A fully automatic tensile testing machine and testing method for metal sheets

Abstract

The present invention discloses a fully automatic tensile testing machine and testing method for metal plates. The present invention utilizes non-contact speckle patterning in combination with a video extensometer and a reflector to monitor the deformation state of a sample in a tensile test in real time. Non-contact optical projection forms a traceable array of optical feature points in the sample coating area, thereby avoiding the local hardening, corrosion or microcrack induction on the surface of the metal material caused by the use of chemicals such as ink and paint in traditional spray speckle patterning. During the tensile test, the two-dimensional position change of the sample is recorded to construct a spatially discrete displacement field. Based on the displacement field, the local strain tensor of the feature point is calculated in real time, and its strain curve varying with time is extracted. Suspected initial deformation points are then identified by setting a strain time threshold. The initial deformation point is accurately determined from the suspected initial deformation point using a scoring mechanism, and the microcrack initiation area and the necking starting point are more accurately identified, thereby improving the accuracy of the results of the metal plate tensile test.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material performance testing, and in particular to a full-automatic tensile testing machine and a testing method for metal plates. Background Art

[0002] Metal materials, due to their excellent mechanical properties, thermal conductivity, and machinability, are widely used in aerospace, automotive, construction, energy equipment, and other fields. Tensile testing, one of the most basic and commonly used mechanical testing methods, is crucial for accurately assessing their mechanical properties. It can determine key parameters such as yield strength, tensile strength, elastic modulus, and elongation after fracture, making it an indispensable step in materials research and development, structural design, and quality control.

[0003] Traditional metal tensile testing generally uses mechanical loading devices combined with extensometers, displacement sensors, and other means to collect mechanical data. Representative test standards include ISO 6892, ASTM E8 / E8M, and GB / T 228. These tests require spraying speckles onto the metal specimen so that the deformation path, local strain concentration, necking development, and even fracture propagation can be determined based on the degree of deformation of the speckles during the tensile test. However, the use of chemicals such as inks and paints in traditional spraying speckles can cause localized hardening, corrosion, or microcracks on the metal surface, which in turn affects the accuracy of the test results.

[0004] Therefore, it is necessary to design a fully automatic tensile testing machine and testing method for metal plates 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 a testing method for metal plates.

[0006] To achieve the above object, the present invention adopts the following technical solution: a tensile test method for metal plates, comprising the following steps:

[0007] Step S1, applying a coating on the coating area of the sample to be observed, and clamping it in a tensile testing machine;

[0008] Step S2: forming a lateral optical observation path by a laterally arranged plane reflector optical system;

[0009] Step S3, non-contact projecting a randomly distributed high-density speckle pattern onto the sample coating area and recording it as a feature point;

[0010] Step S4: start the testing machine to stretch the specimen, and dynamically obtain a deformation image sequence of characteristic points of the coating area during the entire loading process;

[0011] Step S5: performing image processing on the image sequence, extracting two-dimensional displacement information of optical speckle feature points, and constructing a spatial displacement field of feature points in the sample coating area;

[0012] Step S6: Based on the extracted characteristic point displacement data, the full-field strain tensor distribution of the sample coating area at each loading stage is calculated, and the initial deformation point is identified through the characteristic displacement mutation.

[0013] In a preferred embodiment of the present invention, in step S1, a sample is obtained by punching a metal plate, the edge of the sample is polished, cleaned with 75% alcohol, and dried at room temperature for 5-8 minutes;

[0014] After the drying is completed, the observed coating area of the sample is coated and dried at room temperature for 25-30 minutes.

[0015] In a preferred embodiment of the present invention, in step S2, the three-dimensional coordinates (X, Y, Z) of the center position of the sample coating area are obtained, wherein the X axis is the stretching direction, the Y axis is the sample width direction, and the Z axis is the sample thickness direction;

[0016] Place the plane reflector at a 45° angle on the back of the sample to reflect the deformation information on the back of the sample.

[0017] In a preferred embodiment of 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. The formed light spots are used as feature points to construct a traceable optical feature point array.

[0018] In a preferred embodiment of the present invention, in step S4, uniaxial tension is applied to the sample at a set loading rate, and the loading method includes: constant displacement loading, staged loading, or stress-strain feedback controlled loading;

[0019] When the specimen is under tension, the initial position of the characteristic point is recorded, and an image sequence of the speckle pattern in the coating area is collected at preset time intervals. Each frame of the image records the actual position of the characteristic point at the corresponding moment.

[0020] In a preferred embodiment of the present invention, in step S4, the image sequence covers the entire stretching stage, including: the initial stage, the elastic stage, the yield stage, the plastic stage and the fracture stage.

[0021] In a preferred embodiment of the present invention, step S5 includes the following sub-steps:

[0022] Step S51: Number each feature point and record its initial position in the initial image in the image sequence. , and the new position of each frame in the image sequence ;

[0023] Step S52: For the feature point, calculate its 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;

[0024] Step S53: Using the coating area of the sample as the observation surface, construct a discrete spatial displacement field consisting of all characteristic point displacement vectors. ,in, Represents the spatial displacement field of all feature points at time t.

[0025] In a preferred embodiment of the present invention, step S6 includes the following sub-steps:

[0026] Step S61: Record the position of the feature points in each frame of the deformed image sequence and calculate the local strain tensor;

[0027] Step S62: Based on the set strain time threshold, screening suspected initial deformation points and constructing a set of suspected initial deformation points;

[0028] Step S63: For the suspected initial deformation point, calculate its deformation start time, strain growth rate and cumulative deformation, and establish a scoring mechanism to determine the initial deformation point.

[0029] In a preferred embodiment of the present invention, in step S62, constructing a set of suspected initial deformation points includes the following sub-steps:

[0030] Step S621: extract the strain sequence of each feature point on each frame of the image and construct a strain-time curve of each feature point;

[0031] Step S622: Set a strain time threshold to determine whether the time period between the initial deformation and the deformation peak of each feature point is within the strain time threshold. If so, the feature point is determined to be a suspected initial deformation point and its starting deformation time is recorded. If not, the feature point is determined not to be a suspected initial deformation point.

[0032] Step S623: Integrate the acquired feature points in the order of the initial deformation time to construct a set of suspected initial deformation points.

[0033] A fully automatic tensile testing machine for metal sheets, and a tensile testing method for metal sheets, comprising: a tensile testing machine control system, and equipped with 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;

[0034] The observation module includes a reflector arranged at the rear side of the clamping position of the tensile testing machine, which captures the strain image of the specimen during the stretching process by constructing an observation path on the back side of the specimen;

[0035] The speckle generation module generates characteristic points to be monitored by projecting random high-density speckle patterns onto the front and back of the sample using a laser projection device;

[0036] The image acquisition module uses a video extensometer to collect images of the sample in real time during the tensile test, and integrates the images in time sequence to form a deformation image sequence;

[0037] 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, and construct a displacement vector field composed of all feature points to form a discrete spatial displacement field data structure;

[0038] The strain field analysis module performs local strain tensor analysis on the position of feature points in each frame image based on the displacement vector obtained by the displacement field calculation module, and establishes the strain tensor using the displacement gradient between adjacent points;

[0039] 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 of the suspected initial deformation points, and establishes a scoring mechanism to determine the true initial deformation point among the suspected initial deformation points.

[0040] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0041] (1) The present invention provides a tensile test method for metal plates, which utilizes non-contact speckle patterning combined with a video extensometer and a reflector to monitor the deformation state of a sample in a tensile test in real time. A traceable array of optical feature points is formed in the sample coating area by non-contact optical projection, thereby avoiding the local hardening, corrosion or microcrack induction on the surface of the metal material caused by the use of ink, paint and other chemicals in traditional spray speckle patterning. During the tensile test, the two-dimensional position change of the sample is recorded to construct a spatially discrete displacement field. Based on the displacement field, the local strain tensor of the feature point is calculated in real time, and its strain curve changing with time is extracted. Then, the 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, and the microcrack initiation area and the necking starting point are more accurately identified, thereby improving the accuracy of the results of the metal plate tensile test.

[0042] (2) The present invention provides a tensile test method for metal plates. By setting a strain time threshold, a number of feature points are classified, and all feature points are sorted by response time. Feature points suspected of initial deformation points are quickly screened out from the feature points to avoid misidentification caused by local stress. Through a scoring mechanism, each suspected deformation point is quantitatively scored from multiple dimensions such as the initial strain moment, strain growth rate, and cumulative deformation. The points are sorted according to the comprehensive score, and finally the feature point where the earliest rapid strain change occurs during the loading process is identified. Thus, the initial local plastic concentration area is quickly located before the deformation extends to the entire area, providing a positioning basis for subsequent centralized sampling, structural analysis, and micro-damage judgment, avoiding indiscriminate processing of full-field data, and thereby improving the efficiency of strain analysis.

[0043] (3) The present invention provides a tensile test method for metal plates. By vectorizing the displacement of all feature points in each frame of the image, a complete strain field of the entire sample coating area at different loading stages can be constructed, which can reflect the strain concentration trend of the local area and help to dynamically analyze the stress transfer path and the local softening area of the material.

[0044] (4) The present invention induces the specimen to fracture in the coating area by coating the coating on the coating area, thereby ensuring the appearance of the initial deformation point in the coating area, helping to control the fracture position to be concentrated in the set speckle detection area, facilitating the video extensometer to accurately capture the strain evolution process, avoiding indiscriminate processing of full-field data, and being able to effectively gather and uniformly conduct heat energy under laser speckle irradiation, avoiding the local hot spots from causing irreversible changes to the microstructure of the base metal, thereby maintaining the structural integrity and original mechanical properties of the specimen body. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments described in the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive efforts.

[0046] Figure 1 is a flow chart of a tensile testing method for metal plates according to the present invention;

[0047] Figure 2 is a schematic diagram of a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0048] The determination of the initial deformation point refers to the point at which, during the tensile loading process, a characteristic point of the specimen produces the fastest strain growth per unit time, and the growth process is concentrated and has strong mutation. This point marks the first location where local instability or necking tendency occurs, and is a sensitive precursor area for the evolution of plastic deformation.

[0049] Specifically, it should be noted that the initial deformation point is not the point where deformation begins earliest, nor the point where all deformation is completed earliest, but the point where the strain grows most rapidly and the deformation process is most concentrated per unit time. Therefore, the determination of the initial deformation point needs to be combined with the time dimension.

[0050] Determining the initial deformation point can provide early warning of the location where material failure is about to occur. It is the key basis for predicting ultimate performance, constructing stress-strain models, and improving material processing technology.

[0051] like Figure 1 and Figure 2 As shown, a tensile test method for metal plates comprises the following steps:

[0052] Step S1, applying a coating on the coating area of the sample to be observed, and clamping it in a tensile testing machine;

[0053] Step S2: forming a lateral optical observation path by a laterally arranged plane reflector optical system;

[0054] Step S3, non-contact projecting a randomly distributed high-density speckle pattern onto the sample coating area and recording it as a feature point;

[0055] Step S4: start the testing machine to stretch the specimen, and dynamically obtain a deformation image sequence of characteristic points of the coating area during the entire loading process;

[0056] Step S5: performing image processing on the image sequence, extracting two-dimensional displacement information of optical speckle feature points, and constructing a spatial displacement field of feature points in the sample coating area;

[0057] Step S6: Based on the extracted characteristic point displacement data, the full-field strain tensor distribution of the sample coating area at each loading stage is calculated, and the initial deformation point is identified through the characteristic displacement mutation.

[0058] In the present invention, in step S1, a sample is obtained by punching a metal plate, the edge of the sample is polished, cleaned with 75% alcohol, and dried at room temperature for 5-8 minutes;

[0059] After the drying is completed, the observed coating area of the sample is coated and dried at room temperature for 25-30 minutes.

[0060] Specifically, a standard stamping process was used to prepare dumbbell-shaped metal sheet specimens according to ISO 6892. After stamping, the edges of the specimens were deburred and polished with metallographic sand to ensure that the edges were smooth and free of crack sources.

[0061] In the present invention, in step S2, the three-dimensional coordinates (X, Y, Z) of the center position of the sample coating area are obtained, wherein the X axis is the stretching direction, the Y axis is the sample width direction, and the Z axis is the sample thickness direction;

[0062] Place the plane reflector at a 45° angle on the back of the sample to reflect the deformation information on the back of the sample.

[0063] By reflecting the back of the specimen through a plane mirror, the video extensometer can monitor both the front and back of the specimen simultaneously, thereby improving the integrity of the strain identification area while reducing system complexity and cost.

[0064] A lateral optical observation path is constructed through a laterally arranged plane mirror optical system to realize the reflective collection of deformation information on the back side of the specimen. By utilizing the refraction of the optical path, the deformation on the back side of the specimen, which was originally difficult to observe directly, can be captured laterally by high-resolution camera equipment, thereby obtaining three-dimensional deformation information of the specimen during the stretching process. By simultaneously combining two-dimensional optical observations of the X-axis (stretching direction) and the Y-axis (width direction), all-round monitoring of the specimen deformation is formed. This not only avoids the image distortion caused by light reflection in traditional direct 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 validity of the entire tensile test data.

[0065] 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. The formed light spots are used as feature points to construct a traceable optical feature point array.

[0066] The characteristic points in the tensile test of metal sheets are determined by non-contact projection speckle mapping on the surface of the sample. It will not generate additional stress concentration on the surface of the sample 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.

[0067] Specifically, the laser beam is expanded into a number of speckles through a laser diffuser and mapped onto the coating area of the sample, and the position of the speckle is used as the feature point;

[0068] The laser light sources are set at two locations, one on the front and one on the back of the sample, and are evenly distributed within the pre-set coating area to enhance image contrast and feature point distribution density.

[0069] In summary, in step S3, a randomly distributed high-density speckle pattern is non-contact projected onto the coated area of the sample to form dense and evenly distributed light 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, it ensures that the speckle pattern is randomly and densely distributed on the sample surface, forming an array of optical feature points with rich details. The feature points can be clearly captured by high-resolution camera equipment during the stretching process, which facilitates accurate identification and matching by image processing software, improves the spatial resolution and tracking accuracy of the feature points, effectively enhances the meticulousness and accuracy of the displacement field and strain field calculation, and ensures that the tiny deformations and strain mutations occurring in the sample during the loading process can be keenly captured.

[0070] In a preferred embodiment of the present invention, in step S4, uniaxial tension is applied to the sample at a set loading rate, and the loading method includes: constant displacement loading, staged loading, or stress-strain feedback controlled loading;

[0071] When the specimen is under tension, a video extensometer is used to record the initial position of the characteristic point. An image sequence of the speckle pattern in the coating area is collected at preset time intervals. Each frame of the image records the actual position of the characteristic point at the corresponding moment.

[0072] In step S4, the image sequence covers the entire stretching stage, including: the initial stage, the elastic stage, the yield stage, the plastic stage and the fracture stage.

[0073] In step S4, a uniaxial tensile load is applied to the specimen by starting the tensile testing machine, and a sequence of deformation images of characteristic points in the coated area of the specimen is dynamically acquired throughout the loading process. Specifically, the testing machine performs constant displacement loading, staged loading, or stress-strain feedback controlled loading at a set loading rate to ensure that the tensile process is controllable and stable. During the loading process, a high-resolution camera is used to continuously capture a sequence of speckle pattern images covering the entire tensile stage (including the initial, elastic, yield, plastic, and fracture stages) at preset time intervals. Each frame of the image records the precise position changes of the characteristic points.

[0074] By continuously and frequently collecting deformation images, it is possible to fully capture the subtle deformation evolution information of the specimen from the initial state to the entire process of destruction, providing continuous data support for subsequent image processing and strain analysis. The acquisition of dynamic image sequences can then identify the state of the specimen at different loading stages, making it easier to identify the yield point of the material, the starting position of local deformation concentration, and the fracture development trend, thereby improving the accuracy and reliability of the tensile test results. At the same time, this non-contact optical observation avoids the specimen damage or interference that may be caused by traditional contact measurement, and realizes real-time and comprehensive monitoring of the deformation behavior of metal sheets.

[0075] In the present invention, step S5 includes the following sub-steps:

[0076] Step S51: Number each feature point and record its initial position in the initial image in the image sequence. , and the new position of each frame in the image sequence ;

[0077] Step S52: For the feature point, calculate its 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;

[0078] Step S53: Using the coating area of the sample as the observation surface, construct a discrete spatial displacement field consisting of all characteristic point displacement vectors. ,in, Represents the spatial displacement field of all feature points at time t.

[0079] In step S5, the two-dimensional displacement information of the speckle feature points in the coating area is extracted by performing image processing on the deformation image sequence collected in step S4, and then a spatial displacement field of the feature points in the sample coating 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 size of the feature point. With the sample coating area as the observation surface, the displacement data of all feature points are integrated to establish a discrete spatial displacement field, which comprehensively reflects the deformation distribution and change trend of different positions on the sample surface.

[0080] By calculating the surface deformation information of the specimen during the tensile test, spatial mapping and dynamic tracking of the local and overall deformation of the metal sheet during the tensile process are achieved. By constructing the characteristic point displacement field, not only the deformation is monitored, but also a basis is provided for the subsequent calculation of the full-field strain tensor and the identification of deformation mutation points.

[0081] In a preferred embodiment of the present invention, step S6 includes the following sub-steps:

[0082] Step S61: Based on the discrete displacement field obtained in step S5, extract the current coordinate position of the feature point in each time frame, record the position of the feature point in each frame of the deformed image sequence, and calculate the local strain tensor;

[0083] Step S62: Based on the set strain time threshold, screening suspected initial deformation points and constructing a set of suspected initial deformation points;

[0084] Step S63: For the suspected initial deformation point, calculate its deformation start time, strain growth rate and cumulative deformation, and establish a scoring mechanism to determine the initial deformation point.

[0085] Constructing a set of suspected initial deformation points includes the following sub-steps:

[0086] Step S621: extract the strain sequence of each feature point on each frame of the image and construct a strain-time curve of each feature point;

[0087] Step S622: Set a strain time threshold to determine whether the time period between the initial deformation and the deformation peak of each feature point is within the strain time threshold. If so, the feature point is determined to be a suspected initial deformation point and its starting deformation time is recorded. If not, the feature point is determined not to be a suspected initial deformation point. If the time period between the initial deformation and the deformation peak of all feature points is not within the strain time threshold, the monitored feature point is determined to be an initial deformation point.

[0088] Step S623: Integrate the acquired feature points in the order of the initial deformation time to construct a set of suspected initial deformation points.

[0089] Among them, the strain time threshold is set, specifically recording the time point after the deformation of the feature point, selecting the feature point at the earliest time point, and taking 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;

[0090] 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;

[0091] Among them, the strain growth rate is based on the strain-time curve, recording the strain of the feature point from the initial deformation to the main deformation peak, as well as the time of deformation in this stage. Among them, the strain of the feature point from the initial deformation to the main deformation peak is the cumulative deformation, specifically: Specifically, where Vi represents the strain growth rate per unit time, reflecting the sudden change of deformation, It represents the strain of the feature point from the initial deformation to the main deformation peak. Indicates the time it takes for the deformation to occur in this stage.

[0092] According to the determined suspected initial deformation point, the suspected initial deformation point is scored ,in Represents the weight coefficient, which is adjusted according to experimental experience. The earlier the deformation time, the higher the score. It represents the strain of the feature point from the initial deformation to the main deformation peak.

[0093] In step S6, by setting the strain time threshold, several feature points are classified, and all feature points are sorted by response time. The feature points with the earliest response and the most active deformation are quickly screened out from a large range, and then the feature points of some suspected initial deformation points are determined to avoid misidentification caused by local stress. Through the scoring mechanism, each suspected deformation point is quantitatively scored from multiple dimensions such as the starting strain time, strain growth rate, and cumulative deformation, and sorted according to the comprehensive score. Finally, the feature point where the earliest rapid strain change occurs during the loading process is identified, so that the initial local plastic concentration area is quickly located before the deformation extends to the entire area, providing a positioning basis for subsequent centralized sampling, structural analysis, and micro-damage judgment, avoiding indiscriminate processing of full-field data, and thereby improving the efficiency of strain analysis.

[0094] It should be noted that the preparation of the coating includes the following steps:

[0095] A. Mix polyether polyol, 1,4-butanediol and dimethylol propionic acid in proportion and heat to 70-80°C, then add diisocyanate dropwise, continue heating to 80-90°C and maintain for 2-2.5 hours to obtain a prepolymer, cool the prepolymer to 45-50°C and add ammonia water for neutralization reaction;

[0096] Slowly add the neutralized prepolymer dropwise into the deionized water with high-speed stirring and continue stirring for 5-10 minutes;

[0097] After the prepolymer is stirred, methyl methacrylate is added, and the temperature is raised to 70-80°C and maintained for 2-3 hours to obtain a polyurethane-acrylate block copolymer emulsion;

[0098] The mass proportions of the copolymer emulsion material are as follows: 15-20% polyether polyol, 5-10% 1,4-butanediol, 10-12% diisocyanate, 2-3% dihydroxymethylpropionic acid, 1-2% ammonia water, 25-30% methyl methacrylate, and 20-30% deionized water, wherein the ammonia water concentration is 28%.

[0099] B. Take 5-10% by mass of carbon black, 10-15% by mass of graphene microplatelets, 2-5% by mass of SiO2, and 2-5% by mass of TiO2, and add them to 70-75% by mass of copolymer emulsion and stir for 20-25 minutes to form a stable homogeneous slurry;

[0100] Spray on the coating area of the sample, control the film thickness in the range of 8-15μm, and dry it with hot air at 70-80℃ for 10-15min to form a coating on the surface of the sample.

[0101] Verify the difference between coated and uncoated specimens in tensile testing.

[0102] The polyether polyol is from Wanhua Chemical Group Co., Ltd., model number is WANNATE® PE-330N;

[0103] 1,4-Butanediol was obtained from Shandong Dongying Haike Chemical Co., Ltd., model BDO-99;

[0104] Dimethylolpropionic acid is from Anhui Xinzhongyuan Chemical Technology Co., Ltd., model number is XZY-DMPA;

[0105] Diisocyanate is from Wanhua Chemical Group Co., Ltd., model number is WANNATE® HT-100;

[0106] Methyl methacrylate was obtained from Sinopec Shanghai Petrochemical Company, model number MMA-99;

[0107] Carbon black was sourced from Jiangxi Black Cat Carbon Black Co., Ltd., model N330;

[0108] Graphene microplatelets were sourced from Changzhou Sixth Element Materials Technology Co., Ltd., model SE1232;

[0109] Nano-SiO2 was obtained from Quecheng Silicon Chemical Co., Ltd., model number Aerosil® 200, particle size 12 nm;

[0110] Nano-TiO2 was sourced from Panzhihua Iron and Steel Group Vanadium and Titanium Resources Co., Ltd., model P25, with a particle size of 21 nm.

[0111] Example 1:

[0112] 15% polyether polyol, 5% 1,4-butanediol, and 2-3% dimethylol propionic acid were mixed in proportion and heated to 75°C, and 12% diisocyanate was added dropwise, and the temperature was continued to rise to 85°C and maintained for 2.5 hours to obtain a prepolymer. The prepolymer was cooled to 50°C, and 2% ammonia water was added for neutralization reaction. The neutralized prepolymer was slowly added dropwise to 30% deionized water stirred at high speed and stirred for 10 minutes. After the stirring was completed, 30% methyl methacrylate was added to the prepolymer, and the temperature was raised to 80°C and maintained for 2 hours to obtain a polyurethane-acrylate block copolymer emulsion;

[0113] Take 10% carbon black, 15% graphene microplatelets, 5% SiO2, and 3% TiO2 by mass, add them to 75% copolymer emulsion and stir for 25 minutes to form a stable homogeneous slurry, spray it on the coating area of the sample, control the film thickness within the range of 9±0.5μm, and dry it with hot air at 80℃ for 15 minutes to obtain a coating;

[0114] Among them, 6061 aluminum alloy plates with a thickness of 2 mm were used, and standard dumbbell-shaped specimens were processed according to ISO 6892-1. The gauge length was 50 mm × 20 mm, and the coating was applied at two-thirds of the specimen length.

[0115] Example 2:

[0116] A 6061 aluminum alloy plate with a thickness of 2 mm was used to process a standard dumbbell-shaped specimen according to ISO 6892-1, with a gauge length of 50 mm × 20 mm.

[0117] Implementation steps:

[0118] 1. Testing the mechanical properties of the samples: Prepare a WDW-100 electronic universal testing machine and an Instron 2620 series dynamic extensometer, clamp the samples in Example 1 and Example 2 for tensile tests, and test the mechanical properties of the samples using GB / T 228.1-2021.

[0119] The performance of the samples in Example 1 and Example 2 in the tensile test was verified, as shown in Table 1.

[0120] Table 1:

[0121]

[0122] As can be seen from Table 1, the coating can induce deformation of the specimen in the coated area, and the coated specimen will not affect the tensile test of the specimen during the tensile test. This is mainly because the coating is locally attached with micro-nanoparticles, and the particles form an irregularly distributed micropore / microcluster structure on the microscale. The slight difference in local stiffness will cause a very small difference in elastic modulus, making this area more prone to micro-yielding or plastic slip than the surrounding matrix under external load, thus manifesting as "strain first" on a macro scale, guiding the priority formation of the necking zone.

[0123] It was verified that the samples in Example 1 and Example 2 absorb heat from laser, as shown in Table 2.

[0124] 2. Heat Dissipation Performance Test: For the samples in Example 1 and Example 2, the sample in Example 1 was continuously irradiated with a laser on the coated area of the sample for 30 seconds, and the sample in Example 2 was continuously irradiated with a laser at the center position of the sample for 30 seconds. The samples were monitored using a FLIR A655sc infrared thermal imager, and the mechanical properties of the samples were simultaneously tested.

[0125] Table 2:

[0126]

[0127] As shown in Table 2, after laser irradiation, the heating rate of Example 1 was low and did not affect the mechanical properties of the sample, while the heating rate of Example 2 was high and its mechanical properties were reduced. This was mainly because graphene microplatelets and carbon black were introduced into the coating applied in Example 1. The former has extremely high in-plane thermal conductivity and can quickly achieve lateral heat diffusion after the heat generated by laser irradiation, while the latter improves the coating's absorption efficiency of the laser, concentrating the heat in the coating area without penetrating into the deep layers of the substrate. At the same time, the composite effect of TiO2 and SiO2 nanoparticles further enhanced the heat reflection and buffering capabilities, forming a stable heat absorption-diffusion-shielding multi-stage thermal control network;

[0128] In the second embodiment, the metal surface directly absorbs the laser energy to form a local hot spot. This hot spot causes grain coarsening, intensified dislocation slip, and even local annealing or ablation, affecting the tensile properties of the metal.

[0129] A fully automatic tensile testing machine for metal sheets, and a tensile testing method for metal sheets, comprising: a tensile testing machine control system, and equipped with 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;

[0130] The observation module includes a reflector arranged at the rear side of the clamping position of the tensile testing machine, which captures the strain image of the specimen during the stretching process by constructing an observation path on the back side of the specimen;

[0131] The speckle generation module generates characteristic points to be monitored by projecting random high-density speckle patterns onto the front and back of the sample using a laser projection device;

[0132] The image acquisition module uses a video extensometer to collect images of the sample in real time during the tensile test, and integrates the images in time sequence to form a deformation image sequence;

[0133] 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, and construct a displacement vector field composed of all feature points to form a discrete spatial displacement field data structure;

[0134] The strain field analysis module performs local strain tensor analysis on the position of feature points in each frame image based on the displacement vector obtained by the displacement field calculation module, and establishes the strain tensor using the displacement gradient between adjacent points;

[0135] 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 of the suspected initial deformation points, and establishes a scoring mechanism to determine the true initial deformation point among the suspected initial deformation points.

[0136] The above description is based on the ideal embodiment of the present invention. Based on the above description, relevant personnel can make various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the content of the specification and must be determined according to the scope of the claims.

Claims

1. A tensile test method for metal sheets, characterized in that: The following steps are involved: Step S1: applying a coating to the coating area of the sample to be observed, and clamping the sample in a tensile testing machine, wherein the coating preparation includes: Mix 15-20% by weight of polyether polyol, 5-10% by weight of 1,4-butanediol, and 2-3% by weight of dimethylolpropionic acid in proportion and heat to 70-80°C. Then, add 10-12% by weight of diisocyanate dropwise and continue heating to 80-90°C and maintain for 2-2.5 hours to obtain a prepolymer. Then, cool the prepolymer to 45-50°C and add aqueous ammonia for neutralization reaction. Slowly add the neutralized prepolymer dropwise into the deionized water with high-speed stirring and continue stirring for 5-10 minutes; After the prepolymer is stirred, 25-30% methyl methacrylate is added, and the temperature is raised to 70-80°C and maintained for 2-3 hours to obtain a polyurethane-acrylate block copolymer emulsion; Take 5-10% carbon black, 10-15% graphene microplatelets, 2-5% SiO2, and 2-5% TiO2 by mass, add them to 70-75% copolymer emulsion and stir for 20-25 minutes to form a stable homogeneous slurry and spray it on the coating area. Control the film thickness to 8-15μm and dry it at 70-80℃ for 10-15 minutes. Step S2: forming a lateral optical observation path by a laterally arranged plane reflector optical system; Step S3, non-contact projecting a randomly distributed high-density speckle pattern onto the sample coating area and recording it as a feature point; Step S4: start the testing machine to stretch the specimen, and dynamically obtain a deformation image sequence of characteristic points of the coating area during the entire loading process; Step S5: performing image processing on the image sequence, extracting two-dimensional displacement information of optical speckle feature points, and constructing a spatial displacement field of feature points in the sample coating area; Step S6: Based on the extracted characteristic point displacement data, the full-field strain tensor distribution of the sample coating area at each loading stage is calculated, and the initial deformation point is identified through the characteristic displacement mutation.

2. A tensile testing method for metal sheets according to claim 1, characterized in that: In step S1, a sample is obtained by punching a metal plate, the edge of the sample is polished, cleaned with 75% alcohol, and dried at room temperature for 5-8 minutes; After the drying is completed, the observed coating area of the sample is coated and dried at room temperature for 25-30 minutes.

3. The tensile testing method for metal sheets according to claim 1, characterized in that: In step S2, the three-dimensional coordinates (X, Y, Z) of the center position of the sample coating area are obtained, wherein the origin of the coordinate system is the center position of the coating area, the X axis is the stretching direction, the Y axis is the sample width direction, and the Z axis is the sample thickness direction. The coordinate system is a fixed global coordinate system; Place the plane reflector at a 45° angle on the back of the sample to reflect the deformation information on the back of the sample.

4. A tensile testing method for metal sheets according to claim 1, characterized in that: 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. The formed light spots are used as feature points to construct a traceable optical feature point array.

5. The tensile testing method for metal sheets according to claim 1, characterized in that: In step S4, uniaxial tension is applied to the sample at a set loading rate, and the loading method includes: constant displacement loading, staged loading or stress-strain feedback control loading; When the specimen is under tension, the initial position of the characteristic point is recorded, and an image sequence of the speckle pattern in the coating area is collected at preset time intervals. Each frame of the image records the actual position of the characteristic point at the corresponding moment.

6. The tensile testing method for metal plates according to claim 1, characterized in that: In step S4, the image sequence covers the entire stretching stage, including: the initial stage, the elastic stage, the yield stage, the plastic stage and the fracture stage.

7. The tensile testing method for metal plates according to claim 1, characterized in that: In the step S5, the following sub-steps are included: Step S51: Number each feature point and record its initial position in the initial image in the image sequence. , and the new position of each frame in the image sequence ; Step S52: For the feature point, calculate its 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: Using the coating area of the sample as the observation surface, construct a discrete spatial displacement field consisting of all characteristic point displacement vectors. ,in, Represents the spatial displacement field of all feature points at time t.

8. The tensile testing method for metal plates according to claim 1, characterized in that: In the step S6, the following sub-steps are included: Step S61: Record the position 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, screening suspected initial deformation points and constructing a set of suspected initial deformation points; Step S63: For the suspected initial deformation point, according to the formula Score and select the feature point with the highest score as the initial deformation point, where Represents the weight coefficient, which is adjusted according to test experience. Vi represents the strain growth rate per unit time, reflecting the mutation of deformation. The earlier the deformation time, the higher the score. It represents the strain of the feature point from the initial deformation to the main deformation peak.

9. A tensile testing method for metal plates according to claim 8, characterized in that: In 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 a strain-time curve of each feature point; Step S622: Set a strain time threshold to determine whether the time between the initial deformation and the deformation peak of each feature point is less than or equal to the strain time threshold. If so, the feature point is determined to be a suspected initial deformation point and its starting deformation time is recorded. If not, the feature point is determined not to be a suspected initial deformation point. Among them, the difference between the initial deformation moment and the deformation peak moment of the feature point at the earliest time point is the strain time threshold; Step S623: Integrate the acquired feature points in the order of the initial deformation time to construct a set of suspected initial deformation points.

10. A fully automatic tensile testing machine for metal sheets, based on the tensile testing method for metal sheets according to any one of claims 1 to 9, characterized in that: include: Tensile testing machine control system, as well as the observation module, speckle generation module, image acquisition module, displacement field calculation module, strain field analysis module, and initial deformation point identification module; The observation module includes a reflector arranged at the rear side of the clamping position of the tensile testing machine, which captures the strain image of the specimen during the stretching process by constructing an observation path on the back side of the specimen; The speckle generation module generates characteristic points to be monitored by projecting random high-density speckle patterns onto the front and back of the sample using a laser projection device; The image acquisition module uses a video extensometer to collect images of the sample in real time during the tensile test, 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, and construct a displacement vector field composed of all feature points to form a discrete spatial displacement field data structure; The strain field analysis module performs local strain tensor analysis on the position of feature points in each frame image based on the displacement vector obtained by the displacement field calculation module, and establishes the 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 of the suspected initial deformation points, and establishes a scoring mechanism to determine the true initial deformation point among the suspected initial deformation points.

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