基于有限元法的飞机蒙皮裂纹加强补片优化设计

胶接增强修复补片的可设计性是胶接技术的优点之一,科学设计飞机结构损伤部位的增强补片是保证修补效果的重要因素。本文利用 ANSYS 软件对飞机2024蒙皮裂纹复合材料增强补片进行三维有限元建模与仿真计算,计算了复合材料单级补片和多级补片边缘胶层最大剪应力,给出了飞机2024蒙皮裂纹单侧修补时的补片优化设计参数。计算表明,用复合材料增强修补铝合金蒙皮裂纹时,补片应设计成多级形式;当补片级数超过5级时,补片边缘的胶层剪应力显著减小,与单级补片相比,其最大剪应力下降了85%。     

复合材料补片参数对裂纹尖端应力强度因子的影响

利用有限元法对复合材料补片修补前后的铝合金薄板的裂纹尖端应力强度因子KI进行研究,分析各类补片参数对裂纹尖端应力强度因子的影响.结果表明,在正确选择复合材料补片的参数后,修补后铝合金板裂纹尖端的应力强度因子有显著地下降.     

Analysis of a kinked crack in an anisotropic material under antiplane deformation

This paper analyzes the asymptotic problem of a kinked crack in an anisotropic material under antiplane deformation. Using the linear transformation method proposed in this paper, a solution to the asymptotic problem of a kinked crack in an anisotropic material can be obtained from the solution of the corresponding isotropic kinked crack problem. The exact solution of the stress intensity factor for the kinked crack in the anisotropic material is obtained from the solution of the isotropic problem. The effect of the kink angle and two anisotropic parameters on the stress intensity factor is discussed for the inclined orthotropic material as well as the anisotropic material. In order to verify the exact solution of the stress intensity factor, numerical calculations are performed by using finite element analysis.     

Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

Stress intensity factors for elliptical arc through cracks in mechanical joints by virtual crack closure technique

The reliable stress intensity factor analysis is required for fracture mechanics design or safety evaluation of mechanical joints at which cracks often initiate and grow. It has been reported that cracks in mechanical joints usually nucleate as corner cracks at the faying surface of joints and grow as elliptical arc through cracks, In this paper, three dimensional finite element analyses are performed for elliptical arc through cracks in mechanical joints. Thereafter stress intensity factors along elliptical crack front including two surface points are determined by the virtual crack closure technique. Virtual crack closure technique is a method to calculate stress intensity factor using the finite element analysis and can be applied to non-orthogonal mesh. As a result, the effects of clearance on the stress intensity factor are investigated and crack shape are then predicted.     

Subsurface crack mechanisms under indentation loading

A linear elastic fracture mechanics analysis of plane-strain indentation of a homogeneous half-space with a subsurface horizontal crack was performed using the finite element method. Stress intensity factor results obtained for an infinite plate with a central crack subjected to far-field tension and a half-space with a frictionless subsurface horizontal crack under a moving surface point load are shown to be in good agreement with corresponding analytical results. Crack mechanism maps illustrating the occurrence of separation, forward and backward slip, stick, and separation at the crack interface are presented for different indentation load positions and crack face friction coefficients. Results for the stresses in the vicinity of the crack tips and the mode I and mode II stress intensity factors are given for different indentation positions, crack face friction coefficients, and both concentrated and distributed surface normal tractions. Although indentation produces a predominantly shear and compressive stress field, mode I loading conditions are shown to occur for certain indentation positions. However, the magnitude of the mode I stress intensity factor is significantly smaller than that of mode II, suggesting that in-plane shear mode crack growth is most likely to occur in the absence of microstructural defects. The significance of crack face friction and sharpness of the indenter on the subsurface shear mode crack propagation rate is interpreted in terms of the mode II stress intensity factor range and material behavior.     

复合材料补片胶接补强修补技术参数分析

本文针对复合材料与金属结构穿透型损伤外贴复合材料补片胶接修补方式 ,采用“双板 -弹簧”有限元修正模型 ,进行了各修补参数 (补片直径、厚度、修补形式以及胶层厚度等 )对修补效率的影响分析。主要结论 :(1)补片直径对修补效率的影响非常小 ;(2 )单面修补取补片厚度为 5 0 %母板厚度 ,双面修补取 80 %是合理的 ;(3)胶层厚度通过影响胶层内最大剪应力大小影响修补效率 ;(4 )修补参数相同情况下 ,被修补结构的尺寸影响修补效率 ;(5 )金属修补与复合材料修补具有相同的曲线规律     

The singular stress field and stress intensity factors of a crack terminating at a bimaterial interface

For the fracture evaluation of inclined cracks terminating at the dissimilar material interface, not only the singularities, but also the detailed stress field and its stress intensity factors are necessary. However, though there are many researches reported on the singularity analysis, the stress field and its stress intensity factors are still not clear. This paper has deduced theoretically the singular stress and displacement fields near the tip of a crack terminating at the interface between bonded dissimilar materials, for both cases of real and oscillatory singularities. From the deduced singular stress field, the stress intensity factors are defined for such a crack, and the corresponding numerical extrapolation methods are also proposed. Through the numerical examinations, it is found that the theoretical stress distributions agree well with the numerical results obtained by the finite element method. Moreover, the proposed extrapolation method shows a good linearity, thus it can be used as an efficient way to determine the characteristics of the stress and displacement fields near the tip of a crack terminating at interface.     

A study on the determination of closing level for finite element analysis of fatigue crack closure

An elastic-plastic finite element analysis is performed to investigate detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using 4-node isoparametric elements can predict fatigue crack closure behavior. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. The crack opening level for the constant mesh size increases linearly from initial crack growth. The crack opening level for variable mesh size, is almost flat after crack tip has passed the monotonic plastic zone. The prediction of crack opening level using the variable mesh size proportioning the reversed plastic zone size with the opening stress intensity factors presents a good agreement with the experimental data regardless of stress ratios.     

某扭转梁扭转疲劳工况应力分析以及结构优化

针对某扭转梁在扭转疲劳试验过程中出现的裂纹现象,采用有限元法对扭转梁进行应力分析,结合计算结果给出优化改进措施,对该扭转梁的横梁加强板进行结构优化,横梁加强板开裂问题得到解决。     

双排交错铆钉连接对接结构裂纹尖端应力强度因子分析

双排交错铆钉连接对接结构是飞机进行损伤容限设计的重要结构件之一,其应力强度因子分析一直受到工程上的广泛关注。文中采用有限元方法,充分考虑不同开裂模式造成裂纹面上铆钉传力作用不同的特点,建立合理的铆钉传力模型,对双排交错铆钉对接结构进行裂尖应力强度因子分析,给出计算曲线。分析中讨论了裂纹垂直与平行于对接缝两种开裂模式以及单向与双向两种受载模式,并分析了铆钉特定传力作用下的柔度效应。所得计算结果及分析结论对飞机结构损伤容限设计具有直接参考价值。     

复合材料低模量蒙皮结构的损伤特性探讨

低模量蒙皮设计概念是改进复合材料结构损伤容限特性的一种有效的设计方法。它不仅可用于改善结构在拉伸、剪切载荷下的损伤容限特性 ,同时也可提高蒙皮的抗冲击损伤特性。本文主要对用低模量蒙皮设计概念设计的复合材料壁板在拉伸和剪切载荷作用下的损伤扩展特性、剩余强度及其设计方法进行探讨。     

Service life estimation of extrusion dies by numerical simulation of fatigue-crack-growth

In this study, fatigue behavior of cold extrusion dies is investigated analytically. Experimental studies and practical experience have shown that fatigue cracks occur at the inlet radius of die shoulders. The highest stress concentration is found also at the same location. For this reason, effective stress intensity factors at different locations of the die-shoulder inlet of a typical axisymmetric extrusion die have been calculated by using the finite element method. The crack growth has been simulated by applying the Paris/Erdoĝan fatigue law to the computed data. Finally, service life of the extrusion die has been estimated from the crack-growth-rate. Agreement of theoretical estimations with available data is found to be satisfactory within the limits of experimental uncertainties. Furthermore the experimental behavior of the crack growth (stable-unstable-stable growth with final fracture) is simulated correctly. It has been shown that the unexplained behavior is caused by decrease of stress concentration with increasing crack-length.     

Growth behavior of short surface fatigue cracks in 2 1/4 Cr-1 Mo steel

Fatigue tests by axial loading (R-0.05) were carried out to investigate short fatigue crack growth behavior in 2 1/4 Cr-1 Mo steel at room temperature using smooth and a small notched flat specimen. All the data of the fatigue crack growth rate in the present tests were analyzed as a function of the stress intensity factor equation in conjunction with crack closure behavior. Analysis was performed accounting for the relation of surface effective stress range,Ua and depth effective stress range,Ub. In the case of isotropic crack growth properties,Ub=(ΔK_ta/ΔK_tb) ·Ua. By use ofUb obtained from the analysis, crack growth rates to surface direction coincide with those of depth direction.     

Experimental and numerical determination of critical stress intensity factor of aluminum curved thin sheets under tensile stress

We determined the fracture toughness of aluminum curved thin sheets using tensile stress tests and finite element method. We applied Linear elastic fracture mechanics (LEFM) and Feddersen procedure to evaluate stress intensity factor of the samples with central wire-cut cracks and fatigue cracks with different lengths to investigate the notch radius effect. Special fixture design was utilized to establish uniform stress distribution at the crack zone. Less than 9 % difference was found between the wire-cut and the fatigue cracked samples. Since generating central fatigue crack with different lengths required so much effort, wire-cut cracked samples were used to determine critical stress intensity factor. Finite element analysis was also performed on one-quarter of the specimen using both the singular Borsum elements and the regular isoparametric elements to further investigate fracture toughness of the samples. It was observed that the singular elements presented better results than the isoparametric ones. A slight difference was also found between the results obtained from finite element method using singular elements and the experimental results.     

Optimisation Method for Determination of Crack Tip Position Based on Gauss-Newton Iterative Technique

In the digital image correlation research of fatigue crack growth rate,the accuracy of the crack tip position determines the accuracy of the calculation of the stress intensity factor,thereby affecting the life prediction.This paper proposes a Gauss-Newton iteration method for solving the crack tip position.The conventional linear fitting method provides an iterative initial solution for this method,and the preconditioned conjugate gradient method is used to solve the ill-conditioned matrix.A noise-added artificial displacement field is used to verify the feasibility of the method,which shows that all parameters can be solved with satisfactory results.The actual stress intensity factor solution case shows that the stress intensity factor value obtained by the method in this paper is very close to the finite element result,and the relative error between the two is only-0.621％;The Williams coefficient obtained by this method can also better define the contour of the plastic zone at the crack tip,and the maximum relative error with the test plastic zone area is-11.29％.The relative error between the contour of the plastic zone defined by the conventional method and the area of the experimental plastic zone reached a maximum of 26.05％.The crack tip coordinates,stress intensity factors,and plastic zone contour changes in the loading and unloading phases are explored.The results show that the crack tip change during the loading process is faster than the change during the unloading process;the stress intensity factor during the unloading process under the same load condition is larger than that during the loading process;under the same load,the theoretical plastic zone during the unloading process is higher than that during the loading process.     

残余应力对对焊接头疲劳性能的影响

利用线弹性断裂力学（ＬＥＦＭ）理论、迭加原理和有限元分析方法,就残余应力对对焊接头的疲劳性能的影响进行了理论分析。采用有限板中边界裂纹及中心穿透裂纹的Ｂｕｅｃｋｎｅｒ及Ｋａｎａｚａｗａ权函数,计算对应对焊接头中的各种残余应力分布的残余应力强度因子。建立了考虑不同残余应力水平影响的表面裂纹疲劳模型,估算对焊接头疲劳寿命及疲劳强度,并与试验数据进行比较验证。     

Repair of notched beam under dynamic load using piezoelectric patch

In this paper, the repair of a cracked beam under an external dynamic load employing the electro-mechanical characteristic of piezoelectric material to induce a local moment is presented. Conceptually, an external voltage is applied to actuate a piezoelectric patch bonded on the beam to effect closure of a crack so that the singularity at the crack tip under dynamic load may be decreased. Globally, this has the effect of altering the resonant frequency of the cracked beam towards that of the healthy beam, which is the criterion used for the repair. To demonstrate the repair methodology, a cantilever beam is used as an illustration, where the repair moment coefficient and the voltage required are mathematically derived. The relationship between repair moment coefficient, crack parameters and length of piezoelectric patch is investigated. The difference between the proposed repair criterion and an earlier published criterion for cracked beam under static load is also shown. A numerical example is used to study the effectiveness of the proposed repair methodology and its results are compared with those from 3-D finite element analyses using ABAQUS 6.4 as one means of verification.     

基于子模型与网格随移技术的叶轮三维裂纹应力强度因子分析

基于断裂力学理论和有限元数值分析方法,针对压气机叶轮由于铸造缺陷和疲劳引起的三维裂纹,应用子模型技术和ParaMesh网格随移技术,给出了压气机叶轮轴孔三维裂纹前沿应力强度因子的求解方法及途径,并对压气机叶轮轴孔三维裂纹的扩展方向和扩展速率进行了分析.压气机叶轮轴孔三维裂纹应力强度因子求解结果表明,叶轮轴孔三维裂纹前沿...     

不同铺层转角T700/E44 复合材料拉伸强度有限元分析

碳纤维复合材料的失效行为与复合材料内部的应力状态有关,不同铺层转角的单向碳纤维复合材料层合板的性能具有明显差异。文中利用HyperWorks 商用有限元软件建立了T700/E44 复合材料层合板拉伸模型,基于Chang-Chang 复合材料失效模型对不同铺层转角复合材料层合板的 X 向及 Y 向拉伸性能进行了数值模拟分析。研究结果表明,复合材料层合板以45° 铺层转角对称结构层合时,复合材料有着最佳的综合拉伸性能。这对高性能雷达中复合材料部件的铺层结构设计具有重要的指导意义。