Strain rate-based ballistic impact property prediction and structural design method for fiber cloth / resin bulletproof material

Abstract

The present invention specifically relates to a strain rate-based ballistic impact property prediction and structural design method for a fiber cloth / resin bulletproof material, comprising the following steps: (1) establishing a model of fiber cloth, resin and a bullet; (2) determining a strain rate on the basis of bulletproof requirements, and establishing relationships between material parameters of the fiber cloth and the resin and the strain rate; (3) determining cohesion parameters related to the strain rate; (4) determining a failure criterion related to the strain rate; (5) determining a bullet velocity and a boundary condition on the basis of ballistic impact requirements, and calculating and analyzing the model by calling a VUMAT subroutine, to predict a damage behavior during the ballistic impact of a pre-designed fiber cloth / resin material; (6) verifying the effectiveness of the model by means of experiments, and correcting the model by correcting a pre-designed structure and optimizing the material parameters; and (7) obtaining a structure of a fiber cloth / resin bulletproof material having required ballistic impact properties. The model of the present invention has a real geometric shape of the fiber cloth / resin bulletproof material.

Description

Ballistic impact performance prediction and structural design method of fiber cloth / resin bullet-proof materials based on strain rate

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application number 202410084554.8, filed with the Patent Office of China on January 19, 2024, entitled "Ballistic Impact Performance Prediction and Structural Design Method of Fiber Cloth / Resin Bulletproof Materials Based on Strain Rate", the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the technical field of fiber-reinforced resin-based composite materials, and in particular to a strain rate-based ballistic impact performance prediction and structural design method for a fiber cloth / resin bulletproof material. Background Art

[0004] Current ballistic material designs are mostly based on energy dissipation, prioritizing protection against penetration and penetrating damage. However, the forces and damage patterns during the initial impact, erosion, and fracture phases of an impact are diverse, and the layer-by-layer coupling effect of shock waves is significant. Consequently, a single energy dissipation design is unable to simultaneously protect against both penetrating and non-penetrating damage, facing challenges such as insufficient protection against multiple impacts and a significant decrease in load-bearing capacity after impact.

[0005] Continuum finite element models are commonly used to simulate the ballistic penetration behavior of composite materials. Progressive damage material models use effective fabric properties to homogenize the composite structure into continuous layers, improving the realism of the results by incorporating experimental data into the continuous layers. However, these models do not include delamination and cracking between fiber cloth and fiber cloth, debonding between fiber cloth and resin matrix, and fiber pullout. Continuum models also ignore fiber cloth-to-fiber cloth delamination, frictional sliding after fiber cloth-to-resin matrix debonding, and tensile straightening of fiber bundle undulations. The geometry of the tow and matrix can be modeled using a mesoscale approach, using a local-global model of the mesoscale structure at the impact location connected to the global continuum to address these issues. In addition, the cohesive zone model approach is often used to simulate fiber cloth-to-matrix debonding and pullout and matrix cracking.

[0006] Composite materials are subject to energy dissipation damage at many length scales. Mesoscale damage modes in composite materials are comprised of microscale damage mechanisms. Under impact loading, mesoscale woven composite structures can experience inter-tow transverse cracking and intra-tow delamination cracking. These transverse cracks and tow delamination cracks meander through the matrix and around the fibers. The microscale damage mechanisms involved in mesoscale cracking include strain-rate-dependent fiber damage, strain-rate-dependent resin matrix failure, and rate-dependent fiber-to-resin matrix interface failure. These mesoscale damage mechanisms can be implemented in finite element analysis.

[0007] Summary of the Invention

[0008] The present invention overcomes the shortcomings of the existing technology and provides a strain rate-based method for predicting the ballistic impact performance and structural design of fiber cloth / resin bullet-proof materials. By using parameters related to strain rate, the error between finite element simulation results and experimental test results is reduced, thereby improving the accuracy and reliability of the simulation results.

[0009] To solve the above technical problems, the technical solution adopted by the present invention is: a strain rate-based method for predicting the ballistic impact performance and structural design of a fiber cloth / resin bullet-proof material, comprising the following steps:

[0010] S1. Based on the structural parameters of the pre-designed fiber cloth / resin bulletproof material, models of the fiber cloth, resin, and bullet are established respectively;

[0011] S2. Testing the tensile mechanical properties of the fiber cloth and resin. Setting the strain rate-related loading rate based on the ballistic performance requirements. Obtaining the tensile properties of the fiber cloth and resin at various loading rates. Establishing the relationships between the material parameters of the fiber cloth and resin and the strain rate.

[0012] S3. Conducting Type I and Type II fracture toughness tests on the fiber cloth / resin bulletproof material, establishing Type I and Type II load-displacement curves, and determining the cohesion parameters related to strain rate;

[0013] S4. Based on the two-dimensional Hashin composite failure criterion, the strain rate-dependent three-dimensional Hashin composite failure criterion is determined and incorporated into the VUMAT subroutine;

[0014] S5. Performing a ballistic impact finite element simulation on the pre-designed fiber cloth / resin bullet-proof material to obtain the corresponding ballistic penetration behavior of the fiber cloth / resin bullet-proof material;

[0015] S6. Experimentally verify the finite element model, prepare a fiber cloth / resin bullet-proof material, and conduct a ballistic impact test to obtain a measured result of the ballistic impact limit velocity; when the difference between the measured value of the ballistic limit velocity of the pre-designed fiber cloth / resin bullet-proof material and the finite element simulation value exceeds 10%, re-enter step S1 and modify the model by changing the pre-designed structure and optimizing material parameters; until the difference between the measured value of the ballistic limit velocity of the fiber cloth / resin bullet-proof material and the finite element simulation value does not exceed 10%, the model is determined to be valid;

[0016] S7. Obtain a fiber cloth / resin bulletproof material structure with desired ballistic impact performance.

[0017] As a further improvement of the technical solution of the present invention, a 1 / 2 model is established in step S1 for ballistic impact finite element simulation, wherein the ballistic impact area is a mesoscale model and the rest is a uniform continuous model to improve the efficiency of the simulation calculation.

[0018] As a further improvement of the technical solution of the present invention, the fiber cloth is a combination of one or more organic synthetic fiber cloths, with a single layer thickness of 0.1 to 2.0 mm and a surface density of 100 to 1000 g / m 2 ; The resin is a mixture of one or more of epoxy resin, unsaturated polyester resin and phenolic resin.

[0019] As a further improvement of the technical solution of the present invention, the organic synthetic fiber cloth is ultra-high molecular weight polyethylene fiber cloth, glass fiber cloth, carbon fiber cloth, basalt fiber cloth, aramid fiber cloth, poly(p-phenylene benzobisoxazole) fiber or nylon fiber cloth.

[0020] As a further improvement of the technical solution of the present invention, the loading rate associated with the ballistic impact set in step S2 is the loading rate corresponding to the high strain rate, and the high strain rate is 1 to 5000s -1 .

[0021] As a further improvement to the technical solution of the present invention, the energy release rate in step S3 is calculated by energy domain integration, and the formula used is:

[0022] Where: J is the energy release rate; is the strain energy density; x1 is the crack propagation direction; n1 is the component of the unit vector x1 perpendicular to the counterclockwise path around the crack tip; T i is the component of the traction vector; u i are the components of the displacement vector; dΓ is the length increment along the contour line.

[0023] As a further improvement to the technical solution of the present invention, the calculation formula for the damage evolution in the strain rate-dependent three-dimensional Hashin composite material failure criterion in step S4 is:

[0024] Where: d I is the damage variable; is the elastic strain in the corresponding direction; is the final failure when the corresponding damage variable reaches 1, ft, fc, mt, and mc represent fiber tension, fiber compression, matrix tension, and matrix compression, respectively;

[0025] The elasticity-stiffness calculation formula is:

[0026] The strength calculation formula is:

[0027] Cohesive unit damage evolution:

[0028] As a further improvement of the technical solution of the present invention, the finite element simulation process in step S5 is:

[0029] (1) Based on the structural parameters of the pre-designed fiber cloth / resin bulletproof material, geometric models of the fiber cloth, resin, and bullet are established respectively; (2) Material parameters related to strain rate are determined and assigned to the geometric model;

[0030] (3) Assemble the fiber cloth, resin, and bullets and perform meshing;

[0031] (4) Adding a strain rate-dependent cohesive force model between the fiber cloth and the resin;

[0032] (5) Determine bullet velocity and boundary conditions based on ballistic impact requirements;

[0033] (6) By calling the VUMAT subroutine to perform calculation and analysis on the model, the velocity-time curve and the energy-time curve of each part are extracted to predict the damage behavior of the pre-designed fiber cloth / resin bullet-proof material during the ballistic impact process.

[0034] As a further improvement to the technical solution of the present invention, in the experimental verification in step S6, the preparation process of the fiber cloth / resin bullet-proof material used for testing is as follows:

[0035] ①Prepare resin glue

[0036] Add resin and curing agent in proportion, mix well and set aside;

[0037] ②Preparation of prepreg by resin transfer molding process

[0038] The resin transfer molding system includes a mold, a vacuum bag arranged on the upper surface of the mold, and a first pipe fitting and a second pipe fitting, one end of the first pipe fitting and the second pipe fitting are respectively connected to the interior of the vacuum bag, and the other end is respectively connected to a glue storage tank or a buffer tank; the buffer tank is also connected to a vacuum pump through a third pipe fitting, and the vacuum pump is connected to the interior of the buffer tank through the third pipe fitting; the liquid outlet of the first pipe fitting in the vacuum bag is provided with a solenoid wrapped with glue suction felt, the solenoid is laid along the edge of the fiber cloth preform, and the tail of the solenoid does not contact the liquid inlet of the second pipe fitting; when the vacuum pump is turned on, the vacuum bag begins to shrink and quickly wraps the fiber cloth preform, and the resin in the glue storage tank is injected into the vacuum bag under vacuum negative pressure, and then impregnates the fiber cloth preform through the solenoid wrapped with glue suction felt, and according to the viscosity-temperature relationship of the resin, the injection rate and injection time are regulated by the injection temperature to fully impregnate the fiber cloth preform, thereby completing the preparation of the prepreg;

[0039] ③Preparation of fiber cloth / resin bulletproof materials by molding process:

[0040] The prepreg prepared in step ② is placed in a mold and compression molded to complete the preparation of the fiber cloth / resin bulletproof material.

[0041] As a further improvement of the technical solution of the present invention, the structure with the required ballistic impact performance obtained in step S7 includes the types of fiber cloth and resin, the number of fiber cloth layers, the thickness of the fiber cloth / resin bulletproof material, and the volume fraction of the fiber cloth in the fiber cloth / resin bulletproof material.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] (1) The model of the present invention has the geometric shape of a real fiber cloth / resin bulletproof material, and the ballistic limit velocity can be predicted more accurately through the strain rate-dependent material constitutive relationship.

[0044] (2) The strain rate-based model of the present invention provides a more accurate deformation mechanism at lower scales, allowing for the study of deformation mechanisms during ballistic impact. Within the framework of material structure design, the mesoscale model can incorporate different resins, fiber cloths of varying morphologies, and varying fiber cloth volume fractions, and quantify energy dissipation. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG1 is a flow chart of the strain rate-based ballistic impact performance prediction and structural design method of fiber cloth / resin bullet-proof materials of the present invention.

[0046] FIG2 is a schematic diagram of a model of the fiber cloth / resin bullet-proof material in Example 1 of the present invention.

[0047] FIG3 is a diagram showing the process of simulating ballistic impact in Example 1 of the present invention.

[0048] FIG4 shows a resin transfer molding system for preparing the measured fiber cloth / resin bullet-proof material according to Example 1 of the present invention.

[0049] FIG5 is a schematic cross-sectional view of the measured and simulated ballistic impact results in Example 1 of the present invention.

[0050] FIG6 is a schematic diagram of back injuries from the measured and simulated ballistic impact results in Example 1 of the present invention.

[0051] FIG7 is a curve showing the relationship between the tensile modulus and the strain rate in Example 1 of the present invention, which is used to assign material properties at high strain rates.

[0052] The following are marked in the figure:

[0053] 1-mold, 2-vacuum bag, 3-first pipe fitting, 4-second pipe fitting, 5-glue storage tank, 6-buffer tank, 7-third pipe fitting, 8-vacuum pump, 9-fiber cloth preform. DETAILED DESCRIPTION

[0054] The present invention will be further described below with reference to specific embodiments.

[0055] Example 1

[0056] A strain rate-based method for predicting ballistic impact performance and structural design of ultra-high molecular weight polyethylene fiber cloth / epoxy bullet-proof materials comprises the following steps:

[0057] S1. Determine the model structure based on the pre-designed structure of ultra-high molecular weight polyethylene fiber cloth / epoxy bullet-proof material, where the fiber bundle is 0.4 mm thick and 0.8 mm wide. The fibers and matrix constituting the bundle are represented as a continuum with a fiber cloth volume fraction of approximately 60%. A 1 / 2 model is established for ballistic impact finite element simulation, where the ballistic impact region is a mesoscale model and the rest is a uniform continuous model.

[0058] S2. Use a universal testing machine to test the tensile mechanical properties of ultra-high molecular weight polyethylene fiber cloth and epoxy resin, set the strain rate corresponding to the ballistic performance requirements, and establish the relationship between the tensile properties and strain rate of ultra-high molecular weight polyethylene fiber cloth and epoxy resin.

[0059] S3. Use a universal testing machine to test the fracture toughness of ultra-high molecular weight polyethylene fiber cloth / epoxy bulletproof material in type I and II. Set different strain rates corresponding to different ballistic impact properties, establish type I and II load-displacement curves, and determine the cohesive stiffness related to the strain rate to be 10 6 MPa, Type I fracture energy is 310J / mm2 , Type II fracture energy is 630J / mm 2 .

[0060] S4. Determine the strain rate-dependent failure criterion of three-dimensional Hashin composite materials and incorporate it into the Abaqus subroutine.

[0061] The calculation formula for damage evolution in the failure criterion of three-dimensional Hashin composite materials is:

[0062] Where: d I is the damage variable; is the elastic strain in the corresponding direction; is the final failure when the corresponding damage variable reaches 1, ft, fc, mt, and mc represent fiber tension, fiber compression, matrix tension, and matrix compression, respectively;

[0063] The elasticity-stiffness calculation formula is:

[0064] The strength calculation formula is:

[0065] Cohesive unit damage evolution:

[0066] S5. Perform finite element simulation on the pre-designed ultra-high molecular weight polyethylene fiber cloth / epoxy bullet-proof material to obtain its ballistic penetration behavior.

[0067] In this embodiment, the composition of the thermosetting resin system includes bisphenol A epoxy resin TS481 and amine curing agent TS487; the ultra-high molecular weight polyethylene fiber cloth is a unidirectional cloth with a thickness of 0.5 mm, a width of 1 mm, and a surface density of 900 g / m 2 The resin transfer molding system shown in FIG5 was used for preparation.

[0068] A hard single-sided mold 1 is used, and eight layers of demoulding cloth and ultra-high molecular weight polyethylene fiber unidirectional cloth are laid on it and fixed to obtain a fiber cloth preform 9, a demoulding cloth, a breathable felt, a glue-absorbing felt, and a vacuum bag 2; the vacuum pump 8 is turned on, the vacuum bag 2 begins to shrink and quickly wraps the fiber cloth preform 9, and the epoxy resin mixture system in the glue storage tank 5 is injected into the vacuum bag 2 under vacuum negative pressure, and then the fiber cloth preform 9 is impregnated through the solenoid wrapped with the glue-absorbing felt. According to the viscosity-temperature relationship of the resin system, the injection rate and injection time are regulated by the injection temperature, and the injection temperature is determined to be 40°C, the injection rate is 200mL / s, and the injection time is 10min; the preparation of ultra-high molecular weight polyethylene fiber cloth / epoxy prepreg is completed.

[0069] The obtained ultra-high molecular weight polyethylene fiber cloth / epoxy prepreg was transferred to a molding machine for compression molding at a molding temperature of 60°C. After gelling for 1 hour, the pressure was increased to 6 MPa and cured for 5 hours to complete the preparation of the ultra-high molecular weight polyethylene fiber cloth / epoxy bullet-proof material.

[0070] S6. Experimentally verify the finite element simulation results; ballistic testing was conducted on the ultra-high molecular weight polyethylene fiber cloth / epoxy bullet-proof material obtained in S5 using 9mm live ammunition at bullet velocities of 200-400 m / s. The ballistic limit velocity of the ultra-high molecular weight polyethylene fiber cloth / epoxy bullet-proof material was ultimately determined to be 221 m / s, while the finite element simulation result was 213 m / s. The difference between the measured and simulated ballistic limit velocities of the ultra-high molecular weight polyethylene fiber cloth / epoxy bullet-proof material was 3%, confirming that the model was valid. The structure of the ultra-high molecular weight polyethylene fiber cloth / epoxy resin bullet-proof material was obtained, wherein the number of ultra-high molecular weight polyethylene fiber cloth layers was 8, the thickness of the fiber cloth / resin bullet-proof material was 4 mm, and the volume fraction of the fiber cloth was 62%.

[0071] Adaptive changes based on actual needs are all within the scope of protection of the present invention. Specific examples are used in the present invention to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core ideas of the present invention. At the same time, for those skilled in the art, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. A method for predicting the ballistic impact performance and structural design of a fiber cloth / resin bulletproof material based on strain rate, characterized in that, It includes the following steps: S1. Establish models of the fiber cloth, resin, and bullet respectively according to the pre-designed structural parameters of the fiber cloth / resin bulletproof material; S2. Test the tensile mechanical properties of the fiber cloth and resin, set the loading rate related to the strain rate according to the bulletproof performance requirements, obtain the tensile properties of the fiber cloth and resin at various loading rates, and establish the relationships between the material parameters of the fiber cloth and resin and the strain rate respectively; S3. Conduct type I and type II fracture toughness tests on the fiber cloth / resin bulletproof material, establish type I and type II load-displacement curves respectively and determine the cohesive force parameters related to the strain rate; S4. Based on the two-dimensional Hashin composite material failure criterion, determine the three-dimensional Hashin composite material failure criterion related to the strain rate and incorporate it into the VUMAT subroutine; S5. Conduct finite element simulation of the ballistic impact on the pre-designed fiber cloth / resin bulletproof material to obtain the corresponding ballistic penetration behavior of the fiber cloth / resin bulletproof material; S6. Conduct experimental verification on the finite element model, prepare the fiber cloth / resin bulletproof material and conduct ballistic impact experiments to obtain the measured results of the ballistic limit velocity; when the difference between the measured value and the finite element simulation value of the ballistic limit velocity of the pre-designed fiber cloth / resin bulletproof material exceeds 10%, re-enter step S1, modify the model by changing the pre-designed structure and optimizing the material parameters; until the difference between the measured value and the finite element simulation value of the ballistic limit velocity of the fiber cloth / resin bulletproof material does not exceed 10%, determine that the model is effective; S7. Obtain the structure of the fiber cloth / resin bulletproof material with the required ballistic impact performance.

2. The method for predicting the ballistic impact performance and structural design of a fiber cloth / resin bulletproof material based on strain rate according to claim 1, characterized in that In step S1, a 1 / 2 model is established for the finite element simulation of the ballistic impact, where the ballistic impact area is a mesoscale model and the rest is a homogeneous continuous model.

3. The method for predicting ballistic impact performance and structural design of fiber cloth / resin bulletproof material based on strain rate according to claim 1, wherein The fiber cloth is a composition of one or more of organic synthetic fiber cloths, with a single-layer thickness of 0.1 to 2.0 mm and a surface density of 100 to 1000 g / m 2 ; The resin is a mixture of one or more of epoxy resin, unsaturated polyester resin, and phenolic resin.

4. The method for predicting the ballistic impact performance and structural design of a fiber cloth / resin bulletproof material based on strain rate according to claim 3, characterized in that, The organic synthetic fiber cloth is ultra-high molecular weight polyethylene fiber cloth, glass fiber cloth, carbon fiber cloth, basalt fiber cloth, aramid fiber cloth, poly(p-phenylene benzobisoxazole) fiber or nylon fiber cloth.

5. The method for predicting ballistic impact performance and structural design of fiber cloth / resin bulletproof material based on strain rate according to claim 1, characterized in that, The loading rate related to ballistic impact set in step S2 is the loading rate corresponding to the set high strain rate, and the high strain rate is 1 to 5000 s -1 .

6. The method for predicting ballistic impact performance and structural design of fiber cloth / resin bulletproof material based on strain rate according to claim 1, characterized in that In step S3, the energy release rate is calculated by energy domain integration, and the formula used is: Where: J is the energy release rate; is the strain energy density; x1 is the crack propagation direction; n1 is the component of the unit vector perpendicular to the counterclockwise path around the crack tip; T i is the component of the traction vector; u i is the component of the displacement vector; dΓ is the length increment along the contour line.

7. The method for predicting the ballistic impact performance and structural design of a fiber cloth / resin bulletproof material based on strain rate according to claim 1, characterized in that, The calculation formula for the damage evolution in the three-dimensional Hashin composite failure criterion related to the strain rate in step S4 is as follows: where: d I is the damage variable; is the elastic strain in the corresponding direction; It is the final failure when the corresponding damage variable reaches 1, and ft, fc, mt, and mc represent fiber tension, fiber compression, matrix tension, and matrix compression respectively.

8. The method for predicting ballistic impact performance and structural design of fiber cloth / resin bulletproof material based on strain rate according to claim 1, characterized in that The finite element simulation process in step S5 is as follows: (1) Establish geometric models of the fiber cloth, resin, and bullet respectively according to the pre-designed structural parameters of the fiber cloth / resin bulletproof material; (2) Determine the material parameters related to the strain rate and assign them to the geometric model; (3) Assemble the fiber cloth, resin, and bullet and conduct mesh generation; (4) Add a cohesive force model related to the strain rate between the fiber cloth and the resin; (5) Determine the bullet velocity and boundary conditions according to the ballistic impact requirements; (6) Calculate and analyze the model by calling the VUMAT subroutine, extract the velocity-time curve and the energy-time curve of each part, and predict the damage behavior during the ballistic impact process of the pre-designed fiber cloth / resin bulletproof material.

9. The method for predicting ballistic impact performance and structural design of fiber cloth / resin bulletproof material based on strain rate according to claim 1, characterized in that In the experimental verification in step S6, the preparation process flow of the fiber cloth / resin bulletproof material for testing is as follows: ① Prepare the resin sizing Add resin and curing agent in proportion, mix evenly and set aside; ② Prepare the prepreg by resin transfer molding process The resin transfer molding system includes a mold (1), a vacuum bag (2) disposed on the upper surface of the mold (1), and a first pipe fitting (3) and a second pipe fitting (4). One ends of the first pipe fitting (3) and the second pipe fitting (4) are respectively connected to the inside of the vacuum bag (2), and the other ends are respectively connected to a resin storage tank (5) or a buffer tank (6); the buffer tank (6) is further connected to a vacuum pump (8) through a third pipe fitting (7), and the vacuum pump (8) is connected to the inside of the buffer tank (6) through the third pipe fitting (7); the liquid outlet of the first pipe fitting (3) in the vacuum bag (2) is provided with a solenoid coil wrapped with a resin absorption felt, the solenoid coil is laid along the edge of the fiber cloth preform (9), and the tail of the solenoid coil does not contact the liquid inlet of the second pipe fitting (4); when the vacuum pump (8) is turned on, the vacuum bag (2) begins to contract and quickly wraps the fiber cloth preform (9), the resin in the resin storage tank (5) is injected into the vacuum bag (2) under vacuum negative pressure, and then impregnates the fiber cloth preform (9) through the solenoid coil wrapped with the resin absorption felt. According to the viscosity-temperature relationship of the resin, the injection rate and injection time are regulated by the injection temperature, so that the resin fully impregnates the fiber cloth preform (9); the preparation of the prepreg is completed; ③ Preparation of fiber cloth / resin bulletproof material by compression molding process: Place the prepreg prepared in step ② in a mold and perform compression molding to complete the preparation of the fiber cloth / resin bulletproof material.

10. The method for predicting ballistic impact performance and structural design of fiber cloth / resin bulletproof material based on strain rate according to claim 1, characterized in that, The structure with the required ballistic impact performance obtained in step S7 includes the types of fiber cloth and resin, the number of fiber cloth layers, the thickness of the fiber cloth / resin bulletproof material, and the volume fraction of the fiber cloth in the fiber cloth / resin bulletproof material.

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