Composite material fracture toughness test method capable of avoiding crack length measurement

Abstract

The invention provides a composite material fracture toughness test method capable of avoiding crack length measurement. The composite material fracture toughness test method comprises the following steps: (1) preparing a compact tensile test piece; (2) mounting a test piece; (3) carrying out tensile loading on the test piece; and (4) processing test data. The method is reasonable in conception, and the fracture toughness and the load have a simple constant coefficient relationship by utilizing a new test piece configuration with a special design, so that in the process of testing the fracture toughness curve, the crack length does not need to be tracked and measured, only load curve information needs to be recorded, and the purpose of reducing the experimental measurement difficulty is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of mechanical property tests of materials, and particularly to a test method for the fracture toughness of composite materials that avoids crack length measurement. Background Art

[0002] Fiber-reinforced composite materials are widely used as the main structural materials in aerospace engineering. How to evaluate the problems of damage initiation, damage evolution, and crack propagation of composite materials is an important research direction in the field of composite mechanics. Although the failure models of composite materials have been continuously improved in terms of physical mechanism and computational efficiency, the damage parameters at different scales, including fracture toughness, are included in these damage models. The acquisition and calibration of a large number of relevant parameters have always been very difficult problems. A large number of specimen-level specimens bring high costs in terms of time and economy to the verification and characterization of composite structures, especially the measurement of the fracture toughness of composite materials, which has high operation difficulty and precision requirements. The main difficulty lies in the measurement of crack length. Due to phenomena such as fiber bridging and local matrix nonlinearity, the fracture of composite materials across layers exhibits the characteristics of a fracture resistance curve, and it is necessary to maintain stable crack propagation during the test and record the load and crack length simultaneously. At the same time, the theory for calculating fracture toughness from the crack length obtained by experimental measurement is complex. The current formulas are only applicable to specific test piece configurations, and it is challenging to calculate the toughness theory for non-standard test pieces.

[0003] Specifically, the test for mode I fracture toughness usually adopts the compact tension test method, referring to the ASTM E1820 test standard (Standard Test Method for Measurement of Fracture Toughness). The basic method is based on a rectangular test piece with a prefabricated crack, and the fracture toughness resistance curve is calculated through the functional relationship between fracture toughness and the load, crack length, and test piece size during crack propagation. That is:

[0004]

[0005] Among them, K is the fracture toughness to be measured, F is the load, t is the thickness of the test piece, w is the width from the tensile loading position to the right boundary of the test piece, and a is the current crack length.

[0006] For ASTM standard test pieces:

[0007]

[0008] It can be seen that the calculation of fracture toughness is a complex relational expression regarding crack length.

[0009] Currently, the widely adopted direct observation methods for crack length include high-resolution cameras, DIC images, and in-situ X-rays. However, at high temperatures, especially extreme temperatures, the measurement results are greatly restricted by factors such as material thermal radiation and the observation window of the equipment. How to avoid the real-time measurement of crack length is one of the most concerned issues in the development of fracture toughness test methods under extreme environmental conditions. Currently, the main methods adopted are the following two categories:

[0010] (1) Compliance back-calculation method. The theoretical relationship between compliance and crack length is obtained through finite element analysis or experimental calibration, so as to back-calculate the crack length value from the compliance.

[0011] (2) Numerical model fitting method. The method of fitting numerical or theoretical calculations with load-displacement data is used to deduce and characterize fracture toughness. The theoretical numerical calculation methods adopted include modified compliance calibration method, virtual crack extension method, cohesive zone model method, continuous damage model, etc.

[0012] (3) Indirect measurement method. This type of method obtains the R curve by establishing the correlation between the R curve and other physical quantities, and then through the test of indirect physical quantities. Starting from the size effect law of composite materials, an indirect fitting method for the R curve is established.

[0013] Traditional test methods are highly difficult due to the control and observation of crack propagation, which not only brings great challenges to the low-cost of composite material calibration tests, but also limits the R & D and design cycle of composite material structural components. Although the above three types of methods avoid the measurement of real-time cracks in principle, there are still problems such as unstable crack propagation and high result dispersion.

[0014] Therefore, it is necessary to propose a simple and efficient experimental test method for composite material fracture toughness parameters. Summary of the Invention

[0015] In view of the technical problems existing in the above background technology, the present invention proposes a test method for composite material fracture toughness that avoids crack length measurement. Its concept is reasonable, and a simple linear relationship between fracture toughness and the load curve during crack propagation of a new configuration test piece is established, significantly reducing the experimental difficulty of the fracture toughness curve.

[0016] To solve the above technical problems, a test method for composite material fracture toughness that avoids crack length measurement provided by the present invention includes the following steps:

[0017] 1) Prepare a compact tension test piece;

[0018] 2) Install the test piece;

[0019] 3) Apply tensile loading to the test piece;

[0020] 4) Test data processing.

[0021] The test method for the fracture toughness of composite materials to avoid crack length measurement, wherein the step 1) specifically includes the following steps:

[0022] 1.1) Prepare a laminate according to the layup of the specimen to be tested;

[0023] 1.2) Cut the prepared laminate into test pieces;

[0024] 1.3) Pre-crack the test piece.

[0025] The test method for the fracture toughness of composite materials to avoid crack length measurement, wherein: in the step 1.1), the process for preparing the laminate adopts any one of the prepreg laying process, vacuum-assisted resin infusion, resin transfer molding process, and autoclave process;

[0026] The left half of the test piece in the step 1.2) is the same in width, but in the right half, the height is constantly changing;

[0027] In the step 1.3), the pre-cracked initial length of the pre-crack is a0 = 30 mm, and any one of the water cutting, diamond wire cutting, and fatigue loading methods is adopted as the crack prefabrication method.

[0028] The test method for the fracture toughness of composite materials to avoid crack length measurement, wherein in the step 1.1), the process for preparing the laminate adopts the prepreg laying process, and the specific process is as follows:

[0029] 1.1.1) Pre-preg cutting

[0030] Cut the prepreg layer by layer according to the laminate angle and size;

[0031] 1.1.2) Pre-preg laying

[0032] Clean the flat mold and apply a suitable release agent; lay the cut prepreg on the mold accurately according to the design requirements, and ensure smooth fitting to avoid air bubbles and wrinkles;

[0033] 1.1.3) Vacuum packaging

[0034] Lay auxiliary materials on the mold with the prepreg laid, cover the vacuum bag on the auxiliary materials, and seal its edge on the mold with a sealing tape, and evacuate the inside of the vacuum bag to vacuum by a vacuum pump;

[0035] 1.1.4) Curing

[0036] Put the vacuum-packed prepreg into an autoclave, curing furnace or other curing equipment for curing;

[0037] 1.1.5) Demoulding

[0038] After curing is completed, demould the laminate from the vacuum bag and take it out.

[0039] The test method for the fracture toughness of composite materials to avoid crack length measurement, wherein the crack prefabrication method in step 1.3) is the fatigue loading method, and the specific process is as follows:

[0040] 1.3.1) Use a machining method to machine a notch with a certain depth and shape on the test piece;

[0041] 1.3.2) On the fatigue testing machine, determine the maximum load, loading frequency and loading method of the fatigue testing machine according to the material, size and loading requirements of the test piece;

[0042] 1.3.3) Start the fatigue testing machine and perform cyclic loading on the test piece according to the set loading parameters;

[0043] 1.3.4) After a certain number of fatigue loadings are completed, clean and mark the test piece.

[0044] The test method for the fracture toughness of composite materials to avoid crack length measurement, wherein the specific process of step 2) is as follows: First, install the upper and lower clamps of the tensile test fixture into the upper and lower chucks of the universal tensile testing machine respectively, and then pass the loading pin through the upper and lower holes of the test piece formed by cutting in step 1.2) and place it at the middle position before and after the tensile test fixture.

[0045] The test method for the fracture toughness of composite materials to avoid crack length measurement, wherein the specific process of step 3) is as follows:

[0046] 3.1) After the test piece is installed, first perform preloading and unloading to eliminate the dead travel of the universal tensile testing machine;

[0047] 3.2) After preloading is completed, start the formal tensile loading of the test piece;

[0048] 3.3) Stop loading after the crack length a expands to less than 45 mm.

[0049] The test method for the fracture toughness of composite materials to avoid crack length measurement, wherein the specific process of the formal tensile loading of the test piece is as follows: The fixed cylinders of the upper and lower clamps of the tensile test fixture are clamped by the upper and lower chucks of a universal tensile testing machine, and the through holes on both sides of the upper and lower clamps of the tensile test fixture are respectively assembled with the loading pins and the test piece; when the universal tensile testing machine is loaded, the load is transmitted to the loading pins through the tensile test fixture and then to the upper and lower hole edges of the test piece, so as to realize the tensile loading of the type I crack.

[0050] The test method for the fracture toughness of composite materials to avoid crack length measurement, wherein the specific process of the step 4) is as follows:

[0051] Calculate the fracture toughness of the test piece by the following formula (4):

[0052]

[0053] In the above formula (4), K is the fracture toughness to be measured, F is the load, t is the thickness of the test piece, and w is the width from the tensile loading position to the right boundary of the test piece;

[0054] The specific process of the fracture toughness curve result calculated by the above formula (4) is as follows: Substitute the load and the specimen size into the above formula (4), and the critical fracture toughness at crack initiation can be obtained. Subsequently, substitute the load data into formula (4) in turn, and several fracture toughness data points along with crack propagation can be calculated; according to the crack length at the end of the loading, the data points can be fitted to obtain the fracture toughness - crack propagation curve.

[0055] Adopting the above technical solution, the present invention has the following beneficial effects:

[0056] The test method for the fracture toughness of composite materials to avoid crack length measurement in the present invention has a reasonable concept, establishes a simple linear relationship between the fracture toughness and the load curve during the crack propagation of the new configuration test piece, and significantly reduces the experimental difficulty of the fracture toughness curve.

[0057] Aiming at the deficiencies of the existing fracture toughness test methods, especially the difficulty in testing under extreme environmental conditions caused by the need to record the crack propagation length for measuring the fracture toughness resistance curve, the present invention proposes a new type of test piece configuration. There is a simple linear relationship between the fracture toughness and the load of the new compact tensile test piece configuration, and it is independent of the crack length; therefore, the fracture toughness curve can be calculated only by measuring the load curve during the crack propagation of the test piece, achieving the purpose of simplifying the test difficulty.

[0058] Compared with the existing test methods, the present invention has the following characteristics and advantages:

[0059] 1) Low demand for measuring equipment: Without the need to measure crack length, the requirement for real-time crack measurement equipment can be avoided, thus simplifying the hardware cost of fracture toughness tests.

[0060] 2) Simple calculation formula: Compared with the fracture toughness test formula in the existing test methods, due to the relationship of constant coefficients in the present invention, the formula for calculating fracture toughness is simplified and does not require complex function calculations.

[0061] 3) Strong adaptability to test environment: The present invention only needs to measure the applied load and has low requirements for the test environment. Especially in high and low temperature tests with limited test space conditions, the existing crack observation means and fracture toughness back-calculation methods in the existing test methods are difficult to apply.

[0062] 4) Low time cost of the test method: Compared with the existing test methods, the loading process of the test piece in the present invention avoids the process of unloading and reloading, the test process is simple and does not need to be adjusted, and the time cost is relatively low. Description of the Drawings

[0063] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are 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.

[0064] Figure 1 Configuration dimension diagram of the fracture toughness test piece involved in the composite material fracture toughness test method for avoiding crack length measurement in the present invention ((a) ASTM standard test piece (b) new configuration provided by the present invention);

[0065] Figure 2 Schematic diagram of the installation of the test piece and the loading fixture involved in the composite material fracture toughness test method for avoiding crack length measurement in the present invention;

[0066] Figure 3 Graph of the function f(a / w) of the fracture toughness and crack length correlation of the test piece involved in the composite material fracture toughness test method for avoiding crack length measurement in the present invention changing with the crack length;

[0067] Figure 4 Loading curve graph of the typical test piece involved in the composite material fracture toughness test method for avoiding crack length measurement in the present invention;

[0068] Figure 5 Fracture toughness curve result calculated by using the composite material fracture toughness test method for avoiding crack length measurement in the present invention.

[0069] Note:

[0070] Figure 2 In Figure 1, 1 is a tensile test fixture, 2 is a test piece, and 3 is a loading pin. Specific implementation manners

[0071] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0072] The present invention will be further explained below in conjunction with specific implementation manners.

[0073] As Figure 1 shown, a test method for the fracture toughness of a composite material to avoid crack length measurement provided in this embodiment includes the following steps:

[0074] S100. Prepare a compact tension test piece

[0075] S110. Prepare a laminate according to the layup of the specimen to be tested. The process methods that can be used include prepreg laying process, vacuum-assisted resin infusion, resin transfer molding process, autoclave process method, etc.; the most representative method is selected as the prepreg laying process in this embodiment, and its main process is as follows:

[0076] S1101. Cut the prepreg

[0077] Cut the prepreg layer by layer according to the laminate angle and size;

[0078] S1102. Lay the prepreg

[0079] Clean the flat mold and apply a suitable release agent; lay the cut prepreg accurately on the mold according to the design requirements, and ensure it is flat and well-fitted to avoid air bubbles and wrinkles;

[0080] S1103. Vacuum packaging

[0081] On the mold with the prepreg laid, lay auxiliary materials such as release cloth, breather felt, and absorbent felt in sequence. Finally, cover the vacuum bag on the auxiliary materials and seal its edge on the mold with sealing tape, and evacuate the inside of the vacuum bag to vacuum through a vacuum pump;

[0082] S1104. Curing

[0083] Put the vacuum-packed prepreg into an autoclave, curing furnace or other curing equipment for curing, and control parameters such as temperature and time;

[0084] S1105, Demolding and removal

[0085] After curing is completed, demold and remove the laminate from the vacuum bag.

[0086] S120. Cut the obtained laminate into the test piece configuration as shown in Figure 1 (b); compared with the ASTM standard test piece shown in Figure 1 (a), the left half and width of the test piece proposed by the present invention are the same, but in the right half, the height is constantly changing.

[0087] S130. Pre-crack, that is, pre-crack a crack with an initial length of a0 = 30 mm; the crack pre-cracking methods that can be used are water jet cutting, diamond wire cutting, and fatigue loading method, and try to form a narrow and sharp crack as much as possible; the most representative crack pre-cracking method in this embodiment is the fatigue loading method, and its main process includes:

[0088] S1301. Use mechanical processing method to machine a notch with a certain depth and shape on the test piece;

[0089] S1302. On the fatigue testing machine, determine the maximum load, loading frequency, and loading method of the testing machine according to the material, size, and loading requirements of the test piece;

[0090] S1303. Start the fatigue testing machine and perform cyclic loading on the test piece according to the set loading parameters;

[0091] S1304. After a certain number of fatigue loadings are completed, clean and mark the test piece.

[0092] S200. Installation of test piece

[0093] First, install the upper and lower clamps of the tensile test fixture 1 in the upper and lower chucks of the universal tensile testing machine respectively, and then pass the loading pin 3 through the upper and lower through holes of the test piece 2 cut and formed in S102 as shown in Figure 2 (b) and place it at the middle position in front of and behind the tensile test fixture 1; the schematic diagram of the test piece 2 installed on the tensile test fixture 1 is as shown in Figure 1 (b); as shown in Figure 2 .

[0094] S300. Perform tensile loading on the test piece

[0095] S301. After the test piece 2 is installed, preloading and unloading are first carried out. Preloading means applying a small initial load (far lower than the elastic limit of the material) to the test piece 2 before the formal tensile test starts. Unloading means gradually reducing the load to zero to eliminate the idle stroke of the universal tensile testing machine. The preloading load should be as small as possible to avoid significant crack propagation.

[0096] S302. After the preloading is completed, the formal tensile loading of the test piece 2 starts. During the tensile process, the universal tensile testing machine uses a loading speed of 0.5 mm / min to ensure a relatively stable crack propagation process. During the entire loading process, the load data is recorded.

[0097] The specific process of the formal tensile loading of the above-mentioned test piece 2 is as follows: The fixed cylinders of the upper and lower clamps of the test fixture 1 are clamped by the upper and lower chucks of the universal tensile testing machine. The through holes on both sides of the upper and lower clamps of the test fixture 1 are respectively assembled with the loading pin 3 and the test piece 2. When the tensile testing machine is loaded, the load is transmitted to the loading pin 3 through the test fixture 1 and then to the hole edge of the test piece 2, thereby realizing the tensile loading process of the type I crack.

[0098] S303. After the crack has extended to a certain extent (the extended crack length a < 45 mm), stop loading.

[0099] S400. Test data processing

[0100] For the designed test piece configuration, f(a / w) is almost a constant within a certain crack length range, as Figure 3 shown:

[0101] f(a / w) = 15.5 (3);

[0102] The calculation formula for the fracture toughness of the test piece is:

[0103]

[0104] However, it should be noted that the best applicable range of the constant in the above formula (3) is the crack length a < 45 mm, and its schematic diagram is as Figure 3 shown; that is, within the range where the crack extends by 15 mm, calculating with the constant f(a / w) has a relatively high accuracy. In the above formula (4), K is the fracture toughness to be measured, F is the load, t is the thickness of the test piece, and w is the width from the tensile loading position to the right boundary of the test piece.

[0105] The loading curve of the typical test piece is as Figure 4As shown, the specific process of obtaining the results of the fracture toughness curve calculated by the above formula (4) is as follows: First, from the inflection point of the load-displacement curve, the crack initiation load of 80.7 N is obtained; substituting the load and the specimen dimensions (t = 3 mm, w = 51 mm) into formula (4), the critical fracture toughness K ini = 58.4 MPa·mm 1 / 2 can be obtained; subsequently, substituting the load data into formula (4) in sequence, several fracture toughness data points during crack propagation can be calculated; according to the crack length at the end of loading, the data points can be fitted to obtain Figure 5 the fracture toughness-crack propagation curve shown.

[0106] The concept of the present invention is reasonable, establishing a simple linear relationship between the fracture toughness and the load curve during the crack propagation of the new configuration test piece, significantly reducing the experimental difficulty of the fracture toughness curve.

[0107] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite material fracture toughness test method avoiding crack length measurement, characterized in that: The following steps are involved: 1) Prepare compact tensile test pieces; 2) Installation of test pieces; 3) Apply tensile loading to the test piece; 4) Experimental data processing.

2. The composite material fracture toughness test method for avoiding crack length measurement as claimed in claim 1, characterized in that: The step 1) specifically comprises the following steps: 1.1) Prepare laminated boards according to the sample layup to be tested; 1.2) Cut the prepared laminate into test pieces; 1.3) Pre-create cracks on the test piece.

3. The composite material fracture toughness test method for avoiding crack length measurement as claimed in claim 1, characterized in that: The process for preparing the laminate in step 1.1) is any one of a prepreg placement process, a vacuum assisted resin infusion process, a resin transfer molding process and an autoclave process; The test pieces in step 1.2) are identical in left and width, but in right half, the height is constantly changing; The prefabricated crack in step 1.3) has an initial length of a0=30 mm, and the crack prefabrication method used is any one of water jet cutting, diamond wire cutting and fatigue loading.

4. The composite material fracture toughness test method for avoiding crack length measurement as claimed in claim 1 or 3, characterized in that In the step 1.1), the process for preparing the laminate adopts a prepreg laying process, and the specific process is as follows: 1.1.1) Prepreg cutting Cut each layer of the prepreg in turn according to the angle and size of the laminate; 1.1.2) Prepreg Lamination Clean the flat plate mold and apply a suitable release agent; accurately lay the cut prepreg on the mold according to the design requirements, and ensure a smooth fit to avoid bubbles and wrinkles; 1.1.3) Vacuum packaging On the mold with prepreg laid out, lay out auxiliary materials, cover the auxiliary materials with a vacuum bag, and seal its edges on the mold with sealing tape, and evacuate the vacuum bag with a vacuum pump; 1.1.4) Curing Put the vacuum-packed prepreg into an autoclave, curing oven or other curing equipment for curing; 1.1.5) Remove from the mold After curing is complete, the laminate is demoulded and removed from the vacuum bag.

5. The composite material fracture toughness test method for avoiding crack length measurement as claimed in claim 1 or 3, characterized in that , the crack prefabrication method in step 1.3) is fatigue loading method, and the specific process is as follows: 1.3.1) Use mechanical processing methods to process a notch of a certain depth and shape on the test piece; 1.3.2) On the fatigue testing machine, determine the maximum load, loading frequency and loading method of the fatigue testing machine according to the material, size and loading requirements of the test piece; 1.3.3) Start the fatigue testing machine and perform cyclic loading on the test piece according to the set loading parameters; 1.3.4) After a certain number of fatigue loadings, the test pieces are cleaned and marked.

6. The composite material fracture toughness test method for avoiding crack length measurement as claimed in claim 1, characterized in that: The specific process of step 2) is as follows: first, the upper clamp and the lower clamp of the tensile test fixture are respectively installed in the upper and lower chucks of the universal tensile testing machine, and then the loading pin is passed through the upper and lower holes of the test piece cut and formed in step 1.2), and is placed in the middle position before and after the tensile test fixture.

7. The composite material fracture toughness test method for avoiding crack length measurement as claimed in claim 1, characterized in that: The specific process of step 3) is as follows: 3.1) After the test piece is installed, preload and unload it first to eliminate the idle travel of the universal tensile testing machine; 3.2) After the preloading is completed, the formal tensile loading of the test piece begins; 3.3) Stop loading after the crack length a<45mm.

8. The composite material fracture toughness test method for avoiding crack length measurement as claimed in claim 7, characterized in that: The specific process of the formal tensile loading of the test piece is as follows: the fixed cylinders of the upper and lower clamps of the tensile test fixture are clamped by the upper and lower clamps of the universal tensile testing machine, and the through holes on both sides of the upper and lower clamps of the tensile test fixture are assembled with the loading pins and the test piece respectively; when the universal tensile testing machine is loaded, the load is transmitted to the loading pins through the tensile test fixture, and then transmitted to the upper and lower hole edges of the test piece, thereby realizing the tensile loading of the I-type crack.

9. The composite material fracture toughness test method for avoiding crack length measurement as claimed in claim 1, characterized in that: The specific process of step 4) is as follows: The fracture toughness of the test piece is calculated by the following formula (4): In the above formula (4), K is the fracture toughness to be measured, F is the load, t is the thickness of the specimen, and w is the width from the tensile loading position to the right edge of the specimen; The specific process of calculating the fracture toughness curve using the above formula (4) is as follows: Substituting the load and sample size into the above formula (4) can obtain the critical fracture toughness of crack initiation, and then substituting the load data into the formula (4) in turn to calculate a number of fracture toughness data points as the crack propagates; according to the crack length when the loading is finally stopped, the data points can be fitted to obtain the fracture toughness-crack extension curve.

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