Expression method of intensity variation characteristic

Abstract

This invention relates to a method for expressing the physical properties of an elastic material when there is a change in strength in a plastic region. The method includes: a region setting step, wherein the region setting step defines a base region indicating the relationship between strain and stress in the first material and a variation region indicating the portion of stress change according to strain caused by factors induced by the pre-straining and heat treatment of the second material in a first material; and a simulation step, wherein the simulation step expresses the relationship between stress change and pre-strain in the variation region as a Boltzmann function, and derives the relationship between strain and stress in the second material as the sum of stresses according to strain in the base region and the variation region.

Description

Technical Field

[0001] This disclosure relates to a method for expressing the strength change characteristics of elastic materials during plastic deformation, used to accurately obtain stress-strain curves as equations for heat-treated materials. Background Technology

[0002] The plastic deformation that occurs in a part during part forming occurs before the plastic deformation that occurs during a collision, and is called pre-strain.

[0003] In the painting process, drying is performed through baking (heat treatment) to improve paint adhesion. For example, automotive parts undergo baking after assembly to enhance paint adhesion during the painting process. Typically, unformed automotive steel exhibits minimal strength change during baking; however, when plastic deformation is introduced through forming before baking, the material's strength usually increases significantly. Since such strength changes affect crash performance, automakers must consider the changes in physical properties caused by pre-strain and baking during vehicle design and crash analysis. However, because it is difficult to derive equations for the physical properties caused by pre-strain and baking, it is necessary to analyze crashes by expressing relatively accurate physical properties, even in such cases.

[0004] (Patent Document 1) KR10-2020-0118162 (September 15, 2020) Summary of the Invention Technical issues

[0005] One aspect of this disclosure is to provide a method for expressing the strength change characteristics of a material that has undergone pre-straining and baking treatment using equations when plastic deformation is present. Solution to the problem

[0006] To achieve the above objectives, this disclosure provides a method for expressing intensity variation characteristics as follows.

[0007] According to one aspect of this disclosure, a method is provided for expressing strength variation characteristics in a first material and a second material, the second material being obtained by subjecting the first material to pre-straining and heat treatment. The method includes: a region setting step that sets a base region indicating the relationship between strain and stress in the first material, and a variation region indicating stress variation segments based on strain caused by factors resulting from the pre-straining and heat treatment of the second material; and a simulation step that expresses the relationship between stress variation in the variation region and pre-strain as a Boltzmann function, and derives the relationship between strain and stress in the second material as the sum of stresses based on strain in the base region and the variation region. Beneficial effects of the invention

[0008] According to one aspect of this disclosure, the structure described above can express the strength changes in the material caused by pre-straining and baking treatment of the substrate relatively accurately.

[0009] Furthermore, by enabling collision analysis that takes into account changes in material strength (such as during automotive painting processes), the design accuracy of finished products such as automobiles can be improved. Attached Figure Description

[0010] Figure 1 True stress-true strain curves are shown for materials with pre-strain and baking treatment (heat treatment).

[0011] Figure 2 A flowchart illustrating a method for expressing intensity variation characteristics according to an example embodiment of the present disclosure is shown.

[0012] Figure 3 The relationship between true strain and true stress, expressed by an equation, is shown in an example embodiment of this disclosure.

[0013] Figure 4 The increase in material strength based on the prestrain is shown when heat treatment is performed while changing the prestrain at an effective plastic strain of 0.2.

[0014] Figure 5 A graph illustrating the Boltzmann function is shown.

[0015] Figure 6 The relationship between elongation and stress of a material with an upper yield point and a lower yield point is shown.

[0016] Figure 7 A graph is shown comparing test results obtained under varying pre-strain and heat treatment conditions of a material with equations expressed according to exemplary embodiments of this disclosure.

[0017] Figure 8 It shows that it will be different from Figure 7 A graph comparing test results obtained under varying pre-strain and heat treatment conditions of a certain material with equations expressed according to exemplary embodiments of this disclosure.

[0018] Figure 9 It shows that it will be different from Figure 7 and Figure 8 A graph comparing test results obtained under varying pre-strain and heat treatment conditions of a certain material with equations expressed according to exemplary embodiments of this disclosure.

[0019] Figure 10 The shape of the cap-shaped sample is shown schematically.

[0020] Figure 11 This shows the use of, for example Figure 10 The chart shown is a graph of the load curves when the cap-shaped specimen is subjected to a crush test, which compares three cases: the case of the first material, the case considering only work hardening, and the case considering the equations derived using the expression method according to the exemplary embodiments of this disclosure.

[0021] Figure 12 This shows the use of, for example Figure 10 The graph shown is a chart of the energy absorption curves of a cap-shaped specimen when subjected to a crush test, which compares three cases: the case of the first material, the case considering only work hardening, and the case considering the equations derived using the expression method according to the exemplary embodiments of this disclosure. Best practice

[0022] Specific exemplary embodiments of the present disclosure will now be described with reference to the accompanying drawings. However, it should be noted that the spirit of the present disclosure is not limited to the exemplary embodiments described herein, and that those skilled in the art can readily implement regressive inventions or other exemplary embodiments contained within the spirit of the present disclosure by adding, modifying, and deleting components within the same spirit of the present disclosure; however, these are to be construed as being included within the spirit of the present disclosure. Detailed Implementation

[0023] In addition, when describing the contents of this disclosure, "~unit" or "~module" can be implemented in various ways, such as a processor, program instructions executed by a processor, software module, microcode, computer program product, logic circuit, application-specific integrated circuit, firmware, etc.

[0024] The methods disclosed in the exemplary embodiments of this application can be implemented directly by a hardware processor, or they can be implemented and executed through a combination of hardware and software modules of the processor. The software modules can be located in conventional storage media, such as random access memory, flash memory, read-only memory, programmable read-only memory, electrically erasable programmable memory, or registers. The storage medium is located in memory, and the processor reads the information stored in the memory and, in conjunction with the hardware, completes the above-described methods. To avoid repetition, detailed descriptions are omitted here.

[0025] In the implementation process, each of the above methods can be implemented by a hardware logic integrated circuit or by software instructions. The methods disclosed in the example embodiments of this application can be implemented directly by a hardware processor, or implemented and executed by a combination of hardware and software modules of the processor. The software modules can be located in conventional storage media, such as random access memory, flash memory, read-only memory, programmable read-only memory, electrically erasable programmable memory, or registers. The storage medium is located in memory, and the processor reads the information stored in the memory and, in conjunction with the hardware, implements the above methods.

[0026] That is, those skilled in the art will understand that each exemplary unit and algorithm step described in the exemplary embodiments disclosed in this specification can be implemented by combining electronic hardware or by combining computer software and electronic hardware. Whether such functionality is implemented in hardware or software depends on the specific application and design constraints of the technical method. Those skilled in the art can use different methods to implement the described functionality for each specific application, but such implementations should not be considered beyond the scope of this application.

[0027] In the examples provided in this application, it should be understood that the disclosed apparatus and methods can be implemented in other ways. For example, the exemplary embodiments of the above apparatus are merely illustrative. For example, the division of units is merely a logical functional division, and other division schemes may exist in actual implementations. For example, multiple units or components may be combined or integrated into another system, or some features may be omitted or not performed. On the other hand, the coupling, direct coupling, or communication connection between each other shown or discussed may be an indirect coupling or communication connection through an interface, device, or unit, and may be electrical, mechanical, or other forms.

[0028] As described above, the units described as separate components can be physically separate, and the components represented as units may or may not be physical units; that is, they can be located in one place or distributed across multiple network units. Depending on the actual needs, some or all of the units can be selected to achieve the purpose of the solution in this example embodiment.

[0029] That is, each functional unit in each example embodiment of this application can be integrated into a single processing unit, each unit can exist independently, or two or more units can be integrated into a single unit.

[0030] When the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the portions of the technical solutions in this application that substantially contribute to the prior art, or portions of the above technical solutions, can be implemented in the form of software products. This computer software product is stored in a storage medium and includes instructions that cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in each example embodiment of this application. The aforementioned storage medium includes various media capable of storing program code, such as USB storage devices, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or CD-ROMs.

[0031] Figure 1 It is a true stress-true strain graph, and shows the tensile curves under conditions of strength variation due to raw materials, pre-strain, and baking treatment.

[0032] All results were obtained using specimens of the same material and thickness (980HB steel, 1.6 t thick specimens). The figures are shown below: untreated case (L1); case with 0% prestrain and heat treatment (L2); case with 2% prestrain and heat treatment (L3); case with 5% prestrain and heat treatment (L4); case with 8% prestrain and heat treatment (L5); and case with 10% prestrain and heat treatment (L6).

[0033] Typically, the strength change is minimal when heat treatment is performed without pre-strain; however, when the pre-strain is 2%, yield point elongation occurs, resulting in a significant increase in strength in the initial stage of deformation, after which the strength exhibits a value similar to that of the raw material.

[0034] As the prestrain increases, a very large upper yield stress appears in the transition zone from the elastic to the plastic region, and the subsequent work hardening curve also shows a value exceeding the predetermined level compared to the raw material. Overall, the upper yield stress appears significantly upon transitioning to the plastic region, after which the increase gradually decreases. Prestrain of 2% or less results in a work hardening curve that approximates the work hardening curve of the raw material, while prestrain of 5% or more results in a work hardening curve that is larger than the work hardening curve of the raw material.

[0035] like Figure 1 As shown, even when using the same material, the characteristics of the material, especially the physical properties caused by changes in strength, can change due to pre-straining or heat treatment processes. Therefore, a method to accurately express changes in strength is needed.

[0036] Figure 2 This is a flowchart of a method for expressing intensity change characteristics according to an exemplary embodiment of the present disclosure.

[0037] The method for expressing intensity change characteristics according to the exemplary embodiments of this disclosure includes a region setting step (S200) and a simulation step (S400).

[0038] This disclosure relates to a method for predicting the strength variation characteristics of a second material, wherein the second material is obtained by pre-straining and heat-treating a first material.

[0039] In the region setting step (S200), a basic region D1 is set to indicate the relationship between strain and stress caused by the first material (see...). Figure 3 ), and define the variation region D2 representing the stress variation segment based on strain caused by factors such as pre-strain and heat treatment of the second material (see Figure 3 ).

[0040] The first material involves the strain and stress of the raw materials, and the relationship between strain and stress can be determined using prior experimental data. Therefore, by retrieving or receiving pre-set data, information about the basic region can be obtained without conducting experiments.

[0041] For example, in the region setting step (S200), the correction region D3 can also be set based on the increase in initial strength and decrease in stress caused by the yield point elongation of the second material (see...). Figure 3 The correction region D3 can also be simulated and expressed by equations in the subsequent correction step (S600).

[0042] The simulation step (S400) is as follows: the relationship between stress change and pre-strain in the variation region D2 is expressed as a Boltzmann function, and the relationship between strain and stress of the second material is derived as the sum of strain-based stresses in the base region D1 and the variation region D2.

[0043] The Boltzmann function relates to a probability distribution in statistical mechanics or mathematics that provides the probability of a system being in a particular state as a function of the energy and temperature of that state. Such an equation is used in this disclosure to represent intensity variations with respect to pre-strain and heat treatment.

[0044] For example, a first charting step (S300) may be included before the simulation step (S400).

[0045] The first chart step (S300) represents the stress change in the variation region D2 caused by pre-strain and heat treatment as a first chart, and an equation for the variation region D2 can be derived from the first chart.

[0046] In the first graphing step (S300), a graph showing the relationship between the stress change of the second material and the pre-strain is provided, the graph having a first slope, a second slope and a third slope, wherein the absolute value of the second slope is greater than the absolute values ​​of the first slope and the third slope.

[0047] In the simulation step (S400), the variation region D2 can be derived as an equation based on the third prestrain variable and the second slope, the first stress value and the second stress value, where the third prestrain variable is the intermediate value between the first prestrain variable at the beginning of the second slope and the second prestrain variable at the end of the second slope, the first stress value is the stress value for the first prestrain variable, and the second stress value is the stress value for the second prestrain variable. Subsequently, the relationship between strain and stress change in the base region D1 is combined with the relationship between prestrain rate and stress change in the variation region D2. When combining these two regions, the prestrain rate in the variation region D2 can be linearly combined by treating the prestrain rate together with the strain.

[0048] According to an example implementation of this disclosure, a correction step (S600) may also be included after the simulation step (S400).

[0049] In the correction step (S600), by expressing the initial strength increase based on the yield point elongation of the second material and the subsequent stress reduction as the maximum increase in stress and an exponential function, an equation for the correction region D3 is derived, and the relationship between strain and stress of the second material can be derived as the sum of the basic region D1 and the change region D2 in the simulation step S400 and the sum of the equations for the correction region D3.

[0050] The relationship between strain and stress of the second material in the simulation step (S400) can be expressed as the sum of the base region D1 and the variation region D2, as shown in Equation 1 below. Subsequently, in the correction step (S600), the relationship between strain and stress of the second material can be derived by summing the equations for the correction region D3.

[0051] Equation 1:

[0052] In equation 1 above, The relationship between strain and stress in a second material is expressed based on the stress value derived from the strain. This represents the relationship between strain and stress in the base region D1, i.e., the stress value based on the strain of the first material. The stress value represents the prestrain value that varies according to the prestrain and heat treatment of the second material in the variation region D2, and This represents the stress value, based on the strain in the correction region D3, relative to the initial strength increase due to the elongation of the yield point of the second material and the subsequent stress reduction.

[0053] Therefore, by deriving each equation into a linear equation as shown in Equation 1 above, the relationship between strain and stress of the second material can be easily simulated and obtained using the above equations based on experimental or pre-stored data.

[0054] According to an example implementation of this disclosure, a second charting step (S500) may be included before the above-described correction step (S600).

[0055] The second graphing step (S500) can be as follows: deriving a graph with a fourth slope, an upper yield point and a first elongation as the elongation rate at the upper yield point, a lower yield point and a second elongation as the elongation rate at the lower yield point, based on the stress variation relationship of the yield point elongation of the second material. Since the second graphing step only needs to be performed before the correction step, it can be performed at any position after the region setting step (S200) and before the correction step (S600).

[0056] The graph expressed in the second graphing step (S500) is the process of deriving a graph that can simulate the stress reduction after the upper yield point using an exponential function.

[0057] The method for expressing intensity change characteristics according to the example embodiments of this disclosure 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 to receive the pre-input relationship between stress and strain of a first material as the value of the base region, wherein data obtained through experiments can be received, or values ​​previously stored in a server, etc.

[0060] The second input step can receive a graph of the true strain-true stress relationship of the second material obtained experimentally, or the graph can be retrieved if it is stored on a server or similar entity. Alternatively, on a server or similar entity, data for the second material required under the corresponding conditions can be linearly derived from the first material and multiple data obtained by pre-straining and heat-treating the first material, and this derivation can be presented as a graph.

[0061] According to an example embodiment of this disclosure, based on the information input in the input step (S100), the strength variation characteristics of the elastic material for the second material can be expressed as a specific equation through the region setting step (S200), the simulation step (S400), etc.

[0062] According to an example embodiment of this disclosure, an expression step (S700) may be included, which derives a graph of the sum derived after the simulation step (S400) or the correction step (S600). By including the expression step (S700), the accuracy of the equation can be visually displayed to users, etc.

[0063] The following description will describe the specific details of the simulation step (S400) and the calibration step (S600).

[0064] Figure 3 A graph showing true strain and true stress expressed by equations according to an example embodiment of this disclosure is shown.

[0065] The first line, represented by a thick solid line, is a graph of the true strain and true stress of the first material. Since there was no pre-straining or heat treatment process, it represents the stress at the yield point without significant strength changes. The subsequent diagram of the plastic zone .

[0066] The second line, represented by the dashed line, is the prestressed value. And the value of the yield stress The situation.

[0067] The third line, represented by a solid line, represents the prestrain value. The situation.

[0068] The fourth line, represented by the double-dotted line, represents the prestressed strain. And the value of the yield stress is The situation.

[0069] Based on the representation method of this disclosure, according to the third line description, the segment separated by the first line becomes the base region D1, and the largest segment that can be covered by linearly deforming the first line, excluding the base region D1, and having the trend of the first line, can be set as the variation region D2. Furthermore, the remaining region can be set as the correction region D3, in which the stress reduction after the upper yield point and upper yield stress can be expressed as an exponential function.

[0070] When the stress at the upper yield point is arbitrarily expressed as At that time, the response should be... place, have , and The value of . Subsequently, in a portion of the segment forming the correction region D3, the stress at the strain point can be expressed as , and The sum. Furthermore, in the section where the correction region D3 is not formed, the stress can be expressed as... and sum.

[0071] Figure 4 This is a graph showing the change in strength when heat treatment is performed while changing the pre-strain of the same material. Measurements were taken for 980HB steel under the same conditions with an effective plastic strain of 0.2. Figure 5 It is a graph illustrating the Boltzmann function.

[0072] Reference Figure 4 and Figure 5 Equation 1 describes the simulation steps (S400) of an example implementation of this disclosure.

[0073] from Figure 4 As can be seen from the graphs, when pre-strain is present, the increase in strength exhibits a rapid change at approximately 2% to 5% of the pre-strain. Therefore, a method is needed to accurately represent changes in physical properties, particularly within this range.

[0074] According to the exemplary implementation of this disclosure, such as Figure 4 The strength changes shown can be represented by, under conditions of rapid changes due to pre-strain and heat treatment processes, as described above. Figure 5 The Boltzmann function shown is used for simulation.

[0075] Figure 5This is a graph showing the Boltzmann function, and the existence of transition segments (i.e., segments where the y-axis value changes rapidly according to the x-axis) can be represented as an equation. The Boltzmann function is expressed by the following Equation 2.

[0076] Equation 2:

[0077] Here, y1 represents the y value before the transition segment, y2 represents the y value after the transition segment, x0 represents the x-axis value at the midpoint between y1 and y2 (i.e., the midpoint of the transition segment), and k represents the slope of the transition segment.

[0078] When applied to, for example Figure 4 When the strength changes caused by pre-strain and heat treatment are present in the form of a graph, it can be derived as shown in Equation 1 below.

[0079] Formula 1:

[0080] (Here, The strain and stress in the variation region are represented as the increase in strength caused by pre-straining and heat treatment. Indicates the first stress value; Indicates the second stress value; Indicates the third prescribing variable; Variables that represent the predictive variable; (This represents the maximum value of the pre-existing variable; and K1 represents the value of the second slope.)

[0081] According to the exemplary implementation of this disclosure, This represents the first stress value, and The second stress value can also be expressed as a linear equation based on the prestress. This can be represented by the following equations 3 and 4.

[0082] Equation 3:

[0083] Equation 4:

[0084] In equations 3 and 4 Indicates the first stress value. This represents the second stress value. This indicates the variable representing the predicate variable. represents the maximum value of the pre-dependent variable, and a, b, c, and d represent coefficients.

[0085] When Equations 3 and 4 are substituted into Equation 1 above, Equation 1 is derived as a function of the stress value of the prestressed variable.

[0086] In the simulation step (S400), the relationship between the prestrain and stress in the variable region can be derived as described above. Furthermore, by associating the prestrain of the variable region with an elongation, the equation derived in the simulation step (S400) or the graph expressed by the equation can be expressed as a linear sum of the equations or graphs of the base region, as an equation related to the elongation and stress of the second material.

[0087] Figure 6 The relationship between elongation and stress is shown to describe the upper and lower yield points.

[0088] Depending on the material, there may be cases where both an upper and lower yield point exist. Typically, for hard and tough materials, an upper yield point will form, followed by a lower yield point due to a decrease in strength, and the material may exhibit a fracture mode.

[0089] When the first material is transformed into the second material through pre-straining and heat treatment, the second material is harder and tougher than the first material, and therefore has an upper yield point, and may form a stress-reducing section after the upper yield stress.

[0090] According to an exemplary embodiment of this disclosure, when such an upper yield point is obtained due to pre-straining or heat treatment, the region can be represented as Figure 3 The correction region D3 in the text.

[0091] For example, for the correction region D3, an equation can be derived in which the initial strength increase and stress decrease based on the yield point elongation of the second material are expressed as the increased stress at the upper yield point and the stress decrease represented by an exponential function.

[0092] Alternatively, the equation can be derived as relating to the third elongation, the fourth slope, and the upper yield point, where the third elongation is the intermediate value between the first and second elongations. For specific expression, it can be represented as Equation 2 below.

[0093] Formula 2:

[0094] Here, This represents the strain and stress in the correction zone caused by the increase in initial strength due to yield point elongation. This indicates the increased stress value at the upper yield point. Indicates the third elongation. This represents the variable that acts as the prestrain variable when prestrain is present. t1 represents the maximum value of the pre-dependent variable, k2 represents the fourth slope, and t1 represents the coefficient.

[0095] For example, It can be expressed according to Equation 3 below.

[0096] Formula 3:

[0097] Here, K represents the increased stress value at the upper yield point. BH , and m represent coefficients, This represents the variable that acts as the prestrain variable when prestrain is present, and This represents the maximum value of the pre-existing variable.

[0098] When we substitute equation 3 into equation 2 above... In the middle, Equation 2 is transformed into an equation about the stress in the correction region D3 based on the increase in initial strength caused by yield point elongation, where the prestress is the variable.

[0099] By adding the equation derived for the correction region D3 to the sum of the base region D1 and the variation region D2 obtained in the simulation step (S400) above, the true stress value for the prestress can be obtained. In this case, the prestress can be the true strain.

[0100] In the above method, the strength variation characteristics in the plastic deformation region of the elastic material are derived into a specific equation to express the material strength variation according to the pre-strain of the raw material and the baking treatment relatively accurately. For example, in the automotive painting process, by applying this equation, the material strength variation can be taken into account to perform collision analysis, thereby providing the effect of improving the design accuracy of finished products such as automobiles.

[0101] Additionally, according to exemplary embodiments of this disclosure, a recording medium containing a computer-readable program for performing the above-described methods can be provided.

[0102] Figures 7 to 9 The model and experimental values ​​of the physical properties of the second material under various conditions are shown when each first material is different.

[0103] Figures 7 to 9 The graphs are shown based on experiments of plastic deformation, in which the first material obtained by different treatments of 980 MPa steel is subjected to pre-strain and heat treatment under various conditions. The graphs obtained from the experiments are shown as lines, and the graphs drawn based on the equations derived by this method are shown in both line and shape according to line type.

[0104] exist Figure 7 and Figure 8In this diagram, L1 represents the case where the pre-strain is 10%, L2 represents the case where the pre-strain is 8%, L3 represents the case where the pre-strain is 5%, L4 represents the case where the pre-strain is 2%, and L5 represents the case where the pre-strain is 0%. Heat treatment is performed on L1 to L5. L6 is a graph of the first material, which is entered in the input step (S100).

[0105] exist Figure 9 In the diagram, L1 represents the case with a pre-strain of 10%, L2 represents the case with a pre-strain of 8%, L3 represents the case with a pre-strain of 5%, and L4 represents the case with a pre-strain of 2%. Heat treatment was performed on L1 to L4. L5 is a graph of the first material, which is input in the input step (S100).

[0106] from Figures 7 to 9 It can be seen that even with various conditions and formats, almost similar results can be obtained when plotting with experimental values ​​and values ​​obtained by means of the methods described in this disclosure.

[0107] According to an exemplary embodiment of this disclosure, an expression step (S700) is also included to express the summation equation derived after the simulation step (S400) or the correction step (S600) as follows: Figures 7 to 9 The diagrams shown enhance intuitive understanding and facilitate user verification of the accuracy of the equations derived from this disclosure.

[0108] Figure 10 A cap-shaped sample is schematically shown, and it can be obtained by examining items such as... Figure 10 The specimen shown was crushed to analyze the load and absorbed energy.

[0109] Figure 11 Is it used as Figure 10 The load curve of the cap-shaped specimen during the crush test is shown, and Figure 12 Is it used as Figure 10 The energy curve of the cap-shaped specimen during the crush test is shown.

[0110] According to an example embodiment of this disclosure, the analysis results of the collision load and absorbed energy in a vehicle considering both pre-strain and heat treatment versus considering only work hardening (heat treatment) are shown.

[0111] exist Figure 11 In the diagram, the dashed lines are graphs analyzed using the first material, the double-dash lines are graphs analyzed considering pre-strain and heat treatment according to exemplary embodiments of the present disclosure, and the solid lines are load curves considering only work hardening as in conventional methods.

[0112] exist Figure 12In the diagram, the dashed line is a graph analyzing the pre-strain and heat treatment according to the exemplary embodiments of this disclosure, the solid line is a curve of the energy absorbed during work hardening only, as in conventional methods, and the double-dotted line is a curve of the energy absorbed as analyzed as a first material.

[0113] Unlike conventional methods, when the analysis is performed using the equations described above according to the example embodiments of this disclosure, a larger load and a higher energy absorption rate can be observed, and the collision of a vehicle can be analyzed more accurately.

[0114] Although exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations may be made without departing from the scope of this disclosure as defined by the appended claims.

Claims

1. A method for expressing the strength variation characteristics of a first material and a second material, wherein the second material is obtained by subjecting the first material to pre-strain and heat treatment, the method comprising: The region setting step sets a base region indicating the relationship between strain and stress in the first material, and a variation region indicating the stress variation segment according to strain caused by factors such as pre-straining and heat treatment of the second material. as well as The simulation step expresses the relationship between stress change and pre-stress in the variation region as a Boltzmann function, and derives the relationship between strain and stress of the second material as the sum of stress in the base region and the variation region based on strain.

2. The expression method according to claim 1, further comprising, before the simulation step: The first charting step involves plotting a chart with a first slope, a second slope, and a third slope for the second material based on the stress change according to the pre-strain. The absolute value of the second slope is greater than the first slope and the third slope; In the simulation step, the variation region is derived as an equation based on a third prestressed variable and the second slope, a first stress value, and a second stress value. The third prestressed variable is the intermediate value between the first prestressed variable at the beginning of the second slope and the second prestressed variable at the end of the second slope. The first stress value is the stress value for the first prestressed variable, and the second stress value is the stress value for the second prestressed variable.

3. The expression method according to claim 2, wherein, In the region setting step, a correction region is also set based on the increase in initial strength and decrease in stress caused by the yield point elongation of the second material, and The expression method further includes a correction step after the simulation step, the correction step being: The equation for the correction region is derived by expressing the initial strength increase and stress decrease caused by the elongation of the yield point of the second material as a maximum increase in stress and an exponential function. as well as The relationship between strain and stress of the second material is derived as the sum of the base region, the variation region, and the correction region in the simulation step.

4. The expression method according to claim 3, further comprising, before the correction step: The second graphing step involves deriving a graph based on the stress variation relationship of the yield point elongation of the second material, which has a fourth slope, an upper yield point and a first elongation as the elongation rate at the upper yield point, a lower yield point and a second elongation as the elongation rate at the lower yield point. In the correction step, the correction region is derived by an equation related to the third elongation, the fourth slope, and the upper yield point, wherein the third elongation is the intermediate value between the first elongation and the second elongation.

5. The expression method according to claim 2, wherein, In the simulation step, the region of change is expressed by Equation 1 below. Formula 1: in, The strain and stress in the changed region are represented as the increase in strength caused by the pre-strain and the heat treatment; This represents the first stress value; This represents the second stress value; This refers to the third prestressed variable; The variable representing the predicted response variable; K1 represents the maximum value of the prestressed variable, and K1 represents the value of the second slope.

6. The expression method according to claim 4, wherein, In the correction step, the correction region is expressed by the following Equation 2. Formula 2: in, This represents the strain and stress in the correction region resulting from the increase in initial strength caused by the elongation at the yield point. This indicates the increased stress value at the upper yield point. This indicates the third elongation. This indicates the variable that serves as the prestress variable when the prestress is present. t1 represents the maximum value of the pre-dependent variable, k2 represents the fourth slope, and t1 represents the unknown.

7. The expression method according to claim 6, wherein, In the correction step, in Equation 2 This can be expressed according to Equation 3 below. Formula 3: in, K represents the increased stress value at the upper yield point. BH and Represents the coefficient. This indicates the variable that serves as the prestress variable when the prestress is present. represents the maximum value of the predicted variable, and m represents an unknown.

8. The expression method according to claim 1, wherein, The predicted strain is 2% to 5%.

9. A computer-readable recording medium having a program recorded thereon for performing the method according to any one of claims 1 to 8.

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