JP7405119B2 - Biaxial stress testing device and biaxial stress testing method - Google Patents

Description

The present invention relates to a biaxial stress testing device and a biaxial stress testing method, and particularly to a biaxial stress testing device and a biaxial stress testing method that apply a compressive load to a cross-shaped plate-shaped test piece from at least one axial direction.

BACKGROUND ART Conventionally, as a material test for metal materials, an FLD (Forming Limit Diagram) test has been conducted to measure the area in which a thin metal plate can be press-formed without breaking it (see Non-Patent Document 1, etc.). By using the forming limit diagram obtained from the FLD test, it is possible to objectively and quantitatively evaluate the risk of overhang cracks, drawing cracks, etc. that occur during press forming of thin metal sheets. Become.

In addition, as a material test for metal materials, material tests are also conducted in which a compressive load is applied to a thin metal plate (Patent Document 1, Patent Document 2, Patent Document 3, etc.).
For example, in the material test disclosed in Patent Document 3, by applying a compressive load in two in-plane directions to a thin test piece of a thin metal sheet, the material properties of the thin metal sheet under biaxial compressive stress are evaluated. It is said that it can be measured with high accuracy. By measuring the material properties of these thin metal sheets under compressive stress through material testing in which a compressive load is applied to these thin metal sheets, it is possible to contribute to improving the prediction accuracy of CAE analysis (press forming simulation) of the press forming process. It is expected.

Generally, when a thin metal plate is subjected to a compressive load during press forming, a phenomenon occurs in which the thin metal plate suddenly deforms out of plane and protrudes from the target shape of the press formed product (out-of-plane buckling), causing wrinkles ( (hereinafter sometimes referred to as "press wrinkles") may occur. There are two mechanisms by which such press wrinkles occur: (1) elastic or plastic buckling of the thin metal sheet, and (2) excessive or uneven material inflow into the thin metal sheet. being classified.

Among these, the elastic or plastic buckling phenomenon (1) is related to, for example, shrinkage flange during the press forming process of a press-formed product having a hat cross-section and having a top plate, a vertical wall, and a flange. There are cases where deformation is caused by compressive stress generated in the flange, compressive stress generated in the vertical wall due to material flowing into the flange, and non-uniform deformation of a thin metal plate due to uneven loads, non-axisymmetric loads, etc. There are cases where this is caused by
On the other hand, it is said that (2) over-thickness and under-thickness occur in areas where the shape of the press-formed product changes rapidly (see Non-Patent Document 2).

Further, press wrinkles are more likely to occur as the thin metal sheet is thinner and the strength is higher. In stretch molding, excess material on the stretch molding surface due to material flowing into the processed area from the surrounding area tends to be a direct factor, making it difficult to obtain a press-formed product with the target shape while preventing the occurrence of press wrinkles. There was a problem.

Therefore, in order to obtain a press-formed product that prevents the occurrence of press wrinkles, we must clarify the factors that cause wrinkles during the press-forming process, and adjust the press-forming conditions according to the material properties of the thin metal sheet and the target shape of the press-formed product. Need to decide.

A method for predicting the occurrence of press wrinkles is to perform a press forming simulation using an elastic-plastic finite element analysis method, etc., and display the press-formed product during and after forming, determined by the press forming simulation, on a computer screen. In this case, there is a technology that applies shading according to the degree of compressive stress or compressive strain to visually determine the presence or absence of wrinkles.

Furthermore, several techniques have been proposed to estimate the mechanism of wrinkle generation based on strain, stress, etc. calculated by press forming simulation, and to obtain an index for quantitatively determining the presence or absence of wrinkle generation.
For example, in Patent Document 4, the equivalent stress and equivalent strain of each element in the press forming process are determined by press forming simulation of a plate-shaped material based on the elasto-plastic finite element method, and the plate-shaped material is processed based on the obtained equivalent strain. If the difference between the equivalent stress obtained from the hardening curve and the equivalent stress obtained by press forming simulation is large, it is assumed that buckling has occurred at the position of the element in question, and the difference is determined as a wrinkle evaluation parameter to evaluate the wrinkles. Techniques for evaluating the presence or absence of occurrence have been disclosed.
Furthermore, Patent Document 5 discloses that when a press-formed product having a top plate portion and a vertical wall portion curving outward is subjected to shrinkage flange forming by form (bending) molding, a phenomenon occurs at the tip of the vertical wall portion. A technique has been disclosed in which the presence or absence of wrinkles in a press-molded product is determined in advance based on whether or not the compressive strain exceeds the wrinkle generation limit strain.
Furthermore, Patent Document 6 describes that the radius of curvature or curvature after unloading (mold release) is estimated based on the bending stress in the thickness direction cross section of the material to be formed at the bottom dead center in press forming, and A technique has been disclosed for predicting the occurrence of wrinkles in a material to be formed during press molding according to the radius of curvature or the curvature.

Patent No. 6246074 JP 2016-3951 Publication JP 2019-35603 Publication Japanese Patent Application Publication No. 11-319971 JP 2017-100165 Publication JP2007-229761A

ISO 12004-2:2008, "Metallic materials - Sheet and strip - Determination of forming-limit curves - Part 2: Determination of forming-limit curves in the laboratory", 2008 Edited by Thin Steel Sheet Forming Technology Study Group, Press Forming Difficulty Handbook 4th Edition, p.226, Nikkan Kogyo Shimbun, (2017)

All of the techniques disclosed in Patent Documents 4 to 6 assume the mechanism of wrinkle generation in advance, and obtain a judgment index regarding the presence or absence of wrinkle generation based on the mechanism. However, the boundary value (critical value) of the judgment index for the presence or absence of wrinkles may lack objectivity because it is determined by visual inspection and sensory judgment of the presence or absence of wrinkles in a press-formed product by experiment or press-forming simulation. That was a problem.
In addition, the technologies disclosed in Patent Documents 4 to 6 are aimed at specific press molding methods (such as foam molding) and press-formed products of specific shapes, and are Since the state of strain and stress differs depending on the shape of the material, the problem is that the indicators for determining the presence or absence of wrinkles required by these techniques are not versatile.

Therefore, similar to the FLD test for fracture described above, it is easy to objectively and quantitatively determine the presence or absence of wrinkles, and it is possible to universally determine the occurrence of wrinkles regardless of the press forming method or the shape of the press-formed product. There was a need for a test method that could evaluate the
However, the FLD test determines the starting point (critical strain, etc.) at which rupture (cracking) occurs in a thin metal sheet during the press forming process, and out-of-plane buckling that occurs when a compressive load is applied to the thin metal sheet during the press forming process. It is not possible to determine the origin of wrinkles caused by this.

In addition, the techniques disclosed in Patent Documents 1 to 3 are all for performing material tests by suppressing out-of-plane buckling when applying a compressive load to a thin metal sheet. It was not possible to determine the out-of-plane buckling deformation under stress, that is, the index (strain, etc.) that is the starting point of wrinkle generation.
Furthermore, most of the deformation that a blank undergoes during press forming using a thin metal plate as a material (blank) occurs in a biaxial stress state where a compressive load is applied in at least one of the in-plane biaxial directions. Therefore, in order to evaluate the presence or absence of wrinkles during press forming, it was necessary to find an index that is the starting point of wrinkles in a biaxial stress state where a compressive load is applied in at least one in-plane direction.

The present invention has been made in order to solve the above-mentioned problems, and it applies a compressive load to a cross-shaped plate-shaped specimen from at least one axis, and determines the index that is the starting point of wrinkle generation in a biaxial stress state. The purpose of the present invention is to provide a biaxial stress testing device and a biaxial stress testing method that can determine .

The inventor reproduced the occurrence of wrinkles (out-of-plane buckling) in a state of biaxial stress using a plate-shaped test piece of a thin metal plate, and determined the index that is the starting point of wrinkle generation objectively and without relying on sensory evaluation. We worked hard to find a method that would allow for quantitative measurement.

As mentioned above, the deformation that a blank undergoes during press forming using a thin metal sheet as a material (blank) is not an in-plane uniaxial deformation, but mostly biaxial compression or a combination of compression and tension. Therefore, in order to evaluate the presence or absence of press wrinkles (out-of-plane buckling) that occur during press forming, the inventor conducted a biaxial in-plane compression test method disclosed in Patent Document 3 mentioned above. We focused on the fact that it is possible to change the deformation state in various ways by varying the load and strain ratios applied in two in-plane directions of the test piece.

Therefore, in the biaxial in-plane compression test method disclosed in Patent Document 3, the inventor provided an opening in the pressing surface of the central mold, and developed It is recalled that by causing directional deformation in the opening, out-of-plane buckling, that is, wrinkling, occurs under various compressive stress states, and it becomes possible to measure the index that is the starting point of wrinkling. It's arrived.
The present invention has been made based on the above considerations, and has the following configuration.

(1) The biaxial stress test device according to the present invention is a biaxial stress test device that applies a compressive load in at least one in-plane direction to a cross-shaped plate-shaped specimen in two in-plane directions,
a rectangular center portion facing the measurement portion where the cross shapes intersect in the plate-like test piece; and a comb-like first comb-teeth portion formed at the edges of the four sides of the center portion; A pair of central molds that sandwich both sides of a plate-shaped test piece,
a second comb-teeth portion disposed on the four sides of the central mold and meshing with the first comb-teeth portion; and a holding portion for holding the cross-shaped piece of the plate-like test piece. and a side mold having;
The present invention is characterized in that an opening is formed in the central portion of one of the pair of central molds to induce out-of-plane buckling of the measuring portion.

(2) In the items described in (1) above,
The present invention is characterized in that the opening of the central mold is provided with a displacement meter that measures the displacement of the measurement portion of the plate-shaped test piece in an out-of-plane direction.

(3) The biaxial stress testing method according to the present invention applies a compressive load in at least an in-plane uniaxial direction to the plate-shaped test piece using the biaxial stress testing apparatus described in (1) or (2) above. It is something that makes
sandwiching both sides of the measurement part of the plate-shaped test piece between the pair of central molds;
Each of the four pieces of the plate-like test piece is held by the four side molds, and the second comb tooth portion of the side mold is attached to the first comb tooth portion of the central mold. mesh,
Applying a compressive load in at least an in-plane uniaxial direction to the measurement part via the piece part,
The invention is characterized in that out-of-plane buckling of the measuring section is induced in the opening of the central mold.

(4) In the items described in (3) above,
Obtaining the relationship between the strain of the measuring part and the compressive load in the process of applying a compressive load in at least an in-plane uniaxial direction to the measuring part,
Calculating the first derivative of strain based on the relationship between the acquired strain and compressive load,
Determine the strain at which the first derivative of the calculated strain becomes maximum as the critical strain for stable behavior against out-of-plane buckling of the measurement part,
In the relationship between the acquired strain and the compressive load, the strain at which the polarity of the strain increment is reversed with respect to the increment of the compressive load is determined as the wrinkle generation starting strain in the measuring section.

In the present invention, a rectangular central part facing the measurement part where the cross shapes of the plate-shaped test piece intersect, and a comb-shaped first comb-teeth part formed at the edges of the four sides of the central part. a pair of central molds that sandwich both surfaces of the plate-shaped test piece; and second comb-shaped comb teeth disposed on four sides of the central mold and meshed with the first comb-teeth portions. a side mold having a holding part that holds a cross-shaped piece of the plate-shaped test piece; By forming an opening that can induce out-of-plane buckling of the central mold, a compressive load in at least an in-plane uniaxial direction is applied to the plate-shaped test piece. A material test can be performed to detect the starting point of wrinkle generation by inducing out-of-plane buckling of the measurement part in the opening.
In addition, in the present invention, the relationship between strain and compressive load is acquired in the process of applying compressive load to a cross-shaped plate-like test piece, and the starting point of wrinkle generation is determined from the relationship between the acquired strain and compressive load. By determining the strain, it is possible to evaluate the presence or absence of out-of-plane buckling based on the strain that is the starting point of wrinkle generation, so it is possible to evaluate the presence or absence of wrinkles in press forming simulations of press-formed products without relying on visual sensory evaluation. Can be evaluated objectively.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating the configuration of a biaxial stress testing apparatus according to an embodiment of the present invention ((a) central mold and side mold, (b) central mold). FIG. 2 is a diagram showing an example of the shape of a cross-shaped plate-like test piece to be tested in an embodiment of the present invention ((a) a plan view, (b) an enlarged view of a corner of the cross). FIG. 2 is a diagram illustrating the holding mechanisms of the central mold and side molds of the biaxial stress testing apparatus according to the embodiment of the present invention ((a) state in which the holding mechanisms are installed in the central mold and side molds). , (b) holding mechanism). 1 is a plan view of a biaxial stress testing apparatus according to an embodiment of the present invention. In the biaxial stress test method according to the embodiment of the present invention, when a biaxial compressive load is applied to the measuring part of a cross-shaped plate-like test piece, the strain, compressive load, and wrinkle height generated in the measuring part are measured. It is a graph showing a relationship. This is an example of a strain-load diagram when a compressive load is applied in two in-plane directions (X-axis direction and Y-axis direction) to a cross-shaped plate-shaped test piece. It is an example of a strain-load diagram when changing the load ratio in the in-plane biaxial direction applied to the measurement part of a cross-shaped plate-like test piece in the biaxial stress test method according to the embodiment of the present invention. ((a) Load ratio -1:-4, (b) Load ratio -2:-1, (c) Load ratio -1:-1). FIG. 2 is a diagram illustrating a press-formed product that is a molding target of a press-forming simulation and a mold used for press-molding the press-formed product in an example. In the examples, the shading diagram of the press-formed product obtained by press-forming simulation, the strain in the side part of the press-formed product at a portion where the compressive load in the in-plane biaxial directions is equal, and the strain obtained by the biaxial stress test method. - An explanatory diagram showing the correspondence with the load diagram ((a) press-formed product with a forming height of 40 mm, (b) press-formed product with a forming height of 30 mm, (c) press-formed product with a forming height of 20 mm, (d) Strain-load diagram). FIG. 1 is a cross-sectional view of a biaxial stress testing apparatus according to an embodiment of the present invention, and is an explanatory diagram showing a state in which a displacement meter for measuring displacement in an out-of-plane direction of a measurement part of a plate-shaped test piece is installed (FIG. 1 (A-A sectional view in (b)).

Hereinafter, prior to explaining the biaxial stress testing device and the biaxial stress testing method according to the embodiment of the present invention, a plate-shaped test piece used as a test object in the present embodiment will be explained.

<Plate-shaped test piece>
As shown in FIG. 2 as an example, the plate-shaped test piece 100 has a cross shape in two in-plane axes, and includes a rectangular measurement section 101 that is the center of the cross shape, and two in-plane directions from the four sides of the measurement section 101. It has four pieces 103 extending in the axial direction.

The measurement unit 101 induces out-of-plane buckling in a portion facing an opening 17 provided in a central portion 13a of a central mold 11a, which will be described later, in a biaxial stress test, and serves as a measurement target for strain, stress, etc. It is a part. Although FIG. 2 shows an example in which the rectangular measuring section 101 is square, in the present invention, the measuring section may be rectangular.

The piece part 103 consists of a pair of piece parts 103a and a pair of piece parts 103b that face each other with the measuring part 101 in between, and the direction in which the pair of piece parts 103a extends with the measuring part 101 in between and the pair of piece parts 103b are perpendicular to the extending direction in the measuring section 101, and these directions correspond to in-plane biaxial directions in which a predetermined load is applied to the measuring section 101 of the plate-shaped test piece 100.

That is, a biaxial compression test is performed in which the measuring section 101 is placed in a biaxially compressed state by applying a compressive load in two in-plane directions to the measuring section 101 via the pair of pieces 103a and the pair of pieces 103b. be able to.

Alternatively, the measuring section 101 can be uniaxially compressed by applying a compressive load toward the measuring section 101 through one piece 103a and applying a tensile load to the measuring section 101 through the other piece 103b.・Can perform uniaxial compression and other axial tension tests with other axial tension conditions.

By attaching a strain gauge to the measurement section 101, the strain of the measurement section 101 in a biaxial stress state (biaxial compression state, uniaxial compression/other axis tension state) can be measured.

Note that the plate-shaped test piece 100 shown in FIG. 2 has a circular notch 105 in which each corner of a cross shape is circularly cut out, and a measuring part 101 is surrounded and adjacent to each other, as described in the following reference literature. A plurality of hole-shaped portions 107 are provided on each line connecting the centers of the circular notch portions 105.
(Reference: Japanese Patent Application Publication No. 2019-35603)

The circular notch 105 prevents the pieces 103a and 103b from bulging and the corners from folding when a biaxial compressive load is applied to the measurement unit 101 through each of the pieces 103a and 103b. This makes it possible to cause the measuring section 101 to generate compressive strain required for measuring material properties.

The hole-shaped portion 107 is for dispersing local stress concentration in the measuring portion 101 and reducing stress variations.

However, the biaxial stress testing device and the biaxial stress testing method according to the present invention are not limited to using the plate-shaped test piece 100 having the shape shown in FIG. The shape and dimensions of the plate-like test piece may be changed as appropriate depending on the conditions (load ratio, etc.) of the biaxial stress test, regardless of the presence or absence of the test piece.

<Biaxial stress test device>
The biaxial stress test device 1 according to the embodiment of the present invention applies a compressive load in at least one in-plane direction to a cross-shaped plate-like test piece 100 in two in-plane directions, as shown in FIG. 2 as an example. As shown in FIGS. 1 and 3 as an example, this device includes a central mold 11, a side mold 21, and a holding mechanism 31. Each configuration of the biaxial stress testing apparatus 1 will be described below.

≪Central mold≫
As shown in FIGS. 1(b) and 4, the central mold 11 has a rectangular central part 13 facing the measurement part 101 where the cross shape of the plate-shaped test piece 100 intersects, and four edges of the central part 13. It is composed of a pair of central molds 11a and 11b that sandwich both surfaces of the plate-shaped test piece 100.

The first comb-teeth portion 15 includes a pair of first comb-teeth portions 15a and a pair of first comb-teeth portions 15b formed on the edges of two opposing sides of the rectangular central mold 11.
The direction in which the pair of opposing first comb teeth portions 15a are arranged and the direction in which the pair of opposing first comb teeth portions 15b are arranged are orthogonal to each other in the central portion 13. These directions are, as shown in FIG. 4, an in-plane biaxial direction in which a compressive load or a tensile load is applied to the measuring section 101 via the pair of pieces 103a or the pair of pieces 103b of the plate-shaped test piece 100. Corresponds to the direction.

Furthermore, an opening 17 is provided in the central portion 13a of one of the pair of central molds 11, 11a, to provide a degree of freedom in deforming the shape of the measuring part 101, thereby inducing out-of-plane buckling. is formed.
In addition, in order to prevent the plate-shaped test piece 100 from moving in the in-plane direction, the central mold 11 has four corners secured by positioning pins 19 with holding mechanisms 31 (see FIGS. 1(b) and 3). 3) is fixed to the base portion 33.

≪Side mold≫
As shown in FIGS. 1A and 4, the side mold 21 includes second comb-shaped combs disposed on the sides of the four sides of the central mold 11 and meshing with the first comb-teeth portions 15. It has a tooth part 23 and a holding part 25 that holds the cross-shaped piece part 103 of the plate-shaped test piece 100, and a pair of side molds 21a disposed with the central mold 11 in between. It consists of a pair of side molds 21b.

The side mold 21a includes a second comb tooth part 23a that engages with the first comb tooth part 15a of the central mold 11 in a removable manner, and a holding part 25a that holds the piece part 103a of the plate-shaped test piece 100. have

The side mold 21b includes a second comb tooth portion 23b that engages with the first comb tooth portion 15b of the central mold 11 in a removable manner, and a holding portion 25b that holds the piece portion 103b of the plate-shaped test piece 100. have

In this way, the side mold 21 moves while the second comb tooth portion 23 meshes with the first comb tooth portion 15, thereby applying a compressive load to the plate-shaped test piece 100 via the piece portion 103. Buckling in the piece portion 103 can be suppressed.

In addition, in this embodiment, the side mold 21a and the side mold 21b are installed on rollers 27a and 27b, respectively, provided on the upper surface of the base portion 33 of the holding mechanism 31, and the second comb tooth portion 23a and 23b can move in a uniaxial in-plane direction while being engaged with the first comb tooth portions 15a and 15b of the central mold 11, respectively.

≪Holding mechanism≫
The holding mechanism 31 serves as a clamping force imparting means, and is capable of inserting and removing the second comb tooth portion 23a that meshes with the first comb tooth portion 15a and the second comb tooth portion 23b that meshes with the first comb tooth portion 15b. A predetermined clamping force is applied to the plate-shaped test piece 100 in the thickness direction, and as shown in FIG. 3, it includes a base portion 33, a top plate portion 35, and bolts 37.

In the holding mechanism 31, by tightening the four corners of the base part 33 and the top plate part 35 with bolts 37 with a predetermined tightening force, the first comb tooth part 15 and the second comb tooth part 23 which are meshed can be inserted and removed. This is preferable because it is possible to apply a predetermined clamping force to the plate-shaped test piece 100 in the thickness direction of the plate-shaped test piece 100, and to reliably prevent buckling deformation in the piece portion 103.

The biaxial stress testing apparatus 1 according to the present invention has a displacement meter 51 installed in the opening 17 of the central mold 11a to measure the displacement in the out-of-plane direction of the measurement part 101 of the plate-shaped test piece 100, as described later. (FIG. 10, AA sectional view in FIG. 1(b)) is preferably provided. This makes it possible to measure the displacement in the out-of-plane direction of the measuring section 101 in a biaxial stress state in which a compressive load is applied to the cross-shaped plate-shaped test piece 100 at least in one axial direction within the plane. The origin of the occurrence of 53 can be directly and quantitatively confirmed. Note that a laser displacement meter or a contact type displacement meter can be used as the displacement meter.

<Biaxial stress test method>
The biaxial stress testing method according to the embodiment of the present invention uses the biaxial stress testing apparatus 1 shown in FIG. 100 is subjected to a compressive load in at least one in-plane uniaxial direction.
Hereinafter, a case will be described in which a compressive load in at least one in-plane direction is applied to the plate-shaped test piece 100 using the biaxial stress testing apparatus 1.

First, as shown in FIG. 1(b), both sides of the measurement part 101 where the cross shapes of the plate-shaped test piece 100 intersect are sandwiched between a pair of central molds 11a and 11b.

Next, as shown in FIG. 1(a), a side mold 21a and a side mold 21b are arranged on the four sides of the central mold 11, and a plate-shaped test is performed using the holding parts 25a and 25b. While holding the piece 103a and the piece 103b of the piece 100, as shown in FIG. , the second comb teeth 23b of the side molds 21b are engaged with the first comb teeth 15b of the central mold 11.
Then, as shown in FIG. 3, the center die 11 and the side die 21 are held between the central die 11 and the side die 21 with a predetermined clamping force in the thickness direction of the plate-shaped test piece 100, as shown in FIG.

Subsequently, as shown in FIG. 4, the side mold 21a is moved in the in-plane uniaxial direction while the second comb teeth 23a is engaged with the first comb teeth 15a, and the first comb teeth 15b is fitted with the second comb teeth 23a. The side mold 21b is moved in an in-plane uniaxial direction while the two comb tooth portions 23b are engaged with each other.

Here, since the side mold 21a and the side mold 21b can be moved along the in-plane biaxial directions of the measuring section 101 without interfering with each other, the piece 103a and the piece 103b can be moved. A compressive load acts on the measurement unit 101 from at least one in-plane uniaxial direction. Since the opening 17 is provided in the central portion 13a of the central mold 11a, out-of-plane buckling can be induced in the portion of the measurement portion 101 of the plate-shaped test piece 100 that faces the opening 17.

≪Measurement of strain that is the starting point of wrinkle formation≫
By the biaxial stress testing method according to the present embodiment, it is possible to determine the strain at which wrinkles occur in the measurement portion 101 of the plate-shaped test piece 100 (wrinkle generation starting point strain).
Hereinafter, as an example, a procedure for determining the wrinkle generation starting strain in the measurement part 101 when a biaxial compressive load is applied to the measurement part 101 of the plate-shaped test piece 100 will be described.

In the following explanation, the plate-shaped test piece 100 is prepared using a steel plate with a tensile strength of 270 MPa class and a plate thickness of 1.2 mm, and the size of the measurement part 101 in the plate-shaped test piece 100 is 30 mm x 30 mm, and the circular notch 105 is used. The radius _r1 of the hole 107 is 2.5 mm, the long axis length 2r _x is 5.0 mm, the short axis length 2r _y is 1.0 mm, and the opening 17 in the central mold 11a is circular with a diameter of 25 mm. This is the case.

(Obtaining the relationship between strain and compressive load)
First, a strain gauge is attached to the position of the opening 17 of the measurement part 101 of the plate-shaped test piece 100, avoiding the central part, and both sides of the plate-shaped test piece 100 are attached using a pair of central molds 11a and 11b. Insert. Here, two uniaxial strain gauges are attached to the measurement part 101 to measure the strain in each of the two in-plane directions in which a compressive load is applied to the plate-shaped test piece 100. Note that instead of the two uniaxial strain gauges, a biaxial strain gauge in which two sensors are attached perpendicularly to each other may be used.

Next, a biaxial compressive load is applied to the measuring part 101 using the side mold 21 through the piece part 103 of the plate-shaped test piece 100, and the strain in the measuring part 101 and the compressive load applied to the measuring part 101 are Get the relationship. Note that the compressive load may be a value on one axis, or may be an average value or a sum of two axes.
Then, a strain-load diagram shown in FIG. 5 is created based on the relationship between the acquired strain and compressive load. In FIG. 5, the horizontal axis represents the load applied to the plate-shaped test piece 100 (plus: tensile load, minus: compressive load) and the first derivative of strain with respect to the compressive load, and the left vertical axis represents the strain measured by the strain gauge (plus represents the tensile strain, and the negative represents the compressive strain), and the right vertical axis represents the wrinkle height, which is the amount of deformation in the thickness direction (out-of-plane direction) of the measuring section 101.
FIG. 6 is an example of strain-load diagrams created for each of the two in-plane directions (X-axis direction and Y-axis direction).

(Obtaining the critical strain for stable behavior)
As shown in FIG. 5, when a compressive load is gradually applied to the measurement unit 101 (proceeding to the left side of the horizontal axis in FIG. 5), compressive strain is accumulated downward on the vertical axis, but eventually the compressive load is applied. The accumulation of compressive strain slows down, and then the compressive strain reaches a (negative) maximum value, after which out-of-plane buckling occurs and the accumulated compressive strain is suddenly released to the tensile side.

Therefore, we consider the compressive strain at the point when the accumulation of compressive strain starts to slow down before it reaches its (negative) maximum value in response to a compressive load as the compressive strain that is the starting point that indicates the first sign of out-of-plane buckling. In the present invention, the compressive strain that becomes the starting point is defined as the critical strain for stable behavior.

In this embodiment, the first derivative of compressive strain is calculated based on the strain-load diagram shown in FIG. It is determined as the critical strain for stable behavior against out-of-plane buckling.

As shown in FIG. 5, the stable behavior limit strain is determined by installing a displacement meter 51 in the opening 17 of the central mold 11 and determining the height of the out-of-plane buckling 53, which is the displacement of the surface of the measuring part 101 in the out-of-plane direction ( The results of measuring the wrinkle height (Fig. 10) show that the strain at which the wrinkle height starts to change rapidly coincides with the strain.

(Obtaining the wrinkle onset strain)
If a compressive load is still applied to the measuring part 101 even if the strain in the measuring part 101 exceeds the stable behavior limit strain, the accumulated compressive strain is released all at once, clear out-of-plane buckling occurs, and it becomes tensile strain. .

At this time, the strain measured by the strain gauge shows the (minus) maximum value, and the strain increment with respect to the increment of the compressive load (to the left of the horizontal axis in Figure 5) changes from negative (compression) to positive (tension). Invert. Therefore, in the present invention, the strain at which the polarity of the strain increment with respect to the load increment of the compressive load is reversed is defined as the wrinkle generation start strain at which out-of-plane buckling (wrinkle) begins to occur.
Then, in the strain-load diagram shown in FIG. 5, the strain at the time when the polarity of the increment of strain with respect to the increment of compressive load is reversed is determined as the strain at which wrinkle generation starts.

Further, as shown in FIG. 5, the strain at which wrinkles start to appear coincides with the strain at which the wrinkle height measured by the displacement meter starts to rise at an accelerated rate. Therefore, the strain at which wrinkles start to appear corresponds to an index that indicates the timing at which slightly visible wrinkles suddenly change and turn into clear wrinkles during the actual forming process of a press-formed product.

As described above, according to the biaxial stress testing device and the biaxial stress testing method according to the present embodiment, when applying a compressive load in at least one in-plane direction to the cross-shaped plate-shaped test piece 100, the plate-like test piece Out-of-plane buckling can be induced in the portion of the measurement unit 101 of 100 facing the opening 17, and the starting point of wrinkle generation can be detected.

Furthermore, in the biaxial stress test method according to the present embodiment, the relationship between the strain and the compressive load in the process of applying the compressive load to the plate-shaped test piece 100 is acquired, and the relationship between the acquired strain and the compressive load is obtained. By determining the strain that is the starting point of wrinkle generation (stable behavior limit strain and wrinkle initiation strain) based on the The presence or absence of out-of-plane buckling can be objectively evaluated based on the strain.

Note that in the above explanation, an equal compressive load in two axial directions is applied to the measurement part of a cross-shaped plate-like test piece, but the present invention applies biaxial compressive deformation (for example, in the X-axis direction). load ratio in the Y-axis direction -1:-1, -2:-1, etc.), uniaxial compression/uniaxial tension (e.g. -2:+1, etc.), uniaxial compression (e.g. -1:0) , 0:-1) as various biaxial stress states,
Wrinkles (out-of-plane buckling) can be generated in the measuring portion even when the loads in the two in-plane directions applied to the measuring portion are different.

As a specific example of a strain-load diagram at different load ratios, Fig. 7 shows the compressive loads in the in-plane biaxial directions applied to the measurement part of the plate-shaped specimen at the load ratios of -1:-4, -2: The relationship between strain and compressive load when -1 and -1:-1 is shown.

As described above, according to the present invention, the strain that is the starting point of wrinkle generation (stable behavior limit strain, wrinkle By determining the strain at which wrinkles begin to occur, it is possible to determine a general-purpose index (wrinkle generation condition) that is the starting point of wrinkle generation, depending on the press forming method and the shape of the press-formed product.

Furthermore, by reproducing the strain conditions of areas where clear wrinkles have occurred in the press forming simulation of press-formed products using the biaxial stress test method according to this embodiment, clear wrinkles can be avoided during the press-forming process of the press-formed products. Unstable behavior immediately before the occurrence of wrinkles, that is, an index that is the starting point of wrinkle generation (stable behavior limit strain, wrinkle initiation strain) can be confirmed using a plate-shaped test piece using an actual thin metal plate. Then, by reflecting the results of the biaxial stress test in the press forming simulation, it becomes possible to accurately determine the risk of wrinkle generation in the press forming simulation.

In addition, in the biaxial stress testing apparatus according to the present embodiment, the shape of the opening 17 provided in the central portion 13a of one central mold 11a is preferably circular, as shown in FIG. This is because by making the opening 17 circular, the influence of the directionality when the measuring section 101 is deformed can be ignored.
Further, when forming the opening 17, it is preferable that the thickness of the central portion 13 be tapered in diameter from the lower surface side toward the upper surface side, as shown in FIG. 1(b). Thereby, out-of-plane buckling of the measurement portion 101 of the plate-shaped test piece 100 can also be confirmed visually.

Further, the opening does not have to be circular with a specific diameter, and may be a square, rectangle, polygon, or a combination of a polygon and an arc shape, and even if it has such a shape, the measurement part 101 This poses no problem in identifying the origin of out-of-plane buckling induced by

Further, by adjusting the size of the opening 17, the ease with which out-of-plane buckling 53 occurs can be adjusted. The smaller the opening 17, the less likely out-of-plane buckling will occur, and the larger the opening 17, the more likely out-of-plane buckling will occur. The size of the opening 17 is determined by performing a simulation in which a biaxial stress state is generated in the measuring part 101 of the plate-shaped test piece 100, and based on the analysis conditions and results of the simulation. It may be determined by taking into consideration the restraint situation in which the surface is to be restrained. Further, the size may be determined by taking into consideration the size of the area where clear wrinkles have occurred through a press forming simulation of a press formed product.

In addition, it is preferable to provide the displacement meter 51 (FIG. 10) in the opening 17 of the central mold 11 in the biaxial stress testing apparatus according to the present invention.
By providing the displacement meter 51, it is possible to measure the displacement of the measurement unit 101 in the out-of-plane direction in a biaxial stress state, and it is possible to directly and quantitatively measure the occurrence of out-of-plane buckling.
Further, the displacement meter 51 is not particularly limited as long as it does not interfere with measuring the strain of the measurement unit 101, and for example, a laser displacement meter, a contact type displacement meter, etc. can be used.

In the biaxial stress test method according to the present invention, since the out-of-plane deformation of the plate-shaped test piece is a buckling phenomenon, the thickness of the plate-shaped test piece has a large effect. Therefore, it is preferable that the thickness of the plate-shaped test piece be adjusted to the thickness of a press-molded blank plate in which the occurrence of wrinkles is desired to be reproduced.
It should be noted that the material strength of the plate-shaped specimen also affects the out-of-plane buckling of the plate-shaped specimen, but the effect is less compared to the thickness of the plate-shaped specimen and the size of the hole diameter of the opening provided in the central mold. Since it is small, the present invention does not particularly limit the material strength of the plate-shaped specimen.

Furthermore, in the above explanation, the strain was measured using a strain gauge attached to the measurement part 101 in order to capture the strain that becomes the starting point of out-of-plane buckling, but the biaxial stress test method according to the present invention The present invention is not limited to this, and the strain in the measurement unit 101 may be measured from the opening using a digital image correlation method.
Furthermore, the present invention is not limited to measuring the strain of the measuring part 101, but can also detect the starting point of out-of-plane buckling by measuring the displacement of the measuring part 101 in the out-of-plane direction of a plate-shaped test piece. It's okay.

In addition, in the above explanation, the relationship between strain and compressive load was obtained and a strain-load diagram was created. However, the present invention calculates the compressive stress from the measured compressive load. The strain that becomes the starting point of wrinkle generation may be determined based on the acquired relationship between strain and compressive stress.

Experiments and analyzes were conducted to verify the effects of the biaxial stress testing device and biaxial stress testing method according to the present invention, and will be described below.

In this example, we performed a biaxial stress test in which a compressive load was applied in two in-plane directions to a cross-shaped plate specimen to determine the strain that becomes the starting point of wrinkle generation, and The determination of the occurrence of wrinkles was verified by press forming simulation of the press formed product 200 shown in FIG. 8 based on the strain that becomes the starting point.

<Biaxial compression test>
First, using the biaxial stress testing apparatus 1 shown in FIGS. 1 and 3, a biaxial stress test was performed on the cross-shaped plate specimen 100 shown in FIG. The strain in the measurement part 101 when the pressure was applied and the compressive load applied to the plate-shaped test piece 100 were measured.

For the plate-shaped test piece 100, a steel plate with a tensile strength of 270 MPa class and a plate thickness of 1.2 mm was used as the test material, the measurement part size was 30 mm x 30 mm, and it was formed in the central mold 11a of the biaxial stress test device 1. The opening 17 was circular with a diameter of 25 mm.

Then, the plate-shaped test piece 100 is sandwiched between the pair of central molds 11a and 11b of the biaxial stress testing apparatus 1, and the pieces 103a and 103b are held in the holding parts of the side molds 21a and 21b, respectively. 25a and 25b, and a compressive load was applied to the measuring section 101 from two in-plane directions via the pieces 103a and 103b. Here, the direction in which the compressive load is applied by the side mold 21a is the X-axis direction, the direction in which the compressive load is applied by the side mold 21b is the Y-axis direction, and the compressive loads in the X-axis direction and the Y-axis direction are made equal ( Load in the X-axis direction: Load in the Y-axis direction = -1: -1 (minus indicates compressive load).

FIG. 6 shows a strain-load diagram showing the relationship between the measured strain and the compressive loads in the X-axis direction and the Y-axis direction.
Furthermore, Figure 9 shows the first derivative of strain and strain increment calculated for the strain-load diagram in the Y-axis direction, and the stable behavior limit strain and wrinkle initiation strain determined from the calculated first derivative of strain and strain increment. show.
The stable behavior strain was the strain at which the first derivative of strain reached its maximum value in the strain-load diagram, and was -1353με at a load of -6.9kN.
The strain at which wrinkles start is the strain at which the increment of strain in the strain-load diagram is reversed, that is, the strain-load diagram becomes convex downward and reaches its maximum (minus) value, and is -1671 με at a load of -7.2 kN. there were.

<Determination of wrinkle occurrence by press forming simulation>
Next, a press molding simulation was performed using FEM analysis of the press molded product 200 having the overhang molded surface portion 201 and the side surface portion 203 shown in FIG. 8, and the presence or absence of wrinkles in the press molded product 200 was determined. In addition, in FIG. 8, only the left half of the press-formed product 200 is shown so that the state of the punch 211 can be seen. In the press molding simulation, the press molded product 200 extends around the entire circumference of the die 213.

As shown in FIG. 8, the press-formed product 200 is stretch-molded using a mold 210 equipped with a punch 211, a die 213, and a holder 215, and the blank has a tensile strength of 270 MPa and a plate thickness of 1.2 mm. A circular steel plate was used.
Furthermore, the molding height of the press-formed product 200 was changed to 20 mm, 30 mm, and 40 mm, and the presence or absence of wrinkles due to the difference in molding height was determined.
Table 1 shows the blank material, mold conditions, and FEM analysis conditions.

The aforementioned FIG. 9 shows a shading diagram of the press-formed product 200 obtained by press-forming simulation.
No wrinkles were observed on the side surface 203 of the press-formed product 200 when the molding height was 20 mm (Fig. 9(c)) and 30 mm (Fig. 9(b)), but when the molding height was 40 mm (Fig. 9(a)) ), wrinkles were observed on the side surface 203.

Furthermore, in the shading diagram of the press-formed product 200 shown in FIG. 9, the portions in the side surface portion 203 under the equibiaxial compressive load condition where the compressive loads in the two in-plane directions are equal are indicated by ○ marks. We verified the correspondence between the strain in the area and the determination of the presence or absence of wrinkles determined by the biaxial stress test described above.

As shown in FIG. 9, as the molding height of the press-formed product 200 increases to 20 mm, 30 mm, and 40 mm, the position of the portion subject to the equibiaxial compressive load condition changes in the molding height direction.

The strain when the molding height was 20 mm was -228με, and its absolute value was smaller than the stable behavior limit strain (=-1353με) determined by the biaxial stress test. Therefore, it is determined that there is no sign that strain will occur in the region. This determination coincided with the fact that no wrinkles were observed in the shading diagram of the press-formed product 200 obtained by press-forming simulation.

The strain when the molding height was 30 mm was -1230 μ, and its absolute value was smaller than the stable behavior limit strain (=-1353 με) determined by the biaxial compression test. Therefore, as with the molding height of 20 mm, it is determined that there is no sign that strain will occur in this area. This judgment was consistent with the fact that no wrinkles were observed in the shading diagram of the press-formed product obtained by press-forming simulation.

The strain when the molding height is 40mm is -1660μ, which is larger in absolute value than the stable behavior limit strain (=-1353με) determined by biaxial compression test, and almost the same as the wrinkle generation limit strain (=-1671με). It was a value. Therefore, it is determined that wrinkles will occur in the region. This determination coincided with the fact that wrinkles were also observed in the shading diagram of the press-formed product 200 obtained by press-forming simulation.

As described above, it has been shown that the presence or absence of wrinkles in a press-formed product can be determined based on the strain that is the starting point of wrinkle generation (stable behavior limit strain, wrinkle generation start strain) determined by the biaxial stress test method according to the present invention.

1 Biaxial stress test device 11 Central mold 11a Central mold 11b Central mold 13 Central part 13a Central part 15 First comb tooth part 15a First comb tooth part 15b First comb tooth part 17 Opening part 19 Positioning pin 21 Side Part mold 21a Side mold 21b Side mold 23 Second comb tooth part 23a Second comb tooth part 23b Second comb tooth part 25 Holding part 25a Holding part 25b Holding part 27a Roller 27b Roller 31 Pressing mechanism 33 Base part 35 Top plate part 37 Bolt 51 Displacement meter 53 Out-of-plane buckling (out-of-plane deformation)
100 Plate test piece 101 Measuring part 103 Piece part 103a Piece part 103b Piece part 105 Circular notch part 107 Hole-shaped part 200 Press molded product 201 Overhang molded surface part 203 Side part 210 Mold 211 Punch 213 Die 215 Holder

Claims (4)

A biaxial stress test device that applies a compressive load in at least one in-plane direction to a cross-shaped plate specimen in two in-plane directions,
a rectangular center portion facing the measurement portion where the cross shapes intersect in the plate-like test piece; and a comb-like first comb-teeth portion formed at the edges of the four sides of the center portion; A pair of central molds that sandwich both sides of a plate-shaped test piece,
a second comb-teeth portion disposed on the four sides of the central mold and meshing with the first comb-teeth portion; and a holding portion for holding the cross-shaped piece of the plate-like test piece. and a side mold having;
A biaxial stress testing device, characterized in that an opening is formed in the central portion of one of the pair of central molds to induce out-of-plane buckling of the measuring portion. 2. The biaxial stress testing device according to claim 1, wherein the opening of the central mold is provided with a displacement meter for measuring displacement in an out-of-plane direction of the measuring portion of the plate-shaped test piece. . A biaxial stress testing method in which a compressive load in at least an in-plane uniaxial direction is applied to the plate-shaped test piece using the biaxial stress testing apparatus according to claim 1 or 2,
sandwiching both sides of the measurement part of the plate-shaped test piece between the pair of central molds;
Each of the four pieces of the plate-like test piece is held by the four side molds, and the second comb tooth part of the side mold is attached to the first comb tooth part of the central mold. mesh,
Applying a compressive load in at least an in-plane uniaxial direction to the measurement part via the piece part,
A biaxial stress testing method, comprising inducing out-of-plane buckling of the measuring section in the opening of the central mold. Obtaining the relationship between the strain of the measuring part and the compressive load in the process of applying a compressive load in at least an in-plane uniaxial direction to the measuring part,
Calculating the first derivative of strain based on the relationship between the acquired strain and compressive load,
Determine the strain at which the first derivative of the calculated strain becomes maximum as the critical strain for stable behavior against out-of-plane buckling of the measurement part,
4. The biaxial method according to claim 3, wherein, in the relationship between the acquired strain and the compressive load, a strain at which the polarity of the strain increment is reversed with respect to the increment of the compressive load is determined as the wrinkle generation starting strain in the measuring section. Stress test method.