A method for determining equivalent fracture resistance index of metal materials

Abstract

The present invention relates to the field of material testing technology, and provides a method for determining the equivalent fracture resistance index of a metal material. Based on a uniaxial tensile test of a round bar specimen, the stress data, strain data, and fracture strength of the material in the uniform plastic deformation stage and the non-uniform plastic deformation stage are recorded, and the equivalent yield strength is obtained by substituting the data into a stress-strain model. The equivalent strength ratio is obtained based on the fracture strength and the equivalent yield strength, and is used as a fracture resistance index in engineering design. The present invention analyzes the fracture resistance index through the equivalent yield strength and proposes the concept of an equivalent strength ratio. The fracture resistance analysis of the material can be performed while fully considering the strength change state of the material in the yield stage, thereby enabling designers to more accurately grasp the fracture performance of the material and provide a guarantee for the safety of engineering design.

Description

Technical Field

[0001] The present invention relates to the technical field of material testing, and in particular to a method for determining an equivalent fracture resistance index of a metal material. Background Art

[0002] The uniaxial tensile test of round bar specimens is one of the oldest basic mechanical property tests for metal materials. During the test, metal materials generally undergo the entire process of elastic deformation, uniform plastic deformation, non-uniform plastic deformation, and fracture. In theory, this test can measure the stress-strain data of the metal material's elastic deformation, plastic deformation, and fracture strength. However, due to the lack of an established stress-strain function for the non-uniform plastic deformation stage and the lack of a technology to identify the onset of fracture, existing standards related to the uniaxial tensile test of round bar specimens can only measure engineering mechanical parameters such as the yield strength, tensile strength, cross-sectional reduction, and elongation of metal materials, as well as stress and strain data in the elastic deformation stage and uniform plastic deformation stage. The value of the uniaxial tensile test of round bar specimens has not been fully utilized.

[0003] Chinese patent CN202410154117.9 discloses a method for testing the fracture strain of metal materials under high temperature conditions, comprising: preparing different types of tensile specimens; pre-treating the tensile specimens; subjecting the pre-treated tensile specimens to vacuum high temperature testing and obtaining the surface fracture strain and the core fracture strain; wherein obtaining the surface fracture strain is specifically: obtaining it through vacuum high temperature tensile testing and DIC; and obtaining the core fracture strain is specifically: obtaining it through a hybrid numerical-experimental method. The beneficial effect of the present invention is that it can reduce costs while increasing the temperature tolerance of the speckle, thereby accurately describing the damage and fracture behavior inside the material. This patent solves the problem of difficulty in measuring the fracture strain when cracks initiate in the core of steel by a hybrid numerical-experimental method, but it cannot accurately obtain the stress and strain at each moment in the entire plastic strain process, nor does it consider the influence of the strength change in the yield stage on the fracture properties of the material, and cannot comprehensively measure the fracture properties. Summary of the Invention

[0004] In view of this, the present invention aims to provide a method for determining the equivalent fracture resistance index of metal materials. Combining the I1 fracture criterion and the stress-strain relationship of round bar tensile specimens in the necking deformation stage, a new method for determining the equivalent fracture resistance index is proposed, aiming to provide support for more accurate fracture resistance design.

[0005] The present invention discloses a method for determining an equivalent fracture resistance index of a metal material, comprising the following steps:

[0006] Step S1: performing a uniaxial tensile test on a round bar specimen, and recording multiple sets of test data in real time during the test process, wherein the test data at least includes stress data and strain data in a uniform plastic deformation stage and a non-uniform plastic deformation stage;

[0007] Step S2: Substitute the stress and strain data of the uniform plastic deformation stage and the non-uniform plastic deformation stage obtained in step S1 into (1) to fit and obtain the corresponding parameters and equivalent yield strength :

[0008] (1)

[0009] in, is the Mises equivalent stress during the plastic deformation of the specimen, is the equivalent plastic strain during the plastic deformation of the specimen, and Obtained through step S1, is the equivalent yield strength, k1 and k2 are strength coefficients, n1 and n2 are strain hardening coefficients, , k1, k2, n1, n2 are all parameters obtained by fitting formula (1), e is a constant;

[0010] Step S3: Obtain the fracture strength I of the material during the test b , calculate the equivalent strength ratio of the fracture performance index .

[0011] Furthermore, the method further includes the following steps:

[0012] Step S4: Substitute the fitting parameters obtained in step S2 into equation (1) to obtain the stress-strain relationship model of the material.

[0013] Furthermore, the acquisition of the test data in step S1 includes at least the following steps:

[0014] Step S11: In the uniaxial tensile test of the round bar specimen, the tensile axial force F during the test is recorded in real time. z , and record the change parameters of the sample diameter, which at least include the minimum cross-sectional radius of the necking bottom perpendicular to the central axis of the sample , the maximum limit value of the cross-sectional radius perpendicular to the central axis of the specimen , the tangent slope of the inflection point of the projection curve in the imaging direction of the necking deformation contour , the cross-sectional radius perpendicular to the central axis at the inflection point of the projection curve in the imaging direction of the necking deformation contour , the distance between the section perpendicular to the central axis at the inflection point of the projection curve of the necking deformation contour imaging direction and the minimum section of the necking bottom ;

[0015] Step S12: In the uniform plastic deformation stage, according to the F obtained in step S11 z and diameter change parameters, and substitute them into formula (2) to obtain the stress data in the uniform plastic deformation stage:

[0016] (2)

[0017] in, It is the instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen during the test;

[0018] Step S13: In the non-uniform plastic deformation stage, according to the F obtained in step S11 z 、 、 、 、 、 , substituted into formula (3) to calculate the stress data of the non-uniform plastic deformation stage:

[0019] (3).

[0020] Furthermore, the acquisition of the test data in step S1 further includes:

[0021] Step S14: In the uniform plastic deformation stage, according to the diameter change parameter obtained in step S11, substitute it into formula (4) to calculate the strain data of the plastic deformation stage:

[0022] (4)

[0023] in, is the cross-sectional area perpendicular to the central axis of the specimen before the test, It is the instantaneous cross-sectional average stress of the smallest cross-section perpendicular to the central axis of the specimen during the test, and in the uniform plastic deformation stage , E is the elastic modulus of the material;

[0024] Step S15: In the non-uniform plastic deformation stage, according to the F obtained in step S11 z 、 、 、 、 、 , substitute into formula (5) to obtain the corresponding strain data:

[0025] (5)

[0026] in, It is the instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen when the axial force is at its maximum value during the test. It is the maximum axial force that the specimen bears during the test.

[0027] Furthermore, during the round bar tensile test data acquisition process, the cross-sectional radius of the specimen perpendicular to the central axis is acquired through real-time image data acquisition of the specimen profile in two directions, and the instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen during the test is obtained. Calculated by formula (6):

[0028] (6)

[0029] in, is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen during the test in the first imaging direction, The instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen during the test in the second imaging direction;

[0030] The instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen when the axial force is at its maximum value during the test Calculated by formula (7):

[0031] (7)

[0032] in, is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen when the axial force in the first imaging direction is at its maximum value during the test. It is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen when the axial force is maximum during the test in the second imaging direction.

[0033] Furthermore, the two directions of collecting the real-time image data of the sample contour are perpendicular to each other.

[0034] Furthermore, in step S11, the minimum cross-sectional radius of the necking bottom perpendicular to the central axis of the sample is , the maximum limit value of the cross-sectional radius perpendicular to the central axis of the specimen , the tangent slope of the inflection point of the projection curve in the imaging direction of the necking deformation contour Ways to obtain include:

[0035] Step S111: Set the imaging direction projection curve of the sample profile at the necking bottom in the necking stage to be S-shaped, and establish a mathematical model of the imaging direction projection curve as shown in formula (8):

[0036] (8)

[0037] Among them, r is the cross-sectional radius perpendicular to the central axis at any point on the specimen contour surface, z is the distance between the cross section at any point and the minimum cross section at the necking bottom, z1, p1, z2, and p2 are the shape characteristic parameters to be determined;

[0038] Step S112: Measure the test data of the sample with necking deformation. The test data includes at least the cross-sectional radius r and the cross-sectional distance z. There are multiple measurement points. Substitute the measured test data into formula (8) for fitting to determine r n 、r c and the values ​​of shape characteristic parameters z1, p1, z2, p2;

[0039] Step S113: The r determined in step S112 is n 、r c Substituting the values ​​of the shape characteristic parameters z1, p1, z2, and p2 into formula (9), we can obtain the mathematical model of the tangent slope of any point on the projection curve in the imaging direction of the contour in the plane formed by the projection curve and the central axis:

[0040] (9)

[0041] Among them, K t (z) is the slope of the tangent line at the point where the distance z between the section on the projection curve in the imaging direction of the profile and the minimum section of the necking bottom is in the plane formed by the projection curve and the central axis;

[0042] Step S114: Obtain the tangent slope according to step S113, and use linear search to determine the maximum value of the tangent slope under a certain accuracy. , the maximum value is the tangent slope of the inflection point of the projection curve in the imaging direction of the contour;

[0043] The linear search includes: setting a series of z values, the difference between two adjacent z values ​​is the search step Δz, Δz is the search accuracy, and calculating the tangent slope k corresponding to each z value according to formula (9): t , find k t The maximum value is .

[0044] Furthermore, after step S114, the following steps are performed:

[0045] Step S115: Substitute the z value corresponding to the maximum slope obtained in step S114 into formula (8) to calculate the cross-sectional radius r of the inflection point perpendicular to the central axis. ip .

[0046] Furthermore, the breaking strength I in step S3 b Obtained by the following steps:

[0047] Step S31: Record the tensile axial force F during the test in step S11 at a certain frequency in real time. z ;

[0048] Step S32: After the specimen breaks, substitute the specimen diameter change parameters at the n sampling points before the force value drops sharply into formula (10) to calculate the instantaneous stress first invariant at the center of the minimum cross section perpendicular to the central axis of the specimen during the n test processes before the force value drops sharply: :

[0049] (10)

[0050] Step S33: Take the average value of the n first stress invariants obtained in step S32, and determine the average value as the fracture strength I of the material being measured. b .

[0051] Furthermore, the certain frequency in step S31 is 5 to 100 times per second, and the value range of n in step S32 is 1 to 5.

[0052] Compared with the prior art, the method for determining the equivalent fracture resistance index of a metal material described in the present invention has the following advantages:

[0053] This paper analyzes fracture performance indicators using equivalent yield strength and introduces the concept of equivalent strength ratio. This allows for fracture performance analysis while fully considering the changing state of material strength during the yield phase. This allows designers to more accurately understand the fracture properties of materials, ensuring the safety of engineering designs. The analysis method provided by this invention has the advantages of a clear physical mechanism, a concise mathematical model, and high analytical accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0055] Figure 1 is the equivalent stress during the uniaxial tensile test of the round bar sample in the embodiment of the present invention Equivalent total strain Schematic diagram of the change relationship;

[0056] Figure 2Schematic diagram showing the comparison between the Mises equivalent stress and equivalent plastic strain curve model fitted by the power function model and the measured values ​​in an embodiment of the present invention;

[0057] Figure 3 1 is a comparison diagram of the Mises equivalent stress and equivalent plastic strain curve model fitted by the double e function model and the measured values ​​in an embodiment of the present invention;

[0058] Figure 4 Schematic diagram of the comparison of the accuracy of the Mises equivalent stress finite element analysis value and the derived value in an embodiment of the present invention;

[0059] Figure 5 Schematic diagram of the accuracy comparison between the finite element analysis value and the derived value of the equivalent plastic strain in an embodiment of the present invention;

[0060] Figure 6 Schematic diagram of collecting real-time image data of a sample profile in two directions in an embodiment of the present invention;

[0061] Figure 7 The sample image is obtained when real-time image data of the sample contour is collected from two directions in an embodiment of the present invention;

[0062] Figure 8 Schematic diagram of an embodiment of the present invention when the minimum cross-sectional area of ​​the necking portion is an ellipse;

[0063] Figure 9 Schematic diagram of calculation error analysis when using formula (6) to calculate the area when the minimum cross-section of the necking part is an ellipse in an embodiment of the present invention;

[0064] Figure 10 Schematic diagram of the shape of a round bar specimen during the necking deformation stage of a uniaxial tensile test in an embodiment of the present invention;

[0065] Figure 11 Schematic diagram of the sample shape and rectangular coordinate system construction of a round bar sample in the uniaxial tensile necking deformation stage in an embodiment of the present invention;

[0066] Figure 12 Schematic diagram of characteristic parameters of sample shape during the necking deformation stage in an embodiment of the present invention;

[0067] Figure 13 Schematic diagram of the accuracy comparison between the finite element analysis value and the derived value of the first stress invariant in an embodiment of the present invention;

[0068] Figure 14 Schematic diagram of the effect of crack radius on the maximum value of the first invariant of stress at the crack front during a round bar tensile test according to an embodiment of the present invention;

[0069] Figure 15Schematic diagram of stress distribution characteristics during crack propagation in a round bar tensile test according to an embodiment of the present invention;

[0070] Figure 16 Schematic diagram of the change of axial force with loading displacement during a round bar tensile test according to an embodiment of the present invention;

[0071] Figure 17 Schematic diagram of the relationship between equivalent plastic strain and the first invariant of stress during the round bar tensile test according to an embodiment of the present invention. DETAILED DESCRIPTION

[0072] In order to make the technical means, objectives and effects of the present invention easier to understand, embodiments of the present invention are described in detail below with reference to specific figures.

[0073] It should be noted that all terms used in the present invention to indicate direction and position, such as "up", "down", "left", "right", "front", "back", "vertical", "horizontal", "inside", "outside", "top", "low", "lateral", "longitudinal", "center", etc., are only used to explain the relative positional relationship and connection status between the various components in a certain specific state (as shown in the accompanying drawings). They are only for the convenience of describing the present invention, and do not require that the present invention must be constructed and operated in a specific orientation. Therefore, they cannot be understood as limiting the present invention. In addition, the descriptions of "first", "second", etc. in the present invention are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features.

[0074] In the description of the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood broadly. For example, they may refer to fixed, detachable, or integral connections; mechanical connections; direct connections or indirect connections through an intermediary; and internal communication between two components. Those skilled in the art will understand the specific meanings of these terms in the present invention based on the specific circumstances.

[0075] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, illustrative uses of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0076] The present invention discloses a method for determining an equivalent fracture resistance index of a metal material, comprising the following steps:

[0077] Step S1: performing a uniaxial tensile test on a round bar specimen, and recording multiple sets of test data in real time during the test process, wherein the test data at least includes stress data and strain data in a uniform plastic deformation stage and a non-uniform plastic deformation stage;

[0078] Step S2: Substitute the stress and strain data of the uniform plastic deformation stage and the non-uniform plastic deformation stage obtained in step S1 into (1) to fit and obtain the corresponding parameters and equivalent yield strength :

[0079] (1)

[0080] in, is the Mises equivalent stress during the plastic deformation of the specimen, is the equivalent plastic strain during the plastic deformation of the specimen, and Obtained through step S1, is the equivalent yield strength, k1 and k2 are strength coefficients, n1 and n2 are strain hardening coefficients, , k1, k2, n1, n2 are all parameters obtained by fitting formula (1), e is a constant;

[0081] Step S3: Obtain the fracture strength I of the material during the test b , calculate the equivalent strength ratio of the fracture performance index .

[0082] In the prior art, the breaking strength I b and yield strength The intensity ratio obtained by the ratio As an indicator of fracture resistance, it is used to design the fracture resistance of metal structures, such as CN118136190A. In fact, most metal materials have a yielding phenomenon during the stretching process, and the strength of the material will decrease to a certain extent after yielding, such as Figure 1 Therefore, the use of strength ratio for fracture resistance design cannot more effectively determine the fracture performance of the material. In this case, the researchers of this application proposed the equivalent yield strength The concept of equivalent yield strength It represents the strength of the material after yielding, and then the equivalent strength ratio is calculated, which provides more accurate support for the design of the material fracture performance. In addition, within a period of time after the metal material yields, the diameter of the specimen does not change significantly, and its Mises equivalent stress and equivalent plastic strain cannot be accurately calculated. The stress data and strain data of the elastic plastic deformation stage and the subsequent calculable non-elastic plastic deformation stage can be substituted into formula (1) for fitting to obtain the corresponding parameters. Then, by substituting the parameters into formula (1), the relationship model of the stress and strain change in the full plastic deformation stage of the metal material can be obtained. In the actual experiment, the applicant used more than two models to fit the stress-strain relationship of the four materials in the full plastic deformation stage. The results after fitting are as follows: Figure 2 、 Figure 3 As shown, it can be seen that the curve model after fitting with the power function has a good degree of coincidence with the measured values ​​(colored dots) in the elastic-plastic deformation stage, but in the inelastic-plastic deformation stage, the fitting curve model deviates greatly from the measured values ​​and cannot accurately represent the stress-strain relationship of the material in the inelastic-plastic deformation stage. The curve model formed by the double e function fitting proposed in this application has a very high degree of coincidence with the measured values ​​in both the elastic-plastic stage and the inelastic-plastic stage, which helps to accurately deduce the stress-strain relationship of the metal material in the full plastic deformation stage, especially the yield stage, and can also more accurately obtain the equivalent yield strength. The value of enables designers to more accurately determine the fracture properties of the material, providing assurance for the safety of engineering design. It should be noted that the stress and strain data in the uniform plastic deformation stage and the non-uniform plastic deformation stage in step S1 can be detected using existing technologies or new, more accurate methods. The fracture strength in step S3 can be obtained using existing technologies or other methods, which will not be elaborated here.

[0083] Furthermore, the method further includes the following steps:

[0084] Step S4: Substitute the fitting parameters obtained in step S2 into equation (1) to obtain the stress-strain relationship model of the material.

[0085] By substituting the corresponding parameters obtained by fitting in step S2 into formula (1), the stress-strain relationship model of the current material in the full plastic deformation stage can be obtained. It can also be used as a reference for fracture resistance performance indicators in the material fracture resistance design process. For example, during the design process, it can be input as a stress-strain relationship model into the simulation software to perform corresponding material fracture resistance performance analysis, etc., which will not be elaborated here.

[0086] As one of the optional embodiments, the acquisition of test data in step S1 includes at least the following steps:

[0087] Step S11: In the uniaxial tensile test of the round bar specimen, the tensile axial force F during the test is recorded in real time. z , and record the change parameters of the sample diameter, which at least include the minimum cross-sectional radius of the necking bottom perpendicular to the central axis of the sample , the maximum limit value of the cross-sectional radius perpendicular to the central axis of the specimen , the tangent slope of the inflection point of the projection curve in the imaging direction of the necking deformation contour , the cross-sectional radius perpendicular to the central axis at the inflection point of the projection curve in the imaging direction of the necking deformation contour , the distance between the section perpendicular to the central axis at the inflection point of the projection curve of the necking deformation contour imaging direction and the minimum section of the necking bottom ;

[0088] Step S12: In the uniform plastic deformation stage, according to the F obtained in step S11 z and diameter change parameters, and substitute them into formula (2) to obtain the stress data in the uniform plastic deformation stage:

[0089] (2)

[0090] in, It is the instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen during the test;

[0091] Step S13: In the non-uniform plastic deformation stage, according to the F obtained in step S11 z 、 、 、 、 、 , substituted into formula (3) to calculate the stress data of the non-uniform plastic deformation stage:

[0092] (3).

[0093] The Mises equivalent stress of the sample in the uniform plastic deformation stage can be accurately calculated through step S12, and the Mises equivalent stress of the sample in the non-uniform plastic strain stage can be accurately calculated through step S13. The comparison between the Mises equivalent stress analysis value in the non-uniform plastic strain stage obtained by finite element analysis and the Mises equivalent stress value in the non-uniform plastic strain stage derived by formula (3) is shown in Figure 4 , it can be seen that the Mises equivalent stress analysis value obtained by formula (3) in the non-uniform plastic strain stage has extremely high accuracy, which helps to accurately calculate the equivalent yield strength , to improve the accuracy and safety of material analysis and design.

[0094] Furthermore, the acquisition of the test data in step S1 further includes:

[0095] Step S14: In the uniform plastic deformation stage, according to the diameter change parameter obtained in step S11, substitute it into formula (4) to calculate the strain data of the plastic deformation stage:

[0096] (4)

[0097] in, is the cross-sectional area perpendicular to the central axis of the specimen before the test, It is the instantaneous cross-sectional average stress of the smallest cross-section perpendicular to the central axis of the specimen during the test, and in the uniform plastic deformation stage , E is the elastic modulus of the material;

[0098] Step S15: In the non-uniform plastic deformation stage, according to the F obtained in step S11 z 、 、 、 、 、 , substitute into formula (5) to obtain the corresponding strain data:

[0099] (5)

[0100] in, It is the instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen when the axial force is at its maximum value during the test. It is the maximum axial force that the specimen bears during the test.

[0101] The equivalent plastic strain of the sample in the uniform plastic deformation stage can be accurately calculated through step S14, and the equivalent plastic strain of the sample in the non-uniform plastic deformation stage can be accurately calculated through step S15. The comparison between the equivalent plastic strain analysis value of the non-uniform plastic deformation stage obtained by finite element analysis and the equivalent plastic strain value of the non-uniform plastic deformation stage derived by formula (5) is shown in FIG. Figure 5 , it can be seen that the equivalent plastic strain derived value in the non-uniform plastic deformation stage obtained by formula (5) has extremely high accuracy, which helps to accurately calculate the equivalent yield strength , to improve the accuracy and safety of material analysis and design.

[0102] As an optional embodiment of the present invention, during the round bar tensile test data collection process, the cross-sectional radius of the sample perpendicular to the central axis is obtained by real-time image data collection of the sample profile in two directions, and the instantaneous minimum cross-sectional area perpendicular to the central axis of the sample during the test is obtained. Calculated by formula (6):

[0103] (6)

[0104] in, is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen during the test in the first imaging direction, It is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the sample during the test in the second imaging direction. It should be noted that the real-time image data collection of the sample contour can also be carried out in more than three directions, which will not be described in detail here. It should be noted that when testing in two directions, each direction is individually fitted and determined. 、 、 、 The diameter-related variation parameters are calculated separately, and then the corresponding Mises equivalent stress and equivalent plastic strain are calculated and averaged. When calculating, the area-related parameters are obtained using formula (6) or formula (7).

[0105] Preferably, Figure 6 As shown in the figure, the two directions of collecting the real-time image data of the sample contour are perpendicular to each other. The real-time image of the sample contour collected in two directions is as follows: Figure 7 The study found that the minimum cross-section perpendicular to the central axis during the necking deformation stage is circular in some cases and elliptical in some cases, such as Figure 8 As shown, the circle can actually be regarded as a special ellipse with equal major and minor axes, so that a certain angle deviation is maintained between the two directions of real-time image data acquisition of the specimen contour, which facilitates more realistic acquisition of the shape parameters of the minimum cross section perpendicular to the central axis during the necking deformation stage, and facilitates more accurate acquisition of the corresponding minimum cross section area, thereby ensuring the accuracy of the corresponding stress and strain data measurement. Figure 8 Where x and y are the x and y axes in the coordinate system established with the intersection of the central axis and the minimum cross section as the origin, x´ and y´ are two mutually perpendicular data collection directions, r1 is the radius value collected in the first direction, r2 is the radius value collected in the second direction, a is the length of the major semi-axis, and b is the length of the minor semi-axis. Figure 9 The diagram below shows the relationship between the minimum value of the ratio of the cross-sectional area measurement to the true area and the ratio of the major axis to the minor axis of the ellipse, where S t is the measured area calculated by assuming an ellipse with major and minor semi-axes a and b respectively, determined by two mutually perpendicular detection directions. S is the actual area of ​​the ellipse with major and minor semi-axes a and b respectively. S can be obtained through corresponding mathematical derivation. t Minimum value of / S:

[0106]

[0107] like Figure 9As shown in Figure 2, when the length ratio of the major axis to the minor axis is 0.8, the error between the cross-sectional area obtained by using formula (6) and the actual area is less than or equal to 2.5%, indicating that the above method can more accurately obtain the corresponding area data to ensure the accuracy of stress and strain calculation.

[0108] The necking shape characteristic parameters are determined by using the images obtained in two directions and the four contour lines of the upper and lower parts of the minimum necking section. The stress and strain parameters are determined by calculating the shape characteristic parameters of the four contour lines and averaging them to reduce the test error.

[0109] In addition, the instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen when the axial force is at its maximum value during the test Calculated by formula (7):

[0110] (7)

[0111] in, is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen when the axial force in the first imaging direction is at its maximum value during the test. is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen when the axial force is at its maximum value during the test in the second imaging direction. It should be noted that the real-time image data collection of the specimen contour can also be performed in more than three directions, which will not be elaborated here.

[0112] Optionally, in step S11, the minimum cross-sectional radius of the necking bottom perpendicular to the central axis of the sample is , the maximum limit value of the cross-sectional radius perpendicular to the central axis of the specimen , the tangent slope of the inflection point of the projection curve in the imaging direction of the necking deformation contour Ways to obtain include:

[0113] Step S111: Set the imaging direction projection curve of the sample profile at the necking bottom in the necking stage to be S-shaped, and establish a mathematical model of the imaging direction projection curve as shown in formula (8):

[0114] (8)

[0115] Among them, r is the cross-sectional radius perpendicular to the central axis at any point on the specimen contour surface, z is the distance between the cross section at any point and the minimum cross section at the necking bottom, z1, p1, z2, and p2 are the shape characteristic parameters to be determined;

[0116] Step S112: Measure the test data of the sample with necking deformation. The test data includes at least the cross-sectional radius r and the cross-sectional distance z. There are multiple measurement points. Substitute the measured test data into formula (8) for fitting to determine r n 、r c and the values ​​of shape characteristic parameters z1, p1, z2, p2;

[0117] Step S113: The r determined in step S112 is n 、r c Substituting the values ​​of the shape characteristic parameters z1, p1, z2, and p2 into formula (9), we can obtain the mathematical model of the tangent slope of any point on the projection curve in the imaging direction of the contour in the plane formed by the projection curve and the central axis:

[0118] (9)

[0119] Among them, K t (z) is the slope of the tangent line at the point where the distance z between the section on the projection curve in the imaging direction of the profile and the minimum section of the necking bottom is in the plane formed by the projection curve and the central axis;

[0120] Step S114: Obtain the tangent slope according to step S113, and use linear search to determine the maximum value of the tangent slope under a certain accuracy. , the maximum value is the tangent slope of the inflection point of the projection curve in the imaging direction of the contour;

[0121] The linear search includes: setting a series of z values, the difference between two adjacent z values ​​is the search step Δz, Δz is the search accuracy, and calculating the tangent slope k corresponding to each z value according to formula (9): t , find k t The maximum value is .

[0122] It should be understood that the tangent slope k is calculated based on a series of z values. t , which is the tangent slope of a series of equidistant sections with an interval of Δz. The maximum tangent slope with an accuracy of Δz can be obtained by the above method. , and the inflection point position is determined based on the maximum value. It should be noted that the linear search can be calculated using existing general data processing software, such as Excel, etc., or a corresponding calculation program can be compiled for calculation, which will not be further described or limited here.

[0123] For example, the search step size Δz can be set to 0.01 mm to obtain the maximum tangent slope with an accuracy of 0.01 mm. This can be used to perform the next calculation, thereby obtaining the required parameters reflecting the location of the necking deformation inflection point, thereby more accurately describing the contour of the necking deformation. It should be understood that the specific search step size can be set according to needs and is not limited to the accuracy provided in this application. It can also take values ​​between 0.1 mm and 0.001 mm, and is not limited here.

[0124] Figure 10 This is a typical specimen shape in the necking stage of a circular tensile test. At this time, the specimen shape is a rotating body with the central axis of the tensile direction as the rotation axis, and the projection curve of the free surface imaging direction is Figure 11 The outline shown in the figure can be approximately considered that the shapes of the two sides of the minimum cross-section of the necking bottom are symmetrical about the minimum cross-section. The outline of one side of the cross-section presents an "S"-shaped feature z, and the tangent of the outline at the minimum cross-section position (i.e. Figure 10 The tangent of the "neck bottom" shown in the figure is parallel to the central axis. Assuming that the intersection of the minimum section and the rotation axis is the origin, and the distance between the minimum section and the rotation axis is z, then the distribution function of the section radius r perpendicular to the rotation axis with respect to the corresponding section position z is the contour line function of the necking specimen, that is, the projection curve function of the imaging direction of the free surface. From the above-mentioned necking shape characteristics, it can be seen that the distribution function of r with respect to z should satisfy the shape of "S", the tangent at the z=0 position is parallel to the z axis, that is, the first-order derivative of r with respect to z when z=0 According to this mathematical characteristic, Figure 11 In the coordinate system shown in Figure 1, the mathematical model of the necking profile is established as shown in formula (8). This mathematical model can well describe the necking shape profile of the round rod specimen, as shown in Figure 1. Figure 12 As shown. Among them, Figure 11 In the coordinate system, the central axis is the coordinate z-axis, and any two radial lines that are perpendicular to each other and intersect at the center of the circle in the minimum cross-section of the necking bottom are the coordinate x-axis and y-axis.

[0125] After step S114, the following steps are performed:

[0126] Step S115: Substitute the z value corresponding to the maximum slope obtained in step S114 into formula (8) to calculate the cross-sectional radius r of the inflection point perpendicular to the central axis. ip .

[0127] like Figure 12 As shown in the figure, the parameter determination of the inflection point position can more accurately describe the contour of the necking deformation. Compared with the existing technology, it greatly improves the analysis accuracy of the contour and facilitates subsequent analysis and modeling. The minimum cross-sectional radius r of the round bar specimen can be obtained through the above steps. c, the maximum limit value of the cross-sectional radius r n , inflection point tangent slope , the distance z between the section at the inflection point and the minimum section ip (z value corresponding to the maximum slope), cross-sectional radius r at the inflection point ip Shape characteristic parameters such as the above are helpful for the accurate characterization of the necking deformation contour of the specimen and the subsequent modeling analysis. It should be noted that in the prior art, the maximum limit value of the cross-section radius perpendicular to the central axis is r n It cannot be directly measured and is usually replaced by the radius measurement value of the gauge point, but the accuracy of this value is extremely low. In this application, r is proposed. n The calculation method provides a more accurate reference for subsequent research. The slope of the inflection point tangent , the distance z between the section at the inflection point and the minimum section ip , the cross-sectional radius r at the inflection point ip Conventional analysis involves photography and hand-drawn point tracing, which is time-consuming and laborious, and the number of points drawn is limited, resulting in low accuracy. However, the linear search calculation method employed in this application allows for the direct and rapid acquisition of these parameters, with accuracy far exceeding that achieved by hand-drawn point tracing in existing techniques. This technical solution not only provides a method for rapidly calculating these shape feature parameters, but also significantly improves their accuracy.

[0128] As an example of the present invention, the number of measurement points in step S112 is 10 or more.

[0129] Among them, the breaking strength I in step S3 b Obtained by the following steps:

[0130] Step S31: Record the tensile axial force F during the test in step S1 at a certain frequency in real time. z ;

[0131] Step S32: After the specimen breaks, substitute the specimen diameter change parameters at the n sampling points before the force value drops sharply into formula (10) to calculate the instantaneous stress first invariant at the center of the minimum cross section perpendicular to the central axis of the specimen during the n test processes before the force value drops sharply: :

[0132] (10)

[0133] Step S33: Take the average value of the n first stress invariants obtained in step S32, and determine the average value as the fracture strength I of the material being measured. b .

[0134] Through step S33, the first stress invariant of the specimen in the necking deformation stage can be accurately calculated. The comparison between the stress first invariant analysis value obtained by finite element analysis and the stress first invariant value derived by formula (10) is shown in Figure 13 , it can be seen that the derived value of the first stress invariant obtained by formula (10) has extremely high accuracy, which helps to obtain the fracture strength of the material more accurately, thereby improving the accuracy and safety of material analysis and design.

[0135] Optionally, the certain frequency in step S31 is 5 to 100 times / second, preferably 10 to 90 times / second, and the value range of n in step S32 is 1 to 5, preferably 3.

[0136] In this embodiment, finite element simulation of the uniaxial tensile fracture process of a round bar specimen was carried out based on the I1 fracture criterion. The results show that the crack first forms at the center of the smallest cross section during the necking deformation stage. At the moment of crack formation, it becomes unstable and expands into a circular crack of a certain size, then stops expanding. Under the subsequent continuously increasing displacement load, the crack expands step by step, as shown in FIG. Figure 14 and Figure 15 As shown in Figure 2, the formation and instability of the crack are accompanied by a sharp decrease in the measured values ​​of the axial force and the first invariant of stress, as shown in Figure 2. Figure 16 and 17 As shown, d is the circular crack radius and ΔL is the loading displacement during the test. Therefore, this feature is used to determine the fracture strength I of the material. b , that is, the fracture moment is determined by combining the steep drop point of the axial force and the fracture photo of the specimen, and the first invariant value of the core stress at that moment is calculated It can be determined as the fracture strength of the material I b Considering the frequency and error of data acquisition, it is recommended to use the data of the first n acquisition points before the force value drops sharply to calculate the first invariant value of stress. And calculate the average value, which is determined as the breaking strength I of the tested material b , preferably three detection points are selected. This setting can detect the first stress invariant at n sampling moments before the sample breaks. The specific detection value is slightly lower than the actual value, but the lower detection value can significantly improve the performance margin during engineering design, thereby ensuring the performance of engineering materials, as well as their safety and service life. Among them, the force value drop refers to the tensile axial force F corresponding to the latter sampling point between two adjacent sampling points. z Compared with the tensile axial force F at the previous collection point z The value of has decreased by more than 10%.

[0137] As an optional example, the material of the round rod sample is a metal material that undergoes necking deformation during the uniaxial stretching process of the round rod sample.

[0138] Furthermore, the material of the round bar sample is one of steel, aluminum alloy, copper alloy, and titanium alloy that undergoes necking deformation during the uniaxial stretching of the round bar sample.

[0139] Furthermore, the material of the round bar sample is low alloy steel which undergoes necking deformation during the uniaxial stretching process of the round bar sample.

[0140] It should be noted that before the present invention, there was no technology in the existing technology that could measure the stress-strain relationship of the sample in the yield stage. The present invention establishes a corresponding analysis model based on the parameters that can be measured during the test process, and realizes the analysis and derivation of the stress-strain relationship during the full plastic deformation process of the uniaxial tensile test of the round bar sample of the metal material. It can also set the corresponding fracture resistance performance index according to the situation of the strength drop in the yield stage, and provide its measurement method, which is of great significance to the study of the stress-strain relationship and fracture strength of the material.

[0141] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for determining the equivalent fracture resistance index of a metal material, characterized in that: The following steps are involved: Step S1: performing a uniaxial tensile test on a round bar specimen, and recording multiple sets of test data in real time during the test process, wherein the test data at least includes stress data and strain data in a uniform plastic deformation stage and a non-uniform plastic deformation stage; Step S2: Substitute the stress and strain data of the uniform plastic deformation stage and the non-uniform plastic deformation stage obtained in step S1 into (1) to fit and obtain the corresponding parameters and equivalent yield strength : （1） in, is the Mises equivalent stress during the plastic deformation of the specimen, is the equivalent plastic strain during the plastic deformation of the specimen, and Obtained through step S1, is the equivalent yield strength, k1 and k2 are strength coefficients, n1 and n2 are strain hardening coefficients, , k1, k2, n1, n2 are all parameters obtained by fitting formula (1), e is a constant; Step S3: Obtain the fracture strength I of the material during the test b , calculate the equivalent strength ratio of the fracture performance index .

2. The method for determining the equivalent fracture resistance index of a metal material according to claim 1, wherein: The following steps are also included: Step S4: Substitute the fitting parameters obtained in step S2 into equation (1) to obtain the stress-strain relationship model of the material.

3. The method for determining the equivalent fracture resistance index of a metal material according to claim 1, wherein: The acquisition of test data in step S1 includes at least the following steps: Step S11: In the uniaxial tensile test of the round bar specimen, the tensile axial force F during the test is recorded in real time. z , and record the change parameters of the sample diameter, which at least include the minimum cross-sectional radius of the necking bottom perpendicular to the central axis of the sample , the maximum limit value of the cross-sectional radius perpendicular to the central axis of the specimen , the tangent slope of the inflection point of the projection curve in the imaging direction of the necking deformation contour , the cross-sectional radius perpendicular to the central axis at the inflection point of the projection curve in the imaging direction of the necking deformation contour , the distance between the section perpendicular to the central axis at the inflection point of the projection curve of the necking deformation contour imaging direction and the minimum section of the necking bottom ; Step S12: In the uniform plastic deformation stage, according to the F obtained in step S11 z and diameter change parameters, and substitute them into formula (2) to obtain the stress data in the uniform plastic deformation stage: （2） in, It is the instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen during the test; Step S13: In the non-uniform plastic deformation stage, according to the F obtained in step S11 z 、 、 、 、 、 , substituted into formula (3) to calculate the stress data of the non-uniform plastic deformation stage: （3）。 4. The method for determining the equivalent fracture resistance index of a metal material according to claim 3, wherein: The acquisition of the test data in step S1 further includes: Step S14: In the uniform plastic deformation stage, according to the diameter change parameter obtained in step S11, substitute it into formula (4) to calculate the strain data of the plastic deformation stage: （4） in, is the cross-sectional area perpendicular to the central axis of the specimen before the test, It is the instantaneous cross-sectional average stress of the smallest cross-section perpendicular to the central axis of the specimen during the test, and in the uniform plastic deformation stage , E is the elastic modulus of the material; Step S15: In the non-uniform plastic deformation stage, according to the F obtained in step S11 z 、 、 、 、 、 , substitute into formula (5) to obtain the corresponding strain data: （5） in, It is the instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen when the axial force is at its maximum value during the test. It is the maximum axial force that the specimen bears during the test.

5. The method for determining the equivalent fracture resistance index of a metal material according to claim 4, wherein: During the round bar tensile test data acquisition process, the cross-sectional radius of the specimen perpendicular to the central axis is acquired through real-time image data acquisition of the specimen profile in two directions. The instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen during the test is Calculated by formula (6): （6） in, is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen during the test in the first imaging direction, The instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen during the test in the second imaging direction; The instantaneous minimum cross-sectional area perpendicular to the central axis of the specimen when the axial force is at its maximum value during the test Calculated by formula (7): （7） in, is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen when the axial force in the first imaging direction is at its maximum value during the test. It is the instantaneous minimum cross-sectional radius perpendicular to the central axis of the specimen when the axial force is maximum during the test in the second imaging direction.

6. The method for determining the equivalent fracture resistance index of a metal material according to claim 5, wherein: The two directions of real-time image data acquisition of the specimen profile are perpendicular to each other.

7. The method for determining the equivalent fracture resistance index of a metal material according to claim 3, wherein: In step S11, the minimum cross-sectional radius of the necking bottom perpendicular to the central axis of the sample is , the maximum limit value of the cross-sectional radius perpendicular to the central axis of the specimen , the tangent slope of the inflection point of the projection curve in the imaging direction of the necking deformation contour Ways to obtain include: Step S111: Set the imaging direction projection curve of the sample profile at the necking bottom in the necking stage to be S-shaped, and establish a mathematical model of the imaging direction projection curve as shown in formula (8): （8） Among them, r is the cross-sectional radius perpendicular to the central axis at any point on the specimen contour surface, z is the distance between the cross section at any point and the minimum cross section at the necking bottom, z1, p1, z2, and p2 are the shape characteristic parameters to be determined; Step S112: Measure the test data of the sample with necking deformation. The test data includes at least the cross-sectional radius r and the cross-sectional distance z. There are multiple measurement points. Substitute the measured test data into formula (8) for fitting to determine r n 、r c and the values ​​of shape characteristic parameters z1, p1, z2, p2; Step S113: The r determined in step S112 is n 、r c Substituting the values ​​of the shape characteristic parameters z1, p1, z2, and p2 into formula (9), we can obtain the mathematical model of the tangent slope of any point on the projection curve in the imaging direction of the contour in the plane formed by the projection curve and the central axis: （9） Among them, K t (z) is the slope of the tangent line at the point where the distance z between the section on the projection curve in the imaging direction of the profile and the minimum section of the necking bottom is in the plane formed by the projection curve and the central axis; Step S114: Obtain the tangent slope according to step S113, and use linear search to determine the maximum value of the tangent slope under a certain accuracy. , the maximum value is the tangent slope of the inflection point of the projection curve in the imaging direction of the contour; The linear search includes: setting a series of z values, the difference between two adjacent z values ​​is the search step Δz, Δz is the search accuracy, and calculating the tangent slope k corresponding to each z value according to formula (9): t , find k t The maximum value is .

8. The method for determining the equivalent fracture resistance index of a metal material according to claim 7, wherein: After step S114, the following steps are performed: step S115: Substitute the z value corresponding to the maximum slope obtained in step S114 into formula (8) to calculate the cross-sectional radius r of the inflection point perpendicular to the central axis. ip .

9. The method for determining the equivalent fracture resistance index of a metal material according to claim 3, wherein: The breaking strength I in step S3 b Obtained by the following steps: Step S31: Record the tensile axial force F during the test in step S11 at a certain frequency in real time. z ; Step S32: After the specimen breaks, substitute the specimen diameter change parameters at the n sampling points before the force value drops sharply into formula (10) to calculate the instantaneous stress first invariant at the center of the minimum cross section perpendicular to the central axis of the specimen during the n test processes before the force value drops sharply: : （10） Step S33: Take the average value of the n first stress invariants obtained in step S32, and determine the average value as the fracture strength I of the material being measured. b .

10. The method for determining the equivalent fracture resistance index of a metal material according to claim 9, wherein: The certain frequency in step S31 is 5 to 100 times per second, and the value range of n in step S32 is 1 to 5.

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