CN110618023A - Test method for obtaining large bidirectional strain by utilizing male die bulging based on thinned test piece - Google Patents
Test method for obtaining large bidirectional strain by utilizing male die bulging based on thinned test piece Download PDFInfo
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- CN110618023A CN110618023A CN201910894356.7A CN201910894356A CN110618023A CN 110618023 A CN110618023 A CN 110618023A CN 201910894356 A CN201910894356 A CN 201910894356A CN 110618023 A CN110618023 A CN 110618023A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/02—Details
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/02—Details
- G01N3/06—Special adaptations of indicating or recording means
- G01N3/068—Special adaptations of indicating or recording means with optical indicating or recording means
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
Abstract
The invention provides a test method for acquiring large bidirectional strain by utilizing male die bulging based on a thinning test piece, which is characterized in that speckles are uniformly sprayed on the manufactured thinning test piece and then the manufactured thinning test piece is fixed between a female die and a blank holder; starting a high-speed camera to photograph and store the photographed image in a computer, and identifying and tracking the speckle points of the test piece by combining a digital image processing method to determine the speckle quality; applying a blank holder force, starting the male die to expand the thinned test piece until the test piece cracks; and recording the dynamic process of the test piece from the time of fracture to the time of fracture, acquiring a displacement field of a large strain area of the test piece, calculating a strain gradient and further acquiring a strain value. The invention also provides a manufacturing method of the thinning test piece. When a male die bulging test is carried out to obtain large bidirectional strain, the invention can enable the generation of the test piece crack to be very close to or even reach the vault position, obviously reduce or even eliminate the problem of double-neck shrinkage symmetrical to the vault, and enable the measurement of the strain pair to be more accurate.
Description
Technical Field
The invention relates to the technical field of sheet plastic forming, and relates to the measurement of a patent classification number G01; testing; G01N testing or analyzing a material by means of determining a chemical or physical property of the material; G01N3/00 tests the strength characteristic of the solid material by using mechanical stress; G01N19/00 materials were tested by mechanical means; in particular to a test method for obtaining large bidirectional strain by utilizing male die bulging and thinning test pieces.
Background
In the field of sheet plastic forming, plastic instability behavior is always an important research hotspot, and especially with continuous promotion of a plurality of new processes, new structures, sheets made of new materials and some new forming processes, accurate determination of instability points is a key point for obtaining accurate results in theoretical analysis and finite element analysis.
In order to determine the maximum strain which can be achieved before the sheet material is unstable, a plurality of research and evaluation methods are provided from the aspects of theory, experiment and the like, wherein the most practical significance and the most extensive application are that the maximum strain which can be achieved before the sheet material is unstable in deformation constitutes the concept of a forming limit diagram in the 60 th century of the 20 th century. The allowable values of two main strains measured in different stress states are determined by taking a smaller main strain (secondary strain) as an abscissa and a larger main strain (main strain) as an ordinate, and connecting lines of some points are determined to be forming limit curves.
The Nakazima method is the most extensive test method for testing the forming performance of the plate at present, and the maximum strain reached before the plate is unstable is obtained by utilizing the bulging of the hemispherical male die, but in the actual test, because the accurate judgment of the time and the position of the occurrence of the plastic instability of the plate is difficult, the maximum strain of the forming limit of the plate is often expressed by adopting the strain at the fracture part of the plate.
The bulging test of hemispherical male die, although widely applied and continuously improved by users so far, includes changing the shape of the test piece, adjusting the lubrication mode, using advanced measurement method, etc., the bulging test of male die still has certain limitations:
1) in the bulging test of the male die, the crack generation position of the test piece cannot be very close to the dome, particularly, in order to obtain the bulging test with large bidirectional strain, the crack of the test piece is often generated at one fourth or one third of the bulging height, the problem of double-neck shrinkage symmetrical to the dome is obviously caused, and the precision of obtaining the bidirectional strain is reduced.
2) In the male die bulging test, due to the limitation of geometric constraint, a large strain gradient exists in a test piece, so that a large difference exists between the measurement value and the acceptable value of the measured strain pair.
3) Complex loading cannot be implemented.
In summary, it is necessary to provide a test method to solve the deficiencies of the bulging test of the hemispherical punch.
Disclosure of Invention
According to the technical problems that in the bulging test for obtaining the large bidirectional strain, the position where the crack is generated is difficult to approach the arch crown and the measurement of the strain gradient is limited, the test method for obtaining the large bidirectional strain by utilizing the bulging of the male die and the thinning of the test piece is provided. According to the invention, when the large bidirectional strain is obtained in the male die bulging test of the thinning test piece, the crack of the thinning test piece is very close to or even reaches the vault position, so that the problem of double-neck shrinkage symmetrical to the vault is remarkably reduced or even eliminated, and the strain pair is more accurately measured.
The technical means adopted by the invention are as follows:
a thinning test piece is characterized by being prepared by the following steps:
and S1, placing the non-thinned test piece into a die for bulging test, and measuring and recording the horizontal length from the crack generation position (cracks can be observed by visual inspection) of the test piece to the center of the arch of the test piece.
S2, selecting a thinning surface (the sheet metal test piece has an upper surface and a lower surface, and one surface can be selected for thinning due to symmetry) of the test piece to be thinned, taking the center point of the surface as the center of a circle, taking the horizontal length from the crack generating position of the test piece measured by S1 to the center of the test piece as a radius, and defining the area to be thinned by drawing a rule;
and S3, thinning the defined thinning area to obtain a thinning test piece.
Further, in step S3, spherical thinning is performed with the scribe line as a boundary, and the thinned spherical portion is to completely cover the region to be thinned.
Further, under the prerequisite that the attenuate sphere part just covered the attenuate region completely, through the minimum thickness who sets for the test piece attenuate region, change the size of attenuate sphere radius to obtain the attenuate to the regional not equidimension of test piece attenuate.
Further, the minimum thickness of the thinned area of the test piece is set to be more than or equal to one half of the non-thinned thickness.
The invention also discloses a test method for acquiring large bidirectional strain by utilizing male die bulging based on the thinned test piece, which is characterized by comprising the following steps of:
s11, uniformly spraying speckles on the thinning test piece, and fixing the thinning test piece between the female die and the blank holder;
s12, adjusting a light source, starting a high-speed camera to shoot, storing the shot in a computer, capturing and identifying the speckle points of the thinning test piece by combining a digital image processing method (DIC, and selecting software compiled by different companies according to needs) and determining the speckle quality;
s13, applying a blank holder force to the blank holder, and starting the male die to expand the thinning test piece until the thinning test piece cracks;
and S14, recording the dynamic process of the thinning test piece from the time before the thinning test piece is broken to the time when the thinning test piece is broken through a computer, obtaining the displacement field of the large strain area of the thinning test piece, calculating the strain gradient, and further obtaining the strain value.
Further, in the step S11, when the speckles are sprayed, it is ensured that the surface of the thinned test piece is smooth, and the black paint is uniformly sprayed at the front and the back to avoid large particles.
Further, in step S13, a blank-holding force is applied to ensure that the thinned specimen does not move radially.
Further, in step S14, capturing an image of the fracture moment of the thinned test piece, and acquiring a regional strain field by tracking the motion or coordinates of the scattered spots corresponding to the surface of the thinned test piece before and after deformation, thereby calculating the bidirectional strain value near the fracture position.
Compared with the prior art, the invention has the following advantages:
1. the test method provided by the invention can obviously reduce the problem of double necks symmetrical to the vault in the bulging process of the test piece, so that the crack of the thinned test piece is very close to or even reaches the vault, and the accuracy of the strain on measurement is improved.
2. The test method provided by the invention can control the thinning degree of the test piece by designing the minimum thickness of the thinned area of the test piece, and can effectively obtain the most applicable test piece thinning scheme.
3. The test method provided by the invention can meet the design requirements of different plate thinning test pieces, and can be extended to the application of the forming limit test of plates with different widths.
Based on the reasons, the invention can be widely popularized in the fields of sheet plastic forming processing, detection and the like.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a schematic diagram of a method for obtaining large bidirectional strain by using male die bulging and thinning a test piece according to the present invention.
FIG. 2 is a schematic diagram of a test piece to be thinned defining a thinning area according to the present invention, wherein (a) is a schematic diagram of a horizontal distance from a crack of the test piece to a dome, and (b) is a schematic diagram of a selected thinning area.
FIG. 3 is a schematic diagram of thinning a test piece to be thinned according to the present invention.
FIG. 4 is a schematic view of a thinned test piece according to the present invention.
FIG. 5 is a schematic diagram of a finite element simulation result of large bidirectional strain obtained by using male die bulging and thinning test pieces according to the present invention.
In the figure: 1. thinning the test piece; 2. a female die; 3. a blank holder; 4. a male die; 5. a high-speed camera; 6. a light source.
Detailed Description
It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The relative arrangement of the components and steps, the numerical expressions and numerical values set forth in these embodiments do not limit the scope of the present invention unless specifically stated otherwise. Meanwhile, it should be understood that the sizes of the respective portions shown in the drawings are not drawn in an actual proportional relationship for the convenience of description. Techniques, methods, and apparatus known to those of ordinary skill in the relevant art may not be discussed in detail but are intended to be part of the specification where appropriate. Any specific values in all examples shown and discussed herein are to be construed as exemplary only and not as limiting. Thus, other examples of the exemplary embodiments may have different values. It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, further discussion thereof is not required in subsequent figures.
In the description of the present invention, it is to be understood that the orientation or positional relationship indicated by the directional terms such as "front, rear, upper, lower, left, right", "lateral, vertical, horizontal" and "top, bottom", etc., are generally based on the orientation or positional relationship shown in the drawings, and are used for convenience of description and simplicity of description only, and in the absence of any contrary indication, these directional terms are not intended to indicate and imply that the device or element so referred to must have a particular orientation or be constructed and operated in a particular orientation, and therefore should not be considered as limiting the scope of the present invention: the terms "inner and outer" refer to the inner and outer relative to the profile of the respective component itself.
Spatially relative terms, such as "above … …," "above … …," "above … …," "above," and the like, may be used herein for ease of description to describe one device or feature's spatial relationship to another device or feature as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device in the figures is turned over, devices described as "above" or "on" other devices or configurations would then be oriented "below" or "under" the other devices or configurations. Thus, the exemplary term "above … …" can include both an orientation of "above … …" and "below … …". The device may be otherwise variously oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
It should be noted that the terms "first", "second", and the like are used to define the components, and are only used for convenience of distinguishing the corresponding components, and the terms have no special meanings unless otherwise stated, and therefore, the scope of the present invention should not be construed as being limited.
The invention provides a thinning test piece 1 which is prepared by the following steps:
and S1, placing the non-thinned test piece into a die for bulging test, and measuring and recording the horizontal length S from the crack generation position of the test piece to the center of the vault of the test piece, as shown in figure 2 (a).
S2, selecting a thinning surface of the test piece to be thinned, using the center point of the surface as the center of a circle, using the horizontal length S between the crack generating position of the test piece measured in S1 and the center of the test piece as the radius, and defining the area to be thinned by drawing a rule, as shown in FIG. 2 (b);
and S3, thinning the defined thinning area, and performing spherical thinning by taking the scribing line as a boundary, wherein the thinning sphere part completely covers the area to be thinned. On the premise that the thinning sphere part just completely covers the thinning area, setting the minimum thickness t of the thinning area of the test piece, wherein the minimum thickness t of the thinning area of the test piece is more than or equal to one half of the non-thinned thickness; and changing the radius R of the thinning sphere so as to obtain different degrees of thinning of the thinning area of the test piece (as shown in figure 3), and finally obtaining the thinning test piece (as shown in figure 4).
As shown in fig. 1 and 5, the invention also discloses a test method for obtaining large bidirectional strain by using male die bulging based on the thinned test piece, which comprises the following steps:
s11, uniformly spraying speckles on the thinning test piece 1, and fixing the thinning test piece 1 at the middle position between the female die 2 and the blank holder 3; when the speckle sprays, guarantee that attenuate test piece surface is bright and clean, white lacquer is evenly sprayed behind black lacquer in the front, avoids appearing the large granule.
S12, adjusting the light source 6, starting the high-speed camera 5 to shoot and store the shot in a computer, capturing and identifying speckle points of the thinning test piece 1 by combining a digital image processing method (DIC), and determining the speckle quality;
s13, applying a blank holder force to the blank holder 3, wherein the blank holder force must be applied to ensure that the clamped part of the thinning test piece 1 cannot move radially, and then starting the male die 4 to bulge the thinning test piece 1 until the thinning test piece 1 cracks;
and S14, recording the dynamic process of the thinning test piece 1 from the cracking to the cracking through a computer, obtaining the displacement field of the large strain area of the thinning test piece 1, calculating the strain gradient and further obtaining the strain value. The instantaneous image of rupture of the thinning test piece 1 is captured, the regional strain field is obtained by tracking the motion or the coordinates of scattered spots corresponding to the surface of the thinning test piece 1 before and after deformation, and then the bidirectional strain value close to the rupture position is calculated.
Example 1
The method of the invention is adopted to carry out finite element test on the Q195 low-carbon steel test piece, and comprises the following steps:
and S1, performing a male die bulging simulation test on the non-thinned test piece, establishing a model with reference to the female die 2, the blank holder 3 and the male die 4 of the die in FIG. 1, wherein the outer diameter of the male die 4 is 100mm, the inner diameter of the blank holder 3 and the female die 2 is 106mm, the size of the test piece is 180 x 2mm, guiding the test piece into the die for testing until cracks are generated, measuring the horizontal length S between the crack generation position of the test piece and the vault to be 29.5mm as shown in FIG. 2(a), then selecting a thinning surface of the test piece to be thinned, and dividing a thinning area as shown in FIG. 2 (b).
S2, thinning the test piece, setting the minimum thickness t of the thinned test piece 1 to be 1.25mm, and ensuring that the thinned sphere part just completely covers the thinned area, so as to obtain the thinned sphere with the radius R equal to 813mm as shown in figure 3, and further obtain the shape of the thinned test piece as shown in figure 4.
And S3, guiding the manufactured thinning test piece 1 into a die model (such as a die shown in figure 1) to perform a convex die bulging test, stopping until the test piece cracks, and observing the strain state and the crack generation position of the test piece.
As a result, as shown in FIG. 5, the crack zone of the test piece almost reaches the dome position, the problem of double-neck shrinkage symmetrical to the dome is reduced, the phenomenon of serious double-vertex in the section strain distribution diagram is avoided, and the precision of strain pair measurement is improved.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
Claims (8)
1. A thinning test piece is characterized by being prepared by the following steps:
and S1, placing the non-thinned test piece into a die for bulging test, and measuring and recording the horizontal length from the crack generation position of the test piece to the center of the vault of the test piece.
S2, selecting a thinning surface of the test piece to be thinned, using the center point of the surface as the center of a circle, using the horizontal length from the crack generating position of the test piece measured in S1 to the center of the test piece as a radius, and defining the area to be thinned by drawing a rule;
and S3, thinning the defined thinning area to obtain a thinning test piece.
2. The thinning test piece according to claim 1, wherein in the step S3, spherical thinning is performed with the scribe line as a boundary, and the thinning sphere portion is to completely cover the region to be thinned.
3. The thinning test piece of claim 2, wherein on the premise that the thinning sphere part just completely covers the thinning area, the radius of the thinning sphere is changed by setting the minimum thickness of the thinning area of the test piece, so as to obtain different degrees of thinning of the thinning area of the test piece.
4. The thinning test piece of claim 3 wherein the minimum thickness of the set test piece thinned region is equal to or greater than one-half of the non-thinned thickness.
5. A test method for obtaining large bidirectional strain by punch bulging based on a thinned test piece as claimed in any one of claims 1 to 4, characterized by comprising the following steps:
s11, uniformly spraying speckles on the thinning test piece, and fixing the thinning test piece between the female die and the blank holder;
s12, adjusting a light source, starting a high-speed camera to photograph and store the photographed image in a computer, capturing and identifying speckle points of the thinning test piece by combining a digital image processing method, and determining the quality of the speckles;
s13, applying a blank holder force to the blank holder, and starting the male die to expand the thinning test piece until the thinning test piece cracks;
and S14, recording the dynamic process of the thinning test piece from the cracking to the cracking, obtaining the displacement field of the large strain area of the thinning test piece, calculating the strain gradient and further obtaining the strain value.
6. The test method for obtaining large two-way strain based on thinning test piece by male die bulging as claimed in claim 5, wherein in step S11, when speckle is sprayed, the surface of the thinning test piece is ensured to be smooth, and the black paint is uniformly sprayed before and after the white paint, so as to avoid large particles.
7. The test method for obtaining large two-way strain based on thinning test piece by male die bulging as claimed in claim 5, wherein in step S13, the edge pressing force is applied to ensure that the thinning test piece does not move radially.
8. The test method for acquiring the large bidirectional strain based on the thinning test piece by utilizing the convex die bulging as claimed in claim 5, wherein in the step S14, an image of the thinning test piece at the cracking moment is captured, an area strain field is acquired by tracking the movement or coordinates of scattered spots corresponding to the surface of the thinning test piece before and after deformation, and then a bidirectional strain value near the cracking position is calculated.
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