KR20230064980A - Method for evaluating the stability of the target structure - Google Patents

Abstract

Disclosed is a method for the stability of a target structure based on a J-integral value calculated for the target structure, which can perform deformation and safety diagnosis of a structure more precisely. The method for the stability of a target structures comprises: a first step of deriving a correlation between mechano-luminescence (ML) intensity and effective strain by using a specimen, which is manufactured with the same materials as the target structure and is applied with ML paint scattered with ML powder; a second step of applying the ML paint on the target structure and then measuring the ML intensity; and a third step of applying the ML intensity to the correlation to derive the effective strain for the target structure, and by using this, calculating a J-integral value for the target structure.

Description

Method for evaluating the stability of the target structure {Method for evaluating the stability of the target structure}

The present invention relates to a method for evaluating the stability of a target structure in which elasto-plastic deformation occurs.

The movement or strain of structures and products is one of the important indicators of safety. Excessive deformation or material deformation limits can cause life and property damage, so methods for evaluating the stability of structures are being actively studied.

Infrared thermography has been widely used in industrial applications as a full-field and non-contact method to visualize changes in the stress/strain field, but has limitations that limit its application to laboratories only because it needs to maintain a constant temperature.

The digital image correlation method has been used exclusively in industrial applications to measure both in-plane and out-of-plane strains. This digital image correlation method (DIC) can determine the strain/stress tensor, but has a disadvantage in that it is difficult to select the location of the crack tip and the part where the strain concentration occurs.

In addition, since most structural materials undergo plastic deformation at the crack tip, it is necessary to quantify the plastic strain field using a non-contact, full-field, high strain-sensitive, economically inexpensive and simple method.

In order to overcome the limitations of the above-described existing stability evaluation methods, a method using mechano-luminescence (ML) has been proposed. The method using pressure light has advantages such as full inspection, immediate response, non-contact measurement, and low crack measurement ratio, as well as stress distribution in the area close to the crack tip based on fracture mechanics. Therefore, technical problems of conventional methods can be avoided.

However, all safety evaluations based on pressure light have a problem in that it is difficult to derive an accurate relationship between pressure light and strain due to the complexity of nonlinear plasticity and locally different strain.

One object of the present invention is to provide a method for precisely measuring not only the general deformation of a target structure where elasto-plastic deformation occurs, but also the complex deformation or strain rate of cracks, and evaluating the stability of the target structure based on the calculated J-integral value. is to do

The present invention is a method for evaluating the stability of a target structure based on the J-integral value calculated for the target structure, and a pressure paint made of the same material as the target structure and in which mechano-luminescence (ML) powder is dispersed is A first step of deriving a correlation between the pressure light intensity and an effective strain using the applied specimen, a second step of measuring the pressure light intensity after applying the pressure light paint to the target structure, and the pressure light intensity A third step of deriving an effective strain for the target structure by applying to the correlation, and calculating a J-integral value for the target structure using this.

In one embodiment, the first step may include measuring the intensity of the pressure light emitted from the test site of the specimen by light irradiation before deformation, and digital image correlation using the pressure powder in a random pattern after the specimen is deformed. measuring displacement and strain data after deformation of the test site using a method, measuring the pressure light intensity emitted after deformation from the test site tracked through the digital image correlation method by light irradiation, and measuring the pressure light intensity before the deformation Calculating the difference from the intensity, and using the effective strain derived from the strain of the test site measured through the digital image correlation method and the difference between the pressure light intensity before and after the strain, It may include deriving a correlation between the strains.

In one embodiment, in the third step, the J-integral value for the target structure may be calculated using Equation 1 below.

[Equation 1]

는 등가 변형률,

는 재료상수, E는 탄성계수,

는 항복응력,

는

, n은 가공경화계수,

은 재료 상수, r은 균열 팁 극 좌표계 반지름, J는 J-integral, 및

는 각도 의존 무차원 등가 변형률을 나타낸다.)(In Equation 1 above,

is the equivalent strain,

is the material constant, E is the elastic modulus,

is the yield stress,

Is

, n is the strain hardening factor,

is the material constant, r is the crack tip polar coordinate system radius, J is J-integral, and

represents the angle-dependent dimensionless equivalent strain.)

In one embodiment, the pressure light intensity may be measured through a camera and a multi-channel data link device.

In one embodiment, the pressure paint may include an epoxy resin and a SAO-based pressure powder.

In one embodiment, the target structure may include a bridge, a large machine tool, or a building made of a material capable of plastic deformation.

According to the method for evaluating the stability of a target structure of the present invention, not only general deformation but also complex deformation or strain of cracks are precisely measured using Mechano-luminescence (ML) and Digital image correlation method, and the result is measured. Through this, the J-integral value can be calculated and the deformation and safety diagnosis of the structure can be performed more precisely.

In addition, according to the present invention, cracks of the target structure can be visually confirmed. This provides an environment in which the difficulty of selecting the part where the concentration of deformation occurs when using only the digital image correlation method can be more easily found through pressure paint.

1 is a flowchart illustrating a method for evaluating the stability of a target structure according to an embodiment of the present invention.
FIG. 2 shows pressure light emission images of a test piece according to an embodiment of the present invention, and a correlation graph between pressure light intensity and effective strain derived through the pressure light emission images.
Figure 3 is a 2D, 3D graph showing the surface brightness of the specimen image to which the pressure paint is applied.
4 is an image of a test piece according to an embodiment of the present invention (left), a contour image showing pressure light intensity (ML contour, middle), and a contour image showing equivalent strain of the test piece (right), and J-integral values are calculated using these images. It is a diagram schematically showing the calculation method.
5 is a schematic view of manufacturing a pressure paint according to an embodiment of the present invention.
6 is a photographic image of a test piece after a tensile test for three loads (upper side), a contour image showing the equivalent strain of a test piece according to an embodiment of the present invention (middle), and a contour image showing the strain of a test piece according to Comparative Example 1 ( lower side).
7 is an image showing an equivalent strain data acquisition section (black) of test pieces according to Examples and Comparative Example 1 of the present invention.
8 is an image schematically showing a method for analyzing the equivalent strain of a test piece according to Comparative Example 2 of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Since the present invention may have various changes and various forms, specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to a specific form disclosed, and should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. Like reference numerals have been used for like elements throughout the description of each figure.

Terms used in this application are only used to describe specific embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this application, terms such as "comprise" or "have" are intended to designate that there is a feature, step, operation, component, part, or combination thereof described in the specification, but one or more other features or steps However, it should be understood that it does not preclude the possibility of existence or addition of operations, components, parts, or combinations thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the related art, and unless explicitly defined in the present application, they should not be interpreted in an ideal or excessively formal meaning. don't

1 is a flowchart illustrating a method for evaluating the stability of a target structure according to an embodiment of the present invention.

Referring to FIG. 1 , the present invention includes first to third steps, and the stability of the target structure can be evaluated based on the J-integral value calculated for the target structure. The target structure may include, but is not limited to, a bridge, a large machine tool, a building, etc. made of a material capable of plastic deformation.

In the present invention, first, a correlation between mechano-luminescence intensity and equivalent strain (effective strain) is derived using a specimen made of the same material as the target structure and coated with mechano-luminescence (ML) powder-dispersed mechano-luminescence paint. The first step can be performed. Here, the pressure paint is a functional material that emits fluorescence when a mechanical stress is applied, and known pressure paint materials may be applied without limitation. Preferably, the pressure-sensitive paint may include an epoxy resin and a SAO-based pressure-sensitive powder.

In one embodiment, the first step may be performed including the following steps. (See Fig. 2)

First, a step of measuring the intensity of pressure light emitted from the test site of the specimen before deformation by light irradiation is performed.

Next, after deforming the specimen, a step of measuring displacement and strain data after deformation of the test site is performed using a digital image correlation method using the press powder in a random pattern.

The digital image correlation method is a method of measuring displacement and strain data after deformation by assigning a random pattern to the surface of the specimen and comparing the image before and after deformation to calculate the pattern shift. The present invention uses a random pattern of pressed powder, and the random pattern of the pressed powder shows a difference in brightness in a minute range. Referring to FIG. 3 , when the degree of brightness of the camera image picture is expressed in units of pixels, it can be seen that the brightness is slightly different at each point.

As such, the present invention can measure displacement and strain data after deformation of the test site, as shown in FIG. 2, using a digital image correlation method using pressurized powder in a random pattern.

Specifically, referring to FIG. 2 , it is possible to check the result of measuring strains in the xx, xy, and yy axes and u and v displacement data using the digital image correlation method. The ε _xx , ε _xy , and ε _yy strains in this direction can be expressed by substituting them into Equation 1 below to calculate the effective strain (ε _e ).

[Equation 1]

는 등가 변형률,

는 x축 변형률,

는 y축 변형률,

는 z축 변형률이며,

(

는 푸아송비를 나타낸다),

는 xy축 전단변형률을 나타낸다.][In Equation 1 above,

is the equivalent strain,

is the x-axis strain,

is the y-axis strain,

is the z-axis strain,

(

represents the Poisson's ratio),

represents the shear strain on the xy axis.]

Next, a step of measuring the pressure light intensity emitted after deformation from the test site tracked through the digital image correlation method by light irradiation and calculating a difference from the pressure light intensity before the deformation is performed.

Specifically, referring to FIG. 2 , the pressure light intensity difference before and after deformation of the test site can be calculated through u and v displacement data measured using the digital image correlation method. In one embodiment, the pressure light intensity may be measured through a camera and a multi-channel data link device.

Then, using the difference between the effective strain derived from the strain of the test site measured through the digital image correlation method and the pressure light intensity before and after the deformation, to derive a correlation between the pressure light intensity and the equivalent strain do the steps

The present invention can derive an accurate correlation between pressure light intensity and equivalent strain through a first step using digital image correlation (DIC) and light emission of pressure paint.

Meanwhile, in the present invention, after the first step, the second step of measuring the pressure light intensity after applying the pressure light paint to the target structure may be performed.

Specifically, as shown in FIG. 4 , after measuring the pressure intensity of the target structure, it may be displayed as a contour image. Here, the pressure light intensity may be measured through a camera and a multi-channel data link device.

Next, a third step of deriving an effective strain for the target structure by applying the pressure light intensity to the correlation, and calculating a J-integral value for the target structure using this result may be performed. there is.

Specifically, referring to FIG. 4, the brightness of each pixel is replaced by an equivalent strain by applying the pressure light intensity contour image of the target structure to the correlation, and then substituting this into Equation 1 to obtain the J-integral value for the target structure. can be calculated.

[Formula 1]

는 등가 변형률,

는 재료상수, E는 탄성계수,

는 항복응력,

는

, n은 가공경화계수,

은 재료 상수, r은 균열 팁 극 좌표계 반지름, J는 J-integral, 및

는 각도 의존 무차원 등가 변형률을 나타낸다.)(In Equation 1 above,

is the equivalent strain,

is the material constant, E is the elastic modulus,

is the yield stress,

Is

, n is the strain hardening factor,

is the material constant, r is the crack tip polar coordinate system radius, J is J-integral, and

represents the angle-dependent dimensionless equivalent strain.)

According to the present invention, the degree of cracking of the target structure can be determined through the calculated J-integral value of the target structure, and the stability of the target structure can be evaluated based on this. Here, the J-integral value means a value that quantifies and displays the strain field in the plastic region.

According to the method for evaluating the stability of a target structure of the present invention, not only general deformation but also complex deformation or strain of cracks are precisely measured using Mechano-luminescence (ML) and Digital image correlation method, and the result is measured. Through this, the J-integral value can be calculated and the deformation and safety diagnosis of the structure can be performed more precisely.

In addition, according to the present invention, cracks of the target structure can be visually confirmed. This provides an environment in which the difficulty of selecting the part where the concentration of deformation occurs when using only the digital image correlation method can be more easily found through pressure paint.

In addition, since the present invention uses pressure-sensitive paint, it has the advantage of being able to visually observe the degree of strain even in a special environment such as a dark or dark room such as a tunnel or a sewer.

Hereinafter, examples and experimental examples of the present invention will be described in detail. However, the examples described below are only reference materials for explaining the effectiveness of the present invention, and the scope of the present invention should not be construed as being limited to the following examples.

[Example: J-integral analysis using digital image correlation (DIC) and pressure paint (ML)]

23 wt% of SAO powder (SrAl ₂ O ₃ :Eu) and 77 wt% of epoxy resin were mixed using a planetary mixer for 10 minutes. Thereafter, the mixture was left in a vacuum environment to remove air bubbles, and then applied to the area of interest of the test piece (AL 7075-T6, tensile test piece), and the pressure paint was cured and dried in an oven or at room temperature. (See Fig. 5)

After fastening the test piece to which the pressure paint was applied to a universal tensile tester, UV was exposed to the area of interest to which the pressure paint was applied. After continuously exposed to UV for 5 minutes to give sufficient energy, a tensile test was performed at a cross head speed of 0.05 mm/s in a UV irradiation environment.

During the tensile test, a high-speed camera was used to acquire pictures of the specimen, and the force and displacement of the tester were synchronized with the camera image using a multi-channel data link. Thereafter, strains in the xx, xy, and yy axes and u and v displacements of the obtained deformation pictures of the specimen were measured through a digital image correlation method. (See Fig. 2) Here, the open source 'Ncorr' used in the Matlab program was used for the strain, and the directional ε _xx , ε _xy, ε _yy strains were substituted into Equation 1 to obtain the effective strain (ε _e ) was calculated.

On the other hand, for the change in pressure light intensity, after determining an ROI of a certain size in the photograph before deformation of the test piece, and then finding the position of the ROI in the photograph before deformation through the u, v displacement values measured through digital image correlation, in the photograph after deformation, , and the pressure light intensity change was measured by calculating the difference in brightness of the ROI before and after deformation.

Thereafter, a correlation (ML-Effective strain master curve) between the equivalent strain and the pressure light intensity was derived using the derived equivalent strain and the difference between the pressure light intensity before and after deformation. The derived correlation (ML-Effective strain master curve) is shown in FIG. 2 .

Next, in order to evaluate crack characteristics, pressure paint was applied using a CTS test piece having a crack in the middle, and then UV was exposed to the region of interest where the pressure paint was applied. After continuously exposed to UV for 5 minutes to give sufficient energy, a tensile test was performed at a cross head speed of 0.05 mm/s in a UV irradiation environment. When performing the tensile test, images were acquired at three different loads before failure. (See Fig. 6)

Next, the difference in pressure light brightness of the image before and after deformation was obtained, and after correction, it was expressed in the form of contour lines. The corresponding pressure light brightness contour image was applied to the correlation (ML-Effective strain master curve) derived through the tensile test to replace the brightness of each pixel with an equivalent strain. (See Figs. 4 and 6)

Then, the equivalent strain obtained in a certain area around the crack tip was substituted into Equation 1 below to calculate the J-intergral value.

[Formula 1]

는 등가 변형률,

는 재료상수, E는 탄성계수,

는 항복응력,

는

, n은 가공경화계수,

은 재료 상수, r은 균열 팁 극 좌표계 반지름, J는 J-integral, 및

는 각도 의존 무차원 등가 변형률을 나타낸다.)(In Equation 1 above,

is the equivalent strain,

is the material constant, E is the elastic modulus,

is the yield stress,

Is

, n is the strain hardening factor,

is the material constant, r is the crack tip polar coordinate system radius, J is J-integral, and

represents the angle-dependent dimensionless equivalent strain.)

<Comparative evaluation of J-integral value according to crack analysis method>

For the evaluation of the J-integral value calculated by using ML and DIC together according to an embodiment of the present invention, the J-integral value using DIC (Comparative Example 1) using the image obtained from the tensile test, using ML J-integral value (reference value) analysis using J-integral value (Comparative Example 2) and finite element analysis (FEM) was additionally performed. The specific method is as follows.

[Comparative Example 1: J-integral analysis using DIC]

After applying pressure paint using a CTS test piece with a crack in the middle, UV was exposed to the area of interest where the pressure paint was applied. After continuously exposed to UV for 5 minutes to give sufficient energy, a tensile test was performed at a cross head speed of 0.05 mm/s in a UV irradiation environment. When performing the tensile test, images were acquired at three different loads before failure. (See Fig. 6)

Then, the strain was obtained using the digital image correlation method (DIC) using the acquired image. Specifically, the strain of the test piece was obtained using the open source 'ncorr' using images before and after deformation.

Next, as shown in FIG. 7, by substituting the strain into Equation 1 based on the part where the change in the equivalent strain slope occurs based on 0 ° in the region of [-60 ° ~ 60 °] in the region opposite to the crack (black color) Equivalent strain data were obtained. Then, the equivalent strain obtained in a certain area around the crack tip was substituted into Equation 1 to calculate the J-intergral value.

[Comparative Example 2: J-integral analysis using ML]

After applying pressure paint using a CTS test piece with a crack in the middle, UV was exposed to the area of interest where the pressure paint was applied. After continuously exposed to UV for 5 minutes to give sufficient energy, a tensile test was performed at a cross head speed of 0.05 mm/s in a UV irradiation environment. When performing the tensile test, images were acquired at three different loads before failure.

Then, based on the crack tip, the change in brightness of the image before and after deformation was calculated to obtain equivalent strain data of the corresponding specimen. Then, the equivalent strain obtained in a certain area around the crack tip was substituted into Equation 1 to calculate the J-intergral value. (See Fig. 8)

[Reference value: Analysis using finite element (FEM)]

A simulation was performed using the 'ABQUS' program using the properties of al7075 used in the tensile test of the CTS specimen with a crack in the middle.

Table 1 below shows the J-integral values of the CTS test pieces derived according to the methods of Example 1 and Comparative Examples 1 and 2 for three loads, respectively. In Table 1, the FEM analysis data was used as the reference value (FEM - Reference).

Referring to Table 1, in the case of the J-integral value calculated using only ML (Comparative Example 2), the relationship between strain and pressure is not clear in the preceding tensile test, which shows a very large difference from the FEM analysis data. .

On the other hand, the J-integral value calculated using DIC obtained using pressure light paint (Comparative Example 1) and the J-integral value calculated using pressure light and DIC simultaneously according to an embodiment of the present invention are similar to the FEM analysis data. value was shown.

In particular, the J-integral value calculated according to the embodiment of the present invention showed a value closer to the reference value than the J-integral value (ML, DIC) according to Comparative Examples 1 and 2. This is a result showing that crack analysis according to an embodiment of the present invention solves the difficulty of strain analysis in complex deformation, which is a disadvantage of digital image correlation (DIC), and shows excellent performance.

Although the above has been described with reference to preferred embodiments of the present invention, those skilled in the art can variously modify and change the present invention without departing from the spirit and scope of the present invention described in the claims below. You will understand that you can.

Claims (6)

In the method for evaluating the stability of a target structure based on the J-integral value calculated for the target structure,
A first step of deriving a correlation between mechano-luminescence intensity and equivalent strain (effective strain) using a specimen made of the same material as the target structure and coated with mechano-luminescence (ML) powder-dispersed mechano-luminescence paint;
a second step of measuring pressure intensity after applying the pressure light paint to the target structure; and
A third step of deriving an effective strain for the target structure by applying the pressure light intensity to the correlation, and calculating a J-integral value for the target structure using this; a target structure comprising: stability evaluation method.
According to claim 1,
The first step is
measuring the intensity of pressure light emitted from the test site of the specimen before deformation by light irradiation;
Deforming the specimen and then measuring displacement and strain data after deformation of the test site using a digital image correlation method using the pressurized powder in a random pattern;
measuring the pressure light intensity emitted after deformation from the test site tracked through the digital image correlation method by light irradiation, and calculating a difference between the pressure light intensity and the pressure light intensity before the deformation; and
Deriving a correlation between the pressure light intensity and the equivalent strain using an effective strain derived from the strain of the test site measured through the digital image correlation method and a difference between the pressure light intensity before and after the strain; Characterized in that it comprises a, stability evaluation method of the target structure.
According to claim 1,
In the third step, the J-integral value for the target structure is calculated using Equation 1 below, a method for evaluating the stability of the target structure:
[Equation 1]

(In Equation 1 above,

is the equivalent strain,

is the material constant, E is the elastic modulus,

is the yield stress,

Is

, n is the strain hardening factor,

is the material constant, r is the crack tip polar coordinate system radius, J is J-integral, and

represents the angle-dependent dimensionless equivalent strain.)
According to claim 1,
Characterized in that the pressure light intensity is measured through a camera and a multi-channel data link device, the stability evaluation method of the target structure.
According to claim 1,
The method for evaluating stability of a target structure, characterized in that the pressure paint includes an epoxy resin and an SAO-based pressure light powder.
According to claim 1,
The target structure is a stability evaluation method of a target structure, characterized in that it includes a bridge, a large machine tool or a building made of a material capable of plastic deformation.

Images