Method and device for characterizing the time-varying properties of the mechanical properties of a composite material during the curing process

Abstract

The application provides a kind of composite material curing process mechanics performance time-varying characteristic characterization method and device, the method comprises the following steps: step one: obtaining the reaction enthalpy released by unit mass of the measured prepreg complete curing;Step two: obtain a plurality of compression samples and DSC samples;Step three: using composite material repair instrument simultaneously to cure compression sample and DSC sample, terminate curing process at different preset time for each group of samples, and record the temperature T of each group of samples at the time of curing termination;Step four: for each group of samples, the compression modulus of compression sample after step three at corresponding temperature T is measured by using mechanical testing machine, and the curing degree of DSC sample after step three is obtained;Step five: according to a plurality of curing degrees and compression modulus, the modulus of resin under different curing degrees is obtained by using self-consistent field micromechanics model, and then the mechanics performance time-varying characteristic of composite material curing process can be obtained.The application fully combines the characteristics of composite material repair instrument, which can terminate the curing process of composite material safely and controllably at any time, the mechanical properties in the thickness direction of composite material, and the easy implementation of compression test, solves the problems of difficult processing of mechanical samples of composite material under different curing degrees and different temperatures, difficult measurement of mechanical properties, and realizes the accurate measurement of the mechanics performance time-varying characteristic of composite material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of composite material mechanical property characterization, in particular to a method and device for characterizing the time-varying characteristics of the mechanical properties of a composite material during the curing process. BACKGROUND

[0002] During the curing process of a thermosetting resin-based composite material, it will undergo a transition from a viscous flow state to a rubbery state and then to a glassy state, and its mechanical properties will change with the curing process. The time-varying characteristics of the mechanical properties of such composite materials are one of the main reasons for the generation of residual stress and curing deformation of components. Therefore, accurate characterization of the relationship between the mechanical properties of the composite material and the curing degree is the basis for achieving precise manufacturing of composite materials.

[0003] The curing degree of a composite material is usually measured using a DSC (Differential Scanning Calorimeter), while the measurement of its time-varying characteristics of mechanical properties generally includes an empirical method and a DMA (Dynamic Mechanical Analysis) measurement method. On the one hand, the empirical method is based on the mechanical properties of the uncured and fully cured composite materials, combined with empirical formulas and self-consistent field micromechanics models to back-calculate the mechanical properties of the uncured composite material, and then fitted through a related time-varying model, such as the method used in the Chinese patent with publication number CN 110197008B and publication date December 25, 2020. However, there is a certain error between the results calculated based on the empirical formula and the actual situation. On the other hand, since the DMA test cannot provide accurate modulus values, the DMA measurement method obtains the change of the modulus of the composite material with the curing process through DMA testing, and then back-calculates the time-varying characteristics of the mechanical properties of the composite material with the help of a self-consistent field micromechanics model. This method introduces some errors when back-calculating the properties of the resin in the low modulus region.

[0004] The difficulty of characterizing the time-varying characteristics of the mechanical properties of a prepreg during the curing process lies in the fact that it is difficult to prepare mechanical test samples of different curing degrees due to the limitations of autoclave equipment. Therefore, a method for characterizing the time-varying characteristics of the mechanical properties of a thermosetting composite material is proposed, which has important engineering significance for the analysis and solution of typical manufacturing defects of composite materials. SUMMARY

[0005] The mechanical properties of a composite material evolve with the curing process. In order to accurately characterize the change of the mechanical properties of a composite material at different curing degrees, the present application provides a method and device for characterizing the time-varying characteristics of the mechanical properties of a composite material during the curing process. The composite material repair instrument solves the problem of difficult preparation of mechanical test samples of different curing degrees, combines differential scanning calorimetry and a mechanical testing machine, and realizes the direct measurement of the time-varying characteristics of the mechanical properties of the composite material based on a self-consistent model, ensuring the accuracy of the results.

[0006] The first aspect of the present application provides a method for characterizing time-varying characteristics of mechanical properties of a composite material during a curing process, comprising:

[0007] Step 1: obtaining the reaction enthalpy released per unit mass of the to-be-tested prepreg during complete curing;

[0008] Step 2: obtaining a plurality of compression samples and DSC samples;

[0009] Step 3: simultaneously curing the compression samples and the DSC samples by using a composite material repair instrument, terminating the curing process of each group of samples at different preset times, and recording the temperature T of each group of samples at the time of termination of the curing;

[0010] Step 4: for each group of samples, measuring the compression modulus of the compression sample after step 3 at the corresponding temperature T by using a mechanical testing machine, and obtaining the degree of curing of the DSC sample after step 3;

[0011] Step 5: according to the degrees of curing and the compression moduli of the plurality of groups, the modulus of the resin at different degrees of curing is obtained by using a self-consistent field mesoscopic mechanics model, and then the time-varying characteristics of the mechanical properties of the composite material during the curing process can be obtained.

[0012] Optionally, the reaction enthalpy H released per unit mass of the to-be-tested prepreg during complete curing is obtained, comprising:

[0013] The reaction enthalpy H released per unit mass of the to-be-tested prepreg during complete curing is tested and calculated according to ASTM E2160.

[0014] Optionally, the degree of curing of the DSC sample after step 3 is obtained, comprising:

[0015] The DSC sample is subjected to dynamic DSC scanning to obtain the residual reaction enthalpy ΔH per unit mass of the sample, and the degree of curing α of the DSC sample is calculated according to α=(1-ΔH) / H.

[0016] Optionally, the compression sample is obtained, comprising:

[0017] The prepreg is laid according to a symmetric [0 / 90] ns layering sequence to form a prepreg blank, and after sufficient cold drawing and compaction, the prepreg blank is edge trimmed to obtain a compression sample;

[0018] The size of the compression sample is 50mm*50mm, and the layering information [0 / 90] ns n is a positive integer not less than 5.

[0019] Optionally, the DSC sample is obtained, comprising:

[0020] 5-25 milligrams of the to-be-tested prepreg are placed in an aluminum crucible to prepare a DSC sample.

[0021] Optionally, the mechanical testing machine is equipped with a temperature control box, and when the mechanical testing machine is used to measure the compression modulus, the temperature control box controls the temperature to be T.

[0022] Optionally, the compression modulus of the compression sample after step 3 at the corresponding temperature T is measured by using the mechanical testing machine, comprising:

[0023] The compression test is performed on the compression sample to obtain a force-displacement curve, and a group of forces F and compression displacements l0 are obtained from the linear segment of the initial compression stage in the force-displacement curve.

[0024] According to the force F and the compression displacement l0, the formula is used to obtain the compression modulus E.

[0025] Wherein, S represents the effective compression area, and l represents the original thickness of the sample.

[0026] Optionally, a plurality of groups of compression samples and DSC samples are obtained, comprising:

[0027] At least 5 groups of compression samples and DSC samples are obtained.

[0028] Optionally, after obtaining the curing degree of the DSC sample after step 3, the method further comprises:

[0029] It is judged whether the number of samples with curing degree exceeding the preset gel point is less than 3.

[0030] If yes, a new sample group and a different preset time are added.

[0031] The second aspect of the application provides a device for characterizing the time-varying characteristics of the mechanical properties of a composite material during the curing process, which is used to perform the method according to any one of the first aspect.

[0032] The application provides a method and device for characterizing the time-varying characteristics of the mechanical properties of a composite material during the curing process, which fully combines the features of the composite material repair instrument, i.e., the composite material curing process can be safely and controllably terminated at any time, the mechanical properties in the thickness direction of the composite material, and the compression test is easy to implement, solves the problems of difficult processing of mechanical samples of the composite material and difficult measurement of mechanical properties at different curing degrees and different temperatures, and realizes accurate measurement of the time-varying characteristics of the mechanical properties of the composite material. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The dynamic DSC scanning curve of the prepreg system measured in the embodiment 2 of the application;

[0034] Figure 2 The test measurement and data fitting results of the time-varying characteristics of the resin modulus in the embodiment 2 of the application;

[0035] Figure 3A flow chart of a method for characterizing the time-varying mechanical properties of a thermoset resin based composite during curing. DETAILED DESCRIPTION

[0036] The application will be further described in detail with reference to the accompanying drawings.

[0037] Example 1

[0038] As shown in the accompanying drawings, the application provides a method for characterizing the time-varying mechanical properties of a thermoset resin based composite during curing, comprising the following steps: Figure 3 Step 1: Measurement of the enthalpy of curing reaction. According to ASTM E2160, the enthalpy of reaction H released by the complete curing of a unit mass of the pre-preg to be tested is tested and calculated.

[0039] Step 2: Preparation of a thickness direction compression sample. The pre-preg is laid according to the symmetric [0 / 90] ns layup sequence to form a pre-preg blank, and after ensuring sufficient cold drawing and compaction, the pre-preg blank is edge trimmed to prepare a compression sample.

[0040] Step 3: Preparation of a DSC (Differential Scanning Calorimeter) sample. An appropriate amount of the pre-preg to be tested is placed in an aluminum crucible to prepare a DSC sample, and the DSC sample should contain 5-25 milligrams of the pre-preg sample.

[0041] Step 4: Preparation of samples at different curing degrees. According to the recommended curing process parameters of the pre-preg supplier, the composite repair instrument is used to simultaneously cure the compression sample prepared in Step 2 and the DSC sample prepared in Step 3, and the curing process is terminated at an appropriate time, and the temperature T at the time of termination of curing is recorded.

[0042] Step 5: Characterization of the compression properties of the pre-preg blank at different curing degrees. The compression modulus of the compression sample prepared in Step 4 at temperature T is measured using a mechanical testing machine.

[0043] Step 6: Curing degree characterization. Dynamic DSC scanning is performed on the DSC sample in Step 4 to obtain the residual reaction enthalpy ΔH per unit mass of the sample, and the curing degree of the DSC sample in Step 4 is calculated as α = (1-ΔH) / H.

[0044] Step 7: Repeat Steps 1-6 to obtain the compression modulus of the composite in the thickness direction at different curing degrees.

[0045] Step 8: According to the compression modulus of the composite in the thickness direction at different curing degrees obtained in Step 7, the modulus of the resin at different curing degrees is back calculated using the self-consistent field micromechanics model, and the change relationship between the engineering constants of the composite and the curing degree can be obtained.

[0046]

[0047] ​Further, in step 2, the recommended size of the thickness direction compression sample is 50mm*50mm, and the ply information is [0 / 90] ns wherein n should be no less than 5.

[0048] Further, in step 5, the testing machine should be equipped with a temperature control box.

[0049] Further, in step 5, the temperature control box of the testing machine should be set in advance and preheated to temperature T before the end of step 4.

[0050] Further, in step 5, the compression sample should be quickly prepared after the end of step 4, and safety protection should be done during the sample transfer process.

[0051] Further, in step 5, the compression elastic modulus should be calculated according to the force-displacement curve obtained by compression test, taking the linear segment of the initial compression stage, and the calculation method is as follows: wherein F represents force, S represents effective compression area, L0 represents the compression amount of the material, and L represents the original thickness of the sample.

[0052] Further, in step 7, the number of repetitions of steps 1-6 should not be less than 5 times.

[0053] Further, in step 7, when each repetition reaches step 4, a curing process termination time different from the previous repeated test should be selected. It is recommended to reasonably select the curing process termination time according to the measured curing kinetics characteristics of the prepreg, with the goal of equal curing degree interval.

[0054] Further, in step 7, the number of samples with different curing degrees after the prepreg system reaches the gel point should be no less than 3.

[0055] Example 2

[0056] The present application provides a method for characterizing the time-varying properties of the mechanical properties of a thermosetting resin-based composite material during the curing process, comprising the following steps:

[0057] Step 1: Measurement of the enthalpy of the curing reaction. Taking T800 grade carbon fiber unidirectional prepreg as the object, the dynamic DSC scanning curve of the prepreg system is measured according to the ASTM E2160 test method, as shown in Figure 1 The integral of the exothermic peak gives the reaction enthalpy H released by complete curing per unit sample mass, which is 114.38J / g.

[0058] Step 2: Preparation of thickness direction compression sample. According to the symmetric [0 / 90] 10s plying sequence, the prepreg is laid to form a prepreg blank, and after ensuring sufficient cold drawing and compaction, the prepreg blank is edge trimmed to prepare a compression sample.

[0059] Step 3: DSC sample preparation. Take a certain amount of the prepreg to be tested and place it in an aluminum crucible to prepare the DSC sample.

[0060] Step 4: Sample preparation of different curing degrees. According to the recommended curing process parameters of the prepreg supplier, simultaneously cure the compression sample prepared in step 2 and the DSC sample prepared in step 3 using a composite repair instrument, and terminate the curing process at an appropriate time, and record the temperature T at the time of termination of curing.

[0061] Step 5: Compression performance characterization of prepreg with different curing degrees. The compression modulus of the compression sample prepared in step 4 at temperature T is measured using a mechanical testing machine.

[0062] Step 6: Curing degree characterization. Dynamic DSC scanning is performed on the DSC sample in step 4 to obtain the residual reaction enthalpy ΔH per unit mass of the sample, and the curing degree of the DSC sample in step 4 is calculated according to α = (1-ΔH) / H.

[0063] Step 7: Repeat steps 1-6 nine times to obtain the compression modulus of the composite material in the thickness direction at nine different curing degrees.

[0064] It can be understood that the curing processes with different stopping times can also be performed simultaneously when the curing equipment is sufficient.

[0065] Step 8: According to the compression modulus of the composite material in the thickness direction at different curing degrees obtained in step 7, the modulus of the resin at different curing degrees is obtained by back calculation using the self-consistent field micromechanics model. The expression of the self-consistent field micromechanics model is:

[0066]

[0067]

[0068] In the above formula,

[0069]

[0070] E is the elastic modulus, G is the shear modulus, and v is the Poisson's ratio; subscripts 1, 2, and 3 represent directions, where 1 is the fiber direction, 2 is the in-plane direction perpendicular to the fiber, and 3 is the out-of-plane direction perpendicular to the fiber; subscripts r and f represent the resin and the fiber; V f is the fiber volume fraction; k r and k f are the volume moduli of the resin and the fiber.

[0071] The mechanical parameters of carbon fibers (Table 1), the fiber volume fraction of 0.56, and the compression modulus data of the resin in the thickness direction at different curing degrees are substituted into the self-consistent field micromechanics model to obtain the elastic modulus of the resin at different curing degrees, as shown in Table 2.

[0072] Carbon fiber mechanical property parameters of the measured prepreg systems in Table 1

[0073] Parameter Value GPa Parameter Value Parameter Value Gpa e1f 250 v12f 0.2 g12f 15 e2f 25 v13f 0.2 g13f 12 e3f 25 v23f 0.35 g23f 5

[0074] Table 2: Thickness direction compression modulus of the measured prepreg systems at different degrees of cure and the back-calculated resin modulus

[0075] Compression modulus / MPa 34.2 61.5 111.9 121.4 125.6 153.2 334 2904 8000 Degree of cure of the specimen 0 0.0015 0.0068 0.048 0.087 0.24 0.56 0.66 1 Back-calculated resin modulus / MPa 8.5 15 25 30 31 39 85 850 3000

[0076] From the back-calculated resin modulus data, it can be seen that the modulus of the resin is small and changes little before the gel point (corresponding to a degree of cure of about 0.53), so the CHILE (a) model is selected for fitting here, as shown in Figure 2 The change relationship between the degree of cure and the resin modulus E r is:

[0077]

[0078] The fitting results of the CHILE (a) model are substituted into the self-consistent field micromechanics model to obtain the mechanical properties of the composite material at any degree of cure.

[0079] Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for characterizing the time-varying mechanical properties of composite materials during the curing process, characterized in that, The method comprises: Step 1: obtaining the released reaction enthalpy per unit mass of the to-be-tested prepreg for complete curing; Step 2: obtaining a plurality of compression samples and DSC samples; Step 3: simultaneously curing the compression samples and the DSC samples by using a composite material repair instrument, and terminating the curing process of each group of samples at different preset times, and recording the temperature T of each group of samples at the time of termination of the curing; Step 4: for each group of samples, measuring the compression modulus of the compression sample after step 3 at the corresponding temperature T by using a mechanical testing machine, and obtaining the degree of curing of the DSC sample after step 3; Step 5: according to the degrees of curing and the compression moduli of the plurality of groups, the modulus of the resin at different degrees of curing is obtained by using a self-consistent field mesoscopic mechanics model, and then the time-varying characteristics of the mechanical properties of the composite material in the curing process are obtained; obtaining the released reaction enthalpy H per unit mass of the to-be-tested prepreg for complete curing, comprising: testing and calculating the released reaction enthalpy H per unit mass of the to-be-tested prepreg for complete curing according to ASTM E2160; obtaining the degree of curing of the DSC sample after step 3, comprising: performing dynamic DSC scanning on the DSC sample to obtain the residual reaction enthalpy ΔH per unit mass of the sample, and calculating the degree of curing α of the DSC sample according to α = (1-ΔH) / H; measuring the compression modulus of the compression sample after step 3 at the corresponding temperature T by using a mechanical testing machine, comprising: performing compression test on the compression sample to obtain a force-displacement curve, and obtaining a group of force F and compression displacement l0 from the linear section of the initial compression stage in the force-displacement curve; According to the force F and the compression displacement lo; the formula Obtaining the compression modulus E; wherein S represents the effective compression area, and l represents the original thickness of the sample.

2. The method of claim 1, wherein the time-varying mechanical property of the composite curing process is characterized by, obtaining the compression sample, comprising: The prepreg is laid up according to the symmetric [0 / 90] ns The prepreg is laid up according to the symmetric [0 / 90] ns The prepreg is laid up according to the symmetric [0 / 90] ns The prepreg is laid up according to the symmetric [0 / 90] ns The prepreg is laid up according to the symmetric [0 / 90] ns The prepreg is laid up according to the symmetric [0 / 90] ns The size of the compression sample is 50mm*50mm, and the layer information is [0 / 90] ns where n is a positive integer not less than 5.

3. The method of claim 1, wherein the method is characterized by, obtaining the DSC sample, comprising: placing 5-25 milligrams of the to-be-tested prepreg in an aluminum crucible to prepare the DSC sample.

4. The method of claim 1, wherein the method is characterized by, The mechanical testing machine is equipped with a temperature control box, and when the compression modulus is measured by using the mechanical testing machine, the temperature control box controls the temperature to be T.

5. The method of claim 1, wherein the method is characterized by, obtaining a plurality of compression samples and DSC samples, comprising: obtaining not less than 5 groups of compression samples and DSC samples.

6. The method of claim 1, wherein the method is characterized by, After obtaining the degree of curing of the DSC sample after step 3, the method further comprises: determining whether the number of samples with a degree of curing exceeding the preset gel point is less than 3; if yes, adding a new sample group and different preset times.

7. A device for characterizing the time-varying behavior of mechanical properties during a composite curing process, characterized in that, A device for performing the method according to any one of claims 1-6.

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