基于多边形比例边界有限元的复合材料裂纹扩展模拟

该文采用近年提出的多边形比例边界有限元(Polygon Scaled Boundary Finite Elements,PSBFE),结合基于拓扑的局部网格重剖分方法,首次模拟了层状复合材料交界面、不同弹性模量的圆形夹杂对复合材料裂纹扩展的影响。结果表明,该文方法可以有效模拟复合材料的裂纹扩展,算例的结果同现有文献的实验数据和数值模拟结果吻合良好,采用不同网格密度和不同裂纹扩展步长对计算结果影响不大。基于SBFEM的PSBFE可以半解析求解裂纹尖端应力奇异性,具有比FEM更高的精度。另一方面,同现有基于SBFEM的裂纹扩展方法相比,基于拓扑的局部网格重剖分的PSBFE可以处理任意复杂的二维模型,具有更好的通用性。     

无网格法模拟复合型疲劳裂纹的扩展

本文提出了用无网格Galerkin法模拟构件在复合变形作用下疲劳裂纹扩展路径并预估其疲劳寿命的方法。该法能够自然模拟疲劳裂纹的扩展,不需要网格重构,避免了裂纹扩展过程中的精度受损。应用无网格数值结果计算了J积分和应力强度因子IK和IIK;按照最大周向应力理论获得了裂纹扩展偏斜角。基于最小应变能密度因子理论,确定了裂纹扩展量aD,并能获得疲劳载荷的循环周数ND。文末对数值模拟结果和实验拟合结果进行了对照。     

受压弯作用构件半椭圆表面裂纹的疲劳扩展分析

本文给出了半椭圆表面裂纹疲劳扩展的一种有限元仿真分析方法,并对潜艇锥柱结合壳焊趾处压弯联合交变载荷作用下的裂纹扩展进行了计算分析。该方法利用有限元分析计算裂纹前沿应力强度因子,采用Paris公式预测裂纹扩展速率及扩展量,对有限元网格随着裂纹的扩展进行自动重构,从而模拟分析裂纹的疲劳扩展过程。考察了两种初始尺寸半椭圆表面裂纹的情况,计算了裂纹尺寸、应力强度因子随裂纹扩展的变化历程。仿真计算结果表明,初始裂纹尺寸对于疲劳扩展的影响主要体现在初、中期的扩展上,对后期的扩展速率、裂纹形态影响不大;当考虑了焊接区应力集中效应后,扩展的速度提高,总的疲劳扩展寿命下降。本文方法和程序可用于其他较复杂构件表面裂纹的扩展分析。     

某型航空发动机篦齿盘裂纹的原位涡流检测

针对某型航空发动机篦齿盘上产生的裂纹缺陷,在不拆分发动机的前提下,提出采用涡流检测的方法对篦齿盘进行原位无损检测．设计完成了一套可用于篦齿盘裂纹原位检测的涡流无损检测系统．采用正交型锁相放大器对涡流检测信号进行处理,提高了信号检测精度同时达到了抑制干扰的作用．在标准检测试件和模拟试件上分别进行了试验,试验结果表明,该涡流检测系统可以实现航空发动机篦齿盘裂纹缺陷的原位检测,并且可以定性判断裂纹的深度．     

Cu/WCp叠层功能梯度材料疲劳裂纹扩展速率的数值模拟

本研究基于FRANC2D(Fracture Analysis Code in 2 Dimensions)二维断裂分析有限元软件,并结合大型有限元软件ABAQUS,对Cu/WC_p双层及多层功能梯度材料的疲劳裂纹扩展进行数值模拟研究,控制疲劳裂纹沿不同梯度方向扩展,计算出裂纹扩展中的应力强度因子幅(ΔK),绘制出疲劳裂纹扩展速率曲线,以及裂纹扩展速率和裂尖距界面距离l的关系曲线(da/dN-l),将模拟计算结果与试验结果进行对比分析。研究表明:结合FRANC2D和ABAQUS的数值模拟方法,在比较复杂的叠层功能梯度材料有限元模型建立中具有很大的优势,可以快速地计算功能梯度材料的应力强度因子值;发现材料梯度层间界面的存在以及材料梯度含量的变化,对功能梯度材料裂纹扩展的整个阶段都存在很大影响;梯度层的数量对功能梯度材料的疲劳裂纹扩展速率也有一定影响。     

软弱夹层对岩石爆破裂纹扩展影响的研究

为研究平行于炮孔的软弱夹层对岩石爆破裂纹扩展的影响，通过制作含不同软弱夹层的水泥砂浆试块进行爆破模拟试验，并用高速摄影仪对试块爆破裂纹扩展过程进行观测。同时，通过有限元分析软件ANSYS/LS-DYNA对含不同软弱夹层试块的爆破裂纹扩展起因进行数值模拟分析。结果表明：随着软弱夹层强度的降低，软弱夹层侧爆炸反射应力波强度逐渐增加、透射应力波强度逐渐降低，这使得软弱夹层侧裂纹扩展速度逐渐增加、裂纹扩展也越充分。同时，随着软弱夹层强度的降低，爆炸应力波在软弱夹层侧的衰减速度逐渐加快。试验结果与数值模拟结果具有较强的一致性，表明数值模拟对实践具有一定的指导意义。     

发展基于CB壳单元的扩展有限元模拟三维任意扩展裂纹

该文提出了一种新的基于连续体壳单元的扩展有限元格式,以用于对曲面上任意形状裂纹的扩展问题进行模拟。扩充形函数的构造和应力强度因子的计算都是基于三维实体单元进行,因此可以模拟复杂的三维断裂情况,壳体厚度的变化也可以得到考虑。三维应力强度因子的计算公式被引入到这种方法中。为模拟裂纹扩展,三维最大能量释放率准则被用作裂纹扩展准则。计算结果显示了曲面上的裂纹扩展路径可以与网格无关,并且由于在裂纹尖端的单元设置了具有奇异性的形函数,裂尖应力场被精确捕捉,从而证明了这种方法的优越性。     

基于水平集算法的扩展有限元方法研究

扩展有限元是一种以单位分解思想为基础,在常规有限元位移中加入跳跃函数和渐近位移场函数,以处理不连续问题的数值方法。将水平集算法应用到裂纹界面的描述及加强单元类型的判别,并与扩展有限元相结合,用于分析材料断裂问题。相比传统有限元,有限元网格与裂纹面位置相互独立,不需满足裂纹为单元边、裂尖为单元节点和在裂纹附近进行高密度的...     

修正GTN模型及其在预测硅钢冷轧边裂中的应用

考虑剪应变对微孔洞损伤演化的影响, 对GTN损伤模型的损伤演化机制进行修正, 建立了适用于不同应力三轴度水平的损伤模型. 结合隐式应力更新算法和显式有限元计算, 采用VUMAT子程序实现了修正GTN模型在有限元软件ABAQUS中的数值计算. 通过模拟纯剪切和剪切-拉伸两组试样的损伤演化和断裂行为, 验证了修正GTN模型在不同应力三轴度承载条件下的有效性. 运用修正GTN损伤模型模拟含边部缺口的带钢在轧制过程中裂纹的萌生和扩展行为, 模拟结果与实验相一致, 表明该模型可有效地用于带钢缺陷在轧制过程中扩展行为的分析和预测. 模拟和实验结果表明, 带钢边部缺口在轧制过程中, 缺口前沿和后沿均会萌生裂纹, 且后沿裂纹扩展更为明显.     

压缩载荷下闭合裂纹的曲线分支裂纹模型研究

目前远场压应力作用下分支裂纹扩展过程中,其应力强度因子的计算模型中,经常将曲线分支裂纹简化为直线分支裂纹处理,忽略了分支裂纹扩展路径的曲线效应。基于这个考虑,在远场压应力作用下曲线分支裂纹扩展路径数学描述的基础上,建立了曲线分支裂纹模型,该模型可以考虑分支裂纹扩展过程中的曲线效应,进而计算出了曲线分支裂纹扩展过程中应力强度因子。基于ABQUS二次开发实现了对裂纹扩展过程的数值模拟,并将数值计算得出的应力强度因子与所建立的曲线分支裂纹模型解进行对比分析,验证了该模型的可行性,同时将曲线分支裂纹模型解与原有的直线分支裂纹模型解进行了对比,表明了建立曲线分支裂纹模型的必要性。     

Examination of the fatigue crack growth equations

Most of the crack growth equations proposed so far correlate the crack growth rate (da/dN or da/dt) with crack tip parameters such as the stress intensity factor (SIF) or energy release rate (ERR). In our previous works, an experimental setup was designed to examine the applicability and the boundary of the functional relationship between da/dN and the crack tip parameters, particularly, ERR. In the present paper, the variation of the ERR along the experimentally observed curvilinear crack trajectories is obtained by means of the finite element method. The analysis shows that the Paris-Erdogan type of laws are applicable until the crack tip is located outside the strong crack-defect interaction region (SI region). A functional relationship between da/dN and ERR breaks down within this region. This suggests the existence of additional crack tip parameters that are not accounted for within conventional fracture mechanics. An approach to modeling the observed phenomenon is discussed following the concept of the Crack Layer theory.     

A study of the propagation of an embedded crack in a composite laminate of finite thickness

In this paper, the mechanism for the propagation of a crack embedded in the central layer of a composite laminate is studied based on energy criteria. As the crack may propagate in the transverse direction or in the tunneling direction, the critical stresses for these two propagations are presented. The theoretical analysis is applied to a fibre-reinforced composite laminate. Critical stresses for the initiation of crack propagation are calculated taking into account the influence of the adjacent ply angle and the crack size. It is found that for the considered typical fibre-reinforced composite laminate, the critical stress for the transverse propagation is less than that for the tunneling propagation. Therefore, transverse cracking is a more possible fracture mode in composite laminates with initial crack-like defects. This finding supports the calculation of the in situ transverse strength based upon the transverse crack propagation.     

Characteristics of fatigue crack propagation behaviour as identified by hysteresis loop at the crack tip

Fatigue crack propagation tests have been carried out under various load conditions. Hysteresis loops denoting the relationship between load and strain at the crack tip are obtained by using local compliance measurement. Crack growth acceleration, delayed retardation and non‐propagation phenomena are investigated by considering the variation of hysteresis loop expansion and hysteresis loop tail. Based on the physical meaning of hysteresis loops, two types of crack closure are ascertained and the effect of crack closure on fatigue crack propagation is studied. Results show that change of the effective amplitude of the stress intensity factor at the crack tip is the reason that crack propagation rates vary.     

A model for evaluating the effect of fatigue crack repair by the infiltration method

Infiltration of foreign materials into a fatigue crack has previously been shown to be able to retard the crack and extend fatigue life. Most of the related studies were empirical and phenomenological in nature. To aid engineering decisions, it would be advantageous if the possible outcome of a repair can be evaluated beforehand. To this end, a crack closure model taking into account the additional closure effect of the infiltrant has been developed and verified against experimental results. With this model, the sensitivity of the repair effect to various parameters such as mechanical properties of the infiltrant, depth of penetration and infiltration load level can be assessed.     

Experimental investigations on the growth of small fatigue cracks in naval steel

The concept of damage tolerance is now largely employed to evaluate the fatigue life of structures. However, part of this fatigue relies on the initiation and growth of small cracks. The fatigue behaviour of a naval structural steel (S355NL) was investigated. In order to characterize the behaviour of short and long cracks, tests were performed under constant amplitude loading for several load ratios between –1.0 and 0.5. A major part of fatigue life is constituted by short crack initiation and propagation.     

A numerical study on the effect of symmetric crack flank holes on fatigue life extension of a SENT specimen

The crack growth retardation is studied theoretically by drilling two symmetric holes at appropriate locations along the crack flanks. A fatigue crack growth code for two‐dimensional elastic problems is developed to approve the effectiveness of this method. The crack growth retardation was examined using a parametric study both on the arrangements of the flank holes and the diameters of holes. The numerical results reveal that for some cases, the presence of flank holes significantly decreases the stress concentration around the crack tip and its stress intensity factor. The best positions for crack flank holes to provide the highest crack growth retardation are shown to be where the line connecting the two centres of holes passes the crack tip. A larger hole diameter and a closer distance between the flank holes and the crack tip result in lower crack growth rate and higher fatigue life.     

残余应力场中疲劳裂纹闭合模型的扩充

在残余应力场中一般采用残余应力强度因子法预测疲劳裂纹的扩展,但它未考虑残余压应力对裂纹的闭合效应。本文以八十年代初J、C、Newman提出的疲劳裂纹闭合模型为基础,加以修正和扩充使之适用于单边裂纹体,并在原有模型的裂纹面接触应力上迭加残余应力参量。通过实验测定了40Cr钢高温回火态试样在有和无残余应力场时的裂纹闭合力和扩展速率,结果表明,计算值与实验值基本一致。扩充模型尚存在些问题有待改进。     

An improved crack tracking algorithm with self-correction ability of the crack path and its application in a continuum damage model

In this paper, a new procedure for predicting the crack propagation and fracture behavior in quasi-brittle materials is presented based on continuum damage mechanics. Several crack tracking algorithms are widely used for failure analysis, among which the local tracking algorithm is very simple and easy to implement with low computational cost. However, in the prediction of crack growth with the traditional local tracking algorithm, it can be often seen that the sharp changes in the crack path such as U-turns can occur. An improved local tracking algorithm with self-correction ability of the crack path is proposed to overcome these difficulties of the traditional crack tracking algorithms. A continuum damage model, in addition to the present crack tracking algorithm, can greatly enhance the performance of failure prediction in quasi-brittle materials. The present approach has the advantages that it can well predict the crack propagation path and can avoid mesh size and mesh bias dependence without the embedment of discontinuities or nodal enrichment. The present model is incorporated into ANSYS by the ANSYS Parameter Design Language to simulate the initiation and propagation of the discrete crack in engineering applications. The numerical results by this model are compared with the ones by the extended finite element method and experiments, and good agreements are achieved.     

Fatigue crack closure: a review of the physical phenomena

Plasticity‐induced, roughness‐induced and oxide‐induced crack closures are reviewed. Special attention is devoted to the physical origin, the consequences for the experimental determination and the prediction of the effective crack driving force for fatigue crack propagation. Plasticity‐induced crack closure under plane stress and plane strain conditions require, in principle, a different explanation; however, both types are predictable. This is even the case in the transition region from the plane strain to the plane stress state and all types of loading conditions including constant and variable amplitude loading, the short crack case or the transition from small‐scale to large‐scale yielding. In contrast, the prediction of roughness‐induced and oxide‐induced closures is not as straightforward.     

Characterization of the durability of adhesive bonds

The deterioration of adhesive bonds as a result of exposure to aggressive service environments is a problem for both metallic and composite bonded joints in aircraft structures. In this context airworthiness issues arising from the in service growth of small naturally occurring defects in adhesively bonded repairs has led to the need to develop a fracture mechanics based approach for setting the inspection intervals associated with the growth of such naturally occurring defects. This involves issues associated with both durability and damage tolerance that are associated with such small defects because it is well known that, for metals, the use of long crack data can lead to erroneous non‐conservative estimates. As such a means for assessing the issues related to such small naturally occurring defects is urgently needed. In this context the Boeing Wedge test, and its related variants, plays a central role in not only qualitatively assessing environmental performance but also in ranking surface treatments. However, this method has the problem that the initial defect introduced during wedge testing may not be representative of the small initial defects that can arise in service. Thus the relative ranking of various surface treatments obtained using wedge tests may be misleading. To this end, the paper presents a simple fracture mechanics based method that has the potential to account for the discrepancy in the initial defect size. This approach resembles crack growth laws developed to account for the growth of both long and short cracks in metals, environmentally enhanced crack growth in metals and delamination growth in composites.