Method and system for measuring type II layered crack length and fracture toughness of composite material

Abstract

The invention discloses a method and a system for measuring the II-type layered crack length and fracture toughness of a composite material, belongs to the field of mechanical property testing of composite materials, and aims to solve the problems that the conventional ENF test depends on optical equipment, neglects a damage mechanism and is insufficient in precision due to lack of correction capability. The method comprises the following steps: preparing an ENF test piece and recording initial parameters; the universal testing machine and the acoustic emission equipment synchronously acquire load-displacement data and acoustic emission signals; defining an initial parameter based on a peak load, calculating a bending modulus, and obtaining initial / steady-state fracture toughness and an equivalent fracture area length by combining a flexibility beam theory and piecewise fitting; classifying damage events by using a clustering algorithm, quantifying an energy slope, and calculating an initial-steady-state toughness ratio; and iteratively correcting and optimizing the initial fracture toughness and the initial parameter until the error reaches the standard, and outputting a multi-dimensional parameter. The method does not need optical observation, quantifies the damage contribution, improves the measurement precision through iterative correction, provides reliable data support for the safety evaluation of the composite material structure, and is high in practicability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of mechanical property testing of composite materials, and particularly relates to a method and system for measuring the length of a type II delamination crack and the fracture toughness of a composite material. BACKGROUND

[0002] Type II delamination is one of the most common damage forms of a composite material laminated structure, and the accurate measurement of the crack length and fracture toughness thereof is a core link for evaluating the service safety of a composite material structure.

[0003] An ENF (End Notched Flexure) experiment is the most widely used characterization method for type II delamination testing due to its simple operation. However, the ENF test has a core technical limitation: it is difficult to accurately measure the crack length, especially the determination of the initial crack starting time, which is difficult to achieve through conventional optical observation methods. This problem directly leads to the deviation in the determination of the "crack initiation load / displacement", and further leads to the decline in the calculation accuracy of subsequent key mechanical parameters such as the bending modulus and the fracture toughness.

[0004] In view of the above limitation, the prior art mainly determines the crack length and fracture toughness of a type II ENF delamination of a composite material through two methods, but both methods have obvious defects: one is the digital image correlation technology (DIC): this method needs to rely on optical equipment such as a high-speed camera, and indirectly locates the crack tip by monitoring the surface deformation field of the test piece and using the displacement of the upper and lower arms of the ENF sample to separate the data. On the one hand, the crack front is easy to tilt or bend towards the high stress area, resulting in deviation in the observed crack length; on the other hand, the initial crack initiation needs to be determined by optical images manually, which inevitably introduces artificial errors, and the accuracy of the optical equipment is extremely high. The second is the equivalent crack length method: this method calculates the equivalent crack propagation length based on the compliance change (through load-displacement curve fitting) of the ENF sample, and although it does not need real-time optical observation, it only relies on macroscopic load-displacement data and does not consider the contribution of different damage mechanisms such as "matrix crack, fiber-matrix debonding, and fiber fracture" to the fracture toughness during the delamination process of the composite material, resulting in deviation in the calculation of the initial fracture toughness; at the same time, this method lacks an iterative correction mechanism based on the damage mechanism, and cannot compensate for the cumulative error caused by the deviation in the determination of the initial parameters, and cannot completely describe the whole-stage fracture resistance evolution law of "initial crack → crack propagation → steady-state propagation".

[0005] In summary, the existing methods either rely on expensive optical equipment, or ignore the quantitative contribution of damage mechanisms, or lack high-precision correction capabilities, and cannot cover the characterization needs of the full evolution of cracks, making it difficult to meet the engineering application needs of "no optical dependence, quantitative damage mechanism contribution, high-precision calculation, full-stage characterization". Therefore, there is an urgent need for a composite II-type delamination crack length and fracture toughness measurement method that does not rely on optical equipment, quantifies the contribution of damage mechanisms to fracture toughness, and improves calculation accuracy through correction mechanisms. SUMMARY

[0006] Therefore, the purpose of the present application is to provide a composite II-type delamination crack length and fracture toughness measurement method and system, which addresses the technical pain points of existing composite II-type ENF (end notched flexure) delamination measurement, which relies on optical equipment and is prone to errors, ignores the quantitative contribution of damage mechanisms such as matrix cracking, fiber-matrix debonding, and fiber fracture, lacks high-precision iterative correction capabilities, and cannot cover full-stage characterization. By synchronously collecting load-displacement data and acoustic emission signals with a universal testing machine and acoustic emission equipment, clustering algorithms quantify the contribution of various damage mechanisms to fracture toughness, and iterative correction based on the initial-steady toughness ratio, the full-stage efficient and accurate measurement of II-type delamination crack length, initial fracture toughness, and steady-state fracture toughness is achieved.

[0007] To achieve the above-mentioned purposes, the technical solutions adopted are as follows:

[0008] The application discloses a composite material II type layered crack length and fracture toughness measurement method, which comprises the following steps: S1, preparing an ENF test piece for II type layering test, measuring and recording the initial crack length, thickness, width and span of the ENF test piece; S2, performing II type layering test on the ENF test piece on a universal testing machine according to a test standard, recording the load and corresponding displacement data in the II type layering expansion process of the ENF test piece, and simultaneously collecting acoustic emission signals in the II type layering expansion process by using an acoustic emission device; S3, defining the peak load and corresponding displacement in the load and corresponding displacement data recorded in step S2 as the II type layered crack initiation load and initiation displacement, respectively; based on the II type layered crack initiation load and initiation displacement, combining the initial crack length, thickness, width and span of the ENF test piece in step S1, the bending modulus of the ENF test piece is calculated; S4, obtaining the load and corresponding displacement data after the II type layered crack initiation load and initiation displacement, and calculating the II type layered crack fracture toughness by using a beam theory based on flexibility; segment fitting is performed on the II type layered crack fracture toughness according to the crack expansion evolution law, and the II type layered crack initial fracture toughness, II type layered crack steady-state fracture toughness and equivalent fracture region length are obtained; S5, classifying the acoustic emission signals collected in step S2 by using a clustering algorithm, and obtaining the matrix crack damage event, fiber-matrix debonding damage event and fiber fracture damage event; the II type layered crack steady-state expansion stage is identified by using the peak load, the time evolution law of the cumulative acoustic emission energy of the matrix crack damage event, fiber-matrix debonding damage event and fiber fracture damage event in the II type layered crack steady-state expansion stage is analyzed by using a linear fitting method, and the cumulative acoustic emission energy slope of each type of damage event is obtained; based on the cumulative acoustic emission energy slope of each type of damage event, the II type layered crack initial-steady-state toughness ratio is calculated; S6, correcting the II type layered crack initial fracture toughness obtained in step S4 by using the II type layered crack initial-steady-state toughness ratio calculated in step S5, and obtaining the first corrected II type layered crack initial fracture toughness; based on the first corrected II type layered crack initial fracture toughness, the first corrected II type layered crack initial load is calculated, and the first corrected II type layered crack initiation load and initiation displacement are determined by combining the load and corresponding displacement data recorded in step S2; S7, based on the load and corresponding displacement data after the first corrected II type layered crack initiation load and initiation displacement, the calculation and segment fitting logic of step S4 is repeated, and the first corrected II type layered crack initial fracture toughness, II type layered crack steady-state fracture toughness and equivalent fracture region length are obtained; the correction logic of step S6 is repeated again, and the second corrected II type layered crack initial fracture toughness is calculated.S8. Repeat steps S6 to S7 for iterative correction, calculating the relative error of the initial fracture toughness of the type II delamination crack after the two corrections, until the relative error meets the preset error threshold. Output the final type II delamination crack length, initial fracture toughness of the type II delamination crack, steady-state fracture toughness of the type II delamination crack, and equivalent fracture region length.

[0009] This method integrates acoustic emission technology with flexibility-based beam theory. Through processes such as preparing standardized specimens, simultaneously acquiring load displacement data and acoustic emission signals, segmented fitting of fracture toughness, quantification of damage mechanism contributions, and iterative correction, it avoids the equipment dependence and human error caused by optical observation of cracks in traditional methods. It can accurately obtain the length of type II delamination cracks, initial fracture toughness, steady-state fracture toughness, and equivalent fracture region length, fully describe the evolution of fracture resistance from delamination to instability, and quantify the contribution of different damage mechanisms to energy dissipation, providing a comprehensive and reliable mechanical basis for the optimized design of composite materials.

[0010] In this invention, firstly, an ENF specimen for type II delamination testing is prepared, and its initial crack length, thickness, width, and span are measured and recorded. A type II delamination test is then performed on the ENF specimen using a universal testing machine, with load-displacement data recorded simultaneously. Acoustic emission signals during the delamination propagation process are also acquired in real time using an acoustic emission device. Secondly, the peak load and its corresponding displacement in the load-displacement data are defined as the initial load and initial displacement of the type II delamination crack, respectively. After calculating the flexural modulus based on the initial parameters of the specimen, the load-displacement data segment following this initial parameter is selected. The fracture toughness of the type II delamination crack is calculated using beam theory based on flexibility, and the fracture toughness is piecewise fitted according to the crack propagation evolution law to obtain the initial fracture toughness, steady-state fracture toughness, and equivalent fracture region length. Then, a clustering algorithm is used to classify acoustic emission signals into three types of damage events: matrix cracks, fiber-matrix debonding, and fiber fracture. The steady-state crack propagation stage is identified by peak load. The cumulative acoustic emission energy-time curves of each type of damage event in this stage are linearly fitted to obtain the slope of the cumulative acoustic emission energy, and the initial-steady-state toughness ratio is calculated accordingly. Subsequently, the initial fracture toughness is corrected using the initial-steady-state toughness ratio to obtain the corrected initial fracture toughness. Based on the corrected initial fracture toughness, a new initial crack load is calculated, and the displacement corresponding to the new load is determined by combining the original load-displacement data. The load and displacement at crack initiation are then updated. Using the updated initiation parameters as a new benchmark, the corresponding data segments are selected, and the process of "calculating fracture toughness - piecewise fitting of initial fracture toughness, steady-state fracture toughness, and equivalent fracture region length" is repeated. The initial fracture toughness is then corrected again using the initial-steady-state toughness ratio. Finally, the above iterative correction process is repeated, and the relative error is calculated according to the ratio of the absolute value of the difference between the initial fracture toughness after the two corrections to the initial fracture toughness after the previous correction, until the error meets the preset threshold. Finally, the accurate type II delamination crack length, initial fracture toughness, steady-state fracture toughness and equivalent fracture region length are output.

[0011] Furthermore, in step S3, the calculation of the flexural modulus of the ENF specimen includes: calculating the delamination crack initiation compliance of the ENF specimen based on the load and displacement at the initiation of the type II delamination crack, using the following formula:

[0012] ;

[0013] Based on the delamination crack initiation compliance of the ENF specimen, the modified delamination crack initiation compliance of the ENF specimen is calculated using the following formula:

[0014] ;

[0015] Based on the modified delamination crack initiation compliance of the ENF specimen, the flexural modulus of the ENF specimen is calculated using the following formula:

[0016] ;

[0017] In the formula, The delamination crack initiation compliance of the ENF specimen; The displacement at the initiation of a type II delamination crack; The load at the initiation of a type II delamination crack; The modified delamination crack initiation compliance for ENF specimens; The span of the ENF specimen; The shear modulus of the ENF specimen; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The flexural modulus of the ENF specimen; The initial crack length of the ENF specimen.

[0018] In this invention, by using the calculation formulas for delamination crack initiation flexibility, modified delamination crack initiation flexibility, and flexural modulus, combined with the load displacement data at the initiation of type II delamination crack and the key parameters of the specimen, the flexural modulus of the ENF specimen can be accurately derived. The flexural modulus serves as the basis for the subsequent calculation of core parameters such as fracture toughness and equivalent crack length, and its calculation accuracy directly ensures the reliability and precision of the subsequent results of the entire measurement method.

[0019] Further, in step S4, the calculation of the fracture toughness of the type II delamination crack includes: based on the acquired load at the initiation of the type II delamination crack and the load and its corresponding displacement data after the initiation, the real-time compliance during the steady-state propagation process of the type II delamination crack is calculated, as follows:

[0020] ;

[0021] Based on the real-time compliance during the Type II delamination steady-state propagation process, and combined with the modified delamination crack initiation compliance calculation of the ENF specimen in step S3, the equivalent delamination crack length during the Type II delamination steady-state propagation process is calculated using the following formula:

[0022] ;

[0023] Based on the equivalent delamination crack length during the type II delamination steady-state propagation process, the type II delamination fracture toughness is calculated using the following formula:

[0024] ;

[0025] In the formula, For the real-time compliance during the type II hierarchical steady-state expansion process; This represents the real-time displacement during the type II layered steady-state expansion process. For the real-time load during the type II layered steady-state expansion process; The span of the ENF specimen; The shear modulus of the ENF specimen; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The equivalent delamination crack length during the type II delamination steady-state propagation process; The modified delamination crack initiation compliance for ENF specimens; The initial crack length of the ENF specimen; It exhibits type II delamination crack fracture toughness; The bending modulus of the ENF specimen.

[0026] In this invention, based on real-time load displacement data during the steady-state propagation process of type II delamination, the fracture toughness can be accurately calculated without directly observing the crack morphology by using real-time compliance, equivalent delamination crack length, and fracture toughness calculation formulas. This effectively avoids observation errors caused by crack tip tilting and bending, and significantly improves the accuracy and practicality of type II delamination crack fracture toughness calculation.

[0027] Furthermore, in step S5, the formula for calculating the initial-steady-state toughness ratio of the type II delamination crack is as follows:

[0028] ;

[0029] In the formula, The initial-steady-state toughness ratio is that of a type II delamination crack. The slope of the cumulative acoustic emission energy of the matrix crack damage event; The slope of the cumulative acoustic emission energy of the fiber-matrix debonding damage event; ( =1,2,3) represent the cumulative acoustic emission energy slopes for matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events, respectively.

[0030] In this invention, by integrating the cumulative acoustic emission energy slopes of three types of damage events—matrix cracking, fiber-matrix debonding, and fiber fracture—a scientific method for calculating the initial-steady-state toughness ratio is constructed. This not only provides a reliable quantitative basis for subsequent correction of initial fracture toughness but also enables accurate assessment of the contribution weight of different damage mechanisms in the type II delamination process, enriching the mechanical analysis dimensions of composite material delamination damage.

[0031] Furthermore, in step S6, the corrected formula for the initial fracture toughness of type II delamination cracks is as follows:

[0032] ;

[0033] The initial load of the modified type II delamination crack is calculated based on the modified initial fracture toughness, using the following formula:

[0034] ;

[0035] In the formula, For the first The initial fracture toughness of the second-corrected type II delamination crack; The initial-steady-state toughness ratio is that of a type II delamination crack. For the first The corrected steady-state fracture toughness of type II delamination crack; For the first The revised initial load for type II delamination crack; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The initial crack length of the ENF specimen; The bending modulus of the ENF specimen.

[0036] In this invention, the initial fracture toughness is corrected by using the initial fracture toughness correction formula and the correction initial load derivation formula. The initial fracture toughness is then corrected by using the initial-steady-state toughness ratio. At the same time, the corrected initial load is accurately calculated, which can effectively correct the deviation of the initial fracture toughness. This provides key support for determining accurate crack initiation parameters and further improves the accuracy and consistency of the results of the entire measurement system.

[0037] Furthermore, in steps S4 and S7, the calculation formula for piecewise fitting is as follows:

[0038] ;

[0039] In the formula, The equivalent crack length is Type II delamination crack fracture toughness at that time; These are the initial constants for piecewise fitting; For the first The corrected steady-state fracture toughness of type II delamination crack; For the first The initial fracture toughness of the second-corrected type II delamination crack; The equivalent delamination crack length during the type II delamination steady-state propagation process; The initial crack length of the ENF specimen; For the first The length of the equivalent fracture region after the second correction.

[0040] In this invention, the corresponding relationship between the equivalent crack length and fracture toughness in different intervals is accurately characterized by a piecewise fitting formula. This clearly distinguishes the fracture toughness variation law between the crack initiation stage and the steady-state propagation stage, thereby accurately extracting the initial fracture toughness, steady-state fracture toughness and the equivalent fracture region length. This fully presents the fracture resistance evolution process of type II delamination cracks, providing detailed data support for in-depth analysis of the delamination characteristics of composite materials.

[0041] Furthermore, in step S8, the formula for calculating the relative error is as follows:

[0042] ;

[0043] In the formula, The relative error in the initial fracture toughness of the type II delamination crack after the two corrections; For the first The initial fracture toughness of the second-corrected type II delamination crack; For the first The initial fracture toughness of the second-corrected type II delamination crack.

[0044] In this invention, the relative error calculation formula can quantify the degree of deviation of the initial fracture toughness after two corrections, providing a clear and quantifiable judgment standard for the termination of iterative correction, ensuring that the final output of the initial fracture toughness and related parameters meets the preset accuracy requirements, avoiding result deviations caused by excessive or insufficient iteration, and ensuring the reliability of the measurement results.

[0045] Further, in step S6, determining the initial load and initial displacement of the first corrected type II delamination crack includes the following steps: based on the load and corresponding displacement data of the ENF specimen during the type II delamination propagation process recorded in step S2, construct a continuous load-displacement curve; take the initial load of the first corrected type II delamination crack calculated in step S6 as the initial load of the first corrected type II delamination crack; on the load-displacement curve, obtain the displacement corresponding to the initial load of the first corrected type II delamination crack as the initial displacement of the first corrected type II delamination crack.

[0046] In this invention, based on the continuous load-displacement curve recorded in the test, the corresponding crack initiation displacement is accurately located by combining the corrected initial load. This avoids the subjective error of traditional manual determination of crack initiation point and can obtain accurate and objective corrected crack initiation parameters, laying a solid foundation for subsequent iterative correction and accurate calculation of various mechanical parameters.

[0047] Furthermore, the shear modulus of the ENF specimen The methods for obtaining the parameters include: measuring the interlaminar shear modulus or in-plane shear test of the composite material corresponding to the ENF specimen, or using the interlaminar shear modulus parameters published in the material handbook of the composite material.

[0048] In this invention, the shear modulus is defined. Multiple reliable acquisition methods are available, supporting both experimental measurement to obtain accurate parameters of the suitable specimens and the use of publicly available data from material handbooks. This ensures the accuracy and flexibility of the calculation of the compliance of the modified delamination crack initiation, and provides a key guarantee for the accurate derivation of subsequent core parameters such as bending modulus and fracture toughness.

[0049] In summary, this invention employs acoustic emission technology to collect acoustic emission signals, such as amplitude and energy, in real time during the delamination process of Type II ENF composite specimens in delamination testing. First, a clustering algorithm is used to accurately identify the damage modes during delamination. Then, based on the clustering analysis results, cumulative energy characteristics are used to assist in determining the damage initiation time. Simultaneously, combined with a beam theory method based on flexibility, the equivalent crack length and interlaminar fracture toughness are calculated, effectively avoiding the errors introduced by traditional methods that rely on optical observation of crack length and manual determination of crack initiation. This invention is applicable to typical Type II ENF specimens of continuous fiber reinforced composite laminates, and can accurately determine the delamination initiation state, crack length, and interlaminar fracture toughness during its delamination propagation process.

[0050] This invention also discloses a system for implementing the method for measuring the length and fracture toughness of type II delamination cracks in composite materials disclosed herein, comprising: a specimen processing and parameter module for preparing ENF specimens for type II delamination testing and measuring and recording the initial crack length, thickness, width, and span of the ENF specimens; and a loading and synchronous acquisition module, including a universal testing machine and an acoustic emission device: the universal testing machine is used to perform type II delamination testing on the ENF specimens according to the type II delamination test standard and record the load and its corresponding displacement data during the delamination propagation process; the acoustic emission device is used to work synchronously with the universal testing machine to acquire real-time data on the delamination propagation of the ENF specimens. The process includes acoustic emission signals; a fracture toughness calculation module, used to extract peak load and corresponding displacement from load and corresponding displacement data, which are used as the initial load and initial displacement of the type II delamination crack, respectively; and to calculate the bending modulus of the ENF specimen based on the initial load, initial displacement, and the initial crack length, thickness, width, and span of the ENF specimen; it is also used to obtain the load and corresponding displacement data after the initial load and initial displacement, calculate the fracture toughness of the type II delamination crack using beam theory based on flexibility, and perform piecewise fitting of the fracture toughness according to the crack propagation evolution law to obtain the initial fracture toughness and steady-state fracture toughness of the type II delamination crack. The system includes a component for determining the length of the equivalent fracture region; an acoustic emission signal processing and toughness ratio calculation module, used to classify acoustic emission signals using a clustering algorithm to obtain matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events; and a module for identifying the steady-state propagation stage of type II delamination cracks using peak load, analyzing the temporal evolution of accumulated acoustic emission energy for various damage events in this stage using a linear fitting method, obtaining the slope of accumulated acoustic emission energy for various damage events, and then calculating the initial-steady-state toughness ratio of the type II delamination crack; and an iterative correction module, used to correct the initial fracture toughness using the initial-steady-state toughness ratio to obtain the corrected initial fracture toughness. Toughness; the modified initial fracture toughness is used to calculate the modified initial load of the type II delamination crack, and the load and displacement at the start of the modified type II delamination crack are determined by combining the load and its corresponding displacement data; it is also used to repeatedly execute the logic of calculating the fracture toughness based on the modified initial parameters and fitting it piecewise, and modifying the initial fracture toughness based on the toughness ratio, and calculate the relative error of the initial fracture toughness after the two modifications, until the relative error meets the preset error threshold; the result output module is used to output the final type II delamination crack length, the initial fracture toughness of the type II delamination crack, the steady-state fracture toughness of the type II delamination crack, and the equivalent fracture region length.

[0051] This system integrates the entire process of specimen processing, loading and acquisition, parameter calculation, signal processing, iterative correction and result output through modular design. It realizes the collaborative work of universal testing machine and acoustic emission equipment, and automates data acquisition, analysis and parameter calculation. It not only improves the measurement efficiency of parameters related to type II delamination cracks in composite materials, but also reduces human intervention error through standardized processes, ensuring the consistency and accuracy of measurement results, and adapting to the actual application needs of mechanical property testing of composite materials.

[0052] The beneficial effects of this invention are as follows:

[0053] (1) The present invention replaces the optical equipment such as high-speed cameras required by digital image correlation technology (DIC) with acoustic emission equipment. Damage monitoring is achieved through acoustic emission signal acquisition and analysis, without the need to directly observe the crack morphology. At the same time, the crack initiation parameters are defined by peak load and corresponding displacement, avoiding the crack length observation deviation caused by crack tip tilting / bending in DIC technology, as well as the subjective error of manually judging the initial crack through optical images, thus reducing equipment cost and operation complexity.

[0054] (2) Compared with the equivalent crack length method that only relies on macroscopic load-displacement data, this invention classifies acoustic emission signals into three types of damage events: matrix crack, fiber-matrix debonding, and fiber fracture through a clustering algorithm. Then, the cumulative acoustic emission energy slope of each type of damage event is obtained through linear fitting, and the initial-steady-state toughness ratio is calculated. This invention is the first to realize the quantification of the energy dissipation contribution of different damage mechanisms during the delamination process of composite materials, and fundamentally solves the problem of the equivalent crack length method ignoring the damage mechanism, which leads to the deviation in the calculation of initial fracture toughness.

[0055] (3) In view of the lack of correction mechanism in the equivalent crack length method, the present invention uses the initial-steady-state toughness ratio to iteratively correct the initial fracture toughness. By calculating the relative error of the initial fracture toughness after the two corrections, until the preset threshold is met, the subsequent cumulative error caused by the deviation of the initial crack initiation parameter is effectively compensated, and the calculation accuracy of key mechanical parameters such as bending modulus and fracture toughness is significantly improved.

[0056] (4) This invention calculates fracture toughness based on the flexibility-based beam theory and fits the crack propagation law in segments to simultaneously obtain the initial fracture toughness, steady-state fracture toughness and equivalent fracture region length. It fully describes the evolution law of fracture resistance in the whole stage from "initial crack → crack propagation → steady-state propagation", overcomes the shortcomings of existing methods (DIC technology, equivalent crack length method) that cannot cover the full stage characterization, and meets the needs of composite material structure service safety assessment for full-process mechanical data.

[0057] The following describes in detail the method and system for measuring the length and fracture toughness of type II delamination cracks in composite materials according to the present invention, with reference to the embodiments shown in the accompanying drawings and the reference numerals. Attached Figure Description

[0058] Figure 1 This is a flowchart of the steps for measuring the length and fracture toughness of type II delamination cracks in composite materials according to the present invention.

[0059] Figure 2 This is a schematic diagram of the ENF specimen test structure of the present invention;

[0060] Figure 3 This is a load-displacement curve of the ENF specimen type II layered test of the present invention;

[0061] Figure 4 This is a scatter plot of the acoustic emission signal clustering and classification results of this invention;

[0062] Figure 5 This is a graph showing the cumulative acoustic emission energy-time evolution and linear fitting curves for various damage events according to the present invention. Figure 6 This is a graph showing the changes in fracture toughness (R curve) during the iterative correction process of this invention.

[0063] In the figure: 1. Loading roller; 2. Pre-crack; 3. Specimen body; 4. Support roller. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with embodiments of this invention. Obviously, the described embodiments are one embodiment of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0065] like Figure 1As shown, this invention discloses a method for measuring the length and fracture toughness of type II delamination cracks in composite materials, comprising the following steps: S1, preparing an ENF specimen for type II delamination testing, measuring and recording the initial crack length, thickness, width, and span of the ENF specimen; S2, performing a type II delamination test on the ENF specimen on a universal testing machine according to the testing standard, recording the load and its corresponding displacement data during the type II delamination propagation process of the ENF specimen, and simultaneously acquiring acoustic emission signals during the type II delamination propagation process in real time using an acoustic emission device; S3, defining the peak load and its corresponding displacement in the load and displacement data recorded in step S2 as the load and displacement at the initiation of the type II delamination crack, respectively. Based on the initial load and displacement of the type II delamination crack, combined with the initial crack length, thickness, width, and span of the ENF specimen in step S1, the flexural modulus of the ENF specimen is calculated; S4, the load and corresponding displacement data after the initial load and displacement of the type II delamination crack are obtained, and the fracture toughness of the type II delamination crack is calculated using beam theory based on flexibility; the fracture toughness of the type II delamination crack is piecewise fitted according to the crack propagation evolution law to obtain the initial fracture toughness, steady-state fracture toughness, and equivalent fracture region length of the type II delamination crack; S5, the acoustic emission signals collected in step S2 are classified using a clustering algorithm to obtain matrix crack damage events and fiber-matrix debonding losses. Damage events and fiber fracture damage events; the steady-state propagation stage of type II delamination cracks is identified by peak load; the temporal evolution of accumulated acoustic emission energy of matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events during the steady-state propagation stage of type II delamination cracks is analyzed using linear fitting method, and the slope of accumulated acoustic emission energy for each type of damage event is obtained; based on the slope of accumulated acoustic emission energy for each type of damage event, the initial-steady-state toughness ratio of type II delamination cracks is calculated; S6, using the initial-steady-state toughness ratio of type II delamination cracks calculated in step S5, the initial fracture toughness of type II delamination cracks obtained in step S4 is corrected to obtain the first corrected initial fracture toughness of type II delamination cracks. Based on the initial fracture toughness of the first-corrected Type II delamination crack, calculate the initial load of the first-corrected Type II delamination crack. Combine the load and its corresponding displacement data recorded in step S2 to determine the initial load and initial displacement of the first-corrected Type II delamination crack. S7: Based on the initial load and initial displacement of the first-corrected Type II delamination crack and the subsequent load and its corresponding displacement data, repeat the calculation and piecewise fitting logic of step S4 to obtain the initial fracture toughness of the first-corrected Type II delamination crack, the steady-state fracture toughness of the Type II delamination crack, and the equivalent fracture region length. Then repeat the correction logic of step S6 to calculate the initial fracture toughness of the second-corrected Type II delamination crack.S8. Repeat steps S6 to S7 for iterative correction, calculating the relative error of the initial fracture toughness of the type II delamination crack after the two corrections, until the relative error meets the preset error threshold. Output the final type II delamination crack length, initial fracture toughness of the type II delamination crack, steady-state fracture toughness of the type II delamination crack, and equivalent fracture region length.

[0066] Based on the above-disclosed steps, this invention integrates acoustic emission technology with flexibility-based beam theory. Through processes such as preparing standardized specimens, simultaneously acquiring load displacement data and acoustic emission signals, segmented fitting of fracture toughness, quantification of damage mechanism contributions, and iterative correction, it avoids the equipment dependence and human error caused by optical observation of cracks in traditional methods. It can accurately obtain the length of type II delamination cracks, initial fracture toughness, steady-state fracture toughness, and equivalent fracture region length, fully describe the evolution of fracture resistance from delamination to instability, and quantify the contribution of different damage mechanisms to energy dissipation, providing a comprehensive and reliable mechanical basis for the optimized design of composite materials.

[0067] In a preferred embodiment, step S3, the calculation of the flexural modulus of the ENF specimen includes: calculating the delamination crack initiation compliance of the ENF specimen based on the load and displacement at the initiation of the type II delamination crack, using the following formula:

[0068] ;

[0069] Based on the delamination crack initiation compliance of the ENF specimen, the modified delamination crack initiation compliance of the ENF specimen is calculated using the following formula:

[0070] ;

[0071] Based on the modified delamination crack initiation compliance of the ENF specimen, the flexural modulus of the ENF specimen is calculated using the following formula:

[0072] ;

[0073] In the formula, The delamination crack initiation compliance of the ENF specimen; The displacement at the initiation of a type II delamination crack; The load at the initiation of a type II delamination crack; The modified delamination crack initiation compliance for ENF specimens; The span of the ENF specimen; The shear modulus of the ENF specimen; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The flexural modulus of the ENF specimen; The initial crack length of the ENF specimen is given. In this embodiment, by clarifying the calculation logic and formulas for the delamination crack initiation compliance, the corrected delamination crack initiation compliance, and the flexural modulus, and combining the load-displacement data at the initiation of type II delamination cracks and the key parameters of the specimen, the flexural modulus of the ENF specimen can be accurately derived. As the basis for the subsequent calculation of core parameters such as fracture toughness and equivalent crack length, the accuracy of the flexural modulus calculation directly ensures the reliability and accuracy of the subsequent results of the entire measurement method.

[0074] In a preferred embodiment, step S4, the calculation of the fracture toughness of the type II delamination crack, includes: based on the acquired load at the initiation of the type II delamination crack and the load and its corresponding displacement data after the initiation displacement, calculating the real-time compliance during the steady-state propagation process of the type II delamination crack, as shown in the following formula:

[0075] ;

[0076] Based on the real-time compliance during the Type II delamination steady-state propagation process, and combined with the modified delamination crack initiation compliance calculation of the ENF specimen in step S3, the equivalent delamination crack length during the Type II delamination steady-state propagation process is calculated using the following formula:

[0077] ;

[0078] Based on the equivalent delamination crack length during the type II delamination steady-state propagation process, the type II delamination fracture toughness is calculated using the following formula:

[0079] ;

[0080] In the formula, For the real-time compliance during the type II hierarchical steady-state expansion process; This represents the real-time displacement during the type II layered steady-state expansion process. For the real-time load during the type II layered steady-state expansion process; The span of the ENF specimen; The shear modulus of the ENF specimen; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The equivalent delamination crack length during the type II delamination steady-state propagation process; The modified delamination crack initiation compliance for ENF specimens; The initial crack length of the ENF specimen; It exhibits type II delamination crack fracture toughness; The bending modulus of the ENF specimen is given. In this embodiment, based on real-time load-displacement data during the steady-state propagation process of type II delamination, the fracture toughness can be accurately calculated without directly observing the crack morphology by using explicit formulas for real-time compliance, equivalent delamination crack length, and fracture toughness calculation. This effectively avoids observation errors caused by crack front tilting and bending, and significantly improves the accuracy and practicality of type II delamination crack fracture toughness calculation.

[0081] In a preferred embodiment, the formula for calculating the initial-steady-state toughness ratio of the type II delamination crack in step S5 is as follows:

[0082] ;

[0083] In the formula, The initial-steady-state toughness ratio is that of a type II delamination crack. The slope of the cumulative acoustic emission energy of the matrix crack damage event; The slope of the cumulative acoustic emission energy of the fiber-matrix debonding damage event; ( =1,2,3) represent the cumulative acoustic emission energy slopes of matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events, respectively. In this embodiment, by integrating the cumulative acoustic emission energy slopes of the three types of damage events—matrix crack, fiber-matrix debonding, and fiber fracture—a scientific method for calculating the initial-steady-state toughness ratio is constructed. This not only provides a reliable quantitative basis for subsequent correction of initial fracture toughness but also enables accurate assessment of the contribution weight of different damage mechanisms in the type II delamination process, enriching the mechanical analysis dimensions of composite material delamination damage.

[0084] In a preferred embodiment, in step S6, the modified formula for the initial fracture toughness of the type II delamination crack is as follows:

[0085] ;

[0086] The initial load of the modified type II delamination crack is calculated based on the modified initial fracture toughness, using the following formula:

[0087] ;

[0088] In the formula, For the first The initial fracture toughness of the second-corrected type II delamination crack; The initial-steady-state toughness ratio is that of a type II delamination crack. For the first The corrected steady-state fracture toughness of type II delamination crack; For the first The revised initial load for type II delamination crack; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The initial crack length of the ENF specimen; The bending modulus of the ENF specimen is given. In this embodiment, by using a clear initial fracture toughness correction formula and a modified initial load derivation formula, the initial fracture toughness is directionally corrected using the initial-steady-state toughness ratio. At the same time, the modified initial load is accurately calculated, which can effectively correct the deviation of the initial fracture toughness. This provides key support for determining accurate crack initiation parameters and further improves the accuracy and consistency of the results of the entire measurement system.

[0089] In a preferred embodiment, the calculation formula for piecewise fitting in steps S4 and S7 is as follows:

[0090] ;

[0091] In the formula, The equivalent crack length is Type II delamination crack fracture toughness at that time; These are the initial constants for piecewise fitting; For the first The corrected steady-state fracture toughness of type II delamination crack; For the first The initial fracture toughness of the second-corrected type II delamination crack; The equivalent delamination crack length during the type II delamination steady-state propagation process; The initial crack length of the ENF specimen; For the first The corrected equivalent fracture region length. In this embodiment, the piecewise fitting formula accurately describes the correspondence between the equivalent crack length and fracture toughness in different intervals, clearly distinguishing the fracture toughness variation law between the crack initiation stage and the steady-state propagation stage. Thus, the initial fracture toughness, steady-state fracture toughness, and equivalent fracture region length are accurately extracted, fully presenting the fracture resistance evolution process of type II delamination cracks, and providing detailed data support for in-depth analysis of the delamination characteristics of composite materials.

[0092] In a preferred embodiment, in step S8, the formula for calculating the relative error is as follows:

[0093] ;

[0094] In the formula, The relative error in the initial fracture toughness of the type II delamination crack after the two corrections; For the first The initial fracture toughness of the second-corrected type II delamination crack; For the first The initial fracture toughness of the type II delamination crack after correction. In this embodiment, the deviation of the initial fracture toughness after the two corrections can be quantified by a clear relative error calculation formula. This provides a clear and quantifiable criterion for terminating the iterative correction, ensuring that the final output of the initial fracture toughness and related parameters meets the preset accuracy requirements. It avoids result deviations caused by excessive or insufficient iteration, thus ensuring the reliability of the measurement results.

[0095] In a preferred embodiment, step S6, determining the initial load and displacement of the first corrected Type II delamination crack, includes the following steps: constructing a continuous load-displacement curve based on the load and corresponding displacement data recorded in step S2 during the Type II delamination propagation process of the ENF specimen; using the initial load of the first corrected Type II delamination crack calculated in step S6 as the initial load of the first corrected Type II delamination crack; and obtaining the displacement corresponding to the initial load of the first corrected Type II delamination crack on the load-displacement curve as the initial displacement of the first corrected Type II delamination crack. In this embodiment, based on the continuous load-displacement curve recorded in the experiment, combined with the corrected initial load, the corresponding crack initiation displacement is accurately located, avoiding the subjective error of traditional manual determination of the crack initiation point. This allows for accurate and objective corrected crack initiation parameters, laying a solid foundation for subsequent iterative corrections and precise calculation of various mechanical parameters.

[0096] In this embodiment of the invention, the shear modulus of the ENF specimen... The methods for obtaining the shear modulus include: measuring it through interlaminar shear modulus tests or in-plane shear tests on the corresponding composite material of the ENF specimen, or using the interlaminar shear modulus parameters disclosed in the material handbook of the composite material. In this embodiment, the shear modulus is specified. Multiple reliable acquisition methods are available, supporting both experimental measurement to obtain accurate parameters of the suitable specimens and the use of publicly available data from material handbooks. This ensures the accuracy and flexibility of the calculation of the compliance of the modified delamination crack initiation, and provides a key guarantee for the accurate derivation of subsequent core parameters such as bending modulus and fracture toughness.

[0097] The testing method of this invention is based on acoustic emission-assisted technology and flexibility beam theory, and is used to calculate the length of type II delamination cracks and interlaminar fracture toughness of composite materials. Compared with the prior art, it has the following advantages: First, determining the crack length does not require visual observation of the crack, which greatly reduces the reliance on high-precision optical equipment; second, by combining acoustic emission monitoring signals with delamination damage quantification, the acoustic emission (AE) energy release rate is dynamically tracked, which can completely describe the evolution of fracture resistance from the initiation, propagation to instability of type II delamination; third, the contribution of different damage mechanisms to the total energy dissipation can be quantified, providing a more comprehensive mechanical basis for the optimized design of composite materials.

[0098] The following detailed description of the testing process and verification results of the present invention is provided through specific embodiments:

[0099] Example 1

[0100] In this embodiment, an ENF test specimen was prepared using a carbon fiber / epoxy resin matrix composite orthogonal laminate. The length of its type II delamination crack and its fracture toughness were measured using the method of this invention. The specific steps are as follows:

[0101] Step 1: Design and fabricate ENF specimens for type II delamination testing of composite materials, with the following layup method: The specimen structure is as follows Figure 2 As shown. Figure 2 In the diagram, 1 represents the loading roller, used to apply a vertically downward bending load to the specimen body; 2 represents the pre-existing crack (the actual crack processed during specimen preparation), indicating the initial crack length of the ENF specimen. Defined as the distance from the center of loading roller 1 to the tip of the pre-existing crack 2; 3 is the main body of the specimen, made of orthogonal laminate of carbon fiber / epoxy resin matrix composite material; 4 is the support roller, symmetrically arranged on both sides of the loading roller, used to support the specimen and cooperate to form a bending loading condition. Measure and record key parameters: the initial crack length of the ENF specimen is... =30mm, specimen thickness 3.6mm (i.e., half thickness) 1.8mm), specimen width 25.72mm; span (The modified compliance formula has been substituted into the calculation according to the test standard settings).

[0102] Step 2: According to the Type II delamination test standard, perform a Type II delamination test on the ENF specimen on a universal testing machine with a loading rate of 1 mm / min, and record the load during the delamination propagation process of the ENF specimen. Displacement Data, construct load-displacement curves (such as...) Figure 3 (As shown in the figure); simultaneously, acoustic emission signals during the layered expansion process are collected in real time using acoustic emission equipment for subsequent damage mechanism classification.

[0103] Step 3: Extract the peak load and corresponding displacement from the load-displacement curve, and define them as the layering initiation parameters: initial load. 984.942 N, initial displacement 2.469, Calculate the flexural modulus based on the key parameters of the ENF specimen. :

[0104] (1)

[0105] The flexural modulus was calculated. =67591.720MPa.

[0106] in, To correct the delamination crack initiation compliance, =0.002446mm / N.

[0107] Step 4: Calculate the type II delamination fracture toughness using beam theory based on flexibility. :

[0108] (2)

[0109] The initial fracture toughness was obtained by fitting. =1126.038J / m 2 Steady-state fracture toughness =1234.817J / m 2 Equivalent fracture region length =3.910mm.

[0110] Step 5: Use a clustering algorithm to classify the acoustic emission signals collected from the Type II layered ENF specimen, resulting in three types of damage events: matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events. Figure 4 As shown.

[0111] Existing research indicates that crack propagation enters a steady state when the load reaches its peak. During the steady-state stage of crack propagation, the accumulated acoustic emission energy exhibits a linear increasing trend. Linear fitting methods are used to analyze the temporal evolution of accumulated acoustic emission energy for various damage events during the steady-state crack propagation stage, yielding the slope of accumulated acoustic emission energy for each damage event. like Figure 5 As shown. Figure 5 The coordinate axis represents the cumulative acoustic emission energy (unit: mV). The x-axis represents the loading time (in seconds); where line a corresponds to the cumulative acoustic emission energy-time evolution curve and linear fitting result of the matrix crack damage event, and the fitting equation is y=254.629x-75105.717 (y is the cumulative acoustic emission energy, in mV). (ms; x is the loading time, in seconds). After adjustment, the coefficient of determination Adj R-Square = 0.988, and its fitting slope is... =254.629mv ms / s; line b corresponds to the cumulative acoustic emission energy-time evolution curve and linear fitting results of the fiber-matrix debonding damage event. The fitting equation is y = 2.692x - 708.9, and the adjusted coefficient of determination Adj R-Square = 0.969. The fitting slope is... =2.692mv ms / s; Line c corresponds to the cumulative acoustic emission energy-time evolution curve and linear fitting results of the fiber fracture damage event. The fitting equation is y=76.344x-22001.398, and the adjusted coefficient of determination Adj R-Square=0.988. The fitting slope is... =76.344mv ms / s. These slopes are used to quantitatively assess the contribution weights of different damage mechanisms to steady-state fracture toughness. The initial-steady-state toughness ratio is calculated. :

[0112] (3)

[0113] in =254.629mv ms / s, =2.692mv ms / s, =76.344mv ms / s, =0.771.

[0114] Step 6: Utilize the calculated initial-steady-state toughness ratio The initial fracture toughness is corrected to obtain the first corrected initial fracture toughness. :

[0115] (4)

[0116] The initial fracture toughness after the first correction was calculated. 928.924 J / m 2 .

[0117] Calculate the modified initial load using the modified initial fracture toughness. :

[0118] (5)

[0119] The initial load of the first correction was calculated. 907.012N.

[0120] Step 7: Calculate the corrected initial crack load. The corrected crack initiation point was determined using the load-displacement curves recorded in the experiments, and the interlaminar fracture toughness was recalculated. Use the following formula for fitting:

[0121] (6)

[0122] The corrected steady-state fracture toughness was obtained by fitting. J / m 2 Equivalent fracture region length =2.031mm.

[0123] Step 8: Utilize the calculated initial-steady-state toughness ratio The initial fracture toughness is iteratively corrected to obtain the second corrected initial fracture toughness. =959.642J / m 2 The relative error is calculated based on the initial fracture toughness obtained from the two corrections. If the relative error is less than a preset threshold (usually set to ≤1%), the iteration terminates; otherwise, step six is ​​repeated for the next correction. The formula for calculating the relative error is as follows:

[0124] (7)

[0125] The relative error after the first iteration was calculated. 3.969% (greater than the threshold, further iterations are needed). After three rounds of iteration (refer to the steps above for the iteration process), the final interlaminar performance parameters are obtained (as shown in Table 1). The fracture toughness evolution curve (R curve) corresponding to the iteration process is as follows: Figure 6 As shown. Figure 6 The ordinate represents the type II delamination fracture toughness. (Unit: J / m) 2The horizontal axis represents the layer length a (unit: mm); where: line d corresponds to the initial fracture toughness evolution curve before iteration, which shows a rapid increase in fracture toughness in the early stage, reflecting the preliminary calculation results of fracture toughness under the initial parameters; line e corresponds to the fracture toughness evolution curve after the first iteration; line f corresponds to the fracture toughness evolution curve after the second iteration; line g corresponds to the fracture toughness evolution curve after the third iteration; as can be seen from the curve trend, the fluctuation of the fracture toughness curve gradually decreases as the number of iterations increases; lines f and g almost overlap, indicating that the fracture toughness curve has become stable and the iteration results meet the preset relative error threshold.

[0126] After the final iteration converges, the output interlayer performance parameter is: equivalent fracture region length (final crack length). =2.231mm, initial fracture toughness =969.985J / m 2 Steady-state fracture toughness J / m 2 .

[0127] Table 1 shows the calculation results of type II delamination crack length and fracture toughness of composite materials based on acoustic emission assistance proposed in this invention. From the perspective of error, the calculation formulas for delamination crack length and interlaminar fracture toughness provided by this invention show excellent consistency with traditional methods.

[0128] Table 1 Crack length and interlaminar fracture toughness determined by conventional methods and the method of this invention.

[0129]

[0130] This invention also discloses a system for implementing the method for measuring the length and fracture toughness of type II delamination cracks in composite materials disclosed herein, comprising: a specimen processing and parameter module for preparing ENF specimens for type II delamination testing and measuring and recording the initial crack length, thickness, width, and span of the ENF specimens; and a loading and synchronous acquisition module, including a universal testing machine and an acoustic emission device: the universal testing machine is used to perform type II delamination testing on the ENF specimens according to the type II delamination test standard and record the load and its corresponding displacement data during the delamination propagation process; the acoustic emission device is used to work synchronously with the universal testing machine to acquire real-time data on the delamination propagation of the ENF specimens. The process includes acoustic emission signals; a fracture toughness calculation module, used to extract peak load and corresponding displacement from load and corresponding displacement data, which are used as the initial load and initial displacement of the type II delamination crack, respectively; and to calculate the bending modulus of the ENF specimen based on the initial load, initial displacement, and the initial crack length, thickness, width, and span of the ENF specimen; it is also used to obtain the load and corresponding displacement data after the initial load and initial displacement, calculate the fracture toughness of the type II delamination crack using beam theory based on flexibility, and perform piecewise fitting of the fracture toughness according to the crack propagation evolution law to obtain the initial fracture toughness and steady-state fracture toughness of the type II delamination crack. The system includes a component for determining the length of the equivalent fracture region; an acoustic emission signal processing and toughness ratio calculation module, used to classify acoustic emission signals using a clustering algorithm to obtain matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events; and a module for identifying the steady-state propagation stage of type II delamination cracks using peak load, analyzing the temporal evolution of accumulated acoustic emission energy for various damage events in this stage using a linear fitting method, obtaining the slope of accumulated acoustic emission energy for various damage events, and then calculating the initial-steady-state toughness ratio of the type II delamination crack; and an iterative correction module, used to correct the initial fracture toughness using the initial-steady-state toughness ratio to obtain the corrected initial fracture toughness. Toughness; the modified initial fracture toughness is used to calculate the modified initial load of the type II delamination crack, and the load and displacement at the start of the modified type II delamination crack are determined by combining the load and its corresponding displacement data; it is also used to repeatedly execute the logic of calculating the fracture toughness based on the modified initial parameters and fitting it piecewise, and modifying the initial fracture toughness based on the toughness ratio, and calculate the relative error of the initial fracture toughness after the two modifications, until the relative error meets the preset error threshold; the result output module is used to output the final type II delamination crack length, the initial fracture toughness of the type II delamination crack, the steady-state fracture toughness of the type II delamination crack, and the equivalent fracture region length.

[0131] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0132] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment includes only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for measuring the length and fracture toughness of type II delamination cracks in composite materials, characterized in that, Includes the following steps: S1, Prepare an ENF specimen for type II delamination test, and measure and record the initial crack length, thickness, width and span of the ENF specimen; S2, according to the test standard, the ENF specimen is subjected to a type II delamination test on a universal testing machine, and the load and its corresponding displacement data during the type II delamination expansion process of the ENF specimen are recorded. At the same time, acoustic emission signals during the type II delamination expansion process are collected in real time using an acoustic emission device. S3, define the peak load and its corresponding displacement in the load and displacement data recorded in step S2 as the load and displacement at the initiation of type II delamination crack, respectively; based on the load and displacement at the initiation of type II delamination crack, and combined with the initial crack length, thickness, width and span of the ENF specimen in step S1, calculate the bending modulus of the ENF specimen. S4. Obtain the load and displacement data after the initial displacement of the type II delamination crack, and calculate the fracture toughness of the type II delamination crack using the beam theory based on flexibility. Perform segmented fitting on the fracture toughness of the type II delamination crack according to the crack propagation evolution law to obtain the initial fracture toughness, steady-state fracture toughness and equivalent fracture region length of the type II delamination crack. S5, a clustering algorithm is used to classify the acoustic emission signals collected in step S2 to obtain matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events; the steady-state propagation stage of the type II delamination crack is identified by the peak load; the temporal evolution law of the cumulative acoustic emission energy of the matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events in the steady-state propagation stage of the type II delamination crack is analyzed by a linear fitting method to obtain the cumulative acoustic emission energy slope of each type of damage event; based on the cumulative acoustic emission energy slope of each type of damage event, the initial-steady-state toughness ratio of the type II delamination crack is calculated. S6. Using the initial-steady-state toughness ratio of the type II delaminated crack calculated in step S5, the initial fracture toughness of the type II delaminated crack obtained in step S4 is corrected to obtain the first corrected initial fracture toughness of the type II delaminated crack. Based on the initial fracture toughness of the first corrected Type II delamination crack, calculate the initial load of the first corrected Type II delamination crack. Combine the load and its corresponding displacement data recorded in step S2 to determine the initial load and initial displacement of the first corrected Type II delamination crack. S7. Based on the load at the initiation of the Type II delamination crack and the load and corresponding displacement data after the initiation displacement after the first correction, repeat the calculation and piecewise fitting logic of step S4 to obtain the initial fracture toughness, steady-state fracture toughness and equivalent fracture region length of the Type II delamination crack after the first correction; then repeat the correction logic of step S6 to calculate the initial fracture toughness of the Type II delamination crack after the second correction. S8. Repeat steps S6 to S7 for iterative correction, calculate the relative error of the initial fracture toughness of the type II delamination crack after the two corrections, until the relative error meets the preset error threshold, and output the final type II delamination crack length, initial fracture toughness of the type II delamination crack, steady-state fracture toughness of the type II delamination crack, and equivalent fracture region length.

2. The method according to claim 1, characterized in that, In step S3, the calculation of the bending modulus of the ENF specimen includes: Based on the load and displacement at the initiation of the type II delamination crack, the delamination crack initiation compliance of the ENF specimen is calculated using the following formula: ； Based on the delamination crack initiation compliance of the ENF specimen, the modified delamination crack initiation compliance of the ENF specimen is calculated using the following formula: ； Based on the modified delamination crack initiation compliance of the ENF specimen, the flexural modulus of the ENF specimen is calculated using the following formula: ； In the formula, The delamination crack initiation compliance of the ENF specimen; The displacement at the initiation of a type II delamination crack; The load at the initiation of a type II delamination crack; The modified delamination crack initiation compliance for ENF specimens; The span of the ENF specimen; The shear modulus of the ENF specimen; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The flexural modulus of the ENF specimen; The initial crack length of the ENF specimen.

3. The method according to claim 2, characterized in that, In step S4, the calculation of the fracture toughness of the type II delamination crack includes: Based on the acquired load and displacement data at the initiation of the type II delamination crack, and the subsequent load and corresponding displacement data, the real-time compliance during the steady-state propagation process of the type II delamination crack is calculated using the following formula: ； Based on the real-time compliance during the Type II delamination steady-state propagation process, and combined with the corrected delamination crack initiation compliance calculation of the ENF specimen in step S3, the equivalent delamination crack length during the Type II delamination steady-state propagation process is calculated using the following formula: ； Based on the equivalent delamination crack length during the type II delamination steady-state propagation process, the type II delamination fracture toughness is calculated using the following formula: ； In the formula, For the real-time compliance during the type II hierarchical steady-state expansion process; This represents the real-time displacement during the type II layered steady-state expansion process. For the real-time load during the type II layered steady-state expansion process; The span of the ENF specimen; The shear modulus of the ENF specimen; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The equivalent delamination crack length during the type II delamination steady-state propagation process; The modified delamination crack initiation compliance for ENF specimens; The initial crack length of the ENF specimen; It exhibits type II delamination crack fracture toughness; The bending modulus of the ENF specimen.

4. The method according to claim 3, characterized in that, In step S5, the formula for calculating the initial-steady-state toughness ratio of the type II delamination crack is as follows: ； In the formula, The initial-steady-state toughness ratio is that of a type II delamination crack. The slope of the cumulative acoustic emission energy of the matrix crack damage event; The slope of the cumulative acoustic emission energy of the fiber-matrix debonding damage event; ( =1,2,3) represent the cumulative acoustic emission energy slopes for matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events, respectively.

5. The method according to claim 4, characterized in that, In step S6, the corrected formula for the initial fracture toughness of the type II delamination crack is as follows: ； The initial load of the modified Type II delamination crack is calculated based on the modified initial fracture toughness, using the following formula: ； In the formula, For the first The initial fracture toughness of the second-corrected type II delamination crack; The initial-steady-state toughness ratio is that of a type II delamination crack. For the first The corrected steady-state fracture toughness of type II delamination crack; For the first The revised initial load for type II delamination crack; The width of the ENF specimen; The thickness is half the thickness of the ENF specimen; The initial crack length of the ENF specimen; The bending modulus of the ENF specimen.

6. The method according to claim 1, characterized in that, In steps S4 and S7, the calculation formula for the piecewise fitting is as follows: ； In the formula, The equivalent crack length is Type II delamination crack fracture toughness at that time; These are the initial constants for piecewise fitting; For the first The corrected steady-state fracture toughness of type II delamination crack; For the first The initial fracture toughness of the second-corrected type II delamination crack; The equivalent delamination crack length during the type II delamination steady-state propagation process; The initial crack length of the ENF specimen; For the first The length of the equivalent fracture region after the second correction.

7. The method according to claim 1, characterized in that, In step S8, the formula for calculating the relative error is as follows: ； In the formula, The relative error in the initial fracture toughness of the type II delamination crack after the two corrections; For the first The initial fracture toughness of the second-corrected type II delamination crack; For the first The initial fracture toughness of the second-corrected type II delamination crack.

8. The method according to claim 1, characterized in that, In step S6, determining the initial load and initial displacement of the type II delamination crack after the first correction includes the following steps: Based on the load and corresponding displacement data of the ENF specimen type II layered expansion process recorded in step S2, a continuous load-displacement curve is constructed. The first corrected initial load of the type II delamination crack calculated in step S6 is used as the first corrected load at the initiation of the type II delamination crack. On the load-displacement curve, the displacement corresponding to the load at the initiation of the first corrected type II delamination crack is obtained, and it is taken as the displacement at the initiation of the first corrected type II delamination crack.

9. The method according to claim 2, characterized in that, The shear modulus of the ENF specimen The methods for obtaining the parameters include: measuring the interlaminar shear modulus or in-plane shear test of the composite material corresponding to the ENF specimen, or using the interlaminar shear modulus parameters disclosed in the material handbook of the composite material.

10. A system for implementing the method for measuring the length and fracture toughness of type II delamination cracks in composite materials according to any one of claims 1-9, characterized in that, include: The specimen processing and parameter module is used to prepare ENF specimens for type II delamination testing and to measure and record the initial crack length, thickness, width and span of the ENF specimens. The loading and synchronous acquisition module includes a universal testing machine and an acoustic emission device: the universal testing machine is used to perform a Type II delamination test on the ENF specimen according to the Type II delamination test standard, and record the load and its corresponding displacement data during the delamination expansion process; the acoustic emission device is used to work synchronously with the universal testing machine to acquire the acoustic emission signals of the ENF specimen during the delamination expansion process in real time. The fracture toughness calculation module is used to extract the peak load and corresponding displacement from the load and its corresponding displacement data, which are respectively used as the load and displacement at the initiation of the type II delamination crack; and to calculate the bending modulus of the ENF specimen based on the initiation load, initiation displacement, and the initial crack length, thickness, width, and span of the ENF specimen; it is also used to obtain the load and corresponding displacement data after the initiation load and initiation displacement, calculate the fracture toughness of the type II delamination crack using the beam theory based on flexibility, and perform piecewise fitting of the fracture toughness according to the crack propagation evolution law to obtain the initial fracture toughness, steady-state fracture toughness, and equivalent fracture region length of the type II delamination crack; The acoustic emission signal processing and toughness ratio calculation module is used to classify the acoustic emission signals using a clustering algorithm to obtain matrix crack damage events, fiber-matrix debonding damage events, and fiber fracture damage events; and to identify the steady-state propagation stage of type II delamination cracks through the peak load, and to analyze the time evolution law of the cumulative acoustic emission energy of various damage events in this stage using a linear fitting method, to obtain the slope of the cumulative acoustic emission energy of various damage events, and then to calculate the initial-steady-state toughness ratio of type II delamination cracks. An iterative correction module is used to correct the initial fracture toughness using the initial-steady-state toughness ratio to obtain the corrected initial fracture toughness. The initial load of the modified type II delamination crack is calculated based on the modified initial fracture toughness, and the load and displacement at the initiation time of the modified type II delamination crack are determined by combining the load and its corresponding displacement data. It is also used to repeatedly execute the logic of calculating fracture toughness based on the modified initial parameters and fitting it in segments, and modifying the initial fracture toughness based on the toughness ratio, and to calculate the relative error of the initial fracture toughness after the two modifications, until the relative error meets the preset error threshold. The results output module is used to output the final type II delamination crack length, initial fracture toughness of type II delamination crack, steady-state fracture toughness of type II delamination crack, and equivalent fracture region length.

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