A method for assessing internal failure of orthotropic anisotropic laminates based on strain measurement

Abstract

This invention provides a method for assessing internal failure of orthotropic laminates based on strain measurement, relating to the field of orthotropic composite materials technology. The method includes the following steps: S1, fabricating a strain gauge test model of the laminate; S2, connecting the strain gauge test model to a data acquisition instrument, conducting a working condition test on the structure under test, and using a host computer to calculate the stress in each layer and principal direction of each angle within the laminate; S3, acquiring the strength limit of the orthotropic single-layer laminate material, and based on the strength limit of the orthotropic single-layer laminate material and the stress in each layer and principal direction of each angle within the laminate, performing failure assessment for each ply at different angles. Due to the special characteristics of orthotropic laminates, different ply failures can occur under different stress states. This invention provides a method that can calculate the failure assessment of ply in different directions within the laminate structure based on the surface strain measurement values ​​of the composite laminate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of orthotropic composite materials, in particular, and more particularly to a method for evaluating internal failure of orthotropic laminated plates based on measured strain. BACKGROUND

[0002] With the rapid development of new material technology, composite materials have been widely used in production and life. Especially orthotropic composite materials, due to their high specific strength, specific modulus, corrosion resistance and high temperature resistance, etc., have been widely used in aerospace, mechanical manufacturing and rail transportation. Failure evaluation refers to the failure of a single layer under load. The remaining layers may continue to bear load, but the failure layer has completely failed. Currently, the method for measuring surface strain of orthotropic composite materials for failure evaluation mainly relies on the maximum strain evaluation criterion. However, this method can only be used for surface layer failure evaluation and cannot be used for multi-angle failure evaluation analysis of internal layers.

[0003] The difficulty of stress analysis of internal layers and angles lies in that the internal strain and stress of the structure cannot be directly measured, and due to the multi-angle and multi-layer structure characteristics of the orthotropic material laminated plate structure, the internal strain and stress have a specific direction. The existing technology can only evaluate the failure of the surface layer or simulate the failure analysis through finite element modeling, and cannot perform failure analysis of different layers and angles based on surface strain measurement results. SUMMARY

[0004] Therefore, the present application aims to provide a method for evaluating internal failure of orthotropic laminated plates based on measured strain, to solve the technical problem that the existing technology cannot evaluate the failure of each layer in the internal structure of the composite laminated plate based on surface strain measurement.

[0005] The technical means adopted by the present application are as follows:

[0006] A method for evaluating internal failure of orthotropic laminated plates based on measured strain, comprising the following steps:

[0007] S1, making a laminated plate strain gage test model;

[0008] S2, connecting the laminated plate strain gage test model to the acquisition instrument, and performing working condition test on the measured structure, and calculating the stress of each layer and angle of the laminated plate by the host computer;

[0009] S3, collecting strength limit values of the orthotropic single-layer plate material, and performing failure evaluation of the angle-ply based on the strength limit values of the orthotropic single-layer plate material and the stress of each layer and each angle main direction inside the laminate.

[0010] Further, S1 specifically comprises the following steps:

[0011] S11, laminate pretreatment;

[0012] S111, selecting a cuboid laminate, using sandpaper to roughen the surface of the laminate, and removing oil stains and attachments on the surface of the laminate; making the surface of the laminate flat and forming intersecting lines at an angle of 45° with the longitudinal direction of the patch direction;

[0013] S112, cleaning the surface of the laminate treated in S111;

[0014] S12, pasting strain gauges;

[0015] S121, pasting a first strain gauge on the surface of the laminate, the center line of the first strain gauge being parallel to the fiber direction;

[0016] S122, pasting a second strain gauge on the surface of the laminate, the angle between the center line of the second strain gauge and the center line of the first strain gauge being 45°;

[0017] S123, pasting a third strain gauge on the surface of the laminate, the center line of the first strain gauge being perpendicular to the center line of the third strain gauge, and the angle between the center line of the second strain gauge and the center line of the third strain gauge being 45°;

[0018] S13, connecting the strain gauges with the connecting wires, and connecting the other end of the lead wire with the terminal with surface tinning.

[0019] Further, S2 specifically comprises the following steps:

[0020] S21, in the state that the measured surface layer strain gauge is static, zeroing the strain signal, and then loading the working condition of the measured surface layer strain gauge, and collecting the strain values of each strain gauge at this moment when the test loading reaches the specified load value;

[0021] S22, linearly calculating the collected strain values to obtain the surface layer main direction linear strain value at the measurement position of the strain gauge;

[0022] S23, calculating the strain values of each layer and each angle main axis direction inside the laminate based on the surface layer main direction linear strain value;

[0023] S24, calculating the stiffness coefficient based on the engineering elastic constant of the laminate material, and solving the stiffness matrix based on the stiffness coefficient;

[0024] S25, based on the strain value of each layer of the laminate and the stiffness matrix of each angle principal axis direction, the stress value of each layer of the laminate and each angle principal direction is calculated.

[0025] Further, in S22, the calculation formula for linear calculation of the collected strain value is:

[0026]

[0027] Wherein, ε0, ε 45 and ε 90 are the strain values collected by the data acquisition channel, ε x is the strain value in the fiber direction, ε y is the strain value perpendicular to the fiber direction, γ xy is the shear direction strain value, and α is the included angle value between the three direction strain gauges.

[0028] Further, in S23, the formula for calculating the strain value of each angle and each layer of the laminate is as follows:

[0029]

[0030] Wherein, ε1, ε2, γ 12 are the single layer plate principal axis strain values, and θ is the angle of the single layer plate off-axis direction strain to the fiber direction main direction strain.

[0031] Further, S24 specifically includes the following steps:

[0032] The two-dimensional strain-stress relationship of each layer of the laminate structure can be represented as:

[0033] σ k = Q ij ε k

[0034] Wherein, σ k is the stress of the kth layer main direction, Q ij is the two-dimensional stiffness matrix of the single layer plate, and ε k is the strain of the kth layer main direction.

[0035] The two-dimensional stiffness matrix Q ij of the single layer plate is represented as:

[0036]

[0037]

[0038]

[0039]

[0040] Q 66 = G 12

[0041] wherein E1 is the fiber direction elastic modulus, E2 is the transverse fiber direction elastic modulus, v 12 is the Poisson's ratio of the stress in the fiber direction elastic modulus direction when a transverse strain is generated in the transverse fiber direction elastic modulus direction, v 21 is the Poisson's ratio of the stress in the transverse fiber direction elastic modulus direction when a transverse strain is generated in the fiber direction elastic modulus direction, G 12 is the shear modulus.

[0042] Further, in S25, the stress value formula of each layer and each angle main direction inside the laminate is calculated as follows:

[0043]

[0044] wherein ε k1 is the main direction strain value of the kth layer along the fiber direction; ε k2 is the main direction strain value of the kth layer transverse to the fiber direction; γ k12 is the in-plane shear strain value of the kth layer; σ k1 is the main direction stress value of the kth layer along the fiber direction; σ k2 is the main direction stress value of the kth layer transverse to the fiber direction; τ k12 is the in-plane shear stress value of the kth layer.

[0045] Further, S3 specifically comprises the following steps:

[0046] S31, statistics of the strength limit values of the orthotropic single-layer plate material, including the tensile strength value along the fiber direction, the compressive strength value along the fiber direction, the tensile strength value transverse to the fiber direction, the compressive strength value transverse to the fiber direction, and the in-plane shear strength value;

[0047] S32, based on the stress of each layer and each angle main direction inside the laminate and the strength limit values of the orthotropic single-layer plate material, the failure index F(σ) is calculated, and the calculation formula is as follows:

[0048]

[0049] wherein σ1 is the stress value along the fiber direction; σ2 is the stress value transverse to the fiber direction; τ 12 is the stress value in the shear direction; is the tensile strength value along the fiber direction; is the compressive strength value along the fiber direction; is the tensile strength value transverse to the fiber direction; is the compressive strength value in the fiber direction; is the in-plane shear strength value; is the dimensionless correlation term;

[0050] S33, the failure index F(σ) is compared with the numerical value 1, if F(σ)>1, it is determined that the layer fails, if F(σ)<1, it is determined that the layer does not fail.

[0051] Compared with the prior art, the present application has the following advantages:

[0052] The present application obtains the surface strain of the orthotropic composite laminate by measurement, and the failure of each angle and each layer in the structure is evaluated by a specific calculation method. Since the orthotropic composite laminate is made of single-layer plates with multiple angles and multiple layers, under the action of external load, the internal structure may completely fail in one or several layers, and the remaining layers may continue to bear the load and do not necessarily fail completely. This internal partial complete failure cannot be directly measured from the surface. The present application proposes a failure evaluation method for each layer in the internal structure based on the surface strain measurement, which can obtain the failure of each layer under the action of external load through the surface strain information. The user needs to select a specified type of strain gauge, paste it according to the specified direction in the pasting method, and enter the relevant parameters of the strain gauge in the data acquisition software to obtain effective surface strain information. The linear and bias main direction strain of each layer is calculated using the surface strain and angle information, and the two-dimensional bias stiffness matrix is calculated by combining the engineering constants of the material to obtain the main direction stress information of each angle and each layer. The complete failure of each angle and each layer in the internal structure is evaluated using the main direction stress of each layer and each direction and the strength parameters of each direction. BRIEF DESCRIPTION OF DRAWINGS

[0053] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0054] Figure 1 is the flow chart of the method of the present application.

[0055] Figure 2 is the schematic diagram of the principal stress direction of isotropic material.

[0056] Figure 3 is the schematic diagram of the principal stress direction of orthotropic material.

[0057] Figure 4Strain gauge with 45° angle of the present application.

[0058] Figure 5 Strain gauge with 45° angle of the present application.

[0059] Figure 6 Strain gauge with 45° angle of the present application.

[0060] Figure 7 Strain gauge with 45° angle of the present application.

[0061] Figure 8 Strain gauge with 45° angle of the present application.

[0062] Figure 9 Strain gauge with 45° angle of the present application.

[0063] Figure 10 Strain gauge with 45° angle of the present application. DETAILED DESCRIPTION

[0064] In order to make the personnel in the art better understand the present application scheme, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by the person skilled in the art without making creative efforts should belong to the protection scope of the present application.

[0065] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and do not necessarily describe a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not necessarily limit to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0066] As shown in Figure 1 The present application provides a method for evaluating internal failure of an orthotropic laminated plate based on measured strain, which is used to evaluate complete failure of a certain layer or a plurality of layers in the laminated plate

[0067] Orthotropic laminates have their own structural particularity, which is different from the previous isotropic metal plates. Due to the difference of actual working conditions, the isotropic metal plates are more concerned about the principal stress of the plate without specific direction requirements. The orthotropic material emphasizes the stress change in the main direction of the fiber. The main direction stress diagram of different materials is shown in FIG. 2 and Figure 3 .

[0068] Since the method is based on the surface strain measurement to calculate the stress of each layer and each angle of the internal structure of the laminate, the selection of the surface strain gauge, the pasting angle and method need to be clarified first. This will be described in detail below.

[0069] Due to the different angle of the laminated structure, the main direction strain (ε x , ε y ) of each layer in the laminate cannot be directly measured. The strain gauge needs to be pasted on the surface.

[0070] The surface of the laminate needs to be roughened using sandpaper, removing oil stains and attachments, etc. The surface needs to be flat, forming a 45° angle cross line with the pasting direction longitudinal.

[0071] The roughened surface needs to be cleaned. Select a three-way strain gauge with a suitable size and a 45° angle. The strain gauge connected by lead wire 1 and the strain gauge connected by lead wire 2 have a 45° angle, and the strain gauge connected by lead wire 2 and the strain gauge connected by lead wire 3 have a 45° angle. The strain gauge with a 45° angle is shown in FIG. Figure 4 .

[0072] When pasting the strain gauge on the surface of the laminate, attention should be paid to the pasting direction. Different pasting directions will get completely different internal main direction stress calculation results. When pasting, the strain gauge connected by lead wire 1 is defined as the strain gauge pasted along the fiber direction, and the strain gauges connected by lead wire 2 and lead wire 3 are pasted in turn along the clockwise direction. The pasting direction of the three-way strain gauge is shown in FIG. Figure 5 .

[0073] Hold the three groups of lead wires of the strain gauge, evenly apply the adhesive to the glue side of the strain gauge substrate, form a thin glue layer on the surface, and the liquid should cover the glue side of the substrate. When applying the adhesive, it should be done quickly and not for too long. Align the strain gauge connected by lead wire 1 with the fiber direction, and align the strain gauge connected by lead wire 3 with the direction perpendicular to the fiber. Hold the three-way strain gauge with non-sticky plastic film with the other hand by pressing the thumb, and make the strain gauge uniformly stressed as a whole during the pressing process, keep it in the compacted state before the adhesive solidifies, until the strain gauge is firmly bonded to the surface of the measured structure.

[0074] After the three-way strain gauge adhesive is completely solidified, start from one side of the plastic film, slowly remove the plastic film, and then pull out one of the lead wires in lead wire 1 by hand. Slowly pull it out of the overflowed solidified adhesive to separate the lead wire from the surface of the test piece. Repeat the operation for the other lead wires.

[0075] After the lead wire is pulled out, it needs to be fixed. Use a plastic sheet as the base, and a terminal with tin on the surface. The three-way strain gauge corresponds to three terminal ends. The terminal is evenly coated with adhesive on the side to be applied, and is pasted on the lead wire side of the corresponding strain gauge. With the thumb pad with a plastic film, press the terminal to the surface of the measured structure with the thumb, and the force on each part of the terminal surface is uniform. After the adhesive is solidified, slowly lift the thumb. Repeat the operation for the other terminal corresponding to the strain gauge.

[0076] After the terminal and the adhesive on the surface of the measured structure are completely solidified, remove the plastic film and prepare to connect the terminal and the lead wire. When connecting, use a soldering iron heated to the rated temperature to melt the tin on the surface of the terminal. Place one of the strain gauge lead wires 1 in the melted surface tin, lift the soldering iron, and keep the lead wire in the surface tin. After the surface tin is solidified, cut off the excess lead wire with scissors. Repeat the operation for the other lead wire.

[0077] After the strain gauge lead wire is fixed, it needs to be connected to the acquisition instrument. Each strain gauge in the three-way strain gauge needs to be connected to the data acquisition channel of the acquisition instrument through the connecting wire. One end of the connecting wire is connected to the terminal by melting the surface tin of the terminal. The other side is connected to the acquisition instrument through a special connector. The connection diagram of the three-way strain gauge lead wire and the terminal is shown in Figure 6 .

[0078] Connect the acquisition instrument to the upper computer. Connect the acquisition instrument and the upper computer through a network cable. After the communication connection is successful, input the sensitivity coefficient and resistance value of the strain gauge used for testing in the upper computer. In the upper computer, name the strain value acquisition channel connected by lead wire 1 as S0, the strain value acquisition channel connected by lead wire 2 as S 45 , and the strain value acquisition channel connected by lead wire 3 as S 90 .

[0079] After the internal strain test debugging of the upper computer is completed and the confirmation signal is normal, the working condition test of the measured structure can be prepared. In the state of the measured structure being static, zero the strain signal in the upper computer, and then load the working condition of the measured structure. When the test load reaches the specified load value, collect the strain values of each strain gauge at this moment. The strain value collected by channel S0 is named ε0, the strain value collected by channel S 45 is named ε 45 , and the strain value collected by channel S 90The collected strain value is named ε 90 .

[0080] The collected strain value is linearly calculated to calculate the linear strain value (i.e. the strain value of the fiber direction ε x , the strain value perpendicular to the fiber direction ε y , and the shear direction strain value γ xy ) at the measurement point of the strain gauge. The included angle between the three-direction strain gauges is α, and the calculation formula is as follows:

[0081]

[0082] The measured ε0, ε 45 , and ε 90 are substituted into the above formula, and the strain values of the measurement points ε x , ε y , and γ xy are solved by solving the equation set. The main direction strain calculation diagram of the surface layer is shown in Figure 7 .

[0083] Since the three-direction strain gauges are pasted in the clockwise direction along the fiber direction, ε x is the fiber direction strain value ε y of the surface single-layer plate, ε2 is the strain value perpendicular to the fiber direction of the surface single-layer plate, and γ xy is the shear direction strain value γ 12 of the surface single-layer plate.

[0084] After the main direction strain values ε1, ε2, and γ 12 of the surface measurement points are solved, the main direction strain of the internal angle and the single-layer plate of each layer can be calculated. According to the deformation continuity characteristics of the orthotropic material laminated plate, for other layers, the main axis direction strain ε1, ε2, and γ 12 calculated by the surface layer can be converted into the off-axis direction strain ε x , ε y , and γ xy of other different angle layers.

[0085] The angle of the off-axis direction strain of the single-layer plate is turned to the main direction strain of the fiber direction, which is θ (i.e. the turning angle of the x-axis to the 1-axis). The clockwise turning angle is positive, and the counterclockwise turning angle is negative. The relationship between the two coordinates is shown in Figure 8 .

[0086] The off-axis direction strain ε x , ε y , and γ xy of the different angle layers and the turning angle θ are substituted into the formula to calculate the main axis direction strain of the different angle layers, and the calculation formula is as follows:

[0087]

[0088] Substitute calculation can be obtained single layer board main shaft direction strain value ε1, ε2, γ 12 . Different angle of the layer repeat this process, substitute the corresponding angle can be. Based on the surface layer main direction strain internal single layer board main shaft direction strain calculation diagram as Figure 9 shown.

[0089] Thus can be based on the surface layer of the measured strain of orthotropic laminated plate structure of the internal angle of the main shaft direction strain calculation.

[0090] Here has been according to the specific surface layer strain paste method, measured surface layer of the main shaft direction strain value, and calculated the internal layer, the main shaft direction strain value of each angle. For the calculation of internal stress value of each layer, each angle, need to establish the effective relationship between the surface layer strain measurement value and the calculated stress value, the internal main direction strain value is the most effective bridge. Based on the main direction strain of each layer, the surface layer strain measurement and the internal stress of each layer, each angle will be established in the following effective calculation relationship.

[0091] Orthotropic material laminated plate structure is by single layer board according to the specific direction of the stack, single layer board has the attribute of the material itself, respectively along the fiber direction 1 axis elastic modulus E1, perpendicular to the fiber direction 2 axis elastic modulus E2, 1 direction on the stress in the 2 direction of the transverse strain when the poisson's ratio v 12 , 2 direction on the stress in 1 direction of the transverse strain when the poisson's ratio v 21 , shear modulus G 12 .

[0092] For the two-dimensional strain-stress relationship of each layer of the laminated structure can be expressed as:

[0093] σ k = Q ij ε k

[0094] σ k , the k layer main direction stress; Q ij , the two-dimensional stiffness matrix of single layer board; ε k , the k layer main direction strain.

[0095] Q ij , the two-dimensional stiffness matrix of single layer board, through the engineering material constant of single layer board describes the relationship between the main direction strain and the main direction stress. The two-dimensional stiffness matrix of orthotropic material single layer board is Q 11 , Q 12 , Q 22 , Q 66A 3x3 matrix consisting of four coefficients. Two-dimensional stiffness matrix Q of a single ply ij is expressed as:

[0096]

[0097] Since the Poisson's ratio of orthotropic single ply has a relationship with elastic modulus , the orthotropic single ply in plane stress problem can be simplified to four independent elastic constants E1, E2, υ 12 and G 12 . The value of Q 12 can be calculated from E1, E2, υ 12 and G ij .

[0098] Q ij is expressed as follows in terms of engineering elastic constants:

[0099]

[0100]

[0101]

[0102] Q 66 = G 12

[0103] From the above formula, each coefficient of two-dimensional stiffness of a single ply can be solved, and the two-dimensional stiffness matrix Q ij is obtained by assembling the coefficients.

[0104] From the above, the principal stress values of each layer can be solved based on the measured values of surface strain and the principal strain-principal stress relationship of orthotropic composite single ply. The calculation formula is as follows:

[0105]

[0106] ε k1 is the principal strain value of the kth layer along the fiber direction; ε k2 is the principal strain value of the kth layer perpendicular to the fiber direction; γ k12 is the shear strain value of the kth layer in the plane; σ k1 is the principal stress value of the kth layer along the fiber direction, unit: MPa; σ k2 is the principal stress value of the kth layer perpendicular to the fiber direction, unit: MPa; τ k12 is the shear stress value of the kth layer in the plane, unit: MPa.

[0107] From this, the principal stress of each layer, i.e. σ k1 , σ k2 , τk12 .

[0108] Based on the specific strain gauge pasting method, the principal direction strain and principal direction stress of each layer are calculated, and then the failure assessment of each angle layer is carried out. Due to the complexity of the calculation formula, it is necessary to combine the electronic form to carry out the assessment calculation.

[0109] First, the strength limit values of the orthotropic single-layer plate material are statistically analyzed, including the tensile strength value along the fiber direction, the compressive strength value along the fiber direction, the tensile strength value perpendicular to the fiber direction, the compressive strength value perpendicular to the fiber direction and the in-plane shear strength value.

[0110] The statistical strength limit values are respectively input into different cells in the electronic form, the format of the cell is selected as a number, and the strength value name is labeled in the front column cell. The numerical value unit of the strength is uniformly adjusted to MPa.

[0111] The principal direction stress value of the single-layer plate calculated by the same method is input into the electronic form. The corresponding parameters are input into the electronic form as shown in Table 1.

[0112] Table 1

[0113]

[0114] The formula is edited in the electronic form, and the edited formula is as follows:

[0115]

[0116] In the formula: σ1 is the stress value along the fiber direction, unit: MPa; σ2 is the stress value perpendicular to the fiber direction, unit: MPa; τ 12 is the shear direction stress value, unit: MPa; is the tensile strength value along the fiber direction, unit: MPa; is the compressive strength value along the fiber direction, unit: MPa; is the tensile strength value perpendicular to the fiber direction, unit: MPa; is the compressive strength value perpendicular to the fiber direction, unit: MPa; is the in-plane shear strength value, unit: MPa; is a dimensionless related action term.

[0117] Table 2

[0118]

[0119] The corresponding cell is called to perform formula calculation, as shown in Table 2. The calculation result F(σ) is compared with the numerical value 1. If the result F(σ) > 1, it is determined that the layer is failed, and if the result F(σ) < 1, it is determined that the layer is not failed.

[0120] The failure evaluation method of the single-layer plate based on the surface layer measured strain and the angle of each layer is detailed above.

[0121] Embodiment

[0122] As shown in the following table, the strain measurement values of 0°, 45° and 90° are taken as examples to detail the algorithm of the principal strain of each layer: Figure 10

[0123] Firstly, the three-way strain gauges are pasted along the clockwise direction of the surface layer fiber of the laminate, and the measured strain values are ε 0° = ε α1 = 300με, ε 45° = ε α2 = -300με, ε 90° = ε α3 = 400με. Combined with the known angle values of the directions α1, α2, α3, the principal strain is calculated by substituting into the formula:

[0124]

[0125] The calculated principal strain is ε x = ε1= 300με, ε y = ε2= 400με, γ xy = γ 12 = 1300με. ε1, ε2, γ 12 are the principal axis strain values of the surface layer, and the off-axis strain of the 45° off-axis or the principal strain of the other 45° layer, i.e. ε1= ε 45-x , ε2= ε 45-y , γ 12 = γ 45-xy , θ = 45°, are substituted into the following formula.

[0126]

[0127] The calculated off-axis strain of the 45° off-axis or the principal strain of the other 45° layer is ε 45-1 = 1000, ε 45-2 = -300, γ 45-12 = 100. Substituted into the following formula to calculate the stiffness coefficient.

[0128]

[0129] ​

[0130] Calculate stiffness coefficient Q 11 = 121637.03, Q 12 = 2517.89, Q 22 = 9952.12, Q 66 = 5000. Substitute each stiffness coefficient into the stiffness matrix, and combine the principal axis strain to calculate the principal axis stress of the 45° layer. Note that the principal axis strain value unit needs to be converted from micro-strain to strain before calculation (i.e. multiply the principal axis strain value by 10 -6 after calculation).

[0131]

[0132] The calculated principal axis stress σ1= 37.50 MPa, σ2= 4.74 MPa, τ 12 = 6.5 MPa.

[0133] Similarly, substitute the principal axis strain of the 45° direction layer into the formula to calculate the principal axis stress of the layer, and obtain the principal axis stress σ 45-1 = 120.88 MPa, σ 45-2 = -0.47 MPa, σ 45-12 = 0.5 MPa. In this way, the principal axis stress of each angle layer can be calculated.

[0134] Basic material parameters: fiber direction tensile strength, perpendicular fiber direction tensile strength, fiber direction compression strength, perpendicular fiber direction compression strength, in-plane shear strength. Substitute the basic material parameters and the principal axis stress into the composite failure index calculation formula:

[0135]

[0136] In the formula: σ1 is the stress value along the fiber direction, unit: MPa; σ2 is the stress value perpendicular to the fiber direction, unit: MPa; τ 12 is the shear direction stress value, unit: MPa; is the tensile strength value along the fiber direction, unit: MPa; is the compression strength value along the fiber direction, unit: MPa; is the tensile strength value perpendicular to the fiber direction, unit: MPa; is the compression strength value perpendicular to the fiber direction, unit: MPa; is the in-plane shear strength value, unit: MPa; is the dimensionless related action term.

[0137] The calculated failure index is -0.0651 < 1. Therefore, the 45° layer is not failed.

[0138] With this method, the failure evaluation of each angle layer based on the strain value calculated from the surface measurement can be performed.

[0139] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not limited to them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for evaluating internal failure of a cross-ply laminated plate based on measured strain, characterized by, Comprise the following steps: S1, making laminated plate strain gauge test model; S2, connecting the laminated plate strain gauge test model to the acquisition instrument, and performing the working condition test of the measured structure, and calculating the stress of each layer and each angle main direction of the laminated plate by the upper computer; S21, in the state that the measured surface layer strain gauge is static, zero setting of the strain signal is performed, then the working condition loading of the measured surface layer strain gauge is performed, when the test loading reaches the specified load value, the strain values of each strain gauge at this moment are collected; S22, linear calculation is performed on the collected strain values to obtain the surface layer main direction linear strain value at the measurement position of the strain gauge, and the formula is: wherein, S23, based on the surface layer main direction linear strain value, the strain values of each layer and each angle main axis direction of the laminated plate are calculated, and the formula is: 0, S24, the stiffness coefficient is calculated based on the engineering elastic constant of the laminated plate material, and the stiffness matrix is solved based on the stiffness coefficient; 45 and The two-dimensional strain-stress relationship of each layer of the laminated plate structure can be represented as: 90 is a strain value collected by a data collection channel, is a strain value in the fiber direction, is a strain value perpendicular to the fiber direction, is a strain value in the shear direction, α is an angle value between three-direction strain gauges. S25, based on the strain values of each layer and each angle main axis direction of the laminated plate and the stiffness matrix, the stress values of each layer and each angle main direction of the laminated plate are calculated, and the formula is: wherein, is the strain value in the main direction of the single-layered board, S3, collecting the strength limit value of the orthotropic single-layer plate material, and performing failure evaluation of each angle layer based on the strength limit value of the orthotropic single-layer plate material and the stress of each layer and each angle main direction of the laminated plate. is the angle of the strain in the off-axis direction of the single-layered board to the main direction of the fiber direction strain; S1 specifically comprises the following steps: S11, laminated plate pretreatment; wherein, is the principal directional stress of the kth layer, is the two-dimensional stiffness matrix of the single layer plate, is the principal directional strain of the kth layer; Two-dimensional stiffness matrix of a single-ply panel is represented as: wherein E1 is the modulus of elasticity in the fiber direction, E2 is the modulus of elasticity perpendicular to the fiber direction, v 12 v is the Poisson's ratio of the stress in the modulus of elasticity in the fiber direction to the lateral strain in the modulus of elasticity perpendicular to the fiber direction, v 21 G is the Poisson's ratio of the stress in the modulus of elasticity perpendicular to the fiber direction to the lateral strain in the modulus of elasticity in the fiber direction, G 12 G is the shear modulus; S111, selecting a cuboid laminated plate, roughening the surface of the laminated plate by using sandpaper, and removing oil stains and attachments on the surface of the laminated plate; the surface of the laminated plate is flat, and cross lines are formed at an angle of 45° with the longitudinal direction of the patch direction; wherein, is the principal direction strain value of the kth layer along the fiber direction; is the principal direction strain value of the kth layer perpendicular to the fiber direction; is the shear strain value in the plane of the kth layer; is the principal direction stress value of the kth layer along the fiber direction; is the principal direction stress value of the kth layer perpendicular to the fiber direction; is the shear stress value in the plane of the kth layer; S112, cleaning the surface of the laminated plate treated in S111; 2. The method for evaluating internal failure of a cross- anisotropic laminated plate based on measured strain according to claim 1, characterized by, S12, sticking strain gauges; S121, sticking a first strain gauge on the surface of the laminated plate, and the center line of the first strain gauge is parallel to the fiber direction; S122, sticking a second strain gauge on the surface of the laminated plate, and the angle between the center line of the second strain gauge and the center line of the first strain gauge is 45°; S123, sticking a third strain gauge on the surface of the laminated plate, and the center line of the first strain gauge is perpendicular to the center line of the third strain gauge, and the angle between the center line of the second strain gauge and the center line of the third strain gauge is 45°; S13, connecting the strain gauges with the connecting wires, and connecting the other end of the lead wire with the terminal with surface tinning. S3 specifically comprises the following steps: S31, statistics is performed on the strength limit value of the orthotropic single-layer plate material, and the strength limit value of the orthotropic single-layer plate material includes the tensile strength value along the fiber direction, the compression strength value along the fiber direction, the tensile strength value perpendicular to the fiber direction, the compression strength value perpendicular to the fiber direction, and the in-plane shear strength value; ​ ​ 3. The method for evaluating internal failure of a cross- anisotropic laminated plate based on measured strain according to claim 1, characterized by, ​ ​ S32、Based on the stress of each layer and each angle main direction inside the laminate and the strength limit value of the orthotropic single-layer plate material, the damage index is calculated The calculation formula is as follows: wherein, is the stress value in the fiber direction; is the stress value perpendicular to the fiber direction; is the stress value in the shear direction; is the tensile strength value in the fiber direction; is the compressive strength value in the fiber direction; is the tensile strength value perpendicular to the fiber direction; is the compressive strength value perpendicular to the fiber direction; is the in-plane shear strength value; is the dimensionless correlation term; S33, the damage index is compared with a value 1 If the value is greater than 1, the layer is determined to be failed, if the value is less than 1, the layer is determined to be not failed. >1 then the layer is determined to be failed, if the value is less than 1, the layer is determined to be not failed. <1 then the layer is determined to be failed, if the value is less than 1, the layer is

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