Full-automatic tensile testing machine for sheet metal and testing method

Abstract

The invention discloses a full-automatic tensile testing machine and a testing method for a metal plate, which are characterized in that the deformation state of a sample in a tensile test is monitored in real time by combining non-contact speckles with a video extensometer and a reflector, and a traceable optical feature point array is formed in a sample coating area through non-contact optical projection; according to the method, local hardening, corrosion or microcrack induction caused by chemical substances such as printing ink and paint used in traditional speckle spraying to the surface of a metal material is avoided, two-dimensional position changes of a sample are recorded in the tensile test process, so that a spatial discrete displacement field is constructed, then based on the displacement field, the local strain tensor of a feature point is calculated in real time, and the local strain tensor of the metal material is calculated in real time. According to the method, a strain curve changing along with time is extracted, a suspected initial deformation point is distinguished by setting a strain time threshold, the initial deformation point is accurately determined from the suspected initial deformation point by a scoring mechanism, a microscopic crack starting area and a necking starting point are more accurately identified, and the result accuracy of a metal plate tensile test is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material property testing, and particularly to a fully automatic tensile testing machine and testing method for metal sheets. Background Art

[0002] Due to their good mechanical properties, thermal conductivity, and workability, metal materials are widely used in fields such as aerospace, automotive manufacturing, construction engineering, and energy equipment. To accurately evaluate their mechanical properties, the tensile test, as one of the most basic and commonly used mechanical testing methods, can obtain key parameters such as the yield strength, tensile strength, elastic modulus, and elongation after fracture of the material, and is an essential and important link in the processes of material research and development, structural design, and quality control.

[0003] Traditional metal tensile tests generally use mechanical loading devices in combination with extensometers, displacement sensors, etc. to collect mechanical data. Its representative test standards include ISO 6892, ASTM E8 / E8M, and GB / T 228, etc. It is necessary to spray speckles at the position of the metal specimen so that the deformation path, local strain concentration, necking development, and even fracture propagation of the material can be judged according to the deformation degree of the speckles during the tensile test. However, the use of chemical substances such as ink and paint in traditional speckle spraying causes local hardening, corrosion, or microcrack induction on the surface of the metal material, which will affect the accuracy of the test results in the metal tensile test.

[0004] Therefore, it is necessary to design a fully automatic tensile testing machine and testing method for metal sheets to solve the above problems. Summary of the Invention

[0005] The present invention overcomes the deficiencies of the prior art and provides a fully automatic tensile testing machine and testing method for metal sheets.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a tensile testing method for metal sheets, comprising the following steps: Step S1, coat a coating on the coating area for specimen observation and clamp it on the tensile testing machine; Step S2, form a lateral optical observation path through a lateral arrangement of a plane mirror optical system; Step S3, non-contact project a randomly distributed high-density speckle pattern onto the coated area of the specimen, denoted as characteristic points; Step S4, start the testing machine to stretch the specimen and dynamically obtain a sequence of deformation images of the characteristic points in the coated area during the entire loading process; Step S5, perform image processing on the image sequence, extract the two-dimensional displacement information of the optical speckle characteristic points, and construct a spatial displacement field of the characteristic points in the coated area of the specimen; Step S6: Based on the extracted feature point displacement data, calculate the full-field strain tensor distribution in the coated area of the specimen at each loading stage, and identify the initial deformation point through the mutation of the characteristic displacement.

[0007] In a preferred embodiment of the present invention, in the step S1, a specimen is obtained by stamping a metal sheet, the edge of the specimen is polished, cleaned with 75% alcohol, and dried at room temperature for 5 - 8 minutes. After drying the specimen, a coating is applied to the observed coated area and dried at room temperature for 25 - 30 minutes.

[0008] In a preferred embodiment of the present invention, in the step S2, the three-dimensional coordinates (X, Y, Z) of the center position of the coated area of the specimen are obtained, where the X-axis is the tensile direction, the Y-axis is the width direction of the specimen, and the Z is the thickness direction of the specimen. A plane mirror is arranged at a 45° angle on the back of the specimen to reflect the deformation information on the back of the specimen.

[0009] In a preferred embodiment of the present invention, in the step S3, a randomly distributed high-density speckle pattern is non-contact projected onto the coated area of the specimen to form dense light spots in the coated area, and the formed light spots are used as feature points to construct a traceable optical feature point array.

[0010] In a preferred embodiment of the present invention, in the step S4, a uniaxial tensile action is applied to the specimen at a set loading rate, and the loading methods include: constant displacement loading, staged loading, or stress-strain feedback control loading. During the tensile process of the specimen, record the initial positions of the feature points, and collect the image sequence of the speckle pattern in the coated area at a preset time interval. Each frame of the image records the actual positions of the feature points at the corresponding moment.

[0011] In a preferred embodiment of the present invention, in the step S4, the image sequence covers the entire tensile stage, including: initial stage, elastic stage, yield stage, plastic stage, and fracture stage.

[0012] In a preferred embodiment of the present invention, in the step S5, it includes the following sub-steps: Step S51: Number each feature point and record its initial position in the initial image of the image sequence , as well as the new positions of each frame of the image in the image sequence ; Step S52: For the feature points, calculate the displacement vector from the initial frame to the current frame , where t represents the current t moment and i represents the i-th feature point; Step S53: Taking the coated area of the specimen as the observation surface, construct a discrete spatial displacement field composed of all feature point displacement vectors , where represents the spatial displacement field of all feature points at time t.

[0013] In a preferred embodiment of the present invention, in the said step S6, it includes the following sub-steps: Step S61: Record the positions of the feature points in each frame of the deformed image sequence, and calculate the local strain tensor; Step S62: Based on the set strain time threshold, screen out the suspected initial deformation points and construct a set of suspected initial deformation points; Step S63: For the suspected initial deformation points, calculate their deformation start time, strain growth rate and cumulative deformation amount, and construct a scoring mechanism to determine the initial deformation points.

[0014] In a preferred embodiment of the present invention, in the said step S62, constructing a set of suspected initial deformation points includes the following sub-steps: Step S621: Extract the strain sequence of each feature point on each frame of the image, and construct the strain time curve of each feature point; Step S622: Set the strain time threshold, and determine whether the time period of each feature point between the initial deformation and the deformation peak is within the strain time threshold. If it exists, determine that this feature point is a suspected initial deformation point and record its starting deformation moment. If it does not exist, determine that this feature point does not belong to the suspected initial deformation points; Step S623: Integrate the obtained several feature points in the order of the starting deformation moment to construct a set of suspected initial deformation points.

[0015] A fully automatic tensile testing machine for metal sheets, and the said tensile testing method for metal sheets includes: a tensile testing machine control system, and an observation module, a speckle generation module, an image acquisition module, a displacement field calculation module, a strain field analysis module, and an initial deformation point identification module carried thereon; The observation module includes a reflector arranged at the rear of the clamping position of the tensile testing machine, and captures the strain images during the tensile process of the specimen by constructing an observation path on the back of the specimen; The speckle generation module projects a random high-density speckle pattern onto the front and back of the specimen through a laser projection device to generate feature points to be monitored; The image acquisition module uses a video extensometer to collect the images of the specimen in real time during the tensile test process, and integrates the images in time order to form a deformed image sequence; The displacement field calculation module is used to number each feature point in the image sequence, record its initial and new positions in each frame, calculate the two-dimensional displacement vector of the feature point from the initial image to the current image, construct a displacement vector field composed of all feature points, and form a discrete spatial displacement field data structure; The strain field analysis module is based on the displacement vectors obtained by the displacement field calculation module, performs local strain tensor analysis on the positions of feature points in each frame of the image, and establishes a strain tensor by using the displacement gradient between adjacent points; The initial deformation point identification module constructs a strain-time curve for each feature point, sets a strain-time threshold, identifies suspected initial deformation points, calculates the deformation start time, strain growth rate, and cumulative deformation amount for the suspected initial deformation points, and establishes a scoring mechanism to determine the true initial deformation points among the suspected initial deformation points.

[0016] The present invention solves the defects existing in the background technology and has the following beneficial effects: (1) The present invention provides a tensile test method for metal sheets. By using non-contact speckle combined with a video extensometer and a reflector to real-time monitor the deformation state of the specimen in the tensile test, a traceable optical feature point array is formed in the coated area of the specimen through non-contact optical projection, avoiding the local hardening, corrosion or microcrack induction caused by chemical substances such as ink and paint in traditional sprayed speckles on the surface of the metal material, and recording the two-dimensional position change of the specimen during the tensile test process, thereby constructing a spatially discrete displacement field. Then, based on this displacement field, the local strain tensor of the feature points is calculated in real time, and the strain curve changing with time is extracted. Furthermore, by setting a strain-time threshold, suspected initial deformation points are distinguished, and the initial deformation points are accurately determined from the suspected initial deformation points through a scoring mechanism, more accurately identifying the microscopic crack initiation area and the necking start point, and improving the result accuracy of the tensile test of metal sheets.

[0017] (2) The present invention provides a tensile test method for metal sheets. By setting a strain-time threshold, several feature points are classified, the response times of all feature points are sorted, and the feature points suspected of being initial deformation points are quickly screened out from the feature points, avoiding misidentification caused by local stress. And through a scoring mechanism, each suspected deformation point is quantitatively scored from multiple dimensions such as the starting strain moment, strain growth rate, and cumulative deformation amount, and sorted according to the comprehensive score. Finally, the feature point with the earliest rapid strain change during the loading process is identified, so as to quickly locate the initial local plastic concentration area before the deformation spreads to the entire area, providing a positioning basis for subsequent concentrated sampling, structural analysis and microscopic damage judgment, avoiding the indiscriminate processing of the whole-field data, and thus improving the efficiency of strain analysis.

[0018] (3) The present invention provides a tensile test method for metal sheets. By vectorizing the displacements of all feature points in each frame of the image, a complete strain field of the entire specimen coating area at different loading stages can be constructed, which can reflect the strain concentration trend in the local area and is helpful for dynamically analyzing the stress transfer path and the local softening area of the material.

[0019] (4) By coating a coating on the coating area of the specimen, the present invention induces the specimen to fracture within the coating area, thereby ensuring the appearance of the initial deformation point in the coating area. This is helpful for controlling the fracture position to concentrate on the set speckle detection area, facilitating the accurate capture of the strain evolution process by the video extensometer, avoiding the indiscriminate processing of the full-field data, and being able to effectively aggregate and uniformly conduct heat energy under the laser speckle irradiation, avoiding irreversible changes in the microstructure of the base metal caused by local hot spots, so as to maintain the structural integrity and original mechanical properties of the specimen body. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings; Figure 1 It is a flowchart of a tensile test method for metal sheets according to the present invention; Figure 2 It is a schematic diagram of a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] The determination of the initial deformation point refers to the point where, during the tensile loading process, a certain feature point of the specimen has the most rapid strain growth per unit time, and this growth process is concentrated and has a strong mutation. This point marks the earliest occurrence position of local instability or necking tendency and is a sensitive precursor area for the evolution of plastic deformation.

[0022] Specifically, it should be noted that the initial deformation point is not the point where the deformation starts earliest, nor the point where all deformations are completed earliest, but the point where the strain grows most rapidly per unit time and the deformation process is most concentrated. Therefore, the determination of the initial deformation point needs to consider the time dimension.

[0023] Determining the initial deformation point can early warn the position where the material is about to be damaged, and is the key basis for predicting the ultimate performance, constructing the stress-strain model, and improving the material processing technology.

[0024] As Figure 1 and Figure 2 shown, a tensile test method for metal sheets includes the following steps: Step S1: Coat the coating on the coated area of the specimen for observation, and clamp it on the tensile testing machine. Step S2: Construct a lateral optical observation path through a laterally arranged planar mirror optical system. Step S3: Non - contact project a randomly distributed high - density speckle pattern onto the coated area of the specimen, denoted as characteristic points. Step S4: Start the testing machine to stretch the specimen, and dynamically obtain the deformation image sequence of the characteristic points in the coated area during the whole loading process. Step S5: Perform image processing on the image sequence, extract the two - dimensional displacement information of the optical speckle characteristic points, and construct the spatial displacement field of the characteristic points in the coated area of the specimen. Step S6: Based on the extracted displacement data of the characteristic points, calculate the full - field strain tensor distribution in the coated area of the specimen at each loading stage, and identify the initial deformation point through the sudden change of the characteristic displacement.

[0025] In the present invention, in the step S1, the specimen is obtained by stamping a metal sheet, the edge of the specimen is polished, cleaned with 75% alcohol, and dried at room temperature for 5 - 8 min. For the specimen after drying, coat the coating on the coated area for its observation, and dry it at room temperature for 25 - 30 min.

[0026] Specifically, the standard stamping process is adopted. According to ISO 6892, dumbbell - shaped metal sheet specimens are prepared. After stamping, use metallographic sand to deburr and polish the edge of the specimen to ensure that the edge is smooth and there is no crack source.

[0027] In the present invention, in the step S2, obtain the three - dimensional coordinates (X, Y, Z) of the center position of the coated area of the specimen, where the X - axis is the tensile direction, the Y - axis is the width direction of the specimen, and the Z is the thickness direction of the specimen. Arrange the planar mirror at a 45° angle on the back of the specimen to reflect the deformation information on the back of the specimen.

[0028] By reflecting the back of the specimen through the planar mirror, the video extensometer can simultaneously monitor the front and back of the specimen, thereby improving the integrity of the strain recognition area and reducing the system complexity and cost at the same time.

[0029] A lateral optical observation path is constructed through a laterally arranged plane reflector optical system to realize the reflective collection of the deformation information on the back side of the specimen. The refraction of the optical path allows the deformation on the back side of the specimen, which was originally difficult to observe directly, to be captured laterally by a high-resolution camera device, thereby obtaining the three-dimensional deformation information of the specimen during the stretching process. By combining the two-dimensional optical observations of the X-axis (stretching direction) and the Y-axis (width direction), an all-round monitoring of the specimen deformation is formed, which not only avoids the image distortion caused by light reflection in traditional frontal viewing, but also greatly improves the image clarity and the recognition rate of feature points, ensuring the accuracy of subsequent feature point tracking and strain analysis, thereby improving the reliability and effectiveness of the entire tensile test data.

[0030] In the present invention, in step S3, a randomly distributed high-density speckle pattern is non-contact projected onto the sample coating area to form dense light spots in the coating area, and the formed light spots are used as feature points to construct a traceable optical feature point array.

[0031] The characteristic points in the metal sheet tensile test are determined by non-contact projection speckle mapping on the surface of the sample. It will not produce additional stress concentration on the sample surface or change its friction, thermal conductivity, optical and other properties, and avoids the local hardening, corrosion or microcracks caused by the use of chemicals such as ink and paint in traditional spray speckle.

[0032] Specifically, the laser beam is expanded into a number of speckles through a laser diffuser, and mapped on the coating area of ​​the sample, and the position of the speckle is used as a characteristic point; The laser light source is set at two locations, one at the front area and the other at the back area of ​​the sample, and is evenly distributed in the pre-set coating area to enhance the image contrast and feature point distribution density.

[0033] In summary, in step S3, the randomly distributed high-density speckle pattern is non-contact projected onto the sample coating area to form dense and evenly distributed spots, which are used as feature points for subsequent tracking and deformation analysis. The non-contact application of the speckle pattern is achieved through the projection device, which avoids mechanical interference or damage to the sample surface. At the same time, the speckle pattern is randomly and densely distributed on the sample surface to form an optical feature point array with rich details. The feature points can be clearly captured by high-resolution camera equipment during the stretching process, which is convenient for image processing software to accurately identify and match, improves the spatial resolution and tracking accuracy of the feature points, and effectively enhances the meticulousness and accuracy of the displacement field and strain field calculation, ensuring that the tiny deformation and strain mutation occurring in the sample during the loading process can be keenly captured.

[0034] In a preferred embodiment of the present invention, in the step S4, a uniaxial tensile action is applied to the specimen at a set loading rate, and the loading methods include: constant displacement loading, staged loading, or stress-strain feedback control loading; During the tensile process of the specimen, a video extensometer is used to record the initial positions of the characteristic points, and an image sequence of the speckle pattern within the coated area is collected at a preset time interval, and each frame of the image records the actual positions of the characteristic points at the corresponding moment.

[0035] In the step S4, the image sequence covers the entire tensile stage, including: the initial stage, the elastic stage, the yield stage, the plastic stage, and the fracture stage.

[0036] In the step S4, a uniaxial tensile load is applied to the specimen by starting a tensile testing machine, and at the same time, an image sequence of the deformation of the characteristic points in the coated area of the specimen during the entire loading process is dynamically obtained. Specifically, the testing machine performs constant displacement loading, staged loading, or stress-strain feedback control loading at the set loading rate to ensure that the tensile process is controllable and stable. During the loading process, a high-resolution imaging device is used to continuously collect an image sequence of the speckle pattern covering the entire tensile stage (including the initial, elastic, yield, plastic, and fracture stages) at a preset time interval, and each frame of the image records the precise position changes of the characteristic points.

[0037] By continuously and frequently collecting deformation images, the subtle deformation evolution information of the specimen from the initial state to the failure process can be completely captured, providing continuous data support for subsequent image processing and strain analysis. Among them, the acquisition of the dynamic image sequence can further identify the state of the specimen at different loading stages, facilitating the identification of the yield point of the material, the starting position of local deformation concentration, and the fracture development trend, improving the accuracy and reliability of the tensile test results. At the same time, this non-contact optical observation avoids the possible specimen damage or interference caused by traditional contact measurement, realizing real-time and comprehensive monitoring of the deformation behavior of metal sheets.

[0038] In the present invention, in the step S5, the following sub-steps are included: Step S51: Number each characteristic point and record its initial position in the initial image of the image sequence , as well as the new position of each frame of the image in the image sequence ; Step S52: For the characteristic points, calculate the displacement vector from the initial frame to the current frame , where t represents the current t moment and i represents the i-th characteristic point; Step S53: Taking the coated area of the specimen as the observation surface, construct a discrete spatial displacement field composed of all the characteristic point displacement vectors , where, Denote the spatial displacement field of all feature points at time t.

[0039] In step S5, by performing image processing on the sequence of deformed images collected in step S4, the two-dimensional displacement information of the speckle feature points within the coated area is extracted, and then the spatial displacement field of the feature points in the specimen coated area is constructed. Specifically: Each feature point is numbered, and its initial position in the initial image and its new position in each frame of the entire image sequence are accurately recorded. Subsequently, the displacement vector of each feature point from the initial frame to the current frame is calculated to obtain the two-dimensional motion trajectory and displacement magnitude of the feature points. Taking the specimen coated area as the observation surface, by integrating the displacement data of all feature points, a discrete spatial displacement field is established to comprehensively reflect the deformation distribution and change trend at different positions on the specimen surface.

[0040] By calculating the surface deformation information of the specimen during the tensile test, the local and overall deformations of the metal sheet during the tensile process are spatially mapped and dynamically tracked. Through the construction of the feature point displacement field, not only the monitoring of deformation is achieved, but also a basis is provided for the subsequent calculation of the full-field strain tensor and the identification of deformation mutation points.

[0041] In a preferred embodiment of the present invention, in step S6, the following sub-steps are included: Step S61: Based on the discrete displacement field obtained in step S5, the current coordinate positions of the feature points are extracted in each time frame, the positions of the feature points in each frame of the sequence of deformed images are recorded, and the local strain tensor is calculated. Step S62: Based on the set strain time threshold, the suspected initial deformation points are screened to construct a set of suspected initial deformation points. Step S63: For the suspected initial deformation points, calculate their deformation start time, strain growth rate, and cumulative deformation amount, and construct a scoring mechanism to determine the initial deformation points.

[0042] Constructing a set of suspected initial deformation points includes the following sub-steps: Step S621: Extract the strain sequence of each feature point in each frame of the image to construct the strain-time curve of each feature point. Step S622: Set the strain time threshold to determine whether the time period of each feature point from initial deformation to the deformation peak is within the strain time threshold. If it exists, this feature point is determined as a suspected initial deformation point, and its starting deformation moment is recorded. If it does not exist, this feature point is determined not to belong to the suspected initial deformation points. If the time periods of all feature points from initial deformation to the deformation peak are not within the strain time threshold, the monitored feature points are determined as initial deformation points. Step S623: Integrate the obtained several feature points in the order of the starting deformation moment to construct a set of suspected initial deformation points.

[0043] Among them, a strain time threshold is set, specifically recording the time points after the feature points are deformed, selecting the feature points at the earliest time point, and using the time point of the feature point at the deformation peak and the time period between the initial deformation and the deformation peak of the feature point as the strain time threshold; Regarding the strain time curve, the image sequence of the entire loading process is processed, the two-dimensional coordinates of each feature point in each frame are tracked, and the local strain value of each feature point is calculated using the relative displacement of the speckle pattern, thereby obtaining the strain curve of the feature point changing with time; Among them, regarding the strain growth rate based on the strain time curve, recording the strain of the feature point from the initial stage of deformation to the main deformation peak, and the time experienced by the deformation in this stage, where the strain of the feature point from the initial stage of deformation to the main deformation peak is the cumulative deformation amount, specifically , specifically, where Vi represents the strain growth rate per unit time, reflecting the sudden change of deformation, represents the strain of the feature point from the initial stage of deformation to the main deformation peak, represents the time experienced by the deformation in this stage.

[0044] According to the determined suspected initial deformation points, by scoring the suspected initial deformation points , where represents the weight coefficient, adjusted according to experimental experience, indicates that the earlier the deformation time, the higher the score, represents the strain of the feature point from the initial stage of deformation to the main deformation peak.

[0045] In step S6, by setting the strain time threshold, several feature points are classified, the response times of all feature points are sorted, the feature points with the earliest response and the most active deformation are quickly screened out from a large range, and then some feature points of the suspected initial deformation points are determined, avoiding misidentification caused by local stress, and through a scoring mechanism, each suspected deformation point is quantitatively scored from multiple dimensions such as the starting strain time, strain growth rate, and cumulative deformation amount, and sorted according to the comprehensive score, and finally the feature points with the earliest rapid strain change during the loading process are identified, so as to quickly locate the initial local plastic concentration area before the deformation spreads to the entire area, providing a positioning basis for subsequent concentrated sampling, structural analysis, and microscopic damage judgment, avoiding the indiscriminate processing of the whole-field data, and thus improving the efficiency of strain analysis.

[0046] It should be noted that the preparation of the coating includes the following steps: A. Mix polyether polyol, 1,4 - butanediol, and dimethylolpropionic acid in proportion and heat to 70 - 80 °C. Then, dropwise add diisocyanate and continue to heat up to 80 - 90 °C, and maintain for 2 - 2.5 h to obtain a prepolymer. Cool the prepolymer to 45 - 50 °C and add ammonia water for neutralization reaction. Slowly drop the neutralized prepolymer into deionized water under high - speed stirring and continuously stir for 5 - 10 min. After stirring, add methyl methacrylate to the prepolymer and heat up to 70 - 80 °C, and maintain for 2 - 3 h to obtain a polyurethane - acrylate block copolymer emulsion. Among them, the mass parts of the materials for preparing the copolymer emulsion are: 15 - 20 parts of polyether polyol, 5 - 10 parts of 1,4 - butanediol, 10 - 12 parts of diisocyanate, 2 - 3 parts of dimethylolpropionic acid, 1 - 2 parts of ammonia water, 25 - 30 parts of methyl methacrylate, and 20 - 30 parts of deionized water. The concentration of ammonia water is 28%.

[0047] B. Take 5 - 10 parts of carbon black, 10 - 15 parts of graphene micro - sheets, 2 - 5 parts of SiO₂, 2 - 5 parts of TiO₂, and add them to 70 - 75 parts of the copolymer emulsion and stir for 20 - 25 min to form a stable homogeneous slurry. Spray it on the coated area of the specimen, control the film thickness in the range of 8 - 15 μm, and hot - air dry at 70 - 80 °C for 10 - 15 min to form a coating on the surface of the specimen.

[0048] Verify the differences between the specimens with the coated coating and the specimens without the coated coating in the tensile test.

[0049] The polyether polyol is from Wanhua Chemical Group Co., Ltd., with the model WANNATE® PE - 330N. The 1,4 - butanediol is from Shandong Dongying Haike Chemical Co., Ltd., with the model BDO - 99. The dimethylolpropionic acid is from Anhui Xinzhongyuan Chemical Technology Co., Ltd., with the model XZY - DMPA. The diisocyanate is from Wanhua Chemical Group Co., Ltd., with the model WANNATE® HT - 100. The methyl methacrylate is from Sinopec Shanghai Petrochemical Company, with the model MMA - 99. The carbon black is from Jiangxi Black Cat Carbon Black Co., Ltd., with the model N330. The graphene micro - sheets are from Changzhou Sixth Element Materials Technology Co., Ltd., with the model SE1232. The nano - SiO₂ is from Qucheng Silicon Chemical Co., Ltd., with the model Aerosil® 200 and a particle size of 12 nm. The nano-TiO2 is sourced from Pangang Group Vanadium Titanium Resources Co., Ltd., with the model P25 and a particle size of 21 nm.

[0050] Example 1:

[0051] Take 15% polyether polyol, 5% 1,4-butanediol, and 2-3% dimethylolpropionic acid, mix them in proportion and heat to 75°C, then dropwise add 12% diisocyanate, continue to heat up to 85°C, and keep for 2.5 h to obtain a prepolymer. Cool the prepolymer to 50°C, add 2% ammonia water for neutralization reaction, slowly drop the neutralized prepolymer into 30% deionized water under high-speed stirring, continuously stir for 10 min. After the stirring is completed, add 30% methyl methacrylate, and heat up to 80°C, keep for 2 h to obtain a polyurethane-acrylate block copolymer emulsion; Take 10% carbon black, 15% graphene microflakes, 5% SiO2, 3% TiO2 by mass, and add them to 75% copolymer emulsion and stir for 25 min to form a stable homogeneous slurry, spray it on the coating area of the specimen, control the film thickness in the range of 9 ± 0.5 μm, and dry it with hot air at 80°C for 15 min to obtain a coating; Among them, use 6061 aluminum alloy plates with a thickness of 2 mm, process dumbbell-shaped specimens according to the ISO 6892-1 processing standard, the gauge section is 50 mm × 20 mm, and the coating is applied at two-thirds of the length of the specimen.

[0052] Example 2:

[0053] Use 6061 aluminum alloy plates with a thickness of 2 mm, process dumbbell-shaped specimens according to the ISO 6892-1 processing standard, and the gauge section is 50 mm × 20 mm.

[0054] Implementation steps: 1. Detect the mechanical properties of the specimens: Prepare a WDW-100 type electronic universal testing machine and an Instron 2620 series dynamic extensometer, clamp the specimens in Example 1 and Example 2 respectively for tensile tests, detect the mechanical properties of the specimens, and use GB / T 228.1-2021.

[0055] Verify the performance of the specimens in Example 1 and Example 2 during the tensile test, as shown in Table 1.

[0056] Table 1:

[0057] As can be seen from Table 1, coating the specimen can induce deformation in the coated area, and the coated specimen will not affect the tensile test of the specimen during the tensile test. The main reason is that the coating locally adheres to micro-nano particles, and the particles form a microporous / micro-cluster structure with irregular distribution at the microscale. The slight difference in local stiffness will cause a very small difference in elastic modulus, making this area more likely to undergo micro-yield or plastic slip under the external load than the surrounding matrix. Thus, it shows "strain precedence" macroscopically and guides the preferential formation of the necking area.

[0058] Verify the laser heat absorption of the specimens in Example 1 and Example 2, as shown in Table 2.

[0059] 2. Heat dissipation performance test: For the specimens in Example 1 and Example 2, continuously irradiate the coated area of the specimen in Example 1 with laser for 30 s, and continuously irradiate the central position of the specimen in Example 2 with laser for 30 s. Monitor through a FLIR A655sc infrared thermal imager and simultaneously detect the mechanical properties of the specimen.

[0060] Table 2:

[0061] As shown in Table 2, after laser irradiation, the heating rate of Example 1 is low and does not affect the mechanical properties of the specimen, while the heating rate of Example 2 is high and its mechanical properties are reduced. The main reason is that graphene microflakes and carbon black are introduced into the coating in Example 1. The former has extremely high in-plane thermal conductivity and can quickly achieve lateral heat diffusion after heat is generated by laser irradiation. The latter improves the laser absorption efficiency of the coating, making the heat concentrated in the coating area without penetrating deep into the matrix. At the same time, the combined effect of TiO2 and SiO2 nanoparticles further enhances the heat reflection and buffering capabilities, forming a stable multi-level thermal control network of heat absorption - diffusion - shielding; In Example 2, the metal surface directly absorbs laser energy, forming local hot spots. This hot area causes grain coarsening, increased dislocation slip, and even local annealing or melting, affecting the tensile properties of the metal.

[0062] A fully automatic tensile testing machine for metal sheets, and the described tensile test method for metal sheets includes: a tensile testing machine control system, and an observation module, a speckle generation module, an image acquisition module, a displacement field calculation module, a strain field analysis module, and an initial deformation point identification module carried thereon; The observation module includes a reflector arranged behind the clamping position of the tensile testing machine, and captures the strain image during the tensile process of the specimen by constructing an observation path on the back of the specimen; The speckle generation module projects a random high-density speckle pattern onto the front and back of the specimen through a laser projection device to generate characteristic points to be monitored; The image acquisition module collects the images of the specimen in real time during the tensile test through a video extensometer, and integrates the images in time sequence to form a deformation image sequence; The displacement field calculation module is used to number each feature point in the image sequence, record its initial and new positions in each frame, calculate the two-dimensional displacement vector of the feature point from the initial image to the current image, construct a displacement vector field composed of all feature points, and form a discrete spatial displacement field data structure; The strain field analysis module performs local strain tensor analysis on the positions of feature points in each frame of image based on the displacement vectors obtained by the displacement field calculation module, and establishes a strain tensor using the displacement gradient between adjacent points; The initial deformation point identification module constructs a strain-time curve for each feature point, sets a strain-time threshold, identifies suspected initial deformation points, calculates the deformation start time, strain growth rate, and cumulative deformation amount of the suspected initial deformation points, and establishes a scoring mechanism to determine the true initial deformation points among the suspected initial deformation points.

[0063] Based on the ideal embodiments of the present invention as inspiration, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of the present invention. The technical scope of the present invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A tensile test method for metal sheets, characterized in that, It includes the following steps: Step S1: Coat the coating on the coated area of the specimen for observation, and clamp it on the tensile testing machine; Step S2: Construct a lateral optical observation path through a laterally arranged planar mirror optical system; Step S3: Non-contact project a randomly distributed high-density speckle pattern onto the coated area of the specimen, denoted as characteristic points; Step S4: Start the testing machine to stretch the specimen, and dynamically obtain the deformation image sequence of the characteristic points in the coated area during the whole loading process; Step S5: Perform image processing on the image sequence, extract the two-dimensional displacement information of the optical speckle characteristic points, and construct the spatial displacement field of the characteristic points in the coated area of the specimen; Step S6: Based on the extracted characteristic point displacement data, calculate the full-field strain tensor distribution in the coated area of the specimen at each loading stage, and identify the initial deformation point through the mutation of characteristic displacement; 2. The tensile test method for metal sheets according to claim 1, wherein: In the said Step S1, obtain the specimen by stamping a metal sheet, polish the edge of the specimen, clean it with 75% alcohol, and dry it at room temperature for 5 - 8 min; For the specimen after drying, coat the coating on the observed coated area and dry it at room temperature for 25 - 30 min.

3. A tensile test method for metal sheets according to claim 1, characterized in that: In the said Step S2, obtain the three-dimensional coordinates (X, Y, Z) of the center position of the coated area of the specimen, where the X-axis is the tensile direction, the Y-axis is the width direction of the specimen, and the Z is the thickness direction of the specimen; Arrange the planar mirror at a 45° angle on the back of the specimen to reflect the deformation information on the back of the specimen.

4. A tensile test method for metal sheets according to claim 1, characterized in that: In the said Step S3, non-contact project a randomly distributed high-density speckle pattern onto the coated area of the specimen, form dense light spots in the coated area, use the formed light spots as characteristic points, and construct a traceable optical characteristic point array.

5. A tensile test method for metal sheets according to claim 1, characterized in that: In the said Step S4, apply uniaxial tensile action to the specimen at a set loading rate, and the loading methods include: constant displacement loading, staged loading, or stress-strain feedback control loading; During the tensile process of the specimen, record the initial positions of the characteristic points, collect the image sequence of the speckle pattern in the coated area at a preset time interval, and each frame of the image records the actual positions of the characteristic points at the corresponding moment.

6. A tensile test method for metal sheets according to claim 1, characterized in that: In the said Step S4, the image sequence covers the entire tensile stage, including: initial stage, elastic stage, yield stage, plastic stage, and fracture stage.

7. A tensile test method for metal sheets according to claim 1, characterized in that: In the said Step S5, it includes the following sub-steps: Step S51: Number each feature point and record its initial position in the initial image of the image sequence, as well as its new position in each frame of the image sequence. ; Step S52: For the feature points, calculate the displacement vector from the initial frame to the current frame , where t represents the current time t, and i represents the i-th feature point; Step S53: Taking the coated area of the specimen as the observation surface, construct a discrete spatial displacement field composed of all the displacement vectors of the feature points , where represents the spatial displacement field of all the feature points at time t.

8. A tensile test method for metal sheets according to claim 1, characterized in that: In the said Step S6, it includes the following sub-steps: Step S61: Record the positions of the characteristic points in each frame of the deformation image sequence, and calculate the local strain tensor; Step S62: Based on the set strain time threshold, screen the suspected initial deformation points and construct a set of suspected initial deformation points; Step S63: For the suspected initial deformation points, calculate their deformation start time, strain growth rate, and cumulative deformation amount, and construct a scoring mechanism to determine the initial deformation points.

9. A tensile test method for metal sheets according to claim 8, characterized in that: In the said Step S62, to construct a set of suspected initial deformation points, it includes the following sub-steps: Step S621: Extract the strain sequence of each characteristic point on each frame of the image and construct the strain time curve of each characteristic point; Step S622: Set a strain time threshold, and determine whether the time period of each feature point between the initial deformation and the deformation peak is within the strain time threshold. If it exists, determine that this feature point is a suspected initial deformation point and record its starting deformation time. If it does not exist, determine that this feature point does not belong to the suspected initial deformation points; Step S623: Integrate the obtained several feature points in the order of the starting deformation time to construct a set of suspected initial deformation points.

10. An automatic tensile testing machine for metal sheets, based on the tensile testing method for metal sheets according to any one of claims 1-9, characterized in that, Including: A tensile testing machine control system, and an observation module, a speckle generation module, an image acquisition module, a displacement field calculation module, a strain field analysis module, and an initial deformation point identification module carried thereon; The observation module includes a reflector arranged at the rear of the clamping position of the tensile testing machine, and captures the strain image during the tensile process of the specimen by constructing an observation path on the back of the specimen; The speckle generation module projects a random high-density speckle pattern onto the front and back of the specimen through a laser projection device to generate feature points to be monitored; The image acquisition module uses a video extensometer to collect the images of the specimen in real time during the tensile test, and integrates the images in time order to form a deformation image sequence; The displacement field calculation module is used to number each feature point in the image sequence, record its initial and new positions in each frame, calculate the two-dimensional displacement vector of the feature point from the initial image to the current image, construct a displacement vector field composed of all feature points, and form a discrete spatial displacement field data structure; The strain field analysis module performs local strain tensor analysis on the positions of feature points in each frame of image based on the displacement vectors obtained by the displacement field calculation module, and establishes a strain tensor using the displacement gradient between adjacent points; The initial deformation point identification module constructs a strain time curve for each feature point, sets a strain time threshold, identifies suspected initial deformation points, calculates the deformation start time, strain growth rate, and cumulative deformation amount of the suspected initial deformation points, and establishes a scoring mechanism to determine the true initial deformation points among the suspected initial deformation points.

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