KR20230057571A - Residual stress measurement method using ultrasonic sensor - Google Patents

Abstract

According to the present invention of the present application, in a method of measuring a residual stress of a specimen by measuring an ultrasonic time difference using an existing LCR method, regarding an initial density, a longitudinal wave velocity propagating parallel to a load could be calculated only by knowing a secondary elastic constant, a tertiary elastic constant, a Poisson's ratio, and a dimensionless acoustoelastic constant for an LCR wave measured by an uniaxial tensile test. The present application solves these problems by providing an invention that can measure residual stress using only LCR waves, thereby capable of providing a method by which residual stress can be measured using simple calculations. By measuring the velocity of the LCR wave that is concentrated near the surface of the specimen and moving along the surface at high velocity by the above configuration, the velocity is compared with the velocity when the stress is 0 so that the magnitudes of tensile stress, compressive stress, and stress can be measured by velocity changes.

Description

Residual stress measurement method using ultrasonic sensor{.}

The present invention relates to a method for measuring residual stress in a metal material. More specifically, it relates to a method for measuring the residual stress of a metal material using ultrasonic waves.

As a prior art prior to the invention of the present application, a technology related to a residual stress measuring system has been disclosed. In this technology, a portable flaw detector having an ultrasonic probe that directly contacts the test piece and measures the time value of ultrasonic waves transmitted/reflected by the test piece; an ultrasonic amplifier that transmits and amplifies the time value of the ultrasonic probe; A terminal for calculating and displaying the residual stress value of the test piece by calculating the time value received from the ultrasonic amplifier through a microprocessor having a residual stress calculation formula, and the flaw detector has a lower surface in contact with the test piece, A lower horizontal bar in which an ultrasonic transducer is detachably provided, an upper horizontal bar spaced apart from the upper portion of the lower horizontal bar and serving as a handle, and a vertical support bar vertically connecting and supporting one end of the upper horizontal bar and the lower horizontal bar are integrally formed. A jig body made of; an ultrasonic transducer provided detachably to the lower horizontal bar to output/receive ultrasonic waves to the test piece to measure a time value; Disclosed is a technology including a connector that is directly connected to the ultrasonic amplifier and wired to transmit the time value of the ultrasonic amplifier to the terminal.

In another prior art, ultrasonic waves are oscillated from the oscillator 9 toward the inside of the material to be measured 2 through the waveguide 1 brought into close contact with the surface of the material to be measured 2, and the reflected wave from the material to be measured 2 is transmitted through the receiver 10. After reception, by sliding the oscillation position of the ultrasonic wave little by little and measuring the direction in which the received reflected wave takes the maximum value, the direction of the maximum principal stress σ1 of the material to be measured 2 is specified, and the direction of the principal stress σ2 orthogonal to this is determined. In addition, with respect to each direction of the direction of the maximum principal stress σ1 and the direction of the principal stress σ2 orthogonal to this direction, the oscillation angle is changed at the same position as the sound speeds V1 and V2 at the oscillation angle θ of the primary input reflected wave of the ultrasonic wave, and birefringence Residual stress (σ1-σ2) is calculated from the sound velocity difference (V1-V2) and (V1'-V2') by measuring the sound velocities V1' and V2' at the oscillation angle θ' of the secondary reflected wave based on the phenomenon. A stress measurement method is disclosed.

Registered Patent Publication KR 1955440 B1 Japanese Laid-Open Patent Publication 1995-229798

는 초기 밀도,

은 하중에 평행하게 전파하는 종파속도,

와

는 2차 탄성 상수,

과

은 3차 탄성 상수,

는 포아송 비,

은 단축 방향 인장시험으로 측정하는 L_CR 파에 대한 무차원 음향탄성상수를 모두 알아야 계산할 수 있었다. 본 출원 발명은 이러한 문제점을 L_CR 파만을 이용하여 잔류응력을 측정할 수 있는 식(2)를 포함하는 발명을 제공함으로써 간단한 계산만으로 잔류응력이 측정 가능한 방법을 제공하고자 한다.In the present application, the method for measuring the residual stress of the specimen by measuring the ultrasonic time difference using the conventional L _CR method is

is the initial density,

is the longitudinal wave velocity propagating parallel to the load,

and

is the second order elastic constant,

class

is the third order elastic constant,

is Poisson's ratio,

could be calculated by knowing all the dimensionless acoustic elastic constants for the L _CR wave measured by the uniaxial tensile test. The present invention aims to provide a method for measuring residual stress with a simple calculation by providing an invention including Equation (2) capable of measuring residual stress using only L _CR waves to solve these problems.

In order to solve the above problems, the following problem solving means are provided.

In the method for measuring residual stress using an ultrasonic sensor,

Step 1: Preparing a stress-free specimen and

Step 2: preparing ultrasonic transducers and transducers; and

Step 3: Fixing the ultrasonic transducer and transducer prepared in step 2 to a wedge for generating L _CR waves, and

Step 4: The wedge angle of the third step is calculated using the longitudinal wave velocity of the specimen prepared in the first step and the longitudinal wave velocity of the wedge using Snell's law to calculate the wedge angle of the ultrasonic transducer and the transducer, and

를 측정하는 단계 및Step 5: Time until a signal is generated from the ultrasonic transducer and the transducer receives the signal using a wedge equipped with the ultrasonic transducer and the transducer in a state where the stress is zero in the specimen

Step of measuring and

를 측정하는 단계 및Step 6; Time until the ultrasonic transducer generates a signal and the transducer receives the signal according to the change in stress while increasing the stress on the specimen

Step of measuring and

Step 7: repeating the step of measuring the signal according to three or more stresses in step 6, and

Step 8: Obtaining a graph of measurement time (t) vs. stress (PMa) according to the stress measured in Step 6 using the relationship of Equation (2) below, and

Step 9: Provides a residual stress measuring method using an ultrasonic probe, characterized in that it comprises the step of obtaining a relational expression of measurement time (t) vs. stress (PMa) from the graph.

식 (2)

Equation (2)

here,

은 종파의 속도

speed of the silver sect

은 속도변화

is the speed change

는 응력변화

is the stress change

는 탄성계수

is the elastic modulus

은 단축 방향 인장시험으로 측정하는 L_CR 파에 대한 무차원 음향탄성상수(acoustic-elastic constant)

is the dimensionless acoustic-elastic constant for the L _CR wave measured by the uniaxial tensile test

잔류응력이 없을 때 초음파트랜스듀서와 설정거리만큼 이격된 상태에서 초음파 신호가 도달하는 시간

When there is no residual stress, the time required for the ultrasonic signal to arrive at a distance from the ultrasonic transducer by the set distance

잔료응력이 있는 상태에서 초음파트랜스듀서와 설정거리만큼 이격된 상태에서 초음파 신호가 도달하는 시간과

와의 차이

The time for the ultrasonic signal to arrive in the state of being separated by the set distance from the ultrasonic transducer in the presence of residual stress

difference with

In addition, in the residual stress measuring method using an ultrasonic sensor,

Step 1: Preparing a specimen of unknown initial stress and

Step 2: preparing ultrasonic transducers and transducers; and

Step 3: Fixing the ultrasonic transducer and transducer prepared in step 2 to a wedge for generating L _CR waves, and

Step 4: The wedge angle of the third step is calculated using the longitudinal wave velocity of the specimen prepared in the first step and the longitudinal wave velocity of the wedge using Snell's law to calculate the wedge angle of the ultrasonic transducer and the transducer, and

를 측정하는 단계 및Step 5: Time until a signal is generated by the ultrasonic transducer and the transducer receives the signal using a wedge equipped with the ultrasonic transducer and the transducer in a state in which no additional stress is applied to the specimen

Step of measuring and

를 측정하는 단계 및Step 6; Time until the ultrasonic transducer generates a signal and the transducer receives the signal according to the change in stress while increasing the stress by 100 MPa on the specimen

Step of measuring and

Step 7: While repeating the process of step 6 above, a graph of measurement time (t) vs. stress (PMa) according to stress was drawn using the relationship between the residual stress and the _LCR wave speed in Equation (3) below, and three points were drawn. A repeat measurement step of repeating the measurement until the above points form a straight line, and

을 상기 관계식에 대입하여 응력을 계산하는 단계를 포함하여 구성되는 것을 특징으로 하는 초음파탐촉자를 이용한 잔류응력측정방법을 제공한다.Step 8: If three or more points form a straight line in the graph above, the relationship between measurement time (t) and stress (PMa) is obtained, and

It provides a residual stress measuring method using an ultrasonic probe, characterized in that it comprises the step of calculating the stress by substituting into the above relational expression.

식 (3)

Equation (3)

In addition, the material of the wedge provides a residual stress measuring method using an ultrasonic probe, characterized in that PMMA (poly-methyl-meta-acrylate).

In addition, the ultrasonic transducer and the transducer provides a residual stress measuring method using an ultrasonic transducer, characterized in that 2.25MHz.

According to the present invention, even if the ultrasonic longitudinal wave incident on the sample is refracted at an angle greater than or equal to the first critical angle (the total reflection condition of the wavelength in Snail's law), it does not disappear completely and is concentrated near the surface, By measuring the speed of the _LCR wave traveling along the surface at speed, it is effective to measure the magnitude of tensile stress, compressive stress, and stress by comparing the speed when the stress is zero. If it is a specimen of the same material, it is effective to measure the stress accumulation by the user's use only by measuring the speed change according to the stress of the specimen in the laboratory and measuring the speed of the _LCR wave in the field.

Figure 1 shows the principle of measuring the speed of the L _CR wave of the present application.
Figure 2 is a photograph of the configuration of the experimental apparatus for measuring the relationship between the stress and the speed of the L _CR wave of the present application.
Figure 3 is a photograph of the configuration of the ultrasonic measurement transmitter and receiver for measuring the speed of the L _CR wave of the present application invention.
4 is a measurement result of measuring the speed of the L _CR wave with the experimental apparatus of the present application.
Figure 5 is the result of measuring the speed of the LCR wave according to the stress with the experimental device of the present application.

The present invention relates to a technique for measuring residual stress of a material using an ultrasonic sensor.

For this, L _CR (Longitudinal Critically Refracted) waves were used. As shown in FIG. 1, even when an ultrasonic longitudinal wave incident on a sample is refracted at an angle greater than or equal to the first critical angle, a wave that does not disappear completely and is concentrated near the surface and moves along the surface at high speed is referred to as L _CR wave.

The L _CR wave is not a surface wave, but a type of bulk wave that propagates directly under the surface of the material (sub-surface). There is little change depending on the state of the organization.

In the invention of the present application, the stress state inside the material was grasped using these L _CR waves.

The change in ultrasonic velocity according to the strain rate of the material is called relative sensitivity, and is expressed as in Equation (1) below.

(1)

(One)

in equation (1)

는 초기 밀도

is the initial density

은 하중에 평행하게 전파하는 종파속도

is the longitudinal wave velocity propagating parallel to the load

와

는 2차 탄성 상수

and

is the second elastic constant

과

은 3차 탄성 상수

class

is the tertiary elastic constant

는 포아송 비

is Poisson's ratio

은 단축 방향 인장시험으로 측정하는 L_CR 파에 대한 무차원 음향탄성상수(acoustic-elastic constant)

is the dimensionless acoustic-elastic constant for the L _CR wave measured by the uniaxial tensile test

Equation (1) may represent a stress change equation as shown in Equation (2) below in view of a change in ultrasonic transit time ( dt/t ₀ ).

(2)

(2)

in equation (2)

은 종파의 속도

speed of the silver sect

은 속도변화

is the speed change

는 응력변화

is the stress change

는 탄성계수

is the elastic modulus

은 단축 방향 인장시험으로 측정하는 L_CR 파에 대한 무차원 음향탄성상수(acoustic-elastic constant)

is the dimensionless acoustic-elastic constant for the L _CR wave measured by the uniaxial tensile test

잔류응력이 없을 때 초음파트랜스듀서와 설정거리만큼 이격된 상태에서 초음파 신호가 도달하는 시간

When there is no residual stress, the time required for the ultrasonic signal to arrive at a distance from the ultrasonic transducer by the set distance

잔료응력이 있는 상태에서 초음파트랜스듀서와 설정거리만큼 이격된 상태에서 초음파 신호가 도달하는 시간과

와의 차이

The time for the ultrasonic signal to arrive in the state of being separated by the set distance from the ultrasonic transducer in the presence of residual stress

difference with

은 응력과 속도뿐만 아니라 L_CR 파의 전달시간의 변화의 함수로 나타낼 수 있다.In FIG. 1, when the distance of the receiving ultrasonic transducer is fixed, the propagation time of the _LCR wave decreases in compressive stress and increases in tensile stress. At this time, the acoustic elastic constant

can be expressed as a function of the change in the propagation time of the _LCR wave as well as the stress and velocity.

와

는 상수 이므로Re-arranging equation (2),

and

is a constant, so

(3)

(3)

In Equation (3), when the stress is 0, the change in the ultrasonic transit time ( dt/t ₀ ) is 0, and when the stress increases or decreases, the change in the ultrasonic transit time ( dt/t ₀ ) of the L _CR wave becomes linear. Since it changes, the residual stress of the sample can be measured using this. Equation (3) shows the relationship between the stress and the speed of the _LCR wave, and the residual stress of the specimen can be measured only by measuring the speed of the _LCR wave by applying implantation. Existing Barkhausen noise and the like could only measure metal specimens, but this measurement method has the effect of measuring residual stress of non-metals as well.

(Example 1)

과 응력변화

의 관계를 실험으로 증명하고자, 인장실험과 초음파 속도 측정 실험을 한 결과를 비교하였다.of equation (3)

and stress change

In order to prove the relationship between the results of the tensile test and the ultrasonic velocity measurement test, the results were compared.

As shown in FIG. 2, the tensile force test of the metal material was performed using UTM. The metal specimen was produced with a length of 150 mm, a width of 25 mm, and a thickness of 3 mm. To measure the L _CR wave, a 2.25 MHz ultrasonic transducer was used, the wedge angle was set to about 28.11 degrees, and the transducer was configured as shown in FIG. 3 at a distance of 29.3 mm from the ultrasonic transducer.

The wedging angle was calculated by applying Snell's law to the longitudinal wave velocity of the metal specimen and the longitudinal wave velocity of the material of the wedging.

The graph of FIG. 4 shows the experimental results measured when the tensile force is 0 with the experimental apparatus configured in this way.

( t ₀ ) of about 11.3 us was measured.

The results of the experiment while increasing the tensile force are shown in the graph of FIG. 5 . It can be seen that the propagation time of the _LCR wave increases as the tensile force increases. In Example 1, the frequency of ultrasonic waves was selected as 2.25 MHz in order to measure the metal specimen, but it is of course possible to measure by changing the frequency according to the type of material.

(Example 2)

Residual stress measurement method using ultrasound

Step 1: Preparing a stress-free specimen

Step 2: Preparing ultrasonic transducers and transducers

Step 3: Fixing the ultrasonic transducer and transducer prepared in step 2 to a wedge for generating L _CR waves

Step 4: The wedge angle of the third step is the longitudinal wave velocity of the specimen prepared in the first step and the longitudinal wave velocity of the wedge using Snell's law Calculating the wedge angle of the ultrasonic transducer and the transducer

를 측정하는 단계Step 5: Time until a signal is generated from the ultrasonic transducer and the transducer receives the signal using a wedge equipped with the ultrasonic transducer and the transducer in a state where the stress is zero in the specimen

steps to measure

를 측정하는 단계Step 6; Time until the ultrasonic transducer generates a signal and the transducer receives the signal according to the change in stress while increasing the stress on the specimen

steps to measure

Step 7: Repeat measurement step of measuring the signal according to three or more stresses in the process of step 6 above.

Step 8: Obtaining a graph of measurement time (t) vs. stress (PMa) according to the stress measured in step 6 using the relationship of Equation (2)

Step 9: Provides a residual stress measuring method using an ultrasonic probe, characterized in that it comprises the step of obtaining a relational expression of measurement time (t) vs. stress (PMa) from the graph.

(Example 3)

Residual stress measurement method using ultrasound

Step 1: Preparing a specimen of unknown initial stress

Step 2: Preparing ultrasonic transducers and transducers

Step 3: Fixing the ultrasonic transducer and transducer prepared in step 2 to a wedge for generating L _CR waves

Step 4: The wedge angle of the third step is the longitudinal wave velocity of the specimen prepared in the first step and the longitudinal wave velocity of the wedge using Snell's law Calculating the wedge angle of the ultrasonic transducer and the transducer

를 측정하는 단계Step 5: Time until a signal is generated by the ultrasonic transducer and the transducer receives the signal using a wedge equipped with the ultrasonic transducer and the transducer in a state in which no additional stress is applied to the specimen

steps to measure

를 측정하는 단계Step 6; Time until the ultrasonic transducer generates a signal and the transducer receives the signal according to the change in stress while increasing the stress by 100 MPa on the specimen

steps to measure

Step 7: While repeating the process of step 6 above, a graph of measurement time (t) vs. stress (PMa) according to stress was drawn using the relationship between the residual stress and the _LCR wave speed in Equation (3) below, and three points were drawn. Iterative measurement step in which measurements are repeated until the above points form a straight line

을 상기 관계식에 대입하여 응력을 계산하는 단계를 포함하여 구성되는 것을 특징으로 하는 초음파탐촉자를 이용한 잔류응력측정방법을 제공한다.Step 8: If three or more points form a straight line in the graph above, the relationship between measurement time (t) and stress (PMa) is obtained, and

It provides a residual stress measuring method using an ultrasonic probe, characterized in that it comprises the step of calculating the stress by substituting into the above relational expression.

(Example 4)

The material of the wedge uses poly-methyl-meta-acrylate (PMMA).

(Example 5)

The ultrasonic transducer and transducer use 2.25 MHz.

(Example 6)

The longitudinal wave velocity (Vs) of the specimen and the longitudinal wave velocity (Vw) of the wedge material are used to create an angle at which total reflection can occur at the interface between the specimen and the wedge using Snail's law of total reflection, thereby forming L just below the surface of the specimen. The wedge angle is calculated using the arcsin(Vw/Vs) equation so that _{the CR} wave can occur.

The configuration of the invention for exhibiting the above functional effects is as follows.

In the method for measuring residual stress using an ultrasonic sensor,

Step 1: Preparing a stress-free specimen and

Step 2: preparing ultrasonic transducers and transducers; and

Step 3: Fixing the ultrasonic transducer and transducer prepared in step 2 to a wedge for generating L _CR waves, and

Step 4: The wedge angle of the third step is calculated using the longitudinal wave velocity of the specimen prepared in the first step and the longitudinal wave velocity of the wedge using Snell's law to calculate the wedge angle of the ultrasonic transducer and the transducer, and

를 측정하는 단계 및Step 5: Time until a signal is generated from the ultrasonic transducer and the transducer receives the signal using a wedge equipped with the ultrasonic transducer and the transducer in a state where the stress is zero in the specimen

Step of measuring and

를 측정하는 단계 및Step 6; Time until the ultrasonic transducer generates a signal and the transducer receives the signal according to the change in stress while increasing the stress on the specimen

Step of measuring and

Step 7: repeating the step of measuring the signal according to three or more stresses in step 6, and

Step 8: Obtaining a graph of measurement time (t) vs. stress (PMa) according to the stress measured in Step 6 using the relationship of Equation (2) below, and

Step 9: Provides a residual stress measuring method using an ultrasonic probe, characterized in that it comprises the step of obtaining a relational expression of measurement time (t) vs. stress (PMa) from the graph.

식 (2)

Equation (2)

here,

은 종파의 속도

speed of the silver sect

은 속도변화

is the speed change

는 응력변화

is the stress change

는 탄성계수

is the elastic modulus

은 단축 방향 인장시험으로 측정하는 L_CR 파에 대한 무차원 음향탄성상수(acoustic-elastic constant)

is the dimensionless acoustic-elastic constant for the L _CR wave measured by the uniaxial tensile test

잔류응력이 없을 때 초음파트랜스듀서와 설정거리만큼 이격된 상태에서 초음파 신호가 도달하는 시간

When there is no residual stress, the time required for the ultrasonic signal to arrive at a distance from the ultrasonic transducer by the set distance

잔료응력이 있는 상태에서 초음파트랜스듀서와 설정거리만큼 이격된 상태에서 초음파 신호가 도달하는 시간과

와의 차이

The time for the ultrasonic signal to arrive in the state of being separated by the set distance from the ultrasonic transducer in the presence of residual stress

difference with

In addition, in the residual stress measuring method using an ultrasonic sensor,

Step 1: Preparing a specimen of unknown initial stress and

Step 2: preparing ultrasonic transducers and transducers; and

Step 3: Fixing the ultrasonic transducer and transducer prepared in Step 2 to a wedge for generating LCR waves, and

Step 4: The wedge angle of the third step is calculated using the longitudinal wave velocity of the specimen prepared in the first step and the longitudinal wave velocity of the wedge using Snell's law to calculate the wedge angle of the ultrasonic transducer and the transducer, and

를 측정하는 단계 및Step 5: Time until a signal is generated by the ultrasonic transducer and the transducer receives the signal using a wedge equipped with the ultrasonic transducer and the transducer in a state in which no additional stress is applied to the specimen

Step of measuring and

를 측정하는 단계 및Step 6; Time until the ultrasonic transducer generates a signal and the transducer receives the signal according to the change in stress while increasing the stress by 100 MPa on the specimen

Step of measuring and

Step 7: While repeating the process of step 6 above, a graph of measurement time (t) vs. stress (PMa) according to stress was drawn using the relationship between the residual stress and the LCR wave speed in Equation (3) below, and three or more points were drawn. A repeat measurement step of repeating the measurement until the points form a straight line, and

을 상기 관계식에 대입하여 응력을 계산하는 단계를 포함하여 구성되는 것을 특징으로 하는 초음파탐촉자를 이용한 잔류응력측정방법을 제공한다.Step 8: If three or more points form a straight line in the graph above, the relationship between measurement time (t) and stress (PMa) is obtained, and

It provides a residual stress measuring method using an ultrasonic probe, characterized in that it comprises the step of calculating the stress by substituting into the above relational expression.

식 (3)

Equation (3)

In addition, the material of the wedge provides a residual stress measuring method using an ultrasonic probe, characterized in that PMMA (poly-methyl-meta-acrylate).

In addition, the ultrasonic transducer and the transducer provides a residual stress measuring method using an ultrasonic transducer, characterized in that 2.25MHz.

Claims (4)

In the method for measuring residual stress using an ultrasonic sensor,
Step 1: Preparing a stress-free specimen and
Step 2: preparing ultrasonic transducers and transducers; and
Step 3: Fixing the ultrasonic transducer and transducer prepared in Step 2 to a wedge for generating LCR waves, and
Step 4: The wedge angle of the third step is calculated using the longitudinal wave velocity of the specimen prepared in the first step and the longitudinal wave velocity of the wedge using Snell's law to calculate the wedge angle of the ultrasonic transducer and the transducer, and
Step 5: Time until a signal is generated from the ultrasonic transducer and the transducer receives the signal using a wedge equipped with the ultrasonic transducer and the transducer in a state where the stress is zero in the specimen

Step of measuring and
Step 6; Time until the ultrasonic transducer generates a signal and the transducer receives the signal according to the change in stress while increasing the stress on the specimen

Step of measuring and
Step 7: repeating the step of measuring the signal according to three or more stresses in step 6, and
Step 8: Obtaining a graph of measurement time (t) vs. stress (PMa) according to the stress measured in Step 6 using the relationship of Equation (2) below, and
Step 9: A method for measuring residual stress using an ultrasonic probe, characterized in that it comprises the step of obtaining a relational expression of measurement time (t) vs. stress (PMa) from the graph.

Equation (2)
here,

speed of the silver sect

is the speed change

is the stress change

is the elastic modulus

is the dimensionless acoustic-elastic constant for the L _CR wave measured by the uniaxial tensile test

When there is no residual stress, the time required for the ultrasonic signal to arrive at a distance from the ultrasonic transducer by the set distance

The time for the ultrasonic signal to arrive in the state of being separated by the set distance from the ultrasonic transducer in the presence of residual stress

difference with In the method for measuring residual stress using an ultrasonic sensor,
Step 1: Preparing a specimen of unknown initial stress and
Step 2: preparing ultrasonic transducers and transducers; and
Step 3: Fixing the ultrasonic transducer and transducer prepared in Step 2 to a wedge for generating LCR waves, and
Step 4: The wedge angle of the third step is calculated using the longitudinal wave velocity of the specimen prepared in the first step and the longitudinal wave velocity of the wedge using Snell's law to calculate the wedge angle of the ultrasonic transducer and the transducer, and
Step 5: Time until a signal is generated by the ultrasonic transducer and the transducer receives the signal using a wedge equipped with the ultrasonic transducer and the transducer in a state in which no additional stress is applied to the specimen

Step of measuring and
Step 6; Time until the ultrasonic transducer generates a signal and the transducer receives the signal according to the change in stress while increasing the stress by 100 MPa on the specimen

Step of measuring and
Step 7: While repeating the process of step 6 above, a graph of measurement time (t) vs. stress (PMa) according to stress was drawn using the relationship between the residual stress and the _LCR wave speed in Equation (3) below, and three points were drawn. A repeat measurement step of repeating the measurement until the above points form a straight line, and
Step 8: If three or more points form a straight line in the graph above, the relationship between measurement time (t) and stress (PMa) is obtained, and

Residual stress measuring method using an ultrasonic probe, characterized in that it comprises the step of calculating the stress by substituting into the relational expression.

Equation (3) According to claim 1 or 2,
Residual stress measuring method using an ultrasonic probe, characterized in that the material of the wedge is PMMA (poly-methyl-meta-acrylate). According to claim 1 or 2,
Residual stress measuring method using an ultrasonic transducer, characterized in that the ultrasonic transducer and the transducer is 2.25MHz.

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