KR20220116974A - Method of measuring fracture toughness considering the plastic rotational factor based on double clip gauge - Google Patents

Abstract

The present invention relates to a method for measuring fracture toughness based on a double clip gauge, considering a plastic rotational factor. The method for measuring fracture toughness by using a testing device, comprising a seating unit on which a predetermined specimen is seated, a load applying unit for applying a predetermined tensile load to an upper end of the seated specimen, and a displacement measurement unit for measuring displacement information on cracks generated in the specimen by the load, comprises the steps of: preparing a specimen having a notch according to predetermined standards and seating the same on the seating unit; installing a jig having a seating surface having a predetermined height at the upper end of the specimen, and installing the displacement measuring unit at a position adjacent to the entrance of the notch; forming a preliminary fatigue crack extending inward from an inner end of the notch by applying a repeated tensile load, having an amplitude, to the upper end of the notch by using the load applying unit; calculating the amount of displacement in a plastic zone based on a front end of the preliminary fatigue crack based on a result of measuring the amount of displacement for the entrance of the preliminary fatigue crack by applying a quasi-static tensile load thereto with the displacement measurement unit contacting the seating surface of the jig; and calculating a plastic rotation coefficient value and a predicted fracture toughness value for the plastic zone based on the calculated displacement amount in the plastic zone, the seating surface, and the top surface of the specimen. Accordingly, the method can correct the plastic rotation coefficient value that affects the plastic deformation occurring at the front end of the preliminary fatigue crack of the specimen when calculating the predicted fracture toughness value, thereby measuring the plastic deformation more sensitively compared to conventional methods.

Description

Method of measuring fracture toughness based on double clip gauge considering plastic rotation coefficient

The present invention relates to a method for measuring fracture toughness based on a double clip gauge considering the plastic rotation coefficient contributing to the measurement of plastic deformation when measuring the fracture toughness value of a test piece based on a universally used standard.

Since unexpected damage to large structures causes serious losses, structural integrity evaluation of various techniques is being conducted to prevent damage. In order to evaluate the soundness of a structure in use, it is important to accurately know information about stress applied to the structure, information about defects that exist inside the structure, and information about the physical properties of the structure material.

In particular, when designing ships and offshore plants, the classification and shipowners are required to evaluate the soundness of the applied steel materials and welds.

Fracture toughness evaluation is a correct test method that measures the size and propagation direction of a crack by mechanically analyzing the stress and strain around the crack after forming a pre-crack in the specimen by making a specimen of a specific standard shape and size. It is a method to obtain the conditions under which failure occurs through the test after establishing

In this fracture toughness evaluation, plane strain fracture toughness (J _IC ) and elastoplastic equivalent stress factor (Stress Intensity Factor, K _JC ) is used as an important fracture toughness parameter.

Representative fracture toughness test methods include a compact tension test with fatigue precracking and a three-point-single edge notched bend test. For this experiment, a number of specimens are taken from the original plate in a predetermined direction and size (refer to ASTM standards for load direction and crack propagation direction), and the degree of crack propagation is grasped while actually breaking them.

Meanwhile, conventionally, a method of evaluating fracture toughness according to the result of measuring Crack Tip Opening Displacement (CTOD) using a single clip gauge based on the British Standard (BS) has been mainly used. As CTOD-related studies become more active, new methods such as improvement of the definition of CTOD, measurement of crack opening displacement, and application of SENT specimens with reduced restraint effects are being introduced.

However, in the case of the CTOD measurement method using a single clip gauge based on the British Standard Standard (BS), which is currently generalized, there is a limitation in that it is impossible to accurately measure the plastic deformation of a material or there is a large deviation in fracture toughness.

US 10-1254547 B1 US 10-1289905 B1

It is an object of the present invention to solve the above problems, and when measuring the fracture toughness value for a test piece based on a universally used standard, the fracture toughness measurement based on the double clip gauge considering the plastic rotation coefficient contributing to the plastic deformation measurement The purpose is to provide a method.

A method for measuring fracture toughness according to an aspect of the present invention for achieving the above object includes a seating part on which a predetermined test piece is seated, a load applying part for applying a predetermined tensile load to the upper end of the seated test piece, and the load In the method for measuring fracture toughness using a test apparatus including a displacement measuring unit for measuring displacement information about cracks generated in the test piece by Step, installing a jig having a seating surface of a predetermined height on the upper end of the test piece, installing the displacement measuring unit at a position adjacent to the notch entrance, and repeating a constant amplitude at the upper end of the notch using the load applying unit Forming a preliminary fatigue crack extending inward from the inner end of the notch by applying a tensile load, and applying a quasi-static tensile load with the displacement measuring part in contact with the seating surface of the jig to provide the preliminary fatigue crack Calculating the amount of displacement in the plastic region based on the tip of the preliminary fatigue crack based on the result of measuring the amount of displacement for the entrance of and calculating a plastic rotation coefficient value and a fracture toughness predicted value for the plastic region, respectively, based on the height difference of .

Preferably, the jig has a first seating surface and a second seating surface corresponding to two preset height values different from each other, and the calculating of the displacement includes placing the contact end of the displacement measuring unit with the first seating surface. The first entrance displacement value and the second entrance displacement value corresponding to the opening displacement for the entrance of the preliminary fatigue crack by applying a quasi-static tensile load by the load applying unit while in contact with the surface and the second seating surface, respectively. The first inlet displacement for the tensile load based on a predetermined plastic hinge model so as to be divided into an elastic region and a plastic region based on a yield point corresponding to an elastic limit of tensile stress for a predetermined material, respectively. deriving a load-displacement curve according to the value and the second inlet displacement value, the slope of the elastic region according to the derived load-displacement curve, and the first and second seating surfaces of the test piece Based on the first leading edge value and the second leading edge value corresponding to the height difference between the top surfaces, the first tip displacement value and the second tip displacement value corresponding to the opening displacement amount in the plastic region with respect to the tip of the preliminary fatigue crack It characterized in that it comprises the step of calculating the tip displacement value.

In addition, between the step of forming the preliminary fatigue crack and the step of calculating the amount of displacement, the step of measuring the initial length of the notch, the width value of the test piece, and the load line displacement value for the test piece, respectively, further comprising, Calculating the predicted value includes the measured initial length of the notch, the width value of the test piece, and the load line displacement value, and the calculated first tip displacement value, second tip displacement value, first tip edge value, and Calculating the plastic rotation coefficient based on a second tip edge value, and a plurality of fracture mechanics parameters including the calculated plastic rotation coefficient, the first tip displacement value, and the second tip displacement value in a pre-stored CTOD prediction equation It characterized in that it comprises the step of calculating the fracture toughness predicted value based on the results respectively applied to.

According to the present invention, there is an effect that can measure the plastic deformation more sensitively than in the conventional case by correcting the plastic rotation coefficient value that affects the plastic deformation occurring at the pre-crack tip of the test piece when calculating the fracture toughness predicted value.

In addition, according to the present invention, due to the nature of the fracture toughness test, repeated measurement of the same steel material is required several times, and since the deviation of the result calculated for each number of measurements is stably calculated within a short time compared to the prior art, unnecessary time and manpower related to the test are required. It has the effect of reducing consumption.

In addition, according to the present invention, by improving the sensitivity and accuracy of plastic deformation for a test piece used for general purpose when measuring the fracture toughness of a material, the structural integrity of shipbuilding and offshore structures in which large-scale capital is invested can be reliably evaluated to improve economic feasibility. There is an achievable effect.

1 is a flowchart illustrating a method for measuring fracture toughness based on a double clip gauge considering a plastic rotation coefficient according to an embodiment of the present invention;
Figure 2 is a view showing an example of a test piece that is seated in the test apparatus in the seating step of Figure 1,
Figure 3 is a view showing a state in which the jig is installed on the upper end of the test piece by the installation step of Figure 1,
4 is a view showing a state in which the displacement measuring unit is installed in a position adjacent to the notch inlet of the test piece by the installation step of FIG. 1;
FIG. 5 is a view for separately explaining an elastic region and a plastic region in the load-displacement curve derived in the load-displacement curve derivation step of FIG. 1;
6 is a view for explaining a process of calculating a tip displacement value using a triangular similarity ratio in the tip displacement amount calculation step of FIG. 1;
7 is a comparative graph showing the first load-displacement curve corresponding to the displacement value according to the load measured a plurality of times based on one tip edge value using a conventional single clip gauge divided by measurement times;
8 is a second load-displacement corresponding to the displacement value according to the load measured a plurality of times based on the two front edge values preset according to each of the first and second conditions in the load-displacement curve derivation step of FIG. 1; After deriving the curve and the third load-displacement curve, respectively, it is a comparative graph showing by dividing by measurement times,
9 is a diagram for explaining the process of deriving the tip displacement value for the tip of the preliminary fatigue crack based on each of the second load-displacement curve and the third load-displacement curve in the tip displacement calculation step of FIG. 1;
10 is a view for comparing the plastic rotation coefficient values proposed for each research analysis result based on the conventional finite element analysis;
11 is a view for comparing the range and deviation of fracture toughness values measured a plurality of times according to each measurement method of the prior art and the present invention.

Specific details including the problems to be solved for the present invention as described above, the means for solving the problems, and the effects of the invention are included in the embodiments and drawings to be described below. Advantages and features of the present invention, and a method of achieving the same, will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. Like reference numerals refer to like elements throughout.

1 is a flowchart illustrating a method for measuring fracture toughness based on a double clip gauge considering a plastic rotation coefficient according to an embodiment of the present invention, and FIG. 2 is a view showing an example of a test piece seated in the test apparatus in the seating step of FIG. 3 is a view showing a state in which the jig is installed on the upper end of the test piece by the installation step of FIG. 1, and FIG. 4 shows a state in which the displacement measuring unit is installed in a position adjacent to the notch inlet of the test piece by the installation step of FIG. FIG. 5 is a diagram for separately explaining an elastic region and a plastic region in the load-displacement curve derived in the load-displacement curve derivation step of FIG. 1, and FIG. 6 is a triangle similarity ratio in the tip displacement calculation step of FIG. It is a diagram for explaining the process of calculating the tip displacement value using

The fracture toughness measurement method according to the present invention includes a seating part on which a predetermined test piece is seated, a load applying part for applying a predetermined tensile load to the upper end of the seated test piece, and displacement for cracks generated in the test piece by the load. It is preferably carried out using a test apparatus including a displacement measuring unit for measuring information.

At this time, the test apparatus is not specified according to the driving method or shape, and it only needs to include the basic components (seating part, load applying part, and displacement measuring part) that are essential to a general material testing apparatus, so a detailed description thereof will be omitted. do.

Hereinafter, a method for measuring fracture toughness based on a double clip gauge considering a plastic rotation coefficient according to a preferred embodiment of the present invention will be described with reference to the above drawings.

First, a test piece 10 having a notch 22 according to a preset standard is prepared and seated on a seating part of the test apparatus (S100).

Here, the test specimen 10 prepared in step S100 may include a compact tension specimen (CT) among CTOD specimens manufactured based on the BS 7448 standard standardized by the British Standards Association.

Specifically, the test piece 10 is provided as a flat plate of a predetermined size and thickness, as shown in FIG. 2 , a pair of through holes 21 having a predetermined diameter at the upper end are formed spaced apart by a predetermined interval, and the upper center It may be in a state in which the notch 22 in the form of being recessed by a predetermined depth in the vertical direction and narrowing in a V-shape at the inner end of the notch 22 may be formed.

At this time, the test piece 10 is the thickness of the test piece 10 so as to have a predetermined proportional value based on the value of the width (C) based on the BS 7448 standard described above for the steel material used for the manufacture of the welded structure, the through hole ( After designing the diameter of 21), the initial length (a ₀ ) of the notch 22, and the like, it can be manufactured by electric discharge machining of the steel material accordingly.

Next, a jig (J) having a predetermined seating surface is installed on the upper end of the test piece 10, and a displacement measuring unit is installed at a position adjacent to the notch inlet of the test piece 10 (S200).

Here, the jig (J) is provided as a pair and may be designed to have seating surfaces corresponding to two preset height values different from each other.

For example, referring to Figure 3, the first jig (J1) and the second jig (J2) are respectively installed at a predetermined distance spaced apart positions on the top surface of the test piece 10, at the center line 31 of the notch 22 The distance to the center of the jig (J) is 'y', and the distances to the first jig (J1) and the second jig (J2) based on the center line 31 of the notch are 'x1' and 'x2', respectively, The distance between the top surface of the test piece 10 and the first seating surface 41 of the jig (J) is 'z1', and between the top surface of the test piece 10 and the second seating surface 42 of the jig (J) The distance is defined as 'z2'.

Table 1 below shows the first height value and the second height value of the jig (J) by condition for each of the two cases.

case 1 case 2 h1 = z1 = 2mm h1 = z1 = 2mm h2 = z2 = 6mm h2 = z2 = 10mm

Here, z1 is a first height value, z2 is a second height value, h1 is a first leading edge value, and h2 is a second leading edge value. The values of h1 and z1 are the same, and the values of h2 and z2 are the same.

Here, the displacement measuring unit may include a double clip gauge having a pair of contact ends, as shown in FIG. It may be to measure the displacement information of the test piece 10 in a state in contact with any one of the seating surface and the second seating surface.

Although not described in FIG. 1, immediately after step S200, the initial length (a ₀ ) of the notch 22 and the width value (C) of the test piece 10 and the load line displacement for the test piece 10 ( The method may further include (not shown) measuring each Load Line Displacement (LLD) value (W).

Next, a preliminary fatigue crack extending inward from the inner end of the notch 22 is formed by applying a repeated tensile load to the upper end of the notch 22 using the load applying unit of the test apparatus (S300).

Here, the load applying unit of the test apparatus is electrically connected to the through hole 21 formed at the upper end of the test piece 10 prepared in step S100 to apply a repetitive tensile load of a certain amplitude to the through hole 21, but the tensile load The size can be kept constant until the crack in question is reached.

At this time, the initial length (a ₀ ) of the notch 22 is defined as the vertical distance from the center of the through hole 21 of the test piece 10 to the lowest end of the notch 22 as shown in FIG. 2 .

Next, the amount of displacement generated by applying a quasi-static tensile load to the preliminary fatigue crack formed in step S300 is calculated (S400).

Specifically, in step S400, the amount of inlet displacement (v1, v2) for the preliminary fatigue crack caused by applying a quasi-static tensile load by the load applying unit of the test apparatus is measured using the displacement measuring unit installed in the step S200 and then measured. It may be to calculate the amount of displacement (vp1, vp2) in the plastic region with respect to the tip of the preliminary fatigue crack according to the applied inlet displacement amount to the preset plastic hinge model.

Here, the step S400 may include an entrance displacement measurement step (S410), a load-displacement curve derivation step (S420), and a tip displacement amount calculation step (S430) as shown in FIG. 1 .

In the entrance displacement measurement step (S410), the first entrance displacement value V1 and the second entrance displacement value V2 corresponding to the opening displacement with respect to the entrance of the preliminary fatigue crack are respectively measured using the displacement measurement unit.

Here, in step S410, the first entrance displacement with respect to the leading edge value h1 corresponding to the height z1 based on the first seating surface after the displacement measuring unit is brought into contact with the first seating surface of the jig J After measuring the value V1 and contacting the displacement measuring part with the second seating surface of the jig J, the second value for the leading edge value h2 corresponding to the height z2 based on the second seating surface The inlet displacement value (V2) can be measured.

In the load-displacement curve deriving step (S420), the first entrance displacement value (V1) and the second entrance displacement value (V2) measured in the entrance displacement amount measurement step (S410) for the tensile load based on a preset plastic hinge model A load-displacement curve (P-V1-V2) is derived according to

Here, the load-displacement curve is a plastic displacement section (Vp) and an elastic displacement section (Ve) for the plastic region Ap based on the yield point corresponding to the elastic limit of the tensile stress for a given material as shown in FIG. 5 . ) may include a curve graph that can be distinguished by

In this regard, the conventional CTOD prediction equation based on the BS 7448 standard derived according to the result of applying the plastic hinge model based on FIG. 5 is as shown in Equation 1 below.

Here, δ _pl is the fracture toughness (CTOD) value, r _p is the plastic rotation coefficient, W is the load line displacement (LLD) value for the specimen 10, a ₀ is the initial length of the notch, and Vp is the opening displacement value at the entrance. indicates.

At this time, the plastic rotation coefficient (r _p ) is considered to have a fixed value through research analysis based on finite element analysis.

For example, referring to FIG. 10, the r _p value is regarded as "0.46" in the universally used standardized BS 7448 standard. "0.52" suggested by Tomoya is considered as the r _p value.

The tip displacement calculation step (S430) corresponds to the amount of opening displacement in the plastic region with respect to the tip (tip) of the preliminary fatigue crack based on the slope of the elastic region according to the load-displacement curve derived by the step S420. A first tip displacement value Vp1 and a second tip displacement value Vp2 are calculated.

Here, in step S430, as shown in FIG. 6, the first tip displacement value (Vp1) and the second The tip displacement value (Vp2) can be calculated, and the CTOD prediction equation based on the BS 8571 standard derived accordingly is shown in Equation 2 below.

Here, δ _pl is the fracture toughness (CTOD) value, V1 is the first entry displacement value, V2 is the second entry displacement value, a ₀ is the initial length of the notch, and z1 and z2 are the first tip corresponding to the jig (J). The edge value and the second leading edge value are shown.

Next, a plastic rotation coefficient (r _p ) and fracture toughness (CTOD) predicted values are calculated based on the displacement amount calculated in step S400 and a preset tip edge value (S500).

Here, the step S500 includes a plastic rotation coefficient calculation step (S510) and a fracture toughness predicted value calculation step (S520) as shown in FIG. 1 .

In the plastic rotation coefficient calculation step (S510), the first tip displacement value and the second tip displacement value (Vp1, Vp2) calculated by the step S430, and the jig (J) installed on the upper end of the test piece 10 in the step S200 ) on the basis of the first leading edge value (h1) and the second leading edge value (h2) corresponding to the heights (z1, z2) based on the first seating surface and the second seating surface, respectively, the plastic rotation coefficient (r _p ) can be calculated.

Here, the step S510 may be calculated according to Equation 3 below, in which Equations 1 and 2 described above are summarized as Equations related to the plastic rotation coefficient r _p .

Here, Vp1 is a first tip displacement value, Vp2 is a second tip displacement value, h1 is a first tip edge value, h2 is a second tip edge value, and z has the same value as h1.

In the fracture toughness predicted value calculation step (S520), the first tip displacement value (Vp1) and the second tip displacement value (Vp2) calculated by the step S430 and the plastic rotation coefficient (r _p ) calculated by the step S510 are A fracture toughness (CTOD) predicted value is calculated based on the results of applying each to a plurality of fracture mechanics parameters included in the pre-stored CTOD prediction equation.

Here, the predicted fracture toughness (CTOD) value (δ) is the plastic fracture toughness value (δ _pl ) and the elastic fracture toughness value (δ) for fracture toughness in each of the plastic region (Vp) and the elastic region (Ve) shown in FIG. 5 . _el ) can be calculated by summing the two values as shown in Equation 4 below.

In this case, the plastic fracture toughness value (δ _pl ) may be calculated according to Equation 1 above, and the elastic fracture toughness value (δ _el ) may be calculated according to Equation 5 below.

Here, K is the plane strain fracture toughness value, υ is the Poisson's ratio, E is the modulus of elasticity, and σ _YS is the yield strength of the material with respect to temperature and load rate.

As described above, according to the method for measuring fracture toughness according to the present invention, the method for measuring fracture toughness according to the present invention is performed multiple times by using the above characteristic in that it has the characteristic of measuring plastic deformation more sensitively than in the conventional case. The results of comparing the analysis results performed repeatedly with the conventional analysis results are as follows.

In this regard, FIG. 7 is a comparative graph showing the first load-displacement curve corresponding to the displacement value according to the load measured a plurality of times based on one tip edge value using a conventional single clip gauge divided by measurement times. and FIG. 8 is a second load corresponding to the displacement value according to the load measured a plurality of times based on the two tip edge values preset according to each of the first and second conditions in the load-displacement curve derivation step of FIG. 1 . - It is a comparative graph showing the displacement curve and the third load-displacement curve after deriving each of the measurement times, and FIG. 9 is the second load-displacement curve and the third load-displacement curve in the tip displacement calculation step of FIG. It is a diagram for explaining the process of deriving the tip displacement value for the tip of the preliminary fatigue crack based on each.

First, Table 2 below summarizes the fracture toughness (CTOD) values calculated according to Equations 1, 4 and 5 corresponding to the first load-displacement curve shown in FIG. .

test number CTOD [mm] #One 0.61 #2 0.63 #3 0.67 #4 0.68 #5 0.76 #6 0.77

For example, referring to Table 2, it can be seen that the CTOD value has a value in the range of 0.61 mm to 0.77 mm and the deviation is 0.16 mm, and when the plastic rotation coefficient (r _p ) value based on the standard BS 7448 is applied In that the fracture toughness (CTOD) value of (#2) was 0.63 mm, which is the second lowest value, it can be judged that the CTOD value deviation was large due to the lack of sensitivity to plastic deformation in the case of the conventional method.

Next, in the case of the present invention, as the steps S100 to S420 are repeated 6 times based on the first condition (Case 1) and the second condition (Case 2) described in Table 1, FIG. 8 As a result of deriving a second load-displacement curve and a third load-displacement curve divided by each measurement cycle as shown in Fig. 9, and applying a triangular similarity ratio to the load-displacement curve based on Fig. The first tip displacement value Vp1 and the second tip displacement value Vp2 may be calculated.

When analyzing the results according to FIG. 8, the maximum load (Fm) is about 55 kN to 60 kN, and a sudden drop in load and an increase in displacement immediately after the yield point of the material (pop-in) did not occur separately, The difference between the first inlet displacement value (V1) and the second inlet displacement value (V2) is '0.31mm' in the case of the first condition (Case 1) and '0.43 in the case of the second condition (Case 2) based on the average value. mm', it can be seen that the height value z2 corresponding to the second seating surface of the jig J increases by about 27%.

Therefore, as the installation height difference of the clip-type gauge increases, the difference in the entry displacement ( _CMOD ) occurs. will do

Next, Table 3 below shows the second load-displacement curve according to the first condition (Case 1) shown in FIG. 8 and the third load-displacement curve according to the second condition (Case 2), respectively. Corresponding to Equation 3, the plastic rotation coefficient (r _p ) values calculated according to the measurement times are summarized and shown in a table.

test number r _p Case 1 Case 2 #One 0.63 0.80 #2 0.64 0.76 #3 0.55 0.75 #4 0.68 0.75 #5 0.67 0.80 #6 0.61 0.79

For example, referring to Table 3, the plastic rotation coefficient (r _p ) under the first condition (Case 1) ranges from '0.63' to '0.68', and the firing rate under the second condition (Case 2). As the rotation coefficient r _p is expressed as a value of '0.63' to '0.68', the first height value z1 and the second It can be seen that the greater the difference (z2-z1) of the height value (z2) is, the greater the plastic rotation coefficient (r _p ) value is calculated.

In addition, when compared with the plastic rotation coefficient (r _p ) value proposed for each research analysis result based on the conventional finite element analysis shown in FIG. 10 , the first condition (Case 1) and the second condition (Case 1) Among 2), it can be determined that a more reasonable result for the plastic rotation coefficient (r _p ) value is obtained when the second height value z2 is smaller than the first condition (Case 1).

Next, with reference to FIG. 11 , the results of comparing the ranges and deviations of fracture toughness values measured a plurality of times according to each measurement method of the prior art and the present invention are as follows.

Specifically, in FIG. 11, a first graph 11 showing the range and deviation values of the fracture toughness (CTOD) values for each measurement cycle described in Table 3 corresponding to the first load-displacement curve shown in FIG. 7 . ) and the second load-displacement curve according to the first condition (Case 1) shown in FIG. 8 and the third load-displacement curve according to the second condition (Case 2) in Table 3 corresponding to each The range and deviation of the calculated fracture toughness (CTOD) value after calculating the fracture toughness (CTOD) value by applying the plastic rotation coefficient (r _p ) value for each measurement cycle described above to Equations 1, 4, 5 A second graph 12 and a third graph 13 are shown respectively.

Analysis of the results according to FIG. 11 shows that the range of fracture toughness (CTOD) values according to the conventional method is '0.6 mm' to '0.75 mm' and the generated deviation is '0.15 mm', whereas that of the present invention Since the range of the fracture toughness (CTOD) value according to the first condition (Case 1) is '0.66 mm' to '0.69 mm' and the generated deviation is '0.03 mm', the fracture toughness (CTOD) calculated by the present invention ( It can be seen that the deviation of the _CTOD ) value is reduced by about 5 times compared to the deviation of the CTOD value of the conventional method. It can be judged that the CTOD deviation value is reduced by increasing the plastic deformation sensitivity.

Accordingly, according to the present invention described above, when calculating the fracture toughness predicted value, the plastic rotation coefficient value that affects the plastic deformation occurring at the pre-crack tip of the test piece is corrected, so that the plastic deformation can be measured more sensitively than in the conventional case. have.

In addition, according to the present invention, due to the nature of the fracture toughness test, repeated measurement of the same steel material is required several times, and since the deviation of the result calculated for each number of measurements is stably calculated within a short time compared to the prior art, unnecessary time and manpower related to the test are required. consumption can be reduced.

In addition, according to the present invention, by improving the sensitivity and accuracy of plastic deformation for a test piece used for general purpose when measuring the fracture toughness of a material, the structural integrity of shipbuilding and offshore structures in which large-scale capital is invested can be reliably evaluated to improve economic feasibility. can be obtained

As mentioned above, although the present invention has been described in detail through preferred embodiments, the present invention is not limited thereto and may be practiced in various ways within the scope of the claims.

In particular, since the foregoing has rather broadly described the features and technical strengths of the present invention in order to better understand the claims of the present invention to be described later, the concept and specific embodiments of the present invention described above are suitable for carrying out purposes similar to the present invention. It should be recognized by those skilled in the art that it can be used immediately as a basis for designing or modifying other shapes for

In addition, the embodiment described above is only one embodiment according to the present invention, and it can be implemented in various modifications and changed forms within the scope of the technical spirit of the present invention by those of ordinary skill in the art. You will understand. Accordingly, the disclosed embodiments are to be considered in an illustrative rather than a restrictive view, and such various modifications and changes are also indicated in the claims of the present invention described above as falling within the scope of the technical spirit of the present invention, and equivalent scope All differences therein should be construed as being included in the present invention.

10: test piece
21: through hole
22: notch
41: first seating surface
42: second seating surface

Claims (3)

A seating part on which a predetermined test piece is seated, a load applying part for applying a predetermined tensile load to the upper end of the seated test piece, and a displacement measuring part for measuring displacement information about cracks generated in the test piece by the load. In the fracture toughness measurement method using a test device,
preparing a notched test piece according to a preset standard and placing it on the seating part;
installing a jig having a seating surface of a predetermined height on the upper end of the test piece, and installing the displacement measuring unit at a position adjacent to the notch inlet;
forming a preliminary fatigue crack extending inward from the inner end of the notch by applying a repetitive tensile load of a predetermined amplitude to the upper end of the notch using the load applying unit;
The displacement amount in the plastic region based on the tip of the preliminary fatigue crack based on the result of measuring the displacement amount at the entrance of the preliminary fatigue crack by applying a quasi-static tensile load with the displacement measuring part in contact with the seating surface of the jig calculating ; and
Calculating, respectively, a plastic rotation coefficient value and a predicted fracture toughness value for the plastic region based on the calculated displacement amount in the plastic region and the height difference between the seating surface and the top surface of the test piece; A method for measuring fracture toughness based on a double clip gauge considering the plastic rotation coefficient. According to claim 1,
The jig is to have a first seating surface and a second seating surface corresponding to two preset height values different from each other,
Calculating the displacement amount includes:
Corresponding to the amount of opening displacement for the entrance of the preliminary fatigue crack by applying a quasi-static tensile load by the load applying unit in a state in which the contact end of the displacement measuring unit is in contact with the first and second seating surfaces, respectively. measuring a first entrance displacement value and a second entrance displacement value, respectively;
The first inlet displacement value and the second inlet displacement value for the tensile load based on a predetermined plastic hinge model so that it can be divided into an elastic region and a plastic region based on a yield point corresponding to an elastic limit of tensile stress for a predetermined material. deriving a load-displacement curve according to and
The inclination of the elastic region according to the derived load-displacement curve, the first leading edge value and the second leading edge corresponding to the difference in height between the first and second seating surfaces, respectively, and the top surface of the test piece Based on the value, calculating a first tip displacement value and a second tip displacement value corresponding to the amount of opening displacement in the plastic region with respect to the tip of the preliminary fatigue crack; A method for measuring fracture toughness based on a double clip gauge considered. 3. The method of claim 2,
Between the step of forming the preliminary fatigue crack and the step of calculating the amount of displacement,
Measuring the initial length of the notch, the width value of the test piece, and the load line displacement value for the test piece, respectively; further comprising,
Calculating the predicted value includes:
Based on the measured initial length of the notch, the width value of the test piece, and the load line displacement value, and the calculated first tip displacement value, the second tip displacement value, the first tip edge value, and the second tip edge value calculating the plastic rotation coefficient; and
Calculating a fracture toughness predicted value based on the result of applying the calculated plastic rotation coefficient, the first tip displacement value, and the second tip displacement value to a plurality of fracture mechanics parameters included in the pre-stored CTOD prediction equation, respectively; A method for measuring fracture toughness based on a double clip gauge considering the plastic rotation coefficient, characterized in that.

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