A method for reducing the crystallinity of two-dimensional photoelastic samples through structural fine-tuning

Abstract

This invention discloses a method for reducing the crystallinity of a two-dimensional photoelastic sample through structural fine-tuning, belonging to the field of two-dimensional photoelastic sample preparation technology. The method includes: first constructing a two-dimensional photoelastic sample composed of particles of equal or near-equal size; while maintaining the overall boundary conditions and basic structural framework of the sample, fine-tuning the particle structure formation conditions to perturb the internal microstructure of the sample, disrupting the continuity of the regular lattice structure, and reducing the crystallinity of the sample; the structural fine-tuning includes at least one of the following: adjusting the particle size composition to reduce the non-uniformity coefficient of the two-dimensional photoelastic sample. C u The value ranges from 1.06 to 1.20. All circular two-dimensional photoelastic particles are replaced with non-circular photoelastic particles to disrupt the periodic contact relationship and orientation consistency between particles. This method effectively reduces the crystallization effect of two-dimensional photoelastic samples, improving the representativeness, stability, and repeatability of photoelastic mechanical test and microstructural analysis results.

Description

Technical Field

[0001] This invention relates to the field of testing methods for particulate materials and the field of microstructure control technology, and more specifically to a method for reducing the crystallinity of two-dimensional photoelastic samples through structural fine-tuning. Background Technology

[0002] Two-dimensional photoelastic specimens are widely used in the study of micromechanical behavior of granular materials, structural evolution analysis of granular systems, and related physical simulation tests because they can intuitively characterize the distribution of contact forces, force chain evolution characteristics, and local stress transmission paths within granular systems. They have important application value in the fields of granular materials mechanics, basic research in geotechnical engineering, and experimental research on discrete media.

[0003] In existing two-dimensional photoelastic experiments, to simplify the sample preparation process, reduce particle parameter dispersion, and facilitate image recognition and result analysis, spherical photoelastic particles with uniform or highly concentrated particle sizes are typically used to construct two-dimensional samples. During the laying, perturbation, compaction, or loading processes, these samples tend to form local or overall regular arrangements between particles, resulting in a significant crystallization effect. This crystallization effect leads to a high degree of order in the particle arrangement within the two-dimensional photoelastic sample, thereby altering the contact relationships, coordination structures, and contact force transmission paths between particles. This causes the experimental results to deviate to some extent from the true mechanical response of the random particle system. For two-dimensional photoelastic experiments that focus on contact force distribution, force chain evolution, local structural reorganization, and microscopic topological features, excessively high sample crystallinity not only affects the representativeness of the sample's mechanical behavior but also reduces the consistency of structural states between different samples, thus impacting the comparability, stability, and repeatability of the experimental results.

[0004] In existing technologies, the handling of crystallization phenomena in two-dimensional photoelastic specimens or in photoelastic specimens largely relies on empirical perturbations during specimen preparation, adjustments to manual sample placement methods, or post-experimental structure identification and result screening. These methods typically lack clear parameter control guidelines, making it difficult to actively, stably, and repeatedly regulate the local ordered structure of the specimen while maintaining the overall boundary conditions and basic structural framework of the specimen. Especially for two-dimensional photoelastic specimens, excessive adjustment of the particle structure, while potentially weakening crystallization, can also significantly alter the overall skeletal structure, density, or contact mechanical characteristics of the specimen, thus affecting the comparability of experimental results.

[0005] In addition, existing research focuses more on the identification and description of crystallization phenomena and the analysis of their impact on experimental results. However, there is a lack of systematic, clear and easy-to-implement technical solutions on how to actively weaken the formation conditions of regular arrangement structures in two-dimensional photoelastic samples by fine-tuning the structural formation conditions such as particle size composition and particle geometry during the sample preparation stage, thereby reducing crystallinity from the source. Summary of the Invention

[0006] The purpose of this invention is to address the problems existing in the prior art by providing a method for reducing the crystallinity of two-dimensional photoelastic samples through structural fine-tuning.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for reducing the crystallinity of a two-dimensional photoelastic sample through structural fine-tuning includes the following steps: S1. Two-dimensional particles are prepared using photoelastic materials. First, a two-dimensional photoelastic sample composed of particles of equal or near-equal size is constructed. S2. While maintaining the overall boundary conditions and basic structural framework of the two-dimensional photoelastic sample, fine-tune the particle structure formation conditions of the two-dimensional photoelastic sample to perturb the internal microstructure of the sample, disrupt the continuity of the regular lattice structure, and reduce the crystallinity of the sample; wherein, the fine-tuning of the particle structure formation conditions includes at least one of the following: (1) By adjusting the particle size composition, the non-uniformity coefficient Cu of the two-dimensional photoelastic sample is made to be 1.06~1.20; (2) Replace all the circular two-dimensional photoelastic sample particles with non-circular photoelastic particles to disrupt the periodic contact relationship and orientation consistency between the particles.

[0009] The beneficial effects of adopting the above technical solution are as follows: the particles inside the two-dimensional photoelastic sample, which is composed of particles of equal or near-equal size, form a regular arrangement structure and are in a highly crystalline state. Under the premise that the overall sample system and boundary conditions remain basically unchanged, by finely adjusting the particle structure formation conditions of the highly crystalline sample, the microstructure inside the sample is disturbed, the dispersion of particle size distribution is changed, and the conditions for the formation of regular geometric coordination relationships between particles are destroyed, thereby weakening the formation and expansion of regular lattice structure in the sample.

[0010] Preferably, in step S2, the non-uniformity coefficient of the two-dimensional photoelastic sample is adjusted by changing the particle size distribution. C u The inhomogeneity coefficient of the two-dimensional photoelastic sample constructed from non-circular photoelastic particles is 1.06–1.10. C u It ranges from 1.00 to 1.10.

[0011] Preferably, in step S2, the non-uniformity coefficient of the two-dimensional photoelastic sample is adjusted by changing the particle size distribution. C u The inhomogeneity coefficient of the two-dimensional photoelastic sample constructed from non-circular photoelastic particles is 1.08–1.10. C u It ranges from 1.04 to 1.08.

[0012] Preferably, in step S2, the non-uniformity coefficient of the two-dimensional photoelastic sample is adjusted by changing the particle size distribution. C u The inhomogeneity coefficient is 1.10, representing the coefficient of uniformity of a two-dimensional photoelastic sample constructed from non-circular photoelastic particles. C u It is 1.08.

[0013] Preferably, the non-circular photoelastic particles are one or both of elliptical and polygonal particles.

[0014] Preferably, the initial relative density of the two-dimensional photoelastic sample is 60~100%.

[0015] The beneficial effects of adopting the above technical solution are: adjusting the relative density of the two-dimensional photoelastic sample, reducing the degree of geometric constraint of the particle system, limiting the possibility of particles forming a stable and regular arrangement structure, thereby weakening the overall continuity of the crystal structure.

[0016] Preferably, the sphericity S of the non-circular photoelastic particles is 0.80~0.90.

[0017] Preferably, the non-uniformity coefficient of the two-dimensional photoelastic sample C u The value is determined in the following way: Define particle size uniformity coefficient η This is used to characterize the degree to which the particle size deviates from the constant diameter state: (1) Define near-isodiameter crystal breaking efficiency E This is used to characterize the degree of crystallinity reduction caused by per unit particle size disturbance. (2) The particle size uniformity coefficient η From the smallest particle size in the sample r min With maximum particle size r max Confirmed; the near-equal diameter crystal breaking efficiency E Crystallinity of a system of perfectly uniform particle size C 0. Crystallinity of the current particle system C and particle size uniformity coefficient η To be determined jointly; The near-isodiameter crystal breaking efficiency of the two-dimensional photoelastic sample was calculated. E Selecting near-equal diameter crystal breaking efficiency compared to C u When =1, the range of non-uniformity coefficients corresponding to the intervals with significant changes is increased.

[0018] EThis actually reflects the efficiency with which minute changes in particle size disrupt the ordered structure of a particle system. E When the particle size difference is large, it indicates that even when the particle size difference is small and the particles still maintain near-monodispersive characteristics, a small gradation disturbance can significantly reduce the crystallinity of the sample, indicating that the system has a high structural sensitivity to particle size disturbances; conversely, when the particle size difference is small, it indicates that even when the particle size difference is small and the particles still maintain near-monodispersive characteristics, a small gradation disturbance can significantly reduce the crystallinity of the sample, indicating that the system has a high structural sensitivity to particle size disturbances; E When the particle size is small, it indicates that a larger difference in particle size is required to have a significant impact on the crystal structure. Through analysis... E The variation of gradation parameters can further identify the region in the particle system where the crystalline structure is most sensitive to particle size disturbances. E The area near the maximum value is the region where the spherical particle system is most sensitive to particle size disturbances.

[0019] As can be seen from the above technical solutions, compared with the prior art, the method for reducing the crystallinity of two-dimensional photoelastic samples by structural fine-tuning disclosed in this invention has the following significant advantages: This invention enables proactive control over the formation of crystallization effects in two-dimensional photoelastic samples. Starting with structural formation conditions such as particle size distribution, relative density, and particle shape, the invention regulates the sample preparation process, weakening the conditions for the formation of regularly arranged structures at the source, thus avoiding reliance solely on post-hoc identification or empirical manipulation to control crystallization.

[0020] Achieving parameterization and repeatability of crystallinity control. The structural state of samples under different gradation parameters and initial relative density conditions is analyzed to determine the parameter range for reducing crystallinity, providing a clear basis for parameter selection in the preparation of two-dimensional photoelastic samples and improving the repeatability and stability of the method.

[0021] This invention is applicable to the structural control of two-dimensional photoelastic specimens under high density conditions. Addressing the issue of crystalline structures easily forming in two-dimensional granular systems under high relative density conditions, this invention effectively suppresses crystalline structures while maintaining a high density, thus expanding the applicability of two-dimensional photoelastic specimens under different operating conditions. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0023] Figure 1 This is a diagram of the particle size distribution scheme for the two-dimensional photoelastic sample in Example 1. Figure 2 for C u=1 Comparison of crystalline particles extracted from circular and elliptical two-dimensional photoelastic samples; where (a) is the circular two-dimensional photoelastic sample constructed in Example 1, and (b) is the elliptical two-dimensional photoelastic sample constructed in Example 2. Figure 3 The graph shows the relationship between crystallinity and non-uniformity coefficient for different two-dimensional photoelastic samples in Examples 1 and 2. Figure 4 The graph shows the relationship between near-uniformity crystal breaking efficiency and non-uniformity coefficient for different two-dimensional photoelastic samples in Examples 1 and 2. Figure 5 Example 1 C u =1 and C u Comparison of two-dimensional photoelastic samples loaded at =1.1; Figure 6 The graph shows the relationship between the non-uniformity coefficient and peak intensity of the two-dimensional photoelastic sample in Example 1 under different initial relative densities. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1 The method for reducing the crystallinity of two-dimensional photoelastic samples through structural fine-tuning includes the following steps: The crystallinity of the two-dimensional photoelastic sample was controlled by adjusting the particle size distribution. First, two-dimensional photoelastic samples were constructed in both a spherical monodisperse system and a spherical polydisperse system. In the monodisperse system, the particle size was uniform, while in the polydisperse system, the particle size was distributed within a preset range. The gradation distribution curves are shown below. Figure 1 As shown.

[0026] Based on the aforementioned particle gradation scheme, and while maintaining other experimental conditions, experiments were conducted on samples under different gradation conditions to obtain information on the initial spatial position of the particles and the contact relationships between them. By comparing and analyzing the structural states of the samples under different gradation conditions, the influence of particle gradation structure on the formation trend of crystal structure was evaluated. This embodiment demonstrates that, without altering the overall sample system, the formation of regularly arranged structures in the sample can be suppressed simply by adjusting the particle gradation structure.

[0027] Example 2 The method for reducing the crystallinity of two-dimensional photoelastic samples through structural fine-tuning includes the following steps: The crystallinity of two-dimensional photoelastic samples was controlled by altering the particle geometry. Two-dimensional photoelastic samples were constructed using elliptical monodisperse and elliptical polydisperse systems, with gradation curves identical to those in Example 1. The sphericity of the elliptical particles was also controlled. S The value was set at 0.85 to disrupt the periodic contact relationship and orientation consistency between particles. By comparing and analyzing the structural states of samples under different particle shape conditions, the inhibitory effect of particle geometry changes on the formation of regular arrangement structures can be evaluated.

[0028] Test case (1) Determination of the relationship curve between crystallinity and non-uniformity coefficient Structural determination method for crystallinity This method evaluates the crystallinity of two-dimensional photoelastic samples based on the local structural characteristics of particles. First, the bond orientation order parameter of the particles is calculated to characterize the density and orientational order of the local particle arrangement. This bond orientation order parameter is used to evaluate the hexagonal symmetry of the particle arrangement and is a commonly used indicator for characterizing locally ordered structures in two-dimensional particle systems.

[0029] For particles i Its bond orientation order parameter is calculated based on its neighboring particles, where N ( i ) represents the number of neighboring particles. θ ij Indicates particles i With neighboring particles j The line between them is relative to x The included angle along the positive axis. Based on this, particles in the sample are screened according to preset structural criteria. When the bond orientation order parameter of a particle simultaneously meets the corresponding threshold condition, it is defined as a crystalline particle.

[0030] The ratio of the number of crystalline particles to the total number of particles in the sample is used as the crystallinity index of the sample.

[0031] Figure 2 Examples 1 and 2 are given. C u A comparison of the extracted crystalline particles in round and elliptical particle samples when the ratio is 1, where... Figure 2 (a) in the sample is a round particle sample. Figure 2 (b) shows an elliptical particle sample; the blue portion represents crystalline particles. The sphericity of the round particles is 1, and the sphericity of the elliptical particles is... S The value is 0.85. Sphericity is calculated as the ratio of the largest inscribed circle radius of the particle to the outermost circumscribed circle radius. Figure 2As shown in (a~b), under the same two-dimensional sample conditions, particle geometry has a significant impact on the crystallinity of the sample. For spherical particles, due to their perfect rotational symmetry, regular hexagonal close-packed structures are easily formed between particles, with highly consistent local coordination relationships, resulting in a higher proportion of ordered regions in the sample and higher crystallinity. In contrast, when irregularly shaped particles (such as elliptical particles) are used, the orientation of the long axis of the particles has directional differences, making it difficult for adjacent particles to form periodic, repeatable contact configurations. The continuous expansion of the regular lattice structure is significantly disrupted, leading to a significant reduction in ordered regions in the sample and a significant decrease in crystallinity. From a microstructural perspective, irregular particle shapes weaken the conditions for the formation of a stable lattice structure by introducing orientation inconsistencies and contact geometric mismatches, making the particle arrangement more disordered. Therefore, in the preparation of two-dimensional photoelastic samples, introducing irregular particles can be used as an effective structural control method to reduce the crystallinity of the sample and suppress the crystallization effect.

[0032] Under the condition of an initial relative density of 100%, the crystallinity of the two-dimensional photoelastic samples in Examples 1 and 2 is as follows: Figure 3 As shown.

[0033] Comparative analysis results show that as the particle non-uniformity coefficient increases to 1.2, the crystallinity of the sample first shows a significant decreasing trend; when the non-uniformity coefficient further increases, the change in crystallinity gradually slows down.

[0034] (2) Determination of the relationship curve between near-equal diameter crystal breaking efficiency and non-uniformity coefficient Define particle size uniformity coefficient This is used to characterize the degree to which the particle size deviates from the constant diameter state: (1), The near-isomorphic crystal breaking efficiency is defined based on the change in crystallinity of the sample: (2), In formula (1), η r is the particle size uniformity coefficient. min r represents the minimum particle size in the sample. max This represents the maximum particle size in the sample. In formula (2), E For near-equivalent crystal breaking efficiency, C 0 represents the crystallinity of a system with completely uniform particle size. C It represents the crystallinity of the current particle system. η The particle size uniformity coefficient; The physical meaning of this index lies in measuring the degree of crystallinity reduction caused by perturbation of unit particle size while maintaining approximately uniform particle size. Among these, the molecular...C 0- C This represents the amount of weakening of the crystal structure caused by particle size perturbation, while the denominator 1- η This reflects the extent to which the particle size deviates from the uniform diameter state. Therefore, E This actually reflects the efficiency with which minute changes in particle size disrupt the ordered structure of a particle system. E When the particle size difference is large, it indicates that even when the particle size difference is small and the particles still maintain near-monodispersive characteristics, a small gradation disturbance can significantly reduce the crystallinity of the sample, indicating that the system has a high structural sensitivity to particle size disturbances; conversely, when the particle size difference is small, it indicates that even when the particle size difference is small and the particles still maintain near-monodispersive characteristics, a small gradation disturbance can significantly reduce the crystallinity of the sample, indicating that the system has a high structural sensitivity to particle size disturbances; E When the particle size is small, it indicates that a larger difference in particle size is required to have a significant impact on the crystal structure. Through analysis... E By studying the variation of gradation parameters, we can further identify the range in the particle system where the crystalline structure is most sensitive to particle size disturbances.

[0035] Under the condition of an initial relative density of 100%, the inhomogeneity coefficient of the two-dimensional photoelastic samples in Examples 1 and 2 C u The values ​​are 1, 1.02, 1.04, 1.06, 1.08, 1.1, 1.2, 1.4, and 1.6.

[0036] The graphs showing the variation of near-uniform diameter crystal breaking efficiency of the two-dimensional photoelastic samples in Examples 1 and 2 with gradation are shown below. Figure 4 As shown.

[0037] from Figure 4 As can be seen, with the increase of the non-uniformity coefficient, the near-uniformity crystal breaking efficiency of the spherical particle system decreases. E Increased then decreased, and in C u The value reaches its maximum near 1.1. This indicates that when the particle system still maintains near-monodispersive characteristics, even small perturbations in particle size distribution can have the most significant disruptive effect on the regular arrangement structure of spherical particles. C u When the particle size distribution is 1, the particle size is completely uniform, and the system easily forms a stable hexagonal close-packed structure. The crystal lattice has high structural stability, so small perturbations are unlikely to significantly change the crystallization state. With... C u As the particle size increases slightly to approximately 1.1, the difference in particle size begins to introduce geometric mismatch, disrupting the periodic contact relationships of the original crystal lattice structure. This leads to rapid decay of the crystal structure, resulting in higher crystal-breaking efficiency. C u As the particle size continues to increase, the system gradually transitions from a near-monodisperse to a polydisperse structure, the particle arrangement becomes more disordered, and the crystalline structure is significantly weakened. At this point, further increasing the gradation has a relatively weaker effect on crystallinity. EIt shows a downward trend.

[0038] from Figure 4 Medium-sized spherical particle system C u The range of 1.06 to 1.20 and the range of 1.0 to 1.10 for irregular particle systems represent the range where two-dimensional photoelastic sample systems are more sensitive to particle size perturbations. Within this range of non-uniformity coefficients, small perturbations can significantly alter the crystallization state.

[0039] C u When the particle size distribution is 1.1, the deviation of the maximum or minimum particle size from the average particle size in the particle system is approximately 10%. This deviation is still within the allowable error range for particle size commonly found in geotechnical engineering tests, therefore the particle system as a whole can still be considered a near-monodispersed particle system. Figure 5 Example 1 C u =1 and C u Comparison of two-dimensional photoelastic samples loaded at =1.1, from Figure 5 The observed changes in crystal structure mainly stem from the influence of minute particle size perturbations on particle packing structure, rather than significant differences in gradation.

[0040] Numerical samples were constructed using the discrete element method under the same experimental conditions as the circular particle photoelastic sample. Calculations were then performed, combining peak intensity results under different inhomogeneity coefficients and relative density conditions (e.g., ...). Figure 6 As shown in the figure, it can be observed that after the initial relative density of the circular particle photoelastic sample exceeds 60%, the difference in peak intensity between samples with different inhomogeneity coefficients becomes increasingly larger. The closer the inhomogeneity coefficient is to 1, the higher the peak intensity, indicating that the crystallinity of the traditional monodisperse two-dimensional photoelastic sample increases after the initial relative density exceeds 60%. With the increase of the inhomogeneity coefficient, the peak intensity of the sample gradually decreases, and the change tends to stabilize after the inhomogeneity coefficient reaches a certain range. Figure 6 It can be seen that when C u When the value reaches 1.1, the peak intensity of the sample essentially stops decreasing. This reflects that the influence of the crystal structure on the macroscopic mechanical response gradually weakens.

[0041] The above analysis shows that by appropriately expanding the particle size distribution range or reducing the initial relative density of the sample, the degree of crystallization in the two-dimensional photoelastic sample can be effectively weakened, thereby reducing the influence of crystallization on mechanical behavior. Furthermore, since irregularly shaped particles can disrupt the periodic contact relationship and orientation consistency between particles, their introduction can also be an effective means of reducing the crystallinity of the two-dimensional photoelastic sample.

[0042] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since it corresponds to the method disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section. The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of reducing crystallinity in a two-dimensional photoelastic sample by structural fine tuning, characterized by, Includes the following steps: S1. Two-dimensional particles are prepared using photoelastic materials. First, a two-dimensional photoelastic sample composed of particles of equal or near-equal size is constructed. S2. While maintaining the overall boundary conditions and basic structural framework of the two-dimensional photoelastic sample, fine-tune the particle structure formation conditions of the two-dimensional photoelastic sample to perturb the internal microstructure of the sample, disrupt the continuity of the regular lattice structure, and reduce the crystallinity of the sample; wherein, the fine-tuning of the particle structure formation conditions includes at least one of the following: (1) By adjusting the particle size composition, the non-uniformity coefficient Cu of the two-dimensional photoelastic sample is made to be 1.06 to 1.20; (2) Replace all the circular two-dimensional photoelastic sample particles with non-circular photoelastic particles to make the non-uniformity coefficient Cu of the two-dimensional photoelastic sample 1.00 to 1.10, so as to destroy the periodic contact relationship and orientation consistency between the particles. The inhomogeneity coefficient Cu of the two-dimensional photoelastic sample is determined in the following way: Define a particle size uniformity coefficient η to characterize the degree to which particle size deviates from the uniform diameter state: （1） The near-isomorphic crystallization efficiency E is defined to characterize the degree of crystallinity reduction caused by per unit particle size disturbance. （2） The particle size uniformity coefficient η is determined by the minimum particle size rmin and the maximum particle size rmax in the sample; the near-equal diameter crystal breaking efficiency E is jointly determined by the crystallinity C0 of the completely equal diameter particle system, the crystallinity C of the current particle system, and the particle size uniformity coefficient η. By calculating the near-isodiameter crystal breaking efficiency E of the two-dimensional photoelastic sample, the range of non-uniformity coefficients corresponding to the interval in which the near-isodiameter crystal breaking efficiency significantly changes compared to Cu=1 is selected.

2. The method of claim 1, wherein, In step S2, the non-uniformity coefficient Cu of the two-dimensional photoelastic sample is adjusted to be 1.06 to 1.10 by adjusting the particle size composition.

3. The method of claim 1, wherein, In step S2, the non-uniformity coefficient Cu of the two-dimensional photoelastic sample is adjusted to be 1.08 to 1.10 by adjusting the particle size composition, and the non-uniformity coefficient Cu of the two-dimensional photoelastic sample constructed by non-circular photoelastic particles is 1.04 to 1.

08.

4. The method of claim 1, wherein, Non-circular photoelastic particles are one or both of elliptical and polygonal particles.

5. The method of claim 1, wherein, The initial relative density of the two-dimensional photoelastic sample is 60-100%.

6. The method according to claim 1, characterized in that, The sphericity S of non-circular photoelastic particles is 0.8~0.9.

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