A method for determining warm press parameters for dry fiber preform thickness control

Abstract

The application provides a warm-pressing parameter determination method for dry fiber preform thickness control, which comprises the following steps: a single equation form dry fiber preform creep / recovery material model is proposed, creep / recovery experiments are conducted, strain-time curves of test pieces under two test conditions are obtained, model parameter identification is conducted according to the curves, the single equation form dry fiber preform creep / recovery material model after identification is obtained, and finally, the single equation form dry fiber preform creep / recovery material model is used to predict the dry fiber preform thickness under corresponding creep stress and experimental preform temperature. The single equation form dry fiber preform creep / recovery material model established by the application can accurately describe the time-dependent thickness creep / recovery behavior of the dry fiber fabric preform, and the comparison between the model prediction result and the newly designed experimental data shows that the experimental and model prediction curves are basically consistent, thereby proving the effectiveness of the method.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of testing the quasi-static mechanical behavior of composite materials, and in particular to a method for determining warm compaction parameters for dry fiber preform thickness control, which can predict the thickness creep / recovery behavior of dry fiber preforms under different creep stresses and different preforming temperatures during the preforming process. BACKGROUND

[0002] Dry fiber automated placement / liquid molding technology is a low-cost non-heat tank molding process that has developed rapidly in the past decade. The performance of parts manufactured using this molding process is comparable to that of parts manufactured using heat tank molding, and it is suitable for the overall manufacturing of large and complex composite structures. This process combines the advantages of automated placement and liquid molding, avoiding the problem of preform preparation during liquid molding, and improving the weight reduction efficiency, product accuracy and surface quality of composite structures. More and more countries and organizations have launched multiple projects to conduct related research, and Russia has applied this molding technology to the central wing box, wing wall skin and spar structure of the MC-21 single-aisle passenger aircraft.

[0003] The dry fiber preforming process is the basic process of automated placement molding, and the quality of the preform has a significant impact on the mechanical properties of the manufactured composite parts. During compaction, the fiber tows are squeezed together and act as elastic bending beams, and the fiber tows slide against each other and rearrange, generating friction. The above-mentioned combined effects result in complex viscoelastic behavior of the dry fiber preform during compaction. The viscoelastic response of the dry fiber preform includes creep, recovery and stress relaxation. When the load that causes creep is removed, the material exhibits instantaneous elastic recovery and time-dependent hysteresis recovery, which is generally referred to as creep recovery.

[0004] It is of great significance to study the creep / recovery behavior of dry fiber preforms: on the one hand, only by accurately analyzing and describing the change of strain with time during unloading can the nonlinear elastic, viscoelastic, elastoplastic or viscoelastic-plastic deformation behavior during loading be distinguished; on the other hand, the creep / recovery behavior has a significant impact on the subsequent resin infiltration process. The mutual extrusion of fiber tows during preforming causes changes in the cross-sectional shape and fiber orientation of the fiber tows, thereby changing the fiber tow path and spatial distribution of the preform, affecting the formability of the part and the permeability of the resin flow stage boundary, and the inhomogeneity of the macrostructure and microfiber configuration also leads to complex resin flow and infiltration behavior, which easily leads to defects such as high porosity and dry spots in the formed parts. Therefore, it is of great significance to study the viscoelastic behavior of dry fiber preforms during the preforming process.

[0005] Most of the existing models focus on the creep or recovery behavior, some models provide two different formulas for the creep / recovery process, and the corresponding equation coefficients are different in the creep and recovery processes. But in practice, the two stages cannot always be clearly separated, especially the smooth transition from the creep stage to the recovery stage is difficult to handle, and most of the proposed models have not been applied and verified. In addition, the existing models do not analyze the change of the material thickness strain with time after unloading stress under different creep stresses and preforming preforming temperatures. Therefore, it is necessary to study the use of a single equation to comprehensively describe the material creep and recovery behavior for predicting the creep / recovery behavior of the preform under different creep stresses and preforming preforming temperatures. SUMMARY

[0006] To solve the problems existing in the prior art, the present application provides a warm pressing parameter determination method for dry fiber preform thickness control. First, the creep / recovery experiment of the test piece is carried out to obtain the strain-time curve data of the test piece under different stresses at room temperature and the same stress under different preforming temperatures. Then, the single equation form of the dry fiber preform creep / recovery material model is established and the parameters are fitted. Finally, the obtained model is used to realize good prediction of the dry fiber preform creep / recovery under different creep stresses and preforming temperatures.

[0007] The technical scheme of the present application is:

[0008] The warm pressing parameter determination method for dry fiber preform thickness control comprises the following steps:

[0009] Step 1: The creep / recovery experiment of the test piece is carried out, including the creep / recovery experiment under different stresses at room temperature and the creep / recovery experiment under the same stress at different preforming temperatures; the strain-time curve of the test piece under the two test conditions is obtained, and the creep / recovery experiment process is divided into three stages: loading stage, creep stage and unloading response stage;

[0010] Step 2: The single equation form of the dry fiber preform creep / recovery material model is established as follows:

[0011]

[0012] Where σ is the set creep stress, T is the experimental preforming temperature, t0 is the unloading time, E1 is the elastic modulus in the Maxwell unit, E2 is the elastic modulus in the Kevin unit, η1 is the viscosity coefficient, τ1 is the relaxation time, ε N is the non-mechanical strain, H(x) is the step function, a is a function related to the creep stress and the preforming temperature, ε(t) is the strain value, which represents the dry fiber preform thickness;

[0013] According to the creep / recovery experimental data obtained in step 1, the following process is used for model parameter identification, so as to obtain the identified single equation form of the dry fiber preform creep / recovery material model:

[0014] Step 2.1: According to the experimental data obtained in step 1, the parameters E1 and ε N , where the non-mechanical strain ε N The calculation formula is:

[0015] ε l - ε e = ε N

[0016] ε l is the maximum strain value in the loading stage, ε e is the elastic strain, which is the difference of the linear part of the strain value in the unloading response stage; and the elastic modulus E1 in the Maxwell unit is calculated by the formula:

[0017]

[0018] Step 2.2: According to the experimental data obtained in step 1, the formula

[0019]

[0020] The creep compliance J(t) curve is calculated;

[0021] Step 2.3: The parameters E2, τ1 and a are respectively expressed as a function expression about the creep stress σ and the experimental preforming temperature T, and the parameters E1 and ε N obtained in step 2.1 are substituted into the creep compliance J(t) curve, and the creep compliance J(t) curve obtained in step 2.2 is fitted to obtain the coefficients in the function expression of the parameters E2, τ1 and a respectively expressed as a function expression about the creep stress σ and the experimental preforming temperature T;

[0022] Step 3: Using the single equation form of the dry fiber preform creep / recovery material model obtained in step 2, the dry fiber preform thickness is predicted under the corresponding creep stress and experimental preforming temperature.

[0023] Further, in step 2.3, the parameters E2, τ1 and a are respectively expressed as a polynomial function expression about the creep stress σ and the experimental preforming temperature T:

[0024]

[0025] In the formula, the coefficients c 11 , c 12 , c 13 , c 14 , c15 ,c 16 ,c 21 ,c 22 ,c 23 ,c 24 ,c 25 ,c 26 ,c 31 ,c 32 ,c 33 ,c 34 ,c 35 ,c 36 Parameter obtained by fitting of the creep compliance J(t).

[0026] Further, in the single equation form of the dry fiber preform creep / recovery material model of step 2, the parameter η1 is not less than 1000.

[0027] Further, the dry fiber preform is a CF3031 dry fiber preform.

[0028] Advantages

[0029] The present application provides a warm pressing parameter determination method for dry fiber preform thickness control, by providing a single equation form of the dry fiber preform creep / recovery material model, and using the creep / recovery experiments under different stresses at room temperature and the creep / recovery experiments under different preforming temperatures and the same stress, obtaining the strain-time change curve of the test piece under the two test conditions, identifying the model parameters according to the curve, thereby obtaining the identified single equation form of the dry fiber preform creep / recovery material model, and finally using the single equation form of the dry fiber preform creep / recovery material model to predict the dry fiber preform thickness under the corresponding creep stress and experimental preforming temperature.

[0030] The present application overcomes the problem that the conventional model cannot smoothly transition from the creep stage to the recovery stage in the creep / recovery process, and the single equation form of the dry fiber preform creep / recovery material model established can accurately describe the time-dependent thickness creep / recovery behavior of the dry fiber fabric preform, and by comparing the model prediction results with the newly designed experimental data, the results show that the experimental and model prediction curves are basically consistent, proving the effectiveness of the method.

[0031] Additional aspects and advantages of the application will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and / or can be learned by practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0032] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the following description, taken in conjunction with the accompanying drawings, in which:

[0033] Figure 1 Parameters E1, ε N η1 identification diagram

[0034] Figure 2 Three-dimensional fitting plot of creep compliance at room temperature under different creep stresses

[0035] Figure 3 Three-dimensional fitting diagram of creep compliance under the same creep stress at different preforming temperatures

[0036] Figure 4 Comparison of predicted curves and experimental curves Detailed Implementation

[0037] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0038] This embodiment uses dry fiber fabric CF3031 as an example to illustrate the method for determining the temperature and pressure parameters for thickness control during the preforming process of dry fiber preforms.

[0039] Step 1: Conduct creep / recovery tests on the test specimens.

[0040] The fiber reinforcement material used is CF3031 carbon fiber woven fabric produced by Weihai Guangwei Company. This fabric has a 3×1 twill weave, a filament bundle size of 3K, and an area weight of 220±7 g / m². 2 The sample was cut into squares with a single-layer side length of 50mm using a CNC cutting machine and laid out at 0° to form a 24-layer preform stack.

[0041] During the experiment, the fabric laminate sample was placed between two circular steel pressure plates, each 100 mm in diameter. A WDW-100 microcomputer-controlled electronic universal testing machine was used to conduct creep / recovery tests on the specimens, including tests at room temperature (25℃) under different stresses (0.3 MPa, 0.5 MPa, 0.8 MPa) and at different pre-forming temperatures (40℃, 50℃, 60℃) under the same stress (0.3 MPa). A displacement load was applied to the specimens at a loading rate of 1 mm / min, and a sensor with a range of 100 kN recorded the strain change of the specimens under load over time, obtaining strain-time curves for the specimens under both test conditions.

[0042] like Figure 1The creep / recovery experiment process is divided into three stages: loading stage, creep stage and unloading response stage. The loading stage: during the process of stress loading to the predetermined load, the sample mainly occurs elastic deformation not more than 6 minutes; the creep stage: the sample mainly bears constant load for about 3 hours, accompanied by viscoelastic deformation and viscoplastic deformation; the unloading recovery stage: after loading for 3 hours, unloading starts, about 150 seconds, the sample mainly occurs elastic recovery; after unloading, the sample enters the free state, a load of about 10N is applied for 1 hour, and the sample mainly occurs viscoelastic deformation.

[0043] Step 2: Establish a single equation form of the dry fiber preform creep / recovery material model.

[0044] Based on the comparison of the fitting effects of the existing models, the Burgers model with the best fitting effect is first selected. The traditional Burgers model creep strain calculation formula is as follows:

[0045]

[0046]

[0047] Wherein E1 and E2 can be explained as linear spring constant or Young's modulus, and η1 and η2 are called viscosity coefficient. τ1=η2 / E2 is the relaxation time. The improved Burgers model introduces the creep stress σ and the preforming temperature T as the variables of the empirical constitutive equation coefficients, and introduces the non-mechanical strain generated due to material characteristics, fiber interstitial pores and the like. The total strain of the creep / recovery is described as follows:

[0048] ε(t)=ε1(t)+ε2(t-t0)H(t-t0)+ε N

[0049] Wherein, define ε1(t) is the strain increased from the compaction process to the end of unloading σ, according to the superposition principle, the strain generated at t0 unloading is equivalent to the strain ε2(t-t0) generated by applying the reverse force σ, then ε1(t)-ε2(t-t0) is the total strain of the unloading response stage, and H(x) is the step function, the function expression is as follows:

[0050]

[0051] Through the analysis of experimental data and fitting results, the material model based on the superposition principle method will overestimate the recovery ability of the material. On the one hand, the sizing agent on the surface of the CF3031 dry fiber fabric is a thermosensitive viscoelastic material, and the sizing agent gradually changes from a solid state to a molten state as the preforming temperature increases, resulting in mutual adhesion between the dry fiber fabric layers. On the other hand, the increase of creep stress will exacerbate the nesting behavior between the fiber bundles, so that the strain generated by unloading at t0 cannot be recovered during the recovery process, and the equivalent strain generated by the force σ with the same size and opposite direction as the creep stress. In order to consider the influence of different stresses and preforming temperatures on the recovery behavior, the coefficient a is introduced, which is defined as a function of the creep stress σ and the preforming temperature T. Based on the above analysis, the single equation form of the CF3031 dry fiber preform creep / recovery material model is as follows:

[0052]

[0053] Where σ is the set creep stress, T is the experimental preforming temperature, t0 is the unloading time, E1 is the elastic modulus in the Maxwell unit, E2 is the elastic modulus in the Kevin unit, η1 is the viscosity coefficient, τ1 is the relaxation time, ε N is the non-mechanical strain, H(x) is the step function, a is a function related to the creep stress and the preforming temperature, ε(t) is the strain value, which represents the thickness of the dry fiber preform;

[0054] According to the creep / recovery experimental data obtained in step 1, the following process is used for model parameter identification, so as to obtain the identified single equation form of the dry fiber preform creep / recovery material model:

[0055] Step 2.1: According to the experimental data obtained in step 1, the parameters E1 and ε N are calculated, where the non-mechanical strain ε N is calculated by the following formula:

[0056] ε l -ε e = ε N

[0057] ε l is the maximum strain value in the loading stage, ε e is the elastic strain, which is the difference of the linear part of the strain value in the unloading response stage; and the elastic modulus E1 in the Maxwell unit is calculated by the following formula:

[0058]

[0059] Using the creep / recovery experimental data under different creep stresses and different preforming temperatures for calculation, it is found that E1 and ε N tend to be consistent.

[0060] In addition, for the parameter η1, the size of η1 is found to be related to the slope of the fitting curve marked with an ellipse in the figure, and as η1 increases, the angle between the fitting curve and the x-axis in this part becomes smaller, and as time increases, this part of the curve gradually tends to be parallel to the x-axis. Therefore, as long as η1 is taken as a large value, generally not less than 1000. Figure 2

[0061] In this embodiment, the final calculation is

[0062]

[0063] Step 2.2: In the above model, the parameters E2, τ1, and a are quantities related to the creep stress σ and the experimental preforming temperature T. In order to obtain the expression coefficients of the three parameters, first, the creep stress σ and T are normalized, and the creep compliance J(t) curves are calculated through experimental data under different creep stresses at room temperature and different preforming temperatures under the same stress. The creep compliance J(t) calculation formula is as follows:

[0064]

[0065] Step 2.3: Express the parameters E2, τ1, and a as functions of the creep stress σ and the experimental preforming temperature T, respectively, and substitute the parameters E1 and ε N obtained in step 2.1 into the creep compliance J(t) curve, and according to the least squares principle, the creep compliance J(t) under different creep stresses at room temperature and different preforming temperatures under the same stress is fitted respectively, to obtain the coefficients in the function expression of the parameters E2, τ1, and a as functions of the creep stress σ and the experimental preforming temperature T. Figure 2 and Figure 3 are the three-dimensional fitting graphs of the creep compliance under different creep stresses at room temperature and the three-dimensional fitting graphs of the creep compliance under the same creep stress at different preforming temperatures, respectively.

[0066] In this embodiment, the parameters E2, τ1, and a are expressed as polynomial function expressions of the creep stress σ and the experimental preforming temperature T, respectively:

[0067]

[0068] In the formula, the coefficients c 11 ,c 12 ,c 13 ,c 14 ,c 15 ,c 16 ,c 21 ,c 22 ,c 23 ,c 24 ​,c 25 ,c 26 ,c 31 ,c 32 ,c 33 ,c 34 ,c 35 ,c 36 The parameters obtained by fitting the creep compliance J(t) are finally obtained as the polynomial function expression

[0069]

[0070] In order to verify the accuracy of the nonlinear viscoelastic constitutive model of the CF3031 dry fiber fabric preform, the CF3031 dry fiber fabric preform sample in the same form as the experiment above is used to design the experiment with creep holding stress σ, experimental environment preforming temperature T, and unloading time t0 as variables. Figure 4 The comparison results of the model prediction results and the experimental data are shown, and the experimental and model prediction curves are basically consistent. The results show that the material model in the form of a single equation can realize good prediction of the creep / recovery of the dry fiber preform under different creep stresses and preforming temperatures, which is convenient enough for the prediction of the nonlinear viscoelastic behavior of the material in the actual preforming process.

[0071] Finally, using the single equation form of the dry fiber preform creep / recovery material model obtained above, the dry fiber preforming thickness can be predicted under the corresponding creep stress and experimental preforming temperature.

[0072] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present application, and those of ordinary skill in the art can make changes, modifications, replacements and variations to the above embodiments without departing from the principles and purposes of the present application within the scope of the present application.

Claims

1. A method for warm press parameter determination for dry fiber preform thickness control, characterized by: The method comprises the following steps: Step 1: performing a creep / recovery experiment on the test piece, including a creep / recovery experiment under different stresses at room temperature and a creep / recovery experiment under the same stress at different preforming temperatures; obtaining a strain-time curve of the test piece under the two test conditions, and dividing the creep / recovery experiment process into three stages: a loading stage, a creep stage and an unloading response stage; Step 2: establishing a single-equation form of a dry fiber preform creep / recovery material model as follows: where σ is the set creep stress, T is the experimental preform temperature, t0 is the unloading time, E1 is the elastic modulus in Maxwell unit, E2 is the elastic modulus in Kevin unit, η1 is the viscosity coefficient, τ1 is the relaxation time, ε N is the non-mechanical strain, H(x) is the step function, a is a function related to the creep stress and preform temperature, ε(t) is the strain value, which embodies the dry fiber preform thickness; According to the creep / recovery experiment data obtained in step 1, the following process is used for model parameter identification, so as to obtain the identified single-equation form of the dry fiber preform creep / recovery material model: Step 2.1 : Calculate the parameters E1 and ε from the experimental data obtained in Step 1 N where the non-mechanical strain ε N The calculation formula is: e l - e e = e N ε l is the maximum strain value of the loading stage, ε e is the elastic strain, which is the difference of the linear part of the strain value in the unloading response stage; and the elastic modulus E1 in the Maxwell unit is calculated as follows: Step 2.2: according to the experimental data obtained in step 1, the creep compliance J(t) curve is calculated according to the formula Step 3: using the single-equation form of the dry fiber preform creep / recovery material model obtained in step 2, the dry fiber preform thickness is predicted under the corresponding creep stress and experimental preforming temperature. Step 2.3: Express the parameters E2, τ1, a as functions of the creep stress σ and the experimental preforming temperature T, respectively, and substitute the parameters E1 and ε N into the creep compliance J(t) curve and fit the creep compliance J(t) curve obtained in step 2.2 to obtain the coefficients in the expressions for the parameters E2, τ1, a as functions of the creep stress σ and the experimental preforming temperature T, respectively; In step 2.3, the parameters E2, τ1 and a are respectively expressed as a polynomial function expression about the creep stress σ and the experimental preforming temperature T:

2. A method for determining warm pressing parameters for controlling the thickness of dry fiber preforms according to claim 1, characterized in that: In the single-equation form of the dry fiber preform creep / recovery material model of step 2, the parameter η1 is not less than 1000. where the coefficient c 11 12 13 14 15 16 21 22 23 24 25 26 31 32 33 34 35 36 Parameters obtained by fitting the creep compliance J(t).​​​​​​​​​​​​​​​​​ 3. A method for determining warm pressing parameters for dry fiber preform thickness control according to claim 1, characterized in that: The dry fiber preform is a CF3031 dry fiber preform.

4. The method for determining warm pressing parameters for dry fiber preform thickness control according to claim 1, wherein: ​

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