JP2011043452A - Bending limit strain measuring method, bending crack determination method, and bending crack determination program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for measuring accurately a bending limit strain when performing bending deformation having strain gradient in the plate thickness direction to a plate material; and to provide a bending crack determination method and a program therefor, capable of predicting and determining effectively bending breaking from a forming analysis result by a finite element method by utilizing a measured value. <P>SOLUTION: A bending limit strain is measured by putting a ruling line on a cross section of the plate material, and a bending forming portion and an elongation strain of an outermost layer bending center part are extracted from a forming analysis output result of a bending forming product made of the plate material, and existence of a bending crack is determined by comparing the elongation strain with the bending limit strain. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  In the present invention, when performing press forming processing including bending on a plate-shaped material, it is possible to accurately predict in advance whether or not molding is possible, and to reduce costs by shortening the production preparation period, and further improve the moldability. The present invention relates to a bending limit strain measurement method used for obtaining, a bending crack determination method using the same, and a bending crack determination program.

  In the case of manufacturing a pressed part, it is generally performed that the forming prediction is examined in advance by a finite element method or the like at the part design stage. At this time, it is possible to predict to some extent the breakage of drawing or stretch forming by judgment based on a forming limit diagram (FLD). However, it has been pointed out that molding is not stable at the stage of press molding because the material, molding conditions and the like fluctuate. For this purpose, a technique for analyzing a variation factor with respect to a standard value by a computer program and feeding back to a press forming method is disclosed (Patent Document 1).

  In contrast to the deformation mode that undergoes uniform deformation in the plate thickness direction, such as drawing and stretch forming, bending deformation with a strain gradient in the plate thickness direction as shown in Fig. 1 is used to determine the fracture in the conventional FLD forming analysis. This is one of the molding methods that cannot be performed. Furthermore, with the recent increase in molding of difficult-to-work materials such as high-strength steel plates, aluminum materials, and magnesium materials, breakage due to bending has become a problem.

  In order to accurately determine the fracture associated with such bending by forming analysis, in the forming analysis such as the finite element method, in order to accurately determine the strain distribution in the thickness direction, the number of element divisions in the thickness direction is determined using the Solid element. Although it is necessary to increase it, it is computationally expensive and not practical. Furthermore, the definition of the fracture limit strain is not clear, and it is very difficult to predict the bending fracture by forming analysis such as a finite element method.

  On the other hand, hole expansion molding is a molding mode in which fracture determination by conventional FLD is difficult because the limit strain is affected by the cross-sectional properties of the material or the strain gradient in the vicinity of the crack, but the prediction technology by the finite element method is disclosed (Patent Document 2). Moreover, the prediction index of a forge crack is also disclosed also about the prediction of the crack in forging (patent document 3).

  In general, for press-molded parts, not only molding mainly for bending molding, but also bending of the ridgeline part, bending bending back deformation when passing through the die shoulder R, etc. in ordinary deep drawing, overhang molding, composite molding, etc. There is always a bending part. However, at the mold design stage, fractures due to stretch deformation due to drawing can be predicted by FLD and finite element analysis, but bending fracture due to deformation of the bending part has hardly been accurately predicted. It was.

  One of the causes is not only the scribed circle diameters of 5mm and 10mm that are usually used, but even the smallest 1mm grid (Non-Patent Document 1), in the case of severe bending such as 1mmR and 0.5mmR, It was difficult to measure the fracture limit strain of the material with sufficient accuracy because the dimensions were too large for the measurement of the distribution, which was not suitable for the measurement. Even if the fracture limit strain was measured, there was no algorithm for judging bending fracture in the finite element analysis, and there was no way to use the molding analysis result effectively for bending fracture.

  Furthermore, the conventional strain measurement method has a small ductility that is pasted along a curved surface in order to measure the distance between the crease lines on the curved surface by scoring crease lines on the outer surface of the plate-like material. Transfer the ruled lines to cello tape (registered trademark) or resin film, measure the distance between the fine wires in a state where they are affixed to a flat surface, and calculate the strain by the method of calculating the strain or 3D image processing. It is generally done. However, in these methods, an artificial error is generated at the time of tape transfer, and expansion / contraction of the tape remains as an error. In addition, in the case of image processing, there is a drawback that the measurement error becomes large when the depth of focus of the subject is deep like bending deformation. For this reason, the bending limit strain of the plate material could not be measured accurately.

JP 2006-75884 A JP 2009-61477 JP 2006-231384 A

"Press Forming Difficulty Handbook 3rd Edition": Thin Steel Sheet Forming Technology Study Group, Nikkan Kogyo Shimbun (P135)

  Accordingly, an object of the present invention is to accurately measure the bending limit strain when bending deformation having a strain gradient in the plate thickness direction is performed on a plate-shaped material, and molding by the finite element method using the measured value. It is to provide a bending crack determination method capable of effectively predicting and determining a bending fracture from an analysis result, and a program therefor.

  The method of measuring the bending limit strain of the present invention made to solve the above-mentioned problem is to perform a V-bending test or an L-bending test using a plate-shaped test piece in which a scored line in the thickness direction is put in the cross section. The elongation strain of the bending curved surface at the bending center portion of the outermost layer surface of the test piece formed at the bending radius immediately before cracking is measured, and the bending limit strain of the plate material is measured.

  As in claim 2, the elongation strain on the outermost layer surface is preferably measured by reading a line marked in advance on the cross section of the test piece with an magnifying projector or a photograph and converting the coordinate value into strain. Further, as in claim 3, it is preferable that the crease lines are scribed with a line interval of 50 microns or more and less than 1,000 microns using an ultrafine laser or ink jet having a line width of 1 micron or more and less than 50 microns.

  Further, as described in claim 4, the elongation strain of the outermost layer surface is calculated from a value obtained by scoring a ruled line on the cross section of the test piece, reading the coordinates of the intersection of the ruled line with the outer surface of the test piece after forming, Further, as in claim 5, the elongation strain is calculated from the coordinates of the intersection of the ruled line marked on the cross section of the test piece and the outer surface of the test piece. From the above, it is preferable to calculate using an elongation strain calculation formula calculated in consideration of the curvature.

  Moreover, the bending crack determination method of the present invention made to solve the above-mentioned problem is to measure the bending limit strain of a plate material by the method according to any one of claims 1 to 5, and is made of this plate material. The bending analysis part of the bending molded part and its outermost bending center strain are extracted from the bending analysis output result, and the elongation strain is compared with the bending limit strain to determine the presence or absence of bending cracks. It is what.

  As in claim 7, it is preferable that the molding analysis output result includes data on the contact coordinates, the stress value of the numerical integration point, and the strain value. Further, as in claim 8, it is preferable to extract the bending site by evaluating the difference between the elongation strain of the outermost layer surface and the elongation strain of the innermost layer and determining the bending radius. In addition, as described in claim 9, the determination of the bending crack is based on the result of molding analysis, and calculates the outermost layer elongation strain of the bending part, calculates the bending radius of the bending part, and corresponds to the analysis element size used in the analysis. Then, it is preferable to carry out a comparative study by converting the bending limit evaluation value.

  Further, as in claim 10, based on the elongation strain distribution in the vicinity of the bending center determined in advance, the outermost layer elongation strain that changes according to the element dimensions is determined, and the dependence of the inter-score distance is determined based on experimental data. In this case, it is preferable to convert to the analysis element size used in the analysis. In this case, as described in claim 11, when extrapolating the dependency of the distance between the scores based on the experimental data, Accordingly, it is preferable to reflect the influence of the strain gradient in the thickness direction by correcting the influence of the strain gradient in the thickness direction or extrapolating a correction curve at an intermediate bending radius.

  In addition, the bending crack determination program of the present invention made to solve the above-described problems is a computer program for making a bending crack determination of a bent molded article made of a plate material, bending strain limit of the plate material, and bending. A means for inputting a molding analysis output of a molded product, a means for extracting a bending molding part from the molding analysis output result, and an extension strain at the bending center portion of the outermost layer of the extracted bending molding part is corrected according to the bending radius. And a means for comparing the corrected elongation strain with the bending limit strain to perform crack determination.

  According to the method for measuring the bending limit strain of the present invention, by directly reading the ruled line of the fine line marked on the cross section of the material after forming, the error factor in the conventional method is eliminated, and the bending of the plate-like material is performed. The critical strain can be measured accurately. In addition, according to the bending crack determination method and the program of the present invention, it is possible to predict cracks occurring in the bent portion of a press-molded bent product by the finite element method, and it is possible to shorten the die manufacturing period and cost. It becomes.

It is the figure which represented typically the bending process which has a strain distribution in a plate | board thickness direction applied to this invention. In FIG representing the method of measuring the flexural strain at break of the present invention, it is a diagram of the intersection p _1, p _2, p ₃ of the central portion sectional ruffled written filament and the outer surface of the near bend schematically illustrating. It is explanatory drawing of the algorithm of the program which estimates a bending shaping | molding fracture | rupture from a shaping | molding analysis result. FIG. 5 is a diagram for converting according to dimensions when the element dimensions used in the analysis are different from the distance between scores when the fracture limit strain is obtained, and a diagram showing that the degree of influence depending on the distance between scores differs depending on the bending radius. It is the figure which showed the relationship of the distance dependence between grades when the raw material board thickness used for the analysis differs from a reference | standard board thickness, and is a figure for converting the fracture limit strain with this relation. An example showing the state after forming a line with a die shoulder of R0.5mm and a die shoulder of R3.0mm with a line width of 20 microns and a distance between ultrafine wires of 200 microns. In this example, the coordinate values of p ₁ , p ₂ , and p ₃ are obtained as strain distributions using Formulas 1 to 5 for two steel types of 590 MPa. For comparison, this is an example of strain distribution in the thickness direction when bending analysis is performed with a 200-micron solid element. This is an example in which the fracture limit of the bent portion was evaluated by forming analysis using a 590 MPa material, and the fracture risk portion was shown. The figure converted when the element size used for the analysis is different from the distance between the scores when the fracture limit strain is obtained is a figure showing that the peak shape of the elongation strain differs depending on the bending radius.

The present invention is described in further detail below.
(Measurement method of bending limit strain)
In the present invention, in order to accurately measure the bending limit strain of a plate-shaped material that is a material of a bent molded product, a V-bending test or an L-bending test is performed using the same plate-shaped test piece as the plate-shaped material. . In the cross section corresponding to the bent portion of the test piece, a ruled line in the thickness direction is provided as shown in FIG. Then, the bending test is repeated until cracking while gradually reducing the bending radius. And the elongation distortion of the bending curved surface in the bending center part of the outermost layer surface of the test piece shape | molded with the bending radius just before reaching a crack is measured, and it is set as the bending limit distortion of a plate-shaped raw material.

  It is preferable that the ruled line is an ultrathin line marked by a method such as laser marking, ink jet, or electrolytic etching with a line width of 1 to 20 microns. If the line width is less than 1 micron, it cannot be kept straight due to the roughness of the material cross section. On the other hand, if it is 20 microns or more, the line width with respect to the width of the ultra-thin lines is too large and the measurement error is close to 10%. Furthermore, it is preferable that the width of the fine wires is 50 microns or more and 1,000 microns or less. If the width between the extra fine wires is 50 microns or less, the difference from the extra fine wire width becomes smaller and the measurement error becomes larger. In addition, when the line width is 1,000 microns or more, the strain measurement at severe bending radius such as 1mmR and 0.5mmR is not suitable for measuring the fracture limit strain because the distance between points is too large.

  The conventional strain measurement method is a low-ductility cellophane tape (registered trademark) pasted along a curved surface in order to measure the distance between the crease lines on the curved surface of the material. ) And the method of calculating the strain by transferring the ruled lines to the resin film and measuring the distance between the ultrathin wires in a state of being applied to a flat surface, or calculating the strain by three-dimensional image processing. However, in these methods, an artificial error is generated at the time of tape transfer, and expansion / contraction of the tape remains as an error. In addition, in the case of image processing, there is a drawback that the measurement error becomes large when the depth of focus of the subject is deep like bending deformation. On the other hand, according to the present invention, it is possible to eliminate an error factor in the conventional method by directly reading the ruled line of the fine line marked on the cross section of the material after forming.

  The elongation strain on the outermost layer surface can be measured by reading a line marked in advance on the cross section of the test piece with an magnifying projector or a photograph, and converting the coordinate value into strain. Further, the elongation strain on the outermost layer surface can be obtained by reading the coordinates of the intersection point of the ruled line with the outer surface of the test piece after forming and calculating from the value. The elongation strain can be calculated using an elongation strain calculation formula calculated from the coordinate position of the three points in consideration of the curvature. This will be described in detail as follows.

The elongation strain in the measuring method of bending limit strain according to the present invention is measured at three points of intersection coordinates (x, y) between the score line and the outer surface after bending, and the elongation strain ε _{p1 to p2} is calculated by the following procedure. To do. The strain calculation method of the present invention reads the (x, y) coordinates of the three points p ₁ , p ₂ , and p ₃ at the intersection of the cross-section of the bent product and the outermost layer surface as shown in FIG. The elongation strain of the curved surface part from ₁ to p ₂ is calculated.

First, a distance W _{1-3 obtained} by connecting p ₁ and p ₃ with a straight line is obtained by the following equation (1).

Next, when the perpendicular distance d between the line connecting p ₁ and p ₃ and p ₂ is solved as a geometric problem, the following equation (2) is obtained.

Based on this d, the curvature r from p ₁ to p _{3 is} calculated by the following equation (3).

Based on this curvature r, the lengths L _p1 to _p2 of the curved surface from p ₁ to p ₂ are _obtained by the following equation (4).

From this L _{P1 to P2,} the initial p ₁ and elongation strain from the straight line distance L ₀ of p ₂ ε _p1-p2, calculated by the formula for a number of 5.

  Thus, in the present invention, in the simple bending test such as the V-bending test or the flange-up test, the outermost layer elongation strain of the molded product formed with the minimum bending radius that can be formed without breaking is used as the breaking limit bending strain. .

(Bending crack judgment method and its program)
Next, an analysis method for determining a bending crack in the present invention will be described. For this purpose, a computer program having the algorithm shown in FIG. 3 is used.

  This program includes an input means 10 for inputting a bending limit strain of a plate-like material and a molding analysis output of a bent molded product, an extracting means 20 for extracting a bending molding site from the molding analysis output result, and an extracted bending molding. Correction means 30 for correcting the elongation strain of the outermost layer bending center portion of the part according to the bending radius, and determination means 40 for performing crack determination by comparing the corrected elongation strain with the bending limit strain, The computer has a function to execute each of these means.

  First, the forming analysis output data output by a numerical analysis program such as the finite element method is input in the input means 10. The input means 10 also inputs the bending limit strain of the plate-like material obtained by the above-described method.

  Next, the extraction part 20 extracts the bending part of the bending molded product which is the analysis target part. As shown in Fig. 3, the bending part is extracted by calculating the outer layer strain for each element, calculating the bending R from the difference in strain between the front and back surfaces of the bent molded product, and calculating the bending R and the thickness ratio R / t. Includes steps. In addition, after extracting a bending part, only a site | part whose R / t is 10 or less is used for a fracture | rupture determination. This is because when R / t> 10, the bending radius is sufficiently large, and it is not necessary to determine the crack, and the condition for causing the bending crack is not satisfied.

  Next, in order to compare the fracture limit bending strain obtained by the experiment and the outermost layer elongation strain of the analysis result, the correction means 30 determines the elongation strain of the analysis result according to the distance between the scores according to the element dimensions used for the analysis. To convert.

  The operation to convert the distance between the scores according to the element dimensions used in this analysis is the fracture limit of the distance between the scores obtained in the experiment in order to show the distribution of the fracture limit bending strain in bending forming with a peak in the bending center. This is because, when the strain and the element dimensions used in the analysis are different, it is necessary to make the distance between the scores the same as the distance between the scores obtained through experiments in order to compare the analysis results.

  FIG. 4 shows a change in the bending limit strain obtained by the experiment depending on the distance between the scores. In this experiment, the distance between the crease lines is set to 200 microns. Therefore, when the element size is set to 2.0 mm or 4.0 mm in the forming analysis, the bending strain elongation strain value of the analysis result depends on the bend radius. It is necessary to compare it with the value of the bending limit strain obtained by experiment after reflecting it in the relationship. If this operation is not performed, the bending strain elongation value of the analysis result will be lower than the actual value, and the bending crack determination cannot be performed accurately.

  Furthermore, the dependency of the bend limit strain on the distance between the scores is that the value of the bend limit strain is affected by the strain gradient in the thickness direction, and as shown in FIG. (Because of radius 0.5mmR, 1.0mmR, etc.), the bending limit strain is abruptly increased, so it is necessary to perform correction depending on the distance between scores (correction of the effect of strain gradient) according to the bending radius.

  In addition, the curve depending on the inter-score distance of the breaking limit bending strain is different in the peak shape of the bending limit strain at the bending center depending on the thickness of the target material. It is necessary to convert according to / t. As an example, FIG. 5 shows a correction diagram using a 590 MPa DP material.

  After performing such correction, the determination means 40 compares the corrected elongation strain with the bending limit strain to perform crack determination. Furthermore, the fracture determination location of the bending site is displayed by post processing.

As described above, according to the bending crack determination method of the present invention, based on the molding analysis output by the numerical analysis program such as the finite element method and the bending limit strain obtained by the experiment, the bending crack of the bending molded product is performed. The presence or absence of can be accurately determined.
Examples of the present invention are shown below.

[Example 1]
A ruled line was drawn on the cross section of the plate-like material near the center of bending. FIG. 6 shows a state after bending of a sample having a line width of 20 microns and a ruled line with a line interval of 200 microns by laser marking. Measure the coordinate points of the intersections p ₁ , p ₂ , and p ₃ between the marked fine line and the outer surface with an magnifying projector, and obtain the elongation strain ε _p1-p2 using the formulas 1-5 above. FIG. 7 shows an example of measuring the strain distribution along the bending center. The elongation strain shows a peak at the center of bending, and the peak value of the elongation strain at which cracking occurs is the fracture limit strain. In addition, by calculating the value of the fracture limit bending strain when the distance between grades is changed with the position of the peak value in the vicinity of the bending center as the origin, a conversion diagram dependent on the distance between grades is created as shown in FIG. can do.

  Fig. 8 shows a bending analysis model in which bending is performed using a solid element and discretized at 200 microns in the thickness direction, length direction, and depth direction, and the equivalent plasticity after bending at a bending radius of 0.5 mm. The strain distribution is shown. As can be seen from the figure, in the thickness direction, tensile strain is distributed in the outermost layer with a peak at the bending center, and plastic strain due to compression is generated in the innermost layer.

  In ordinary forming analysis, analysis by the Shell element is common due to the calculation cost. Therefore, the outermost layer compressive strain and innermost layer compressive strain can be calculated by setting the plate thickness direction integration point of the numerical integration point to 5 or more. Can improve the accuracy. Here, a shell element with a side of 2 mm is subjected to molding analysis at 7 numerical integration points, the outermost layer strain is obtained from the numerical calculation result file, the bending part is determined by a program having the algorithm of FIG. FIG. 9 shows the result of determining the fracture by comparing the fracture limit strains of the bent parts using a conversion diagram of the distance between the four grades. As a result, it was possible to detect severely bent parts at the mold design stage, which contributed to shortening the mold production period.

[Example 2]
In the bending limit strain measurement method of the present invention, when determining the bending crack limit strain from the experimental results, various changes are made in the line width and the distance between the ruled lines using conventional methods such as electrolytic etching, stamping, laser marking, and inkjet. Table 1 shows the determination results when the elongation strain of the bending portion having a bending radius of 1.0 mm was measured.

  The reading error was marked with ○ within ± 5%, Δ within ± 10%, and x otherwise. In addition, the determination of the bending section strain was made positive with the value of the outermost layer elongation strain obtained by the primary solution as positive, within ± 10% from this value, Δ within ± 20%, and x otherwise. Comprehensive judgment was judged by the ratio of ○ or △, and when there was at least one x, it was taken as x.

  As a result, conventional electrolytic etching and stamps have a large line width and are not suitable for measurement of bending limit strain, and can be used for measurement of bending limit strain by appropriately setting a marking line by laser marking or inkjet. It turns out that you can.

[Example 3]
Next, as Example 3, the bending strain distribution in the vicinity of the bending center corresponding to FIG. 7 was measured with the bending radius changed, and is shown in FIG. Since the strain peak varies depending on the bending radius as shown in this figure, it is preferable to convert the bending limit strain according to the bending radius as shown in FIG.

DESCRIPTION OF SYMBOLS 10 Input means 20 Extraction means 30 Correction means 40 Determination means

Claims (12)

  Bending center part of the outermost layer surface of the test piece formed with a bending radius just before reaching the crack by performing a V-bending test or L-bending test using a plate-shaped test piece with a score line in the thickness direction in the cross section The bending limit strain measurement method characterized by measuring the elongation strain of the bending curved surface in to obtain the bending limit strain of the plate material.   2. The elongation strain on the outermost layer surface is measured by reading a line marked in advance on the cross section of the test piece with an magnifying projector or a photograph, and converting the coordinate value into strain. Bending limit strain measurement method.   2. The bending limit according to claim 1, wherein the ruled lines are marked with a line interval of 50 μm or more and less than 1,000 μm using an ultra-fine laser or ink jet having a line width of 1 μm or more and less than 50 μm. Strain measurement method.   The elongation strain of the outermost layer surface is obtained by scoring a crease line on the cross section of the test piece, reading the coordinates of the intersection of the crease line with the outer surface of the test piece after forming, and calculating from the value. The bending limit strain measuring method according to claim 3.   The elongation strain is calculated from the coordinates of the intersection of the scribe line marked on the cross section of the test piece and the outer surface of the test piece using an elongation strain calculation formula that is calculated considering the curvature from the coordinate position of the three points. The bending limit strain measuring method according to claim 4, wherein the bending limit strain measuring method is calculated.   The bending limit strain of the plate-shaped material is measured by the method according to any one of claims 1 to 5, and the bending-formed portion and its outermost layer bending center portion are obtained from the result of molding analysis output of the bent-shaped product made of the plate-shaped material. A bending crack determination method characterized by extracting the elongation strain of the sample and comparing the elongation strain with the bending limit strain to determine the presence or absence of a bending crack.   7. The bending crack determination method according to claim 6, wherein the forming analysis output result includes data of contact coordinates, a stress value of a numerical integration point, and a strain value.   8. The bending crack determination method according to claim 7, wherein the extraction of the bending portion is performed by evaluating a difference between an elongation strain of the outermost layer surface and an elongation strain of the innermost layer and determining a bending radius.   Judgment of bending cracks is based on the results of molding analysis, and calculates the outermost layer elongation strain of the bending part, calculates the bending radius of the bending part, and converts the bending limit evaluation value according to the analysis element size used in the analysis. The method for determining a bending crack according to claim 6, wherein the method is performed by comparative examination.   Based on the elongation strain distribution in the vicinity of the bending center determined in advance, the outermost layer elongation strain that changes according to the element dimensions is extrapolated based on the experimental data on the dependency of the distance between the scores, and the analysis used in the analysis The bending crack determination method according to claim 9, wherein the method is converted into an element size.   When extrapolating the dependency of the distance between scores based on experimental data, the effect of the strain gradient in the thickness direction is corrected according to the bending radius, or the correction curve is extrapolated at an intermediate bending radius. The bending crack determination method according to claim 10, wherein an influence of a directional strain gradient is reflected. In order to make a bending crack judgment of a bent molded product made of a plate-like material,
Means for inputting the bending limit strain of the plate-like material and the molding analysis output of the bent molded product;
Means for extracting a bending part from a molding analysis output result;
Means for correcting the elongation strain at the outermost layer bending center portion of the extracted bending forming portion according to the bending radius;
Means for determining the crack by comparing the corrected elongation strain with the bending limit strain;
Bending crack judgment program to make it function.