Device and method for measuring shrinkage rate of thermosetting material based on digital image correlation method

Abstract

The invention relates to the technical field of thermosetting material testing, in particular to a thermosetting material shrinkage rate measuring device and method based on a digital image correlation method. The device comprises a data acquisition unit and a data processing unit, the data acquisition unit comprises a heating platform, a temperature detection device, a temperature controller, a structure or a material (such as a lubricating medium) for reducing out-of-plane deformation and an image acquisition device, and the data processing unit can calculate the curing shrinkage rate and thermal expansion coefficients in different curing states according to temperature and image data. The thermal expansion coefficient and the curing shrinkage rate of the decoupled sample can be obtained after a single experiment, the time for switching multiple devices and repeatedly loading the sample is saved, the curing shrinkage strain and the thermal expansion strain of a plurality of samples can be measured at the same time, the measurement efficiency is remarkably improved, and the device and the method are suitable for rapid detection of batch samples.

Description

Technical Field

[0001] This invention relates to the field of thermosetting materials testing technology, specifically to a device and method for measuring the shrinkage rate of thermosetting materials based on digital image correlation. Background Technology

[0002] During the curing process, thermosetting materials experience thermal shrinkage and curing shrinkage due to temperature and degree of cure changes, resulting in residual stress and deformation that affect the quality of thermosetting material products. Accurately measuring the shrinkage rate of thermosetting materials helps in calculating the thermal strain and curing shrinkage strain of components during finite element molding simulation, thereby predicting the curing deformation of thermosetting materials.

[0003] However, during the molding process of thermosetting materials, the curing reaction is usually accompanied by thermal expansion due to temperature rise. This causes the curing shrinkage and thermal expansion of thermosetting materials to be coupled and difficult to separate. Furthermore, current devices for measuring the shrinkage rate of thermosetting materials typically can only measure a single material in a single direction, and cannot simultaneously measure the curing shrinkage and thermal shrinkage of multiple materials in different directions. Therefore, this application proposes a device and method for real-time measurement of the shrinkage rate of thermosetting materials to address the above problems. Summary of the Invention

[0004] One object of the present invention is to solve at least the above-mentioned problems and / or defects. In order to solve the above problems, the present invention provides a device and method for measuring the shrinkage rate of thermosetting materials based on digital image correlation.

[0005] To achieve the aforementioned objectives of the present invention: In a first aspect, the present invention provides a measuring device, comprising a data acquisition unit and a data processing unit; the data acquisition unit includes: a heating platform supporting one or more test samples for heating the test samples; a structure or device placed on the heating platform for reducing out-of-plane deformation of the test samples; a temperature controller for executing a preset three-stage temperature program of heating, holding, and cooling; a temperature detection device for detecting real-time temperature data of the test samples; and an image acquisition device for acquiring projected image data of the surface of the test samples on the plane of the heating platform; the data processing unit is integrated into a computer and communicatively connected to the temperature detection device and the image acquisition device; the data processing unit is used to acquire curing degree data and strain data of the test samples based on the acquired temperature data and image data; and to calculate the curing shrinkage rate and the coefficient of thermal expansion of the test samples in different curing states based on the strain data, temperature data, and curing degree data at each temperature stage.

[0006] In one embodiment of the present invention, the material of the test sample may be a thermosetting resin, or a reinforced composite material composed of or at least partially composed of a thermosetting resin. To ensure measurement accuracy, the test sample is constructed as a thin sheet of uniform thickness during measurement, and the height of the sheet along the thickness direction should be sufficiently small, controlled to be less than 0.5 mm, to reduce the risk of out-of-plane deformation of the test sample during curing.

[0007] In one embodiment of the present invention, the structure or material for reducing out-of-plane deformation of the sample under test can be a low-friction interface component disposed between the sample under test and the heating platform. The low-friction interface component can reduce the interface frictional resistance between the sample under test and the heating platform through physical isolation, material lubrication, etc., to ensure measurement accuracy. Its specific forms include, but are not limited to, lubricating media, low-friction coatings, or heating-resistant films.

[0008] Furthermore, the lubricating medium includes, but is not limited to, any one or a combination of Teflon, polyimide, etc.

[0009] In one embodiment of the present invention, the data processing unit is an independent software and / or hardware system, and the data processing unit is configured to: perform digital image correlation analysis on image data to calculate the strain data of the sample under test in the plane; calculate the degree of curing data of the sample under test based on a pre-stored curing kinetic model and real-time temperature data; and calculate the curing shrinkage rate and the coefficient of thermal expansion of the sample under test in different curing states by combining the strain data, temperature data and degree of curing data.

[0010] Furthermore, the data processing unit includes a curing degree calculation module, an image strain analysis module, and a data fitting module. The curing degree calculation module receives the collected temperature data and calls a pre-stored curing kinetic model to calculate the curing degree data of the tested sample. The image strain analysis module receives the collected image data and uses a digital image correlation algorithm to calculate the strain data of the tested sample in the orthogonal direction within the plane at each stage. The data fitting module performs fitting by associating the curing degree data, strain data, and temperature data of the tested sample to obtain the curing shrinkage rate and thermal expansion coefficient of the tested sample. Here, "within the plane" refers to a two-dimensional plane parallel to the surface of the tested sample and the surface of the heating platform; "orthogonal direction" refers to two mutually perpendicular coordinate axes in a two-dimensional coordinate system established within the plane.

[0011] In one embodiment of the present invention, the image acquisition device includes a camera, a fixing device, and an adjustable supplementary lighting device. The camera is used to acquire images of the sample under test at different temperature stages at regular intervals. The fixing device is used to fix the camera above the heating platform and keep the camera position unchanged to ensure that the camera captures the projected image data of the surface of the sample under test on the plane where the heating platform is located. The light intensity of the adjustable supplementary lighting device is adjustable to ensure that the outline of the sample under test is clear.

[0012] In one embodiment of the present invention, the heating platform can carry multiple test samples, and the image acquisition device can acquire images of multiple test samples in two orthogonal directions in a plane in parallel to simultaneously calculate the curing shrinkage rate and thermal expansion coefficient of multiple test samples, thereby significantly improving the measurement efficiency.

[0013] In one embodiment of the present invention, the heating rate during the heating stage is confirmed by differential scanning calorimetry, specifically including measuring the degree of curing of the sample under test when it reaches the holding temperature at different heating rates, selecting the heating rate when the degree of curing is less than 3% when entering the holding stage as the heating rate of the temperature controller; at the same time, the curing kinetic equation of the thermosetting resin at this heating rate is extracted.

[0014] In one embodiment of the present invention, the temperature controller has a preset three-stage temperature program of heating, holding and cooling as follows: Heating stage: a relatively fast heating rate is adopted to suppress significant curing reaction during the heating process; Holding stage: the temperature is maintained at the preset curing temperature, and the holding time ensures that the sample under test is completely cured before entering the cooling stage; Cooling stage: set to natural cooling.

[0015] In one embodiment of the present invention, the data processing unit calculates the curing shrinkage rate and the coefficient of thermal expansion of the tested sample under different curing states based on the strain data, temperature data and curing degree data at each temperature stage: the coefficient of thermal expansion of the tested sample when the curing degree is close to 0% and 100% is obtained in the heating stage and cooling stage respectively, and the curing shrinkage coefficient of the tested sample is obtained in the heat preservation stage.

[0016] In a second aspect, the present invention provides a measurement method using the above-described measuring device, comprising: S11. Prepare one or more sets of test sample materials into thin sheets and place them on a heating platform for heat treatment; S12. Acquire real-time temperature data of the sample under test during the three stages of heating, holding and cooling, and collect image data of the sample surface in the orthogonal direction in the plane during the three temperature stages. S13. Obtain curing degree data based on curing kinetic equation and real-time temperature data; calculate strain data of the tested sample in two orthogonal directions in the plane at each temperature stage based on digital image correlation method and image data. S14. Based on the strain data, temperature data, and degree of curing data obtained at each temperature stage, calculate the curing shrinkage rate and the coefficient of thermal expansion for one or more tested samples under different curing states.

[0017] As a preferred embodiment, the step of acquiring strain data of the test sample in two orthogonal directions in a plane based on digital image correlation and image data includes the following steps: S21. Use the first image of each temperature stage as the reference image and the remaining images as comparison images; S22. Set the area where the sample is located in the image as the region of interest, and set coordinate axes in two orthogonal directions in the strain direction of interest. S23. Track the displacement changes of regions of interest in an image using feature matching algorithms; S24. After subpixel-level displacement field calculation, the strain field in the plane of the measured sample along the coordinate axis in two orthogonal directions at each temperature stage is obtained by quantization through the strain-displacement differential relationship. S25. Export the obtained strain data; S26. Remove outlier data points from the strain data; S27. Take the average value of the strain data of different samples as the average strain at the current moment; S28. Check whether the average strain calculation of the two orthogonal directions in the plane of all the tested samples has been completed. Repeat the calculation until the average strain data of all stages are obtained. S29. Obtain the strain curves of the two orthogonal directions in the plane to be extracted at different temperature stages as a function of time.

[0018] As a preferred embodiment, the calculation of the curing shrinkage rate and the coefficient of thermal expansion of the tested sample under different curing states based on the obtained strain data, temperature data, and degree of curing data at each temperature stage includes: A. Based on the strain and temperature change data during the heating stage, the thermal expansion strain is extracted through the linear relationship between strain and temperature, and the thermal expansion coefficient of the uncured state is calculated. B. Based on the strain and curing degree data during the heat preservation stage, chemical shrinkage strain is extracted through the linear relationship between strain and curing degree, and the curing shrinkage coefficient is calculated. C. Based on the strain and temperature change data during the cooling stage, the thermal expansion strain is extracted through the linear relationship between strain and temperature, and the thermal expansion coefficient of the fully cured state is calculated.

[0019] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The present invention uses a preset three-stage temperature program of heating, holding and cooling, combined with the independent analysis of the strain field of each stage by digital image correlation method, so that the strain in the heating stage only reflects thermal expansion, the strain in the holding stage only reflects curing shrinkage, and the strain in the cooling stage only reflects thermal expansion after complete curing, thereby effectively separating the thermal expansion and curing shrinkage during the curing process of thermosetting materials.

[0020] (2) Compared with traditional measurement methods, this invention does not require the installation of sensors on the surface of the sample to measure the curing shrinkage rate and thermal expansion strain of thermosetting materials, so as to avoid the additional stress and deformation interference introduced by contact measurement, and ensure that the strain measurement results reflect the thermal expansion and curing shrinkage of the material itself, thus ensuring the reliability of the measurement data.

[0021] (3) The present invention can obtain the thermal expansion coefficient and curing shrinkage rate of the tested sample after a single experiment. Compared with the current method of measuring the thermal expansion coefficient using a thermomechanical analyzer (TMA) and measuring the curing shrinkage rate by the plunger method or volume dilatometer, it saves the time of switching multiple devices and repeated sample loading. At the same time, the present invention is based on the digital image correlation method. Compared with the current method of testing a single sample with a single set of equipment, it can simultaneously measure the curing shrinkage strain and thermal expansion strain of multiple samples, which significantly improves the overall efficiency and is suitable for rapid detection of batch samples.

[0022] (4) By controlling the sample to be tested into a thin sheet, the thickness of the sheet is much smaller than the plane size, so the strain can be approximated as mainly occurring in the plane, and the deformation in the out-of-plane direction can be ignored, which facilitates accurate measurement of plane strain data by the DIC method; and by further eliminating out-of-plane deformation through the lubricating medium, the strain measurement results are ensured to truly reflect the response of thermal expansion or curing contraction, thus improving the measurement accuracy.

[0023] (5) The core equipment of the present invention is a camera, a common computer and a heating platform. Compared with the traditional measurement method using devices such as DMA and TMA, the equipment cost is significantly reduced. At the same time, the requirements for the experimental environment are relatively relaxed, and there is no need for strict experimental environment requirements, which expands the scope of application and is suitable for various scenarios such as laboratory research and development and industrial production. Attached Figure Description

[0024] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0025] Figure 1A schematic diagram of a device for measuring the shrinkage rate of thermosetting materials based on digital image correlation method provided by the present invention; Figure 2 This is a schematic diagram of the data processing unit structure provided by the present invention; Figure 3 A schematic flowchart of the shrinkage rate measurement method for thermosetting materials based on digital image correlation provided by the present invention; Figure 4 The strain field data processing flowchart provided by this invention; Figure 5 This is a graph showing the results of the thermal expansion coefficient measurement in this invention; Figure 6 This is a graph showing the results of the curing shrinkage coefficient measurement for this invention. Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] The terms "first," "second," and "third" used in this invention are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first," "second," or "third" may explicitly or implicitly include multiple such features. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. All directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of this application are only used to explain the relative positional relationships and movements between components in a specific posture (as shown in the figures). If the specific posture changes, the directional indications also change accordingly. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or modules is not limited to the listed steps or modules, but may optionally include steps or modules not listed, or may optionally include other steps or modules inherent to these processes, methods, products, or devices.

[0028] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in multiple embodiments of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0029] Example 1, please refer to Figure 1 A measuring device, comprising a data acquisition unit 100 and a computer 400; The data acquisition unit 100 includes: a heating platform 110, a temperature controller 120, a lubricating medium 130, a temperature detection device 140, and an image acquisition device 150; wherein, The heating platform 110 is used to heat the sample 300 under test; the temperature controller 120 is used to control the heating platform 110 to execute a preset temperature program to provide the sample 300 under test with the ambient temperature required for the curing reaction. The lubricating medium 130 is located between the heating platform 110 and the sample 300 under test. By reducing the friction between the sample 300 under test and the heating platform 110, the out-of-plane deformation of the sample 300 under test during the curing reaction is reduced, thus ensuring the accuracy of the measurement. The temperature detection device 140 includes a temperature sensor and a data acquisition card; the temperature sensor may be a thermocouple, which is attached to the surface of the heating platform to measure the real-time temperature of the sample being tested; the data acquisition card reads the temperature of the thermocouple and connects to the computer 400 to record the temperature during the experiment. Image acquisition device 150 includes camera 151 and adjustable supplementary lighting device 152. Camera 151 is used to acquire images of the sample under test during the curing reaction process. The front of the camera lens is kept parallel to the surface of the heating platform so that the camera can capture the projected image data of the surface of the sample under test on the plane of the heating platform. The adjustable supplementary lighting device 152 is set as a light source at a suitable angle and direction around the heating platform to provide adjustable intensity illumination to the surface of the heating platform 110, ensuring that the outline of the sample being tested is clear. The computer 400 is communicatively connected to the data acquisition module 100, and receives real-time temperature data and image data acquired by the data acquisition unit 100, which is used to calculate the curing shrinkage rate and thermal expansion coefficient of the thermosetting material based on the real-time temperature data and image data. The communication connection can be a wireless connection module, such as a Bluetooth communication component, a WiFi communication component, or a 4G / 5G communication component, or a wired connection module, such as an optical fiber communication module or a cable communication module. This application does not impose any specific restrictions.

[0030] It is worth noting that the preset temperature program executed by the temperature controller includes three stages: heating, holding, and cooling, in order to separate the thermal expansion and curing shrinkage of thermosetting materials during the heating and curing process. Specifically: the heating rate during the heating phase is pre-calibrated using differential scanning calorimetry; Select a heating rate at which the degree of curing of the sample is less than or equal to 3% when it enters the heat preservation stage. This can be approximated as the curing reaction not having started significantly during the heating stage, and the strain during the heating stage only reflects the thermal expansion of the sample. During the heat preservation stage, the temperature is kept constant at the curing temperature of the sample being tested. During this process, the degree of curing gradually increases without temperature change, and the thermal strain is constant. At this time, the strain change mainly comes from the volume shrinkage of the curing reaction. The cooling stage is set to natural cooling. The heat preservation time of the aforementioned heat preservation stage ensures that the sample under test is completely cured when entering the cooling stage. The curing time can be determined according to the curing temperature of the sample under test. Therefore, the cooling stage only reflects the thermal expansion of the sample under test after it is completely cured.

[0031] It should be noted that the computer 400 in this embodiment is equipped with a data processing unit 200, which is used to calculate the curing shrinkage rate and thermal expansion coefficient of thermosetting materials based on real-time temperature data and image data. The data processing unit 200 may be an independent software and / or hardware system, and this embodiment does not impose specific limitations on it. Specifically, please see Figure 2 The data processing unit 200 includes a curing degree calculation module 210, an image strain analysis module 220, and a data module 230. The curing degree calculation module 210 receives the collected temperature data and calls the pre-stored curing kinetic model to calculate the curing degree data of the sample under test. The image strain analysis module 220 receives the acquired image data and uses the digital image correlation algorithm (DIC) to calculate the strain data in the orthogonal direction in the plane of the tested sample at each stage. The data fitting module 230 fits the curing shrinkage rate and thermal expansion coefficient of the tested sample by associating the curing degree data, strain data and temperature data of the tested sample.

[0032] The pre-stored curing kinetic model refers to the mathematical model that corresponds to the relationship between the degree of curing of the sample under test and temperature and time. It is used to calculate the change in the degree of curing of the sample under test during the measurement experiment. Both the model and the heating rate can be pre-calibrated by differential scanning calorimetry, which can be referred to ASTM E2070 for details, so they will not be elaborated further.

[0033] Furthermore, when measuring thermosetting materials using the above-mentioned apparatus, the following steps are included: S11. Prepare the test sample by preparing the thermosetting material to be tested, such as thermosetting resin or fiber-reinforced composite material composed of thermosetting resin, into a sheet structure with a thickness controlled within 0.5 mm to reduce the risk of out-of-plane deformation during the curing process of the test sample. S12. Determine the curing kinetic model and heating rate of the sample under test, including measuring the degree of cure of the sample material at different heating rates when reaching the holding temperature using differential scanning calorimetry, and selecting the heating rate when the degree of cure is less than or equal to 3% at the beginning of the holding stage as the heating rate of the temperature controller; simultaneously, extract the curing kinetic equation of the thermosetting resin at this heating rate: S13. Apply or lay a layer of high-temperature resistant lubricating medium, such as Teflon or polyimide, evenly on the surface of the heating platform to reduce friction between the sample and the platform. Place the sample flat on the lubricating medium and attach the thermocouple to the surface of the heating platform to ensure the real-time and accuracy of temperature detection. S14. Start the heating platform to heat the sample under test, and execute the preset three-stage temperature program through the temperature controller: heating stage, heating to the curing temperature corresponding to the sample under test at the calibrated heating rate; heat preservation stage, maintaining the curing temperature at a constant temperature until the sample under test is completely cured; cooling stage, turning off the heating after the sample under test is completely cured, and allowing it to cool naturally to room temperature. S15. At the beginning of the heating stage in step S14, the image acquisition device and temperature detection device are turned on to take pictures and detect the temperature at regular intervals, so as to obtain the projected images and temperature data of each temperature stage until the end of the experiment. The acquired data is recorded and stored in the computer. The image data and temperature data are processed by the data processing unit to obtain the curing shrinkage rate of the tested sample (based on the change of strain with curing degree during the heat preservation stage) and the coefficient of thermal expansion before curing (uncrosslinked) and after curing (fully crosslinked).

[0034] Example 2, please refer to Figure 1 To make the present invention clearer, the present invention also provides a method for measuring the shrinkage rate of thermosetting materials based on digital image correlation, comprising the following steps: S100. Prepare the test sample by preparing the thermosetting resin prepreg into a sheet with a thickness controlled below 0.5 mm. S200, Setting up the experimental system, including: Hardware setup and connection: Install heating platform 110 and temperature controller 120 to ensure stable output of preset temperature program; Attach a temperature sensor, such as a thermocouple, to the surface of the heating platform and connect a data acquisition card to a computer 400. The camera 151 is mounted above the heating platform 110, with the lens of the camera 151 directly opposite the heating platform 110 and parallel to the heating platform 110. The camera 151 is connected to the computer 400. Adjust the angle and light intensity of the adjustable supplementary lighting device by 152 degrees to make the outline of the sample being tested clear and imaged. Lubricating medium 130 is evenly spread on heating platform 110. The specific lubricating medium 130 is selected according to the material of the sample being tested and the heating temperature. Software and parameter presets: First, the heating rate of the temperature controller 120 and the curing kinetic equation of the sample material under test at this heating rate are confirmed by differential scanning calorimetry. Set the image acquisition frequency, temperature acquisition time interval, and digital image correlation analysis parameters (such as region of interest, measurement direction, etc.).

[0035] S300, Heating the prepreg using a heating platform, specifically including: S301, During the heating stage, multiple test samples are heated at the heating rate calibrated in step S200; S302. During the heat preservation stage, the temperature is kept constant to ensure that the sample under test undergoes a complete curing reaction. S303, during the cooling phase, natural cooling.

[0036] S400. During the experiment, digital images of the sample under test are acquired at regular intervals: Starting from the heating stage, the camera acquires surface images of multiple samples at a preset frequency of step 200, covering the entire heating, holding and cooling stages until the end of the experiment. S500: Select the region of interest and set the measurement direction, including: S501. Use two-dimensional digital image correlation (DIC) software to analyze the projected images of each temperature stage, taking the first image of each temperature stage as the reference image and the remaining images as the current images. S502. Set the region where each sample is located in the image as the region of interest, and establish a two-dimensional orthogonal coordinate system in the plane, with the two directions of the coordinate system being the X direction and the Y direction, respectively. S503. Set digital image processing parameters, analyze images at different temperature stages, and calculate the X-direction strain field and Y-direction strain field within the region of interest. For S600 and digital image related data processing, please refer to [link / reference]. Figure 5 ,include: S601. Export the strain data obtained from DIC calculation; S602, Remove abnormal data points; S603. Take the average value of the strain data of each tested sample as the average strain. S604. Check whether the strain calculation of all the tested samples has been completed. If not, repeat the above steps S601 to S603 until the average strain data of all heating stages are obtained. S605, Obtain the strain variation curves with time at different temperature stages; for reference, see [link to relevant documentation]. Figure 5 (a) and Figure 6 (a), Figure 5 (a) and Figure 6 (a) The strain-time curves of the tested sample during the heating stage and the strain-time curves during the heat preservation stage are respectively. S606A. The coefficient of thermal expansion of the uncured sample is calculated based on the average strain and temperature change during the heating stage. By observing the changes in strain in the X and Y directions over time and combining them with the heating curve, the relationship between thermal strain and temperature can be obtained. The data is then fitted using formula (1).

[0037] In the formula, It is the thermal strain during the heating phase. This is the coefficient of thermal expansion when the degree of curing of the tested sample is 0. The transverse and axial coefficients of thermal expansion of the tested sample in the uncured state can be obtained from the slopes of the X-axis and Y-axis strains, such as... Figure 5 As shown in (b).

[0038] S606B. The curing shrinkage coefficient of the tested sample is calculated based on the average strain and curing degree change during the heat preservation stage. By observing the changes in strain in the X and Y directions over time and combining them with the curing degree curve, the relationship between curing shrinkage strain and curing degree can be obtained. The data is fitted using formula (2).

[0039] In the formula, It is the curing shrinkage strain. This is the curing shrinkage coefficient. The curing shrinkage coefficients in the X and Y directions of the tested sample during the curing process in the heat preservation stage can be obtained from the slopes of the strain in the X and Y directions. Figure 6 As shown in (b).

[0040] S606C, the coefficient of thermal expansion of the tested sample when fully cured is calculated based on the average strain during cooling and the temperature change. The relationship between thermal strain and temperature can be obtained by combining the changes in transverse and axial strain over time with the cooling curve. The data is fitted using formula (3):

[0041] In the formula, It is the thermal strain during the cooling phase. It is the coefficient of thermal expansion when the degree of curing of the tested sample is 1. The coefficients of thermal expansion in the X and Y directions of the tested sample under fully cured conditions can be obtained from the slopes of the X-direction strain and the Y-direction strain.

[0042] According to the above embodiments disclosed in this invention, the present invention can achieve the following technical effects: (1) The present invention uses a preset three-stage temperature program of heating, holding and cooling, combined with the independent analysis of the strain field of each stage by digital image correlation method, so that the strain in the heating stage only reflects thermal expansion, the strain in the holding stage only reflects curing shrinkage, and the strain in the cooling stage only reflects thermal expansion after complete curing, thereby effectively separating the thermal expansion and curing shrinkage during the curing process of thermosetting materials.

[0043] (2) Compared with traditional measurement methods, this invention does not require the installation of sensors on the surface of the sample to measure the curing shrinkage rate and thermal expansion strain of thermosetting materials, so as to avoid the additional stress and deformation interference introduced by contact measurement, and ensure that the strain measurement results reflect the thermal expansion and curing shrinkage of the material itself, thus ensuring the reliability of the measurement data.

[0044] (3) The present invention can obtain the thermal expansion coefficient and curing shrinkage rate of the tested sample after a single experiment. Compared with the current method of measuring the thermal expansion coefficient using a thermomechanical analyzer (TMA) and measuring the curing shrinkage rate by the plunger method or volume dilatometer, it saves the time of switching multiple devices and repeated sample loading. At the same time, the present invention is based on the digital image correlation method. Compared with the current method of testing a single sample with a single set of equipment, it can simultaneously measure the curing shrinkage strain and thermal expansion strain of multiple samples, which significantly improves the overall efficiency and is suitable for rapid detection of batch samples.

[0045] (4) By controlling the sample to be tested into a thin sheet, the thickness of the sheet is much smaller than the plane size, so the strain can be approximated as mainly occurring in the plane, and the deformation in the out-of-plane direction can be ignored, which facilitates accurate measurement of plane strain data by the DIC method; and by further eliminating out-of-plane deformation through the lubricating medium, the strain measurement results are ensured to truly reflect the response of thermal expansion or curing contraction, thus improving the measurement accuracy.

[0046] (5) The core equipment of the present invention is a camera, a common computer and a heating platform. Compared with the traditional measurement method using devices such as DMA and TMA, the equipment cost is significantly reduced. At the same time, the requirements for the experimental environment are relatively relaxed, and there is no need for strict experimental environment requirements, which expands the scope of application and is suitable for various scenarios such as laboratory research and development and industrial production.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A measuring device, characterized in that, Includes a data acquisition unit and a data processing unit; The data acquisition unit includes: A heating platform that holds one or more samples to be tested and is used to heat the samples. Temperature controller is used to control the heating platform to execute a preset temperature program; Structures or materials placed on a heating platform to reduce out-of-plane deformation of the sample under test, including but not limited to any one of lubricating media, coatings or films with low coefficient of friction; Temperature detection device, used to detect the real-time temperature data of the sample being tested; An image acquisition device is used to acquire projected image data of the surface of the sample under test onto the plane of the heating platform; The data processing unit is integrated into a computer and is communicatively connected to the temperature detection device and the image acquisition device; the data processing unit is used for: Based on the collected temperature and image data, the curing degree and strain data of the tested sample are calculated. Based on the strain data, temperature data, and degree of curing data at each temperature stage, the curing shrinkage rate and the coefficient of thermal expansion of the tested sample under different curing states are calculated.

2. The measuring device according to claim 1, characterized in that, The data processing unit is configured as follows: Digital image correlation analysis is performed on the image data to calculate the strain data of the tested sample in the plane; The degree of curing of the tested sample is calculated based on the pre-stored curing kinetics model and real-time temperature data. By combining strain data, temperature data, and degree of cure data, the curing shrinkage rate and the coefficient of thermal expansion of the tested sample under different curing states are calculated.

3. The measuring device according to claim 2, characterized in that, The data processing unit includes: The curing degree calculation module is used to receive temperature data and call the curing kinetics model to calculate the curing degree. The image strain analysis module is used to analyze image data using the digital image correlation algorithm and calculate the strain data in two orthogonal directions within the plane of the sample under test. The data fitting module is used to correlate curing degree data, strain data, and temperature data to obtain the curing shrinkage rate and coefficient of thermal expansion.

4. The measuring device according to claim 1, characterized in that, The image acquisition device includes a camera and an adjustable supplementary lighting device; the camera is mounted directly above the sample being tested, and the area directly opposite the camera is parallel to the surface of the heating plane; the adjustable supplementary lighting device is used to provide a stable light source to ensure that the sample image captured by the camera has a clear outline.

5. The measuring device according to claim 1, characterized in that, The temperature measurement unit includes a temperature sensor and a data acquisition card; the temperature sensor is attached to the surface of the heating platform; the data acquisition card is used to collect the temperature data from the temperature sensor in real time and transmit the temperature data to the computer.

6. The measuring device according to claim 1, characterized in that, The heating platform can support multiple test samples, and the image acquisition device can acquire image data of multiple test samples in two orthogonal directions in a plane to calculate the curing shrinkage rate and thermal expansion coefficient of each test sample in parallel.

7. The measuring device according to claim 1, characterized in that, In the temperature program executed by the temperature controller, the heating rate during the heating phase is such that the degree of curing of the sample being tested is ≤3% when it enters the heat preservation phase; and the duration of the heat preservation phase is such that the sample being tested is fully cured when it enters the cooling phase.

8. A method for measuring the shrinkage rate of thermosetting materials based on digital image correlation, characterized in that, The method is applied to the measuring apparatus according to any one of claims 1-7, and the measuring method includes: S11. Prepare one or more sets of test sample materials into thin sheets and place them on a heating platform for heat treatment; S12. Acquire real-time temperature data of the sample under test during the three stages of heating, holding and cooling, and collect image data of the sample surface in the orthogonal direction in the plane during the three temperature stages. S13. Obtain curing degree data based on the curing kinetic equation and real-time temperature data of the sample under test; Based on digital image correlation and image data, the strain data of the tested sample in two orthogonal directions in the plane at each temperature stage are calculated. S14. Based on the strain data, temperature data, and degree of curing data obtained at each temperature stage, calculate the curing shrinkage rate and the coefficient of thermal expansion of the tested sample under different curing states.

9. The measurement method according to claim 8, characterized in that, The method of calculating strain data of the tested sample in two orthogonal directions in the plane at each temperature stage based on digital image correlation and image data includes the following steps: S21. Use the first image of each temperature stage as the reference image and the remaining images as comparison images; S22. Set the area where the sample is located in the image as the region of interest, and set coordinate axes in two orthogonal directions in the strain direction of interest. S23. Track the displacement changes of regions of interest in an image using feature matching algorithms; S24. After subpixel-level displacement field calculation, the strain field in the plane of the measured sample along the coordinate axis in two orthogonal directions at each temperature stage is obtained by quantization through the strain-displacement differential relationship. S25. Export the obtained strain data; S26. Remove outlier data points from the strain data; S27. Take the average value of the strain data of different samples as the average strain at the current moment; S28. Check whether the average strain calculation of the two orthogonal directions in the plane of all the tested samples has been completed. Repeat the calculation until the average strain data of all temperature stages are obtained. S29. Obtain the strain curves of the tested sample in two orthogonal directions in the plane as a function of time at different temperature stages.

10. The measurement method according to claim 8, characterized in that, Based on the obtained strain data, temperature data, and degree of cure data at each temperature stage, the curing shrinkage rate and thermal expansion coefficient of the tested sample under different curing states are calculated, including: A. Based on the strain and temperature change data during the heating stage, the thermal expansion strain is extracted through the strain-temperature relationship, and the thermal expansion coefficient of the uncured state is calculated. B. Based on the strain and curing degree data during the heat preservation stage, chemical shrinkage strain is extracted through the linear relationship between strain and curing degree, and the curing shrinkage coefficient is calculated. C. Based on the strain and temperature change data during the cooling stage, the thermal expansion strain is extracted through the linear relationship between strain and temperature, and the thermal expansion coefficient of the fully cured state is calculated.

Images