Method for comprehensively evaluating curing degree of thermosetting composite material based on differential scanning calorimetry

Abstract

The invention discloses a method for comprehensively evaluating the curing degree of a thermosetting composite material based on a differential scanning calorimetry, which comprises the following steps of: carrying out layered sampling according to the layering design of a to-be-detected thermosetting composite material workpiece, and taking a prepreg of the to-be-detected thermosetting composite material workpiece as a reference substance; all samples are crushed and fully mixed for differential scanning calorimetry, and the curing degree of the to-be-detected thermosetting composite material part is comprehensively evaluated. According to the method, the problem of sample heterogeneity existing in detection of the curing degree of the thermosetting composite material by a differential scanning calorimetry method is solved, and a rapid and accurate detection method is provided for detection of the curing degree of the thermosetting composite material. Meanwhile, layered sampling is carried out on the formed thermosetting composite material, the temperature program of the prepreg takes-50 DEG C as the initial temperature, the heat flow difference between the thermosetting composite material and the reference substance is fully measured, the detection accuracy is ensured, and the method has good application value and prospects.

Description

Technical Field

[0001] The invention relates to a method for comprehensively evaluating the curing degree of a thermosetting composite material based on differential scanning calorimetry. Background Art

[0002] Thermosetting composite materials are high-performance materials that are based on thermosetting resins and are compounded with reinforcing fibers, such as carbon fiber / epoxy resin, glass fiber / phenolic resin, etc. During the curing process, the resin forms a three-dimensional network structure through cross-linking reaction, and its curing degree (curing degree) directly affects the mechanical properties, heat resistance and long-term stability of the material. Therefore, accurate measurement of the curing degree of thermosetting composite materials is crucial for their research and development, production process control and finished product quality assessment.

[0003] Unlike thermoplastic resins that can be heated and cooled multiple times and repeatedly molded, thermosetting resins are a special type of polymer material that forms a highly cross-linked, irreversible three-dimensional network structure through chemical reactions under the action of heating or curing agents. That is, once the thermosetting resin forms a three-dimensional cross-linked network structure through a curing reaction, it is insoluble, infusible, and irreversible, which gives it the characteristics of high heat resistance and high mechanical strength.

[0004] The curing temperature of thermosetting resins varies depending on the type of resin, formulation and application scenario. Generally speaking, the curing temperature of phenolic resins is usually above 150°C; the curing temperature range of epoxy resins is relatively wide, between room temperature and 200°C; the curing of unsaturated polyester resins is usually carried out between room temperature and 60°C through initiators and accelerators; the curing temperature of melamine formaldehyde resins is usually between 130 and 150°C; the curing temperature of urea formaldehyde resins is generally between 120°C and 140°C, and the curing temperature of silicone resins is between 150°C and 200°C. In summary, the curing temperature of most thermosetting resins is below 300°C, but in some special applications, there may be situations where higher curing temperatures are required. For example, in the fields of aerospace and high-end electronics, the curing temperature of some high-performance thermosetting resins may exceed 300°C. For example, the curing temperature of polyimide resins is usually between 300°C and 350°C.

[0005] At present, the methods for testing the curing degree of thermosetting resin materials include physical methods, chemical methods and thermal analysis methods. Among them, physical methods include ultrasonic detection method, dynamic mechanical analysis method (DMA), etc., chemical methods are mainly Fourier transform infrared spectroscopy (FTIR), and thermal analysis methods include differential scanning calorimetry (DSC) dielectric analysis method (DEA), etc. However, most of these methods are aimed at thermosetting resin materials, and for thermosetting composite materials composed of thermosetting resin and reinforcing fiber, there are certain detection limitations and inaccuracies. For example, the infrared spectroscopy method evaluates the curing degree by detecting the changes in functional groups in the resin. For thermosetting composite materials, it is difficult to completely extract all the target functional groups in the material, and the test results are far from the actual results; for another example, the dielectric analysis method (DEA) evaluates the curing degree by monitoring the changes in the dielectric constant of the resin during the crosslinking process, and the dielectric signal is related to the fluidity and crosslinking density of the resin. For thermosetting composite materials, there is no prepreg and parts that cannot be applied.

[0006] The ultrasonic testing method uses the propagation speed or attenuation of ultrasonic waves in the material to evaluate the degree of curing. It can more accurately detect the degree of curing of thermosetting composite materials. This method is non-destructive and can be used for online monitoring, so it has a certain application space. However, ultrasonic testing requires standard comparison test blocks, and the preparation of test blocks takes time. The testing procedure is also relatively complicated, which is inconvenient to use.

[0007] Differential Scanning Calorimetry (DSC) is a common thermal analysis method with the advantages of high sensitivity, wide application range and simple operation. It is widely used in the detection of the degree of cure of thermosetting resin materials. The detection principle of DSC is to measure the relationship between the power difference (in the form of heat) input to the workpiece sample and the uncured resin (reference material) and the temperature under the program control temperature, and analyze the thermal effect of the curing reaction by detecting the changes in the heat release or heat absorption of the material during the curing process, so as to determine the degree of cure. That is, the test results of differential scanning calorimetry are also affected by the test samples. At present, differential scanning calorimetry is mostly based on pure thermosetting resin materials as the detection basis. However, as mentioned above, thermosetting composite materials are composed of thermosetting resin and reinforcing fibers. The samples are heterogeneous, which easily leads to poor repeatability of the test results, and the reinforcing fiber complex components may also interfere with the measurement signal. Therefore, DSC is not used for the detection of the degree of cure of thermosetting composite materials.

[0008] In addition, in order not to damage the structure and performance of the parts, the existing thermosetting composite parts inspection is mostly carried out by sampling and testing from the residual area of ​​the parts. However, small samples may not fully represent the performance and curing degree of the entire material system, and the accuracy of the test results is lacking. Furthermore, the existing differential scanning calorimetry method is used to detect the curing degree of thermosetting composite materials. It is usually to place the sample and the reference material under the same temperature program and measure the heat flow difference between the two to determine the thermal enthalpy of the resin material, so as to quantitatively evaluate the curing degree of the resin. However, for composite materials, the reference material should be selected as prepreg. Since the storage temperature of prepreg is relatively low in actual production, and the sample is stored at room temperature, the same temperature program will inevitably lead to the heat change of the thawing process of the prepreg being ignored, which will also lead to inaccurate detection.

[0009] Therefore, how to use differential scanning calorimetry to detect the degree of curing of thermosetting composite parts and accurately evaluate the various material properties quickly, accurately and at low cost is an urgent problem to be solved in the engineering application of thermosetting composites. Summary of the invention

[0010] In view of the above-mentioned problems existing in the existing differential scanning calorimetry method for detecting the curing degree of thermosetting composite materials, the present invention provides a method for comprehensively evaluating the curing degree of thermosetting composite materials based on differential scanning calorimetry, which accurately detects the curing degree of thermosetting composite materials through the homogenization treatment of samples and the differentiated setting of temperature programs, and realizes accurate evaluation of various properties of materials. The specific technical scheme is as follows:

[0011] A method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry comprises the following steps:

[0012] 1) Randomly select at least three thermosetting composite material products to be tested from the same batch, and take samples from different areas of the selected thermosetting composite material products by stratified sampling method, and crush and fully mix them as samples of thermosetting composite material products to be tested;

[0013] 2) randomly selecting thermosetting composite material prepregs used to make the thermosetting composite material product to be tested in step 1), and randomly sampling and crushing them as thermosetting composite material reference samples;

[0014] 3) Performing differential scanning calorimetry tests on the thermosetting composite material product samples and the thermosetting composite material prepreg samples, respectively, and obtaining the total reaction enthalpy ΔHt and the residual reaction enthalpy ΔHr by integrating the reaction exothermic peak areas;

[0015] 4) Calculating the degree of curing of the thermosetting composite material product to be tested according to the degree of curing formula; the degree of curing formula is:

[0016]

[0017] Where:

[0018] C is the degree of curing of the thermosetting composite material to be tested, %;

[0019] ΔH t is the reaction enthalpy of thermosetting composite prepreg, J / g;

[0020] ΔH r is the residual reaction enthalpy of the thermosetting composite material to be tested, J / g.

[0021] In the aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, the step 1) of sampling from different areas of the selected thermosetting composite material part by the layered sampling method is specifically as follows: according to the design and layer thickness of the thermosetting composite material part to be tested, the thermosetting composite material part to be tested is divided into different areas, and sampling is performed in each area at least three times, which are used as the thermosetting composite material samples to be tested.

[0022] The aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry divides the thermosetting composite material product to be tested into different areas, and the ply thicknesses sampled in the area differ by ≤20 plies, preferably one sample is sampled for every 10 plies.

[0023] In the aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, the thermosetting composite material sample and the thermosetting composite material reference sample are crushed to a particle size of no more than 100 meshes, preferably 120-150 meshes.

[0024] In the aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, the mass of the sample used for the differential scanning calorimetry test in step 3) is controlled within the range of 15 to 25 mg, preferably 20 mg.

[0025] In the aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, in step 3), the test parameters of the differential scanning calorimetry test on the thermosetting composite material sample are:

[0026] Starting temperature: 40°C;

[0027] End temperature: 350℃;

[0028] Heating rate: 5-20°C / min, preferably 10°C / min;

[0029] Nitrogen flow rate: 40-60 mL / min, preferably 50 mL / min.

[0030] In the aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, in step 3), the test parameters of the differential scanning calorimetry test on the thermosetting composite material prepreg sample are:

[0031] Starting temperature: -50℃;

[0032] End temperature: 350℃;

[0033] Heating rate: 5-20°C / min, preferably 10°C / min;

[0034] Nitrogen flow rate: 40-60 mL / min, preferably 50 mL / min.

[0035] In the aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, before performing the differential scanning calorimetry test in step 3), there is also a step of drying the crushed thermosetting composite material product sample and the thermosetting composite material prepreg sample, that is, placing the evenly mixed thermosetting composite material product sample and the thermosetting composite material prepreg sample in a dryer respectively to remove moisture or solvent interference.

[0036] In the aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, the thermosetting resin of the thermosetting composite material product includes any one of epoxy resin, phenolic resin, bismaleimide resin, and benzoxazine resin.

[0037] In the aforementioned method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, the prepreg of the thermosetting composite material product includes any one of unidirectional tape prepreg, fabric prepreg, and glass fiber prepreg.

[0038] The beneficial effects of the present invention are as follows:

[0039] 1) The present invention measures the exothermic enthalpy of a thermosetting resin prepreg and a cured thermosetting composite material based on differential scanning calorimetry, and calculates the degree of cure of the thermosetting composite material. The present invention is applicable to various types of thermosetting resin prepregs such as thermosetting epoxy resins, phenolic resins, bismaleimide resins, benzoxazine resins, and is also applicable to prepregs with reinforcing phases such as carbon fiber, glass fiber, and ceramic fiber, thereby solving the problem of lack of an accurate method for detecting the degree of cure of thermosetting composite materials.

[0040] 2) The method of the present invention crushes the thermosetting composite material sample and its prepreg reference material for detection, thereby avoiding the problem of sample inhomogeneity and realizing that the degree of cure of the thermosetting resin prepreg to be tested can be calculated by the DSC device; at the same time, during the detection of the present invention, multi-point sampling of the formed thermosetting composite material is carried out, and emphasis is placed on the sampling of the part shape and the layer thickness, and mixed detection and evaluation are performed for comprehensive evaluation, thereby providing a quick and effective analytical means for realizing the evaluation of the degree of cure of the composite material.

[0041] 3) The method of the present invention uses the same batch of thermosetting resin prepregs of thermosetting composite parts as reference materials, and sets the temperature program of the prepregs to -50°C as the starting temperature and the ending temperature to 350°C, so as to fully measure the heat flow difference between the two and ensure the detection accuracy.

[0042] In general, the method of the present invention realizes the evaluation of the curing degree of thermosetting composite materials by differential scanning calorimetry, and the test method is simple, the calculation is convenient, the application range is wide, and it has good application value and prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of the tenon component structure prepared from thermosetting T800 grade carbon fiber epoxy resin prepreg;

[0044] Figure 2 Schematic diagram of sampling and partitioning of the tenon component structure prepared from thermosetting T800 grade carbon fiber epoxy resin prepreg;

[0045] Figure 3 This is the peak area diagram of the heat release reaction of the thermosetting T800 grade carbon fiber epoxy resin prepreg in the differential scanning calorimetry test;

[0046] Figure 4 This is the peak area diagram of the differential scanning calorimetry test reaction exotherm of the tenon part prepared with thermosetting T800 grade carbon fiber epoxy resin prepreg. DETAILED DESCRIPTION

[0047] The following will be combined with the embodiments to clearly and completely describe the technical solution of the present invention. Obviously, the described embodiments are only preferred embodiments of the present invention, not all embodiments, and are not intended to limit the present invention in other forms. Any technician familiar with the profession may use the disclosed technical content to make changes or modifications. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

[0048] Example 1

[0049] This embodiment is a method for comprehensively evaluating the degree of curing of a thermosetting composite material based on differential scanning calorimetry, comprising the following steps:

[0050] 1) Randomly select at least three thermosetting composite parts to be tested from the same batch, and sample from different areas of the selected thermosetting composite parts by layered sampling method, specifically: divide the thermosetting composite parts to be tested into different areas according to the design and ply thickness of the thermosetting composite parts to be tested, and randomly sample at least three times in each area. Then crush them to a particle size of not less than 100 mesh, preferably 120-150 mesh, and fully mix them as samples of thermosetting composite parts to be tested. When the thermosetting composite parts to be tested are divided into different areas, the ply thickness of the samples sampled in the area differs by ≤10 plies, preferably 10 plies are sampled once, to ensure the comprehensiveness of the sampling, and to avoid the different shapes and ply thicknesses of the thermosetting composite parts, resulting in unrepresentative sampling, to ensure the accuracy of the precise measurement.

[0051] 2) Randomly select the thermosetting composite material prepreg used to make the thermosetting composite material product to be tested in step 1), randomly sample, crush, and crush the particle size to be not less than 100 mesh, preferably 120-150 mesh, as the thermosetting composite material reference sample. Solve the problem of sample inhomogeneity, so that the DSC equipment can detect and calculate the degree of curing of the thermosetting resin prepreg to be tested.

[0052] 3) Differential scanning calorimetry tests were performed on thermosetting composite parts and thermosetting composite prepreg samples, and the total reaction enthalpy ΔHt and the residual reaction enthalpy ΔHr were obtained by integrating the reaction exothermic peak area. This achieved differential scanning calorimetry to measure the degree of cure of thermosetting composites, solved the problem of lack of accurate methods for testing the degree of cure of thermosetting composites, and provided a quick and effective analytical method for evaluating the degree of cure of composites.

[0053] The test parameters for differential scanning calorimetry tests on thermosetting composite samples were:

[0054] Starting temperature: 40°C;

[0055] End temperature: 350℃;

[0056] Heating rate: 5-20°C / min, preferably 10°C / min;

[0057] Nitrogen flow rate: 40-60 mL / min, preferably 50 mL / min.

[0058] The test parameters for the differential scanning calorimetry test on the thermosetting composite prepreg samples were:

[0059] Starting temperature: -50℃;

[0060] End temperature: 350℃;

[0061] Heating rate: 5-20°C / min, preferably 10°C / min;

[0062] Nitrogen flow rate: 40-60 mL / min, preferably 50 mL / min.

[0063] 4) Calculating the degree of curing of the thermosetting composite material product to be tested according to the degree of curing formula; the degree of curing formula is:

[0064]

[0065] Where:

[0066] C is the degree of curing of the thermosetting composite material to be tested, %;

[0067] ΔH t is the reaction enthalpy of thermosetting composite prepreg, J / g;

[0068] ΔH r is the residual reaction enthalpy of the thermosetting composite material to be tested, J / g.

[0069] In the method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry described in this embodiment, the mass of the sample used for the differential scanning calorimetry test is controlled within the range of 15 to 25 mg, preferably 20 mg. Before conducting the differential scanning calorimetry test, the crushed thermosetting composite material part sample and the thermosetting composite material prepreg sample need to be dried, that is, the evenly mixed thermosetting composite material part sample and the thermosetting composite material prepreg sample are placed in a dryer to remove moisture or solvent interference. Usually, the cold Tg starting front position of the prepreg is 0°C, and it is selected to extend 50°C according to experience. Therefore, the starting temperature starts from -50°C in order to observe the influence of this data on the exothermic peak area; and there is no cold Tg in the composite parts, so room temperature is selected to start.

[0070] The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry described in this embodiment, the thermosetting resin of the thermosetting composite material part includes any one of epoxy resin, phenolic resin, bismaleimide resin, and benzoxazine resin, and the prepreg of the thermosetting composite material part includes any one of unidirectional tape prepreg, fabric prepreg, and glass fiber prepreg, has a wide range of applications and has good application value and prospects.

[0071] Example 2

[0072] This embodiment adopts the method for comprehensively evaluating the curing degree of thermosetting composite materials based on differential scanning calorimetry described in Example 1, and comprehensively evaluates the curing degree of blade tenon parts (autoclave molding) prepared from thermosetting T800 grade carbon fiber epoxy resin prepreg by differential scanning calorimetry. Figure 1 As shown, according to the ply thickness, the blade tenon structural component can be divided into a contact area, a transition area and a root reinforcement area, wherein the ply thickness of the transition area and the root reinforcement area and the contact area is very large on site, and different thickness areas are very likely to be incompletely cured during the curing process, which can easily lead to errors when evaluating the degree of curing. In this embodiment, the method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry described in the present invention is used to evaluate the degree of curing, and an ultrasonic detection method is used to verify the detection effect of the method of the present invention. The specific process is as follows:

[0073] 1) Preparation of blade tenon parts

[0074] Three blade tenon parts made of thermosetting T800 carbon fiber epoxy resin prepreg from the same batch were selected as parallel repeated tests. The tenon contact area (630-560 layers), transition area (560-370 layers), and root reinforcement area (370-160 layers) of the blade tenon parts were divided into three corresponding areas, with a difference of about 150 layers in each area. Figure 2 As shown, samples are taken in each area respectively, and the sampling thickness differs by 10 plies. For example, the tenon contact area is sampled at 560 plies, 570 plies, 580 plies, 590 plies, 600 plies, 610 plies, 620 plies and 630 plies respectively; the transition area and root reinforcement area are sampled in the same way.

[0075] Then the samples were mixed and crushed to a particle size of about 120 mesh, placed in a dryer, and vacuum dried at 50°C for 2 hours to remove moisture and solvent interference, and set aside.

[0076] 2) Preparation of reference materials for tenon parts

[0077] The same batch of thermosetting T800 grade carbon fiber epoxy resin prepreg used to prepare the above-mentioned tenon parts was used as a reference, and three samples were taken for parallel repeated tests. The thermosetting T800 grade carbon fiber epoxy resin prepreg was crushed to a particle size of about 120 mesh, placed in a dryer, and vacuum dried for 2 hours to remove moisture and solvent interference, and then set aside.

[0078] 3) Conduct differential scanning calorimetry test

[0079] The reduction method is to weigh about 20 mg of the thermosetting T800 grade carbon fiber epoxy resin prepreg sample with a weighing accuracy of 0.1 mg, put it into an aluminum crucible, and gently press to seal it (to avoid air residue). At the same time, prepare an empty crucible as a reference test.

[0080] The test was carried out according to ASTM E2160, and the experimental parameters were as follows:

[0081] Starting temperature: -50°C; End temperature: 350°C; Heating rate: 10°C / min; Nitrogen flow rate: 50mL / min;

[0082] After the scan is completed, the baseline of the reference experiment is deducted according to the software, and the total reaction enthalpy ΔHt of the thermosetting T800 grade carbon fiber epoxy resin prepreg sample is obtained by integrating the reaction exothermic peak area of ​​the prepreg. The spectrum is shown in Figure 3 shown.

[0083] Weigh 20 mg of tenon sample made of thermosetting T800 grade carbon fiber epoxy resin prepreg to an accuracy of 0.1 mg by the reduction method, put it into an aluminum crucible, and seal it with light pressure (to avoid air residue). At the same time, prepare an empty crucible as a reference test.

[0084] The test was carried out according to ASTM E2160, and the experimental parameters were as follows:

[0085] Starting temperature: 40℃; ending temperature: 350℃; heating rate: 10℃ / min; nitrogen flow rate: 50mL / min; after the scan, the baseline of the reference experiment was deducted according to the software, and the total residual reaction enthalpy ΔHr was obtained by integrating the reaction exothermic peak area of ​​the composite laminate. The spectrum is shown in Figure 4 shown.

[0086] Repeat the above operation to test the remaining 2 groups of samples.

[0087] 4) Calculate the degree of curing of the composite material according to the formula, calculate the single value and average value of the degree of curing, and retain 2 decimal places for the test results. The calculation formula is as follows:

[0088]

[0089] Where:

[0090] ΔH t is the reaction enthalpy of the uncured prepreg, J / g;

[0091] ΔH r is the residual reaction enthalpy of the cured laminate, J / g;

[0092] C is the degree of curing of the composite material, %.

[0093] Comparative Example 1: No zoning is performed when preparing the tenon parts, random sampling is performed in the remaining areas, and no crushing treatment is performed. Similarly, the corresponding reference material, thermosetting T800 grade carbon fiber epoxy resin prepreg, is not crushed when prepared, and the remaining steps are consistent with Example 2.

[0094] Comparative Example 2: The preparation of the tenon component and the thermosetting T800 grade carbon fiber epoxy resin prepreg are consistent with those in Example 2, but when conducting the differential scanning calorimetry test, the starting temperature of the thermosetting T800 grade carbon fiber epoxy resin prepreg is 40°C, which is consistent with the differential scanning calorimetry test parameters of the tenon component sample.

[0095] Comparative Example 1 and Comparative Example 2 were also repeated 3 times.

[0096] Verification test: Verify the curing degree of the blade tenon by ultrasonic testing

[0097] Thermosetting T800-grade carbon fiber epoxy resin prepreg from the same batch as the composite tenon to be tested was selected, and the composite laminate was prepared using the same curing process as Example 2 as a standard test block. The thickness of the standard test block was set according to the characteristic thickness of the tenon area to be tested, that is, 50 composite laminates were prepared according to the thickness of the regional plies divided by the tenon, wherein laminates 1-8 were the characteristic thickness of the corresponding tenon contact area (630 to 560 layers of plies), laminates 9-28 were the characteristic thickness of the transition area (560 to 370 layers of plies), and press plates 29-50 were the characteristic thickness of the root reinforcement area (370 to 160 layers of plies).

[0098] The prepared composite laminate was used as a standard test block, and the composite tenons from the same batch to be tested in Example 2 were used as samples.

[0099] Ultrasonic C scanning USL was used, the probe frequency was 1MHz to 2MHz, and water was used as a coupling agent. The sound velocity and attenuation values ​​of several standard test blocks were measured respectively as the benchmarks of the three areas of the tenon, and the propagation time and attenuation value of the sound wave in the material were recorded. In order to improve the accuracy, multiple measurements were made at different positions in each area of ​​the standard test block and the tenon, and the average value was taken. In order to avoid the temperature change affecting the sound velocity and attenuation, during the detection process, the detection environment temperature was set at a constant temperature of 22℃, and finally the sound velocity, energy attenuation and curing degree were calculated according to the test results.

[0100] The speed of sound formula is: Where: d is the material thickness and t is the propagation time.

[0101] The energy attenuation of ultrasound in a material is usually expressed in decibels (dB). The attenuation calculation formula is:

[0102] attenuation Where V1 is the incident wave amplitude and V2 is the transmitted wave amplitude.

[0103] The sound velocity and attenuation values ​​measured by the tenon are compared with the average data measured by the standard test block sample, and the corresponding ratio can be used to evaluate the degree of curing. The formula is as follows:

[0104]

[0105] The actual curing degree of the tenon is 1. The test data are summarized in Table 1, and the average values ​​of all test results are retained to 2 decimal places.

[0106] Table 1. Summary of T800 carbon fiber parts and prepreg curing test data

[0107]

[0108] It can be seen from the test results in Table 1 that the test results of the test method of the present invention are consistent with the ultrasonic test results, while the tests of Comparative Examples 1 and 2 have some deviations. Among them, in Comparative Example 1, no zoning was performed when preparing the tenon parts, only the residual area was randomly sampled, and the samples were not crushed. The test result was 0.45% higher than the actual degree of cure, while the test result of Comparative Example 2, in which the starting temperature of the thermosetting T800 grade carbon fiber epoxy resin prepreg was consistent with that of the tenon part sample, was 0.04% lower than the actual degree of cure. It can be seen that factors such as sampling, sample homogenization, and the detection temperature of the prepreg all have an impact on the test results, especially sampling and sample homogenization have a great impact on the inspection results.

[0109] In general, the present invention measures the curing degree of thermosetting resin pre-composite materials by differential scanning calorimetry through zone sampling and sample crushing, providing a quick and effective analytical means for evaluating the curing degree of composite materials, and the method is applicable to various types of thermosetting resin prepregs such as thermosetting epoxy resins, phenolic resins, bismaleimide resins, benzoxazine resins, and prepregs with reinforcing phases such as carbon fiber, glass fiber, and ceramic fiber, and has a wide range of applications; the method of the present invention uses the same batch of thermosetting resin prepregs of thermosetting composite parts as reference materials, and sets the temperature program of the prepregs to -50°C as the starting temperature, and sets the ending temperature to 350°C, so as to fully measure the heat flow difference between the two and ensure the detection accuracy, and the test method is simple, the calculation is convenient, the application range is wide, and the method has good application value and prospects.

[0110] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations within the meaning and scope of the elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0111] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in the embodiments may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry, characterized in that: The steps include: 1) Randomly select at least three thermosetting composite material products to be tested from the same batch, and take samples from different areas of the selected thermosetting composite material products by stratified sampling method, and crush and fully mix them as samples of thermosetting composite material products to be tested; 2) randomly selecting thermosetting composite material prepregs used to make the thermosetting composite material product to be tested in step 1), and randomly sampling and crushing them as thermosetting composite material reference samples; 3) Performing differential scanning calorimetry tests on the thermosetting composite material product samples and the thermosetting composite material prepreg samples, respectively, and obtaining the total reaction enthalpy ΔHt and the residual reaction enthalpy ΔHr by integrating the reaction exothermic peak areas; 4) Calculating the degree of curing of the thermosetting composite material product to be tested according to the degree of curing formula; the degree of curing formula is: Where: C is the degree of curing of the thermosetting composite material to be tested, %; ΔH t is the reaction enthalpy of thermosetting composite prepreg, J / g; ΔH r is the residual reaction enthalpy of the thermosetting composite material to be tested, J / g.

2. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 1, characterized in that: The step 1) of sampling from different areas of the selected thermosetting composite material product by the layered sampling method is specifically as follows: according to the design and layer thickness of the thermosetting composite material product to be tested, the thermosetting composite material product to be tested is divided into different areas, and sampling is performed in each area for no less than three times, and the samples are used as the thermosetting composite material samples to be tested.

3. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 2, characterized in that: The thermosetting composite material product to be tested is divided into different areas, and the ply thicknesses sampled in the areas differ by ≤20 plies.

4. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 1, characterized in that: The thermosetting composite material sample and the thermosetting composite material reference sample are crushed to a particle size not greater than 100 meshes.

5. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 1, characterized in that: The mass of the sample used for differential scanning calorimetry in step 3) is controlled within the range of 15 to 25 mg.

6. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 1, characterized in that: In step 3), the test parameters of the differential scanning calorimetry test on the thermosetting composite material sample are: starting temperature: 40°C; End temperature: 350℃; Heating rate: 5-20℃ / min; Nitrogen flow rate: 40~60mL / min.

7. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 1, characterized in that: In step 3), the test parameters of the differential scanning calorimetry test on the thermosetting composite prepreg sample are: Starting temperature: -50℃; End temperature: 350℃; Heating rate: 5-20℃ / min; Nitrogen flow rate: 40~60mL / min.

8. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 1, characterized in that: Before performing the differential scanning calorimetry test in step 3), there is also a drying step for the crushed thermosetting composite material product sample and the thermosetting composite material prepreg sample, that is, the evenly mixed thermosetting composite material product sample and the thermosetting composite material prepreg sample are placed in a dryer to remove moisture or solvent interference.

9. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 1, characterized in that: The thermosetting resin of the thermosetting composite material product includes any one of epoxy resin, phenolic resin, bismaleimide resin and benzoxazine resin.

10. The method for comprehensively evaluating the degree of curing of thermosetting composite materials based on differential scanning calorimetry according to claim 1, characterized in that: The prepreg of the thermosetting composite material product includes any one of unidirectional tape prepreg, fabric prepreg and glass fiber prepreg.

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