CN115728155A - Device for measuring stress in dielectric material - Google Patents

Abstract

An apparatus for stress measurement in a dielectric material, comprising: the device comprises a transmitting antenna, a first lens, a first polarizer, a second lens, a loading device, a third lens, a second polarizer, a fourth lens and a receiving antenna; the transmitting antenna transmits time-domain terahertz waves to enable the time-domain terahertz waves to sequentially pass through the first lens, the first polarizer, the second lens, the loaded sample, the third lens, the second polarizer and the fourth lens and then to be received by the receiving antenna; the terahertz wave focusing lens comprises a first lens, a second lens, a third lens and a fourth lens, wherein the first lens is used for modulating scattered terahertz waves into parallel, the second lens is used for modulating the parallel terahertz waves into focusing, the third lens is used for modulating the scattered terahertz waves into parallel, and the fourth lens is used for modulating the parallel terahertz waves into focusing; wherein the polarization direction of the first polarizer and the polarization direction of the second polarizer are orthogonal to each other.

Description

Device for measuring stress in dielectric material

Technical Field

The invention belongs to the technical field of material testing characterization, and particularly relates to a device for measuring stress in a dielectric material.

Background

In measuring stress in an opaque dielectric, the prior art is limited to single point measurements in that it is difficult to obtain full field stress due to its complex experimental steps, and thus difficult to achieve fast full field measurements.

Internal stresses in materials during processing and use are one of the factors in material failure and destruction, particularly in multilayer bonded structures. The method can be used for visually acquiring the magnitude and the direction of the internal stress by using a polarimeter to observe the transparent material, and cannot be used for acquiring the internal stress of the opaque material. The internal stress of an opaque material can be obtained by a small pore method, but this is a lossy method.

The terahertz wave has good permeability for most dielectric materials, so that a nondestructive testing method for measuring the internal stress of the opaque dielectric material can be established based on the terahertz wave. In recent years, due to the trend of miniaturization and low cost of terahertz systems, more and more industrial and civil product pipelines are matched with terahertz imaging systems to complete rapid nondestructive testing. The terahertz nondestructive testing is widely applied to the testing of plastic materials, ceramic materials, semiconductor materials and the like. The plastic material changes from isotropic to anisotropic in a state where strain occurs upon bearing a load and exhibits a phenomenon of birefringence to light, and the internal stress of the opaque plastic material can be measured by a terahertz wave. Ceramic matrix composites are commonly used to make thermal barrier coatings for aircraft engine blade surfaces. Terahertz waves can penetrate through the thermal barrier coating and detect the thickness of various components in the coating; the internal stress can also be measured by terahertz waves. Stress-strain in semiconductors can enhance chip performance, such as strained silicon technology by physically stretching or compressing the silicon crystal, which in turn increases carrier mobility and enhances transistor performance. The stress in the semiconductor can also be measured by terahertz waves.

Therefore, a device capable of rapidly measuring stress at each point in a lightproof electrolyte sample is urgently needed to be provided, on one hand, a high requirement is provided for measurement efficiency, the calculation amount cannot be large, and otherwise, full-field stress distribution is difficult to obtain in a rapid point-by-point scanning mode; meanwhile, high requirements are also put forward on the measurement accuracy, and the sample cannot be damaged. At present, a testing or characterization method which can meet various requirements of rapidness, no damage, accuracy and the like is not seen.

Disclosure of Invention

In order to partially solve the above technical problem to some extent, the present invention provides an apparatus for stress measurement in a dielectric material, comprising: the device comprises a transmitting antenna, a first lens, a first polarizer, a second lens, a loading device, a third lens, a second polarizer, a fourth lens and a receiving antenna; the transmitting antenna transmits time-domain terahertz waves to enable the time-domain terahertz waves to sequentially pass through the first lens, the first polarizer, the second lens, the loaded sample, the third lens, the second polarizer and the fourth lens and then to be received by the receiving antenna; the terahertz wave focusing lens comprises a first lens, a second lens, a third lens and a fourth lens, wherein the first lens is used for modulating scattered terahertz waves into parallel, the second lens is used for modulating the parallel terahertz waves into focusing, the third lens is used for modulating the scattered terahertz waves into parallel, and the fourth lens is used for modulating the parallel terahertz waves into focusing; wherein the polarization direction of the first polarizer and the polarization direction of the second polarizer are orthogonal to each other.

Further, the polarization direction of the first polarizer coincides with the polarization direction of the transmitting antenna, and the polarization direction of the second polarizer coincides with the polarization direction of the receiving antenna.

Furthermore, the high-reliability frequency range of the terahertz waves emitted by the transmitting antenna is 0.2-2.5 THz.

Further, the transmitting antenna, the first lens, the first polarizer, the second lens, the loading device, the third lens, the second polarizer, the fourth lens and the receiving antenna are all arranged in a dark field.

Further, the loading device is configured to apply pressure to the event.

Further, the loading device is a four-point bending loading device.

Further, the loading device is a diameter disc loading device.

Further, a force sensor is arranged in the loading device to measure the magnitude of the loading load.

Furthermore, the loading device is capable of obtaining stress angle and size analytic solutions of each point of the loaded sample.

Further, the sample was measured in a static loading state.

The beneficial effects of the invention include but are not limited to: the apparatus and system are suitable for measuring internal stress fields in various dielectric materials. The method provides a novel nondestructive non-contact detection means for obtaining the internal stress field of the material aiming at the internal stress characterization problem of the dielectric material. The method may facilitate obtaining stress field information within the material, including a distribution of primary stress differences and a distribution of first primary stresses. The difference in principal stresses is one of the important criteria for the failure behaviour of a material. Based on the distribution of the main stress difference, the specific position of the stress concentration phenomenon in the material can be predicted. More importantly, the method is a qualitative and quantitative characterization method, and the magnitude of the main stress difference can be used for checking whether the design of a specific structure is reasonable, whether a specific part is qualified and whether the part is safe under a specific service conditioner. The main stress direction has important significance on the design of the structure, particularly the design optimization of the fiber reinforced material. The method can also specifically detect whether the fiber reinforcement direction arrangement of the material is reasonable.

Drawings

Some specific embodiments of the invention will be described in detail hereinafter, by way of illustration and not limitation, with reference to the accompanying drawings. The same reference numbers in the drawings identify the same or similar elements or components. Those skilled in the art will appreciate that the drawings are not necessarily drawn to scale.

FIG. 1 shows the basic structure and composition of an apparatus for stress measurement in dielectric materials according to an embodiment of the present invention;

FIG. 2 shows polarization directions and principal stress directions, including a first polarization direction and a second polarization direction;

FIG. 3 illustrates the main functional units of a four-point bend load test apparatus used in an embodiment in accordance with the invention;

FIG. 4 shows top and side views of a specimen subjected to four-point bend loading in an embodiment in accordance with the invention, from which ROI is the measurement area;

fig. 5 shows terahertz signals of 6 measurement points in the ROI region of the sample subjected to four-point bending loading and the main stress difference of the corresponding points, so as to reflect the change of the terahertz signals under different stresses according to the embodiment of the present invention;

fig. 6 shows an amplitude distribution of a signal of a terahertz wave of an ROI region of a specimen subjected to four-point bending loading, that is, a difference between a peak value and a valley value of the terahertz wave signal, in an embodiment according to the present invention;

FIG. 7 shows a fitted line representing stress optical coefficients as a function of the measured refractive index difference and the calculated principal stress difference, and as a function of the refractive index difference and the principal stress difference at different points;

FIG. 8 illustrates an experimental setup for determining or calibrating optical stress coefficients of a sample-a diametric disk test experimental setup, according to another embodiment of the present invention;

FIG. 9 illustrates a loading condition of a disk-shaped specimen and a ROI test area when an experimental apparatus for determining or calibrating an optical stress coefficient of the specimen according to another embodiment of the present invention is operated;

fig. 10 (a) and (b) respectively show a full-field distribution of an analytical solution of a principal stress direction in a test area (i.e., an analytical solution of a principal stress direction at each point) and an analytical solution of a principal stress difference corresponding thereto when a diametric compressive disk is loaded according to an embodiment of the present invention;

FIGS. 11 (a) and (b) respectively show the amplitude (distribution) of the terahertz wave signal passing through different points in the test region when the diametric compression disk is loaded according to the embodiment of the invention, wherein FIG. 11a shows

In FIG. 11b

Fig. 12 (a) and (b) respectively show a full-field distribution of a test result of a main stress direction in a test area when a radial compression disk is loaded (i.e., an experimental solution of the main stress direction at each point) and an experimental solution of a corresponding main stress difference of the test result, which are calculated by a terahertz test method according to an embodiment of the present invention in combination with a measured optical stress coefficient;

fig. 13 (a) and (b) respectively show a comparison of an experimental solution and a theoretical solution calculated by the terahertz test method according to the embodiment of the present invention in combination with the measured optical stress coefficient.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the relevant invention and not restrictive of the invention. It should be noted that, for convenience of description, only the portions related to the present invention are shown in the drawings. It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present invention will be described in detail below with reference to the accompanying drawings in conjunction with embodiments.

It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. Further, it is also noted that terms used herein such as front, back, up, down, left, right, top, bottom, front, back, horizontal, vertical, and the like, to denote orientation, are used merely for convenience of description to facilitate understanding of relative positions or orientations, and are not intended to limit the orientation of any device or structure.

It should be noted that, according to the embodiment of the present invention, the principal stress directions and the magnitude degree of the difference thereof (qualitatively, rather than precisely and quantitatively) at different positions of the sample to be measured can be characterized only by calculating the refractive index difference between the two principal stress directions (two principal optical axis directions), where "characterization" means that the stress/stress difference distribution is generally estimated, which is very efficient. However, it should be understood that the present application is not limited in this respect.

Further, as shown in fig. 1, a polarization time domain terahertz system/apparatus (polarization sensitive THz-TDS system/device) 100 for testing a dielectric material sample 105 includes a transmitting antenna 109, a first lens 108, a first polarizer 107, a second lens 106, a third lens 104, a second polarizer 103, a fourth lens 102, a receiving antenna 101, and a loading device 110 for performing a four-point bending loading on the sample 105, wherein the loading device 110 includes a loading frame 20, a loading seat clamp 201, a fixing seat clamp 202, a loading seat 203, a fixing seat 204, a force sensor 205, and the like shown in fig. 3 to implement the four-point bending loading on the rectangular sample 105 and measure the magnitude of the loading force, and the four-point bending experiment and related apparatuses are conventional technical means and apparatuses in experimental mechanics, and the structure and operation thereof are not repeated in this disclosure. However, it should be understood that the present application is not limited in this respect.

It should be noted that the experiment according to the embodiment of the present invention is generally performed in a dark field environment, where there is no terahertz emission source other than the transmitting antenna 109 in the environment, and the amplitude of the terahertz signal finally received by the receiving antenna 101 from the transmitting antenna 109, the first lens 108, the first polarizer 107, the second lens 106, the third lens 104, the second polarizer 103, and the fourth lens 102 is low and close to the oscillation amplitude of noise. It should be noted that the terahertz wave described here does not undergo modulation by the force specimen 105. However, it should be understood that the present application is not limited in this respect.

Further, the source terahertz wave emitted by the transmitting antenna 109 is divergent, a light beam modulated into a parallel beam by the first lens 108 formed as a convex lens after being subjected to first parallel modulation so as to be incident on the first polarizer 107 and then modulated into a focus by the second lens 106 formed as a convex lens is incident on an arbitrary region of the sample 105 in the four-point bending load state, and after the light beam (i.e., terahertz wave) passes through the sample 105, the light beam is modulated into a parallel beam by the third lens 104 formed as a convex lens, and then subjected to second polarization modulation by the second polarizer 103, and then subjected to focus modulation by the fourth lens 102 formed as a convex lens, and received as a first signal by the receiving antenna 101. It should be noted that in the above-described measuring apparatus and measuring process, the polarization directions of the first polarizer 107 and the second polarizer 103 are orthogonal to each other. However, it should be understood that the present application is not limited in this respect.

Subsequently, the polarization direction of the first polarizer 107 is adjusted and changed to be different from the original polarization direction, the polarization direction of the second polarizer 103 is adjusted to be still orthogonal to the adjusted polarization direction of the first polarizer 107, and then the above test process is repeated, so that the incident light still passes through the same area of the above first test, and the signal received by the receiving antenna 101 after the second test is recorded as a second signal. However, it should be understood that the present application is not limited in this respect.

In the above system, terahertz waves (light beams) are transmitted and received by two rotatable photosensitive antennas, the polarization directions of which are set to be orthogonal. The first polarizer 107 and the second polarizer 103 are partially arranged to obtain a high extinction ratio (extinction ratio), and their polarization directions are set to coincide with the transmitting antenna 109 and the receiving antenna 101, that is, the polarization direction of the first polarizer 107 coincides with the polarization direction of the transmitting antenna 109, and the polarization direction of the second polarizer 103 coincides with the polarization direction of the receiving antenna 101. The second lens 106 functions to focus the terahertz wave in the form of a spot on and through the sample 105. The spot diameter is about 5mm. The highly reliable frequency range of the above system is 0.2 to 2.5THz. Actually, a terahertz wave is focused and modulated to form a light spot area on the surface of a test piece; the terahertz wave field measurement method has the advantages that due to the limitation of terahertz wave wavelength and technology, light spots formed by focusing of far-field terahertz waves through the lens are about 3mm-7mm, preferably 4mm-6mm, and further preferably 5mm, so that field information of a larger area can be obtained through point-by-point scanning imaging of the two-dimensional displacement platform, and accordingly, the time-domain terahertz wave field measurement method has the advantage that the field range of measurement is large.

Further, the loading device 110 described above is a device capable of performing both four-point bending loading and uniaxial pressing loading as shown in fig. 3, and is equipped with a sensor 205 to measure the value of the loading force with a maximum measurement value of 2000N and a test accuracy of 0.6N, wherein the driving device for loading is two stepping motors, not shown, with a maximum scanning range (maximum scanning range) of 50mm × 50mm and a repeated positioning accuracy of 2 μm. However, it should be understood that the present application is not limited in this respect.

The propagation of terahertz waves can be represented by Jones Matrix (Jones Matrix), and the electric field signal of the polarized terahertz waves emitted by the transmitting antenna can be represented as

Wherein f and t are frequency and time, respectively,

is the angle between the polarization direction and the horizontal direction, delta ₀ Is the initial phase of the terahertz wave.

After the terahertz wave passes through the first polarizer 107, the loaded sample 105 and the second polarizer 103 in sequence, the electric field signal of the terahertz wave finally received by the receiving antenna 101 can be expressed as:

wherein the content of the first and second substances,

matrix array

Sum matrix

Is a Jones matrix of the first polarizer 107 and the second polarizer 103, and embodies the influence of the first polarizer 107 and the second polarizer 103 on the terahertz wave signal (in the disclosure, similar terms such as terahertz, terahertz wave and the like have the same meaning if not specially explained), J _θ The Jones matrix of the sample subjected to the loading effect reflects the loading effect and the influence of the generated stress on the terahertz waves, theta is a first main stress direction, a vector R represents a terahertz receiving antenna and can only receive the edges in the electric field signals

The signal portion of the directional polarization. However, it should be understood that the present application is not limited in this respect.

FIG. 2 shows the polarization direction and the principal stress direction, combining equation 1 and equation 2,E ₁ Can be simplified as:

when a terahertz wave passes through the sample 105 under load, it is split (distorted) into two polarized terahertz waves under the effect of stress birefringence. The different propagation speeds cause a phase difference between the two polarized terahertz waves. After both polarized terahertz waves pass through the sample, they will be synthesized into a terahertz signal E after passing through the second polarizer 103 ₁ ，E ₁ The amplitude of (d) can be expressed as:

wherein, delta ₁ -δ ₂ When the thickness d of the sample is constant, δ is the phase difference caused by the stress between the two principal optical axes ₁ -δ ₂ Can be expressed by formula as

Where f is the frequency of the terahertz wave, c is the speed of light, Δ n is the difference in refractive index between the two principal optical axes due to stress birefringence, which can be expressed as

Δn＝C·Δσ (10)

Wherein C is a stress optical coefficient, and the principal stress difference is Δ σ = σ ₁ -σ ₂ By combining the above formulas, it can be seen that the amplitude a includes information of the first principal stress direction θ and the principal stress difference Δ σ. Thus, two measurements of the amplitude a are necessary and sufficient to solve the equation with two unknowns θ and Δ σ. If it can be measured separately from the experiment

And pi/4, the amplitude a of the terahertz wave passing through the sample 105, the first principal stress direction θ and the principal stress difference Δ σ can be calculated by the following equations (11) and (12):

in the same way, by calculating

The time amplitude A can also calculate the main stress difference delta sigma

To obtain more accurate results, measurements may be taken

Multiple sets of principal stresses delta at different valuesσ and the principal stress differences are averaged to obtain a more accurate final result.

Next, in order to further obtain the accurate acquisition of the principal stress difference by the refractive index difference between the principal optical axes, the recorded stress optical coefficients can be obtained by table lookup or the stress optical coefficients C of different samples can be measured by calibration experiments. However, it should be understood that the present application is not limited in this respect.

One example of measuring the stress optical coefficient C of a sample of Material

The sample used in this example was made of PTFE (polytetrafluoroethylene, abbreviated as PTFE), a four-point bending experiment was performed to determine the stress optical coefficient C thereof, and the measurement region (i.e., ROI region) having a size of 28mm × 2mm and a scanning step of 0.5mm is shown in the dotted line portion in fig. 4. The direction of the principal stresses in this region is constant, but the magnitude varies.

Based on the theory of elasticity mechanics, the difference of the principal stresses in the ROI area along the x-axis direction can be calculated by the following formula (14)

Where l, d and h are the geometry of the bent test piece 105 shown in fig. 4 and p is the static pressure, the values of the relevant parameters for this test piece are listed in table 1. As can be seen from the above equation (14), the main stress difference Δ σ is proportional to the x-coordinate value, in the measurement region, the first main stress direction θ is always pi/2, and the stress modulates the terahertz time-domain signal passing through the sample according to the stress optical effect. Fig. 5 shows experimental data of a bent sample, in which terahertz time-domain signals are measured at 6 points along the x-axis direction, respectively, with a 2.5mm interval between each point. The waveforms of the passing terahertz waves at these points are shown in fig. 5, and the amplitude of the visible signal is significantly modulated by the stress. Next, terahertz time-domain signals of all points in the measurement region are measured, and the amplitude thereof, i.e., the difference between the maximum value and the minimum value of the waveform, is calculated by a peak-trough difference (peak-trough difference). The distribution of the amplitudes in the measurement region is given in fig. 6, which shows that the amplitudes increase from left to right due to the increasing primary stress difference Δ σ. However, it should be understood that the present application is not limited in this respect.

TABLE 1 four-point bending test parameters

In the bending experiment according to the embodiment of the present invention, the polarization direction angle of the first polarizer 107

Is set to pi/4 in view of A- _φ＝π/4 Can be measured in a bending experiment (i.e., the amplitude of a signal after a terahertz wave passes through a sample when phi = pi/4), a refractive index difference Δ n caused by stress birefringence can be determined, in the above formula (10), a stress optical coefficient C is a ratio between the refractive index difference Δ n and a principal stress difference Δ σ, fig. 7 shows results of Δ n and Δ σ measured by an experiment according to an embodiment of the present invention, a correlation coefficient C is obtained by linear fitting according to the results of the experiment measurement, and a final measured stress optical correlation coefficient C of a PTFE material used is-2.4 × 10 ^-10 Pa ^-1 . However, it should be understood that the present application is not limited in this respect.

After the stress optical coefficient C is measured, the stress optical coefficient C can be used for calculating the stress field from the amplitude field through formulas (10) - (12), so that the stress distribution of the whole field of the sample to be measured can be quickly, accurately and quantitatively calculated. However, it should be understood that the present application is not limited in this respect.

Second example for measuring stress optical coefficient C of material sample

The specimen 30 used in accordance with this embodiment of the present invention was made of PTFE and a radial compression disk experiment was used to determine its stress optical coefficient C.

Fig. 8 shows a core unit of an experimental apparatus used in the test, which includes a loading frame 40, and a fixed seat 404, a fixed seat clamp 402, a force sensor 405, a test sample 30, a loading seat 403, and the like, which are disposed on the loading frame, wherein the test sample 30 is loaded by the loading seat clamp 401 and the fixed seat clamp 402, and the force sensor 405 is used for measuring the magnitude of the loading pressure load p. The red line portion in fig. 9 shows a measurement area which is a circle having a diameter of 45mm and whose scanning step size is 0.5mm. However, it should be understood that the present application is not limited in this respect.

TABLE 2 parameters of diameter-measuring pressed disk experiment

Based on the theory of elastic mechanics, the analytic solution of the stress field on the radial compression disk can be expressed as

Where p is the applied load and r and d are the radius and thickness of the test piece. The first principal stress direction θ and the principal stress difference Δ σ may be calculated as:

fig. 10a and b show an analytical solution of the primary stress direction θ and the primary stress difference Δ σ of the diametric compression disc specimen 30.

The experiment of penetrating the specimen 30 with terahertz performed in the time-domain terahertz system in the above-described embodiment was repeated, and the distribution of the amplitude a of terahertz waves was measured while the polarization angle of the polarizer was set to different angles; are respectively at

And

under the condition (2), terahertz signals of all points on the radial compression disc sample 30 are scanned, and the loading is static in the scanning process. Fig. 11a and b show amplitude distributions of terahertz waves passing through the disc sample 30 measured at different settings. Assuming that the value of C is known, the first principal stress direction θ and the principal stress difference Δ σ can be calculated according to equations (11) to (13). To determine the value of C, an expression of an error function H (C) is defined as

Wherein, delta sigma _i Δ σ as an experimental result calculated by the equations (11) to (13) _i0 And (3) the main stress difference analytic solution is given by the formula (17), and n is the number of the measuring points in the experimental area. When the error function takes a minimum value, C at this time is the stress optical coefficient of the material. The stress field is calculated from the magnitude field by equations (11) to (13), and a stress distribution map of the full field is obtained, as shown in fig. 12. However, it should be understood that the present application is not limited in this respect.

Comparing the experimental actual measurement result shown in fig. 12 with the theoretical analytical solution shown in fig. 10, it can be found that both the measured values of the main stress difference and the distribution of the main stress direction are close to the theoretical values, and there is a large error only at the abrupt change of the main stress direction. The error is caused by the fact that the size of a terahertz wave focusing light spot is large, and the representation of a stress field sudden change position is limited by the limitation of spatial resolution. Therefore, the experimental measurement value is basically consistent with the theoretical value, and the effectiveness of the method can be verified.

Further, as shown in fig. 13a and b, an average error between a test result obtained by the terahertz wave penetration amplitude characterization method of the diameter log experiment according to the second embodiment of the present invention and a theoretical analytical solution is smaller, the analytical solution is obtained under an ideal condition, and an actual test condition receives a plurality of external influence factors such as a temperature field, a magnetic field/electric field, a humidity field, etc., so that the test result can be considered to be closer to a real condition, and it can be seen that the method, the system and/or the apparatus of the present invention have practicability and industrial application value.

The above description is only a preferred embodiment of the application and is illustrative of the principles of the technology employed. It will be appreciated by a person skilled in the art that the scope of the invention as referred to in the present application is not limited to the embodiments with a specific combination of the above-mentioned features, but also covers other embodiments with any combination of the above-mentioned features or their equivalents without departing from the inventive concept. For example, the above features may be replaced with (but not limited to) features having similar functions disclosed in the present application.

Claims (10)

1. An apparatus for stress measurement in a dielectric material, comprising: the device comprises a transmitting antenna, a first lens, a first polarizer, a second lens, a loading device, a third lens, a second polarizer, a fourth lens and a receiving antenna;

the transmitting antenna transmits time-domain terahertz waves to enable the time-domain terahertz waves to sequentially pass through the first lens, the first polarizer, the second lens, the loaded sample, the third lens, the second polarizer and the fourth lens and then to be received by the receiving antenna;

the terahertz wave focusing device comprises a first lens, a second lens, a third lens and a fourth lens, wherein the first lens is used for modulating scattered terahertz waves into parallel, the second lens is used for modulating the parallel terahertz waves into focusing, the third lens is used for modulating the scattered terahertz waves into parallel, and the fourth lens is used for modulating the parallel terahertz waves into focusing;

wherein the polarization direction of the first polarizer and the polarization direction of the second polarizer are orthogonal to each other.

2. The apparatus for stress measurement in dielectric materials of claim 1, wherein the polarization direction of the first polarizer coincides with the polarization direction of the transmitting antenna and the polarization direction of the second polarizer coincides with the polarization direction of the receiving antenna.

3. The apparatus for stress measurement in dielectric materials according to claim 1, wherein the terahertz waves emitted by the transmitting antenna have a highly reliable frequency in the range of 0.2 to 2.5THz.

4. The apparatus for stress measurement in a dielectric material of claim 1, wherein the transmitting antenna, the first lens, the first polarizer, the second lens, the loading device, the third lens, the second polarizer, the fourth lens, and the receiving antenna are all disposed in a dark field.

5. The apparatus for stress measurement in a dielectric material of claim 1, wherein the loading device is configured to apply pressure to the event.

6. The apparatus for stress measurement in a dielectric material of claim 5, wherein the loading device is a four-point bending loading device.

7. The apparatus for stress measurement in a dielectric material of claim 5, wherein the loading device is a diametric disk loading device.

8. An apparatus for stress measurement in a dielectric material as defined in claim 5, wherein a force sensor is provided in the loading means to measure the magnitude of the loading load.

9. The apparatus for stress measurement in a dielectric material according to any one of claims 5 to 8, wherein the loading device is a loading device that can obtain an angular and magnitude analytical solution of stress at each point of the loaded specimen.

10. An apparatus for stress measurement in a dielectric material according to any of claims 5 to 8, wherein the measurement is performed with the sample under static loading.

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