A damage tolerance assessment method for composite L-shaped plates with delamination defects

Abstract

The present invention discloses a damage tolerance assessment method for composite L-shaped plates containing delamination defects, which belongs to the technical field of comprehensive evaluation systems for correcting numerical models with experimental data, and includes: preparing defect-free L-shaped laminate specimens, performing loading tests on the specimens, and recording experimental data; constructing a numerical model of an L-shaped laminate containing cohesive units, correcting the numerical model of the L-shaped laminate, and obtaining a critical failure position based on the stress cloud map and the crack initiation position; designing the size and position of prefabricated delamination defects based on the critical failure position; optimizing the numerical model of the L-shaped laminate to obtain an interlaminar damage model of the L-shaped laminate that is highly sensitive to the defect position; and analyzing the influence of the delamination defect position on the bearing capacity and failure model of the laminate. The present invention adopts the above-mentioned damage tolerance assessment method for composite L-shaped plates containing delamination defects, which is low in cost, does not require the preparation of a large number of defective specimens, and has high loading test efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of comprehensive evaluation systems for correcting numerical models using experimental data, and in particular to a damage tolerance evaluation method for an L-shaped composite material plate containing delamination defects. Background Art

[0002] Advanced fiber-reinforced resin-based composite materials are widely used in the aviation field due to their light weight and high strength. Their use in civil aircraft has become an important indicator of advancement. As typical components of civil aircraft, L-shaped composite components are widely present in wing spars, ribs and long stringer structures, and are one of the most common connectors in composite aircraft. When subjected to bending loads, the bearing capacity of arc-shaped or large curvature sections (often called R zones), especially R zones containing defects, is an important basis for determining the structural design value of L-shaped composite components. In the preparation process of L-shaped composite components, defects such as delamination, wrinkles, resin enrichment, uneven thickness, fiber breakage and matrix cracking will inevitably appear in the R zone. These defects will reduce the bearing capacity of the composite component to a certain extent. The damage tolerance design criteria for aviation composite materials stipulate that defects and damage are allowed in composite structures, but under the action of design loads, defects or damage cannot expand.

[0003] While numerous experimental and simulation studies have investigated the failure modes and load-bearing capacity of L-shaped composite components, relatively few studies have examined the residual load-bearing capacity, failure modes, and mechanisms of defective L-shaped composite components. These reports primarily focus on three main types of defects: interlaminar delamination, wrinkling, and resin accumulation.

[0004] Wimmer was the first to use a research method that combines simulation and experiment to study the effect of delamination defect size on delamination growth. In terms of simulation, virtual crack closure technology (VCCT) was combined with implicit finite element analysis to simulate the growth process of delamination damage. The simulation results showed that for delamination defects as small as 1mm, unstable delamination growth occurred, and when the delamination expanded to a certain size, it turned into stable growth; for delamination defects of 3mm, the delamination growth under displacement load control was stable, while the delamination under force load control was unstable growth; and for delamination defects of 5mm, the delamination growth was always stable. In terms of experiment, thin film Teflon strips were buried in pre-designed lamination positions to form delamination defects. The experimental results showed the same pattern as the simulation.

[0005] after The work of

[15] introduced the cohesive element method proposed by Dugdale into the simulation of L-shaped laminates for the first time, and used it to simulate the interlaminar delamination occurring in the arc region of the L-shaped laminates. Since then, the method of arranging cohesive elements between layers has been widely adopted to establish the interlaminar damage model of L-shaped laminates. This work is the first to dynamically analyze delamination growth in L-shaped laminates. The results show that under lateral bending loading, the right crack tip propagates under shear, while the left crack tip propagates under opening. The shear-dominated crack on one side grows an order of magnitude faster than the opening-dominated crack on the other side, demonstrating good agreement between the experimental and simulation results.

[0006] Woo innovated his modeling approach by incorporating the geometric characteristics of the specimen's defects into the finite element mesh. The finite element model predicted delamination behavior. The predicted load-displacement curves better matched the experimental results than models that ignored the defect geometry, validating the accuracy of the proposed numerical modeling. The results showed that in the absence of voids or when the central defect size was very small, the center of the 5 / 6 interface was the primary location for delamination development. As the defect size increased, the centers of the 6 / 7 and 7 / 8 interfaces became the primary locations for defect development. As the defect location moved toward the center of the R zone, the maximum load decreased. Conversely, for delamination defects at the same location, the larger the size, the smaller the maximum load.

[0007] Qian used manual laying and laid a 2mm wide and 0.16mm thick polyvinyl chloride film at the center of the corner. He designed three defect configurations: layered, stacked, and layered and stacked to simulate the defects caused by the lateral deviation of the fiber bundle during the automatic fiber placement (AFP) process. The results showed that the layered defect would significantly reduce the in-plane tensile strength ILTS (-31.2%).

[0008] Currently, most studies on delamination defects in the R zone of L-shaped laminates use a combination of experiments and calculations. The conclusions show that the presence of delamination defects significantly reduces the interlaminar strength of L-shaped laminates and is sensitive to the location of the defect. These conclusions are one-sided and focus on the impact of a defect at a specific point in the R zone on the L-shaped laminate. In reality, delamination defects can occur at any location in the R zone. To systematically study the impact of delamination defects at all possible locations on the load-bearing capacity of L-shaped composite laminates, a large number of defective specimens must be prepared and loaded for testing, which undoubtedly consumes a lot of manpower and financial resources. Summary of the Invention

[0009] The purpose of the present invention is to provide a damage tolerance assessment method for an L-shaped composite plate containing a delamination defect. The method has low cost, does not require the preparation of a large number of defective specimens, has high loading test efficiency, and can comprehensively study the load-bearing capacity of the L-shaped laminate composite material when the delamination defect is located at a certain position.

[0010] To achieve the above object, the present invention provides a method for evaluating damage tolerance of a composite L-shaped plate containing delamination defects, comprising the following steps:

[0011] S1. Prepare defect-free L-shaped laminate specimens, perform loading tests on the specimens, and record stress distribution cloud maps, crack initiation locations at critical moments, and displacement-load curves.

[0012] S2. Construct a numerical model of an L-type laminate containing cohesive elements, and use the data obtained in S1 to modify the numerical model of the L-type laminate. The critical failure position is obtained based on the stress cloud step and the crack initiation position.

[0013] S3. Design the size and location of prefabricated delamination defects based on the critical failure location;

[0014] S4. Based on the experimental results in S1, the numerical model of the L-type laminate is further optimized to obtain an L-type laminate interlaminar damage model with high sensitivity to defect locations;

[0015] S5. Analyze the influence of delamination defect location on the load-bearing capacity and failure model of the laminate.

[0016] Preferably, the specific operation of S1 is: first prepare an L-shaped composite laminate by manual paving and autoclave molding, design and prefabricate three major types of delamination defects according to the stress distribution characteristics of the defect-free L-shaped composite laminate model: ply position, circumferential position, and defect size, then perform four-point bending loading on the specimen, use a high-speed camera to monitor the initiation and expansion of delamination damage on the specimen measuring surface, and record the load-displacement curve and stress distribution cloud map.

[0017] Preferably, the sample size in S1 is: the inner radius of the R zone is 6.4 mm, the right-angle loading arm is 90 mm, and the width is 25 mm.

[0018] Preferably, the numerical model of the L-type laminate in S2 is a finite element model with an inner radius of 6.4 mm, an outer radius of 10.72 mm, a thickness of 4.32 mm, a total of 24 layers, and a thickness of 0.18 mm per layer. In terms of comprehensive calculation accuracy and efficiency, the numerical model of the L-type laminate has a total of 177,968 nodes and 87,734 units, including 79,776 C3D8R units and 7,958 COH3D8 units. The grid is encrypted in the R zone, the loading arm transition zone, and the loading contact zone. For the boundary conditions in the simulation experiment, an analytical rigid cylinder is used. The loading rod is simulated, and frictionless surface-to-surface contact conditions are used to simulate the contact between the fixture and the specimen, as well as the interaction between the interfaces after failure. The two rigid cylinders on the lower side are completely fixed, and the two rigid cylinders on the upper side are bound together with another rigid cylinder directly above the corner through MPC multi-point constraints. A downward displacement load of 10 mm is applied, and the laminate is constrained symmetrically about the plane perpendicular to the X-axis to completely limit its displacement in the Z and X directions. The laminate with equivalent prepreg parameters is used as the model material parameters to construct a numerical model of the L-shaped laminate.

[0019] Preferably, in the experiment of S3, a method of embedding a polytetrafluoroethylene film is used to prefabricate the delamination defect, and a method of deleting the cohesive force is used in the simulation model.

[0020] Preferably, in S5, the influence of the delamination defect position on the bearing capacity and failure mode of the L-shaped laminate is analyzed by arranging delamination defect envelope R zones that are evenly distributed and of the same size.

[0021] Therefore, the present invention adopts the above-mentioned damage tolerance assessment method for L-shaped composite plates containing delamination defects, which has low cost, does not require the preparation of a large number of defective specimens, has high loading test efficiency, and can comprehensively study the load-bearing capacity of L-shaped laminate composite materials when the delamination defect is located at a certain position.

[0022] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the sample size of an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing delamination defects according to the present invention;

[0024] Figure 2 This is a schematic diagram of prefabricated defect paving of an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing delamination defects according to the present invention;

[0025] Figure 3 This is a curing process curve diagram of an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing delamination defects according to the present invention;

[0026] Figure 4 This is a diagram of an L-shaped laminate specimen according to an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing delamination defects of the present invention;

[0027] Figure 5 This is an assembly diagram of a numerical model of an L-shaped laminated plate according to an embodiment of a damage tolerance assessment method for an L-shaped composite plate containing delamination defects of the present invention;

[0028] Figure 6 This is a mesh division diagram of a numerical model of an L-shaped laminated plate according to an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing delamination defects of the present invention;

[0029] Figure 7 It is a cohesive force unit position diagram of an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing delamination defects according to the present invention;

[0030] Figure 8 A triangle traction separation law TSL diagram of an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing a delamination defect according to the present invention;

[0031] Figure 9 This is a cloud diagram of radial stress distribution of a defect-free specimen of an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing delamination defects according to the present invention;

[0032] Figure 10 This is a representative defect point map of an embodiment of a damage tolerance assessment method for a composite L-shaped plate containing delamination defects according to the present invention;

[0033] Figure 11 It is a defect lattice envelope diagram of an embodiment of a damage tolerance assessment method for a composite L-shaped plate containing delamination defects according to the present invention;

[0034] Figure 12 This is a force-displacement curve comparison diagram of an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing delamination defects according to the present invention;

[0035] Figure 13 This is a comparison diagram of a force-displacement curve of a prefabricated delamination specimen and a simulation result of a delamination defect in an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing a delamination defect of the present invention;

[0036] Figure 14 It is a defect position-limit load surface diagram of an embodiment of a damage tolerance assessment method for an L-shaped composite material plate containing a delamination defect of the present invention. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0038] Unless otherwise defined, technical or scientific terms used in the present invention shall have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs.

[0039] The present invention provides a damage tolerance assessment method for a composite L-shaped plate containing delamination defects, comprising the following steps:

[0040] S1. Prepare defect-free L-shaped laminate specimens, conduct loading tests on the specimens, and record stress distribution cloud maps, crack initiation positions at critical moments, and displacement load curves. The specific operations are as follows: first, prepare L-shaped composite laminates by manual paving and autoclave molding. According to the stress distribution characteristics of the defect-free L-shaped composite laminate model, design and prefabricate three major types of delamination defects: ply position, circumferential position, and defect size. Then, perform four-point bending loading on the specimens, and use a high-speed camera to monitor the initiation and expansion of delamination damage on the specimen measuring surface, and record the load-displacement curve and stress distribution cloud map. The specimen dimensions are: the inner radius of the R zone is 6.4 mm, the right-angle loading arm is 90 mm, and the width is 25 mm.

[0041] S2. Construct a numerical model of an L-type laminate containing cohesive elements, and use the data obtained in S1 to modify the numerical model of the L-type laminate. The critical failure position is obtained based on the stress cloud step and the crack initiation position.

[0042] The numerical model of the L-type laminate is a finite element model with an inner radius of 6.4 mm, an outer radius of 10.72 mm, a thickness of 4.32 mm, a total of 24 layers, and a thickness of 0.18 mm per layer. In terms of comprehensive calculation accuracy and efficiency, the numerical model of the L-type laminate has a total of 177,968 nodes and 87,734 units, including 79,776 C3D8R units and 7,958 COH3D8 units. The grid is encrypted in the R area, the loading arm transition area, and the loading contact area. For the boundary conditions in the simulation experiment, an analytical rigid cylinder is used to simulate the loading. The rod is loaded, and frictionless surface-to-surface contact conditions are used to simulate the contact between the fixture and the specimen, as well as the interaction between the interfaces after failure. The two rigid cylinders on the lower side are completely fixed, and the two rigid cylinders on the upper side are bound together with another rigid cylinder just above the corner through MPC multi-point constraints. A downward displacement load of 10 mm is applied, and the laminate is constrained symmetrically about the plane perpendicular to the X-axis to completely limit its displacement in the Z and X directions. The laminate with equivalent prepreg parameters is used as the model material parameters to construct a numerical model of the L-shaped laminate.

[0043] S3. Based on the critical failure location, the size and location of the prefabricated delamination defect are designed. In the experiment, the delamination defect is prefabricated by burying polytetrafluoroethylene film, and the cohesive force is deleted in the simulation model.

[0044] S4. Based on the experimental results in S1, the numerical model of the L-type laminate is further optimized to obtain an L-type laminate interlaminar damage model with high sensitivity to defect locations;

[0045] S5. The influence of the delamination defect location on the load-bearing capacity and failure model of the laminate is analyzed by arranging evenly distributed and uniformly sized delamination defect envelope R zones.

[0046] Example 1

[0047] The present invention provides a damage tolerance assessment method for a composite L-shaped plate containing delamination defects, comprising the following steps:

[0048] S1. Prepare defect-free L-shaped laminate specimens, conduct loading tests on the specimens, and record stress distribution cloud maps, crack initiation positions at critical moments, and displacement load curves. The specific operations are as follows: first, prepare L-shaped composite laminates by manual paving and autoclave molding. According to the stress distribution characteristics of the defect-free L-shaped composite laminate model, design and prefabricate three major types of delamination defects: ply position, circumferential position, and defect size. Then, perform four-point bending loading on the specimens, use a high-speed camera to monitor the initiation and expansion of delamination damage on the specimen measuring surface, and record the load-displacement curve and stress distribution cloud map.

[0049] The specimen dimensions are: 6.4mm inner radius of the R zone, 90mm right-angle loading arm, and 25mm width. The specimen material is T700-grade carbon fiber prepreg provided by Jiangsu Hengshen Co., Ltd., model EV201-35%-12KHF10-U-200gsm-1000, with a resin content of 35±3% and a fiber areal density of 200±10%. The material parameters provided by the prepreg manufacturer are shown in Table 1. The main experimental instruments used during specimen preparation and testing are shown in Table 2.

[0050] Table 1 Prepreg material parameters

[0051] Material properties Numerical <![CDATA[E 11 / GPa]]> 126 <![CDATA[E 22 / GPa]]> 11 <![CDATA[E 33 / GPa]]> 11 <![CDATA[μ 12 ]]> 0.29 <![CDATA[μ 13 ]]> 0.29 <![CDATA[μ 23 ]]> 0.40 <![CDATA[G 12 / GPa]]> 6.6 <![CDATA[G 13 / GPa]]> 6.6 <![CDATA[G 23 / GPa]]> 3.9

[0052] Table 2 Main experimental instruments used in sample preparation and experimental testing

[0053] Experimental instruments Manufacturer Device Model Autoclave Lontek RG-21 vacuum pump Zhejiang Qiujing Technology 2XZ-2 cutting machine BOSUN ZIE(T)-350Ae(b) angle grinder BOSUN BS-2 Belt grinder Suzhou Tianyiwei Electric HBS78B Micro-CT BRUKER Skyscan-1273 Electronic universal testing machine MTS MTS370 SLR camera Nikon D90 High-speed camera Zhongzheng Instruments ZZLJ-2000H

[0054] The sample was prepared by hand paving method, and the paving scheme for the defect-free sample was [0] 24 , where 0° is along the outer curvature direction, the specimen has 24 layers, and the thickness after curing is 4.32 mm. The main operating steps for manually laying specimens include: cutting prepregs, mold pretreatment, laying prepregs, and vacuum pre-compacting. In order to reduce bubbles and bulges generated during laying, vacuum pre-compacting is performed every 6 layers of prepreg. An L-shaped composite laminate with delamination defects is prepared by laying polytetrafluoroethylene film on the prepreg. The production process is as follows: When laying the laminate, when the number of prepreg layers reaches the number of layers that require prefabricated defects, use a paper cutter to cut the polytetrafluoroethylene into strips of preset width, lay them flat on the curved part of the L-shaped sample, and confirm the position of the defect by the scale pre-marked on the mold, such as Figure 2 As shown in the figure, the mold used to prepare the specimen is an L-shaped Invar female mold with an inner chamfer radius of 10.72 mm.

[0055] The hand-laid prepreg is cured by the autoclave molding process. The curing process curve used by the autoclave is as follows: Figure 3 As shown, Figure 3(a) is the curing temperature curve. Figure 3 (b) is the curing pressure curve. After the process curve is completed in the autoclave, the mold is taken out and demoulding can be carried out after a period of cooling. After the laminate motherboard is cured and formed, the scraps are cut off using a cutting machine. Six identical samples can be cut from a prepared motherboard. The actual sample of the L-shaped laminate is as follows: Figure 4 shown.

[0056] S2. Construct a numerical model of an L-type laminate containing cohesive elements, and use the data obtained in S1 to modify the numerical model of the L-type laminate. The critical failure position is obtained based on the stress cloud step and the crack initiation position.

[0057] The numerical model of the L-shaped laminate is a finite element model. The model assembly diagram is shown in Figure 5 As shown, the model mesh is divided as Figure 6 As shown, the model has an inner radius of 6.4 mm, an outer radius of 10.72 mm, a thickness of 4.32 mm, a total of 24 layers, and each layer is 0.18 mm thick. In terms of comprehensive calculation accuracy and efficiency, the numerical model of the L-shaped laminate has a total of 177,968 nodes and 87,734 units, including 79,776 C3D8R units and 7,958 COH3D8 units. The grid is encrypted in the R area, the loading arm transition area, and the loading contact area. For the boundary conditions in the simulation experiment, an analytical rigid cylinder is used to simulate the loading rod, and an unsigned matrix is ​​used. The surface-to-surface contact condition of friction simulates the contact between the fixture and the specimen, as well as the interaction between the interfaces after failure. The two rigid cylinders on the lower side are completely fixed, and the two rigid cylinders on the upper side are bound together with another rigid cylinder directly above the corner through MPC multi-point constraints. A downward displacement load of 10 mm is applied, and the laminate is constrained symmetrically about the plane perpendicular to the X-axis to completely limit its displacement in the Z and X directions. The laminate, after equivalent calculation of prepreg parameters, is used as the model material parameters to construct a numerical model of the L-shaped laminate.

[0058] In order to ensure the reproducibility of the simulation experiment to the greatest extent, the laminate parameters calculated equivalently to the prepreg parameters are used as the model material parameters. The radial tensile strength is measured by the DCB experiment. Table 3 shows the relevant parameters.

[0059] Table 3 Equivalent laminate parameters

[0060] Material parameters Numerical <![CDATA[E 11 / GPa]]> 126 <![CDATA[E 22 / GPa]]> 11 <![CDATA[E 33 / GPa]]> 11 <![CDATA[μ 12 ]]> 0.29 <![CDATA[μ 13 ]]> 0.29 <![CDATA[μ 23 ]]> 0.40 <![CDATA[X t / MPa]]> 1950 <![CDATA[X c / MPa]]> 1480 <![CDATA[Y t / MPa]]> 48

[0061] Zero-thickness cohesive elements are arranged between the layers of the L-shaped composite material in the arc region to simulate the delamination damage between the layers. Figure 7 , a Python-based parametric modeling method is used to assist in improving modeling efficiency and reducing the probability of manual modeling errors.

[0062] Cohesive Zone Element (CZM) is a finite element modeling technique that uses crack propagation caused by gradual separation of material interfaces to simulate material damage. Discontinuities in the material are simulated using a special type of viscous finite element, where zero-thickness cohesive zones are inserted between regular volume finite elements. Cohesive zones should be inserted where delamination is likely to occur, i.e., between layers of a laminate, e.g. Figure 7 shown.

[0063] During loading, the mechanical behavior of the laminate elements remains elastic-plastic. However, the cohesive elements between the laminate elements exhibit unique mechanical behavior defined by a specific traction separation law (TSL). When the conditions set by the traction separation law are met, these zero-thickness cohesive elements represent structural discontinuities by acquiring a finite volume.

[0064] Once the deformation exceeds the limit of the traction-separation law, the cohesive unit no longer transmits any force and is essentially equivalent to a void space. Different researchers have proposed several traction-separation law models, including exponential, cubic polynomial, trapezoidal, triangular, linear and polynomial softening, and hardening-softening.

[0065] Triangle TSL such as Figure 8 As shown in Figure 2, the traction separation law is given by the maximum interface traction force (T max ), cohesive stiffness (K eff ), initial damage separation (δ0), final damage separation (δ f ) and fracture energy (G C ). The maximum traction is the traction that the cohesive element bears before the crack occurs. The initial slope is the effective cohesive stiffness (K eff The fracture energy, or the area under the curve, is defined as the energy dissipated by the creation of a new crack surface. The initial and final failure displacements determine the initiation and propagation of fracture, respectively.

[0066] Through the double cantilever beam test (DCB), all the above parameters can be obtained directly or indirectly, and the maximum interface traction T max The value of T will have a certain impact on the prediction results. max The value of will affect the accuracy and convergence of the calculation results.

[0067] Another important parameter to determine is the cohesive stiffness. If the number of cohesive elements is large or the thickness is limited, the cohesive stiffness can have a significant impact on the overall elastic response, i.e., an additive compliance effect. Since the cohesive elements only need to simulate fracture, their impact on the overall stiffness should be minimized.

[0068] When the number of cohesive elements inserted in the model is large and the cohesive elements are inserted in multiple layers, K effThe value of becomes particularly important. In order to minimize the effect of the additional flexibility, K is set relative to the stiffness of the laminate element. eff It should be as large as possible, however, if the value is too large, it may increase the numerical error and cause problems in the convergence of the solution, so a series of preliminary analyses are performed and the cohesive stiffness value is selected that is close in value to the elastic response of the laminate element.

[0069] S3. Based on the critical failure location, the size and location of the prefabricated delamination defect are designed. In the experiment, the delamination defect is prefabricated by burying polytetrafluoroethylene film, and the cohesive force is deleted in the simulation model.

[0070] S4. Based on the experimental results in S1, the numerical model of the L-type laminate is further optimized to obtain an L-type laminate interlaminar damage model with high sensitivity to defect locations.

[0071] First, according to Figure 9 The radial stress distribution cloud diagram of the defect-free model shown in the figure sets six representative defect points with a defect size of 2 mm. Figure 10 shown.

[0072] Then, the results are compared with the loading test results of the defective specimen to verify the sensitivity of the numerical model to the defect location. The model is further modified to make it more sensitive to the defect location, thereby increasing the credibility of the defect lattice envelope simulation results in the next step.

[0073] S5. The influence of the delamination defect location on the load-bearing capacity and failure model of the laminate is analyzed by arranging evenly distributed and uniformly sized delamination defect envelope R zones.

[0074] In order to study the effect of defects at different locations on the bearing capacity of L-type laminates in a more comprehensive and systematic manner, 96 defect points with uniform distribution and size are set to envelop the entire R area, such as Figure 11 shown.

[0075] The force-displacement curve collected by the loading platform and the force-displacement curve obtained by simulation are as follows: Figure 12 As shown in the figure, the elastic response of the model is basically consistent with that of the sample, and the curves are well matched. The model has initially acquired the ability to predict the delamination behavior of defect-free samples.

[0076] The force-displacement curve of the prefabricated delaminated specimen collected by the loading platform and the simulation results with delaminated defects are shown as follows: Figure 13 As shown in the figure, the maximum load of the characteristic point defect specimen predicted by the model has a small error with the experimental results, with a degree of agreement of more than 97%. The model is highly sensitive to the defect location.

[0077] After comparing the defect location simulation with the experiment, it is shown that the model is highly sensitive to the defect location. The limit load corresponding to each defect point on the envelope surface is extracted and plotted into a scatter plot with the plane absolute coordinates of the defect point. The graph is then normalized into a surface diagram of defect location-limit load, as shown in the figure. Figure 14 shown.

[0078] The results show that when defects of the same size appear in different locations, the ultimate load-bearing capacity varies by about 37%. The closer the defect location is to the 45° angle of the 9th or 10th floor, the lower the ultimate load. In the long high-risk area between the 9th and 10th floors (10-35°), if a defect occurs, the ultimate load will drop by about one-third.

[0079] Therefore, the present invention adopts the above-mentioned damage tolerance assessment method for L-shaped composite plates containing delamination defects, which has low cost, does not require the preparation of a large number of defective specimens, has high loading test efficiency, and can comprehensively study the load-bearing capacity of L-shaped laminate composite materials when the delamination defect is located at a certain position.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A damage tolerance assessment method for a composite L-shaped plate containing delamination defects, characterized by: The following steps are involved: S1. Prepare defect-free L-shaped laminate specimens, perform loading tests on the specimens, and record stress distribution cloud maps, crack initiation locations at critical moments, and displacement-load curves. S2. Construct a numerical model of an L-type laminate containing cohesive elements, and use the data obtained in S1 to modify the numerical model of the L-type laminate. The critical failure position is obtained based on the stress cloud step and the crack initiation position. The numerical model of the L-type laminate in S2 is a finite element model with an inner radius of 6.4 mm, an outer radius of 10.72 mm, a thickness of 4.32 mm, a total of 24 layers, and a thickness of 0.18 mm per layer. In terms of comprehensive calculation accuracy and efficiency, the numerical model of the L-type laminate has a total of 177,968 nodes and 87,734 units, including 79,776 C3D8R units and 7,958 COH3D8 units. The grid is encrypted in the R area, the loading arm transition area, and the loading contact area. For the boundary conditions in the simulation experiment, an analytical rigid cylinder simulation is used. The loading rod uses frictionless surface-to-surface contact conditions to simulate the contact between the fixture and the specimen, as well as the interaction between the interfaces after failure. The two rigid cylinders on the lower side are completely fixed, and the two rigid cylinders on the upper side are bound together with another rigid cylinder directly above the corner through MPC multi-point constraints. A downward displacement load of 10 mm is applied, and the laminate is constrained symmetrically about the plane perpendicular to the X-axis to completely limit its displacement in the Z and X directions. The laminate with equivalent prepreg parameters is used as the model material parameters to construct a numerical model of the L-shaped laminate. S3. Design the size and location of prefabricated delamination defects based on the critical failure location; S4. Based on the experimental results in S1, the numerical model of the L-type laminate is further optimized to obtain an L-type laminate interlaminar damage model with high sensitivity to defect locations; S5. Analyze the influence of delamination defect location on the load-bearing capacity and failure model of the laminate.

2. The damage tolerance assessment method for a composite L-shaped plate containing delamination defects according to claim 1, characterized in that: The specific operation of S1 is as follows: first, an L-shaped composite laminate is prepared by manual laying and autoclave molding. According to the stress distribution characteristics of the defect-free L-shaped composite laminate model, three major types of delamination defects are designed and prefabricated: layup position, circumferential position, and defect size. Then, the specimen is subjected to four-point bending loading, and a high-speed camera is used to monitor the initiation and expansion of delamination damage on the specimen measuring surface, and the load-displacement curve and stress distribution cloud map are recorded.

3. The damage tolerance assessment method for a composite L-shaped plate containing delamination defects according to claim 1, characterized in that: The specimen dimensions in S1 are: inner radius of R zone is 6.4 mm, right-angle loading arm is 90 mm, and width is 25 mm.

4. The damage tolerance assessment method for a composite L-shaped plate containing delamination defects according to claim 1, characterized in that: In the S3 experiment, the delamination defect was prefabricated by burying polytetrafluoroethylene film, and the cohesive force was deleted in the simulation model.

5. The damage tolerance assessment method for a composite L-shaped plate containing delamination defects according to claim 1, characterized in that: In S5, the influence of the delamination defect position on the bearing capacity and failure mode of the L-type laminate is analyzed by arranging the delamination defect envelope R zone with uniform distribution and the same size.

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