A method for characterizing the influence of boundary effect and size effect on the mechanical properties of unidirectional fiber composites

Abstract

The present invention discloses a method for characterizing the influence of boundary effect and size effect on the mechanical properties of unidirectional fiber composites, including: S1: fabricating a model unidirectional fiber composite composed of a single-mode optical fiber and a corresponding matrix; S2: performing optical fiber high-spatial-resolution strain testing on the model unidirectional fiber composite, and extracting data such as the damage critical cluster at instability, the cross-sectional position where the damage critical cluster is located, and the mechanical properties of the model unidirectional fiber composite; S3: repeating S1-S2, calculating the probabilities of the damage critical clusters controlling material failure being at the boundary and inside, and determining the corresponding relationship between the position where the damage critical cluster is located and the mechanical properties; S4: changing the cross-sectional size and length of the model unidirectional fiber composite, and repeating S1-S3 to obtain the evolution relationship of the mechanical properties of the model unidirectional fiber composite with the cross-sectional size and length. The present invention can realize the characterization of the influence law of boundary effect and size effect on the mechanical properties of unidirectional fiber composites under different load conditions.

Description

Technical Field

[0001] The present invention belongs to the technical field of critical damage characterization and mechanical property evaluation of fiber composites, and particularly relates to a method for characterizing the influence of boundary effect and size effect on the mechanical properties of unidirectional fiber composites. Background Technique

[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. At the same time, after the unidirectional fiber breaks, due to the bonding effect of the matrix, the broken fibers can continue to bear the load after leaving the break point for a certain range.

[0003] Generally speaking, the mesoscopic damage of unidirectional fiber composites includes the following categories: 1. debonding between fiber and matrix; 2. matrix cracking; 3. fiber fracture. These mesoscopic damages continuously evolve under the action of load, environment and their coupling, and finally, due to the fiber fracture in the composite material, the formation of damage critical clustering controls the failure of the composite material.

[0004] A large number of fiber filaments in the unidirectional fiber composite bear the load along the fiber length direction. The fiber filaments break under the action of the load, and a broken filament clustering is formed in a small area. Due to the load redistribution caused by the broken filament clustering, the surrounding unbroken fiber filaments are damaged more severely and gradually form a relatively large damage clustering. When the number of broken filaments in this area reaches a critical value, the damage of the unidirectional fiber composite becomes unstable and the whole unidirectional fiber composite fails.

[0005] For the broken filaments inside the unidirectional fiber composite material, the number of fiber filaments around the break point that can participate in the load redistribution is relatively large; however, for the broken filaments on the surface of the unidirectional fiber composite material, since the fiber filaments are at the boundary of the composite material, the number of fiber filaments around the break point that can participate in the load redistribution is relatively small. Therefore, the stress concentration on the unbroken fibers around the surface break is more significant, and it is easier to form broken filament clustering locally at the surface break point. Especially when the cross-sectional shape of the unidirectional fiber composite material is rectangular or has sharp angles, once there is a broken filament in the corner area, the number of fiber filaments around that can participate in the load redistribution is relatively small, and the unidirectional fiber composite material is more likely to form broken filament clustering first at this part and gradually evolve into damage critical clustering to control the failure of the unidirectional fiber composite material, that is, the boundary effect will affect the mechanical properties of the unidirectional fiber composite material. As the cross-section of the unidirectional fiber composite material increases, the number of fiber filaments inside increases, the influence of the boundary effect decreases, the possibility of defects in the internal fiber filaments increases, and as the length increases, the possibility of defects in the unidirectional fiber composite material along the length direction increases. For unidirectional fiber composite materials with the same cross-sectional size, their mechanical properties decrease as the length increases, and the unidirectional fiber composite material exhibits an obvious size effect.

[0006] In the prior art, there are strict requirements for unidirectional fiber composite specimens based on optical imaging methods, and it is impossible to characterize the influence of the boundary effect on the failure cause of unidirectional fiber composite materials. At present, there is no effective method to characterize and evaluate the contribution of the boundary effect to the formation of damage critical clustering of unidirectional fiber composite materials, and it is impossible to verify the influence of the boundary effect on the mechanical properties of unidirectional fiber composite materials. Therefore, it is urgent to quantitatively evaluate the influence of the boundary effect on the initial formation of damage clustering and its mechanical properties of unidirectional fiber composite materials through effective characterization and testing methods, so as to provide support for the prediction of the mechanical properties of unidirectional fiber composite materials. Summary of the Invention

[0007] The present invention provides a method for characterizing the influence of the boundary effect and the size effect on the mechanical properties of unidirectional fiber composite materials, and solves the technical problem that it is difficult to quantitatively evaluate the boundary effect of unidirectional fiber composite materials on the formation of damage clustering and damage evolution in the prior art by optical testing.

[0008] To solve the above technical problems, the present invention provides the following technical solutions:

[0009] A method for characterizing the influence of the boundary effect and the size effect on the mechanical properties of unidirectional fiber composite materials includes the following steps:

[0010] Step S1: Fabricate a model unidirectional fiber composite material composed of single-mode optical fibers and corresponding matrices;

[0011] Step S2: Using millimeter-level high-spatial-resolution strain sensing technology, perform high-spatial-resolution strain testing on the unidirectional fiber composite material of the model. According to the strain and strain gradient changes on all the optical fibers within the unidirectional fiber composite material of the model, extract data such as the damage critical clusters, the cross-sectional positions where the damage critical clusters are located, and the mechanical properties of the unidirectional fiber composite material of the model when the unidirectional fiber composite material of the model loses its stable bearing capacity;

[0012] Step S3: Repeat Steps S1 - S2, extract a series of the data described in Step S2, calculate the probabilities of the damage critical clusters that control the failure of the unidirectional fiber composite material of the model being at the boundary and inside, and determine the corresponding relationships between the damage critical clusters being at the boundary and inside and their mechanical properties;

[0013] Step S4: While keeping other parameters of the unidirectional fiber composite material of the model unchanged, change the cross-sectional dimensions and length, repeat Steps S1 - S3, obtain the evolution relationship of the mechanical properties of the unidirectional fiber composite material of the model with the cross-sectional dimensions and length, and determine the influences of the boundary effect and size effect on the mechanical properties of the unidirectional fiber composite material.

[0014] Further, Step S1 includes:

[0015] 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;

[0016] Step S12: Use single-mode optical fibers as substitutes for the unidirectional fibers in the prototype unidirectional fiber composite material, and based on the cross-sectional shape, basic mechanical parameters, and unidirectional fiber volume content in Step S11, according to the similarity principle, determine the corresponding matrix and the basic mechanical properties of the fiber-matrix interface, and determine the cross-sectional dimensions of the unidirectional fiber composite material of the model;

[0017] Step S13: Fabricate the unidirectional fiber composite material of the model composed of the single-mode optical fibers and the corresponding matrix.

[0018] Further, in Step S1, the unidirectional fiber composite material of the model is fabricated using a compression molding vacuum injection or pultrusion process. The cross-sectional shape of the unidirectional fiber composite material of the model is the same as that of the prototype unidirectional fiber composite material, and the cross-sectional dimensions are determined according to the similarity principle.

[0019] 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 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 fibers.

[0020] Further, step S2 includes:

[0021] Step S21: After the model unidirectional fiber composite material is formed, initial calibration is performed on each optical fiber to determine the relative position of the optical fiber in the composite material and its initial state;

[0022] Step S22: Make a tooling and design load steps according to the load action to be characterized. Apply the load step by step. Using the millimeter-level high-spatial-resolution strain sensing technology, obtain the strain change and / or optical path 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;

[0023] Step S23: Continue to apply the load according to the load step, judge whether new break points are generated, and evaluate the stress redistribution and influence range;

[0024] Step S24: Repeat step S23 until the model composite material loses its stable load-bearing capacity, and extract the damage critical clustering, the cross-sectional position where the damage critical clustering is located, and the mechanical properties of the unidirectional fiber composite material when the unidirectional fiber composite material loses its stable bearing capacity.

[0025] Further, in the initial calibration in step S21, the positions of sudden changes in the strain of all optical fibers in the unidirectional fiber composite material and their relative positions in the cross-section are determined by means of local temperature rise and fall; the relative positions of the fiber filaments on all the boundaries of the unidirectional fiber composite material are obtained by means of slight extrusion.

[0026] Further, evaluating the stress redistribution and influence range includes:

[0027] If new break points are generated, it is necessary to judge whether the new break points occur near the existing break points and the cross-section boundary, and whether they are affected by the existing break points or boundary effects, etc.;

[0028] If no new break points are generated, re-evaluate the influence range of the existing break points, determine the influence range of the break points and the strain concentration under the current load, and judge whether damage such as debonding between the fiber and the matrix occurs on both sides of the break points according to the influence range of the break points and the strain concentration;

[0029] If matrix cracking occurs but 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. The local matrix cracking range can be determined according to the position and quantity of the increased strain on the optical fibers;

[0030] If the stress on the optical fiber exceeds the tensile strength of the part where the matrix cracking occurs, the optical fiber breaks, and the break point position and the load redistribution caused by the broken wire are determined;

[0031] If the optical fiber breaks, the fiber and the matrix at the break point will debond under the action of the load step. Then, 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.

[0032] Furthermore, to determine the break point position, it 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.

[0033] Furthermore, the present invention provides a device for the above method, including: a rigid sheath, a high-spatial-resolution optical frequency domain reflectometry fiber demodulator, a fatigue testing machine, and a fixture. The high-spatial-resolution optical frequency domain reflectometry fiber demodulator is connected to the single-mode optical fiber within the unidirectional fiber composite specimen fixed on the fatigue testing machine. The fatigue 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 fiber of the model unidirectional fiber composite.

[0034] Furthermore, the present invention provides a method for using the above device, including the following steps:

[0035] Step S1’: Fabricate a unidirectional fiber composite specimen and determine the free segment length of the unidirectional fiber composite specimen;

[0036] Step S2’: Connect the two ends outside the free segment to the clamping through a rigid sheath;

[0037] Step S3’: Connect each single-mode optical fiber on both sides outside the clamping segment to the high-spatial-resolution optical frequency domain reflectometry fiber demodulator

[0038] Step S4’: Fix the unidirectional fiber composite specimen on the fixture of the fatigue testing machine through the clamping segment:

[0039] Step S5’: Calibrate the initial state of the unidirectional fiber composite specimen;

[0040] Step S6’: Adopt millimeter-level high-spatial-resolution strain sensing technology to perform high-spatial-resolution strain testing on the optical fiber of the model unidirectional fiber composite. According to the strain and strain gradient changes on all the optical fibers within the model unidirectional fiber composite, extract data such as the damage critical clustering when the unidirectional fiber composite loses its stable bearing capacity, the cross-sectional position where the damage critical clustering is located, and the mechanical properties of the unidirectional fiber composite;

[0041] Step S7’: Repeat steps S1’ - S6’, extract the data described in step S6’ in a series, calculate the probabilities of the damage critical clustering controlling the failure of the model unidirectional fiber composite at the boundary and inside, and determine the corresponding relationship between the position where the damage critical clustering is located and the mechanical properties;

[0042] Step S8’: Change the cross-sectional size and length of the model unidirectional fiber composite material, repeat Steps S1’ - S7’, obtain the evolution relationship of the mechanical properties of the model unidirectional fiber composite material with respect to the cross-sectional size and length, and determine the influence of boundary effects and size effects on the mechanical properties of the unidirectional fiber composite material.

[0043] The beneficial effects brought by the technical solution provided by the present invention at least include:

[0044] The above method uses 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 (realized by the optical frequency domain reflectometry (OFDR) principle), effectively solving the technical problem in the prior art that the optical method cannot characterize the influence of boundary effects on unidirectional fiber composite materials. It can realize the characterization of the influence of boundary effects on the mechanical properties of unidirectional fiber composite materials under different working conditions, determine the influence of boundary effects on the critical damage clustering of unidirectional fiber composite materials, reveal the control effect of boundary effects of unidirectional fiber composite materials on their failure and the influence on their mechanical properties under different load conditions, and the method is simple and easy to implement. Description of the Drawings

[0045] 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 drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0046] Figure 1 It is a flowchart of a method for characterizing the influence of boundary effects and size effects on the mechanical properties of unidirectional fiber composite materials provided by an embodiment of the present invention;

[0047] Figure 2 It is a schematic diagram of the similarity principle parameters in an embodiment of the present invention, where (a) is a schematic diagram of the parameters of the prototype unidirectional carbon fiber composite material; (b) is a schematic diagram of the parameters of the model unidirectional optical fiber composite material using a single-mode optical fiber to replace the carbon fiber;

[0048] Figure 3 It is a test device for characterizing and evaluating the influence of boundary effects and size effects on the mechanical properties of unidirectional fiber composite materials in an embodiment of the present invention;

[0049] Figure 4 It is a specimen of unidirectional fiber composite material in an embodiment of the present invention;

[0050] Figure 5 It is a schematic connection diagram of a single-mode optical fiber and a high-spatial-resolution optical frequency domain reflectometry fiber demodulator in the unidirectional fiber composite material in an embodiment of the present invention;

[0051] Figure 6 The cross-section of the clamping section of the unidirectional fiber composite specimen according to the embodiment of the present invention, where (a) is a circular cross-section and (b) is a rectangular cross-section;

[0052] Figure 7 The schematic diagram of the influence result of the boundary effect of the unidirectional fiber composite material according to the embodiment of the present invention, where (a) is the initial intact state of the circular cross-section, (a)' is the initial intact state of the rectangular cross-section; (b) is the local wire breakage damage of the circular cross-section, (b)' is the local wire breakage damage of the rectangular cross-section; (c) is the damage accumulation process of the circular cross-section, (c)' is the damage accumulation process of the rectangular cross-section; (d) is the formation of damage clustering of the circular cross-section, (d)' is the formation of damage clustering of the rectangular cross-section. Specific embodiments

[0053] 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.

[0054] As Figure 1 shown, the embodiment of the present invention provides a method for characterizing the influence of boundary effect and size effect on the mechanical properties of unidirectional fiber composite materials, including:

[0055] Step S1: Fabricate a model unidirectional fiber composite material composed of a single-mode optical fiber and a corresponding matrix;

[0056] Step S2: Adopt a 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, extract data such as the damage critical clustering when the model unidirectional fiber composite material loses its stable bearing capacity, the cross-section position where the damage critical clustering is located, and the mechanical properties of the model unidirectional fiber composite material;

[0057] Step S3: Repeat steps S1-S2, extract a series of data described in step S2, calculate the probabilities of the damage critical clustering that controls the failure of the model unidirectional fiber composite material being at the boundary and inside, and determine the corresponding relationship between the damage critical clustering being at the boundary and inside and its mechanical properties;

[0058] Step S4: Without changing other parameters of the model unidirectional fiber composite material, change the cross-section size and length, repeat steps S1-S3, obtain the evolution relationship of the mechanical properties of the model unidirectional fiber composite material with the cross-section size and length, and determine the influence of the boundary effect and size effect on the mechanical properties of the unidirectional fiber composite material.

[0059] Next, in combination with Figures 2 - 7 , a method for characterizing and evaluating the mesoscopic damage of unidirectional fiber composite materials according to the embodiment of the present invention will be described in detail.

[0060] For the sake of illustration, it is assumed that the fibers are uniformly 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 content V f :

[0061]

[0062] When the diameters (d p , d m ) of the carbon fibers and single-mode optical fibers and the fiber volume content V f are known, the relationship between the geometric dimensions of the two can be determined:

[0063]

[0064] 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:

[0065]

[0066] 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.

[0067] 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 derived:

[0068]

[0069] In the formula: τ represents the interfacial shear strength between the fiber and the matrix base surface.

[0070] 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.

[0071] 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 the fiber volume content is selected to be 60%. Arrange the relative positions of the fiber filaments, and use compression molding and vacuum injection molding to fabricate the fiber filaments with the coating removed into a unidirectional fiber composite specimen 1 with a rectangular cross-section of 5 mm in width and 1 mm in thickness, or a circular cross-section with a diameter of 2 mm.

[0072] The free segment length of the unidirectional fiber composite specimen 1 is set to 500 mm. As Figures 3 - 6 shown, at both ends outside the free segment, rigid sheaths 2 are used to design corresponding clamping segments through bonded anchoring. A length is reserved for each fiber filament 3 on both sides outside the clamping segment to ensure connection with the high-spatial-resolution optical frequency domain reflectometry fiber demodulator 4. Fix the fabricated unidirectional fiber composite to the fixture 7 of the fatigue testing machine 6 through the clamping segment 5.

[0073] Before starting the fatigue test, calibrate each optical fiber inside the unidirectional fiber composite through both ends of the unidirectional fiber composite to determine the initial state. Determine the positions of all optical fiber strain mutations inside the unidirectional fiber composite and their relative positions in the cross-section by means of local heating and cooling. Obtain the relative position information of the fiber filaments on all boundaries of the unidirectional fiber composite through a slight extrusion method, accurate to 1 mm, and determine the relative position relationship sensed by all optical fibers in the free segment.

[0074] Set the fatigue load amplitude. After 10,000 cycles per cycle, stop the fatigue test and hold the load at the mean load. Collect the strain in all optical fibers and determine the break point positions, where the spatial resolution of strain collection is 1 mm. Theoretically, if all optical fibers are in good condition, the strain on each optical fiber inside should be basically consistent with the applied strain. If the strain gradient changes on a certain optical fiber, it is necessary to judge whether matrix cracking has occurred.

[0075] If, in this state, a certain optical fiber breaks and cannot form an optical path, its break position can be judged based on the collected data. Judge whether the break point is at the cross-section boundary, and determine the strain concentration coefficient and load redistribution according to the strain gradient changes on other optical fibers at this position and in the adjacent area. Judge the influence range of the broken wire at this cross-section and its influence range along the length direction, and judge whether debonding occurs at the break point position. Repeat the above process to evaluate whether new break points occur, whether the occurrence positions are at the cross-section boundary, and evaluate the stress redistribution and its influence range.

[0076] If no new breakpoints are generated, re-evaluate the influence range of the existing breakpoints to determine the influence range of the breakpoints and the strain concentration at the current fatigue cycle number. Based on the influence range of the breakpoints and the strain concentration, judge whether damage such as debonding between fibers and the matrix occurs on both sides of the breakpoints.

[0077] If new breakpoints are generated, it is necessary to judge whether the new breakpoints occur near the existing breakpoints and the cross-section boundary, and whether they are affected by the existing breakpoints or boundary effects, 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 them; if the new breakpoints are far from the existing breakpoints, the evaluation process is similar to the above evaluation of a single breakpoint. By analogy, evaluate the positions of subsequent breakpoints and their influence ranges to determine the damage evolution of unidirectional fiber composites during fatigue.

[0078] If matrix cracking occurs and the nearby optical fibers do not break, due to the local strain concentration caused by matrix cracking, bridging is formed at the edges of the matrix cracking area, and the strain on the optical fibers increases. The local matrix cracking range can be determined according to the positions and quantities of the increased strain on the optical fibers. If the stress on the optical fibers exceeds the tensile strength of this part due to matrix cracking, the optical fibers will break. The breakpoint positions and the load redistribution caused by the broken wires can be determined according to the aforementioned broken wires.

[0079] If debonding occurs between the fibers and the matrix at the breakpoint under the action of fatigue load, 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.

[0080] When a unidirectional fiber composite reaches the critical state of failure under a certain load condition, a large number of broken wires are formed in a certain local area. Judge whether they belong to the same damage influence area according to the breakpoint positions of these fibers, mark the breaking order of the fiber filaments on the cross-section of this area, judge whether the damage area is at the cross-section boundary of the unidirectional fiber composite, determine whether the damage area is affected by the boundary effect according to the broken wire order, and determine the number of broken wires in this area and the stress distribution and stress gradient on the adjacent unbroken optical fibers. The total number of broken wires in this damage influence area is the damage critical clustering, and the stress distribution and stress gradient on the adjacent unbroken optical fibers are the stress redistribution characteristics of the damage critical clustering.

[0081] The failure of unidirectional fiber composites under fatigue loading is controlled by the breakage of the fibers within them. When the fatigue cycle reaches the fatigue life of the unidirectional fiber composites and failure occurs, the damage critical cluster is determined based on the local damage area that controls the failure of the unidirectional fiber composites formed during the previous fatigue cycle (the previous 10,000 fatigue cycles). Then, it is judged whether it is affected by the boundary effect according to the damage evolution process. The contribution of the boundary effect is determined according to the relationship between the breakpoints formed by the critical damage cluster and the cross-section boundary. In theory, the fatigue cycle interval can be further reduced to more accurately determine the sequential evolution relationship of the breakpoints formed by the damage critical cluster on the cross-section, especially the stress redistribution characteristics near the interface.

[0082] If the unidirectional fiber composites do not break after 2 million fatigue cycles, the fatigue test is stopped and strain-controlled stepwise loading is adopted. When loading 500με each time, the strain in all optical fibers is collected, and the position of the breakpoints and their relationship with the interface are determined. The damage characterization process similar to the above fatigue loading step is repeated for each strain loading step. When the unidirectional fiber composites break when reaching their fatigue residual strength, the influence and contribution of the boundary effect on the remaining strength failure of the unidirectional fiber composites after 2 million fatigue cycles are determined.

[0083] It is also possible to repeat the above strain loading step after a certain number of fatigue cycles to determine the influence and contribution of the boundary effect on the fatigue residual strength failure of the unidirectional fiber composites at that fatigue cycle. Through the above process, the influence and contribution of the boundary effect during the failure of the unidirectional fiber composites under fatigue loading are realized. Repeating the above test can obtain the influence law of the boundary effect on the fatigue life or fatigue residual strength of the unidirectional fiber composites under a given working condition (given upper and lower limits of fatigue load, given number of fatigue cycles).

[0084] The damage critical cluster formed by the same unidirectional fiber composites under the same load working condition may be inside the cross-section or at the cross-section edge, and the probability of it being at the cross-section edge is relatively large. Due to the certain discreteness of the performance of unidirectional fiber composites, the influence law of the interface effect of unidirectional fiber composites can be determined through the test of a certain number of unidirectional fiber composites. Repeating the above-mentioned test process, the damage critical clusters of a certain number of unidirectional fiber composites, the cross-section positions where the damage critical clusters are located, and the mechanical properties of the unidirectional fiber composites are obtained. The probability characteristics of the damage critical clusters at the boundary and inside are obtained respectively, and the probabilities of the damage critical clusters at the boundary and inside when the unidirectional fiber composites fail under the current cross-section can be obtained, as well as the mechanical properties of the unidirectional fiber composites corresponding to when the damage critical clusters are at the boundary and inside, including fatigue life, fatigue residual strength, etc., to establish the mapping relationship between the mechanical properties of the unidirectional fiber composites and the damage critical clusters.

[0085] Damage evolution is asFigure 7 As shown, white represents intact fibers, gray and black represent broken fibers, and black is the broken fibers coplanar with the current cross-section. The failure of unidirectional fiber composites under tensile action is controlled by the breakage of the internal fibers. The unidirectional fiber composites start from the initial intact state, form local filament breakage damage under load, and through the process of damage accumulation, form damage clustering. When the number of breakpoints of 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 local broken fibers and damaged fibers. 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 filaments in the damage influence area, the damage critical clustering 8 of unidirectional fiber composites under tensile load can be evaluated.

[0086] Based on the above process, while further keeping the fiber volume content and cross-sectional shape unchanged, by changing the cross-sectional size and length of the unidirectional fiber composite, repeating the above test process and analysis method, the evolution relationship of the mechanical properties of the model unidirectional fiber composite with the cross-sectional size and length can be further obtained, and the influence of boundary effect and size effect on the mechanical properties of the unidirectional fiber composite can be determined.

[0087] 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 described in 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. A method for characterizing the influence of boundary effect and size effect on the mechanical properties of unidirectional fiber composites, characterized in that, it comprises the following steps: Step S1: Fabricate a model unidirectional fiber composite composed of single-mode optical fibers and corresponding matrix; Step S2: Conduct fiber high-spatial-resolution strain testing on the model unidirectional fiber composite, and extract data such as the damage critical clustering, the cross-sectional position where the damage critical clustering is located, and the mechanical properties of the model unidirectional fiber composite when the model unidirectional fiber composite loses its stable bearing capacity according to the strain and strain gradient changes on all the optical fibers in the model unidirectional fiber composite; Step S3: Repeat Steps S1 - S2, extract a series of data described in Step S2, calculate the probabilities of the damage critical clustering that controls the failure of the model unidirectional fiber composite being at the boundary and inside, and determine the corresponding relationship between the position where the damage critical clustering is located and the mechanical properties; Step S4: Change the cross-sectional size and length of the model unidirectional fiber composite, repeat Steps S1 - S3, obtain the evolution relationship of the mechanical properties of the model unidirectional fiber composite with the cross-sectional size and length, and determine the influence of the boundary effect and size effect on the mechanical properties of the unidirectional fiber composite.

2. The method according to claim 1, characterized in that, Step S1 includes: Step S11: Analyze the load action and cross-sectional shape of the prototype unidirectional fiber composite, determine the basic mechanical parameters, and the unidirectional fiber volume content; Step S12: Use single-mode optical fibers as substitutes for the unidirectional fibers in the prototype unidirectional fiber composite, and according to the cross-sectional shape, basic mechanical parameters, and unidirectional fiber volume content in Step S11, determine the corresponding matrix and the basic mechanical properties of the fiber-matrix interface according to the similarity principle, and determine the cross-sectional size of the model unidirectional fiber composite; Step S13: Fabricate the model unidirectional fiber composite composed of the single-mode optical fibers and the corresponding matrix.

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

4. The method according to claim 2, characterized in that, 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 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.

5. The method according to claim 1, characterized in that, Step S2 includes: Step S21: Conduct initial calibration on each optical fiber after the model unidirectional fiber composite is formed to determine the relative position and initial state of the optical fibers in the composite; Step S22: Fabricate a tooling fixture and design load steps according to the loads to be characterized. Apply the loads step by step, and use a millimeter-level high-spatial-resolution strain sensing technique to obtain the strain changes and / or optical paths of each measuring point on each optical fiber in real time, determine the break point positions, and evaluate the stress redistribution and influence range near the break points; Step S23: Continue to apply loads according to the load steps, determine whether new break points occur, 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 extract the damage critical clusters when the unidirectional fiber composite material loses its stable load-bearing capacity, the cross-sectional positions where the damage critical clusters are located, and the mechanical properties of the unidirectional fiber composite material.

6. The method according to claim 5, wherein, in the initial calibration in Step S21, the positions of all optical fiber strain mutations in the unidirectional fiber composite material and their relative positions in the cross-section are determined by means of local temperature rise and fall; the relative positions of the fiber filaments on all the boundaries of the unidirectional fiber composite material are obtained by means of slight extrusion.

7. The method according to claim 5, wherein, evaluating the stress redistribution and influence range includes: if new break points occur, it is necessary to determine whether the new break points occur near the existing break points and the cross-section boundary, and whether they are affected by the existing break points or boundary effects, etc.; if no new break points occur, re-evaluate the influence range of the existing break points, determine the influence range of the break points and the strain concentration at the current load, and judge whether damage such as debonding between the fiber and the matrix occurs on both sides of the break points according to the influence range of the break points and the strain concentration; if matrix cracking occurs while the nearby optical fibers do not break, due to local strain concentration caused by matrix cracking, bridging is formed at the edges of the matrix cracking area, and the strain on the optical fibers increases. The local matrix cracking range can be determined according to the positions and quantities of the increased optical fiber strain; if matrix cracking causes the stress on the optical fiber to exceed the tensile strength of the part where it is located, the optical fiber breaks, and the break point position and the load redistribution caused by the broken wire are determined; if the optical fiber breaks and the fiber and the matrix at the break point are debonded 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 surrounding unbroken optical fibers increases with the increase of the debonding length.

8. The method according to claim 5, wherein, determining the break point positions 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 due to the inability to form an optical path.

9. An apparatus for the method according to any one of claims 1-8, characterized in that it includes: a rigid sheath, a high-spatial-resolution optical frequency domain reflectometry optical fiber demodulator, a fatigue testing machine and a 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 fatigue testing machine. The fatigue testing machine applies loads 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.

10. A method for using the apparatus according to claim 9, characterized in that it includes the following steps: Step S1’: Fabricate a unidirectional fiber composite specimen and determine the free - segment length of the unidirectional fiber composite specimen; Step S2’: Connect the two outer ends of the free segment to the clamp through a rigid sheath; Step S3’: Connect each single - mode optical fiber on both sides outside the clamped segment to a high - spatial - resolution optical frequency domain reflectometry fiber demodulator; Step S4’: Fix the unidirectional fiber composite specimen to the fixture of a fatigue testing machine through the clamped segment; Step S5’: Calibrate the initial state of the unidirectional fiber composite specimen; Step S6’: Use the millimeter - level high - spatial - resolution strain sensing technology to conduct 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, extract data such as the damage - critical clustering when the unidirectional fiber composite loses its stable bearing capacity, the cross - sectional position where the damage - critical clustering is located, and the mechanical properties of the unidirectional fiber composite; Step S7’: Repeat steps S1’ - S6’, extract a series of data described in step S6’, calculate the probabilities of the damage - critical clustering that controls the failure of the model unidirectional fiber composite being at the boundary and inside, and determine the corresponding relationship between the location of the damage - critical clustering and the mechanical properties; Step S8’: Change the cross - sectional dimensions and length of the model unidirectional fiber composite, repeat steps S1’ - S7’, obtain the evolution relationship of the mechanical properties of the model unidirectional fiber composite with the cross - sectional dimensions and length, and determine the influence of the boundary effect and size effect on the mechanical properties of the unidirectional fiber composite.

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