Terahertz plasticity coefficient and elasticity coefficient measurement method and system for polymer materials

Abstract

The invention provides a method for measuring a terahertz plastic coefficient and an elastic coefficient of a polymer material, comprising the following steps: obtaining a theoretical model of elastic strain and plastic strain modulation of a refractive index according to a refractive index change caused by elastic deformation and a refractive index change caused by plastic deformation at a terahertz frequency, and establishing an error function according to the theoretical model; performing loading-unloading cyclic uniaxial stretching on a sample, and obtaining a plurality of groups of refractive index variation data in different loading states when a polarization direction of a terahertz time-domain spectroscopy system is consistent with a stretching direction and a plurality of groups of refractive index variation data in different loading states when a polarization direction of the terahertz time-domain spectroscopy system is perpendicular to the stretching direction; calculating a minimum value of the error function through an iterative algorithm according to the acquired data and in combination with an error function; and calculating a terahertz elastic coefficient and a terahertz plastic coefficient according to the minimum value of the error function, so that the measurement result is accurate and the error is small.

Description

Technical Field

[0001] The present invention belongs to the technical field of material mechanics, and relates to a method and system for measuring terahertz plastic coefficient and elastic coefficient of a polymer material, and in particular to a method and system for measuring terahertz-plastic coefficient and terahertz-elastic coefficient of a polymer material based on terahertz time-domain spectroscopy. Background Art

[0002] Terahertz time-domain spectroscopy can be used to detect the internal parameters of dielectric materials, such as the refractive index inside the material. The refractive index change affected by the stress of the material can be calculated based on the stress-refractive index theory by the change in the refractive index of the material. To characterize stress using terahertz time-domain spectroscopy, it is necessary to build a polarized terahertz time-domain spectroscopy system. In 2007, Japanese scholar Shin-ichiro Ebara et al. constructed a polarized terahertz time-domain spectroscopy system and experimentally measured the refractive index change caused by stretching polytetrafluoroethylene materials. In 2011, Tsuguhiro Takahashi et al. improved the experimental system to extract the internal stress information of the material through the change in the intensity of the terahertz transmission signal. In recent years, Wang Zhiyong et al. from Tianjin University have used terahertz time-domain spectroscopy technology for mechanical measurements. In 2014, Song Wei et al. rebuilt the traditional THz-TDS system and proposed the experimental photoelastic measurement principle in the terahertz band based on the law of stress optics. They used experiments to verify the photoelastic theory in the terahertz band and measured the internal stress of polytetrafluoroethylene materials using the improved experimental system. In 2021, Kang Kai, Wang Zhiyong and others proposed a method for measuring internal stress fields based on terahertz time-domain spectroscopy, and in 2022 combined it with the CT algorithm to realize three-dimensional characterization of internal stress.

[0003] The above research work is all aimed at measuring the elastic stress of polymer materials. When the stress on the material exceeds the yield limit and the material undergoes plastic deformation, the plane stress characterization method based on photoelasticity is no longer applicable. The greater the plastic deformation, the greater the error of this stress characterization method. Summary of the invention

[0004] In view of the above problems, the present invention provides a method and system for measuring the terahertz plastic coefficient and elastic coefficient of a polymer material to solve the above or other former problems existing in the prior art.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for measuring the terahertz plastic coefficient and elastic coefficient of a polymer material, wherein the terahertz-plastic coefficient and the terahertz-elastic coefficient are measured when the polymer material undergoes elastic-plastic deformation, and the method comprises the following steps:

[0006] Under terahertz frequency, a theoretical model of refractive index modulation by elastic strain and plastic strain is obtained according to the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation, and an error function is established according to the theoretical model;

[0007] The sample is subjected to a loading-unloading cyclic uniaxial stretching to obtain a plurality of groups of refractive index variation data in different loading states when the polarization direction of the terahertz time-domain spectroscopy system is consistent with the stretching direction and a plurality of groups of refractive index variation data in different loading states when the polarization direction of the terahertz time-domain spectroscopy system is perpendicular to the stretching direction;

[0008] According to the acquired data and in combination with the error function, the minimum value of the error function is calculated by an iterative algorithm;

[0009] The terahertz-elastic coefficient and the terahertz-plastic coefficient are calculated according to the minimum value of the error function.

[0010] Furthermore, the loading-unloading cycle includes loading, unloading, reloading and re-unloading in sequence. During the loading process and the reloading process, the limit load is greater than the elastic limit stress of the sample, so that the sample enters an elastic-plastic deformation state.

[0011] Furthermore, when the polymer material undergoes elastic-plastic deformation, the total change in refractive index is calculated as:

[0012] Δn=C e ·ε e +C p ·ε p

[0013] Where Δn is the total change in refractive index, ε e is the elastic strain, ε p is the plastic strain, C e is the terahertz-elastic coefficient, C p is the terahertz-plasticity coefficient.

[0014] Furthermore, the theoretical model for obtaining elastic strain and plastic strain according to the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation includes:

[0015] Calculate the refractive index change caused by elastic deformation along the two principal optical axes of the sample under plane stress state;

[0016] Calculate the refractive index change caused by plastic deformation along the two principal optical axes of the sample under plane stress state;

[0017] The theoretical model of refractive index modulation by elastic strain and plastic strain is obtained according to the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation.

[0018] Furthermore, the calculation formulas for the refractive index changes caused by elastic deformation along the two main optical axes are:

[0019] Δn e1 =C e1 ·ε e1 +C e2 ·ε e2

[0020] Δn e2 =C e1 ·ε e2 +C e2 ·ε e1

[0021] Among them, Δn e1 and Δn e2 are the refractive index changes along the two principal optical axes, C e1 and C e2 are the terahertz photoelastic coefficients in the two principal stress directions, ε e1 and ε e2 is the principal elastic strain.

[0022] Furthermore, the calculation formulas for the refractive index changes caused by plastic deformation along the two main optical axes are:

[0023] Δn p1 =C p1 ·ε p1 +C p2 ·ε p2

[0024] Δn p2 =C p1 ·ε p2 +C p2 ·ε p1

[0025] Among them, Δn p1 and Δn p2 are the refractive index changes along the two principal optical axes, C p1 and C p2 is the terahertz plasticity coefficient, ε p1 and ε p2 is the principal plastic strain.

[0026] Furthermore, the theoretical model of elastic strain and plastic strain modulation on refractive index is:

[0027]

[0028] Among them, Δn 1 and Δn 2 is the total refractive index variation along the principal optical axis.

[0029] Furthermore, the calculation formula of the error function is:

[0030]

[0031] Among them, Δn i represents the measured refractive index change at the ith point, C e is the terahertz-elastic coefficient, C p is the terahertz-plasticity coefficient.

[0032] Furthermore, during the loading-unloading cyclic uniaxial stretching of the sample, the calculation formula of the refractive index of the sample is:

[0033]

[0034] Where c is the speed of light, f is the THz frequency, d is the thickness of the sample, and φ(f) is the phase of the THz time-domain spectrum.

[0035] Furthermore, the load-unload cycle includes loading, unloading, reloading and re-unloading in sequence.

[0036] A terahertz-plastic coefficient and terahertz-elastic coefficient measurement system for polymer materials comprises a terahertz time-domain spectroscopy system, a loading device and an image acquisition device. The loading device is arranged in the terahertz time-domain spectroscopy system, the loading device performs uniaxial stretching on the sample, the terahertz time-domain spectroscopy system is used to generate terahertz waves, the terahertz waves act on the sample, and the image acquisition device acquires an image of the sample during the stretching process.

[0037] By adopting the above technical scheme, based on terahertz time-domain spectroscopy, a theoretical model of elastic strain and plastic strain is extracted and created according to the change of the refractive index of the polymer material in the terahertz frequency band, and an error function is established according to the theoretical model of elastic strain and plastic strain to describe the error between the theoretical value and the measured value of the refractive index of the deformation modulated terahertz frequency band, and the change of the refractive index of multiple groups of polymer materials in different loading states when the polarization direction is consistent with the stretching direction and when the polarization direction is perpendicular to the stretching direction is actually measured, and the terahertz-elastic coefficient and terahertz-plastic coefficient of the polymer material are calculated in combination with the error function, and the measurement result is accurate with small error. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic structural diagram of a polymer material terahertz-plastic coefficient and terahertz-elastic coefficient measurement system according to an embodiment of the present invention;

[0039] Figure 2a is a stress-strain curve in a horizontal polarization direction of an embodiment of the present invention;

[0040] Figure 2bis a stress-strain curve in a vertical polarization direction according to an embodiment of the present invention;

[0041] Figure 3a 1 is a stress-strain curve and a refractive index change (loading part) in a horizontal polarization direction of an embodiment of the present invention;

[0042] Figure 3b 1 is a stress-strain curve and a refractive index change (reloading part) in a horizontal polarization direction of an embodiment of the present invention;

[0043] Figure 4a 1 is a stress-strain curve and a refractive index change (loading part) in a vertical polarization direction of an embodiment of the present invention;

[0044] Figure 4b 1 is a stress-strain curve and a refractive index change (reloading part) in a vertical polarization direction according to an embodiment of the present invention.

[0045] In the figure:

[0046] 1. Receiving antenna 2. Polarizer 3. Transmitting antenna

[0047] 4. Image acquisition device 20, polarizing filter 21, lens

[0048] 5. Samples DETAILED DESCRIPTION

[0049] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0050] Figure 1 A structural schematic diagram of an embodiment of the present invention is shown. This embodiment relates to a method and system for measuring the terahertz plastic coefficient and elastic coefficient of a polymer material, which is used to measure the terahertz-elastic coefficient and terahertz-plastic coefficient of the polymer material based on terahertz time-domain spectroscopy, obtain a theoretical model of elastic strain and plastic strain from the change of the refractive index of the polymer material in the terahertz frequency band, and establish an error function based on the theoretical model to describe the error between the theoretical value and the measured value of the refractive index in the deformation-modulated terahertz frequency band, measure multiple groups of data under horizontal polarization and vertical polarization, and calculate the terahertz-elastic coefficient and terahertz-plastic coefficient in combination with the error function, so that the elastic coefficient and plastic coefficient of the polymer material in the elastic-plastic deformation stage can be measured, the measurement is accurate, and the error is small.

[0051] A method for measuring terahertz plastic coefficient and elastic coefficient of a polymer material is provided. The terahertz-plastic coefficient and the terahertz-elastic coefficient are measured when the polymer material undergoes elastic-plastic deformation. According to the modulation principle of the elastic strain and plastic strain of the polymer material on the refractive index under elastic-plastic deformation, the terahertz-elastic coefficient and the terahertz-plastic coefficient of the polymer material based on terahertz time-domain spectroscopy are measured. The method comprises the following steps:

[0052] Within the elastic range, the elastic deformation of the polymer material will linearly modulate the refractive index. However, when the deformation of the polymer material exceeds the elastic range, the polymer material will simultaneously produce elastic deformation and plastic deformation. Therefore, the polymer material is used as sample 5, and the sample 5 is placed in a terahertz time-domain spectroscopy system. The terahertz time-domain spectroscopy system is used to generate and detect terahertz waves. The terahertz waves gather on the sample 5 and will produce refraction when passing through the sample 5. The sample 5 is set on a loading device, and the loading device applies tension to the sample 5. The sample 5 is subjected to a loading-unloading cyclic uniaxial stretching, and the sample 5 undergoes elastic-plastic deformation.

[0053] Under terahertz frequency, theoretical models of elastic strain and plastic strain are obtained according to the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation, and an error function is established according to the theoretical model.

[0054] Specifically, when the sample 5 is subjected to a loading-unloading cyclic uniaxial stretching, the limit load is greater than the elastic limit stress of the sample 5, so that the sample 5 enters an elastic-plastic deformation state. When the polymer material undergoes elastic-plastic deformation, the total strain calculation formula is:

[0055] ε=ε e +ε p

[0056] Among them, ε e is the elastic strain, ε p is the plastic strain and ε is the total strain.

[0057] Elastic strain and plastic strain modulate the refractive index linearly respectively. The calculation formulas for the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation are:

[0058] Δn e =C e ·ε e

[0059] Δn p =C p ·ε p

[0060] Among them, Δn e is the refractive index change caused by elastic deformation, Δn pis the refractive index change caused by plastic deformation, C e is the terahertz-elastic matrix, C p is the terahertz-plasticity coefficient matrix.

[0061] The refractive index change Δn caused by elastic deformation e and the refractive index change Δn caused by plastic deformation p Calculate the total change in refractive index. The total change in refractive index is calculated as: Δn = Δn e +Δn p , the total change in refractive index Δn is the change in refractive index caused by elastic deformation Δn e and the refractive index change Δn caused by plastic deformation p Therefore, according to the above calculation formulas for the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation, the calculation formula for the total refractive index change can be obtained as follows:

[0062] Δn=C e ·ε e +C p ·ε p

[0063] Where Δn is the total change in refractive index, ε e is the elastic strain, ε p is the plastic strain, C e is the terahertz-elastic coefficient, C p is the terahertz-plasticity coefficient.

[0064] Sample 5 produces double refraction when refracted. The theoretical model of elastic strain and plastic strain is extracted and created according to the refractive index changes of elastic deformation and plastic deformation on the two main optical axes, including:

[0065] Calculate the refractive index change of sample 5 caused by elastic deformation along the two principal optical axes under plane stress: refractive index change Δn e Only the elastic strain ε e Modulation, the principal optical axis direction of the birefringence of the polymer material generated by elastic strain is along the direction of elastic strain. For most polymer materials, the principal optical axis direction of birefringence is the direction of the plane principal stress. Under the plane stress state, the calculation formulas for the refractive index changes caused by elastic deformation along the two principal optical axes are:

[0066] Δn e1 =C e1 ·ε e1 +C e2 ·ε e2

[0067] Δn e2 =C e1 ·εe2 +C e2 ·ε e1

[0068] Among them, Δn e1 and Δn e2 are the refractive index changes along the two principal optical axes, C e1 and C e2 are the terahertz photoelastic coefficients in the two principal stress directions, ε e1 and ε e2 is the principal elastic strain.

[0069] Calculate the refractive index change caused by plastic deformation along the two principal optical axes of sample 5 under plane stress: refractive index change Δn p Modulated only by plastic strain, the principal optical axis direction of plastic strain birefringence is along the principal strain direction. Under plane stress state, the calculation formulas for the refractive index changes caused by plastic deformation along the two principal optical axes are:

[0070] Δn p1 =C p1 ·ε p1 +C p2 ·ε p2

[0071] Δn p2 =C p1 ·ε p2 +C p2 ·ε p1

[0072] Among them, Δn p1 and Δn p2 are the refractive index changes along the two principal optical axes, C p1 and C p2 is the terahertz plasticity coefficient, ε p1 and ε p2 is the principal plastic strain.

[0073] The total refractive index change along the two main optical axes is calculated based on the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation: the principal elastic strain direction and the principal plastic strain direction are parallel, so the modulation model of elastic strain and plastic strain on the refractive index can be derived, that is, the calculation formulas for the total refractive index change along the two main optical axes are:

[0074]

[0075] Among them, Δn 1 and Δn 2 is the total refractive index variation along the principal optical axis.

[0076] The error function is established based on the theoretical model of elastic strain and plastic strain. Specifically, the error function is established based on the total refractive index change: the total refractive index change along the two main optical axes takes into account photoelasticity and photoplasticity to describe the change of refractive index under elastic-plastic deformation. In the calculation formula of the total refractive index change along the two main optical axes, during the uniaxial stretching of the polymer material, its plastic strain ε p1 , ε p2 and elastic strain ε e1 , ε e2 It can be measured. Using a terahertz time-domain spectrometer, the refractive index change Δn of the polymer in the terahertz band can be measured in two cases: the polarization direction is consistent with the stretching direction and the polarization direction is perpendicular to the stretching direction. 1 and Δn 2 If a set of terahertz-elastic coefficients C is given e and the terahertz-plasticity coefficient C p , the error between the theoretical value and the measured value of the refractive index change can be calculated, and the total error of multiple measurements can be calculated to obtain the error function. The calculation formula of the error function is:

[0077]

[0078] Among them, Δn i represents the measured refractive index change at the ith point, C e is the terahertz-elastic coefficient, C p is the terahertz-plasticity coefficient.

[0079] After the error function is established, the changes in the refractive index of multiple groups of different loading states when the polarization direction of the terahertz time-domain spectroscopy system is consistent with the stretching direction and the changes in the refractive index of multiple groups of different loading states when the polarization direction of the terahertz time-domain spectroscopy system is perpendicular to the stretching direction are obtained respectively, and the multiple groups of measured data in the parallel polarization direction and the perpendicular polarization direction are brought into the error function, and the minimum value of the error function is calculated by an iterative algorithm to obtain the minimum value of the error function;

[0080] The terahertz-elastic coefficient and the terahertz-plastic coefficient are calculated according to the minimum value of the error function. When the error function reaches the minimum value, the terahertz-elastic coefficient C e and the terahertz-plasticity coefficient C p The value of is the terahertz-elastic coefficient and terahertz-plastic coefficient of the polymer material.

[0081] The above-mentioned loading-unloading cycle includes loading, unloading, reloading and re-unloading in sequence, wherein during the loading process and the reloading process, the limit load should exceed the elastic limit stress of the polymer material, so that the material enters an elastic-plastic deformation state, generating elastic deformation and plastic deformation.

[0082] A method for measuring the terahertz plasticity coefficient and elasticity coefficient of a polymer material is provided by using a terahertz plasticity coefficient and elasticity coefficient measuring system for polymer material. Figure 1 As shown, it includes a terahertz time-domain spectroscopy system, a loading device and an image acquisition device 4. The loading device is arranged in the terahertz time-domain spectroscopy system. The loading device performs uniaxial stretching on the sample 5. The terahertz time-domain spectroscopy system is used to generate terahertz waves. The terahertz waves act on the sample 5. The image acquisition device 4 acquires an image of the sample 5 during the stretching process.

[0083] The above-mentioned terahertz time-domain spectroscopy system is a polarization-sensitive terahertz time-domain spectroscopy system, comprising two rotatable photoconductive antennas and two polarizers 2, the photoconductive antennas are used to generate and detect terahertz waves, the two photoconductive antennas are a transmitting antenna 3 and a receiving antenna 1, two polarizers 2 are arranged between the transmitting antenna 3 and the receiving antenna 1, the polarizer 2 can make the optical radiation beam become polarized radiation with only one polarization state, the polarizer 2 comprises a polarizer 20 and lenses 21 arranged on both sides of the polarizer 20, a sample 5 is arranged between the two polarizers 2, the transmitting antenna 3, polarizer I, polarizer II and the receiving antenna 1 are all arranged in parallel, and the terahertz wave is focused on the sample 5. The highly reliable frequency range of the terahertz time-domain spectroscopy system is 0.2-2.5THz, and a terahertz time-domain spectrometer is used.

[0084] The above-mentioned loading device is used to provide uniaxial tensile testing. The loading value is measured by a force sensor capable of measuring up to 2000N with an accuracy of 0.6N. The loading device is a commercially available product and is selected according to actual needs.

[0085] The above-mentioned image acquisition device 4 is a CCD camera, which acquires the image of the sample 5 during the stretching process, especially the image of the gauge point marked on the sample, processes the acquired image, and uses the digital image correlation (DIC) method to calculate the tensile strain under the uniaxial tensile test.

[0086] The terahertz time-domain spectroscopy system is a polarization-sensitive terahertz time-domain spectroscopy system. Through the force sensor and CCD camera on the loading device, the stress and strain of the sample 5 can be measured. The calculation formula of the plastic strain is:

[0087]

[0088] Where E is the elastic modulus of the polymer material and σ is the nominal stress in the measuring section calculated from the force measured by the force sensor.

[0089] Then the calculation formula for elastic strain is:

[0090]

[0091] where ε is the total strain measured by the CCD camera.

[0092] Sample 5 undergoes refraction during uniaxial stretching in the terahertz time-domain spectroscopy system. The calculation formula for the refractive index of sample 5 is:

[0093]

[0094] Wherein, c is the speed of light, f is the frequency of terahertz, d is the thickness of sample 5, and φ(f) is the phase of terahertz time-domain spectroscopy. The refractive index n(f) of the sample can be measured by terahertz time-domain spectroscopy using the above formula.

[0095] When the terahertz time-domain spectroscopy system is a polarization-sensitive terahertz time-domain spectroscopy system, the terahertz-elastic coefficient and the terahertz-plastic coefficient of the polymer material are measured as follows:

[0096] A cyclic uniaxial load is applied to sample 5, and the cyclic process is loading-unloading-reloading-reloading. During the loading process and the reloading process, the limit load should exceed the elastic limit stress of the polymer material, so that the material enters an elastoplastic deformation state. First, the polarization direction of the terahertz time-domain spectroscopy system is adjusted to be consistent with the stretching direction. During the loading stage and the reloading stage, the change in the refractive index (1THz) of multiple groups (such as 20 groups) of polymer materials in different loading states is measured. Then the polarization direction of the terahertz time-domain spectroscopy system is adjusted to be perpendicular to the stretching direction. Repeat the above operation and measure the change in the refractive index (1THz) of another multiple groups (such as 20 groups) of polymer materials in different loading states. Finally, multiple groups (40 groups) of data in these two cases are substituted into the error function, and the total error is obtained through an iterative algorithm. At this time, the terahertz-elastic coefficient C e and the terahertz-plasticity coefficient C p The value of is the terahertz-elastic coefficient and terahertz-plastic coefficient of the polymer material.

[0097] For example, sample 5 is a standard tensile specimen with a size of 60 mm × 24 mm × 4 mm, and the material is polytetrafluoroethylene (PTFE). Cyclic uniaxial loading is applied to measure the elastic and plastic coefficients of sample 5. The stress-strain curves when the polarization direction is consistent with the tensile direction (horizontal polarization direction) and when the polarization direction is perpendicular to the tensile direction (vertical polarization direction) are shown in Figure 2.

[0098] After the coefficient calibration, the validity of the theoretical model was confirmed experimentally. The refractive index change at terahertz frequency was measured during the loading-unloading cycle of polytetrafluoroethylene sample 5, as shown in Figures 3 and 4, which show the refractive index change produced under horizontal polarization and vertical polarization, respectively. The measured values ​​of the refractive index change are very close to the theoretical values. In the elastic stage, the refractive index change was found to be linearly modulated by strain, but this linear trend collapsed in the plastic stage. As shown in the nonlinear relationship between the refractive index change and strain in Figures 3 and 4, it is necessary to model terahertz-elasticity and terahertz-plasticity. In addition, the trends of the refractive index change during the loading and reloading parts are similar. Specifically, in the initial stage of loading and unloading, the refractive index is linearly modulated by the elastic strain, while in the subsequent stage, the refractive index is modulated by both plastic and elastic strains. After the loading and unloading cycles, the material of sample 5 is strengthened, but the elastic and plastic coefficients remain unchanged.

[0099] The measurement results of polytetrafluoroethylene material are that the terahertz-elastic and terahertz-plastic coefficients are Ce and 1 =-0.014, Ce 2 =-0.114 and Cp 1 =0.130, Cp 2 =0.046.

[0100] By adopting the above technical scheme, based on terahertz time-domain spectroscopy, a theoretical model of elastic strain and plastic strain is extracted and created according to the change of the refractive index of the polymer material in the terahertz frequency band, and an error function is established according to the theoretical model of elastic strain and plastic strain to describe the error between the theoretical value and the measured value of the refractive index of the deformation modulated terahertz frequency band, and the change of the refractive index of multiple groups of polymer materials in different loading states when the polarization direction is consistent with the stretching direction and when the polarization direction is perpendicular to the stretching direction is actually measured, and the terahertz-elastic coefficient and terahertz-plastic coefficient of the polymer material are calculated in combination with the error function, and the measurement result is accurate with small error.

[0101] The embodiments of the present invention are described in detail above, but the contents described are only preferred embodiments of the present invention and cannot be considered to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A method for measuring the terahertz plastic coefficient and elastic coefficient of a polymer material, characterized by: The measurement of the terahertz-plastic coefficient and the terahertz-elastic coefficient when the polymer material undergoes elastic-plastic deformation comprises the following steps: Under terahertz frequency, a theoretical model of refractive index modulation by elastic strain and plastic strain is obtained according to the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation, and an error function is established according to the theoretical model; The sample is subjected to load-unload cyclic uniaxial stretching to obtain multiple sets of refractive index change data in different loading states when the polarization direction of the terahertz time-domain spectroscopy system is consistent with the stretching direction and multiple sets of refractive index change data in different loading states when the polarization direction of the terahertz time-domain spectroscopy system is perpendicular to the stretching direction; Calculating the minimum value of the error function through an iterative algorithm based on the acquired data and in combination with the error function; Calculating the terahertz-elastic coefficient and the terahertz-plastic coefficient according to the minimum value of the error function; The theoretical model for obtaining elastic strain and plastic strain according to the refractive index change caused by elastic deformation and the refractive index change caused by plastic deformation includes: Calculate the refractive index change caused by elastic deformation along the two principal optical axes of the sample under plane stress state; Calculate the refractive index change caused by plastic deformation along the two principal optical axes of the sample under plane stress state; A theoretical model of refractive index modulation by elastic strain and plastic strain is obtained according to the refractive index change caused by the elastic deformation and the refractive index change caused by the plastic deformation.

2. The method for measuring the terahertz plastic coefficient and elastic coefficient of polymer materials according to claim 1, characterized in that: The loading-unloading cycle includes loading, unloading, reloading and re-unloading in sequence. During the loading process and the reloading process, the limit load is greater than the elastic limit stress of the sample, so that the sample enters an elastic-plastic deformation state.

3. The method for measuring the terahertz plastic coefficient and elastic coefficient of a polymer material according to claim 1 or 2, characterized in that: When a polymer material undergoes elastic-plastic deformation, the total change in refractive index is calculated as: ， Where Δn is the total change in refractive index, ε e is the elastic strain, ε p is the plastic strain, C e is the terahertz-elastic coefficient, C p is the terahertz-plasticity coefficient.

4. The method for measuring the terahertz plastic coefficient and elastic coefficient of polymer materials according to claim 1, characterized in that: The calculation formulas for the refractive index changes caused by elastic deformation along the two principal optical axes are: ， Among them, Δn e1 and Δn e2 are the refractive index changes along the two principal optical axes, C e1 and C e2 are the terahertz photoelastic coefficients in the two principal stress directions, ε e1 and ε e2 is the principal elastic strain.

5. The method for measuring the terahertz plastic coefficient and elastic coefficient of a polymer material according to claim 1 or 4, characterized in that: The calculation formulas for the refractive index changes caused by plastic deformation along the two principal optical axes are: ， Among them, Δn p1 and Δn p2 are the refractive index changes along the two principal optical axes, C p1 and C p2 is the terahertz plasticity coefficient, ε p1 and ε p2 is the principal plastic strain.

6. The method for measuring the terahertz plastic coefficient and elastic coefficient of polymer materials according to claim 5, characterized in that: The theoretical model of elastic strain and plastic strain modulation of refractive index is: ， where Δn1 and Δn2 are the total refractive index changes along the principal optical axis.

7. The method for measuring terahertz plastic coefficient and elastic coefficient of polymer materials according to claim 6, characterized in that: The calculation formula of the error function is: ， Among them, Δn i represents the measured refractive index change at the ith point, C e is the terahertz-elastic coefficient, C p is the terahertz-plasticity coefficient.

8. The method for measuring the terahertz plastic coefficient and elastic coefficient of polymer materials according to claim 1, characterized in that: During the loading-unloading cyclic uniaxial stretching of the sample, the calculation formula of the refractive index of the sample is: ， Where c is the speed of light, f is the frequency of terahertz, d is the thickness of the sample, (f) is the phase of the terahertz time-domain spectrum.

9. A polymer material terahertz-plastic coefficient and terahertz-elastic coefficient measurement system, characterized in that: The measurement is performed using the terahertz plastic coefficient and elastic coefficient measurement method of a polymer material as described in any one of claims 1 to 8, comprising a terahertz time-domain spectroscopy system, a loading device and an image acquisition device, wherein the loading device is arranged in the terahertz time-domain spectroscopy system, the loading device performs uniaxial stretching on the sample, the terahertz time-domain spectroscopy system is used to generate terahertz waves, the terahertz waves act on the sample, and the image acquisition device acquires an image of the sample during the stretching process.

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