A method for obtaining in-situ interfacial mode ii fracture toughness of three-dimensional woven composites

Abstract

The application discloses a method for obtaining in-situ interfacial mode II fracture toughness of three-dimensional woven composite material, the internal yarn weaving structure of the three-dimensional woven composite material comprises warp yarns and weft yarns, and space interlocking exists between the two kinds of yarns, the warp yarn fluctuation direction is the 1 direction of the local coordinate system of the composite material plate, the weft yarn fluctuation direction is the 2 direction, and the thickness direction is the 3 direction, and the method comprises the following steps: step 1, a small-size off-axis compression test piece is designed, and a pre-prepared crack is cut at a specified position; and step 2, a high-fidelity finite element model is established according to the structure of the test piece. The cutting position and size design of the test piece proposed in the application make the test piece have independent and simple interface structures, which is beneficial to the test of the interface mechanical properties. The notch position and length of the test piece end can ensure the stable expansion of the mode II crack along the interface of the test piece. The test piece is small in size and simple in structure, which is beneficial to the establishment of a high-fidelity full-size finite element model, and the modeling efficiency and accuracy are improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of mechanical property testing of composite materials, and relates to a method for testing the interfacial mechanical properties between components (fiber bundle / fiber bundle or fiber bundle / matrix) of three-dimensional woven composite materials, in particular to a method for obtaining the two-type fracture toughness of the in-situ interface of three-dimensional woven composite materials by combining test and simulation. BACKGROUND

[0002] In recent years, fiber-reinforced composites have been widely used in aerospace, shipbuilding, automotive and sports due to their excellent specific modulus, specific strength and other mechanical properties. Compared with traditional laminated composites, three-dimensional woven composites have better delamination resistance and impact resistance, and have broad application prospects. The spatial structure of three-dimensional woven composite preforms is complex and diverse, which makes the overall material exhibit diverse mechanical properties. A large number of experimental studies have shown that the interfacial debonding between component materials is the core failure mode in three-dimensional woven composites, which significantly affects the allowable load and damage mechanism of the material. Therefore, how to obtain the interfacial mechanical properties of three-dimensional woven composites is of great significance to the evaluation of the overall performance of the material.

[0003] Due to the novelty of three-dimensional woven composites, the test methods suitable for the interlaminar property testing of traditional laminated composites are no longer applicable to three-dimensional woven composites. Studies have shown that the traditional two-type fracture toughness testing method can cause the broken yarns of three-dimensional woven composites to fluctuate, resulting in a measurement result that is more than ten times higher. The interface structure of three-dimensional woven composites is very complex and small in size, which brings great difficulty to the design of test pieces. Test pieces of conventional size cannot extract independent interface structures, so they are not suitable for interfacial mechanical property testing. Small-size test pieces can extract independent interface structures, but the position selection of the test pieces and the processing precision of the test pieces need to be carefully studied. These works bring great difficulty to the interfacial mechanical property testing, and therefore there is currently no research on the method for obtaining the two-type fracture toughness of the interface of three-dimensional woven composites. However, this parameter is of great significance to the performance evaluation of the material and the simulation research of three-dimensional woven composites, so it is necessary to carry out research on the two-type fracture toughness of the interface of three-dimensional woven composites. SUMMARY

[0004] In view of the fact that the method for obtaining the two-type fracture toughness of the interface of three-dimensional woven composites is currently in a blank state, the present application provides a method for obtaining the two-type fracture toughness of the in-situ interface of three-dimensional woven composites. In this method, in-situ means in-situ sampling, and the interface refers to the interface between component materials, i.e. the fiber bundle / fiber bundle interface or the fiber bundle / matrix interface.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A method for obtaining in-situ interface mode II fracture toughness of a three-dimensional woven composite material, wherein the internal yarn weaving structure of the three-dimensional woven composite material includes warp yarns and weft yarns, and there is spatial interlocking between the two yarns. The warp yarn fluctuates in a direction 1 of a local coordinate system of the composite material plate, the weft yarn fluctuates in a direction 2 of the local coordinate system of the composite material plate, and the thickness direction is a direction 3 of the local coordinate system of the composite material plate, comprising the following steps:

[0007] Step 1: Design a small-sized eccentric compression test specimen and cut a prefabricated crack at a specified location. The cutting position of the specimen is determined according to the wave structure of the fiber bundle in the material. The specific steps are as follows:

[0008] Step 1-1: In a three-dimensional woven composite material plate, find a weft yarn section in a plane formed by directions 1 and 3, and cut a thin plate that does not exceed the width of the weft yarn section according to the position of the weft yarn section;

[0009] Step 1-2: After removing the thin plate, cut an eccentric compression specimen from the thin plate. Make a prefabricated crack at one end of the specimen. The prefabricated crack must be cut along the cross section of the material.

[0010] Step 2: Establish a high-fidelity finite element model based on the specimen structure. The specific steps are as follows:

[0011] Step 2-1: Take a photo of the specimen surface to obtain the geometric information of the specimen surface;

[0012] Step 2-2: Based on the geometric information of the specimen surface, a geometric model is established to restore the specimen structure;

[0013] Step 2-3: Mesh the geometry and simulate the crack propagation process using virtual crack closure technology in the interface region connected by the pre-existing cracks.

[0014] Step 2-4: Add material properties to the fiber bundle and matrix respectively, and modify the main material directions of the warp and weft yarns in different directions to ensure the accuracy of the finite element model;

[0015] Step 3: Compare the crack initiation load in the test with the crack initiation load in the finite element model to obtain the mode II fracture toughness of the specimen interface. The specific steps are as follows:

[0016] Step 3-1: Obtain the strain and displacement information near the prefabricated crack tip of the specimen;

[0017] Step 3-2: Assemble the compression fixture and the specimen, and place the assembled compression fixture and specimen on the compression disc of the quasi-static testing machine;

[0018] Step 3-3: After preloading, start compression test using displacement control testing machine until obvious crack propagation on the surface of the test piece is observed, and output the load, displacement and time information during the test by the quasi-static testing machine;

[0019] Step 3-4: Observe the crack propagation on the surface of the test piece and record the time of crack initiation;

[0020] Step 3-5: According to the recorded time of crack initiation, find the compression load corresponding to the crack initiation by combining the load-time information output by the quasi-static testing machine;

[0021] Step 3-6: Apply the same boundary conditions to the finite element model as the test piece, and determine the accuracy of the finite element model by comparing the displacement field of the test piece surface with the displacement field of the finite element model;

[0022] Step 3-7: After verifying the accuracy of the finite element model, adjust the bimaterial fracture toughness value of the interface in the virtual crack closure technique, and compare the crack initiation load in the finite element model with the crack initiation load observed in the test;

[0023] Step 3-8: When the results of the finite element model are close to the test results (deviation within 1%), the bimaterial fracture toughness input in the finite element model is the bimaterial fracture toughness of the three-dimensional woven composite interface.

[0024] Compared with the prior art, the present application has the following advantages:

[0025] 1. The cutting position and size of the test piece designed in the present application have independent and simple interface structure inside the test piece, which is beneficial to the test of the interface mechanical properties.

[0026] 2. The notch position and length at the end of the test piece can ensure the stable propagation of the bimaterial crack along the interface of the test piece.

[0027] 3. The test piece has small size and simple structure, which is beneficial to the establishment of high-fidelity full-size finite element model, and improves the modeling efficiency and accuracy.

[0028] 4. The combination of test and simulation is beneficial to obtaining more accurate bimaterial fracture toughness of the interface. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 It is a typical internal yarn weaving structure of three-dimensional woven composite material;

[0030] Figure 2 It is a typical test piece cutting position;

[0031] Figure 3 It is an assembly example of the test piece and the clamp;

[0032] Figure 4 The process for obtaining the interfacial mode II fracture toughness;

[0033] Figure 5 The typical test results and the comparison analysis of the finite element model;

[0034] Figure 6 The typical crack propagation phenomenon. DETAILED DESCRIPTION

[0035] The technical solutions of the present application are further described below in conjunction with the drawings, but are not limited thereto, and any modification or equivalent replacement to the technical solutions of the present application without departing from the spirit and scope of the technical solutions of the present application shall be encompassed in the protection scope of the present application.

[0036] The present application provides a method for obtaining in-situ interfacial mode II fracture toughness of three-dimensional woven composite materials, and the research object of the method is three-dimensional woven composite materials, such as Figure 1 As shown in the figure, the internal yarn weaving structure of the three-dimensional woven composite material includes warp yarns and weft yarns, and there is spatial interlocking between the two kinds of yarns. The warp yarns fluctuate obviously, and the weft yarns are relatively straight. The fluctuation direction of the warp yarns is the 1 direction of the local coordinate system of the composite material plate, the fluctuation direction of the weft yarns is the 2 direction, and the thickness direction is the 3 direction.

[0037] Figure 2 The typical cutting position of the off-axis compression specimen. In the three-dimensional woven composite material plate, a weft yarn section is found in the 1-3 plane, and a thin plate not exceeding the width of the weft yarn section is cut according to the position of the weft yarn section. After removing the thin plate, an off-axis compression specimen is cut on the thin plate, a prefabricated crack is made at one end of the specimen, and the prefabricated crack must be cut along the material section.

[0038] Figure 3 The assembly example of the specimen and the compression clamp. The compression clamp includes a clamp body, a cover plate and a cylindrical guide rail, the clamp body is composed of an upper clamp and a lower clamp, the upper clamp and the lower clamp are connected through two cylindrical guide rails, wherein: the opposite surfaces of the upper clamp and the lower clamp are provided with through holes, the two ends of the cylindrical guide rail are located in the through holes, the through holes and the surface of the cylindrical guide rail should be tightly matched and have small friction force, so that the test result is accurate; the surface of the compression clamp in contact with the specimen needs to be polished. The clamp body and the cover plate are connected through bolts to press the specimen tightly, limit the out-of-plane displacement of the specimen, and prevent the specimen from buckling. The parallelism between the parallel surfaces of the upper clamp and the lower clamp needs to be strictly guaranteed.

[0039] Figure 4 The process for obtaining the interfacial mode II fracture toughness, specifically comprising the following steps:

[0040] Step 1: Cut a specimen from the 3D woven composite panel along the plane formed by direction 1 and direction 3.

[0041] Step 2: Take a picture of the specimen surface to obtain the geometry information of the specimen.

[0042] Step 3: Spray speckles on the specimen surface and use digital image correlation technique (or use a crack meter) to obtain the strain and displacement information near the pre-existing crack tip. Assemble the grips with the specimen and place the assembled grips and specimen on the compression disc of the testing machine. After preloading, start the compression test using the displacement control testing machine until obvious crack propagation is observed on the specimen surface. Use the testing machine to output the load, displacement, and time information during the test.

[0043] Step 4: Observe the crack propagation on the specimen surface and determine the reasonable crack propagation location as shown in FIG. 1. The crack must propagate along the interface from the pre-existing crack tip. Record the time of crack initiation by digital image correlation analysis (or by observing the results of the crack meter). If other damage modes or crack propagation at other locations occurs during the test before the crack at the pre-existing crack tip, this experimental method is not suitable for the 3D woven composite material of this material system. Figure 6

[0044] Step 5: Based on the recorded crack initiation time, combine the load-time information output by the testing machine to find the compression load corresponding to the crack initiation.

[0045] Step 6: Based on the geometry information of the specimen surface, establish a finite element model that restores the structure of the specimen. Mesh the geometric model and use virtual crack closure technique to simulate the crack propagation process in the interface region connected to the pre-existing crack. Add material properties to the fiber bundles and matrix respectively, and modify the material principal direction of the warp and weft yarns in different directions.

[0046] Step 7: Apply consistent boundary conditions to the finite element model. First, compare the displacement field of the specimen surface with the displacement field of the finite element model to determine the accuracy of the finite element model. After verifying the accuracy of the model, adjust the interfacial mode II fracture toughness value in the virtual crack closure technique, and compare the crack initiation load in the finite element model with the crack initiation load observed in the test.

[0047] Step 8: Figure 5 A typical test result is compared with the calculation result of the finite element model. When the input fracture toughness value of the finite element model is 0.5 N / mm, the crack initiation load in the finite element model is consistent with the test result. At this time, the input interfacial mode II fracture toughness of the finite element model is the mode II fracture toughness of the 3D woven composite interface.​

Claims

1. A method for obtaining in-situ interface mode II fracture toughness of a three-dimensional woven composite material, characterized in that The internal yarn weaving structure of the three-dimensional woven composite material includes warp yarns and weft yarns, and there is spatial interlocking between the two yarns. The warp yarn fluctuation direction is the 1 direction of the local coordinate system of the composite material plate, the weft yarn fluctuation direction is the 2 direction of the local coordinate system of the composite material plate, and the thickness direction is the 3 direction of the local coordinate system of the composite material plate, including the following steps: Step 1: Design a small-sized eccentric compression test piece and cut a prefabricated crack at the specified position. The specific steps are as follows: Step 1-1: In a three-dimensional woven composite material plate, find a weft yarn section in a plane formed by directions 1 and 3, and cut a thin plate that does not exceed the width of the weft yarn section according to the position of the weft yarn section; Step 1-2: After removing the thin plate, cut an eccentric compression specimen from the thin plate. Make a prefabricated crack at one end of the specimen. The prefabricated crack must be cut along the cross section of the material. Step 2: Establish a high-fidelity finite element model based on the specimen structure. The specific steps are as follows: Step 2-1: Take a photo of the specimen surface to obtain the geometric information of the specimen surface; Step 2-2: Based on the geometric information of the specimen surface, a geometric model is established to restore the specimen structure; Step 2-3: Mesh the geometry and simulate the crack propagation process using virtual crack closure technology in the interface region connected by the pre-existing cracks. Step 2-4: Add material properties to the fiber bundle and matrix respectively, and modify the main material directions of the warp and weft yarns in different directions to ensure the accuracy of the finite element model; Step 3: Compare the crack initiation load in the test with the crack initiation load in the finite element model to obtain the mode II fracture toughness of the specimen interface. The specific steps are as follows: Step 3-1: Obtain the strain and displacement information near the prefabricated crack tip of the specimen; Step 3-2: Assemble the compression fixture and the specimen, and place the assembled compression fixture and specimen on the compression disc of the quasi-static testing machine; Step 3-3: After preloading, start the compression test using a displacement-controlled testing machine until obvious crack propagation is observed on the specimen surface. Use a quasi-static testing machine to output information such as load, displacement, and time during the test. Step 3-4: Observe the crack propagation on the specimen surface and record the time of crack initiation; Step 3-5: Based on the recorded crack initiation time and the load-time information output by the quasi-static testing machine, find the compression load corresponding to the crack initiation; Step 3-6: Apply boundary conditions consistent with those of the test piece to the finite element model, and determine the accuracy of the finite element model by comparing the displacement field of the specimen surface with the displacement field of the finite element model; Step 3-7: After verifying the accuracy of the finite element model, adjust the mode II fracture toughness value of the interface in the virtual crack closure technique and compare the crack initiation load in the finite element model with the crack initiation load observed in the experiment; Step 3-8: When the results of the finite element model are close to the test results, the interface mode II fracture toughness input into the finite element model is the mode II fracture toughness of the interface of the three-dimensional woven composite material.

2. The method for obtaining in-situ interface mode II fracture toughness of a three-dimensional woven composite material according to claim 1, characterized in that In step 3-1, speckles are sprayed on the surface of the specimen, and digital image correlation technology or a crack meter is used to obtain strain and displacement information near the prefabricated crack tip.

3. The method for obtaining in-situ interface mode II fracture toughness of a three-dimensional woven composite material according to claim 1, characterized in that In step 3-2, the compression fixture includes a fixture body, a cover plate and a cylindrical guide rail. The fixture body consists of an upper fixture and a lower fixture. The upper fixture and the lower fixture are connected by two cylindrical guide rails, and the fixture body and the cover plate are connected by bolts.

4. The method for obtaining in-situ interface mode II fracture toughness of a three-dimensional woven composite material according to claim 1, characterized in that In the steps 3-8, the deviation between the results of the finite element model and the test results is within 1%.

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