Expression method of strength variation property

Abstract

The present invention relates to a method for expressing physical properties when there is a change in strength in the plastic region of an elastic material, the method including: a region setting step of setting, in a first material and a second material obtained by pre-straining and heat-treating the first material, a basic region indicating the relationship between strain and stress in the first material and a change region indicating a portion of the change in stress according to strain due to factors caused by pre-straining and heat treatment of the second material; and a simulation step of expressing the relationship between stress change and pre-strain amount in the change region as a Boltzmann function, and deriving the relationship between strain and stress due to the second material as the sum of the stress according to strain in the basic region and the change region.

Description

Method of expressing strength change properties

[0001] The present invention relates to a method for expressing the change in strength properties in plastic deformation of an elastic material, which seeks to accurately obtain a stress and strain curve as a formula for a material subjected to heat treatment.

[0002] The plastic deformation that occurs in a part during molding is called pre-strain because it occurs before the plastic deformation that occurs during a collision.

[0003] During the finishing process, a baking process (heat treatment) is performed to improve paint adhesion. For example, automobile parts undergo a baking process after assembly to strengthen the paint adhesion during the painting process. While the strength of automotive steel in its original state generally does not change significantly during the baking process, the strength of the material generally increases significantly after the baking process after plastic deformation during the forming process. Since this strength change affects crashworthiness, it is necessary for automobile manufacturers to consider the changes in material properties due to pre-strain and baking processes in crash analysis during vehicle design. However, it is difficult to derive equations for the material properties due to pre-strain and baking processes, so even in these cases, it is necessary to express the material properties relatively accurately to analyze crashworthiness.

[0004] (Patent Document 1) KR10-2020-0118162 (2020.09.15)

[0005] The present invention is intended to solve the above problem, and provides a method for expressing a change in strength properties by expressing it as a formula when there is plastic deformation in a material having a pre-deformation and a sub-process.

[0006] In order to achieve the above purpose, the present invention provides a method for expressing strength change properties formed as follows.

[0007] A method for expressing strength change properties according to an embodiment of the present invention comprises: a first material; and a second material obtained by pre-straining and heat-treating the first material, wherein the first material is subjected to a pre-strain process, and a region setting step for setting a basic region for the relationship between strain and stress by the first material and a change region representing a portion of stress change according to strain due to factors caused by pre-straining and heat treatment of the second material; and a simulation step for expressing the relationship between stress change according to pre-strain amount in the change region as a Boltzmann function, and deriving the relationship between strain and stress by the second material as a sum of stresses according to strain rates in the basic region and the change region.

[0008] The present invention provides an effect of relatively accurately expressing the change in strength of a material according to the deformation of the raw material and the firing process through the above structure.

[0009] Additionally, it can perform crash analysis by considering changes in the strength of materials, such as changes in the paint shop process of automobiles, thereby improving the design accuracy of finished products such as automobiles.

[0010] Figure 1 shows a true stress-true strain diagram for a material with pre-deformation and a small-scale process (heat treatment).

[0011] Figure 2 illustrates a flowchart of a method for expressing changes in strength properties according to one embodiment of the present invention.

[0012] Figure 3 illustrates the relationship between true strain and true stress expressed in a formula according to one embodiment of the present invention.

[0013] Figure 4 shows the increase in strength of a material according to pre-strain when heat treatment is performed while varying the pre-strain at an effective plastic strain of 0.2.

[0014] Figure 5 shows a graph for explaining the Boltzmann function.

[0015] Figure 6 shows the relationship between elongation and stress of a material having an upper yield point and a lower yield point.

[0016] Figure 7 shows a graph comparing the test results when the conditions for deformation and heat treatment of a material are changed with an equation expressed according to an embodiment of the present invention.

[0017] FIG. 8 is a graph comparing the test results for a case where the conditions for deformation and heat treatment of a material different from the material of FIG. 7 are changed with an equation expressed according to an embodiment of the present invention.

[0018] FIG. 9 is a graph comparing the test results for a case where the conditions for deformation and heat treatment of a material different from those of FIGS. 7 and 8 are changed with an equation expressed according to an embodiment of the present invention.

[0019] Figure 10 schematically illustrates the shape of a hat-shaped specimen.

[0020] Fig. 11 is a graph showing a load diagram when a collapse test is performed using a hat-shaped specimen like Fig. 10, comparing the case of the first material, the case where only work hardening is considered, and the case where it is considered using an expression method derived according to an embodiment of the present invention.

[0021] Fig. 12 is a graph showing an absorption energy diagram in the case of a collapse test using a hat-shaped specimen like Fig. 10, comparing the case of the first material, the case of considering only work hardening, and the case of considering an equation derived using an expression method according to an embodiment of the present invention.

[0022] Hereinafter, specific embodiments of the present invention will be described with reference to the attached drawings. However, the spirit of the present invention is not limited to the presented embodiments, and those skilled in the art who understand the spirit of the present invention will be able to easily propose other regressive inventions or other embodiments included within the scope of the spirit of the present invention by adding, modifying, or deleting other components within the scope of the same spirit. However, this will also be considered to be included within the scope of the spirit of the present invention.

[0023] Additionally, in describing the present invention, 'part' or 'unit' can be implemented in various ways, for example, by a processor, program instructions executed by a processor, a software module, microcode, a computer program product, a logic circuit, an application-specific integrated circuit, firmware, etc.

[0024] The content of the method disclosed in the embodiments of the present application can be directly implemented in a hardware processor, or can be implemented and performed by a combination of hardware and software modules within the processor. The software module can be stored in a conventional storage medium such as a random access memory, flash memory, read-only memory, programmable read-only memory, electrically erasable programmable memory, register, etc. The storage medium is located in a memory, and the processor reads the information stored in the memory and combines it with the hardware to complete the content of the above-described method. To avoid duplication, a detailed description is omitted herein.

[0025] In the implementation process, each content of the above-described method can be completed by a hardware logic integrated circuit of a processor or by instructions in the form of software. The content of the method disclosed in the embodiments of the present application can be directly implemented by a hardware processor, or can be implemented and performed by a combination of hardware and software modules of the processor. The software module can be stored in a conventional storage medium such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory, an electrically erasable programmable memory, a register, etc. The storage medium is located in a memory, and the processor reads the information stored in the memory and combines it with the hardware to complete the content of the above-described method.

[0026] That is, those skilled in the art will appreciate that the exemplary units and algorithm steps described in the embodiments disclosed herein can be implemented using electronic hardware or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art may implement the described functions using different methods for each specific application, but such implementations should not be considered outside the scope of the present application.

[0027] It should be understood that the disclosed devices and methods may be implemented in other ways, as described in the embodiments herein. For example, the device embodiments described above are merely exemplary, and the division of the units, for example, is merely a logical functional division, and other division methods may exist in actual implementations. For example, multiple units or assemblies may be combined or integrated into another system, or certain features may be ignored or not performed. On the other hand, the coupling or direct coupling or communication connection between the units shown or discussed may be an indirect coupling or communication connection through some interface, device, or unit, and may be electrical, mechanical, or in other forms.

[0028] The units described above as separate components may be physically separate, and the components represented as units may or may not be physical units, i.e., located in one location or distributed across multiple network units. Depending on actual needs, some or all of these units may be selected to achieve the objectives of the present embodiment.

[0029] That is, each functional unit in each embodiment of the present application may be integrated into one processing unit, each unit may exist alone, or two or more units may be integrated into one unit.

[0030] If the above function is implemented in the form of a software function unit and sold or used as an independent product, it can be stored in a single computer-readable storage medium. Based on this understanding, the technical solution of the present application, or a part that essentially contributes to the prior art or a part of the technical solution, can be implemented in the form of a software product, and the computer software product is stored in a single storage medium and includes a small number of instructions to cause a single computer device (which may be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the method described in each embodiment of the present application. The above-mentioned storage medium includes various media capable of storing program code, such as a USB memory, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or a CD-ROM.

[0031] Figure 1 is a graph of true stress-true strain, showing a tensile diagram in cases where there is a change in strength according to the raw material, pre-deformation, and bending process.

[0032] All were derived using specimens formed with the same material and thickness (980HB steel, specimen formed with 1.6t), and are shown as cases without processing (L1), with 0% pre-strain and heat treatment processing (L2), with 2% pre-strain and heat treatment processing (L3), with 5% pre-strain and heat treatment processing (L4), with 8% pre-strain and heat treatment processing (L5), and with 10% pre-strain and heat treatment processing (L6).

[0033] In general, if only heat treatment is performed without pre-deformation, the change in strength is minimal, but if the pre-deformation is 2%, the strength increases significantly in the early stage of deformation as yield point elongation occurs, and thereafter, the strength has a value similar to that of the original material.

[0034] As the pre-strain increases further, the upper yield stress becomes very large in the transition section from elasticity to plasticity, and the subsequent work hardening curve also has a value that is significantly larger than that of the raw material. Overall, the upper yield stress becomes large when moving to the plastic section, and the amount of increase thereafter gradually decreases. When the pre-strain is less than 2%, the work hardening curve is close to that of the raw material, and when the pre-strain is 5% or more, the work hardening curve is larger than that of the raw material.

[0035] As shown in Figure 1, even if the same material is used, the material properties, especially the physical properties according to the change in strength, change due to deformation or heat treatment processing, so a method for accurately expressing these changes is necessary.

[0036] Figure 2 is a flowchart for a method for expressing strength change properties according to one embodiment of the present invention.

[0037] A method for expressing strength change properties according to one embodiment of the present invention includes a region setting step (S200) and a simulation step (S400).

[0038] The present invention relates to a method for predicting strength change properties of a first material and a second material obtained by pre-straining and heat-treating the first material.

[0039] The region setting step (S200) sets a basic region (D1, see Fig. 3) for the relationship between the strain and stress by the first material, and a change region (D2, see Fig. 3) that represents the change in stress according to the strain due to factors such as pre-deformation and heat treatment of the second material.

[0040] The first material deals with strain and stress in raw materials. Using previously tested data, the relationship between strain and stress can be determined. Therefore, by loading or inputting preset data, information on basic areas can be obtained without experimentation.

[0041] For example, in the region setting step (S200), a compensation region (D3, see Fig. 3) can be further set according to the initial strength increase and stress decrease according to the yield point elongation of the second material. This compensation region (D3) can be additionally simulated and expressed as a formula in the following compensation step (S600).

[0042] The simulation step (S400) is a step in which the relationship between the stress change according to the amount of deformation in the change area (D2) is expressed as a Boltzmann function, and the relationship between the strain rate and stress by the second material is derived as the sum of the stress according to the strain rate of the basic area (D1) and the change area (D2).

[0043] The Boltzmann function is a probability distribution in statistical mechanics or mathematics that provides the probability that a system is in a specific state as a function of the energy and temperature of that state. In the present invention, this equation is applied to represent changes in strength due to deformation and heat treatment.

[0044] For example, a first graph step (S300) may be further included prior to the simulation step (S400).

[0045] The first graph step (S300) represents the pre-deformation amount and stress change according to heat treatment in the change area (D2) as a first graph, and an equation for the change area (D2) can be derived from this first graph.

[0046] In the first graph step (S300), a graph is drawn having a first slope, a second slope, and a third slope for the stress change relationship according to the amount of deformation of the second material, and the second slope has an absolute value greater than the first slope and the third slope.

[0047] And, in the simulation step (S400), the change area (D2) can be derived as an equation for the third pre-strain amount, which is an intermediate value between the first pre-strain amount at which the second slope starts and the second pre-strain amount at which the second slope ends, the second slope, the first stress value, which is a stress value for the first pre-strain amount, and the second stress value, which is a stress value for the second pre-strain amount. After that, the relationship between the strain and stress change for the basic area (D1) and the relationship between the pre-strain and the stress change in the change area (D2) are combined. When the two areas are combined, the pre-strain in the change area (D2) can be linearly combined as seen from the strain.

[0048] According to one embodiment of the present invention, a correction step (S600) may be further included after the simulation step (S400).

[0049] The correction step (S600) derives an equation for the correction region (D3) by expressing the initial strength increase and the subsequent stress decrease according to the yield point elongation of the second material as a maximum increase stress and an exponential function, and the relationship between the strain rate and stress of the second material can be derived as the sum of the basic region (D1) and the change region (D2) of the simulation step (S400) and the equation for the correction region (D3).

[0050] As expressed by the following mathematical expression 1, in the simulation step (S400), the relationship between the strain and stress of the second material can be expressed as the sum of the basic region (D1) and the change region (D2). Thereafter, in the correction step (S600), the relationship between the strain and stress of the second material can be derived by combining the expressions for the correction region (D3).

[0051] Mathematical formula 1: σ = σ AR + △σ shift + △σ peak

[0052] In the above mathematical expression 1, σ is the stress value according to the strain in the relationship between the strain and stress of the second material, and σ AR is the stress value according to the strain of the first material as a relationship between the strain and stress of the basic area (D1), and △σ shift is the value of stress according to the amount of pre-deformation changed according to the pre-deformation and heat treatment of the second material in the above change area (D2), and △σ peak is the stress value for the initial strength increase and the subsequent stress decrease according to the yield point elongation of the second material according to the strain rate of the above compensation area (D3).

[0053] Therefore, by deriving each equation as a linear equation, such as the mathematical equation 1 above, the relationship between the strain rate and stress of the second material according to the experiment or previously stored data can be easily obtained by simulating it as a mathematical equation.

[0054] According to one embodiment of the present invention, a second graph step (S500) may be further included prior to the correction step (S600).

[0055] The second graph step (S500) may be a step for generating a graph derived from the relationship of stress change according to the yield point elongation of the second material, the fourth slope, the upper yield point and the first elongation which is the elongation at the upper yield point, and the lower yield point and the second elongation which is the elongation at the lower yield point. Since the second graph step is sufficiently performed before the correction step, it may be performed anywhere after the area setting step (S200) and before the correction step (S600).

[0056] The graph expressed in the second graph step (S500) above is a process of deriving a graph so as to be able to simulate the decrease in stress after the upper yield stress with an upper yield point and an exponential function.

[0057] A method for expressing strength change properties according to one embodiment of the present invention may further include an input step (S100) before the region setting step (S200).

[0058] The input step (S100) may include a first input step and a second input step.

[0059] The first input step is a step for inputting in advance the relationship between stress and strain for the first material as a value for the basic area, and data from experiments can be input, or values ​​previously stored on a server, etc. can be input.

[0060] The second input step may input a graph obtained through an experiment on the true strain-true stress relationship for the second material, or, if the graph is stored on a server or the like, retrieve it. Alternatively, data for the desired second material under the given conditions may be linearly derived from a plurality of data on the first material and its pre-strained and heat-treated state on a server or the like, and then derived as a graph.

[0061] According to one embodiment of the present invention, based on the information input in the input step (S100), the change in strength properties of the elastic material for the second material can be expressed in a specific formula through the above-described area setting step (S200), simulation step (S400), etc.

[0062] According to one embodiment of the present invention, an expression step (S700) for graphically deriving an equation for the sum derived after the simulation step (S400) or the correction step (S600) may be further included. By including the expression step (S700), the accuracy of the equation, etc. can be intuitively displayed to users, etc.

[0063] The following description will explain the specific contents of the above-mentioned simulation step (S400) and correction step (S600).

[0064] Figure 3 illustrates a graph of true strain and true stress expressed by a formula according to one embodiment of the present invention.

[0065] The first line, expressed as a thick solid line, is a graph of the true strain and true stress of the first material. Since there is no pre-deformation or heat treatment process, there is no particular change in strength and the yield point σ Y Graph showing the subsequent plasticity region (σ AR )am.

[0066] The second line, represented by a dashed line, has a deformation of ε p BH1 In this case, the yield stress is σ Y,BH1 has the value of .

[0067] The third line, represented by a solid line, has a deformation of ε p BH2 This is the case when there is that much.

[0068] The fourth line, expressed as a dashed line, is a pre-deformation of ε p BH3 In this case, the yield stress is σ Y,BH3 has the value of .

[0069] If the expression method according to the present invention is explained based on the third line, the section divided by the first line becomes the basic area (D1), and the maximum section that can be covered by linearly deforming the first line, excluding the basic area (D1), has the tendency of the first line, can be set as the change area (D2). In addition, the other area can be set as the compensation area (D3) in which the stress decrease after the upper yield stress can be expressed as an upper yield point and an exponential function.

[0070] The stress at the yield point at this time is arbitrarily σ Y,BH2 Expressed as σ Y,BH2 is ε pBH2 σ at the strain rate of AR and △σ shift Wow △σ PO It has the value of . In some sections where the correction area (D3) is formed thereafter, the stress is σAR and △σ shift and △σ peak can be expressed as the sum of . And, in the section where the compensation area (D3) is not formed, the stress is σ AR and △σ shift can be expressed as the sum of .

[0071] Figure 4 is a graph illustrating the change in strength when heat-treated with different pre-deformations of the same material. Measurements were made under identical conditions with an effective plastic strain of 0.2 for 980HB steel. Figure 5 is a graph illustrating the Boltzmann function.

[0072] In order to explain Equation 1 of the simulation step (S400) according to one embodiment of the present invention, reference is made to FIGS. 4 and 5.

[0073] As can be seen in the graph of Figure 4, when there is pre-strain, the increase in strength has the characteristic of changing rapidly at a pre-strain of approximately 2 to 5%. Therefore, a method is needed to accurately express the changes in material properties, especially in this range.

[0074] According to one embodiment of the present invention, when there is a rapid change in strength according to the deformation and heat treatment process as in FIG. 4, the change in strength can be simulated using the form of a Boltzmann function as in FIG. 5.

[0075] Figure 5 is a graph for explaining the Boltzmann function, and can be expressed as an equation in the case where there is a transition section, which is a section where the value of the y-axis changes abruptly according to a change in the x-axis. The Boltzmann function is expressed as in the mathematical equation 2 below.

[0076] Mathematical formula 2:

[0077] Here, y1 is the y value before the transition section, y2 is the y value after the transition section, x0 is the x-axis value at the point midway between y1 and y2, which is the midpoint of the transition section, and k is the slope of the transition section.

[0078] If this is applied to a case where there is a change in strength due to deformation and heat treatment in the form of a graph as in Fig. 4, it can be derived as in Equation 1 below.

[0079] Equation 1:

[0080] Here, Δσ shift is the value for the increase in strength due to deformation and heat treatment, and represents the strain and stress for the above-mentioned change area, and σ Δ,min is the first stress value, and σ Δ,max is the second stress value, and ε tran1 is the third example variation, and ε p BH is a variable as a pre-deformation quantity, and ε BH,Max is the maximum value of the pre-deformation amount, and K1 is the value of the second slope.

[0081] According to one embodiment of the present invention, σ Δ,min is the first stress value and the σ Δ,max The second stress value can also be expressed as a linear formula according to the amount of deformation. This can be expressed as in the following mathematical expressions 3 and 4.

[0082] Mathematical formula 3:

[0083] Mathematical formula 4:

[0084] In equations 3 and 4, σ Δ,min is the first stress value, and σ Δ,max is the second stress value, and ε p BH is a variable as a pre-deformation quantity, and ε BH,Max is the maximum value of the pre-deformation, and a, b, c, and d are coefficients.

[0085] By substituting mathematical expressions 3 and 4 into the above equation 1, equation 1 is derived as a function of the stress value according to the amount of pre-deformation.

[0086] In the simulation step (S400), the relationship between the pre-strain and stress in the change region can be derived using the method described above. Furthermore, the pre-strain in the change region can be expressed as a type of elongation, using the equation derived in the simulation step (S400) or the graph expressed by the equation, as a linear sum of the equations or graphs for the basic region, to express the elongation and stress of the second material.

[0087] Figure 6 illustrates the relationship between elongation and stress to explain the upper and lower yield points.

[0088] Depending on the material, there may be an upper yield point and a lower yield point. Generally, in the case of a hard and tough material, an upper yield point and a lower yield point are formed as the strength decreases, and the material may break.

[0089] When the first material is transformed into a second material through deformation and heat treatment, the second material becomes harder and tougher than the first material, so it may have an upper yield point and a section in which the stress decreases after the upper yield stress.

[0090] According to one embodiment of the present invention, when the yield point as described above is obtained through deformation or heat treatment, the above region may be represented as a correction region (D3) of FIG. 3.

[0091] For example, for the compensation area (D3), the initial strength increase and stress decrease according to the yield point elongation of the second material can be derived as an equation expressing the increased stress at the upper yield point and an exponential function.

[0092] Alternatively, a third elongation, which is an intermediate value between the first elongation and the second elongation, and an equation relating the fourth slope and the upper yield point can be derived. Specifically, this can be expressed as Equation 2 below.

[0093] Equation 2:

[0094] Here, Δσpeak represents the strain and stress for the above compensation region according to the increase in initial strength by yield point elongation, and Δσ p0 is the increased stress value at the above yield point, and ε tran2 is the third elongation, and ε p BH is a variable that represents the amount of variation in the case of a variation, and ε BH,Max is the maximum value of the pre-deformation, k2 is the fourth slope, and t1 represents the coefficient.

[0095] For example, Δσ p0 can be expressed according to the following equation 3.

[0096] Equation 3:

[0097] Here, Δσ p0 is the increased stress value at the above yield point, K BH Wow ε B0 and m are coefficients, ε p BH is a variable that represents the amount of variation in the case of a variation, and ε BH,Max is the maximum value of the pre-deformation amount.

[0098] The above equation 3 is Δσ of the above equation 2 p0 When substituted into , Equation 2 is transformed into an equation for the stress in the above compensation area (D3) according to the increase in initial strength by yield point elongation with the pre-strain as a variable.

[0099] By adding the equation derived for the correction area (D3) to the sum of the basic area (D1) and the change area (D2) obtained in the above simulation step (S400), the true stress value for the pre-strain amount can be obtained, and at this time, the pre-strain amount can be the true strain rate.

[0100] In the above manner, by deriving a specific formula for the strength change properties in the plastic deformation zone of an elastic material, the strength change of the material according to the pre-deformation and firing process of the raw material can be expressed relatively accurately, and by applying this, it is possible to perform a crash analysis by considering the strength change of the material that has changed, such as in the paint shop process of an automobile, thereby improving the design accuracy of finished products such as automobiles.

[0101] In addition, according to one embodiment of the present invention, there may be a recording medium recording a computer-readable program for executing the above method.

[0102] Figures 7 to 9 illustrate the property models and experimental values ​​under various conditions of the second material when the first material is different.

[0103] Figures 7 to 9 show graphs drawn as lines according to experiments on plastic deformation of the first material obtained by performing different treatments on 980 MPa steel, subjected to pre-deformation and heat treatment under various conditions, and graphs drawn according to the formula derived by this method according to the type of the line are displayed together as lines and shapes.

[0104] In Figures 7 and 8, L1 represents a case where the pre-strain amount is 10%, L2 represents a case where the pre-strain amount is 8%, L3 represents a case where the pre-strain amount is 5%, L4 represents a case where the pre-strain amount is 2%, and L5 represents a case where the pre-strain amount is 0%. L1 to L5 were subjected to heat treatment. L6 is a graph input in the input step (S100) as the first material.

[0105] And in Fig. 9, L1 is when the pre-deformation amount is 10%, L2 is when the pre-deformation amount is 8%, L3 is when the pre-deformation amount is 5%, and L4 is when the pre-deformation amount is 2%, and L1 to L4 were subjected to heat treatment. L5 is a graph input in the input step (S100) as the first material.

[0106] As can be seen in Figures 7 to 9, even though there are various conditions in various types of graphs, when plotted using experimental values ​​and the method according to the present invention, almost similar results can be derived.

[0107] According to one embodiment of the present invention, an expression step (S700) for expressing the sum formula derived after the simulation step (S400) or the correction step (S600) as a graph such as FIGS. 7 to 9 is further included, thereby enhancing intuitive understanding and making it easy for the user to confirm the accuracy of the formula derived according to the present invention.

[0108] Fig. 10 is a schematic diagram of a hat-shaped specimen. A specimen like Fig. 10 can be used to analyze load and absorbed energy by collapsing.

[0109] Fig. 11 is a load diagram for a collapse test using a hat-shaped specimen like Fig. 10, and Fig. 12 is an energy diagram for a collapse test using a hat-shaped specimen like Fig. 10.

[0110] According to one embodiment of the present invention, the results of analysis of collision load and absorbed energy in an automobile are shown in the case of considering deformation and heat treatment and in the case of considering only work hardening (heat treatment).

[0111] In Fig. 11, the dashed-dotted line is a graph analyzed as the first material, the two-dotted line is a graph analyzed considering pre-deformation and heat treatment according to one embodiment of the present invention, and the solid line is a load diagram when only work hardening is considered as before.

[0112] In Fig. 12, the dashed-dotted line is a graph analyzed by considering deformation and heat treatment according to an embodiment of the present invention, the solid line is a graph of absorbed energy when only work hardening is considered as before, and the two-dotted line is a graph of absorbed energy analyzed as the first material.

[0113] When analyzed as described above using a formula according to an embodiment of the present invention, it provides the effect of allowing for a more accurate analysis of a car collision by showing a greater load and energy absorption rate than before.

[0114] Although the present invention has been described above with reference to examples, the present invention is not limited to the above-described examples, and it goes without saying that modifications can be made and implemented by those skilled in the art without changing the technical idea of ​​the present invention as claimed in the claims.

Claims

1. In the first material and the second material that is pre-strained and heat-treated from the first material, A region setting step for setting a basic region for the relationship between the strain and stress by the first material and a change region representing a portion of the stress change according to the strain due to factors such as pre-deformation and heat treatment of the second material; and A method for expressing strength change properties, comprising: a simulation step in which the relationship between the stress change according to the amount of deformation in the above change area is expressed as a Boltzmann function, and the relationship between the strain and stress by the second material is derived as the sum of the stress according to the strain in the basic area and the above change area.

2. In paragraph 1, Before the above copying step, The stress change relationship according to the amount of deformation of the second material has a first slope, a second slope, and a third slope. Further comprising a first graph step for showing a graph in which the second slope has a larger absolute value than the first slope and the third slope; In the above simulation step, A method for expressing strength change properties by deriving the change area as an equation for a third pre-strain amount that is an intermediate value between a first pre-strain amount at which the second slope starts and a second pre-strain amount at which the second slope ends, a first stress value that is a stress value for the second slope, and a second stress value that is a stress value for the second pre-strain amount.

3. In paragraph 2, In the above region setting step, a compensation region is further set according to the initial strength increase and stress decrease according to the yield point elongation of the second material, After the above copying step, The initial strength increase and stress decrease according to the yield point elongation of the above second material are expressed as the maximum increased stress and an exponential function to derive an equation for the above compensation area. A method for expressing strength change properties, further comprising a correction step in which the relationship between the strain and stress of the second material is derived as the sum of the basic area and the change area of ​​the simulation step and the sum of the correction area.

4. In paragraph 3, Before the above correction step, A second graph step for generating a graph derived from the relationship of stress change according to the yield point elongation of the second material with the fourth slope and the first elongation, which is the elongation at the upper yield point and the upper yield point, and the second elongation, which is the elongation at the lower yield point and the lower yield point; further comprising; In the above correction step, A method for expressing the change in strength properties of an elastic material, wherein the above correction region is derived from an equation relating to a third elongation which is an intermediate value between the first elongation and the second elongation, the fourth slope, and the upper yield point.

5. In paragraph 2, A method for expressing strength change properties, characterized in that in the above simulation step, the change area is expressed by the following equation 1. Equation 1: (Here, Δσ shift is the value for the increase in strength due to deformation and heat treatment, and represents the strain and stress for the change area, and σ Δ,min is the first stress value, and σ Δ,max is the second stress value, and ε tran1 is the third example variation, and ε p BH is a variable as a pre-determinant, and ε BH,Max is the maximum value of the pre-deformation amount, and K1 is the value of the second slope.) 6. In paragraph 4, A method for expressing strength change properties, characterized in that in the above correction step, the correction area is expressed by the following equation 2. Equation 2: (Here, Δσ peak represents the strain and stress for the above compensation region due to the increase in initial strength by the yield point elongation, and Δσ p0 is the increased stress value at the above yield point, and ε tran2 is the third elongation, and ε p BH is a variable that represents the amount of variation in the case of a variation, and ε BH,Max is the maximum value of the pre-deformation, k2 is the fourth slope, and t1 represents an unknown.) 7. In paragraph 6, In the above correction step, Δσ of the above equation 2 p0 A method of expressing strength change properties according to the following Equation 3. Equation 3: (Here, Δσ p0 is the increased stress value at the above yield point, K BH Wow ε B0 is the coefficient, and ε p BH is a variable that represents the amount of variation in the case of a variation, and ε BH,Max is the maximum value of the pre-deformation, and m is an unknown number.) 8. In paragraph 1, A method for expressing strength change properties in which the above deformation amount is 2 to 5%.

9. A recording medium recording a computer-readable program for executing any one of the methods of clauses 1 to 8.

Images