Method for establishing fracture model of metal hot forming under complex stress state and system thereof
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- CHINA NAT PETROLEUM CORP
- Filing Date
- 2022-08-25
- Publication Date
- 2026-05-29
AI Technical Summary
In the hot forming process of complex components, metallic materials are prone to ductile fracture under low/negative stress triaxial conditions, making it difficult to predict fracture behavior and affecting formability and reliability.
By extracting characteristic components of complex structures and determining their stress state changes, a sample and test device that can reproduce complex stress states are constructed to perform fracture simulation. Combined with fracture tests at different temperatures and strain rates, a fracture model is established to predict cracking defects.
It provides a scientific basis for quickly determining a reasonable forming process, saving experimental costs and time, and accurately simulating the fracture failure of metallic materials under complex stress states.
Smart Images
Figure CN117672419B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the technical field of metal plastic forming, specifically to a method and system for establishing a fracture model of metal hot forming under complex stress conditions. Background Technology
[0002] In the hot forming process of complex components, such as multi-cavity components under multi-directional loading (where the punches simultaneously or sequentially load the blank in the axial and radial (lateral) directions), complex stress states always exist and change, and are all in low / negative stress triaxial states, specifically combinations of compression-shear, pure shear, and tension-shear stress states. Simultaneously, due to the poor plasticity of the material, deformation is severely uneven, making ductile fracture prone to occur, which seriously restricts the formability and reliability of the components. The fracture of metallic materials is highly sensitive to stress state, deformation temperature, and strain rate, making the fracture behavior extremely complex and difficult to predict. Therefore, how to construct a testing device to realize complex stress state transitions and establish a corresponding hot forming fracture model has become a crucial issue. Summary of the Invention
[0003] To address the problem of cracking during metal hot forming under complex stress conditions with low / negative stress triaxiality in existing technologies, this invention provides a method and system for establishing a fracture model for metal hot forming under complex stress conditions. Based on the material stress, strain, temperature, and deformation rate of weak areas during the forming process of complex components, cracking defects during metal hot forming under complex stress conditions can be predicted, providing a scientific basis for quickly determining a reasonable forming process and saving experimental costs and time.
[0004] This invention is achieved through the following technical solution:
[0005] A method for establishing a fracture model of metal hot forming under complex stress states includes the following steps:
[0006] The characteristic components of the complex component are extracted during the forming process, and the stress state changes of the characteristic components during the forming process are determined. The values of the stress triaxiality distribution and Lode angle parameter distribution in the weak area of the characteristic component before and after deformation are obtained.
[0007] Based on the values of the stress triaxiality distribution and Lode angle parameter distribution of the characteristic component before and after deformation, a sample and test device that can reproduce complex stress states are constructed. The structural parameters of the sample are obtained by simulating the stress state of the weak area of the corresponding characteristic component before and after deformation through fracture simulation.
[0008] Fracture tests were conducted on the specimens and test equipment of the characteristic components under complex stress conditions at different temperatures and strain rates to obtain the fracture displacement and fracture strain of the specimens under different deformation temperatures, strain rates and stress conditions.
[0009] The critical damage values of different fracture models under different stress states are determined, and the basic fracture model is obtained based on the critical damage values. The influence of fracture displacement and fracture strain of the sample under different deformation temperatures, strain rates, and stress states on the critical damage values is introduced to obtain the fracture model of the hot forming of metallic materials under complex stress states.
[0010] Preferably, the characteristic components of the complex component are extracted during the forming process, and the stress state of the characteristic components is extracted in the weak area of the forming process, wherein the stress state includes tensile-shear stress, pure shear stress and compressive-shear stress.
[0011] Preferably, based on the stress state analysis during the forming process of the characteristic component, combined with the values of stress triaxiality and Lode angle parameters and the test conditions of material fracture performance, a numerical simulation method is used to obtain the sample and test device.
[0012] Furthermore, the specimen is placed in the test device to perform a fracture simulation at room temperature. The stress state and strain change state of the deformation zone during the forming process of the characteristic component are analyzed. The specimen and its size are optimized accordingly until the stress of the specimen is concentrated in the measurement area of the test device, and the values of the stress triaxiality and Lode angle parameters in the measurement area of the test device reach stability. Then, the specimen and its size are obtained for the fracture test.
[0013] Preferably, the fracture test of the specimen at different temperatures and strain rates in the test apparatus is performed by loading the specimen with a universal testing machine until it fractures.
[0014] Furthermore, the process of loading the test sample within the test apparatus includes a pre-loading stage and a main loading stage;
[0015] Preloading stage: The specimen is loaded in the test device until the deformation zone of the specimen is completely in a yield state, stress concentration occurs at both ends of the specimen, the stress triaxiality of the deformation zone of the specimen drops to a negative value and remains within a stable value, the specimen undergoes elastic deformation, and at the end of the preloading stage, the material of the specimen enters the plastic deformation stage.
[0016] Main loading stage: Loading is applied to the middle region of the sample until shear deformation occurs in the deformation zone of the sample, leading to fracture.
[0017] Furthermore, during the preloading stage, the specimen is loaded from top to bottom at a constant strain rate using a universal testing machine within the testing apparatus until the deformation zone is completely in a yielding state.
[0018] Preferably, the critical damage values of different fracture models under different stress states are determined, and the average value, coefficient of variation, and deviation of the critical damage values of different fracture models under different stress states are calculated.
[0019] Furthermore, the average value, coefficient of variation, and deviation of the critical damage value under different stress states are determined. The fracture model with the lowest average value, coefficient of variation, and deviation of the critical damage value under different stress states is used as the basic fracture model. The fracture displacement and fracture strain of the sample under different deformation temperatures, strain rates, and stress states are introduced. Based on the influence on the critical damage value, the fracture model of the metallic material under complex stress state hot forming is obtained.
[0020] A fracture model establishment system for metal hot forming under complex stress states, including
[0021] The first processing module is used to extract the characteristic components of the complex component during the forming process, determine the stress state changes of the characteristic component during the forming process, and obtain the values of the stress triaxiality distribution and Lode angle parameter distribution of the weak area of the characteristic component before and after deformation.
[0022] The second processing module is used to construct a sample and test device that can reproduce complex stress states based on the values of the stress triaxiality distribution and Lode angle parameter distribution of the characteristic component before and after deformation. The structural parameters of the sample are obtained by simulating the stress state of the weak area of the corresponding characteristic component before and after deformation through fracture simulation.
[0023] The third processing module is used to conduct fracture tests under complex stress conditions at different temperatures and strain rates based on the sample of the characteristic component and the test device, and to obtain the fracture displacement and fracture strain of the sample under different deformation temperatures, strain rates and stress conditions.
[0024] The fourth processing module is used to determine the critical damage value of different fracture models under different stress states, and to obtain the basic fracture model based on the critical damage value. It introduces the influence of fracture displacement and fracture strain of the sample under different deformation temperatures, strain rates, and stress states on the critical damage value, so as to obtain the fracture model of the metal material under complex stress state thermoforming.
[0025] Compared with the prior art, the present invention has the following beneficial technical effects:
[0026] This invention provides a method for establishing a fracture model of metal hot forming under complex stress conditions. Based on the evolution of stress state in weak areas during the forming process of characteristic components, a sample structure and test device that can reproduce this stress state are constructed. Based on the material temperature and deformation rate during the forming process of complex components, simulated fracture tests are conducted. By combining the parameters obtained from the simulated fracture tests with the basic fracture model determined from the critical damage values of different fracture models under different stress states, a fracture model of metal materials under complex stress conditions can be obtained. The fracture model of metal hot forming under complex stress conditions can predict cracking defects during the metal hot forming process under complex stress conditions, providing a scientific basis for quickly determining a reasonable forming process and saving test costs and time.
[0027] Furthermore, by constructing a specimen structure and test device that can reproduce complex stress states, the design of specimen structural parameters through fracture simulation can correspond to the stress state of characteristic components before and after deformation. By simulating the influence of different stress states on fracture failure, the fracture failure of metallic materials under complex stress states can be accurately simulated. Attached Figure Description
[0028] Figure 1 This is a flowchart of the method for establishing a fracture model of metal hot forming under complex stress conditions in this invention;
[0029] Figure 2 A system structure diagram is established for the fracture model of metal hot forming under complex stress conditions in this invention.
[0030] Figure 3 This is a schematic diagram of multi-cavity component multi-directional loading forming in this invention.
[0031] Figure 4 This is a schematic diagram showing the structure and dimensions of the complex multi-cavity component in this invention;
[0032] Figure 5 This is a schematic diagram of the intersection area between the main tube cavity and the branch tube cavity in this invention;
[0033] Figure 6 This is a schematic diagram of the force analysis of the intersection zone of multi-cavity components in this invention;
[0034] Figure 7 This is a schematic diagram of the sample structure and dimensions in this invention;
[0035] Figure 8 This is a physical diagram of the experimental apparatus in this invention;
[0036] Figure 9 These are the load-displacement curves under different loading conditions in this invention;
[0037] Figure 10This illustrates the relationship between the critical fracture value and the Z-parameter in this invention.
[0038] In the figure: 1-Upper die; 2-Lower die; 3-Blank; 4-Main tube punch; 5-Branch tube punch; 6-Main tube cavity; 7-Branch tube cavity; 8-Test device; 9-Sample; 10-Preload upper die; 11-Preload lower pad; 12-Main load upper pad; 13-Main load lower pad. Detailed Implementation
[0039] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.
[0040] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.
[0041] The present invention will now be described in further detail with reference to the accompanying drawings:
[0042] This invention provides a method and system for establishing a fracture model of metal hot forming under complex stress conditions. Based on the material stress, strain, temperature and deformation rate of the weak area during the forming process of complex components, cracking defects during metal hot forming under complex stress conditions can be predicted, providing a scientific basis for quickly determining a reasonable forming process and saving experimental costs and time.
[0043] Specifically, according to Figure 1 As shown, the method for establishing a fracture model of metal hot forming under complex stress state includes the following steps:
[0044] S1, extract the characteristic components of the complex component during the forming process, determine the stress state changes of the characteristic components during the forming process, and obtain the values of the stress triaxiality distribution and Lode angle parameter distribution of the weak area of the characteristic component before and after deformation;
[0045] Specifically, the characteristic components of complex components are extracted during the forming process, and the stress states are extracted from the stress concentration areas and weak areas of the characteristic components during the forming process. The stress states include tensile-shear stress, pure shear stress, and compressive-shear stress.
[0046] S2. Based on the values of the stress triaxiality distribution and Lode angle parameter distribution of the characteristic component before and after deformation, construct a sample 9 and test device 8 that can reproduce this stress state, and obtain the structural parameters of the sample 9 by simulating the stress state of the weak area of the corresponding characteristic component before and after deformation through fracture simulation.
[0047] Specifically, the sample 9 is placed in the test device 8 to perform a fracture simulation at room temperature. The stress state and strain change state of the deformation zone during the forming process of the characteristic component are analyzed. The sample 9 and its size are optimized accordingly until the stress of the sample 9 is concentrated in the measurement area of the test device 8, and the values of the stress triaxiality and Lode angle parameters in the measurement area of the test device 8 reach stability. Then the sample 9 to be fractured and its size are obtained.
[0048] S3, based on the specimen 9 of the characteristic component and the test device 8, conduct fracture tests under complex stress conditions at different temperatures and strain rates to obtain the fracture displacement and fracture strain of the specimen 9 under different deformation temperatures, strain rates and stress conditions.
[0049] Specifically, the fracture test of sample 9 at different temperatures and strain rates in the test apparatus 8 was carried out by loading sample 9 with a universal testing machine until it fractured.
[0050] The process of loading the sample 9 into the test device 8 includes a preloading stage and a main loading stage;
[0051] Preloading stage: The specimen 9 is loaded in the test device 8 until the deformation zone of the specimen 9 is completely in the yield state, stress concentration occurs at both ends of the specimen 9, the stress triaxiality of the deformation zone of the specimen 9 drops to a negative value and remains within a stable value, the specimen 9 undergoes elastic deformation, and at the end of the preloading stage, the material of the specimen 9 enters the plastic deformation stage.
[0052] Main loading stage: Loading is applied to the middle region of sample 9 until shear deformation occurs in the deformation zone of sample 9, leading to fracture.
[0053] During the preloading stage, the specimen 9 is loaded from top to bottom at a constant strain rate by a universal testing machine in the testing device 8 until the deformation zone is completely in a yielding state.
[0054] Load-displacement curves of specimen 9 were obtained under different deformation temperatures, strain rates, and stress states. Three tests were conducted for each set of forming parameters to verify repeatability. The load-displacement curve of the median fracture displacement was used to obtain the corresponding fracture displacement. Numerical simulation was used to obtain the fracture strain under the same deformation conditions.
[0055] In this invention, the upper and lower top surfaces of the sample 9 are respectively provided with two pairs of grooves in the middle part. The two pairs of grooves on the upper and lower top surfaces are arranged opposite each other. The grooves are inclined and the inclination angle is set according to different stress states. The two sides of the sample 9 are inclined and the sample 9 is fitted with the inner wall of the test device 8.
[0056] The test device 8 designed in this invention has a U-shaped outer ring concave mold structure with blocks at both ends. In the preloading stage, the preloading lower pad 11, the sample 9, and the preloading upper mold 19 are placed sequentially from bottom to top in the groove of the test device 8. The top two ends of the preloading upper mold 19 are placed under the blocks at the top of both ends of the test device 8. In the main loading stage, the main loading lower pad 13, the sample 9, and two main loading upper pads 12 are placed sequentially from bottom to top in the groove of the test device 8. The two main loading upper pads 12 are respectively located at the top two ends of the sample 9, and the tops of the two main loading upper pads 12 are placed under the blocks at the top of both ends of the test device 8.
[0057] S4. Determine the critical damage value of different fracture models under different stress states, and obtain the basic fracture model based on the critical damage value. Introduce the influence of fracture displacement and fracture strain of sample 9 under different deformation temperatures, strain rates, and stress states on the critical damage value, so as to obtain the fracture model of the metal material under complex stress state thermoforming.
[0058] Specifically, the critical damage values of different fracture models under different stress states are determined, and the average value, coefficient of variation, and deviation of the critical damage values of different fracture models under different stress states are calculated. The fracture model with the lowest average value, coefficient of variation, and deviation of the critical damage value is determined as the basic fracture model, and the influence of fracture displacement and fracture strain of sample 9 under different deformation temperatures, strain rates, and stress states on the critical damage value is introduced.
[0059] The formulas for calculating the average value, coefficient of variation, and deviation of the critical damage value for different fracture models under different stress states are as follows:
[0060] Average value of critical damage:
[0061] ;
[0062] Coefficient of variation of critical damage value:
[0063] ;
[0064] Deviation of critical damage value:
[0065] ;
[0066] in, This represents the average value of the critical damage values; denoted as the critical damage value for different fracture models, where i = 1, 2, 3...; MD is the coefficient of variation of the critical damage value; The deviation from the critical damage value; C max C represents the maximum critical damage value. min is the minimum critical damage value; n is the number of fracture models.
[0067] according to Figure 2 As shown, the present invention also provides a fracture model establishment system for metal hot forming under complex stress conditions, including a first processing module, a second processing module, a third processing module and a fourth processing module;
[0068] The first processing module is used to extract the characteristic components of the complex component during the forming process, determine the stress state changes of the characteristic component during the forming process, and obtain the values of the stress triaxiality distribution and Lode angle parameter distribution of the weak area of the characteristic component before and after deformation.
[0069] The second processing module is used to construct a specimen sample 9 and a test device 8 that can reproduce complex stress states based on the values of the stress triaxiality distribution and Lode angle parameter distribution of the characteristic component before and after deformation, and to obtain the structural parameters of the specimen sample 9 by simulating the stress state of the weak area of the corresponding characteristic component before and after deformation through fracture simulation.
[0070] The third processing module is used to conduct fracture tests under complex stress conditions at different temperatures and strain rates based on the sample 9 of the characteristic component and the test device 8, and to obtain the fracture displacement and fracture strain of the sample 9 under different deformation temperatures, strain rates and stress conditions.
[0071] The fourth processing module is used to determine the critical damage value of different fracture models under different stress states, and to obtain the basic fracture model based on the critical damage value. The influence of fracture displacement and fracture strain of sample 9 under different deformation temperatures, strain rates, and stress states on the critical damage value is introduced to obtain the fracture model of the metal material under complex stress state thermoforming.
[0072] Example
[0073] This embodiment employs a method for establishing a metal hot forming fracture model under complex stress conditions with low / negative stress and triaxiality. The specific steps are as follows:
[0074] Step 1: Analyze the service stress state of the target deformable structural component.
[0075] This includes: extracting characteristic components of complex components, analyzing stress concentration areas and weak areas during the forming process of the characteristic components, clarifying the stress state changes during the forming process of the characteristic components, and obtaining the stress triaxiality distribution before and after deformation.
[0076] In this embodiment, the feature component to be formed is a complex multi-cavity component, specifically two main cavities and two branch cavities (four-way body). The multi-directional loading technology used is a plastic forming technology that actively loads and forms the component from multiple directions simultaneously. Figure 3 This is a schematic diagram of multi-cavity component multi-directional loading forming. First, the upper die 1 and the lower die 2 are closed to form a closed mold cavity. Then, by controlling the movement of the main punch 4 and the branch punch 5, they can be made to move together or separately to actively load the heated blank 3 from different directions in multiple directions, forming a multi-cavity component with cavities or branches in different directions in one go.
[0077] Schematic diagram of the structure and dimensions of complex multi-cavity components, as shown below Figure 4 As shown, the inner diameter of the main pipe =42mm, outer diameter of main pipe =60mm, branch pipe inner diameter =20mm, outer diameter of branch pipe =40mm, main tube cavity depth =55mm, branch tube cavity depth =35mm, overall length of component =180mm, horizontal distance between the centers of the two branch pipes =60mm, distance between the top surface of the branch pipe and the center line of the main pipe =60mm, the included angle between the two branch pipes = Forging forming with reserved thickness =5mm, the original blank for forming this component is a cylindrical blank with a diameter of 60mm. The length of the original blank is calculated from the final volume of the formed component based on the principle of constant volume. 148mm.
[0078] During the multi-directional loading forming process, the tensile stress concentration area and weak area of the four-piece structure are the intersection area of the main tube cavity 6 and the branch tube cavity 7, such as Figure 5 As shown in the box, the material in this region mainly experiences three stress states: tension-shear, pure shear, and compression-shear, as follows. Figure 6 As shown, where Figure 6 In the diagram, (a) represents tensile-shear stress; (b) represents pure shear stress; and (c) represents compressive-shear stress. Finite element numerical simulation was used to obtain the characteristic points P1~P4 during the multi-directional loading forming process of the four-piece structure. Figure 5 The stress triaxiality range is -1.0 to 0.3. All three stress states are low stress triaxiality, and a reliable experimental device needs to be designed to study the ductile fracture behavior of the material under different stress states.
[0079] Step 2: Design of test specimens and apparatus for testing the fracture properties of materials under complex stress conditions
[0080] Considering the stress triaxiality range of -1.0 to 0.3 at the intersection point of the main tube cavity and branch tube cavity during multi-directional loading forming process, and taking into account the simplicity and applicability of the experiment, this invention constructs and designs specimen sample 9, which is a trapezoidal groove shape (TGS). The main dimensions of the cross-section are as follows: Figure 7 As shown. After simulating a set of specific dimensional parameters and establishing the corresponding finite element model, the deformation process was initially simulated, and the dimensional parameter values were continuously adjusted. When the groove part underwent local uniform deformation and the triaxial stress stabilized and approached zero, the final TGS specimen structure dimensional parameters were determined, namely, the specimen sample 9 has a width of 11 mm, a length L1 = 23 mm, a groove spacing L2 = 12 mm, a height H = 6 mm, a groove bottom spacing C = 2 mm, and a groove width W = 1.5 mm. The trapezoidal angle α was set to 87.5°, firstly to facilitate the placement of the specimen into the test device 8, and secondly to ensure that the specimen sample 9 is subjected to the pre-compression stress of the outer ring during the pre-loading stage. Groove angle β Designed to be 80°, 90°, 100° or 110°, different stress states can be generated in the groove bridge of the specimen by changing the groove angle β, namely tension-shear (80°), pure shear (90°) and compression-shear (100°, 110°) stress states. It is easy to achieve positive, zero and negative triaxial stress states, which provides a good condition for studying ductile fracture under different stress states.
[0081] The test apparatus of this invention consists of a U-shaped outer ring concave mold, a preloaded upper mold, a main loading upper mold, and upper / lower pads. Figure 8 All materials were H13 mold steel. A universal testing machine was used to load the specimens until fracture; the loading process consisted of two steps: pre-loading and main loading.
[0082] a) Preloading stage: Place the sample in a U-shaped die, and then press the preloaded upper die down by Δh at room temperature (20℃), as shown. Figure 8As shown in (a), the lower surface of the specimen is flush with the upper surface of the lower pad, and a constant strain rate is applied. The initial stress state of the bridge deformation zone of the trapezoidal groove component is determined by Δh. Finite element simulation shows that when Δh = 0.5 mm, the deformation zone is completely in a yield state, stress concentration occurs at both ends of the groove, the stress triaxiality of the deformation zone drops to a negative value and remains stable, the stress triaxiality at the edge is -0.4, and the stress triaxiality in the middle part is about -0.6. The specimen undergoes elastic deformation. At the end of the preloading stage, the material in the groove deformation zone begins to enter the plastic deformation stage, which can ensure the initial plastic deformation conditions of the main loading stage and is beneficial to ensuring the stability of subsequent tension-shear, pure shear, and compression-shear composite forming.
[0083] b) Main loading stage: The upper mold for main loading acts only on the central region of the specimen, causing shear deformation in the deformation zone of the grooved bridge until fracture. To ensure the stability of the deformation, upper pads need to be added above both sides of the specimen, such as... Figure 8 As shown in (b).
[0084] Step 3: Fracture tests at different temperatures and strain rates under complex stress conditions
[0085] Using the test apparatus 8 and sample 9 described in step two, with β designed to be 80°, 90°, 100°, or 110°, at five temperatures (250, 300, 350, 400, 450°C) and four strain rates (0.04, 0.1, 0.4, 1 s⁻¹), the test was conducted at five temperatures (250, 300, 350, 400, 450°C) and four strain rates (0.04, 0.1, 0.4, 1 s⁻¹). 1) Fracture tests were conducted. Three tests were performed for each set of forming parameters to verify repeatability. The load-displacement curve of the median fracture displacement was used. Load-displacement curves under different loading conditions are shown below. Figure 9 ,in Figure 9 In Figure (a), the temperature is 300℃ and the strain rate is 0.04s. -1 (b) is at a temperature of 400℃ and a strain rate of 0.04s. -1 (c) is at a temperature of 250℃ and a strain rate of 0.4 s⁻¹. -1 (d) is at a temperature of 300℃ and a strain rate of 0.4 s⁻¹. -1 (e) is at a temperature of 350℃ and a strain rate of 0.4 s⁻¹. -1 (f) is at a temperature of 450℃ and a strain rate of 0.4s. -1 ;
[0086] This embodiment primarily studies the plastic deformation and fracture of materials; therefore, the load-displacement curve has been modified, retaining only the plastic stage to obtain the corresponding fracture displacement. Numerical simulation is used to obtain the fracture strain under the same deformation conditions.
[0087] Step 4: Determine the hot forming fracture model under complex stress conditions
[0088] Based on the cumulative ductility model established by the continuous accumulation of stress and strain in a continuous medium, this embodiment analyzes the applicability of existing fracture models based on six commonly used ductile fracture models. Specifically, it calculates the critical damage value C of each fracture model under different stress states, and calculates the average value, coefficient of variation, and deviation of C. The calculation formulas are as follows:
[0089] Average value of critical damage:
[0090] ;
[0091] Coefficient of variation of critical damage value:
[0092] ;
[0093] Deviation of critical damage value:
[0094] ;
[0095] in, This represents the average value of the critical damage values; denoted as the critical damage value for different fracture models, where i = 1, 2, 3...; MD is the coefficient of variation of the critical damage value; The deviation from the critical damage value; C max C represents the maximum critical damage value. min is the minimum critical damage value; n is the number of fracture models.
[0096] In this embodiment, AA7075 aluminum alloy was used, with a temperature of 300℃ and a strain rate of 0.4s. -1 The applicability of the six ductile fracture criteria to predict material fracture under different deformation paths was evaluated by loading fractures at β angles of 80° / 90° / 100°. Figure 9 The experimental fracture displacements of the 80°, 90°, and 100° specimens in section (d) were 1.56, 2.18, and 2.55 mm, respectively. The average fracture strains extracted from the characteristic points of the groove bridge section of the trapezoidal specimens after numerical simulation were 0.68, 1.01, and 1.3 mm, respectively. The critical damage value C under different stress states was also determined. MD and DF are shown in Table 1.
[0097]
[0098] Table 1 Damage C values under different forming paths
[0099] By comparing the data of six fracture criteria, the MD and DF values of the Brozzo, Oyane, and McClintockt fracture criteria are all lower than those of the other three. Therefore, these three relatively accurate fracture criteria can be selected for further research. The C values of the Rice and Oyane fracture criteria are consistent under both compressive-shear and pure shear (low-stress triaxiality) stress states, and can basically achieve unified prediction between the two.
[0100] In summary, under the loading and forming conditions presented in this paper, the MD and DF values of the critical damage value C calculated using the Oyane fracture criterion are relatively small, the damage evolution law is consistent with the actual situation, and the C value is consistent under both compression-shear and pure shear (low stress triaxiality) stress states. Therefore, the Oyane criterion can be selected as the basic fracture criterion to further realize the fracture prediction of AA7075 under complex stress states during hot forming.
[0101] Furthermore, the effects of temperature and strain rate on the critical damage value C are introduced to realize a fracture model for hot-formed metallic materials under complex stress states. Specifically:
[0102] Continuum damage mechanics describes the mechanical behavior of materials with microscopic defects by introducing a "damage variable." The damage evolution equation for elastoplastic materials is derived from the energy potential function, and its expression is:
[0103] (1)
[0104] in, denoted as , where is the damage evolution rate; Y is the damage strain energy release rate; and S is the damage intensity parameter, which normalizes Y and can be determined in experiments. The equivalent strain rate.
[0105] If Y in the formula is related to the flow stress function Therefore, S should also be a function related to strain rate and deformation temperature:
[0106] (2)
[0107] To simplify the calculation process, remove the negative sign from equation (1) and integrate it to obtain the expression for the damage variable:
[0108]
[0109] In the formula, t is the deformation time.
[0110] Expressing Y in the above equation using a stress function, the damage variable can be expressed as:
[0111]
[0112] In the formula, For fracture strain The corresponding critical damage value, if considered as an inherent material property independent of forming conditions and deformation path, is normalized using equation (4) to equation (3), then the damage model... D Represented as:
[0113]
[0114] According to the definition of this model, when D When the value is 1, the material begins to fracture and fail. To calculate... D To determine the value, the function must first be determined. The form of S is related to the deformation temperature and strain rate, and can be reflected by the Zener-Hollomon parameter. Therefore, the S-function can be expressed as... The damage model can be transformed into the following form at a certain deformation temperature and strain rate:
[0115]
[0116] Thus, by introducing the principles of continuous damage mechanics, commonly used ductile fracture criteria can be linked to plastic deformation process parameters.
[0117] set up ,in C The critical damage value is calculated based on the appropriate basic fracture model. It is a function of the Z-parameters. When C Once the relationship with the Z parameter is determined, the function of H(Z) can be obtained.
[0118] Based on the improved Oyane fracture criterion, the influence of deformation temperature and strain rate on damage values can be incorporated into fracture prediction. Under high-temperature forming conditions, experimental fracture displacements under different deformation parameters (…) Exp. d f Fracture strain calculated by numerical simulation Sim.ε f ), average stress triaxiality ( Sim. η ) and Oyane fracture value ( Sim. C As shown in Table 2, the relationship between the critical fracture value C and the Z factor can be determined using mathematical fitting methods. Figure 10 From (a), we can see that the critical value for fracture is... C It increases with increasing temperature. Regarding fracture strain... ε f ,along with lnZ The increase is generally downward. Figure 10 (b) is similar to the law of conventional plastic deformation. Figure 10 (c) indicates Cand lnZ It is linear under tension-shear stress, but under pure shear and compression-shear stress states, C and lnZ The relationship is no longer a simple linear one; the relationship between the two can be obtained by data fitting using the least squares method.
[0119] In summary, this invention provides a method for establishing a fracture model of metal hot forming under complex stress states. Based on the evolution of stress states during the forming process of complex components, and according to the material stress, strain, temperature, and deformation rate of weak areas in the forming process, a sample and testing device capable of reproducing complex stress states are constructed. The structural parameters of the sample are designed through fracture simulation. Simulated fracture tests are conducted on the sample and testing device. The parameters obtained from the simulated fracture tests are combined with the basic fracture model determined from the critical damage values of different fracture models under different stress states to obtain a fracture model of metal materials under complex stress states during hot forming. This fracture model can predict cracking defects during the hot forming process of metal under complex stress states, providing a scientific basis for quickly determining a reasonable forming process and saving experimental costs and time. In the sample and testing device for fracture simulation of characteristic components, the design of the sample can correspond to the stress states of the characteristic components before and after deformation. By simulating the influence of different stress states on fracture failure, the fracture failure of metal materials under complex stress states can be accurately simulated.
[0120] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.
Claims
1. A method for establishing a fracture model of metal hot forming under complex stress states, characterized in that, Includes the following steps: The characteristic components of the complex component are extracted during the forming process, and the stress state changes of the characteristic components during the forming process are determined. The values of the stress triaxiality distribution and Lode angle parameter distribution in the weak area of the characteristic component before and after deformation are obtained. Based on the values of the stress triaxiality distribution and Lode angle parameter distribution before and after deformation of the characteristic component, a sample (9) and test device (8) that can reproduce complex stress states are constructed. The structural parameters of the sample (9) are obtained by simulating the stress state before and after deformation of the weak area of the corresponding characteristic component through fracture simulation. Fracture tests were conducted on the specimen (9) of the characteristic component and the test device (8) under complex stress conditions at different temperatures and strain rates to obtain the fracture displacement and fracture strain of the specimen (9) under different deformation temperatures, strain rates and stress conditions. Determine the critical damage value of different fracture models under different stress states, and obtain the basic fracture model based on the critical damage value. Introduce the influence of fracture displacement and fracture strain on the critical damage value of the sample (9) under different deformation temperatures, strain rates, and stress states, so as to obtain the fracture model of the metal material under complex stress state thermoforming.
2. The method for establishing a fracture model of metal hot forming under complex stress state according to claim 1, characterized in that, The characteristic components of complex components are extracted during the forming process. The stress state of the characteristic components is extracted from the weak areas during the forming process. The stress state includes tensile-shear stress, pure shear stress and compressive-shear stress.
3. The method for establishing a fracture model of metal hot forming under complex stress state according to claim 1, characterized in that, Based on the stress state analysis during the forming process of the characteristic component, combined with the values of stress triaxiality and Lode angle parameters and the test conditions of material fracture performance, the sample (9) and test device (8) are obtained by numerical simulation method.
4. The method for establishing a fracture model of metal hot forming under complex stress conditions according to claim 3, characterized in that, The sample (9) is placed in the test device (8) for fracture simulation at room temperature. The stress state and strain change state of the deformation zone during the forming process of the characteristic component are analyzed. The size of the sample (9) and the sample (9) are optimized accordingly until the stress of the sample (9) is concentrated in the measurement area of the test device (8) and the values of the stress triaxiality and Lode angle parameters of the measurement area of the test device (8) are stable. Then the size of the sample (9) and the sample (9) to be fractured are obtained.
5. The method for establishing a fracture model of metal hot forming under complex stress state according to claim 1, characterized in that, The fracture test of the specimen (9) at different temperatures and strain rates in the test apparatus (8) was carried out by loading the specimen (9) with a universal testing machine until it fractured.
6. The method for establishing a fracture model of metal hot forming under complex stress state according to claim 5, characterized in that, The process of loading the test sample (9) into the test apparatus (8) includes a preloading stage and a main loading stage; Preloading stage: The specimen (9) is loaded in the test device (8) until the deformation zone of the specimen (9) is completely in the yield state, stress concentration occurs at both ends of the specimen (9), the stress triaxiality of the deformation zone of the specimen (9) drops to a negative value and remains within a stable value, the specimen (9) undergoes elastic deformation, and at the end of the preloading stage, the material of the specimen (9) enters the plastic deformation stage. Main loading stage: Loading is applied to the middle region of the specimen (9) that has entered the plastic deformation stage until shear deformation occurs in the deformation zone of the specimen (9) until fracture.
7. The method for establishing a fracture model of metal hot forming under complex stress state according to claim 6, characterized in that, During the preloading stage, the specimen (9) is loaded from top to bottom at a constant strain rate in the test device (8) by a universal testing machine until the deformation zone is completely in a yielding state.
8. The method for establishing a fracture model of metal hot forming under complex stress state according to claim 1, characterized in that, Determine the critical damage values of different fracture models under different stress states, and calculate the average value, coefficient of variation, and deviation of the critical damage values of different fracture models under different stress states.
9. The method for establishing a fracture model of metal hot forming under complex stress state according to claim 8, characterized in that, The average value, coefficient of variation and deviation of critical damage value under different stress states are determined. The fracture model with the lowest average value, coefficient of variation and deviation of critical damage value under different stress states is used as the basic fracture model. The fracture displacement and fracture strain of sample (9) under different deformation temperature, strain rate and stress state are introduced. Based on the influence on critical damage value, the fracture model of hot forming of metal material under complex stress state is obtained.
10. A fracture model establishment system for metal hot forming under complex stress states, characterized in that, include The first processing module is used to extract the characteristic components of the complex component during the forming process, determine the stress state changes of the characteristic component during the forming process, and obtain the values of the stress triaxiality distribution and Lode angle parameter distribution of the weak area of the characteristic component before and after deformation. The second processing module is used to construct a sample (9) and test device (8) that can reproduce complex stress states based on the values of stress triaxiality distribution and Lode angle parameter distribution before and after deformation of the characteristic component. The structural parameters of the sample (9) are obtained by simulating the stress state before and after deformation of the corresponding weak area of the characteristic component through fracture simulation. The third processing module is used to conduct fracture tests under complex stress conditions at different temperatures and strain rates based on the sample (9) of the characteristic component and the test device (8), and to obtain the fracture displacement and fracture strain of the sample (9) under different deformation temperatures, strain rates and stress conditions. The fourth processing module is used to determine the critical damage value of different fracture models under different stress states, and to obtain the basic fracture model based on the critical damage value. The influence of the fracture displacement and fracture strain of the sample (9) under different deformation temperatures, strain rates and stress states on the critical damage value is introduced to obtain the fracture model of the metal material under complex stress state.