Method and device for testing triaxial mechanical property of sample piece

Abstract

The invention relates to the technical field of mechanical testing, in particular to a triaxial mechanical property testing method and device for a sample piece. The method comprises the following steps: determining a buckling critical load of a sample piece according to characteristic parameters of the sample piece; inputting the buckling critical load into a transverse clamping device so as to apply a transverse clamping force to the sample piece through the transverse clamping device; controlling an axial force applying device to apply an axial force to the sample piece; in the process of applying the axial force, parameters of the transverse clamping device subjected to reverse action of the sample piece are collected in real time; the transverse clamping force is adjusted in real time according to the reverse action parameters; acquiring an image of the surface of the sample piece in real time; and identifying speckle deformation on the surface of the sample piece in the image. According to the invention, the mechanical property of the material under the negative stress triaxial degree can be accurately tested and obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of mechanical testing, and in particular to a method and device for testing the triaxial mechanical properties of a sample. Background Art

[0002] In the context of global energy conservation and emission reduction, automobile lightweight technology has become a focus of major OEMs. The application of lightweight and high-strength materials (such as high-strength steel, aluminum alloy, magnesium alloy, etc.) is an important method to improve the level of automobile lightweighting. Material properties are directly related to component performance and are essential and important data in the process of automobile design, manufacturing and numerical simulation. Mechanical properties tests are required to accurately obtain the mechanical properties parameters of materials.

[0003] The mechanical properties test of metal materials usually adopts the mechanical properties test method of uniaxial positive tension, which can characterize the yield, hardening and fracture behavior of materials under uniaxial tension conditions (positive stress triaxiality). However, the mechanical properties test of metal materials under reverse compression (negative stress triaxiality) is mainly based on the round bar compression test method, and there is a lack of a reliable and accurate mechanical properties test method for thin metal materials under negative stress triaxiality.

[0004] Automobile parts are mostly made of thin plate materials. Collision safety requires that parts have good performance under compressive crushing conditions. Accurately testing and obtaining the mechanical properties of materials under negative stress triaxiality is particularly critical to improving the collision safety performance of parts.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] The purpose of the present invention is to provide a method and device for testing the triaxiality mechanical properties of a sample, which is suitable for thin plate materials and can accurately test and obtain the mechanical properties of the material under negative stress triaxiality.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a method for testing the triaxial mechanical properties of a sample, comprising: Determining the critical buckling load of the sample according to the characteristic parameters of the sample; inputting the critical buckling load into a lateral clamping device to apply a lateral clamping force to the sample through the lateral clamping device; Controlling the axial force applying device to apply axial force to the sample; During the application of the axial force, the reverse action parameters of the sample on the lateral holding device are collected in real time; and the lateral clamping force is adjusted in real time according to the reverse action parameters; During the application of the axial force, an image of the surface of the sample is collected in real time; The speckle deformation on the sample surface in the image is identified.

[0008] In a second aspect, the present invention provides a triaxial mechanical properties testing device for a sample, comprising: A lateral clamping device, used to apply a lateral clamping force to the sample; An axial force applying device, used for applying an axial force to the sample; An image acquisition device, used for acquiring an image of the surface of the sample in real time during the application of the axial force; A controller is used to determine the critical buckling load of the sample according to the characteristic parameters of the sample; input the critical buckling load into the lateral holding device to apply a lateral clamping force to the sample through the lateral holding device; control the axial force applying device to apply an axial force to the sample; during the application of the axial force, collect the reverse action parameters of the lateral holding device affected by the sample in real time; adjust the lateral clamping force in real time according to the reverse action parameters; and identify the surface speckle deformation of the sample in the image.

[0009] Compared with the prior art, the present invention has the following beneficial effects: 1. Ensure the accuracy of the loading force in the test by measuring the critical buckling load of the sample to avoid premature instability of the sample. Ensure the reliability of the test results by calculating the Young's modulus and section moment of inertia based on the material.

[0010] 2. Intelligent clamping and dynamic adjustment: High-precision electric cylinder and controller automatically adjust the clamping force to keep the specimen stable and reduce human error. The dynamic adjustment mechanism automatically adjusts the support force according to the specimen force and deformation to improve test accuracy.

[0011] 3. The present invention simulates complex negative stress triaxiality conditions by applying axial force and lateral clamping force, which is closer to practical application than traditional uniaxial testing and improves data practicality.

[0012] 4. The present invention uses a DIC device and a high-magnification optical magnification device to monitor the deformation of the sample surface in real time, avoiding the error of traditional contact sensors. The image processing algorithm enhances the image to ensure that the strain data is accurate and reliable.

[0013] 5. The fixture of the present invention includes multiple detachable parts, that is, a modular structure, which can automatically adjust the structure of the fixture to adapt to different samples, improve the versatility and flexibility of the system, and cover a wide range of materials and structural forms.

[0014] 6. The present invention uses DIC technology and image processing algorithms to quickly extract and enhance strain data, improve test efficiency and analysis accuracy, and reduce human errors.

[0015] 7. The present invention is suitable for negative stress triaxiality testing of metal plates and also supports unidirectional compression testing to meet diverse application requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 It is a structural schematic diagram of a triaxial mechanical properties testing device for a sample provided by the present invention; Figure 2 This is a structural schematic diagram of another triaxial mechanical properties testing device for a sample provided by the present invention. Figure 3 It is a structural schematic diagram of the clamp provided by the present invention; Figure 4 It is a flow chart of a method for testing the triaxial mechanical properties of a sample provided by the present invention; Figure 5 It is a structural schematic diagram of a controller provided by the present invention; Among them, 1-lateral holding device, 2-axial force applying device, 3-image acquisition device, 4-controller, 5-optical magnification device, 6-clamp, 7-polytetrafluoroethylene film, 8-support end, 9-sample. DETAILED DESCRIPTION

[0018] The following is a description of exemplary embodiments of the present application in conjunction with the accompanying drawings, including various details of the embodiments of the present application to facilitate understanding, which should be considered as merely exemplary. Therefore, it should be recognized by those of ordinary skill in the art that various changes and modifications can be made to the embodiments described herein without departing from the scope and spirit of the present application. Similarly, for the sake of clarity and conciseness, the description of well-known functions and structures is omitted in the following description.

[0019] The present invention is further described in detail below with reference to the embodiments.

[0020] Example 1 Figure 1 This is a schematic diagram of the structure of a triaxial mechanical properties testing device for a sample provided by the present invention, which is suitable for the scenario of negative stress triaxial mechanical properties testing of metal plates. Of course, the present invention can also be applied to negative stress triaxial mechanical properties testing of other plates.

[0021] See also Figure 1The device provided by the present invention comprises a lateral holding device 1, an axial force applying device 2, an image acquisition device 3 and a controller 4. The controller 4 is connected to the lateral holding device 1, the axial force applying device 2 and the image acquisition device 3, and can be connected by wire or wirelessly. Each component is described in detail below.

[0022] The axial force applying device 2 is used to apply axial force to the sample, see Figure 1 Assuming that the sample 9 is a thin sheet of metal sample, the axial direction is perpendicular to the upper and lower bottom surfaces of the sample. One end of the sample can be fixed to the support end 8, and an axial force is applied along the axial direction at the other end of the sample (which can be called the loading end). The magnitude of the axial force is controlled by the controller and can be determined according to the test requirements.

[0023] The lateral holding device 1 is used to apply a lateral clamping force to the sample 9. There are two lateral holding devices 1, one for applying a lateral clamping force to the left side of the sample 9, and the other for applying a lateral clamping force to the right side of the sample 9. Optionally, the two lateral holding devices 1 apply a lateral clamping force to the sample simultaneously. The lateral holding device 1 and the axial force applying device 2 in this embodiment are used to simulate the negative stress triaxiality condition on the sample 9, see Figure 1 The markings of axial 1, axial 2 and axial 3 in the figure. Figure 1 The letter "F" and the arrow next to the letter "F" represent the force and the direction in which the force is applied.

[0024] The function of the lateral holding device 1 is to clamp the sample to prevent the sample 9 from deformation, displacement and other unstable conditions during the application of axial force. At the beginning of the test, an initial lateral clamping force needs to be provided to the lateral holding device 1 to prevent the sample from becoming unstable prematurely; during the application of axial force, the lateral holding device 1 will adjust the clamping force adaptively according to the condition of the sample 9.

[0025] Specifically, the initial lateral clamping force 1 is provided by the controller 4. The controller 4 determines the critical buckling load of the sample 9 according to the characteristic parameters of the sample 9, and inputs the critical buckling load into the lateral holding device 1 so as to apply the lateral clamping force to the sample through the lateral holding device 1. The lateral holding device 1 can be an electric cylinder, which takes the critical buckling load as the target clamping force and drives the electric cylinder to move toward the sample. The force sensor between the sample and the electric cylinder transmits the clamping force detected in real time to the electric cylinder, and the electric cylinder stops moving when it determines that the current clamping force reaches the target clamping force, and maintains the current position. Among them, the critical buckling load refers to the critical value at which the sample 9 changes from the original straight line state to the curved state when the load reaches a certain size when applied to the sample. By applying the critical buckling load to the lateral direction of the sample 9, the sample can be clamped to the maximum extent.

[0026] During the application of axial force, the sample 9 may deform or move, and the initial lateral clamping force can no longer keep the sample 9 balanced. In this case, the sensor built into the electric cylinder device collects the reverse action parameters of the sample on the lateral holding device 1 in real time. In one embodiment, during the application of axial force, the reverse displacement of the lateral holding device 1 caused by the reverse action of the sample 9 is collected in real time through the displacement sensor built into the electric cylinder device, that is, the displacement away from the sample 9. In another embodiment, during the application of axial force, the reverse force of the sample 9 on the lateral holding device 1 is collected in real time through the force sensor built into the electric cylinder device, that is, the force directed from the sample 9 to the lateral holding device 1. These reverse action parameters are collected in real time and transmitted to the controller 4 in real time.

[0027] The controller 4 adjusts the lateral clamping force in real time according to the reverse action parameter. Specifically, a feedback mechanism is used to automatically adjust the magnitude of the lateral clamping force so that the sample 9 maintains lateral balance. The adjusted lateral clamping force is input into the lateral holding device 1, and the lateral holding device 1 applies the adjusted lateral clamping force to the sample.

[0028] Specifically, in one embodiment, the controller 4 receives the reverse displacement caused by the reverse action of the sample 9 on the lateral holding device 1. If the reverse displacement is not 0 (the position of the lateral holding device 1 when applying the critical buckling load to the sample is recorded as 0 displacement), the reverse displacement is used as a deviation, and the PID algorithm is used to calculate the magnitude of the lateral clamping force. The reverse displacement at the next moment is collected, and the deviation and the lateral clamping force are updated. This cycle is repeated for multiple rounds to make the deviation 0 or close to 0. In another embodiment, the controller 4 receives the reverse force of the sample 9 on the lateral holding device 1. If the reverse force is inconsistent with the critical buckling load, the difference between the reverse force and the critical buckling load is used as a deviation, and the PID algorithm is used to calculate the magnitude of the lateral clamping force. The reverse force at the next moment is collected, and the deviation and the lateral clamping force are updated. This cycle is repeated for multiple rounds to make the deviation 0 or close to 0.

[0029] The image acquisition device 3 is used to acquire images of the surface of the sample 9 in real time during the application of the axial force; and to send the real-time acquired images (with timestamps) to the controller 4. Optionally, the image acquisition device 3 can be a digital image correlation (DIC) device, which is an easy-to-use optical device for measuring the surface deformation of an object. The device tracks the changes in the gray value pattern in adjacent small areas called pixel subsets during the deformation process for measurement.

[0030] Optional, see Figure 2, an optical magnifying device 3, such as a high-magnification optical magnifying device, is provided between the sample 9 and the image acquisition device 3, for magnifying the surface of the sample 9 during the application of the axial force. In this way, after the surface of the sample 9 is magnified during the application of the axial force, a digital image correlation device is used to collect an image of the surface of the sample 9 in real time. This embodiment can capture the speckle deformation on the surface of the sample 9 in real time, ensuring accurate monitoring of the tiny deformation of the sample 9 during the loading process; the digital imaging technology (DIC) is used to monitor the strain and deformation of the sample 9 in real time, avoiding the errors that may be caused by traditional contact measurement.

[0031] The controller 4 identifies the surface speckle deformation of the sample in each image. Specifically, the controller 4 receives images in real time and performs enhancement processing on each image, such as adjusting the brightness, contrast, saturation, hue, etc. of the image to increase clarity and reduce noise. The image area that needs to be specifically analyzed is selected, and the strain at the scattered point position is calculated according to the relative position change of the scattered points of the sample in the image, so as to obtain the strain field data of the selected area of ​​the image. The strain here usually adopts equivalent strain; according to the strain field data obtained from the image, the change of strain on both sides of the sample during the reverse loading process is observed. When the strain values ​​of the two sides are opposite, it is considered that the sample has buckled.

[0032] Optionally, the device further comprises a clamp 6 located between the lateral holding device 1 and the sample 9. Since there are two lateral holding devices 1, the clamp 6 also has two clamps. The clamp 6 comprises a plurality of detachable parts, that is, the clamp 6 is modularly designed. The plurality of detachable parts can be used to adapt to samples 9 of different shapes and sizes. Figure 2 and Figure 3 , the fixture 6 is U-shaped and can be disassembled and spliced ​​from the dotted line to accommodate samples of different sizes. Figure 3 The fixture 6 should be close to the surface of the sample, which will inevitably block part of the surface of the sample 9. However, in order to conveniently capture the speckle deformation on the surface of the sample, it is necessary to make part of the surface of the sample 9 exposed, and the exposed part faces the image acquisition device 3. The position and size of the exposed part can be changed by disassembling the detachable parts of the fixture 6, and the direction of the image acquisition device 3 can be moved, so as to observe the surface of the sample in different directions.

[0033] Optional, see Figure 1 There is a polytetrafluoroethylene film 7 between the fixture 6 and the sample 9; there are polytetrafluoroethylene film 7 and lubricating grease between the axial force applying device 2 and the sample 9. The function of the polytetrafluoroethylene film 7 and lubricating grease is to reduce the influence of friction.

[0034] The embodiment of the present invention also provides a method for testing the triaxial mechanical properties of a sample, which is applicable to the triaxial mechanical properties testing device of the sample provided in the above embodiment and is executed by a controller, see Figure 4, specifically including the following steps.

[0035] S110. Determine the critical buckling load of the sample according to the characteristic parameters of the sample.

[0036] First, record the size of the original sample materials, bundle the samples together, spray one side of the sample with matte white paint evenly, and then spray matte black spots evenly on the other side.

[0037] Optionally, the critical buckling stress of the sample is calculated according to the Young's modulus, section moment of inertia, length coefficient and effective length of the sample; the critical buckling load of the sample is calculated according to the critical buckling stress and the cross-sectional area of ​​the sample. For details, refer to the following formula.

[0038] ; ; ; Where E is Young's modulus, I is the section moment of inertia, K is the length factor, and L is the effective length (i.e. Figure 1 The length of the sample from top to bottom); h is the width of the sample (i.e. Figure 1 The width of the sample perpendicular to the paper surface), b is the thickness of the sample (i.e. Figure 1 The sample in this embodiment is negatively loaded, that is, one end is fixed and the other end is free. The length coefficient K is a constant of 2. A is the transverse cross-sectional area of ​​the sample. is the critical buckling stress, is the critical buckling load.

[0039] S120, inputting the critical buckling load into the lateral holding device to apply a lateral clamping force to the sample through the lateral holding device.

[0040] High-precision electric cylinders are installed at both ends of the specimen, and the lateral clamping force of the electric cylinders is automatically adjusted through an intelligent device to ensure stable clamping of the specimen.

[0041] The controller has a dynamic adjustment mechanism that can automatically adjust the lateral clamping force according to the stress and deformation of the sample to ensure that the sample is evenly stressed during the entire test process.

[0042] S130, controlling the axial force applying device to apply axial force to the sample.

[0043] S140. During the application of the axial force, the reverse action parameters of the sample on the lateral holding device are collected in real time; and the lateral clamping force is adjusted in real time according to the reverse action parameters.

[0044] S150. During the application of the axial force, an image of the surface of the sample is collected in real time.

[0045] S160: Identify the surface speckle deformation of the sample in the image.

[0046] Compared with the prior art, the present invention has the following beneficial effects: 1. Ensure the accuracy of the loading force in the test by measuring the critical buckling load of the sample to avoid premature instability of the sample. Ensure the reliability of the test results by calculating the Young's modulus and section moment of inertia based on the material.

[0047] 2. Intelligent clamping and dynamic adjustment: High-precision electric cylinder and controller automatically adjust the clamping force to keep the specimen stable and reduce human error. The dynamic adjustment mechanism automatically adjusts the support force according to the specimen force and deformation to improve test accuracy.

[0048] 3. The present invention simulates complex negative stress triaxiality conditions by applying axial force and lateral clamping force, which is closer to practical application than traditional uniaxial testing and improves data practicality.

[0049] 4. The present invention uses a DIC device and a high-magnification optical magnification device to monitor the deformation of the sample surface in real time, avoiding the error of traditional contact sensors. The image processing algorithm enhances the image to ensure that the strain data is accurate and reliable.

[0050] 5. The fixture of the present invention includes multiple detachable parts, that is, a modular structure, which can automatically adjust the structure of the fixture to adapt to different samples, improve the versatility and flexibility of the system, and cover a wide range of materials and structural forms.

[0051] 6. The present invention uses DIC technology and image processing algorithms to quickly extract and enhance strain data, improve test efficiency and analysis accuracy, and reduce human errors.

[0052] 7. The present invention is suitable for negative stress triaxiality testing of metal plates and also supports unidirectional compression testing to meet diverse application requirements.

[0053] The controller in this embodiment includes at least one processor; and a memory connected to the at least one processor in communication. The memory stores instructions that can be executed by the at least one processor, and the instructions are executed by the at least one processor so that the at least one processor can perform the above-mentioned triaxial mechanical properties test method of the sample. At least one processor in the controller can perform the above-mentioned method, and thus has at least the same advantages as the above-mentioned method.

[0054] Optionally, the controller also includes interfaces for connecting various components, including high-speed interfaces and low-speed interfaces. The various components are connected to each other using different buses and can be installed on a common mainboard or installed in other ways as needed. The processor can process instructions executed in the controller, including instructions stored in or on a memory to display graphical information of a GUI (Graphical User Interface) on an external input / output device (such as a display device coupled to an interface). In other embodiments, if necessary, multiple processors can be used together with multiple memories, and / or multiple buses can be used together with multiple memories. Similarly, multiple electronic devices can be connected (for example, as a server array, a group of blade servers, or a multi-processor system), and each device provides some necessary operations. Figure 5 A processor 301 is taken as an example.

[0055] The memory 302 is a computer-readable storage medium that can be used to store software programs, computer executable programs and modules, such as program instructions / modules corresponding to the controller method in the embodiment of the present invention. The processor 301 executes various functional applications and data processing of the device by running the software programs, instructions and modules stored in the memory 302, that is, implements the above-mentioned controller method.

[0056] The memory 302 may mainly include a program storage area and a data storage area, wherein the program storage area may store an operating system and at least one application required for a function; the data storage area may store data created according to the use of the terminal, etc. In addition, the memory 302 may include a high-speed random access memory, and may also include a non-volatile memory, such as at least one disk storage device, a flash memory device, or other non-volatile solid-state storage device. In some instances, the memory 302 may further include a memory remotely arranged relative to the processor 301, and these remote memories may be connected to the device via a network. Examples of the above-mentioned network include, but are not limited to, the Internet, an intranet, a local area network, a mobile communication network, and combinations thereof.

[0057] The controller may further include: an input device 303 and an output device 304. The processor 301, the memory 302, the input device 303 and the output device 304 may be connected via a bus or other means. Figure 5 The example of connecting through bus is taken in the following.

[0058] The input device 303 can receive input digital or character information, and the output device 304 can include a display device, an auxiliary lighting device (e.g., LED), and a tactile feedback device (e.g., a vibration motor), etc. The display device can include, but is not limited to, a liquid crystal display (LCD), a light emitting diode (LED) display, and a plasma display. In some embodiments, the display device can be a touch screen.

[0059] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps recorded in this application can be executed in parallel, sequentially or in different orders, as long as the expected results of the technical solution disclosed in this application can be achieved, and this document is not limited here.

[0060] The above specific implementations do not constitute a limitation on the protection scope of this application. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions and improvements made within the spirit and principles of this application should be included in the protection scope of this application.

Claims

1. A method for testing the triaxial mechanical properties of a sample, characterized in that: include: Determining the critical buckling load of the sample according to the characteristic parameters of the sample; inputting the critical buckling load into a lateral clamping device to apply a lateral clamping force to the sample through the lateral clamping device; Controlling the axial force applying device to apply axial force to the sample; During the application of the axial force, the reverse action parameters of the sample on the lateral holding device are collected in real time; and the lateral clamping force is adjusted in real time according to the reverse action parameters; During the application of the axial force, an image of the surface of the sample is collected in real time; The speckle deformation on the sample surface in the image is identified.

2. The method according to claim 1, characterized in that: Determining the critical buckling load of the sample according to the characteristic parameters of the sample, including: Calculating the critical buckling stress of the sample according to the Young's modulus, the section moment of inertia, the length coefficient and the effective length of the sample; The critical buckling load of the sample is calculated according to the critical buckling stress and the cross-sectional area of ​​the sample.

3. The method according to claim 1, characterized in that During the application of the axial force, the reverse action parameters of the sample on the lateral holding device are collected in real time, including: During the application of the axial force, the reverse displacement of the lateral holding device caused by the reverse action of the sample is collected in real time by means of a displacement sensor; or During the application of the axial force, the reverse force of the sample exerted on the lateral holding device is collected in real time through the force sensor.

4. The method according to claim 3, characterized in that Adjusting the lateral clamping force in real time according to the reverse action parameter comprises: A feedback mechanism is used to automatically adjust the magnitude of the lateral clamping force so that the sample can maintain lateral balance.

5. The method according to claim 4, characterized in that During the application of the axial force, an image of the surface of the sample is collected in real time, including: During the application of the axial force, the surface of the sample is magnified, and then a digital image correlation device is used to collect images of the surface of the sample in real time.

6. The method according to claim 5, characterized in that Identifying the surface speckle deformation of the sample in the image includes: Performing enhancement processing on the image; Identifying strain field data in the image by an image recognition algorithm; The mechanical properties of the sample are analyzed based on the strain field data.

7. A triaxial mechanical properties testing device for a sample, characterized in that: include: A lateral clamping device, used to apply a lateral clamping force to the sample; An axial force applying device, used for applying an axial force to the sample; An image acquisition device, used for acquiring an image of the surface of the sample in real time during the application of the axial force; A controller is used to determine the critical buckling load of the sample according to the characteristic parameters of the sample; input the critical buckling load into the lateral holding device to apply a lateral clamping force to the sample through the lateral holding device; control the axial force applying device to apply an axial force to the sample; during the application of the axial force, collect the reverse action parameters of the lateral holding device affected by the sample in real time; adjust the lateral clamping force in real time according to the reverse action parameters; and identify the surface speckle deformation of the sample in the image.

8. The device according to claim 7, characterized in that Also includes: An optical magnifying device, located between the sample and the image acquisition device, for magnifying the surface of the sample during the application of the axial force; The image acquisition device is a digital image correlation device.

9. The device according to claim 7, characterized in that Also includes: A fixture located between the lateral holding device and the sample; The fixture includes a plurality of detachable parts, through which samples of different shapes and sizes can be adapted; The fixture allows a portion of the sample surface to be exposed, with the exposed portion facing the image acquisition device.

10. The device according to claim 9, characterized in that A polytetrafluoroethylene film between the fixture and the sample; a polytetrafluoroethylene film and grease between the axial force applying device and the sample; The lateral holding device is an electric cylinder.

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