CN113820052B - Characterization method for stress in dielectric material - Google Patents
Characterization method for stress in dielectric material Download PDFInfo
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Abstract
A method for characterizing stress in a dielectric material, comprising: the source terahertz wave is subjected to first modulation by parallel modulation, first front polarization modulation and focusing modulation, passes through a region of the sample, and is received as a first signal by parallel modulation, first rear polarization modulation and focusing modulation; performing second modulation on the source terahertz wave through parallel modulation, second front polarization modulation and focusing modulation, enabling the source terahertz wave to pass through the region of the sample along the same direction of the terahertz wave after first modulation, and receiving the source terahertz wave as a second signal through the parallel modulation, second rear polarization modulation and focusing modulation; the difference in amplitude of the first signal and the second signal is calculated to obtain the direction of the two principal optical axes in the region and the refractive index difference in the directions of the two principal optical axes to characterize the intensity of the principal stress difference in the region and its direction.
Description
Technical Field
The invention belongs to the technical field of material test characterization, and particularly relates to a characterization method for stress in a dielectric material.
Background
In measuring stress in opaque dielectrics, 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 it is difficult to achieve rapid full field measurements.
Internal stress of the material is one of the factors of material failure and failure during material processing and use, particularly in multi-layer bonded structures. The transparent material is observed by using a 'polarimeter', the magnitude and the direction of the internal stress can be intuitively obtained, and the method cannot be used for obtaining the internal stress of the opaque material. The internal stress of the opaque material can be obtained by the small pore method, but this is a detrimental method.
Terahertz waves have good permeability for most dielectric materials, so a nondestructive testing method for measuring the internal stress of an opaque dielectric material can be established based on the terahertz waves. 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 finish rapid nondestructive detection. Terahertz nondestructive testing is widely applied to the detection of plastic materials, ceramic materials, semiconductor materials and the like. The plastic material changes from isotropy to anisotropy in a state of being strained by a load and exhibits a phenomenon of birefringence to light, and an internal stress of the opaque plastic material can be measured by terahertz waves. Ceramic matrix composites are commonly used to manufacture 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 stress inside it can also be measured by terahertz waves. Stress strain in semiconductors can improve chip performance, such as strained silicon technology physically stretching or compressing silicon crystals, thereby increasing carrier mobility and enhancing transistor performance. The stress in the semiconductor can also be measured by terahertz waves.
Therefore, a method capable of rapidly measuring stress at each point in an opaque electrolyte sample is needed to be provided, on one hand, high requirements are put on measurement efficiency, the calculated 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 on the measurement accuracy, and the sample cannot be damaged. At present, a testing or characterization method capable of meeting the requirements of rapidness, no damage, accuracy and the like is not yet seen.
Disclosure of Invention
In order to solve the above technical problems, in part, the present invention proposes a method for characterizing stress in a dielectric material, comprising: a, carrying out first modulation on a source terahertz wave through parallel modulation, first front polarization modulation and focusing modulation, enabling the source terahertz wave to pass through a region of a sample, and then receiving the source terahertz wave as a first signal through the parallel modulation, the first rear polarization modulation and the focusing modulation; b, performing second modulation on the source terahertz wave through parallel modulation, second front polarization modulation and focusing modulation, enabling the source terahertz wave to pass through the region of the sample along the same direction of the terahertz wave after first modulation, and receiving the source terahertz wave as a second signal through the parallel modulation, second rear polarization modulation and focusing modulation; calculating the amplitude difference of the first signal and the second signal, thereby obtaining the directions of two main optical axes in the region and the refractive index difference in the directions of the two main optical axes so as to represent the intensity and the direction of the main stress difference in the region; wherein the modulation directions of the first front polarization modulation and the first rear polarization modulation are orthogonal to each other, and the modulation directions of the second front polarization modulation and the second rear polarization modulation are also orthogonal to each other.
Further, the first front polarization modulation is different from the second front polarization modulation in polarization direction.
Further, both step a and step b are performed in a dark field environment.
Further, the source terahertz wave is a terahertz wave emitted via a time-domain terahertz system, the frequency of which is 0.2-3THz.
Further, the region is a circular region with a diameter of not more than 3mm-7 mm.
Further, the principal stress difference in the region of the sample is Δσ=Δn/C, where Δσ is the principal stress difference in the first principal stress direction and the second principal stress direction, Δn is the refractive index difference in the two principal optical axis directions, and C is the stress optical coefficient of the material under the action of the terahertz wave, where the directions of the two principal stresses coincide with the directions of the two principal optical axes.
Further, the amplitude of the electric field signal of the terahertz wave received after passing through the sample is
Wherein f is the frequency of the source terahertz wave, d is the thickness of the test piece, c is the speed of light, θ is the direction of the first principal stress in the test piece,
is the direction of front polarization modulation; the direction theta of the first principal stress and the principal stress difference delta sigma are measured, and the terahertz wave received after passing through the sampleThe amplitude of the electric field signal is an experimental measurement.
Further, in the case that the first polarization modulation direction is 0 and the second polarization modulation direction is pi/4, the amplitude of the first signal is denoted as A 1 The amplitude of the second signal is A 2 The (first) principal stress direction θ is
Alternatively, the primary stress difference may be passed.
Obtaining the product.
Further, the stress optical coefficient C of the material under the action of the terahertz wave can be determined through table lookup or calibration experiments; alternatively, the stress optical coefficient C of the material is measured by the following calibration experiment: modulating the source terahertz wave through parallel modulation, polarization modulation before calibration and focusing modulation, enabling the source terahertz wave to pass through a region of a sample, and receiving the source terahertz wave as a calibration signal through the parallel modulation, the polarization modulation after calibration and the focusing modulation; step b, performing the calibration measurement in the step a on a test piece with a known stress distribution area, wherein the known stress distribution area is a pure bending area loaded by four-point bending or a uniform stress area loaded by stretching; c, calculating amplitude differences of the calibration signals, so as to obtain directions of two main optical axes in the area and refractive index differences in the directions of the two main optical axes, and calculating a stress optical coefficient C of the material by combining the known stress distribution in the step b; wherein the modulation directions of the polarization modulation before calibration and the polarization modulation after calibration are orthogonal to each other, and the polarization modulation before calibration is 45 degrees with the horizontal direction.
Further, the known principal stress difference delta sigma, the first principal stress direction theta and the electric field signal A of the received terahertz wave are obtained, and the stress optical coefficient C is
The beneficial effects of the invention include, but are not limited to: the method, apparatus and system are suitable for measuring internal stress fields in various dielectric materials. The method provides a nondestructive non-contact novel 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 primary stresses. The primary stress difference is one of the important criteria for material failure behavior. Based on the distribution of the primary stress differences, specific locations within the material where stress concentration occurs 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 specific parts are qualified and whether safety is ensured under specific service agents. The main stress direction is of great importance for the design of the structure, especially for the design optimization of the fiber reinforced material. The method can also specifically detect whether the fiber reinforcement direction of the material is reasonably arranged.
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Some specific embodiments of the invention will be described in detail hereinafter by way of example and not by way of limitation with reference to the accompanying drawings. The same reference numbers will be used throughout the drawings to refer to the same or like parts or portions. It will be appreciated by those skilled in the art that the drawings are not necessarily drawn to scale.
Fig. 1 shows a basic structure and composition of a polarized time-domain terahertz system according to an embodiment of the present invention;
FIG. 2 shows polarization direction and principal stress direction, including a first polarization direction and a second polarization direction;
FIG. 3 shows the main functional units of a four-point bend loading test apparatus used in accordance with an embodiment of the present invention;
FIG. 4 shows a top view and a side view of a sample loaded with four-point bends, with ROI being the measurement area, according to an embodiment of the present invention;
FIG. 5 shows the terahertz signals of 6 measurement points in the ROI area of a sample subjected to four-point bending loading and the main stress differences of the corresponding points so as to embody the change of the terahertz signals under different stress effects in the embodiment of the invention;
fig. 6 shows an amplitude distribution of signals of terahertz waves of an ROI region of a sample subjected to four-point bending loading, i.e., differences between peaks and valleys of the terahertz wave signals, in accordance with an embodiment of the present invention;
FIG. 7 shows a fitted line representing the stress optical coefficient fitted from the measured refractive index difference and the calculated principal stress difference, and from the refractive index difference and the principal stress difference at different points;
FIG. 8 shows an experimental setup for determining or calibrating the optical stress coefficient of a sample-a radial compression disk test experimental setup according to another embodiment of the invention;
FIG. 9 shows the loading of a disk-shaped specimen and the ROI test area during operation of an experimental setup for determining or calibrating the optical stress coefficient of the specimen according to another embodiment of the present invention;
fig. 10 (a) and (b) respectively show the full field distribution of the analytical solutions of the principal stress directions in the test area when the radial compression disk is loaded (i.e., the analytical solutions of the principal stress directions at each point) and the analytical solutions of the corresponding principal stress differences thereof according to an embodiment of the present invention;
fig. 11 (a) and (b) respectively show amplitudes (distributions) of terahertz wave signals passing through the radius-pressed disk at different points in the test area when loaded according to an embodiment of the present invention, in which fig. 11a
In FIG. 11b
Fig. 12 (a) and (b) respectively show the full-field distribution of the test result of the principal stress direction in the test area (i.e., the experimental solution of the principal stress direction at each point) and the experimental solution of the corresponding principal stress difference of the radial compression disk when loaded, which are calculated by combining the terahertz test method of the embodiment of the present invention with the measured optical stress coefficient;
fig. 13 (a) and (b) show the comparison of the experimental solution calculated by the terahertz test method of the embodiment of the present invention with the measured optical stress coefficient, respectively, with the theoretical solution.
Detailed Description
The invention is described in further detail below with reference to the drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be noted that, for convenience of description, only the portions related to the invention are shown in the drawings. It should be noted that, in the case of no conflict, the embodiments and features in the embodiments may be combined with each other. The invention will be described in detail below with reference to the drawings in connection with embodiments.
It should be noted that, in the case of no conflict, the embodiments and features in the embodiments may be combined with each other. Furthermore, it is noted that terms such as front, back, upper, lower, left, right, top, bottom, front, back, horizontal, vertical, and the like used herein are merely used for ease of description to aid in understanding the relative position or orientation and are not intended to limit the orientation of any apparatus or structure.
It should be noted that, according to the embodiment of the present invention, the degree (qualitative, rather than precise, quantitative) of the principal stress direction and the difference thereof at different positions of the sample to be measured can be represented by calculating the refractive index difference between the two principal stress directions (the directions of the two principal optical axes), where the meaning of "representation" is to generally estimate the stress/stress difference distribution, 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 polarized time domain terahertz system/device (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 and a receiving antenna 101, and a loading device 110 for performing four-point bending loading on the sample 105, where the loading device 110 includes a loading frame 20, a loading base fixture 201, a fixing base fixture 202, a loading base 203, a fixing base 204, a force sensor 205, and the like as shown in fig. 3, to implement four-point bending loading on the rectangular sample 105 and measure loading force, and in view of the fact that the four-point bending experiment and related devices are conventional technical means and common devices in experimental mechanics, the structure and operation of the present disclosure will not be repeated. However, it should be understood that the present application is not limited in this respect.
It should be noted that experiments according to embodiments of the present invention are generally performed in a dark field environment, which means that there is no terahertz emission source other than the emission antenna 109 in the environment, and the terahertz wave is received from the emission 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, which are eventually received by the receiving antenna 101, has a low amplitude, close to the oscillation amplitude of noise. It should be noted that the terahertz waves described herein are not modulated by the force test piece 105. However, it should be understood that the present application is not limited in this respect.
Further, the source terahertz wave emitted from the transmitting antenna 109 is divergent, modulated into a parallel beam by the first lens 108 formed as a convex lens, then incident on the first polarizer 107, modulated into a focused beam by the second lens 106 formed as a convex lens, incident on an arbitrary region of the sample 105 in a four-point bending loading state, modulated into a parallel beam by the third lens 104 formed as a convex lens after passing through the sample 105, polarized modulated into a second polarization by the second polarizer 103, and received as a first signal by the receiving antenna 101 after being focused modulated by the fourth lens 102 formed as a convex lens. It should be noted that in the above 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 to be different from the original polarization direction, and the polarization direction of the second polarizer 103 is adjusted to remain orthogonal to each other with respect to the polarization direction of the adjusted first polarizer 107, and then the above-described test process is repeated so that the incident light passes through the same area as in the above-described first test, and the signal received by the receiving antenna 101 after the second test is recorded as the 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, and the polarization directions of the two antennas are set to be orthogonal. The first polarizer 107 and the second polarizer 103 are provided in part for the purpose of obtaining a high extinction ratio (extinction ratio) and their polarization directions are set to coincide with the transmitting antenna 109 and the receiving antenna 101, i.e., 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 diameter of the spot is about 5mm. The high reliable frequency range of the system is 0.2-2.5 THz. Actually, the terahertz wave is focused and modulated to form a light spot area on the surface of the test piece; the far-field terahertz wave is focused by the lens to form a light spot of about 3mm-7mm, preferably 4mm-6mm and further preferably 5mm, so that the field information of a larger area can be obtained through the two-dimensional displacement platform point-by-point scanning imaging.
Further, the loading device 110 described above is a device capable of 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, the maximum measurement value of which is 2000N, and the test accuracy of which is 0.6N, wherein the driving device for loading is two stepping motors, not shown, the maximum scanning range (maximum scanning range) of which is 50mm×50mm, and the repeated positioning accuracy of which is 2 μm. However, it should be understood that the present application is not limited in this respect.
The propagation of terahertz wave can be represented by Jones Matrix, and the electric field signal of polarized terahertz wave emitted by the transmitting antenna can be represented as
Wherein f and t are frequency and time respectively,
delta is the angle between the polarization direction and the horizontal direction 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 this order, the electric field signal of the terahertz wave finally received by the receiving antenna 101 can be expressed as:
wherein,,
matrix array
Sum matrix->
For the jones matrix of the first polarizer 107 and the second polarizer 103, the influence of the first polarizer 107 and the second polarizer 103 on the terahertz wave signal (in the present disclosure, the terahertz, terahertz wave, terahertz light wave, and the like, the same meaning as not specifically interpreted) is embodied, J θ In order to represent the influence of the loading and the generated stress on the terahertz wave for the Jones matrix of the sample subjected to the loading, θ is the first principal stress direction, and the vector R represents the terahertz receiving antenna which can only receive the edges +_ in the electric field signal>
Signal portions polarized in direction. 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 the terahertz wave passes through the sample 105 subjected to loading, it is split (split) into two polarized terahertz waves by 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 terahertz signal E after passing through the second polarizer 103 1 ，E 1 The magnitude of (c) can be expressed as:
wherein delta 1 -δ 2 For the phase difference caused by stress between the two principal optical axes, delta when the thickness d of the specimen is constant 1 -δ 2 Can be expressed as by the formula
Where f is the frequency of terahertz wave and c is the speed of light Δn is the refractive index difference between the two principal optical axes caused by stress birefringence, which can be expressed as
Δn＝C·Δσ (10)
Wherein C is the stress optical coefficient, the principal stress difference Δσ=σ 1 -σ 2 As can be seen by combining the above formulas, the amplitude a contains information of the first principal stress direction θ and the principal stress difference Δσ. Thus, the measurement of the amplitude a twice is necessary and sufficient to solve the equation with two unknowns θ and Δσ. If can be measured from experiments separately
And the amplitude a of the terahertz wave passing through the specimen 105 at pi/4, the first principal stress direction θ and the principal stress difference Δσ can be calculated by the following formulas (11) and (12):
similarly, by calculation
The amplitude A at the moment can also calculate the main stress difference delta sigma
To obtain more accurate results, measurements may be taken
Is a plurality of sets of principal stresses delta sigma at different values and averages the principal stress differences to obtain a more accurate final result.
Next, in order to further obtain the principal stress difference precisely from the refractive index difference between the principal optical axes, the stress optical coefficients C of the different samples can be measured either by looking up a table to obtain already recorded stress optical coefficients or by calibration experiments. However, it should be understood that the present application is not limited in this respect.
One example of determining the stress optical coefficient C of a sample of material
The sample used in this example was made of PTFE (polytetrafluoroethylene, poly tetra fluoroethylene, abbreviated as PTFE) and a four-point bending test was employed to determine its stress optical coefficient C, and the broken line portion in fig. 4 shows a measurement region (i.e., ROI region) having dimensions of 28mm×2mm and a scanning step length of 0.5mm. The direction of the principal stress is constant in this region, but the magnitude is varied.
Based on the elastic mechanics theory, the principal stress difference in the x-axis direction in the ROI region can be calculated by the following formula (14)
Where l, d and h are the geometry of the curved sample 105 shown in FIG. 4, p is the static pressure, and the values of the relevant parameters for this sample are listed in Table 1. As can be seen from equation (14) above, the primary stress difference Δσ is proportional to the x-coordinate value, and in the measurement region, the first primary 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 spacing of 2.5mm 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 obviously modulated by the stress. Next, terahertz time-domain signals of all points in the measurement region are measured, and the amplitude thereof, that is, the difference between the maximum value and the minimum value of the waveform is calculated by the peak-to-valley difference (peak-trough difference). The distribution of the amplitude in the measurement area is given in fig. 6, which shows that the amplitude increases from left to right due to the increasing main stress difference Δσ. However, it should be understood that the present application is not limited in this respect.
TABLE 1 four-point bend experimental parameters
In the bending experiment according to the embodiment of the present invention, the polarization direction angle of the first polarizer 107
Set to pi/4 in view of A| φ＝π/4 Can be measured in a bending experiment (i.e., amplitude of a signal of terahertz wave after passing through a sample at phi=pi/4), the refractive index difference Δn generated by stress birefringence can be determined, and in the above formula (10), the stress optical coefficient C is the ratio between the refractive index difference Δn and the main stress difference Δσ, and fig. 7 shows the results of experimentally measured Δn and Δσ according to an embodiment of the present invention, the correlation coefficient C is obtained by linear fitting based on the results of experimentally measured results, and the final measured stress optical correlation coefficient C of the PTFE material used is-2.4x10 -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 obtained through rapid and accurate quantitative calculation. However, it should be understood that the present application is not limited in this respect.
Second embodiment of measuring stress optical coefficient C of material sample
The test specimen 30 used in accordance with this embodiment of the present invention was made of PTFE and a radial compression disc test was used to determine its stress optical coefficient C.
Fig. 8 shows a core unit of an experimental apparatus used for the test, which includes a loading frame 40, a fixing base 404 provided to the loading frame, a fixing base jig 402, a force sensor 405, a sample 30, a loading base 403, and the like, wherein the sample 30 is loaded by the loading base jig 401 and the fixing base jig 402, and the force sensor 405 is used to measure the magnitude of the loading pressure load p. The red line portion in fig. 9 shows a measurement area, which is a circle with 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 experiment parameters of diameter-pressed disc
Based on the elastic mechanics theory, 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 principal stress difference Δσ may be calculated as:
fig. 10a and b show the analytical solutions for the first principal stress direction θ and principal stress difference Δσ of the radial compression disc sample 30.
The experiment of the terahertz penetration sample 30 performed in the time-domain terahertz system in the above-described embodiment was repeated, and the distribution of the amplitude a of the terahertz wave was measured while setting the polarization angle of the polarizer to different angles; respectively are provided withAt the position of
And
the terahertz signals at various points on the radial compression disk sample 30 are scanned, and the loading is static during the scanning process. Fig. 11a and b show the amplitude distribution of the terahertz wave passing through the disk sample 30 measured at different settings. If the value of C is assumed to be known, the first principal stress direction θ and the principal stress difference Δσ can be calculated according to equations (11) - (13). To determine the value of C, the expression of the error function H (C) is defined as
Wherein Δσ i For the experimental results calculated by formulas (11) - (13), Δσ i0 The principal stress difference analytical solution given by the formula (17) is given, and n is the number of measurement points in the experimental area. When the error function takes the minimum value, C is the stress optical coefficient of the material. The stress field is then calculated from the magnitude field by equations (11) - (13) to obtain a full field stress profile, 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 the measured values of the main stress difference and the distribution of the main stress direction are close to the theoretical values, and only a large error exists at the abrupt change of the main stress direction. The error is due to the fact that the terahertz wave focusing light spot size of the method is large, and the characterization of the abrupt change of the stress field is limited by spatial resolution. It can be seen that the experimental measurement value is substantially consistent with the theoretical value, and the validity of the method can be verified.
Further, as shown in fig. 13a and b, according to the second embodiment of the present invention, the average error between the test result obtained by the terahertz wave transmission amplitude characterization method of the radius original disc experiment and the theoretical analysis solution is smaller, the analysis solution is obtained under ideal conditions, the 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 is more similar to the actual condition, and the method, the system and/or the device of the present invention have practical and industrial application values.
The foregoing description is only of the preferred embodiments of the present application and is presented as a description of the principles of the technology being utilized. It will be appreciated by persons skilled in the art that the scope of the invention referred to in this application is not limited to the specific combinations of features described above, but it is intended to cover other embodiments in which any combination of features described above or equivalents thereof is possible without departing from the spirit of the invention. Such as the above-described features and technical features having similar functions (but not limited to) disclosed in the present application are replaced with each other.
Claims (9)
1. A method for characterizing stress in a dielectric material, comprising:
a, carrying out first modulation on a source terahertz wave through parallel modulation, first front polarization modulation and focusing modulation, enabling the source terahertz wave to pass through a region of a sample, and then receiving the source terahertz wave as a first signal through the parallel modulation, the first rear polarization modulation and the focusing modulation;
b, performing second modulation on the source terahertz wave through parallel modulation, second front polarization modulation and focusing modulation, enabling the source terahertz wave to pass through the region of the sample along the same direction of the terahertz wave after first modulation, and receiving the source terahertz wave as a second signal through the parallel modulation, second rear polarization modulation and focusing modulation;
calculating the amplitude difference of the first signal and the second signal, thereby obtaining the directions of two main optical axes in the region and the refractive index difference in the directions of the two main optical axes so as to represent the intensity and the direction of the main stress difference in the region;
wherein the modulation directions of the first front polarization modulation and the first rear polarization modulation are orthogonal to each other, and the modulation directions of the second front polarization modulation and the second rear polarization modulation are also orthogonal to each other;
the first front polarization modulation is different from the second front polarization modulation in polarization direction.
2. The characterization method of claim 1 wherein step a and step b are both performed in a dark field environment.
3. The characterization method of claim 1 wherein the source terahertz wave is a terahertz wave emitted via a time-domain terahertz system, the frequency of which is 0.2-3THz.
4. The characterization method of claim 1 wherein the region is a circular region having a diameter of 3mm to 7 mm.
5. The characterization method according to claim 1, wherein the principal stress difference of the region of the specimen is Δσ = Δn/C, wherein Δσ is the principal stress difference in the first principal stress direction and the second principal stress direction, Δn is the refractive index difference in the two principal optical axis directions, and C is the stress optical coefficient of the material under the action of the terahertz wave, wherein the directions of the two principal stresses coincide with the directions of the two principal optical axes.
6. The characterization method according to any one of claims 1 to 5, wherein the amplitude of the electric field signal of the terahertz wave received after passing through the sample is
Wherein f is the frequency of the source terahertz wave, d is the thickness of the test piece, c is the speed of light, θ is the direction of the first principal stress in the test piece,
is the direction of front polarization modulation; the direction theta and the main stress difference delta sigma of the first main stress are measured to be measured, and the first main stress is received after passing through the sampleThe amplitude of the electric field signal of the terahertz wave is an experimental measurement quantity.
7. The characterization method of claim 6 wherein the amplitude of the first signal is noted as A for a first polarization modulation direction of 0 and a second polarization modulation direction of pi/4 1 The amplitude of the second signal is A 2 The direction theta of the first main stress is
Alternatively, the primary stress difference may be determined by
Obtaining the product.
8. The characterization method according to any one of claims 5 to 7, wherein the stress optical coefficient C of the material under the action of the terahertz wave can be determined by a look-up table or a calibration experiment; alternatively, the stress optical coefficient C of the material is measured by the following calibration experiment:
modulating the source terahertz wave through parallel modulation, polarization modulation before calibration and focusing modulation, enabling the source terahertz wave to pass through a region of a sample, and receiving the source terahertz wave as a calibration signal through the parallel modulation, the polarization modulation after calibration and the focusing modulation;
step b, performing the calibration measurement in the step a on a test piece with a known stress distribution area, wherein the known stress distribution area is a pure bending area loaded by four-point bending or a uniform stress area loaded by stretching;
c, calculating amplitude differences of the calibration signals, so as to obtain directions of two main optical axes in the area and refractive index differences in the directions of the two main optical axes, and calculating a stress optical coefficient C of the material by combining the known stress distribution in the step b;
wherein the modulation directions of the polarization modulation before calibration and the polarization modulation after calibration are orthogonal to each other, and the polarization modulation before calibration is 45 degrees with the horizontal direction.
9. The characterization method according to claim 8, wherein the known principal stress difference Δσ, the first principal stress direction θ and the electric field signal a of the received terahertz wave are obtained, and the stress optical coefficient C is
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