An improved composite material laminate type III delamination testing device and method

Abstract

The present invention relates to an improved device and method for testing Type III delamination of composite material laminates. The test specimen used is a circular multidirectional laminate made of unidirectional / fabric prepreg with optimized ply sequence, which can ensure the smooth expansion of delamination. The specific steps include: manufacturing an improved composite material laminate Type III delamination edge annular torsion test fixture; preparing edge annular crack specimens with specific shapes and ply sequences; testing the basic mechanical properties of the laminate; using displacement-controlled loading on a hydraulic servo torsion testing machine to obtain an effective load-displacement curve; determining the initial slope or critical torque of the load-displacement curve, and substituting it into the formula to calculate the Type III fracture toughness. The improved test device has obvious effects and can obtain pure Type III delamination that is uniform along the leading edge. The designed circular fixture and test specimen are convenient for clamping and have good centering; the device is easy to manufacture and test, and the data required for the test are easy to obtain. The present invention can characterize the Type III delamination expansion behavior of composite material laminates of different material systems.

Description

Technical Field

[0001] The present invention relates to an improved Type III delamination testing apparatus and method for composite laminates, suitable for characterizing the Type III delamination propagation behavior of composite laminates widely used in aerospace and other fields. Using the improved testing apparatus to conduct Type III delamination tests on multi-directional laminates can yield more stable delamination propagation. Background Art

[0002] Composite materials have been increasingly used in aerospace and other engineering fields to achieve structural weight reduction and other purposes. The low strength and toughness of the matrix material and the lack of reinforcement in the thickness direction make laminated fiber reinforced polymer (FRP) composites prone to delamination failure. The critical strain energy release rate, i.e., interlaminar fracture toughness (G C ) is used to characterize the ability of composite laminates to resist delamination damage. In order to design laminates with better mechanical properties and better damage resistance, it is necessary to focus on the delamination damage problem generated in the actual structure and obtain data that can characterize the interlayer performance of the laminate, especially the G that controls the delamination expansion. C .

[0003] Delamination damage in composite materials is generally classified into three basic forms: opening (Type I), slip shear (Type II), and tear shear (Type III). Delamination damage in actual engineering structures often occurs as a combination of these three types. Type I and II delamination, including mixed types, have been extensively studied. Type I double cantilever beam (DCB), Type II end notch bending (ENF), and mixed mode bending (MMB) tests are widely accepted, and relevant standards have been established for testing fracture toughness, such as ASTM D5528-13, ASTM D7905-14, and ASTM D6671-06. However, research on Type III delamination is quite limited, and a comprehensive standard for determining Type III delamination damage is still lacking. The greatest challenge in developing a Type III delamination standard is designing a test protocol that can achieve pure Type III delamination while maintaining a uniform distribution of Type III strain energy release rates at the delamination front.

[0004] With the increasing thickness of composite structures and the emergence of more complex structures that need to withstand torsion, bending, and compression (such as engine fan blades and helicopter main shaft rotors), significant Mode III delamination has occurred in these structures. However, the lack of accurate and reliable Mode III fracture toughness data has become a bottleneck restricting the development of composite structure design. Therefore, it is necessary to establish a test method for pure Mode III delamination fracture toughness. This is of great significance for accurately characterizing the mechanical properties of composite interlaminar layers, thereby guiding mixed-mode delamination experimental research and helping to improve the level of damage tolerance design of composite structures. Summary of the Invention

[0005] The technical problem solved by the present invention is to design a test device to obtain pure type III stratification and establish a corresponding test method.

[0006] Generally speaking, there are two requirements in the test process of pure type III delamination. First, the type I and type II strain energy release rate (SERR) components G observed at the delamination front of the specimen are I and G II The delamination must be negligible. Existing test methods, such as MSCB and ECT, can obtain a pure Type III region by configuring the load at the center of the specimen's delamination front. However, excess Type II components often appear at the specimen's edge. This edge effect is caused by the lack of a closed delamination front. Therefore, a simple Type III delamination test cannot produce pure Type III delamination. The present invention relates to an improved edge annular torsion test that uses closed, unbounded laminates for testing, eliminating this edge effect.

[0007] The second is the type III strain energy release rate G at the front of the delamination expansion Ⅲ The change of G at the crack initiation should be as small as possible. Ⅲ The average value can be regarded as the fracture toughness, and only then can we obtain effective data that can characterize the delamination. Type III delamination test can use unidirectional or multidirectional laminates. When using full unidirectional laminates, it is inevitable that the G of the delamination front will be Ⅲ The use of multi-directional laminates can optimize the ply to obtain a G with a smaller change in the front edge of the delamination. Ⅲ The present invention relates to a multi-directional laminate with optimized ply order for type III delamination test, which can make the G Ⅲ The average value can be used to visualize fracture toughness and save material costs;

[0008] The present invention takes into account the geometric factors of the device and the specimen on the delamination front G Ⅲ The influence of the distribution of the test device and the test specimens used in the design are more conducive to achieving pure type III delamination. Figure 2 The figure shows the distribution of the SERR components of the delamination front obtained by the test device of the present invention and the original test device (the test device with subscript A is the original test device, and the test device with subscript B is the improved test device of the present invention). It can be seen from the figure that the new test device eliminates the influence of type I and type II delamination. The calculated SERR of type I and type II are close to 0, and there is no edge effect. More importantly, the G obtained by the new test device is Ⅲ The distribution along the front edge of the delamination (0° to 360°) is more uniform, forming a smooth and non-oscillatory curve, which can achieve a more stable delamination expansion;

[0009] In addition, the present invention can achieve uniform torque transmission, and the outer diameters of the torsion disc and the disc-shaped test piece are the same, so that the centrifugal force is significantly improved;

[0010] The present invention relates to the G of ordinary fiber rather than fabric reinforced laminates. ⅢC The calculation takes into account the influence of orthotropic anisotropy and gives its calculation expression; the fracture toughness G based on the flexibility method is also proposed. ⅢC The new test device designed also takes into account the influence of processing and clamping errors that are easy to occur during the test process, and provides the allowable range of various errors;

[0011] In summary, the present invention aims at measuring G IIIC In order to overcome the shortcomings of the above, an improved test method is proposed. The improved edge ring torsion test method is suitable for engineering applications, can ensure the accurate test of mode III fracture toughness, and better guarantee the safety of the designed structure.

[0012] The technical solution of the present invention is an improved device and method for testing Type III delamination of composite laminates, which is implemented in the following steps:

[0013] Step 1: Make a Type III delamination test fixture for composite laminates. The fixture diagram is shown in the attached figure. Figure 3 , 5, 6; the top end (1) of the fixture loading head is flat and can be directly clamped on the torsion testing machine; different from the traditional square loading plate and test piece, the loading plate (2) of the torsion device is designed to be disc-shaped; the test piece (3) used is also designed to be disc-shaped;

[0014] Step 2: Prepare a Type III delaminated specimen for testing. The specimen must meet the requirements of symmetry, balance, and quasi-isotropy, and there must be no elastic coupling. The disc-shaped specimen (3) used includes a multi-directional laminate made of unidirectional prepreg; the ply layup order of the specimen is: Fabric prepreg preparation: [0 / θ / θ / 0 / θ / θ / [0 / θ / θ / 0] n / θ / θ / 0 / θ / θ / 0] 2T ;Unidirectional prepreg preparation: [0 / +θ / -θ / 0 / -θ / +θ / [0 / -θ / +θ / 0] n / -θ / +θ / 0 / -θ / +θ / 0] 2T, where n = 2, 3, 4, and θ can be 30°, 45°, or 60°. The specimen preparation process of standards such as ASTM D5528-13 / ASTM D7905-14 / ASTM D6671-06 can be referred to to prepare a composite laminate III layered specimen. The specimen consists of an upper and lower sub-layer plate with a diameter of D. There is an annular initial crack in the middle of the two sub-layer plates. The initial crack is formed by inserting a circular polytetrafluoroethylene film with a diameter of d in the middle of the layer plate. Its geometric features are shown in the attached figure. Figure 5 As shown; the conditions for the prepared specimen to be considered as a valid specimen are: the eccentricity between the center of the PTFE film hole and the center of the specimen does not exceed 2mm, the ellipticity of the PTFE film hole after curing is not greater than 0.05, and there are no other obvious defects, otherwise it will cause inaccurate test results;

[0015] Step 3: Measure the mechanical properties of the composite laminate according to relevant test standards such as GB / T 30970-2014 and ASTM D5379 / D379M-12 to obtain the out-of-plane interlaminar shear modulus C 44 and C 55 ;

[0016] Step 4: Install the fixture on the hydraulic servo torsion testing machine and perform the edge ring torsion test; the clamping order is to install the lower loading head first and then the upper loading head to eliminate the influence of the device's own weight; record the relevant load-displacement curve, obtain the relationship between the torque T acting on the specimen and the torsion angle α between the upper and lower sub-layers, and record the peak torque T at the descending point on the load-displacement curve C ;

[0017] Step 5: Calculate the Mode III interlaminar fracture toughness G ⅢC , the calculation formula is:

[0018]

[0019] The calculation formula is derived from the relationship between the stress intensity factor and energy release rate of orthotropic materials. III The calculation method is derived from the analytical solution of the stress intensity factor for an infinitely long cylinder with a diameter of D and an initial annular crack of diameter d in the middle, with both ends subjected to torsion. The calculation formula can be found in the relevant stress intensity factor manual. Where:

[0020]

[0021]

[0022] For tests in which the variation patterns of specimen compliance C and initial crack length (a = Dd) during the loading process are obtained, if there are at least 5 valid data points, the flexibility method expressed as follows can also be used to calculate the mode III interlaminar fracture toughness:

[0023]

[0024] This formula is derived from the flexibility method of the specimen under torsion under constant displacement conditions, so it is required to load the specimen in a displacement-controlled manner, where C is the flexibility of the specimen under torsion (the inverse of the slope of the load-displacement curve).

[0025] The advantages of the present invention compared with the prior art are:

[0026] (1) In view of the lack of accurate testing of mode III delamination fracture toughness in existing testing standards and the problems existing in existing testing methods, an improved testing device and method are proposed to achieve the problem of accurate testing of mode III fracture toughness that needs to be solved urgently in engineering.

[0027] (2) This invention uses a closed, unbounded edge ring torsion test to conduct Type III delamination testing, which can characterize the Type III delamination expansion behavior of composite laminates under different material systems. The designed circular fixture has the same outer diameter as the specimen, which facilitates specimen clamping and alignment. The device is simple to manufacture, the testing process is easy, and the required test data is readily available.

[0028] (3) The improved test method of the present invention has been verified by experiments, and numerical simulations show that the improved test method can reduce G Ⅲ The irregular changes in the specimen geometry at the delamination front are also closer to the simplified model used in the formula for calculating the Type III energy release rate.

[0029] (4) The multi-directional ply layup sequence is optimized, which saves material costs while achieving a more stable layer expansion.

[0030] The test method for mode III interlaminar fracture toughness of composite laminates proposed in the present invention can effectively provide interlaminar fracture toughness parameters, provide data support for structural design, and effectively ensure structural safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart of an improved composite material laminate type III delamination testing device and method according to the present invention;

[0032] Figure 2 The distribution law of the energy release rate at the delamination front edge obtained by the improved type III delamination test device and the original test device;

[0033] Figure 3 The improved type III delamination test device and the schematic diagram of its operation under torsion;

[0034] Figure 4 is a schematic diagram of the shape and geometric properties of the test piece used in the improved test device;

[0035] Figure 5 It is a three-view drawing of the improved type III delamination test device;

[0036] Figure 6 It is a three-view drawing of the loading ring on the improved Type III delamination test device;

[0037] Figure 7 is an example graph of the effective load-displacement curve obtained during the test;

[0038] Figure 8 Schematic diagram of the compliance and fitting results under different pre-crack lengths.

[0039] Explanation of symbols in the figure: 1. Clamping end of the upper loading head, 2. Bolt connection hole between the loading head and the loading ring, 3. Fiber-reinforced composite disc-shaped specimen, 4. Loading ring, 5. Clamping end of the lower loading head, 6. Upper sub-layer of the specimen, 7. Polytetrafluoroethylene film, 8. Circular hole on the polytetrafluoroethylene film, 9. Lower sub-layer of the specimen. DETAILED DESCRIPTION

[0040] The present invention is described in further detail below:

[0041] The present invention relates to an improved device and method for testing type III delamination of composite material laminates, and the specific implementation steps are as follows:

[0042] Step 1: Preparation of fixtures and test pieces. The fixtures required for the test must be manufactured by professional manufacturers and surface treated. For example, the test piece can be made of T700 / QY9511 and T00 / X850 prepregs, and other fiber reinforced composite materials are also applicable. The geometric dimensions are as shown in the attached Figure 3As shown. The specimen consists of an upper sub-layer (6), a lower sub-layer (9) and a polytetrafluoroethylene film (7) in the middle. A hole (8) with a diameter of d is cut out in the center of a square polytetrafluoroethylene film with a thickness of less than 25 μm and a side length greater than D, and then inserted into the middle plane of the laid prepreg sheet to form a prefabricated crack. Then, it is cured into a prepreg sheet in a curing oven according to the recommended procedure of the prepreg supplier. Ultrasonic C scanning is used to check whether the polytetrafluoroethylene film hole in the middle of the cured plate is strictly centered and has no shape changes, and to check other possible defects in the manufacture of the layer. It is required that the eccentricity between the center of the polytetrafluoroethylene film hole and the center of the specimen does not exceed 2 mm, the ellipticity of the polytetrafluoroethylene film hole after curing is not greater than 0.05, and there are no other obvious defects to be considered a valid specimen. A water-cooled high-speed diamond tool is used to cut the required shape and size of the specimen from the specimen where no defects are detected, and the final diameter D and thickness h of the specimen are measured and recorded. The edges of the specimens were then manually polished using sandpaper to remove any microstructural damage that may have been caused during the cutting process and to make the sharp edges as smooth as possible. The mechanical properties of the composite laminates were measured with reference to relevant test standards such as GB / T 30970-2014 and ASTM D5379 / D379M-12, and the out-of-plane interlaminar shear modulus C was obtained. 44 and C 55 .

[0043] Step 2: Connect the fixture and the specimen. Before testing, apply a thin layer of white correction fluid or paint to the side of the specimen. At the same time, use a marker to draw vertical lines along the thickness direction in an easily observable area to make it easier to monitor the torsion position between the upper and lower layers. Apply glue evenly to the protruding cone surface of the loading ring (4) designed on the test device, and carefully attach the specimen between the two loading rings. After checking for alignment, let it stand in a ventilated and dry place for more than 12 hours. After checking the quality of the glued area, fix the upper and lower loading rings with the specimen to the loading head (1) with bolts.

[0044] Step 3: Install the fixture on the testing machine. Install the clamp on the hydraulic servo torsion testing machine, align the top of the loading head and the testing machine clamp, and then clamp it. In this process, it is necessary to clamp the lower loading head (5) first and then clamp the upper loading head to avoid damaging the specimen due to the tension generated by the weight of the device. After clamping the upper loading head, use the force control of the torsion testing machine and clear the torque and axial force to zero. In order to maintain a better delamination effect and avoid the influence of friction on the delamination performance, apply a tensile force of no more than 50N in the axial direction. After preparation, use a mobile microscope located on one side of the specimen to locate the vertical line in the thickness direction and monitor the angle of rotation between the upper and lower layers.

[0045] Step 4: Perform a torsion test. After debugging the testing machine, use displacement control. Use a low loading rate of 0.1° / min to ensure that the layer expansion is slow and stable. Use the sensor provided by the testing machine to continuously record the angle α and torque T of the loading point at a certain time interval and draw a load-displacement curve. The more complete load-displacement curve obtained from the test should be as shown in the attached figure. Figure 5 As shown, a linear load-displacement curve can be observed at the beginning, and the peak torque T can be clearly defined. C If the load-displacement curve becomes flat, it may be caused by slippage at the bolt connection. The slippage here needs to be subtracted when calculating the torsion angle. If the load-displacement curve is not flat, the torque corresponding to the intersection of the 95% initial slope straight line and the load-displacement curve can be taken as T C .

[0046] Step 5: If the load-displacement curve obtained under the same initial crack length (Dd) is obtained, take its peak torque T C And substitute the following semi-analytical formula to calculate the type III stress intensity factor K III , where f is the shape factor, a parameter related to the relative ratio (d / D).

[0047]

[0048]

[0049] Since the delamination failure of CFPR laminates is the brittle fracture failure of the matrix, it can be calculated based on the G ⅢC With K III The relational calculation G ⅢC .

[0050]

[0051] Step 6: After the test is completed, remove the fixture and the test piece attached to it, make a record and soak it in the debonding agent. After a period of time, the fixture and test piece can be separated and the surface of the fixture can be cleaned to ensure that there is no excess glue residue before the next test.

[0052] Step 7: Repeat steps 1-6 to obtain at least five sets of valid data, and take the average value as the mode III fracture toughness G of the material system. ⅢC .

[0053] In step 5, if the load-displacement curves under different initial crack lengths (a=Dd) are obtained, the slope K of each load-displacement curve is obtained, and its reciprocal C is taken as the flexibility of the specimen. A trend graph of the specimen flexibility C changing with a is drawn and a linear fit is performed (requires no less than 5 sets of data), as shown in the attached figure. Figure 8As shown, and by calculating the slope of the fitting line, we can get The obtained value can also be substituted into the following formula to calculate the mode III fracture toughness.

[0054]

[0055] Some parts of the present invention are well known to those skilled in the art and are not described in detail.

[0056] The above description is only part of the specific implementation methods of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any person familiar with the art within the technical scope disclosed in the present invention should be covered by the protection scope of the present invention.

Claims

1. An improved method for testing type III delamination of composite laminates, characterized in that The following steps are involved: Step 1: manufacturing a Type III delamination test fixture for composite laminates, wherein the top end (1) of the fixture loading head is flat and clamped on a torsion testing machine; unlike conventional square loading discs and test specimens, the loading disc (2) of the torsion device is designed to be disc-shaped; the test specimen (3) used is also designed to be disc-shaped to reduce the irregular variation of the distribution of the Type III strain energy release rate at the delamination front; Step 2: Prepare a Type III layered specimen for testing. The specimen must meet the requirements of symmetry, balance, and quasi-isotropy, and there is no elastic coupling. The disc-shaped specimen (3) used includes a multi-directional layer made of unidirectional prepreg. The ply layup order of the specimen is: Fabric prepreg preparation: [0 / θ / θ / 0 / θ / θ / [0 / θ / θ / 0] n / θ / θ / 0 / θ / θ / 0] 2T ;Unidirectional prepreg preparation: [0 / +θ / -θ / 0 / -θ / +θ / [0 / -θ / +θ / 0] n / -θ / +θ / 0 / -θ / +θ / 0] 2T , where n = 2, 3, or 4, and θ is 30°, 45°, or 60°; referring to the specimen preparation process in ASTM D5528-13 / ASTM D7905-14 / ASTM D6671-06, composite laminate III layered specimens were prepared. The specimens consisted of upper and lower sub-laminae with a diameter of D. An annular initial crack was formed between the two sub-laminae by inserting a circular polytetrafluoroethylene film with a diameter of d between the laminates. The prepared specimens were considered valid if the eccentricity between the center of the polytetrafluoroethylene film hole and the center of the specimen did not exceed 2 mm, the ellipticity of the polytetrafluoroethylene film hole after curing was no greater than 0.05, and there were no other obvious defects, otherwise inaccurate test results would result. Step 3: Measure the mechanical properties of the composite laminate according to the relevant test standards GB / T 30970-2014 and ASTM D5379 / D379M-12 to obtain the out-of-plane interlaminar shear modulus C 44 and C 55 ; Step 4: Install the fixture on the hydraulic servo torsion testing machine and perform the edge ring torsion test; the clamping order is to install the lower loading head first and then the upper loading head to eliminate the influence of the device's own weight; record the relevant load-displacement curve, obtain the relationship between the torque T acting on the specimen and the torsion angle α between the upper and lower sub-layers, and record the peak torque T at the descending point on the load-displacement curve C ; Step 5: Calculate the Mode III interlaminar fracture toughness G ⅢC ; G ⅢC and stress intensity factor K Ⅲ The relationship between G and ⅢC The calculation formula is: in: f is the shape factor, which is calculated as: For tests in which the variation patterns of specimen compliance C and initial crack length a during loading are obtained, where a = Dd, and if there are at least 5 valid data points, the mode III interlaminar fracture toughness is calculated using the compliance method expressed in the following formula:

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