A method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composites

Abstract

The present invention discloses a method for characterizing and evaluating microscopic damage in unidirectional fiber composites. The method comprises: Step S1: preparing a model unidirectional fiber composite composed of a single-mode optical fiber and a corresponding matrix; Step S2: using millimeter-level high-spatial-resolution strain sensing technology to perform optical fiber high-spatial-resolution strain testing on the model unidirectional fiber composite. Based on the strain and strain gradient changes across all optical fibers within the model unidirectional fiber composite, microscopic damage in the unidirectional fiber composite under different conditions is characterized and evaluated. This method can be effectively applied to characterizing and evaluating the entire process of damage initiation, development, and evolution in unidirectional fiber composites.

Description

Technical Field

[0001] The present invention belongs to the technical field of local damage characterization of fiber composites and mechanical property evaluation of fiber composites, and particularly relates to a method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composites. Background Art

[0002] Fiber composites are obtained by laying 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 fibers therein. 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. [[ID=X]]

[0003] Generally speaking, the mesoscopic damage of unidirectional fiber composites includes the following categories: 1. fiber-matrix debonding; 2. matrix cracking; 3. fiber fracture. These mesoscopic damages continuously evolve under the action of load, environment and their coupling, and ultimately cause the failure of unidirectional fiber composites. The evolution of mesoscopic damage directly affects the macroscopic mechanical properties of unidirectional fiber composites.

[0004] At present, optical imaging methods are mainly used for the characterization and evaluation of mesoscopic damage of composites. However, based on optical imaging methods, there are strict requirements for unidirectional fiber composite specimens, and it is difficult to realize the real-time in-situ characterization of the evolution of mesoscopic damage of unidirectional fiber composites under tensile load, fatigue load, hygrothermal and ultraviolet aging. Summary of the Invention

[0005] The present invention provides a method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composites to solve the technical problem that optical testing in the prior art is difficult to characterize and evaluate the mesoscopic damage of unidirectional fiber composites in real time, in-situ and quantitatively.

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

[0007] A method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composites, comprising:

[0008] Step S1, fabricating a model unidirectional fiber composite composed of a single-mode optical fiber and a corresponding matrix;

[0009] Step S2, adopting a millimeter-level high-spatial-resolution strain sensing technology to perform high-spatial-resolution strain testing on the model unidirectional fiber composite, and characterizing and evaluating the mesoscopic damage of the unidirectional fiber composite under different load conditions according to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite.

[0010] Optionally, the steps of S1:

[0011] Step S11: Analyze the load action and cross-sectional shape of the prototype unidirectional fiber composite material, determine the basic mechanical parameters, and the unidirectional fiber volume content;

[0012] Step S12: Use single-mode optical fibers as substitutes for the unidirectional fibers in the prototype unidirectional fiber composite material. Each optical fiber simultaneously has the functions of bearing force and high-spatial-resolution strain sensing. According to the similarity principle, determine the corresponding matrix, the basic mechanical properties of the fiber-matrix interface, and the cross-sectional dimensions of the model unidirectional fiber composite material based on the cross-sectional shape, basic mechanical parameters, and unidirectional fiber volume content in step S11;

[0013] Step S13: Fabricate the model unidirectional fiber composite material composed of the single-mode optical fibers and the corresponding matrix.

[0014] Optionally, in step S1, the model unidirectional fiber composite material is fabricated by using a compression molding vacuum injection or pultrusion process. 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.

[0015] Optionally, 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 material, the interfacial bonding strength between the fibers and the matrix, and the matrix elastic modulus; the basic mechanical properties in step S12 include at least one of the following: the elastic modulus of the matrix, and the bonding strength between the matrix and the single-mode optical fiber.

[0016] Optionally, the steps of S2:

[0017] Step S21: After the model unidirectional fiber composite material is formed, perform initial calibration on each optical fiber to determine the relative position and initial state of the optical fibers in the composite material;

[0018] Step S22: Fabricate a tooling and design load steps according to the load action to be characterized. Apply the load step by step. Use 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;

[0019] Step S23: Continue to apply the load according to the load step, judge whether there are new break points generated, and evaluate the stress redistribution and influence range;

[0020] Step S24: Repeat step S23 until the model composite material loses its stable load-bearing capacity, and determine the damage evolution process of the unidirectional fiber composite material.

[0021] Optionally, in the initial calibration in step S21, the position of the sudden change in optical fiber strain is determined by heating and cooling a local small area, and the relative positional relationship of all the sensing optical fibers in the free section is determined.

[0022] Optionally, the determination of the damage evolution process of the unidirectional fiber composite material specifically includes:

[0023] If a new break point occurs, judge the generation position of the new break point, and evaluate the influence of the existing break point, the influence range and the strain concentration situation through the changes in strain and strain gradient;

[0024] If no new break point occurs, re-evaluate the influence range of the existing break point, determine the influence range and strain concentration situation of the break point under the current load condition 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;

[0025] If matrix cracking occurs but the nearby optical fiber does not break, due to local strain concentration caused by matrix cracking, bridging is formed at the edge of the matrix cracking area, the strain on the optical fiber increases, and the local matrix cracking range is determined according to the position and number of the increased strain on the optical fiber;

[0026] If matrix cracking causes the stress on the optical fiber to exceed the tensile strength of the part, the optical fiber breaks, and the break point position and the load redistribution caused by the broken wire are determined;

[0027] If the optical fiber breaks and debonding occurs between the fiber and the matrix at the break point under the action of the load step, 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.

[0028] Optionally, the determination of the break point position specifically includes: when the stress at a certain point in the optical fiber exceeds its tensile strength, breakage occurs at that point, and then that point is directly determined as the broken wire position because an optical path cannot be formed.

[0029] On the other hand, a device for the above method is provided, including a rigid sheath, a testing machine, a fixture and a high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator. 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 a unidirectional fiber composite material specimen fixed on the testing machine, the testing machine applies a load step to the model unidirectional fiber composite material, and the high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator performs high-spatial-resolution strain testing on the model unidirectional fiber composite material.

[0030] On the other hand, a method for using the above device is provided, including the following steps:

[0031] Step S1’: Fabricate a model unidirectional fiber composite specimen and determine the free - segment length of the model unidirectional fiber composite specimen;

[0032] Step S2’: Bond and anchor the clamping segments at both ends outside the free segment to a rigid sheath;

[0033] Step S3’: Fix the unidirectional fiber composite specimen to the fixture of a testing machine through the rigid sheath;

[0034] Step S4’: Connect each single - mode optical fiber on both sides outside the clamping segment to a high - spatial - resolution optical frequency domain reflectometry fiber demodulator;

[0035] Step S5’: Calibrate the initial state of the unidirectional fiber composite specimen;

[0036] Step S6’: Adopt a millimeter - level high - spatial - resolution strain sensing technology to perform fiber 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, characterize and evaluate the mesoscopic damage of the unidirectional fiber composite under different load conditions.

[0037] The beneficial effects brought by the technical solution provided by the present invention at least include:

[0038] The above - mentioned method effectively solves the technical problem that in the prior art, the optical method cannot characterize and evaluate the mesoscopic damage of unidirectional fiber composites. It can realize the characterization and evaluation of the mesoscopic damage evolution of unidirectional fiber composites under different loads, environments and their coupling conditions, and provides ideas and methods for the characterization and evaluation of the whole process of the occurrence, development and evolution of damage in unidirectional fiber composites. Description of the Drawings

[0039] 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, without creative efforts, other drawings can be obtained based on these drawings.

[0040] Figure 1 It is a flowchart of a method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composites provided by an embodiment of the present invention;

[0041] Figure 2 It is a schematic diagram of similar principle parameters in an embodiment of the present invention, where (a) is a schematic diagram of the parameters of a prototype unidirectional carbon fiber composite; (b) is a schematic diagram of the parameters of a model unidirectional fiber composite using single - mode optical fibers instead of carbon fibers;

[0042] Figure 3Schematic diagram of the mesoscopic damage evaluation test device for unidirectional fiber composite specimens in the embodiments of the present invention;

[0043] Figure 4 Unidirectional fiber composite specimen in the embodiments of the present invention;

[0044] Figure 5 Schematic diagram of the connection between the single-mode optical fiber and the high-spatial-resolution optical frequency domain reflectometry demodulator in the unidirectional fiber composite in the embodiments of the present invention;

[0045] Figure 6 Schematic diagram of the clamping section of the unidirectional fiber composite specimen in the embodiments of the present invention;

[0046] Figure 7 Schematic diagram of the damage evolution process and the formation of damage critical clusters in the unidirectional fiber composite, where (a) is the initial intact state; (b) is the local wire breakage damage; (c) is the damage accumulation process; (d) is the formation of damage clusters. Detailed implementation manners

[0047] 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.

[0048] As Figure 1 shown, the embodiments of the present invention provide a method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composites, including:

[0049] Step S1, fabricating a model unidirectional fiber composite composed of a single-mode optical fiber and a corresponding matrix;

[0050] Step S2, adopting a millimeter-level high-spatial-resolution strain sensing technology to perform optical fiber high-spatial-resolution strain testing on the model unidirectional fiber composite, and characterizing and evaluating the mesoscopic damage of the unidirectional fiber composite under different conditions according to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite.

[0051] Next, with reference to Figures 2 - 7 , a method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composites in the embodiments of the present invention will be described in detail.

[0052] For ease of explanation, it is assumed that the fibers are uniformly arranged in a quadrilateral shape in the unidirectional fiber composite (actually randomly arranged), and the basic geometric and mechanical parameters of the prototype unidirectional carbon fiber composite to be studied and the designed model unidirectional fiber composite using a single-mode optical fiber instead of carbon fiber are as Figure 2 shown. According to the similarity principle, to ensure the geometric similarity of the unidirectional carbon fiber composite to be studied and the designed unidirectional fiber composite using a single-mode optical fiber instead of carbon fiber, the two use the same fiber volume content Vf:

[0053]

[0054] When the diameters (d p , d m ) of carbon fiber and single-mode optical fiber and the fiber volume fraction V f are known, the relationship between the geometric dimensions of the two can be determined:

[0055]

[0056] To ensure that the material properties of the two unidirectional fiber composites are similar, the elastic modulus ratio of the fiber to the matrix should satisfy:

[0057]

[0058] In the formula: E is the elastic modulus, with the subscript p representing the unidirectional carbon fiber composite under study, m representing the unidirectional fiber composite using single-mode optical fiber as a substitute; the superscript F represents the fiber, and M represents the matrix.

[0059] 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:

[0060]

[0061] In the formula: τ represents the interfacial shear strength between the fiber and the matrix base surface.

[0062] According to 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 fiber as a substitute according to geometric similarity.

[0063] Select a single-mode optical fiber and remove the outermost coating layer of the optical fiber by chemical or physical methods, and retain the core and cladding inside the single-mode optical fiber as the fiber filaments in the unidirectional fiber composite. The outer diameter of the cladding is about 125 μm. According to the similarity principle, use bisphenol A epoxy resin as the matrix material, select a fiber volume fraction of 60%, arrange the relative positions of the fiber filaments, and use compression molding and vacuum injection molding to make the above-mentioned fiber filaments without the coating layer into a circular cross-section model unidirectional fiber composite 1 with a diameter of 2 mm.

[0064] The tensile free segment length of the model unidirectional fiber composite 1 is 500 mm. As Figures 3 - 6 shown, at both ends outside the free segment, rigid sheaths 2 are used to design corresponding clamping segments 3 through adhesive anchoring. Fix the rigid sheaths 2 at both ends of the model unidirectional fiber composite 1 on the fixture 5 of the testing machine 4, and reserve a section of length for each fiber filament 6 on both sides outside the clamping segment to ensure connection with the high-spatial-resolution optical frequency domain reflectometry fiber demodulation instrument 7.

[0065] Before starting the experiment, calibrate each optical fiber in the model unidirectional fiber composite through both ends of the model unidirectional fiber composite to determine the initial state. Determine the positions of all strain mutations of the optical fibers in the model unidirectional fiber composite by means of local small-area temperature rise and fall, accurate to 1 mm, and determine the relative position relationship sensed by all optical fibers in the free section. To characterize the damage evolution of the model unidirectional fiber composite during unidirectional tension, use load steps for step-by-step loading. When the strain is controlled to 500 με in each load step, collect the strain in all optical fibers, where the spatial resolution of strain collection is 1 mm.

[0066] If, under a certain loading strain state, 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 optical fiber demodulator 7 (the high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator 7 uses millimeter-level high-spatial-resolution strain sensing technology and is realized by the optical frequency domain reflectometry (OFDR) principle), and according to this position, and the strain and strain gradient changes on other optical fibers in the adjacent area, determine the strain concentration coefficient and the load redistribution situation, and judge the influence range of the broken wire in the influence range of this cross-section and the influence range along the length direction.

[0067] Continue to increase the load steps, and evaluate whether new breakpoints occur, evaluate the stress redistribution and its influence range in a similar process as above. 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 ranges of the two breakpoints and the strain concentration 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.

[0068] If no new breakpoints occur, re-evaluate the influence range of the existing breakpoints, determine the influence range of the breakpoints and the strain concentration under the current load conditions, and judge whether damage such as debonding between the fiber and the matrix occurs on both sides of the breakpoints according to the influence range and strain concentration of the breakpoints; if matrix cracking occurs and the nearby optical fibers do not break, due to local strain concentration caused by matrix cracking, bridging is formed on the optical fibers at the edge of the matrix cracking area, and the strain on the optical fibers increases. Determine the local matrix cracking range according to the position and quantity of the increased strain on the optical fibers.

[0069] If the stress on the optical fiber exceeds the tensile strength at its location due to matrix cracking, the optical fiber breaks. Determine the break point location and the load redistribution caused by the broken wire. If the optical fiber breaks and the fiber and matrix at the break point debond under the action of the load step, the strain within the debonded length range of the optical fiber tends to zero. At the same time, the length of the strain concentration area of the unbroken optical fibers around increases with the increase of the debonded length. The properties of the constituent materials of unidirectional fiber composites are random, and there is a certain discreteness in the tensile strength and fatigue performance of different parts of the same optical fiber, and the same is true for different optical fibers. The bonding properties between the fiber and the matrix at different parts and the mechanical properties of the matrix itself are different. Therefore, under the action of a certain load step in the same unidirectional fiber composite, the occurrence and development of mesoscopic damage at different internal parts are also different, showing a certain discreteness, ultimately controlling the failure of the unidirectional fiber composite and forming damage clustering locally within it. Since the axial load of the unidirectional fiber composite is considered to be borne entirely by the fiber filaments, the damage development and evolution of the unidirectional fiber composite are mainly controlled by the breakage of the internal fibers and ultimately form a damage critical clustering in a local area.

[0070] The damage evolution is as Figure 7 shown. White represents intact fibers, gray and black represent broken fibers, and black is the broken fiber coplanar with the current cross-section. The failure of the unidirectional fiber composite under tensile action is controlled by the breakage of its internal fibers. The unidirectional fiber composite starts from an initially intact state, forms local broken wire damage under the load, and through the process of damage accumulation, forms damage clustering. When the number of break points in the entire unidirectional fiber composite continuously increases to reach the critical damage state, it finally breaks. The critical clustering represents the maximum local damage before failure, including the number of locally broken and damaged fibers. Since the break points 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 within the damage influence area, the damage critical clustering 8 of the unidirectional fiber composite under tensile load can be evaluated.

[0071] Through the above process, the characterization and evaluation of the damage of the unidirectional fiber composite under tensile load are realized.

[0072] The embodiment of the present invention also provides a device for the above method, including a rigid sheath, a testing machine, a fixture, and a high-spatial-resolution optical frequency domain reflectometry fiber demodulator. The fixture is set on the testing machine, the rigid sheath can be fixed on the testing machine through the fixture, and the high-spatial-resolution optical frequency domain reflectometry fiber demodulator is connected to the unidirectional fiber composite specimen fixed on the testing machine. The testing machine applies a load to the model unidirectional fiber composite, and the high-spatial-resolution optical frequency domain reflectometry fiber demodulator performs high-spatial-resolution strain testing on the optical fibers of the model unidirectional fiber composite.

[0073] The embodiment of the present invention also provides a usage method of the above device, including the following steps:

[0074] Step S1': Fabricate a model unidirectional fiber composite specimen and determine the free segment length of the model unidirectional fiber composite specimen;

[0075] Step S2': Bond and anchor the clamping segments at both ends outside the free segment to a rigid sheath;

[0076] Step S3': Fix the unidirectional fiber composite specimen to the fixture of a testing machine through the rigid sheath;

[0077] Step S4': Connect each fiber filament on both sides outside the clamping segment to a high-spatial-resolution optical frequency domain reflectometry fiber optic demodulator;

[0078] Step S5': Use millimeter-level high-spatial-resolution strain sensing technology to conduct fiber optic high-spatial-resolution strain testing on the model unidirectional fiber composite, and characterize and evaluate the mesoscopic damage of the unidirectional fiber composite under different conditions according to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite.

[0079] The above is the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composites, characterized in that: Including: Step S1, manufacturing a model unidirectional fiber composite material composed of a single-mode optical fiber and a corresponding matrix, the steps of S1: Step S11, analyzing the load action and cross-sectional shape of the prototype unidirectional fiber composite material, determining the basic mechanical parameters, and the volume content of unidirectional fibers; Step S12: Using a single-mode optical fiber as a substitute for the unidirectional fiber in the prototype unidirectional fiber composite material, each optical fiber simultaneously has the functions of bearing force and high-spatial-resolution strain sensing, and based on the cross-sectional shape, basic mechanical parameters, and volume content of unidirectional fibers in Step S11, according to the similarity principle, determining the corresponding matrix, and the basic mechanical properties of the fiber-matrix interface, and determining the cross-sectional size of the model unidirectional fiber composite material; Step S13: Manufacturing the model unidirectional fiber composite material composed of the single-mode optical fiber and the corresponding matrix; Step S2, using a millimeter-level high-spatial-resolution strain sensing technology to perform optical fiber high-spatial-resolution strain testing on the model unidirectional fiber composite material, and characterizing and evaluating the mesoscopic damage of the unidirectional fiber composite material under different load conditions according to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite material; The basic mechanical parameters determined in Step S11 include at least one of the following: the tensile strength, fatigue performance, elastic modulus of the fiber in the composite material, the interfacial bonding strength between the fiber and the matrix, and the matrix elastic modulus; The basic mechanical properties in Step S12 include at least one of the following: the elastic modulus of the matrix, and the bonding strength between the matrix and the single-mode optical fiber.

2. The method according to claim 1, characterized in that, In Step S1, the model unidirectional fiber composite material is manufactured by a compression molding vacuum injection or pultrusion process, and 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.

3. The method according to claim 1, characterized in that The steps of S2: Step S21, after the model unidirectional fiber composite material is formed, performing initial calibration on each optical fiber to determine the relative position and initial state of the optical fibers in the composite material; Step S22: Manufacturing a tooling and designing load steps according to the required load action to be characterized, applying loads step by step according to the load steps, using 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, judging the break point position, and evaluating the stress redistribution and influence range near the break point; Step S23: Continuing to apply loads according to the load steps, judging whether new break points are generated, and evaluating the stress redistribution and influence range; Step S24: Repeating Step S23 until the model composite material loses its stable load-bearing capacity, and determining the damage evolution process of the unidirectional fiber composite material.

4. The method according to claim 3, wherein In the initial calibration in Step S21, the position of the sudden change in the optical fiber strain is determined by locally heating and cooling a small area, and the relative position relationship of all the sensing optical fibers in the free section is determined.

5. The method according to claim 3, wherein In Step S24, the determination of the damage evolution process of the unidirectional fiber composite material specifically includes: If new break points are generated, judging the generation position of the new break points, and evaluating the influence situation, influence range, and strain concentration situation affected by the existing break points through the changes in strain and strain gradient; If no new breakpoints are generated, re-evaluate the influence range of the existing breakpoints. Determine the influence range of the breakpoints and the strain concentration under the current load condition 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 breakpoint. If matrix cracking occurs while the nearby optical fiber does not break, due to the local strain concentration caused by matrix cracking, bridging is formed at the edge of the matrix cracking area for the optical fiber, and the strain on the optical fiber increases. Determine the local matrix cracking range according to the position and quantity of the increased strain on the optical fiber. If the stress on the optical fiber exceeds the tensile strength of the corresponding part due to matrix cracking, the optical fiber breaks, and determine the breakpoint position and the load redistribution caused by the broken wire. If the optical fiber breaks and the fiber and the matrix at the breakpoint debond under the action of the load step, 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.

6. The method according to claim 3, characterized in that, The determination of the breakpoint position specifically includes: when the stress at a certain point in the optical fiber exceeds its tensile strength, breakage occurs at that point, and then that point is directly determined as the broken wire position because no light path can be formed.

7. An apparatus for the method according to any one of claims 1-6, characterized in that, It includes a rigid sheath, a testing machine, a fixture, and a high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator. 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 a unidirectional fiber composite material specimen fixed on the testing machine, the testing machine applies a load step to the model unidirectional fiber composite material, and the high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator performs high-spatial-resolution strain testing on the optical fibers of the model unidirectional fiber composite material.

8. A method for using the device according to claim 7, characterized in that, It includes the following steps: Step S1’: Fabricate a model unidirectional fiber composite material specimen and determine the free segment length of the model unidirectional fiber composite material specimen. Step S2’: Bond and anchor the clamping segments at both ends outside the free segment to the rigid sheath. Step S3’: Fix the unidirectional fiber composite material specimen on the fixture of the testing machine through the rigid sheath. Step S4’: Connect each single-mode optical fiber on both sides outside the clamping segment 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’: Adopt a millimeter-level high-spatial-resolution strain sensing technology to perform high-spatial-resolution strain testing on the optical fibers of the model unidirectional fiber composite material, and characterize and evaluate the mesoscopic damage of the unidirectional fiber composite material under different load conditions according to the strain and strain gradient changes of all the optical fibers within the model unidirectional fiber composite material.

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