the specific part and has a correlation with the residual stress value by a sound elasticity method.
A calibration coefficient for converting the converted stress value into the residual stress value is calculated based on the residual stress value measured in the residual stress value measuring step and the converted stress value measured in the converted stress value measuring step. Calibration coefficient calculation process and
It includes a calibration step of calibrating each converted stress value obtained by measuring a plurality of parts of the object to be inspected by the sound elasticity method to a residual stress value based on the calibration coefficient.
In the step of measuring the residual stress value, as the residual stress value, the X-ray diffraction method is performed on a plurality of parts in a depth range in which the ultrasonic waves used in the sound elasticity method propagate from the surface of the specific part. A method for measuring residual stress, which measures the average value of each residual stress value measured in. It is a method of measuring the residual stress of the object to be inspected .
A residual stress value measuring step of measuring a residual stress value which is a residual stress value of a specific part of the test object or a test body made of the same material as the test object by an X-ray diffraction method .
A conversion stress value measurement step of measuring a conversion stress value which is a stress value of the specific part and has a correlation with the residual stress value by a sound elasticity method .
A calibration coefficient for converting the converted stress value into the residual stress value is calculated based on the residual stress value measured in the residual stress value measuring step and the converted stress value measured in the converted stress value measuring step. Calibration coefficient calculation process and
It includes a calibration step of calibrating each converted stress value obtained by measuring a plurality of parts of the object to be inspected by the sound elasticity method to a residual stress value based on the calibration coefficient .
In the converted stress value measuring step, a plurality of values are measured by the sound elasticity method for different depths of the specific portion by using a plurality of ultrasonic waves having different frequencies as the converted stress values. Based on each measured value, the stress value function indicating the relationship between the depth of the specific part and the converted stress value is obtained, and
In the calibration coefficient calculation step, a residual stress measuring method for calculating the calibration coefficient based on the residual stress value and the converted stress value when the depth is zero in the stress value function . In the residual stress measuring method according to claim 1 or 2.
Wherein in the residual stress measuring step and the conversion stress value measuring step, the Ru is measured the specific site of the test subject, the residual stress measuring method.

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