A method for predicting the strength of Z-pin three-dimensional reinforced composites

Abstract

The present invention discloses a method for predicting the strength of Z-pin three-dimensional reinforced composite materials, which belongs to the field of composite material mechanical property analysis methods. Based on the complex problem of strength prediction and analysis of Z-pin three-dimensional reinforced composite laminates, the method proposes a multi-scale analysis method from micro to macro to predict the mechanical properties of Z-pin three-dimensional reinforced composite laminates. The method has a clear physical mechanism and can better predict its strength and failure process, providing strong support for its preparation process guidance.

Description

Technical Field

[0001] The invention belongs to the field of composite material mechanical property analysis methods, and specifically relates to a method for predicting the strength of a Z-pin three-dimensional reinforced composite material. Background Art

[0002] With the widespread application of fiber-reinforced composite materials in various fields, higher requirements are placed on their performance. Traditional composite laminates have weak tensile and compressive properties in the vertical direction and interlaminar shear properties, and poor impact resistance, which seriously limits the application of composite materials. At present, the main ways to improve the interlaminar performance of composite laminates are three-dimensional weaving, suture toughening and Z-pin reinforcement. Compared with the first two methods, Z-pin reinforcement technology is simple, cheaper, and has higher processing efficiency. It is now widely used in various industries.

[0003] The implantation of Z-pins brings about changes in the local structure of composite laminates: the number of pins in the z-direction is increased, which causes the buckling of the in-plane fibers and forms a resin-rich area between the pins and the surrounding in-plane fibers. Z-pin reinforcement technology can improve the impact resistance of composite materials, thereby improving the performance of composite laminate structures, but there are also problems such as reducing the in-plane tensile and compression properties of the laminates and the fatigue life of the laminates. In addition, the change in microstructure makes the traditional composite laminate theory no longer applicable, and the finite element method based on the single-layer uniform assumption can no longer predict the macroscopic properties of Z-pin three-dimensional reinforced composite laminates. Therefore, in order to solve these problems, it is challenging to establish a strength analysis method for Z-pin three-dimensional reinforced composite laminates based on physical mechanisms and with high prediction accuracy. Summary of the invention

[0004] The purpose of the present invention is to propose a multi-scale analysis method from micro to macro to predict the mechanical properties of Z-pin three-dimensional reinforced composite laminates (ZPLs), targeting the complex problem of strength prediction and analysis of Z-pin three-dimensional reinforced composite laminates (ZPLs). This method has a clear physical mechanism and can better predict its strength and failure process, providing strong support for its preparation process guidance.

[0005] The technical solution of the present invention is as follows:

[0006] A method for predicting the strength of a Z-pin three-dimensional reinforced composite material comprises the following steps:

[0007] S1: Establish the unit cell geometry of a single layer of a Z-pin three-dimensional reinforced composite laminate at the microscopic scale;

[0008] S2: At the mesoscopic scale, based on the unit cell geometry at the microscopic scale, the failure criteria of the fiber, the failure criteria of the matrix, and the failure criteria of the interface between the fiber and the matrix are defined respectively. In the finite element method, multiple adjacent unit cells are further constructed into unit cell models with different fiber volume fraction distributions and orientations. Periodic boundary conditions are applied to the unit cell model to solve the fiber bundle strength, and the stress-strain curves under different fiber volume fraction distributions and orientations are obtained.

[0009] S3: On a macroscopic scale, a finite element model of the entire Z-pin three-dimensional reinforced composite laminate is further established based on the unit cell model on a mesoscopic scale, and the effect of different microstructures in the Z-pin three-dimensional reinforced composite laminate on the macroscopic material strength is predicted by setting failure criteria;

[0010] In the finite element model, three different material models, failure criteria and damage evolution models are used for three different regions within the unit cell, namely, the fiber-containing region, the resin-rich region and the interlayer interface; the 3D Hashin criterion is used for the fiber-containing region, the maximum tensile and compressive stress and maximum shear stress criteria are used for the resin-rich region, and the maximum quadratic nominal stress criterion is used for the interlayer interface.

[0011] Preferably, the application of the periodic boundary condition is implemented in the form of a script.

[0012] Preferably, the material definition in the finite element model is implemented by the Abaqus UMAT subroutine, and the material direction of the fiber deformation area is assigned to each unit by a script.

[0013] Preferably, after the finite element model is established, the influence of the microstructure including the Z-pin diameter, the needle distance and the deflection angle on the macroscopic mechanical properties can be analyzed under the set failure criterion.

[0014] Preferably, the method is used to implement failure evolution analysis of a unit cell of a Z-pin three-dimensional reinforced composite laminate by assuming linear damage evolution.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] The present invention provides a method for analyzing the multi-scale failure strength of a Z-pin three-dimensional reinforced composite material. Based on the complex problem of ZPLs strength prediction and analysis, the method proposes a multi-scale analysis method from micro to macro to predict the mechanical properties of ZPLs. The method has a clear physical mechanism and can better simulate the whole process of ZPLs damage initiation, evolution and final failure, and can more accurately predict its strength and failure process, providing strong support for its preparation process guidance.

[0017] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A flowchart of the basic steps of a method for predicting the strength of a Z-pin three-dimensional reinforced composite material provided by the present invention;

[0019] Figure 2 A schematic diagram of the geometric configuration of a quarter unit cell of a ZPLs monolayer provided in an embodiment of the present invention;

[0020] Figure 3 An analysis process for multi-scale failure strength of a Z-pin three-dimensional reinforced composite material provided by the present invention;

[0021] Figure 4 A typical ply finite element model diagram of ZPLs provided in an embodiment of the present invention; wherein (a) resin-rich region, (b) fiber denaturation region, (d) Pin region, (e) a combined structure of the resin-rich region and the Z-pin region;

[0022] Figure 5 The damage cloud maps of various regions of the quasi-isotropic symmetrical ply ZPLs that can be obtained by the present invention under the x-axis tensile condition; (a) interface damage cloud map (a total of 7 layers of interfaces, from left to right, from top to bottom, the order is the 1st to the 7th layer of interfaces), (b) resin-rich area damage cloud map (tensile failure), (c) fiber-containing area failure cloud map (matrix tensile failure);

[0023] Figure 6 The stress-strain curve diagram of the ZPLs of the quasi-isotropic symmetrical plies obtainable by the present invention under the x-axis tensile condition;

[0024] Figure 7 The unit cell geometry model and mesh division of unidirectional ply ZPLs corresponding to different Z-pin diameters that can be obtained by the present invention; wherein (a) the unit cell geometry model corresponding to R = 0.1 mm, (b) the unit cell mesh division corresponding to R = 0.1 mm, (c) the unit cell geometry model corresponding to R = 0.2 mm, (d) the unit cell mesh division corresponding to R = 0.2 mm;

[0025] Figure 8The macro strength of unidirectional ply ZPLs corresponding to different Z-pin diameters that can be obtained by the present invention; wherein (a) X-direction tensile strength of ZPLs with different Z-pin diameters, (b) X-direction compressive strength of ZPLs with different Z-pin diameters, (c) in-plane shear strength of ZPLs with different Z-pin diameters, (d) interlaminar shear strength of ZPLs with different Z-pin diameters;

[0026] Fig. 9 The macroscopic strength of unidirectional ply ZPLs corresponding to different Z-pin implantation distances that can be obtained by the present invention, including (a) X-direction tensile strength at different Z-pin implantation distances, (b) X-direction compressive strength at different Z-pin implantation distances, (c) in-plane shear strength at different Z-pin implantation distances, and (d) interlaminar shear strength at different Z-pin implantation distances. DETAILED DESCRIPTION

[0027] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation mode of the present invention is described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0028] like Figure 1 As shown, in a preferred embodiment of the present invention, a method for predicting the strength of Z-pin three-dimensional reinforced composite materials is provided, which includes steps S1 to S3. This method spans three scales, namely, microscale, mesoscale and macroscale, and predicts the strength of fiber bundle scale and the strength of ZPLs macroscale respectively. The parameterized finite element model established by this method can analyze the influence of microstructure (Z-pin diameter, needle spacing and deflection angle, etc.) on its macroscopic performance. A specific multi-scale analysis process can be seen in Figure 2 For ease of understanding, the specific implementation process of each step in the method for predicting the strength of Z-pin three-dimensional reinforced composite materials is described in detail below:

[0029] S1: On a microscopic scale, the unit cell geometry of a single layer of Z-pin three-dimensional reinforced composite laminates (hereafter referred to as ZPLs) is established.

[0030] The unit cell geometry needs to be constructed according to the specific ZPLs form. The specific construction method of the unit cell geometry at the microscopic scale in this embodiment is described in detail below:

[0031] In this embodiment, the unit cell geometry of the ZPLs monolayer is a 1 / 4 unit cell model, which is defined as follows: Figure 3 As shown, where: L is 1 / 2 of the longitudinal needle spacing, H is 1 / 2 of the transverse needle spacing, l is 1 / 2 of the fiber deformation zone length, w is 1 / 2 of the fiber deformation zone width, R is the short semi-axis of the elliptical Pin needle, and R' is the long semi-axis.

[0032] Based on the experimental study of the microstructure near the sewing hole, the following two basic assumptions are made before establishing the theoretical model:

[0033] 1) Due to the addition of sewing thread, the fiber is bent and deformed in the deformation zone. The shape of the fiber in this area can be described by the following cosine function:

[0034]

[0035] Where a and b are two parameters describing the fiber morphology. m is the ordinate of the point on the boundary between regions II and III and can be defined by:

[0036]

[0037] 2) Due to the bending deformation of the fiber, the volume content of the fiber in the deformation zone is non-uniformly distributed. The fiber is densest at the edge of the sewing hole, and the farther away from the sewing hole, the sparser the fiber is. At the boundary between area II and area I, the fiber volume content is the smallest, which is the fiber volume content in the undeformed area. Assume that the fiber volume content changes linearly along the y-axis, that is:

[0038] f(0,y)=cy+d (3)

[0039] Where f(0,y) is the fiber volume content at point (0,y) on the y-axis, and c and d are unknown parameters.

[0040] Sewing only changes the distribution of fibers, but does not change the total number of fibers, that is, the fiber volume does not change before and after sewing. From hypothesis 2), we can get:

[0041]

[0042] Then the fiber volume content f(0,R) at point (0,R) can be expressed as:

[0043]

[0044] Where f 0 is the fiber volume content of the undeformed zone, that is, the average fiber volume content of the composite material. From assumption 2), we know that the fiber volume content at the boundary (0,w) is f(0,w) = f 0, together with formula (5), the unknown parameters in function (3) can be determined:

[0045]

[0046] Then the fiber volume content at any point on the y-axis is obtained:

[0047]

[0048] Assume that the intersection of the fiber passing through a point (x, y) in the deformation zone and the y-axis is (0, y 0 ), and the intersection point with the straight line x=l is (l,y l ). Since the bending of the fiber does not change the volume of the fiber, the geometric relationship between the two intersection points can be expressed by the following formula:

[0049]

[0050] The two intersection points are points on the same fiber and should satisfy the shape function (1) of the fiber at the same time, and then determine the two parameters of the shape function:

[0051]

[0052] Since the point (x, y) is also a point on the fiber, it satisfies the fiber equation (1), and the ordinate y of the intersection of the fiber and the y-axis can be obtained: 0 :

[0053]

[0054] Since sewing does not change the volume of the fiber, the volume of the fiber remains unchanged in the micro-segment around the curve defined by equation (1), and its mathematical expression is:

[0055] f(x,y)dy=f(0,y 0 )dy 0 (R≤y 0 ≤w) (11)

[0056] get:

[0057]

[0058] Combining equations (7), (10) and (12), the fiber volume content at any point in the deformation zone is obtained:

[0059]

[0060] To describe the deflection angle of the fiber, we have the following relationship:

[0061]

[0062] According to equations (9), (10) and (14), the fiber deflection angle φ can be expressed as:

[0063]

[0064] At this point, the fiber deflection angle and fiber volume content at any point in the fiber deformation zone are determined by explicit mathematical expressions, in which three mesostructure parameters are used: deformation zone length, deformation zone width, and sewing hole minor axis.

[0065] The fiber deflection angles and volume contents in other regions are as follows:

[0066] Region I:

[0067]

[0068] Region III and Region IV:

[0069]

[0070] S2: At the mesoscopic scale, based on the unit cell geometry at the microscopic scale, the failure criteria of the fiber, the failure criteria of the matrix, and the failure criteria of the interface between the fiber and the matrix are defined respectively. In the finite element method, multiple adjacent unit cells are further constructed as unit cell models with different fiber volume fraction distributions and orientations. Periodic boundary conditions are applied to the unit cell model to solve the fiber bundle strength and obtain the stress-strain curves under different fiber volume fraction distributions and orientations. The purpose of this step is to assign material parameters and directions to each material point according to the distribution and orientation of different fiber volume fractions in the unit cell of ZPLs.

[0071] In the unit cell model, it is necessary to consider all other unit cells around the unit cell that may affect its performance, and the specific number and range need to be determined according to actual conditions. The specific approach at the mesoscopic scale in this embodiment is described in detail below:

[0072] 1. Fiber failure and its evolution

[0073] The fiber failure modes are fiber tensile failure, compression failure, and longitudinal and transverse shear failure. After any mode failure, the stiffness is reduced to zero. The stiffness matrix can be specifically expressed as follows:

[0074]

[0075] Among them, the elastic modulus E i , shear modulus G ij and Poisson's ratio v ij Characterizes the stiffness matrix of the material, 1, 2 and 3 represent the main directions of the material, where direction 1 is the direction of the fiber, and directions 2 and 3 represent two orthogonal directions perpendicular to the fiber.

[0076]

[0077] In the formula, σ 1 is the normal stress in the fiber direction; τ 12 , τ 13 are the shear stresses in two directions respectively; t , X c and S L are the tensile, compressive and longitudinal and transverse shear strengths of the fiber respectively; d t , d c and d s are the tensile, compressive and shear damage variables respectively; d is the total damage variable, d = 0 means no failure occurs, d = 1 means complete failure occurs.

[0078] 2. Matrix failure and its evolution

[0079] The matrix failure modes include tensile failure, compression failure and shear failure. After any mode failure, the stiffness matrix is ​​reduced to zero, and the stiffness evolution is as follows:

[0080]

[0081] in

[0082]

[0083] In the formula, σ I , σ III are the first and third principal stresses of the matrix; is the maximum shear stress on the matrix; X t , X c and S L are the tensile, compressive and shear strengths of the matrix respectively; d t , d c and d s are the tensile, compressive and shear damage variables respectively; d is the total damage variable. Like the fiber, d = 0 means no failure and d = 1 means complete failure.

[0084] 3. Interface failure and its evolution

[0085] The interface failure is simulated using Cohesive units, and the constitutive model uses the Traction-separation constitutive model. The failure criterion uses the maximum quadratic nominal stress criterion, that is:

[0086]

[0087] where t n is the normal component of the force vector, i.e., the normal normal stress, and t s , t t are two shear stress components on the interface; is the normal tensile strength, are the two shear strengths of the interface respectively.

[0088] The damage evolution of the interface adopts linear damage evolution, and the evolution method is shown in the following formula:

[0089]

[0090] where D is the damage variable, which is defined by the following formula:

[0091]

[0092] In the formula, δ m is the distance between the material points on both sides of a point on the interface, i.e.:

[0093]

[0094] where δ n is the normal component of the separation distance, δ s and δ t are the two tangential components of the separation distance, is the separation distance at initial damage, is the distance at complete damage, is the current maximum separation amount. It can be seen from the definition of the damage amount that D = 0 means that no interface failure has occurred, 0 < D < 1 means that failure has occurred but it can still bear load and has not completely failed, and D = 1 means that the interface has lost its load-bearing capacity and has completely failed.

[0095] 4. Finite element model

[0096] The establishment of the finite element model includes five steps: establishing a geometric model, meshing, defining material properties, applying boundary conditions, solving and post-processing. The specific implementation of each step belongs to the existing technology of finite element modeling. For the geometric model, the fiber volume fraction in the fiber deformation area changes with the position, and the volume fraction ranges from about 50% to 85%. Therefore, it is necessary to establish a single cell model of the fiber bundle under different volume fractions and orientations. Assume that the fibers are closely arranged in a hexagonal manner, that is, each fiber is evenly surrounded by 6 fibers. In this way, a single cell model of a mesoscopic fiber bundle can be established based on the single cell geometry. Before the single cell meshing, the geometric segmentation is performed to facilitate the use of the structured grid with the best calculation accuracy for meshing. Secondly, the stiffness and strength properties of the fiber and matrix resin, the stiffness and strength properties of the interface are input. Finally, periodic boundary conditions are applied to the entire single cell model for solution. Among them, the application of periodic boundary conditions is realized in the form of scripts.

[0097] S3: At the macroscopic scale, a finite element model of the entire Z-pin three-dimensional reinforced composite laminate is further established based on the unit cell model at the mesoscopic scale, and the influence of different microstructures in the Z-pin three-dimensional reinforced composite laminate on the final macroscopic mechanical properties, i.e., material strength, is predicted by setting failure criteria. Thus, the material strength of ZPLs under different microstructures can be predicted through simulation, providing a data basis for the optimization of the material preparation process.

[0098] In the finite element model, three different material models, failure criteria and damage evolution models are used for the three different regions of the unit cell, namely the fiber-containing region, the resin-rich region and the interlayer interface; the 3DHashin criterion is used for the fiber-containing region, the maximum tensile and compressive stress and maximum shear stress criteria are used for the resin-rich region, and the maximum quadratic nominal stress criterion is used for the interlayer interface. Specifically, the definition of materials in the finite element model is implemented through the Abaqus UMAT subroutine, and the material direction of the fiber deformation region is assigned to each unit through a script.

[0099] The specific method in this embodiment on a macro scale is described in detail below:

[0100] 1. Fiber-containing area

[0101] For the fiber-containing area (i.e., fiber deformation area, fiber non-deformation area, and Pin area), the failure criterion adopts the three-dimensional Hashin criterion. Hashin classifies the failure modes of unidirectional composite materials into four types, namely, fiber direction tensile failure, fiber direction compression failure, matrix tensile failure, and matrix tensile failure. The initial failure criterion is:

[0102] 1) Tensile failure in fiber direction

[0103]

[0104] 2) Compression failure in fiber direction

[0105]

[0106] 3) Matrix tensile failure

[0107]

[0108] 4) Matrix compression failure

[0109]

[0110] In the above types Indicates equivalent stress, subscripts 1, 2, and 3 represent the fiber direction, the direction perpendicular to the fiber in a single layer, and the thickness direction of the ply (the same below). 11 , σ 22 , σ 33 , τ 12 , τ 13 , τ 23 are the components of the independent stress tensor; X T , X C , Y T , Y C , S T , S L They are respectively the tensile and compressive strength in the fiber direction, the tensile and compressive strength perpendicular to the fiber direction, the in-plane shear strength perpendicular to the fiber, and the out-of-plane shear strength. α represents the contribution of shear stress to the initial failure in the fiber direction, which can be taken as 1.

[0111] Once the initial criterion occurs, the material stiffness is reduced, that is, damage evolution occurs. The stiffness reduction is also calculated using the damage amount. Assuming that the stiffness is reduced linearly, the reduced stiffness can be expressed as:

[0112]

[0113] in

[0114]

[0115] Where d f , d m , d s are the damage variables in the fiber direction, matrix direction and the total damage variable, respectively. ft , d fc , d mt , d mcThey represent the damage caused by tension and compression in the fiber direction and tension and compression in the matrix direction, respectively. The damage is defined by the following formula:

[0116]

[0117] where δ I，eq is the equivalent displacement, is the equivalent displacement at the initial damage, are the equivalent displacements when fully damaged, and they are calculated by the following formulas:

[0118]

[0119] Among them G I is the fracture energy, F I is the failure criterion of the fiber-containing area; measured by experiment; σ I，eq is the equivalent stress, is the equivalent stress at the initial failure; the definitions of equivalent displacement and equivalent stress are as follows:

[0120] 1) Fiber stretching

[0121]

[0122]

[0123] 2) Fiber compression

[0124] δ fc，eq =L c <-ε 11 > (36)

[0125]

[0126] 3) Matrix stretching

[0127]

[0128]

[0129] 4) Matrix compression

[0130]

[0131]

[0132] In the above formulas, L c represents the characteristic length of the unit, ε 11 , ε 22 , ε 33 , ε 12 , ε 13 , ε 23are independent components of the strain tensor.

[0133] The stiffness reduction shown in formula (30) is a linear reduction, and the corresponding constitutive relationship between equivalent stress and equivalent displacement is a bilinear constitutive relationship.

[0134] 2. Resin-rich area

[0135] The maximum tensile stress criterion, the maximum compressive stress criterion, and the maximum shear stress criterion are also used to determine whether failure has occurred. Specifically, the matrix failure and its evolution in S2 can be seen.

[0136] 3. Interface between layers

[0137] For the interfaces between the single layers, the maximum quadratic nominal stress criterion based on the Cohesive unit is also adopted. The interface failure and its evolution in S2 can be seen in detail.

[0138] 4. Finite element model

[0139] According to the geometric parameters determined by the unit cell geometry established in S1, the finite element model of ZPLs is established. Taking the quasi-isotropic symmetric ply ([0° / 45° / -45° / 90°]s) as an example, the finite element model is as follows: Figure 4 As shown in FIG. 4 , finite element modeling includes five steps: establishing a geometric model, meshing, defining material properties, applying boundary conditions, solving, and post-processing. The specific implementation of each step belongs to the existing technology of finite element modeling.

[0140] The above method for predicting the strength of Z-pin three-dimensional reinforced composite materials starts from the micro-scale fibers and matrices, and uses the finite element method to analyze the strength of fiber bundles at different fiber volume fractions, as well as the influence of the interface between the fiber matrix on the strength of the fiber bundle. In addition, this method can obtain the mesoscopic properties of ZPLs through micro-scale analysis, that is, the distribution of modulus and strength in different regions within the unit cell. By using this information, combined with the 3D Hashin failure criterion, the maximum secondary nominal stress failure criterion of the interface, and the maximum tensile and compressive stress and maximum shear stress criteria in the resin-rich area, the macroscopic strength information of ZPLs can be predicted. Moreover, in the above method, the failure evolution analysis of the ZPLs unit cell can be realized through the assumption of linear damage evolution.

[0141] In this embodiment, the tensile condition of quasi-isotropic symmetrical plies ZPLs is taken as an example to demonstrate the technical effect. In this example, multi-scale progressive damage analysis is performed, and the respective failure processes and stress-strain curves are given. The material parameters of the finite element model are shown in Tables 1 to 4.

[0142] Table 1 Fiber stiffness and strength information

[0143]

[0144] Table 2 Matrix material parameters

[0145]

[0146] Table 3 Material parameters of the interlayer interface

[0147]

[0148] Table 4 Material parameters of the pin interface

[0149]

[0150] After analysis, the failure mode of ZPLs of quasi-isotropic symmetrical plies under x-axis tensile conditions is: first, the interlayer interface fails, then the resin-rich area fails, and finally the matrix fails in the fiber deformation area. The cloud diagram of interface failure is shown in Figure 5 (a), damage cloud map of resin-rich area Figure 5 (b), failure cloud map of fiber-containing area Figure 5 (c), longitudinal tensile stress-strain curve is shown in Figure 6 shown.

[0151] In addition, by parametric modeling of ZPLs, the influence of microstructure including Z-pin diameter, needle distance and deflection angle on macroscopic mechanical properties can be analyzed. In this embodiment, the control variable method is used to study the influence of Z-pin diameter and implantation density of ZPLs on the macroscopic strength of ZPLs. The results of the two aspects are shown as follows:

[0152] 1. Two Z-pin diameters, R = 0.1 mm and R = 0.2 mm, were selected respectively. The unit cell geometry model and finite element mesh corresponding to the two Z-pin diameters are shown in Figure 7 As shown. After calculation, the macroscopic strength of ZPLs under different Z-pin diameters is shown in Figure 8 As shown. From the results, we can see that (1) at the same implantation density, the larger the Z-pin diameter, the smaller the in-plane tensile, compressive and shear strengths. This is because the implantation of the Z-pin causes uneven fiber distribution and introduces resin-rich areas, which causes in-plane stress concentration and ultimately leads to a decrease in strength. (2) The effect of the Z-pin diameter on the interlaminar shear strength is small and can be ignored, but it can be seen that the larger the pin diameter, the greater the interlaminar fracture toughness (area under the stress-strain curve). It can be seen that compared with R=0.2mm, the use of a Z-pin with R=0.1mm can improve the in-plane strength without affecting the interlaminar strength.

[0153] 2. When the same Z-pin diameter (R = 0.1 mm) is set, two different needle distances of L = 6.6 mm and L = 3.3 mm are selected respectively, and the macroscopic strength information under different Z-pin implantation needle distances is obtained. Fig. 9 As shown in the figure. The analysis results show that increasing the density of Z-pin implantation (reducing the pin spacing) will significantly reduce the in-plane tensile and compressive strength. On the contrary, the in-plane shear strength after densification is improved, but the magnitude is small. As for the interlayer shear strength, as the Z-pin implantation density increases, the interlayer shear strength will also increase accordingly, as shown in the figure. Fig. 9 As shown, compared with L=6.6mm, L=3.3mm increases the interlaminar shear strength by 6.3%.

[0154] The above-described embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A method for predicting the strength of a Z-pin three-dimensional reinforced composite material, characterized in that: The following steps are involved: S1: Establish the unit cell geometry of a single layer of a Z-pin three-dimensional reinforced composite laminate at the microscopic scale; S2: At the mesoscopic scale, based on the unit cell geometry at the microscopic scale, the failure criteria of the fiber, the failure criteria of the matrix, and the failure criteria of the interface between the fiber and the matrix are defined respectively. In the finite element method, multiple adjacent unit cells are further constructed into unit cell models with different fiber volume fraction distributions and orientations. Periodic boundary conditions are applied to the unit cell model to solve the fiber bundle strength, and the stress-strain curves under different fiber volume fraction distributions and orientations are obtained. S3: On a macroscopic scale, a finite element model of the entire Z-pin three-dimensional reinforced composite laminate is further established based on the unit cell model on a mesoscopic scale, and the effect of different microstructures in the Z-pin three-dimensional reinforced composite laminate on the macroscopic material strength is predicted by setting failure criteria; In the finite element model, three different material models, failure criteria and damage evolution models are used for three different regions within the unit cell, namely, the fiber-containing region, the resin-rich region and the interlayer interface; the 3DHashin criterion is used for the fiber-containing region, the maximum tensile and compressive stress and maximum shear stress criteria are used for the resin-rich region, and the maximum quadratic nominal stress criterion is used for the interlayer interface.

2. A method for predicting the strength of a Z-pin three-dimensional reinforced composite material according to claim 1, characterized in that: The application of the periodic boundary conditions is achieved in the form of a script.

3. The method for predicting the strength of a Z-pin three-dimensional reinforced composite material according to claim 1, characterized in that: The material definition in the finite element model is implemented through the Abaqus UMAT subroutine, and the material direction of the fiber deformation area is assigned to each unit through a script.

4. The method for predicting the strength of a Z-pin three-dimensional reinforced composite material according to claim 1, characterized in that: After the finite element model is established, the influence of the microstructure including the Z-pin diameter, the needle distance and the deflection angle on the macroscopic mechanical properties is analyzed under the set failure criterion.

5. The method for predicting the strength of a Z-pin three-dimensional reinforced composite material according to claim 1, characterized in that: This method is used to realize the failure evolution analysis of Z-pin three-dimensional reinforced composite laminate unit cell through the assumption of linear damage evolution.

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