A method for damage testing and characterization of distributed unidirectional fiber composite materials

Abstract

This invention discloses a method for distributed unidirectional fiber composite material damage testing and characterization. The method comprises the following steps: Step S1: selecting fibers and a matrix, determining the fiber volume content, and controlling the positional distribution of a single-mode sensing optical fiber within the unidirectional fiber composite material; then forming the fibers, matrix, and single-mode sensing optical fiber into a unidirectional fiber composite material; Step S2: employing millimeter-level high-spatial-resolution strain sensing technology to perform optical fiber high-spatial-resolution strain testing on the unidirectional fiber composite material, collecting optical fiber single-mode sensing optical fiber strain, and performing damage testing and characterization on the unidirectional fiber composite material. This method can be effectively applied to characterize and evaluate the entire process of damage initiation, development, and evolution in unidirectional fiber composite materials.

Description

Technical Field

[0001] The present invention belongs to the fields of fiber composite material damage testing and fiber composite material mechanical property evaluation, and particularly relates to a method for distributed unidirectional fiber composite material damage testing and characterization. Background Art

[0002] Fiber composites are made by layering unidirectional fibers at different angles. Therefore, the mechanical properties of unidirectional composites are fundamental to their performance. Unidirectional composites offer advantages such as high tensile strength, corrosion resistance, and high specific strength. Unidirectional composites primarily utilize the excellent tensile properties of their fibers. After a unidirectional fiber breaks, the bond between the fibers and the matrix allows the broken fibers to continue to carry loads for a certain distance from the breakpoint.

[0003] Generally speaking, microscopic damage in unidirectional fiber composites can be categorized as follows: 1. Fiber-matrix debonding; 2. Matrix cracking; and 3. Fiber breakage. These microscopic damages evolve continuously under the combined effects of load, environment, and their interaction, ultimately leading to fiber breakage within the unidirectional fiber composite and the formation of critical damage clusters that control its failure.

[0004] Existing optical imaging methods have strict requirements for unidirectional fiber composite material specimens. The current related testing means and characterization methods cannot meet the needs of unidirectional fiber composite damage evolution testing, damage clustering evaluation and damage characterization methods. It is urgent to quantitatively evaluate the damage evolution process of unidirectional fiber composites through effective testing and characterization methods. Summary of the Invention

[0005] The present invention provides a new characterization method to solve the technical problem in the prior art that optical testing is difficult to achieve continuous distributed testing and characterization of the damage evolution process of unidirectional fiber composite materials under different loads, different environments and different load and environment coupling conditions.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0007] A method for damage testing and characterization of distributed unidirectional fiber composite materials, comprising the following steps:

[0008] Step S1: Selecting fibers and a matrix, determining the volume content of the fibers, and controlling the position distribution of the single-mode sensing optical fiber in the unidirectional fiber composite material, and molding the fibers, the matrix, and the single-mode sensing optical fiber into a unidirectional fiber composite material;

[0009] Step S2: using millimeter-level high spatial resolution strain sensing technology to perform optical fiber high spatial resolution strain testing on the unidirectional fiber composite material, collecting optical fiber single-mode sensing optical fiber strain, and testing and characterizing the damage of the unidirectional fiber composite material.

[0010] Furthermore, the single-mode sensing optical fibers are distributed at the center, edge and interior of the cross section of the unidirectional fiber composite material. The single-mode sensing optical fibers are distributed at equal intervals along the circumferential direction at the edge and interior of the cross section of the unidirectional fiber composite material.

[0011] Furthermore, in step S1, the unidirectional fiber composite material is manufactured by compression molding vacuum injection molding or pultrusion molding process.

[0012] Furthermore, step S2 includes:

[0013] Step S21: performing initial calibration on the single-mode sensing optical fiber embedded in the unidirectional fiber composite material to determine the relative position and initial state of the sensing optical fiber;

[0014] Step S22: Set the load step and apply the load, collect the strain in the single-mode sensing optical fiber, calculate the average strain of all embedded single-mode sensing optical fibers within the entire length of the unidirectional fiber composite material per unit resolution length, and calculate the change in cross-sectional stiffness at different positions;

[0015] Step S23: Repeat step S22 until the unidirectional fiber composite material loses its stable bearing capacity, determine the stiffness degradation rate of different load steps according to the strain and strain gradient change rate between different load steps, and determine the damage evolution of the unidirectional fiber composite material, the critical damage clustering of the local area of ​​maximum damage, and the stiffness degradation rate according to the location of the strain mutation and its influence range.

[0016] Furthermore, in step S21, initial calibration is performed to determine the position of the optical fiber strain mutation by heating and cooling a small local area, and to determine the relative position relationship of all optical fiber sensing in the free section.

[0017] Furthermore, step S23 includes: determining the damage size based on the average strain of all sensing optical fibers in the strain concentration area by the ratio of the current stiffness to the initial stiffness, and determining the stiffness degradation and rate of the area; determining the damage evolution of the unidirectional fiber composite material and the critical clustering of damage in the local area of ​​maximum damage based on the location of the strain mutation and its impact range.

[0018] At the same time, the present invention provides a device for the above method, including a rigid sheath, a high spatial resolution optical frequency domain reflection fiber optic demodulator, a fatigue testing machine, and a clamp, wherein the clamp is arranged on the fatigue testing machine, the rigid sheath can be fixed to the testing machine through the clamp, the high spatial resolution optical frequency domain reflection fiber optic demodulator is connected to the single-mode sensing optical fiber in the unidirectional fiber composite material specimen fixed on the testing machine, the fatigue testing machine applies a load to the unidirectional fiber composite material, and the high spatial resolution optical frequency domain reflection fiber optic demodulator performs high spatial resolution optical fiber strain testing on the unidirectional fiber composite material.

[0019] The present invention further provides a method for using the above device, comprising the following steps:

[0020] Step S1': preparing a unidirectional fiber composite material specimen and determining the free section length of the unidirectional fiber composite material specimen;

[0021] Step S2': bonding and anchoring the clamping sections at both ends of the free section to the rigid sheath;

[0022] Step S3': fixing the unidirectional fiber composite material specimen on a fixture of a testing machine through a rigid sheath;

[0023] Step S4': connecting each pre-buried single-mode sensing optical fiber on both sides outside the clamping section to a high spatial resolution optical frequency domain reflection optical fiber demodulator;

[0024] Step S5': calibrating the initial state of the unidirectional fiber composite material specimen;

[0025] Step S6': applying load to the unidirectional fiber composite material specimen by step-by-step loading, collecting strain information in all single-mode sensing optical fibers, and performing damage testing and characterization of the unidirectional fiber composite material.

[0026] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects:

[0027] In the above scheme, the present invention effectively solves the technical problem that optical methods in the existing technology cannot characterize and evaluate the microscopic damage of unidirectional fiber composite materials based on the function of high spatial resolution strain sensing of optical fiber (implemented by the principle of optical frequency domain reflectometry (OFDR)). It can realize the characterization of the microscopic damage evolution of unidirectional fiber composite materials under different working conditions, and provide solutions and methods for the characterization and evaluation of the entire process of damage occurrence, development and evolution of unidirectional fiber composite materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 A flow chart of a method for damage testing and characterization of distributed unidirectional fiber composite materials provided in an embodiment of the present invention;

[0030] Figure 2 A distributed unidirectional fiber composite material damage testing and characterization test device provided in an embodiment of the present invention;

[0031] Figure 3 A unidirectional fiber composite material specimen provided by an embodiment of the present invention;

[0032] Figure 4 A schematic diagram showing the connection between a single-mode optical fiber embedded in a unidirectional fiber composite and a high spatial resolution optical frequency domain reflection optical fiber demodulator provided by an embodiment of the present invention;

[0033] Figure 5 Schematic diagram of the layout of sensing optical fibers in a unidirectional fiber composite material provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0034] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0035] like Figure 1 As shown, an embodiment of the present invention provides a method for damage testing and characterization of a distributed unidirectional fiber composite material, comprising:

[0036] Step S1: Selecting fibers and a matrix, determining the volume content of the fibers, and controlling the position distribution of the single-mode sensing optical fiber in the unidirectional fiber composite material, and molding the fibers, the matrix, and the single-mode sensing optical fiber into a unidirectional fiber composite material;

[0037] Step S2: using millimeter-level high spatial resolution strain sensing technology to perform optical fiber high spatial resolution strain testing on the unidirectional fiber composite material, collecting optical fiber single-mode sensing optical fiber strain, and testing and characterizing the damage of the unidirectional fiber composite material.

[0038] The following combination Figure 2-Figure 5 , a method for damage testing and characterization of distributed unidirectional fiber composite materials according to an embodiment of the present invention is described in detail.

[0039] Taking a unidirectional carbon fiber composite reinforcement with a circular cross-section diameter of 4 mm as an example, T700SC carbon fiber is used, the fiber volume content is controlled to 66%, and bisphenol A epoxy resin is used as the matrix material. During the pultrusion process, 10 single-mode sensing optical fibers 1 are embedded throughout the reinforcement, of which 6 are located at a distance of 0.2 mm from the edge of the circular cross-section to the surface, with an interval of π / 3 radians, 3 are located at a distance of 1 mm from the edge of the circular cross-section to the surface, with an interval of 2π / 3 radians, and 1 is located at the center of the cross-section.

[0040] The functions and effects achieved by the present invention are illustrated below using the entire process of damage evolution, critical damage clustering, and determination of critical section stiffness when the unidirectional fiber composite material is subjected to a uniform tensile-tensile fatigue load, with the carbon fiber breakpoints continuously increasing, damage continuously accumulating, and ultimately failing.

[0041] The free section length of unidirectional fiber composite specimen 2 is 100 mm. Figure 2-Figure 5As shown, the two ends of the free section are designed with corresponding clamping sections 4 through bonding and anchoring with rigid sheaths 3. A length is reserved for each pre-buried single-mode sensing optical fiber 1 on both sides of the clamping section to ensure connection with a high spatial resolution optical frequency domain reflection optical fiber demodulator 5. The prepared unidirectional fiber composite specimen 2 is fixed to the fixture 7 of the fatigue testing machine 6 through the clamping section 4.

[0042] Before starting the fatigue test, the sensing optical fiber embedded in the unidirectional fiber composite was calibrated at both ends of the unidirectional fiber composite to determine the initial state. The position of the optical fiber strain mutation in the unidirectional fiber composite and its relative position in the cross section were determined by local heating and cooling. The spatial resolution was accurate to 1mm, and the relative position relationship of all optical fiber sensing in the free section and its initial strain were determined.

[0043] The fatigue load amplitude was set to 10,000 cycles, and the fatigue test was stopped after each cycle. The load was maintained at the upper stress limit, and the strain within each sensing fiber was collected with a spatial resolution of 1 mm. By calculating the average strain measured by 10 single-mode sensing fibers per millimeter over the entire length of the unidirectional carbon fiber composite, any changes in cross-sectional stiffness at different locations were evaluated.

[0044] If, under this condition, local strain concentration (strain gradient) occurs on a fiber, it indicates fiber damage at the strain concentration location. The stiffness degradation is estimated based on the average strain across all fibers at that location. Similarly, the location of the sudden strain change and its impact range are evaluated to determine the damage evolution of the unidirectional fiber composite during fatigue. Under fatigue loading, the failure of the unidirectional fiber composite is controlled by the breakage of its fibers. When the number of fatigue cycles reaches the fatigue life of the unidirectional fiber composite and failure occurs, the critical damage cluster and the stiffness degradation rate of the local area with the maximum damage formed during the previous fatigue cycle (the previous 10,000 cycles) are determined. The development history of the damage in the local area is then determined based on the strain changes in each sensing fiber.

[0045] If the unidirectional fiber composite material does not break after 2 million fatigue cycles, the fatigue test is terminated and strain-controlled step loading is performed. The strain and strain gradient on each sensing fiber are collected every 500 με. Local damage areas are identified and the stiffness degradation and its rate of change are calculated. A damage characterization process similar to the fatigue loading step described above is repeated for each strain loading step. When the unidirectional fiber composite material reaches its residual fatigue strength, it breaks. The damage evolution process at the time of residual strength failure of the controlled unidirectional fiber composite material after 2 million fatigue cycles is determined. Alternatively, the strain loading step described above can be repeated after a certain number of fatigue cycles to determine the damage evolution and stiffness degradation at the time of residual fatigue strength failure of the unidirectional fiber composite material at that number of fatigue cycles.

[0046] When the stressed fibers in the unidirectional fiber composite reach their ultimate strength or ultimate strain, the fibers break. Due to the bonding effect of the matrix, the axial load of the broken fibers at the break point is transferred to the surrounding unbroken fibers through shear stress, resulting in stress concentration on the unbroken fibers at the break point, increasing the tensile stress of the surrounding unbroken fibers and the risk of damage. Under the action of external loads, environment or their coupling, new breakpoints are constantly formed in the unidirectional fiber composite or new breakpoints are further formed near the existing breakpoints due to stress redistribution. As the number of broken fibers in a certain area continues to increase, the stiffness of the area decreases, the strain and change rate sensed by the sensing optical fiber in the area change, and the strain gradient and change rate of the area and the nearby area also change. Based on the strain data of sensing optical fibers with different cross-sections, the location with a large number of broken fibers in the cross-section of the area can be determined. The stiffness degradation of the area can be determined based on the average strain of all sensing optical fibers in the area. The damage magnitude is determined by the ratio of the current stiffness to the initial stiffness. As the damage continues to develop and evolve, the number of broken fibers in a certain area of ​​the unidirectional fiber composite gradually increases, forming a critical damage cluster and causing damage. The location of the damaged area is determined based on the strain values ​​of each sensing optical fiber in this area, and it is determined whether the damage is formed at the cross-sectional edge of the unidirectional fiber composite. The critical state of cross-sectional stiffness degradation during damage is determined based on the strain, strain gradient, and its change rate of the sensing optical fiber in this area before damage. This characterizes the damage evolution and critical damage of the unidirectional fiber composite during damage, and further estimates the critical damage cluster in this area based on the stiffness degradation.

[0047] At the same time, the present invention provides a device for the above method, including a rigid sheath, a high spatial resolution optical frequency domain reflection fiber optic demodulator, a fatigue testing machine, and a clamp, wherein the clamp is arranged on the fatigue testing machine, the rigid sheath can be fixed to the testing machine through the clamp, the high spatial resolution optical frequency domain reflection fiber optic demodulator is connected to the single-mode sensing optical fiber in the unidirectional fiber composite material specimen fixed on the testing machine, the fatigue testing machine applies a load to the unidirectional fiber composite material, and the high spatial resolution optical frequency domain reflection fiber optic demodulator performs high spatial resolution optical fiber strain testing on the unidirectional fiber composite material.

[0048] The present invention further provides a method for using the above device, comprising the following steps:

[0049] Step S1': preparing a unidirectional fiber composite material specimen and determining the free section length of the unidirectional fiber composite material specimen;

[0050] Step S2': bonding and anchoring the clamping sections at both ends of the free section to the rigid sheath;

[0051] Step S3': fixing the unidirectional fiber composite material specimen on a fixture of a testing machine through a rigid sheath;

[0052] Step S4': connecting each pre-buried single-mode sensing optical fiber on both sides outside the clamping section to a high spatial resolution optical frequency domain reflection optical fiber demodulator;

[0053] Step S5': calibrating the initial state of the unidirectional fiber composite material specimen;

[0054] Step S6': applying load to the unidirectional fiber composite material specimen by step-by-step loading, collecting strain information in all single-mode sensing optical fibers, and performing damage testing and characterization of the unidirectional fiber composite material.

[0055] The above process allows for the testing and characterization of unidirectional fiber composite damage under fatigue loading. Since unidirectional fiber composite damage exhibits a certain degree of discreteness, repeating the above test can yield probabilistic characteristics of characteristic parameters characterizing damage and stiffness degradation under specified operating conditions (given upper and lower limits of fatigue load and a given number of fatigue cycles).

[0056] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for damage testing and characterization of distributed unidirectional fiber composite materials, characterized by: The following steps are involved: Step S1: Selecting fibers and a matrix, determining the volume content of the fibers, and molding the fibers, the matrix, and the single-mode sensing optical fiber into a unidirectional fiber composite material; The single-mode sensing optical fibers are distributed at the center, edge, and interior of the cross section of the unidirectional fiber composite material. The single-mode sensing optical fibers are distributed at equal intervals along the circumferential direction at the edge and interior of the cross section of the unidirectional fiber composite material. Step S2 includes: Step S21: performing initial calibration on the single-mode sensing optical fiber embedded in the unidirectional fiber composite material to determine the relative position and initial state of the sensing optical fiber; Step S22: Set the load step and apply the load, collect the strain in the single-mode sensing optical fiber, calculate the average strain of all embedded single-mode sensing optical fibers within the entire length of the unidirectional fiber composite material per unit resolution length, and calculate the change in cross-sectional stiffness at different positions; Step S23: Repeat step S22 until the unidirectional fiber composite material loses its stable bearing capacity, determine the stiffness degradation rate of different load steps according to the strain and strain gradient change rate between different load steps, and determine the damage evolution of the unidirectional fiber composite material and the critical damage clustering of the local area of ​​maximum damage according to the location of the strain mutation and its influence range.

2. The method according to claim 1, wherein: In step S1, the unidirectional fiber composite material is manufactured by using a compression molding vacuum injection molding or pultrusion molding process.

3. The method according to claim 1, wherein: In step S21, initial calibration is performed to determine the position of the optical fiber strain mutation by heating and cooling a small local area, and to determine the relative position relationship of all optical fiber sensing in the free section.

4. The method according to claim 1, wherein: Step S23 includes: determining the damage size based on the average strain of all sensing optical fibers in the strain concentration area by the ratio of the current stiffness to the initial stiffness, and determining the stiffness degradation and rate of the area; determining the damage evolution of the unidirectional fiber composite material and the critical clustering of damage in the local area of ​​maximum damage based on the location of the strain mutation and its impact range.

5. A device for the method according to any one of claims 1 to 4, characterized in that: It includes a rigid sheath, a high spatial resolution optical frequency domain reflection fiber optic demodulator, a fatigue testing machine, and a fixture, wherein the fixture is set on the fatigue testing machine, the rigid sheath can be fixed to the testing machine through the fixture, the high spatial resolution optical frequency domain reflection fiber optic demodulator is connected to the single-mode sensing optical fiber in the unidirectional fiber composite material specimen fixed on the testing machine, the fatigue testing machine applies a load to the unidirectional fiber composite material, and the high spatial resolution optical frequency domain reflection fiber optic demodulator performs optical fiber high spatial resolution strain testing on the unidirectional fiber composite material.

6. A method for using the device according to claim 5, characterized in that: The following steps are involved: step S1': preparing a unidirectional fiber composite material specimen and determining the free section length of the unidirectional fiber composite material specimen; Step S2': bonding and anchoring the clamping sections at both ends of the free section to the rigid sheath; Step S3': fixing the unidirectional fiber composite material specimen on a fixture of a testing machine through a rigid sheath; Step S4': connecting each pre-buried single-mode sensing optical fiber on both sides outside the clamping section to a high spatial resolution optical frequency domain reflection optical fiber demodulator; Step S5': calibrating the initial state of the unidirectional fiber composite material specimen; Step S6': applying load to the unidirectional fiber composite material specimen by step-by-step loading, collecting strain information in all single-mode sensing optical fibers, and performing damage testing and characterization of the unidirectional fiber composite material.

Images