Rheological model establishment method and rheological test analysis method for ultra-high-filled wood-plastic composites

Abstract

The invention relates to a method for establishing a rheological model and a method for rheological testing and analyzing ultra-high-filled wood-plastic composite materials, and belongs to the technical field of material analysis. The invention aims to solve the problem that it is currently impossible to obtain rheological property data of wood-plastic composite materials filled with high wood fiber content. The invention is aimed at the melt of ultra-high-filled wood-plastic composite materials, and the continuous deformation process of the melt during uniaxial compression is analogized to a creep process with continuously changing stress; the compression process is differentiated and decomposed into multiple processes with very short duration, and the stress of each process after differentiation is approximately constant; then, according to the Boltzmann superposition principle, the strain generated by each process is superimposed using the creep expression to obtain the strain expression of the compression process with continuously changing stress, that is, the compression rheological model. Then, the compression rheological model is used to realize the rheological testing and analysis of highly-filled wood-plastic composite materials. It is mainly used for the establishment of rheological models and rheological testing and analysis of ultra-high-filled wood-plastic composite materials.

Description

Technical Field

[0001] The invention relates to a modeling and testing analysis method for a filled wood-plastic composite material, and belongs to the technical field of material analysis. Background Art

[0002] At present, the wood fiber filling content of mainstream wood-plastic composites (WPC) on the market is about 65%. Further development of wood-plastic composites (HWPC) filled with high wood fiber content can not only significantly reduce the amount of petroleum-based polymers and reduce carbon emissions, but also effectively reduce production costs and increase corporate profits. It is an important development direction for the WPC industry in the future. However, high wood fiber content will bring a series of problems to the production of WPC: uneven mixing of raw materials, poor melt fluidity, increased viscosity, difficulty in molding, etc., which will directly affect the production efficiency and quality of the product. These problems are closely related to the processing characteristics of HWPC, that is, the rheological properties of the melt. Therefore, if you want to efficiently produce HWPC products with stable quality, you must first master the rheological properties of HWPC.

[0003] At present, the commonly used conventional rheological testing instruments include torque rheometer, rotational rheometer, capillary rheometer, etc., but these instruments can only perform rheological characterization on some composite materials with low filling content and good fluidity. Taking WPC as an example, when the wood powder content exceeds 60%, the melt properties change from "liquid-like" to "solid-like" or even to "quasi-solid", the fluidity becomes worse, and the structure becomes more complex. The above conventional rheological testing instruments have certain limitations when performing rheological tests on HWPC, such as: 1. The material will slip in the chamber of the torque rheometer, be coated on the rotor and cannot be mixed and exchanged, and cannot be melted and blended; 2. Due to the extremely strong "quasi-solid" characteristics in the molten state, the HWPC sample will slip between the splint when tested using a rotational rheometer (even if it is a serrated splint, it will only leave scratches on the specimen, and it cannot perform effective rheological tests); 3. Due to the poor fluidity of HWPC materials and strong "solid-like" characteristics, the die mouth will be damaged when tested using a capillary rheometer. Therefore, it is impossible to obtain accurate rheological property data such as viscosity, modulus, and damping of HWPC using conventional rheological testing instruments. Summary of the invention

[0004] The present invention aims to solve the problem that it is currently impossible to obtain rheological property data of wood-plastic composite materials filled with high wood fiber content.

[0005] The method for establishing a rheological model of an ultra-high-filled wood-plastic composite material comprises the following steps:

[0006] For the melt of ultra-high-filled wood-plastic composite materials, the continuous deformation process of the melt during uniaxial compression is analogized to a creep process with continuously changing stress; the compression process is differentiated and decomposed into multiple processes with very short duration, and the stress of each process after differentiation is approximated to be constant; then, according to the Boltzmann superposition principle, the strain generated by each process is superimposed using the creep expression to obtain the strain expression of the compression process with continuously changing stress, that is, the compression rheological model.

[0007] Furthermore, the specific process of establishing the rheological model is as follows:

[0008] First, assume that at t a Increase the load Δσ at any time a , the load is the applied stress, then the load Δσ a From a The strain generated at t is:

[0009] ε(t a →t)=Δσ a J(tt a )

[0010] Where, J is the creep compliance;

[0011] During creep:

[0012]

[0013] j=1 / E1

[0014] ψ(t)=(1-e -t / τ ) / E3

[0015] Where, τ is the relaxation time of the Maxwell model;

[0016] By analogy, the strain generated by each load added before time t at time t is:

[0017] ε(t1→t)=Δσ1J(t-t1)

[0018] ε(t2→t)=Δσ2J(t-t2)

[0019] ε(t3→t)=Δσ3J(t-t3)

[0020] …

[0021] ε(t n →t)=Δσ n J(tt n )

[0022] The total deformation caused by the additional stress at time t is:

[0023]

[0024] Among them, σ is stress, ε is strain, E is modulus, η is viscosity, and the subscripts of the parameters are used to indicate the corresponding parameters at each stage of applied load;

[0025] The compression rheological model is established based on the strain generated by each load at time t and the total deformation caused by the additional stress at time t.

[0026] Furthermore, a compression rheological model of the unloading process of the uniaxial compression experiment is established according to the strain generated by each load at time t and the total deformation generated by the additional stress at time t. The specific process includes the following steps:

[0027] During the unloading process, for the process in which the stress decreases from the maximum value to 0, the unloading process time is divided into n time periods, and the load change time is recorded as t0, t1, t2, t3...t n , the corresponding load increase at each time point is recorded as σ0, σ1, σ2, σ3...σ n ; Where t0 = 0, σ0 = 0;

[0028] According to the above analysis, the strain generated by the load increased at each time during the entire unloading process is:

[0029]

[0030]

[0031]

[0032] …

[0033]

[0034] The unloading stage of the uniaxial compression experiment of UWFPE melt is divided into an elastic response part and a viscoelastic response part; the strain of the elastic response part is:

[0035]

[0036] The strain of the viscoelastic response part:

[0037]

[0038] Then the compression rheological expression of the compression unloading stage is obtained:

[0039] ε(t)=ε(e)+ε(ve)

[0040] Among them, ε(e) is the elastic deformation and ε(ve) is the viscoelastic deformation.

[0041] Furthermore, a compression rheological model of the uniaxial compression test loading process is established according to the strain generated by each load at time t and the total deformation generated by the additional stress at time t. The specific process includes the following steps:

[0042] The strain analysis in the loading stage is expressed using the Burgers four-element model;

[0043] Apply a constant stress σ to the Burgers model, then the strain of the model at time t is:

[0044]

[0045] The analog creep process in the loading stage is divided into elastic response part, viscoelastic response part and viscous response part. Among them, the strain of the elastic response part is:

[0046]

[0047] The strain of the viscoelastic response part:

[0048]

[0049] Strain in the viscous response part:

[0050]

[0051] The compression rheological expression of the UWFPE melt uniaxial compression test in the loading stage is:

[0052] ε(t)=ε(e)+ε(ve)+ε(v).

[0053] The rheological test analysis method of highly filled wood-plastic composite materials includes the following steps:

[0054] First, the viscosity value of the viscous deformation stage of the Maxwell model is obtained, and the initial value of the viscosity of the viscous elements of the Maxwell model in the Burgers model is obtained;

[0055] The Burgers model is composed of the Maxwell model and the Kelvin model in series;

[0056] Then, the modulus and viscosity values ​​in the Kelvin model are obtained, and the initial viscosity values ​​of some viscous components of the Kelvin model in the Burgers model are obtained;

[0057] The initial values ​​of the compression rheological expressions in the loading and unloading stages are determined according to the initial values ​​of the Burgers model. Then, the nonlinear regression calculation of the compression rheological expressions in the loading and unloading stages containing the initial values ​​is performed using SAPSS combined with experimental data to obtain the true value of each parameter in the compression rheological expressions in the loading and unloading stages during the compression process of the highly filled wood-plastic composite melt. The rheological test analysis of the highly filled wood-plastic composite material is realized based on the compression rheological expressions.

[0058] Furthermore, the viscosity value of the viscous deformation stage of the Maxwell model is obtained through a uniaxial compression creep test of the melt and / or a stress relaxation test of the melt.

[0059] Furthermore, the process of obtaining the viscosity value of the viscous deformation stage of the Maxwell model through the uniaxial compression creep experiment of the melt includes the following steps:

[0060] The constant load mode in the universal mechanical testing machine is used to record the time and displacement data of the upper platen during the creep process in real time for calculating the creep process. The viscosity value is calculated based on the slope of the linear creep stage of the creep process of the melt under a fixed axial load.

[0061] Furthermore, the viscosity value of the viscous deformation stage of the Maxwell model is taken as the average value of the viscosity values ​​of the viscous deformation stage of the Maxwell model obtained from the uniaxial compression creep experiment of the melt under different constant load modes.

[0062] Furthermore, the process of obtaining the viscosity value of the viscous deformation stage of the Maxwell model through the stress relaxation experiment of the melt includes the following steps:

[0063] The Maxwell model is used to simulate the stress relaxation process. According to the creep expression of the Maxwell model, the strain in the stress relaxation process remains unchanged, that is, dε / dt=0, then:

[0064]

[0065] Among them, σ is stress, ε is strain, t is time; E is modulus, η is viscosity;

[0066] Separating the variables in the above formula yields:

[0067]

[0068] The boundary conditions for solving this differential equation are: t = 0, σ = σ0; t = t, σ = σ(t); and then we get:

[0069]

[0070] Wherein, e is a natural number;

[0071] Then the stress relaxation expression of the Maxwell model is obtained as follows:

[0072] σ(t)=σ0e -t / τ τ=η / E

[0073] Where, τ is the relaxation time of the Maxwell model;

[0074] The viscosity of the melt is calculated based on the stress relaxation expression of the Maxwell model.

[0075] Furthermore, the process of obtaining the modulus and viscosity values ​​in the Kelvin model includes the following steps:

[0076] Apply a constant stress σ to the Kelvin model, then the strain of the model at time t is:

[0077]

[0078] Among them, τ d =η 3 / E3 represents the relaxation time of the Kelvin model;

[0079] According to the components of the entire Burgers model, the elastic element in series and the elastic element in parallel are parts with the same properties; according to the creep expression of the Kelvin model, its viscosity value η3=τ d E3; The viscosity value of the viscous element in the Kelvin model is obtained based on the relaxation time of the Kelvin model and the modulus of the parallel elastic element part.

[0080] Beneficial effects:

[0081] The present invention can realize modeling and analysis of the rheological properties of wood-plastic composite materials filled with high wood fiber content, thereby obtaining rheological property data of the wood-plastic composite materials filled with high wood fiber content, thereby effectively providing reference and guidance for the processing and production of wood-plastic composite materials filled with high wood fiber content, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Figure 1 Schematic diagram of the model involved in the compression rheological model of the uniaxial compression process;

[0083] Figure 2 It is a superimposed schematic diagram of the creep process;

[0084] Figure 3 The creep process of UWFPE melt under two different fixed axial loads;

[0085] Figure 4It is the stress relaxation process of UWFPE melt under a fixed compression deformation of 0.45 mm for 30 minutes;

[0086] Figure 5a and Figure 5b It is the loading stage of the uniaxial cyclic compression process of UWFPE melt under the compression speed of 2mm / min ( Figure 5a ) and uninstallation phases ( Figure 5b )'s stress-strain curves of the compression rheology model calculation results and experimental results;

[0087] Figure 6 Correction curve of loading calculation results for UWFPE melt during uniaxial compression. DETAILED DESCRIPTION

[0088] In order to fully explain the present invention, the basis of the model selected in the present invention is first explained. The present invention compares the continuous deformation process of the UHWPC melt in the uniaxial compression process to a creep process with continuously changing stress, and the Boltzmann superposition principle is a method that can continuously superimpose the history of load on material deformation, so it can be used to process the stress and strain data of the UHWPC melt in the uniaxial compression process to comply with the calculation of the creep model. The present invention uses a combination of the Boltzmann superposition principle and the creep model to establish a compression rheological model of the UWFPE melt in the uniaxial compression process. The compression rheological model of the uniaxial compression process involves models such as Figure 1 As shown, Figure 1 Where σ is stress, ε is strain, E is modulus, and η is viscosity; the stress in the Maxwell model is σ, the corresponding strains of the elastic element and viscous element are ε1 and ε2 respectively, the modulus of the elastic element is E1, and the viscosity of the viscous element is η2; similarly, the strain and modulus of the elastic element in the Kelvin model are ε3 and E3 respectively, and the viscosity of the viscous element is η3; and the Burgers model is composed of a Maxwell model and a Kelvin model connected in series. Specific implementation method one:

[0090] This embodiment is a method for establishing a rheological model of an ultra-high-filled wood-plastic composite material, comprising the following steps:

[0091] 1. The continuous deformation process of UHWPC melt during uniaxial compression is analogous to a creep process with continuous stress change;

[0092] 2. Differentiate the compression process and decompose it into multiple processes with very short duration;

[0093] 3. Since the duration of each process after differentiation is very short, the stress of each process after differentiation can be approximately considered as invariant, that is, each process is approximately considered as a creep process with invariant stress and very short duration.

[0094] 4. According to the Boltzmann superposition principle, the strains generated by each process are superimposed using the creep expression, and the strain expression of the compression process with continuously changing stress can be obtained, that is, the compression rheological model.

[0095] Specifically, in the experiment with continuously changing load, consider the case of having several step-increasing load programs: at times t1, t2, t3... t n Extra stresses Δσ1, Δσ2, Δσ3... Δσ n Are respectively applied to the specimen, then the strain generated by each process during which the extra stress persists can be calculated using the creep model. According to the content of the Boltzmann superposition principle, the strain generated in the specimen at time t should be the sum of the strains generated by all the loads increased before time t, as Figure 2 Shown.

[0096] First, assume that at time t a The load (stress) Δσ a Is increased, where t a <t, then the strain generated by the load Δσ a From t a To t is:

[0097] ε(t a →t) = Δσ a J(t - t a )

[0098] Where J is the creep compliance;

[0099] During the creep process:

[0100]

[0101] j = 1 / E1

[0102] ψ(t) = (1 - e -t / τ ) / E3

[0103] Where τ is the relaxation time of the Maxwell model.

[0104] And so on, the strains generated by each load increased before time t at time t are successively:

[0105] ε(t1→t) = Δσ1J(t - t1)

[0106] ε(t2→t)=Δσ2J(t-t2)

[0107] ε(t3→t)=Δσ3J(t-t3)

[0108] …

[0109] ε(t n →t)=Δσ n J(tt n )

[0110] Then the total deformation caused by these additional stresses at time t is:

[0111]

[0112] Based on the above process, the present invention divides the compression process into two parts: loading phase and unloading phase for modeling:

[0113] a. Establish the compression rheological model of the unloading process of uniaxial compression experiment:

[0114] Since the unloading process of the uniaxial compression test is carried out at a certain deformation recovery rate, that is, the return rate of the mechanical testing machine is certain, the unloading process is different from the traditional creep recovery process of instantaneously removing all loads.

[0115] In this experiment, UWFPE was subjected to uniaxial compression test in the melt state, so structural changes occurred during the compression process. The structural characteristics during the unloading process are determined by the structural characteristics of the material at the final moment of the loading process, which will lead to obvious differences in the parameters of the analog creep expression during the loading process and the unloading process.

[0116] During the unloading process, the stress decreases from the maximum value to 0, and the strain recovery process is analyzed. By analogy, this unloading process at a certain deformation recovery rate is analogized to a creep process with continuous change of reverse stress: (1) The continuous reduction process of stress is analogized to the continuous increase process of stress; (2) The continuous recovery process of strain is analogized to the continuous increase process of strain. Since the strain in the unloading process is the strain recovered within the unloading time, there is no plastic strain in the strain, and it is necessary to use the combined model of the elastic element in the three-parameter solid model and the Kelvin model for calculation.

[0117] According to the Boltzmann superposition principle, the deformation history of the UWFPE melt generated by the loads added at different times needs to be added to calculate the overall strain of the sample, so it is necessary to artificially set the time t of different load additions. x; The present invention utilizes the function of real-time recording of force, time and displacement of the mechanical testing machine during the experiment. The default interval time of collecting two adjacent data points in the software is the interval time of adding different loads. The CMT5504 universal mechanical testing machine used in this experiment records the interval time of two adjacent data points as 0.03s during the experiment.

[0118] First, the unloading process is divided into n time periods of 0.03s. According to the Boltzmann superposition principle, it is approximately assumed that the load in each time period is constant, that is, the stress continuous change characteristics of the uniaxial cyclic compression process are decomposed into n loads that can last for 0.03s according to the differential principle. The load change time is recorded as t0, t1, t2, t3...t n , the corresponding load increase at each time point compared to 0.03s before is recorded as σ0, σ1, σ2, σ3...σ n . Where t0=0,σ0=0.

[0119] According to the above analysis, the strain generated by the load increased at each time during the entire unloading process is:

[0120]

[0121]

[0122]

[0123] …

[0124]

[0125] According to the characteristics of the formula, the analog creep calculation formula of the unloading stage of the UWFPE melt uniaxial compression experiment can be divided into two parts, one is the elastic response part and the other is the viscoelastic response part; among them,

[0126] Elastic response part:

[0127]

[0128] Viscoelastic response part:

[0129]

[0130] As mentioned above, the analog creep process of the unloading stage of the uniaxial compression experiment of UWPC melt can be divided into the elastic response part and the viscoelastic response part, that is, the compression rheological expression:

[0131] ε(t)=ε(e)+ε(ve)

[0132] Among them, ε(e) is the elastic deformation and ε(ve) is the viscoelastic deformation.

[0133] b. Establish the compression rheological model of the loading process of uniaxial compression experiment:

[0134] The analysis method and data processing of the analog creep process in the loading stage of the UWFPE melt uniaxial compression experiment are the same as those in the unloading stage, which are described in detail in the above unloading stage and will not be described in detail here. According to the analysis of the UWFPE melt uniaxial compression experiment process, irreversible viscous strain occurs in the loading stage, so the strain analysis in the loading stage needs to be expressed by the Burgers four-element model.

[0135] Apply a constant stress σ to the Burgers model, then the strain of the model at time t is:

[0136]

[0137] According to the analysis of the unloading stage, the analog creep process in the loading stage can be divided into three parts: elastic response part, viscoelastic response part and viscous response part; among them,

[0138] Elastic response part:

[0139]

[0140] Viscoelastic response part:

[0141]

[0142] The strain of the viscous response part is calculated as follows:

[0143]

[0144] It should be noted here that the components represented by the relevant parameters in the strain calculation formula need to correspond to the components used in the Burgers model.

[0145] Therefore, the compression rheological expression of the UWFPE melt in the loading stage of the uniaxial compression experiment is:

[0146] ε(t)=ε(e)+ε(ve)+ε(v)

[0147] The present invention can realize modeling of the rheological properties of wood-plastic composite materials filled with high wood fiber content. Specific implementation method 2:

[0149] This embodiment is a rheological test analysis method for a highly filled wood-plastic composite material, which specifically includes the following steps:

[0150] First, the viscosity value of the viscous deformation stage of the Maxwell model is obtained, and the initial value of the viscosity of the viscous elements of the Maxwell model in the Burgers model is obtained;

[0151] The Burgers model is composed of the Maxwell model and the Kelvin model in series;

[0152] Then, the modulus and viscosity values ​​in the Kelvin model are obtained, and the initial viscosity values ​​of some viscous components of the Kelvin model in the Burgers model are obtained;

[0153] The initial values ​​of the compression rheological expressions in the loading and unloading stages were determined according to the initial values ​​of the Burgers model. Then, SAPSS was used in combination with experimental data to perform nonlinear regression calculations on the compression rheological expressions in the loading and unloading stages containing the initial values. The true value of each parameter in the compression rheological expressions in the loading and unloading stages during the compression process of UHWPC was obtained, and the rheological test analysis of highly filled wood-plastic composites was realized based on the compression rheological expressions.

[0154] In order to determine the specific compression rheological expression of each specimen, it is necessary to use the multivariate nonlinear regression calculation function in the SPSS software to solve the compression rheological expression parameters of the UWFPE melt uniaxial compression process. Therefore, the initial values ​​of each parameter in the expression are required as the input values ​​of the regression calculation. Therefore, it is necessary to first obtain the initial values ​​of each parameter in the compression rheological expression.

[0155] S1. Determine the initial value of the compression rheological model expression:

[0156] (a) Conducting uniaxial compression creep test on UWFPE (UWFPE is UHWPC based on high-density polyethylene) melt:

[0157] The constant load mode of the universal mechanical testing machine is used to record the time and displacement data of the upper platen during the creep process in real time for the calculation of the creep process. The creep time is 30 minutes.

[0158] Figure 3 The creep process of UWFPE melt under two different fixed axial loads. Figure 3 The black dashed line in the figure represents the linear creep stage of the UWFPE melt. The linear creep process is the response of the viscous element to a fixed load, which is manifested as a linear relationship between strain and time. Therefore, the viscosity value of this process can be calculated using the data of the linear creep stage. Since the creep experiment is carried out in the molten state of UWFPE, the viscous response will be more obvious than that of solid UWFPE, so the Maxwell model can be used to solve the viscosity of the series viscous element. According to the strain calculation formula of the Maxwell model, Figure 3 The slope of the black dashed line is the ratio of stress to viscosity, so viscosity is equal to the ratio of stress to slope.

[0159] Apply a constant stress σ to the Maxwell model, then the strain at time t is:

[0160]

[0161] Using the linear creep data to perform a linear fit, we can obtain Figure 3 The slope of the black dashed line in the figure is shown in Table 1. The viscosity of UWFPE at fixed loads of 2000N and 3000N can be obtained by calculation, as shown in Table 1. The viscosity of the viscous deformation stage calculated under the two fixed loads is 10 10 Although the calculated viscosities under the two loads are slightly different in value, this difference is negligible under such a large order of magnitude. It can be considered that the viscosity calculated by creep in the viscous deformation stage is of the order of 10 10 Pa·s. The specific value of viscosity in the viscous deformation stage is the average value of the calculation results of the two loads.

[0162] Table 1 Viscosity calculated using linear creep stage data of UWFPE melt

[0163]

[0164] In order to reduce errors, the present invention selects to obtain viscosity values ​​under fixed loads of 2000N and 3000N. In the subsequent S2, the average value of the viscosity values ​​obtained from the creep data under the two fixed loads is used as the initial value of the viscosity in the regression calculation; while in the actual rheological test process, only one fixed load can be used to obtain the viscosity value; at the same time, it should be noted that when obtaining the corresponding viscosity value under the fixed load, it is not limited to 2000N and / or 3000N, and other loads can be used as long as the experimental materials and experimental conditions permit.

[0165] It should also be noted that in order to verify that the viscosity value obtained by the present invention is appropriate, a viscosity value is obtained by performing a stress relaxation experiment on a UWFPE melt, and verification is performed by using the viscosity value. In fact, in the subsequent S2, the corresponding viscosity value can also be used as the initial value of the viscosity in the regression calculation; it can also be the average value of the viscosity value obtained by the creep data under a fixed load in S1 and the viscosity value obtained by the stress relaxation experiment on the UWFPE melt.

[0166] (b) Stress relaxation experiment of UWFPE melt:

[0167] Stress relaxation refers to the phenomenon that the total deformation of the specimen remains unchanged, but as time goes on, the plastic deformation continues to increase, the elastic deformation decreases accordingly, and the stress slowly decreases over time. Its essence is that the molecular chain slowly slides (viscosity) over time and gradually moves in the direction of the external force to reduce or eliminate the internal stress, that is, the process of transformation from elastic strain to viscous strain. Stress relaxation and creep are essentially the same. Stress relaxation can be regarded as a "multi-stage" creep with continuously decreasing stress, so the process of stress relaxation can also be used to calculate the viscosity in the viscous strain stage.

[0168] The Maxwell model can be used to simulate a simple stress relaxation process. According to the creep expression of the Maxwell model, the strain in the stress relaxation process remains unchanged, that is, dε / dt=0, then:

[0169]

[0170] Among them, σ is stress, ε is strain, t is time, E is modulus, and η is viscosity.

[0171] Separating the variables in the above formula yields:

[0172]

[0173] This is a variable separable differential equation. The boundary conditions for solving this differential equation are: t = 0, σ = σ0; t = t, σ = σ(t). Further, we can get:

[0174]

[0175] Wherein, e is a natural number;

[0176] Therefore, the stress relaxation expression of the Maxwell model can be obtained as follows:

[0177] σ(t)=σ0e -t / τ τ=η / E

[0178] Here, τ is the relaxation time of the Maxwell model.

[0179] Figure 4 This is the 30-minute stress relaxation process of UWFPE melt under a fixed compression deformation of 0.45 mm. As shown in the figure, according to the relaxation expression of the Maxwell model, it can be calculated that the approximate relaxation time of UWFPE melt under a compression deformation of 0.45 mm is 698 s.

[0180] The viscosity of the UWFPE melt can be calculated based on the stress relaxation expression of the Maxwell model, as shown in Table 2. The viscosity values ​​calculated using the modulus obtained from the loading stage data and the modulus obtained from the stress relaxation data are at the same order of magnitude, but there is a slight difference from the viscosity value obtained using the creep data, which may be caused by the different calculation methods of the two. However, in the case of this experiment, this difference is allowed, and it can still be considered that the viscosity value of the UWFPE melt obtained by creep and stress relaxation in the plastic strain stage is on the order of 10 9 ~10 10 Pa·s.

[0181] Table 2 Calculation of viscosity using modulus and relaxation data of UWFPE melt loading stage

[0182]

[0183] The viscosity values ​​obtained by creep test and stress relaxation test are close, which verifies the correctness of the method of determining melt viscosity value by creep test and stress relaxation test. In the subsequent calculation, for the convenience of calculation, we take the average of the two viscosity values ​​obtained by creep test with load of 2000N and 3000N as the initial value of viscosity of UWFPE melt, that is, the initial value of viscosity of series viscosity element in Maxwell model is 10 10 Pa·s order of magnitude.

[0184] S2. Determine the initial values ​​of component parameters in the Kelvin model:

[0185] The viscosity value of the viscous element in the Burgers model is obtained by creep experiment and / or stress relaxation experiment. In this embodiment, the average value of the viscosity value obtained by creep data under two fixed loads is used as the initial value of viscosity in the regression calculation. The initial value of the modulus is obtained by regression calculation using the effective value in the loading stage, and the Burgers model still requires the initial values ​​of the other two component parameters, namely the initial values ​​of the modulus and viscosity in the Kelvin model.

[0186] Apply a constant stress σ to the Kelvin model, then the strain of the model at time t is:

[0187]

[0188] Among them, τ d =η 3 / E3 represents the relaxation time of the Kelvin model.

[0189] According to the components of the entire Burgers model, the elastic elements in series and the elastic elements in parallel can be considered to be parts with the same properties, so the values ​​of E1 and E3 are by default in the same order of magnitude, and the initial values ​​of E1 and E3 can be set to the same value in the regression calculation. According to the creep expression of the Kelvin model, the exponential parameter τ in the formula d =η3 / E3, which represents the relaxation time of the model, that is, the relaxation time of viscoelasticity, so the viscosity value η3 = τ d E3. The viscosity value of the viscous element in the Kelvin model can be obtained by knowing only the relaxation time of the Kelvin model and the modulus of the parallel elastic element in the model.

[0190] The strain experienced during the unloading phase of the uniaxial cyclic compression test represents the deformation that can be recovered. Due to the extremely strong solid-like properties of UWFPE, the plastic strain generated during the unloading phase can be ignored, that is, the entire unloading process can be approximately regarded as a process of viscoelastic strain recovery. The creep expression of the Burgers model can be used to remove the series viscosity element to express it and calculate the relaxation time of the Kelvin model. According to the experimental data, the relaxation time of the Kelvin model of the WFPE70 (UHWPC with 70% wood powder content) sample during the unloading phase is 10 1 Therefore, the viscosity value of the viscous element in the Kelvin model of the WFPE70 specimen (the parallel viscous element in the Burgers model) can be determined to be 10 7 Pa·s order of magnitude.

[0191] At this point, the initial values ​​of the compression rheological expressions in the loading and unloading stages are determined. Then, the SAPSS software is used in combination with the experimental data to perform nonlinear regression calculations on the compression rheological expressions in the loading and unloading stages containing the initial values, and the true values ​​of each parameter in the compression rheological expression in each stage can be obtained.

[0192] After the initial values ​​of the compression rheological expressions in the loading stage and the unloading stage are determined, the rheological test analysis of the highly filled wood-plastic composite material is realized according to the compression rheological expressions.

[0193] In order to verify the feasibility and effectiveness of the present invention, the present invention is verified below.

[0194] Validation of the compression rheology model: Figure 5a and Figure 5bIt is a stress-strain curve diagram of the calculation results of the compression rheology model and the experimental results of the loading and unloading stages of the uniaxial cyclic compression process of UWFPE melt under the condition of a compression speed of 2mm / min. It can be seen from the figure that the calculation results of UWFPE melt using the Boltzmann superposition principle and the three-parameter model (hereinafter referred to as the calculation results) are in good agreement with the experimental data in the unloading stage, while the calculation results combined with the Burgers model are different from the experimental data in the loading stage, as shown by the arrows (blue) in the figure.

[0195] The difference between the calculated data curve and the experimental data curve during the loading phase is caused by two reasons: (1) the concave part of the actual compression curve (“virtual strain” ε u ); (2) The compression rheology model is analogous to the creep model. After the compression process begins, the sample will reach the instantaneous modulus stage of the creep model after a period of time due to the existence of the "virtual strain" part. That is, the compression rheology model analogous to the creep model will not have the "virtual strain" data during calculation, so there is a strain difference between the two calculation methods, and this strain difference is the "virtual strain" ε u The calculation result curve of the loading stage is calculated according to ε u The value is corrected (translated to the left along the horizontal axis ε u ) after that, it almost coincides with the experimental data curve, such as Figure 6 As shown, it is proved that the present invention is correct.

[0196] After correction, the compression rheological equations of WFPE70 in the loading stage are:

[0197]

[0198] Among them, ε(t) is the strain at time t; σ(t) is the stress at time t, in MPa; t is the time, in s.

[0199] Figure 6 The calculated result curve after correction is almost the same as the experimental result curve, which shows that the Boltzmann superposition principle is applicable to the superposition processing of data in the uniaxial compression process of UWFPE melt. At the same time, the analogy method of dividing the compression process into two parts, the loading stage and the unloading stage, and considering both stages as the creep process with continuous stress change is correct. Through this method, the uniaxial compression process of UWFPE melt is well simulated, and the parameters of the compression rheological equation in the compression process can be obtained, and the rheological behavior of the whole process is discussed and analyzed.

[0200] The present invention may also have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art may make various corresponding changes and modifications based on the present invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the present invention.

Claims

1. A method for establishing a rheological model of ultra-high-filled wood-plastic composite materials, characterized in that: The following steps are involved: For the melt of ultra-high-filled wood-plastic composites, the continuous deformation process of the melt during uniaxial compression is analogized to the creep process of continuous stress change; Differentiate the compression process and decompose it into several processes with very short duration, and approximate the stress of each process after differentiation to be constant; Then, according to the Boltzmann superposition principle, the strain generated in each process is superimposed using the creep expression to obtain the strain expression of the compression process with continuous stress change, that is, the compression rheological model; the specific process of establishing the rheological model is as follows: First, assume that at t a Increase the load Δσ at any time a , the load is the applied stress, then the load Δσ a From a The strain generated by t is: ε(t a →t)=Δσ a J(tt a ), where J is the creep compliance; During creep: j=1 / E1 ψ(t)=(1-e -t / τ ) / E3 Where, τ is the relaxation time of the Maxwell model; By analogy, the strain generated by each load added before time t at time t is: ε(t1→t)=Δσ1J(t-t1) ε(t2→t)=Δσ2J(t-t2) ε(t3→t)=Δσ3J(t-t3) … ε(t n →t)=Δσ n J(t-t n ) The total deformation caused by the additional stress at time t is: Among them, σ is stress, ε is strain, E is modulus, η is viscosity, and the subscripts of the parameters are used to indicate the corresponding parameters at each stage of applied load; A compression rheological model is established based on the strain generated by each load at time t and the total deformation generated by the additional stress at time t; a compression rheological model of the unloading process of a uniaxial compression experiment is established based on the strain generated by each load at time t and the total deformation generated by the additional stress at time t. The specific process includes the following steps: During the unloading process, for the process in which the stress decreases from the maximum value to 0, the unloading process time is divided into n time periods, and the load change time is recorded as t0, t1, t2, t3...t n , the corresponding load increase at each time point is recorded as σ0, σ1, σ2, σ3...σ n ; Where t0 = 0, σ0 = 0; According to the above analysis, the strain generated by the load increased at each time during the entire unloading process is: … The unloading stage of the uniaxial compression experiment of UWFPE melt is divided into an elastic response part and a viscoelastic response part; the strain of the elastic response part is: The strain of the viscoelastic response part: Then the compression rheological expression of the compression unloading stage is obtained: ε(t)=ε(e)+ε(ve) Among them, ε(e) is the elastic deformation and ε(ve) is the viscoelastic deformation.

2. The method for establishing a rheological model of an ultra-high-filled wood-plastic composite material according to claim 1, characterized in that: The compression rheological model of the uniaxial compression test loading process is established according to the strain generated by each load at time t and the total deformation generated by the additional stress at time t. The specific process includes the following steps: The strain analysis in the loading stage is expressed using the Burgers four-element model; Apply a constant stress σ to the Burgers model, then the strain of the model at time t is: The analog creep process in the loading stage is divided into elastic response part, viscoelastic response part and viscous response part. Among them, the strain of the elastic response part is: The strain of the viscoelastic response part: Strain in the viscous response part: The compression rheological expression of the UWFPE melt uniaxial compression test in the loading stage is: ε(t)=ε(e)+ε(ve)+ε(v).

3. The rheological test analysis method of highly filled wood-plastic composite materials is characterized by: The following steps are involved: First, the viscosity value of the viscous deformation stage of the Maxwell model is obtained, and the initial value of the viscosity of the viscous elements of the Maxwell model in the Burgers model is obtained; The Burgers model is composed of the Maxwell model and the Kelvin model in series; Then, the modulus and viscosity values ​​in the Kelvin model are obtained, and the initial viscosity values ​​of some viscous components of the Kelvin model in the Burgers model are obtained; The initial values ​​of the compression rheological expressions in the loading stage and the unloading stage are determined according to the initial values ​​of the Burgers model, and then the nonlinear regression calculation of the compression rheological expressions in the loading stage and the unloading stage containing the initial values ​​is performed using SPSS combined with experimental data to obtain the true value of each parameter in the compression rheological expressions in the loading stage and the unloading stage in the compression process of the high-filled wood-plastic composite material melt, and the rheological test analysis of the high-filled wood-plastic composite material is realized according to the compression rheological expressions; the compression rheological expressions in the loading stage and the unloading stage are the compression rheological expressions obtained in the loading process and the compression rheological expressions obtained in the unloading process in the method for establishing the rheological model of the ultra-high-filled wood-plastic composite material according to claim 2.

4. The rheological test analysis method for highly filled wood-plastic composite materials according to claim 3, characterized in that: The viscosity value of the viscous deformation stage of the Maxwell model is obtained through a uniaxial compression creep test of the melt and / or a stress relaxation test of the melt.

5. The rheological test analysis method for highly filled wood-plastic composite materials according to claim 4, characterized in that: The process of obtaining the viscosity value of the viscous deformation stage of the Maxwell model through a uniaxial compression creep experiment of the melt includes the following steps: The constant load mode in the universal mechanical testing machine is used to record the time and displacement data of the upper platen during the creep process in real time for calculating the creep process. The viscosity value is calculated based on the slope of the linear creep stage of the creep process of the melt under a fixed axial load.

6. The rheological test analysis method for highly filled wood-plastic composite materials according to claim 5, characterized in that: The viscosity value of the viscous deformation stage of the Maxwell model is taken as the average value of the viscosity value of the viscous deformation stage of the Maxwell model obtained by the uniaxial compression creep experiment of the melt under different constant load modes.

7. The rheological test analysis method for highly filled wood-plastic composite materials according to claim 4, characterized in that: The process of obtaining the viscosity value of the viscous deformation stage of the Maxwell model through a melt stress relaxation experiment includes the following steps: The Maxwell model is used to simulate the stress relaxation process. According to the creep expression of the Maxwell model, the strain in the stress relaxation process remains unchanged, that is, dε / dt=0, then: Among them, σ is stress, ε is strain, t is time; E is modulus, η is viscosity; Separating the variables in the above formula yields: The boundary conditions for solving this differential equation are: t = 0, σ = σ0; t = t, σ = σ(t); and then we get: Wherein, e is a natural number; Then the stress relaxation expression of the Maxwell model is obtained as follows: σ(t)=σ0e -t / τ τ=η / E Where, τ is the relaxation time of the Maxwell model; The viscosity of the melt is calculated based on the stress relaxation expression of the Maxwell model.

8. The rheological test analysis method for a highly filled wood-plastic composite material according to claim 4, 5, 6 or 7, characterized in that: The process of obtaining the modulus and viscosity values ​​in the Kelvin model consists of the following steps: Apply a constant stress σ to the Kelvin model, then the strain of the model at time t is: Among them, τ d =η 3 / E3 represents the relaxation time of the Kelvin model; According to the components of the entire Burgers model, the elastic element in series and the elastic element in parallel are parts with the same properties; according to the creep expression of the Kelvin model, its viscosity value η3=τ d E3; The viscosity value of the viscous element in the Kelvin model is obtained based on the relaxation time of the Kelvin model and the modulus of the parallel elastic element part.

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