A method for evaluating the corrosion fatigue life of metal-composite bonded structures

Abstract

The present invention relates to the technical field of composite material fatigue life assessment, and specifically to a method for assessing the corrosion fatigue life of a metal-composite adhesively bonded structure, comprising the following steps: establishing a finite element model of the metal-composite adhesively bonded structure after pre-corrosion; establishing a theoretical model of progressive fatigue damage of metal plates, composite plates, and adhesive layers; and constructing a method for assessing the fatigue life of a metal-composite adhesively bonded structure after pre-corrosion. By considering the effects of the corrosive environment on the metal plates, composite plates, and adhesive layers, the present invention combines the theoretical model of progressive fatigue damage of the metal plates, composite plates, and adhesive layers with finite element analysis technology, thereby enabling effective assessment of the corrosion fatigue life and progressive damage expansion process of the metal-composite adhesively bonded structure, thereby achieving the purpose of considering the effects of the corrosive environment and fatigue load on the composite plates and adhesive layers.

Description

Technical Field

[0001] The present invention relates to the technical field of composite material fatigue life assessment, in particular to a corrosion fatigue life assessment method for a metal-composite material adhesively bonded structure. Background Art

[0002] With the increasing demand for lightweight, high-strength, and long-life structures in advanced aircraft, lightweight alloys (such as aluminum and titanium alloys) and composites have become important aerospace structural materials. Joining metal and composite structures is unavoidable during structural assembly. Adhesive bonding, which bonds the two materials together using adhesives, prevents damage to the composite's internal fibers and overcomes the stress concentration issues associated with mechanical connections. Therefore, it is widely used in metal-composite joints. During service, bonded metal-composite structures must withstand not only long-term fatigue loads but also harsh environmental conditions such as temperature, humidity, and corrosion. Metals are susceptible to electrochemical corrosion in humid environments, producing large amounts of corrosion products under the influence of corrosive media. While composite materials and adhesive layers offer good corrosion resistance, they are susceptible to hygroscopic aging in humid environments. Furthermore, the significant difference in the wet expansion coefficients of fibers and resins can generate significant internal wet stresses. These factors can affect the fatigue performance and damage mechanisms of bonded metal-composite structures, posing a serious threat to aircraft safety and reliability. Therefore, it is crucial to pay close attention to the corrosion fatigue behavior of bonded metal-composite structures.

[0003] However, when existing technologies are actually used, research on the corrosion fatigue performance and life assessment methods of connection structures is mostly focused on metal-metal connection structures, and there is less relevant research on metal-composite connection structures. A few existing connection structure corrosion fatigue life assessment methods find it difficult to consider the impact of corrosion environment and fatigue load on composite material plates and adhesive layers. Summary of the Invention

[0004] The purpose of the present invention is to provide a corrosion fatigue life assessment method for metal-composite adhesively bonded structures to solve the problem that it is difficult to consider the effects of the corrosion environment and fatigue load on the composite material plate and adhesive layer.

[0005] To achieve the above object, the present invention provides the following technical solution: a method for evaluating the corrosion fatigue life of a metal-composite bonded structure, comprising the following steps:

[0006] S1. Establish a finite element model of the metal-composite bonded structure after pre-corrosion, specifically:

[0007] S11. Based on the scanning measurement and statistical analysis results of the corrosion depth of the metal plate surface, the corrosion damage is explicitly modeled and a finite element model of the metal plate with a random corrosion depth surface is established;

[0008] S12. Based on the geometric dimensions and moisture absorption measurement results of the composite material plate, implicit modeling of moisture absorption aging damage is performed to establish a finite element model of the composite material plate that reduces the material strength performance;

[0009] S13. Insert zero-thickness cohesive elements at the joints between the metal and composite plate assemblies to simulate the adhesive layer. Based on the corrosion environment and duration, the adhesive layer material properties are discounted to implicitly model the moisture absorption and aging damage of the adhesive layer.

[0010] S2. Establish a theoretical model of progressive fatigue damage of metal plates, composite materials and adhesive layers, specifically:

[0011] S21. For the metal plate part, the progressive fatigue cumulative damage theoretical model of the material is calculated according to the Miner linear cumulative damage model. It is assumed that the stiffness and strength properties of the material do not change during fatigue loading, and the cumulative damage coefficient reaching 1 is used as the fatigue failure criterion;

[0012] S22. For the composite plate, a theoretical model of progressive fatigue cumulative damage is calculated based on the residual strength model. The residual strength of the material is gradually degraded during fatigue loading. The strength value in the Tsai–Wu failure criterion is replaced by the fatigue residual strength value. When the criterion judgment coefficient reaches 1, fatigue failure is considered to have occurred. In other words, the criterion judgment coefficient is used as the fatigue failure criterion for the composite plate.

[0013] S23. For the adhesive layer, a two-parameter fatigue cumulative damage model is introduced into the bilinear cohesive constitutive relation to establish a cohesive fatigue damage model. Based on this model, a theoretical model for the progressive fatigue cumulative damage of the material is calculated. The residual stiffness of the material is gradually degraded during fatigue loading, and the cumulative damage coefficient reaching 1 is used as the fatigue failure criterion.

[0014] S3. Construct a fatigue life assessment method for pre-corroded metal-composite bonded structures, specifically:

[0015] The established theoretical model of progressive fatigue damage of metal plates, composite plates, and adhesive layers is combined with finite element analysis technology. Fatigue loads are applied to the established finite element model of metal-composite adhesively bonded structures. Through iterative calculations of stress / strain analysis of each unit, fatigue damage accumulation, and failure judgment, the fatigue life and progressive damage expansion process of the adhesively bonded structure are evaluated.

[0016] Preferably, the scanning measurement of the metal plate surface corrosion depth in step S11 adopts non-contact three-dimensional topography scanning technology to obtain high-precision corrosion surface topography data, and the statistical analysis in step S11 adopts Weibull distribution fitting.

[0017] Preferably, the fatigue load in step S3 is applied by constant amplitude loading, and the number of loading cycles is recorded until the entire bonded structure is destroyed.

[0018] Preferably, in step S3, finite element analysis software is used to perform iterative calculations to simulate the progressive damage expansion process of the metal-composite bonded structure under a corrosive environment and fatigue load, and output fatigue life assessment results.

[0019] Preferably, during the iterative calculation process of step S3, the units that meet the fatigue failure criterion are deleted from the finite element model, and the iterative calculation is continued on the remaining units until the entire structure is destroyed.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The present invention considers the influence of the corrosive environment on metal plates, composite plates and adhesive layers, combines the theoretical model of progressive fatigue damage of metal plates, composite plates and adhesive layers with finite element analysis technology, and can realize effective evaluation of the corrosion fatigue life and progressive damage expansion process of metal-composite adhesive structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a system block diagram of a corrosion fatigue life assessment method for a metal-composite bonded structure according to the present invention. DETAILED DESCRIPTION

[0023] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0024] See also Figure 1 The present invention provides a technical solution: a method for evaluating the corrosion fatigue life of a metal-composite bonded structure, comprising the following steps:

[0025] S1. Establish a finite element model of the metal-composite bonded structure after pre-corrosion:

[0026] S11. For the metal plate, based on the surface morphology scan results of the metal plate after the pre-corrosion test, it can be generally assumed that the corrosion depth on the rough surface after corrosion follows a Weibull distribution. Mathematical statistics methods are used to calculate the probability distribution parameters of the corrosion depth. Then, based on the geometric dimensions of the metal plate and the statistical analysis results of the corrosion depth, the corrosion damage is explicitly modeled, and a finite element model of the metal plate with a random corrosion depth surface is established.

[0027] S12. For the composite material plate, due to the excellent corrosion resistance of the composite material, it is believed that it will not produce damage such as corrosion pits in a corrosive environment. Therefore, a finite element model of the composite material plate is established based on the geometric dimensions of the composite material plate. However, since the composite material is sensitive to a humid environment and is prone to a decrease in bearing capacity after moisture absorption, the material strength performance is reduced based on the measurement results of the moisture absorption of the composite material plate after the pre-corrosion test, and the moisture absorption aging damage of the composite material plate is implicitly modeled.

[0028] S13. Assemble the finite element models of the metal plate and the composite plate, and simulate the adhesive layer by inserting zero-thickness cohesive elements at the joints. Similarly, since adhesives are prone to moisture absorption and aging damage in corrosive and humid environments, the material properties of the adhesive layer are reduced according to the environmental conditions and duration of the pre-corrosion test, and the moisture absorption and aging damage of the adhesive layer are implicitly modeled.

[0029] S2. Establish theoretical models of progressive fatigue damage of metal plates, composite materials and adhesive layers:

[0030] S21. For the metal plate part, the Miner linear cumulative damage model is used to calculate the material's progressive fatigue cumulative damage theoretical model. The formula is:

[0031]

[0032] Where D m is the fatigue cumulative damage coefficient of the metal plate;

[0033] Since the damage propagation of the connection structure during fatigue will lead to stress / strain redistribution, the fatigue stress level of the element will change even under constant amplitude fatigue external load. i and N i are the number of loading cycles and fatigue life under the i-th stress level, respectively, where the fatigue life N i It can be determined by the fatigue stress-life (SN) curve model, and the model expression can be written as:

[0034] (S-S0) m N=C (2)

[0035] Where S is fatigue stress, N is fatigue life, S0 is fitting fatigue limit, and m and C are model parameters.

[0036] It is generally believed that the stiffness and strength properties of metal materials do not change during fatigue cyclic loading. According to the linear cumulative damage theory, when the cumulative damage coefficient D m When it reaches 1, fatigue failure is considered to have occurred, that is, D m =1 as the fatigue failure criterion for metal plates.

[0037] S22. For composite material plates, the residual strength model is used to calculate the material's progressive fatigue cumulative damage theoretical model. The fatigue cumulative damage coefficient based on the residual strength can be defined as

[0038]

[0039] Where D c is the fatigue cumulative damage coefficient of the composite plate, R(n) is the residual strength of the material, and R0 is the initial strength of the material.

[0040] The strength properties of composite materials will gradually degrade under fatigue cyclic loading. The residual strength R(n) after degradation can be determined by the fatigue residual strength model. The model expression can be written as

[0041] n=A(SK) p [R0-R(n)] q (4)

[0042] Where n is the number of loading cycles, and A, K, p, and q are model parameters.

[0043] Similarly, since the fatigue stress level of the unit will change, by transforming Equation (4), the relationship between the residual strength before and after a loading cycle at any stress level can be obtained, that is,

[0044]

[0045] The residual strength of the composite material is gradually degraded according to formula (5).

[0046] Since composite materials are anisotropic materials, their strength properties in various directions (including longitudinal, transverse, and shear directions) are different. Therefore, it is necessary to degrade the residual strength values ​​in various directions under fatigue loading according to formula (5), and then calculate the fatigue cumulative damage coefficient in various directions according to formula (3).

[0047] In addition, based on the Tsai–Wu failure criterion to judge fatigue failure, the expression can be written as

[0048]

[0049] Where,

[0050]

[0051] is the Tsai–Wu criterion judgment coefficient, σ 11 , σ 22 and σ 33 is the normal stress, σ 12 , σ 13 and σ 23is the shear stress, X T and X C are longitudinal tensile strength and compressive strength, Y T and Y C are the transverse tensile strength and compressive strength, S 12 、S 13 and S 23 It is worth noting that these strength values ​​will gradually degrade with the increase of the number of loading cycles. Therefore, the strength values ​​in the Tsai–Wu criterion are replaced by fatigue residual strength values.

[0052] When the criterion judgment coefficient When it reaches 1, fatigue failure is considered to have occurred, that is, the criterion judgment coefficient is used. as a fatigue failure criterion for composite plates.

[0053] S23. For the adhesive layer, a bilinear model is used to characterize the continuous damage constitutive relation of the cohesive unit under quasi-static loading, where the damage initiation criterion adopts the quadratic stress criterion and the damage evolution law adopts a linear model.

[0054] On this basis, the two-parameter progressive fatigue cumulative damage theoretical model is introduced into the bilinear cohesive constitutive relation, and the cohesive progressive fatigue cumulative damage theoretical model is established to calculate the fatigue damage of the adhesive layer material, namely:

[0055]

[0056] Where D a is the fatigue cumulative damage coefficient of the adhesive layer, σ e and σ ec is the equivalent stress and its critical value, γ and α are model parameters.

[0057] Similarly, since the fatigue stress level of the unit will change, by transforming Equation (8), we can obtain the relationship between the cumulative damage coefficient before and after a loading cycle at any stress level, that is,

[0058]

[0059] The fatigue damage of the adhesive layer material is gradually accumulated according to formula (9).

[0060] During the fatigue damage accumulation process, the residual stiffness performance of the material will gradually degrade, and the stiffness degradation model can be written as

[0061]

[0062] Where K is the residual stiffness of the material, K0 is the initial stiffness of the material, δ0 and δ f are the damage initiation displacement and failure displacement, respectively.

[0063] In addition, when the cumulative damage coefficient D a When it reaches 1, fatigue failure is considered to have occurred, that is, D a =1 is used as the fatigue failure criterion for the adhesive layer.

[0064] S3. Establish a fatigue life assessment method for metal-composite bonded structures after pre-corrosion:

[0065] S31. Combine the theoretical model of progressive fatigue damage with finite element analysis technology, apply fatigue load to the established finite element model of metal-composite bonded structure, simplify one load cycle in the actual loading process to apply the maximum load, calculate the stress / strain distribution results, accumulate the fatigue damage of each unit, and judge whether the unit fails according to the fatigue failure criterion. If a failed unit occurs, it will be killed. Repeat the calculation and record the number of loading cycles until the overall damage of the bonded structure occurs.

[0066] S32. The number of loading cycles recorded when the overall failure of the bonded structure occurs is the fatigue life estimation result. This method can also be used to obtain the progressive expansion process of fatigue damage in the metal-composite bonded structure after pre-corrosion.

[0067] The present invention includes three core steps: first, a finite element model of the metal-composite adhesive bonded structure after pre-corrosion is established, which takes into account the random corrosion depth of the metal plate surface, the moisture absorption and aging damage of the composite plate, and the performance reduction of the adhesive layer; second, a theoretical model of progressive fatigue damage of the metal plate, the composite plate, and the adhesive layer is established, and the Miner linear cumulative damage model, the residual strength model, and the two-parameter fatigue cumulative damage model are used to calculate the fatigue cumulative damage of each part, and the corresponding fatigue failure criteria are defined; finally, a fatigue life assessment method for the metal-composite adhesive bonded structure after pre-corrosion is constructed, combining the theoretical model with the finite element analysis technology, and through the iterative calculation of the stress / strain analysis of each unit, fatigue damage accumulation, and failure judgment, an effective assessment of the fatigue life and progressive damage expansion process of the adhesive structure is achieved.

[0068] Specifically, this invention considers the effects of the corrosive environment on metal plates, composite plates, and adhesive layers. Through explicit and implicit modeling, corrosion damage and hygroscopic aging damage are incorporated into the finite element model. Furthermore, different fatigue damage calculation models and failure criteria are employed for metal plates, composite plates, and adhesive layers to more accurately reflect their fatigue behavior in actual working environments. By combining finite element analysis with fatigue damage accumulation calculations, a precise assessment of the corrosion fatigue life of metal-composite bonded structures is achieved, thereby achieving the goal of considering the effects of the corrosive environment and fatigue loads on composite plates and adhesive layers, and providing important technical support for related engineering applications.

[0069] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0070] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for evaluating the corrosion fatigue life of a metal-composite bonded structure, characterized by: The following steps are involved: S1. Establish a finite element model of the metal-composite bonded structure after pre-corrosion, specifically: S11. Based on the scanning measurement and statistical analysis results of the corrosion depth of the metal plate surface, the corrosion damage is explicitly modeled and a finite element model of the metal plate with a random corrosion depth surface is established; S12. Based on the geometric dimensions and moisture absorption measurement results of the composite material plate, implicit modeling of moisture absorption aging damage is performed to establish a finite element model of the composite material plate that reduces the material strength performance; S13. Insert zero-thickness cohesive elements at the joints between the metal and composite plate assemblies to simulate the adhesive layer. Based on the corrosion environment and duration, the adhesive layer material properties are discounted to implicitly model the moisture absorption and aging damage of the adhesive layer. S2. Establish a theoretical model of progressive fatigue damage of metal plates, composite materials and adhesive layers, specifically: S21. For the metal plate part, the progressive fatigue cumulative damage theoretical model of the material is calculated according to the Miner linear cumulative damage model. It is assumed that the stiffness and strength properties of the material do not change during fatigue loading, and the cumulative damage coefficient reaching 1 is used as the fatigue failure criterion; S22. For the composite plate, a theoretical model of progressive fatigue cumulative damage is calculated based on the residual strength model. The residual strength of the material is gradually degraded during fatigue loading. The strength value in the Tsai–Wu failure criterion is replaced by the fatigue residual strength value. When the criterion judgment coefficient reaches 1, fatigue failure is considered to have occurred. In other words, the criterion judgment coefficient is used as the fatigue failure criterion for the composite plate. S23. For the adhesive layer, a two-parameter fatigue cumulative damage model is introduced into the bilinear cohesive constitutive relation to establish a cohesive fatigue damage model. Based on this model, a theoretical model for the progressive fatigue cumulative damage of the material is calculated. The residual stiffness of the material is gradually degraded during fatigue loading, and the cumulative damage coefficient reaching 1 is used as the fatigue failure criterion. S3. Construct a fatigue life assessment method for pre-corroded metal-composite bonded structures, specifically: The established theoretical model of progressive fatigue damage of metal plates, composite plates, and adhesive layers is combined with finite element analysis technology. Fatigue loads are applied to the established finite element model of metal-composite adhesively bonded structures. Through iterative calculations of stress / strain analysis of each unit, fatigue damage accumulation, and failure judgment, the fatigue life and progressive damage expansion process of the adhesively bonded structure are evaluated.

2. The corrosion fatigue life assessment method for metal-composite bonded structures according to claim 1, characterized in that: The scanning measurement of the corrosion depth of the metal plate surface in step S11 adopts non-contact three-dimensional topography scanning technology to obtain high-precision corrosion surface topography data, and the statistical analysis in step S11 adopts Weibull distribution fitting.

3. The corrosion fatigue life assessment method for metal-composite bonded structures according to claim 1, characterized in that: The fatigue load in step S3 is applied in a constant amplitude loading manner, and the number of loading cycles is recorded until the entire bonded structure is destroyed.

4. The corrosion fatigue life assessment method for a metal-composite adhesively bonded structure according to claim 1, characterized in that: In step S3, finite element analysis software is used to perform iterative calculations to simulate the progressive damage expansion process of the metal-composite bonded structure under a corrosive environment and fatigue load, and output fatigue life assessment results.

5. The corrosion fatigue life assessment method of a metal-composite adhesively bonded structure according to claim 1, characterized in that: During the iterative calculation process of step S3, the units that meet the fatigue failure criterion are deleted from the finite element model, and the iterative calculation is continued for the remaining units until the entire structure is destroyed.

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