In-situ characterization and analysis method for mesoscopic damage and durability of unidirectional fiber composites under the coupled action of load and environment

Abstract

The present invention discloses a method for in-situ characterization and analysis of mesoscopic damage and durability of unidirectional fiber composites under the coupled action of load and environment, including S1: fabricating a model unidirectional fiber composite composed of single-mode optical fibers and corresponding matrices; S2: applying the coupled action of load and environment, conducting fiber optic high-spatial-resolution strain testing on the model unidirectional fiber composite, and in-situ characterizing the mesoscopic damage evolution of the unidirectional fiber composite according to the changes in strain and strain gradient; S3: repeating S1 and S2 to obtain a series of damage critical clusters and durability data, and obtaining the durability evolution law through statistical analysis; S4: selecting matrices with different resistance to environmental corrosion, repeating S1-S3, and obtaining the influence law of different matrices on the durability of unidirectional fiber composites. This method can be effectively applied to the characterization, analysis and evaluation of the mesoscopic damage evolution of unidirectional fiber composites under the coupled action of load and environment.

Description

Technical Field

[0001] The present invention belongs to the fields of durability damage characterization of fiber composites and mechanical property evaluation of fiber composites, and particularly relates to a method for in-situ characterization and analysis of mesoscopic damage and durability of unidirectional fiber composites under the coupled action of load and environment. Background Art

[0002] Fiber composites are obtained by laminating unidirectional fiber composites at different angles. Therefore, the mechanical properties of unidirectional fiber composites are the basis of the mechanical properties of fiber composites. Unidirectional fiber composites have the advantages of high tensile strength, high corrosion resistance, and high specific strength. Unidirectional fiber composites mainly utilize the excellent tensile properties of the internal fibers. After the unidirectional fibers break, due to the bonding effect of the matrix, the broken fibers can continue to bear load within a certain range after leaving the break point.

[0003] Under the action of a specific environment, the matrix of unidirectional fiber composites may be eroded by the environment, such as hygrothermal aging, ultraviolet aging, etc., resulting in matrix cracking or reduced bonding performance, forming local mesoscopic damage, which causes the fibers and the matrix to be unable to work together. As the service time increases, the mesoscopic damage continues to evolve and develop, ultimately leading to the failure of unidirectional fiber composites. Therefore, under the coupled action of load and environment, the mechanical properties of unidirectional fiber composites degrade severely. The mesoscopic damage of unidirectional fiber composites includes the following categories: 1. debonding between fibers and the matrix; 2. matrix cracking; 3. fiber fracture. These mesoscopic damages continue to evolve under the action of load, environment, and their coupling, and ultimately, due to the fracture of the fibers in the unidirectional fiber composites and the formation of damage critical clustering, the failure of the unidirectional fiber composites is controlled.

[0004] In the prior art, the optical imaging method has strict requirements for unidirectional fiber composite specimens, and it is impossible to realize the in-situ characterization of the evolution of mesoscopic damage of unidirectional fiber composites under the coupled action of load and environment, and it is impossible to determine the characteristics of the strength, fatigue life, and fatigue residual strength damage critical clustering of unidirectional fiber composites under the coupled action of load and environment. At present, there is no effective method to determine the evolution law of mesoscopic damage of unidirectional fiber composites under the coupled action of load and environment. Summary of the Invention

[0005] The present invention provides a method for in-situ characterization and analysis of mesoscopic damage and durability of unidirectional fiber composites under the coupled action of load and environment, and solves the technical problem that it is difficult to realize the in-situ characterization of the evolution of mesoscopic damage of unidirectional fiber composites and the quantitative evaluation of critical damage under the coupled action of load and environment in the prior art by optical testing.

[0006] To solve the above technical problems, the present invention provides the following technical solutions:

[0007] A mesoscopic damage and durability in-situ characterization and analysis method for unidirectional fiber composites under the coupled action of load and environment, characterized by comprising the following steps:

[0008] Step S1: Fabricate a model unidirectional fiber composite composed of single-mode optical fibers and the corresponding matrix;

[0009] Step S2: Apply the coupled action of load and environment, and use millimeter-level high-spatial-resolution strain sensing technology to conduct high-spatial-resolution strain testing on the model unidirectional fiber composite. According to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite, conduct in-situ characterization of the mesoscopic damage evolution of the unidirectional fiber composite under the coupled action of load and environment;

[0010] Step S3: Repeat Step S1 and S2 to obtain a series of damage critical clusters and their durability data of the unidirectional fiber composite under the coupled action of load and environment. Determine the probability distribution functions and their parameters of the aging life, damage critical clusters, and their remaining mechanical properties through statistical analysis to obtain the durability evolution law;

[0011] Step S4: Select matrices with different environmental corrosion resistance to fabricate model unidirectional fiber composites, and repeat Steps S1 - S3 to obtain the influence law of different matrices on the durability of unidirectional fiber composites.

[0012] Further, Step S1 includes:

[0013] Step S11: Analyze the load and environmental actions, cross-sectional shape and size of the prototype unidirectional fiber composite to determine the basic mechanical parameters and the unidirectional fiber volume content;

[0014] Step S12: Use single-mode optical fibers as substitutes for the unidirectional fibers in the prototype unidirectional fiber composite, and according to the similarity principle, determine the basic mechanical properties, environmental aging resistance of the required matrix and the fiber-matrix interface, select a suitable matrix, and determine the cross-sectional size of the model unidirectional fiber composite;

[0015] Step S13: Fabricate a model unidirectional fiber composite composed of single-mode optical fibers and the selected matrix.

[0016] Further, the basic mechanical parameters determined in Step S11 include at least one of the following: the tensile strength, fatigue performance, elastic modulus of the fibers in the composite, the interfacial bonding strength between the fibers and the matrix, and the matrix elastic modulus; the basic mechanical properties of the matrix in Step S12 include at least one of the following: the matrix elastic modulus and the bonding strength between the matrix and the single-mode optical fiber.

[0017] Further, Step S2 includes:

[0018] Step S21: After the model unidirectional fiber composite material is formed, perform initial calibration on each optical fiber to determine the relative position of the optical fiber in the composite material and its initial state;

[0019] Step S22: Design load steps and environmental conditions, then apply the load and the environmental conditions. Adopt the millimeter-level high-spatial-resolution strain sensing technology to obtain the strain changes and / or optical paths of each measuring point on each optical fiber in real time, judge the break point position, and evaluate the stress redistribution and influence range near the break point;

[0020] Step S23: Continue to apply the load and the environmental conditions according to the load steps and environmental conditions, judge whether new break points occur, and evaluate the stress redistribution and influence range;

[0021] Step S24: Repeat Step S23 until the model composite material loses its stable bearing capacity, and conduct in-situ characterization of the mesoscopic damage evolution of the unidirectional fiber composite material under the coupled action of load and environment.

[0022] Furthermore, in the initial calibration of Step S21, determine the position of the sudden change in the optical fiber strain by means of local small-area temperature rise and fall, and determine the relative position relationship of all the sensing optical fibers in the free section.

[0023] Furthermore, in Step S1, use the compression molding vacuum injection or pultrusion process to fabricate the model unidirectional fiber composite material. The cross-sectional shape of the model unidirectional fiber composite material is the same as that of the prototype unidirectional fiber composite material, and the cross-sectional dimensions are determined according to the similarity principle.

[0024] Furthermore, evaluating the stress redistribution and influence range includes:

[0025] If no new break points occur, re-evaluate the influence range of the existing break points, determine the influence range and strain concentration of the break points under the current load conditions through the changes in strain and strain gradient, and judge whether damage such as debonding between the fiber and the matrix occurs on both sides of the break point;

[0026] If new break points occur, judge the generation position of the new break points, and evaluate the influence situation, influence range and strain concentration of the existing break points through the changes in strain and strain gradient.

[0027] If the environmental action causes the degradation of the matrix performance, when the matrix on the surface of the unidirectional fiber composite material cracks under the coupled action of load and environment while the nearby optical fibers do not break, due to the local strain concentration caused by the matrix cracking, the optical fibers at the edge of the matrix cracking area form a bridge connection, and the strain on the optical fibers increases. The local matrix cracking range can be determined according to the position and quantity of the increased strain on the optical fibers;

[0028] If the matrix cracking causes the stress on the optical fiber to exceed the tensile strength of this part, the optical fiber breaks, and determine the break point position and the load redistribution caused by the broken wire;

[0029] If the fiber and the matrix at the break point undergo debonding under the coupled action of load and environment, the strain within the debonding length range of the optical fiber tends to zero, and at the same time, the length of the strain concentration region of the surrounding unbroken optical fibers increases with the increase of the debonding length.

[0030] Meanwhile, the present invention provides a device for the above method, including a rigid sheath, a testing machine, an environmental chamber, a high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator, and a fixture. The fixture is arranged on the testing machine, the rigid sheath can be fixed on the testing machine through the fixture, the high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator is connected to the single-mode optical fiber in the unidirectional fiber composite specimen fixed on the testing machine, the free section part of the unidirectional fiber composite specimen is placed in the environmental chamber, the testing machine applies a load to the model unidirectional fiber composite, the environmental chamber applies a working condition environment to the unidirectional fiber composite, and the high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator conducts high-spatial-resolution strain testing on the optical fiber of the model unidirectional fiber composite.

[0031] A method for using a device includes the following steps:

[0032] Step S1: Fabricate a unidirectional fiber composite specimen and determine the free section length of the unidirectional fiber composite specimen;

[0033] Step S2: Bond and anchor the clamping sections at both ends outside the free section to the rigid sheath;

[0034] Step S3: Fix the unidirectional fiber composite specimen on the fixture of the testing machine through the rigid sheath, and place the free section of the unidirectional fiber composite specimen in the environmental chamber;

[0035] Step S4: Connect each single-mode optical fiber on both sides outside the clamping section to the high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator;

[0036] Step S5: Calibrate the initial state of the unidirectional fiber composite specimen;

[0037] Step S6: Apply a load and a working condition environment to the unidirectional fiber composite specimen, adopt a millimeter-level high-spatial-resolution strain sensing technology to conduct high-spatial-resolution strain testing on the optical fiber of the model unidirectional fiber composite, and conduct in-situ characterization of the mesoscopic damage evolution of the unidirectional fiber composite under the coupled action of load and environment according to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite;

[0038] Step S7’: Repeat steps S1’ - S6’ to obtain a series of damage critical clusters and durability data of the unidirectional fiber composite under the coupled action of load and environment, determine the probability distribution function and its parameters of the aging life, damage critical cluster size and its remaining mechanical properties through statistical analysis, and obtain the durability evolution law of the unidirectional fiber composite under the coupled action of load and environment;

[0039] Step S8': Select matrices with different environmental corrosion resistance to fabricate model unidirectional fiber composites, repeat Steps S1'-S7' to obtain the influence law of different matrices on the durability of unidirectional fiber composites, determine the probability distribution functions and their parameters of the aging life, damage critical clustering and mechanical properties of unidirectional fiber composites under different matrices, and conduct durability analysis of unidirectional fiber composites under the coupled action of load and environment.

[0040] The technical solution provided by the present invention has at least the following beneficial effects compared with the prior art:

[0041] In the above solution, through similarity design, the present invention uses a single-mode optical fiber as a substitute for the unidirectional fiber in the prototype unidirectional fiber composite. Each optical fiber simultaneously has the functions of bearing force and high-spatial-resolution strain sensing (realized by the optical frequency domain reflectometry (OFDR) principle), effectively solving the technical problem in the prior art that the optical method cannot characterize and evaluate the damage evolution method of unidirectional fiber composites under the coupled action of load and environment. It can realize in-situ characterization of the damage evolution of unidirectional fiber composites under different coupled actions of load and environment, and provide ideas and methods for determining the damage critical clustering and durability evaluation when controlling the failure of unidirectional fiber composites under the coupled action of load and environment. Description of the Drawings

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0043] Figure 1 It is a flowchart of a method for in-situ characterization and analysis of mesoscopic damage and durability of unidirectional fiber composites under the coupled action of load and environment provided by an embodiment of the present invention;

[0044] Figure 2 It is a schematic diagram of similarity principle parameters in an embodiment of the present invention, where (a) is a schematic diagram of parameters of a prototype unidirectional carbon fiber composite; (b) is a schematic diagram of parameters of a model unidirectional optical fiber composite using a single-mode optical fiber to replace carbon fiber;

[0045] Figure 3 It is an in-situ characterization test device for mesoscopic damage and durability of unidirectional fiber composites under the coupled action of load and environment in an embodiment of the present invention;

[0046] Figure 4 It is a unidirectional fiber composite specimen in an embodiment of the present invention;

[0047] Figure 5 Schematic diagram of the connection between a single-mode optical fiber in a unidirectional fiber composite material and a high-spatial-resolution optical frequency domain reflectometry fiber demodulator according to an embodiment of the present invention;

[0048] Figure 6 Schematic diagram of the clamping section of a unidirectional fiber composite material specimen according to an embodiment of the present invention;

[0049] Figure 7 Schematic diagram of the critical damage clustering of a unidirectional fiber composite material under the coupled action of load and environment according to an embodiment of the present invention, where (a) is the initial intact state; (b) is local wire breakage damage; (c) is the damage accumulation process; (d) is the formation of damage clustering. Detailed implementation manners

[0050] To make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will be described in detail with reference to the accompanying drawings and specific embodiments.

[0051] As Figure 1 shown, an embodiment of the present invention provides a method for in-situ characterization and analysis of mesoscopic damage and durability of a unidirectional fiber composite material under the coupled action of load and environment, including the following steps:

[0052] Step S1: Fabricate a model unidirectional fiber composite material composed of a single-mode optical fiber and a corresponding matrix;

[0053] Step S2: Apply the coupled action of load and environment, and use millimeter-level high-spatial-resolution strain sensing technology to perform high-spatial-resolution strain testing on the model unidirectional fiber composite material. According to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite material, in-situ characterization of the mesoscopic damage evolution of the unidirectional fiber composite material under the coupled action of load and environment is carried out;

[0054] Step S3: Repeat steps S1 and S2 to obtain a series of critical damage clustering and durability data of the unidirectional fiber composite material under the coupled action of load and environment. Through statistical analysis, the probability distribution functions and their parameters of the aging life, critical damage clustering and their remaining mechanical properties are determined to obtain the durability evolution law;

[0055] Step S4: Select matrices with different environmental corrosion resistance to fabricate model unidirectional fiber composite materials, and repeat steps S1 - S3 to obtain the influence law of different matrices on the durability of the unidirectional fiber composite material.

[0056] Next, in combination with Figures 2 - 7 , a method for characterizing and evaluating the mesoscopic damage of a unidirectional fiber composite material according to an embodiment of the present invention will be described in detail.

[0057] For ease of explanation, it is assumed that the fibers are evenly arranged in a quadrilateral pattern within the unidirectional fiber composite (in reality, they are randomly arranged). The basic geometric and mechanical parameters of the prototype unidirectional carbon fiber composite under study and the designed model unidirectional fiber composite using single-mode optical fibers to replace carbon fibers are as Figure 2 shown. According to the similarity principle, to ensure geometric similarity between the unidirectional carbon fiber composite to be studied and the designed unidirectional fiber composite using single-mode optical fibers to replace carbon fibers, both use the same fiber volume fraction V f :

[0058]

[0059] When the diameters (d p , d m ) of the carbon fibers and single-mode optical fibers and the fiber volume fraction V f are known, the relationship between the geometric dimensions of the two can be determined:

[0060]

[0061] To ensure material property similarity between the two unidirectional fiber composites, the ratio of the elastic moduli of the fiber and the matrix should satisfy:

[0062]

[0063] In the formula: E represents the elastic modulus, with the subscript p indicating the unidirectional carbon fiber composite under study and m indicating the unidirectional fiber composite using single-mode optical fibers as a replacement; the superscript F represents the fiber and M represents the matrix.

[0064] For unidirectional tension, based on the similarity of the ineffective recovery length, the similarity relationship of the fiber-matrix interface properties of the two unidirectional fiber composites can be deduced:

[0065]

[0066] In the formula: τ represents the interfacial shear strength between the fiber and the matrix base surface.

[0067] Based on the above similarity relationship of the fiber-matrix interface properties, select a suitable matrix for making the unidirectional optical fiber composite, and determine the geometric dimensions of the unidirectional fiber composite using single-mode optical fibers as a replacement according to geometric similarity.

[0068] Select a single-mode optical fiber and remove the outermost coating layer of the optical fiber by chemical or physical methods. Retain the core and cladding inside the single-mode optical fiber as the fiber filaments in the unidirectional fiber composite material. The outer diameter of the cladding is about 125 μm. According to the similarity principle, bisphenol A epoxy resin is used as the matrix material, and a fiber volume content of 60% is selected. Arrange the relative positions of the fiber filaments, and use compression molding and vacuum injection molding to fabricate the above-mentioned fiber filaments with the coating removed into a unidirectional fiber composite material 1 with a circular cross-section of 2 mm in diameter.

[0069] The tensile free segment length of the unidirectional fiber composite material 1 is 500 mm. As Figures 3 - 6 shown, at both ends of the free segment, rigid sheaths 2 are used to design corresponding clamping segments 7 through bonded anchoring. Fix the rigid sheaths 2 at both ends of the unidirectional fiber composite material 1 to the fixture 8 of the testing machine 3. Place the free segment of the unidirectional fiber composite material in the hygrothermal aging environment chamber 4. Reserve a section of length for each single-mode optical fiber 5 on both sides outside the clamping segment 7 to ensure connection with the high-spatial-resolution optical frequency domain reflectometry fiber demodulator 6.

[0070] Before starting the test, calibrate each optical fiber inside the unidirectional fiber composite material through both ends of the unidirectional fiber composite material to determine the initial state. Determine the positions of all fiber strain mutations inside the unidirectional fiber composite material by means of local small-area temperature rise and fall, accurate to 1 mm, and determine the relative position relationship sensed by all the optical fibers in the free segment. To characterize the damage evolution of the unidirectional fiber composite material, stretch the unidirectional fiber composite material specimen to the specified stress by the testing machine 3 and keep the load unchanged. The environment chamber 4 provides a hygrothermal aging environment.

[0071] Set the sampling period of the optical frequency domain reflectometry fiber demodulator to 24 hours. If a certain optical fiber breaks and cannot form an optical path, the fracture position can be judged according to the data collected by the high-spatial-resolution optical frequency domain reflectometry fiber demodulator 6, and the strain concentration coefficient and load redistribution situation can be determined according to the strain and strain gradient changes on other optical fibers at this position and in the adjacent area, and judge the influence range of the broken wire in the cross-section and the influence range along the length direction.

[0072] Repeat the above loading steps of load and environmental conditions, and evaluate whether there are new breakpoints, evaluate the stress redistribution and its influence range. If there are no new breakpoints, re-evaluate the influence range of the existing breakpoints, determine the influence range of the breakpoints and the strain concentration situation under the current load condition, and judge whether there is damage such as debonding between the fiber and the matrix on both sides of the breakpoints according to the influence range of the breakpoints and the strain concentration situation; if new breakpoints occur, it is necessary to judge whether the new breakpoints occur near the existing breakpoints and whether they are affected by the existing breakpoints, etc.; if the new breakpoints are affected by the existing breakpoints, it is necessary to further determine the influence range of the two breakpoints and the strain concentration situation of the intact fibers around; if the new breakpoints are far from the existing breakpoints, the evaluation process is similar to the above evaluation of a single breakpoint.

[0073] And so on, the damage evolution of unidirectional fiber composites under the coupled action of tensile stress and hygrothermal aging environment is determined. Since the failure of unidirectional fiber composites under tensile stress is controlled by the breakage of the internal fibers, when the unidirectional fiber composites fail, the number of breakpoints corresponding to the maximum damage local area formed by the previous load and environmental coupling loading cycle can be used to determine the damage critical clustering of unidirectional fiber composites under the coupled action of tensile stress and hygrothermal aging environment.

[0074] If the unidirectional fiber composites do not break during the specified test duration, the test is stopped, and strain-controlled step-by-step loading is adopted. When loading 500με each time, the strain in all optical fibers is collected and the breakpoint position is determined. Each step of strain loading repeats a similar damage characterization process as described above. When the unidirectional fiber composites break when reaching their remaining strength, the damage critical clustering that controls the failure of the unidirectional fiber composites at the specified tensile test duration is determined and compared with the damage critical clustering of the unidirectional fiber composites under tensile strength failure in a non-corrosive environment to determine the influence of the corrosive environment on the damage critical clustering.

[0075] If the environmental action causes the degradation of the matrix performance, the matrix on the surface of the unidirectional fiber composites cracks under the coupled action of load and environment while the nearby optical fibers do not break. Due to the local strain concentration caused by the matrix cracking, bridging is formed at the edges of the optical fibers in the matrix cracking area, and the strain on the optical fibers increases. The local matrix cracking range can be determined according to the position and number of the increased strain on the optical fibers.

[0076] If the stress on the optical fiber exceeds the tensile strength at this part due to the matrix cracking, the optical fiber breaks. Then, the position of the broken wire can be directly determined because no light path can be formed at this place. Due to the broken wire, local load redistribution occurs in this area, and the strain of the unbroken optical fibers around the breakpoint increases.

[0077] If the fiber and the matrix at the breakpoint are debonded under the coupled action of load and environment, the strain within the debonding length range of the optical fiber tends to zero, and at the same time, the length of the strain concentration area of the unbroken optical fibers around increases with the increase of the debonding length.

[0078] The damage evolution is as Figure 7As shown, white represents intact fibers, gray and black represent broken fibers, and black are the broken fibers coplanar with the current cross-section. The failure of unidirectional fiber composites under the coupling of load and environment is controlled by the breakage of its internal fibers. The unidirectional fiber composites start from the initial intact state, gradually form local wire breakage damage, and through the process of damage accumulation, form damage clustering. When the number of breakpoints in the whole unidirectional fiber composite continuously increases to reach the critical damage state, it finally breaks. The critical clustering represents the maximum local damage before failure, that is, the total number of broken wires in the damage influence area. Since the breakpoints of different fibers may not be coplanar, it is a small area in the local length direction. Represented by the total number of broken wires in the damage influence area, the damage critical clustering 9 of unidirectional fiber composites under tensile load can be evaluated.

[0079] Repeating the above tests can obtain the aging life of the unidirectional fiber composites when they fail under a certain number of specified working conditions (given tensile stress and aging environment), or the remaining strength and its corresponding damage critical clustering under a specified test duration. By statistical analysis, the probability distribution functions and their parameters of the aging life, damage critical clustering and their remaining mechanical properties are determined, and the durability evolution law of unidirectional fiber composites under the coupling of load and environment is obtained. Theoretically, if there is no corrosion environment, the unidirectional fiber composites will not fail under the given tensile stress condition. According to the aging life under the coupling of load and environment, the influence of the erosion environment on the durability life of unidirectional fiber composites can be determined; if the unidirectional fiber composites do not fail under the coupling of load and environment, the durability degradation law of the unidirectional fiber composites under the coupling of this corrosion environment and load can be determined by comparing the remaining strength of the unidirectional fiber composites under the specified test duration with the ultimate strength without corrosion environment. Both of the above situations reflect the influence of the matrix on the durability of unidirectional fiber composites.

[0080] Further, on the premise of meeting geometric similarity and material property similarity, unidirectional fiber composites are made with matrices having different corrosion resistance, and the above test, analysis and evaluation processes are repeated to determine the probability distribution functions and their parameters of the aging life, damage critical clustering and their mechanical properties of unidirectional fiber composites with different matrices, obtain the corrosion sensitivity data of various matrices under different environmental and load couplings, and determine the durability of unidirectional fiber composites under the coupling of load and environment. Through the above process, the in-situ characterization of damage evolution and the evaluation of durability of unidirectional fiber composites under the coupling of tensile stress and hygrothermal aging environment are realized.

[0081] The above is the preferred implementation manner of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An in-situ characterization and analysis method for mesoscopic damage and durability of unidirectional fiber composites under the coupled action of load and environment, characterized in that, Including: Step S1 includes: S11: Analyze the load and environment coupling effect, cross-sectional shape and size of the prototype unidirectional fiber composite material, determine the basic mechanical parameters, and the unidirectional fiber volume content; the basic mechanical parameters include at least one of the tensile strength, fatigue performance, elastic modulus of the fibers in the composite material, the interfacial bonding strength between the fibers and the matrix, and the matrix elastic modulus; S12: Use single-mode optical fiber as a substitute for the unidirectional fiber in the prototype unidirectional fiber composite material, and according to the similarity principle, determine the basic mechanical properties, environmental aging resistance of the required matrix and the interface between the fiber and the matrix, select a suitable matrix, and determine the cross-sectional size of the model unidirectional fiber composite material; the basic mechanical properties include at least one of the matrix elastic modulus and the bonding strength between the matrix and the single-mode optical fiber; S13: Fabricate a model unidirectional fiber composite material composed of single-mode optical fiber and the selected matrix; Step S2: Apply the load and environment coupling effect, use millimeter-level high-spatial-resolution strain sensing technology to conduct high-spatial-resolution strain testing on the model unidirectional fiber composite material, and based on the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite material, conduct in-situ characterization of the mesoscopic damage evolution of the unidirectional fiber composite material under the load and environment coupling effect; Step S3: Repeat Step S1 and S2 to obtain a series of damage critical clusters and their durability data of the unidirectional fiber composite material under the load and environment coupling effect, and through statistical analysis, determine the probability distribution function and its parameters of the aging life, damage critical clusters, and their remaining mechanical properties, and obtain the durability evolution law; Step S4: Select matrices with different environmental corrosion resistance to fabricate model unidirectional fiber composite materials, and repeat Steps S1 - S3 to obtain the influence law of different matrices on the durability of the unidirectional fiber composite material.

2. The method according to claim 1, characterized in that, Step S2 includes: Step S21: After the model unidirectional fiber composite material is formed, conduct initial calibration on each optical fiber to determine the relative position and initial state of the optical fibers in the composite material; Step S22: Design the load steps and environmental conditions, then apply the load and the working environment, use millimeter-level high-spatial-resolution strain sensing technology to obtain the strain changes and / or optical paths of each measurement point on each optical fiber in real time, judge the break point position, and evaluate the stress redistribution and influence range near the break point; Step S23: Continue to apply the load and the working environment according to the load steps and environmental conditions, judge whether there are new break points generated, and evaluate the stress redistribution and influence range; Step S24: Repeat Step S23 until the model composite material loses its stable load-bearing capacity, and conduct in-situ characterization of the mesoscopic damage evolution of the unidirectional fiber composite material under the load and environment coupling effect.

3. The method according to claim 2, characterized in that, In the initial calibration in Step S21, determine the position of the sudden change in the optical fiber strain by means of local small-area temperature rise and fall, and determine the relative position relationship of all the sensing optical fibers in the free section.

4. The method according to claim 1, characterized in that, In Step S1, use the compression molding vacuum injection or pultrusion process to fabricate the model unidirectional fiber composite material. The cross-sectional shape of the model unidirectional fiber composite material is the same as that of the prototype unidirectional fiber composite material, and the cross-sectional size is determined according to the similarity principle.

5. The method according to claim 2, characterized in that, Evaluating the stress redistribution and influence range includes: If no new breakpoints occur, re-evaluate the influence range of existing breakpoints, determine the influence range of breakpoints and strain concentration under the current load condition through strain and strain gradient changes, and judge whether fiber-matrix debonding damage occurs on both sides of the breakpoints; If new breakpoints occur, determine the generation position of the new breakpoints, and evaluate the influence, influence range and strain concentration of the existing breakpoints through strain and strain gradient changes; If the environmental action causes the degradation of the matrix performance, when the matrix on the surface of the unidirectional fiber composite material cracks under the coupled action of load and environment while the nearby optical fibers do not break, due to the local strain concentration caused by the matrix cracking, the optical fibers at the edge of the matrix cracking area form a bridge connection, and the strain on the optical fibers increases. The local matrix cracking range can be determined according to the position and number of the increased strain on the optical fibers; If the stress on the optical fiber exceeds the tensile strength of this part due to the matrix cracking, the optical fiber breaks, and determine the breakpoint position and the load redistribution caused by the broken wire; If fiber-matrix debonding occurs at the breakpoints under the coupled action of load and environment, the strain within the debonding length range of the optical fiber tends to zero, and at the same time, the length of the strain concentration area of the surrounding unbroken optical fibers increases with the increase of the debonding length.

6. A device for the method according to any one of claims 1 - 5, characterized in that, Including: A rigid sheath, a testing machine, an environmental chamber, a high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator, and a fixture. The fixture is arranged on the testing machine. The rigid sheath can be fixed on the testing machine through the fixture. The high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator is connected to the single-mode optical fiber in the unidirectional fiber composite material specimen fixed on the testing machine. The free section of the unidirectional fiber composite material specimen is placed in the environmental chamber. The testing machine applies load to the model unidirectional fiber composite material, the environmental chamber applies working conditions environment to the unidirectional fiber composite material, and the high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator conducts high-spatial-resolution strain testing on the optical fibers of the model unidirectional fiber composite material.

7. A method for using the device according to claim 6, characterized in that, Including the following steps: Step S1’: Fabricate an unidirectional fiber composite material specimen and determine the free section length of the unidirectional fiber composite material specimen; Step S2’: Bond and anchor the clamping sections at both ends outside the free section to the rigid sheath; Step S3’: Fix the unidirectional fiber composite material specimen on the fixture of the testing machine through the rigid sheath, and place the free section of the unidirectional fiber composite material specimen in the environmental chamber; Step S4’: Connect each single-mode optical fiber on both sides outside the clamping section to the high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator; Step S5’: Calibrate the initial state of the unidirectional fiber composite material specimen; Step S6’: Apply load and working conditions environment to the unidirectional fiber composite material specimen, adopt the millimeter-level high-spatial-resolution strain sensing technology to conduct high-spatial-resolution strain testing on the optical fibers of the model unidirectional fiber composite material, and conduct in-situ characterization of the mesoscopic damage evolution of the unidirectional fiber composite material under the coupled action of load and environment according to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite material; Step S7’: Repeat steps S1’ - S6’ to obtain a series of damage critical clusters and durability data of unidirectional fiber composites under the coupled action of load and environment. Determine the probability distribution functions and their parameters of the aging life, damage critical cluster size, and their remaining mechanical properties through statistical analysis, and obtain the durability evolution law of unidirectional fiber composites under the coupled action of load and environment; Step S8’: Select matrices with different environmental corrosion resistance to fabricate model unidirectional fiber composites. Repeat steps S1’ - S7’ to obtain the influence law of different matrices on the durability of unidirectional fiber composites. Determine the probability distribution functions and their parameters of the aging life, damage critical cluster size, and their mechanical properties of unidirectional fiber composites with different matrices, and conduct durability analysis of unidirectional fiber composites under the coupled action of load and environment.

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